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 face and a second face; an anode catalyst layer on the first face of the membrane, the anode catalyst layer comprising an oxygen evolution reaction catalyst; and a porous web of polymer fibres in contact with the anode catalyst layer, the polymer fibres comprising a conductive metal additive.

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 (known as a polymer electrolyte 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 of high conductivity and excellent OER catalytic activity and are preferred materials for the oxygen evolution reaction on the anode side of a water electrolyser.

[0008] Other OER catalyst materials have been developed, however such materials often provide a lower conductivity and I or reduced OER catalyst activity in comparison with iridium and I or ruthenium-containing catalyst materials.

[0009] Due to the scarcity of key elements, such as iridium, and the growing demand for electrolytically produced hydrogen there is a need to reduce the amount of catalytic metal present in the anode layer of electrolyser CCMs. However, reduction in the catalytic metal 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. This is particularly true for supported catalysts.

[0010] It is also desirable to reduce the thickness of the polymer electrolyte 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 which can cause degradation in contact with other electrolyser components, in particular the porous transport layers 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 that lrC>2 on a nanostructured textile may be used as the anode layer for a water electrolyser. This nanostructured textile is positioned between a PEM and a gas diffusion layer (GDL).

[0012] It is further described in US2022 / 0307141 (HAHN SCHICKARD GES FUER ANGEWANDTE FORSCHUNG E V) that an intermediate layer comprising ceramic or metallic nanofibers may be positioned between a catalytically active layer and a transport layer.

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

[0014] Summary of the invention

[0015] The present inventors have identified that the use of a porous web of polymer fibres modified with a conductive metal additive can help to facilitate the use of low anode catalyst loadings in electrolyser CCMs and I or the use of less conductive catalyst materials, such as supported catalysts. Such layers offer high conductivity in combination with good mechanical durability and flexibility and may be advantageously used as a component of a membrane-electrode assembly for a water electrolyser. Therefore, in a first aspect of the invention, there is provided a membrane-electrode assembly for a water electrolyser, the membrane-electrode assembly comprising: (i) a polymer electrolyte membrane with a first face and a second face;

[0016] (ii) an anode catalyst layer on the first face of the membrane, the anode catalyst layer comprising an oxygen evolution reaction catalyst; and

[0017] (iii) a porous web of polymer fibres in contact with the anode catalyst layer, the polymer fibres comprising a conductive metal additive.

[0018] Preferably, the oxygen evolution reaction catalyst is an iridium-containing catalyst.

[0019] The porous web may be advantageously positioned between the anode catalyst layer and a porous transport layer (PTL) in a water electrolyser. Therefore, in a second aspect of the invention, there is provided a water electrolyser comprising:

[0020] (i) a membrane-electrode assembly according to the first aspect;

[0021] (ii) a porous transport layer in contact with the porous web of polymer fibres.

[0022] Description of the Figures

[0023] Figure 1 shows schematic representation of a membrane electrode assembly incorporating a porous web of polymer fibres.

[0024] Figure 2 shows a scanning electron microscopy (SEM) image of a porous web of polymer fibres sputter-coated with gold.

[0025] Figure 3 shows scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM / EDX) analysis of a porous web of polymer fibres sputter-coated with gold.

[0026] Figure 4 shows post-mortem X-ray microscopy images. Figure 4a shows a CCM operated without a porous web; Figure 4b shows a CCM operated with the use of a porous web as described herein.

[0027] Detailed Description

[0028] Preferred and / or optional features of the invention will now be set out. Any of the preferred and / or optional features of any aspect may be combined, either singly or in combination, with any other preferred and / or optional features of any aspect of the invention unless the context demands otherwise. The present invention provides a membrane-electrode assembly (MEA) for a water electrolyser. Suitably, the MEA is for a proton exchange membrane water electrolyser or an anion exchange membrane water electrolyser.

[0029] The MEA comprises an ion-conducting membrane (a polymer electrolyte membrane). Preferably, the polymer electrolyte membrane is a proton-exchange membrane (PEM) or an anion-exchange membrane (AEM).

[0030] 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).

[0031] 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).

[0032] 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 pm offering a reduction in degradation on the anode side of the CCM 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. It may be preferred that the polymer electrolyte membrane comprises at least one polymeric reinforcement layer. Such reinforcement layers 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 layer. Suitably, the polymer electrolyte membrane has one polymeric reinforcement layer 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.

