Electrochemical system comprising a gallium redox mediator and uses thereof

Abstract

The present invention refers to an electrochemical system comprising: i. an electrolyte, preferably a liquid electrolyte, more preferably an aqueous electrolyte, comprising a stabilizing anion, wherein said electrolyte comprises > 10 mol / mol % of water; ii. a redox mediator electrode comprising Ga(0) or alloys thereof; iii. a cathode; iv. an anode; and v. a wavefunction generator to alternately polarize the electrical connection between the redox mediator electrode and the cathode or anode; wherein the redox mediator electrode is electrically connected with the cathode and the anode, provided that the anode and the cathode are not electrically connected with each other. The gallium-based redox mediator electrode permits the nearly complete reversibility between dissolution and electroplating of gallium, thus cathodic and anodic reactions can be carried out in an alternating manner by electrically connecting the redox mediator electrode with the cathode or the anode. The present invention also refers to a method for the electrochemical production of H2, and oxidized species, such as O2 and / or Cl2 or H+, with the electrochemical system of the invention. Therefore, the present invention may find application in fuel production, e.g. in combination with fuel cells or internal combustion engines, or in chemical reactions such as hydrogenation reactions, reversible H2 production and H2 oxidation, hydrotreating reactions, hydrocracking reactions, hydroisomerisation reactions, oil hydrofinishing reactions, reforming reactions, Fischer-Tropsch reactions and methanol to olefin reactions.

Description

[0001] ELECTROCHEMICAL SYSTEM COMPRISING A GALLIUM REDOX MEDIATOR

[0002] AND USES THEREOF

[0003] FIELD OF THE INVENTION

[0004] The present invention relates to the field of electrochemistry, particularly to the electrochemical production of H2, and O2 and / or other oxidized species from water by using an electrochemical system comprising a gallium redox mediator. The present invention also relates to the uses of said electrochemical system in various fields such as fuel production, reversible H2 production and H2 oxidation, and storage and bulk chemicals production.

[0005] BACKGROUND

[0006] Conventional water splitting electrolyzers produce H2 and O2 simultaneously posing safety risks which increase the manufacturing cost of electrolyzers due to the potentially hazardous formation of H2-O2 mixtures and reactive oxygen species. Decoupled electrolysis occurs when a mediator with an appropriate redox potential is employed so that, rather than direct generation of H2, O2 generation is coupled with the reduction of the mediator. Similarly, H2 production can be performed, independently from the O2 evolution, by coupling H2 generation to the reoxidation of the mediator. Decoupled water electrolysis is a relatively recent concept that offers a solution to the gas crossover and several additional issues as it allows O2 and H2 to be generated at different rates, at different times (whenever required) and even in different electrochemical cells.

[0007] With each half-reaction occurring separately in space and time, the rate of the HER (hydrogen evolution reaction) depends on the rate of oxidation of the redox mediator, rather than on the rate of the OER (oxygen evolution reaction). When these mediators also buffer the pH during electrolysis, they are known as electron-coupled proton buffers (ECPBs). A suitable redox mediator should thus have a fast and reversible redox wave, should be stable to repeated redox cycling, and ideally should be cheap and easy to obtain. The first practical realization of decoupled oxygen and hydrogen evolution from electrolytic water splitting using such an ECPB was disclosed by M. D. Symes and L. Cronin (WO2013068754A1; Nature Chemistry 2013, vol. 5, 403-409). These authors used the polyoxometalate phosphomolybdic acid ([H3PM012O40]) to decouple the OER from the HER under acidic conditions.

[0008] Different redox mediators (molecular and solid-state mediators) working in electrolytes within a pH range from strongly acidic to strongly alkaline conditions have been subsequently found.

[0009] Taking inspiration from redox flow cell devices, different molecular ion-based redox mediators, such as V (III) / V (II), Ce (III) / Ce (IV) and Fc(CN),<' / Fc(CN),, . have been used in an attempt to decouple water electrolysis. Long term instability of porous structure (e.g. carbon) used as solid electrodes in ion based redox mediators has been observed.

[0010] Redox decouple electrolysers relying on the electrochemical redox process of solid metal oxides such as nickel (oxy)hydroxides and Fe (oxy)hydroxides show good stabilities and efficiencies over multiple cycles, however the change of phase of the oxides during cycling results in changes of the physicochemical and electronic properties of the material and possible degradation.

[0011] As an example, nickel (oxy)hydroxide as a redox mediator has been used to decouple the hydrogen and oxygen production in alkaline water electrolysis, wherein the hydrogen production occurs at the cathode by water reduction, and the anodic Ni(0H)2 is simultaneously oxidized into NiOOH while separate oxygen production involves a cathodic NiOOH reduction and an anodic OH oxidization (L. Chen et al., Nature Communications 2016, 7, Article number: 11741). Recently, H2-Pro has launched into the market E-TAC (Electrochemical, Thermally Activated Chemical), which provides a decoupled method for splitting water based on a Ni(0H)2 / Ni00H redox mediator.

[0012] Xiao and co-workers explored the use of iron oxides as a solid-state redox mediator, in place of the nickel (oxy)hydroxides used by L. Chen et al.

[0013] WO2016079746Aldescribes a system and amethod for generating hydrogen gas from an aqueous solution. Said system comprises i) a first compartment with a working electrode for reducing water in response to an applied voltage to generate hydrogen and a redox-active electrode capable of reversibly undergoing oxidation and reduction and ii) a second compartment with a working electrode for generating oxygen and redox-active electrode electrically connectable to the redoxactive electrode in the first compartment. The oxidation / reduction preferably takes place on a solid electrode based on the redox pair Ni(0H)2 / Ni00H which may display the stability issues common for solid electrodes.

[0014] Nevertheless, most of the redox mediators currently used, including the ones mentioned above, further suffer from i) low solubility of the redox species in water, ii) poor long-term stability under operation conditions and / or iii) pH instability.

[0015] BRIEF DESCRIPTION OF THE INVENTION

[0016] The inventors have found a system that comprises elemental gallium or gallium alloys as redox mediators for decoupled electrochemical reactions, particularly enabling the non-simultaneous production of H2 and oxidized species (such as O2) from the electrochemical splitting of water and / or other species dissolved in aqueous electrolytes. In particular, the inventors have found that elemental gallium and gallium-based alloys dissolve spontaneously in contact with aqueous electrolyte into Ga3+cations, participating in a Ga(0) / Ga(III) redox cycle in electrolytic reactions, particularly in water splitting reactions.

[0017] The present invention mainly refers to an electrochemical system and a method for carrying out an electrochemical process.

[0018] Thus, in a first aspect, the invention refers to an electrochemical system comprising: i. an electrolyte, preferably a liquid electrolyte, more preferably an aqueous electrolyte comprising a stabilizing anion, wherein said electrolyte comprises > 10 mol / mol % of water; ii. a redox mediator electrode comprising Ga(0) or alloys thereof; iii. a cathode; iv. an anode; and v. a wave function generator to alternately polarize the electrical connection between the redox mediator electrode and the cathode or anode; wherein the redox mediator electrode is electrically connected with the cathode and the anode, provided that the anode and the cathode electrode are not electrically connected with each other.

[0019] A second aspect refers to a method for the electrochemical production of reduced and oxidized species comprising the steps of: i) providing the electrochemical system according to the first aspect of the invention; ii) applying an anodic potential to the redox mediator electrode, electrically connected to the cathode, so as to produce IT; iii) applying a cathodic potential to the redox mediator electrode, electrically connected to the anode, so to produce one or more oxidized species; and iv) optionally, repeating steps ii) and iii) at least two times; wherein steps ii) and iii) are not carried out simultaneously.

