Catalyst-coated membranes for water electrolysis

Abstract

A catalyst-coated membrane (10) for a water electrolyser is provided. The catalyst-coated membrane comprises a polymer electrolyte membrane with an anode catalyst layer (12) on a first side of the membrane (14). The anode catalyst layer (12) comprises an oxygen evolution reaction catalyst containing at least one noble metal at a loading of the oxygen evolution reaction catalyst, based on the noble metal content, of less than or equal to 0.6 mg / cm2 . The polymer electrolyte membrane comprises a membrane layer comprising dispersed platinum group metal-containing nanoparticles (20), a nanoparticle stabilising agent and an ion-conducting polymer.

Description

Field of the Invention This invention relates to catalyst-coated membranes for water electrolysis. In particular, this invention relates to catalyst-coated membranes for proton exchange membrane (PEM) water electrolysis. Background Solid polymer electrolyte membranes, such as proton exchange membranes (PEMs) or anion exchange membranes (AEMs), may be employed for water electrolysis in combination with anode and cathode catalyst layers which are positioned on opposite sides of the membrane. In some cases, the anode catalyst layer and / or the cathode catalyst layer are applied to a face of the membrane to form a catalyst-coated membrane (CCM). In other cases, the respective catalyst layers may be applied to other components, such as transport layers, and the catalyst layers compressed against the membrane during assembly and subsequent use of the electrolysis cell. Hydrogen evolution reaction (HER) catalysts are used in such electrolyser cathode catalyst layers, for example, HER catalysts comprising platinum, such as platinum on a carbon support. Oxygen evolution reaction (OER) catalysts are used in electrolyser anode catalyst layers, with noble metal-containing catalysts, such as iridium or ruthenium-containing catalysts, offering a particularly good balance between OER activity and stability under electrolysis conditions. Separate film layers, typically formed from non-ion conducting polymers, may be positioned around the edge region of a CCM, 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 or elastomeric gaskets. An adhesive layer may be present on one or both surfaces of the seal film 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 features of which depend on the final MEA application and stack configuration. Such transport layers allow the reactants to reach the electrocatalyst layer and products to leave. It is desirable to reduce the thickness of polymer electrolyte membranes to minimise electronic and ionic resistance. However, it is also important to minimise any hydrogen crossover through the membrane, to avoid hydrogen mixing with oxygen and associated safety concerns. For water electrolysers, it is beneficial to maintain low levels of hydrogen crossover even at high pressure differentials across the membrane. The use of high pressures during electrolyser operation is advantageous as it reduces the requirement for compression of the generated hydrogen and reduces operating costs. It is known to produce proton exchange membranes comprising recombination catalysts which catalyse the reaction between hydrogen and oxygen to form water and therefore reduce levels of hydrogen crossover. For example, it is described in WO / 2024 / 003540 (Johnson Matthey Hydrogen Technologies Limited) that electrolyte membranes may be provided with a recombination catalyst layer. The Examples disclose the use of unsupported particles of platinum black dispersed in a membrane layer. Due to the relative scarcity and cost of platinum group metals it is desirable to reduce the amount of such metals in electrolyser components. There is a particular focus on the reduction of the amount of noble metal, such as iridium, present in water electrolyser anode catalyst layers, for example to less than or equal to 0.6 mg of noble metal per cm2. Routes to enable such reduced loading include the use of thin printed or coated catalyst layers, and I or the use of nanostructured catalysts. For example, it is described in US20230374679A1 (Greenerity GmbH) that catalyst-coated membranes may be produced by dispersing a pulverulent noble-metal-containing catalyst together with an ionomeric binder in an organic solvent or in a mixture of water and one or more organic solvents. Using a printing or coating device the resulting ink may be coated directly onto a membrane surface or initially onto a decal. The liquid medium is then evaporated affording a thin electrode layer with an areal weight of the noble-metal-containing catalyst being less than or equal to 0.6 mg / cm2. It is also known to provide a thin film coating comprising a noble-metal-containing catalyst, for example via vacuum deposition onto the surface of the electrolyte membrane, or onto a support substrate, for example Ir-nanostructured thin film materials (Ir-NSTF, 3M Corporation). The reduction in thickness and I or increase in porosity of such low iridium layers can lead to an increase in hydrogen crossover through the CCM. In order to mitigate the increased hydrogen crossover for particular operating conditions, an option would be to increase the recombination catalyst loading in the electrolyte membrane, or to increase the thickness of the electrolyte membrane. However, using such strategies will lead to a reduction in CCM performance for a given total PGM loading. There remains a need to further enhance and develop catalyst coated membranes for water electrolysers, in particular which enhance CCM performance at low iridium loadings. Summary of the invention The present inventors have identified that stabilised dispersions of platinum group metalcontaining nanoparticles may advantageously combined with ion-conducting polymers, and the resulting inks utilised to form polymer