[0033] The polymer electrolyte membrane may suitably contain two or more polymeric reinforcement layers. Suitably, the polymer electrolyte membrane has two polymeric reinforcement layers 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 during operation in cases when there is a large difference in the gas pressure on opposite sides of the membrane.

[0034] 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).

[0035] 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.

[0036] The membrane-electrode assembly comprises an anode catalyst layer on the first face of the polymer electrolyte membrane. The anode catalyst layer comprises an oxygen evolution reaction (OER) catalyst. The type of OER catalysts is not particularly limited in the current invention so long as the catalyst materials are sufficiently stable under PEM or AEM electrolyser operating conditions. Such materials are known to the skilled person. Typically, the OER catalyst comprises a transition metal, for example a noble metal (Rh, Ru, Rh, Pd, Ag, Os, Ir, Pt, Au) or one or more of Ni, Fe, Cu and Co. Preferably, for PEM applications, the OER catalyst comprises iridium and 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.

[0037] 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.

[0038] Preferably, the anode catalyst layer comprises an iridium-containing catalyst. The iridium- containing catalyst is typically a doped or undoped iridium oxide (IrOx) material or iridium black. The term “iridium oxide (IrOx) material” as used herein refers to a material which may be entirely lrC>2, or may comprise one or more oxidic iridium species, such as lrC>2, lr2C>3, or iridium oxyhydroxide, alone or in combination with metallic iridium. Iridium oxyhydroxides are iridium compounds having both oxo (lr=O) and hydroxo (Ir-OH) functionalities and may have a composition which can be represented, for example, by the following formula: lrOx(OH)ywherein 1 < x < 2 and 0 < y < 2, and 3 < 2x+y < 4. The iridium- containing catalyst may comprise iridium metal.

[0039] It may be preferred that the OER catalyst (preferably an iridium-containing catalyst) is on a catalyst support, for example an inorganic metal oxide support, for example a transitional metal oxide or oxide of a main group metal, such as TiO2, AI2O3, ZrO2 or mixtures thereof. If the OER catalyst is a supported catalyst, the loading of catalyst metal (such as iridium) on the support material is suitably in the range of and including 10 to 90 wt%, preferably in the range of and including 15 to 75 wt% of the total weight of the supported catalyst. The incorporation of a porous web of polymer fibres offers particular benefits in combination with supported catalyst materials, enabling their use without excessive ohmic losses.

[0040] The anode catalyst layer typically comprises additional components, such as an ionconducting polymer, to improve ionic conductivity within the layer. Typically, the anode catalyst layer has a thickness less than or equal to 20 pm, such as less than or equal to 10 pm, for example in the range of and including 0.5 to 10 pm.

[0041] The anode and 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 catalyst layers may be applied by directly coating the membrane, or the catalyst layers may be formed on a suitable backing material and then applied to the membrane using a decal process.

[0042] It may be preferred that the anode catalyst layer is a vapor deposited catalyst layer. In such cases the OER catalyst (such as an iridium-containing catalyst) is deposited on the surface of the polymer electrolyte membrane using a vapour deposition process, such as physical vapour deposition (PVD), for example sputtering (such as described in “Sputtered iridium oxide films as electrocatalysts for water splitting via PEM electrolysis”, Slavcheva, E., Electrochi mica Acta, Vol 52, Issue 12, 2007, 3889-3894). The use of the porous web offers mechanical support when using thin catalyst layers, such as those formed through vapour deposition, and offers protection from contact with the relatively coarse structure of typical electrolyser porous transport layers.

[0043] The skilled person will understand that the loading (weight per unit area of the anode catalyst layer) of the OER catalyst will depend, for example, on the catalyst activity of the OER catalyst and the loading may be varied accordingly.

[0044] 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 / cm2, or 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.

[0045] In the case that the OER catalyst is an iridium-containing catalyst, the membrane-electrode assembly as described herein advantageously offers electrochemical performance benefits at low iridium loadings. Preferably the mass per unit area of iridium in the anode catalyst layer is less than or equal to 0.6 mgir / cm2. It may be preferred that the weight per unit area of iridium in the anode catalyst layer is less than or equal 0.55 mgir / cm20.50 mgir / cm2, 0.45 mgir / cm20.40 mgir / cm20.35 mgir / cm2, 0.30 mgir / cm2, or 0.25 mgir / cm2. The lower limit of iridium loading in the current invention is not particularly limited, but the weight per unit area of iridium in the anode catalyst layer may be greater than 0.05 mgir / cm2, or 0.10 mgir / cm2. The iridium loading may be determined by x-ray fluorescence (XRF) analysis.

