Water electrolyser membrane

Abstract

The present disclosure provides a dispersion-cast ion-conducting membrane for a proton exchange membrane water electrolyser, the membrane comprising a dispersion cast ion- conducting polymer layer which layer comprises an ion-conducting polymer, the ion- conducting polymer layer having a maximum burst strength of greater than or equal to 10.5 MPa.

Description

[0001] Water Electrolyser Membrane

[0002] Summary

[0003] The present disclosure provides membranes for proton exchange membrane water electrolysers. In particular, membranes which have a high maximum burst strength.

[0004] Background

[0005] The electrolysis of water to produce high purity hydrogen and oxygen can be carried out in both alkaline and acidic electrolyte systems. Those electrolysers that employ a solid protonconducting 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] A catalyst-coated membrane (CCM) may be employed within the stack of a water electrolyser. CCMs comprises an electrolyte membrane, such as a PEM or AEM, with at least one of an anode catalyst layer and a cathode catalyst layer coated on a face of the membrane. Typically for PEMWEs, cathode catalyst materials comprise platinum. Anode catalysts for PEMWEs typically comprise iridium or iridium oxide (IrOx) materials, or oxides containing both iridium and ruthenium.

[0007] To form a water electrolyser, additional layers are added either side of a CCM to make an assembly, sometimes referred to as a membrane electrode assembly (MEA). These additional layers may include a porous transport layer (PTL) on the anode side and a gas diffusion layer (GDL) on the cathode side of the CCM. These layers may or may not be directly attached to the CCM. Other components may include bipolar plates and current collector plates. Stacks of such assemblies make up an electrolyser system including power and control systems.

[0008] Electrolyte membranes, such as PEMs and AEMs, are also used in fuel cells. In proton exchange membrane fuel cells (PEMFC) the membrane is proton-conducting, and protons, produced at the anode, are transported across the membrane to the cathode, where they combine with oxygen to form water.

[0009] PEMWEs are subject to high pressure during real-world use and so the membranes must be mechanically durable enough to withstand this pressure over a long period of real-world use. There remains a need to further develop such membranes, especially those which are dispersion-cast rather than extruded. Extruded membranes have traditionally been used in PEMWE applications. Unfortunately, membranes are very difficult to manufacture using extrusion processes, especially at thicknesses below about 80 microns. Such thinner membranes equate to increased proton conductivity and overall system efficiency; both necessary for PEMWE to be economical at scale. Membranes can also be manufactured by dispersion-casting the membrane rather than by extrusion. Such casting processes not only allow for a wide range of thicknesses but also offer many additional advantages including, among others, the use of a wide variety of reinforcements, additives, and the ability to design multilayer constructions that would be difficult to generate by extrusion.

[0010] Summary of the Invention

[0011] Surprisingly, the present inventors found that a minimum processing temperature for unreinforced, dispersion-cast membranes produces membranes that have improved mechanical properties relative to extruded and reinforced membranes. The minimum processing temperature is related to the glass transition temperature of the ion-conducting polymer. The mechanical properties of the membranes are quantified by measuring the resistance to bursting caused by pressurised water at 80°C, called the membrane burst strength. Therefore, the inventors have provided new, highly mechanically durable, dispersion-cast proton exchange membrane water electrolyser membranes.

[0012] Accordingly, the present disclosure provides a dispersion-cast ion-conducting membrane for a proton exchange membrane water electrolyser, the membrane comprising a dispersion cast ion-conducting polymer layer which layer comprises an ion-conducting polymer, the ionconducting polymer layer having a maximum burst strength of greater than or equal to 10.5 MPa.

[0013] The disclosure also provides a process of preparing a dispersion-cast ion-conducting membrane for a proton exchange membrane water electrolyser, the membrane comprising a dispersion cast ion-conducting polymer layer which layer comprises an ion-conducting polymer, the process comprising the steps of: i) providing a dispersion of the ion-conducting polymer and a solvent; ii) casting an ion-conducting polymer layer from the dispersion; and iii) annealing the ion-conducting polymer layer; wherein the step of annealing comprises heating the ion-conducting polymer layer casted in step ii) at a temperature which is at least about 65°C greater than the glass transition temperature, Tg, of the ion-conducting polymer.

[0014] The disclosure also provides an ion-conducting membrane for a proton exchange membrane water electrolyser which is obtainable by this process, suitably obtained by this process.

