Photo-electrochemical device comprising an electron storage material, use of an electron storage material or of a device comprising it, method for photocatalytic production of a product, autophotorechargable battery, method for detecting oxygen, method for detecting light, and method for detecting an analyte

Abstract

The present invention relates to photo-Elecerochemical devices comprising an electron storage material (ESM), the use of an electron storage material or of a device comprising it, a method for the photocatalytic production of a product, an autophotorechargable Battery, a method for detecting oxygen, a method for detecting light, and a method for detecting an Analyte. The ESM has a structure containing oxygen and at least one selected from the transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides, and is capable of producing electron-hole pairs when irradiated by light. The thereby generated electrons can be stored in the ESM and can be extracted after irradiation has stopped.

Description

Photo-Electrochemical Device comprising an Electron Storage Material, Use of an Electron Storage Material or of a device comprising it, Method for photocatalytic production of a product, Autophotorechargable Battery, Method for detecting oxygen, Method for detecting Light, and Method for detecting an Analyte

[0001] The present invention relates to the utilization of solar energy using an Electron Storage Material (in the following referred to as ESM), and specifically to a photo-electrochemical device comprising an ESM, the use of an ESM or of a device comprising it, a method for the photocatalytic production of a product, an auto-photorechargable battery, a method for detecting oxygen, a method for detecting light, and a method for detecting an analyte.Background of the Invention

[0002] There is a high interest in the utilization of solar power, in particular as a replacement of fossil fuels in order to reduce carbon dioxide emissions to fight global warming. Over the last decades, attempts have been made to utilize solar power not only as a source of heat but also as a source of electric power, the most prominent being solar cells relying mainly on silicon materials as semiconductor, converting light into electricity.

[0003] There are however certain applications and devices where electricity cannot be used or is associated with certain limitations, so that the generation of electricity alone is not sufficient to reduce or eliminate carbon dioxide emissions. For example, in steel production, carbon (coke) is used as a reducing agent, generating large amounts of carbon dioxide, and here hydrogen gas can be used as a replacement.

[0004] Some devices and apparatuses are also difficult to electrify, such as airplanes, and for these the use of so-called "e-fuels" is contemplated. Such e-fuels are synthesized from educts including hydrogen. Hydrogen may also directly be used as power source by combustion, as a reducing agent in manufacturing processes, or as fuel in in fuel cells.

[0005] It is thus clear that besides the generation of electricity from renewable sources, also hydrogen is key in the transformation to a climate-friendly and sustainable society. Carbon dioxide emissions are only reduced if the hydrogen is produced without emitting carbon dioxide, such as by electrolysis of water employing electricity obtained from solar cells ("green hydrogen").

[0006] Current problems of solar cells include that they generate electricity only while irradiated, that their output varies significantly depending on season, daytime and weather conditions, and that they are unable to store the solar energy. Modern society however requires energy not only during daytime and in varying amounts on demands. Energy storage is thus essential.

[0007] Storage of electric energy via secondary batteries or in converted forms from such as by pumped hydroelectric energy storage requires large capacities and is always associated with conversion losses. Also, battery production causes environmental loads, and batteries suffer from performance degradation over time, limited cycle life and limited storage time. A conceivable solution to these problems is to store the energy in a form where it can be readily transformed into either electricity of hydrogen, or in the form of green hydrogen.

[0008] Nature has solved some of these problems by decoupling the absorption of light, the generation of energy thereby and the energy storage into separate reactions in photosynthesis. Photosynthesis is a complex process by which plants convert solar (light) energy into chemical energy stored in glucose. This process occurs in two main stages: the light reaction and the dark reaction (also known as the Calvin cycle or light-independent reaction). During the light reaction, NADPH is formed by a sequence of steps, and the NADPH then provides the energy for the dark reaction. Also, light and dark reactions occur at different locations in the plant. Thereby, the conversion from solar energy to chemical energy is affected. However, the dark reaction only occurs upon supply of ATP / NADPH, so that the light and dark reactions in photosynthesis are not fully decoupled in time and space.

[0009] It would hence be highly desirable to provide novel devices and materials that are not only capable of converting electromagnetic radiation (i.e., light, mainly in the visible and UV spectrum) into electricity, but which also are able to store the irradiation energy such as to produce electricity after irradiation has stopped, thereby decoupling the irradiation step from the production of electricity (thereby providing for a "solar battery"). A further advantage would be given if these devices were capable of converting the stored solar energy into chemical energy, preferably on demand and in a time-delayed or time-decoupled manner relative to thelight irradiation ("solar battolyzer", combining a solar battery with an electrolyzer), such as by generating hydrogen by electrolysis, most preferably upon demand.

[0010] In the more recent past, certain materials were developed that are capable of decoupling irradiation and electricity and / or hydrogen generation on a time scale by forming long-lasting photoreduced states.

[0011] The first demonstration of a photo-electrochemical reaction combining solar energy conversion and storage at a single device level is reported back in 1976 by Cahen and co-workers who studied polycrystalline chalcogenide electrodes in S / S2- electrolyte (Nature 1976, 261, 403).

[0012] H. Tributsch, Applied Physics 1980, 23, 61-71, discusses the concept of photo intercalation and its possible application in solar energy devices. More particularly, these publications describe the potential use of ZrSe2 and other metal compounds for converting and simultaneously storing solar energy by means of light driven electrochemical reactions producing intercalation compounds of layer-type semiconducting material. G. Betz and H. Tributsch, Progress in Solid State Chemistry 1985, 16, 195-290, lists further metal compounds as potential materials for such use.

[0013] Y. Arora et al., Scientific Reports 2018, 8, Article No. 12752, describe studies with the aim of developing a solar battery. In this work, solar energy capture and storage are coupled using a single bi-functional material. The electroactive semiconductors BiVO4 (n-type) and CO3O4 (p- type) have been separately evaluated for their energy storage capability in the presence and absence of visible radiation. Each of these materials is described to function as a light harvester and to have faradaic capability. The authors describe an enhancement of ca. 30% of the discharge capacity of BiVO4 in the presence of light.

[0014] S. N. Lou et al., Advanced Energy Materials 2017, 7, 1700545 relates to a solarintercalation battery, which is able to both harvest and store solar energy within an electrode. More particularly, the authors describe a solar-rechargeable sodium-ion intercalation battery derived from a stand-alone MOO3 photoanode that possesses the dual functionalities of solar energy harvesting and energy storage. MOO3 is found to transform, via a two-phase reaction mechanism, initially into a sodium bronze phase, Nag 33MOO3, followed by the formation of solid solutions, NaxMoO3 (0.33 < x < 1.1), on further photointercalation.

[0015] V. W. Lau et al describe in Angew. Chem. Int. Edition 2017 (56) 510-514 the dark photocatalysis of solar energy in carbon nitride for time-delayed hydrogen generation. Here, it is described that upon solar irradiation highly reductive radicals are formed within a cyanamide-functionalized polymeric network of heptazine units in the presence of an electron donor. The radicals have a lifetime that allows to yield molecular hydrogen in the dark, enabling a temporal decoupling of the light and dark reaction of the photocatalytic hydrogen production. The photocatalyst used in this study is a cyanamide-functionalized heptazine-based polymer obtained by ionothermal treatment of melon (also known as graphitic carbon nitride) with KSCN. The polymer changes colour from yellow to blue when irradiated in the presence of certain electron donors in an oxygen-free environment. The light-induced transient species has a lifetime from tens of minutes to hours after cessation of irradiation. The authors attribute the radicals to a polymer of heptazine units with cyanamide side groups. The blue state is capable of reducing protons to molecular hydrogen by a hydrogen evolution reaction (HER) in the presence of platinum as a catalyst, thereby allowing "dark" photocatalysis for hydrogen generation. This blue state is not observed for melon alone or in the heptazine polymer alone in a control experiment, showing that the long-lived excited state is caused by an interaction between heptazine polymer and melon. The HER can be repeated over 15 cycles, but a decreasing amount of dark hydrogen formed.

[0016] A related study was described by Podjaski et al in Adv. Mater. 2018 (30) 1705477. Here, 2D cyanamide-functionalized polyheptazine imide (NCN-PHI) is described, which is said to allow for the first time the synergistic coupling of two key functions of energy conversion within one single material: light harvesting and electrical energy storage. This "solar battery" material relies on charge storage based on photoreduction of the carbon nitride backbone and charge compensation by adsorption of (additional) alkali metal ions within the NCN-PHI layers. The photoreduced carbon nitride is described as a solar battery anode operating as a pseudocapacitor, which can store light-induced charge in the form of long-lived "trapped" electrons for hours. The authors conclude that the feasibility of light-induced electrical energy storage and release on demand by a one-component light-charged battery anode is demonstrated, which provides a sustainable solution to overcome the intermittency of solar radiation.

[0017] Even though the results reported by Lau and Podjaski are significant, there are certain limitations and drawbacks. The need to rely on polyheptazine imide (polymer) may reduce the chemical robustness of the system and its stability towards irradiation, as even though polyheptazine imide is known as a photocatalyst, polyheptazine imide salts are sometimes described as exhibiting photocatalytic instability, even though their stability is quite high for a polymer. The observed decay in productivity after 15 cycles of dark hydrogen generation also casts doubts on the suitability for long-term applications. Further limitations include poor electronic conductivity, a relatively high self-discharge rate, limited photoelectron stability in the dark, and a lack of functionality and functionalizability of carbon nitrides in general: PHI is alsodifficult to functionalize through organic synthesis and to process by liquid, solid state or gas phase techniques.

[0018] WO 2020 / 143912 Al describes an electrochemical device, comprising a negative electrode comprising a nitrogen-containing electron storage material, a positive electrode, and an electrolyte, wherein the nitrogen-containing electron storage material has a two-dimensional or a three-dimensional covalent structure, contains heptazine and / or triazine moieties, and is capable of intercalating and de-intercalating cations. In one embodiment, the nitrogencontaining electron storage material comprises up to 40 wt.-% of at least one member selected from the list consisting of S, B, P, O, I, F, Cl, Se, Te, Si, Ge, As, Sb, alkali cations, alkaline earth cations and transition metal cations. Further described is a photorechargeable battery comprising a negative electrode comprising a substrate with a surface and a layer comprising a nitrogen-containing electron storage material provided on the surface of the substrate, and a positive electrode comprising a substrate with a surface and a layer comprising a hole storage material provided on the surface of the substrate, and a photovoltaic element sandwiched between the layer of the nitrogen-containing electron storage material and the layer of the hole storage material, wherein the photovoltaic element is capable of charging the electrodes upon illumination, and wherein the nitrogen-containing electron storage material has a two- dimensional or a three-dimensional covalent structure, contains heptazine and / or triazine moieties, and is capable of intercalating and de-intercalating cations. Further described is an autophotorechargeable battery comprising a negative electrode comprising a substrate with a surface and a layer comprising a nitrogen-containing electron storage material provided on the surface of the substrate, a positive electrode comprising a substrate with a surface, and a layer comprising a hole storage material provided on the surface of the substrate, and an electrolyte, wherein the nitrogen-containing electron storage material has a two-dimensional or a three- dimensional covalent structure, is capable of intercalating and de-intercalating cations, and has a band gap in the range of 0.5 to 3.5 eV. Also described are methods for harvesting and storing light, for detecting or removing oxygen, and for detecting light. The document also describes experiments relating to hydrogen evolution after irradiation.

[0019] In another aspect, it is also known that certain polymorphs of TiO2 are capable of photocatalytically producing molecular hydrogen in e.g. a water-methanol medium using a platinum catalyst, see e.g. P. Afanasiev, Appl. Cat. A, General, 598 (2020) 117548. In this study, it was also shown that hydrogen can be generated in the dark upon addition of Pt / TiO2 suspension to previously illuminated TiO2-

[0020] C. Andriamiadamanana et al describe in Adv. Sustainable Systems 2018, 2, 1700166 the light-induced charge separation in LiTiOxnanoparticles, which is described as an attempt toprovide for solar-rechargeable lithium ion batteries. By bandgap excitation, a quantitative Li-ion de-insertion reaction can be caused by free holes generated. It is described that halfelectrochemical cells containing these mesoporous lithiated TiO2 can be fully oxidized in 1 h of light exposure.

[0021] One disadvantage of this system is that it relates to lithium ion batteries, which are associated with an environmental burden due to the use of potentially critical raw materials such as lithium. It is based on the concept of lithium de-insertion, and requires wavelengths of below 370 nm (bandgap of 3.30 eV). Upon absorption, the electrode gives rise to the formation of electron-hole pairs, and because the particles are of nanometer scale, the generated photocarriers accelerate the kinetics of electrochemical electron / ion transfer. The generated photoelectrons are scavenged by the electrolyte when associated to a metallic Li electrode but can be collected through an external circuit by the development of an adequate host counter electrode having a conduction band lying below the one of the discharged Li TiC>2 determined at =-4.79 eV vs vacuum.

[0022] Thus, electrical energy is stored in this system by lithium de-insertion, forming a photocathode storing holes (not electrons). Further, the shape of the material in the form of nanoparticles imposes certain limitations on the device design and their utilization, and the system relies on an organic LiPFg EC / DMC electrolyte which leaves room for improvements in terms of stability, safety and environment friendliness. Further drawbacks of the LixTiO2 system a 3D structure which can only take up and release substantial amounts of Li if it is nanostructured; a low photocurrent, and low cycle stability, i.e. a decrease in 60% of capacity after 10 cycles. Additional challenges and limitations arise from a long photocharging time up to 1 h to form fully oxidized TiC>2, and a poor dark stability.

[0023] A. Paolella et al report in Nat Commun 8: 14643 (2017) the direct photo-oxidation of lithium iron phosphate nanocrystals in the presence of a dye as a hybrid photo-cathode in a two- electrode system, with lithium metal as anode and lithium hexafluorophosphate in carbonate- based electrolyte; a configuration corresponding to lithium ion battery charging. Here, dyesensitization generates electron-hole pairs with the holes aiding the delithiation of lithium iron phosphate at the cathode and electrons utilized in the formation of a solid electrolyte interface at the anode via oxygen reduction. Lithium iron phosphate acts effectively as a reversible redox agent for the regeneration of the dye. A drawback of this system is the long photocharging time of up to 1.5 days to fully charge the electrode, limiting its practical applications, and the need to rely on dye for light absorption.

[0024] W. Yang et al describe the post-illumination activity of E^WOg in the dark from the photocatalytic "memory" effect (see W. Yang es al, Journal of Advanced Ceramics 2021 10(2): 355-367). The authors state that due to the dual requirements of light absorption and energy storage / release functions, most previously reported photocatalysts with the photocatalytic "memory" effect were composite photocatalysts of two phase components, which may lose their performance due to gradually deteriorated interface conditions during their applications. The pure phase E^WOg was found to possess the photocatalytic "memory" effect through the trapping and release of photogenerated electrons by the reversible chemical state change of W component in the (WO^- layers. When the illumination was switched off, E^WOg microspheres continuously produced H2O2 in the dark as those trapped photogenerated electrons were gradually released to react with O2 through the two-electron O2 reduction process, resulting in the continuous disinfection of Escherichia coli bacteria in the dark through the photocatalytic "memory" effect. However, this system does not allow control of the release of photoelectrons, the photopotential is low at -0.065V (vs Ag / AgCI), and the charge stability in the dark after illumination is poor.

[0025] S. Amthor et al describe in Nature Chemistry Vol. 14, March 2022, 321-327 a fully integrated photochemical molecular dyad composed of a ruthenium-complex photosensitizer covalently linked to a Dawson polyoxometalate that acts as an electron-storage site and hydrogen-evolving catalyst. Visible-light irradiation of the system in solution leads to charge separation and electron storage on the polyoxometalate, effectively resulting in a liquid fuel. In contrast to related, earlier dyads, this system enables the harvesting, storage and delayed release of solar energy. On demand hydrogen release is possible by adding a proton donor to the dyad solution. The PS-POM dyad is formed by reaction of a phosphonate-functionalized photosensitzier la with a lacuna

[0026] Here, in the light reaction, a covalent PS-POM dyad is chemically reduced under irradiation (electron storage step). In the dark reaction, the reduced PS-POM dyad can be reacted with protons in solution, leading to the light-independent, delayed release of solar hydrogen. This enables the temporal and spatial decoupling of light harvesting, chargeseparation, charge storage and hydrogen release. However, this system cannot be used in aqueous / protic media, as this would trigger an immediate proton reduction. Control of this system for "on-demand" hydrogen evolution is thus difficult. Further, this system requires a dedicated dye sensitizer. It is therefore not a simultaneously light absorbing and charge storing system.

[0027] As derivable from the above, several publications describe the separation of the light and the dark reactions, wherein the light reaction leads to formation of photoreduced electron / photooxidized hole states having sufficient longevity to allow subsequent release to cause e.g. hydrogen formation by water electrolysis / splitting. However, the documents often do not describe a full photo-electrochemical device but only a half-cell. Also, several of the documents relate to materials that are either complex to prepare, cannot be appropriately functionalized, cannot be processed adequately, such as by exfoliation and re-stacking into robust and smooth films, or are two-component systems which require an additional photosensitizer, or which require organic components that may be more prone to decay and stability problems than purely inorganic materials, or which may not possess the required long-term stability required for outdoor installations (similar to solar cells). The systems described above often lack high photostability and chemical stability, long charge trapping stability (i.e. show a shorter lifetime of photoreduced / oxidized states), a desirably high photopotential and adequate cycling stability.