electrolyte membranes with an excellent dispersion of nanoparticles. Such membranes have been found to provide an enhanced reduction of hydrogen crossover levels as demonstrated in Example 3. The present inventors have further identified that such membranes offer particular advantages when incorporated into a catalyst-coated membrane for a water electrolyser with a low-iridium anode layer. Therefore, in a first aspect of the invention there is provided a catalyst-coated membrane for a water electrolyser, the catalyst-coated membrane comprising a polymer electrolyte membrane with an anode catalyst layer on a first side of the membrane, the anode catalyst layer comprising an oxygen evolution reaction catalyst containing at least one noble metal, wherein: (i) the loading of the oxygen evolution reaction catalyst, based on the noble metal content, is less than or equal to 0.6 mg / cm2; (ii) the polymer electrolyte membrane comprises a membrane layer comprising dispersed platinum group metal-containing nanoparticles, a nanoparticle stabilising agent and an ionconducting polymer. In a second aspect of the invention, there is provided a water electrolyser comprising a catalyst-coated membrane according to the first aspect. Brief description of the Figures Figure 1 shows a schematic representation of an example of a catalyst-coated membrane. Figure 2 shows the results of hydrogen cross-over testing of catalyst-coated membranes incorporating PVP-stabilised Pt nanoparticles and Pt-black particles. 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 catalyst-coated membranes for a water electrolyser, such as a proton exchange membrane (PEM) water electrolyser or an anion exchange membrane (AEM) water electrolyser, and in particular for PEM water electrolyser applications. The catalyst-coated membrane is provided with an anode layer on a first side of the membrane. The anode layer may be provided over substantially all of the first side of the membrane or may, for example, be provided as a patch with a region of polymer electrolyte membrane surrounding the patch upon which no anode layer is applied. The anode catalyst layer comprises an oxygen evolution reaction catalyst containing at least one noble metal. As used herein, the term noble metal refers to ruthenium, rhodium, palladium, silver, osmium, iridium, platinum and gold. Preferably, due to high OER activity, the oxygen evolution reaction catalyst comprises iridium and I or ruthenium. More preferably, the OER catalyst comprises iridium. Preferably, due to the combination of activity and stability, the oxygen evolution reaction catalyst is selected from oxides of iridium and I or ruthenium, and mixtures and alloys thereof, for example iridium oxide (IrOx), ruthenium oxide (RuOx), iridium ruthenium oxide (IrRuOx), and iridium metal oxides or ruthenium metal oxides, preferably where the metal is a transition metal. The OER catalyst may be present, for example, in the form of particles or a thin film coated onto a substrate. It may be preferred that the anode catalyst layer comprises an ion-conducting polymer and particles of the OER catalyst. Suitably, the anode catalyst layer comprises an ion-conducting polymer and particles of the OER catalyst and has a thickness less than or equal to 3.5 gm, such as less than or equal to 3.0 pm, less than or equal to 2.5 pm, or preferably less than or equal to 2.0 pm. The lower limit of thickness of the anode catalyst layer is not particularly limited and will depend on the catalyst particle size and catalyst loading, but suitably the anode catalyst layer comprises an ion-conducting polymer and particles of the OER catalyst and has a thickness of greater than or equal to 0.5 pm, such as in the range of 0.5 to 3.5 pm. In some cases, the OER catalyst may be supported on a particulate inorganic support, for example on particles of a transition metal oxide, such as titanium oxide, cerium oxide, or zirconium oxide. The OER catalyst may be provided as a thin film coating on a substrate. The term “thin film” takes its conventional meaning in the art, which will be understood by a skilled person. Suitably, the thin film coatings of the present invention have a thickness of no more than 1000 nm. The thin film coatings typically have a thickness of at least 60 nm, at least 100 nm, or preferably at least 200 nm. The thin film may be provided directly onto the surface of the polymer electrolyte membrane (i.e. the substrate is the polymer electrolyte membrane), or the substrate may be either inorganic, such as a ceramic material (for example ceramic fibres), or organic, for example a polyaromatic compound, such as a perylene. The thickness of the anode layer may be measured by scanning electron microscopy (SEM). SEM analysis is carried out on cross sections of the coating and the thickness measured at multiple (for example 10) points. The thickness values are then determined by calculating the arithmetic mean of the measured values. The loading of the oxygen evolution reaction catalyst, based on the noble metal content, is less than or equal to 0.6 mg / cm2. The loading of the OER catalyst will be understood by the skilled person to mean weight in mg of noble metal (present in the catalyst) present in a geometic area (cm2) of the anode layer of the CCM. Preferably, the loading of the oxygen evolution reaction catalyst, based on the noble metal content, is less than or equal to 0.5 mg / cm2, 0.4 mg / cm2, or 0.3 mg / cm2. Suitably, the loading of the oxygen evolution reaction catalyst is at least 0.05 mg / cm2,0.10 mg / cm2, or 0.15 mg / cm2, for example in the range of and including 0.05 to 0.6 mg / cm2, or 0.10 to 0.5 mg / cm2. The anode catalyst layer is provided on a first side of a polymer electrolyte membrane. The layers may be provided by directly coating the membrane or, for example, by decal transfer. For example, it is described in US2023374679A1 at paragraph

