Catalyst for stable aqueous electrolysis, method for the preparation thereof and related

Abstract

The invention relates to a catalyst for stable electrolysis of aqueous systems, to a method for the production thereof, and to the associated electrochemical implementation. The present invention provides a catalyst for a cation exchange membrane water electrolyser, the catalyst being a layered metal oxide represented by ABxOy, x = 1 or 2 and y = 2, 3 or 4, where A is selected from non-platinum group transition metals and B is selected from chalcogenide elements and group VI transition metal elements.

Description

[0001] Acknowledgements

[0002] The project that generated these results received support from the “la Caixa” Foundation (ID 100010434) and from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 847648. The fellowship number is LCF / BQ / PI21 / 11830021. Technical Field

[0003] The present invention relates to a catalyst for aqueous electrolysis, a method for manufacturing the same, a catalyst-coated film including the catalyst, and a proton exchange membrane electrolyzer cell including the catalyst-coated film. Background Art

[0004] The growing global energy demand, combined with the urgent need to mitigate climate change, has accelerated the development of sustainable and clean energy technologies to replace fossil fuels. Water electrolysis (WE) for synthesizing hydrogen (H2) and other solar fuels has emerged as a promising strategy for producing clean energy carriers from water and low-carbon electricity, providing a pathway for the decarbonization of global industries such as energy, transportation, manufacturing, and agriculture.

[0005] Oxidizing water to oxygen is a key process in solar-fuel systems and the gateway to energy-efficient production of H2 and other emerging solar fuels. Catalysts that promote the oxygen evolution reaction (OER) must not only be active and stable under relevant operating conditions; they must also be sustainable: that is, they should not rely on rare or critical elements, which are an obstacle to the ultimate large-scale deployment of these technologies.

[0006] Among different water electrolysis technologies, proton exchange membrane (PEMWE), in which the cathode and anode electrodes are tightly connected by a proton-conductive membrane, exhibits advantages in terms of productivity (high current density operation), energy efficiency, stability, and the levelized cost of hydrogen compared to diaphragm and alternative anion-conduction-based options (NPTL 1 to 3). However, PEMWE operation requires strongly acidic conditions at the anode – an environment that is highly challenging for catalyst stability. To date, only iridium oxide catalysts have sufficient activity and stability under these conditions, raising questions about the prospects for deploying this technology on a multi-GW scale given the limited global reserves of iridium (Ir), one of the rarer critical raw materials (NPTL 4).

[0007] Ruthenium-based alternatives have shown promising activity but suffer from strong metal dissolution in acidic media inherent to the lattice oxygen evolution reaction mechanism. Thus, there is an urgent need to develop effective and stable iridium-free anodes for PEMWE. Unfortunately, only a few recent instances have translated the discovery of fundamental systems into practical PEMWE (NPTL 5). In this prominent instance, stable operation was demonstrated at 200 mA / cm 2 2.

[0008] PTL 1 discloses a noble-metal-free electrocatalyst composition for acidic media such as, for example, acidic electrolytes. The noble-metal-free electrocatalyst is composed of non-Pt group material (PGM) elements, i.e., free of Ru, Rh, Pd, Os, Ir, and Pt. The non-noble metal is a non-noble metal oxide and is generally in the form of any configuration of solid or hollow nanomaterials such as, for example, nanoparticles, nanocrystalline films, nanorods, nanoshells, nanosheets, nanotubes, nanoplates, nanospheres, and nanowhiskers or a combination of a large number of nanoscale structural embodiments. Optionally, the noble-metal-free electrocatalyst composition includes dopants such as, but not limited to, halogens. Acidic media include the oxygen reduction reaction (ORR) in proton exchange membrane (PEM) fuel cells, as well as direct methanol fuel cells and the oxygen evolution reaction (OER) in PEM-based water electrolysis and metal-air batteries, and hydrogen production by solar- and power-driven water splitting. The disclosed catalyst does not provide active control of OH - and water fragments and lacks stability at high-density proton environments and high current densities.

[0009] PTL 2 discloses a method for improving the oxygen evolution reaction performance of hydroxides by surface modification with anion exchange. The method is mainly applicable to improving the oxygen evolution reaction performance of hydroxides such as iron, cobalt, and nickel or hydrotalcite / hydrotalcite materials mixed with metals. The anions on the surface of the hydroxide catalyst are replaced by anions in a salt solution. Similarly, the disclosed catalyst also does not provide active control of OH - and water fragments and lacks stability in high-density proton environments.

[0010] PTL 3 and PTL 4 disclose an electrode and an oxygen evolution catalyst, respectively.

[0011] NPTL 6 and NPTL 7 disclose information on electrocatalytic reactions.

[0012] The provision of an Ir-free, stable, and active catalyst that realizes the promise of PEMWE (i.e., stable operation at high current densities) has not been demonstrated.

[0013] Citation List

[0014] Non-Patent Literature (NPTL)

[0015] NPTL 1M. Carmo, D. L. Fritz, J. Mergel, D. Stolten, A comprehensive review on PEM water electrolysis. Int J Hydrogen Energy. 38, 4901 - 4934 (2013).

[0016] NPTL 2C. J. T. H. Kwan, A. Bonakdarpour, D. P. Wilkinson, P. Strasser, The Stability Challenges of Oxygen Evolving Catalysts: Towards a Common Fundamental Understanding and Mitigation of Catalyst Degradation. Angew. Chem. Int. Ed. 56, 5994 - 6021 (2017).

[0017] NPTL 3L. An, C. Wei, M. Lu, H. Liu, Y. Chen, G. G. Scherer, A. C. Fisher, P. Xi, Z. J. Xu, C.-H. Yan, Recent Development of Oxygen Evolution Electrocatalysts in Acidic Environment. Advanced Materials. 33, 2006328 (2021).

[0018] NPTL 4P. C. K. Vesborg, T. F. Jaramillo, Addressing the terawatt challenge: scalability in the supply of chemical elements for renewable energy. RSC Adv. 2, 7933 - 7947 (2012).

