Catalyst-coated polymer electrolyte membranes and methods for their manufacture

Abstract

A method for the manufacture of catalyst-coated polymer electrolyte membranes (CCMs) for water electrolysis is described. The CCMs may comprise a proton exchange membrane (PEM) or an anion exchange me

Description

Field of the Invention This invention relates to catalyst-coated polymer electrolyte membranes for water electrolysis and to methods for their manufacture. Background Catalyst-coated polymer electrolyte membranes (CCMs) are a key component found in polymer electrolyte membrane water electrolysers. Such CCMs comprise a solid electrolyte membrane, such as a proton exchange membrane (PEM) or an anion exchange membrane (AEM), with an anode catalyst layer and / or a cathode catalyst layer applied to a face of the membrane, the anode catalyst layer and cathode catalyst layer being applied to opposite faces of the membrane. For water electrolyser applications, hydrogen evolution reaction (HER) catalysts are used in such cathode catalyst layers, for example HER catalysts comprising platinum, such as platinum on a carbon support. Oxygen evolution reaction (OER) catalysts are utilised in electrolyser anode catalyst layers. For PEM water electrolyser applications, suitable OER catalysts comprise iridium and / or ruthenium, for example oxides of iridium, or oxides containing both iridium and ruthenium. For AEM water electrolyser applications, non-platinum group metal OER catalysts may also be used, such as alloys and oxides of nickel, cobalt, iron, and copper. Separate seal material layers, typically formed from non-ion conducting polymers, may be positioned around the edge region of a COM, for example on exposed surfaces of the polymer electrolyte membrane where no electrocatalyst is present (but will also often overlap on to the edge of the electrocatalyst layer) to provide a seal to prevent escape of reactant and product gases, to reinforce and strengthen the edge of the CCM and provide a suitable surface for supporting subsequent components such as sub-gaskets. An adhesive layer may be present on one or both surfaces of the seal material layer. CCMs may be incorporated into a membrane electrode assembly (MEA), which is essentially composed of five layers. The central layer is the polymer electrolyte membrane. On either side of the polymer electrolyte membrane there is an electrocatalyst layer, containing an electrocatalyst designed for the specific electrolytic reaction. Finally, adjacent to each electrocatalyst layer there is a transport layer, the composition and properties of which depend on the final MEA application and stack configuration. Such layers allow the reactants to reach the electrocatalyst layer and products to leave. It is known to form CCMs by sequential deposition of layers. EP2774203B1 describes a method comprising the steps of a) preparing a first catalyst layer on a supporting substrate; b) coating the first catalyst layer with an ionomer dispersion to form an ionomer membrane in contact with the first catalyst layer; and c) applying a second catalyst layer on top of the ionomer membrane. In the Examples, a roll of Pt-alloy cathode electrode on a carrier substrate is coupled and made to adhere to a still wet ionomer impregnated ePTFE film. There are significant manufacturing efficiency advantages associated with the manufacture of CCMs via sequential deposition of layers (such processes may be known as additive layer manufacturing), for example relating to the reduction in handling steps and the number of required backing materials. However, the formation of a polymer electrolyte membrane on the surface of a catalyst layer provides significant technical challenges. This is particularly the case when manufacturing catalyst-coated membranes for water electrolysers which typically require multiple, sequential, membrane formation steps due to the relative thickness of electrolyser membranes in comparison with fuel cell membranes, and the desire to place additives, such as recombination catalysts, and polymer reinforcements at particular locations in the membranes to optimise performance and durability. For example, it has been found that significant and problematic bubbling or blistering may be observed during membrane formation, in particular during the formation of a second membrane layer (for example by cast dispersion of membrane ion-conducting polymer) onto a first membrane layer that itself has been formed on a catalyst layer. Incursion of the membrane ion-conducting polymer into the first catalyst layer during membrane manufacture can also have a detrimental impact on electrolyser CCM performance. There remains a need to further enhance and develop methods for the production of catalyst-coated polymer electrolyte membranes, in particular methods which enable sequential deposition of CCM components whilst maintaining technical performance. Summary of the invention The present inventors have found that by controlling the properties of the first-formed catalyst layer, manufacturing issues relating to blistering or bubble formation during additive layer manufacturing of electrolyser CCMs can be significantly reduced or eliminated. Therefore, in a first aspect of the invention there is provided a method of manufacturing a catalyst-coated polymer electrolyte membrane for a water electrolyser, the method comprising the steps of: (a) providing a first catalyst layer on a support substrate, the first catalyst layer comprising a platinum-containing catalyst on a carbon support material and a catalyst layer ion-conducting polymer; (b) forming a polymer electrolyte membrane on the first catalyst layer by a method comprising the steps of: (i) coating the first catalyst layer with a dispersion of a membrane ion-conducting polymer to form a first polymer electrolyte membrane layer; (ii) depositing one or more additional polymer electrolyte membrane layers on the first polymer electrolyte membrane layer; and wherein the first catalyst layer has an expected effective platinum surface area in the range of and including 5 to 200 cm2Pt / cm2 and a carbon content in the range of and including 30 to 60 wt%. The present inventors have also identified electrolyser CCMs with properties that enable efficient manufacture via an additive layer process (with associated reduction in manufacturing waste and increases in manufacturing efficiency) and which have an electrochemical performance which testing indicates at least matches that of CCMs produced by decal transfer of catalyst layers onto a membrane. Therefore, in a second aspect of the invention there is provided a catalyst-coated polymer electrolyte membrane (CCM) for a water electrolyser, the catalyst coated membrane comprising a first catalyst layer on a first face of a polymer electrolyte membrane and a second catalyst layer on the opposite second face of the polymer electrolyte membrane, and wherein: (i) the first catalyst layer comprises a platinum-containing catalyst on a carbon support material and a catalyst layer ion-conducting polymer; (ii) the first catalyst layer has an electrochemical platinum surface area in the range of and including 5 to 100 cm2Pt / cm2 and a carbon content in the range of and including 30 to 60 wt%; (iii) the first catalyst layer has a higher gloss than the second catalyst layer. The catalyst coated polymer electrolyte membranes of the second aspect are obtainable by, or may be obtained, by the method of the first aspect. In a third aspect of the invention