[0020] Lastly, a third aspect of the invention refers to the use of the electrochemical system according to the first aspect of the invention for fuel production, optionally in combination with fuel cells or internal combustion engines, or in chemical reactions such as hydrogenation reactions, H2 oxidation reaction, hydrotreating reactions, hydrocracking reactions, hydroisomerisation reactions, oil hydrofinishing reactions, reforming reactions, Fischer-Tropsch reactions and methanol to olefin reactions.

[0021] The present invention as defined in the various aspects defined above offers the following advantages over the prior art: 1) Reversibility. The working concept moves away from oxide based solid redox mediators that degrade overtime due to the partially reversible change of crystalline structure of the oxides. On the contrary, the present invention provides decoupled electrolyzers where a gallium-based redox mediator goes through a reversible process of dissolution / electroplating with efficiencies reaching 100%. Further improvements of the reversibility of gallium oxidation and reduction (electroplating) can be appreciated by applying short AC pulses with controlled amplitude, frequency and duty cycle.

[0022] 2) Low energy consumption. Due to the low oxidation potential of gallium (-0.53 V vs NHE), in the case of HER reactions the decoupled hydrogen evolution take place either at very low potentials or even spontaneously without the need of an external power source. In the full reversible cycle of dissolution / electroplating, only the electroplating step is performed with the application of external electrochemical potential.

[0023] 3) Kinetics of H2 production in HER. Since the dissolution of elemental gallium and gallium- based alloys into Ga3+cations is much faster than the oxygen evolution reaction, the H2 production in gallium-based decoupled electrolyzers for water splitting is faster than in conventional electrolyzers (PEMEL, AEMEL and AEL). At the same time, since the above-mentioned dissolution process is faster than the electron transfer in an inner sphere process, such as in the phase transformation of metal solid oxides, the H2 evolution in an electrochemical system such as in the present invention is also faster than those using oxide-based solid mediators.

[0024] 4) Simplification of the electrolyzer setups. With the implementation of short AC pulses as already mentioned, the decoupled electrolyzer of the invention might avoid the use of an ion exchange membrane or diaphrams unlike in the case of conventional electrolyzers and soluble ion-based electrolyzers.

[0025] DESCRIPTION OF THE FIGURES

[0026] Figure 1. Simplified scheme of the decoupled electrolyzer implementing Ga and Ga-based metal alloys as redox mediator (non-limiting example). The scheme displays: (a) a liquid gallium redox mediator electrode, (b) a cathode electrode , (c) an anode electrode, (d) a liquid electrolyte, each of (a), (b), (c) and (d) contained in (i) an electrochemical compartment, (e) a wavefunction generator with an optional power supply, (g) and (h) 2 gas outlets (such as for H2 and O2 exit) and (f) a syringe to dispense the liquid gallium redox mediator and establish the electrical connection with the gallium redox mediator. Figure 2. Simplified scheme of the alternating decoupled electrolysis by using a Ga or a Ga-based metal alloy as redox mediator for the switchable H2 and O2 production.

[0027] Figure 3. Current response of the Ga electrode as described in example 5 during the application of program Pl (A) and P2(B).

[0028] DETAILED DESCRIPTION OF THE INVENTION

[0029] All terms as used herein in this application, unless otherwise stated, shall be understood in their ordinary meaning as known in the art. Other more specific definitions for certain terms as used in the present application are as set forth below and are intended to apply uniformly throughout the specification and claims unless an otherwise expressly set out definition provides a broader definition.

[0030] Throughout the description and claims the word “comprises" and variations of the word, are not intended to exclude other technical features, additives, components or steps. Furthermore, the word “comprise” encompasses the cases of “consist of’ and “consists essentially of’. Additional objects, advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention.

[0031] “Decoupled electrolyzer” (also “decoupled water electrolyzer”) refers to an electrolyzer where the hydrogen evolution reaction (HER) and the oxygen evolution reaction from water (or an aqueous solution) do not take place simultaneously as in common electrolyzers. Instead the reactions are separated in time using a redox mediator.

[0032] A “wavefunction generator” is meant as an electronic circuit element used for generating waveforms, of defined shape, potential amplitude, frequency and duty cycle within other apparatus, and also in a function generator instrument. In the present case, it is used for alternating the electrical connection between the redox mediator electrode and the cathode or the anode. It is also known as “waveform generator” or “signal generator”.

[0033] A “redox mediator” is used herein as a material (element, compound, alloy, mineral and the likes) that can go through reversible or quasi-reversible oxidation / reduction process.

[0034] The term “oxide-based solid redox mediators” refers to solid electrodes where the composition is mainly a monoatomic, binary or ternary oxide. During the redox process the material oxidize from a lower oxidation state to a higher oxidation state, and vice versa, without changing the solid physical state (i.e. by dissolution of the oxide). According to this definition, the pair Ni(0H)2 (solid) NiOOH (solid) is an example of oxide-based solid redox mediator.

[0035] The term “ion-based redox mediator” refers to charged species (anions or cations) dissolved or homogeneously dispersed in an electrolyte. During a redox cycle, such charged species go from a lower oxidation state to a higher oxidation state, and vice versa, without change in the liquid physical state (i.e. ions in solution). According to this definition, the pair V+2(Solution) V+3(solution) is an example of ion-based redox mediator.

[0036] “Ga-based or Ga alloys electrode” refers to an electrode where the main component is metallic gallium, preferably comprising metallic gallium in at least 60 wt. % of the total weight of the electrode. Common, commercial alloys referred to in the text are Gain, GaSn and GalnSn (also called Galistan), however other gallium alloys would be suitable for the present invention (see embodiments below).

[0037] “Gallium electroplating” refers to the reduction process of gallium ions into metallic gallium via the application of an external electrochemical potential.

[0038] In the context of the present invention, the term “stabilizing anion” refers to an anion that allows gallium to be in the aqueous electrolyte as stable Ga3+cations (in a solvated form or as a soluble complex). Conversely, anions that cause Ga3+to precipitate in the aqueous electrolyte do not fall within the definition of “stabilizing anion”.

[0039] For the purposes of the invention, any ranges given include both the lower and the upper endpoints of the range. Ranges or values given, such as temperatures, times, molar ratio, volume ratio and the like, should be considered approximate when they are defined by the term “about” (i.e. with a 5% margin of variation around indicated point).

[0040] Electrochemical system

[0041] In a first aspect, the invention refers to an electrochemical system comprising: i. an electrolyte, preferably a liquid electrolyte, more preferably an aqueous electrolyte, comprising a stabilizing anion wherein said electrolyte comprises > 10 mol / mol % of water; ii. a redox mediator electrode comprising Ga(0) or alloys thereof; iii. a cathode; iv. an anode; and v. a wave function generator to alternately polarize the electrical connection between the redox mediator electrode and the cathode or anode; wherein the redox mediator electrode is electrically connected with the cathode and the anode, provided that the anode and the cathode electrode are not electrically connected with each other.

[0042] The electrochemical system of the first aspect may include a cell, battery apparatus, reactor or general setup comprising three (redox mediator, cathode and anode) or more electrodes spatially separated and distributed throughout one or more ionically-conductive media, or electrolytes, placed in a compartment of the electrochemical system while also being in electrical contact with one another according to the first aspect via a separate current path (optionally containing a potentiostat). The electrodes in an electrochemical system undergo oxidation and reduction reactions, with movement of electrons producing current traveling through the current path simultaneously with movement of ions through the one or more ionically-conductive media producing an overall balance of charge transfer within the system.

[0043] The electrochemical system of the present invention comprises an electrolyte, preferably a liquid electrolyte, more preferably an aqueous electrolyte, comprising a stabilizing anion wherein said electrolyte comprises > 10% mol / mol of water.