[0019] NPTL 5L.Chong, G.Gao, J.Wen, H.Li, H.Xu, Z.Green, D.Sugar, A.J.Kropf, W.Xu, X.-M.Lin, H.Xu, L.-W.Wang, D.-J.Liu, La- and Mn-doped cobalt spinel oxygen evolution catalyst for proton exchange membrane electrolysis.Science.380,609-616(2023).

[0020] NPTL 6Tian Tian et al.; ELECTROCHINICA ACTA, Elsevier, Amsterdam; Vol.224, December 1, 2016; 551-560

[0021] NPTL 7Nguyen Anh Quoc Khuong et al.; APPLIED CATALYSIS B.ENVIRONMENTAL; Elsevier, Amsterdam; Vol.324, December 6, 2022

[0022] Patent Literature (PTL)

[0023] PTL 1 US 2021184226 A1

[0024] PTL 2 CN 110394179 A

[0025] PTL 3 US 4 426 269 A

[0026] PTL 4 JP 6 671924 B2 Summary of the Invention

[0027] Problems to be Solved by the Invention

[0028] The basic problem of this application is to provide a catalyst for water splitting that has both activity and stability in a proton exchange membrane water electrolysis system and does not rely on rare platinum group metals (PGMs).

[0029] Solutions for Solving the Problems

[0030] The present invention solves this objective by providing the catalyst according to the present invention. Preferred embodiments are defined in the specification. The present invention further provides the subject matter defined in the present invention.

[0031] Therefore, the present invention provides a non-iridium catalyst that combines activity and stability at a current density of 1 A / cm 2 in PEMWE operating under industrial conditions. Contrary to conventional catalyst design strategies based on doping aimed at controlling the electronic properties of the catalyst, the inventors sought to jointly address the water and oxide structures, an avenue that has been hitherto under-explored, to achieve activity and stability in strong acids.

[0032] The inventors designed a hierarchical strategy in a crystalline metal oxide system preferably containing cobalt metal oxide, whereby a high-valence sacrificial element such as W would exchange with water / hydroxide ions.

[0033] Experiments showed that this anion exchange led to the capture and stabilization of water and water fragments in the hierarchical catalyst. X-ray photoelectron spectroscopy showed a lower anion content in the hierarchical samples, and the features in the O2 1s peak were compatible with different local oxide networks. Thermogravimetric analysis combined with mass spectrometry revealed higher water content, water stability, and unique water desorption characteristics compared to standard non-hierarchical materials. Infrared spectroscopy revealed the confined nature of water in the hierarchical catalyst.

[0034] The resulting catalyst achieved a reduced overpotential in a PEMWE system under industrial conditions, a current density of 1.8 A·cm -2 at 2 V, and stable operation up to 1 A·cm -2 stabilized at 1.74 V. Thus, this is the first demonstration of stable operation in a PGM-free system in the range of 1 A·cm -2 .

[0035] These findings provide avenues for high-performance, sustainable water electrolysis and other supported solar fuel technologies. More broadly, the present invention provides options for designing durable PGM-free electrocatalysts, highlighting the potential of addressing electrolyte structures to break the conventional performance trade-offs in electrochemical systems.

[0036] Effects of the Invention

[0037] A PGM-free catalyst designed using an anion exchange hierarchical strategy is reported. This enables non-iridium catalysts to achieve record productivity and energy efficiency and demonstrates for the first time iridium-free operation at PEMWE-relevant current densities. Description of the Drawings

[0038] Figure 1A Diagrams of PEMWE and catalyst coated membrane (CCM), cathode and anode gas diffusion layers (GDL). Figure 1BPEMWE-based polarization curves (without resistivity or iR correction) of CWO-del-48, CWO, commercial Co3O4, and IrO2 used as anode materials, respectively. The PEMWE-based polarization curves were compared with the reported optimal PGM-free La, Mn co-doped porous cobalt spinel fiber catalysts.

[0039] Figure 1C After overnight conditioning at 1.7 V, in PEMWE, at a temperature of 80 °C, chronopotentiometric stability tests of CWO-del-48 were carried out at current densities of 0.2 A·cm -2 and 1.0 A·cm -2 for 278 h and 250 h, respectively.

[0040] Figure 1D Performance comparison of cell voltage versus current density, and Figure 1E Durability comparison of total charge versus current density of CWO-del-48 with other state-of-the-art PGM-free materials in PEMWE to date.

[0041] Figure 2 Ex situ Raman spectra of the synthesized CWO and layered products (24 h and 48 h) in the upper figure showed a regular red shift of the vibration peaks related to the original CWO to CWO-del-48. The intensity ratios of Co-O and W-O peaks as a function of the delamination time are shown in the inset.

[0042] Figure 2 iDPC-STEM micrograph of CWO-del-48 in the middle figure. The absence of O atoms (circled in white) in the atomic array of Co, W, and O indicates O defects generated by the leaching of WO4 2- due to the alkali treatment of CWO.

[0043] Figure 2 In situ Raman spectra of CWO and CWO-del-48 catalysts at 1.7 V vs. RHE in the lower figure, indicating the participation of Co(III), Co(IV), and Co peroxide species as active OER species.

[0044] Figure 3A Comparison of linear sweep voltammetry of different catalysts, as well as commercial IrO2 and Co3O4, without iR correction in 0.5 M H2SO4 electrolyte at 5 mV·s -1 −1. Figure 3B Comparison of overpotential ranges of CWO and different delaminated materials at a current density of 10 mA·cm -2 −2. Figure 3CIn an H-cell established using Nafion 117 as the membrane, in 0.5 M H2SO4 electrolyte, at a constant current density of 10 mA·cm -2 , chronopotentiometric stability tests of different catalysts were carried out. Detailed implementation mode

[0045] The present invention provides a layered catalyst for aqueous electrolysis, particularly an effective and stable anode electrode that can be used for proton exchange membrane water electrolysis. The layered metal oxide catalyst is represented by AB x O y , where x = 1 or 2 and y = 2, 3 or 4, and A is selected from non-platinum group transition metals, and B is selected from chalcogens and Group VI transition metals.

