Membrane-electrode assembly for a water electrolyser

Abstract

A membrane-electrode assembly for a water electrolyser is provided. The membrane-electrode assembly comprises a polymer electrolyte membrane with a first major surface and a second major surface, and an anode component in contact with the first major surface of the polymer electrolyte membrane. The anode component comprises (i) a porous framework of polymer fibres at least partially coated with a metal-containing thin film; and (ii) an oxygen evolution reaction (OER) catalyst supported on the porous framework of polymer fibres.

Description

[0001] MEMBRANE-ELECTRODE ASSEMBLY FOR A WATER ELECTROLYSER

[0002] Field of the Invention

[0003] The present invention relates to membrane-electrode assemblies for use in a water electrolyser, such as a proton exchange membrane water electrolyser.

[0004] Background of the Invention

[0005] The electrolysis of water to produce hydrogen and oxygen can be carried out in both alkaline and acidic electrolyte systems. Those electrolysers that employ a solid proton-conducting polymer electrolyte membrane, or proton exchange membrane (PEM), are known as proton exchange membrane water electrolysers (PEMWEs). Those electrolysers that utilise a solid anion-conducting polymer electrolyte membrane, or anion exchange membrane (AEM), are known as anion exchange membrane water electrolysers (AEMWEs).

[0006] Catalyst-coated membranes (CCMs) may be employed within such water electrolysers. Such CCMs comprise an ion-conducting membrane, such as a PEM or 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.

[0007] For water electrolyser applications, hydrogen evolution reaction (HER) catalysts are used in 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. Iridium and I or ruthenium-containing catalysts are well known for their properties as excellent OER catalysts and are preferred materials for the oxygen evolution reaction on the anode side of a water electrolyser.

[0008] Minimising resistance in electrolysis cells is crucial for enhancing energy efficiency, reducing power consumption and improving overall performance. Higher resistance leads to increased energy loss as heat, requiring more voltage to drive the reaction, and hindering the cell's ability to operate effectively at high current densities. The resistance of an electrolysis cell has a contribution from each of the cell components, including catalyst layers and the polymer electrolyte membrane.

[0009] Due to the scarcity of metals, such as iridium, and the growing demand for electrolytically produced hydrogen there is a need to reduce the amount of catalyst metal present in the anode layer of electrolyser CCMs. However, reduction in catalyst loading can lead to poor electrochemical performance. When the anode loading of an electrolyser is thrifted, it has been observed that losses due to the reduction of in-plane conductivity can be more important than the inevitable kinetic losses.

[0010] It is also desirable to reduce the thickness of the ion-conducting membranes used in water electrolysers to minimise electronic and ionic resistance and therefore to increase efficiency. However, as the membrane is thinner, distortion of the membrane may be observed at operating pressure differentials across the CCM. This distortion can cause degradation in contact with other electrolyser components, in particular the porous transport layers (PTLs) provided on the anode side of an electrolysis cell.

[0011] It is described in Higashi, S., Beniya, A.; Applied Catalysis B, Environmental 321 (2023) 122030 (“Higashi”) that conductive I rC>2 nanostructured textiles made solely of lrC>2 may be used as the anode layer of an electrolyser. In the Examples, the I rC>2 nanostructured textiles are prepared by I rC>2 deposition onto nanofibres formed from a water-soluble polymer (PVP) which is removed by water dissolution to form the desired IrOx nanostructure.

[0012] It is described in JP2023174340 (TOPPAN HOLDINGS INC) and W02024106060A1 (TOPPAN HOLDINGS INC), and US2023317968A1 (TOPPAN INC) that polymer fibres may be added to electrolyser anode layers incorporating IrOx. The addition of such fibres can lead to a reduction in anode layer conductivity.

[0013] There remains a need to develop membrane-electrode assembly structures for water electrolysers which facilitate the use of low catalyst loadings in the anode layer and I or thinner ion-conducting (polymer electrolyte) membranes, whilst maintaining or improving electrochemical performance and durability.

[0014] Summary of the invention

[0015] The present inventors have identified certain electrolyser anode configurations comprising a porous framework of polymer fibres that offer benefits with regards to one or more of the aforementioned problems. The porous framework acts as the anode component of the electrolysis cell and comprises polymer fibres provided with both a partial coating of a metalcontaining thin film and OER catalyst supported on the polymer fibres. Such a configuration successfully enables the lateral and through-plane conductivity of the anode component to be tuned, and therefore the anode layer resistance to be reduced, without increasing OER catalyst loading and facilitates excellent electrochemical performance at low OER catalyst loadings. It is considered by the present inventors that the presence of the porous framework of polymer fibres also offers durability benefits, for example due to a reduction in degradation arising from interactions between metallic porous transport layers and the polymer electrolyte membrane during electrolyser operation, for example due to extrusion of the membrane into the porous transport layer which can cause electroflaking, membrane cracking and delamination.

[0016] Therefore, in a first aspect of the invention, there is provided a membrane-electrode assembly for a water electrolyser, the membrane-electrode assembly comprising a polymer electrolyte membrane with a first major surface and a second major surface, and an anode component in contact with the first major surface of the polymer electrolyte membrane, the anode component comprising, or consisting essentially of:

[0017] (i) a porous framework of polymer fibres at least partially coated with a metal-containing thin film; and

[0018] (ii) an oxygen evolution reaction (OER) catalyst supported on the porous framework of polymer fibres.

[0019] When incorporated into a water electrolysis cell, such membrane-electrode assemblies show significantly reduced anode layer resistance even at low anode catalyst loadings. Preferably, the OER catalyst comprises a noble metal, more preferably iridium and I or ruthenium, and the loading of the OER catalyst, based on the total noble metal content of the OER catalyst, is preferably less than or equal to 0.6 mg / cm2(such as less than or equal to 0.6 mgir+Ru / cm2).

[0020] In a second aspect of the invention, there is provided a water electrolyser comprising: (i) a membrane-electrode assembly according to the first aspect; and (ii) a porous transport layer in contact with the anode component.