[0028] There is thus a need for novel devices, materials, and methods, which preferably overcome one or more of the disadvantages of the prior art.Summary of the InventionThe present invention includes the following embodiments:1. A photo-electrochemical device, comprising a first electrode containing an electron storage material, ESM, wherein the ESM has a structure containing oxygen and at least one selected from the transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides, and wherein the ESM is capable of producing electron-hole pairs when irradiated by light; a second electrode, optionally containing a reduction reaction catalyst-containing layer; an electrolyte containing a to-be-oxidized reactant and a to-be-reduced reactant, the electrolyte being in contact with the ESM and the second electrode, preferably the reduction reaction catalyst-containing layer if present; andan electrical connection between the first electrode and the second electrode for transferring electrons from the first to the second electrode.2. A photo-electrochemical device, comprising a first electrode containing an electron storage material, ESM, wherein the ESM has a structure containing oxygen and at least one selected from the transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides, and wherein the ESM is capable of producing electron-hole pairs when irradiated by light; an oxidation reaction catalyst-containing layer, arranged to receive holes from the ESM; a second electrode, optionally containing a reduction reaction catalyst-containing layer; and an electrolyte containing a to-be-oxidized reactant and a to-be-reduced reactant, the electrolyte being in contact with the oxidation reaction catalyst-containing layer and the second electrode, preferably with the reduction reaction catalyst-containing layer if present; an electrical connection between the first electrode and the second electrode for transferring electrons from the first to the second electrode.3. The photo-electrochemical device according to embodiment 0 or 2, wherein the electrical connection comprises a potentiostat configured to amplify a first voltage between the first electrode and the second electrode.4. The photo-electrochemical device according to any one of embodiments 1 to 3, wherein the electrical connection comprises a switching device for electrically connecting or disconnecting the first electrode and the second electrode, and / or an electrical load.5. The photo-electrochemical device according to any one of embodiments 1 to 4, further comprising: a layer containing a hole transport material, HTM, provided between the first electrode and the oxidation reaction catalyst-containing layer, the HTM-containing layer being arranged to transfer holes from the first electrode to the oxidation reaction catalyst-containing layer.6. The photo-electrochemical device according to embodiment 5, further comprising: a layer containing a hole storage material, HSM, the HSM-containing layer being arranged to receive holes from the HTM-containing layer.7. The photo-electrochemical device according to embodiment 6, further comprising: a voltage sensor configured to measure a second voltage between the first electrode and the HSM-containing layer,wherein the switching device is configured to be in a closed state when the second voltage is above a predetermined threshold value.8. The photo-electrochemical device according to any one of embodiments Fehler! Verweisquelle konnte nicht gefunden werden. to 7, wherein the first electrode further contains a transparent conductive material electrically connected to the electrical connection.9. The photo-electrochemical device according to embodiment 8, wherein the transparent conductive material is a fluorine-doped tin oxide, FTO, indium-doped tin oxide, ITO, or graphene.10. The photo-electrochemical device according to embodiment 8 or 9, wherein the transparent conductive material is formed as a thin film and the ESM is provided on a surface of the film.11. A photo-electrochemical device, comprising: a layer containing an electron storage material, ESM, wherein the ESM has a structure containing oxygen and at least one selected from the transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides, and wherein the ESM is capable of producing electron-hole pairs when irradiated by light; a reduction reaction catalyst-containing layer, arranged to receive electrons from the ESM- containing layer; a layer containing a hole storage material, HSM; an oxidation reaction catalyst-containing layer, arranged to receive holes from the HSM; a layer containing a hole transport material, HTM, and provided between the ESM- containing layer and the HSM-containing layer, the HTM-containing layer being arranged to transfer holes from the ESM-containing layer to the HSM-containing layer; and an electrolyte containing a to-be-oxidized reactant and a to-be-reduced reactant, the electrolyte being in contact with the oxidation reaction catalyst-containing layer and the reduction reaction catalyst-containing layer.12. The photo-electrochemical device according to embodiment 11, further comprising: an electrical connection electrically connecting the ESM-containing layer and the HSM- containing layer, wherein the electrical connection comprises an electrical load.13. The photo-electrochemical device according to embodiment 5 or 11, wherein the HTM is a conducting material, preferably a conducting polymer.14. The photo-electrochemical device according to any one of embodiments 1 to 13, wherein the electrolyte is a liquid electrolyte.15. The photo-electrochemical device according to embodiment 14, wherein the liquid electrolyte is water or an aqueous medium.16. The photo-electrochemical device according to embodiment 6 or 11, wherein the HSM contains a conducting polymer, a metal, a semiconductor or a mixture thereof.17. The photo-electrochemical device according to any one of the preceding embodiments, wherein the ESM is not MOO3.18. The photo-electrochemical device according to any one of the preceding embodiments, wherein the ESM is not TiC>2 or lithiated TiC>2, i-e., LixTiC>2 (with 0<x<l), and / or wherein the ESM is not TiC>2 or lithiated TiC>2, i-e., LixTiC>2 (with 0<x<l) wherein the TiC>2 is of anatase and / or rutile phase.19. The photo-electrochemical device according to any one of the preceding embodiments, wherein the ESM is an oxide, oxynitride or oxysulfide of the at least one of the transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides.20. The photo-electrochemical device according to any one of the preceding embodiments, wherein the ESM is an oxide of at least one transition metal in Groups 4, 5, 6 and 7 of the Periodic Table of the Elements or of cerium.21. The photo-electrochemical device according to any one of the preceding embodiments, wherein the structure of the ESM is composed of oxygen and two or more selected from transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides, wherein the structure of the material is composed of oxygen and one transition metal selected from Zr, Hf, V, Ta, Nb, Cr, W, Mn and Re, or wherein the ESM is CeC>2, and wherein the ESM material that is composed of oxygen and one transition metal selected from Zr, Hf, V, Ta, Nb, Cr, W, Mn and Re or which is CeC>2 is optionally doped with one or more other metals, such as one, two or three of selected from the lanthanides and transition metal elements, e.g. one, two, three or more selected from La, Y, Sm, Yb, Sc, Mg, Ca, Co, Ni, Zr, Ti, Hf, Eu, Gd, Pr and Sn, such as Y, La, Zr, and Sc.22. The photo-electrochemical device according to any one of the preceding embodiments, wherein the at least one selected from transition metals and lanthanides forming the ESM includes at least one of Nb and W, and preferably includes Nb.23. The photo-electrochemical device according to any one of the preceding embodiments, wherein the optical absorption spectrum of the ESM in the range of 400 to 700 nm changes upon illuminating the ESM, which change is preferably an increase in absorbance by at least 5%, and which change is further preferably maintained for at least 15 minutes after the illumination has stopped.24. The photo-electrochemical device according to any one of the preceding embodiments, wherein the ESM has and / or is able to form color centers.25. The photo-electrochemical device according to embodiment 24, wherein the color centers are atomic scale imperfections in the ESM selected from the group consisting of impurities, vacant lattice sites, interstitial atoms and ions or combinations thereof.26. The photo-electrochemical device according to any one of the preceding embodiments, wherein the ESM is an oxide of the at least one of transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides, and further comprises a cation of a main group metal of the Periodic Table of the Elements.27. The photo-electrochemical device according to embodiment 26, wherein the at least one selected from transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides has a d^ electronic configuration and the main group metal cation has an electronic s^d^O configuration.28. The photo-electrochemical device according to embodiment 27, wherein the main group metal cation is Sn^+or Bi^+, preferably Sn^+.29. The photo-electrochemical device according to any one of embodiments 26 to 28, wherein the ESM exhibits a bandgap which is the difference in the energy levels between the conduction band minimum and the valence band maximum, wherein the conduction band minimum is formed by a d^ electronic configuration of the at least one selected from the transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides, and wherein the valence band maximum is formed by hybridized main group metal s^ and oxygen 2p orbitals.30. The photo-electrochemical device according to any one of the preceding embodiments, wherein the ESM has or is capable of forming oxygen vacancies in the structure.31. The photo-electrochemical device according to embodiment 30, wherein upon illumination with light oxygen vacancies are formed in the structure of the ESM; preferably upon irradiation at 340 nm at 20 mW / crn^ for 1 minute.32. The photo-electrochemical device according to any one of the preceding embodiments, wherein the ESM has a bandgap in the visible or near UV range.33. The photo-electrochemical device according to any one of the preceding embodiments, wherein the photoelectrons are trapped in the material locally in the form of polarons and are as such stored in the ESM, wherein the energy level of the polarons is below the conduction band minimum and above the valence band maximum of the ESM.34. The photo-electrochemical device according to any one of the preceding embodiments, wherein the photoelectrons have, under oxygen-free conditions, a lifetime of at least 4 hours, preferably at least 8 hours and more preferably at least 12 hours, in the ESM.35. The photo-electrochemical device according to any one of the preceding embodiments, wherein the ESM is a semiconductor with a bandgap in the range of 0.5 to 4 eV.36. The photo-electrochemical device according to any one of the preceding embodiments, wherein the ESM is capable of photointercalating and photodeintercalating cations or anions.37. The photo-electrochemical device according to embodiment 36, wherein the ESM is capable of intercalating and de-intercalating cations, in particular alkali metal cations and protons.38. The photo-electrochemical device according to any one of the preceding embodiments, wherein the ESM has a 2D or 3D crystal structure, preferably selected from Bronze phase, Magneli phase, Dion-Jacobsen phase, Ruddlesden-Popper phase, and Wadsley-Roth phase .39. The photo-electrochemical device according to any one of the preceding embodiments, wherein the ESM comprises nanosheets.40. The photo-electrochemical device according to any one of the preceding embodiments, wherein the ESM is in the form of exfoliated nanosheets.41. The photo-electrochemical device according to embodiment 39 or 40, wherein the nanosheets have a thickness in the range of 0.5 to 2.5 nm.42. The photo-electrochemical device according to any one of the preceding embodiments, wherein the ESM comprises structural units of oxygen selected from the group consisting of octahedral, tetrahedral, cubic, trigonal prismatic, trigonal bipyramidal structural units, and distorted versions thereof, the structural units of oxygen having the transition metals in their centers, wherein the structural units of oxygen are preferably selected from the group consisting of octahedral and tetrahedral structural units and distorted versions thereof, and wherein the structural units of oxygen are more preferably octahedral structural units.43. The photo-electrochemical device according to any one of the preceding embodiments,divalent cation; wherein the ESM is optionally doped with one or more metals, such as one, two or three of selected from the lanthanides and transition metal elements, e.g. one, two, three or more selected from La, Y, Sm, Yb, Sc, Mg, Ca, Co, Ni, Zr, Ti, Hf, Eu, Gd, Pr and Sn, such as Y, La, Zr, and Sc.44. The photo-electrochemical device according to embodiment 43, wherein the monovalent cation M+is selected from the group consisting of organic cations, such as tetramethyl ammonium and tetrabutyl ammonium, H+, and alkali metals, such as Li+, and wherein the divalent cation is selected from the group consisting of Zn2+and alkaline earth metals, such as Mg2+, Ca2+, Sr2+and Ba2+.45. The photo-electrochemical device according to any one of the preceding embodiments, wherein the ESM is a single-phase material.46. A use of an electron storage material, ESM, as defined in any one of embodiments 1 to 45 in a combined solar battery and electrolyzer.47. A method for photocatalytic production of a product comprising the following steps: illuminating an electron storage material, ESM, with light to create photogenerated electrons in the ESM, storing the photogenerated electrons in the ESM, andreacting a to-be-reduced reactant with the stored photogenerated electrons, optionally in the presence of a reduction catalyst, to form a product, wherein the ESM has a structure containing oxygen and at least one selected from the transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides, provided that the ESM is not TiC>2, E^WOg and WO20O56, and with the further proviso that the ESM satisfies at least one of the following requirements (i) to (iv):(i) it does not includeanions;(ii) it does not include a polyoxometalate, POM;(iii) it does not include a photosensitizer including a (4,4'-di-tert-butyl 2,2'- bipyridine)2 Ru (diethyl((4'-methyl-(2,2'-bipyridine)-4-yl)methyl)phosphonate) moiety;(iv) it does not include an organic photosensitizer.48. The method according to embodiment 47, wherein the ESM satisfies requirement (i).49. The method according to embodiment 47, wherein the ESM satisfies requirement (ii).50. The method according to embodiment 47, wherein the ESM satisfies requirement (iii).51. The method according to embodiment 47, wherein the ESM satisfies requirement (iv).52. The method according to any one of embodiments 47 to 51, wherein the ESM is not MoO^.53. The method according to any one of embodiments 47 to 52, wherein the ESM is as defined in any one of embodiments 16 to 44.54. The method according to any one of embodiments 47 to 53, wherein the photogenerated electrons are stored in the ESM until they are extracted by adding the reduction catalyst and react with the to-be-reduced reactant to form the product.55. The method according to any one of embodiments 47 to 54, wherein the reduction catalyst is added after the illumination has stopped.56. The method according to any one of embodiments 47 to 55, wherein a to-be-oxidized reactant is present when the ESM is illuminated with light, which to-be-oxidized reactant quenches the holes that are formed along with the photogenerated electrons in the ESM upon illumination of the material, to yield a product of the to-be-oxidized reactant.57. The method according to embodiment 56, wherein the to-be-oxidized reactant is an alcohol, a sugar or water.58. The method according to any one of embodiments 47 to 57, wherein the to-be-reduced reactant is H2O, oxygen, CO2, CO, nitrogen, NOg’ or an acid.59. The method according to any one of embodiments 47 to 58, wherein the to-be-reduced reactant is H2O and the product is hydrogen.60. The method according to any one of embodiments 47 to 58, wherein the to-be-reduced reactant is oxygen and the product is H2O2 and / or H2O.61. The method according to any one of embodiments 47 to 58, wherein the to-be-reduced reactant is CO2 and the product is selected from the group consisting of CO, formaldehyde, formic acid, acetic acid, methanol, ethanol, methane, ethylene and mixtures thereof.62. The method according to any one of embodiments 47 to 58, wherein the to-be-reduced reactant is nitrogen and the product is NH3.63. The method according to any one of embodiments 47 to 62, wherein the ESM is present in the form of particles that are dispersed in an electrolyte.64. An electrochemical device, comprising a negative electrode comprising an electron storage material, ESM, a positive electrode, and an electrolyte, optionally containing a to-be-reduced reactant and / or a to-be-oxidzed reactant, ,divalent cation, wherein the ESM is optionally doped with one or more metals, such as one, two or three of selected from the lanthanides and transition metal elements, e.g. one, two, three or more selected from La, Y, Sm, Yb, Sc, Mg, Ca, Co, Ni, Zr, Ti, Hf, Eu, Gd, Pr and Sn, such as Y, La, Zr, and Sc.65. A photorechargeable battery comprisinga negative electrode comprising a substrate with a surface and a layer comprising an electron storage material, ESM, provided on the surface of the substrate, and a positive electrode comprising a substrate with a surface and a layer comprising a hole storage material provided on the surface of the substrate, and a photovoltaic element sandwiched between the layer comprising the ESM and the layer of the hole storage material, wherein the photovoltaic element is capable of charging the electrodes upon illumination, and wherein the ESM is selected from the group consisting of CeC>2, M+fNbWOg)’,lent cation and wherein X2+is a divalent cation; wherein the ESM is optionally doped with one or more metals, such as one, two or three of selected from the lanthanides and transition metal elements, e.g. one, two, three or more selected from La, Y, Sm, Yb, Sc, Mg, Ca, Co, Ni, Zr, Ti, Hf, Eu, Gd, Pr and Sn, such as Y, La, Zr, and Sc.66. An autophotorechargeable battery comprising a negative electrode comprising a substrate with a surface and a layer comprising an electron storage material, ESM, provided on the surface of the substrate, a positive electrode comprising a substrate with a surface, and a layer comprising a hole storage material provided on the surface of the substrate, and an electrolyte, wherein the ESM is selected from the group consisting of CeO2, M+fNbWOg)’, x2+[(NbWO6)’]2,divalent cation.67. The autophotorechargeable battery according to embodiment 66, which is a autophotorechargeable battery wherein both steps of light harvesting and electrical energy storage occur within the ESM.68. A method for detecting oxygen, the method including the following steps: providing an electrochemical device according to embodiment 64, charging the ESM comprised in the negative electrode with electrons to give a charged ESM, bringing the charged ESM in contact with a fluid or gas suspected to contain oxygen, andanalyzing the state of the charged ESM by visual detection or by measuring the change of the electrical potential of the device before, during and / or after bringing it in contact with the fluid or gas, wherein the charging of the ESM is preferably done by illumination or by applying an electric charging current.69. A method for detecting light, the method including the following steps: providing an electrochemical device according to embodiment 64, illuminating the ESM comprised in the negative electrode with light for a predetermined time in the presence of a to-be-oxidized reactant, TBOR, which is present in the electrolyte, to create electron-hole pairs in the ESM, wherein the holes are quenched by reacting with the TBOR and the electrons are stored in the ESM, and wherein after stopping illuminating the ESM, a property of the ESM that is or that correlates with the amount of electrons in the ESM is measured and correlated with the illumination intensity of the light, wherein said property that correlates with the amount of electrons or which is the amount of electrons in the ESM is preferably measured by spectroscopic detection or by electrochemical detection.70. A method for detecting an analyte, the method including the following steps: providing an electrochemical device according to embodiment 64, illuminating the ESM comprised in the negative electrode for a predetermined time to create electron-hole pairs therein while the ESM is in contact with a test fluid or gas containing the analyte during the illumination, wherein the analyte is a to-be-oxidized reactant, TBOR, capable of quenching the holes of the electron-hole pairs, and after stopping illuminating the ESM, measuring a property that is or that correlates with the amount of electrons in the ESM, wherein the property in the ESM is preferably measured by spectroscopic detection or by electrochemical detection.71. The method according to embodiment 69 or 70, wherein the amount of electrons in the ESM or a property that correlates therewith is measured by measuring a change in absorbance of the ESM or by measuring fluorescence emission of the ESM.72. The method according to embodiment 69 or 70, wherein the amount of electrons in the ESM or a property that correlates therewith is measured by measuring a change of the electrochemical potential or a change of the impedance of the ESM, or by applying a discharging current and quantifying the amount of discharged electrons.71. The method according to embodiment 70, wherein the sensitivity of the electrochemical device for detecting the analyte is adjusted by the duration of the step of illuminating the ESM or by the intensity of said illumination.72. A use of the electrochemical device according to embodiment 64 as an oxygen detector or a light detector.73. A use of the electrochemical device according to embodiment 64 for detecting an analyte, which is a TBOR.74. A use of a material for generating electron-hole pairs by illuminating the material and storing electrons generated in the same material, wherein the material is the electron storage material as defined in any one of embodiments 47 to 53.75. The use according to embodiment 74, wherein the material is illuminated with light comprising wavelengths in the visible range of the spectrum.Brief Description of the DrawingsFig. 1 shows a photo-electrochemical device according to one embodiment of the present invention.Fig. 2 shows a photo-electrochemical device according to an embodiment of the present invention.Fig. 3 shows a photo-electrochemical device according to an embodiment of the present invention.Fig. 4 shows a photo-electrochemical device according to one embodiment of the present invention.Fig. 5 relates to Example 1 and shows a) a schematic illustration of the liquid exfoliation process; b) and c) XPS data for Ca 2p and Nb 3d; d) ex-situ UV-vis absorbance spectra upon 340 nm UV irradiation for different irradiation times in the presence of MeOH as TBOR; the insert shows the magnified region between 570 and 580 nm; e) the development of absorbance at 575 nm over time of irradiation at 340 nm; f) the UV-Vis transmittance and absorbance spectrums of CNO film on FTO substrate and g) its corresponding Tauc plot; h) operando UV-Vis transmittance spectral changes under electric charging from OCP to -1 V and j) electric discharging from -0.95 V to -0.2 V; inserts in h) and j) are the magnified region between 570 nm and 580 nm; i) the transmittance intensity at 575 nm under different applied bias for electric charging and (k) electric discharging in an oxygen-free 0.1 M LiC I electrolyte; I) the photocharging profiles of aCNO photoanode under 340 nm UV irradiation and m) its corresponding electric discharging profiles under different current densities.Fig. 6 relates to Example 1 and shows the PXRD patterns of layered KCa2Nb3O^Q, protonated H Ca2Nb30iQ and exfoliated CNO nanosheets powders, together with a reference diffractogram.Fig. 7 relates to Example 1 and shows the results of a HER test with and without a delay between the end of irradiation and the addition of Pt catalyst.Fig. 8 relates to Example 1 and shows a) the development of the electrode potential over time after irradiation for 1; and b) the dark and light CV at a scan rate of 10 mV s'l; the electrolyte for dark CV is oxygen-free 0.1 M LiCI, the electrolyte for light CV is oxygen-free 0.1 IVI LiCI with the presence of MeOH as TBOR,Fig. 9 relates to Example 1 and shows a) the observed potential (vs. Ag / AgCI) after different irradiation times; b) the electric discharge capacity under different current densities after 5 min irradiation; c) the open circuit potential profiles under different photocharging time and (d) corresponding capacity at a discharge current density of 4 mA g'l.Fig. 10 relates to Example 1 and shows a) the cycling stability under light irradiation followed by electric discharge; the electrolyte is oxygen-free 0.1 IVI LiCI and MeOH (10 vol%); b) the chronoamperometry (CA) photocurrents of CNO photoanode under 340 nm UV irradiation in 0.1 M LiCI with the presence of MeOH and TEOA; c) the photocurrents at different potentials under 340 nm UV irradiation in the presence of MeOH and TEOA; d) linear sweep voltammetry of CNO photoanode under 340 nm UV irradiation in the presence of MeOH and in the dark; and e)the photocurrent stability measurement under 340 nm UV irradiation with the presence of MeOH donor. The applied potential is 0 V (vs Ag / AgCI).Fig. 11 relates to Example 2 and outlines the synthesis of the obtained 2D (TBA)Ti5NbO^4 material.Fig. 12a relates to Example 2 and shows the TiOg or NbOg octahedra in the material.Fig. 12b relates to Example 2 and shows XRD data.Fig. 13 relates to Example 2 and shows the typical morphology of the KTNO crystal (left) and its corresponding HRTEM image with FFT inset (right).Fig. 14a relates to Example 2 and shows the UV-Vis transmission and absorbance spectra of a TNO film on FTO substrate; the Tauc plot of a TNO thin film; c) the development of absorbance at 575 nm over irradiation time at 340 nm; and d) the development of the UV-Vis spectrum over time of irradiation at 340 nm at 20 mW / cnFig. 15a relates to Example 2 and shows the responsive photocurrent in oxygen-free aqueous IM LiCI with the presence of MeOH (10 vol%) as TBOR under 365 nm and 405 nm UV irradiation.Fig. 15b relates to Example 2 and shows the summary of first cycle J ph under different irradiation wavelengths of 340, 365 nm, and 405 nm, respectively, each at 20 mW cm'2Fig. 16 relates to Example 2 and shows the representative (a) first cycle and (b) three cycles of photocurrent profiles under 340 nm UV irradiation with different power intensities in oxygen-free IM LiCI in the presence of MeOH (10 vol%) as TBOR.Fig. 17a relates to Example 2 and illustrates electrochemical impedance spectroscopy (EIS) measurements under different applied bias.Fig. 17b relates to Example 2 and shows a Bode-type diagram.Fig. 18a and 18b illustrate the general presumed principle of photocharging and electric discharging with reference to the ESM material of Example 2.Fig. 19 relates to Example 2 and shows the electric discharging potential provides under different current densities after 5 minutes irradiation.Fig. 20a relates to Example 2 and shows the photocharging profiles under different irradiation time, and Fig. 20b the corresponding electric discharging profiles at a current density of 24 mA g'l. Inserts show the OCP under illumination at the beginning, i.e. up to 1.5 min.Fig. 21 relates to Example 2 and shows the open circuit potential stability of two independent samples during and after light illumination in oxygen-free aqueous IM LiCI with the presence of MeOH (10 vol%) as TBOR.Fig. 22 relates to Example 2 and shows the cycling stability of (a) photocharging for 5 min and (b) electrical discharging at a current density of 24 mA g'l.Fig. 23 relates to Example 2 and shows the CV scanning of the TNO electrode between -0.8 V to 0.3 V under dark and 340 nm UV irradiation in aqueous 1 M LiCI electrolyte in the presence of MeOH (10 vol%) as TBOR.Fig. 24 relates to Example 2 and shows a comparison of capacity under dark and light-assisted discharge at a current density of 24 mA g'l.Fig. 25a and b relate to Example 2. Fig. 25a shows light and dark LSV scanning at a sweep rate of 10 mV s'l under 340 nm, 365 nm and 405 nm UV irradiation, respectively, and Fig. 25b shows the responsive photocurrent in oxygen-free IM LiCI with the presence of MeOH (10 vol%) under 340 nm, 365 nm and 405 nm UV irradiation, respectively.Fig. 25c relates to Example 2 and shows the J ph under different applied potentials and light irradiation in the presence of MeOH (10 vol%).Fig. 25d relates to Example 2 and shows the relationship between J ph and light intensity.Fig. 25e relates to Example 2 and shows the development of J ph over time under 340 nm at a light intensity of 20 mW cm'2 and an applied potential of 0 V.Fig. 26 illustrates a three-electrode setup that was used to evaluate the capacity performance of the TNO photoanode.Fig. 27 relates to Example 2 and illustrates the maximum photocharging capacity when the photoanode was irradiated for different times;Fig. 28 relates to Example 2 and shows electrochemical impedance spectroscopy (EIS) measurements in the dark and under light irradiation for different times;Fig. 29 shows the Bode plot obtained for the material of Example 2;Fig. 30 show light charging and electric discharging cycling measurement performed on the electrode obtained in Example 2;Fig. 31 relates to Example 2 and shows the generated hydrogen amount without (a) and with (b) delay between the end of irradiation and the addition of catalyst for the HER reaction:Fig. 32 relates to Example 3 and illustrates the manufacturing process;Fig. 33 relates to Example 3 and illustrates the obtained crystal structure;Fig. 34 relates to Example 3 and shows XRD data for HNbWOg and LiNbWOg;Fig. 35 relates to Example 3 and shows SEM images of (a) LiNbWOe and (b) HNbWOe-xH2O powders,Fig. 36 relates to Example 3 and shows Nb 3d and W 4f XPS spectra of the material obtained;Fig. 37 relates to Example 3 and shows UV Vis Absorbance and transmission data;Fig. 38 relates to Example 3 and shows the Tauc plot;Fig. 39 relates to Example 3 and shows the UV-Vis absorbance data after different times of irradiation;Fig. 40 relates to Example 3 and shows the absorbance at 575 nm relative to the illumination time;Fig. 41 relates to Example 3 and shows the effect of pH-dependent photocurrent measurement;Fig. 42 illustrates a three-electrode setup;Fig. 43 relates to Example 3 and shows the photocurrents of NbWO6 photoanodes under (a) 1 sun and (b) 365 nm UV illumination in oxygen-free IM LiCI in the presence of MeOH;Fig. 44 relates to Example 3 and shows the photocurrents of NbWOg photoanodes in oxygen-free (a) IM LiCI in the presence of 10 mM 4-MBA and (c) IM LiCI electrolyte in pure H2O under 1 sun illumination;Fig. 45 relates to Example 3 and shows the photocurrents of NbWOg electrodes in oxygen-free (a) IM LiCI in the presence of 10 mM 4-MBA, and (c) IM LiCI electrolyte in H2O under 365 nm UV illumination. The first off / on / off cycles of (a, c) are shown in (b, d), respectively.Fig. 46 relates to Example 3 and shows the UV-vis absorbance spectra of oxygen-free NbWOg nanosheet suspension in the presence of (a) H2O, (b) 4-MBA, and (c) MeOH; the insert in (c) shows the magnified region between 570 nm to 580 nm; Fig. 46 d) summarizes the dependence of absorption intensity at a wavelength of 575 nm as a function of the different TBORs;Fig. 47 relates to Example 3 and shows a CV sweep in IM LiCI and MeOH electrolyte under 365 nm UV illumination and continuous Ar and oxygen purging at a scan rate of 10 mV s'l;Fig. 48 relates to Example 3 and shows dark CV sweep measurement, indicating reversible electron storage and release in continuous Ar purging, and the oxygen reduction reaction (ORR) in oxygen rich environment.Fig. 49 relates to Example 3 and shows the OCP of NbWOg electrodes in oxygen-rich 1 M LiCI in the presence of MeOH as TBOR under 1 sun and 365nm UV illumination;Fig. 50 relates to Example 3 and shows the development of J PH over time under different types of irradiation;Fig. 51 relates to Example 3 and shows linear sweep voltammetry (LSV) under different illumination conditions;Fig. 52 relates to Example 3 and shows photocurrents at different potentials under 1 sun respectively 365 nm UV illumination;Fig. 53 relates to Example 3 and shows the capacity under (a) different discharge current densities after 10 min light illumination and (b) different illumination times under discharge current densities of 0.48 and 4.8 mA g'l for 1 sun and 365 nm UV, respectively;Fig. 54 relates to Example 3 and shows the electrical discharge profiles at different discharge current densities after illumination under (a) 1 sun and (b) 365 nm UV light for 10 min in IM LiCI and MeOH electrolyte under continuous Ar purging;Fig. 55 relates to Example 3 and shows the OCP stability during and after light illumination;Fig. 56 relates to Example 3 and shows the cycling stability of the photoanode under 1 sun and 365 nm UV illumination, followed by electric discharge; solid and hollow symbols represent capacity and cycling capacity retention, respectively;Fig. 57 relates to Example 3 and shows the Nyquist plot of NbWOg photoanode under 1 sun illumination;Fig. 58 shows the Bode plot for Example 3;Fig. 59 relates to Example 3 and shows electric discharge profiles after (a) 1 sun illumination for 10 min and (b) 365 nm UV illumination for 5 min. The discharge current densities are (a) 4.8 mA g1and (b) 24 mA g’1;Fig. 60 relates to Example 3 and shows the cycling stability of the photoanode under 1 sun and 365 nm UV illumination, followed by electric discharge;Fig. 61 a and b relate to Example 3 and show the dark hydrogen generation as a function of time in HER experiments;Fig. 62 a and b relates to Example 3 and show a) the hydrogen amount as a function of illumination time, where b) shows the long-term measurement for dark hydrogen generation; Fig. 62c is a histogram plot that shows the turnover frequency (TOF) after 365 UV illumination for 30 min without delay time and with 1 h delay time;Fig. 63 relates to Example 3 and shows the illumination time-dependent UV-vis spectrum.Fig. 64 and 65 illustrate devices, methods and uses according to embodiments of the present invention.Fig. 66 relates to the UV-vis spectra of pristine Nbi4WsO44 powder in the dark and the mixture with LiCI and 4-methylbenzyl alcohol under UV irradiation.Fig. 67 shows the normalized Kubelka-Munk absorbance (F(R)) spectra of a NbieWsOss powder mixture with LiCI and 4-methylbenzyl alcohol in the dark and upon 365 nm UV irradiation for 10 min.Fig. 68 shows the normalized Kubelka-Munk absorbance (F(R)) spectra of a NbisWieOgs powder mixture with LiCI and 4-methylbenzyl alcohol in the dark and upon 365 nm UV irradiation for 10 min.Fig. 69 shows the galvanostatic charge and discharge profile of a TiNb2O? electrode at 1C rate in the dark and under 365 nm UV irradiation. The electrode was prepared by mixing TiNb2O?: carbon black: polyvinylidene fluoride at a mass ratio of 8:1:1.Detailed Description of the InventionDefinitions

[0029] In the following, the terms used in the present invention will be explained in more detail. If the definition of a term should depart from a meaning known to a skilled person, the definition given in the present invention prevail.

[0030] In certain aspects, the claimed subject matter is defined by properties or parameters. The method used for determining these properties and parameters are not particularly limited, and any method known to a skilled person can be used, including those that are defined in standards such as DIN or ASTM. If different methods should lead to different results, the test methods described herein may be used. The test methods employed in the examples can be used for all embodiments, unless this is clearly inapplicable from the context.

[0031] Generally, the properties and parameters defining certain aspects of the present invention are to be determined at 25°C and at 1 atm pressure, unless it is clear from the context that different conditions are to be used (e.g., in a DSC measurement).

[0032] When ranges for properties or parameters are identified, such as from 5 to 10, the ranges include the upper and the lower limit and any value in between. If restricted to values or ranges are indicated, the respective limits or values can be combined.

[0033] The term "comprises" is open ended and requires the presence of the respective component or element, but does not exclude the presence of other components or elements. The term also encompasses the more restricted meanings "consisting essentially of" and "consisting of'. Here, a composition or member that "consists of" the respective component or element does not contain any other component or element. Were a composition or member "consists essentially of" the component or element, additional components or elements are not excluded, yet these are generally present in an amount of 20% by weight or less, such as 10% by weight or less, e.g. 5% by weight or less, 3% by weight or less, such as 1% by weight or less, relative to the total weight of the composition or member. In the case of an ESM as defined below, an ESM that comprises or essentially consists of the recited components may thus include additional components, such as doping elements and optional unavoidable impurities, which doping elements and optional unavoidable impurities are present in a total amount within the compositional limits above. Put differently, the amount of a doping element is typically 20% by weight or less, such as 10% by weight or less, e.g. 5% by weight or less, 3% by weight or less, such as 1% by weight or less, relative to the total weight of the composition of the ESM. The doping elements are typically one or more metals, such as one or more transition metals or lanthanides, preferably Sc, Y and the lanthanides, preferably those with element numbers 57 - 64, such as Y, La, Sc, Zr, Gd, Eu and Pr.

[0034] The term "the structure of the ESM is composed of oxygen and two or more selected from the transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides" indicates that the crystal structure of the ESM is formed by oxygen and the respective transition metals only. This does not exclude the presence of intercalated or adsorbed ions, e.g. cations that are present to balance a negative charge of the ESM or of doping elements. Without limitation, the presence of such intercalated or adsorbed ions, e.g. cations, and doping elements may be limited to 20% by weight or less, such as 10% by weight or less, e.g. 5% by weight or less, relative to the total weight of the ESM, and the term includes the possibility that such ions or dopants are completely absent.

[0035] The terms "light", "irradiation" and "illumination" are used interchangeably to refer to electromagnetic radiation, and include UV, visible and IR wavelength regions. In the present invention, the UV radiation refers to electromagnetic radiation having a wavelength of 400 or less, such as from 180 - 400 nm or from 220 to 400 nm. UV includes both near UV (UVA) having a wavelength of 315 - 400 nm and mid UV (UVB) having a wavelength of 280 to 314nm. The "visible" wavelength region extends from more than 400 nm to 700 nm, and the IR wavelength region extends from greater than 700 to 1.500 nm, such as from 700 to 950 nm.

[0036] The term "photo-electrochemical device" denotes a device that is capable of absorbing electromagnetic radiation in the UV, visible or IR wavelength region to form a photoreduced state originating from an electron (also referred to as "photogenerated electron") stored on / trapped by a material, and to utilize the photogenerated electron for reducing a to-be- reduced reactant. The photogenerated electron forming the photoreduced state originates from an electron-hole pair that is formed in the device upon absorption of the electromagnetic radiation.

[0037] A "to-be-reduced reactant" (TBRR) is a chemical species or compound that is reduced by the operation of the photo-electrochemical device by the photogenerated electrons. Here, reduced means the uptake of one or more, typically one or two electrons.

[0038] A "to-be-oxidized reactant" (TBOR) is a chemical species or compound that is utilized in the photo-electrochemical device to donate electrons, thereby being oxidized. This species or compound may be referred to as TBOR or as sacrificial electron donor (SED), the terms being synonymous.

[0039] An electron storage material (ESM) denotes a material which is capable of forming a photoreduced state by receiving an electron (also referred to as photoelectron or photogenerated electron) originating from an electron / hole pair that is formed upon absorption of electromagnetic radiation in the visible or UV range (also referred to as "light reaction"), which is able to maintain the photoreduced state for a period of time, and which is able to provide electrons for a subsequent electrochemical reaction (also referred to as "dark reaction"). Put differently, the ESM is a material that is capable of forming an electron / hole pair upon absorption of electromagnetic radiation in the visible or UV range, which is able to store the photogenerated electron for a period of time as a trapped charge, and from which the photogenerated electron can subsequently be released or extracted. The photoelectron may take the form of a polaron, such as a small (Holstein) polaron or large (Frohlich) polaron, typically a small (Holstein) polaron. The subsequent electrochemical reaction is generally a reduction reaction of a TBRR as defined above, which typically occurs using a reduction catalyst, e.g. at a RCCL as defined below. The ability of a material to act as ESM can be assessed by methods such as open-circuit potential (OCP) and UV-Vis spectra measurements. As shown in the examples, the OCP of an ESM increases during light irradiation in the UV and / or visible spectrum and will maintain the increased OCP for a period of time after end of the irradiation. Also, UV-Vis absorbance increases during light irradiation, as will be explained in more detail below. This increased UV-Vis absorbance may be observed as a change in color upon irradiation, often creating a blue to greenish color that is believed to be a characteristic of trapped photoelectrons.

[0040] A hole storage material (HSM) denotes a material which is capable of forming a photooxidized state by receiving a hole (also referred to as photohole or photogenerated hole) originating from an electron / hole pair that is formed upon absorption of electromagnetic radiation in the visible or UV range (also referred to as "light reaction"), which is able to maintain the photooxidized state for a period of time, and which is to provide holes for a subsequent electrochemical reaction (also referred to as "dark reaction"). The photohole may take the form of a polaron, such as a small (Holstein) polaron or large (Frohlich) polaron, typically a small (Holstein) polaron. The subsequent electrochemical reaction is generally an oxidation reaction of a TBOR as defined above as defined above, which typically occurs at a OCCL as defined below.

[0041] A "reduction-catalyst containing layer" (RCCL) is a layer that comprises a reduction catalyst.

[0042] An "oxidation-catalyst containing layer" (OCCL) is a layer that comprises an oxidation catalyst.

[0043] An electrode contains a current collector, which may also be denoted as "material connected to an electrical connection". The current collector is typically made from a material having electrical conductivity, defined as a conductivity of 10^ S / m or more. The electrode current collector material is not particularly limited and includes metals such as copper and aluminum, but also oxides such as transparent conductive oxides (TCO), including indium tin oxide (ITO), fluorine-doped tin oxide (FTO), niobium doped anatase (NTO) or doped zinc oxide. The current collector may also be formed by a conductive polymer or carbon-based materials, such as carbon nanotubes or graphene. In certain embodiments, the current collector is transparent or translucent as is the case for TCOs, in particular in the first electrode containing the ESM.

[0044] An electrical connection is a connection having electric conductivity, defined as conductivity of 10^ S / m or more. In one embodiment, an electrical connection is formed by a metal (e.g. Cu or Al) wire, which may optionally have an insulating layer formed from a thermoplastic polymeric material, e.g. polyolefin.

[0045] In the present invention, the term "transition metal" denotes an element in the d-block of the Periodic Table of the Elements (Groups 3 - 12), not including the lanthanides, and preferred embodiments thereof include the elements found in the first two rows of the Periodic Table (elements Sc to Zn and Y to Cd), more preferably the second row (Y to Cd). In each row, those in Groups 3 - 10 or 3 to 9 are preferred.

[0046] The lanthanides include the elements numbers 57 - 71 and 89 - 103, with those of numbers 57-71 (La to Lu) being preferred, and those of numbers 57 - 64 being most preferred.

[0047] The transition metals of Groups 4, 5, 6 and 7 present in the ESM can be found in the Periodic Table of the Elements (PSE), and include the following: a. Group 4: Ti, Zr, and Hf, preferably Hf and Zr; b. Group 5: V, Nb, and Ta;, preferably Nb; c. Group 6: Cr, Mo, and W, and preferably W; d. Group 7: Mn and Re, preferably Mn.

[0048] An electrolyte in the present invention is not particularly limited and may have any physical state including solid, but is typically a liquid or gel-like material. The electrolyte is preferably a liquid material, more preferably an aqueous and / or alcoholic solution.

[0049] A hole transport material (HTM) is a material capable of transporting holes. Suitable materials are well known to a skilled person and can be found in the literature, such as in the literature relating to OLEDs (organic light emitting devices) or DSSCs (dye-sensitized solar cells), but will also be discussed below. A material may simultaneously be a HTM and HSM, and both may be selected from the same materials. However, HTM and HSM may also be different materials from each other, e.g., having a different chemical composition.

[0050] The term "visual detection" encompasses both observation with the naked eye as well as examination of material properties that are reflected in the material's interaction with electromagnetic radiation, such as by UV-Vis spectrometry.Photo-Electrochemical Devices

[0051] In one embodiment (Embodiment 1) as defined in Claim 1, the present invention relates to a photo-electrochemical device, comprising a first electrode containing an electron storage material, ESM, wherein the ESM has a structure containing oxygen and at least one of the transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides, and wherein the ESM is capable of producing electron-hole pairs when irradiated by light and storing the thereby formed photogenerated electrons for a period of time; a second electrode, optionally containing a reduction reaction catalyst-containing layer;an electrolyte containing a to-be-oxidized reactant and a to-be-reduced reactant, the electrolyte being in contact with the ESM and the second electrode, preferably the reduction reaction catalyst-containing layer if present; an electrical connection between the first electrode and the second electrode for transferring electrons from the first to the second electrode.

[0052] In another embodiment (Embodiment 2) as defined in Claim 2, the present invention relates to a photo-electrochemical device, comprising a first electrode containing an electron storage material, ESM, wherein the ESM has a structure containing oxygen and at least one selected from the transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides and wherein the ESM is capable of producing electron-hole pairs when irradiated by light; an oxidation reaction catalyst-containing layer, arranged to receive holes from the ESM; a second electrode, optionally containing a reduction reaction catalyst-containing layer; and an electrolyte containing a to-be-oxidized reactant and a to-be-reduced reactant, the electrolyte being in contact with the oxidation reaction catalyst-containing layer and the second electrode, preferably the reduction reaction catalyst-containing layer if present; an electrical connection between the first electrode and the second electrode for transferring electrons from the first to the second electrode.

[0053] In a further embodiment (Embodiment 3) as defined in Claim 11, the present invention relates to a photo-electrochemical device, comprising: a layer containing an electron storage material, ESM, wherein the ESM has a structure containing oxygen and at least one selected from the transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides, and wherein the ESM is capable of producing electron-hole pairs when irradiated by light; a reduction reaction catalyst-containing layer, arranged to receive electrons from the ESM- containing layer; a layer containing a hole storage material, HSM; an oxidation reaction catalyst-containing layer, arranged to receive holes from the HSM; a layer containing a hole transport material, HTM, and provided between the ESM-containing layer and the HSM-containing layer, the HTM-containing layer being arranged to transfer holes from the ESM-containing layer to the HSM-containing layer; and an electrolyte containing a to-be-oxidized reactant and a to-be-reduced reactant, the electrolyte being in contact with the oxidation reaction catalyst-containing layer and the reduction reaction catalyst-containing layer.

[0054] Optional and / or preferred embodiments of the devices are specified in claims 3 - 10 and 12 and 45. In the embodiments, the same terms denote the same materials, and the explanations, definitions and preferred features of the materials such as ESM, HTM, electrode, electrolyte, TBRR, TBOR, HSM, current collector, etc. are given comprehensively and generally apply to all embodiments, except the opposite is explicitly stated or it is clear from the context.