[0046] Preferred embodiments are described in other aspects related to the method of manufacturing the catalyst and the electrochemical implementation of the catalyst.

[0047] The present invention is mainly described herein with respect to aqueous electrolysis leading to the production of hydrogen, i.e., water oxidation. However, it is obvious that the catalysts of the present invention can also be employed in other electrochemical reactions, particularly redox reactions, including but not limited to CO2 electroreduction, CO electroreduction, oxygen electroreduction, and electroreduction of nitrogen-containing materials such as nitrogen, nitrate, nitrite, nitric acid, and combinations thereof.

[0048] The inventors surprisingly found that by laminating the metal oxide structure, a stable and active catalyst for the desired electrochemical applications can be obtained, which is superior to other known catalysts without platinum group metals and closely mimics the properties of known Ir-containing catalysts.

[0049] For example, the lamination can be observed by the shift of the 2θ value of the peaks in the powder XRD pattern (measured as described herein), such as the shift of the peaks in the powder XRD pattern. For the purposes of the present invention, a shift of at least 0.001° in the 2θ value of the peaks in the powder XRD pattern is considered representative of the desired lamination. The peaks in the XRD pattern are at least one relevant peak for the crystal structure of the non-laminated metal oxide, which is typically one of the 6 most prominent peaks detected. The lamination that can be detected by evaluation relative to the powder XRD pattern of the non-laminated metal oxide can be caused by generating defects and / or strain in the crystal structure of the metal oxide by treating the metal oxide with an alkali.

[0050] In an embodiment, this alkali treatment of the metal oxide causes a partial element B to be removed from the metal oxide AB x O yMesoporous leaching occurs and causes it to be locally replaced by water and anionic substances, resulting in an electrochemically stable structure. Depending on the material selection, this process may cause the above-mentioned shift in the XRD pattern. However, this leaching or anion exchange process can also be determined by other methods, such that the desired stratification can also be determined, for example, by Raman infrared analysis (where the leaching of element B results in a red shift (see Figure 2 )) and the different ratios of the characteristics of A oxide and B oxide; or by elemental analysis, for example, using the XPS method, where the leaching of element B causes a shift in the ratio A / B of the metal oxides. Therefore, another method to determine that the desired stratification has been achieved is to analyze the elemental composition to determine the ratio A / B. The shift of this ratio relative to the starting material indicates stratification. This shift is preferably at least 5%, more preferably at least 10%.

[0051] Element A in the metal oxide is preferably selected from the group consisting of Mn, Co, Ni or Cu, more preferably selected from Co. B is preferably selected from the group consisting of S, Mo and W, more preferably selected from W. Preferably, AB x O y is CoWO4.

[0052] The stratified metal oxide can be prepared by treatment with an alkali. It is preferably selected from alkali metal salts, especially alkali metal hydroxides, such as LiOH, NaOH and KOH, and mixtures thereof. The preferred alkali is KOH.

[0053] The treatment of the metal oxide with an alkali is carried out in a solution of the alkali, preferably an aqueous solution. The alkali treatment solution can also contain other components to tailor the stratification process, such as other solvents and / or additives. Examples thereof are water-miscible solvents, such as alcohols, ketones, etc., and water-soluble additives.

[0054] This method for stratifying the metal oxide can be carried out by the exchange between the lattice oxygen anions of the metal oxide represented by B x O y z- (where B is defined as above) and the OH - / H2O substances, where x = 1 or 2, y = 2, 3 or 4 and z = 2. This method can include the following steps:

[0055] i) Immerse the metal oxide in an aqueous solution of KOH;

[0056] ii) Continuously stir the impregnated material to obtain a stratified product;

[0057] iii) Wash and centrifuge the stratified product;

[0058] iv) Anneal the stratified product to obtain the final catalyst.

[0059] The annealing step is preferably carried out at a temperature of 70 to 120 °C, more preferably at a temperature of 80 to 110 °C, and most preferably at a temperature of 90 °C to 100 °C.

[0060] The present invention also provides a catalyst-coated film comprising the catalyst according to the present invention as an anode catalyst. The film preferably further comprises a cathode catalyst and a polymer electrolyte. The cathode catalyst preferably contains 40 to 70 wt.% of Pt, more preferably contains 60 wt.% of Pt. The polymer electrolyte preferably contains a perfluorosulfonic acid / polytetrafluoroethylene copolymer. Such a film can be used in a proton exchange membrane water electrolysis cell. The cell preferably contains Pt-coated Ti as an anode current collector, and further preferably contains a graphite plate as a cathode current collector.

[0061] In the present invention, it has been demonstrated that the control of the water structure and oxide species in the layered metal oxide lattice produces an active and stable PEMWE. This is achieved by implementing a layering strategy, whereby when a high-valence sacrificial element such as S, Mo, or W is introduced into the AB x O y crystal structure, it can be selectively removed during the subsequent water / hydroxide - B x O y z- anion exchange process. This results in the layering of water and hydroxide species in the metal oxide defect network and their subsequent capture and stabilization, which the inventors have regulated to improve activity and stability.

[0062] To introduce and stabilize OH - / H2O into the lattice of A oxide (A: preferably Mn, Co, Ni, Cu), the inventors designed a layering strategy based on the exchange between lattice oxygen anions (B x O y z- , B: S, Mo, W) and OH - / H2O species, as follows:

[0063] AB x O y +mH2O+nOH - → (B x O y ) 1-q (H2O) m (OH) n +q B x O y z- (1)

[0064] The metal oxide employed according to the present invention provides a layered substance, wherein the oxygen anion and OH -The water species have sufficient binding energy, and this condition promotes their sacrificial leaching, enabling the host lattice to accommodate OH - / H2O species to saturate the resulting oxygen anion vacancies. This in turn provides the desired activity and stability after decomposition as described herein.

[0065] The metal oxide AB employed in the present invention x O y can be synthesized using a hydrothermal reaction. To effect delamination (B x O y z- →OH - / H2O anion exchange), an alkali treatment of dispersing the resulting AB x O y material in 0.1 M MOH aqueous solution was successfully explored. The inventors investigated the effects of cations (Li + to Cs + )), solvents (H2O, DMSO, NMP), and pH in this method.