[0021] In a third aspect of the invention, there is provided a process for the manufacture of a membrane-electrode assembly, such as an MEA of the first aspect, the process comprising the steps of: (i) providing an anode component comprising a porous framework of polymer fibres at least partially coated with a metal-containing thin film, and with an oxygen evolution reaction (OER) catalyst supported on the porous framework of polymer fibres; (ii) contacting the porous framework of polymer fibres with a polymer electrolyte membrane to form the membrane-electrode assembly.

[0022] Description of the Figures Figure 1 shows schematic representation of a membrane electrode assembly incorporating a porous framework of polymer fibres.

[0023] Figure 2 shows the results of electrochemical testing of the membrane-electrode assembly formed in Example 4.

[0024] Figure 3 shows the results of electrochemical testing of the membrane-electrode assembly formed in Example 8.

[0025] Detailed Description

[0026] 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.

[0027] The present invention provides a membrane-electrode assembly (MEA) for a water electrolyser. Suitably, the MEA is for a proton-exchange membrane (PEM) water electrolyser, or for an anion-exchange membrane (AEM) water electrolyser. Preferably, the MEA is for a PEM water electrolyser.

[0028] In the case that the polymer electrolyte membrane is a proton-exchange membrane (PEM), the membrane is typically formed from a perfluorinated sulfonic acid (PFSA) ionomer, a partially-fluorinated sulfonic acid ionomer, non-fluorinated sulfonic acid ionomer (such as a non-fluorinated hydrocarbon sulfonic acid ionomer), or mixtures thereof. Suitable membranes are available commercially, and include Nation (RTM) membranes (Chemours) and Pemion (RTM) membranes (lonomr Innovations).

[0029] In the case that the polymer electrolyte membrane is an anion-exchange membrane (AEM), the membrane is typically formed from a hydroxide-conducting polymer, for example an ionconducting polymer comprising quaternary ammonium functional groups. Suitable membranes are available commercially and include Aemion (RTM) membranes (lonomr Innovations) and PiperlON (RTM) membranes (Versogen).

[0030] Typically, the thickness of the polymer electrolyte membrane is less than or equal to about 200 pm, such as less than or equal to 150 pm, or less than or equal to 100 pm. The membrane electrode assembly configuration as described herein offers particular benefits in combination with a polymer electrolyte membrane with a thickness less than or equal to 100 m offering a reduction in mechanical degradation on the anode side of the membrane during use through mitigation of the potential for the membrane to distort in contact with the relatively course structure of typical porous transport layers, especially at high differential pressures. Suitably, the polymer electrolyte membrane has a thickness of at least about 8 pm. It may be preferred that the polymer electrolyte membrane has a thickness of at least about 10 pm, at least about 12 pm, at least about 15 pm, or at least about 20 pm. The thickness of the membrane may be determined by analysis of a scanning electron microscope (SEM) image of a cross section of the membrane (suitably dried at 0% relative humidity). It may be preferred that the membrane has a thickness in the range of and including 8 pm to 200 pm, or 10 to 100 pm.

[0031] It may be preferred that the polymer electrolyte membrane comprises at least one polymeric reinforcement component. Such reinforcement components are known to the skilled person and may, for example, be suitably formed from expanded polytetrafluoroethylene (ePTFE) or polybenzimidazole (PBI). The polymer electrolyte membrane may suitably contain one polymeric reinforcement component. Suitably, the polymer electrolyte membrane has one polymeric reinforcement component and has a thickness in the range of and including 10 pm to 60 pm. Such membrane materials offer an excellent combination of durability and conductivity.

[0032] The polymer electrolyte membrane may suitably contain two or more polymeric reinforcement components. Suitably, the polymer electrolyte membrane has two polymeric reinforcement components and has a thickness in the range of and including 50 pm to 100 pm. Such membranes offer an excellent combination of durability and strength whilst maintaining high ion conductivity. Such membranes may be of particular utility in cases when during operation there is a large difference in the gas pressure on opposite sides of the membrane.

[0033] In some other embodiments the polymer electrolyte membrane comprises a reinforcing component in the form of a woven fabric, such as a woven fabric formed from polymer threads, such as ePTFE or PEEK threads. Suitable materials are described in US11742507B2 (AGC INC).

[0034] The polymer electrolyte membrane 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 and manganese oxides. A recombination catalyst catalyses the reaction of H2 and O2 to form H2O. Suitable recombination catalysts can comprise a metal (such as platinum). Particularly suitable polymer electrolyte membranes incorporating a recombination catalyst are described in W02023052750A1 (Johnson Matthey Public Limited Company) which is incorporated herein by reference. Polymer electrolyte membranes incorporating an additive, such as a recombination catalyst, may alternatively be formed by applying a layer incorporating the additive and an ionconducting polymer to the surface of a polymer electrolyte membrane. In such cases, this layer is considered to be part of the polymer electrolyte membrane in the MEA configurations described herein, i.e. in such cases the first major surface of the polymer electrolyte membrane is considered to be the surface of the membrane layer incorporating the additive that has been applied to the polymer electrolyte membrane.

[0035] The polymer electrolyte membrane has a first major surface and a second major surface. The MEA comprises an anode component in contact with the first major surface of the polymer electrolyte membrane. The anode component comprises a porous framework of polymer fibres. The polymer fibres are at least partially coated with a metal-containing thin film, i.e. a thin film coating is present on the surface of the polymer fibres forming the porous framework. Preferably, the polymer fibres are partially coated with the metal-containing thin film. It may be preferred that some regions of the porous framework have a substantially continuous coating of the metal-containing thin film (such as regions adjacent to each major surface of the porous framework), whilst other regions have a partial coating or are not coated. This enables the amount of metal in the anode component to be thrifted whilst maximising conductivity of the anode component.

[0036] 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 2000 nm, typically no more than 1750 nm, or preferably no more than 1500 nm, no more than 1000 nm, no more than 500 nm, or no more than 300 nm. The thin film coatings typically have a thickness of at least 10 nm, or preferably at least 20 nm, at least 30 nm, or at least 40 nm. Accordingly, the thin film coatings of the invention may have a thickness in the range of and including 10 to 2000 nm, preferably 20 to 1000 nm, 30 to 500 nm, or 40 to 300 nm. The thickness of the thin film coating may be measured by scanning electron microscopy (SEM). SEM analysis is carried out on cross sections of the structure 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.