there is provided a membrane electrode assembly comprising a catalyst-coated polymer electrolyte membrane according to the second aspect and a transport layer. In a fourth aspect of the invention, there is provided a water electrolyser comprising a catalyst-coated polymer electrolyte membrane according to the second aspect, or a membrane electrode assembly according to the third aspect. Brief description of the Figures Figure 1 shows a schematic representation of patches of the first catalyst layer on a support substrate. Figure 2 shows a schematic representation of a method of manufacturing a catalyst-coated polymer electrolyte membrane. Figure 3 shows a schematic representation of a water electrolyser. Figure 4 shows an image of a membrane formed in Example 3. Figure 5 shows an image of a catalyst-coated polymer electrolyte membrane formed in Example 6. Figure 6 shows images of opposite sides of a catalyst-coated polymer electrolyte membrane obtained by a method as set out herein. Detailed Description Preferred and / or optional features of the invention will now be set out. Any of the preferred and / or optional features of any aspect may be combined, either singly or in combination, with any other preferred and / or optional features of any aspect of the invention unless the context demands otherwise. The present invention provides a method of manufacturing a catalyst-coated polymer electrolyte membrane for a water electrolyser. Suitably, the catalyst-coated polymer electrolyte membrane is a catalyst-coated proton exchange membrane for a proton exchange membrane water electrolyser (PEMWE) or a catalyst-coated anion exchange membrane for an anion exchange membrane water electrolyser (AEMWE). Preferably, the catalyst-coated polymer electrolyte membrane is a catalyst-coated proton exchange membrane. As used herein the term “catalyst-coated polymer electrolyte membrane” refers to a polymer electrolyte membrane with a first face and a second face, and which has a first catalyst layer on the first face and, optionally, a second catalyst layer on the second opposite face. The method comprises step (a) providing a first catalyst layer on a support substrate. The support substrate provides support for the catalyst-coated polymer electrolyte membrane during manufacture and, if not immediately removed, can provide support and strength during any subsequent storage and / or transportation. The material from which the support substrate is made should provide the required support, be able to withstand the process conditions involved in producing the catalyst-coated polymer electrolyte membrane and be able to be easily removed without damage to the catalyst-coated polymer electrolyte membrane. Examples of materials suitable for use as a support substrate include a fluoropolymer, such as polytetrafluoroethylene (PTFE), ethylene tetrafluoroethylene (ETFE), perfluoroalkoxy polymer (PFA), fluorinated ethylene propylene (FEP - a copolymer of hexafluoropropylene and tetrafluoroethylene), polyolefins, such as biaxially oriented polypropylene (BOPP), polyesters, such as polyethylene naphthalate (PEN), and poly(phenylene sulfide) (PPS). Other examples include laminates, multi-layer extrusions and coated films / foils capable of retaining their mechanical strength / integrity at elevated temperatures, for example temperatures up to 200 °C. Examples include laminates of: poly(ethylene-co-tetrafluoroethylene) and polyethylene naphthalate (PEN); polymethylpentene (PMP) and PEN; polyperfluoroalkoxy (PFA) and polyethylene terephthalate (PET) and polyimide (PI). The laminates can have two or more layers, for example ETFE-PEN-ETFE, PMP-PEN-PMP, PFA-PET-PFA, PEN-PFA, FEP-PI-FEP, PFA-PI-PFA and PTFE-PI-PTFE. The layers may be bonded using an adhesive, such as acrylic or polyurethane. A PEN support substrate may be particularly preferred providing a suitable balance between adhesion to catalyst layers and to formed membrane. The first catalyst layer on the support substrate comprises a platinum-containing catalyst on a carbon support material. Suitably, the catalyst comprises metal particles on the carbon support material. Typically, the loading of metal particles on the carbon support material in the range of and including 10 to 90 wt%, preferably in the range of and including 15 to 70 wt%, of the total weight of the supported catalyst. The platinum-containing catalyst may be alloyed with other precious metals or base metals. Preferably, that the catalyst in the first catalyst layer on the support substrate is platinum on a carbon support material (Pt / C). The first catalyst layer on the support substrate also comprises a catalyst layer ion-conducting polymer. The catalyst layer ion-conducting polymer can be a proton-conducting polymer or an anion-conducting polymer, such as a hydroxyl anion-conducting polymer. Preferably catalyst layer ion-conducting polymer is a proton-conducting polymer. Typically, the catalyst layer ion-conducting polymer comprises sulfonic acid groups. Suitably, the catalyst layer ionconducting polymer is a perfluorinated sulfonic acid ionomer, or a partially-fluorinated or nonfluorinated hydrocarbon sulfonic acid ionomer. Typically, the catalyst layer ion-conducting polymer comprises sulfonic acid groups and the ion-conducting polymer has an equivalent weight of about 1100 or less, typically about 900 or less, suitably about 850 or less. Typically, the catalyst layer ion-conducting polymer comprises sulfonic acid groups and has an equivalent weight of at least about 450. The equivalent weight of the ion-conducting polymer may be readily measured using an acid titration following a hydroxide exchange. For example, a sample may be vacuum dried at about 110 °C for 16 hours to obtain about 2g of the dried material. The material may then be immersed in about 30 mL of a 0.1N NaOH solution to substitute sodium ions for protons in the sample. Then titration by neutralisation is carried out, for example using 0.1N hydrochloric acid, to determine the number of exchangeable protons, and therefore the EW may be calculated. The first catalyst layer on the support substrate has an expected effective platinum surface area (eEPSA) in the range of and including 5 to 200 cm2Pt / cm2 of the geometric area of the catalyst layer. An eEPSA of greater than 200 cm2Pt / cm2 has been found to lead to manufacturing issues during additive layer CCM formation. An eEPSA less than 5 cm2Pt / cm2 typically leads to a reduction in electrochemical performance of the formed CCM. Preferably, the eEPSA is in the in the range of and including 10 to 190 cm2Pt / cm2, 20 to 180 cm2Pt / cm2, 30 to 170 cm2Pt / cm2, 40 to 160 cm2Pt / cm2, or most preferably 50 to 150 cm2Pt / cm2. The eEPSA is calculated from the CO metal area (COMA) of the platinum-containing catalyst on a carbon support material and the loading of platinum in the first catalyst layer using the formula: eEPSA = loading of platinum in the first catalyst layer (mgPt / cm2) * COMA of the platinum-containing catalyst (m2 / gpt) * 10. The COMA of the platinum-containing catalyst on a carbon support material is measured using carbon monoxide chemisorption. The loading of platinum in the first catalyst layer (mgPt / cm2) can be determined using inductively coupled plasma mass spectrometry (ICP-MS). Preferably, the first catalyst layer on the support substrate has a loading of platinum less than or equal to 0.25 mg / cm2 such as less than or equal to 0.20 mg I cm2. Suitably, the first catalyst layer has a loading of platinum of