[0044] The electrolyte may be a solid or a liquid electrolyte, preferably the electrolyte is a liquid electrolyte wherein the liquid electrolyte comprises > 10% mol / mol of water. The value 10% mol / mol of water refers to 10 mol of water per 100 mol of components comprised in the electrolyte.

[0045] In an embodiment, the water content in the electrolyte, preferably a liquid electrolyte, more preferably an aqueous electrolyte, is more than 10% mol / mol, more than 20% mol / mol, more than 30% mol / mol, more than 40% mol / mol, more than 50% mol / mol, more than 60 % mol / mol, more than 70% mol / mol, more than 80% mol / mol, more than 90% mol / mol.

[0046] In an embodiment, the water content in the electrolyte, preferably a liquid electrolyte, more preferably an aqueous electrolyte, is comprised between 10% mol / mol and 99% mol / mol, between 20% mol / mol and 98% mol / mol, between 30% mol / mol and 97% mol / mol, between 40% mol / mol and 96% mol / mol, between 50% mol / mol and 95% mol / mol.

[0047] In an alternative embodiment, the gravimetric water content in the electrolyte, preferably a liquid electrolyte, more preferably an aqueous electrolyte, is more than 10 wt.%, more than 20 wt.%, more than 30 wt.%, more than 40 wt.%, more than 50 wt.%, more than 60 wt.%, more than 70 wt.%, more than 80 wt.%, or more than 90 wt.% with respect to the total weight of the electrolyte. In an embodiment, the gravimetric water content in the electrolyte, preferably a liquid electrolyte, more preferably an aqueous electrolyte, is comprised between 10 wt.% and 99 wt.%, between 20 wt.% and 98 wt.%, between 30 wt.% and 97 wt.%, between 40 wt.% and 96 wt.%, between 50 wt.% and 95 wt.% with respect to the total weight of the electrolyte.

[0048] In an alternative embodiment, the water content in the electrolyte, preferably a liquid electrolyte, more preferably an aqueous electrolyte, in volumetric terms, is more than 5%, more than 10%, more than 20%, more than 30%, more than 40%, more than 50%, more than 60%, more than 70%, more than 80%, or more than 90% with respect to the total volume of the electrolyte. It would be understood that in volumetric terms, 10% water refers to 10 mL of water per 100 mL of electrolyte.

[0049] In a more particular embodiment, the liquid electrolyte is an aqueous electrolyte. An aqueous electrolyte, as generally used in the art, is an electrolyte comprising water as solvent / reactant and additionally comprising co-solvents and / or solutes. The aqueous electrolyte may be any aqueous solution which has the ability to oxidize the gallium electrode and provides species for the cathodic and the anodic reactions, in particular protons (H+) and hydroxyls (OH ) for H2 and O2 formation. Another feature of the aqueous electrolyte is that it has so-called mobile charges, i.e. it must have the ability to transfer charges between the electrodes of the electrochemical system of the first aspect. Such mobile charges are suitably cations and anions. The electrolyte, preferably a liquid electrolyte, more preferably an aqueous electrolyte may further comprise a co-solvent, a salt, an acid, a base, or a combination thereof.

[0050] When the electrolyte is a liquid electrolyte, preferably an aqueous electrolyte, examples of suitable co-solvents are those that are miscible with water in the weight proportions specified above and under ambient temperature and pressure, and most importantly they do not undergo oxidation. Typical co-solvents include alcohols (methanol, ethanol, isopropanol, butanol, glycerol and the like), ketones (acetone, methyl ethyl ketone and the like), ethers (diethyl ether, tetrahydrofuran, dioxane, 1,3 -dioxolane, anisole, dimethoxyethane and the like), esters (ethyl acetate, methyl acetate, and the like), organic acids (acetic acid, formic acid, propionic acid and the like), amides (acetamide, dimethylformamide, dimethylacetamide and the like), amines (dimethylamine, trimethylamine, ethylamine, diethylamine, trimethylamine and the like), sulfoxides (dimethyl sulfoxide, dipropyl sulfoxide, and the like), acetonitrile, pyridine, nitromethane, nitroethane, sulfolane, trimethyl phosphate, triethyl phosphate, polyethylene glycol (PEG), dimethylacetal, dimethyl carbonate, propylene carbonate and ionic liquids. Suitable ionic liquids are those including an anion selected OH', F , Cl', Br , NOs', CIO4 , SO42; more preferably, ionic liquids are selected from l-Butyl-3-methylimidazolium chloride ([BMIM]C1), l-Ethyl-3- methylimidazolium chloride ([EMIM]C1), l-Hexyl-3-methylimidazolium chloride ([HMIM] Cl), l-Octyl-3-methylimidazolium chloride ([OMIM] Cl), l-Butyl-3-methylimidazolium sulfate ([BMIM]SO4), l-Ethyl-3-methylimidazolium sulfate ([EMIM] SO4), l-Hexyl-3- methylimidazolium sulfate ([HMIM] SO4), l-Octyl-3-methylimidazolium sulfate ([OMIM] SO4). In an embodiment, the co-solvent is selected from alcohols, ketones, ethers, esters, organic acids, amides, amines, and sulfoxides, preferably according to the specific examples above for each class of co-solvent, acetonitrile, pyridine, nitromethane, nitroethane, sulfolane, trimethyl phosphate, triethyl phosphate, polyethylene glycol (PEG), dimethylacetal, dimethyl carbonate, propylene carbonate, ionic liquids, preferably the ionic liquids according to the above list, or a combination thereof.

[0051] Preferably, the co-solvents, if present, are alcohols selected from methanol, ethanol, isopropanol, butanol, glycerol or combinations thereof. The optimal ratio between water and the optional co-solvent(s) may vary depending on the application; in an embodiment, said (volumetric) ratio is comprised between 5 / 95 and 99 / 1, preferably between 50 / 50 and 99 / 1, more preferably between 75 / 25 and 99 / 1, even more preferably between 90 / 10 and 99 / 1.

[0052] The electrolyte, preferably a liquid electrolyte, more preferably an aqueous electrolyte, comprises a stabilizing anion which enables metallic gallium dissolution and Ga3+ions stabilization in the aqueous electrolyte. Anions where the interaction with gallium ions result in the precipitation of gallium compounds due to poor solubility should be avoided. In an embodiment, the electrolyte, preferably a liquid electrolyte, more preferably an aqueous electrolyte, comprises a stabilizing anion which does not cause the precipitation of Ga3+compounds. The precipitation of Ga3+compounds after dissolution strongly affects the reversibility of the system. In an embodiment of the first aspect, the stabilizing anion is selected from OH', F , Cf, Br , NOs', ClOf, C2O42, SO42, or combinations thereof.

[0053] In a preferred embodiment, the stabilizing anion is selected from Cf, NOs', CIO4 , SO42, or combinations thereof.

[0054] In a particular embodiment, the stabilizing anion is OH'.

[0055] In a particular embodiment, the stabilizing anion is Cf.

[0056] In a particular embodiment, the stabilizing anion is NOs'.

[0057] In a particular embodiment, the stabilizing anion is CIO4 .

[0058] In a particular embodiment, the stabilizing anion is SO42.

[0059] The concentration of the stabilizing anion may vary within a wide range. In a preferred embodiment, the concentration of the stabilizing anion is comprised between 0.01 M and 10.0 M; preferably, between 0.05 M and 5 M; more preferably, between 0.1 M and 2.0 M.

[0060] The source of the stabilizing anion may be any suitable salt known in the art, preferably metal salts of the stabilizing anion (such as sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium salts of the stabilizing anions), acids containing the stabilizing anion (such as, HC1, HBr, HNO3, HCIO4, H2C2O4, H2SO4), bases containing the stabilizing anion (such as OH ) or combinations thereof.