[0053] that anode inks may be applied to a PTFE substrate via a spiral applicator and then dried to form anode layer decal which are then used to form catalyst-coated membranes with iridium loadings between 0.16 mg / cm2 and 2 mg / cm2 in Example 1. The anode catalyst may also be formed by vapour deposition techniques, such as physical vapour deposition (PVD), i.e. the layer is vapour-deposited. A description of methods for forming anode layers via vapour deposition may be found in US patent application 63 / 626,568 (Johnson Matthey Hydrogen Technologies Limited) which is incorporated herein by reference. The polymer electrolyte membrane comprises a membrane layer comprising dispersed platinum group metal-containing nanoparticles. The term nanoparticle as used herein relates to a particle with a particle size in the range of and including 1 to 100 nm. The average particle size of the platinum-group metal containing nanoparticles in the polymer electrolyte membrane may be determined by transmission electron microscopy (TEM), for example analysing a cross-section of the membrane by TEM and, from the resulting image, measuring the size of a population of (e.g. 100) particles by image analysis and then calculating the average (mean) value. Suitably, the platinum group metal-containing nanoparticles have an average size less than 50 nm, such as in the range of and including 1 to 50 nm. It may be preferred that the platinum group metal-containing nanoparticles have an average size in the range of and including 1 to 40 nm, 1 to 30 nm, 1 to 20 nm, or 1 to 10 nm. The platinum group metals are platinum, palladium, iridium, rhodium, ruthenium, and osmium. Suitably, the platinum group metal-containing nanoparticles comprise platinum group metal, typically platinum or palladium, or consists essentially of platinum group metal, typically platinum or palladium i.e. the nanoparticles are platinum group metal nanoparticles, typically platinum nanoparticles or palladium nanoparticles. Alternatively, the platinum group metalcontaining nanoparticles may be platinum group metal alloyed with one or more of the following elements: i) another platinum group metal; ii) gold; iii) base metals, such as iron, nickel, cobalt or chromium. For example, a platinum-palladium alloy, a platinum-iridium alloy, a platinum cobalt alloy or a platinum-ruthenium alloy. The platinum group metal-containing nanoparticles are suitably recombination catalyst nanoparticles, i.e. they are a catalyst which catalyses the reaction between hydrogen gas and oxygen gas to form water. Preferably, the platinum group metal-containing nanoparticles are unsupported. The term unsupported will be readily understood by the skilled person. For example, it will be understood that the platinum group metal-containing nanoparticles are not bound or fixed to a solid catalyst support, such as a carbon support, by physical or chemical bonds, e.g. by way of ionic or covalent bonds, or non-specific interactions such as Van der Waals forces. The use of unsupported nanoparticles offers increased membrane stability during electrochemical operation, avoiding routes of degradation via corrosion or other chemical or electrochemical reactions of the support, and enables greater dispersion within the membrane. Preferably, the polymer electrolyte membrane has a platinum group metal-containing nanoparticle (e.g. platinum) loading in the range of and including 1 to 30 |ig / cm2, such as in the range of and including 5 and 25 p.g / cm2, or in the range of and including 8 and 15 gg / cm2. It has been found that this range of catalyst loading provides a suitable balance between reducing the level of hydrogen crossover during use and the cost associated with the inclusion of catalyst in the membrane. The catalyst loading may be determined by inductively coupled plasma mass spectrometry (ICP-MS). The membrane layer also comprises a nanoparticle stabilising agent. Suitable platinum group metal-containing nanoparticle stabilising agents are selected from those agents which interact with the nanoparticle surface, preventing aggregation and coalescence of the nanoparticles, and enabling the formation of a nanoparticle dispersion. Such stabilisation of the nanoparticle surface is typically through interaction between the nanoparticle with a polar functional group of the stabilising agent. Typically, the stabilising agent comprises an amide, carboxylic acid, sulphonic acid, amine, alcohol, or ether functional group. The stabilising agent may comprise an amide or an ether functional group. The stabilising agent may suitably comprise a tertiary amide group. Suitably, the stabilising agent does not have acidic functional groups. Suitably, the stabilising agent does not have sulphonic acid functional groups. Preferably, the nanoparticle stabilising agent is water-soluble, such as a water-soluble polymer. Preferably, the nanoparticle stabilising agent has a water solubility at 25 °C of at least 1 mg / mL, typically at least 10 mg / mL, more typically at least 100 mg / mL. The stabilising agent may advantageously have a greater hydrophobicity and I or a lower water uptake value than the ion-conducting polymer present in the membrane layer. Suitably, the stabilised dispersion comprises a polymeric nanoparticle stabilising agent. The stabilised dispersion may comprise a polymeric nanoparticle stabilising agent with a greater hydrophobicity and / or a lower water uptake value than the ion-conducting polymer present in the membrane layer. The polymeric nanoparticle stabilising agent may suitably have a lower weight average molecular weight than the ion-conducting polymer present in the membrane layer. Without being bound by theory, the use of a nanoparticle stabilising agent with a higher hydrophobicity and I or lower water uptake value than the membrane ionconducting polymer has the potential to improve recombination catalyst efficiency by increasing the rate of hydrogen gas access to the surface of the recombination-catalyst nanoparticles and facilitating removal of formed water from the