[0066] This revealed that KOH is preferably used as the base and water as the solvent for delamination. Studies have shown that, contrary to the Co control, the AB x O y -del (“-del” refers to the delaminated oxide) samples maintain structural stability after immersion in 0.5 M H2SO4 for 72 h. The powder XRD patterns show a regular shift of the strongest peaks, indicating the generation of defects / strain in the crystal structure while the bulk monoclinic phase remains intact. These are consistent with photoelectron, magnetic, and X-ray absorption spectroscopy studies showing an increase in the formation of oxygen vacancies. Transmission electron micrographs (TEM) and scanning transmission electron micrographs (STEM) show a shape transformation from cubic AB x O y to flaky after alkali treatment. High-resolution TEM images reveal the missing regular (010) crystal plane in AB x O y -del-48, where “-del-48” refers to the sample obtained after 48 h of delamination, indicating stacking fault defects caused by B x O y z- leaching. To evaluate the atomic arrangement of the A and B sites, integrated differential phase contrast (iDPC) STEM images were taken. The AB x O y samples exhibit a regular atomic arrangement of A, B, and O atoms consistent with monoclinic. On the other hand, AB x O y -del-48 reveals a large density of Bx O y z- Vacancy.

[0067] Figure 2 The STEM image of the layered material according to the present invention is shown. This image reveals the empty array positions of tungsten atoms in the pristine metal oxide. Due to the leaching of tungsten atoms from the pristine metal oxide and the different ratios of A-oxide and B-oxide characteristics, the corresponding Raman spectrum shows a red shift. Further Raman spectroscopy ( Figure 2 ) conducted during in-situ electrochemical operation shows the characteristic peaks of peroxide species and -OOH species, demonstrating the active species for the electrochemical reaction during water oxidation.

[0068] Figure 3 shows the voltammetric measurement results using the layered material (CWO-del-48) of the present invention compared with the non-layered material (CWO) and Co3O4 and IrO2. These measurements demonstrate that the material according to the present invention provides properties close to the gold standard IrO2, while being significantly superior to the non-layered material and cobalt oxide.

[0069] The same improvement is shown in Figure 1, which shows the actual performance of the layered material of the present invention in a practical PEMWE. Again, the layered catalyst of the present invention performs significantly better than the comparative materials, while showing similar performance compared to the IrO2 catalyst. This demonstrates that the present invention can actually provide a catalyst that has properties and electrochemical performance similar to those of an Ir-containing catalyst and is free of platinum group metals, especially free of Ir. Figure 1C The high stability of the catalyst of the present invention is shown, which is significantly superior to the stability of other catalysts (see Figure 1E ). Figure 1D It is shown again that the catalyst of the present invention shows comparable properties when compared with IrO2, while performing much better than other known catalysts.

[0070] To obtain more in-depth information about peroxide species and OH -The nature of the active sites after H2O capture was investigated by additional dynamic in-situ (operando) Raman spectroscopy studies before and after the OER onset potential. The peak intensities of both β-CoOOH and Co peroxide increased steadily from the open circuit potential (OCP, 0.2 V vs. RHE) to 1.9 V vs. RHE and disappeared as the potential was cycled back from 1.9 V vs. RHE to OCP. This indicates that both β-CoOOH and Co peroxide are active sites for OER. To study the role of surface oxides and water-hydroxide capture in OER activity, a series of pH-dependent electrochemical studies and dynamic in-situ interfacial water structure evaluations were performed using Raman. The layered samples showed a very strong pH-dependence during OER, with a reaction order (ρ) of -0.81, almost twice that of CWO (ρ = -0.42). This can be explained by the presence of captured water and a higher OH - coverage. The inventors hypothesized a CoOOH-rich arrangement where water is bonded to the A atoms through the O centers of water. This is consistent with the Raman findings and methanol oxidation reaction (MOR) experiments, showing that MOR is superior to water dissociation in CWO-del-48 at different pHs; consistent with a higher amount of surface oxides.

[0071] The catalyst for proton exchange membrane water electrolyzers can be used as defined in the present invention. It can be deposited or coated, for example, at least in part on a support or substrate. The layered catalyst powder can be mixed with other components such as water, alcohols, and / or ionomers to produce an ink for coating on a support or substrate. The alcohol can be selected from the group consisting of ethanol, ethylene glycol, glycerol, isopropyl alcohol, isobutyl alcohol, and decanol. The alcohol is preferably ethanol. The ionomer is a polymer electrolyte, which preferably comprises a perfluorosulfonic acid / polytetrafluoroethylene copolymer.

[0072] The ink can be used to prepare a catalyst-coated film using conventional coating methods such as spraying, spin coating, dip coating, or the decal method. Among these methods, the decal method is preferred. Suitable supports or substrates include a wide variety of those known in the art for use as electrodes, such as, for example, but not limited to, Ti foil, glassy carbon (GC) disk, and inert transfer substrates. The thickness of the resulting film is from 5 μm to 300 μm.

[0073] In such a setup, the catalyst for proton exchange membrane water electrolyzers is used as the anode catalyst. The cathode catalyst consists of carbon black, which can optionally be coated with Pt. The cathode catalyst preferably consists of 40 to 70 wt.% Pt on carbon black, more preferably consists of 45 to 65 wt.% Pt on carbon black, and even more preferably consists of 50 to 60 wt.% Pt on carbon black.

[0074] The catalyst coated membrane can be used in a membrane electrode assembly (MEA). In a preferred membrane electrode assembly comprising a catalyst coated membrane, the membrane is located between an anode and a cathode membrane. To ensure proper adhesion and integration of the catalyst layer, the entire MEA can be hot pressed at a temperature of 90°C to 150°C, preferably 100°C to 140°C, more preferably 110°C to 130°C for less than 10 minutes, preferably less than 8 minutes, more preferably less than 5 minutes, and for 1 minute or more, preferably 2 minutes or more, more preferably 3 minutes or more.