[0061] It was found that the pH of the aqueous electrolyte may influence the rate of dissolution of gallium but is not limiting the performance of the electrochemical system.

[0062] The aqueous electrolyte may additionally comprise a pH adjuster, namely a substance or compound used to modify or regulate the pH (acidity or alkalinity) of said aqueous electrolyte such as the acids or bases mentioned above. Even though in the present invention it is preferred that the pH be acidic (<7) or basic (>7), a pH of about 7 can be also used. This can be easily achieved when using pure water (i.e. water essentially free of mineral ions, microorganisms and contaminants; see: ASTM DI 193-06(2018)) and adding an appropriate source of the stabilizing anion which does not essentially alter the pH of said pure water (such as NaF, NaCl, NaBr, NaNOs, NaCICb and the like).

[0063] The kinetics of the cathodic reaction (particularly, the generation of H2) depend on the kinetics of dissolution of Ga. This dissolution occurs throughout a wide range of pH, however the presence of stabilizing anions derived from acids, particularly those anions derived from inorganic strong acids already mentioned above or superacids, and the presence of OH', particularly derived from alkali metal hydroxides or superbases, make dissolution faster.

[0064] Superacids and superbases are known in the art; typical examples of the former are HF:SbR. HSO3F:SbF5, CH(CF3SO2)3, H(HCBnXn (X = H, Me, halide), HF:BF3, NH(CF3SO2)2, FSO3H , HOSO2CF3while typical examples of the latter include phosphanes and carbodiphosphoranes. Thus, in an embodiment, the aqueous electrolyte further comprises a pH adjuster so as the pH is 8 or higher; preferably 9 or higher; more preferably 10 or higher, even more preferably 11 or higher. In another embodiment, the aqueous electrolyte further comprises a pH adjuster so as the pH is comprised between 8 and 14, preferably between 10 and 14. In a more particular embodiment, the aqueous electrolyte further comprises a metal hydroxide, preferably sodium or potassium hydroxide, as pH adjuster so as the pH of the aqueous electrolyte is comprised between 8 and 14, preferably between 10 and 14. At a basic pH, the main species derived from the dissolution of Ga(0) is Ga(OH)4 .

[0065] In an embodiment, the aqueous electrolyte further comprises a pH adjuster so as the pH is 6 or lower; preferably 5 or lower, more preferably 4 or lower, even more preferably 3 or lower.

[0066] In another embodiment, the aqueous electrolyte further comprises a pH adjuster so as the pH is comprised between 6 and 0, preferably between 4 and 0.

[0067] In an embodiment, the pH adjuster is also the source of the stabilizing anion; preferably, the pH adjuster is a source of Cl' (e.g. HC1), NO3' (e.g. HNO3), or OH' (e.g. NaOH and KOH).

[0068] The electrochemical system of the first aspect comprises ii) a redox mediator electrode comprising Ga(0) or alloys thereof. Compared to other redox mediator electrodes, the redox mediator electrode comprising Ga(0) or alloys thereof in the system of the first aspect allows near perfect reversibility between gallium oxidation and reduction.

[0069] In the context of the present invention, the redox mediator electrode comprising Ga(0) or alloys thereof will have the meaning of metal electrode comprising Ga(0) in its metallic state or alloys thereof, either in solid or liquid physical form. The redox mediator electrode may comprise an alloy of two or more metals (one of them being gallium) or may comprise substantially pure metallic gallium. By substantially pure metallic gallium is meant that the composition comprises more than 98 wt % of gallium. In an embodiment, the redox mediator electrode comprises Ga(0) or alloys thereof, and it is further characterized in that it does not comprise aluminum.

[0070] In an embodiment, the redox mediator electrode consists of substantially pure metallic Ga(0) (as defined above) or alloys thereof.

[0071] Examples of alloys are the following: i. Gallium-Indium Eutectic Alloy. Particularly, an eutectic composition of 75% gallium and 25% indium by weight is known and remains in a liquid state over a wide temperature range. ii. Gallium- Aluminum Alloy; iii. Gallium-Tin Alloy; iv. Gallium-Platinum Alloy. Gallium-platinum alloys are known for their high temperature stability. v. Gallium- Antimony Alloy. This alloy is known for its low melting point. vi. Gallium-Indium-Tin Alloy. A combination of these elements can result in alloys with various melting points and solidification temperatures. vii. Gallium-Bismuth Alloy. These alloys have low melting points. viii. Gallium-Silver Alloy. This alloy exhibits good electrical conductivity while remaining in a liquid state at relatively low temperatures. ix. Gallium-Copper Alloy. Gallium-copper alloys can be tailored to have specific melting points. x. Gallium-Strontium Alloy.

[0072] In an embodiment, the redox mediator electrode comprises metallic Ga(0), Gain, GaAl, GaSn, GalnSn, GaPt, GaSb, GaBi, GaAg, GaCu, GaSr or a combinations thereof. More suitably, the redox mediator electrode comprises the alloy combinations illustrated in the examples.

[0073] In another embodiment, the redox mediator electrode comprises metallic Ga(0), Gain, GaAl, GaSn, GalnSn, GaPt, GaSb, GaBi, GaAg, GaCu, GaSr or a combination thereof, as major component of the redox mediator electrode (>50 wt.%). In another embodiment, the redox mediator electrode comprises metallic Ga(0), Gain, GaSn, GalnSn, GaSb, GaBi, GaSr or a combination thereof, as major component of the redox mediator electrode (>50 wt.%).

[0074] In another embodiment, the redox mediator electrode consists of metallic Ga(0), Gain, GaAl, GaSn, GalnSn, GaPt, GaSb, GaBi, GaAg, GaCu, GaSr or a combination thereof, preferably consists of metallic Ga(0), Gain, GaSn, GalnSn, GaSb, GaBi, GaSr or a combination thereof.

[0075] The physical state of the redox mediator electrode will be defined by the working temperature and the melting point of the composition (metallic gallium or alloy). The gallium redox mediator electrode can be present in any form such as stream fluidic wire, static or dynamic generated liquid drop, solid disk, solid fdm, self-standing nanoparticles or dispersed nanoparticles. The oxidation and electroplating rate also depends on the physical form of the gallium redox mediator electrode.

[0076] The electrochemical system of the first aspect of the invention additionally comprises iii) a cathode and iv) an anode. The cathode and the anode, independently from each other, comprise a metal, an alloy, a carbonaceous material, a perovskite, a metal sulfide, a metal phosphide, or a metal carbide.

[0077] The cathode and anode electrodes can be materials comprising a metal. In an embodiment, the cathode and the anode are, each independent from the other, materials comprising a metal selected from group 3 through group 15 (both ends included) of the periodic table, wherein the groups are numbered according to the system adopted by IUPAC. In a preferred embodiment, suitable metals independently chosen for cathode and anode are Y, Ti, Mn, Fe, Co, Ni, Cu, Zn, Zr, Mo, Ru, Rh, Ag, W, Ir, Pt, Au, As, In, Sn, Sb, or Pb. In another embodiment, the cathode and the anode consist of, each independently from the other, a pure metal selected from the above list. The term “pure metal” refers to a material comprising more than 98 wt % of said metal. More suitably, the metal for cathode and anode are those illustrated in the examples.

[0078] In another embodiment, the cathode and the anode comprise, independently from each other, an alloy of two or more metals or may consist substantially of an alloy of two or more metals. Examples of possible alloys are PtNi, Ptlr, PtRh, PtRu, PtCo, PtMo, PtAu, PtAg, PtRuMo, PtFe, PtCu, NiFe, NiCo, NiFeCo, NiMn, NiMnCo, NiFeMn, stainless steel and its variants.