[0015] The disclosure also provides a catalyst-coated membrane for a water electrolyser, the catalyst-coated membrane comprising an ion-conducting membrane according to the disclosure and a catalyst layer.

[0016] The disclosure also provides a catalyst-coated membrane for a water electrolyser, the catalyst-coated membrane comprising an ion-conducting membrane and a catalyst layer, wherein the catalyst-coated membrane has a maximum burst strength of greater than or equal to 10.5 MPa, suitably greater than or equal to 11.0 MPa.

[0017] The disclosure also provides a proton exchange membrane water electrolyser comprising an ion-conducting membrane according to the disclosure or comprising a catalyst-coated membrane according to the disclosure.

[0018] Brief Description of the Figures

[0019] Figures 1a and b are diagrams of catalyst-coated membranes containing ion-conducting membranes as disclosed herein.

[0020] Figure 2 is a plot of annealing temperature (Tann)(°C) - the glass transition temperature (Tg) (°C) against maximum burst strength (MPa) for a series of ion-conducting membranes.

[0021] Figure 3 is a plot of Tann-Tgagainst maximum burst strength (MPa) for a series of ionconducting membranes.

[0022] Figure 4 is a plot of Tann-Tgagainst in-plane swelling (%) for a series of ion-conducting membranes.

[0023] Figure 5 is a plot of nominal stress at rupture against time to rupture (mins) for a series of ionconducting membranes.

[0024] Figures 6a and b provide diagrams of the equipment used to measure maximum burst strength. Detailed Description of the Invention

[0025] The ion-conducting membranes disclosed herein are dispersion-cast. As is known in the art, such membranes are different from extruded membranes for at least the reason that extruded membranes have anisotropic properties in the x,y, and z- directions due to stretching during the extrusion process. Dispersion-cast membranes do not. As is known by the skilled person, techniques such as WAXD and SAXS can identify the anisotropy along with other methods such as anisotropic swell or tensile properties.

[0026] The ion-conducting polymer is suitably a proton conducting polymer, and in particular a partially- or fully-fluorinated sulphonic acid polymer. Examples of suitable proton-conducting polymers include perfluorosulphonic (PFSA) acid polymers. The ion-conducting polymer optionally can contain partially or fully fluorinated vinyl ethers such is disclosed in US11 ,492, 431. The ion-conducting polymer can also optionally comprise bifunctional ionconducting monomers e.g. in which the side chains contain more than one ionic group as disclosed in US9, 118,043.

[0027] Ion-conducting polymers, in particular partially- or fully-fluorinated sulphonic acid polymers such as PFSAs, may be characterised with respect to the length of the sulphonic acid containing the side chain. A chain length refers to the minimum number of atoms between the end group (the sulphur atom where the ionisable end group is sulphonic acid) and the carbon atom of the repeating unit of the main chain to which the side chain is covalently bonded. Suitably, the ion-conducting polymer may be a long side chain (LSC) ion-conducting polymer or a short side (SSC) chain ion-conducting polymer. In the present disclosure a LSC has more than five atoms between the end group and the carbon atom of the repeating unit of the main chain, and an SSC has five or less atoms, typically less than five atoms, for example less than four atoms or less than three atoms, between the end group and the carbon atom of the repeating unit of the main chain.

[0028] Typically, the side chain is a linear or branched alkyl. One or more non-adjacent non-terminal carbon atoms of the alkyl may be substituted with an oxygen atom. Typically, the side chains are covalently bonded to the main chain via an oxygen atom (oxygen atom substitution, where present, will be appreciated as providing an ether, and no ketone or ester groups, for example, may be present). The side chain may be branched.

[0029] The side chain may suitably be represented by the formula:

[0030] -O-CR2-CR2-O-(CR2)a-SO3H wherein a is 2 or more (preferably up to 6, preferably 2, 3 or 4), and each R is independently selected from the group consisting of H, F, Cl, alkyl, perfluoroalkyl, perchloroalkyl and perfluorochloroalkyl (wherein the alkyl, perfluoroalkyl, perchloroalkyl and perfluorochloroalkyl may have a carbon number of from 1 to 10, such as from 1 to 3, and in some embodiments may be 1), preferably selected from the group consisting of H, F and perfluoroalkyl, for example:

[0031] -O-CX2-CX(CF2X)-O-(CX2)b-(CF2)c-SO3H wherein X is F or Cl, wherein b and c are each greater than 1 (wherein b + c = a).

