Water-electrolyser anode, PEM water electrolyser, method of manufacture and method of water electrolysis

Abstract

A water-electrolyser anode for a proton exchange membrane (PEM) water electrolyser comprises: a transition metal oxychalcogenide catalyst having the formula ABxOy, wherein A is a transition metal and B is a chalcogenide, and wherein 0 < x < 2 and 0 < y < 2. Also provided are a proton exchange membrane (PEM) water electrolyser, a method of water electrolysis, use of a transition metal oxychalcogenide as a catalyst in an oxygen evolution reaction under acidic conditions, and a method of manufacturing an anode for an electrolyser.

Description

[0001] Water-Electrolyser Anode, PEM Water Electrolyser, Method of Manufacture and Method of Water ElectrolysisThe invention relates to providing a water-electrolyser anode for a proton exchangemembrane (PEM) water electrolyser, and to a PEM water electrolyser. The invention furtherrelates to the use of a transition metal oxychalcogenide as a catalyst in an oxygen evolutionreaction under acidic conditions, a method of water electrolysis, and a method of manufacturing an anode for a PEM water electrolyser. Background of the Invention Proton exchange membrane (PEM) water electrolysis enables the production of high-purity hydrogen and oxygen from water, and is considered a highly promising technology for the sustainable production of green hydrogen. While water splitting can be performed both under alkaline conditions, using an alkaline electrolysis cell, or acidic conditions, in a PEM cell, ion- exchange membranes typically perform poorly under the highly alkaline conditions in alkaline electrolysis. PEM water electrolysis under acidic conditions does not suffer from the samemembrane limitations, but so far the commercial applications of PEM water electrolysis havebeen hindered by factors including the high-cost of the electrocatalysts used in PEM cells. Electrocatalytic water splitting using proton exchange membrane (PEM) electrolysers enablehigh current densities and intermittent operation. The hydrogen evolution reaction (HER)using Pt catalysts occurs on the cathode side of a PEM electrolyser. The exceptional activityand single Pt atom HER catalysis makes it viable for large scale use– despite its cost. Onthe anode side, the oxygen evolution reaction (OER) has sluggish kinetics and therefore reduces the overall efficiency of the water splitting process. The high overpotential and zero pH conditions of proton exchange membrane (PEM) water electrolysers means that only a few metal oxides based on iridium and its alloys are sufficiently stable catalysts for the water oxidation reaction on the anode.IrO2 is the best catalyst for OER because it is catalytically active and stable in the highlycorrosive environment required for PEM. However, iridium is rare and expensive and it is challenging to scale down to single atom utilization, which has hindered scale up of PEMelectrolysers for hydrogen production. Inexpensive oxygen evolution reaction (OER)catalysts based on non-iridium oxides are therefore required for acidic water electrolysers. It is an aim of the present invention to provide an effective catalyst for OER in acidic media, which provides a cheaper and more scaleable alternative to existing iridium-based catalysts. Summary of the Invention The invention provides a water-electrolyser anode for a proton exchange membrane (PEM) water electrolyser, a PEM water electrolyser, the use of a transition metal oxychalcogenide as a catalyst in an oxygen evolution reaction under acidic conditions, a method of water electrolysis, and a method of manufacturing an anode for a PEM water electrolyser, as defined in the appended independent claims, to which reference should now be made. Preferred or advantageous features of the invention are set out in dependent subclaims. Water-Electrolyser AnodeAccording to a first aspect of the invention, there is provided a water-electrolyser anode fora proton exchange membrane (PEM) water electrolyser, the anode comprising:a transition metal oxychalcogenide catalyst having the formula ABxOy, wherein A is atransition metal and B is a chalcogenide, and wherein 0 < x < 2 and 0 < y < 2.A PEM water electrolyser cell comprises a cathode electrode in a cathodic chamber on oneside of a proton-exchange membrane, and an anode electrode in an anodic chamber on the opposite side of the proton-exchange membrane. Both the cathodic and anodic chambers of the cell are filled with water (H2O), and a potential difference is applied between the cathode and the anode. Relatively speaking, the anode is subject to positivepotential and the cathode is subject to negative potential. This potential difference drives acurrent flow which causes a hydrogen evolution reaction (HER) to take place at thecathode, so that hydrogen gas (H2) is evolved at the cathode, and an oxygen evolutionreaction (OER) to take place at the anode, so that oxygen gas (O2) is evolved at the anode.During operation, the cathode and the anode experience different electrical potentials, and different electrochemical catalysts are used at the cathode and anode to catalyse the separate HER and OER gas evolution reactions. The gas evolution reactions in the PEMcell take place under highly acidic conditions (pH ≈ 0), which means that very fewcatalysts are sufficiently stable to catalyse the OER on the anode. The conditions faced byanode catalysts in PEM water electrolysis cells are therefore completely different from those faced in alkaline water electrolysis. The present inventors have found that the oxygen evolution reaction (OER) in a PEM water electrolysis cell can be particularly effectively catalysed by an anode which comprises atransition metal oxychalcogenide catalyst having the formula ABxOy, wherein A is atransition metal and B is a chalcogenide, and wherein 0 < x < 2 and 0 < y < 2. A transition metal oxychalcogenide compound is a compound containing at least onetransition metal (designated herein with symbol A), a chalcogenide element (designatedherein with symbol B), and oxygen (O). Instead of “B”, the chalcogenide may instead bedesignated with another representative letter or symbol, for example “Z” or “β”.While transition metal oxides are typically stable, they are also typically electrically insulating, which renders them unusable as catalysts for OERs. Transition metal chalcogenide compounds, on the other hand, are typically insufficiently stable to be used in acidic media.The present inventors have found, however, that transition metal oxychalcogenide catalystshaving the formula ABxOy are both stable in the acidic environment of a PEM water electrolysis cell, and effective at catalysing the oxygen evolution reaction at the anode of such a cell. Unlike their related oxide or chalcogenide counterparts, the oxychalcogenide is advantageously sufficiently conductive to allow OER catalytic activity from uncoordinatedtransition metal sites while remaining stable in the acidic environment. The inventors havedemonstrated transition metal oxychalcogenide catalysts which show an overpotential of~295 ± 8 mV versus reversible hydrogen electrode (RHE) at a current density of 10 mA cm‒2 and Tafel slopes of ~63 ± 5 mV dec‒1. These catalysts exhibit amongst the highestreported mass activity ~103,000 A g‒1at an overpotential of 0.5 V versus RHE and excellent stability (potential change of 14 ± 3 mV after 24 hours at a current density of 50 mA cm‒2) for OER in acid.Transition metal oxychalcogenide catalysts are noble-metal free, and can be manufacturedusing straightforward processes, which makes them a significantly cheaper alternative to the iridium-based catalysts which have until now been the only viable option to catalyse the required oxygen evolution reaction in acidic conditions. These transition metaloxychalcogenide catalysts therefore represent a highly promising route forward towards themainstream adoption of PEM water electrolysis as a mode of hydrogen generation.The anode comprises a transition metal oxychalcogenide catalyst having the formulaABxOy, wherein A is a transition metal and B is a chalcogenide, and wherein 0 < x < 2 and 0 < y < 2.The transition metal A in the transition metal oxychalcogenide ABxOy is preferably one ofHf, Zr, Ta, Ti, Nb, or V.The chalcogen B in the transition metal oxychalcogenide ABxOy is preferably one of S, Seor Te.The transition metal oxychalcogenide may preferably be a transition metal oxysulfide,which the chalcogen B is sulfur, S. By starting with the transition metal disulfide compound, the inventors have found that catalytically-active oxysulfide compounds can be created, for