[0046] Suitably, a cathode catalyst layer is on the second face of the 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.

[0047] The membrane-electrode assembly comprises a porous web of polymer fibres in contact with the anode catalyst layer. The use of a porous web of polymer fibres offers higher in plane conductivity at lower thicknesses and higher porosities, than alternative components, such as particulate interface layers. Such fibres 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.

[0048] The porous web 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). The fibres suitably have a diameter of 30-700 nm, suitably 50-500 nm and preferably 50-300 nm.

[0049] 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.

[0050] 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.

[0051] The length of the fibres is not material to the invention, but each fibre should be 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.

[0052] The polymer fibres are suitable formed from polymers which maintain mechanical stability at the elevated electrolyser operating temperatures and, advantageously, are suitable for formation into the porous web using a spinning technique.

[0053] Preferably, the porous web of polymer fibres 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 porous web of polymer fibres 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.

[0054] Preferably, in the case that the MEA is for a proton exchange membrane electrolyser, the porous web of polymer fibres 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 porous web of polymer fibres exhibits a weight loss of less than 3 wt%, or 2 wt%.

[0055] Preferably, in the case of that the MEA is for an anion exchange membrane electrolyser, the porous web of polymer fibres exhibits a weight loss of less than 5 wt% (such as 0 to 5 wt%) when immersed in 1 M KOH at 80 °C for a period of 16 hours. More preferably, the porous web of polymer fibres exhibits a weight loss of less than 3 wt%, or 2 wt%.

[0056] 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.

[0057] 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 web of polymer fibres 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.

[0058] It may be preferred that the polymer is polybenzimidazole, polybenzaole, polyetherimide, polyetheretherketone, polyamideimide, polyphenylene sulfide, polysulfone and polyimide.

[0059] The polymer fibres comprise a conductive metal additive. The conductive metal additive increases the lateral and through-plane conductivity of the porous web of polymer fibres. Suitably, the conductive metal additive comprises a transition metal, or an oxide or alloy thereof. The conductive metal additive 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 web of polymer fibres as set out herein). It will be understood by the skilled person that the conductive metal additive is preferably a different material from the OER catalyst in the anode layer, facilitating high conductivity of the porous web of polymer fibres whilst enabling thrifting or enhancing the performance of the OER catalyst. Preferably, the porous web of polymer fibres does not comprise iridium or ruthenium.

[0060] Preferably, the conductive metal additive comprises, or consists of, niobium, silver, platinum or gold. These elements provide particular beneficial combinations of high conductivity and resistance to oxidation and dissolution. Suitably, the conductive metal additive comprises, or consists of, one or more of silver, platinum, or gold. Preferably, the conductive metal additive comprises, or consists of, platinum or gold. Such materials provide a suitable balance between electronic conductivity and stability to the acidic environment present during PEMWE operation. A conductive metal additive comprising, or consisting of, platinum may be particularly preferred and additionally offers catalysis of the recombination of oxygen and hydrogen in order to help remove hydrogen that has crossed through the membrane from the cathode.

[0061] In some preferred embodiments, the conductive metal additive comprises a doped or undoped oxide or nitride of a transition metal or a mixture of transition metals. Such additives are selected to increase the conductivity of the porous web of polymer fibres. Suitable materials include Magneli phase Ti4O7, oxides of niobium (for example one or more of Nb2O5, NbO2and NbO) and T N. The skilled person will understand that the loading of the conductive metal additive will depend on the conductivity of the material and can be varied depending on the desired properties of the MEA. By loading of the conductive metal additive, it is meant herein the weight of metal (present in the additive) per unit area of the porous web of polymer fibres. For example, in the case of a platinum additive, the loading of conductive metal additive will be the weight of platinum per unit area of the porous web of polymer fibres.

[0062] Preferably, loading of conductive metal additive 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 conductive metal additive is in the range of and including 0.05 to 0.5 mg I cm2, or 0.05 to 0.4 mg I cm2.