[0055] As will be shown in the Examples, the electrochemical device of the present invention is sensitive towards the presence of molecular oxygen (O2), because molecular oxygen is a good electron acceptor. Accordingly, unless the device or ESM is used as an oxygen sensor and the reaction with molecular oxygen is desired for the proper functioning of the device, care should be taken that after the photocharging process the ESM does not come into contact with molecular oxygen and that the device is encapsulated or otherwise protected against the presence and / or ingress of oxygen. This is because molecular oxygen may react with the trapped photogenerated electrons, thereby lowering or fully eliminating the amount of trapped photogenerated electrons, with the consequence that these are no longer available for e.g. on- demand reduction reactions (e.g. HER reactions) and are unable to contribute to the reduction of the TBRR.

[0056] Accordingly, the devices of the present invention are preferably essentially free of molecular oxygen. Here, the term "essentially free of molecular oxygen" means that in any gas space in the device oxygen is either completely absent or present in an amount of 5 Vol.-% or less, such as 1 Vol% or.- less, preferably 0.1 Vol.% or less, such as 0.01 Vol.-% or less, and that in the liquid components, such as the electrolyte, the content of molecular oxygen it 1,000 ppm or less, such as 500 ppm or less, 100 ppm or less, or 50 ppm or less, and sometimes even 10 ppm or less or 5 ppm or less, such as 1 ppm or less. This definition of the content of molecular oxygen applies to all devices, methods and uses of the present invention.ESM - Structure and Composition

[0057] All devices of the present invention comprise an ESM, and in certain devices the ESM is part of a first electrode. The same ESM is also used in the methods and the uses of the present invention unless additional limitations are specified.

[0058] The ESM used in the present invention has a structure containing oxygen and at least one selected from the transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides, is capable of generating electron-hole pairs upon absorption of electromagnetic radiation in the visible and / or UV range, preferably in the visible range, and is able to trap or store the thereby generated electron (also referred to as photogeneratedelectrons). The hole of the electron-hole pair (also referred to as photogenerated hole or photohole) is typically quenched by reaction with a TBOR. The quenching of the photohole may occur before, while or after the photogenerated electron is trapped or stored within the ESM, and occurs preferably simultaneously with the trapping of the photogenerated electron. An ESM preferably is able to stabilize the photogenerated electrons, for example by adsorbing / photointercalating counter-cations from the electrolyte.

[0059] It is a key finding of the present invention that a variety of materials containing oxygen and at least one selected from the transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides can be used as an ESM. The ability to form electron / hole pairs and to store (or trap) the thereby photogenerated electron is inherently a requirement for an ESM of the present invention. High performance characteristics and a generally higher output (in terms of electric current or conversion rate of a TBRR or TBOR) can be achieved if the ESM satisfies the structural features below, e.g. wherein the ESM is selected from the group consisting of CeO2, M+(NbW06)-, X2+[(NbWO6)-]2, (M+)3(Ti5NbO14)3-, [X2+]3[(Ti5NbO14)3-]2, M+(Ca2Nb3O10)-, X2+[(Ca2Nb3O10)-]2, (M+)2Cs4WnO362-, X2+Cs4WnO362-, (M+)1.6Rb2.4wll°351’6’' (x2+)o.8Rb2.4wll°351'6’' Li4Ti5O12,Nb2°5 and Nb16W5O55, wherein M+ is a monovalent cation and wherein X2+ is a divalent cation. The monovalent cation may be selected from the group consisting of organic cations, in particular tetraalkyl ammonium cations, such as tetramethyl ammonium and tetrabutyl ammonium, ammonium cations (NH4+), H+, and alkali metal cations, such as Li+and Na+and the divalent cation may be selected from the group consisting of Zn2+and alkaline earth metal cations, such as Mg2+, Ca2+, Sr2+and Ba2+

[0060] The ESM thus has a dual functionality, i.e., has the ability to form an electron / hole pair and subsequently forming photogenerated electrons and holes therefrom, combined with the ability of trapping the photogenerated electrons. The electron trapping may be facilitated by ions, in which case this process may be described as an optoionic process.

[0061] It is further a key finding of the present invention that an ESM capable of forming electron / hole pairs and trapping the photogenerated electrons can be prepared on a purely inorganic basis, i.e. without the need to rely on organic compounds as part of the molecular / atomistic structure of the ESM, as ligand, or as additive, such as a sensitizing dye. While the presence of such organic compounds in the ESM is not excluded in the present invention, in one embodiment of the ESM it is a purely inorganic compound. Here, the term "purely inorganic" denotes the absence of compounds having a carbon-hydrogen bond and / or a carbon-carbon bond.

[0062] In one embodiment, the ESM may comprise, consist essentially of, or consist of oxygen and at least one selected from the transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides, and optionally other main group metal cations, which may be selected from the group of alkaline metals and earth alkaline metals, such as lithium, sodium, potassium, rubidium, calcium, magnesium, strontium, and barium. Here, the main group (preferably alkaline metals and earth alkaline) metal is present as cation, the remainder, being formed by oxygen and at least one selected from the transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides, having the corresponding negative charge.

[0063] Thus, in one embodiment, the ESM may comprise, essentially consist of, or consist of an oxide of the at least one selected from the transition metals in Groups 4, 5, 6 or 7 and the lanthanides, and further comprises a cation of a main group metal of the Periodic Table of the Elements. Here, the at least one selected from the transition metals and the lanthanides may have an electronic d° configuration and the main group metal cation may have an electronic s2d10configuration. Examples of such main group metals include in particular Sn^+and Bi^+, preferably Sn^+.

[0064] In one embodiment, the ESM may comprise, consist essentially of or consist of oxygen and at least one selected from the transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides, and optionally one or more types of cations selected from the group consisting of protons (H+), hydronium ions (HgO"1") and quaternary ammonium cations represented by NR4+(wherein R in each case independently is selected from H and alkyl groups), the remainder formed by oxygen and at least one selected from the transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides having the corresponding negative charge. Examples of suitable quaternary ammonium cations are tetraalkyl ammonium cations, where the alkyl group is a branched or linear C^-Cg alkyl group. Specific examples include tetramethyl ammonium, tetraethyl ammonium, tetra-n-propyl ammonium, tetra-n-butyl ammonium (TBA) etc. Other examples include organic compounds having a 5-membered and 6- membered nitrogen-containing ring, such as imidazolium cations.

[0065] In one embodiment, the ESM may comprise, consist essentially of, or consist of oxygen, at least one selected from transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides, a proton or quaternary ammonium cation as described in the preceding paragraph, and one or more main group metal cations, which may be selected from the group of alkaline metals and earth alkaline metals, such as lithium, sodium, potassium, rubidium, calcium, magnesium, barium and strontium. An example of this embodiment is an ESM that comprises, consists essentially of or consists of (TBA)(Ca2Nb3O^Q). Another example is anESM that comprises, consists essentially of or consists of Ti5NbO^4. A further example is an ESM that comprises, consists essentially of or consists of NbWOg.

[0066] In one embodiment, the ESM comprises, essentially consists of, or consists of an oxide, oxynitride or oxysulfide of the at least one selected from transition metals of Groups 4, 5, 6 or 7 of the PSE and the lanthanides. In one embodiment, the ESM comprises, essentially consists of, or consists of an oxide of the at least one selected from transition metals of Groups 4, 5, 6 or 7 of the PSE and the lanthanides. In one of these embodiments, the transition metals of Groups 4, 5, 6 and 7 includes one or more of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, and Re, preferably Hf, Zr, V, Nb, Ta, W, and Mn.

[0067] Generally but also in the embodiments described in the preceding paragraph, the ESM comprises one, two or three, preferably one or two selected from the transition metals of Groups 4, 5, 6 or 7 of the PSE. For example, the transition metal may consist of one or two selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, and Re, preferably Hf, Zr, V, Nb, Ta, W, and Mn. The transition metals of Groups 4, 5, 6 or 7 may thus e.g. be Nb or W, or may be a combination of one of Ti with W and / or Nb. Example of such an ESM include those wherein Nb and / or W, optionally in combination with Ti, are present in a structure comprising NbOg and / or WOg octahedra, such as in (TBA)(NbWOg) and (TBA)3(Ti5NbO^4).

[0068] Thus, in one embodiment, the ESM comprises two or more transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements. In another embodiment, the ESM comprises only one transition metal selected from Zr, Hf, V, Ta, Cr, W, Mn, Nb, and Ti. In both embodiments, preferably the transition metal is or includes Nb, such as a combination of Nb and W, or a combination of Nb, W and Ti.

[0069] It is preferred that in the above embodiment, but also in the embodiment wherein the transition metal is only one of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Tc and Re, preferably Hf, Zr, V, Nb, Ta, W, Mn and Ti, there is at least one main group metal contained. The main group metal may be selected from the group of alkaline metals and earth alkaline metals, such as lithium, sodium, potassium, rubidium, calcium, magnesium, barium and strontium. The amount of the main group metal is not particularly limited, but is typically 30% by weight or less, such as 25% by weight or less of 20% by weight or less, e.g. 15% by weight or less or 10% by weight or less, relative to the total weight of the ESM. Also in this embodiment, the ESM may additionally comprise one or more types of cations selected from the group consisting of protons (H+), hydronium ions (HgO"1") and quaternary ammonium cations represented by NR4+(wherein R in each case independently is selected from H and alkyl groups) as described above.

[0070] Without wishing to be bound by theory, it is believed that the presence of several metals (e.g. a combination of at least one selected from transition metals of Groups 4, 5, 6 or 7 of the PSE and the lanthanides with a main group metal, such as alkaline metal or earth alkaline metal, or the combination of two or three transition metals of Groups 4, 5, 6 or 7 of the PSE and the lanthanides), preferably in the form of an oxide, can facilitate the formation of long-lasting photoelectrons, as in such a mixed metal oxide compound a stabilizing effect on the photoelectron may be exerted. This effect also depends on the structure of the ESM, and suitable materials can be identified amongst those forming e.g. a bronze phase. Bronze oxides (mixed metal oxides) such as tungsten bronzes that contain tungsten, alkali metals / alkaline earth metals or protons, often form frameworks of corner-sharing octahedra. Examples include NaxW03 , KXW03, or KxMoO3 or NaxMoO3, vanadium bronzes such as KXV2O5, titanium bronzes, such as NaxTiO3, niobium bronzes, such as NaxNbO3 (where in each instance x is a number > 0, and typically 3 or less, such as 2 or less) Besides bronze phases, suitable materials can also be identified amongst those forming a Magneli phase, Dion-Jacobson phase, Ruddlesden-Popper phase, and Wadsley-Roth phase. Magneli phases have a stoichiometric composition that can be represented by MnO3n_^ (where M is a transition metal in groups 4, 5, 6, of the Periodic Table of the Elements, such as Ti, V or Nb, and n is an integer >2). Dion- Jacobson phases are a family of layered perovskite-related structures having a stoichiometric composition that can be represented by A'[An_iBnO3n+i], where A' and A are different or identical monovalent cations, e.g. K, Rb, or Ba, B is a transition metal oxide in groups 4, 5,6, or 7 of the Periodic Table or a lanthanide, and n>l, typically >2. Here, n represents also the number of BOg octahedra in the perovskite-layer thickness, a perovskite unit having the fo Wadsley-Roth phases typically have a stoichiometric composition represented by ( where M and M' are the same or different metals selected from transition metals 6, of the Periodic Table of the Elements and the lanthanides, in particular Nb andAn example of a Wadsley-Roth phase is Nb^4W3O44, for which a blue coloration was observed by UV-Vis spectroscopy upon exposure to light, similar as for MxNbWOg.

[0071] Further specific examples include Nb gW^gOg , Mo3Nb2O^4, W3Nb2O^4, MO5O14, TiNb2O7 (Li 3Ti N b20 after Li intercalation), M+^NbOy- (M = alkali metal, in particular Li, Na or K); MNb30g(M = alkali metal, in particular Li, Na or K), CeOy, pure or doped with one or more t metals , preferably transition metals, more preferably selected from the group consisting of Cu, Co, Mn, Fe, Ni, Zr, and Zn; NbyOg, NbOy, Nb^yOyg, NB22O54, MNbOg (M = Li, Na or K); M2+Nb2O6(M2+= Mg, Ca, Cu, Cd, Zn, Ni, Mn, Co or Fe); (NH4)14[NaP5W30O110];H3[PW12O40]; H4[SiW12O40]; (C14H11N4O)2[Mo8O26]; EV2+[Mo9O28] (EV2+= ethyl viologen cation); (aZ-PyWiyOgjJiQ-MoOg; LiMnyC^ (and Co, Ni-substituted variants); LiyMnOg (and Co, Ni-substituted variants) LiMOy (M = Mn, Co, Ni); LiyMnOy (and Co, Ni-substituted variants)NMC / NCA-type cathode materials, e.g. Li(Ni,Mn,Co)O2 Li(Ni,Co,AI)O2; Na2 3(PO4); NagN^fPO / i.^; Na2M3(SO4)4 (M = Mn and / or Fe) and Li2MSiO2 (M = Fe and / or Ni).

[0072] The ESM consists of, essentially consists of or comprises of oxygen and at least one selected from the transition metals in Groups 4, 5, 6, and 7 and the lanthanides, but may optionally also include one or more dopant metals. In the definition of the ESM, the term "dopant" encompasses all metals that are added in an amount that does not alter the basic crystal structure of the ESM material, but which is incorporated into the ESM such as to cause local defects of the crystal structure. Put differently, a "metal dopant" refers to a metallic element that is deliberately incorporated into the oxide matrix in small quantities to modify and enhance the intrinsic properties of the material. The metal dopant is dispersed at the atomic or molecular level within the oxide host lattice, which can result in significant alterations in the electrical, optical, magnetic, or catalytic characteristics of the oxide. The inclusion of the metal dopant may induce changes in the band structure, defect density, or lattice parameters of the oxide, but do not typically alter its crystal structure or phase. The amount of a dopant is typically 10 mol% or less, relative to all atoms forming the crystal structure, such as 5 mol% or less, and a dopant present in such an amount is disregarded in the calculation of the number of elements forming the ESM, such as in the embodiment wherein the ESM comprises one or two of the transition metal elements in groups 4, 5, 6 and 7 . The dopant may be one or more metals, such as one, two or three, selected from the lanthanides and transition metal elements, e.g. one, two, three or more selected from La, Y, Sm, Yb, Sc, Mg, Ca, Co, Ni, Zr, Ti, Hf, Eu, Gd, Pr and Sn, such as Y, La, Zr, and Sc.

[0073] In one embodiment, the ESM comprises, consists essentially of or consists of one or more selected from the group consisting of CeC>2, CeC>2 that is doped with one or more metals, such as one, two or three, selected from the lanthanides and transition metal elements, e.g. one, two, three or more selected from La, Y, Sm, Yb, Sc, Mg, Ca, Co, Ni, Zr, Ti, Hf, Eu, Gd, Pr and Sn, such asM+being a monovalent cation (preferably an alkali metal cation or an ammonium cation, e.g. as represented by formula NR4+as described above, with R being hydrogen or alkyl, such as linear or branched C^.g alkyl) and X2+being a divalent cation (preferably an earth alkaline metal cation). Herein, the monovalent cation M+may be selected from the group consisting of organic cations, e.g. tetraalkyl ammonium, such as tetramethyl ammonium or tetrabutyl ammonium, H+, ammonium (NH4+) and alkali metals, such as Li+, and the divalent cation may be selected from the group consisting of Zn2+and alkaline earth metals, such as Mg2+, Ca2+, Sr2+and Ba2 +

[0074] In one embodiment, which may be combined with the embodiments in the preceding paragraphs, the ESM is a single-phase material. Here, the term "single phase material" denotes a material exhibiting only peaks that can be attributed to a single crystal structure in an X-ray diffractogram, such as a bronze-type structure (space group POg / mmc). In one such embodiment, the single-phase ESM does not contain an organic dye, or any organic substance (substances containing a C-H bond). Here, the ESM may be a completely inorganic material, and is not e.g. an organic-inorganic hybrid as in the case of dye-sensitized oxide materials. Despite being a single phase material, the ESM single phase material may be doped, as outlined above.

[0075] In one embodiment that may be combined with any of the above embodiments, in particular the embodiments wherein the ESM comprises only one, or two or more of the transition metals in Groups 4, 5, 6, and 7, the ESM comprises structural units of oxygen selected from the group consisting of octahedral, tetrahedral, cubic, trigonal prismatic, distorted trigonal bipyramidal structural units, and, tetrahedral, distorted tetrahedral and combinations thereof, the structural units of oxygen having the transition metals in their centers, wherein the structural units of oxygen are preferably selected from the group consisting of octahedral and tetrahedral structural units and distorted versions thereof, and wherein the structural units of oxygen are more preferably octahedral structural units. Examples of such materials include ESMs containing NbOg and / or WOg octahedra in their structure. Without wishing to be bound by theory, it is believed that such structural units may assist in providing a beneficial environment for the generation of electron / hole pairs, and for trapping the thereby photogenerated electrons for sufficient time after illumination has stopped, thereby facilitating the time-separation of the "light" and "dark" reaction. Besides this, such structures may also facilitate attaining preferred properties of the ESM. Such desirable and preferred properties include the ability to form (or the presence of) oxygen vacancies, the ability (or the presence of) polarons, and the presence or adsorption of ions that may lead to opto-ionic (sometimes also called photo-ionic) effects.

[0076] In one embodiment, the ESM is not, or does not comprise, TiC>2, in particular TiC>2 of rutile and / or anatase structure. In one embodiment, the ESM is not, or does not comprise, lithiated TiC>2, i.e., LixTiC>2 (wherein x is not particularly limited, and can e.g. be 0<x<l), in particular wherein the TiC>2 is of anatase and / or rutile structure. In the latter case, materials such as lithiated LTO are not excluded and may be used as ESM.

[0077] In one embodiment, the ESM is not, or does not comprise, MOO3.

[0078] In one embodiment, the ESM has a 2D or 3D crystal structure. Here, a "2D" crystal structure denotes a crystalline material wherein, in an orthogonal x-y-z coordinate system, the extension of a structural motif in one or two dimensions (e.g. x, y) greatly exceed the extensionin one direction (e.g. z), e.g. by a factor of 10 or more, such as 100 or more. Put differently, a 2D crystal structure may be defined as a type of material where the crystal-constituting atoms are arranged in a periodic pattern within a single plane extending in two dimensions (e.g. x and y directions) but with limited thickness in the third dimension (e.g. z direction) that is orthogonal to the plane. This does not just include monolayer-type materials such as graphene, but also a 2D layered materials like a Dion-Jacobson or Ruddlesden-Popper perovskite consisting of more extended 2D "blocks". The extension of the structural motif in the direction orthogonal to the plane (e.g., z) is the smallest amongst the directions x, y and z by a factor of 10 (i.e. lOz < x, y).

[0079] A "3D" crystal structure can be characterized as a spatial arrangement of constituent particles (atoms, ions, or molecules) in a periodic / repeating pattern, extending in all three dimensions (X, Y, and Z axes). A "3D" crystal structure may denote a crystalline material wherein the extension of a structural motif in two dimensions (e.g. x, y) does not exceed the extension in one direction (e.g. z), e.g. by a factor of 10 or more, so that lOz > x, y. Put differently, in a "3D" crystal structure the extension of a structural motif in the three dimensions x, y and z may differ by less than a factor of 10 from each other.

[0080] In one embodiment, a "2D" material may be a nanosheet. Such nanosheets may be formed by exfoliation, as well known to a skilled person. Information on the formation of nanosheets by exfoliation can be found in various literatures.

[0081] Thus, in one embodiment the ESM is or comprises exfoliated nanosheets. The nanosheets may have any thickness, but a thickness in the range of 0.5 nm to 2.5 nm may be preferred in order to obtain the maximum benefits from the limited thickness of the nanosheets. Without wishing to be bound by theory, it is believed that such ESM materials may improve quantum efficiency, charge storage capacity and / or stability, and / or photocharging time, e.g. by providing for an easier and / or faster photo-intercalation of cations and / or a more rapid or more effective quenching of holes originating from the hole-electron pair formed upon absorption of UV or visible light. An ESM in nanosheet form may improve the stabilization of the photogenerated electrons, for example by facilitating adsorption / photointercalation of counter-ions from the electrolyte.

[0082] An ESM in nanosheet form may thus improve the properties of the overall device in terms of the ability to generate electron-hole pairs and / or to store the photogenerated electrons, thereby potentially allowing to obtain improvements with regard to storage efficiency or with regard to the ability to generate a larger amount of the desired chemical species in the subsequent dark reaction.ESM - Properties

[0083] Typically, the illumination of the ESM with electromagnetic radiation, in particular in the visible and / or UV range, leads to a change in the absorption spectrum, which is furthermore typically an increase in absorbance over a relatively broad wavelength range, such as about 400 - 700 nm, that can be observed in UV-VIS spectrometry after a certain time of irradiation, e.g. 30 minutes or less, but preferably less, such as 15 minutes or less, 10 minutes or less, 5 minutes or less, or even 1 minute or less. Without wishing to be bound by theory, it is believed that such an increase in absorbance correlates with the trapping / storing of photogenerated electrons originating from electron / hole pairs formed upon irradiation, and that the duration of time for which the increase in absorbance is observed after the irradiation has stopped correlates with the lifetime of the photogenerated electrons. A faster and / or more persistent (long-lasting) change in the spectrum is believed to indicate fast photocharging, a high efficiency of photoelectron generation and trapping, and reduced loss of photogenerated electrons over time e.g. due to electron-hole recombination or other loss, thereby characterizing a material having preferable properties for use as ESM. A material exhibiting an increase in absorbance after irradiation with UV or preferably visible irradiation, e.g. at 340 nm or higher, and which maintains this increased absorbance over a certain time after the irradiation has stopped (e.g. 30 minutes or more) is thus a good candidate for an ESM. It goes without saying that these properties should be assessed under conditions under which a loss of photogenerated electrons due to e.g. reaction with molecular oxygen can be as far as possible be minimized.

[0084] As will be apparent to the skilled reader, the wavelength of irradiation needs to have an energy of the ESM bandgap or higher. The change in absorption spectrum is often even visible with the naked eye, e.g. by observing after the irradiation a color change from white to blue, indicating that the photogenerated electrons are trapped locally and may form small polarons. These may be stored and subsequently be utilized for the generation of an electric current or for a reduction reaction in a dark reaction. It is further believed that in such materials, the storage time of the trapped photogenerated electrons (e.g. estimated as the time until the absorbance at 400 to 700 nm returns to the absorbance before irradiation) can be improved if the ESM itself contains or can adsorb / photointercalate cations from the electrolyte, as outlined above, as these cations may stabilize the negative charge. This phenomenon is known as electron screening.

[0085] Thus, in one embodiment the ESM may be characterized as a material that after illumination with visible or UV radiation (e.g. at 340 nm or higher, such as 365 nm) for 30 minutes at 200mW / cm2 exhibits an increase in absorbance of an aqueous suspension of the ESM in a solvent system containing 95% by weight water and 5% by weight methanol, determined at an ESM content of 5 mg / ml (e.g. at a wavelength of 570 nm) using a commercial UV-Visspectrometer, or at least 5%, preferably at least 10%, as calculated by the formula (absorbance at 570nm after 30 minute illumination / absorbance at 570nm before UV illumination) x 100. Here, the intensity expressed in mW / crn^ refers to the light intensity on the illuminated surface of the cuvette used for the UV-Vis measurement. Preferably, the specified increase in absorbance is observed after even shorter illumination time than 30 minutes, such as after 15 minutes or less, 10 minutes or less, 5 minutes or less, or even 1 minute or less. As explained above, the observation of in increased absorbance after shorter periods of time is considered to be an indication of a faster photoelectron production efficiency and / or a higher ability to trap the photogenerated electrons. In this test, the aqueous suspension containing methanol acts as TBOR, quenching the hole of the generated electron / hole pair.

[0086] Furthermore, an increase in absorbance of 5% or more, preferably 10% or more, relative to the absorbance before irradiation as expressed by the above formula, is preferably maintained for at least 15 minutes, more preferably at least 30 minutes, most preferably at least 45 minutes after the end of the illumination with radiation at e.g. 340 nm or higher, such as at 365 nm). It is considered that the ability to maintain the increased absorption for a longer period of time reflects an increased ability to stabilize the trapped photogenerated electrons. This higher ability to stabilize may in one embodiment be achieved by using an ESM material that contains cations and / or which is able to adsorb or intercalate cations from the electrolyte, thereby stabilizing the negative charge of the photogenerated electron.

[0087] In another yet related aspect, the ESM is able to form color centers. A color center, also known as an F-center (from the German word "Farbe," meaning color) is a type of point defect in a crystal lattice that can absorb UV and / or visible light and re-emit it. A color center is a point defect in a crystal where an atom or ion is missing, and an electron is trapped in its place. These trapped electrons can absorb certain wavelengths of light, leading to characteristic colors. Color centers can form naturally during the crystal growth process or be introduced intentionally through irradiation, reduction or doping, as well known to a skilled person. The ability to form color centers is often linked or associated with the ability to provide for an increase in absorbance in UV-Vis and / or the presence of oxygen vacancies as described herein. Materials forming color centers can e.g. by identified also by XPS, where in materials such as NbWOg the presence of Nbz*’+and / or W5+can be detected. The formation of such color centers as in NbWOg can also frequently be observed in other tungstates, niobates, niobium tungstates, and titanates such as LTO (Li / jl^O^)- The presence of a color center in these oxides can typically be identified by their characteristic absorbance in UV-vis spectroscopy, as well known to a skilled person.

[0088] Color centers are useful in the context of the present invention, as they are considered to be capable of creating new energy levels within the bandgap. These energy levels can facilitatethe absorption of photons that would otherwise not be absorbed by the material, potentially broadening the range of light wavelengths that the ESM can utilize and increasing efficiency. By introducing intermediate energy levels, color centers may impact the recombination dynamics of electron-hole pairs, potentially extending the lifetime of electron-hole pairs and / or the photogenerated electrons derived therefrom, thereby allowing a longer "storage" of the absorbed light energy as electron-hole pairs or as photogenerated electrons, for allowing the dark reaction to be performed subsequently and preferably on demand. Without wishing to be bound by theory, it is considered that trap states below the conduction band minimum created by oxygen vacancies are one of the main charge trapping centers, typically in conjunction with the transition metal(s) or lanthanide(s). Therefore, the main function of the color center is to trap the photogenerated electron and to stabilize it. Color centers may also facilitate the separation of the electron-hole pair that is formed initially upon absorption of light by the ESM into electron and holes, and may thereby not increase storage time (allowing for a greater time gap between the "light" and the "dark" reactions), but also may improve overall reaction kinetics and the amount of energy stored by a given amount of ESM or a given amount of irradiation energy.

[0089] In one embodiment, the color centers may be formed by atomic scale imperfections in the ESM selected from the group consisting of impurities, vacant lattice sites, surface defects, interstitial atoms and ions, or combinations thereof.

[0090] In the embodiment wherein the ESM comprises or is an oxide of the at least one selected from the transition metals in Groups 4, 5, 6, and 7 and the lanthanides and further comprises a cation of a main group metal of the Periodic Table of the Elements , the ESM may exhibit a bandgap, which is the difference in the energy levels between the conduction band minimum and the valence band maximum, wherein the conduction band minimum is formed by a d° electronic configuration of the at least one transition metal and wherein the valence band maximum is formed by hybridized main group metal s2and oxygen 2p orbitals. The bandgap may have an energy between 0.5 eV and 4.0 eV, preferably between 1.5 and 3.5 eV, such as between 1.5 and 2.5 eV, or between 2.5 and 3.5 eV.

[0091] In one embodiment, the ESM is capable of forming oxygen vacancies in the structure. In the context of the present invention, oxygen vacancies may be considered a type of color center. Oxygen vacancies can be formed in a variety of materials, e.g. as a consequence of a solid-state synthesis at elevated temperatures (such as 700°C or higher) in air, or by using reducing agents such as H2. Examples of materials in which oxygen vacancies have been identified by XPS include NbWOg, Ca2Nb30iQ, Rb2 4W^^O35 and CS4W44O35. Here, XPS can be used to determine the oxidation state of the transition metal, detecting e.g. Nbz*’+and / or W5+in the case of NbWOg.Alternatively, also UV-Vis can be used as set out above in relation to the presence of color centers in general. The ability to form oxygen vacancies is believed to be associated with the following properties:- the presence of a transition metal with variable oxidation states (like Ti, W, Nb, etc), which can stabilize adjacent vacancies;- a high dielectric constant;- bond strength: The bonds between the metal and oxygen atoms can tolerate the removal of oxygen atoms, which helps in stabilizing oxygen vacancies;- polarizable lattices (often structures with polar / ionic bonding);-certain structure types, e.g. perovskites and other crystal structures containing octahedrally coordinated transition metals;- a high mobility of oxide vacancies / oxide ions in the lattice;- good electronic and ionic conductivities (mixed electronic / ionic conductivity);- electrochromic and / or photochromic materials.Based on these properties, suitable candidate materials capable of forming oxygen vacancies can be readily identified by a skilled person. It goes without saying that more suitable ESM materials may simultaneously exhibit more than one, i.e., two or more or even all of the above properties.

[0092] In one embodiment, the ESM has a bandgap in the visible (400 - 700 nm) or near UV (315-400 nm) range, preferably in the visible range. Here, the bandgap can be determined from a Tauc plot as illustrated in the examples and the figures.

[0093] In one embodiment, the ESM has the ability to locally trap photogenerated electrons in the form of polarons and to store them as such in the ESM, wherein the energy level of the polarons is below the conduction band minimum and above the valence band maximum of the ESM. Here, the ability to "trap and store electrons in the form of polarons" means that the presence of the respective polarons is observed for at least 5 minutes, preferably 10 minutes or more after the ESM has been irradiated at 365 nm for 30 minutes at 200mW / cm2, based on a suspension of the ESM in 95wt.% water / 5wt.% methanol at 5 mg / ml (wherein the surface again relates to the illuminated surface of the cuvette used for the measurement). Again, the respective behavior is preferably observed after even shorter irradiation times, such as after 15 minutes or less, 10 minutes or less, 5 minutes or less, or even 1 minute or less, as this is believed to be representative of kinetic aspects of the process for forming and / or storing the photogenerated electrons, such as a higher efficiency of photoelectron generation and / or better ability to stabilize the photogenerated electrons thus formed. Given that molecular oxygen can act as an electron acceptor, it is evident that the respective behavior is to be observed in theabsence of molecular oxygen, e.g. by sufficiently purging the suspension with nitrogen prior to irradiation.