catalyst surface. This offers increased rates of recombination of hydrogen and oxygen and therefore enhanced protection from hydrogen crossover for a given catalyst loading in the membrane. The water uptake value of the membrane ion-conducting polymer and the nanoparticle stabilising agent may be determined by drying a sample of material and then measuring the weight of the sample before and after immersion in water (e.g. at 23°C for 24 hours). For example the water uptake value may be measured by (i) drying a sample in an oven until the weight stabilises; (ii) cooling the sample in a desiccator; (iii) weighing the sample; (iv) immersing the sample in water (e.g. at 23°C for 24 hours); (iv) removing the sample and patting dry with a lint free cloth; and (v) reweighing the samples. Suitably, the polymeric stabilising agent may comprise amide functional groups, such as tertiary amide functional groups, for example pyrrolidone functional groups (such as polyvinylpyrrolidone, or copolymers including vinylpyrrolidone as a first polymerisation unit). Suitably, the stabilising agent is polyvinylpyrrolidone (PVP). The use of PVP has been found to provide excellent dispersion stability in the presence of perfluorosulphonic (PFSA) acid polymers, and greater nanoparticle dispersion stability than nanoparticles with PFSA alone. Suitably, the stabilising agent is PVP with a weight average molecular weight in the range of and including 5,000 to 50,000. Such a range is considered to provide a suitable balance between dispersion stability and ease of polymer processability. Preferably, the polymeric stabilising agent is PVP with a weight average molecular weight in the range of and including 8,000 to 45,000. The membrane layer comprises an ion-conducting polymer. Ion conducting polymers that are suitable for forming polymer electrolyte membranes are known to the skilled person and are available commercially. The ion-conducting polymer can be a proton-conducting polymer or an anion-conducting polymer, such as a hydroxyl anion-conducting polymer. Examples of suitable proton-conducting polymers include perfluorosulphonic acid ionomers (e.g. Nation® (E.l. DuPont de Nemours and Co.), Aciplex® (Asahi Kasei), Aquivion™ (Solvay Speciality Polymers), 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). Examples of suitable anion-conducting polymers include A901 and A201 made by Tokuyama Corporation, Fumasep FAA from FuMA-Tech GmbH, and Aemion polymers from lonomr. In cases in which the catalyst-coated membrane is for a PEM water electrolyser, the ionconducting polymer is suitably a proton conducting polymer, and in particular a partially- or fully-fluorinated sulphonic acid polymer. Examples of suitable proton-conducting polymers include perfluorosulphonic (PFSA) acid polymers. It may be preferred that the ion-conducting polymer is a PFSA polymer and has an equivalent weight (EW) greater than 600 EW, greater than 650 EW, or greater than 700 EW. For example, it may be preferred that the ionconducting polymer is a PFSA polymer with an equivalent weight in the range of and including 600 to 1200 EW, such as in the range of and including 700 to 1000 EW. The platinum group metal-containing nanoparticles are dispersed in a membrane layer. By dispersed in the membrane layer it is meant herein that the nanoparticles are distributed throughout the membrane layer, i.e. they are not located in a discrete region of this layer such as on the surface of a reinforcement component. The skilled person will understand that the term ‘dispersed’ does not preclude clustering of the nanoparticles, although in such cases the clusters are themselves distributed throughout the membrane layer. The membrane layer may be produced from an ink formed from mixing a stabilised dispersion of the platinum group metal-containing nanoparticles with a dispersion of the ion-conducting polymer. The stabilised nanoparticle dispersion comprises solid platinum group metalcontaining nanoparticles in a liquid phase comprising at least one nanoparticle stabilising agent which interacts with the nanoparticles to prevent nanoparticle agglomeration. It will be understood by the skilled person that the stabilising agent used to form the stabilised nanoparticle dispersion is not the same as the ion-conducting polymer with which the nanoparticle dispersion is mixed to form the ink. Typically, the stabilised dispersion is formed in an aqueous medium, such as water. Suitably, the platinum group metal-containing nanoparticles are present in the dispersion in an amount in the range of any including 0.5 to 10 g L’1, for example in the case that the nanoparticles are platinum particles, such particles are typically present in an amount in the range of any including 0.5 to 10 gpt L'1. The nanoparticle concentration may be adjusted using techniques known to the skilled person, such as evaporation or cross-flow filtration. The skilled person will be aware of methods for the production of suitable stabilised nanoparticle dispersions. For example, suitable stabilised dispersions of the platinum group metal-containing nanoparticles may be produced by mixing a suitable catalyst precursor with the stabilising agent in a solvent, such as water and then forming the nanoparticles in situ. For example, in the case of platinum nanoparticles, dispersions may be produced by mixing a platinum precursor, such as chloroplatinic acid (H2PtCle), Pt nitrate or Pt (acac), with the stabilising agent in a solvent, such as water, and then reducing the platinum precursor, for example using sodium borohydride. An example of such a preparation is described in Du, Y.K., Journal of Applied Polymer Science, Vol. 99, 23-36 (2006) which is incorporated herein by reference. The stabilised dispersion of the platinum group metal-containing nanoparticles is mixed with an ion-conducting polymer to form an ink. Typically, this is achieved by forming a dispersion of the ion-conducting polymer and then mixing this dispersion with the stabilised dispersion of nanoparticles. The ion-conducting polymer is typically