[0079] In another embodiment, the cathode and anode comprise, each independent from the other, a carbonaceous material. In a preferred embodiment, said carbonaceous material comprises glassy carbon, carbon Vulcan, graphene, highly oriented pyrolytic graphite, carbon nanotubes or a combination thereof. In a more preferred embodiment, said carbonaceous material consists of glassy carbon, carbon Vulcan, graphene, highly oriented pyrolytic graphite, or carbon nanotubes. In another embodiment, the cathode and anode comprise, each independent from the other, metal oxides, perovskites, metal sulfides, metal phosphides, metal carbides; examples of suitable materials of these classes are LaxCaixMnOs, MoSx, NiPx, and WXC, wherein x is comprised between 0.5 and 2.0.

[0080] The materials chosen for the cathode and anode, should be suitable for the electrochemical reactions in the chosen aqueous electrolyte medium.

[0081] In a particular embodiment, the cathode and the anode are the same electrode. Thus, in this particular case, the electrochemical system comprises: i. an electrolyte, preferably a liquid electrolyte, more preferably an aqueous electrolyte comprising a stabilizing anion, wherein said electrolyte comprises > 10 mol / mol % of water; ii. a redox mediator electrode comprising Ga(0) or alloys thereof; iii. an electrode acting alternatively as cathode and anode; and iv. a wave function generator to alternately polarize the electrical connection between the redox mediator electrode and the cathode or anode; wherein the redox mediator electrode is electrically connected with the electrode.

[0082] However, in a preferred embodiment, the anode and the cathode are independently from each other, i.e., they are independent physical entities and are not electrically connected with each other.

[0083] The electrochemical system may comprise additional electrodes, such as a reference electrode. The choice of a reference electrode depends on factors such as the nature of the electrochemical system, the solvent, the potential range of interest, and the required precision and stability. In an embodiment, the electrochemical system further comprises a reference electrode, preferably a Hg / HgO reference electrode.

[0084] The electrochemical system of the first aspect further comprises: v. a wave function generator to alternately polarize the electrical connection between the redox mediator electrode and the cathode or anode; wherein the redox mediator electrode is electrically connected with the cathode and the anode, provided that the anode and the cathode electrode are not electrically connected with each other. In an embodiment, the electrochemical system further comprises a power supply which is integrated with the wavefunction generator. In another embodiment, the electrochemical system further comprises a power supply, wherein the power supply and the wavefunction generator are separated elements in the system.

[0085] In an embodiment, the wavefunction generator, optionally in combination with a power supply according to the above embodiments, may apply alternate current (AC) or direct current (DC), preferably alternate current (AC).

[0086] Alternating current (AC) voltage enables the oxidation and electroplating (reduction) of Ga reversibly with efficiency of nearly 100%.

[0087] The alternating process of oxidation and reduction is determined by the polarization cycle between the redox mediator electrode and cathode and between the redox mediator electrode and the anode, as well as by the time for each step (oxidation and reduction) and voltages applied. Due to the possible differences in the kinetics, it is preferred that the cycles are asymmetric in order to maintain the equilibrium of dissolution and deposition. It is worth noting that the oxidation phase is feasible even without energy input, i.e., exponentially without polarization, at the cost of slower kinetics. Illustrative conditions will be provided in the examples.

[0088] In an embodiment, the electrochemical system of the first aspect optionally comprises a separator such as an ion membrane or a porous diaphragm.

[0089] In an embodiment, the electrochemical system of the first aspect is suitable for producing gases from the cathodic and / or anodic reaction (e.g. H2 and O2), thereby generating pressure in the electrochemical system. Thus, in an embodiment, the electrochemical system further comprises at least one gas outlet, preferably at least two gas outlets, for the exit of gaseous products. In a most preferred embodiment, the electrochemical system is equipped with one or two gas outlets, preferably two gas outlets. Gas outlets are connected to the compartment with electrodes and electrolyte. Typical gas outlets are tubes or pipes connected to said compartment. Figure 1 shows an embodiment with this particular arrangement.

[0090] When gases are generated in the electrochemical system, said gases may be collected and / or their volume measured after they go through the gas outlet(s). Thus, in an embodiment, the electrochemical system is a closed system. In a particular embodiment, the electrochemical system is a closed system and further comprises at least a gas collector, preferably at least two gas collectors. In a most preferred embodiment, the electrochemical system is equipped with one or two gas collectors, preferably two gas collectors. Said gas collectors are directly connected to the gas outlets through connecting means such as tubes or pipes.

[0091] In a most preferred embodiment, the electrochemical system comprises a first gas outlet with a first end connected to a compartment where the cathode is placed and a second end connected to a first gas collector, and a second gas outlet with a first end connected to a compartment where the anode is placed and a second end connected to a second gas collector. In a preferred embodiment of the latter, the anode and cathode are in the same compartment.

[0092] In a particular embodiment, the electrochemical system of the first aspect is characterized in that: i. the electrolyte is an aqueous electrolyte comprising a stabilizing anion selected from F

[0093] , O’, Br , NOs', CIO4 , C2O42, SO42, or combinations thereof, wherein the pH is comprised between 10 and 14 or between 0 and 4; ii. the redox mediator electrode comprises Ga(0), Gain, GaAl, GaSn, GalnSn, GaPt, GaSb, GaBi, GaAg, GaCu, GaSr or a combination thereof; iii. the cathode comprises platinum; iv. the anode comprises platinum or iridium; v. a separator is optionally present. In a particular embodiment, the electrochemical system of the invention is an electrochemical reactor, more preferably an electrochemical reactor for the production of H2 and O2 from an aqueous electrolyte.

[0094] Figure 1 shows a non-limiting but representative scheme of the electrochemical system of the invention which comprises a compartment (i) of the system with an electrolyte (d), a redox mediator electrode (a), a cathode (b) and an anode (c); (a), (b) and (c) being located at a certain distance between each other, while being in contact with the electrolyte (d) in the compartment (i). A wavefunction generator (e) in an external circuit, optionally combined and / or integrated with a power source and suitable to modulate alternate (AC) or direct currents (DC), with control on potential, current, wave forms with defined shape, frequencies, duty cycle and amplitude, is also provided in the system. The wavefunction generator (e) in the external circuit is responsible for the switching of the electrical connection between the redox mediator electrode and either of the cathode (b) or anode (c). The electrodes (redox mediator electrode (a), cathode (b) and anode (c) may or may not be separated by a separator such as ion membranes or porous diaphragms. The system comprises a gas outlet (g), a gas outlet (h) and optional gas collector(s) (not shown) connected to the compartment (i) with cathode (b) and anode (c) for the removal and optional collection of gases formed during an electrochemical reaction, such as H2 and O2. The invention comprises an external circuit that connects the three electrodes (a), (b) and (c), where the redox mediator electrode (a) is electrically connected to cathode (b) and anode (c) in an independent and alternating manner, but cathode (b) and anode (c) are not connected with each other.

[0095] In another embodiment, the electrochemical system is for the reversible H2 production and H2 oxidation. In this particular case, the anode and the cathode can be (but not necessarily) the same electrode, which acts as cathode for H2 production and as anode for H2 oxidation.

[0096] Electrochemical method

[0097] In a second aspect, the invention refers to a method for the electrochemical production of H2 and oxidized species, said method comprising the steps of: i) providing the electrochemical system according to the first aspect of the invention; ii) applying an anodic potential to the redox mediator electrode of said electrochemical system, electrically connected to the cathode, so as to produce H2; iii) applying a cathodic potential to the redox mediator electrode of said electrochemical system, electrically connected to the anode, so as to produce one or more oxidized species; and iv) optionally, repeating steps ii) and iii) at least two times; wherein steps ii) and iii) are not carried out simultaneously. The method of the second aspect is preferably carried out under inert atmosphere (e.g. under an argon or nitrogen atmosphere) in order to avoid the possible interference of atmospheric air.