[0094] The presence of polarons can be assessed by electron paramagnetic resonance (EPR) by detecting unpaired spins; by conductivity measurements due to the temperature-dependence of charge transport that is characteristic for polarons, by UV-Vis that identifies characteristic absorption bands of polarons, by photoluminescence spectroscopy that analyzes emission changes due to polarons, by IR / Raman spectroscopy that detects lattice distortions caused by polarons, by electron microscopy that visualizes the lattice distortions associated with polarons. In addition, X-ray absorption spectroscopy provides information on the electronic state and local structure indicative of polarons. Mott-Schottky and ultraviolet photoelectron spectroscopy (UPS) can be used to measure the band edges of the materials.

[0095] In one embodiment, the photoelectrons generated in the ESM have, under oxygen-free conditions, a lifetime of at least 4 hours, preferably at least 8 hours and more preferably at least 12 hours, in the ESM. Longer lifetimes can often be obtained if small polarons are formed. The lifetime can be determined by irradiating the ESM at 365 nm for 30 minutes at 200mW / cm2 and measuring the open circuit potential (OCP) over time. Typically, the irradiation is continued until a stable OCP is observed, but in practice 15 minutes may be employed. In the present invention, the lifetime is determined as the time required until the OCP drops to 50% of the value observed at the time the irradiation is stopped (e.g. 15 minutes after start of the irradiation). Again, the presence of cations (either in the ESM or absorbed / intercalated from the electrolyte) may increase in the lifetime of the polarons, possibly due to the formation of polaron-ion complexes.

[0096] In one embodiment, the ESM is a semiconductor having a bandgap in the range of 0.5 to 4.0 eV.

[0097] In one embodiment, the ESM is capable of photointercalating and photodeintercalating cations or anions. Here, the cations or anions are not particularly limited. The cations preferably include protons (H+) and the alkali metals, in particular Li+, and Na+and K+; the anions preferably include the halogens, such as as Cl’ , Br’, and I’, as well as other types of inorganic anions, includingThe ability of photointercalating and photodeintercalating cations or anions may improve the properties of the ESM to store energy provided in the form of electromagnetic radiation due to the stabilizing effect on the charge generated from the electron / hole pair, e.g. the photogenerated electrons, in particular when the ESM is used in combination with an electrolyte containing such ions.Electrodes

[0098] The ESM is typically contained in a first electrode, while the device also contains a second electrode.

[0099] Both the first and second electrode typically contain a current collector as defined above and an electrical connection between the electrodes. Both electrodes typically contain at least one additional layer, as will be described below, in particular a RRCL and an ORCL, respectively.

[0100] The device of the present invention contains at least one first and one second electrode. However, in order to maximize output or to enable performing reactions that require a higher voltage than delivered by one first electrode, one or more first electrodes and / or one or more second electrodes may be connected in parallel and / or in series. Further, the present invention also includes devices that contain one or more additional electrodes besides the first and second electrode, such as present in a three-electrode device, wherein the one more additional electrodes do not need to satisfy the definitions of the first and second electrode of the present invention.Current Collector

[0101] The material of the current collector is not specifically limited as long as it is able to conduct an electric current. While thus metals such as copper or aluminum can be used, the current collector of in particular the first electrode is preferably made from a transparent material, e.g. a transparent conductive oxide, such as ITO or FTO or doped zinc oxide. This allows providing the ESM not only on the site of the current collector facing the direction from which incident light reaches the device, but alternatively or additionally on the opposite side. In the latter case, the incident light transmits through the transparent electrode and reaches the ESM to cause the "light reaction". In addition to transparent conductive oxides, also metals, conductive organic materials and doped semiconductors may be used.

[0102] Before providing the ESM or other layers, the current collector may be treated in an oxygen plasma to activate the surface and make it more hydrophilic. This step can ensure a homogeneous surface wetting of the current director with a deposition suspension and may remove organic contaminants, if present. After plasma treatment, the respective deposition suspension is deposited on the current collector and subsequently dried. Multiple cycles of deposition may be performed and multiple layers of the same or different materials by be deposited on the current collector. Plasma cleaning is only carried out for the substrate prior to first deposition. Deposition may be performed by dip coating, drop casting, spin coating, theLangmuir-Blodgett or doctor blade techniques. Several subsequent steps may be necessary in case of a weak interaction of the electron storage material and the current collector. After deposition, non-coated parts of the current collector may be sealed with epoxy for example, in order to avoid contact of the current collector and the electrolyte.

[0103] To create an electrical connection, typically wires are attached to the current collector. Depending on sample, contacting may be performed before or after thin film deposition. Copper wires may be glued to the current collector by a conductive glue, such as a silver glue, and the joint may be sealed with epoxy glue.

[0104] The shape of the current collector is not specifically limited, and may be any of sheet form, film form or net form.

[0105] In certain embodiments, the first electrode may not only comprise a layer containing an ESM, but also one, two or all of a layer comprising, consisting essentially of, or consisting of a hole transport material, a layer comprising, consisting essentially of, or consisting of a hole storage material, and a layer comprising, consisting essentially of, or consisting of an oxidation reaction catalyst. These materials will be described in the following. Further layers, such as a glass layer or protective layer, may also be present.Hole Transport Material and Hole Storage Material

[0106] Hole transport materials (HTM) are utilized for the task of charge separation and will be discussed in the following.

[0107] The hole transport material is not particularly limited, and use can be made of those that are known from similar applications, such as in OLEDs. The hole transport material is preferably a conducting material, and more preferably a conducting polymer. The hole transport material is preferably a material selected from the group consisting of polynaphthalenes; polyphenylenes; polyphenylene vinylenes; polyparaphenylene; polyparaphenylene sulfide; polyparaphenylene vinylene; polyanilines; polypyrroles; polycarbazoles; polyindoles; polyazepines;, polythiophenes; polyisothianaphthene; polyacetylenes; polyazulenes; poly(3,4- ethylenedioxythiophene) ; carbon nitrides; quinones; copolymers of thiophenes with polyfluorenes, polycarbazoles, polydibenzosiloles, benzothiadiazoles or diketopyrrolopyrroles; copolymers of selenophenes with polyfluorenes, polycarbazoles or polydibenzosiloles; benzothidiazole; diketopyrrolopyrrol; compounds comprising N-O-bonded oxygen as radical center; quinone-based acenes; compounds comprising carbon centered radical cations;compounds comprising tertiary amine centered radicals; poly(9,9-dioctylfluorene-alt- benzothiadiazole) (F8BT), 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD), and additives including tetracyanoquinodimethane (TCNQ) derivatives, ionic liquids, and metal-TFSI complexess, as well as doped variants of all of them. Also used can be semiconductors such as graphene oxide (GO). GO represents a non-stoichiometric material class, synthesized through partial oxidation of graphene by, e.g., using an oxidizing agent such as hydrazine, via thermal annealing reduction, or using a photocatalyst.

[0108] The hole transport materials (HTM) described above can also be used as hole storage materials. The following table provides an overview of suitable classes of materials, as well as the literature citations describing their preparation and properties:Table 1: Classes of hole storage and hole transport materialsReduction-Catalyst containing layer (RCCL)

[0109] A "reduction-catalyst containing layer" (RCCL) is a layer that comprises a reduction catalyst. The reduction catalyst can be selected based on the type of TBRR and desired dark reaction, but typically includes one or more metals or metal compounds that are known to have activity as hydrogenation catalysts, such as metallic (i.e., oxidation state 0) Ir, Ni, Pd, Pt, Rh or Ru, as well as complexes and metalorganic compounds including one or more of these metals. In a preferred aspect, the reduction catalyst is platin or a platin salt or complex. In another preferred aspect, the reduction catalyst is rhodium or a rhodium salt or complex. In another preferred aspect, the reduction catalyst is palladium or a palladium salt or complex.

[0110] Further examples of reduction reaction catalysts, in particular HER catalysts, include the following: Ru / Rh, Ru / RuOx (x = 1-3, e.g. 2), RuO2 / Pt, Rh / Cr2O3, RhCrOx (x = 2-4, e.g. 3), RuO2 / Rh, Co3O4, Pt / PtOx (x= 1-2), TiO2 / CoOOH / RhCrOx (x = 2-4, e.g. 3), trifluoroacetic acid, and chloro(pyridine)cobaloxime. Further examples include transition metal phosphides (MoP, Ni2P, Co2P, CoP); molybdenum carbides, nitrides and sulfides (e.g., Mo2C, Mo2N, MoS2); Ni- based materials such as Ni-Mo alloys, Ni, NigN, Ni3S2, Ni(OH)2, NiCu on carbon; Co-based materials such as CoP, C02P on CO3O4, CoO@Co, Co(OH)2, C0S2; Ni-Co based materials such as Nig 5C0Q 5P; and Fe-based materials such as NigFeN / rGO or NiFe layered double hydroxide.

[0111] Further examples of reduction reaction catalysts, in particular CO2 reduction catalysts, include the following: Cu, Co porphyrins, Co tetraphenylporphyrin, Au, Ru / TiO2, Au-Cu alloys, CUCO2O4, C02N, Ni(OH)2, WS2, TiO2-co(terpyridine)2, Re(bpy)(CO)3CI where bpy = 2,2'- bipyridyl, Pt, Ag, Pd, Rh, Co phthalocyanine, phosphonic-acid functionalized Ni-cyclam, phosphonated cobalt(ii) bis(terpyridine), cobalt bis(terpyridine), (M0)4 / Cu(100) where M is Fe, Co and / or Ni; MofCOjg; ZnAI layered double hydroxide; Fe-porphyrin which is p-terphenyl (TP). Further examples of reduction reaction catalysts, in particular nitrogen reduction catalysts, include the following: Ag, Rh, Bi, TiC>2, BiOCI, 61507!, Bi5O7Br, MOO3, M0S2, FeS2, FeTe2, FeP2-

[0112] In the RCCL, the metals and metal compounds may be present in an amount of from 0.01 to 100% by weight of the RCCL, with 100% by weight denoting the case where the RCCL consist of the metal or metal compound. The content of the metal or metal compound can be appropriately adjusted and can e.g. be from 0.1 to 20% by weight or from 0.5 to 10% by weight of the total weight of the RCCL. The remainder may optionally be formed by a support material, optionally in combination with a binder material.

[0113] The support material may be appropriately selected and includes e.g. carbon based materials such as carbon black, natural and artificial graphite, and carbon nanotubes, alumina, silica, zirconia, titania, and others, of which carbon-based materials, such as Pt / C, may be preferably employed. The binder material is optional and includes generally known polymers such as polyolefins, (meth)acrylic polymers, polyvinyl acetate, polyesters, polyamides, polyurethanes, fluoropolymers, such as based on TFE and TFE / PAVE copolymers, polyorganosiloxanes, etc., but may also include electrically conductive polymers such as polyaniline. The content of the binder is typically 50% by weight or less, such as 25% by weight or less. If present, the amount is typically 2% by weight or more, such as 5% by weight or more.Oxidation-Catalyst containing layer (OCCL)

[0114] An "oxidation-catalyst containing layer" (OCCL) is a layer that comprises an oxidation catalyst. The oxidation catalyst can be selected based on the type of TBOR and desired dark reaction, but typically includes one or more metals or metal compounds that are known to have activity as oxidation catalysts, such as oxidic forms of Pd, Ru, Rh, Cu, Fe, Ni, Mn, Co, V, or Cr as well as complexes and metalorganic compounds thereof. Examples include BiVC>4, Co- polyoxometalates, Mn-based catalyst, Ir(lll) catalysts, Ni / Co / Fe layered double hydroxides, NiFe2C>4 and other spinels, a-Fe2O3, CO3O4, CO2O3, COO2, NiFeAIO4, BiCu2 Og, CO3V2O8, Cu / CuO, ZnO, ZnCo2C>4, Mn02, Mn2O3, and (MFe)- layered double hydroxides where M is Ni, Co or Li.

[0115] In the OCCL, the metals, metal oxides and metal compounds may be present in an amount of from 0.01 to 100% by weight of the OCCL, with 100% by weight denoting the case where the OCCL consist of the metal or metal compound or oxide. The content of the metal, metal compound or oxide can be appropriately adjusted and can e.g. be from 0.1 to 20% by weight or from 0.5 to 10% by weight of the total weight of the OCCL. The remainder may optionally be formed by a support material, optionally in combination with a binder material.

[0116] The catalyst support material may be appropriately selected and includes e.g. carbon based materials such as carbon black, natural and artificial graphite, and carbon nanotubes, alumina, silica, zirconia, titania, and others, of which oxide materials such as silica, alumina, zirconia, and titania are preferred. The binder material is optional and includes generally known polymers such as polyolefins, (meth)acrylic polymers, polyvinyl acetate, polyesters, polyamides, polyurethanes, fluoropolymers, such as based on TFE and TFE / PAVE copolymers, polyorganosiloxanes, etc., but may also include electrically conductive polymers such as polyaniline. The content of the binder is typically 50% by weight or less, such as 25% by weight or less. If present, the amount is typically 2% by weight or more, such as 5% by weight or more.Electron Transport Material Layer (ETML)

[0117] Both the first and second electrode may comprise an electron transport material layer (ETML), which is defined as a layer containing an electron transport material. In one embodiment, an ETML is present between the layer comprising, consisting essentially of, or consisting of the ESM and the current collector of the first electrode. The electron transport material may facilitate the charge extraction from the ESM to the current collector. An electron transport material may also prevent holes created during photoexcitation of electrons in the electron storage material from recombining with electrons on the current collector. Thus, the electron transport material may prevent charge recombination, and thus may improve charge storage. The electron transport material may be a titanium dioxide, or a material different from titanium dioxide, such as a material not containing titanium, the electron transport materials listed in Tables a) and b) may be used in the present invention.Table a) Electron Transport MaterialsTable b) Electron Transport MaterialsTBOR and TBRR, and Electrolyte containing TBOR and TBRR

[0118] In certain embodiments, the device of the present invention includes an electrolyte that comprises a to-be-reduced reactant (TBRR) and a to-be-oxidized reactant (TBOR). However, depending on the application and use, in certain embodiments an electrolyte may contain one of a TBOR a TBRR. A TBOR or TBRR may also be an analyte to be detected by a device or use of the present invention, and a TBOR typically has the function of quenching holes of an electron / hole pair that is formed upon irradiation of an ESM. A TBRR may also be a reaction substrate (starting material) for a dark reaction.

[0119] The TBRR and the TBOR may be the same or different compounds. An example wherein the TBRR and the TBOR are the same compound is wherein both are water, in which case the protons are reduced to form molecular hydrogen (H2) and the oxygen is oxidized to molecular oxygen (O2). The energy required for the water electrolysis originates from the absorption of solar energy by the ESM, causing the formation of electron / hole pairs. The TBRR is reduced at the second electrode, preferably at the RRCL if present, and the TBOR is oxidized at the first electrode, preferably at the OCCL.

[0120] The electrolyte may thus be pure water, but may also be an aqueous or alcoholic solution (e.g. an ethanolic or methanolic solution). The aqueous solution may be formed by water and one or more additives, such as an acid or base adjust the pH, or an inorganic salt.

[0121] The TBOR and the TBRR may be different compounds. For example, the TBRR may still be water (or more specifically, the protons of water), and the TBOR may be an organic or inorganic compound. The TBRR may also be the protons of an acid, in which case the protons may be reduced to form molecular hydrogen in a HER as, or as part of, the dark reaction.

[0122] The TBRR may also be oxygen that is reduced to e.g. H2O or H2O2. The TBRR is however not limited to oxygen, water or protons, and may in principle be any species or compound that can undergo a reduction reaction to form a reduced form of the species or compound. Other examples include CO2, CO, N2, NO2~, NO35 or organic compounds such as monobasic or polybasic carboxylic acids as proton donors.

[0123] In further embodiments, the TBRR is CO2 and the product is selected from the group consisting of CO, formaldehyde, formic acid, acetic acid, methanol, ethanol, methane, ethylene and mixtures thereof. In another embodiment, the TBOR is nitrogen in e.g. molecular form (N2), and the product is NH3.

[0124] The TBOR is a chemical species or compound that is utilized in the photoelectrochemical device to donate electrons, thereby being oxidized. In one example, the TBOR may be as present in water or other compounds, which may then be oxidized to form molecular oxygen (O2) or H2O2.

[0125] In one embodiment, the TBOR is selected from organic compounds, such as those having a hydroxy group, e.g., primary, secondary or tertiary monohydric or polyhydric alcohols, including aliphatic compounds such as methanol, ethanol and ethylene glycol, and aromatic compounds such as phenols, organic acids such as citric acid, as well as sugars, which may be oxidized to the corresponding aldehydes, ketones, acids, or to CO or CO2. In order to reduce carbon oxide emissions, the reaction product of the TBOR is preferably not CO or CO2, but an aldehyde or ketone. It is also within the scope of the present invention to use a material as TBOR that is typically regarded as waste material, such as biomass originating from agriculture or livestock production, or plastic waste, e.g., in the form of microplastic. Other examples of such TBORs are undesired byproducts from chemical or nutritional manufacturing or purification processes.

[0126] In one embodiment, the TBOR is selected from inorganic materials. Examples include halogenide ions such as Cl- or I-, or h’ which may be oxidized, or oxidizable metals cations such

[0127] The concentration of the TBOR and the TBRR in the electrolyte is not particularly limited. In case where the TBOR and TBRR are not the solvent (e.g. in the case where both are not water), the concentration of each of the TBRR and the TBOR may be selected to each to be 0.01 mol / l or higher, such as 0.1 mol / l or higher. In the case where the TBRR or TBOR is an analyte to be detected, the detectable concentration of the TBOR or TBRR can be lower, such as 0.1 mol / l or less, e.g., 0.01 mol / l or less or 0.001 mol / l or less.

[0128] The pH of the electrolyte is not particularly limited, but is preferably 7 or less, such as 6 or less, but typically 0.1 or more, such as 1 or more. A pH in this range may facilitate the HER in the dark reaction, and a too acidic pH may lead to reduced stability and corrosion of the device.Additional Device Elements and Features

[0129] In addition to the electrodes (at least one first and one second electrode, and optionally further electrodes) and the electrolyte described above, the photo-electrochemicaldevice of the present invention may optionally, yet preferably include one or more additional elements (or members). These include without limitation the following:Potentiostat

[0130] In some embodiments, the photo-electrochemical device of the present invention may comprise a potentiostat. A potentiostat is a member adapted to control the voltage between two electrodes in a multiple electrode electrochemical cell, and the potentiostat typically contains internal circuits that allow it to function in this capacity. The circuits generate and / or measure potentials and / or currents. The potentiostat is connected to the electrodes, specifically, the electrode current collector, of the device.

[0131] A potentiostat may be included in order to compensate for fluctuations in the current / voltage generated by the first electrode due to fluctuations in e.g. operating conditions such as temperature and / or fluctuations in solar energy over day- and night-time, thereby ensuring a more stable production efficiency and reaction rate of the "dark" reaction occurring at the second electrode (preferably at the RRCL, if present).

[0132] In one embodiment, a potentiostat may be used to amplify or supplement the current and / or voltage supplied by the first electrode, thereby allowing to increase the production efficiency of the "dark reaction" at the second electrode and / or to enable reactions for which the potential provided by the first electrode is insufficient. The operation of the potentiostat such as a degree of voltage amplification may be controlled by a controller. Still, this allows the exploitation of solar energy by the contribution from the first electrode to the current or voltage supplied to the second electrode / the RCCL.Load

[0133] In certain embodiments, the photo-electrochemical device of the present invention may comprise a load, optionally in addition to a potentiostat. A load is generally defined as an element or device consuming electric energy for operation.

[0134] The load may be of any kind, and examples include an LED, other light or display that indicates the presence of a desired voltage and / or current, the status of the device, or quantitative or qualitative information regarding the production of the desired compounds at the second electrode or RCCL. The load may also be a device operating a moveable device, e.g. an electric motor, driving for instance a physical switch or valve. The load may also include a controller, which controls and / or operates another member of the device, e.g. a switch or valve controlling the inflow or outflow of oxidation or reduction products produced from the TBRRand / or TBOR present in the electrolyte. The load may also include a chargeable battery and / or a capacitor, which is charged by the electric energy provided by the generated by the photoelectrochemical device.Switch

[0135] In certain embodiments, the photo-electrochemical device of the present invention may comprise a switch, optionally in addition to a potentiostat and / or a load. The switch is used to enable / disable an electrical connection, e.g., between the first and second electrode, whereby this connection may also contain a potentiostat and / or load connected in series. The switch may be operated manually or may be operated by a controller. Examples for a switch operated by a controller include a relay, a transistor, and the like.Housing

[0136] Generally, the photo-electrochemical device of the present invention is present in a housing that contains the electrolyte and provides protection against environmental factors. The housing is partially or fully made from a transparent or translucent material such as glass or a polymeric material, such as polyethylene terephthalate (PET) or polycarbonate (PC). The housing may contain electrical connections for a battery, potentiostat, load or switch, as illustrated in the figures.

[0137] The housing may further contain input or output lines or valves that are operated manually or automatically, e.g. a pressure release valve for venting produced hydrogen of oxygen from a "dark" HER or OER reaction. The housing may also contain a connection for a line that is used for re-freshening, supplementing and / or replacing the electrolyte or components thereof, such as to supplement fresh electrolyte, TBOR or TBRR to the system, or for providing fresh catalyst.

[0138] The housing may further be structures such as contain several (i.e. two or more, such as 5 or more or 10 or more) compartments, each of which contains a photo-electrochemical device as described herein. The compartments may be connected in series or in parallel to maximize the production of the reduced compounds (e.g. H2 produced in a HER).Heating and / or Cooling device

[0139] As will be readily apparent to any skilled person, almost any electrochemical reaction is temperature-dependent, and the reactions occurring at the ORCL and RRCL are no exceptions. Further, in addition to requiring the presence of electrons and / or holes, the reduction and / or oxidation reaction rates may be influenced by providing for a suitable reactiontemperature, and generally a temperature in the range of 15 - 95°C, such as from 20 - 90°C or from 30 - 80°C, may be preferable, depending on the type of catalysts, TBOR and TBRR. In practice, often temperatures in the range of 15 - 50°C may be preferable, as in this temperature window a sufficient reaction rate can be ensured while avoiding harsh conditions.

[0140] It is entirely conceivable that the desired temperature is obtained from solar power, i.e. by the natural heating that is observed for any material that is exposed to solar radiation. Depending on the conditions (e.g. during winter time), this may however not be sufficient. Also, when exposed to intense direct sunlight in summer, temperatures may be higher than desired and may cause thermal degradation of the device components and / or evaporation of the electrolyte. Also, the reactions of the TBOR and / or TBRR occurring at the ORCL and RRCL, respectively, may be exothermic or endothermic, thereby altering the temperature of the device / the electrolyte.

[0141] The device of the present invention is therefore optionally equipped with a cooling and / or heating device, such as to achieve and / or maintain a suitable temperature window. Suitable devices are known to a skilled person, and a description thereof will thus be omitted. If the photo-electrochemical device is equipped with a cooling and / or heating device, this may be controlled by a controller and may further include a temperature sensor.Controller

[0142] The device of the present invention may, in certain embodiments, additionally comprise a controller that is adapted to control the operation of the device and / or which monitors the delivered current and / or potential. Such a controller may execute certain operations upon fulfilment of pre-defined conditions, such as a shutdown of the device outside a pre-defined temperature window or upon detection of critical conditions, or activation of a heating or cooling device, if pre-defined temperature conditions are not met.

[0143] Additionally or alternatively, the controller may operate external devices for replenishing, refreshening and / or supplementing the electrolyte or components thereof, such as the TBRR or TBRR, upon detection of a reduction in voltage and / or current and / or production rate of the desired compound, such as molecular hydrogen in a HER.

[0144] The controller may further operate or control a potentiostat and / or switch and / or load and / or heating and / or cooling device, as described above.Voltage Sensor

[0145] In certain embodiment, the device of the present invention may comprise a voltage sensor. Voltage sensors are known in the art and can be appropriately chosen by a skilled person. For example, a voltage sensor may be used to measure a voltage between two electrodes / layers of the photo-electrochemical device or a voltage between an electrode / layer and ground.

[0146] A voltage sensor may be used to monitor or control the proper functioning of the device, e.g. monitoring the voltage / potential that is provided by the device upon irradiation or in the dark. The voltage sensor may be part of or connected to a control device that is adapted to receive a signal related to the detected voltage, and which, depending on the detected voltage, operates the device, for example by operating a switching device. The switching device may operate an electrical switch or a physical entity, such as a valve. For example, the switching device may be configured to be in a closed or opened state when the second voltage is above a predetermined threshold value, e.g. such that if the voltage is above a pre-determined threshold an electrical switch is operated that allows a current to flow to the second electrode for effecting an electrochemical reaction, such as a hydrogen evolution reaction (HER). Also, the voltage sensor signal may be used in connection with the potentiostat described above.

[0147] As one example, the voltage sensor may be used to monitor the voltage delivered by the device of the present invention. If the voltage is below a pre-set threshold, the voltage sensor may communicate with a controller to close a valve, e.g. a valve in a line for transporting generated hydrogen away from the device.

[0148] In one embodiment, a voltage sensor is employed in a device containing a HSM. The voltage sensor is configured to measure a second voltage between the first electrode and the HSM -containing layer, and the device is configured such that the switching device is in a closed state when the second voltage is above a predetermined threshold value. In this manner, the degree of saturation of the HSM material with holes can be determined and the state of the device be identified.Device Embodiments

[0149] In the following, certain device embodiments will be described, partially with reference to the drawings. It is readily apparent that certain changes to the devices may be made within the scope of the appended claims. Further, the specific features of one embodiment (e.g., the design and composition of the first electrode) may be transposed to other embodiments, i.e.may be combined with other embodiments, except where this is clearly not applicable from the overall design of the device embodiment.

[0150] In one embodiment, the device comprises the elements and features as set out in Figure 1. In this embodiment, the device comprises a first and a second electrode that are present in a liquid electrolyte, typically water or an aqueous solution comprising the TBOR and TBRR as described above. The first electrode includes a current collector (here represented by FTO), in this order, on one side of the current collector and in contact therewith, a layer comprising, consisting essentially of, or consisting of the ESM as described above, and a layer comprising, consisting essentially of, or consisting of the oxidation reaction catalyst as described above. On the other side of the current collector, an optional glass layer is provided in contact with the current collector. The second electrode contains a current collector and a reduction reaction catalyst containing layer (RRCL) that comprises, essentially consists of, or consists of a reduction catalyst as described above.

[0151] Further, an electrical connection is provided between the first and second electrode, in this embodiment including a potentiostat and a switch. The electrical connection is arranged to facilitate the transferring of electrons from the first to the second electrode. Other elements that are not shown in Figure 1 may be present, such as a controller or load. Further, while the housing shown here contains only a single photo-electrochemical device, the housing may host a plurality of photo-electrochemical devices.

[0152] The electrolyte comprising the TBOR and TBRR is in contact with at least the ESM and the second electrode, preferably the RRCL if present. Alternatively, for an embodiment comprising an ORCL arranged to receive holes from the ESM, the electrolyte is in contact with the ORCL and the second electrode, preferably with the RRCL if present. In the embodiment shown in Figure 1, the electrolyte is also in contact with the current collector, the ORCL and the glass layer. However, such a configuration is not necessary, and the current collector and / or the ESM may not be in contact with the electrolyte. This is because the reaction of the TBOR occurs at the ORCL, and no contact of the TBOR with the current collector and / or the ESM is required, given that the ESM provides the holes to the ORCL for reaction with the TBOR. Indeed, it may be preferable to design the device such that there is no contact between the electrolyte and the current collector and / or the ESM in order to avoid side reactions or decay of the current collector and / or the ESM by reaction with the electrolyte. This can be realised by surrounding the current collector and / or the ESM with glass or another protective material, such as a (preferably transparent or translucent) polymeric material.

[0153] In the device shown in Figure 1, the light enters the device and transmits through a glass layer and a transparent or at least semi-transparent current collector (FTO) before reaching the ESM, thereby generating electron-hole pairs in the ESM. However, such a design is not necessarily realised, and the light may reach the ESM without transmitting through the transparent electrode and / or an optional glass layer.

[0154] In an embodiment, the device may adopt a configuration as illustrated in Figure 2. Here, the above explanations regarding Figure 1 also apply, the main difference to the device illustrated in Figure 1 being the presence of layer comprising, consisting essentially of, or consisting of an HTM between the layer comprising, consisting essentially of, or consisting of the ESM. The presence of such a HTM layer may facilitate hole transfer to the ORCL, and may thereby facilitate efficacy or turnover of the oxidation reaction of the TBOR at the ORCL.

[0155] In an embodiment, the device may adopt a configuration as illustrated in Figure 3. Here, the above explanations regarding Figure 1 and 2 also apply, the main difference to the device illustrated in Figure 2 being the presence of layer comprising, consisting essentially of, or consisting of an HSM between the layer comprising, consisting essentially of, or consisting of the HTM and the layer comprising, consisting essentially of, or consisting of the ESM. The presence of such a HSM layer may facilitate hole may facilitate storing of holes over an extended period of time such as over multiple hours (e.g. after the illumination has stopped).

[0156] In the devices illustrated in Figures 1, 2 and 3, the first and second electrodes are separated by the electrolyte. However, such a device design is not compulsory, as illustrated in Claim 11. A possible embodiment of the device of Claim 11 is illustrated in Figure 4.

[0157] In the embodiment of Claim 11, the first and second electrodes may be integrated into a single electrode assembly surrounded by the electrolyte. Here, the photo-electrochemical device comprises a layer comprising, consisting essentially of or consisting of an ESM as set out above; an RRCL comprising, consisting essentially of or consisting of a reduction reaction catalyst, arranged to receive electrons from the ESM-containing layer; a layer comprising, consisting essentially of or consisting of an HSM; an ORCL comprising, consisting essentially of, or consisting of an oxidation reaction catalyst, arranged to receive holes from the HSM; a layer containing an HTM provided between the ESM-containing layer and the HSM-containing layer, the HTM- containing layer being arranged to transfer holes from the ESM-containing layer to the HSM- containing layer; and an electrolyte containing a TBOR and a TBRR, the electrolyte being in contact with the ORCL and the RRCL.

[0158] The device as claimed in Claim 11 may further comprise two current collectors as set out above to form the first and second electrode, which are provided between the RRCL and the ESM or within the ESM, and within the HSM or between the HSM and the HTM, respectively. In the same manner as outlined above, a load, switch and / or potentiostat may be electrically connected with the current collectors, e.g. in series or in parallel. The electrode current collectors and the respective connections thus establish an electrical connection between the ESM-containing layer and the HSM-containing layer.