[0063] For example, in the case of a platinum additive, it is preferred that the loading of Pt is less than or equal to 0.5 mgptI cm2of the porous web of polymer fibres, 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 conductive metal additive provides a suitable balance between conductivity and a desire to reduce overall metal use in the MEA.

[0064] The conductive metal additive may be present, for example, in an amount in the range of and including 40 to 95 wt% of the total weight of the porous web of polymer fibres, for example in the range of and including 50 to 80 wt%.

[0065] Suitably, the conductive metal additive is in the form of metal nanoparticles. Suitably, the nanoparticles comprise, or consist of, one or more of silver, platinum, or gold. It may be preferred that the conductive metal additive is selected from silver nanoparticles, platinum nanoparticles, and gold nanoparticles. The nanoparticles may be provided on the surface of the polymer fibres and I or embedded within the polymer fibres.

[0066] It may be preferred that the conductive metal additive is a coating layer on the surface of the polymer fibres. The coating layer may be continuous or discontinuous. Preferably, the coating layer may comprise, or consist of, one or more of silver, platinum, or gold. Typically, the thickness of the conductive metal additive coating layer is in the range of and including 5 to 300 nm.

[0067] Preferably, the conductive metal additive is provided as a metal-containing thin film at least partially coating the polymer fibres, i.e. a thin film coating is present on the surface of the polymer fibres forming the porous web of polymer fibres. Preferably, the polymer fibres are partially coated with the metal-containing thin film. It may be preferred that some regions of the porous web of polymer fibres have a substantially continuous coating of the metalcontaining thin film (such as regions adjacent to each major surface of the porous web of polymer fibres), whilst other regions have a partial coating. This enables the amount of metal in the porous web to be thrifted whilst maximising conductivity. 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, 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, 40 to 500 nm, or 50 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 of the measured values. 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 web of polymer fibres as set out herein). In some preferred embodiments, the metal-containing film comprises, or consists essentially of, niobium, silver, 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. 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 porous web of polymer fibres. Suitable materials include Magneli phase Ti4O7, oxides of niobium (for example one or more of Nb2O5, NbO2and NbO) and TiN.

[0068] The porous web of polymer fibres is in contact with the anode catalyst layer. By “in contact with” it is meant herein that the porous web of polymer fibres is adjacent to the anode catalyst layer without any intermediate layers or other components. In some cases, the porous web of polymer fibres may be partially embedded into the anode catalyst layer. In some cases, the porous web of polymer fibres is not embedded into the anode catalyst layer.

[0069] Whilst the porous web of polymer fibres is in contact with the anode layer, it will be understood that the membrane-electrode assemblies as disclosed herein comprise both (i) an anode catalyst layer; and (ii) a porous web of polymer fibres, i.e. the porous web of polymer fibres is a separate component. The provision of the porous web as a separate component facilitates maximum utilisation of catalyst metal, such as iridium, in the form of a thin coating on the membrane and enables the use of supported iridium catalysts.

[0070] The polymer fibres comprising a conductive metal additive may suitably be formed 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 impluse magnetron sputtering, high power pulsed magnetron sputtering, thermal evaporation, electron beam evaporation, molecular beam epitaxy and pulse laser deposition. Such processes provide the conductive metal additive on the surface of the polymer fibres. The conductive metal additive may be in the form of, for example, a surface coating (such as a thin film coating) or as discrete nanoparticles.

[0071] The polymer fibres comprising a conductive metal additive may also suitably be formed through the inclusion of nanoparticles of the conductive metal additive in a polymer dispersion used to form the porous web, for example in a dispersion used for an electrospinning process. Such processes provide the conductive metal additive as nanoparticles embedded in the polymer fibres.

[0072] The polymer fibres comprising a conductive metal additive may also suitably be formed by coating a pre-formed porous web with a dispersion of a polymer and nanoparticles of the conductive metal additive. The polymer included in the dispersion may be the same or different to the polymer used to form the porous web.

[0073] Suitably, the porous web of polymer fibres has a thickness of less than or equal to 25 pm. Preferably, the porous web of polymer fibres 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. Suitably, the porous web of polymer fibres has a thickness of less than 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 porous web may be determined by SEM analysis of a cross section of the web. The 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.