dispersed in a mixture of an organic solvent and water. For example, the solvent may be a mixture of an alcohol (e.g. ethanol or propanol) and water. The volume ratio of organic solvent, such as ethanol, to water may be in the range of and including 95: 5 to 60: 40, such as in the range of and including 90: 10 to 70: 30. The solvent is formulated for achieving the desired dispersion, coating, and drying characteristics. The ink may also comprise a radical reducing additive (e.g., a peroxide radical reducing additive, such as ceria). For example, the radical reducing additive (such as a cerium containing compound, for example ceria) may be provided in the dispersion at a weight percentage, relative to the weight of ion-conducting polymer, in the range of and including 0.15 wt% to 0.35 wt%, such as in the range of and including 0.20 to 0.30 wt%. The radical reducing agent is typically added to the ink once the stabilised dispersion is mixed with the ionconducting polymer. The ink is then used to form the membrane layer. The membrane layer is typically formed by depositing the ink onto a substrate to form the layer. The ink may be deposited using a slotdie coating process (whereby the dispersion is squeezed out by gravity or under pressure via a slot onto the substrate), knife-coating, bar coating, inkjet printing, curtain coating, spray coating, or casting processes. Preferably, the coating composition can be deposited using slot-die coating, bar coating, or inkjet printing. Deposition using slot-die coating may be particularly preferred. The ink is deposited onto a substrate to form a membrane layer. In some cases, the polymer electrolyte membrane is formed from a single membrane layer. Alternatively, the polymer electrolyte membrane may be formed from two or more layers, such as between two and seven layers. The number of layers will be determined, for example, by the thickness of the desired membrane, and the degree of variation in desired composition across the membrane (for example the membranes may contain one or more layers comprising a reinforcement polymer, such as ePTFE, or an additive, such as a radical reducing additive). Typically, the substrate is a backing sheet, a polymer electrolyte membrane layer, a catalyst layer on a backing sheet, or a catalyst layer on a gas diffusion electrode. It will be understood by the skilled person that the choice of substrate will depend on the structure and stage of production of the membrane. In the case that the membrane is formed of a single membrane layer, or at the start of production of a multi-layer membrane, the substrate is typically a backing layer. The backing layer provides support for the polymer electrolyte membrane during manufacture and if not immediately removed, can provide support and strength during any subsequent storage and / or transport. The material from which the backing layer is made should provide the required support, preferably be compatible with the ink, preferably be impermeable to the ink, be able to withstand the process conditions involved in producing the polymer electrolyte membrane and be able to be easily removed without damage to the polymer electrolyte membrane. Examples of materials suitable for use include a fluoropolymer, such as polytetrafluoroethylene (PTFE), ethylene tetrafluoroethylene (ETFE), perfluoroalkoxy polymer (PFA), fluorinated ethylene propylene (FEP - a copolymer of hexafluoropropylene and tetrafluoroethylene), and polyolefins, such as biaxially oriented polypropylene (BOPP). In some cases, a catalyst layer is provided on a backing layer, for example by printing or using known coating techniques. The coating composition may then be deposited onto the catalyst layer such that the catalyst layer is disposed between the backing layer and the membrane layer formed by depositing the ink. In some cases, typically when the membrane thickness is such that multiple passes are required in order to build up the membrane structure, the substrate is a previously formed membrane layer. It will be understood that the polymer electrolyte membrane may be formed by sequential deposition of layers. As an example, polymer electrolyte membranes may be formed as follows. In the first pass, an ink containing an ion-conducting polymer may be deposited onto a backing layer to form a first ion-conducting polymer layer, which is then dried. In a second pass, an ink is deposited onto the first ion-conducting polymer layer to form a second ion-conducting polymer layer. The second ion-conducting polymer layer is then dried. This sequence of application and drying is continued to produce further ion-conducting polymer layers as required to form the desired membrane structure. It will be understood by the skilled person that the nanoparticle-containing ink as described hereinbefore is used in one or more of the coating passes as required by the final membrane structure. Suitably, the polymer electrolyte membrane has a thickness of less than or equal to 100 pm. It may be preferred that the membrane has a thickness of less than or equal to 95 p.m, 90 gm, or 85 urn. Suitably, the membrane has a thickness of at least 10 pm, such as at least 15 pm, at least 20 pm, at least 25 pm, at least 30 pm or at least 40 pm. Preferably, the membrane has a thickness in the range of and including 10 to 100 pm, such as 15 to 100 pm, 20 to 100 pm, 30 to 100 pm, 30 to 90 pm, or 40 to 90 pm. The polymer electrolyte membrane thickness may be measured by scanning electron microscopy (SEM). SEM analysis is carried out on cross sections of the membrane and the membrane and / or layer thickness measured at multiple (for example 10) points. The thickness values are then determined by calculating the arithmetic mean of the measured values. Suitably, the SEM measurement is carried out on a cross section of the membrane prepared at 0% relative humidity which is embedded in resin, ground and polished. The polymer electrolyte membrane comprises a membrane layer comprising dispersed platinum group metal-containing nanoparticles. It will be understood by