[0075] In another aspect of the present invention, a proton exchange membrane water electrolysis cell is provided. In such a cell, several of the said membrane electrode assemblies are placed between a porous transport layer and a gas diffusion layer.

[0076] The porous transport layer can be characterized by a powder structure, felt, and mesh, and is made of a Ti-based material, preferably Ti coated with Pt. The gas diffusion layer is made of a porous material such as carbon paper or carbon cloth. Among them, carbon paper is preferred.

[0077] The proton exchange membrane water electrolysis cell further comprises an anode current collector and a cathode current collector. Examples of suitable materials and configurations for the current collectors are known in the art and include multiple metal screens, woven metal layers, porous carbon layers, metal or carbon foams, or polymers filled with conductive materials such as metals or carbons. The anode current collector is preferably made of Ti coated with Pt, and the cathode current collector is preferably made of graphite.

[0078] Examples

[0079] Chemicals and electrolytes for synthesis

[0080] Precursors and electrolytes for synthesizing nanocrystals were purchased from Sigma-Aldrich and used without any further purification. The chemicals used were Co(NO3)2·6H2O (solid, ACS reagent, ≥98%), Na2WO4·2H2O (solid, ACS reagent, ≥99%), cetyltrimethylammonium bromide (CTAB, solid, BioXtra, ≥99%), LiOH (solid, reagent grade, 98%), NaOH (solid, reagent grade, ≥98%), KOH (solid, ACS reagent, ≥85%), CsOH·H2O (solid, ≥90%, ≥99.5% metal basis), Vulcan XC72 (conductive carbon black, NG10BEW0938, Nanografi), methanol (liquid, Pharmpur, Scharlab), ethanol (liquid, Pharmpur, Scharlab), isopropanol (liquid, Pharmpur, Scharlab), acetone (liquid, Pharmpur, Scharlab), dimethyl sulfoxide (DMSO, liquid, ACS reagent, ≥99.9%), N-methyl-2-pyrrolidone (NMP, liquid, ACS reagent, ≥99%). Different concentrations of MOH (M = Li, Na, K, and Cs) were prepared by dissolving the solid bases in milli-Q water (18.2 M·Ω). The electrolyte, 0.5 M H2SO4, was prepared by diluting a stock solution of higher-concentration H2SO4 (liquid, ACS reagent, 95 - 98%) in milli-Q water. Commercial iridium oxide (IrO2, Alfa Aesar, Premion, 99.99%) and cobalt oxide (Co3O4, nanopowder, <50 nm particle size, Sigma Aldrich, 99.5%) were used as reference anode materials.

[0081] Synthesis of CoWO4 (CWO) nanorectangles

[0082] In a typical synthesis of CWO nanorods, 2 mmol of Na2WO4·2H2O and 4 mmol of CTAB were placed in a 100 mL beaker. 42 mL of milli-Q water was added thereto and stirred vigorously until a clear solution (Solution A) was formed. Meanwhile, in a 50 mL beaker, 3 mmol of Co(NO3)2·6H2O was placed and 20 mL of milli-Q water was poured therein. Then the solution was stirred to obtain a bright red aqueous Co(II) solution (Solution B). Next, Solution B was added to Solution A, and the entire solution mixture was stirred vigorously to obtain a homogeneous solution. Then the resulting purple solution mixture was transferred to a 100 mL Teflon-lined stainless steel autoclave and maintained at 180 °C for 12 hours. After 12 hours, the autoclave was naturally cooled to room temperature. The blue product was collected and washed several times with milli-Q water and acetone.

[0083] Synthesis of hierarchical CWO

[0084] The hierarchical CWO-del compound was obtained by immersing CWO in 0.1 M aqueous KOH solution for different times (1 h to 48 h) under magnetic stirring. Under continuous stirring, the blue CWO turned into a brown to black product. Then the post-treated product was washed with milli-Q water and acetone and centrifuged five times. Finally, the resulting CWO-del powder was obtained after centrifugation and annealing at 90 °C overnight.

[0085] Preparation of catalyst ink

[0086] To prepare the catalyst ink, 10 mg of the catalyst and 2.5 mg of Vulcan carbon powder were dispersed in a mixed solution of 750 μL of milli-Q water, 150 μL of ethanol, 80 μL of isopropanol, and 20 μL of Nafion solution (5 wt.% in lower aliphatic alcohols). After sonication for 1 h, the catalyst ink was drop-casted on a pre-polished GCE. The catalyst loading on the GCE was ~0.5 mg·cm -2 . For long-term chronopotentiometry (CP), the catalyst ink was sprayed on carbon paper ( MGL370, Toray). The catalyst loading was ~1.4 ± 0.2 mg·cm -2 .

[0087] X-ray diffraction

[0088] Using a Cu-Kα radiation source The phase purity and crystal structure of the synthesized CWO and CWO-del products were characterized by X-ray diffraction (XRD) using a Rigaku Smartlab system. XRD was performed in the range of 10° to 70°. To evaluate the crystallography of the layered products, XRD was carried out on all the layered compounds. A regular change in the 2θ values of the peaks was observed for CWO and the time-dependent layered compounds treated with 0.1 M KOH. The analysis of the XRD patterns and background correction were performed using X’Pert HighScore Plus software.

[0089] Electron microscopy

[0090] The morphology and microstructure of the materials were investigated using a Zeiss Auriga Crossbeam scanning electron microscope (SEM) equipped with a Ga focused ion beam and a JEOL JEM 2010F 200 kV transmission electron microscope (TEM) with a field emission gun and an electron energy loss spectrometer (EELS). Additionally, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), integrated differential phase contrast (iDPC-STEM) images, and energy-dispersive X-ray spectroscopy (EDS) elemental maps were obtained using a ThermoFisher Scientific Spectra 300 microscope operating at 60 kV. The STEM beam was monochromated using TFS Optimono to avoid chromatic aberration when working at low voltages. TEM and STEM images were processed using DigitalMicrograph software from Gatan and Velox software from ThermoFisher Scientific. EDS data were processed using Velox. The iDPC-STEM images were overlapped with the simulated crystal structures obtained from CaRIne crystallography to confirm the positions of the atoms within the lattice. The lattice in the micrographs of the regions of interest in this work was enhanced by using a frequency filter in reciprocal space. First, a spot mask was applied to the diffraction nodes in the corresponding fast Fourier transforms (FFTs). Subsequently, an inverse FFT filter was applied, and the resulting image was overlapped with the original micrograph. All these processes were performed using DigitalMicrograph. For electron microscopy studies (SEM, TEM, and STEM), very dilute solutions of the materials were separately prepared by dispersing them in ethanol. Then, the dispersed ethanol solutions were drop-coated onto Si / SiO2 wafers (for SEM) and C-coated Au grids (for TEM and STEM) and dried overnight under vacuum.