[0032] The side chain may be represented by the formula:

[0033] -O-(CR’2)d-SO3H wherein d is from 1 to 5, preferably 2. Preferably R’ is as described above for R, or is selected from H or F.

[0034] Accordingly, the side chain may be -O-CR2-CR(CR3)-O-(CR2)2-SO3H or -O-(CR’2)2-SO3H, wherein each R and R’ are each independently selected from H and F, preferably wherein R = R’ = F.

[0035] The ion-conducting polymer may be a PFSA polymer having an equivalent weight (EW) (g polymer / mol SO3H) greater than or equal to 500, typically greater than or equal to 700, suitably greater than or equal to 750 EW. The ion-conducting polymer may be a PFSA polymer having an equivalent weight (EW) of less than or equal to 1200. In a specific example, the ionconducting polymer may be an SSC PFSA having an EW in the range of and including 750 to 900 EW. Suitable ion-conducting polymers include perfluorosulphonic acid ionomers such as those with the desired EW available as e.g. Nation® (Chemours Company), Aciplex® (Asahi Kasei), Aquivion™ (Solvay Speciality Polymers), Flemion® (Asahi Glass Co.).

[0036] An ion-conducting polymer layer is typically composed of a single ion-conducting polymer as described herein. An ion-conducting polymer layer may also comprise a blend of ionconducting polymers as described herein, for example a blend of two or more, typically two. ion-conducting polymers. Suitably, one ion-conducting polymer in the blend is a LSC ionconducting polymer and the other is a SSC ion-conducting polymer. As shown in the Examples section, blending ion-conducting polymers allows manipulation of the overall glass transition temperature of the ion-conducting polymer layer without affecting other properties. For example, the mixture can be prepared so it has high processability and also creates an ionconducting polymer layer with high maximum burst strength. The ratio of LSC to SSC ionconducting polymer will be determined by the required properties of the layer. For example, the ratio may be in the range of and including 90:10 to 10:90 by weight. The ratio may also be in the range of and including 90:10 to 50:50 SSC:LSC, for example 90:10:60:40 SSC to LSC by weight which optimises processibility whilst facilitating high maximum burst strength.

[0037] Typically, the ion-conducting membrane has a thickness of less than or equal to 100 microns. Suitably, the membrane has a thickness of less than or equal to 95 microns, 90 microns, or 85 microns. The membrane may have a thickness of less than 60 microns, typically less than 50 microns. Thinner membranes result in lower ion resistance resulting in higher cell electrical efficiency. Thinner membranes are also more conductive and thus increase water electrolyser efficiency. Thinner membranes are also cheaper in terms of materials cost and may be faster to manufacture (relative to a thicker membrane). Therefore, it is a benefit of the present ionconducting membranes that thinner water electrolyser membranes can be prepared, which will retain desired mechanical durability due to higher inherent maximum burst strength. The membrane may have a thickness of at least 10 microns, such as at least 15 microns, at least 20 microns, at least 25 microns, at least 30 microns or at least 40 microns.

[0038] The ion-conducting membrane may comprise one or more ion-conducting polymer layers, suitably two to seven layers, for example one, two, three or four layers, typically one, two or three. The thickness of each individual ion-conducting polymer layer, and the number of layers, will depend on the intended thickness of the ion-conducting membrane and the degree of variation in desired composition across the membrane (for example the membranes may contain one or more layers comprising a reinforcing layer, such as ePTFE or PEEK, or an additive, such as a radical reducing additive, as discussed in more detail below).