the skilled person that the membrane may comprise more than one membrane layer comprising dispersed platinum group metal-containing nanoparticles, such as two or more membrane layer comprising dispersed platinum group metal-containing nanoparticles. It may be preferred that the membrane has a single membrane layer comprising dispersed platinum group metalcontaining nanoparticles. Typically, the or each platinum group metal nanoparticle-containing membrane layer has a thickness in the range of and including 5 to 30 p.m. The dispersion of nanoparticles in a membrane layer of at least 5 p.m offers improved membrane stability benefits in comparison with the use of thinner catalyst layer, e.g. applied to a membrane surface. The use of a membrane layer comprising dispersed nanoparticles with a thickness greater than 30 gm is not required to substantially reduce hydrogen crossover and can provide manufacturing difficulties, in particular when forming non-laminated membrane structures. The thickness of the membrane layer may be determined by SEM analysis of a cross-section of the membrane as hereinbefore described. Preferably, the platinum group metal nanoparticle-containing membrane layer has a thickness in the range of and including 5 to 25 pm, such as between 7 and 15 pm. Such thicknesses offer a suitable balance between the reduction of hydrogen crossover by the formed membrane and manufacturing efficiency. It is preferred that the membrane is formed by methods that do not require lamination steps to form the membrane, for example by depositing multiple layers of ion-conductive polymer on top of each other via a liquid phase deposition process such as printing, spraying, or coating. It is preferred that the membrane is a single coherent polymer film comprising a plurality of ion-conducting polymer layers. The term ‘coherent’ as used herein means that the membrane is free from internal lamination interfaces. Lamination of ion-conductive membranes comprises pressing and / or bonding at least two solid ion-conductive membranes together, such membranes optionally being coated with a catalyst layer. A lamination interface is formed between the two membranes where solid surfaces of the individual membranes are pressed and / or bonded together. Lamination interfaces comprise physical defects. Furthermore, the structural and / or chemical nature of a lamination interface also differs from that of the bulk polymer material. This is because when a solid membrane is formed, the outer surfaces of the solid membrane have surface features which are distinct from those in the bulk material. For example, a hydrophobic skin forms on a surface of a membrane at an air interface. Raman spectroscopy can detect this difference. As such, when two solid membranes are pressed together, the lamination interface formed by the two solid surfaces is distinctive in chemical and / or structural form compared to the bulk of the ion-conductive polymer material. Microscopy and spectroscopy techniques can thus distinguish between lamination interfaces between layers of ion-conductive polymer and interfaces which have been formed via a liquid phase deposition process such as printing, spraying, or coating of layers to build up a multi-layer structure. That is, a non-laminated interface is structurally and / or chemically distinct from a laminated interface and is not just a feature of the manufacturing method. Furthermore, a non-laminated interface can be identified as being non-laminated in a membrane without prior knowledge of the manufacturing method. Examples of analysis techniques for detecting a laminated interface include cross-section SEM. Variations of crystallinity at interfaces can be detected using cross-section TEM. Other techniques for detecting laminated interfaces include 13C / 1H / 19F solid state NMR, neutron diffraction, and / or a combination of two or more of the aforementioned techniques. Due to physical defects and / or chemical variations at lamination interfaces between polymer electrolyte membranes, such interfaces can increase the resistance of a multi-layer ion conductive membrane. As such, it has been found to be advantageous to fabricate a multilayer 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 conductive polymer. Suitably, the membrane layer comprising dispersed platinum group metal-containing nanoparticles is disposed between a first membrane layer adjacent to the first side of the membrane and a second membrane layer adjacent to the second side of the membrane. Suitably, the first and the second membrane layers do not comprise dispersed platinum group metal-containing nanoparticles. It will be understood by the skilled person that the first membrane layer and a second membrane layer may be formed from one or more sub-layers, which may be of the same or different composition. Preferably, the thickness of the first membrane layer is less than the thickness of the second membrane layer. This asymmetry enables the catalyst nanoparticles to be placed closer to the anode than the cathode in a water electrolyser configuration, which is considered beneficial for the reduction in hydrogen crossover. Typically, a reinforcement polymer component (such as ePTFE) and / or a radical reducing agent (e.g. a peroxide radical reducing additive, such as ceria) is present in the first and I or the second membrane layer. Preferably, a reinforcement polymer component, is not present in the or each membrane layer comprising dispersed platinum group metal-containing particles. Typically, a cathode catalyst layer is provided on the second side of the electrolyte membrane (i.e. the opposite side of the membrane to the anode catalyst later. Such cathode catalyst layers comprise a hydrogen evolution reaction catalyst, such as a platinum-based catalyst, for example platinum on a carbon support (Pt / C). Suitably, the cathode catalyst layer comprises a hydrogen evolution reaction catalyst, such as a platinum-containing catalyst and an ionconducting polymer. Such layers may be provided by directly coating the membrane