[0091] Spectral characterization

[0092] In addition, the oxidation states of elements and surface analysis were characterized by X-ray photoelectron spectroscopy (XPS) using SPECS PHOIBOS150. For the XPS experiment, an ethanol solution of the material was drop-coated on a Si / SiO2 wafer and dried overnight under vacuum. CasaXPS software was used for XPS peak fitting and data analysis. The binding energies of all peaks were corrected relative to the C1s peak (284.5 eV). To evaluate the elemental concentrations in different reactions and experiments, inductively coupled plasma optical emission spectrometry (ICP-OES) and mass spectrometry (ICP-MS) were performed using a Perkin Elmer Optima 8300 and an Agilent7800 ICP-MS, respectively. The absorption spectra of the materials were analyzed from the UV-Vis spectra obtained in a ParkinElmerLambda 950 spectrophotometer. An ethanol solution of the 0.1 M sample was prepared to study their optical properties. The indirect optical band gap was calculated by the Tauc plot method. The wavelength (nm) was converted to energy (eV) and the absorption data was converted to (2.303 × energy) 0.5 , with the energy unit in eV. Electron paramagnetic resonance (EPR) measurements were obtained using a Bruker EMX Micro spectrometer with an X-band bridge of 9.1 - 9.9 GHz. The powder sample was poured into a capillary with one side blocked having dimensions of 76 mm (length) × 1.5 mm (outer diameter) × 0.84 mm (inner diameter) and pressed to minimize air voids. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra were obtained from the powder material using an Agilent Cary 630FTIR spectrometer. To minimize the amount of water adsorbed from the air, the powder sample was heated at 120 °C FTIR measurement prior to

[0093] Under standard ring conditions, X-ray absorption data at the Co K-edge was collected on beamline I18 at the Diamond Light Source. Energy selection was performed using a Si(111) monochromator. The beam was focused to 2 × 4 μm. Two N2-filled ion chambers were used to monitor the intensities of the incident and transmitted beams before (I0) and after (I1) the sample. A cobalt foil was used for calibration. Data was collected in fluorescence mode using a four-element Si drift detector (Vortex ME4). Standard Co and the catalyst were measured under the same conditions in air. Demeter software was used for data analysis to subtract the pre-edge background and normalize the spectra after the edge jump.

[0094] Raman spectroscopy measurement

[0095] All Raman spectra were measured using a Renishaw Raman spectrometer equipped with 532 nm and 785 nm lasers. Ex situ Raman spectra were measured using a 532 nm laser through a L20x objective lens. For ex situ Raman spectra, CWO and its derivatives were drop-coated on a silica substrate (Si / SiO2).

[0096] In situ Raman spectra were measured using a 785 nm laser through a customized in situ cell using an immersion objective lens (L63x). Samples were prepared by spraying on a carbon paper ( MGL370, Toray). Potentials were applied using a single-channel autolab204 potentiostat with 0.5 M H2SO4 as the electrolyte and a Pt wire as the counter electrode. All data were obtained with 10% laser power and accumulated 30 times. To study the change in the interfacial water structure, 300 nm Ag-sputtered PTFE (polytetrafluoroethylene) was used as the substrate for surface-enhanced Raman spectroscopy, using a 532 nm laser with a laser power of 0.5% and accumulated 30 times. The broad water peak (O-H) from 3,000 to 3,700 cm -1 was deconvoluted into three contributions using Gaussian fitting in origin software to minimize the random residuals indicating three different types of hydrogen-bonded (n-HB·H2O) water structures: 4-HB·H2O (~3,200 cm -1 ), 3-HB·H2O (~3,400 cm -1 ), and 0-HB·H2O (~3600 cm -1 ). Samples were drop-coated onto an electrode consisting of 300 nm sputtered Ag on a PTFE sheet.

[0097] In situ Raman studies in dynamic fields showed that the peak intensities of both β-CoOOH and Co peroxide increased steadily from the open-circuit potential (OCP, 0.2 V vs. RHE) to 1.9 V vs. RHE and disappeared as the potential was cycled back from 1.9 V vs. RHE to OCP. This indicates that both β-CoOOH and Co peroxide are active sites for OER. To study the role of surface oxides and water-hydroxide trapping in OER activity, a series of pH-dependent electrochemical studies and dynamic in situ interfacial water structure evaluations were performed using Raman. The layered samples showed a very strong pH-dependence during OER, with a reaction order (ρ) of -0.81, almost twice that of CWO (ρ = -0.42). This can be attributed to the presence of trapped water and higher OH -It is explained by coverage. The inventors hypothesized an arrangement rich in CoOOH, where water is bonded to Co atoms through the O center of water. This is consistent with Raman findings and methanol oxidation reaction (MOR) experiments, showing that MOR is superior to water splitting in CWO-del-48 at different pH values; it is consistent with a higher amount of surface oxides.

[0098] Effect of solvents and alkali metal ions on the hierarchy

[0099] CWO was treated with KOH for 48 h to obtain CWO-del-48. To better understand the anion exchange process, time- and concentration-dependent studies were carried out, and samples were collected at different times to characterize them. It was observed that for a reaction time of 12 h, the morphology of the particles remained almost intact. However, during the reaction time, the shape of the nanocrystals changed. After 24 h of KOH treatment, the rectangular CWO transformed into a flaky morphology. During the reaction, the leaching of Co and W from CWO was evaluated using ICP-OES technology. Raman spectra of the products obtained at different times indicated kinetic limitations in W leaching, as the I Co-O / I W-O ratio was almost close to saturation.