[0039] The thickness of the ion-conducting membrane or an ion-conducting polymer layer may be measured using a low force high precision gauge instrument (e.g. VL-50B Litematic™ available from Mitutoyo (UK) Ltd.), which may give a direct reading of the thickness. A motorised spindle is used to take measurement readings with a measuring force of 0.01 N. At least three readings are taken from different locations on the ion-conducting membrane or polymer layer (prior to adding catalyst layers) at a temperature of 20 °C ± 3 °C, and relative humidity (RH) of 30-50%. Thickness of an ion-conducting layer may also be determined by analysis of a scanning electron microscope (SEM) image of a cross section of the layer at 0% relative humidity. Suitably, when the ion-conducting membrane comprises a plurality of ion-conducing layers the membrane is a single coherent polymer film comprising a plurality of the ion-conducting polymer layers. The term ‘coherent’ as used herein means that the membrane is free from internal lamination interfaces. Lamination of ion-conducting membranes comprises pressing and / or bonding at least two solid ion-conducting layers together, such membranes optionally being coated with a catalyst layer. A lamination interface is formed between the two ionconducting layers where solid surfaces of the individual layers are pressed and / or bonded together. Lamination interfaces comprise physical defects. Furthermore, the structural and / or chemical nature of a lamination interface also differs from that of the bulk polymer material. This is because when a solid membrane is formed, the outer surfaces of the solid membrane have surface features which are distinct from those in the bulk material. For example, a hydrophobic skin forms on a surface of a membrane at an air interface. Raman spectroscopy can detect this difference. As such, when two solid ion-conducting layers are pressed together, the lamination interface formed by the two solid surfaces is distinctive in chemical and / or structural form compared to the bulk of the ion-conducting polymer material. Microscopy and spectroscopy techniques can thus distinguish between lamination interfaces between layers of ion-conducting polymer and interfaces which have been formed via a liquid phase deposition process such as printing, spraying, or coating of layers to build up a multilayer structure. That is, a non-laminated interface is structurally and / or chemically distinct from a laminated interface and is notjust a feature of the manufacturing method. Furthermore, a non-laminated interface can be identified as being non-laminated in a membrane without prior knowledge of the manufacturing method. Examples of analysis techniques for detecting a laminated interface include cross-section SEM. Variations of crystallinity at interfaces can be detected using cross-section TEM. Other techniques for detecting laminated interfaces include 13C / 1 H / 19F solid state NMR, neutron diffraction, and / or a combination of two or more of the aforementioned techniques. Due to physical defects and / or chemical variations at lamination interfaces between ion-conducting polymer layers, such interfaces can increase the resistance of a multi-layer ion-conducting membrane. As such, it is advantageous to fabricate a multi-layer ion-conducting membrane by depositing layers of ion-conducting polymer dispersed in a liquid solvent to build up a multi-layer membrane structure rather than via lamination of individual solid layers / membranes of ion-conducting polymer.

[0040] The ion-conducting membrane may comprise one or more reinforcing layer(s). The reinforcing layer can confer further mechanical strength to the ion-conducting membrane. The reinforcing layer is typically in the form of a planar layer, and may include woven materials. The reinforcing layer may be, for example, polytetrafluoroethylene (ePTFE), polyether ether keto (PEEK) or polybenzimidazole (PBI). The reinforcing layer typically comprises a porous reinforcement polymer sheet which is impregnated with ion-conducting polymer. As typical reinforcing layer materials are not conductive to ions, or not sufficiently conductive to ions, the reinforcing layer is thus formed using a porous layer which is impregnated with ion-conducting polymer through the pores of the material to provide ion-conductive paths from one side of the layer to the other side of the layer.

[0041] The ion-conducting membrane may contain more than one, for example two, reinforcing layers each having ion-conducting polymer impregnated in at least a region thereof. In the case of an ion-conducting membrane containing a plurality of ion-conducting polymer layers, when more than one reinforcing layer is present each reinforcing layer will typically be present in a different ion-conducting polymer layer. In some cases, a reinforcing layer may bridge two ionconducting polymer layers.

[0042] The ion-conducting membrane may comprise a radical reducing additive (e.g. peroxide radical reducing additive, such as ceria). It will be noted that peroxide can decompose to form a range of radicals (O, OH, OOH) and the radical reducing additive may reduce the amount of one, more, or all of these radicals. The radical reducing additive may be dispersed within the one or more ion-conducting polymer layer(s). The ion-conducting membrane may also comprise a recombination catalyst. A recombination catalyst is a catalyst which catalyses the reaction between hydrogen gas and oxygen gas to form water, thus minimise any hydrogen crossover through the membrane, to avoid hydrogen mixing with oxygen and associated safety concerns. Suitably, the recombination catalyst comprises platinum or palladium, or consists essentially of platinum or palladium (i.e. the nanoparticles are platinum nanoparticles or palladium nanoparticles). Alternatively, the recombination catalyst may be platinum alloyed with another element, for example a platinum-palladium alloy, a platinum-iridium alloy, a platinum cobalt alloy or a platinum-ruthenium alloy. The recombination catalyst may be dispersed within the one or more ion-conducting polymer layer(s).