[0098] All the embodiments of the first aspect of the invention apply to the electrochemical system of step i) of the method for the electrochemical production of H2 and oxidized species.

[0099] Preferably, steps ii) and iii) are carried out sequentially in the same order as defined in the second aspect.

[0100] This means that when the redox mediator electrode and the cathode are connected, the redox mediator electrode and the anode are not connected; conversely, when the redox mediator electrode and the anode are connected, the redox mediator electrode and the cathode are not connected.

[0101] As previously stated, the power supply of the electrochemical system of the first aspect of the invention generates alternate current (AC) or direct current (DC), preferably alternate current (AC).

[0102] Alternating current (AC) voltage enables the oxidation and electroplating (reduction) of Ga reversibly with efficiency of nearly 100%.

[0103] The more positive is the oxidation potential, the faster are the oxidation rate and the H2 production. When the electroplating potential is more negative, the gallium electroplating starts competing with the hydrogen production on the gallium and the efficiency of the process decreases.

[0104] In an embodiment, the potential applied between the redox mediator electrode and the cathode in step ii) ranges from -1 to -0.3 V vs Hg / HgO for the Ga oxidation.

[0105] In a preferred embodiment, the one or more oxidized species produced in step iii) is selected from O2, CI2, F2, Br2, or a combination thereof, preferably the one or more oxidized species is O2 and / or CI2, more preferably is O2. These oxidized species would result from the electrochemical oxidation of OH", Cl", F , and Br .

[0106] In another preferred embodiment, the oxidized species is H+and it would result from the electrochemical oxidation of the H2 previously produced. In other words, the method of the invention involves the reversible H2 production and H2 oxidation to produce H+as oxidized species.

[0107] In an embodiment, the potential applied between the redox mediator electrode and the anode in step iii) ranges from -1.4 to -1.9V vs Hg / HgO.

[0108] In an embodiment, the applied potentials range from -1 to -0.3 V vs Hg / HgO for the oxidation of Ga and from -1.4 to -I.9V vs Hg / HgO for the electroplating.

[0109] In further embodiments, step ii) is carried out for at least 1 msec, at least 10 msec, at least at least 100 sec, at least 1 sec, at least 30 sec, at least 1 min, at least 2 min, at least 5 min, at least 15 min, at least 30 min, at least 1 hour, at least 2 hours, at least 3 hours, at least 6 hours, at least 12 hours. In further embodiments, step ii) is carried out for an amount of time comprised between 1 msec and 12 hours, preferably between 100 msec and 6 hours, more preferably between 1 sec and 3 hours, even more preferably between 1 min and 3 hours. The latter may also apply to a range of potential from -1 to -0.3 V vs Hg / HgO.

[0110] In further embodiments, step iii) is carried out for at least 1 msec, at least 10 msec, at least at least 100 msec, at least 1 sec, at least 30 sec, at least 1 min, at least 2 min, at least 5 min, at least 15 min, at least 30 min, at least 1 hour, at least 2 hours, at least 3 hours, at least 6 hours, at least 12 hours.

[0111] In further embodiments, step iii) is carried out for an amount of time comprised between 1 msec and 12 hours, preferably between 100 msec and 6 hours, more preferably between 1 sec and 3 hours, even more preferably between 1 min and 3 hours. The latter may also apply to a range of potential from -1.4 to -1.9V vs Hg / HgO.

[0112] In an embodiment, steps ii) and iii) are each carried out for the same amount of time. This scenario is referred to as 50% duty cycle. All the amount of times for step ii) and iii) detailed above apply to the previous embodiment.

[0113] However, it is preferred that the cycles are asymmetric in order to maintain the equilibrium of dissolution and electroplating as the Ga oxidation phase is feasible even without energy input at the cost of slower kinetics.

[0114] In an embodiment, the electrochemical charge in the gallium oxidation and electroplating steps (steps ii and iii) is identical or differs by a margin of 5% while pulse times differ between the two steps; preferably, step ii) is carried out for an equal amount of time or shorter time than step iii). Thus, in a preferred embodiment, steps ii) and iii) are each carried out for a different amount of time; preferably step ii) is carried out for an equal amount of time or shorter time than step iii). When oxidation or reduction steps (step ii and iii of the method of the second aspect) are carried out for a different amount of time, the duty cycle changes to, for example, 10%, 20%, 30%, 40%. All the amount of times for step ii) and iii) detailed above apply to the previous embodiment.

[0115] In an embodiment, the electrochemical charge in the gallium oxidation and electroplating steps (steps ii and iii) is identical or differs by a margin of 5% while potential pulses times differ between the two steps; preferably, the electrochemical charge in the gallium oxidation and electroplating steps (steps ii and iii) is identical or differs by a margin of 5% while step ii) is carried out for an equal amount of time or shorter time than step iii). By reducing gallium ions (gallium electroplating) in a faster manner, it is avoided that they diffuse far away from the metallic gallium and negatively impact the cathode and anode performance.

[0116] The asymmetry in the pulse time for steps ii) and iii) (also referred to as asymmetrical duty cycle) may also be advantageous for a better control of the oxidation and reduction reactions in the method. Thus, the method of the second aspect may involve an electrochemical system according to the first aspect where a separator, such as an ion membrane or a porous diaphragm, is absent, particularly under asymmetrical duty cycle conditions. In a preferred embodiment, the electrochemical system of step i) of the method of the second aspect does not comprise a separator, such as an ion membrane or a porous diaphragm, while the current of gallium dissolution is equal to or below 6 mA / cm2and / or pulses equal to or higher than 1 Hz are used.

[0117] In a particular embodiment, the electrochemical system of step i) of the method of the second aspect does not comprise a separator, such as an ion membrane or a porous diaphragm, while the current of gallium dissolution is 6mA / cm2, the reduction current is 1 mA / cm2, the frequency is 1 Hz, and duty cycle is 14.3% (i.e. 0. 143 s in the oxidation step and 0.857 s in the reduction step). In a particular embodiment, the electrochemical system of step i) of the method of the second aspect does comprise a separator, such as an ion membrane or a porous diaphragm. The separator physically prevents the gallium ions from diffusing away, reaching the other electrodes and possibly inhibit a given reaction (particularly, the hydrogen evolution reaction at the cathode).

[0118] In an embodiment, the method of the second aspect comprises carrying out steps ii) and iii) only once. In further embodiments, steps ii) and iii) are repeated at least two times, at least 3 times, at least 4 times, at least 5 times, at least 6 times, at least 7 times, at least 8 times, at least 9 times, at least 10 times, at least 100 times, at least 1000 times, at least 104times, at least 105times, at least 106times.

[0119] In a specific embodiment, the method comprises repeating step ii) and iii) from 2 to 10 times. All amounts of times detailed above apply to optional step iv).

[0120] In a particular embodiment, the electrochemical method of the invention provides a process to produce H2 and O2 from the water contained in the electrolyte. In particular H2 and O2 are produced in an alternating manner from decoupled water splitting.

[0121] Thus, in a particular embodiment, the invention refers to a method for the electrochemical production H2 and O2 from an aqueous solution comprising the steps of: i) providing the electrochemical system according to the first aspect of the invention; ii) applying an anodic potential to the redox mediator electrode, electrically connected to the cathode, such as to produce H2; iii) applying a cathodic potential to the redox mediator electrode, electrically connected to the anode, such as to produce O2; and iv) optionally, repeating steps ii) and iii) at least two times; wherein steps ii) and iii) are not carried out simultaneously.

[0122] Figure 2 is a schematic representation of the steps to produce hydrogen and oxygen according to the particular embodiment of the invention as mentioned above. When a gallium-based electrode (redox mediator electrode) is connected through an external circuit to a cathode, water splitting on the cathode results in the EE gas formation, while Ga3+cations are released in the aqueous electrolyte. This step will be also named “gallium oxidation” or simply “oxidation”, “gallium dissolution” or simply “dissolution” throughout the text.