[0159] Such a design of the device may provide practical advantages. It is readily apparent that the electrode assembly as shown in e.g. Figure 4 may be activated by dipping into the electrolyte, and such an electrode assembly can be easily removed and replaced from the device. In such a design, the electrode assembly may also easily be provided in a rotatable manner such as to follow the direction of incident light (e.g. over the course of the day), thereby maximising production efficiency.

[0160] In all of the devices shown in the Figures, the electrolyte is provided stationary, i.e. is contained in a closed housing together with the other elements of the photo-electrochemical device and does not flow. As will be readily apparent to a skilled person, such a design is not compulsory, and the electrolyte may be present such as to flow around the respective 1st and 2nd electrodes, respectively the electrode assembly, in all embodiments (not only those shown in the Figures). This may provide practical benefits and may maximise production efficiency, as thereby local concentration gradients can be avoided and the educts and products of the respective reactions at the ORCL and RRCL can be supplemented / replenished or removed, respectively, at an increased rate, thereby allowing continuous operation over extended time periods. In such devices, the TBOR and / or the TBRR may also be stored in separate containers or may be provided or replenished via supply lines that may be controlled manually or automatically by means of a controller. For example, the devices of the present invention can be employed at a site where organic waste material such as biomass or undesired byproducts are produced, and the device of the present invention would then allow turning the waste material into valuable materials, such as molecular hydrogen.

[0161] Further embodiments of devices that can be employed in the present invention, in particular in the method for photocatalytic production of a product or for using the ESM for producing a product photo-catalytically, are shown in Figures 64 and 65. A more detailed description of these devices is given in the disclosure relating to the method for the photocatalytic production of a product as described below.Method for photocatalytic production of a product

[0162] In one aspect, the present invention pertains to a method for the photocatalytic production of a product, comprising the steps of illuminating an electron storage material, ESM, with light to create photogenerated electrons in the ESM, storing the photogenerated electrons in the ESM, and reacting a to-be-reduced reactant with the stored photogenerated electrons, optionally in the presence of a reduction catalyst, to form a product.

[0163] Here, the ESM is defined as described above in relation to the device, including the preferred embodiments thereof. However, in this aspect the ESM is not TiC>2, E^WOg and WO20O56, and with the further proviso that the ESM satisfies at least one of the following requirements (i) to (iv):(i) it does not include (P2W17O61)10- anions;(ii) it does not include a polyoxometalate, POM;(iii) it does not include a photosensitizer including a (4,4'-di-tert-butyl 2,2'-bipyridine)2 Ru (diethyl((4'-methyl-(2,2'-bipyridine)-4-yl)methyl)phosphonate) moiety; and(iv) it does not include an organic photosensitizer.

[0164] Each of the above (i) - (iv) may apply separately, as outlined in claims 48 -52.

[0165] The product produced by the method is not particularly limited, but may preferably be molecular hydrogen (H2) that is obtained by electrolysis of water or aqueous solution, the aqueous solution preferably having a pH of 7 or less, such as 5 or less, and 0 or more, such as 1 or more. Other products that can be produced by the method of the present invention include generally all organic and inorganic compounds that are accessible by an electrochemical reduction reaction, such as benign metals (metals in oxidation state 0), e.g. copper, zinc, aluminium, or other main group metals or transition metals, metal ions that can be stably present in different oxidation states (e.g. Cr in oxidation states +6 and +3, or Mn in oxidation states +7, +4 and +2, cobalt in oxidation states +3 and +2, etc.), and organic compounds such as primary, secondary and tertiary alcohols, sugars, and aldehydes.

[0166] In a specific embodiment, the to-be-reduced reactant is molecular oxygen and the product is H2O2 and / or H2O. In another embodiment, to-be-reduced reactant is CO2 and the product is selected from the group consisting of CO, formaldehyde, formic acid, acetic acid, methanol, ethanol, methane, ethylene and mixtures thereof. In a further embodiment, the to- be-reduced reactant is molecular nitrogen and the product is NH3

[0167] In one embodiment, the method for producing a product may utilize photogenerated electrons that are stored in an ESM until they are extracted. The extraction can be effected by adding a reduction catalyst and reacting the photogenerated electrons with the to-be-reduced reactant to form the product, e.g. by adding a platinum-based catalyst to a medium containing an ESM containing photogenerated electrons. In other embodiments, the electrons can be extracted by closing an electrical switch, thereby providing a reduction potential to an electrode that is in contact with a TBRR-containing medium, such as water or an aqueous medium.

[0168] The method for producing a product may be performed using photogenerated electrons that are stored in an ESM while the ESM is under light illumination, such as by UV or visible light, preferably natural light (sunlight). In other embodiments, the method for producing a product is performed while the ESM is under no illumination, e.g. at night or in the dark. Both embodiments may be combined, allowing prolonged or continuous production of the product, such as over a part of or the entirety of day- and night time.

[0169] In one embodiment, the extraction of photogenerated electrons is thus performed after the ESM has been illuminated, and thereby photo-charged, to provide for photogenerated electrons stored in the ESM. The extraction can e.g. be effected by adding a reduction catalyst after the illumination has stopped, thereby providing the on-demand generation of reduced compounds (e.g. molecular hydrogen) from a photo-charged ESM, as also illustrated in the following examples.

[0170] Further, in all of the above embodiments, the illumination of the ESM (photocharging) may be performed in the presence of an aqueous solution containing protons, alkali metal cations, earth alkaline metal cations, and / or ammonium cations (including NH4+and tetraalkyl ammonium cations, such as TBA). The presence of such cations may lead to photointercalation of the respective cations, stabilizing the trapped negative charge.

[0171] In one embodiment, the ESM is present in the form of particles that are dispersed in an electrolyte. Here, the same description and definition of the electrolyte as given in the description of the devices of the present invention applies. The electrolyte is thus preferably a liquid or gel-type electrolyte containing a TBOR. In this embodiment, the particles may consist of the ESM, or may contain the ESM on a suitable carrier, such as silica, alumina, titania or other inorganic oxides. The carrier may also be or comprise a material as described above for the HTM / HSM, or may be or comprise material as described above for the reduction reaction catalyst or oxidation reaction catalyst. In the case of a supported ESM, the weight percentage of the ESM (relative to the total weight of the particles) may be in the range of from 10-90 wt.%,such as from 20 - 80 wt. %, the remainder comprising a support material and / or an oxidation reaction catalyst as described above.

[0172] The method for photocatalytic producing a product may thus rely on a reduction reaction catalyst and / or an oxidation reaction catalyst. The presence of such catalysts is not mandatory, yet preferred. The catalysts have been described above in connection with the devices and other methods of the present invention, and the same disclosure applies in respect of the method for photocatalytically producing a product. Incidentally, the method for photocatalytically producing a product can be implemented using the devices described above. Alternative examples of devices that can be used to implement the method are shown in Figures 64 and 65.

[0173] Figure 64 shows an implementation of the method utilizing a device wherein the ESM is provided on a support material comprising, or consisting of, a ORC and RRC, suspended in an electrolyte containing a TBOR. Upon illumination with light comprising wavelengths having an energy higher than the bandgap of the ESM, electron / hole pairs are formed, and the holes are quenched by reaction of the TBOR at the ORC, causing the formation of trapped photogenerated electrons within the ESM. Subsequently, e.g. in the dark / after ceasing irradiation, upon addition of TBRR (illustrated by the syringe), the electrons can be extracted and utilized for the production of the desired compound, e.g. molecular hydrogen in a HER.

[0174] Figure 65 shows an implementation of the method utilizing a device wherein the ESM is provided on an oxidation reaction catalyst (ORR) as described above in relation to the device of the present invention, which in this case simultaneously acts as a carrier for the ESM, suspended in an electrolyte containing both an TBRR and TBOR (which may the same or different, as explained above in relation to the device of the invention). Since the TBRR and TBOR have been described above, a detailed description is omitted here.

[0175] In the first step, the ESM on the ORR is illuminated with light comprising wavelengths above the band gap of the ESM to form electron / hole pairs, and the holes are quenched by reaction of the TBOR at the ORR. After illumination, in step 2, the reduction reaction catalyst (RRC) is added, here illustrated by the syringe. The addition of the RRC initiates the extraction of the photogenerated electrons and causes the reduction reaction of the TBRR at the RRC, thereby yielding the desired product. For example, the RRC can be a suspension of platinum, and the desired product can be molecular hydrogen (H2) - In step 3, after forming the desired product, the RRC is filtered off, leaving the device in a state where it again can be photocharged (i.e. returned to step).

[0176] The devices of Figures 64 and 65 are merely illustrative, and of course the concept of the present invention and the method for photocatalytically producing a product can also be implemented using different device designs. For example, a suspension containing ESM particles a TBOR and a TBRR may be pumped towards an irradiation section where irradiation takes place, and then, after formation of the photogenerated electrons, be pumped to a separate section containing a RRC, to extract the photogenerated electrons and to form the desired product. Such a device could then be operated continuously or intermittently.Photo-rechargeable Battery

[0177] In one aspect, the present invention pertains to a photo-rechargeable battery comprising a negative electrode comprising a substrate with a surface and a layer comprising an electron storage material, ESM, provided on the surface of the substrate, and a positive electrode comprising a substrate with a surface and a layer comprising a hole storage material provided on the surface of the substrate, and a photovoltaic element sandwiched between the layer comprising the ESM and the layer of the hole storage material, wherein the photovoltaic element is capable of charging the electrodes upon illumination, and wherein the ESM is selected from the group consisting of CeC>2, M+fNbWOg)’,lent cation and wherein X2+is a divalent cation. As outlined above, M+can be any monovalent cation (preferably an alkali metal cation or an ammonium cation, e.g. as represented by formula NR4+as described above, with R being hydrogen or alkyl, such as linear or branchedalkyl) and X2+can be any divalent cation (preferably an earth alkaline metal cation). Also here, the monovalent cation M+may be selected from the group consisting of organic cations, e.g. tetraalkyl ammonium, such as tetramethyl ammonium or tetrabutyl ammonium, H+, ammonium (NH4+) and alkali metal cations, such as Na+and Li+, and the divalent cation may be selected from the group consisting of Zn2+and alkaline earth metal cations, such as Mg2+, Ca2+, Sr2+and Ba2+Further, the ESM may be optionally doped with one or more metals, e.g. selected from the transition metal and the lanthanides, such as one, two or three selected from the lanthanides and transition metal elements, e.g. one, two, three or more selected from La, Y, Sm, Yb, Sc, Mg, Ca, Co, Ni, Zr, Ti, Hf, Eu, Gd, Pr and Sn, such as Y, La, Zr, and Sc.

[0178] A photorechargeable battery within the meaning of the present invention is a battery that can be charged by exposure to light, without having to apply an external voltage. The photovoltaic element is capable of charging the electrodes upon illumination. Hence, a photovoltaic element within the meaning of the present invention is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect. The photovoltaic effect is defined as the creation of voltage in a material upon exposure to light.

[0179] As ESM used herein, the same disclosure of the ESM used in the other devices of the present invention applies, except that the ESM is limited to compounds that satisfy the above compositional formula. All other properties however also apply, e.g. the preferred ability to form color centers or polarons, the preferred structural motifs, or the bandgap thereof. A detailed description of the ESM is thus omitted, and reference is made to the above instead.

[0180] The negative electrode comprises the ESM and the substrate. The substrate is made from a conductive material, and here the same description of the electrode current collector applies.

[0181] The layer of the ESM may be formed on one side of the negative electrode substrate or may be formed on multiple sides of the substrate of the negative electrode. The negative electrode may also comprise the ESM such that the ESM surrounds the substrate surface in at least part thereof. As will be readily apparent to a skilled reader, the negative electrode will also contain elements for connecting to the battery, such as wiring or tab.

[0182] The layer of the HSM may be formed on one side of the positive electrode substrate or may be formed on multiple sides of the substrate of the positive electrode. The positive electrode may also comprise the ESM such that the ESM surrounds the substrate surface in at least part thereof. As will be readily apparent to a skilled reader, the positive electrode will also contain elements for connecting to the battery, such as wiring or tab.

[0183] The photovoltaic (PV) element is not particularly limited, and any element known in the art can be used. Such elements are well-known and are typically based on silicon, such as monocrystalline or polycrystalline silicon. Also, the use of dye-sensitized solar cells and hybrid perovskite solar cells is encompassed by the present invention, as is the use of thin film solar cells such as based on amorphous or polycrystalline silicon, Cadmium Telluride (CdTe) and Copper Indium Gallium Selenide (CIGS) cells. Further, the PV element may comprise an n-type semiconductor layer and / or a p-type semiconductor layer.

[0184] The photovoltaic element is preferably a semiconductor or semiconductor composite containing a p-n-junction. A p-n junction is a boundary or interface between two types of semiconductor materials, p-type and n-type, inside a single crystal of semiconductor. The photovoltaic element is more preferably a layer structure of p-type and n-type semiconductors, and most preferably a layer structure of a layer of a p-type semiconductor and a layer of an n-type semiconductor. In this configuration, the layer of the n-type semiconductor is in contact with the surface of the layer of the electron storage material opposite to the substrate of the negative electrode, and the layer of the p-type semiconductor is in contact with the surface of the layer of the hole storage material opposite to the substrate of the positive electrode. In this configuration, the p-type semiconductor layer and the n-type semiconductor layer are in contact. An inverted configuration of the photovoltaic element, in which the p-type semiconductor is in contact with the electron storage material and the n-type semiconductor is in contact with hole storage material, with an ohmic contact in-between the n-type and the p- type semiconductor, is also possible (so called "Z-Scheme")

[0185] The p-type and the n-type semiconductor preferably have a lower band gap compared to the electron storage material. By absorption of the sunlight by the p-type and the n-type semiconductor having a lower band gap, the efficiency of visible light absorption and hence charge storage can be improved. Due to the larger energy gap between the electron and hole storage material compared to the p-type and the n-type semiconductor (especially in the Z- scheme approach), a high cell voltage can be maintained.

[0186] The photovoltaic element is provided sandwiched between the layer comprising the ESM and the layer of the hole storage material, such that upon illumination with suitable radiation (e.g. sunlight) the electrodes comprising the ESM or HSM, respectively, are charged and entrap the electron and / or the hole, such as in the form of small polarons. Thereby, the radiation (solar) energy is stored in the form of trapped charges in the ESM and HSM, respectively, and can be released at a later point in time (e.g. after the illumination has stopped).

[0187] The photorechargeable battery may contain the other conventional components of a battery, such as an electrolyte, to which the same description as above applies. However, in this aspect of the present invention, a TBOR and TBRR is not mandatory, as the charges originate from the PV element. Hence, electrolytes generally used in this field can be employed.Auto-photorechargeable BatteryIn one aspect, the present invention pertains to an auto-photorechargeable battery comprising a negative electrode comprising a substrate with a surface and a layer comprising an electron storage material, ESM, provided on the surface of the substrate, a positive electrode comprising a substrate with a surface, and a layer comprising a hole storage material provided on the surface of the substrate, and an electrolyte, wherein the ESM is selected from the group consisting of CeC>2, M+fNbWOg)’,lent cation and wherein X2+is a divalent cation; and wherein the ESM is optionally doped with one or more metals, such as one, two or three, selected from the lanthanides and transition metal elements, e.g. one, two, three or more selected from La, Y, Sm, Yb, Sc, Mg, Ca, Co, Ni, Zr, Ti, Hf, Eu, Gd, Pr and Sn, such as Y, La, Zr, and Sc.

[0188] As outlined above, M+can be any monovalent cation (preferably an alkali metal cation or an ammonium cation, e.g. as represented by formula NR4+as described above, with R being hydrogen or alkyl, such as linear or branchedalkyl) and X2+can be any divalent cation (preferably an earth alkaline metal cation). Also here, the monovalent cation M+may be selected from the group consisting of organic cations, e.g. tetraalkyl ammonium, such as tetramethyl ammonium or tetrabutyl ammonium (TBA), H+, ammonium (NH4+) and alkali metal cations, such as Na+and Li+, and the divalent cation may be selected from the group consisting of Zn2+and alkaline earth metal cations, such as Mg2+, Ca2+, Sr2+and Ba2+-

[0189] As mentioned above, a photorechargeable battery within the meaning of the present invention is a battery that can be recharged by irradiation of visible light, without having to apply a voltage. An autophotorechargeable battery within the meaning of the present invention is a photorechargeable battery, wherein both steps of light harvesting and electrical energy storage occur within the same material, namely in the ESM. Hence, light harvesting and charge storage are combined in the same material. More particularly, in the ESM, photogenerated electrons are stored in a state trapped in the ESM. Thus, the autophotorechargeable battery is able to create a photopotential and to provide a current during, and in particular, after illumination.

[0190] The autophotorechargeable battery of the present invention is distinct form a battery that can only undergo photo-assisted charging and de-charging and cannot be charged by irradiation without applying a voltage. The autophotorechargeable battery of the presentinvention is also distinct form a photorechargeable battery according to the above aspect of the invention, in which light harvesting and electron storage does not take place in the same material.

[0191] The electrode of the autophotorechargeable battery is preferably a first electrode as defined above for the devices of the present invention. More particularly, the ESM used in the autophotorechargeable battery is an ESM as defined above. The broadest and also the more specific and preferred disclosure relating to the ESM in other aspects of the present invention is fully applicable also for the autophotorechargeable battery.

[0192] In another embodiment, the present invention also includes a voltage efficient charging (VEC) solar battery. Here, the same disclosure as above in relation to the auto- photorecharchable battery applies, except that an external voltage is applied. Such an external voltage may be derived from a conventional primary or secondary battery, may derive from a solar cell or other renewable energy source such as wind, or may be derived from a fuel-driven generator or a public electricity grid. More information can be found in ACS Energy Lett. 2023, 8, 8, 3343-3355 entitled "Integrated Solar Batteries: Design and Device Concepts", which is hereby incorporated by reference, in particular in relation to the "VEC mode" described therein.

[0193] A voltage efficient charging battery and the underlying principles are as follows. Absorbed photons in the ESM lead to electron-hole generation, and the photogenerated electrons are stored in the ESM and create a potential (voltage). However, some applications require a higher voltage than provided by the ESM, which can then be supplemented by an external source. In such a battery, charging (by applying an external voltage) and photocharging can occur simultaneously. Photocharging and electric charging can occur in parallel if the photoactive material provides a photocurrent additionally to the applied electric current, thereby potentially allowing achieving the desired potential, e.g. for driving a connected load.Method for detecting oxygen

[0194] In another aspect, the present invention relates to a method for detecting oxygen, the method including the following steps: providing an electrochemical device comprising a negative electrode comprising an ESM, charging the ESM comprised in the negative electrode with electrons to give a charged ESM, bringing the charged ESM in contact with a fluid or gas suspected to contain oxygen, and analyzing the state of the charged ESM by visual detection or by measuring the change of the electrical potential of the device before, during and / or after bringing it in contact with the fluidor gas, wherein the charging of the ESM is preferably done by illumination or by applying an electric charging current; wherein the electrochemical devices comprise a negative electrode comprising an electron storage material, ESM, a positive electrode, and an electrolyte,divalent cation; wherein the ESM is optionally doped with one or more metals, such as one, two or three, selected from the lanthanides and transition metal elements, e.g. one, two, three or more selected from La, Y, Sm, Yb, Sc, Mg, Ca, Co, Ni, Zr, Ti, Hf, Eu, Gd, Pr and Sn, such as Y, La, Zr, and Sc.

[0195] As outlined above, M+can be any monovalent cation (preferably an alkali metal cation or an ammonium cation, e.g. as represented by formula NR4+as described above, with R being hydrogen or alkyl, such as linear or branchedalkyl) and X2+can be any divalent cation (preferably an earth alkaline metal cation). Also here, the monovalent cation M+may be selected from the group consisting of organic cations, e.g. tetraalkyl ammonium, such as tetramethyl ammonium or tetrabutyl ammonium, H+, ammonium (NH4+) and alkali metal cations, such as Na+and Li+, and the divalent cation may be selected from the group consisting of Zn2+and alkaline earth metal cations, such as Mg2+, Ca2+, Sr2+and Ba2+-

[0196] Here, the charging of the ESM can be conducted by applying a current, or preferably by illumination, e.g. with sunlight. This leads to the generation of a (photo)potential at the negative electrode, as explained above in relation to the other aspects of the present invention.

[0197] Further, as also explained above, the ESM used in the present invention is sensitive towards the presence of molecular oxygen, as molecular oxygen is a good electron acceptor. The presence of oxygen and its contact with the ESM thus leads to the extraction of electrons from the ESM, lowering the potential (voltage) of the electrode.

[0198] Further, as also explained above, the ESM is typically a material that undergoes a colour change upon charging due to the trapped photogenerated electrons, indicated typically by a colour change from white to blueish (see also the UV-VIS data in the following examples). This colour change can be observed with both spectroscopic instruments as well as with the naked eye. A change in the colour or other visibly or spectroscopically detectable change in properties, such as a colour change of a charged ESM from blueish to white or yellow, can thus be used as anindicator of the presence of oxygen. Of course, also a reduction in potential (as caused by the uptake of electrons by oxygen) may be used as an indicator for the presence of oxygen.

[0199] Thus, by analyzing the state of the charged ESM, e.g. by visual or spectroscopic detection, or by measuring the change of the electrical potential of the device before, during and / or after bringing it in contact with the fluid or gas, the presence of oxygen can be detected. The visual detection mentioned here includes inspection by the naked eye, and spectroscopic analysis includes e.g. UV-Vis spectrometry.Method for detecting light

[0200] In another aspect, the present invention relates to a method for detecting light, the method including the following steps: providing an electrochemical device comprising a negative electrode comprising an ESM, illuminating the ESM comprised in the negative electrode with light for a predetermined time in the presence of a to-be-oxidized reactant, TBOR, to create electron-hole pairs in the ESM, wherein the holes are quenched by reacting with the TBOR and the electrons are stored in the ESM, and wherein after stopping illuminating the ESM, a property of the ESM that is or that correlates with the amount of electrons is measured and correlated with the illumination intensity of the light, wherein the property that is or correlates with the amount of electrons in the ESM is preferably measured by spectroscopic detection or by electrochemical detection; wherein the electrochemical device comprises a negative electrode comprising an electron storage material, ESM, a positive electrode, and an electrolyte, wherein the ESM is selected from the group consisting of CeC>2, M+fNbWOg)’, X2+[(NbWO6)’]2,divalent cation; wherein the ESM is optionally doped with one or more metals, such as one, two or three, selected from the lanthanides and transition metal elements, e.g. one, two, three or more selected from La, Y, Sm, Yb, Sc, Mg, Ca, Co, Ni, Zr, Ti, Hf, Eu, Gd, Pr and Sn, such as Y, La, Zr, and Sc .

[0201] As outlined above, M+can be any monovalent cation (preferably an alkali metal cation or an ammonium cation, e.g. as represented by formula NR4+as described above, with R being hydrogen or alkyl, such as linear or branched alkyl) and X2+can be any divalent cation (preferably an earth alkaline metal cation). Also here, the monovalent cation M+may be selected from the group consisting of organic cations, e.g. tetraalkyl ammonium, such as tetramethylammonium or tetrabutyl ammonium, H+, ammonium (NH4+) and alkali metal cations, such as Na+and Li+, and the divalent cation may be selected from the group consisting of Zn^+and alkaline earth metal cations, such as

[0202] In the above, the same definition of the TBOR as above applies. Also, the entire disclosure of the ESM also applies, except for the stoichiometric composition of the ESM being limited as set out above. This also applies to the disclosure of the electrolyte, which is given above and which typically contains the TBOR.

[0203] The illumination occurs preferably with visible light, and more preferably with light having a wavelength that contains spectral parts with energies above the band gap of the electron storage material. The band gap of the ESM can be determined as set out in the examples.

[0204] The property that correlates with or is the amount of electrons can be different properties and can be examined by techniques known to a skilled person. The amount of photogenerated electrons is the primary quantity that changes upon illumination, but the readout (i.e. the examined property) can take different forms. For example, it is possible to measure potentiometrically, i.e. via a photopotential, impedimetrically via the resistance, coulometrically - by extracting the charges, or via fluorescence quenching or calorimetrically (via the absorbance).

[0205] That is, as described in detail above, the ESM used in the above aspect is capable of photogenerating and storing electrons, e.g. in a polaron state, thereby creating a potential that can be measured / detected and correlated to illumination duration and / or intensity.Further, the trapped photogenerated electrons lead to a visual change of the material and a change in the UV-Vis spectrum (e.g. at around 500 - 800 nm), as will also be demonstrated below in the examples. Both the visual change and the potential generated can be correlated to the amount and / or intensity of the light irradiated on the ESM, typically after calibration using light of known intensity.

[0206] Instead of observing the potential generated by the ESM or observing the change in spectroscopic properties of the ESM, the light intensity can also be determined by electrochemical detection. As one example, the spectroscopic properties, colour or other detectable properties of the TBOR, e.g. present in the electrolyte, may change upon being oxidized, which change in properties or colour may be used as a proxy for the amount of generated electrons and thus the illumination intensity. Similarly, the photogenerated electrons may be utilized to reduce a TBRR that is present in e.g. the electrolyte, which reduction reactionleads to a change in colour, spectroscopic or other properties of the TBRR that can be detected and correlated to the light intensity.Method for detecting an analyte

[0207] In another aspect, the present invention relates to a method for detecting an analyte, the method including the following steps: providing an electrochemical device comprising a negative electrode comprising an ESM, illuminating the ESM comprised in the negative electrode for a predetermined time to create electron-hole pairs therein while the ESM is in contact with a test fluid or gas suspected to contain the analyte during the illumination, wherein the analyte is a to-be-oxidized reactant, TBOR, capable of quenching the holes of the electron-hole pairs, and after stopping illuminating the ESM, measuring a property of the ESM that is or that correlates with the amount of electrons in the ESM, wherein the property of the ESM is preferably measured by spectroscopic detection or by electrochemical detection or by measuring photovoltage; and wherein the electrochemical devices comprise a negative electrode comprising an electron storage material, ESM, a positive electrode, and an electrolyte,divalent cation; wherein the ESM is optionally doped with one or more metals, such as one, two or three, selected from the lanthanides and transition metal elements, e.g. one, two, three or more selected from La, Y, Sm, Yb, Sc, Mg, Ca, Co, Ni, Zr, Ti, Hf, Eu, Gd, Pr and Sn, such as Y, La, Zr, and Sc .

[0208] As outlined above, M+can be any monovalent cation (preferably an alkali metal cation or an ammonium cation, e.g. as represented by formula NR4+as described above, with R being hydrogen or alkyl, such as linear or branchedalkyl) and X2+can be any divalent cation (preferably an earth alkaline metal cation). Also here, the monovalent cation M+may be selected from the group consisting of organic cations, e.g. tetraalkyl ammonium, such as tetramethyl ammonium or tetrabutyl ammonium, H+, ammonium (NH4+) and alkali metal cations, such as Na+and Li+, and the divalent cation may be selected from the group consisting of Zn2+and alkaline earth metal cations, such as Mg2+, Ca2+, Sr2+and Ba2+-

[0209] In the above formula, the same definition of the TBOR as above applies. Also, the entire disclosure of the ESM also applies, except for the stoichiometric composition of the ESM being limited as set out above. This also applies to the disclosure of the electrolyte, except that is does not contain the TBOR, the TBOR being the analyte that is potentially present in the gas or fluid to be subjected to the method.

[0210] In one embodiment of the method, the property that correlates with the amount of electrons in the ESM is a change in the spectroscopic properties of the ESM, such as the absorbance of the ESM in UV-Vis, or the fluorescence emission of the ESM. Additionally or alternatively, a property that is or that correlates with the amount of electrons in the ESM can be determined by electrochemical detection.

[0211] In one embodiment, the property that is or that correlates with the amount of electrons in the ESM is a change of the electrochemical potential (voltage) or a change in the impedance of the ESM. Alternatively, the amount of electrons may be measured by applying a discharging current and quantifying the amount of discharged electrons.

[0212] In one embodiment, the sensitivity of the electrochemical device for detecting the analyte is adjusted by the duration of the step of illuminating the ESM or by the intensity of said illumination. This is feasible because the amount of photogenerated electrons depends on both the availability and concentration of the TBOR (analyte) quenching the holes and on the amount and rate of electron-hole pair formation, which depends on the illumination intensity and duration. In practice, a given device containing the ESM can be calibrated using known illumination intensities and durations and known concentrations of analyte (TBOR).Use of Devices and ESM

[0213] In a further aspect, the present invention is directed to various uses of the electrochemical devices described above and / or the ESM as described above. More particularly, the electrochemical devices and / or the ESM according to the various aspects of the present invention may be used in or as a photorechargeable battery, in or as an autophotorechargeable battery, in or as an oxygen detector, in the detection of an analyte which is a TBOR, in or as a light detector, or in a or as a material for storing electric or solar energy. Here, all features and definitions provided above also apply to the use, and reference is made to the above disclosure instead of repeating it here.

[0214] Such a use involves illuminating the device or ESM with light comprising wavelengths in the visible spectrum comprising wavelengths having an energy higher than the bandgap of the ESM, thereby creating photogenerated electrons that are trapped in the ESM, such as in the form of small polarons. The photogenerated electrons may then be used for storing electric or solar energy and / or for as an energy source for an electrochemical reaction, such as a HER.Examples

[0215] The following examples illustrate the present invention, yet it goes without saying that the present invention is not limited by the examples. The claimed subject matter is defined by the scope of the appended claims as construed properly by law.Example 1 Device having Ca2Nb30io - based ESM

[0216] Ca2Nb30iQ is a layered Dion-Jacobson type perovskite (denoted as CNO in the following part) having a 2D crystal structure and is able to trap photogenerated electrons for a long time, and which allows extracting the electrons for on-demand HER, e.g. upon the addition of Pt catalyst. Figure 5a shows the crystal structure of 2D CNO nanosheet (tetrabutyl ammonium (TBA) counter cations are not shown) and the basic steps of manufacture. Figures 5b and 5c show the high-resolution XPS spectra of (b) Ca 2p and (c) Nb 3d in the CNO sheet.

[0217] A 2D Ca2Nb30iQ- nanosheet was exfoliated from layered bulk KCa2Nb3O^Q powder via a top-down exfoliation approach (Fig. 5a). The obtained material consists of triple layers of infinite vertex-shared NbOg octahedra with subtle tilting (Fig. 5a).