[0037] The metal-containing thin film increases the lateral and through-plane conductivity of the porous polymer framework. Suitably, the metal-containing thin film comprises a transition metal, or an oxide or alloy thereof. The metal-containing thin film is selected based on its conductivity and stability in the operating conditions of the water electrolyser (as can be determined for example by acid or base stability measurements of the porous polymer framework as set out herein). It will be understood by the skilled person that the metalcontaining thin film is a different material from the OER catalyst, facilitating high conductivity of the porous framework whilst enabling thrifting or enhancing the performance of the OER catalyst. Preferably, the metal-containing thin film does not contain iridium or ruthenium.

[0038] In some preferred embodiments, the metal-containing film comprises, or consists essentially of, niobium, platinum or gold. These elements provide particular beneficial combinations of high conductivity and resistance to oxidation and dissolution. Preferably, the metal-containing film is a platinum thin film.

[0039] In some preferred embodiments, the metal-containing film comprises a doped or undoped oxide or nitride of a transition metal or a mixture of transition metals. Such films are selected to increase the conductivity of the polymer framework. Suitable materials include Magneli phase Ti4O7, oxides of niobium (for example one or more of Nb2Os, NbO2and NbO) and TiN.

[0040] When assembled into an MEA, the anode component comprises a first region adjacent to the polymer electrolyte membrane and a second region distal to the polymer electrolyte membrane. It may be preferred that the amount of the metal-containing thin film is higher in the second region than in the first region. The amount of metal coating in the first region and the second region may be determined by, for example, using scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) analysis of a crosssection of the MEA. Providing a greater amount of the metal-containing thin film in the second region enables overall thrifting of the metal requirement of the MEA, whilst maintaining the required conductivity on the side of the anode component which contacts a transport layer in use.

[0041] The skilled person will understand that the loading of the metal-containing film will depend on the conductivity of the film and can be varied depending on the desired properties of the MEA. By loading of the metal-containing film, it is meant herein the weight of metal (present in the thin film) per unit area of the anode component. For example, in the case of a platinum thin film, the loading of metal containing film will be the weight of platinum per unit area of the anode component.

[0042] Preferably, loading of metal-containing film is less than or equal to 0.5 mg I cm2, such as less than or equal to 0.4 mg I cm2. Suitably, the loading is at least 0.05 mg I cm2. Preferably, the loading of metal-containing film is in the range of and including 0.05 to 0.5 mg I cm2, or 0.05 to 0.4 mg I cm2.

[0043] For example, in the case of a platinum thin film, it is preferred that the loading of Pt is less than or equal to 0.5 mgptI cm2of the anode component, such as less than or equal to 0.4 mg I cm2. Suitably, the loading of Pt is at least 0.05 mgptI cm2. Preferably, the loading of Pt is in the range of and including 0.05 to 0.5 mgpt I cm2, or 0.05 to 0.4 mgpt I cm2. Such a loading of the metal-containing film provides a suitable balance between conductivity and a desire to reduce overall metal use in the MEA.

[0044] An oxygen evolution reaction (OER) catalyst is supported on the porous framework of polymer fibres. By supported on the porous framework it is meant herein that the OER catalyst is bound or fixed to the porous framework by physical or chemical bonds, e.g. by way of ionic or covalent bonds, or non-specific interactions such as Van der Waals forces. It will be understood by the skilled person that supported on the porous framework includes the OER catalyst being fixed or bound to the polymer fibres that form the framework and I or the OER catalyst being fixed or bound to the metal-containing thin film at least partially coating the polymer fibres that form the framework.

[0045] The type of OER catalyst is not particularly limited in the current invention so long as the catalyst materials may be provided in a suitable form to be supported on the porous framework of polymer fibres, for example by wet layer deposition, or by chemical or physical deposition, and be stable under 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 I 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 I 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 porous framework of polymer fibres is at least partially coated with a platinum thin film and the OER catalyst comprises iridium and I or ruthenium. More preferably, the OER catalyst is an oxide of iridium and I or ruthenium.

[0046] 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.

[0047] The OER catalyst is suitably provided in the form of a thin film partially coating the porous framework of polymer fibres. The thin film of OER catalyst is typically provided at least partially on top of the metal-containing thin film, i.e. there is a degree of overlap between the metal-containing thin film and the OER catalyst film. In such cases, in the areas of overlap, the polymer fibre has a metal-containing thin film on its surface and then the OER catalyst is provided on the metal-containing thin film. It will be understood by the skilled person that the OER catalyst may be at least partially present as a thin film directly on the surface of the polymer fibres (e.g. in areas where there is no overlap), or may be present only in areas in which there is a metal-containing thin film.

[0048] Preferably, the polymer fibres have a partial coating of a platinum thin film and a partial coating of a thin film of an OER catalyst (such as an iridium and I or ruthenium-containing thin film).

[0049] Preferably, the thin film of an OER catalyst satisfies the following requirements: (i) the thin film coating has a first region adjacent to the surface of the polymer fibres and a second region distal to the surface of the polymer fibres; and (ii) the porosity of the second region of the thin film is higher than the porosity of the first region of the thin film coating. The thin film coating advantageously has a graded porosity. The thin film coating may consist of the first region and the second region. The thin film coating may have a third region between the first region and the second region. The relative porosity of the first region and of the second region may be determined by SEM analysis of cross sections of the thin film. Parts of each image which correspond to pores or to solid structure may be classified and then the number of pixels in each region of interest counted and used to calculate the relative porosity of regions. Porosity is measured at multiple (for example, 10) points of each region. Providing a first region with a lower porosity provides a high lateral electronic conductivity to the thin film layer whilst maintaining a high surface area in the second region. Thin film coatings (such as IrOx coatings) with graded porosity may be suitably formed by physical vapour deposition. The graded porosity of the thin film may be achieved using a magnetron sputtering process by selection of total chamber pressure, partial pressure of oxygen and pulsed DC power. For example, by flowing into a sputtering chamber Ar and O2 gases, and controlling pumping speed by way of a variable speed vacuum pump or a throttle valve, it is possible to achieve a controllable oxygen partial pressure.