[0043] The ion-conducting membranes as described herein may suitably be used as part of a catalyst-coated membrane. Such catalyst-coated membranes have an anode catalyst layer and / or a cathode catalyst layer applied to a face of the membrane. The membranes also have utility in systems in which one or more of the anode and the cathode catalyst layers are applied to substrates positioned either side of the membrane, such as gas diffusion layers or porous transport layers.

[0044] In a catalyst-coated membrane for a water electrolyser, a cathode catalyst layer may be applied to a surface of the membrane comprising a catalyst for catalysing the hydrogen evolution reaction. Suitably, the cathode catalyst layer comprises platinum, for example a platinum-on-carbon catalyst. The catalyst material can be formulated into a dispersion, printed ex-situ onto a PTFE sheet, and transferred onto the membrane by hot pressing. Alternatively, the dispersion can be directly coated onto the membrane.

[0045] In the case of a catalyst-coated membrane for a water electrolyser, an anode catalyst layer may be applied to a surface of the membrane comprising a catalyst for catalysing the oxygen evolution reaction. In the case that the catalyst-coated membrane is for a PEMWE, it may be typical that the anode catalyst layer comprises iridium, such as iridium oxide or mixed oxides of iridium and another metal or metals.

[0046] The anode material can be formulated into a dispersion, suitably in an ion-conducting polymer, printed ex-situ onto a PTFE sheet, and transferred onto the membrane by hot pressing. Alternatively, the dispersion can be directly coated onto the membrane.

[0047] Figs. 1 , a and b show electrolyser catalyst-coated membranes 1 in which there are two ionconducting polymer layers 4 and 5. With reference to Fig. 1a, the ion-conducting membrane 4 is disposed between a cathode catalyst layer 2 and an anode catalyst layer 3. A reinforcing layer 6 is positioned in ion-conducting polymer layer 5. With reference to Fig.1a, the reinforcing layer 6 bridges ion-conducting polymer layers 4 and 5.

[0048] Also provided is a membrane electrode assembly comprising an ion-conducting membrane as described herein, and a gas diffusion electrode and / or a porous transport layer on a first and / or second face of the ion-conducting membrane. Also provided is a membrane electrode assembly comprising a catalyst-coated membrane and a gas diffusion layer or porous transport layer present on the at least one of the catalyst layers. Gas diffusion layers are suitably based on conventional gas diffusion substrates. Typical substrates include non-woven papers or webs comprising a network of carbon fibres and a thermoset resin. A microporous layer may also be applied to the gas diffusion substrate on the face that will contact the catalyst layer. The microporous layer typically comprises a mixture of a carbon black and a polymer such as polytetrafluoroethylene (PTFE). The porous transport layer is suitably based on conventional porous transport substrates, such as a titanium mesh.

[0049] Also provided is a proton exchange membrane water electrolyser comprising an ionconducting membrane, a catalyst-coated membrane, or a membrane-electrode assembly as described herein.

[0050] The ion-conducting polymer layer has a maximum burst strength of greater than or equal to 10.5 MPa, suitably greater than or equal to 11.0 MPa. Maximum burst strength is normalised for thickness and is an inherent property of an ion-conducting polymer layer. It captures the mechanical durability of a layer under stress from water pressure, and thus captures the mechanical durability of an ion-conducting polymer layer during real-world use of a water electrolyser. Surprisingly, the ion-conducting polymer layers of the present disclosure have a high maximum burst strength even without additional reinforcement. As such, they are inherently durable and will provide exceptionally durable membranes when combined with a reinforcement. The particular benefit of the rupture testing method used herein to provide maximum burst strength is that the maximum stress in the sample is at the centre of a blister, enabling evaluation of membrane mechanical properties in the absence of edge effects such as stress on the membrane due to fixture clamps. Edge effects can also refer to defects on the edge of a tensile sample that can occur during preparation of the sample. These defects, however small, can act as early mechanical failure points. The rupture testing method also constrains the membrane in the in-plane direction and applies a biaxial stress to the membrane, similar to the mechanical conditions a membrane would experience in a water electrolyser or fuel cell. In contrast, tensile testing applies uniaxial stress and stretches the membrane in the in-plane direction. Related tests have been applied to blister testing of membranes for fuel cell applications using temperature and pressurised, humidified gas. The test disclosed herein uses water pressure instead of humidified gas to more closely replicate proton exchange membrane water electrolyser operational conditions.