[0218] Specifically, layered perovskite KCa2Nb3O^Q powder was synthesized as reported in Jacobson, A.J., Johnson, J.W., and Lewandowski, J.T. (1985). Interlayer chemistry between thick transition-metal oxide layers: synthesis and intercalation reactions of K[Ca2Nan_3NbnO3n+1]. Inorganic Chemistry 24, 3727-3729. Briefly, K2CO3 (99+%, Acros), Nb2O5(>99.9%, Roth), and CaCC>3 (99+%, Acros) in a molar ratio of 1.1:3:4 were mixed thoroughly by grinding the mixture with the presence of small amount of MeOH for 40 min. The mixture was calcined at 1200 °C for 12 h in air. The protonated HCa2Nb3O^Q was obtained by treating the KCa2Nb3O^Q powder in concentrated HCI for 3 days at room temperature, followed by washing thoroughly with water by centrifugation and drying at room temperature. The exfoliated (TBA)Ca2Nb3O^Q nanosheets were obtained by adding equimolar amount of tetrabutylammonium hydroxide (TBAOH, 40 wt%, Acros) solution into the protonated HCa2Nb3O^Q suspension, followed by stirring for 7 days at room temperature. The suspension was left overnight unstirred to collectthe top suspension for further use, thereby obtaining an ESM. Figure 6 shows the PXRD patterns of layered KCa2Nb3O^Q, protonated H Ca2Nb3O^Q and exfoliated CNO nanosheets powders.

[0219] A first electrode was prepared by drop casting 50 pL 5 mg / mL CNO nanosheets suspension containing Nation 117 solution (2 vol%) on oxygen plasma cleaned fluorine-doped tin oxide glass (FTO, Sigma-Aldrich) substrates with size of 10 x 12 mm^ and dried on a hot plate at 80 °C, followed by annealing at 200 °C in the air for 1 hour. A 12 mm long copper wire was connected to FTO layer by silver paste to complete the photoanodes fabrication. The silver and FTO contacting area was sealed by epoxy glue (DP410, 3M Scotch-Weld), leaving an active electrode area of approximately 10 x 10 mm^.

[0220] Powder X-ray diffraction (PXRD) was performed on a STOE Stadi P diffractometer (Mo Kai) in a Debye-Scherrer configuration. High-resolution scanning electrom microscopy (SEM) was conducted on Zeiss Merlin. Atomic force microscopy (AFM) was conducted on Bruker Dimension ICON under Peak Force Tapping mode. The AFM data was analyzed by Gwyddion (version 2.59) software. Transmission Electron Microscopy (TEM) was performed on a Philips CM 30 ST microscope (300 kV, LaB6 cathode). Images were taken with a TVIPS TemCam-F216 CMOS Camera. The program EM-Menu 4.0 Extended was used to perform Fast Fourier Transformations (FFT). XPS was performed on Kratos Axis Ultra photoelectron spectrometer with monochromated Al Ka (hv = 1486.6 eV) source. (In-situ) UV-vis spectra was conducted on Agilent Cary 60 spectrophotometer in transmission and absorbance modes. In-situ UV-vis transmittance spectra under different applied bias potential was performed by connecting the potentiostat (Autolab PGSTAT302N, Metrohm) with the electrode which was immersed in oxygen-free 0.1 M LiCI electrolyte.

[0221] X-ray photoelectron spectroscopy (XPS) was conducted to obtain electronic structure of CNO. The Ca 2p (Figure 5b) spectra exhibits the presence of one species only, while the Nb 3d spectra shows the co-existence of Nb in the oxidation of Nb^+and Nbz*’+at a ratio of 20 (Figure 5c). The existence of Nbz*’+is considered to be due to the oxygen vacancies introduced during solid state synthesis (see also Tokumitsu, T. et al. Powder Neutron Diffraction Study of Layered Perovskite, KCa2Nb3O^Q., Journal of the Ceramic Society of Japan 114, 795-797 (2006)).

[0222] When the oxygen-free CNO nanosheets was suspended in water with the presence of the TBOR methanol (MeOH), a color change from white to blue under 340 nm UV irradiation was observed (Fig. 5e, insert). To quantify the color change, ex-situ UV-vis absorbance spectra were conducted (Fig. 5f). Upon 340 nm UV irradiation, the absorbance intensity at 575 nm increased compared with the sample before irradiation (Fig. 5f, insert). As shown in Fig. 5e, the absorbance intensity increases rapidly within 30 min, then reaches a saturation state.

[0223] In addition, the operando UV-vis transmittance spectrum confirms the reversible color change of the CNO electrode. After irradiation the suspension for 2 h, the sample was kept in the dark for ca. 2.75 h. An 4.9% absorbance intensity loss compared with immediately after 2 h irradiation. Even after 20.18 hours in the dark an increase in absorbance was observed (Fig. 5f).

[0224] Fig. 5g shows the UV-vis transmittance and absorbance spectrums of CNO film on FTO substrate and Fig. 5h the corresponding Tauc plot, from which an indirect band gap of 3.52 eV was calculated.

[0225] Fig. 5 i - I show the operando UV-vis transmittance spectral changes under i) electric charging from OCP to -1 V and k) electric discharging from -0.95 V to -0.2 V. Inserts in (i) and (k) are the magnified region between 570 nm and 580 nm. The transmittance intensity at 575 nm under different applied bias for (k) electric charging and (i) electric discharging. The electrolyte is oxygen-free 0.1 IVI LiCI. Here, to quantify the photoanode transmittance against applied potential, the oerando UV-vis transmittance spectrum was conducted under different applied potential. As shown in the figures, the transmittance remains stable at the time of electric charging from OCP to -0.75 V, but starts to decrease when charging to -1 V, suggesting charge storage. Conversely, the transmittance starts to increase during electric discharging from - 0.95 V to -0.75 V, and maintains stable when continuous discharging to more positive potential. The transmittance intensity at 575 nm against applied potential consists with its CV plot. It is noteworthy the electrode transmittance intensity exhibits high reversibility during electric charging and discharging.

[0226] Fig. 5 m and n show the photocharging profiles of the CNO photoanode under 340 nm UV irradiation and its corresponding electric discharging profiles under different current densities. A slow self-discharge process was observed at a low current density of 0.4 mA g'l, which also affects the dark stability of the CNO photoanode after photocharging (Fig. 8). A capacity of 3.4 C g'l was achieved at a current density of 400 mA g'l, with only 12.6% capacity loss compared to the maximum capacity of 3.92 C g'l at 4 mA g'l, indicating a high rate performance of CNO photoanode. Therefore, a discharge current density of 4 mA g'l was used for the other experiments. To investigate the maximum photocharging capacity, the photoanode was irradiated at different time, followed by electric discharging at 4 mA g'l to extract the charge (Fig.9a). Upon irradiation, the open circuit potential (OCP) increases to around -0.7 V within several seconds (Fig. 9b), indicating a pronounced photovoltaic effect of CNO. Further increasing the irradiation time slowly increase the photo-potential, suggesting a saturation of excited electrons after long-term irradiation. The potential against the irradiation time is shown in Fig. 9c, the capacity against irradiation time in figure 9d.

[0227] Fig. 8a shows the open circuit potential (OCP) over the time after photocharging for lh. The OCP reaches around -0.95 V from 0 V after irradiation for 1 h. After the light is switched off, the OCP starts to drop slowly to around -0.8 V after around 4 h in the dark, followed by a quick drop to -0.4 V within 3 h. Then, the OCP drops slowly to 0 V after leaving in the dark for about 24 h. This illustrates the potential to release electrons, e.g. to provide energy for HER or other reduction reaction or to provide an electric current, for a significant time after end of the irradiation period, thereby maximizing the exploitation of solar energy.

[0228] Fig. 8b shows the CV graphs in the dark and under illumination at 340 nm at a scan rate of 10 mV s'l. The electrolyte for dark CV is oxygen-free 0.1 M LiCI, the electrolyte for light CV is oxygen-free 0.1 IVI LiCI with the presence of MeOH as TBOR.Fig. 9a shows the observed potential (vs. Ag / AgCI) after different irradiation times. To investigate the maximum photocharging capacity, the material was irradiated for different times, followed by electric discharging at 4 mA g-1 to extract the charge. Upon irradiation, the open circuit potential (OCP) increases to around -0.7 V within several seconds, indicating a pronounced photovoltaic effect of CNO. Further increasing the irradiation time slowly increased the photo-potential, suggesting a saturation of excited electrons after long-term irradiation. As can be seen, after 1 minute irradiation the potential reaches about -0,7V, and the potential reached after 5, 10, 20, 30, 60 and 120 minutes are similar, in each instance only showing a slight increase upon prolonged irradiation. This shows that large amounts of photogenerated electrons are formed within the first 5 minutes, and that the system then approaches saturation.

[0229] In a further experiment, the cycling stability under light irradiation followed by electric discharge was examined (see Fig. 10a). The cycling stability of the CNO photoanode was investigated by photocharging for 5 min followed by electric discharging at a current density of 4 mA g'l. The photoanode exhibited 92.5% capacity retention after 15 cycles with 0.5% loss per cycle, indicating high stability. The used electrolyte was oxygen-free 0.1 IVI LiCI and MeOH (10 vol%).

[0230] The efficiency of two different TBORs (MeOH and triethanolamine (TEOA)) was investigated by chronoamperometry (CA) measurements under dark / light cycles (see Fig. 10b and c). The photocurrent (J ph) of around 0.7 A g'l in the presence of MeOH donor is higher than TEOA of around 0.3 A g'l, suggesting the higher efficiency of MeOH for the quenching of holes. To further investigate the amount of excited charges per second with the presence of different donors, the J ph under different applied potentials were assessed. The current flow is negligible without light irradiation under an applied potential of 0.3 V for both cases. Under light irradiationand applied potentials, the excited electrons in the polaronic state were extracted to generate the positive current flow. The J ph decreases when further decrease the potential to more negative value due to the decreasing driven force to extract electrons. The J ph with MeOH as TBOR is higher than with TEOA in potential windows between 0.3 V to -0.4 V.

[0231] The linear sweep voltammetry (LSV) was conducted to extract the electrons under light irradiation and in the dark (Fig. lOd). The J ph at 0 V is 0.18 mA under irradiation, which is four magnitudes higher than the dark current of 44 nA. The J ph under irradiation remains stable until -0.4 V, and slowly decrease to 0 until -0.65 V when scanning to more negative potentials due to the decreasing driven force to extract electrons. Moreover, the Jph exhibits high stability which maintains around 0.15 mA after 24 hours irradiation (Fig. lOe).

[0232] The obtained material was tested for its ability to provide electrons for a subsequent HER reaction (Figure 7). The on-demand HER experiments were conducted in a home-made glass reactor with quartz window on top for illumination and thermostated at 25 °C. The reactor was soaked in aqua regia overnight before dark photocatalysis experiment. The 340 nm UV irradiation was provided by Thorlabs M340L4 lamp at an operating current of 700 mA. The CNO nanosheets (20 mg) were dispersed in water (8.8 mL) with the presence of MeOH (1 mL). The headspace of the reactor was evacuated and argon backfilled several times to remove the air. The suspension was stirred at 400 rpm during illumination. For the on-demand hydrogen evolution experiments, the platinum nanoparticles co-catalyst (200 pL of 1000 ppm aqueous colloidal solution, US nanomaterials) was injected into the photocharged suspension. To quantify the hydrogen amount upon adding the Pt co-catalyst, the headspace of the reactor was periodically samples and the amount of evolved gases was quantified by gas chromatography (Shimadzu GC-2030). This gas chromatograph is equipped with a Barrier Discharge Ionization detector (BID) and Thermal conductivity detector (TCD) using argon as the carrier gas. Unless stated differently, evolution of gases in the headspace is measured in a closed system (i.e., batch measuerments).

[0233] Figure 7a shows the hydrogen evolution amount as a function of time in the dark without delay time and Figure 7b with delay time between "light off" and Pt addition. The insert in Figure 7a shows photos of the CNO suspension before and after light irradiation. The light grey region up 1 hour indicates the period of irradiation, the dark grey region (in Figure 7b only) indicates the delay time.

[0234] Without wishing to be bound by theory, the proposed light harvest, conversion and storage mechanisms may be described by Equation (1-4)D + h+(hv)-^ D+(2)MxCa2Nbx(V)Nb3-x(IV)Oio* + yM++ ye~ (hv) -> Mx+yCa2Nbx(V)Nb3-x(IV)Oio (3)Mx+yCa2Nbx(V)Nb3-x(IV)Oio^ MxCa2Nbx(V)Nb3-x(IV)Oio + yM++ ye" (4) where D is the electron donor (TBOR), and M is a cation. During light irradiation, the electron-hole pairs were generated (Equation 1), where the holes migrate to the surface of the materials followed by quenching by the TBOR (Equation 2). The excited electrons in the conduction band (CB) move towards the distorted Nb simultaneously, resulting in the self-trapped charges on the distorted Nb and the formation of polaronic states beneath the conduction band minimum (CBM). A part of the electrons in CB may catalyze water reduction to H2 before moving to distorted Nb. The negatively charged small polarons drive the photointercalation of cation from the electrolyte into the host material (Equation 3). To extract the excited electrons from the polaronic state, Pt nanoparticles catalyst was added to the suspension. The electrons transfer from the polaronic state to the Pt catalyst to split the water into H2 with the cation deintercalation occurring simultaneously (Equation 4).

[0235] During irradiation, the suspension color changed from white to blue, suggesting that the CNO was photoreduced and formed small polarons. A small amount of H2 (0.04 umol) was detected after irradiation for 1 hour even without the presence of a catalyst, suggesting partial photogenerated electrons catalyze water to produce hydrogen.

[0236] Subsequently, a Pt catalyst (200 pL) was added to the photocharged CNO suspension without time delay after the light is off. A maximum of 0.47 pmol H2 was detected after the addition of Pt catalyst after 2 hours (Figure 7a). The suspension color changed from blue to white again, indicating the photogenerated electrons were fully released for H2 evolution.To evaluate the dark stability of photogenerated electrons in the above system, a dark HER was conducted with delay time before adding Pt catalyst (Fig. 7b). After irradiation the CNO suspension for 1 hour, the photocharged suspension was left in the dark for another 1 hour before adding Pt catalyst. A total amount of 0.1 pmol H2 was generated before adding the Pt catalyst. Then Pt catalyst was added into the suspension, and the H2 amount increases rapidly to 0.24 pmol within one hour, then reached a saturation state. The total H2 amount dropped by about 67%, presumably due to background reactions either by oxygen leakage or by electronhole recombination.(v) In summary, Example 1 shows that light harvesting (generation of electron / hole pairs), conversion (to photogenerated electrons) and storage (trapping of photogenerated electrons) was achieved in a2D Ca2Nb3O^Q material was achieved. The stored photogenerated electrons were efficiently transferred to Pt catalyst to generate hydrogen in the dark. This demonstrates the decoupling light and dark reactions in single material.Example 2 - Device having TigNbOj^ - based ESM

[0237] A 2D (TBA^Ti NbO^ nanosheet was prepared as follows:

[0238] A layered bulk KgTisNbO^ (KTNO) powder was synthesized as reported in the literature (see Zhang, N. et al., "A Facile Route to Synthesize the Ti5NbO^4 Nanosheets by Mechanical Cleavage Process"; Journal of the American Ceramic Society 93, 536-540 (2010)). Briefly, K2CO3, Nb2C>5, and WO3 were thoroughly mixed in a molar ratio of 2:4:1 by grinding the mixture for around 15 min. The mixture was first calcined at 400 °C for 4h, then slowly increased to 900 °C and kept for 20 h. The protonated H3Ti5NbO^4 (HTNO) was obtained by treating the bulk KTNO powder into 2 M HCI solution for 3 days and replacing new acid solution everyday by decantation, followed by washing thoroughly with water and drying at room temperature. The exfoliated (TBA^Ti NbO^ (TNO) nanosheets were obtained by adding an equimolar amount of tetrabutyl ammonium hydroxide solution (40 wt%) to the HTNO suspension, followed by shaking for 10 days at room temperature. The suspension was centrifuged at 1500 rpm for 10 min and the top suspension collected for future experiments.

[0239] All the (photo)electrochemistry measurements were performed in a home-made closed glass reactor equipped with a quartz window for light irradiation. The Ag / AgCI electrode (saturated KCI, RE-1CP) was used as reference electrode and Au foil was used as counter electrode. Unless otherwise specified, all potentials in this work were measured versus Ag / AgCI. Oxygen-free IM LiCI (Roth) aqueous solution was used as electrolyte. Methanol was used as electron donor. The electrolyte was purged with Ar for at least 30 min prior to every measurement through a porous glass frit to remove dissolved oxygen. The 340 nm UV (M340L4), 365 nm UV (M365LP1) and 405 nm UV (M405LP1-C4) LEDs were provided by Thorlabs at a light intensity of 20 mW cm-2. The light intensity was measured by a calibrated Thorlabs S120VC standard photodiode power sensor and PM100D Thorlabs power meter.

[0240] (Photo)electrochemical measurements were performed on a multichannel potentiostate (Autolab M204, Metrohm). The dark EIS was performed under different appliedbias potentials from 10 kHz to 0.1 Hz. The light EIS was conducted under 340 nm UV LED irradiation for different time from 10 kHz to 0.1 Hz.The real capacitance C' is defined as: where -Z" is the imaginary impedance, OJ is the frequency, Z is the electrochemical impedance.

[0241] The synthesis is outlined in Figure 11. The structure of the TiOg or NbOg octahedrons is shown in Fig. 12a, the XRD patterns of HgTi NbO^ x H2O, KgTi NbO^ and a reference diffractogram (from top to bottom) are shown in Figure 12b. The asterisks represent impurities. XRD analysis and other characterisations were carried out as described above for Example, unless indicated otherwise.

[0242] The crystal structure of TNO consists of blocks of 2 x 3 edge-sharing octahedra, which are joined by Ti / NbOg octahedra with an edge- and corner-sharing. The thickness of obtained monolayer TNO nanosheet is between 1.6 to 1.9 nm which is close to the theoretical value of 1.05 nm, indicating the successfully exfoliation of monolayer TNO. The thickness difference may due to the tetrabutylammonium (TBA+) and water molecules on the surface of TNO. The valence states of Ti and Nb were determined by X-ray photoelectron spectroscopy (XPS) to be Tj4+ and Nb^+, respectively, consistent with the composition.

[0243] The bulk KTNO powder showed needle like morphology. The protonated HTNO powder exhibits the similar needle like morphology, but the HTNO powder shows visible layered structure, suggesting the successful protonation process. AFM images confirm the monolayer of TNO nanosheets with thickness between 1.6 to 1.9 nm. Figure 13 shows the typical morphology of the KTNO crystal (left) and its corresponding HRTEM image with FFT inset (right).

[0244] A first electrode was prepared by drop casting 50 pL of a 1 mg / mL TNO nanosheet suspension on oxygen plasma cleaned fluorine-doped tin oxide glass (FTO) substrates having a size of 10 x 12 mm^,anc| d ryi ng on a hot plate at 60 °C for 15 min, followed by annealing at 300 °C in air. To complete the electrode (photoanode) fabrication, an about 12 mm long metal wire was contacted to FTO slide using silver paste. The contact area was sealed by epoxy, leaving an active electrode area of approximately 10 x 10 mm^

[0245] Fig. 14a shows the UV-Vis transmission and absorbance spectra of TNO film on FTO substrate. Fig. 14b shows the Tauc plot of TNO thin film, which indicates an indirect band gap of 3.53 eV. The UV-vis absorbance spectra confirm that TNO exhibits strong absorption within UV range and low absorption in visible light range due to its large indirect bandgap.

[0246] The photochromic behavior of TNO nanosheets suspension under 340 nm UV (at 20 mW cm’2) irradiation with the presence of MeOH (10 vol%) as TBOR was studied. The TNO suspension changed color from white milky to blue upon light irradiation. The ex-situ UV-vis spectra was recorded to quantify the color intensity, and it was found that the absorbance spectra exhibit broad intensity increase between 400 to 800 nm. The absorbance intensity at 575 nm increases significantly up within 30 min, after which a plateau is observed. This is illustrated in Fig. 14c, and the development of the UV-Vis spectrum over time of irradiation is shown in Fig. 14d.

[0247] The photochromic behaviour can be explained by the following proposed reaction scheme, which is not intended to limit the present invention by theory:MxTi5NbOi4 + hv -> MxTi5NbOi4* + e~ (hv) + h+(hv) (1)D + h+(hv)-^ D+(2)MxTi5NbOi4* + yM++ ye~ (hv) -> Mx+yTi5NbOi4 (3)Mx+yTisNbOw^ MxTisNbOu + yM++ ye" (4) where M represents cations that in this case include Li+, H+ and TBA+; D is the electron donor (TBOR), h is the Planck constant, and v is the frequency.

[0248] It is believed that electron-hole pairs are generated upon above bandgap light irradiation, where the hole migrates to the surface of the materials and is quenched by the TBOR / donor (Equation 2). Meanwhile, the photoexcited electron moves to the conduction band, followed by trapping by metal atoms to form small polarons for charge storage. To stabilize the negative charge, cations intercalate into the host material simultaneously (Equation 3). The trapped charge in polaronic state can be extracted either for short-term direct solar battery by applying a positive current or for long-term solar hydrogen generation in the dark upon the addition of Pt catalyst (Equation 4).

[0249] Fig. 15a shows the responsive photocurrent in oxygen-free aqueous IM LiCI with the presence of MeOH (10 vol%) as TBOR under 365 nm and 405 nm UV irradiation. CA experiments were conducted under an applied potential of 0 V. Fig. 15b provides the summary of first cycle J ph under different irradiation wavelengths, each at 20 mW cm'2.

[0250] Fig. 16 shows the representative (a) first cycle and (b) three cycles of photocurrent profiles under 340 nm UV irradiation with different power intensities in oxygen-free IM LiCI in the presence of MeOH (10 vol%) as TBOR.

[0251] The CV performance of TNO thin film electrode fabricated by drop casting was studied. A broad anodic peak around -0.6 V vs Ag / AgCI (saturated KCI) was found in CV, suggesting the Faradic redox reaction of TNO. Kinetic analysis was conducted to investigate the capacitive and diffusion contributions to the electrochemical performance of TNO electrode through the relation of i=av where i and v are the current and scan rate, respectively, a and b are the adjustable parameters. The b value is close to 0.5 indicating a diffusion dominated process, while the b value is close to 1 indicating the surface capacitive process. The plots of both anodic and cathodic currents exhibit a linear relationship against the scan rate. The b values are 0.54 and 0.52 for anodic and cathodic currents, respectively, suggesting the electrochemical performance is dominated by ions diffusion process.

[0252] To gain insight into the energy storage mechanism in TNO electrode, electrochemical impedance spectroscopy (EIS) measurements under different applied bias (Fig. 17a) were conducted. The nearly vertical Nyquist plots at OCP and -0.2 V in the low-frequency region exhibits the typical capacitive behavior. However, the impedance spectra deviate from the capacitive behavior in the low-frequency region as the state of charge increases from -0.6 V to - 0.8 V. The trend fits well with the intercalation reactions that occur from -0.6 V in cyclic voltammetry (Fig. 17a). The Bode-type was used to understand the energy storage mechanism of TNO photoanode (Fig. 17b). The real capacitance C' shows constant values in the low frequency region from dark OCP to -0.2 V, indicating the capacitive process. However, C' start to increase to higher value as the applied bias increases, indicating the diffusion-dominated process.

[0253] Fig. 18 shows a proposed schematic illustration of a) photocharging and b) electic discharge. Here, the balls having a "+" represent cations, the balls having a represent electrons, the balls having a "h" represent electron / hole pairs, and D represents an electron donor (TBOR). The lines represent the proposed electron, respectively cation, pathways in the material during photocharging (left) and during discharging (right).

[0254] Fig. 19 a shows the electric discharging potential provides under different current densities after 5 minutes irradiation at 340 nm and 20 mW / cn

[0255] Fig. 20a shows the photocharging profiles under different irradiation time, and Fig. 20b the corresponding electric discharging profiles at a current density of 24 mA g'l. Inserts show the OCP under illumination at the beginning, i.e. up to 1.5 min.

[0256] Fig. 21 shows the open circuit potential stability of two independent samples during and after light illumination in oxygen-free aqueous IM LiCI with the presence of MeOH (10 vol%) as TBOR. The illumination time is 30 min. Light source: A.340 nm, 20 mW cm'2. The results show good agreement, illustrating the reproducibility of the electrode. The OCP reaches about -0.7 V after light irradiation for 30 min. When the light is off, the OCP decreased slowly in the dark, suggesting the high photoelectron stability and low recombination rate. The possible reason for OCP drop may be caused by air leakage that may be optimized during manufacture of the device.

[0257] Fig. 22 shows the cycling stability of (a) photocharging for 5 min and (b) electrical discharging at a current density of 24 mA g'l. Remarkably, in Figure 22a the potential increases with cycle number, showing a high stability of the system. Also charge density increases with cycle number.

[0258] Fig. 23 shows the CV scanning of the TNO electrode between -0.8 V to 0.3 V under dark and 340 nm UV irradiation in aqueous 1 M LiCI electrolyte in the presence of MeOH (10 vol%) as TBOR. The arrow indicates the scanning direction.

[0259] Fig. 24 shows a comparison of capacity under dark and light-assisted discharge at a current density of 24 mA g'l. The photocharging time is 10 min. Insert is the corresponding discharge profiles.

[0260] Fig. 25a shows light and dark LSV scanning at a sweep rate of 10 mV s'l under 340 nm, 365 nm and 405 nm UV irradiation, and Fig. 25b shows the responsive photocurrent in oxygen-free IM LiCI with the presence of MeOH (10 vol%) under 340 nm, 365 nm and 405 nm UV irradiation. Note the different Y axis in Fig. 25 and the different behaviour of the photocurrent in dependence on the wavelength. The photocurrent density (JPh) under 340 nm UV irradiation is higher than 365 nm and 405 nm light irradiation, suggesting the above bandgap light irradiation could efficiently create electron-hole pairs. The JPh decrease to 0 pA cm'2by increasing the potential to the negative part due to the decreased drive force to extract electrons. Moreover, the responsive JPh in the presence of MeOH (10 vol%) as TBOR under an applied potential of 0 V was constructed using chronoamperometry (CA) measurement under da rk / light cycles and different light irradiation. Consistent with LSV, the JPh under 340 nm irradiation is higher thanunder 365 nm and 405 nm irradiation, further confirming the efficiency of above bandgap light irradiation.

[0261] To further investigate the photogenerated electrons upon light irradiation, the JPh under different applied potentials and light irradiation in the presence of MeOH (10 vol%) were measured. The dark current at 0.2 V without light irradiation is negligible. Similar to the LSV trend, the JPh increased to 1.46 pA cm-2 at 0.2 V upon 340 nm irradiation, which is almost 4 times and 25 times higher than 365 nm and 405 nm irradiation, respectively (see Fig. 25c). We then estimate the JPh at an applied potential of 0 V under 340 nm UV irradiation at different light intensities. As shown in Fig. 25d, JPh under high light intensity is higher than low intensity irradiation, suggesting high intensity light could excite electron-hole pairs more efficiently than low intensity light. Therefore, 340 nm UV at light intensity of 20 mW cm'2 was used for further study. To evaluate the stability of the photoanode, we measured the Jph under 340 nm UV at light intensity of 20 mW cm-2 and an applied potential of 0 V (Fig. 25e). The TNO photoanode exhibits high photocurrent stability over 24 h, exhibiting highly stable and continuous charge carriers and extraction.

[0262] A three-electrode setup was used to evaluate the capacity performance of the TNO phtoanode (Fig. 26a). The reference and counter electrodes are Ag / AgCI (saturated KCI) and Au plate, respectively. The electron-hole pair was excited under 340 nm UV irradiation on the front side of the TNO photoanode, with Li+ ion intercalation simultaneously (Fig. 18). To extract the photoexcited electrons, a positive current flow was applied. A rate measurement to study the relationship between capacity and discharge current density was conducted (Fig. 19 and b). The TNO photoanode exhibits self-discharge at low current density and kinetic limitation at high current density due to poor electronic conductivity. It is worth noting that TNO photoanode exhibits good rate performance as the capacity only decreases by 18.9% when the current density increase from 2.4 mA g1to 240 mA g1. The maximum capacity was observed at a current density from 2.4 to 24 mA g1. Therefore, a current density of 24 mA g1was used for further measurements.

[0263] To evaluate the maximum photocharging capacity, the photoanode was irradiated for different time, followed by electric discharging to 0.3 V in the dark at a discharge current density of 24 mA g1(Fig. 27). The photo-potential increases almost vertically from 0 V to -0.4 V within 10 s irradiation, and slowly reaches to around -0.8 V during the ensuing irradiation (Fig. 20), suggesting the high photovoltaic effect of TNO photoanode. Similar to the photo-potential upon irradiation, the capacity also exhibits a saturation dependence as a function of irradiation time (Fig. 27). Interestingly, the capacity increases almost linearly with the irradiation time from 1 to 30 min (Fig. 27 insert). Further increasing the irradiation time would lead to a saturatedsteady state, presumably due to balance of photogenerated electrons and electron / hole- recombination.

[0264] To gain deep insight into the TNO photoanode under light irradiation, electrochemical impedance spectroscopy (EIS) measurements in the dark and under light irradiation for different time were conducted (Fig. 28). The nearly vertical EIS curve at dark OCP in the low-frequency region indicates the typical capacitive charge storage mechanism. While the light EIS spectra deviate from the capacitive behavior suggesting different charge storage mechanisms compared with the dark. The fitted equivalent circuit model contains material conductivity (Rc) and charge transfer (RCT) component. Upon light irradiation for 5 min, the Roland Rc decrease more than two orders of magnitude compared with the dark. It is worth noting that the R - decreases accordingly when increasing the irradiation time, indicating continuous photoexcited electrons storage in the polaronic state. The EIS results suggest the above bandgap light irradiation significantly decrease R - and R^. The Bode-typo plot was used to further understand the TNO photoanode under light irradiation (Fig. 29). The real capacitance C' increase significantly under light irradiation compare with the dark, suggesting the ion-diffusion process.

[0265] To reveal the photoanode cycling stability, light charging and electric discharging cycling measurement were performed (Fig. 30). The TNO photoanode was photocharged for 5 min, followed by electric discharging at a current density of 24 mA g-1. The ultimate cycling capacity retention is 123% compared with the first cycle and increases with each cycle, which may suggest an initial activation process. The cycling stability of the system is very high. In addition, the light-assisted electric discharge boosts the capacity 15 times higher than in the dark (Fig. 23, 24).