[0050] The OER catalyst may also be provided in particulate form, for example in the form of OER catalyst nanoparticles (i.e. particles with a size between about 1 and 100 nm) supported onto the polymer framework, such as nanoparticles comprising iridium and I or ruthenium, for example doped or undoped oxide of iridium and I 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.

[0051] As described hereinbefore, when assembled into an MEA, the anode component comprises a first region adjacent to the polymer electrolyte membrane and a second region distal to the polymer electrolyte membrane. It is preferred that the amount of the OER catalyst is higher in the first region than in the second region. The amount of OER catalyst in the first region and in the second region may be determined by, for example, using scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX). Providing, a greater amount of the OER catalyst in the first region enables overall thrifting of the metal (e.g. iridium) requirement of the MEA, whilst maintaining the performance of the catalyst layer.

[0052] The skilled person will understand that the loading (weight per unit area of the anode component) of the OER catalyst will depend, for example, on the catalyst activity of the OER catalyst and the loading may be varied accordingly.

[0053] For example, in the case that the OER catalyst comprises a noble metal, preferably iridium and I or ruthenium, the loading of the OER catalyst, based on the total noble metal content of the OER catalyst, is preferably less than or equal to 0.6 mg / cm2(such as less than or equal to 0.6 mgir+RU / cm2), less than or equal to 0.5 mg / cm2or less than or equal to 0.4 mg / cm2. The loading of the OER catalyst, based on the total noble metal content of the OER catalyst, is typically at least 0.05 mg / cm2, or at least 0.1 mg / cm2. It may be preferred that the loading of the OER catalyst, based on the total noble metal content of the OER catalyst, is in the range of and including 0.05 to 0.6 mg / cm2, such as 0.05 to 0.5 mg / cm2or 0.05 to 0.4 mg / cm2.

[0054] It has been found by the present inventors that the MEA configurations as described herein advantageously facilitate high OER activity at low metal (e.g. Ir+Ru) content. The loading of the OER catalyst may be determined by techniques known to the skilled person, for example X-ray Fluorescence (XRF) analysis.

[0055] The membrane-electrode assembly comprises an anode component. The anode component is in contact with the first major surface of the polymer electrolyte membrane. By “in contact with” it is meant herein that the anode component is adjacent to and touching the first major surface of the polymer electrolyte membrane. In some cases, the anode component may be partially embedded into the polymer electrolyte membrane. In some cases, the anode component is not embedded into the polymer electrolyte membrane.

[0056] The use of an anode component as described herein offers high in plane conductivity at lower thicknesses and higher porosities, than alternative components, such as particulate interface layers. Polymer fibres with a thin film metal coating are desirable as they provide in-plane conductivity between contact points of the porous transport layer positioned on the anode side of the membrane-electrode assembly. The use of polymer fibres also offers advantages over ceramic or metal oxide fibres due to their flexibility and accommodation of expansion and contraction stresses within the membrane-electrode assembly in use.

[0057] The porous framework of polymer fibres is suitably formed from entangled polymer fibres. The fibres may comprise discrete fibres that are entwined. For example, the fibres can cross each other or be twisted with other fibres or itself. Suitably, the fibres have a substantially random orientation in the plane of the anode catalyst layer (i.e. the xy plane). Suitably, the fibres have a diameter in the range of and including 1 to 2500 nm, such as in the range of and including 1 to 1500 nm. Preferably, the fibres are nanofibers (i.e. fibres with a diameter in the range of and including 1 to 1000 nm).

[0058] In some embodiments, the polymer fibres are polybenzimidazole fibres and the fibres have a diameter of 30-700 nm, suitably 50-500 nm and preferably 50-300 nm.

[0059] In some embodiments, the polymer fibres are PVDF-HFP fibres and the fibres have a diameter of 400-2000 nm, suitably 600-1800 nm and preferably 800-1500 nm.

[0060] The length of the fibres is not material to the invention, but each fibre is suitably sufficiently long (for example several millimetres or centimetres) to be entangled, either with one or more other fibres or with itself. The fibres are suitably spun fibres, i.e. the fibres are formed using a spinning technique. Examples of suitable spinning techniques include, but are not limited to, electrospinning, such as melt-electrospinning, and force spinning. Suitably, the porous framework of polymer fibres is a spun porous framework of polymer fibres, such as an electrospun porous framework of polymer fibres. The polymer fibres are suitably formed from polymers which maintain structural stability under electrolyser operating conditions, and which may be formed into a porous framework (for example using a spinning technique).

[0061] The polymer fibres are suitably formed from polymers with low aqueous solubility. The use of a polymer with a low aqueous solubility maintains the structural integrity of the polymer framework upon which the metal thin film and OER catalyst are supported. Preferably, the anode component exhibits a weight loss of less than 5 wt% (such as 0 to 5 wt%) when immersed in water at 80 °C for a period of 16 hours. More preferably, the anode component exhibits a weight loss of less than 3 wt%, or 2 wt%. Such materials show good stability at the elevated temperatures typically utilised during electrolysis.

[0062] Preferably, in the case that the MEA is for a proton exchange membrane electrolyser, the anode component exhibits a weight loss of less than 5 wt% (such as 0 to 5 wt%) when immersed in 0.5 M H2SO4 at 80 °C for a period of 16 hours. More preferably, the anode component exhibits a weight loss of less than 3 wt%, or 2 wt%.

[0063] Preferably, in the case of that the MEA is for an anion exchange membrane electrolyser, the anode component exhibits a weight loss of less than 5 wt% (such as 0 to 5 wt%) when immersed in 1M KOH at 80 °C for a period of 16 hours. More preferably, the anode component exhibits a weight loss of less than 3 wt%, or 2 wt%.

[0064] Preferred polymers include polymers comprising a polymer backbone based on a nitrogencontaining heterocycle, for example polymer backbone comprising benzimidazole, thiadiazole, triazole, oxazole, benzoxazole, thiazole, pyrazole or benzazoles. Polybenzimidazoles may be particularly preferred, for example poly[2,2’-(m-phenylen)-5,5’- bisbenzimidazole. Polymer fibres comprising a polymer backbone based on a nitrogencontaining heterocycle, such as polymer fibres formed from polybenzimidazole, advantageously offer resistance to oxidation and high mechanical stability under typical electrolyser operating conditions.