at least 0.05 mg / cm2. The first catalyst layer on the support substrate has a carbon content in the range of and including 30 to 60 wt%. The carbon content is the wt% of carbon as a proportion of the total weight of the components of the catalyst layer (including, for example, platinum and catalyst layer ion-conducting polymer). A carbon content in the range of and including 30 to 60 wt% in combination with an eEPSA in the range of an including 5 to 200 cm2Pt / cm2 offers an advantageous balance of catalyst layer properties for subsequent membrane formation. The first catalyst layer is provided on a first face of the support substrate. The first catalyst layer may cover substantially the entire first face of the support substrate, or it may be preferred that the first catalyst layer is provided in at least one region of the first face of the support substrate with the first catalyst layer being absent on other regions of the first face of the support substrate. In such cases it will be understood that the dimensions of the one or more regions coated with a first catalyst layer will depend on the configuration of the electrochemical device into which the formed catalyst-coated ion-conducting membrane is designed to be incorporated. Typically, the region(s) coated with the first catalyst layer corresponds to the active catalyst area of the formed catalyst-coated polymer electrolyte membrane. Suitably, such regions are in the shape of a quadrilateral, such as a rectangle or a square, or may be, for example an oval or circle. It may be preferred that the regions coated with a first catalyst layer do not extend to the edges of the support substrate. It will be understood by the skilled person that the present method may be used to prepare a single catalyst-coated polymer electrolyte membrane, in which case a single region (or patch) of catalyst layer may be provided on the support substrate or may be used to manufacture multiple catalyst-coated polymer electrolyte membranes, for example using a roll-to-roll process. In such cases, multiple regions (or patches) of the first catalyst layer may be provided on the support substrate. Preferably, the first catalyst layer is provided in the form of discrete patches on the support substrate. By discrete patches it is meant that the first catalyst layer is present on the first face of the support substrate in two or more regions which are not connected to each other. The provision of the first catalyst layer as discrete patches offers advantages associated with a reduction in the amount of catalyst material used in the manufacturing process and avoids the provision of catalyst layers in areas of the catalyst coated membrane that are not required to be electrochemically active. Typically, the first catalyst layer has a thickness in the range of and including 2 to 20 gm, such as in the range of and including 2 to 10 gm. The first catalyst layer may be prepared using, for example, coating methods such as a slotdie (slot, extrusion) coating process, inkjet printing, gravure printing, curtain coating, spray coating, or a laser direct write process, such as laser induced forward transfer (LIFT). Suitably, step (a) further comprises the sub-steps of: i) depositing a first catalyst dispersion comprising the platinum-containing catalyst on a carbon support material and the catalyst layer ion-conducting polymer onto the support substrate; and (ii) drying the first catalyst dispersion to form the first catalyst layer. Typically, the catalyst dispersion comprises the platinum-containing catalyst material and an ion-conducting polymer dispersed in a solvent, such as water, a polar solvent (other than water), or a mixture of water and a polar solvent (other than water). The polar solvent can be a polar protic solvent. Preferably, the polar solvent is an alcohol, more preferably a C1-4 alcohol, such as be methanol, ethanol, propan-1-ol or propan-2-ol. Drying of the first catalyst dispersion to form the first catalyst layer removes solvent from the layer and it will be understood by the skilled person that the drying temperature may be adjusted depending on the boiling point of the solvent used in the first catalyst dispersion. Typically, the first catalyst dispersion is dried to a temperature of at least 130 °C. The method comprises step (b) forming a polymer electrolyte membrane on the first catalyst layer. This membrane acts as the polymer electrolyte in the manufactured catalyst-coated ion-conducting membrane. It will be understood by the skilled person that the membrane is formed on the opposite face of the first catalyst layer to the face of the first catalyst layer in contact with the support substrate. The polymer electrolyte membrane on the first catalyst layer is formed by a method comprising the step (b) (i) coating the first catalyst layer with a dispersion of a membrane ion-conducting polymer to form a first polymer electrolyte membrane layer. Suitably, the coating in step (b) (i) is carried out by dispersion casting the first polymer electrolyte membrane layer. The dispersion of membrane ion-conducting polymer comprises the ion-conducting polymer dispersed in a solvent, or a mixture of solvents. Advantageously, the solvent (or mixture of solvents) is selected such that rapid solvent evaporation may be achieved. It may be preferred that the solvent (or each solvent in a mixture of solvents) has a boiling point at 1 bar of pressure in the range of and including 60 to 110 °C. It may be preferred that the solvent is water, a polar solvent (other than water), or a mixture of water and a polar solvent (other than water). The polar solvent can be a polar protic solvent. Preferably, the polar solvent is an alcohol, more preferably a C1.4 alcohol, such as methanol, ethanol, or propan-1-ol. It may be preferred that the solvent is a mixture of water and a C1-4 alcohol (such as methanol, ethanol, or propan-1-ol). Typically, the dispersion of membrane ion-conducting polymer used for coating comprises ionconducting polymer in an amount in the range of and including 5 to 25 wt% based on the total weight of components in the dispersion. It will be understood that the amount of ion-conducting polymer in the dispersion can readily be varied to adjust the viscosity of the dispersion to facilitate coating. In the case that the catalyst-coated polymer electrolyte membrane comprises a PEM, the membrane ion-conducting polymer is a proton-conducting polymer, such as an ion-conducting polymer which comprises sulfonic acid groups. The membrane ion-conducting polymer preferably comprises a perfluorinated sulfonic acid (PFSA) ionomer, a partially-fluorinated sulfonic acid ionomer, a non-fluorinated sulfonic acid ionomer (such as a non-fluorinated hydrocarbon sulfonic acid ionomer, or mixtures thereof. The ion-conducting polymer may comprise a blend of proton-conducting polymers. Examples of suitable proton-conducting polymers include perfluorosulphonic acid ionomers (e.g. Nation® (Chemours), Aciplex® (Asahi Kasei), Aquivion® (Syensqo), Flemion® (Asahi Glass Co.), or ionomers based on a sulphonated hydrocarbon such as those available from FuMA-Tech GmbH as the fumapem® P, E or K series of products, JSR Corporation, Toyobo Corporation, and others. In the case that the catalyst-coated polymer electrolyte membrane comprises an AEM, the ion-conducting polymer is an anion-conducting polymer, such as a hydroxy-conducting polymer, for example ion-conducting polymers comprising quaternary ammonium functional groups. Suitable ion-conducting