example, by electrochemical exfoliation and subsequent oxidation.The transition metal oxychalcogenide anode catalyst is advantageously noble-metal-free.Preferably, in the transition metal oxychalcogenide catalyst ABxOy, (x+y) < 2. The transitionmetal oxychalcogenide may thus be a non-stoichiometric compound. This ensures thatthere are some uncoordinated sites on the transition metals in the transition metaloxychalcogenide structure. These lattice vacancies may advantageously enhance thecatalytic activity of the transition metal oxychalcogenide. In particularly preferred embodiments, 1.5 < (x+y) < 1.9, or 1.6 < (x+y) < 1.8. In preferredembodiments, for example, 0.3 < x < 0.7, or 0.4 < x < 0.6. In preferred embodiments, 1.0 <y < 1.4, or 1.1 < y < 1.3.The transition metal oxychalcogenide may comprise a mixture of amorphous (a) andcrystalline (c) phases. The transition metal oxychalcogenide may be a disorderedcompound, in which the solid transition metal oxychalcogenide contains a mixture ofamorphous and crystalline phases.The transition metal oxychalcogenide may be a layered oxychalcogenide, in which parallellayers of transition metal oxychalcogenide lattice are held together by weak van der Waal’s forces.The anode may comprise a plurality of nanoparticles or nanosheets of transition metaloxychalcogenide. The transition metal oxychalcogenide catalyst may take the form of nanosheets of transition metal oxychalcogenide, in which the catalyst nanosheets have a thickness of only a few nanometres, for example less than 5 nm, or 10 nm, or 20 nm. The nanosheets may have an average lateral size which is at least a few micrometres, for example at least 5 µm, or 10 µm, or 20 µm, up to a few hundred micrometres, for example up to 200 µm, or 300 µm. The nanosheets may for example have a thickness:lateral width aspect ratio of at least 1:500, or 1:750, or 1:1000. In a particularly preferred embodiment, the transition metal oxychalcogenide has theformula HfSxOy. Thus the anode comprises a hafnium oxysulfide catalyst. Particularlypreferably the hafnium oxysulfide catalyst is an amorphous / crystalline (a / c) catalyst which contains a mixture of amorphous (a) and crystalline (c) phases. Preferably, in the hafnium oxysulfide catalyst HfSxOy, (x+y) < 2. In particularly preferred embodiments, 1.5 < (x+y) < 1.9, or 1.6 < (x+y) < 1.8. In preferred embodiments, forexample, 0.3 < x < 0.7, or 0.4 < x < 0.6. In preferred embodiments, 1.0 < y < 1.4, or 1.1 < y< 1.3. Metallic Hf readily oxidises to form a stable and electrically insulating HfO2. Hf also forms a layered van der Waals solid in the form of HfS2. Using HfO2 for OER is not possible, however, as it is too insulating for the reaction to proceed. HfS2 can be made conductive byintroduction of S vacancies, which also reduces the coordination of Hf – the active site forOER, but HfS2 is not sufficiently stable in acidic media to allow its use as a catalyst for OERin a PEM cell. The present inventors have found, however, that OER in acidic media is veryeffectively catalysed by hafnium oxysulfide catalyst having the formula HfSxOywherein 0 < x < 2 and 0 < y < 2.As set out below in relation to the Figures, the present inventors have demonstrated hafnium oxysulfide catalyst HfSxOy nanosheets as stable and high-performance catalysts for OER in acidic (pH = 0) media. The nanosheets can be manufactured by electrochemical exfoliation of hafnium disulfide HfS2to form exfoliated nanosheets which are oxidised during synthesis and consist of a mixture of amorphous and crystalline regions (a / c- HfSxOy), which renders the material sufficiently conductive to allow OER catalytic activity from uncoordinated Hf sites while remaining stable in the acidic environment. The a / c- HfSxOycatalysts show an overpotential of ~295 ± 8 mV versus reversible hydrogen electrode (RHE) at a current density of 10 mA cm‒2and Tafel slopes of ~63 ± 5 mV dec‒1. The catalysts exhibit amongst the highest reported mass activity ~103,000 A g‒1at an overpotential of 0.5 V versus RHE and excellent stability (potential change of 14 ± 3 mV after 24 hours at a current density of 50 mA cm‒2) for OER in acid.Using a / c-HfSxOy for OER and 1T-MoS2 or 2H-NbS2‒x for hydrogen evolution reaction wasfound to enable efficient PEM water electrolysis. The present inventors expect that the advantageous catalytic performance of HfSxOy forOER in acidic media is not limited to hafnium oxysulfide, but is also likely to be exhibited byother transition metal oxychalcogenide compounds which physical and electrochemicalproperties similar to hafnium oxysulfide. In another embodiment for example, the transition metal oxychalcogenide may have theformula ZrSxOy. The inventors have found that, analogously to hafnium oxysulfide,zirconium oxysulfide ZrSxOy can be electrochemically exfoliated from ZrS2, leading to atransition metal oxychalcogenide which may be effective for OER in acidic conditions. In yet another embodiment, the transition metal oxychalcogenide may have the formulaTaSxOy. The inventors have found that, analogously to hafnium oxysulfide, tantalumoxysulfide TaSxOy can be electrochemically exfoliated from TaS2, leading to a transitionmetal oxychalcogenide which may be effective for OER in acidic conditions. Tantalumoxysulfide TaSxOy may preferably be formed from TaS2 by thermally assisted electrochemical exfoliation. In yet another embodiment, the transition metal oxychalcogenide may have the formulaTiSxOy. The inventors have found that, analogously to hafnium oxysulfide, titaniumoxysulfide TiSxOy can be electrochemically exfoliated from TiS2, leading to a transitionmetal oxychalcogenide which may be effective for OER in acidic conditions. Titanium oxysulfide TiSxOymay preferably be formed from TiS2by thermally assisted electrochemical exfoliation. In yet another embodiment, the transition metal oxychalcogenide may have the formulaNbSxOy. The inventors have found that, analogously to hafnium oxysulfide, niobiumoxysulfide NbSxOy can be electrochemically exfoliated from NbS2, leading to a transitionmetal oxychalcogenide which may be effective for OER in acidic conditions. Niobium oxysulfide NbSxOymay preferably be formed from NbS2by thermally assisted electrochemical exfoliation. In yet another embodiment, the transition metal oxychalcogenide may have the formulaVSexOy. The inventors have found that, analogously to hafnium oxysulfide, vanadiumoxyselenide VSexOy can be electrochemically exfoliated from VSe2, leading to a transitionmetal oxychalcogenide which may be effective for OER in acidic conditions. Vanadium oxyselenide VSexOy may preferably be formed from VSe2 by thermally assisted electrochemical exfoliation.The anode may comprise a substrate, with the transition metal oxychalcogenide catalystbeing loaded or coated onto the substrate. A variety of substrates may be used for theanode. For example the substrate may be a membrane which is chemically stable in anacidic environment. The catalyst may be coated onto the substrate by, for example, coatingthe substrate in an ink containing the catalyst.The substrate may optionally be a membrane, so that the anode is a catalyst coatedmembrane, comprising a membrane coated in the transition metal oxychalcogenide.The anode may comprise a titanium porous transport layer configured to transportincoming water to the catalyst and substrate, and to enable the product gases to escape from the anode. PEM water electrolyserAccording to a second aspect of the disclosure there is provided a proton exchangemembrane (PEM) water electrolyser comprising:an anode electrode comprising a transition metal oxychalcogenide having the formulaABxOy, wherein A is a transition metal and B is a chalcogenide, and wherein 0 < x < 2 and 0 < y < 2. The PEM water electrolyser may additionally comprise a cathode electrode. A variety of known cathode electrodes may be used, with cathode catalysts known in the art for catalysing the hydrogen evolution reaction on the cathode side of a PEM cell. In a particularly preferred embodiment, for example, the cathode electrode may advantageously comprise 2H-NbS2‒x. As set out below in relation to the Figures, a 2H-NbS2‒xcathode and HfSxOyanode has been found to be particularly effective for a PEM water electrolyser. In an alternative embodiment, the cathode may comprise 1T-MoS2. The anode electrode is preferably an anode according to the first aspect set out above. Features of the invention discussed in relation to the other aspects of the invention mayapply equally to the PEM water electrolyser of the present aspect.Use of a transition metal oxychalcogenide as a catalyst in acidic OERAccording to a