[0065] Other preferred polymers include copolymers of vinylidene fluoride, such as poly(vinylidene fluoride-hexafluoropropylene (PVDF-HFP) which have been found to have a beneficial combination of properties enabling facile formation of a porous framework and resistance to electrolysis conditions. Other suitable polymers include polyetherimide, polyetheretherketone, polyamideimide, polyphenylene sulfide, polysulfone, polyaniline, polyimide, polyvinylidene difluoride, polylactic acid, polycaprolactone, polystyrene, polyurethane, polyacrylonitrile, and polyethylene terepthalate. Preferably, the porous framework of polymer fibres has a modal pore diameter of at least 0.05 pm. The modal pore diameter may be derived from mercury porosimetry at a temperature of 23 °C and applied pressure from 3 to 60,000 psia and is the peak maximum in a plot of dV / dlogP (cm3 / cm2) against pore diameter (nm). It may be preferred that the modal pore diameter of the porous framework of polymer fibres is at least 0.1 pm or at least 0.2 pm. Suitably, the porous framework of polymer fibres has a modal pore diameter in the range of and including 0.05 to 10 pm, such as 0.05 to 2.0 pm, or 0.1 to 1 .0 pm.

[0066] It may be preferred that the porous framework of polymer fibres has a gradient of pore size from one side of the web to the other side, and that the side with the smaller pore size is positioned next to the polymer electrolyte membrane. Such a configuration offers improvements in the transport of liquids and gases to and from the OER catalyst while still maintaining good contact to both a transport layer and membrane surface. Porous framework of polymer fibres with a gradient of pore size may be provided for example by suitable variation of the electrospinning parameters for different layers in the framework. The presence of a gradient of pore size may be determined by microscopy, such as scanning electron microscopy (SEM) or X-ray microscopy (XRM).

[0067] Suitably, the anode component has a thickness of less than or equal to 25 pm. Preferably, the catalysed polymer anode component has a thickness of less than or equal to 20 pm, less than or equal to 15 pm, or less than or equal to 10 pm, such as in the range of and including 0.1 to 10 pm, such as 0.5 to 8 pm, or 1 to 6 pm. The thickness of the anode component may be measured by scanning electron microscopy (SEM). SEM analysis is carried out on cross sections of the structure and the thickness measured at multiple (for example 10) points. The thickness values are then determined by calculating the arithmetic mean of the of the measured values.

[0068] Advantageously, the anode component described herein offers high lateral conductivity. Preferably, the anode component has a sheet resistance less than 200 Q sq-1, less than 150 Q sq-1, or preferably less than 100 Q sq-1. The anode component may have a sheet resistance of greater than 10 Q sq-1, or greater than 15 Q sq-1. Preferably, the anode component has a sheet resistance in the range of and including 10 to 200 Q sq-1, or 15 to 100 Q sq-1. The sheet resistance of the anode component may be determined using a resistivity meter at 23 °C, for example using a Loresta-GX MCP-T700 (NH Instruments). The sheet resistance of the anode component may suitably be varied by adjusting the degree of coating of the polymer fibres with the metal-containing thin film. Preferably, the anode component has a through-plane resistance of less than 0.020 Q cm2. Preferably, the catalysed polymer web has a through-plane resistance of less than 0.010 Q cm2, 0.008 Q cm2, less than 0.006 Q cm2, or less than 0.004 Q cm2, for example in the range of and including 0.001 Q cm2to 0.020 Q cm2. The through-plane resistance may be measured by using a potentiostat at a pressure of 3 bar and 23 °C.

[0069] Suitably, a cathode catalyst layer is provided on the second major surface of the polymer electrolyte membrane. The cathode catalyst layer comprises a hydrogen evolution reaction (HER) catalyst, such as a HER catalyst comprising platinum. It may be preferred that the cathode catalyst layer comprises platinum-on-carbon. The cathode catalyst layer typically comprises additional components, such as an ion-conducting polymer, to improve ionic conductivity within the layer. The cathode layer may be applied to the catalyst-coated membrane using, for example, coating methods such as a slot-die (slot, extrusion) coating process, inkjet printing, gravure printing, curtain coating, or a spray coating process. The cathode catalyst layer may be applied by directly coating the membrane, or the cathode catalyst layer may be formed on a suitable backing material and then applied to the membrane using a decal process.

[0070] Separate film layers, typically formed from non-ion conducting polymers, may be positioned around the edge region of the MEA, for example on exposed surfaces of the polymer electrolyte membrane where no catalyst is present (but will also often overlap on to the edge of the catalyst layers) to provide a seal to prevent escape of reactant and product gases, to reinforce and strengthen the edge of the MEA and provide a suitable surface for supporting subsequent components. An adhesive layer may be present on one or both surfaces of the seal film layer.

[0071] In awater electrolyser, additional transport layers are positioned each side of the membrane electrode assembly to facilitate reagent and product transfer to and from the catalysts, and to provide electrical contact. 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 MEA. 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.

[0072] Suitable transport layers at the anode side of the MEA 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 MEA (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. For PEMWE applications, suitable transport layers comprise titanium. For AEMWE applications, suitable transport layers comprise nickel or stainless steel.

[0073] Suitable transport layers at the cathode side of the MEA 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.

[0074] The membrane-electrode assemblies described herein may be produced by a process comprising the step of (i) providing an anode component comprising a porous framework of polymer fibres at least partially coated with a metal-containing thin film, and with an oxygen evolution reaction (OER) catalyst supported on the porous framework of polymer fibres.

[0075] Typically, the metal-containing thin film and the OER catalyst are deposited sequentially onto the polymer fibres.

[0076] Preferably, step (i) comprises the sub steps of (i) (a) at least partially coating a porous framework of polymer fibres with a metal-containing thin film; and (i) (b) depositing an OER catalyst onto the porous framework of polymer fibres at least partially coated with a metalcontaining thin film.