polymers include Fumion FAA-3 (Fumatech), Aemion ionomers (lonomr), and PiperlON ionomers (Versogen). The first (and subsequent) polymer electrolyte membrane layers are preferably deposited by slot-die coating, knife-coating, bar coating, inkjet printing, gravure printing, or a curtain coating process. Preferably, the first (and subsequent) polymer electrolyte membrane layers are deposited by slot-die coating. Advantageously, solvent is evaporated from the first polymer electrolyte membrane layer prior to further deposition of ion-conducting polymer. This helps to avoid solvent and ion-conducting polymer ingress into the first catalyst layer. Evaporation may be achieved by heating the first polymer electrolyte membrane layer, for example by passing the layer through an oven. Suitably, the first polymer electrolyte membrane may be dried at a temperature, for example, in the range of and including 50 °C to 100 °C, such as in the range of and including 60 °C to 80 °C. The method to form the polymer electrolyte membrane on the first catalyst layer additionally comprises step (b) (ii) depositing one or more additional polymer electrolyte membrane layers on the first polymer electrolyte membrane layer. The one or more additional layers are typically formed by a method comprising the steps of (i) depositing a dispersion of membrane ion-conducting polymer; and (ii) drying the membrane ion-conducting polymer dispersion to form part of the polymer electrolyte membrane. The ion-conducting polymer(s) used to form the one or more additional layers is typically the same as the ion-conducting polymer used for the first polymer electrolyte membrane layer. The ion-conducting polymer used to form the one or more additional layers may however be different from the ion-conducting polymer used for the first polymer electrolyte membrane layer. The ion-conducting polymer used to form the one or more additional layers may suitably be selected from the ion-conducting polymers described hereinbefore in relation to the first polymer electrolyte membrane layer. The ion-conducting polymer is typically deposited as a dispersion in a solvent as previously described hereinbefore in relation to the first polymer electrolyte membrane layer. Preferably, the one or more additional polymer electrolyte layers are dispersion cast. The one or more additional layers are preferably deposited by slot-die coating, knife-coating, bar coating, inkjet printing, gravure printing, or a curtain coating process. Preferably, the one or more additional layers are deposited by slot-die coating. Preferably, step (b) (ii) comprises depositing a plurality of polymer electrolyte membrane layers. For example, step (b) (ii) may comprise the steps of (i) depositing an ion-conducting polymer dispersion onto the first polymer electrolyte membrane layer to form a first additional polymer electrolyte membrane layer; (ii) optionally drying the first additional polymer electrolyte membrane layer; (iii) depositing one or more additional ion-conducting polymer layers onto the first additional polymer electrolyte membrane layer. It is preferred that each polymer electrolyte membrane layer is dried (or partially dried) before the next layer is deposited. Once dried, the first polymer electrolyte membrane layer and the one or more additional layers together form the polymer electrolyte membrane. The polymer electrolyte membrane layer is suitably ion-conducting and electrically insulating. Preferably, the polymer electrolyte membrane formed in step (b) has a thickness at least 20 pm. It may be preferred that the polymer electrolyte membrane has a thickness of at least about 25 pm, at least about 30 pm or at least about 35 pm. Typically, the thickness of the polymer electrolyte membrane formed in step (b) is less than or equal to about 200 pm, such as less than or equal to 150 pm, or preferably less than or equal to 100 pm. Preferably, the polymer electrolyte membrane formed in step (b) has a thickness in the range of including 20 to 100 pm. The thickness of the membrane may be determined by analysis of scanning electron microscope (SEM) images of a cross sections of the membrane (suitably dried at 0% relative humidity) and measured at multiple (for example 10) points. The thickness values are then determined by calculating the arithmetic mean of the measured values. The polymer electrolyte membrane formed in step (b) may comprise one or more polymeric reinforcement components. Such components provide strength and restrict the swelling of the CCM. Typically, such reinforcement components are provided as part of the process of forming a membrane layer, for example by embedding the reinforcement component into one or more membrane layers, for example after a dispersion of ion-conducting polymer is cast and prior to drying the layer. Suitably, the reinforcement components are a porous polymer material, which has the membrane ion-conducting polymer impregnated within the pores in the formed membrane. The reinforcement may be non-woven or woven. The porous polymer material may be a fluoropolymer. The porous polymer material may be selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyethersulfone (PES), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), polyimides (PI), polyetherimide (PEI), poly(aryl ether ketone) (PAEK), poly(aryl ether sulfone), poly(phenylene sulfide) (PPS), polyvinylpyrrolidone (PVP) and polyether ether ketone (PEEK). The porous polymer material may be expanded polytetrafluoroethylene (ePTFE). The porous polymer material may also comprise a polymer backbone based on a nitrogen-containing heterocycle, such as polybenzimidazole. The polymer electrolyte membrane formed in step (b) may comprise one or more additives, for example a radical scavenger (such as ceria), and / or a recombination catalyst. Such additives may be introduced by inclusion in one of the dispersions used for the formation of a polymer electrolyte membrane layer. Suitable radical scavengers are known to those in the art and include metal oxides, such as cerium oxides, manganese oxides, titanium oxides, beryllium oxides, bismuth oxides, tantalum oxides, niobium oxides, hafnium oxides, vanadium oxides and lanthanum oxides, suitably cerium oxides, manganese oxides or titanium oxides, preferably cerium dioxide (ceria). A recombination catalyst catalyses the reaction of H2 and O2 to form H2O. Suitable recombination catalysts can comprise a metal (such as platinum) which may be supported, for example on carbon, or may be unsupported. Suitably, the polymer electrolyte membrane formed in step (b) is a single coherent polymer film. The term ‘coherent’ as used herein means that the membrane is free from internal interfaces between ion-conducting polymer layers, for example formed through lamination or through annealing of membrane layers prior to subsequent membrane layer deposition. Lamination of ion-conducting membranes comprises pressing and / or bonding at least two solid ion-conducting layers together, such membranes optionally being coated with a catalyst layer. Such interfaces may be suitably detected by cross-section SEM. Preferably, polymer electrolyte membrane formed in step (b) is a single coherent polymer film with no internal interfaces between ion-conducting polymer layers detectable by cross-section SEM. Due to physical defects and / or chemical variations at interfaces between ion-conducting polymer layers, such interfaces can increase the resistance of a multi-layer polymer electrolyte membrane. As such, it