third aspect of the disclosure there is provided a use of a transition metaloxychalcogenide as a catalyst in an oxygen evolution reaction under acidic conditions,in which the transition metal oxychalcogenide has the formula ABxOy,wherein A is a transition metal and B is a chalcogenide, and wherein 0 < x < 2 and 0 < y < 2.This aspect of the disclosure may provide a use of a transition metal oxychalcogenide as acatalyst on an anode of a PEM water electrolyser, the catalyst catalysing an oxygenevolution reaction under acidic conditions in the PEM electrolyser.The transition metal oxychalcogenide catalyst may have the formula ABxOy, wherein A is atransition metal and B is a chalcogenide, and wherein 0 < x < 2 and 0 < y < 2.The transition metal A in the transition metal oxychalcogenide ABxOy is preferably one ofHf, Zr, Ta, Ti, Nb or V.The chalcogen B in the transition metal oxychalcogenide ABxOy is preferably one of S, Seor Te. The transition metal oxychalcogenide may preferably be a transition metal oxysulfide, which the chalcogen B is sulfur, S.The transition metal oxychalcogenide catalyst is advantageously noble-metal-free.Preferably, in the transition metal oxychalcogenide catalyst ABxOy, (x+y) < 2. The transitionmetal oxychalcogenide may thus be a non-stoichiometric compound. This ensures thatthere are some uncoordinated sites on the transition metals in the transition metal oxychalcogenide structure. These lattice vacancies may advantageously enhance the catalytic activity of the transition metal oxychalcogenide. In particularly preferred embodiments, 1.5 < (x+y) < 1.9, or 1.6 < (x+y) < 1.8. In preferred embodiments, for example, 0.3 < x < 0.7, or 0.4 < x < 0.6. In preferred embodiments, 1.0 < y < 1.4, or 1.1 < y < 1.3. The transition metal oxychalcogenide may comprise a mixture of amorphous (a) and crystalline (c) phases. The transition metal oxychalcogenide may be a disordered compound, in which the solid transition metal oxychalcogenide contains a mixture of amorphous and crystalline phases. The transition metal oxychalcogenide may be a layered oxychalcogenide, in which parallel layers of transition metal oxychalcogenide lattice are held together by weak van der Waal’s forces.The transition metal oxychalcogenide may be provided as a plurality of nanoparticles ornanosheets. The transition metal oxychalcogenide catalyst may take the form of nanosheets of transition metal oxychalcogenide, in which the catalyst nanosheets have a thickness of only a few nanometres, for example less than 5 nm, or 10 nm, or 20 nm. The nanosheets may have an average lateral size which is at least a few micrometres, for example at least 5 µm, or 10 µm, or 20 µm, up to a few hundred micrometres, for example up to 200 µm, or 300 µm. The nanosheets may for example have a thickness:lateral width aspect ratio of at least 1:500, or 1:750, or 1:1000. In a particularly preferred embodiment, the transition metal oxychalcogenide has theformula HfSxOy. Thus the third aspect may provide a use of a hafnium oxysulfide as acatalyst in an oxygen evolution reaction under acidic conditions. Particularly preferably the hafnium oxysulfide catalyst is an amorphous / crystalline (a / c) catalyst which contains a mixture of amorphous (a) and crystalline (c) phases. Preferably, in the hafnium oxysulfide catalyst HfSxOy, (x+y) < 2. In particularly preferred embodiments, 1.5 < (x+y) < 1.9, or 1.6 < (x+y) < 1.8. In preferred embodiments, for example, 0.3 < x < 0.7, or 0.4 < x < 0.6. In preferred embodiments, 1.0 < y < 1.4, or 1.1 < y < 1.3.In a particularly preferred embodiment, the third aspect may provide use of a hafniumoxysulfide as a catalyst in an oxygen evolution reaction under acidic conditions, thehafnium oxysulfide having the formula HfSxOy, in which 0.3 < x < 0.7 and 1.0 < y < 1.4. The present inventors expect that the advantageous catalytic performance of HfSxOy for OER in acidic media is not limited to hafnium oxysulfide, but is also likely to be exhibited byother transition metal oxychalcogenide compounds which physical and electrochemicalproperties similar to hafnium oxysulfide. Features of the invention discussed in relation to the other aspects of the disclosure mayapply equally to the use of the present aspect.Method of Water ElectrolysisAccording to a fourth aspect of the disclosure there is provided a method of waterelectrolysis, comprising the step of electrolysing water using a proton exchange membrane(PEM) electrolyser, in which a transition metal oxychalcogenide catalyses an oxygenevolution reaction under acidic conditions at an anode of the PEM electrolyser.The method may comprise the step of applying a potential difference between a cathode of the PEM electrolyser and the anode of the PEM electrolyser. Method of Manufacturing Nanoparticles or Nanosheets of a Transition Metal OxideAccording to a further aspect of the disclosure there is provided a method of manufacturingnanoparticles or nanosheets of a transition metal oxide, the method comprising the steps of: electrochemically exfoliating a transition metal dichalcogenide in an electrolyte to form exfoliated nanoparticles or nanosheets. The exfoliated nanoparticles or nanosheets areoxidised to form a transition metal oxide having the formula ABxOy, wherein A is a transitionmetal and B is a chalcogenide, and wherein 0 ≤ x < 2 and 0 < y < 2. The method may bea method of manufacturing an anode for an electrolyser, further comprising the step of depositing the transition metal oxide onto an anode. By using electrochemical exfoliation as a manufacturing method, transition metal oxide nanosheets can be formed with vacancies in the crystal structure. The method may comprise the step of electrochemically exfoliating the transition metal dichalcogenide in an electrolyte with a potential difference having a magnitude of between -2 V and -6 V, or between -2.5 V and -5.5 V, preferably between -3 V and -5 V. The optimalpotential difference for electrochemical exfoliation may vary depending on the transition metal being exfoliated. Electrochemical exfoliation of HfS2 with a potential difference greater in magnitude than–4 V led to the formation of a transition metal oxychalcogenide having the formula ABxOy,wherein A is a transition metal and B is a chalcogenide, and wherein 0 < x < 2 and 0 < y < 2. As described above, this transition metal oxychalcogenide performs very well as a catalyst for OER in acidic conditions. While good results could be achieved over a range of potential differences, and over a range of electrolyte concentrations, the present inventors have found that electrochemical exfoliation of HfS2 with a potential difference lower in magnitude than –4 V led to the formation of amorphous hafnium oxide HfOy, in which x = 0 and y < 2. While HfO2 thin film, which is typically synthesized by atomic layer deposition (ALD), has been widely used as a dielectric layer in electronic devices, HfOy nanosheets are not known in the prior art. The HfOy (y < 2) nanosheets that were synthesized in this way can be used as OER catalyst in acid (as described further below). However, the catalytic performance of HfOy is not as good as that of HfSxOy, due to the poor electrical conductivity and zero crystallinity of HfOy. For Ta-based catalysts, the method may comprise the step of electrochemically exfoliating the transition metal dichalcogenide in an electrolyte with a potential difference having amagnitude of between -2 V and -3 V, preferably between -2 V and -2.5 V, particularlypreferably a magnitude of -2 V. For Ti-based catalysts, the method may comprise the step of electrochemically exfoliating the transition metal dichalcogenide in an electrolyte with a potential difference having amagnitude of between -2 V and -4 V, preferably between -2 V and -3 V, particularlypreferably a magnitude of -2.5 V. For Nb-based catalysts, the method may comprise the step of electrochemically exfoliating the transition metal dichalcogenide in an electrolyte with a potential difference having amagnitude of between -2 V and -5 V, preferably between -2.5 V and -4 V, particularlypreferably a magnitude of -3.5 V. For V-based catalysts, the method may comprise the step of electrochemically exfoliating the transition metal dichalcogenide in an electrolyte with a potential difference having amagnitude of between -3 V and -5.5 V, preferably between -4 V and -5 V, particularlypreferably a magnitude of -4.5 V. For Zr-based catalysts, the method may comprise the step of electrochemically exfoliating the transition metal dichalcogenide in an electrolyte with a potential difference having amagnitude of between -2.5 V and -5 V, preferably between -3 V