[0077] The porous framework of polymer fibres may be suitably formed by a spinning technique. Such a technique forms the web onto a suitable substrate or surface, for example, the porous framework of polymer fibres may be formed using electrospinning. In an example of a suitable process, an electrospinning formulation is provided comprising a polymer, and optionally a second polymer, in a suitable solvent, such as an organic solvent, or suitable solvent mix. The solvent may suitably comprise (or consist of) at least one of N- methylpyrrolidone (NMP), dimethylformamide (DMF), dimethylacetamide (DMAc) and / or dimethylsulphoxide (DMSO), suitably DMAc and / or DMSO. The electrospinning formulation can be a solution or a dispersion. The electrospinning formulation is pushed through a needle using a syringe pump, wherein the needle is maintained at a potential difference with respect to the substrate / surface. Electrospun nanofibres are collected on a substrate (e.g. rotating drum collector) moving translationally and rotationally which is set at some distance from the needle, such as around 10-15 cm from the needle. The fibre morphology is obtained through control of the formulation parameters, such as concentration, whereas framework thickness and uniformity are controlled through deposition time and collector rotation / translation speed.

[0078] The at least partially coating a porous framework of polymer fibres with a metal-containing thin film may suitably carried out using a vapour deposition process, such as either chemical vapour deposition or physical vapour deposition, or alternatively may be suitably formed by electroless plating. Suitable chemical vapour deposition techniques include, but are not limited to, metal organic chemical vapour deposition, atomic layer deposition, plasma enhanced chemical vapour deposition, and plasma enhanced atomic layer deposition. Suitable physical vapour deposition techniques include but are not limited to, magnetron sputtering, DC sputtering, RF sputtering, high power impulse magnetron sputtering (HiPIMs), high power pulsed magnetron sputtering, thermal evaporation, electron beam evaporation, molecular beam epitaxy and pulse laser deposition. High power impulse magnetron sputtering may advantageously be used to increase the deposition penetration of the metal-containing thin film.

[0079] The deposition of an OER catalyst may be suitably carried out by wet layer deposition, or by chemical or physical deposition.

[0080] The membrane-electrode assemblies described herein may be produced by a process comprising the step of (ii) contacting the anode component with a polymer electrolyte membrane to form the membrane-electrode assembly.

[0081] Preferably, polymer electrolyte membrane has a first major surface and a second major surface and step (ii) comprises pressing the anode component onto the first major surface of the polymer electrolyte membrane at an elevated temperature, for example using a hot press or a roller press. Suitably, the pressing is carried out at a temperature in the range of and including 100 to 200 °C, for example 110 to 160 °C. Suitably, pressing is carried out at a pressure in the range of and including 200 to 600 psi. Pressing the catalysed polymer web onto the polymer electrolyte membrane at an elevated temperature ensures high levels of contact between the components. Optionally, the anode component is coated with a layer of ion-conducting polymer prior to pressing onto the membrane.

[0082] The step (ii) of contacting the anode component with a polymer electrolyte membrane may comprise other methods of MEA formation. For example, it may be preferred that the membrane is formed by a process comprising the step of depositing one of layers of ionconducting polymer onto the anode component and drying to form the membrane.

[0083] As an alternative, the anode component may be attached to the membrane, or to a transport layer by an adhesive layer. The adhesive layer may comprise a hot melt adhesive, a pressure sensitive adhesive or a thermosetting adhesive. The adhesive may be polyolefin based, such as a polyethylene or polypropylene adhesive, or other polyolefin material. Suitably, the adhesive layer does not shrink appreciably under manufacturing conditions. It will be understood by the skilled person that such an adhesive layer would only be applied in peripheral regions of the MEA, leaving an electrochemically active area in which the anode component is in direct contact with the first major surface of the polymer electrolyte membrane.

[0084] In a further suitable configuration, the anode component is not attached to either the membrane or a transport layer but is held in contact with both components due to the design and configuration of the electrolysis cell.

[0085] Figure 1 shows a schematic representation of a cross section of an example of a membrane-electrode assembly (10) as described herein. A polymer electrolyte membrane (1) has a first major surface (2). An anode component (3) is provided in contact with the first major surface (2) of the polymer electrolyte membrane (1). A cathode layer (4) is provided on the second major surface (5) of the polymer electrolyte membrane (1). When incorporated into an electrolysis cell, the anode component (3) is arranged between, and in contact with, the first major surface (2) of the polymer electrolyte membrane (1) and a porous transport layer (6). A second transport layer (7) is provided adjacent to the cathode layer (4).

[0086] Examples

[0087] 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. Test procedures

[0088] Aqueous stability testing

[0089] Samples of size 1 cm x 1cm were cut and dried overnight in an oven at 100 °C. The samples were then weighed and added to a vial with deionised water, 0.5 M H2SO4 or 1 M KOH solutions. The vials were then heated on a hotplate for 16 hours to 80 °C, afterwards, the samples were rinsed three times with deionised water and placed in the oven overnight at 100 °C to dry. The samples were then weighed again. A weight change % was calculated by the formula ((initial sample weight - final sample weight) / initial sample weight)*100.

[0090] In-plane sheet resistivity measurement

[0091] The in-plane sheet resistivity was measured at ambient temperature (23 °C) using a Loresta-GX MCP-T700 with a LSP probe from NH instruments. 3 measurements were made over 30 seconds with the average reported.

[0092] Modal pore diameter measurement

[0093] The pore size distribution and pore volume of the porous web may be measured using a mercury porosimeter (Micromeritics, Auto Pore V 9620) with the measurement conducted at room temperature (23 °C) with applied pressure ranged from 3 to 60,000 psia, which corresponds to pore size ranging from around 6 pm to 3 nm.

[0094] The modal pore diameter measurement may be determined as follows. From the pressure vs. intrusion raw data generated by the porosimeter rig itself, the pore size and volume is calculated based on the Washburn equation: D = -4r cos(theta) / P, where D is the pore diameter, r (gamma) is the surface tension of the mercury, theta is the contact angle of mercury, and P is the applied pressure. The differential plot dV / dlogP is used to identify the modal pore diameter peaks, where V is the volume and P is the pressure.