or, for example, by decal transfer as hereinbefore described for the anode layer. An example of a catalyst-coated membrane (10) is shown in Figure 1. The catalyst-coated membrane (10) has an anode layer (12) on a first side (14) of a polymer electrolyte membrane and a cathode layer (16) on a second side (18) of the membrane. The membrane has a layer (20) comprising dispersed platinum group metal-containing nanoparticles which is disposed between first (22) and a second (24) membrane layers which do not include dispersed nanoparticles. Protective film layers, typically formed from non-ion conducting polymers, may be positioned around the edge region of the CCM, 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. An adhesive layer may be present on one or both surfaces of the protective film layer. In a water electrolyser, additional transport layers are positioned each side of the catalyst-coated membrane to facilitate reagent and product transfer to and from the catalyst layers, and to provide electrical contact. The catalyst-coated membrane and transport layer(s) are referred to together as a membrane electrode assembly (MEA). These additional transport layers may be known as porous transport layers or gas diffusion layers. These layers may or may not be directly attached to the CCM. Other components of a water electrolyser may include bipolar plates and current collector plates. Stacks of such assemblies make up an electrolyser system including power and control systems. Suitable transport layers at the anode side of the CCM are known to the skilled person and 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 adjacent to the CCM (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 nonwoven cloths, mesh, foams and sintered compacts of metal-containing particles. ForPEMWE applications, suitable transport layers comprise titanium. For AEMWE applications, suitable transport layers comprise nickel or stainless steel. Suitable transport layers at the cathode side of the CCM 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 from Avcarb Material Solutions, or woven carbon cloths. The carbon paper, web or cloth may be provided with a further treatment prior to being incorporated into as MEA 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. 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 Testing of hydrogen crossover levels The level of hydrogen crossover for each CCM was measured at different pressures using the following method: A water electrolysis cell was prepared incorporating the catalyst-coated membrane to be tested. The cell temperature was held at to 80 °C and the anode and cathode pressure were set to 2 bar. Next, the current density was set to 2 A / cm2. The cathode pressure was increased stepwise from 2 to 6, to 10 bar with a minimal duration of 45 minutes for each step. The % of H2 in the oxygen at the anode gas outlet was measured by a Compact GC 4.0 Gas Chromatograph (GC) from Global Analysis Solutions. Example 1 - Formation of a stabilised dispersion of platinum nanoparticles using PVP and formaldehyde. Pt(NOa)4 (equivalent to 1g of Pt) was added to water (500mL) and stirred. PVP10 (average molecular 10,000, 8.5g) was added followed by the addition of formaldehyde (37% in water, 20.8 g). The mixture was heated to 68 °C and then allowed to cool to room temperature and stirred overnight to form a dispersion. Example 2 - Formation of an ion-conducting polymer inks containing PVP-stabilised nanoparticles A stabilised aqueous dispersion of Pt nanoparticles (formed according to a method analogous to Example 1) was mixed with ethanol and water to create a mixture with a ethanol: water weight ratio of 80:20. Dry ionomer (3M Corp, 800 EW) is added to the mixture to create a dispersion, where the ionomer solids is — 17 %wt. Example 3 - Hydrogen cross-over testing at a range of Pt loadings A series of catalyst-coated membranes were prepared with different loadings of stabilised Pt nanoparticles and the following structure: (1) Pt / C-containing cathode layer (2) ~60 micron PFSA membrane (with two ePTFE reinforcements) (3) ~ 10 micron Pt particle containing membrane layer (4) ~10 micron PFSA membrane layer (5) Iridium oxide (IrOx)-containing anode layer Layers 3 and 4 were applied using film applicators (slot die and baker coater). Layers 1 and 5 were attached to the composite layer 2-3-4 using lamination at a temperature greater than the ionomers transition temperature (160 °C). The layers containing recombination catalyst were prepared with either (i) ion-conducting polymer inks containing PVP-stabilised Pt nanoparticles (“PVP-Pt” in Figure 2) prepared using a method analogous to that of Example 2; or (ii) ion-conducting polymer inks containing unsupported Pt particles (without the use of PVP) (“Pt particles” in Figure 2). The CCMs were tested for hydrogen cross-over as shown in Figure 2. This shows that the use of a stabilised nanoparticle dispersion (Pt-PVP) can achieve almost complete reaction of hydrogen crossing through the membrane under the test conditions even at low platinum loading, with significantly improved performance when considered alongside comparative CCMs not prepared with a stabilised nanoparticle dispersion. Example 4 - Formation of a coherent polymer film comprising a layer incorporating dispersed Pt nanoparticles. An electrolyte membrane with a thickness of approximately 50 gm was produced through series of five coating passes onto a backing substrate using a slot-dye coater. Each pass comprised the deposition of a dispersion of PFSA ionomer in a water-ethanol mixture. Each membrane-sub layer was dried to remove solvent prior to the next coating pass. The coating passes were: (1) Coating pass using a dispersion of ionomer; (2) Coating pass using a dispersion of ionomer; (3) Coating pass using a dispersion of ionomer with incorporation of an ePTFE reinforcement; (4) Coating pass using a dispersion of ionomer and PVP-stabilised platinum nanoparticles; (5) Coating pass using a dispersion of ionomer; The formed membrane was annealed at around 160 °C.