[0123] Then, the gallium-based electrode (redox mediator electrode) is connected through an external circuit to an anode and, by applying a cathodic potential to the gallium-based electrode, the Ga3+cations are reversibly reduced to metallic gallium and the oxidized species, such as O2 and or Cl, formation takes place at the anode (the gallium-based electrode acts as a cathode). As the redox mediator electrode is connected with cathode and anode in an alternating manner, the formation of H2 gas and oxidized species (preferably O2 and or CL) take place at different times and in different places. Step two will be also named “gallium electroplating” or simply “electroplating” throughout the text.

[0124] The two steps above (dissolution and electroplating) can be repeated as many times as desired.

[0125] In another particular embodiment, the invention refers to a method for the electrochemical reversible H2 production and H2 oxidation from an aqueous solution comprising the steps of: i) providing the electrochemical system according to the first aspect of the invention; ii) applying an anodic potential to the redox mediator electrode, electrically connected to the cathode, such as to produce EE; iii) applying a cathodic potential to the redox mediator electrode, electrically connected to the anode, such as to produce H+; and iv) optionally, repeating steps ii) and iii) at least two times; wherein steps ii) and iii) are not carried out simultaneously, and wherein the cathode and the anode can be (but not necessarily) the same electrode.

[0126] Uses

[0127] The products generated by the electrochemical system are relevant to the chemical industry and in the energy field. Particularly, hydrogen and oxygen produced in an alternating manner from decoupled water splitting can be used as fuel, suitably as fuel to generate electricity in a fuel cell, can be used as fuel in internal combustion to generate heat and can be used for chemical synthesis such as hydrogenation reactions, reversible EE production and EE oxidation, hydrotreating reactions, hydrocracking reactions, hydroisomerisation reactions, oil hydrofinishing reactions, reforming reactions, Fischer-Tropsch reactions and methanol to olefin reactions.

[0128] EXAMPLES The following examples are intended to illustrate but not to limit the disclosed embodiments.

[0129] Acronyms used throughout the text:

[0130] PEMEL: Polymer Electrolyte Membrane Electrolyzer

[0131] AEMEL: Anion Exchange Membrane Electrolyzers

[0132] AEL: Alkaline Electrolyzer

[0133] NHE: Hydrogen Electrode Potential

[0134] HER: Hydrogen Evolution Reaction.

[0135] OER: Oxygen Evolution Reaction.

[0136] Reagents and starting materials

[0137] Gain alloy was purchased from Indium corporation and used as such.

[0138] Metal cathode and anode electrodes were provided by Alfa Aesar or Sigma Aldrich with minimum purity of 99.99%.

[0139] NaOH 99.9% was purchased from Sigma-Aldrich.

[0140] HC1 was purchased from Sigma- Aldrich, ACS reagent, 37%.

[0141] Millipore MilliQ system was used for obtaining ultrapure water, 18.2 MQ cm, 3 ppb total organic carbon.

[0142] Characterization

[0143] A potentiostat Autolab N100 and reference Hg / HgO electrodes (Pine research- RREF0038 -4.24 M KOH) were used in in the below examples.

[0144] An ICP-OES Horiba Jobin Yvon Ultima 2 equipped with AS 500 Autosampler and ACTIV Analyst Software 5.4 was used to monitor the gallium dissolution.

[0145] Example 1 : Use of the electrochemical reactor for alternate H2 and O2 evolution

[0146] A liquid drop of eutectic Gain alloy (redox mediator electrode 1) of «4 mm diameter was generated by the help of a syringe connected with a Teflon tube into an electrochemical reactor. The electrochemical reactor contained an aqueous solution (20 mL) containing 1 M NaOH. Said solution was prepared with ultrapure water (Millipore MilliQ system, 18.2 MQ cm, 3 ppb total organic carbon) and NaOH (99.9% from Sigma-Aldrich). Platinum electrode was used as cathode (electrode 2) for H2 gas production. The Pt wire was used as anode (electrode 3) for O2 gas production. The reference electrode was Hg / HgO (Pine research- RREF0038 -4.24 M KOH). The gases were collected in inverted graduated test tubes. The solution was agitated by external means using a rotor and by bubbling Ar gas in the electrolyte.

[0147] The electrochemical H2 and O2 evolution involved two steps: Step 1: an anodic potential of -1.1 V vs Hg / HgO was applied on the Ga electrode connected to electrode 2. In electrode 2, vigorous H2 gas evolution was observed and the volume of the solution inside the test tube was displaced given an indication of the H2 gas generated. A sample of the electrolyte was taken at given times and the content of Ga was measured by ICP-MS. Step 2: Subsequently, a cathodic potential of -1.7 vs Hg / HgO was applied on the Ga electrode connected to electrode 3. In the electrode 3, vigorous O2 gas evolution was observed. A sample of the electrolyte was taken every given time and the content of Ga was measured by ICP-MS.

[0148] The values of electrochemical charge on the anodic and cathodic cycle, volume of electrolyte displaced by H2 and the content of Ga in the electrolyte after the anodic and cathodic cycle are shown in table 1.

[0149] Example 2: Cycling of electrochemical production of H2 and O2. A liquid drop of eutectic Gain alloy (redox mediator electrode 1) of « 4 mm diameter was generated by the help of a syringe connected with a Teflon tube into an electrochemical reactor. The electrochemical reactor contained an aqueous solution containing 1 M NaOH. Said solution was prepared with ultrapure water (Millipore MilliQ system, 18.2 MQ cm, 3 ppb total organic carbon) and NaOH (99.9% from Sigma- Aldrich). Platinum electrode was used as cathode (electrode 2) for H2 gas production. The Pt wire was used as anode (electrode 3) for O2 gas production. The reference electrode was Hg / HgO (Pine research- RREF0038 -4.24 M KOH). Square wave voltage was applied on the Ga electrode with the time / potential programs as described in table 2:

[0150] Table 2. Programs of potential E applied vs Hg / HgO on electrode 1 during Ga oxidation / tE production (step 1) and Ga electroplating / O2 production (step 2)

[0151] In all examples the H2 formation and O2 formation were observed in electrode 2 and electrode 3 together with the dissolution and electroplating, respectively, of gallium at the electrode 1.

[0152] Examples 3 : A liquid drop of eutectic Gain alloy (redox mediator electrode 1) of «5 mm diameter is generated with the help of a syringe connected with a Teflon tube to an electrochemical reactor. The electrochemical reactor contains an aqueous solution containing 1 M HC1. Said solution was prepared with ultrapure water (Millipore MilliQ system, 18.2 MQ cm, 3 ppb total organic carbon) and HC1 (Sigma-Aldrich, ACS reagent, 37%). A platinum electrode was used as cathode (electrode 2) for H2 gas production. Iridium wire was used as anode (electrode 3). The reference electrode used was a Saturated Calomel Electrode (SCE). The gases were collected in inverted graduated test tubes. The solution was agitated by external means using a rotor and by bubling Ar gas in the electrolyte. The experiment involved in two steps:

[0153] Step 1: For 5 minutes at open circuit potential on the Ga electrode connected to electrode 2. In the electrode 2, vigorous H2 gas evolution was observed and the volume of the solution inside the test tube was displaced given an indication of the H2 gas generated. At given times, a sample of the electrolyte was taken and the content of Ga was measured by ICP-MS.

[0154] Step 2: Subsequently, a cathodic potential of -1.0 vs SCE was applied on the Ga electrode connected to electrode 3. In the electrode 3, vigorous O2 gas evolution was observed. In addition, near the Ir electrode a dark yellow color formation was observed. This may be associated to the slow corrosion of iridium and formation of CI2 gas.