[0074] Suitably, the porous web of polymer fibres has a through-plane resistance of less than 0.020 Q cm2. It may be preferred that the porous web of polymer fibres has a through-plane resistance of less than 0.010 Q cm2, less than 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 0.0005 to 0.020 Q cm2Inclusion of a porous web with a through-plane resistance of less than 0.010 Q cm2ensures that this component does not have a significant detrimental impact on through plane cell resistance. The through plane conductivity may be measured by using a potentiostat at a pressure of 3 bar and 22 °C.

[0075] Advantageously, the porous web of polymer fibres described herein offer high lateral conductivity. Preferably, the porous web of polymer fibres 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 porous web of polymer fibres may have a sheet resistance of greater than 5 Q sq-1, or greater than 10 Q sq-1. Preferably, the porous web of polymer fibres has a sheet resistance in the range of and including 5 to 200 Q sq_1 , or 10 to 100 Q sq-1. The sheet resistance of the porous web of polymer fibres may be determined using a resistivity meter at 22 °C, for example using a Loresta-GX MCP-T700 (NH Instruments). The sheet resistance of the porous web of polymer fibres may suitably be varied by adjusting the amount of conductive metal additive provided in the porous web.

[0076] Preferably, the porous web of polymer fibres has a modal pore diameter of at least 0.05 .m. The modal pore diameter may be determined by mercury porosimetry at a temperature of 23 °C and applied pressure from 3 to 60,000 psia. The modal pore diameter 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 web of polymer fibres is at least 0.1 .m or at least 0.2 .m. Suitably, the porous web of polymer fibres has a modal pore diameter in the range of and including 0.05 to 10 .m, such as 0.05 to 2.0 .m, or 0.1 to 1.0 .m.

[0077] Suitably, the porous web of polymer fibres has a modal pore diameter in the range of and including 50 to 2000 nm, such as 100 to 600 nm, or 200 to 500 nm. It may be preferred that the porous web 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 anode catalyst layer. Such a configuration offers improvements in the transport of liquids and gases to and from the anode layer while still maintaining good contact to both the PTL and anode layer. Porous webs 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 web. The presence of a gradient of pore size may be determined by microscopy, such as scanning electron microscopy (SEM) or X-ray microscopy (XRM).

[0078] Depending on the desired configuration, the porous web of polymer fibres may be attached to another component of the membrane-electrode assembly, such as the catalyst coated membrane or a porous transport layer, via 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 that the adhesive layer, if present, does not cover any electrochemically active part of the anode catalyst layer.

[0079] For example, the anode catalyst layer may be provided as a patch on the polymer electrolyte membrane with a surrounding area of polymer electrolyte membrane which has no catalyst layer applied. In such cases, the porous web of polymer fibres may be attached to the polymer electrolyte membrane in the surrounding area via an adhesive layer.

[0080] As another example, the porous web of polymer fibres may be attached to the porous transport layer, for example via an adhesive layer. In such cases, the porous web of polymer fibres may be attached to the PLT via an adhesive layer in peripheral areas of the membrane-electrode assembly which do not align with the electrochemically active areas of the anode catalyst layer.

[0081] In a further suitable configuration, the porous web of polymer fibres is not attached to either the catalyst-coated membrane or the PTL but is held in contact with both components due to the design and configuration of the electrolysis cell. The porous web of polymer fibres may also be contacted with the catalyst layer during routes of manufacture which involve hot-pressing. In such cases, an adhesive layer may not be required. Separate 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 such as sub-gaskets or elastomeric gaskets. An adhesive layer may be present on one or both surfaces of the seal film layer. The membrane-electrode assembly may be arranged such that the porous web of polymer fibres is positioned between a seal film layer and the catalyst-coated membrane.

[0082] When incorporated into an electrolysis cell, the porous web of polymer fibres is in direct contact with a porous transport layer (PTL). Suitable PTLs are known to the skilled person and are typically formed from a metal-based porous structure. Such PTLs must be sufficiently conducting and in a form that is compatible with positioning adjacent to the porous web of polymer fibres (without, for example, sharp edges or protrusions that would damage the membrane during use). Such metal-based porous structures may be in the form of, for example, felts or non-woven cloths, mesh, foams and sintered metal-containing particles. For PEMWE applications, suitable PTLs comprise titanium. For AEMWE applications, suitable PTLs comprise nickel or stainless steel.