and -4.5 V, particularlypreferably a magnitude of -4 V. For Zr-based catalysts, the electrochemical exfoliation may preferably be performed at room temperature. For Hf-based catalysts, the method may comprise the step of electrochemically exfoliating the transition metal dichalcogenide in an electrolyte with a potential difference having amagnitude of between -3.5 V and -6 V, preferably between -4 V and -5.5 V, particularlypreferably a magnitude of -5 V. For Hf-based catalysts, the electrochemical exfoliation may preferably be performed at room temperature.The electrochemical exfoliation may be performed at room temperature (at 20 degreescentigrade). Alternatively, the electrochemical exfoliation may be thermally-assisted, by performing the exfoliation at a temperature above room temperature. To perform thermally- assisted electrochemical exfoliation, the electrolyte is heated up during the electrochemical exfoliation, preferably to a temperature above room temperature and below the boiling point of the electrolyte. The principle and the procedure of thermally-assisted electrochemical exfoliation is otherwise the same as that of non-thermally-assisted electrochemical exfoliation.For Ti-, Ta-, V- and Nb-based catalysts, the exfoliation is preferably performed at atemperature above room temperature.Thermally-assisted exfoliation may be performed at a temperature of at least 40 degreescentigrade, or 50 degrees centigrade, or 70 degrees centigrade. In preferred embodiments, thermally-assisted exfoliation may be performed at an electrolyte temperature of between 50 degrees centigrade and 95 degrees centigrade, or between 70 degrees centigrade and 80 degrees centigrade. The optimal temperature may depend on the transition metal compound being electrochemically exfoliated. Method of Manufacturing an AnodeAccording to a fifth aspect of the disclosure there is provided a method of manufacturing ananode for an electrolyser, the method comprising the steps of:electrochemically exfoliating a transition metal dichalcogenide in an electrolyte with apotential difference greater in magnitude than –2 V to form exfoliated nanoparticles or nanosheets. The exfoliated nanoparticles or nanosheets are oxidised to form a transitionmetal oxychalcogenide having the formula ABxOy, wherein A is a transition metal and B is achalcogenide, and wherein 0 < x < 2 and 0 < y < 2. The method may further comprise thestep of depositing the transition metal oxychalcogenide onto an anode.The method is preferably a method of manufacturing an anode for a PEM waterelectrolyser. The present inventors have found that electrochemical exfoliation of layered transition metal chalcogens (in particular those with relatively weak metal-chalcogen bond strengths) is a harsh process which results in the formation of defect-rich exfoliated nanosheets. While in many applications this would be a drawback, in the present case the inventors have found that the electrochemical exfoliation leads to the formation of chalcogen vacancies in the lattice structure of the transition metal dichalcogenide nanosheets. Theconsiderable loss of chalcogen atoms from the lattice gives exposes the neighboringtransition metal atoms, such that these unsaturated transition metal atoms are then readilyand naturally oxidized in water and air.This oxidation step does not occur in other exfoliation methods (such as chemical reductiveexfoliation, for example), as more gentle exfoliation methods do not create the vacancies in the lattice structure which are required to allow oxidation, and therefore enable formation ofthe transition metal oxychalcogenide from the exfoliated nanoparticles or nanosheets.The method may comprise the step of electrochemically exfoliating the transition metaldichalcogenide in an electrolyte with a potential difference having a magnitude of between -2 V and -6 V, or between -2.5 V and -5.5 V, preferably between -3 V and -5 V. The optimalpotential difference for electrochemical exfoliation may vary depending on the transition metal being exfoliated. For Hf-based catalysts, for example, the method may comprise the step of electrochemically exfoliating the transition metal dichalcogenide in an electrolyte with apotential difference having a magnitude of at least -3.5 V or preferably at least -4 V.Potential differences with lower magnitudes were found to lead to transition metal oxides(with y < 2), rather than oxychalcogenides, as the compound resulting from the exfoliationand oxidation steps, while potential differences having a magnitude of at least -3.5 V produced transition metal oxychalcogenides containing a mixture of amorphous and crystalline structures. For Ta-based catalysts, the method may comprise the step of electrochemically exfoliating the transition metal dichalcogenide in an electrolyte with a potential difference having amagnitude of between -2 V and -3 V, preferably between -2 V and -2.5 V, particularlypreferably a magnitude of -2 V. For Ti-based catalysts, the method may comprise the step of electrochemically exfoliating the transition metal dichalcogenide in an electrolyte with a potential difference having amagnitude of between -2 V and -4 V, preferably between -2 V and -3 V, particularlypreferably a magnitude of -2.5 V. For Nb-based catalysts, the method may comprise the step of electrochemically exfoliating the transition metal dichalcogenide in an electrolyte with a potential difference having amagnitude of between -2 V and -5 V, preferably between -2.5 V and -4 V, particularlypreferably a magnitude of -3.5 V. For Zr-based catalysts, the method may comprise the step of electrochemically exfoliating the transition metal dichalcogenide in an electrolyte with a potential difference having amagnitude of between -2.5 V and -5 V, preferably between -3 V and -4.5 V, particularlypreferably a magnitude of -4 V. For Zr-based catalysts, the electrochemical exfoliation may preferably be performed at room temperature. For Hf-based catalysts, the method may comprise the step of electrochemically exfoliating the transition metal dichalcogenide in an electrolyte with a potential difference having amagnitude of between -3.5 V and -6 V, preferably between -4 V and -5.5 V, particularlypreferably a magnitude of -5 V. For Hf-based catalysts, the electrochemical exfoliation may preferably be performed at room temperature.The electrochemical exfoliation may be performed at room temperature (at 20 degreescentigrade). Alternatively, the electrochemical exfoliation may be thermally-assisted, by performing the exfoliation at a temperature above room temperature. To perform thermally- assisted electrochemical exfoliation, the electrolyte is heated up during the electrochemical exfoliation, preferably to a temperature above room temperature and below the boiling point of the electrolyte. The principle and the procedure of thermally-assisted electrochemical exfoliation is otherwise the same as that of non-thermally-assisted electrochemical exfoliation.For Ti-, Ta-, and Nb-based catalysts, the exfoliation is preferably performed at atemperature above room temperature.Thermally-assisted exfoliation may be performed at a temperature of at least 40 degreescentigrade, or 50 degrees centigrade, or 70 degrees centigrade. In preferred embodiments, thermally-assisted exfoliation may be performed at an electrolyte temperature of between 50 degrees centigrade and 95 degrees centigrade, or between 70 degrees centigrade and 80 degrees centigrade. The optimal temperature may depend on the transition metal compound being electrochemically exfoliated. The electrolyte preferably comprises a solute and a solvent, and the solute preferablycomprises an ammonium cation. The ammonium cations in the solute may advantageouslyact as the intercalant that enables exfoliation. The anion contained in the solute may vary –for example the method has been found to work with solutes such as tetrabutylammonium(TBA) or tetrapropylammonium (TPA). The solvent may be for example propylenecarbonate (PC) or dimethylformamide (DMF).In one preferred example, the electrolyte is 0.01 M tetrabutylammonium (TBA)‒dimethylformamide (DMF). The oxidation of the exfoliated nanoparticles or nanosheets of the transition metaldichalcogenide may occur naturally during and after the electrochemical exfoliation, whenthe exfoliated nanosheets are in contact with air which contains water vapour and oxygen.The method may comprise the steps of rinsing, ultrasonicating and drying the transitionmetal oxychalcogenide. The method may comprise the step of depositing, loading or coating the transition metaloxychalcogenide onto an anode substrate. For example the