is advantageous to fabricate a multi-layer polymer electrolyte membrane by depositing layers of ion-conducting polymer dispersed in a liquid solvent to build up a multi-layer membrane structure rather than via lamination of individual solid layers / membranes of ion-conducting polymer. Typically, the process comprises the step of: (c) heat-treating the polymer electrolyte membrane formed in step (b). Heat treatment of the polymer electrolyte membrane improves the mechanical strength and dissolution resistance of the membrane and can increase the adhesion of the first and second (if present) catalyst layers to the membrane. Typically, the heat treatment is carried out a temperature in the range of and including 110 to 250 °C, such as 140 to 220 °C, or in the range of and including 150 to 180 °C. The formed polymer electrolyte membrane has a first face adjacent to the first catalyst layer and a second, opposite face. The method comprises the optional step of: (d) applying a second catalyst layer to the second face of the polymer electrolyte membrane. It will be understood by the skilled person that the second catalyst layer is applied to the opposite face of the ion-conducting membrane from the first catalyst layer such that, once the second catalyst layer is applied, the polymer electrolyte membrane is positioned between, and typically in direct contact with, the first and the second catalyst layers. Typically, the second catalyst layer comprises an oxygen evolution reaction (OER) catalyst. The type of OER catalyst is not particularly limited in the current invention so long as the catalyst materials are sufficiently stable under PEM or AEM electrolyser operating conditions. Such materials are known to the skilled person. Typically, the OER catalyst comprises a transition metal, for example a noble metal (Rh, Ru, Rh, Pd, Ag, Os, Ir, Pt, Au) or one or more of Ni, Fe, Cu and Co. Preferably, for PEM applications, the OER catalyst comprises iridium and / or ruthenium. Such materials have a particularly good combination of catalyst activity and stability under PEMWE operating conditions. Suitably, the OER catalyst is a doped or undoped oxide, for example a doped or undoped oxide of iridium and / or ruthenium, or an iridium metal oxide, for example a metal oxide material comprising iridium and metal M, wherein M = Ta, Nb, Ti, Rh, Ru, or Pt Preferably, the second catalyst layer comprises an oxygen evolution reaction (OER) catalyst but does not contain any further electrocatalyst materials. Preferably, the second catalyst layer does not contain platinum. Preferably, for AEM applications, the OER catalyst comprises non-noble transition metals, such as Ni, Co, Cu and Fe, for example alloys and oxides of one or more non-noble metal transition metals, such as alloys and oxides of Ni, Co, Cu and Fe. It may be preferred that the OER catalyst (preferably an iridium-containing catalyst) is on a catalyst support, for example an inorganic metal oxide support, for example a transitional metal oxide or oxide of a main group metal, such as TiOz, AI2O3, ZrOz or mixtures thereof. If the OER catalyst is a supported catalyst, the loading of catalyst metal (such as iridium) on the support material is suitably in the range of and including 10 to 90 wt%, preferably in the range of and including 15 to 75 wt% of the total weight of the supported catalyst. The second catalyst layer typically comprises additional components, such as an ionconducting polymer, to improve ionic conductivity within the layer. In some embodiments that second catalyst layer consists essentially of an oxygen evolution reaction catalyst and an ionconducting polymer. The application of the second catalyst layer may occur after step (b), or after optional heat treatment of the membrane (step (c)). It may be preferred that the second catalyst layer is applied after step (c). Applying the second catalyst layer on the surface of the membrane after heat treatment offers advantages associated with reduced membrane swelling and solvent uptake during the application process. Preferably, the second catalyst layer is applied to regions of the ion-conducting membrane which correspond to the regions of the ion-conducting membrane with the first catalyst layer applied to the opposite face. Preferably, the regions of the second catalyst layer are centrally positioned over the regions of the first catalyst layer (but on the opposite face of the polymer electrolyte membrane). The second catalyst layer may be applied to the membrane using, for example, coating methods such as a slot-die (slot, extrusion) coating process, inkjet printing, gravure printing, curtain coating, a spray coating process, or a laser direct write process, such as laser induced forward transfer (LIFT). The catalyst layer may be applied by directly coating the membrane, or the catalyst layer may be formed on a suitable backing material and then applied to the membrane using a decal process. Preferably, the catalyst layer is applied directly to the membrane enabling efficient manufacturing and a reduction in the amount of backing material required during the CCM manufacturing process. Optionally, the method comprises the step of applying a seal material layer to the first face and / or the second face of the catalyst-coated ion-conducting membrane. Typically, the seal material layer is applied after the removal of the catalyst-coated ion-conducting membrane from the support substrate, but the seal material layer may also be applied to one face of the catalyst-coated polymer electrolyte membrane prior to removal from the support substrate, for example after heat-treatment of the membrane and / or after application of the second catalyst layer. Optionally, the method comprises the step of applying a transport layer to a first face and / or a second face of the catalyst-coated polymer electrolyte membrane. Suitable transport layers are known by the skilled person. At the anode (second catalyst layer) side of the catalyst-coated polymer electrolyte membrane, suitable transport layers are typically formed from a metal-based porous structure. Such transport layers must be sufficiently conducting and in a form that is compatible with positioning in close proximity to the catalyst-coated membrane (without, for example, sharp edges or protrusions that would damage the membrane during use). Such metal-based porous structures may be in the form of, for example, felts or non-woven cloths, mesh, foams and sintered metal-containing particles. For PEMWE applications, suitable metal-based porous structures comprise titanium. For AEMWE applications, suitable metal-based porous structures comprise nickel or stainless steel. Suitable transport layers at the cathode (first catalyst layer) side of the catalyst-coated polymer electrolyte membrane, are known to the skilled person and are typically non-woven papers or webs comprising a network of carbon fibres and a thermoset resin binder (e.g. the TGP-H series of carbon fibre paper available from Toray Industries Inc., Japan or the H2315 series available from Freudenberg FCCT KG, Germany, or the Sigracet® series available from SGL Technologies GmbH, Germany or AvCarb® series, or woven carbon cloths). The carbon paper, web or cloth may be provided with a further treatment either to make it more wettable (hydrophilic) or more wet-proofed (hydrophobic). The nature of any treatments will depend on the type of electrochemical device and the operating conditions that will be used. It will be understood by the skilled