[0083] 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.

[0084] MEAs as described hereinbefore may suitably be incorporated into a water electrolyser such as a PEMWE or AEMWE. The MEAs as described herein may also be incorporated into other membrane-based electrolysers requiring oxygen evolution, such as a carbon dioxide electrolyser. Figure 1 shows a schematic representation of a cross section of an example of a membrane-electrode assembly incorporating a porous web of polymer fibres. A proton exchange membrane (1) has an anode catalyst layer (2) on one face and a cathode catalyst layer (3) on the second opposite face. A porous web of polymer fibres (4) as described herein is provided in contact with the anode catalyst layer (2). When incorporated into an electrolysis cell, the porous web of polymer fibres (4) is arranged between, and in contact with, the anode catalyst layer (2) and a porous transport layer (5).

[0085] Examples

[0086] 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.

[0087] 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 at ambient temperature (22 °C) was measured using a Loresta-GX MCP-T700 with a LSP probe from NH instruments. 5 measurements were made over 30 seconds with the average reported.

[0092] Through plane resistivity measurement

[0093] The through-plane resistivity of the gold-coated and platinum-coated porous webs was measured by placing samples of the coated porous webs 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 (22 °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.

[0094] Modal pore diameter measurement

[0095] 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.

[0096] The modal pore diameter measurement may be determined as followed. From the pressure vs. intrusion raw data generated by the porosimeter rig itself, the pore size and volume could be 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.

[0097] Electrochemical testing

[0098] For electrochemical testing, catalyst coated membranes were provided incorporating an 80- micron PFSA membrane with two ePFTE reinforcements, with an anode layer (IrOx, 0.2 mgircnr2) on one face and a cathode layer (Pt / C, 0.4 mgptcm-2) on the other face (each comprising catalyst and ionomer and applied by a decal transfer).

[0099] An electrochemical cell was constructed by combining the CCM with the porous web to be tested to form an MEA. The MEA was then placed 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. The cell was constructed such that the porous web to be tested was positioned between, and in contact with, the anode layer and the PTL.

[0100] The MEA was first conditioned with water flowing across the anode at 80 °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 1 A / cm2in steps of 0.04 A / cm2and then from 1 A / cm2to 4 A / cm2in steps of 0.08 A / cm2. The current density was then decreased from 4 A / cm2to 1 A / cm2in steps of 0.08 A / cm2and then from 1 A / cm2to 0 A / cm2in steps of 0.04 A / cm2. The upward going measurement (low to high current) was used for further analysis.

[0101] Example 1 : Production of a porous web of polybenzimidazole (PBI) fibres

[0102] 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

[0103] 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.

[0104] Example 2: Application of a gold coating onto the porous web of polybenzimidazole fibres

[0105] The PBI web was coated with a gold layer 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 when applied to a flat surface).

[0106] A sample of the gold sputter-coated porous mat was analysed by scanning electron microscopy-energy dispersive X-ray spectroscopy SEM / EDX. The images (Figure 2) indicated that the porous mat had fibres with a diameter of ~ 50 to 300 nm. EDX analysis (Figure 3) indicated a high uniformity in the gold coating.

[0107] The through-plane resistivity of the gold-coated web of polymer fibres was measured as

[0108] O.005 Q cm2.

[0109] The in-plane sheet resistivity was measured as 21 Q sg-1. This indicates that the resistivity of the porous web is significantly lower than a typical water electrolyser anode catalyst layer.

[0110] Example 3: Application of a platinum coating onto the porous web of polybenzimidazole fibres

[0111] The PBI web was coated with a platinum layer using the method set out for gold coating. The through-plane resistivity of the platinum-coated web of polymer fibres was measured as 0.0025 Q cm2. This data indicates that the inclusion of the porous webs will not have a significant detrimental impact on through plane cell resistance, with the platinum coated web providing particularly low through-plane resistivity.

[0112] Example 5: Application of a platinum thin film onto the porous web of polybenzimidazole fibres

[0113] The PBI web 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).

[0114] The Pt-coated PBI web 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.

[0115] Example 6: Production of a porous web of PVDF-HFP fibres

[0116] 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.

[0117] Example 7: Application of a platinum thin film onto the porous web of PVDF-HFP fibres

[0118] The PVDF-HFP web was partially coated with a platinum thin film using a method analogous to that described in Example 5 except that the deposition was carried out on each side for 200 seconds.