transition metaloxychalcogenide may be deposited, loaded or coated onto an electrically-conductive anodesubstrate. The substrate may be, for example, a membrane. The catalyst may be coatedonto the substrate by, for example, coating the substrate in an ink containing the catalyst.In a preferred embodiment, the fifth aspect of the disclosure provides a method ofmanufacturing an anode for an electrolyser, the method comprising the steps of:electrochemically exfoliating a transition metal disulfide in an electrolyte with a potentialdifference greater in magnitude than -2 V to form exfoliated nanoparticles or nanosheets. The exfoliated nanoparticles or nanosheets are oxidised to form a transition metaloxysulfide having the formula ASxOy, wherein A is a transition metal and S is sulfur, andwherein 0 < x < 2 and 0 < y < 2. The method may further comprise the step of depositingthe transition metal oxysulfide onto an anode.In a particularly preferred embodiment, the fifth aspect of the disclosure provides a methodof manufacturing an anode for an electrolyser, the method comprising the steps of:electrochemically exfoliating hafnium disulfide in an electrolyte with a potential differencegreater in magnitude than -2 V to form exfoliated nanoparticles or nanosheets. Theexfoliated nanoparticles or nanosheets are oxidised to form a hafnium oxysulfide havingthe formula HfSxOy, wherein 0 < x < 2 and 0 < y < 2. The method may further comprise thestep of depositing the hafnium oxysulfide onto an anode.The method is preferably a method of manufacturing an anode according to the first aspectset out above. Features of the invention discussed in relation to the other aspects of theinvention, for example the compounds involved in the process, may apply equally to themethod of the present aspect.Catalyst-Containing InkAccording to a sixth aspect of the disclosure there is provided an ink for coating anelectrolyser electrode, the ink comprising a suspension of a transition metaloxychalcogenide catalyst having the formula ABxOy,wherein A is a transition metal and B is a chalcogenide, and wherein 0 < x < 2 and 0 < y < 2.The transition metal oxychalcogenide catalyst is preferably the transition metaloxychalcogenide catalyst described above in relation to the preceding aspects.Providing the catalyst as a suspension in a catalyst ink may advantageously provide aconvenient way of depositing the catalyst onto anode substrates which are chemicallystable in acidic environments. To form such an ink, the transition metal oxychalcogenidecatalyst, which is preferably in the form of nanosheets or nanoparticles as described above, may be dispersed into a solvent to form a suspension. The ink may then be coated onto anode substrates using techniques which are standard in the art. Catalytic inks are known in the art for PEM electrolyser catalysts such as IrO2and Pt, and a variety of solvents suitable for use in catalyst-bearing inks are known in the art. Features described above in relation to any one aspect of the invention are equally applicable to every other aspect of the invention. Brief Description of the Drawings The invention will now be described, by way of example only, by reference to the following figures, in which:Figure 1a is a scanning electron microscopy image of an electrochemically exfoliated a / c-HfSxOy nanosheet with an inset photograph of a suspension of nanosheets;Figure 1b is an atomic force microscope image of an exfoliated Hf-based nanosheet withthe corresponding height profile showing a thickness of ~2.3 nm;Figure 1c shows Raman spectra of c-HfS2, a / c-HfSxOy and a-HfOx;Figures 1d‒f are HAADF-STEM images of a / c-HfSxOy (Figure 1d), c-HfS2 (Figure 1e) anda-HfOx (Figure 1f) for comparison, with inset fast Fourier transformation (FFT) patterns;Figures 2a-f show X-ray photoelectron spectroscopy (XPS) for transition-metal orbitals indifferent transition metals;Figures 2g and 2h show representative XPS for S 2p and Se 3d;Figure 2i shows Hf L3-edge EXAFS spectra of HfS2, HfSxOy with different stoichiometricratios and HfO2;Figure 3a shows polarization curves for a / c-HfSxOy, a-HfOx, c-HfS2 and IrO2after iR correction, for OER activity in 0.5M H2SO4;Figure 3b shows Tafel slopes acquired from Figure 3a;Figure 3c shows Nyquist plots for a / c-HfSxOy, a-HfOx, c-HfS2;Figure 3d shows the results of a chronopotentiometric test of the OER activity of a / c-HfSxOyand a-HfOx over 24 hours at j = 50 mA cm‒2;Figures 3e and 3f show a comparison of mass activity of a / c-HfSxOy at pH = 0 with othernoble-metal-free low-dimensional OER catalysts in basic media (pH = 13 or 14) (Figure 3e)and stability (Δη versus charge density) in acid media for nanosheet catalysts (Figure 3f);Figure 4a is a schematic diagram illustrating a proton exchange membrane electrolyzer forwater splitting;Figures 4b, 4c, 4d show IV curves taken from electrolysers with MSxOy (M = Hf, Zr, Ta) asthe anode for OER and 1T-MoS2 as the cathode for HER ;Figure 5a is an optical microscope image of a device with exfoliated a / c-HfSxOy nanosheetas the channel material and two Cr / Au (5 nm / 50 nm) electrodes;Figure 5b illustrates output characteristic curves of the electrical conductivity of c-HfS2, a / c-HfSxOy and a-HfOx;Figure 6a is an ADF-STEM image of an exfoliated a / c-HfSxOy flake supported by a C filmcovered Cu grid;Figures 6b‒d are EDX maps of Hf (Figure 6b), S (Figure 6c) and O (Figure 6d);Figure 6e is an EDX spectrum of a / c-HfSxOy;Figure 7a is an ADF-STEM image and Figure 7b is a corresponding FFT pattern of a a / c-HfSxOy nanosheet that was exfoliated at ‒5.0 V;Figures 8a-f are ADF-STEM images of the structure of a / c-HfSxOy;Figure 9 illustrates XANES curves of c-HfS2, a / c-HfSxOy and a-HfOx, with an inset closeupview showing the positions near the Hf L3 edge; Figures 10a-c are polarisation slopes, and Figures 10d-f are corresponding Tafel slopes, ofa / c-HfSxOy that is supported by glassy carbon (Figures 10a, 10d), carbon cloth (Figures10b, 10e) and carbon paper (Figures 10c, 10f);Figure 11a is a cyclic voltammetry (CV) curve for c-HfS2;Figure 11b is a CV curve for a-HfOx;Figure 11c is a CV curve for a / c-HfSxOy;Figure 11d plots values derived from (Figures 11a‒c), showing half of the current differences at +0.21 V versus RHE as a function of scan rate;Figure 12a shows the Raman spectrum of 1T-MoS2;Figure 12b is a cathodic polarization curve of 1T-MoS2;Figure 12c is the Tafel slope acquired from Figure 12b;Figure 12d is a Nyquist plot for of 1T-MoS2 fitted using an equivalent circuit.Figures 13a and 13b illustrate anodic polarization curves (Figure 13a) andchronopotentiometric stability measurements (Figure 13b) measured for Hf-based catalysts electrochemically exfoliated at different electrical potentials; Figures 14a and 14b illustrate anodic polarization curves (Figure 14a) and chronopotentiometric stability measurements (Figure 14b) measured for Zr-based catalysts electrochemically exfoliated at different electrical potentials; Figures 15a and 15b illustrate anodic polarization curves (Figure 15a) and chronopotentiometric stability measurements (Figure 15b) measured for Ti-based catalysts electrochemically exfoliated at different temperatures; Figures 16a and 16b illustrate anodic polarization curves (Figure 16a) and chronopotentiometric stability measurements (Figure 16b) measured for Ta-based catalysts electrochemically exfoliated at different temperatures; Figures 17a and 17b illustrate anodic polarization curves (Figure 17a) and chronopotentiometric stability measurements (Figure 17b) measured for Nb-based catalysts electrochemically exfoliated at different temperatures. Experimental Examples In order to prepare a transition metal oxychalcogenide for use as a catalyst, HfS2was exfoliated into thin layers by electrochemical exfoliation in which application of negative potential on HfS2crystals in 0.01 M tetrabutylammonium (TBA)‒dimethylformamide (DMF) leads to intercalation of ammonium ions together with the DMF molecules into HfS2inducing expansion and exfoliation of layers. Figures 1a-1f illustrate microscopic, spectroscopic, and atomic characterization of such electrochemically-exfoliated Hf-based nanosheets.Figure 1a is a scanning electron microscopy image of an electrochemically exfoliated a / c-HfSxOy nanosheet with an inset photograph of a suspension of nanosheets. The sheets arelaterally large with dimensions of tens of microns. The nanosheets can be exfoliated down to a few layers with a typical thickness being ~4.6± 2.3 nm. Figure 1b is an atomic force microscope image of an exfoliated Hf-basednanosheet with the corresponding height profile showing a thickness of ~2.3 nm;Figure 1c shows Raman spectra of c-HfS2, a / c-HfSxOy and a-HfOx. Raman spectroscopy ofcrystalline HfS2 shows peaks at 261 and 335 cm‒1, which correspond to the Eg and A1gvibration modes. These peaks are substantially quenched in the a / c-HfSxOy samples dueto the disorder in the samples and are completely absent in the amorphous HfOx. The peaks are strongly suppressed due to disorder in the a / c-HfSxOy sample.Figures 1d‒f are HAADF-STEM images of a / c-HfSxOy (Figure 1d), c-HfS2 (Figure 1e) anda-HfOx (Figure 1f) for comparison, with inset fast Fourier transformation (FFT) patterns.The fast Fourier transformation (FFT) patterns shown in insets indicate that a / c-HfSxOy contains features of both amorphous and crystalline structures.Based on the electrical measurements shown in Figure 5b, the conductivity of a / c- HfSxOywas found be >2,000 S m‒1, suggesting that the material retains sufficient conductivity for catalysis. The electrochemical exfoliation process is harsh and leads to the formation of sulfur vacancies so that non-stoichiometric HfS2‒xis formed at low S vacancy concentrations and a highly disordered structure is obtained at high S vacancy concentrations. This is comparable to what happens in MoS2where S vacancy concentration of >1015cm‒2leadsto highly disordered structure. The present inventors have found that the disordered HfS2‒xwith high sulfur vacancy concentration readily oxidises (see chemical analysis using electron dispersive spectroscopy, EDX in Figure 6) under ambient conditions to form a mixed amorphous and crystalline structure, as shown in the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and the correspondingdiffraction pattern in Figure 1d. For comparison, Figures 1e and 1f respectively showatomic resolution images of mechanically exfoliated crystalline HfS2 and amorphousHfOx synthesized by progressive reduction of the HfS2 until complete removal of S isaccompanied by oxidation and amorphization. In Figure 1d, it can be seen that Hf atomsare aggregated as indicated by their higher contrast (also shown in Figure 7).Figures 2a-i show X-ray spectroscopic characterisation. Figures 2a‒f show X-rayphotoelectron spectroscopy (XPS) for transition-metal orbitals, where the fitting for each indicate the chemical states. A typical O1s core-level spectrum is also shown in Figure 2f. Figures 2a-2f respectively show XPS data obtained from electrochemically exfoliated samples of HfSxOy, ZrSxOy TiSxOy TaSxOy NbSxOy and VSexOy.Figures 2g and 2h show representative XPS for S 2p and Se 3d.Figure 2i shows Hf L3-edge EXAFS spectra of HfS2, HfSxOy with different stoichiometricratios and HfO2. The X-ray spectroscopic results demonstrate that the electrochemical exfoliation leads to oxidation of the transition metal dichalcogenides and formation of chalcogen vacancies. X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical states of the a / c-HfSxOy. Hf in these samples resides in the dominant +4 state (4f5 / 2: 19.7 eV, 4f7 / 2: 18.0 eV), accompanied by +2 (4f5 / 2: 17.6 eV, 4f7 / 2: 15.9 eV) and 0 (4f5 / 2: 15.8 eV, 4f7 / 2:14.1 eV) states. The Hf2+ and Hf0 states originate from formation of mono- and double-Svacancies, respectively, at neighboring sites. The Hf0 peaks reveal the presence of metallicHf in a / c-HfSxOy. X-ray absorption near edge structure (XANES) measurements show that Hf in a / c-HfSxOyexhibits lower oxidation state compared to those in the c-HfS2 and the a-HfOx (Figure 9). The extended X-ray absorption fine structure (EXAFS) results in Figure 2ishow that HfS2contains only Hf‒S bonds while a-HfOxcontains Hf‒O with small portion of Hf‒Hf bonds. In contrast, Hf‒O, Hf‒S and Hf‒Hf bonds are observed in the a / c-HfSxOysamples. Figure 2f demonstrates that after exfoliation, the starting material of VSe2is oxidized andforms VSexOy. The stability of VSexOy for OER was found to be lower than that of HfSxOy,ZrSxOy and TaSxOy. However, VSexOy is more stable than the starting material of VSe2.Based on the coordination number ratios (set out below in Tables 1 and 2), the chemicalformulae for the samples are estimated to be HfS0.54O1.17 and HfO1.75. Table 1 | Fitting parameters for EXAFS of a / c-HfSxOy. N: Coordination number, R: bond length, σ2: Debye-Waller factor, namely, mean square relative displacement owing to atom vibrations. The amplitude factor (S02) was fixed at 1. The energy correction (ΔE0) was varied as a global fit parameter, returning a value of (‒7 ± 2) eV. The data range was 1.0 ≤ R ≤ 6.0 Å, and the misfit (R-factor) was 1.5%. Table 2 | Fitting parameters for EXAFS of a-HfOy. The S02was fixed at 1. The ΔE0was varied as a global fit parameter, returning a value of (‒ 7.9 ± 3) eV. The data range was 1.0 ≤ R ≤ 6.0 Å, and the R-factor was 1.8%. Figures 3a-f characterise the OER activity of a / c-HfSxOy in 0.5 M H2SO4.OER activity was measured in 0.5 M H2SO4(pH = 0) using a rotating disc electrode (RDE) as set out in McCrory, C. C. L., Jung, S., Peters, J. C. & Jaramillo, T. F. Benchmarking heterogeneous electrocatalysts for the oxygen evolution reaction. J. Am. Chem. Soc.135, 16977–16987 (2013).Figure 3a shows polarization curves for a / c-HfSxOy, a-HfOx, c-HfS2 and IrO2 after iRcorrection, for OER activity in 0.5M H2SO4. The polarization curves [current density versus applied potential (versus a reversible hydrogen electrode, RHE)] of a / c-HfSxOy nanosheetsalong with c-HfS2 and a-HfOx are shown in Figure 3a. Catalytic activity of IrO2 forcomparison is also plotted in Fig.3a. a / c-HfSxOycatalysts show the lowest overpotential and highest current density compared to other Hf based materials. A / c-HfSxOy nanosheets exhibit practical current density of >1,000 mA cm‒2at potentials of <1.8 V versus RHE. The reproducibility of the results is demonstrated via measurements of OER on different substrates (Figure 10). The polarization curves have been iR correctedand normalized to the electrochemical surface area (ECSA) obtained by cyclic voltammetry(CV) at different scan rates (Figure 11). According to measurements on eight different samples, the overpotential (η) of a / c-HfSxOy catalysts was found to be 295.6 ± 8.7 mV versus RHE at a current density of 10 mA cm‒2. The mass activity was measured to be 102,959 ± 7416 A g‒1at η = 0.5 V versus RHE.Figure 3b shows Tafel slopes acquired from Figure 3a, showing that reaction kinetics areimproved for a / c-HfSxOy. Tafel slopes of 63.4 ± 4.5 mV dec‒1 suggest efficient kinetics ofthe OER. These metrics demonstrate that a / c-HfSxOy nanosheets are good catalysts for OER and their performance can be attributed to combination of uncoordinated Hf atoms in the amorphous regions and improved conductivity arising from the crystalline areas (Figure 5).Figure 3c shows Nyquist plots for a / c-HfSxOy, a-HfOx, c-HfS2 showing the lowestelectrochemical impedance for a / c-HfSxOy.The Nyquist plots in Figure 3c confirm lower impedance of a / c-HfSxOy catalysts comparedto a-HfOxand c-HfS2. The lower activity of c-HfS2stems from the fact that Hf atoms are fully coordinated making it unfavorable for adsorption of O-containing intermediate species (i.e., *OH, *O and *OOH).Figure 3d shows the results of a chronopotentiometric test of the OER activity of a / c-HfSxOyand a-HfOx over 24 hours at j = 50 mA cm‒2, showing minimal changes in η. The stability of a / c-HfSxOy (and HfOx for comparison) was monitored by measuring the change in overpotential required to maintain a current density of 50 mA cm‒2with time for 24 hours in pH = 0 media. Only a small change in overpotential (Δη = 14 ± 3 mV) wasmeasured after 24 hours (Figure 3d), suggesting that a / c-HfSxOy catalysts are stable andable to survive in highly acidic media. The chemical states of Hf after the cycling were measured using XPS and found to be remain nearly unchanged.To put these results in context with other reports, the mass activity as a function ofoverpotential is compared in Figure 3e. More specifically, the mass activity of a / c-HfSxOy atpH = 0 is compared with mass activity of noble-metal-free low-dimensional catalysts from the literature for alkaline media due to lack of data for OER in acidic media. The massactivity of a / c-HfSxOy in acid is among the highest reported. In Figure 3f, the stability of a / c-HfSxOy is compared in terms of change in overpotential (Δη) versus charge density (current density × time) to demonstrate how much the catalytic performance is affected by electrons transferred for the oxygen evolution and is closely related to the lifespan of catalysts in practical applications (Table 3). The stability of the a / c-HfSxOy is the best when compared with similar materials (Table 4).