[0266] Dark hydrogen evolution experiments were conducted in a home-made glass reactor with quartz window on top for illumination and thermostated at 25 °C as previous described (see Lau, V. W. et al. Dark Photocatalysis: Storage of Solar Energy in Carbon Nitride for Time-Delayed Hydrogen Generation. Angew Chem Int Ed Engl 56, 510-514 (2017); and Kroger, J. et al. Interfacial Engineering for Improved Photocatalysis in a Charge Storing 2D Carbon Nitride: Melamine Functionalized Poly(heptazine imide). Advanced Energy Materials 11, 2003016 (2020)). The reactor was soaked in aqua regia overnight before conducting the dark photocatalysis experiment. A 340 nm UV LED was used at a light intensity of 16 mW cm'2. The TNO nanosheets was dispersed in water in the presence of MeOH (1 mL). The total volume of suspension was 10 mL. The headspace of the reactor was evacuated and argon backfilled several times to remove the air. The suspension was stirred at 400 rpm during irradiation. For the dark HER, an oxygen- free platinum nanoparticles catalyst (0.2 mL of 1000 ppm aqueous colloidal solution, USNanomaterials) was injected into the photocharged suspension. To measure the amount of H2 generated upon adding the Pt catalyst, the headspace of the reactor was periodically sampled and the amount of evolved gases was quantified by gas chromatography (Shimadzu GC-2030). This gas chromatograph is equipped with a Barrier Discharge Ionization detector (BID) and Thermal conductivity detector (TCD) using argon as the carrier gas. Unless stated differently, evolution of gases in the headspace is measured in a closed system (i.e., batch measurements). Control experiments confirm that no hydrogen was generated in the absence of TNO, light or Pt catalyst.

[0267] On-demand HER from a photocharged TNO suspension in water was investigated. To generate hydrogen, oxygen-free TNO (26 pmol) suspension in the presence of MeOH as TBOR (10 vol%, 24.7 mmol, 950 equiv.) was irradiated under 340 nm for 2 h. After the irradiation, the suspension's color changed from white to light blue, indicating the photoexcited electrons are trapped locally which may form small polarons. The addition of Pt catalyst in the dark results in the hydrogen generation which was identified and quantified by reactor headspace gas chromatography. After the dark reaction, the suspension's color turned back to white, suggesting that the trapped photogenerated electrons were fully extracted.

[0268] A further on-demand HER study was conducted by setting a delay time between "light off" and the addition of Pt catalyst. After irradiation for 2 hour and maintaining in the dark for 30 min, 0.10 pmol hydrogen (maximum rate of 0.23 pmol h-1, maximum turnover frequency of 0.22 h-1) was formed in the dark upon the addition of Pt catalyst (Fig. 31b), which is 22.4% loss compare with the one without delay (Fig. 31a).

[0269] Example 2 thus shows that purely layered 2D titanium niobate enables to harvest light, convert, store and release the charge on-demand, which can spatially decouple light and dark reactions. Without wishing to be bound by theory, it is understood that upon above bandgap light irradiation, photogenerated electrons are trapped in TNO and stabilized via simultaneous photointercalation of Li ions. The photogenerated charge may be trapped in the form of small polarons. The trapped charge can be extracted in the dark as solar battery photoanode and generate hydrogen on-demand upon the addition of Pt catalyst. Specifically, the solar battery exhibits the maximum capacity of 2.2 mAh g'l, and 0.23 pmol h'l hydrogen with 30 min time delay upon the addition of Pt catalyst.Example 3 - Device having NbWOg - based ESM

[0270] Layered a-LiNbWOe powder was synthesized as reported in the literature (see Fourquet, J. L. e.al., LiNbWOg: Crystal structure of its two allotropic forms. Materials Research Bulletin 23, 1163-1170 (1988)). Briefly, Li2CO3(99.999%, Acros), Nb2O5(>99.9%, Roth) and WO3(Aldrich) were mixed thoroughly in a molar ratio of 1:1:2 by grinding the mixture for around 15 min. The mixture was calcined at 760 °C for 24 h in the air. The protonated HNbWOe-xH2O was obtained by treating bulk a-LiNbWOe powder with 0.1 M HCI solution for 3 days and replacing new acid solution every day, followed by washing thoroughly with water and drying at room temperature. The exfoliated NbWOe nanosheets were obtained by adding an equimolar amount of tetrabutylammonium hydroxide (TBAOH, 40 wt%, Acros) solution to the HNbWOe-H2O suspension, followed by stirring for 7 days at room temperature. The suspension was centrifuged at 2,000 rpm for 10 min and collected the top suspension for future experiments.

[0271] First electrodes (photoanodes) were prepared by drop casting 50 pL 5 mg / mL NbWOg nanosheets suspension on oxygen plasma cleaned fluorine-doped tin oxide glass (FTO, Sigma-Aldrich) substrates with size of 10 x 12 mm^, and dried on a hot plate at 60 °C for 15 min, followed by annealing at 200 °C in the air for 1 h. To complete the photoanodes fabrication, about 12 mm long copper wire was connected to FTO layer by silver paste. The contact area was sealed with epoxy glue (DP410, 3M Scotch-Weld), leaving an active electrode area of approximately 10 x 10 mm^.

[0272] Atomic force microscopy (AFM) was performed on Bruker Dimension ICON under Peak Force Tapping mode. The AFM data were analyzed by Gwyddion (version 2.59) software. Scanning electrom microscopy (SEM) was conducted on Zeiss Merlin. Transmission Electron Microscopy (TEM) was performed on a Philips CM 30 ST microscope (300 kV, LaB6 cathode). Images were taken with a TVIPS TemCam-F216 CMOS Camera. The program EM-Menu 4.0 Extended was used to perform Fast Fourier Transformations (FFT). Powder X-ray diffraction (PXRD) was conducted on a STOE Stadi P diffractometer (Ag Kai, Johann-type Gelll monochromator, triple array of Mythen (Dectris) detectors) in a Debye-Scherrer configuration. The samples were sealed in 0.5mm borosilicate glass capillaries (Hilgenberg, glass No. 14), which were spun during the measurements applying a total scan time of 3 hours. Temperature dependent in situ PXRD measurements were performed using the same device. A capillary of HNbWOe-xH2O was heated using with a hot air blower (Large Hot Air Gas Blower DGB0001 FMB Oxford). The sample was heated from 25 °C to 300 °C in 25 K steps applying a heating rate of 3K / min. During isothermal hold periods, RXRD patterns were recorded applying a total scan time of one hour and an isothermal delay of 2 minutes prior to every measurement for ensuring thermal equilibration. XPS was conducted on Kratos Axis Ultra photoelectron spectrometer with monochromated Al Ka (hv = 1486.6 eV) source. Inductively coupled plasma - optical emission spectrometry (ICP-OES) was performed on Varian Vista-PRO (simultaneous ICP-OES spectrometerwith axial plasma (Fa.Varian Darmstadt)). The sample was dissolved in HNO3 (65%), HF (40%) and H3PO4 at 165°C for 35min which was diluted with double distilled water. The microwave digestion with Discover SP-D is from CEM GmbH. ICP-Expert software was employed to analyze the data. (In-situ) UV-vis spectra was performed on Agilent Cary 60 spectrophotometer in transmission and absorption modes. To perform in-situ UV-vis absorption measurement on suspension, the 365 nm UV light was illuminated from the top of quartz cuvette with distance of 20 cm. In-situ UV-vis transmission under different applied bias potential was conducted by connecting the potentiostate (Autolab PGSTAT302N, Metrohm) with the electrode which was immersed in oxygen-free 1 M LiCI electrolyte.

[0273] All the (photo)electrochemistry measurements were performed in a home-made closed glass reactor equipped with a quartz window for light illumination. The Ag / AgCI electrode (saturated KCI, RE-1CP) was used as reference electrode and Au foil was used as counter electrode. Unless otherwise specified, all potentialswere measured versus Ag / AgCI. Oxygen-free IM LiCI (Roth) aqueous solution was used as electrolyte. Methanol, 4-MBA and water were used as TBOR. The electrolyte was purged with Ar for at least 30 min prior to every measurement through a porous glass frit to remove dissolved oxygen. Artificial sunlight was provided either by a Sciencetech LightLine A4 solar simulator (class AAA) fitting the ASTM standard G138 (AM 1.5G). The 365 nm UV illumination was provided by Thorlabs M365LP1-C4 lamp at an operating current of 1700 mA. The sunlight intensity was measured by a calibrated Thorlabs S130C / PM100D thermal power meter. The light intensity for 365 nm UV was measured by a calibrated Thorlabs S120VC standard photodiode power sensor and PM100D Thorlabs power meter.(Photo)electrochemical measurements were performed on a multichannel potentiostate (Autolab M204, Metrohm). The dark EIS was performed under different applied bias potentials from 10 kHz to 0.1 Hz. The light EIS was conducted under 340 nm UV LED irradiation for different time from 10 kHz to 0.1 Hz.

[0274] The real capacitance C' is defined as:where -Z" is the imaginary impedance, OJ is the frequency, Z is the electrochemical impedance.

[0275] Dark hydrogen evolution experiments were conducted in a home-made glass reactor with quartz window on top for illumination and thermostated at 25 °C as previous described (see Lau, V. W. et al. Dark Photocatalysis: Storage of Solar Energy in Carbon Nitride for Time-Delayed Hydrogen Generation. Angew Chem Int Ed Engl 56, 510-514 (2017) and Kroger, J. et al. Interfacial Engineering for Improved Photocatalysis in a Charge Storing 2D Carbon Nitride: Melamine Functionalized Poly(heptazine imide). Advanced Energy Materials 11, 2003016 (2020)). The reactor was soaked in aqua regia overnight before dark photocatalysis experiment. The 365nm UV illumination was provided by Thorlabs M365LP1-C4 lamp at an operating current of 1700 mA. The NbWOe nanosheets (95 mg) was dispersed in water (8 mL) with the presence of MeOH (1 mL). The headspace of the reactor was evacuated and argon backfilled several times to remove the air. The suspension was stirred at 400 rpm during illumination. For the dark hydrogen evolution experiments, the platinum nanoparticles catalysis (1 mL of 1000 ppm aqueous colloidal solution, Aldrich) was injected into the photocharged suspension with different delay time. To measure the amount of hydrogen evolution upon adding the Pt catalysis, the headspace of the reactor was periodically sampled and the amount of evolved gases was quantified by gas chromatography (Shimadzu GC-2030). This gas chromatograph is equipped with a Barrier Discharge Ionization detector (BID) and Thermal conductivity detector (TCD) using argon as the carrier gas. Unless stated differently, evolution of gases in the headspace is measured in a closed system (i.e., batch measurements). Control experiments confirmed that no hydrogen was generated in the absence of NbWOe, light or Pt catalysis.

[0276] Electronic structure calculations were made utilizing the FHI-aims program package. All structural optimizations employed the HSE06 hybrid functional to ensure precise electronic structures. Owing to the size of the models, numerical convergence was attained with a light basis set. The reciprocal space was sampled through a 2 x 2 x 1 Monkhorst-Pack k- point grid. The structures were utterly relaxed until the forces fell below 5 x 10“2eV / k.

[0277] To identify potential polaron formation sites, an initial relaxation by applying PBE+U and implementing a Hubbard U correction on a single atom of the chosen element was excecuted. In this particular case, it was determined that U = 1.2 was sufficient for pre-relaxing the structure towards the desired polaron formation, using a tight basis set. Subsequently, the pre-relaxed structure was subjected to a comprehensive relaxation protocol at the HSE06 level without the +U correction, yielding the final geometry and electronic structure.

[0278] The single-layer LiNbWOg / NbWOg’ structures were derived from the pristine LiNbWOg structure obtained from the Materials Project. (10.17188 / 1278002, see e.g. https: / / www.osti.gov / biblio / 1278002) The two-hydroxyl model was constructed by incorporating two hydrogen atoms symmetrically onto oxygen atoms in the single layer. The oxygen vacancy model was generated by eliminating one oxygen atom bound to one of the tungsten atoms.

[0279] In order to model the negatively charged single-layer NbWOg system under periodic boundary conditions, we employed the virtual crystal approach (VGA) by adjusting the charge of atomic nuclei. A minuscule extra charge qextrawas incorporated into each nucleus. The qextravalue is system-dependent and conforms to Eq. 1. The compensating charge qextraisprimarily governed by three factors: 1. numbers of missing cations ncat, 2. numbers of introduced photoelectrons npe, 3. numbers of atoms shared the counterbalance charge natoms.The missing cations arise from the exfoliation process, during which excess TBAOH reacts with / / NbWO^ and, theoretically, yields TBA+and NbWOg . For instance, the two-hydroxyl model with zero photoelectron has chemical formulaThere are 30 missing cations and, photoelectron and 258 atoms for sharing counterbalance charge, resulting in a qextraequal to 0.1162. Since the NbWOg single layer is isolated by bulky TBA+species, we only modeled the negatively charged single layer within the simulation box.„ extra _ ncat+nVenatoms Eq. 2)

[0280] The general synthesis scheme for the 2D NbWOg nanosheets is illustrated in Fig.32. After exfoliation, monolayer NbWO6 nanosheets were obtained that consist of layers of edge-sharing NbOg and WOg octahedra (Fig. 33).

[0281] The program TOPAS 6.05 was used to refine the recorded XRPD data. The peak profile was described by the fundamental parameter approach implemented into TOPAS 6 and the background modeled by Chebychev polynomials of 6thorder. The pattern of LiNbWOg was subjected to a fully weighted Rietveld refinement? using the dataset of a-LiNbWO6 published by Fourquet et al. as starting model. In the crystal structure of a-LiNbWOg there are three metal position situated all on 2c sites. In their crystal structure refinement Fourquet et al. used an ordered distribution of the cations within the cation substructure, i.e. lithium, tungsten and niobium occupying separate sites. However, they point out the possibility of a cation disorder, which they could not rule out due to the limited quality of the sample and the X-ray and neutron scatting data. Using a structure model of a-LiNbWOg with ordered cations only led to a very poor fit of our diffraction data. An inspection of the Fourier map revealed considerable positive residual electron density at the lithium site, slight positive residual electron density at the niobium sites and considerable negative residual electron density at the tungsten site, indicating occupational disorder among the cations. In the first attempt it was tried to model the disordered cation substructure isotropically, i.e. the lithium site is partially substituted by equivalent amounts of tungsten and niobium, the niobium site is partially substituted by equivalent amounts of tungsten and lithium and so on. The led to a significant improvement in the refinement and yielded an acceptable R-wp value (4.03 %). However there is still some misfit and this model does not properly account for what was observed in the residual electron density map. In X-ray diffraction the scattering power roughly scales with the number of electrons. For the cations we consider: Li+= 2 electrons, Nb^+= 36 electrons and W^+= 68 electrons. The significant positive residual electron density observed for the lithium position in the refinementusing a model with ordered cations, can be explained by the presence of niobium and / or tungsten on this position. The negative residual electron density observed for the tungsten position can be explained by the presence of niobium and / or lithium on this position, with lithium having the stronger impact. For the niobium position a slightly positive residual electron density was observed. If niobium cations (36 electrons) were substituted isotropically by lithium and tungsten cations ((2 electrons + 68 delectrons) / 2 = 35 electrons) this would yield in hardly detectable negative residual electron density. Only an excess of tungsten can lead to a slightly positive residual electron density. Hence, the presence of an anisotropically disordered cation substructure appears to be more suitable. Fortesting this refined was a structure model in which lithium and niobium are partially replaced by tungsten and tungsten is partially replaced by both lithium and niobium. This led to an additional improvement of the fit and a lower R-wp value (3.74 %).

[0282] The XRPD analyses of the LiNbWOg sample showed that the powder mainly consist of a-LiNbWO6 (83.5 wt-%) with minor impurities of |3-LiNbWO6 (8.4 wt-%) and LiNb3O8 (8.1 wt-%). After protonation the diffraction pattern fundamentally changes. The recorded spectra are shown in Fig. 34, showing the powder XRD patterns of LiNbWOg and protonated HNbWO6-xH2O powder including selected reflections indices and corresponding d-spacings f and peak positions of |3-LiNbWO6 and LiNb3O8 (bottom) impurities. Reflections attributed to the impurities are not effected, indicating that neither in |3-LiNbWO6 nor in LiNb3O8 lithium ions are exchanged by protons.

[0283] By protonation, the 001 peak of a-LiNbWOg shifts towards lower diffraction angles corresponding to an increase in the lattice plane distance from 9.27 A to 13.01 A and increases in intensity. This is attributed to the intercalation of water molecules into the structure forming an HNbWO6-xH2O type compound. The slight downshift of non-001 reflections also indicates that the lateral dimension of the layers slightly expanded upon protonation and water intercalation. A LeBail fit of the pattern using space P421m and a- and c- lattice parameters adapted to the peak shift failed. As neither reducing the space group symmetry to P4 as well as reducing the lattice symmetry yielded a suitable fit, it is considered that the protonation leads to a fundamental change in the unit cell metrics. As many reflections seem to be unaffected by protonation (at d ~ 3.3 A, 2.4 A, 1.7 A and 1.5 A, Fig. 34), it is further understood that the setup of the layers is not undergoing a fundamental change during the transition from LiNbWO6 to HNbWO6-xH2O. Due to the pronounced anisotropic peak broadening, which is indicative for structural disorder the powder patterns cannot be indexed. The structural disorder presumably originates from the positional disorder in the cation substructure. As in p-LiNbWOg in which all metal cations are occupationally disorder lithium cations cannot be exchanged by protons, the proton exchange is presumably incomplete in the LiNbWOg.

[0284] Temperature dependent in situ PXRD measurements gave insights into the HNbWOg-xH2O material. Moderate heating at 75 °C leads to a significant change in the diffraction pattern. The basal reflection corresponding to a d-spacing of 13.01 A is shifted towards 10.7 A (Fig. S4b), i.e. the interlayer spacing is significantly contracted. This points to the presence of loosely bound water molecules in-between the layers. In addition, broad peaks situated at 7.8 and 11.2 ° 20 disappear indicating a reduction of structural disorder by the release of water molecules. Further heating above 150 °C leads to a gradual broadening of the basal reflection, which eventually disappears, whereas all other peaks remain sharp. This is most likely attributed to either the gradual release of additional water molecules or the dehydration of hydroxide groups, which leads to an increasing modulation of the interlayer distance and at the end of the process the material loses its layered character. Peaks attributed to the minor impurities LiNbWOg and LiNbgOg can be observed throughout the entire temperature range.

[0285] Fig. 35 shows SEM images of (a) LiNbWOe and (b) HNbWOe-xl-hO powders, which exhibit well-defined layered structures. High-resolution transmission electron microscopy (HRTEM) and Fast Fourier Transform (FFT) of stacked sheets confirm the highly crystalline 2D structure of NbWOg, while AFM reveals a thickness of the NbWO6 monolayer nanosheets between 1.4 and 1.9 nm, which is larger than the theoretical thickness of 7.19 A obtained from the DFT relaxed NbWOg model in the absence of solvent. This discrepancy in thickness can be ascribed to two main factors: First, the presence of charge compensating protons forming hydroxyl groups on both surfaces of the nanosheet, as well as the contribution of other species, here likely TBA+, towards charge compensation. Without wishing to be bound by theory, the hydroxyl-bound tungsten atoms may function as sites accommodating small polarons. Second, hydrogen bonds may be formed between water and the nanosheet surface, which leads to an effective increase in the (solvated) nanosheet thickness.

[0286] The NbWOg surface composition and electronic structure were extracted by X-ray photoelectron spectroscopy (XPS) as depicted in Fig. 36. While the Nb XPS spectrum shows the presence of one Nb^+species only, the W XPS spectrum suggests the co-existence of W in the oxidation states W^+and W^+(ratio of approx. 7:1), where the latter is likely due to native oxygen vacancies introduced during bulk synthesis.

[0287] According to DFT calculations, the resulting symmetry-reduced W^+c^ motifs introduce new states slightly below the conduction band minimum (CBM), which effectively lower the bandgap of an idealized 2D nanosheet from 4.17 eV to 3.64 eV. While such color centers can lead to a faint blue hue of the otherwise colorless and transparent material, the UV- vis absorbance spectrum (Fig. 37) indicates low absorption in the visible range and strongabsorption in the near UV range, consistent with an indirect bandgap of 3.43 eV (derived from the Tauc plot shown in Fig. 38). While the small quantity of intrinsic W^+color centers is insufficient to significantly alter the material's intrinsic color, their presence is thought to be instrumental for the resulting photochromic behaviour under UV light exposure, akin to the situation in WO3.

[0288] The photochromic behaviour of a NbWOg nanosheet suspension under 365 nm UV illumination was examined by adding 10 vol% methanol as TBOR; the relationship between blue color intensity and illumination time is quantified by operando UV-vis spectroscopy (Fig. 39). The NbWOg suspension shows an increase in absorbance intensity in the range of 400 to 800 nm already after 1 min of 365 nm UV illumination. The absorbance intensity at 575 nm increases significantly up to 20 min illumination, after which a plateau is observed (Fig. 40).

[0289] To explain this coloration behaviour, we propose a process similar to other photochromic metal oxides, such as WO3 and MOO3. The proposed energy conversion and storage mechanism is described by Equations (1 - 4):MxNbW06+ hv -> MxNbW06* + e~ (hv) + h+(hv) (1)D + h+(hv)-^ D+(2)MxNbW06* + yM++ ye~ (hv) -> Mx+yNbWy+vWi-y+vlO6(3)Mx+yNbWy+vWi-y+vlO6^ MxNbW06+ yM++ ye~ (4) where IVI is an electrolyte cation from solution and can include H+, Li+, and TBA+; D is the TBOR (electron donor; here: MeOH).

[0290] During photocharging, electron (e-) - hole (h+) pairs are formed upon abovebandgap illumination (Equation 1). While the holes migrate to the surface of the material where they are quenched by the TBOR (Equation 2), the photogenerated electrons in the conduction band get trapped at a defect or transition metal site, forming a color center. Under illumination, these in-gap states form the new (quasi-)Fermi level and drive the photointercalation of cations from the electrolyte into the layered host. The resulting overall increase in the system's energy results in an effective storage of solar energy (Equation 3). During the charge releasing phase, the application of a positive current or Pt catalyst extract the photoexcited electrons from the polaronic states, triggering cation deintercalation (Equation 4).

[0291] To gain a more comprehensive understanding of the charge trapping mechanism in NbWO6, DFT simulations examined possible defects and associated electronic arrangements, which play a pivotal role in the charge trapping performance. In the investigated sample, nativeexfoliated NbWOg nanosheets possess a negative surface charge compensated by surface- adsorbed TBA+ions and protons. To examine the electronic structure and polaron formation of the sample, which contribute to the stabilization and storage of photoelectrons, a series of single-layer NbWOg models was constructed. In order to model charged species under periodic boundary conditions, a minute positive charge is introduced to each nucleus in the models, ensuring charge neutrality within the system.

[0292] The pristine NbWOg single-layer model exhibits a 4.29 eV bandgap, with Nb- related conduction bands displaying lower energies (approximately 2.0 eV) compared to those originating from tungsten. Consequently, when an extra electron is introduced into the system, it localizes on one of the Nb atoms located in the central region of the nanosheet, rather than on W, thus forming a polaronic state 1.09 eV below the CBM. However, the WOg motifs in the outer layer contain small amounts of intrinsic oxygen vacancies and tungsten-based color centers, as evidenced by the XPS measurements (Fig. 36). The resulting lattice distortion promotes the formation of polarons on W, leading to polaronic states with lower energies than those formed on Nb in the pristine model. In addition, native surface hydroxyl groups are assumed to be present since compensation of the inherent negative layer charge of the nanosheets occurs either by TBA+or H+as counter ions, which likewise result in the distortion of the octahedral tungsten environment with concomitant symmetry breaking and a shift of tungsten states to lower energies.

[0293] The calculations show that introducing native hydroxyl groups on each side of the symmetric layer model generates new bands, shifted to 0.28 - 0.39 eV below the CBM. These initially empty states may transform into polaronic states upon the introduction of light-induced electrons into the system (Note that for H+was used as a proxy for Li+ in stabilizing thecolor centers). Since the interaction between surface-bound oxygen and protons is expected to be stronger and more localized than for hydrated Li ions in the electrolyte, our calculations using protons as charge compensating species represent the boundary case of strong polaron— cation interactions while being qualitatively similar to charge screening by alkali metal ions. W-0...H interactions are thus expected to correspond to energetically more stabilized polaron states located at approximately 1.58 - 1.68 eV below the CBM, while weaker alkali-metal— polaron interactions may lead to more shallow polaron / trap states. This hypothesis is corroborated by pH-dependent photocurrent measurements, which indicate significantly enhanced photocurrent under acidic conditions as compared to basic conditions with LiOH as an electrolyte (Fig. 41). Importantly, however, due to the layered 2D morphology of the NbWOg nanosheets, both protons and Li ions can efficiently access surface-oxygen sites and form M...0-W (M = H+, Li+) complexes, compensating the and stabilizing the electronic charge on the color centers.

[0294] A similar local distortion effect can be created in an oxygen vacancy model by assuming oxygen vacancies on the surface, forming a new state 0.53 eV below the CBM. As expected, this new state may transform into a polaronic state upon accommodating photoelectrons on the associated W site, with energies ranging from 1.96 to 1.72 eV below the CBM, depending on whether one or two electrons are localized on a single W site. The second photoelectron may also localize on Nb, as only one W site with an oxygen vacancy exists in this model. However, the polaronic state formed by Nb'V js less stable than the ones fromand by about 0.88 and 0.60 eV, respectively. Overall, the singlet state model (both polarons on W) possesses an energy 0.33 eV lower than the triplet model (one polaron on W and one on Nb). The in-gap states are considered to serve as the origin of the blue color, and the polaron formation observed in these models signifies the presence of versatile sites capable of capturing and storing photogenerated electrons for extended durations. The local distortion induced by hydroxyl groups and oxygen vacancies underlines the potential for tailoring this material to achieve superior performance in terms of augmented capacity and extended storage time.

[0295] To investigate the photoelectrochemical (PEC) properties of the NbWOg electrode (photoanode), a three-electrode setup with Ag / AgCI (saturated KCI) as reference electrode (RE) and Au plate as counter electrode (CE) was employed (Fig. 42). The resulting photocurrents of the NbWOg photoanode in the presence of different TBORs (electron donors) were measured using chronoamperometry (CA) under da rk / light cycles and different illumination conditions. Unless otherwise specified, the light sources used are 1 sun, standard solar illumination (AM 1.5G, 100 mW cm’2), and 365 nm UV LED (A.365 nm, 206 mWFig. 43 shows the photocurrent under (top) 1 sun and (bottom) 365 nm UV illumination. Chronoamperometry experiments were performed with an applied potential of -0.1 V vs. Ag / AgCI (saturated KCI).

[0296] In all cases, the photocurrent density in the presence of MeOH as TBOR (electron donor) is higher than for 4-methyl benzyl alcohol (4-MBA, 10 mM) and H2O, as derivable from the following Figures:

[0297] Fig. 43 shows the photocurrents of NbWOg photoanodes under (a) 1 sun and (b) 365 nm UV illumination in oxygen-free IM LiCI in the presence of MeOH. A pronounced decrease in photocurrent over time is observed under 1 sun illumination, while very little decrease is seen under 365 nm UV illumination. CA experiments were performed under an applied potential of - 0.1 V.

[0298] Fig. 44 shows the photocurrents of NbWOg photoanodes in oxygen-free (a) IM LiCI in the presence of 10 mM 4-MBA and (c) IM LiCI electrolyte in pure H2O under 1 sunillumination. The first off / on / off cycles of (a, c) are shown in (b, d), respectively. CA experiments were performed under an applied potential of -0.1 V.

[0299] Fig. 45 shows the photocurrents of NbWOg electrodes in oxygen-free (a) IM LiCI in the presence of 10 mM 4-MBA, and (c) IM LiCI electrolyte in H2O under 365 nm UV illumination. The first off / on / off cycles of (a, c) are shown in (b, d), respectively. CA experiments were performed with an applied potential of -0.1 V. The photocurrent transient spikes may be caused by the discrepancy between the fast carrier generation, recombination, and slow surface reaction dynamics. They confirm the lower efficiency of 4-MBA and H2O as TBOR compared to MeOH. These results indicate the high efficiency of MeOH for the quenching of holes.

[0300] Fig. 46 shows the UV-vis absorbance spectra of oxygen-free NbWOg nanosheet suspension in the presence of (a) H2O, (b) 4-MBA, and (c) MeOH donors under light illumination for 10 min. The insert in (c) shows the magnified region between 570 nm to 580 nm. Fig. 46 d) summarizes the dependence of absorption intensity at a wavelength of 575 nm as a function of the different TBORs. The absorbance at a wavelength of 575 nm in the presence of MeOH as TBOR under 365 nm UV illumination is higher than for 4-MBA in water, while the absorption intensity in water is the lowest. The UV-Vis absorbance spectra thus confirm the higher hole quenching efficiency of MeOH compared to 4-MBA and water, in line with the more intense blue color of the NbWOg suspension in the presence of MeOH. The color change from light milky to blue is in line with the formation of small polarons as discussed above.

[0301] Note that the presence of oxygen in the electrolyte (or the device) decreases the photocurrent, as oxygen is a potent electron acceptor. This is illustrated in Fig. 47, showing a CV sweep in IM LiCI and MeOH electrolyte under 365 nm UV illumination and continuous Ar and oxygen purging. The scan rate is 10 mV s'l. Fig. 48 shows a dark CV sweep measurement, indicating reversible electron storage and release in continuous Ar purging, and the oxygen reduction reaction (ORR) in oxygen rich environment. Fig. 49 shows the OCP of NbWOg electrodes in oxygen-rich 1 M LiCI in the presence of MeOH as TBOR under 1 sun and 365nm UV illumination. The electrode was illuminated for 10 min and left in the dark for 1 h. The presence of oxygen would scavenge photo-generated electrons from NbWOg, which decreases the OCP of the electrodes during light charge. The OCP drops dramatically when light charging is stopped, indicating the electron scavenging side reaction by oxygen which quickly consumes the photogenerated electrons. These results highlight the importance of oxygen-free electrolyte, respectively an oxygen-free device. Interestingly, however, the photocurrent even reaches 40 pA cm’2 under 365 UV illumination in spite the presence of oxygen, which we rationalize with a dynamic equilibrium between electron generation and consumption. Notably, the photocurrent under 365 nm UV illumination reaches a steady state immediately after turning on the light andis stable for more than 120 min, suggesting efficient and continuous charge carrier generation and extraction. (Fig. 50).