person that the support substrate is removed from the formed catalyst-coated polymer electrolyte membrane prior to application of a transport layer on the cathode side of the catalyst-coated polymer electrolyte membrane. Figures 1 and 2 depict exemplary methods of the present invention. The dimensions (e.g. thickness) of each layer are not drawn to scale for the sake of clarity. It will be clear to the skilled person that although the process described below is with reference to the manufacture to a continuous roll of multiple catalyst-coated polymer electrolyte membrane components, the basic process could be applied to the manufacture of an individual catalyst-coated polymer electrolyte membrane component. Figure 1 shows a plan view of a section of a support substrate (1) provided with patches of the first catalyst layer (2). The first catalyst layer (2) comprises a platinum-containing catalyst on a carbon support material and a catalyst layer ion-conducting polymer. Figure 2 shows a schematic of a process of forming a catalyst-coated polymer electrolyte membrane viewed as a cross section along line a-a indicated on Figure 1. In step (i) the support substrate (1) is provided with patches of the first catalyst layer (2). In step (ii), the first catalyst layer (2) is coated with a dispersion of a membrane ion-conducting polymer to form a first polymer electrolyte membrane layer (3). In step (iii) a polymer electrolyte membrane (4) is formed by depositing one or more additional polymer electrolyte membrane layers on the first polymer electrolyte membrane layer (3) (noting that the line indicating the interface between layer (3) and (4) is for representative purposes only and an interface is typically not visible between the membrane layers). In step (iv) patches of a second catalyst layer (5) are applied to the opposite face of the polymer electrolyte membrane (4) to the first catalyst layer (2). The product may be further processed, for example though removal of the support substrate (1), cutting the coated membrane to form individual CCMs, the application of seals, etc. It will be understood that the first catalyst layer (2) may be provided as patches as shown in Figures 1 and 2 or may, for example, be provided in other configurations, such as provided over substantially all of the surface of the support substrate (1), or as a central continuous strip in the longitudinal direction of the support substrate (1). The first polymer electrolyte membrane layer (3) may be provided as a continuous layer over substantially all of the surface of the first catalyst layer (2) and any regions of the surface of the support substrate (1) where the first catalyst layer (2) is absent or may, for example, only be provided in regions where the first catalyst layer (2) is present. One or more polymer reinforcements may be incorporated into the membrane (not shown in Figure 2), for example by introducing an ePTFE web during the deposition of an additional polymer electrolyte membrane layer. As an alternative to the process shown in Figure 2, no second catalyst layer (5) may be provided (for example for applications where the catalyst-coated membrane is used together with a catalyst coated substrate). The present invention also provides catalyst-coated polymer electrolyte membranes (CCMs) for a water electrolyser, for example for a proton exchange membrane water electrolyser (PEMWE) or an anion-exchange membrane water electrolyser (AEMWE). Preferably, the catalyst-coated polymer electrolyte membrane is for a PEMWE. The CCMs are obtained or obtainable by the method as described hereinbefore. The CCMs comprise a first catalyst layer on a first face of a polymer electrolyte membrane and a second catalyst layer on the opposite second face of the polymer electrolyte membrane. The first catalyst layer comprises a platinum-containing catalyst on a carbon support material and an ion-conducting polymer. The first catalyst layer is the cathode layer of the electrolyser CCM. The first catalyst layer has an electrochemical platinum surface area (EPSA) in the range of and including 5 to 100 cm2Pt / cm2 and a carbon content in the range of and including 30 to 60 wt%. Such a combination of properties of the first catalyst layer enables efficient manufacture via an additive layer process enabling an increase in manufacturing efficiency and a reduction in waste, whilst maintaining CCM quality and performance in comparison with prior art methods. The EPSA is the platinum surface area (cm2) per geometric electrode area (cm2) of the first catalyst layer. The EPSA is dependent on the combination of the metal surface area of platinum in the first electrocatalyst component (cm2 Pt / gPt) and the loading of the first electrocatalyst component per geometric area in the first catalyst layer (gPt / cm2 first catalyst catalyst layer) and is measured on using a cyclic voltammetry protocol with carbon monoxide (CO) stripping. Preferably, the EPSA of the first catalyst layer is in the range of and including 10 to 90 cm2Pt / cm2, or more preferably, in the range of and including 20 to 70 cm2Pt / cm2. Preferably, the first catalyst layer of the CCM has a platinum loading of less than or equal to 0.25 mgpt / cm2, such as less than or equal to 0.20 mg / cm2. Suitably, the first catalyst layer has a loading of platinum of at least 0.05 mg / cm2, for example in the range of and including 0.05 mg / cm2 to 0.25 mg / cm2. The first catalyst layer of the CCM has a carbon content in the range of and including 30 to 60 wt%. The carbon content is the wt% of carbon as a proportion of the total weight of the components of the catalyst layer (including, for example, platinum and catalyst layer ionconducting polymer). The first catalyst layer of the CCM comprises a catalyst layer ion-conducting polymer. The nature of the ion-conducting polymer is as hereinbefore described in relation to the catalyst layer ion-conducting polymer used to form the first catalyst layer. The first catalyst layer of the CCM has a higher gloss than the second catalyst layer. Such a configuration reflects the formation of the first catalyst layer on a backing sheet which is later removed and subsequent formation of a second catalyst layer directly on the formed membrane. Therefore such CCMs, in combination with the selection of first catalyst layer properties, may be efficiently produced using an additive manufacturing process, and also advantageously offer rapid determination of the correct orientation of the CCM during incorporation into a water electrolyser stack. Such as configuration differs from CCMs manufactured using other methods, for example CCMs manufactured using decal transfer of both catalyst layers onto a membrane, for which the level of gloss would be expected to be the same for each catalyst layer. Preferably, the first catalyst layer has a gloss of at least 10 gloss units higher than the second catalyst layer, or at least 20 units higher than the second catalyst layer. The gloss value of the catalyst layers may be determined using a gloss meter at an angle of incidence of 60 for example using a Novo Gloss Trio 20 / 60 / 85 Glossmeter (Rhopoint Instruments). Preferably, the gloss value of the first catalyst layer at an angle of incidence of 60 ° is at least 20. A value of at least 20 indicates that the layer has a low degree of surface roughness. The other components of the CCM, such as the polymer electrolyte membrane and the second catalyst layer, are as described hereinbefore during the description of the