[0095] Through plane resistivity measurement

[0096] The through-plane resistivity was measured by placing samples of the anode component on a circular gold coated contact of 1cm2in area. A second gold coated contact of the same size is pressed onto the top of the sample using a pneumatic cylinder. The pressure exerted on the sample is 3 bar. The resistance between the 2 gold contacts is then measured using a Autolab potentiostat at room temperature (23 °C) to ramp the current pass between the contacts between + / - 0.05 A, whilst measuring the voltage using separate sensing wires attached to the contacts. The best fit line to the measured voltage gives the resistance.

[0097] Example 1 : Production of a porous framework of polybenzimidazole (PBI) fibres

[0098] Polybenzimidazole (PBI) solutions having a concentration in the range of 15 to 18.5 % were prepared by heating the appropriate amount of m-polybenzimidazole (PBI-m) powder and N,N-dimethylacetamide (DMAc) solvent in digestion bombs at 200 °C and under autogenous pressure for 24 hrs. For generating nanofibres by electrospinning, the PBI solutions were fed through a syringe connected to a hypodermic needle with blunt tip (22 G) using a pumping system at a feed rate of 0.05-0.15 mL / hr. The needle was held at a distance 10 cm above the drum collector and a potential difference of 10-22 kV was maintained between them. Under this potential difference the PBI solution coming from the needle gets transformed into nanofibres which were collected on the rotating drum collector to obtain nanofibre mats.

[0099] Example 2: Application of a platinum thin film onto the porous framework of polybenzimidazole fibres

[0100] The PBI framework was partially coated with a platinum thin film by sputter deposition (Quorum Q150TS) on both sides. The deposition was carried out at 30 mA for 600 s on each side of the web (settings which provide a metal layer thickness of 200 nm (100 nm on each side) when applied to a flat surface).

[0101] The Pt-coated PBI framework was subjected to aqueous stability testing and showed a weight loss of 3 wt% in water, 0.5 wt% in KOH, and no measured weight loss in H2SO4.

[0102] Example 3: Deposition of IrOx onto the porous framework of polybenzimidazole fibres

[0103] Iridium oxide was deposition onto one side of the Pt-coated PBI framework by reactive sputtering from Ir target in Ar / O2 gas mix of 1 :1 ratio by reactive magnetron sputtering from an Ir metal target in Ar / O2 plasma and at in pulse de regime. A sample with loading of 0.325 mglr / cm2was produced under sputtering conditions of 140W pulse de regime of 150khz, 1.5|JS, a reading voltage 622V, flow rates at 30 seem for inert Ar and reactive O2 gases, a fixed substrate - target distance of 16cm and sputtering pressure 2 - 5E'3mbar, achieved at full pumping power of the turbo.

[0104] Example 4: Formation of a membrane-electrode assembly (MEA) A catalyst-coated membrane was provided incorporating an 80-micron PFSA membrane with two ePFTE reinforcements with a cathode layer comprising a Pt / C catalyst (0.4 mgptcm-2) and a PFSA ionomer provided on one face. A porous framework produced according to the method of Example 3 with an iridium loading of 0.325 mgircm-2was hot pressed onto the opposite face of the membrane to the cathode layer (-170 °C at 400 psi for 2 minutes).

[0105] Example 5: Production of a porous framework of PVDF-HFP fibres

[0106] Poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) solutions having a concentration in the range of 10 to 18 % were prepared by mixing appropriate amount of PVDF-HFP pellets in 7:3 solvent mixture of dimethyformamide (DMF) and acetone at room temperature for 24 hrs. For generating nanofibres by electrospinning, the PVDF-HFP solutions were fed through a syringe connected to a hypodermic needle with blunt tip (22 G) using a pumping system at a feed rate of 0.1 - 1 mL / hr. The needle was held at a distance 10 cm above the drum collector and a potential difference of 5-15 kV was maintained between them. Under this potential difference the PVDF-HFP solution coming from the needle gets transformed into nanofibres which were collected on the rotating drum collector to obtain nanofibre mats.

[0107] Example 6: Application of a platinum thin film onto the porous framework of PVDF-HFP fibres

[0108] The PVDF-HFP framework_was partially coated with a platinum thin film using a method analogous to that described in Example 2 except that the deposition was carried out on each side for 200 seconds.

[0109] The Pt-coated PVDF-HFP framework was subjected to aqueous stability testing and showed a weight loss of 1 wt% in water, 1 wt% in KOH, and no measured weight loss in H2SO4.

[0110] Example 7: Forming an MEA with iridium oxide supported on a porous framework via a wet layer.

[0111] A catalyst ink was prepared by mixing iridium oxide (IrOx) powder and PFSA ionomer (AGO 154) at - 10% ionomer to catalyst ratio in a mixture of ethanol: water (40:60). The mixture was milled and then aged for 12 hours prior to further dilution with water to around 30 wt% of solid material. The catalyst ink was spread on a PTFE substrate using a ~ 10 micron doctor blade and then a PVDF-HFP framework, partially coated with a platinum thin film in accordance with Example 7 was suspended over and contacted with the wet layer, left to fully impregnate, and dried under a ventilation arm. The iridium loading of the formed porous framework was 0.4 mgir cm-2.

[0112] Example 8: Formation of a membrane-electrode assembly (MEA)

[0113] A catalyst-coated membrane was provided incorporating an 80-micron PFSA membrane with two ePFTE reinforcements with a cathode layer comprising a Pt / C catalyst (0.4 mgptcm-2) and a PFSA ionomer provided on one face. A porous framework produced according to the method of Example 7 was hot pressed onto the opposite face of the membrane to the cathode layer (-190 °C at 800 psi for 3 minutes).

[0114] Example 9: Electrochemical testing of the MEA produced in Example 4

[0115] An electrochemical cell was constructed by combining the MEA produced in Example 4 with a platinum-coated titanium porous transport layer (PTL, Bekeart 2GDL40-1 ,0) on the anode side, and a gas diffusion layer (SGL 22BB) on the cathode side.