[0302] Next, linear sweep voltammetry (LSV) under illumination was conducted to extract the photogenerated electrons (Fig. 51). In both cases, the photocurrent density is higher than the dark current, reaching 61pA cm'2 under 365 nm UV illumination. Fig. 52 shows photocurrents at different potentials under 1 sun and 365 nm UV illumination. Blank FTO substrate under 1 sun illumination is used as reference. The electrolyte used for PEC measurements is oxygen-free IM LiCI and MeOH (10 vol%). Scanning towards more negative potentials, the photocurrent slowly decreases until around -0.38 V and -0.40 V under 1 sun and 365 nm UV illumination, respectively.

[0303] To study the charge storage mechanism and solar battery function, the NbWOg solar battery half-cell performance was examined using a three-electrode setup as shown in Fig. 42 to perform photocharging and electric discharging measurements. The photoanode (first electrode) was immersed into oxygen-free 1 M LiCI in the presence of 10 vol% MeOH (TBOR) and a waiting time of around 30 min was applied to achieve equilibration. The photoanode was then photocharged for 10 min under OCP conditions, during which a clear color change was observed, followed by electric discharge at different current densities. Fig. 53 shows the capacity under (a) different discharge current densities after 10 min light illumination and (b) different illumination times under discharge current densities of 0.48 and 4.8 mA g'l for 1 sun and 365 nm UV, respectively. Fig. 54 shows the electrical discharge profiles at different discharge current densities after illumination under (a) 1 sun and (b) 365 nm UV light for 10 min in IM LiCI and MeOH electrolyte under continuous Ar purging. The electrode which was photocharged by 365 nm UV exhibits significantly higher capacity than that illuminated by 1 sun. The considerable reduction in capacity at increased current densities can be attributed to internal resistance. This may be due to the fact that below the percolation threshold, isolated small polarons on distorted W atoms may show low mobilities.

[0304] To evaluate the maximum photocharging capacity, the photoanode was illuminated for different times, followed by electric discharge (Fig. 53b). The OCP increases to about -0.4 V and -0.6 V after 1 sun and 365 nm UV illumination for 1 min, respectively, indicating a pronounced photovoltaic effect of NbWOg. A further increase in illumination time leads to a slow increases in the photopotential; the OCP reaches to around -0.68 V and -0.81 V after 1 sun and 365 nm UV illumination for 120 min, respectively. Subsequently, the photoanode was electrically discharged to 0.2 V. The capacity increases accordingly when increasing the illumination time from 1 to 30 min. Specifically, the capacity reaches to 186.7 pAh g'l and 3.2 mAh g'l after 30 min 1 sun and 365 nm UV illumination, respectively. An apparent plateau is reached when further increasing the illumination time from 30 to 120 min, indicating asaturation of photogenerated electrons after long-term illumination which may due to a steady state of photoexcited electron generation and recombination of charge carriers (see also Fig. 40).

[0305] Furthermore, double reducedstates and Nb'V may be additional photocharging channels that can be accessed as thestates are filled, as suggested by the DFT calculations, leading to an additional enhancement in capacity. The constrained mobility of polarons on the distorted W atoms may hinder their accessibility to Li ions. This limitation is presumably especially prominent when these polarons are deeply embedded within the material, suggesting that there is room for further improvement and highlighting the preference for easily accessible 2D materials.

[0306] Fig. 55 shows the OCP stability during and after light illumination. The photopotential reaches around -0.50 V and -0.70 V under 1 sun and 365 nm UV illumination for 10 min, respectively. The OCP remains stable for at least 2h and 20h under 1 sun and 365 nm UV illumination, respectively, suggesting that charge trapping is highly robust in this system. A slow increase in photopotential after ca. 90 min is probably due to that additionalstates become filled. This long-term stability of the sample in the dark after exposure to UV illumination is likely rooted in the atomistic details of the energy storage mechanism, which appears to be the formation of small polarons on the (distorted) tungsten ions. This mechanism gives rise to more stable polarons compared to polaron formation on niobium by roughly 0.88 eV, as dictated by the density of states (DOS) of the oxygen vacancy model.

[0307] The population of photoelectrons in the conduction band and associated polaronic / trap states will affect the mobility of the charge carriers. To identify the influence of light illumination on the charge transfer and the role of counter ions in NbWOg, EIS was conducted to probe both the ground state potential in the dark and photoreduced states after light illumination. Fig. 56 shows the cycling stability of the photoanode under 1 sun and 365 nm UV illumination, followed by electric discharge. Solid and hollow symbols represent capacity and cycling capacity retention, respectively. The electrolyte used for PEC measurement is oxygen-free IM LiCI and MeOH (10 vol%). Fig. 57 shows the Nyquist plot of NbWOg photoanode under 1 sun illumination.

[0308] Before illumination, the material shows high resistance of 7.72 MQ. Under 1 sun and 365 nm UV illumination, an equivalent circuit composed of a charge transfer resistance (RCT) in parallel to a constant phase element (CPE1), and a resistive element RC which is used to describe the material's conductivity in parallel to CPE2 , while an additional resistor RS is used to describe serious resistance including electrode contact and electrolyte. The RCT for the sample under 1 sun and 365 nm UV illumination decrease to less than 300 Q, suggesting significantlyincreased mobility of the system under light illumination. Notably, RC amounts to around 3.26 kQ under 365 nm illumination, which is five orders of magnitude lower than under 1 sun illumination (around 17.5 kQ). The EIS results therefore suggest that above-bandgap illumination significantly decreases RCT and RC. This drop in resistance can be ascribed to the generation of small polarons under UV illumination. When reaching a percolation threshold upon continued photocharging, the polaron states can condense into polaron bands which act as highly effective charge transporter channels.

[0309] A Bode-type plot is further used to understand the energy storage mechanism of NbWO6 (Fig. 58). The real capacitance C' exhibits almost constant values in the low frequency range in the dark, indicating the capacitive process. In contrast, C' increases significantly under illumination, indicating a diffusion-limited process.

[0310] Next, the cycling stability of the NbWOe photoanode was investigated by photocharging and electric discharging for 30 cycles. Fig. 59 shows Electric discharge profiles after (a) 1 sun illumination for 10 min and (b) 365 nm UV illumination for 5 min. The discharge current densities are (a) 4.8 mA g1and (b) 24 mA g1. The photoanodes under 1 sun and 365 nm UV illumination show 35% and 48% cycling capacity retention after 30 cycles, respectively. However, a significant fraction of the capacity reduction in the UV-illuminated electrode manifests after the initial cycle, possibly attributed to an irreversible reaction resulting from water co-intercalation, causing the nanosheets to swell and eventually detach, leading to permanent capacity loss. Comparing with the second cycle as the reference, the photoanode under 365 nm UV illumination exhibits a capacity retention as high as 76% after 30 cycles. This implies that the electrode maintains its robustness after the first cycle, akin to the behavior of many battery electrode materials, and the sites involved in polaron formation continue to function efficiently as charge storage reservoirs. This is further illustrated in Fig. 60, showing the cycling stability of the photoanode under 1 sun and 365 nm UV illumination, followed by electric discharge. Solid and hollow symbols represent capacity and cycling capacity retention, respectively. The electrolyte used for PEC measurement is oxygen-free IM LiCI and MeOH (10 vol%).

[0311] The ability of NbWOg to store photogenerated electrons stably and reversibly qualifies this material as a solar battery photoanode. In order to explore its potential as a dual charge storing and hydrogen evolving material suitable for solar battolyzers, i.e. to act as an ESM in the context of the present invention, the exploitation of the material's charge trapping capabilities for dark photocatalysis and on-demand HER was evaluated.

[0312] The photogenerated electrons are trapped by the material, and their release is triggered by the addition of a Pt nanoparticle catalyst to generate hydrogen. An aqueous NbWOg (268.3 pmol) suspension was irradiated in the presence of MeOH as TBOR (10 vol%, 24.7 mmol, 92 equiv.) under 365nm UV for 30 min, followed by hydrogen evolution upon the addition of Pt catalyst after one hour delay in the dark. This is illustrated in Figure 61. Fig. 61a shows the dark hydrogen generation as a function of time. The inserts photos show the color change of NbWOg suspension from colorless / white to dark blue upon 365nm UV illumination for 30 min. Fig. 61b illustrates the durability measurement for dark hydrogen evolution. The regions in grey correspond to the light illumination period. Fig. 62 shows the hydrogen amount as a function of illumination time, where Fig. 62b shows the long-term measurement for dark hydrogen generation. The grey regions correspond the light illumination. Fig. 62c is a histogram plot that shows the turnover frequency (TOF) after 365 UV illumination for 30 min without delay time and with 1 h delay time.

[0313] The maximum hydrogen generated is 0.68 pmol (max. TOF 0.13 h'l) which corresponds to a 17% loss compared with hydrogen evolution without a delay (max. TOF 0.17 h' 1) (Fig. 62). As expected, photo(de)charging of NbWOg is accompanied by the material's color change from white to blue (Fig. 61a, insert). Consistent with the time-dependent UV-vis spectrum (Fig. 63), the amount of hydrogen generated saturates after 30 min of illumination, and no significant increase is observed even after two hours illumination (Fig 62).

[0314] Intriguingly, the 2D NbWOg suspension displays durability after the addition of Pt nanoparticles, and consequently enable the on-demand hydrogen generation process multiple times under UV illumination. As shown in Fig. 62b, we first illuminated the NbWOg suspension for 30 min, followed by adding Pt catalyst to produce hydrogen. The hydrogen amount plateaus after around 2 h. We then illuminated the suspension two more times (30 min) without the addition of an additional Pt catalyst, followed by a dark phase, during which the hydrogen amount increased again. All in all, however, the experiments show that the trapped charges are available for efficient hydrogen evolution upon the addition of a co-catalyst.

[0315] It was demonstrated herein that layered 2D niobium tungstate is a novel optoionic material that can be used for a "solar battolyzer" capable of light harvesting, conversion and storage. Light storage in NbWOg proceeds via the formation of small polarons residing on the tungsten sublattice to trap charge as shown in DFT, photointercalation of Li+ into the layered host upon above bandgap illumination, and hole quenching. The trapped charges can be stored in the ESM material (a "solar battolyzer") which can be used both in a solar battery-type setting and allows extraction with a time delay to generate e.g. solar hydrogen on-demand. Without wishing to be bound by theory, it is understood that polaron formation acts as a viablemechanism of charge trapping in such layered oxides, which represents thus a more general design concept for novel optoionic materials.

[0316] A "solar battolyzer" is a powerful concept to mitigate the intermittency of solar irradiation. The device can operate essentially around the clock, leading to a high degree of utilization: either the electricity surplus is stored in electrical storage capacity and hydrogen or electricity is provided from the storage when there is a deficit. In principle the solar battolyser uses a single system instead of two separate ones which is also highly important for economic considerations. Therefore, the utility of combined light harvesting and charge storage via photointercalation is far more general and opens the door to new optoionic device concepts ranging from photo-memory devices to photo-desalination.Industrial Applicability

[0317] The present invention provides a novel system that allows decoupling the light and dark reaction and to store the absorbed photonic energy in the form of trapped charges (photogenerated electrons). The present invention thus allows the industrial application in the fields of solar energy storage and on-demand supply thereof to provide the necessary energy for producing electricity and / or for reducing reducible reactants.

Claims

Claims1. A photo-electrochemical device, comprising a first electrode containing an electron storage material, ESM, wherein the ESM has a structure containing oxygen and at least one selected from the transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides, and wherein the ESM is capable of producing electron-hole pairs when irradiated by light; a second electrode, optionally containing a reduction reaction catalyst-containing layer; an electrolyte containing a to-be-oxidized reactant and a to-be-reduced reactant, the electrolyte being in contact with the ESM and the second electrode, preferably the reduction reaction catalyst-containing layer if present; and an electrical connection between the first electrode and the second electrode for transferring electrons from the first to the second electrode.

2. A photo-electrochemical device, comprising a first electrode containing an electron storage material, ESM, wherein the ESM has a structure containing oxygen and at least one selected from the transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides, and wherein the ESM is capable of producing electron-hole pairs when irradiated by light; an oxidation reaction catalyst-containing layer, arranged to receive holes from the ESM; a second electrode, optionally containing a reduction reaction catalyst-containing layer; and an electrolyte containing a to-be-oxidized reactant and a to-be-reduced reactant, the electrolyte being in contact with the oxidation reaction catalyst-containing layer and the second electrode, preferably with the reduction reaction catalyst-containing layer if present; an electrical connection between the first electrode and the second electrode for transferring electrons from the first to the second electrode.

3. The photo-electrochemical device according to claim 0 or 2, wherein the electrical connection comprises a potentiostat configured to amplify a first voltage between the first electrode and the second electrode.

4. The photo-electrochemical device according to any one of claims 1 to 3, wherein the electrical connection comprises a switching device for electrically connecting or disconnecting the first electrode and the second electrode, and / or an electrical load.

5. The photo-electrochemical device according to any one of claims 1 to 4, further comprising: a layer containing a hole transport material, HTM, provided between the first electrode and the oxidation reaction catalyst-containing layer, the HTM -containing layer being arranged to transfer holes from the first electrode to the oxidation reaction catalyst-containing layer.

6. The photo-electrochemical device according to claim 5, further comprising: a layer containing a hole storage material, HSM, the HSM-containing layer being arranged to receive holes from the HTM-containing layer.

7. The photo-electrochemical device according to claim 6, further comprising: a voltage sensor configured to measure a second voltage between the first electrode and the HSM-containing layer, wherein the switching device is configured to be in a closed state when the second voltage is above a predetermined threshold value.

8. The photo-electrochemical device according to any one of claims Fehler! Verweisquelle konnte nicht gefunden werden. to 7, wherein the first electrode further contains a transparent conductive material electrically connected to the electrical connection.

9. The photo-electrochemical device according to claim 8, wherein the transparent conductive material is a fluorine-doped tin oxide, FTO, indium-doped tin oxide, ITO, or graphene.

10. The photo-electrochemical device according to claim 8 or 9, wherein the transparent conductive material is formed as a thin film and the ESM is provided on a surface of the film.

11. A photo-electrochemical device, comprising: a layer containing an electron storage material, ESM, wherein the ESM has a structure containing oxygen and at least one selected from the transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides, and wherein the ESM is capable of producing electron-hole pairs when irradiated by light; a reduction reaction catalyst-containing layer, arranged to receive electrons from the ESM- containing layer; a layer containing a hole storage material, HSM; an oxidation reaction catalyst-containing layer, arranged to receive holes from the HSM; a layer containing a hole transport material, HTM, and provided between the ESM- containing layer and the HSM-containing layer, the HTM-containing layer being arranged to transfer holes from the ESM-containing layer to the HSM-containing layer; andan electrolyte containing a to-be-oxidized reactant and a to-be-reduced reactant, the electrolyte being in contact with the oxidation reaction catalyst-containing layer and the reduction reaction catalyst-containing layer.

12. The photo-electrochemical device according to claim 11, further comprising: an electrical connection electrically connecting the ESM-containing layer and the HSM- containing layer, wherein the electrical connection comprises an electrical load.

13. The photo-electrochemical device according to claim 5 or 11, wherein the HTM is a conducting material, preferably a conducting polymer.

14. The photo-electrochemical device according to any one of claims 1 to 13, wherein the electrolyte is a liquid electrolyte.

15. The photo-electrochemical device according to claim 14, wherein the liquid electrolyte is water or an aqueous medium.

16. The photo-electrochemical device according to claim 6 or 11, wherein the HSM contains a conducting polymer, a metal, a semiconductor or a mixture thereof.

17. The photo-electrochemical device according to any one of the preceding claims, wherein the ESM is not MOO3.

18. The photo-electrochemical device according to any one of the preceding claims, wherein the ESM is not TiC>2 or lithiated TiC>2, i-e., LixTiC>2 (with 0<x<l), and / or wherein the ESM is not TiC>2 or lithiated TiC>2, i-e., LixTiC>2 (with 0<x<l) wherein the TiC>2 is of anatase and / or rutile phase.

19. The photo-electrochemical device according to any one of the preceding claims, wherein the ESM is an oxide, oxynitride or oxysulfide of the at least one of the transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides.

20. The photo-electrochemical device according to any one of the preceding claims, wherein the ESM is an oxide of at least one transition metal in Groups 4, 5, 6 and 7 of the Periodic Table of the Elements or of cerium.

21. The photo-electrochemical device according to any one of the preceding claims, wherein the structure of the ESM is composed of oxygen and two or more selected from transition metalsof Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides, wherein the structure of the material is composed of oxygen and one transition metal selected from Zr, Hf, V, Ta, Nb, Cr, W, Mn and Re, or wherein the ESM is CeC>2, and wherein the ESM material that is composed of oxygen and one transition metal selected from Zr, Hf, V, Ta, Nb, Cr, W, Mn and Re or which is CeC>2 is optionally doped with one or more other metals, such as one, two or three of selected from the lanthanides and transition metal elements, e.g. one, two, three or more selected from La, Y, Sm, Yb, Sc, Mg, Ca, Co, Ni, Zr, Ti, Hf, Eu, Gd, Pr and Sn, such as Y, La, Zr, and Sc.

22. The photo-electrochemical device according to any one of the preceding claims, wherein the at least one selected from transition metals and lanthanides forming the ESM includes at least one of Nb and W, and preferably includes Nb.

23. The photo-electrochemical device according to any one of the preceding claims, wherein the optical absorption spectrum of the ESM in the range of 400 to 700 nm changes upon illuminating the ESM, which change is preferably an increase in absorbance by at least 5%, and which change is further preferably maintained for at least 15 minutes after the illumination has stopped.

24. The photo-electrochemical device according to any one of the preceding claims, wherein the ESM has and / or is able to form color centers.

25. The photo-electrochemical device according to claim 24, wherein the color centers are atomic scale imperfections in the ESM selected from the group consisting of impurities, vacant lattice sites, interstitial atoms and ions or combinations thereof.

26. The photo-electrochemical device according to any one of the preceding claims, wherein the ESM is an oxide of the at least one of transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides, and further comprises a cation of a main group metal of the Periodic Table of the Elements.

27. The photo-electrochemical device according to claim 26, wherein the at least one selected from transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides has a d^ electronic configuration and the main group metal cation has an electronic s^d -O configuration.

28. The photo-electrochemical device according to claim 27, wherein the main group metal cation is Sn^+or Bi^+, preferably Sn^+.

29. The photo-electrochemical device according to any one of claims 26 to 28, wherein the ESM exhibits a bandgap which is the difference in the energy levels between the conduction band minimum and the valence band maximum, wherein the conduction band minimum is formed by a d^ electronic configuration of the at least one selected from the transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides, and wherein the valence band maximum is formed by hybridized main group metal s^ and oxygen 2p orbitals.

30. The photo-electrochemical device according to any one of the preceding claims, wherein the ESM has or is capable of forming oxygen vacancies in the structure.

31. The photo-electrochemical device according to claim 30, wherein upon illumination with light oxygen vacancies are formed in the structure of the ESM; preferably upon irradiation at 340 nm at 20 mW / cm^ for 1 minute.

32. The photo-electrochemical device according to any one of the preceding claims, wherein the ESM has a bandgap in the visible or near UV range.

33. The photo-electrochemical device according to any one of the preceding claims, wherein the photoelectrons are trapped in the material locally in the form of polarons and are as such stored in the ESM, wherein the energy level of the polarons is below the conduction band minimum and above the valence band maximum of the ESM.

34. The photo-electrochemical device according to any one of the preceding claims, wherein the photoelectrons have, under oxygen-free conditions, a lifetime of at least 4 hours, preferably at least 8 hours and more preferably at least 12 hours, in the ESM.

35. The photo-electrochemical device according to any one of the preceding claims, wherein the ESM is a semiconductor with a bandgap in the range of 0.5 to 4 eV.

36. The photo-electrochemical device according to any one of the preceding claims, wherein the ESM is capable of photointercalating and photodeintercalating cations or anions.

37. The photo-electrochemical device according to claim 36, wherein the ESM is capable of intercalating and de-intercalating cations, in particular alkali metal cations and protons.

38. The photo-electrochemical device according to any one of the preceding claims, wherein the ESM has a 2D or 3D crystal structure, preferably selected from Bronze phase, Magneli phase, Dion-Jacobsen phase, Ruddlesden-Popper phase, and Wadsley-Roth phase .

39. The photo-electrochemical device according to any one of the preceding claims, wherein the ESM comprises nanosheets.

40. The photo-electrochemical device according to any one of the preceding claims, wherein the ESM is in the form of exfoliated nanosheets.

41. The photo-electrochemical device according to claim 39 or 40, wherein the nanosheets have a thickness in the range of 0.5 to 2.5 nm.

42. The photo-electrochemical device according to any one of the preceding claims, wherein the ESM comprises structural units of oxygen selected from the group consisting of octahedral, tetrahedral, cubic, trigonal prismatic, trigonal bipyramidal structural units, and distorted versions thereof, the structural units of oxygen having the transition metals in their centers, wherein the structural units of oxygen are preferably selected from the group consisting of octahedral and tetrahedral structural units and distorted versions thereof, and wherein the structural units of oxygen are more preferably octahedral structural units.

43. The photo-electrochemical device according to any one of the preceding claims, whereindivalent cation; wherein the ESM is optionally doped with one or more metals, such as one, two or three of selected from the lanthanides and transition metal elements, e.g. one, two, three or more selected from La, Y, Sm, Yb, Sc, Mg, Ca, Co, Ni, Zr, Ti, Hf, Eu, Gd, Pr and Sn, such as Y, La, Zr, and Sc.

44. The photo-electrochemical device according to claim 43, wherein the monovalent cation M+is selected from the group consisting of organic cations, such as tetramethyl ammonium and tetrabutyl ammonium, H+, and alkali metals, such as Li+, and wherein the divalent cation is selected from the group consisting of Zn2+and alkaline earth metals, such as Mg2+, Ca2+, Sr2+and Ba2+.

45. The photo-electrochemical device according to any one of the preceding claims, wherein the ESM is a single-phase material.

46. A use of an electron storage material, ESM, as defined in any one of claims 1 to 45 in a combined solar battery and electrolyzer.

47. A method for photocatalytic production of a product comprising the following steps: illuminating an electron storage material, ESM, with light to create photogenerated electrons in the ESM, storing the photogenerated electrons in the ESM, and reacting a to-be-reduced reactant with the stored photogenerated electrons, optionally in the presence of a reduction catalyst, to form a product, wherein the ESM has a structure containing oxygen and at least one selected from the transition metals of Groups 4, 5, 6 or 7 of the Periodic Table of the Elements and the lanthanides, provided that the ESM is not TiC>2, E^WOg and WO20O56, and with the further proviso that the ESM satisfies at least one of the following requirements (i) to (iv):(v) it does not includeanions;(vi) it does not include a polyoxometalate, POM;(vii) it does not include a photosensitizer including a (4,4'-di-tert-butyl 2,2'- bipyridine)2 Ru (diethyl((4'-methyl-(2,2'-bipyridine)-4-yl)methyl)phosphonate) moiety;(viii) it does not include an organic photosensitizer.

48. The method according to claim 47, wherein the ESM satisfies requirement (i).

49. The method according to claim 47, wherein the ESM satisfies requirement (ii).

50. The method according to claim 47, wherein the ESM satisfies requirement (iii).

51. The method according to claim 47, wherein the ESM satisfies requirement (iv).

52. The method according to any one of claims 47 to 51, wherein the ESM is not MOO3.

53. The method according to any one of claims 47 to 52, wherein the ESM is as defined in any one of claims 16 to 44.

54. The method according to any one of claims 47 to 53, wherein the photogenerated electrons are stored in the ESM until they are extracted by adding the reduction catalyst and react with the to-be-reduced reactant to form the product.

55. The method according to any one of claims 47 to 54, wherein the reduction catalyst is added after the illumination has stopped.

56. The method according to any one of claims 47 to 55, wherein a to-be-oxidized reactant is present when the ESM is illuminated with light, which to-be-oxidized reactant quenches the holes that are formed along with the photogenerated electrons in the ESM upon illumination of the material, to yield a product of the to-be-oxidized reactant.

57. The method according to claim 56, wherein the to-be-oxidized reactant is an alcohol, a sugar or water.

58. The method according to any one of claims 47 to 57, wherein the to-be-reduced reactant is H2O, oxygen, CO2, CO, nitrogen, NOg’ or an acid.

59. The method according to any one of claims 47 to 58, wherein the to-be-reduced reactant is H2O and the product is hydrogen.

60. The method according to any one of claims 47 to 58, wherein the to-be-reduced reactant is oxygen and the product is H2O2 and / or H2O.

61. The method according to any one of claims 47 to 58, wherein the to-be-reduced reactant is CO2 and the product is selected from the group consisting of CO, formaldehyde, formic acid, acetic acid, methanol, ethanol, methane, ethylene and mixtures thereof.

62. The method according to any one of claims 47 to 58, wherein the to-be-reduced reactant is nitrogen and the product is NH3.

63. The method according to any one of claims 47 to 62, wherein the ESM is present in the form of particles that are dispersed in an electrolyte.

64. An electrochemical device, comprising a negative electrode comprising an electron storage material, ESM, a positive electrode, andan electrolyte, optionally containing a to-be-reduced reactant and / or a to-be-oxidzed reactant, ,divalent cation, wherein the ESM is optionally doped with one or more metals, such as one, two or three of selected from the lanthanides and transition metal elements, e.g. one, two, three or more selected from La, Y, Sm, Yb, Sc, Mg, Ca, Co, Ni, Zr, Ti, Hf, Eu, Gd, Pr and Sn, such as Y, La, Zr, and Sc.

65. A photorechargeable battery comprising a negative electrode comprising a substrate with a surface and a layer comprising an electron storage material, ESM, provided on the surface of the substrate, and a positive electrode comprising a substrate with a surface and a layer comprising a hole storage material provided on the surface of the substrate, and a photovoltaic element sandwiched between the layer comprising the ESM and the layer of the hole storage material, wherein the photovoltaic element is capable of charging the electrodes upon illumination, and wherein the ESM is selected from the group consisting of CeC>2, M+fNbWOg)’,lent cation and wherein X2+is a divalent cation; wherein the ESM is optionally doped with one or more metals, such as one, two or three of selected from the lanthanides and transition metal elements, e.g. one, two, three or more selected from La, Y, Sm, Yb, Sc, Mg, Ca, Co, Ni, Zr, Ti, Hf, Eu, Gd, Pr and Sn, such as Y, La, Zr, and Sc.

66. An autophotorechargeable battery comprising a negative electrode comprising a substrate with a surface and a layer comprising an electron storage material, ESM, provided on the surface of the substrate, a positive electrode comprising a substrate with a surface, and a layer comprising a hole storage material provided on the surface of the substrate, and an electrolyte, wherein the ESM is selected from the group consisting of CeC>2, M+fNbWOg)’, X2+[(NbWO6)’]2, (M+)3(Ti5NbO14)3’, [X2+]3[(Ti5NbO14)3-]2, M+(Ca2Nb3O10)-, X2+[(Ca2Nb3O10)-]2, (M+)2CS4W11O362-, X2+Cs4WnO362-, (M+)1.6Rb2.4WiiO35l-6-, (X2+)0.8Rb2.4W11O35l-6-,Li4Ti50i2' Nb2C>5 and NbigW5O55, wherein M+is a monovalent cation and wherein X^+is a divalent cation.

67. The autophotorechargeable battery according to claim 66, which is a autophotorechargeable battery wherein both steps of light harvesting and electrical energy storage occur within the ESM.

68. A method for detecting oxygen, the method including the following steps: providing an electrochemical device according to claim 64, charging the ESM comprised in the negative electrode with electrons to give a charged ESM, bringing the charged ESM in contact with a fluid or gas suspected to contain oxygen, and analyzing the state of the charged ESM by visual detection or by measuring the change of the electrical potential of the device before, during and / or after bringing it in contact with the fluid or gas, wherein the charging of the ESM is preferably done by illumination or by applying an electric charging current.

69. A method for detecting light, the method including the following steps: providing an electrochemical device according to claim 64, illuminating the ESM comprised in the negative electrode with light for a predetermined time in the presence of a to-be-oxidized reactant, TBOR, which is present in the electrolyte, to create electron-hole pairs in the ESM, wherein the holes are quenched by reacting with the TBOR and the electrons are stored in the ESM, and wherein after stopping illuminating the ESM, a property of the ESM that is or that correlates with the amount of electrons in the ESM is measured and correlated with the illumination intensity of the light, wherein said property that correlates with the amount of electrons or which is the amount of electrons in the ESM is preferably measured by spectroscopic detection or by electrochemical detection.

70. A method for detecting an analyte, the method including the following steps: providing an electrochemical device according to claim 64, illuminating the ESM comprised in the negative electrode for a predetermined time to create electron-hole pairs therein while the ESM is in contact with a test fluid or gas containing the analyte during the illumination, wherein the analyte is a to-be-oxidized reactant, TBOR, capable of quenching the holes of the electron-hole pairs, and after stopping illuminating the ESM, measuring a property that is or that correlates with the amount of electrons in the ESM, wherein the property in the ESM is preferably measured by spectroscopic detection or by electrochemical detection.

71. The method according to claim 69 or 70, wherein the amount of electrons in the ESM or a property that correlates therewith is measured by measuring a change in absorbance of the ESM or by measuring fluorescence emission of the ESM.

72. The method according to claim 69 or 70, wherein the amount of electrons in the ESM or a property that correlates therewith is measured by measuring a change of the electrochemical potential or a change of the impedance of the ESM, or by applying a discharging current and quantifying the amount of discharged electrons.

73. The method according to any one of claims 70 to 72, wherein the sensitivity of the electrochemical device for detecting the analyte is adjusted by the duration of the step of illuminating the ESM or by the intensity of said illumination.

74. A use of the electrochemical device according to claim 64 as an oxygen detector or a light detector.

75. A use of the electrochemical device according to claim 64 for detecting an analyte, which is a TBOR.

76. A use of a material for generating electron-hole pairs by illuminating the material and storing electrons generated in the same material, wherein the material is the electron storage material as defined in any one of claims 47 to 53.

77. The use according to claim 74, wherein the material is illuminated with light comprising wavelengths in the visible range of the spectrum.

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