method of manufacture. Preferably, the CCM is for a PEMWE and the polymer electrolyte membrane comprises a proton-conducting polymer, for example a proton-conducting polymer comprising sulfonic acid functional groups. Preferably, the polymer electrolyte membrane comprises at least one polymeric reinforcement component. Preferably, the polymer electrolyte membrane has a thickness of at least 20 pm, such as in the range of and including 20 to 100 pm, or 30 to 90 pm. The CCMs are for use in a water electrolyser. Figure 3 shows a schematic representation of an example of a water electrolyser (20) incorporating a CCM. A polymer electrolyte membrane (22) is provided with an anode catalyst layer (24) incorporating an oxygen evolution catalyst and a cathode catalyst layer (26) incorporating a platinum-containing catalyst, the anode and cathode layers being provided on opposite faces of the polymer electrolyte membrane (22). Adjacent to the anode catalyst layer (24) is a transport layer (28) which is typically a metal-based porous structure and may be known as a porous transport layer (PTL). Adjacent to the cathode catalyst layer (26) is a transport layer (30) which is typically a nonwoven paper or web comprising a network of carbon fibres and may be known as a gas diffusion layer (GDL). Adjacent to each transport layer (28, 30) is a bipolar plate (32) which provides flow channels for gases and uniformly distributes water participating in the reaction on the electrode surface. Some components, such as seal material layers and sub-gaskets are not shown in this representation but may be present as understood by the skilled person. The present invention will now be described with reference to the following examples, which are provided to assist with understanding the present invention and are not intended to limit its scope. Examples The following measurement techniques were used: Measurement of gloss - catalyst-coated membranes The gloss measurement of each side of a catalyst-coated polymer electrolyte membrane was carried out using a Novo Gloss Trio 20 / 60 / 85 Glossmeter (Rhopoint Instruments) which was set at a 60 ° angle of incidence. Gas phase CO metal surface area (COMA) testing protocol The COMA was determined using gas phase adsorption of carbon monoxide (CO). The gas phase COMA is determined by reducing the catalyst, in its as-made form prior to incorporation into a catalyst layer, in hydrogen, then titrating aliquots of carbon monoxide gas until the active metal surface is saturated with a chemisorbed CO monolayer, and there is no more uptake. The moles of CO chemisorbed is measured. The measurement is normalised for standard temperature (Lw) and standard pressure (pM) by multiplying by ((Tstd / pstd) x (p / T)) where p and T are the pressure and temperature of the measurement respectively. The moles of CO chemisorbed can then be converted into a metal surface area by assuming the number of atoms / m2 of metal (e.g. 1.25 x 1019 atoms / m2 for Pt) as defined in ‘Catalysis - Science and Technology, Vol. 6, p. 257, Eds J. R. Anderson and M. Boudart. Electrochemical platinum surface area For the EPSA measurement, hydrogen was flowed on the anode side of the MEA and nitrogen on the cathode. Once the cathode side was fully purged of any residual hydrogen the flow was switched to 10% v / v CO in nitrogen for 10 minutes, before switching back to pure nitrogen for a further 10 minutes. With hydrogen still flowing on the anode side, 3 voltammetry cycles at 20 mV / sec were performed from HLIPD potentials to just above the potential at which all the CO is striped from the surface. The charge under the CO stripping peak recorded on the first cycle, relative to the stable baseline from the 2nd / 3rd cycle was then integrated to provide a measurement of the platinum surface area using a conversion factor of 420 C / cm2. The surface area measurement was then normalised to the geometric area of the cathode electrode to provide the EPSA value. Formation of Pt / C containing layers An ink comprising the selected platinum-on-carbon catalyst material (as shown in Table 1, available from Johnson Matthey) was prepared by adding a dispersion of a proton-conducting polymer (PFSA) and catalyst (solids % of total ink weight 10-15%) in a mixture of propanol: water (80:20 - 50:50). The layer was deposited onto a backing substrate using slot-dye coating and then dried to remove solvent. Formation of a polymer electrolyte membrane on the platinum-on carbon layer. A first polymer electrolyte membrane layer was formed by casting a dispersion of ionconducting polymer (800 EW PFSA, 3M) in ethanol-water onto the Pt / C catalyst layer using slot-dye coating and then drying to substantially remove solvent to form a membrane layer with a thickness of approximately 10 pm. A second polymer electrolyte membrane layer was then formed by casting a dispersion of ionconducting polymer (800 EW PFSA, 3M) in ethanol-water onto the first polymer electrolyte membrane layer and then drying to remove solvent. The formed membrane layer was analysed for bubbling or blistering. Table 1 - Pt-C layers trialled in an additive layer process. Ex. no Catalyst Catalyst COMA m2 / gPt Pt loading (mg / cm2) Expected EPSA (cm2Pt / cm2) wt% carbon in layer Blistering? 1 20 wt% Pt on carbon (HS3000) 86 0.08 69 49 No Ex. no Catalyst Catalyst COMA m2 / gPt Pt loading (mg / cm2) Expected EPSA (cm2Pt / cm2) wt% carbon in layer Blistering? 2 40 wt% Pt on carbon (HS4000) 55 0.10 55 41 No 3 50 wt% Pt on carbon (HS21710) 54 0.40 217 35 Yes + 4 60 wt% Pt on carbon (HS9100) 71 0.50 357 30 Yes +++ 5 20 wt% Pt on carbon (HS3000) 86 0.10 86 49 No An Example image of a membrane formed after 3 passes of membrane coating onto a Pt / C layer formed in Example 3 is shown in Figure 4. This shows the formation of bubbles and blisters which would be detrimental to electrochemical performance and durability of a CCM. Example 6: Formation of a catalyst-coated membrane A CCM with a catalyst layer as formed in Example 5 was formed by six coating passes of membrane ion-conducting polymer (800 EW PFSA, 3M), with drying to remove solvent between each pass, the inclusion of an ePTFE reinforcement component in passes 2 and 4, and with annealing of the membrane after drying of the sixth pass, at approximately 160 °C. An anode layer was applied by slot-dye coating a dispersion of iridium oxide and PFSA ionomer onto the formed membrane (on the opposite side to the cathode layer) and then drying to remove solvent. Figure 5 shows an image of a completed CCM, showing no evidence of blistering or bubbling. Example 7 - Gloss measurement Gloss measurement was carried out of an electrolyser CCM produced in accordance with the method as described herein. Gloss measurements of the cathode side of the COM (60°C, 6 measurements, mean value) provided a gloss value of 48. Gloss measurements of the anode side of the CCM (60°C, 4 measurements, mean value) provided a gloss value of 0.9. A further measurement at an angle of 85° (4 measurements, mean value) provided a gloss value of 22. Figure 6 shows images of the anode layer (Figure 6(a)) and cathode layer (Figure 6(b)). The data indicated that CCMs formed by the method as described herein may be produced with a significantly higher gloss value at the (first-deposited) cathode side which is distinct from CCMs produced by other methods, for example decal transfer of both catalyst layers onto a polymer electrolyte membrane, and easy determination of CCM orientation during electrolyser stack assembly.