[0002] Table 3 | Literature survey of mass activities of noble-metal-free low-dimensional electrocatalysts. For each system, the best result is presented. LDHs represent layered double hydroxides. HHTP is 2,3,6,7,10,11- hexahydroxytriphenylene. The prefixes indicate the electronic or elemental doping. The slash means the heterostructure. Table 4 | Literature survey of chronopotentiometric stability test results of noble- metal-free low-dimensional electrocatalysts for OER in 0.5 M H2SO4. The prefixes indicate the phases of the materials. The elements before the slashes show the functionalization or doping agents. PEM Water Electrolyser In order to confirm the effectiveness of such electrochemically-exfoliated nanosheets as catalysts for OER in acidic media, a working PEM water electrolyser was tested with MSxOyanodes and earth-abundant 2H-NbS2‒xor 1T-MoS2as the cathodes). Figures 4a-4d illustrate PEM water electrolysis based on TMD-based nanosheet HER andOER catalysts.Figure 4a is a schematic diagram illustrating a proton exchange membrane electrolyzer forwater splitting. HER properties of 2H-NbS2‒x (Yang, J. et al. Ultrahigh-current-density niobium disulfide catalysts for hydrogen evolution. Nat. Mater.18, 1309–1314 (2019)) and 1T-MoS2 (Voiry, D. et al. Conducting MoS2 nanosheets as catalysts for hydrogen evolution reaction. Nano Lett.13, 6222–6227 (2013)) have previously been demonstrated. The reduced coordination of Nb in 2H-NbS2‒x is confirmed by XPS and HAADF STEM, which makes it active for HER.Figure 4b, 4c and 4d respectively show IV curves taken from electrolysers with MSxOy (M =Hf, Zr, Ta) as the anode for OER and 1T-MoS2 as the cathode for HER, showing the stability of the MSxOy:1T-MoS2 cell for operation over 100 hours.This testing confirmed that electrolysers with an MSxOy anode (M = Hf, Zr, Ta) exhibit goodstability without the use of expensive catalysts for water splitting. Figures 5-12Figures 5 and 5b illustrate the electrical conductivity of a / c-HfSxOy. Figure 5a is an opticalmicroscope image of a device with exfoliated nanosheet as the channel material and twoCr / Au (5 nm / 50 nm) electrodes. Figure 5b shows output characteristic curves of the c-HfS2,a / c-HfSxOyand a-HfOx. Their conductivities are 771.6, 2003.4 and 92.8 S m‒1, respectively.Figures 6a-c show EDX data from a / c-HfSxOy. Figure 6a is an ADF-STEM image of anexfoliated a / c-HfSxOy flake supported by a C film covered Cu grid. Figures 6b‒d are EDXmaps of Hf (Figure 6b), S (Figure 6c) and O (Figure 6d). Figure 6e is an EDX spectrum ofa / c-HfSxOy.Figures 7a and 7b show the effect of cathode potential on amorphization duringelectrochemical synthesis. Figure 7a is an ADF-STEM image and Figure 7b is acorresponding FFT pattern of a nanosheet that was exfoliated at ‒5.0 V. The amorphous- to-crystalline phase areal ratio is lower compared to that in Fig 1e. The complex diffraction points in Figure 7b reveal various lattice directions arising from grain rotations, whichdemonstrate its polycrystallinity.Figures 8a-f shows effects of electron beam on the structure of a / c-HfSxOy. Figures 8a-f areADF-STEM images with a time interval of 30 s each. The negligible change excludes the possibility of amorphization induced by the electron beam.Figure 9 shows the evolution of oxidation states with structural amorphization in e form ofXANES curves of c-HfS2, a / c-HfSxOyand a-HfOx. The inset is a closeup view showing the positions near the Hf L3 edge.Figures 10a-f show the reproducibility of OER measurements. Figures 10a-c arepolarisation slopes, and Figures 10d-f are corresponding Tafel slopes, of a / c-HfSxOy that issupported by glassy carbon (Figures 10a, 10d), carbon cloth (Figures 10b, 10e) and carbonpaper (Figures 10c, 10f). Multiple samples were tested with glassy carbon as the support,which demonstrate variations in the electrocatalytic performance. The grey box in Figure 10d shows where the Tafel slopes are obtained.Figures 11a-d show investigation of the catalyst Cdl. Figures 11a‒c are CV curves of c-HfS2 (Figure 11a), a-HfOx (Figure 11b) and a / c-HfSxOy (Figure 11c) measured in the potential window of 160‒260 mV versus RHE. The scan rates vary from 5 to 120 mV s‒1.Figure 11d shows plots derived from Figures 11a‒c, showing half of the current differencesat +0.21 V versus RHE as a function of scan rate. The double-layer capacitances (Cdl) ofthe three corresponding catalysts are 1.2 mF cm-2, 3.9 mF cm-2, and 10.5 mF cm-2. Foreach catalyst, the scan rates are 5, 20, 40, 60, 80, 100 and 120 mV s–1. The linear fitsindicate the Cdlof the catalysts. Figure 12a to 12d show the characterisation of 1T-MoS2, which is usable as a cathodematerial for HER in water electrolysis. Figure 12a shows the Raman spectrum of 1T-MoS2.Figure 12b is a cathodic polarization curve. Figure 12c is the Tafel slope acquired fromFigure 12b. Figure 12d is a Nyquist plot fitted using an equivalent circuit.Materials Synthesis The following method was used to synthesise the materials characterised in the Figures. a / c-HfSxOyand 2H-NbS2‒xnanosheets were synthesized by electrochemical exfoliation using electrochemical workstation (CHI 760E, Shanghai CH Instrument or ModuLab XM, Solartron Analytical, Ametek Scientific Instruments). Bulk crystals of 1T-HfS2or 2H-NbS2 (HQ graphene) acted as the cathode, a carbon rod was chosen as the anode, and the electrolyte was 0.01 M tetrabutylammonium (TBA)‒dimethylformamide (DMF). The organic cations along with the DMF molecules co-intercalate into layered materials under a negative potential, so that cathodic reduction and exfoliation takes place simultaneously. Mixture of amorphous and crystalline structures are obtained under potentials larger than ‒3.5 V. The amorphization degree increases as the potential decreases. The a / c-HfSxOyshowing optimized OER activities were synthesized at ‒4.0 V. Lower potentials resulted in a-HfOx. The 2H-NbS2‒x was produced at ‒2.8 V. The 1T-MoS2 was produced by chemical exfoliation with n-butyl lithium dissolved in hexane as the intercalation agent, utilizing theprotocol reported previously in Eda, G. et al. Photoluminescence from chemically exfoliatedMoS2. Nano Lett.11, 5111–5116 (2011). To rinse the products, the suspension was rested until the nanosheets completely sedimented, after which the TBA‒DMF or n-butyl lithium‒ hexane was removed and ethanol poured in, followed by gentle shaking of the vial to disperse the nanosheets. The above steps were repeated several times. Then the samples were dispersed by ultrasonication for 20 s, and were dried under ambient conditions before characterization and measurements. Characterization The nanosheets were mechanically exfoliated (c-HfS2) or electrochemically synthesizedand drop-casted (a / c-HfSxOy and a-HfOx) on SiO2 (300 nm) / Si for optical microscopy,scanning electron microscopy, atomic force microscopy, Raman spectroscopy, XANES and EXAFS, on copper grids for ADF-STEM and EDX, and on glassy carbon for XPS. The imaging took place at room temperature using an optical microscope (Olympus DP74), a scanning electron microscope (FEI Verios3, 2.0 kV accelerating voltage), and an ADF- STEM (JEOL ARM-200F STEM) with a CEOS aberration corrector (80 kV accelerating voltage). The Raman spectra were recorded by a confocal Raman microscope (alpha300 R, WITec). A 1 mW, 532 nm (2.33 eV) diode laser was employed for excitation, the spot size of which is 1 μm. The acquisition time was 10 s. The flake thickness was examined by an AFM (Bruker Dimension FastScan) in tapping mode. The XPS data were recorded in ultrahigh vacuum (base pressure of 1 × 10–10mbar). The photon energy was set at 630 eV to probe the core-level spectra that were collected at the normal emission using a VG Scienta R4000 analyzer, and the data were normalized by the photon current. The XANES and EXAFS data were recorded at the Singapore Synchrotron Light Source using a Si(111) double-crystal monochromator (700 MeV accelerating voltage, 200 mA beam current). Electrochemical measurements The catalytic performance was measured in N2-purged aqueous solutions of 0.5 M H2SO4 (Sigma-Aldrich). A three-electrode system was employed (CHI 760E, Shanghai CH Instrument or ModuLab XM, Solartron Analytical, Ametek Scientific Instruments). The catalysts were rinsed using ethanol, after which 20 µL 5% Nafion-117 solution (Sigma- Aldrich) was added into 1 mL of the suspension. Once well dispersed, the mixture was drop-cast onto glassy carbon surface (0.3 cm diameter) of a RDE. The mass loadingdensity of the catalysts was 0.1 mg cmgeo−2. The RDE served as the working electrode at arotation speed of 1,600 r.p.m. A carbon rod and saturated calomel electrode (SCE) were chosen as the counter and reference electrodes, respectively. All the potentials measured against SCE were calibrated to RHE by using the following equation: E(RHE) = E(SCE) + 0.241 V + 0.0591 × pH, and were iR-compensated. To obtain the polarization curves, linear sweep voltammetry (LSV) was conducted under quasi-equilibrium conditions at a scan rate (ks) of 5 mV s−1. ECSA is the ratio of Cdl to CS ( Cdl / Cs , where Cs represents the specificcapacitance). CV was carried out at different ks (5‒120 mV s−1), then Cdl was estimated byderiving the slope value from the linear regression between half of the current densityvariation and the scan rate (i.e., Δjgeo / 2 versus ks). Cs is expressed as ACV / [2 × ks × ΔV× Ageo ], where ACV is the area of the CV curve at a certain ks, ΔV is the potential window,Ageo is the geometric area of the catalyst on the glassy carbon. The stability was investigated by measuring the potential versus RHE while keeping the current density at 50 mA cm‒2. The EIS was performed by sweeping the frequency in the range of 105‒10‒2Hz with 5 mV voltage amplitude. Figures 13-17 Figures 13a, 14a, 15a, 16a and 17a illustrate anodic polarization curves measured fortransition metal oxysulfide catalysts containing different transition metals Hf, Zr, Ti, Ta andNb. Figures 13b, 14b, 15b, 16b and 17b show chronopotentiometric stabilitymeasurements taken for the same catalysts. The hafnium oxysulfide catalysts studied in Figures 13a and 13b were prepared by electrochemical exfoliation at room temperature, as described above, but under the application of different magnitudes of electrical potential.The zirconium oxysulfide catalysts studied in Figures 14a and 14b were prepared using thesame electrochemical exfoliation technique as was used for hafnium in Figures 13a and13b, at room temperature, using different electrolyte concentrations (TBA in DMF).The titanium-based catalysts of Figures 15a and 15b, the tantalum-based catalysts of Figures 16a and 16b, and the niobium-based catalysts of Figures 17a and 17b were prepared by electrochemical exfoliation of their respective disulfides using the sametechnique described above, but with the exfoliation being carried out at different elevatedtemperatures.For Ti-, Ta-, Nb- and V-based catalysts, the electrochemical exfoliation of the bulk crystalswas conducted while the electrolyte is heated up to temperatures above room temperature.A hotplate equipped with a thermocouple is used to achieve this thermally-assisted electrochemical exfoliation.In the same way that is described above for Hf-based compounds, the exfoliation andsubsequent natural oxidation which occurs to the exfoliated nanoparticles means that theZr-, Ti-, Ta-, and Nb- based catalysts formed by the exfoliation process are also oxysulfideswith the formula ABxOy, wherein A is Zr, Ti, Ta, Nb or V and B is sulfur, and wherein 0 < x< 2 and 0 < y < 2, though the precise stoichiometry of these samples has not yet been characterised. The inventors have found that all of these oxysulfide catalysts can work to catalyse OER in acid, but their catalytic performances vary. According to the polarization curves in Figures 13a, 14a, 15a, 16a and 17a, the performance trend for the effectiveness of the tested catalysts at catalysing the oxygenevolution reaction in acidic conditions is expected to be Hf ≈ Zr > Ti ≈ Ta > Nb.Vanadium oxyselenide catalyst was also prepared by thermally-assisted exfoliation of VSe2. The inventors investigated the best process conditions for the electrochemical exfoliation of a variety of transition metal sulfides into transition metal oxychalcogenides. Table 5 sets out the preferred conditions found by the inventors for the applied potential, thetemperature and the concentration of TBA in DMF are specified for each transition metal. As these parameters affect one another, these conditions can be varied while still producing exfoliated catalyst nanosheets suitable for catalysing OER in acidic conditions, and the values in Table 5 are representative examples only.Table 5. Optimized conditions for electrochemical exfoliation of catalysts This project received support from the Engineering and Physical Sciences ResearchCouncil (EPSRC grant no. EP / V012932 / 1). The inventors thank the support from NationalUniversity of Singapore (grants no. E-141-00-0003-03, A-0008379-00-00) and Guangdong Province Natural Science Foundation (grant no.2019A1515011007).