[0119] The Pt-coated PVDF-HFP web 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. Example 8: Testing of a series of porous webs

[0120] A series of porous webs provided with Pt-thin film coatings were produced using methods analogous to that described in Examples 5 to 7. The results of property and electrochemical testing of these materials (in combination with a CCM @ 0.2 mgircm-2) is shown in Table 1 below. Also shown is a comparative example using a CCM @ 0.2 mgircm-2without a porous web of polymer fibres (Comp. Ex.). The data shows that the formation of an MEA using a porous web of polymer fibres as described herein (i) improves electrochemical performance at low OER catalyst loadings; and (iii) provides a reduced high frequency resistance. The porous webs provide very low lateral and through plane resistance values.

[0121] Table 1

[0122] Example 9: Membrane durability testing

[0123] Two CCMs were tested (i) a three-layer catalyst coated membrane consisting of I rOxanode, PFSA membrane and Pt / C cathode; and (ii) a four-layer catalyst coated membrane consisting of the three-layer CCM with an additional Pt-coated porous web of polymer fibres behind the anode. The Pt-coated porous web of polymer fibres was hotpressed onto the anode side of the catalyst coated membrane at 150 °C for 2 minutes, with a pressure of 100 psi. The catalyst coated membrane was placed into an electrochemical cell with a 4.9 cm2active area (2.5 cm diameter circle), with a fibre-type Pt coated Ti porous transport layer (Bekaert) on the anode and a carbon-based gas diffusion layer (AvCarb Material Solutions) on the cathode. Pt-coated titanium meshes were placed behind the anode porous transport layer and cathode gas diffusion layer and the electrochemical cell was pneumatically compressed with 10 bar nitrogen.

[0124] Heated water was flowed to the cell at a rate of 40 mL min-1for 12 hours using a peristaltic pump, with the cell outlet temperature of 60 °C. A polarisation curve is measured by varying the current density from 0 to 3 A cm2(hold for 30 - 60 seconds at each point) and impedance measured at 1 .8 kHz at each current density. Next, the current density was held for 1 hour at 1 A cm2, followed by repeating the polarisation curve. Lastly, impedance was measured at 0.1 , 0.5, 1 , 2 and 3 A cm-2, as well as at a cell voltage of 1.59 V, with an amplitude of 10% from 0.1 Hz - 10 kHz.

[0125] After testing, the electrochemical cells were deconstructed and the CCMs isolated. X-ray microscopy (XRM) analysis showed a significant reduction in the surface roughness of the anode side of the CCM in which a porous web has been present in comparison to the CCM in which a porous web had not been present. This indicates that the porous web is protecting the CCM from membrane roughening as the CCM is pressurised against the porous transport layer. Images of the post-mortem XRM analysis are shown in Fig 4a - CCM without porous web; and Figure 4b CCM with porous web. The images provide evidence that the porous web is acting to protect the membrane during cell operation.

[0126] Example 10: Preparation of a Pt-coated porous web of polymer fibres via electroless deposition.

[0127] A PVDF-HFP porous web of fibres was coated using electroless deposition, with 0.5 mgptcm'2deposited.

[0128] A sample of electrospun PVDF-HFP porous web was cleaned by placing in a solution of 1g / mL NaOH at 80 °C for 1 min. The sample was rinsed 3 times in de-ionised water. The sample was then activated in 10% v / v acetic acid glacial, for 5 min at room temperature. The sample was rinsed 3 times in de-ionised water. At 50-60 °C, the sample was placed for a few seconds in 0.5M NaOH, quick rinse in deionised water, 1 min in Pt salt solution (Pt(NHs)(NO2)2)(1g Pt / L), back to dip in 0.5 M NaOH, repeat 2 more times until visually greyish colouring on polymer.

[0129] A Pt-containing mixture was prepared by mixing 0.5 M sodium formate solution (Alfa

[0130] Aesar sodium formate catalog number A17813) and 5.77 wt % Pt assay, diamminedinitroplatinum(ll) in ammonia solution (JM product number C1031 , pH = c.9)). The mixture was heated to 50 °C and then the PVDF-HFP sample added. The mixture was then heated to 70 °C before the addition of a second aliquot of diamminedinitroplatinum solution (total amount of Pt added 0.5 mgptcm-2of the porous web). The bath was adjusted to pH 11 and then to pH 12 with KOH solution. The reaction was left for 1h 15 min before the reaction was allowed to cool down and then the substrate left overnight in the mixture. The porous web was isolated and then dried.