Claims

1. A method of manufacturing a catalyst-coated polymer electrolyte membrane for a water electrolyser, the method comprising the steps of:(a) providing a first catalyst layer on a support substrate, the first catalyst layer comprising a platinum-containing catalyst on a carbon support material and a catalyst layer ion-conducting polymer;(b) forming a polymer electrolyte membrane on the first catalyst layer by a method comprising the steps of:(i) coating the first catalyst layer with a dispersion of a membrane ionconducting polymer to form a first polymer electrolyte membrane layer;(ii) depositing one or more additional polymer electrolyte membrane layers on the first polymer electrolyte membrane layer;and wherein the first catalyst layer has an expected effective platinum surface area in the range of and including 5 to 200 cm2Pt / cm2 and a carbon content in the range of and including 30 to 60 wt%.

2. A method according to claim 1, wherein the carbon content is in the range of and including 35 to 55 wt%.

3. A method according to claim 1 or claim 2, wherein the first polymer electrolyte membrane layer is dried prior to step (b) (ii).

4. A method according to any one of the preceding claims, wherein the method comprises the additional step of: (c) heat-treating the polymer electrolyte membrane formed in step (b)5. A method according to claim 4, wherein the polymer electrolyte membrane is heat-treated at a temperature in the range of and including 110 to 250 °C, such as 140 to 220 °C, or preferably 150 to 180 °C.

6. A method according to any one of the preceding claims, wherein the polymer electrolyte membrane has a first face adjacent to the first catalyst layer and a second opposite face, and the method comprises the additional step of: (d) applying a second catalyst layer to the second face of the polymer electrolyte membrane.

7. A method according to claim 6, wherein the second catalyst layer comprises an oxygen evolution reaction catalyst, such as an iridium-containing and / or a ruthenium-containing catalyst material.

8. A method according to any one of the preceding claims, wherein the first catalyst layer is provided in the form of discrete patches on the support substrate.

9. A method according to any one of the preceding claims, wherein the formation of a polymer electrolyte membrane in step (b) is carried out by dispersion casting.

10. A method according to any one of the preceding claims, wherein step (a) comprises the sub-steps of (i) depositing a first catalyst dispersion comprising the platinum-containing catalyst on a carbon support material and the catalyst layer ion-conducting polymer onto the support substrate; and (ii) drying the first catalyst dispersion to form the first catalyst layer.

11. A method according to any one of the preceding claims, wherein forming a polymer electrolyte membrane in step (b) comprises the incorporation of a polymeric reinforcement component in one or more of the polymer electrolyte membrane layer(s).

12. A method according to any one of the preceding claims, wherein applying a second catalyst layer to the opposite face of the polymer electrolyte membrane to the first catalyst layer in step (c) comprises the sub-steps of (i) depositing a second catalyst dispersion on the surface of the polymer electrolyte membrane; and (ii) drying the second catalyst dispersion to form the second catalyst layer.

13. A method according to any one of the preceding claims, wherein the method comprises the additional step of removing the support substrate.

14. A method according to any one of the preceding claims, wherein the method comprises the additional step of applying a seal material layer to a first face and / or a second face of the catalyst-coated polymer electrolyte membrane.

15. A method according to any one of the preceding claims, wherein the method comprises the additional step of applying a transport layer to a first face and / or a second face of the catalyst-coated polymer electrolyte membrane.

16. A catalyst-coated polymer electrolyte membrane (CCM) for a water electrolyser, the catalyst coated membrane comprising a first catalyst layer on a first face of a polymer electrolyte membrane and a second catalyst layer on the opposite second face of the polymer electrolyte membrane, and wherein:(i) the first catalyst layer comprises a platinum-containing catalyst on a carbon support material and a catalyst layer ion-conducting polymer;(ii) the first catalyst layer has an electrochemical platinum surface area in the range of and including 5 to 100 cm2Pt / cm2 and a carbon content in the range of and including 30 to 60 wt%;(iii) the first catalyst layer has a higher gloss than the second catalyst layer.

17. A catalyst-coated polymer electrolyte membrane according to claim 16, wherein the second catalyst layer comprises an oxygen evolution reaction catalyst, preferably an oxygen evolution reaction catalyst comprising iridium and I or ruthenium.

18. A catalyst-coated polymer electrolyte membrane according to claim 16 or claim 17 wherein the polymer electrolyte membrane has a thickness of at least 20 p.m, such as in the range of and including 20 to 100 p.m.

19. A catalyst-coated membrane according to any one of claims 16 to 18, wherein the polymer electrolyte membrane is a single coherent polymer film.

20. A catalyst-coated polymer electrolyte membrane, according to any one of claims 16 to 19, wherein the first catalyst layer has a gloss of at least 10 gloss units higher than the second catalyst layer.

21. A catalyst-coated polymer electrolyte membrane (CCM) for an electrolytic cell, obtained or obtainable using the method of any of claims 1 to 15.

22. A membrane-electrode assembly comprising a catalyst-coated polymer electrolyte membrane according to any one of claims 16 to 21 and a transport layer.

23. A water electrolyser comprising a catalyst coated membrane according to any one of claims 16 to 21, or a membrane-electrode assembly according to claim 22.

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