Claims

1. A catalyst-coated membrane for a water electrolyser, the catalyst-coated membrane comprising a polymer electrolyte membrane with an anode catalyst layer on a first side of the membrane, the anode catalyst layer comprising an oxygen evolution reaction catalyst containing at least one noble metal, wherein:(i) the loading of the oxygen evolution reaction catalyst, based on the noble metal content, is less than or equal to 0.6 mg / cm2;(ii) the polymer electrolyte membrane comprises a membrane layer comprising dispersed platinum group metal-containing nanoparticles, a nanoparticle stabilising agent and an ion-conducting polymer.

2. A catalyst-coated membrane according to claim 1, wherein the polymer electrolyte membrane has a cathode layer comprising a hydrogen evolution reaction catalyst on a second side of the membrane.

3. A catalyst-coated membrane according to claim 1 or claim 2, wherein the loading of the oxygen evolution reaction catalyst, based on the noble metal content, is in the range of and including 0.05 to 0.5 mg / cm2.

4. A catalyst-coated membrane according to any proceeding claim, wherein the oxygen evolution catalyst is iridium oxide or an iridium metal oxide.

5. A catalyst-coated membrane according to any proceeding claim, wherein the anode catalyst layer comprises an ion-conducting polymer and particles of the oxygen evolution reaction catalyst.

6. A catalyst-coated membrane according to any one of claims 1 to 4, wherein the anode catalyst layer comprises a thin film coating of the oxygen evolution reaction catalyst on the first side of the membrane.

7. A catalyst-coated membrane according to any one of the preceding claims, wherein the nanoparticle stabilising agent has a higher hydrophobicity and / or a lower water uptake value than the ion-conducting polymer in the membrane.

8. A catalyst-coated membrane according to any one of the preceding claims, wherein the nanoparticle stabilising agent is a polymer, such as a polymer comprising pyrrolidone functional groups.

9. A catalyst-coated membrane according to any one of the preceding claims, wherein the nanoparticle stabilising agent is polyvinylpyrrolidone, or a copolymer including vinylpyrrolidone as a first polymerisation unit.

10. A catalyst-coated membrane according to any one of the preceding claims, wherein the average particle size of the platinum group metal-containing nanoparticles is less than 50 nm.

11. A catalyst-coated membrane according to any one of the preceding claims, wherein the polymer electrolyte membrane comprises a first ion-conducting polymer layer and a second ion-conducting polymer layer, and wherein the membrane layer comprising dispersed platinum group metal-containing nanoparticles is disposed between the first and the second ion-conducting polymer layers.

12. A catalyst-coated membrane according to any one of the preceding claims, wherein the electrolyte membrane is a single coherent polymer film comprising a plurality of ion-conducting polymer layers.

13. A catalyst-coated membrane according to any one of the preceding claims, additionally comprising a protective film layer.

14. A water electrolyser comprising a catalyst-coated membrane according to any one of claims 1 to 13.19

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