[0100] This was shown by XRD powder evaluation, where the 2θ values of the peaks of the material shifted from 30.546 of the non-layered starting material to 30.548 and up to 30.563 for the layered material.

[0101] In addition, the role of the solvent in the delamination process was studied. Instead of an aqueous KOH solution, CWO powder (∼30 mg) was post-treated in DMSO (15 mL) and NMP (15 mL) separately. Compared with treatment in water, the effect of the two solvents on delamination was smaller.

[0102] To evaluate the role of alkali metal cations, delamination experiments were carried out using different types of bases MOH (M = Li, Na, K, and Cs). After 18 h of delamination time, the differences in alkali-treated CWO under different MOH solutions were observed. It was found that the degree of delamination was the highest for LiOH treatment and the lowest for CsOH treatment. Treatment with aqueous NaOH and KOH solutions resulted in a similar degree of delamination. In these studies, the role of cations in the delamination process was obvious. The ionic radius trend of the 4 ions was: r Li+ <r Na+ <r K+ <r Cs+ . The degree of delamination can be explained by the ionic radius of the cations; the smaller the ionic radius, the faster the ionic diffusion rate.

[0103] Electrochemical study

[0104] The electrochemical properties of all catalysts were studied and analyzed in an Autolab M204 equipped with electrochemical impedance spectroscopy (EIS) and a Biologic SP50 electrochemical workstation. For all OER studies, 0.5 M H2SO4 was used as the electrolyte. A graphite rod and a saturated Hg / HgSO4 (MSE) electrode (E MSE = 0.65 V vs. RHE) were used as the counter electrode and reference electrode, respectively, and the catalyst on a glassy carbon electrode (GCE) and / or carbon paper was used as the working electrode. Linear sweep voltammetry (LSV) studies were carried out at a scan rate of 5 mV·s -1 . Chronopotentiometry (CP) tests were performed at a current density of 10 mA·cm -2 in an H-cell using Nafion 117 as the proton exchange membrane. LSV was recorded before the electrode was conditioned at 10 mA·cm -2 for 1 h, and then cyclic voltammetry (CV) was carried out for 20 cycles from 0.65 to 1.6 V vs. RHE at a scan rate of 50 mV·s -1 . All electrochemical studies were conducted with stirring at 600 rpm in 0.5 M H2SO4 electrolyte at room temperature.

[0105] CP tests were carried out in an H-cell device (CS932S sealed H-cell, CorrTest Instruments; 50 mL and 30 mL volumes). The electrode applied potential was converted to the RHE scale using the following equation:

[0106] E RHE = E HG2SO4 + E 0 MSE + (0.0591 × pH)

[0107] where E HG2SO4 is the electrode potential, E 0 MSE = 0.65 V vs. RHE, the pH of 0.5 M H2SO4 = 0, so the electrode potential can be converted to

[0108] E RHE = E HG2SO4 + 0.65 V

[0109] The average Faradic Efficiency for O2 at a current density of 10 mA·cm 2 was 96.6 ± 5.2%; e = the charge of an electron, i.e., 1.602 × 10 -19 C, n c is the number of active sites of the catalyst. Co is considered the active atomic site, and all Co atoms are active. n is calculated as followsc :

[0110]

[0111] where m l is the supported mass of the catalyst, N A is Avogadro's constant = 6.022×10 23 , M is the molar mass of the catalyst, and n m is the number of Co atoms in 1 mol of the catalyst. Based on these assumptions, the determined TOF values are underestimated. For the calculation of n c for all catalysts, the value is 5.889×10 16 .

[0112] Electrochemical impedance spectroscopy (EIS) was performed in a three-electrode cell without any magnetic stirring. In the frequency range from 100 kHz to 0.01 Hz, for each electrode, the EIS spectra were recorded at 1.45 V vs. RHE.

[0113] CCM manufacturing

[0114] Catalyst-coated membrane (CCM) samples were prepared using the method with Nafion 117 as the polymer electrolyte membrane. CWO-del-48 was used as the anode catalyst, while the cathode catalyst consisted of 60 wt.% Pt on Vulcan carbon XC 72R obtained from Fuel Cell Store. As a reference, iridium oxide (Alfa Aesar, Premion, 99.99%) was used for comparison of the anode catalyst.

[0115] To fabricate the CCM samples, the catalyst powder and the ionomer solution (20 wt.% Nafion for the anode and 25 wt.% Nafion for the cathode) were mixed in a solution of water and ethanol to produce an ink. Then the ink was subjected to 30 minutes of ultrasonic homogenization and then sprayed onto an inert transfer substrate using a manual spray gun.

[0116] The membrane was placed between an anode and a cathode decal, and the entire assembly was hot-pressed at 130 °C for 3 minutes to ensure proper adhesion and integration of the catalyst layer. The final loadings of the catalyst layer were: 0.8 mg·cm -2 for Pt / C at the cathode, 1.0 mg·cm -2 for IrO2 at the anode, and 4.0 mg·cm -2 for CWO-del-48 at the anode.

[0117] Cell assembly and PEMWE measurement

[0118] The MEA was placed between a porous transport layer (PTL) made of platinum-plated titanium received from Mainz Hydrogen Energy and a gas diffusion layer (GDL, MGL370, Toray). The PEMWE cell is characterized by using platinum-plated titanium as the anode current collector and a graphite plate as the cathode counterpart. Both sides of the cell contain serpentine flow channels, each covering an area of 4 cm 2 . The cell was compressed with a torque of 7 N·m on each of the 4 bolts. Milli-Q water preheated at 80 °C was continuously pumped into the anode side of the fuel cell at a flow rate of 25 mL·min -1 . The membrane electrode assemblies (MEAs) were conditioned by holding the cell at a constant potential of 1.7 V for 12 h, and the operating temperature was controlled at 80 °C using 2 heating rods. After the conditioning process, polarization tests were performed using linear sweep voltammetry at a scan rate of 5 mV·s -1 . For the durability test of the catalyst, the current density was set at a specific value of 0.2 or 1.0 A·cm -2 , while the cell voltage was continuously monitored over time.