Claims

Claims 1. A water-electrolyser anode for a proton exchange membrane (PEM) waterelectrolyser, the anode comprising: atransition metal oxychalcogenide catalyst having the formula ABxOy,wherein A is a transition metal and B is a chalcogenide, and wherein 0 < x < 2 and 0 < y < 2.

2. The water-electrolyser anode of claim 1, in which A is one of Hf, Zr, Ta, Ti, Nb or V.

3. The water-electrolyser anode of claim 1 or 2, in which B is one of S, Se or Te.

4. The water-electrolyser anode of claim 1, 2 or 3, in which the transition metaloxychalcogenide is a transition metal oxysulfide, in which the chalcogen B is sulfur,S.

5. The water-electrolyser anode of any preceding claim, in which (x+y) < 2, preferablyin which 1.5 < (x+y) < 1.

9.

6. The water-electrolyser anode of any preceding claim, in which 0.3 < x < 0.

77. The water-electrolyser anode of any preceding claim, in which 1.0 < y < 1.

48. The water-electrolyser anode of any preceding claim, in which the transition metaloxychalcogenide is a non-stoichiometric compound.

9. The water-electrolyser anode of any preceding claim, in which the transition metaloxychalcogenide is a layered oxychalcogenide.

10. The water-electrolyser anode of any preceding claim, in which the anode comprisesa plurality of nanosheets of transition metal oxychalcogenide.

11. The water-electrolyser anode of any preceding claim, in which the transition metaloxychalcogenide comprises a mixture of amorphous and crystalline phases.

12. The water-electrolyser anode of any preceding claim, in which the transition metaloxychalcogenide has the formula ZrSxOy, or in which the transition metaloxychalcogenide has the formula TaSxOy, or in which the transition metal oxychalcogenide has the formula TiSxOy, or in which the transition metal oxychalcogenide has the formula NbSxOy, or in which the transition metal oxychalcogenide has the formula VSexOy .

13. The water-electrolyser anode of any of claims 1 to 11, in which the transition metaloxychalcogenide has the formula HfSxOy.

14. The water-electrolyser anode of any preceding claim, in which the anode comprisesa substrate, the transition metal oxychalcogenide catalyst being loaded or coatedonto the substrate.

15. The water-electrolyser anode of claim 14, in which the substrate is a catalyst coatedmembrane, comprising a membrane coated in the transition metal oxychalcogenide.

16. Use of a transition metal oxychalcogenide as a catalyst in an oxygen evolutionreaction under acidic conditions, in which the transition metal oxychalcogenide has the formula ABxOy,wherein A is a transition metal and B is a chalcogenide, and wherein 0 < x < 2 and 0 < y < 2.

17. Use of a transition metal oxychalcogenide as a catalyst according to claim 16, inwhich the transition metal oxychalcogenide is on an anode of a PEM electrolyser.

18. A proton exchange membrane (PEM) water electrolyser comprising:an anode electrode comprising a transition metal oxychalcogenide having theformula ABxOy, wherein A is a transition metal and B is a chalcogenide, and wherein 0 < x < 2 and 0 < y < 2.

19. A method of water electrolysis, comprising the step of electrolysing water using aproton exchange membrane (PEM) electrolyser, in which a transition metal oxychalcogenide catalyses an oxygen evolution reaction under acidic conditions atan anode of the PEM electrolyser.

20. A method of manufacturing an anode for an electrolyser, the method comprising thesteps of: electrochemically exfoliating a transition metal dichalcogenide in an electrolyte with apotential difference greater in magnitude than -2 V to form nanoparticles ornanosheets of the transition metal dichalcogenide which are oxidised to form a transition metal oxychalcogenide having the formula ABxOy,wherein A is a transition metal and B is a chalcogenide, and wherein 0 < x < 2 and 0 < y < 2; and depositing the transition metal oxychalcogenide onto an anode substrate.

21. The method of claim 20, comprising the step of electrochemically exfoliating thetransition metal dichalcogenide in an electrolyte with a potential difference magnitude of between -2 V and -6 V, or between -2.5 V and -5.5 V, preferablybetween -3.5 V and -5 V.

22. The method of claim 20 or 21, in which the electrolyte comprises a solute and asolvent, and in which the solute comprises an ammonium cation.

23. The method of claim 22, in which the solute is tetrabutylammonium (TBA) ortetrapropylammonium (TPA), and / or in which the solvent is propylene carbonate (PC) or dimethylformamide (DMF).

24. The method of any of claims 20 to 23, in which the electrolyte is heated such thatthe electrochemical exfoliation is thermally-assisted electrochemical exfoliation.

25. The method of any of claims 20 to 24, comprising the steps of rinsing,ultrasonicating and drying the transition metal oxychalcogenide.

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