[0131] Pt-coated porous web of polymer fibres had a Pt loading by XRF analysis of 0.52 mgptcm2. The through-plane resistance was measured as 20.5 mOhm.cm2@2bar.

Claims

Claims1 . A membrane-electrode assembly for a water electrolyser, the membrane-electrode assembly comprising:(i) a polymer electrolyte membrane with a first face and a second face;(ii) an anode catalyst layer on the first face of the membrane, the anode catalyst layer comprising an oxygen evolution reaction catalyst; and(iii) a porous web of polymer fibres in contact with the anode catalyst layer, the polymer fibres comprising a conductive metal additive.

2. A membrane-electrode assembly according to claim 1 , wherein the oxygen evolution reaction (OER) catalyst is an iridium-containing catalyst.

3. A membrane-electrode assembly according to claim 1 or claim 2, wherein a cathode catalyst layer is on the second face of the membrane.

4. A membrane-electrode assembly according to any one of the preceding claims, wherein the polymer electrolyte membrane is a proton exchange membrane or an anion exchange membrane.

5. A membrane-electrode assembly according to any one of the preceding claims, wherein the anode catalyst layer comprises a particulate OER catalyst and an ionconducting polymer, such as an iridium-containing catalyst and an ion-conducting polymer.

6. A membrane-electrode assembly according to any one of the preceding claims, wherein the OER catalyst is on a catalyst support, such as a metal oxide support.

7. A membrane-electrode assembly according to one of claims 1 to 3, wherein the anode catalyst layer is a vapor deposited catalyst layer.

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

9. A membrane-electrode assembly according to any one of the preceding claims, wherein the porous web of polymer fibres comprising a conductive metal additive exhibits a weight loss of less than 5 wt% when immersed in water at 80 °C for a period of 16 hours.

10. 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.11 . A membrane-electrode assembly according to any one of claims 1 to 10 wherein the polymer fibres comprise polybenzimidazole (PBI) or poly(vinylidene fluoridehexafluoropropylene) (PVDF-HFP)..

12. A membrane-electrode assembly according to any one of the preceding claims wherein the conductive metal additive is in the form of metal nanoparticles, such as platinum or gold nanoparticles.

13. A membrane-electrode assembly according to any one of claims 1 to 11 wherein the conductive metal additive is a coating layer on the surface of the polymer fibres.

14. A membrane-electrode assembly according to any one of the preceding claims, wherein the conductive metal additive comprises, or consists essentially of, one or more of silver, platinum, niobium or gold.

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

16. A membrane-electrode assembly according to any one of the preceding claims, wherein the conductive metal additive is on the surface of the polymer fibres.

17. A membrane-electrode assembly according to any one of the preceding claims, wherein the porous web of polymer fibres is an electrospun porous web of polymer fibres.

18. A membrane-electrode assembly according to any one of the preceding claims, wherein the OER catalyst is an iridium-containing catalyst and the weight per unit area of iridium in the anode catalyst layer is less than or equal to 0.6 mg / cm2.

19. A membrane-electrode assembly according to any one of the preceding claims wherein the polymer electrolyte membrane has a thickness in the range of and including 10 to 100 pm.

20. A membrane-electrode assembly according to any one of the preceding claims wherein the porous web of polymer fibres has a through-plane resistance of less than 0.010 Q cm2.

21. A membrane-electrode assembly according to any one of the preceding claims comprising a porous transport layer in contact with the porous web of polymer fibres.

22. A membrane-electrode assembly according to any one of the preceding claims wherein the porous web of polymer fibres has a coating comprising the conductive metal additive.

23. A membrane-electrode assembly according to claim 22 wherein the conductive metal additive is a metal-containing thin film at least partially coating the polymer fibres.

24. A membrane-electrode assembly according to any one of the preceding claims wherein the conductive metal additive is formed by gas phase deposition.

25. A water electrolyser comprising:(i) a membrane-electrode assembly according to any one of claims 1 to 24;(ii) a porous transport layer in contact with the porous web of polymer fibres.

Images