[0051] Data is gathered using a rupture testing cell as shown in Figs. 6a and 6b, which is described in the Examples section. Membranes were hydrated and heated by the cell to 80°C for two hours before testing. Then, water pressure was used to pressurise membranes against a disc with a 0.8 mm diameter hole, resulting in deformation and creep of the membrane into the hole. The temperature and water pressure were maintained for the duration of the test, ensuring the membranes did not dry out.

[0052] To measure maximum burst strength, the water pressure was increased at about 1 .5 bar per second until pressure loss associated with rupture of the membrane occurred. The maximum pressure was recorded and converted to burst strength using the following equation: o: burst strength

[0053] Bo: coefficient in Hencky’s solution ~ 1.777

[0054] E: membrane Young’s modulus p: pressure a: hole radius h: membrane thickness

[0055] The Young’s modulus of commercially available membranes is around 35 MPa. This value of 35 MPa was used to convert rupture pressure into burst strength for all commercially available membranes tested. The Young’s modulus of the dispersion-cast membranes described herein was measured as set out in the Examples section.

[0056] The process disclosed herein may be a process of preparing the ion-conducting membrane disclosed herein. In the process, to provide the dispersion in step i) the ion-conducting polymer is typically dispersed in a mixture of an organic solvent and water. For example, the solvent may be a mixture of an alcohol (e.g. ethanol or propanol) and water. The volume ratio of organic solvent, such as ethanol, to water may be in the range of and including 95: 5 to 60: 40, such as in the range of and including 90: 10 to 70: 30. The solvent is formulated for achieving the desired dispersion, coating, and drying characteristics. When an ion-conducting polymer layer comprises a blend of two or more, preferably two, partially- or fully-fluorinated sulphonic acid polymers, step ii) comprises providing a dispersion of the two ion-conducting polymers and a solvent, which ion-conducting polymers are described above.

[0057] The dispersion may also comprise a radical reducing additive (e.g., a peroxide radical reducing additive, such as ceria). For example, the radical reducing additive (such as ceria) may be provided in the dispersion at a weight percentage, relative to the weight of ion-conducting polymer, in the range of and including 0.15 wt% to 0.35 wt%, such as in the range of and including 0.20 to 0.30 wt%. The radical reducing agent is typically added to the dispersion once the stabilised dispersion is mixed with the ion-conducting polymer. The dispersion may also comprise a recombination catalyst such as palladium or platinum nanoparticles. Typically the catalyst nanoparticles are present in the dispersion in an amount, with respect to the total weight of the dispersion components, in the range of and including 0.01 to 0.40 wt%, such as platinum nanoparticles in the range of and including 0.01 to 0.40 wt% Pt.

[0058] The process comprises the step of (ii) casting an ion-conducting polymer layer from the dispersion. The ion-conducting polymer layer is typically formed by depositing a dispersion onto a substrate to form the layer. The casting composition may be deposited using a slot-die coating process (whereby the dispersion is squeezed out by gravity or under pressure via a slot onto the substrate), knife-coating, bar coating, inkjet printing, curtain coating, or spray coating. Typically, the casting composition can be deposited using slot-die coating, bar coating, or inkjet printing, more typically slot-die coating.

[0059] The casting composition is deposited onto a substrate to form an ion-conducting polymer layer. In some cases, the ion-conducting membrane is formed from a single ion-conducting polymer layer. Alternatively, the ion-conducting membrane may be formed from two or more layers, such as between two and seven layers. The number of layers will be determined, for example, by the thickness of the desired membrane, and the degree of variation in desired composition across the membrane (for example the membranes may contain one or more layers comprising a reinforcing layer, such as ePTFE or PEEK, or an additive, such as a radical reducing additive).