[0116] The MEA was first conditioned with water flowing across the anode at 60 °C for 12 hours. Then the polarisation measurement was performed. Anode and cathode pressures were kept equal at atmospheric pressure. The current density was increased from 0 A / cm2to 0.15 A / cm2in 9 incremental steps and then from 0.15 A / cm2to 0.5 A / cm2in steps of 0.05 A / cm2, followed by an increase from 0.5 A / cm2to 3 A / cm2in steps of 0.1 A / cm2. The current density was then decreased from 3 A / cm2to 0.5 A / cm2in steps of 0.1 A / cm2and then from 0.5 A / cm2to 0.15 A / cm2in steps of 0.05 A / cm2and then 0.15 to 0 A / cm2in 9 incremental points. The upward going measurement (low to high current) was used for further analysis.

[0117] The results for the material produced in Example 4 and a Comparative Example are shown in Figure 2. The Comparative Example comprised the same cathode and membrane components as Example 4, but with the anode component replaced with a standard anode layer formed of IrOx particles dispersed in a PFSA ionomer at a loading of 0.2 mgircm'2. The results show that the electrochemical performance of the material produced in Example 4 matches that of the Comparative Example showing efficient oxygen evolution, whilst showing a reduction in high frequency resistance (HFR).

[0118] Example 10: Electrochemical testing of the MEA produced in Example 8 The MEA produced in Example 8 was tested using a method as set out in Example 9 (except that the gas diffusion layer was AvCarb MGL370 on the cathode side). The results for the MEA produced in Example 8 and a Comparative Example are shown in Figure 3. The Comparative Example comprised the same cathode and membrane components as Example 8, but with the anode component replaced with a standard anode layer formed of

[0119] IrOx particles dispersed in a PFSA ionomer at a loading of 0.4 mgircm-2without the porous framework of polymer fibres. The results show that the Example 8 has a significant a reduction in high frequency resistance (HFR) in comparison with the Comparative Example.

Claims

Claims1. A membrane-electrode assembly for a water electrolyser, the membrane-electrode assembly comprising a polymer electrolyte membrane with a first major surface and a second major surface, and an anode component in contact with the first major surface of the polymer electrolyte membrane, the anode component comprising:(i) a porous framework of polymer fibres at least partially coated with a metalcontaining thin film; and(ii) an oxygen evolution reaction (OER) catalyst supported on the porous framework of polymer fibres.

2. A membrane-electrode assembly according to claim 1 , wherein a cathode catalyst layer is on the second major surface of the polymer electrolyte membrane.

3. A membrane-electrode assembly according to claim 1 or claim 2, wherein the anode component has a through-plane resistance of less than 0.020 Q cm2.

4. A membrane-electrode assembly according to any one of claims 1 to 3, wherein the anode component has a sheet resistance of less than 200 Q sq-1.

5. A membrane-electrode assembly according to any one of the preceding claims, wherein the polymer fibres are nanofibers.

6. A membrane-electrode assembly according to any one of the preceding claims, wherein the anode component exhibits a weight loss of less than 5 wt% when immersed in water at 80 °C for a period of 16 hours.

7. A membrane-electrode assembly according to any one of the preceding claims, wherein the polymer comprises a polymer backbone based on a nitrogen-containing heterocycle, preferably benzimidazole.

8. A membrane-electrode assembly according to any one of claims 1 to 6, wherein the polymer is polybenzimidazole (PBI) or poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP).

9. A membrane-electrode assembly according to any one of the preceding claims, wherein the metal-containing thin film comprises, or consists essentially of, one or more of gold, niobium or platinum.

10. A membrane-electrode assembly according to any one of the preceding claims, wherein the metal-containing thin film comprises, or consists essentially of, a doped or undoped oxide or nitride of a transition metal or a mixture of transition metals.

11. A membrane-electrode assembly according to any one of the preceding claims, wherein the anode component has a weight per unit area of metal present in the metal-containing thin film of less than or equal to 0.5 mg I cm2, such as in the range of and including 0.1 to 0.5 mg I cm2.

12. A membrane-electrode assembly according to any one of the preceding claims wherein the OER catalyst comprises a noble metal, preferably iridium and I or ruthenium.

13. A membrane-electrode assembly according to claim 12, wherein the anode component has a loading of the OER catalyst, based on the noble metal content, of less than or equal to 0.6 mg / cm2, such as in the range of and including 0.05 to 0.6 mg / cm2.

14. A membrane-electrode assembly according to any one of the preceding claims, wherein the OER catalyst is in the form of a thin film partially coating the porous framework of polymer fibres.

15. A membrane-electrode assembly according to claim 14, wherein the thin film satisfies the following requirements:(i) the thin film coating has a first region adjacent to the surface of the polymer fibres and a second region distal to the surface of the polymer fibres;(ii) the porosity of the second region of the thin film is higher than the porosity of the first region of the thin film coating.

16. A membrane-electrode assembly according to any one of claims 1 to 13, wherein the OER catalyst is in the form of nanoparticles.

17. A membrane-electrode assembly according to any one of the preceding claims, wherein the anode component comprises a first region adjacent to the polymer electrolyte membrane and a second region distal to the polymer electrolyte membrane, and wherein the amount of OER catalyst in the first region is higher than the amount of OER catalyst in the second region.

18. A membrane-electrode assembly according to any one of the preceding claims, wherein the anode component comprises a first region adjacent to the polymer electrolyte membrane and a second region distal to the polymer electrolyte membrane, and wherein the amount of the metal-containing thin film is higher in the second region than in the first region.

19. A membrane-electrode assembly according to any one of the preceding claims, comprising a seal component provided around the edges of the membraneelectrode assembly.

20. A membrane-electrode assembly according to any one of the preceding claims, comprising a transport layer in contact with the anode component.

21. A water electrolyser comprising a membrane-electrode assembly according to any one of claims 1 to 19, and a porous transport layer in contact with the anode component.

22. A process for the manufacture of a membrane-electrode assembly according to any one of claims 1 to 20, the process comprising the steps of:(i) providing an anode component comprising a porous framework of polymer fibres at least partially coated with a metal-containing thin film, and with an oxygen evolution reaction (OER) catalyst supported on the porous framework of polymer fibres;(ii) contacting the anode component with a polymer electrolyte membrane to form the membrane-electrode assembly.

23. A process according to claim 22, wherein step (ii) comprises pressing the anode component onto the first major surface of the polymer electrolyte membrane at an elevated temperature.

Images