[0119] Electrochemical performance

[0120] The polarization curves of the CWO-del catalyst were compared with commercial Co3O4 and IrO2 ( Figure 3A ). At a current density of 10 mA·cm -2 , a minimum overpotential of 276 mV was obtained for CWO-del-48 (without iR correction), while at a similar loading, the overpotentials of commercial Co3O4 and IrO2 were 380 mV and 247 mV, respectively. Statistical analysis and extended measurements confirmed the reproducibility of these trends. At a current density of 10 mA·cm -2 , the Faraday efficiency for O2 evolution was 96.6 ± 5.2%. Tafel analysis revealed a slope of 85 mV·dec for CWO-del-48 -1 , a slope of 63 mV·dec relative to IrO2 -1 , and a slope of 227 mV·dec for CWO -1 . This demonstrates the improved OER kinetics of CWO-del-48 relative to the reference samples, approaching the OER kinetics of IrO2. The double-layer capacitance representing the electrochemically active surface area (ECSA) increased with increasing delamination time and reached the highest in CWO-del-48. In a 10 mA·cm -2 H-cell, CWO-del-48 showed the highest stability (>175 h) compared to commercial Co3O4 and CWO ( Figure 3C)。The leaching of Co from CWO-del-48 at different time intervals during a 100 h stability test in an H cell at 10 mA·cm -2 was evaluated using inductively coupled plasma mass spectrometry (ICP-MS). The results showed that Co leaching was negligible (1.5 ppm after 100 h).

[0121] The CWO-del catalyst was implemented in a PEMWE system, and the cell performance was investigated under industrial operating settings, including a temperature of 80 °C and high current densities of 0.2 A·cm -2 -1 A·cm -2 . The polarization curve of the CWO-del-48 cell reached a nominal current density of 1.8 A·cm -2 at 2 V; the productivity was three times higher compared to the previously best PGM-free anode catalyst. During electrolysis at a fixed current density of 0.2 A·cm -2 , the voltage range (1.53 - 1.56 V) was 130 mV lower than that of the previously best Co-based PEMWE and matched that of Ir black (1.50 V) with a four-fold lower load. This demonstrated the potential of the CWO-del catalyst compared to the well-established iridium oxide, as well as its lower cost and potentially higher availability of Co and W.

[0122] This performance (~1.52 V at 0.2 A·cm -2 ) was maintained for at least over 278 h of continuous operation (limited by pump failure). The stability of the CWO-del-48 catalyst was further challenged at 1 A·cm -2 - a benchmark that has been difficult to achieve for PGM-free catalysts to date. The PEMWE cell exhibited very stable performance, with a stable cell voltage of ~1.74 V (limited by pump failure) during a 250 h durability study. This was the first successful implementation in a PGM-free PEMWE under such industrial parameters and conditions.

Claims

1. A catalyst for a cation exchange membrane water electrolyzer, the catalyst being composed of AB x O y The layered metal oxide represented by x=1 or 2 and y=2, 3 or 4, wherein A is selected from non-platinum group transition metals, and B is selected from chalcogen elements and transition metal Group VI elements.

2. The catalyst according to claim 1, wherein the stratification is represented by at least one of: - at least one peak in the powder XRD pattern is shifted in 2θ relative to a non-layered metal oxide of the same composition; and / or - a red shift of the A oxide species and an increase in the ratio of the A oxide species to the B oxide species observed in the Raman spectrum; and / or - The loss of B-site anionic species determined by XPS was between 0.5% and 99%.

3. The catalyst according to claim 1 or 2, wherein A is selected from the group consisting of Mn, Co, Ni or Cu; and / or, wherein B is selected from the group consisting of S, Mo and W.

4. The catalyst according to any one of claims 1 to 3, wherein the metal oxide is CoWO4.

5. A catalyst according to any one of the preceding claims, wherein the shift in the powder XRD pattern is Peak shift.

6. A method for preparing a catalyst according to any one of claims 1 to 5, the method comprising treating a metal oxide with a base.

7. The process according to claim 6, wherein the base is selected from alkali metal salts such as LiOH, NaOH and KOH, and mixtures thereof.

8. The method according to claim 6 or 7, comprising the following steps: i) immersing the metal oxide in an aqueous solution of alkali; ii) continuously stirring the impregnated product to obtain a layered product; iii) washing and centrifuging the layered product; iv) annealing the layered product to obtain the final catalyst.

9. The method of claim 8, wherein the annealing step is performed at a temperature of 70°C to 120°C. 10 . A catalyst coated membrane comprising the catalyst according to claim 1 as an anode catalyst.

11. The catalyst coated membrane of claim 10, further comprising a cathode catalyst and an ionomer.

12. The catalyst coated membrane of claim 11, wherein the cathode catalyst comprises 40 wt.% to 70 wt.% Pt; and / or wherein the ionomer comprises perfluorosulfonic acid / polytetrafluoroethylene copolymer.

13. A proton exchange membrane water electrolyzer cell, comprising at least one catalyst coated membrane according to any one of claims 10 to 12; preferably, wherein at least one catalyst coated membrane is placed between a porous transport layer and a gas diffusion layer; and / or, wherein the porous transport layer is composed of a Ti-based material; and / or, wherein the gas diffusion layer is composed of a material selected from carbon paper or carbon cloth; and / or, wherein the cell comprises Pt-coated Ti as an anode collector; and / or, wherein the cell further comprises a graphite plate as a cathode collector.

14. An electrochemical system using the catalyst according to any one of claims 1 to 5 as an anode active material for water oxidation and other redox operations.

15. The system of claim 14, wherein the redox operation is selected from cathode CO2 electroreduction, CO electroreduction, oxygen electroreduction, electroreduction of nitrogen-containing species such as nitrogen, nitrites, nitrates, nitric acid, and combinations thereof.

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