[0155] The values of electrochemical charge on the anodic and cathodic cycle and volume of electrolyte displaced by H2 after the anodic and cathodic cycle are included in table 3. Table 3: Values of electrochemical charge on the anodic and cathodic cycle, volume of electrolyte displaced by H2 after the anodic and cathodic cycle according to example 3.

[0156] Example 4: Example 3 was repeated wherein the following aqueous electrolyte compositions were used: 0.5 HC1, 0.01 M HC1, 0.1 HC1O4, 0.1 M HNOs, 0.1 M H2SO4, 0.1 HC1O4+ 0.1 M HC1. In all examples the H2 formation was observed in electrode 2. At electrode 3, O2, CE or a mixture of O2+CI2 at different O2 / CI2 ratios was formed during step 2 depending on the specific electrolyte composition. Example 5: A liquid drop of eutectic Gain alloy of about 4 mm diameter is generated by the help of a syringe connected with a Teflon tube into an electrochemical reactor. The electrochemical reactor contains an aqueous solution (10 mL) containing 1 M NaOH. Said solution was prepared with ultrapure water (Millipore MilliQ system, 18.2 MQ cm, 3 ppb total organic carbon) and NaOH (99.9% from Sigma- Aldrich). Platinum electrode was used as electrode 2 for H2gas production. Platinum wire was used as electrode 3 for O2gas production. Hg / HgO (Pine research- RREF0038 -4.24 M KOH) was used as a reference electrode.

[0157] Square wave voltage was applied on the Ga electrode with the time / potential programs as described in table 4: Table 4: Programs of potential applied E vs Hg / HgO on electrode 1 during Ga oxidation / EE production (step 1) and Ga electroplating / O2 production (step 2)

[0158] Figure 3 shows the examples indicating the oxidation and electroplating currents of the gallium.

[0159] Example 6: Use of the electrochemical reactor for reversible H2 production and H2 oxidation

[0160] A disk of solid Gallium (redox mediator electrode 1) of «3 mm diameter connected through a Copper wire for electrical connection and held into an electrochemical reactor. The electrochemical reactor contained an aqueous solution (20 mb) containing 1 M NaOH. Said solution was prepared with ultrapure water (Millipore MilliQ system, 18.2 MQ cm, 3 ppb total organic carbon) and NaOH (99.9% from Sigma- Aldrich). Platinum electrode was used as electrode 2 reversibly for H2 gas production and also H2 oxidation. The reference electrode was Hg / HgO (Pine research- RREF0038 -4.24 M KOH). The electrode 2 was placed in an inverted graduated test tube to collect the H2 gas and keep the solution saturated with H2 to be oxidized in step 2 as follow.

[0161] The electrochemical H2 and H2 oxidation involved two steps:

[0162] Step 1: an anodic potential of -1.1 V vs Hg / HgO was applied, during 5 s, on the Ga electrode connected to electrode 2. In electrode 2, vigorous H2 gas evolution was observed and the volume of the solution inside the test tube was displaced given an indication of the H2 gas generated.

[0163] Step 2: Subsequently, a cathodic potential of -1.7 vs Hg / HgO was applied, during 10s, on the Ga electrode connected to electrode 2. In the electrode 2, the oxidation of the H2 generated in the step 1 took place.

[0164] The values of charge on the anodic and cathodic cycle were measured.

Claims

CLAIMS1. An electrochemical system comprising: i. an electrolyte, preferably a liquid electrolyte, more preferably an aqueous electrolyte, comprising a stabilizing anion, wherein said electrolyte comprises > 10 mol / mol % of water with respect to the total moles of the species in the electrolyte; ii. a redox mediator electrode comprising Ga(0) or alloys thereof; iii. a cathode; iv. an anode; and v. a wavefunction generator to alternately polarize the electrical connection between the redox mediator electrode and the cathode or anode; wherein the redox mediator electrode is electrically connected with the cathode and the anode, provided that the anode and the cathode electrode are not electrically connected with each other.

2. The electrochemical system according to claim 1, further comprising at least one gas outlet, preferably at least two gas outlets.

3. The electrochemical system according to claim 1 or 2, wherein the wavefunction generator applies alternate current (AC).

4. The electrochemical system according to any of the preceding claims, wherein the electrolyte, preferably the liquid electrolyte, more preferably the aqueous electrolyte, comprises a stabilizing anion selected from OH', F , Cl', Br , NOs', CIO4 , C2O42, SO42' and combinations thereof.

5. The electrochemical system according to any of the preceding claims, wherein the redox mediator electrode comprises metallic Ga(0), Gain, GaAl, GaSn, GalnSn, GaPt, GaSb, GaBi, GaAg, GaCu, GaSr or a combination thereof.

6. The electrochemical system according to any of the preceding claims, wherein the cathode and the anode, independently from each other, comprise an elemental metal, an alloy, a perovskite, a metal sulfide, a metal phosphide, a metal carbide or a carbonaceous material.

7. The electrochemical system according to claim 6, wherein the elemental metal is selected from Y, Ti, Mn, Fe, Co, Ni, Cu, Zn, Zr, Mo, Ru, Rh, Ag, W, Ir, Pt, Au, As, In, Sn, Sb, and Pb.

8. The electrochemical system according to claim 6. wherein the alloy is selected from PtNi, Ptlr, PtRh, PtRu, PtCo, PtMo, PtAu, PtAg, PtRuMo, PtFe, PtCu, NiFe, NiCo, NiFeCo, NiMn, NiMnCo, NiFeMn and stainless steel.

9. The electrochemical system according to claim 6, wherein the carbonaceous material is selected from glassy carbon, carbon Vulcan, graphene, graphite and carbon nanotubes.

10. The electrochemical system according to any of the preceding claims, wherein: i. the electrolyte, preferably a liquid electrolyte, more preferably an aqueous electrolyte comprises a stabilizing anion selected from F , CT, Br , NOs', ClOf, C2O42; SO42, or combinations thereof, wherein the pH is comprised between 10 and 14 or between 0 and 4; ii. the redox mediator electrode comprises Ga(0), Gain, GaAl, GaSn, GalnSn, GaPt, GaSb, GaBi, GaAg, GaCu, GaSr or a combination thereof; iii. the cathode comprises platinum; iv. the anode comprises platinum or iridium.

11. Method for the electrochemical production of reduced and oxidized species comprising the steps of: i) providing the electrochemical system according to any one of claims 1 to 10; ii) applying an anodic potential to the redox mediator electrode, electrically connected to the cathode, such as to produce H2; iii) applying a cathodic potential to the redox mediator electrode, electrically connected to the anode, such as to produce one or more oxidized species; and iv) optionally, repeating steps ii) and iii) at least two times; wherein steps ii) and iii) are not carried out simultaneously.

12. The method according to claim 11, wherein the anodic potential ranges from -1 to -0.3 V vs Hg / HgO and the cathodic potential ranges from -1.4 to -1.9V vs Hg / HgO.

13. The method according to claims 11 or 12, wherein the oxidized species is O2 and / or CI2 or H+.

14. The method according to any one of claims 11 to 13, wherein steps ii) and iii) are each carried out for a different amount of time; preferably wherein step ii) is carried out for an equal amount of time or a shorter time than step iii).

15. Use of the electrochemical reactor according to any of claims 1 to 10 for fuel production, in combination with fuel cells or internal combustion engines, or in chemical reactions such as hydrogenation reactions, reversible H2 production and H2 oxidation, hydrotreating reactions, hydrocracking reactions, hydroisomerisation reactions, oil hydrofinishing reactions, reforming reactions, Fischer-Tropsch reactions and methanol to olefin reactions.

Images