[0060] The casting step may include a step of drying to remove any solvent from the dispersion. Drying may occur at room temperature, for example allowing the solvent to evaporate over time. Typically, such drying may be carried out, for example, by heating, for example to a temperature in the range of and including about 40 to about 120°C. Process step iii) of annealing the ion-conducting polymer layer is a separate step distinct from any drying step and is carried out at a temperature which is at least about 65°C greater than the glass transition temperature, Tg, of the ion-conducting polymer. Tgcan be measure using the protocol described in the Examples section. As a skilled person knows, a step of annealing changes the properties of the ion-conducting polymer, for example such that it becomes less water soluble, for example containing less than 10w% percent by weight of a water soluble fraction. The step of annealing is typically carried out for a time of at least about three minutes, typically at least about ten minutes. The step of annealing is typically carried out for a time of at most about thirty minutes. Surprisingly, this minimum processing temperature for unreinforced, dispersion-cast membranes produces membranes that have improved mechanical properties relative to extruded and reinforced membranes. Accordingly, the inventors have provided to the art ion-conducting membranes for proton exchange membrane water electrolysers having desirable mechanical properties, which will be exceptional when combined with a reinforcing layer.

[0061] Typically, the substrate is a backing sheet, an ion-conducting polymer layer, a catalyst layer on a backing sheet, or a catalyst layer on a gas diffusion electrode. It will be understood by the skilled person that the choice of substrate will depend on the structure and stage of production of the membrane.

[0062] In the case that the membrane is formed of a single ion-conducting polymer layer, or at the start of production of a multi-layer membrane, the substrate is typically a backing layer. The backing layer provides support for the ion-conducting membrane during manufacture and if not immediately removed, can provide support and strength during any subsequent storage and / or transport. The material from which the backing layer is made should provide the required support, suitably be compatible with the dispersion, suitably be impermeable to the dispersion, be able to withstand the process conditions involved in producing the ionconducting membrane and be able to be easily removed without damage to the ion-conducting membrane. Examples of materials suitable for use include a fluoropolymer, such as polytetrafluoroethylene (PTFE), ethylene tetrafluoroethylene (ETFE), perfluoroalkoxy polymer (PFA), fluorinated ethylene propylene (FEP - a copolymer of hexafluoropropylene and tetrafluoroethylene), and polyolefins, such as biaxially oriented polypropylene (BOPP), also polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) typically with a suitable coating such as silicone or fluorinated ethylene propylene (FEP).

[0063] In some cases in which a catalyst-coated membrane is to be produced, a catalyst layer is provided on a backing layer, for example by printing or using known coating techniques. The casting composition may then be deposited onto the catalyst layer such that the catalyst layer is disposed between the backing layer and the ion-conducting polymer layer formed by depositing the dispersion.

[0064] In some cases, typically when the ion-conducting membrane thickness is such that multiple passes are required in order to build up the membrane structure, the substrate is a previously formed ion-conducting polymer layer. It will be understood that the ion-conducting membrane may be formed by sequential deposition of layers. As an example, ion-conducting membranes may be formed as follows. In the first pass, a dispersion containing an ion-conducting polymer may be deposited onto a backing layer to form a first ion-conducting polymer layer, which is then dried. In a second pass, a dispersion is deposited onto the first ion-conducting polymer layer to form a second ion-conducting polymer layer. The second ion-conducting polymer layer is then dried. This sequence of application and drying is continued to produce further ionconducting polymer layers as required to form the desired membrane structure. The step iii) of annealing the ion-conducting polymer layer may be performed after the casting of one or more ion-conducting polymer layer, for example after step ii) of casting each ion-conducting polymer layer, or after all of the ion-conducting polymer layers have been cast.

[0065] The membranes formed by the method as described herein may be used in the production of catalyst-coated membranes. In such cases, the method may comprise the step of forming a catalyst layer on the first and I or the second face of the membrane to form an anode and I or a cathode. The specific type of catalysts for the cathode and anode for a proton exchange membrane water electrolyser are discussed above. Furthermore, the method of deposition can be varied, for example catalyst layers may be transferred to the membrane from a decal, for example by hot pressing, or catalyst dispersions may be directly printed onto the membrane.

[0066] Examples

[0067] Ion-conducting polymer layer casting

[0068] Ion-conducting polymer layers were dispersion-cast using commercially available 720, 790, 870, and 980 EW PFSA Aquivion® ionomers, as well as 1000 PFSA EW Nation™ ionomer, which are provided as aqueous dispersions. The Aquivion® ionomers are short side chain ionomers (length of two atoms), and the Nation™ ionomer is a long side chain ionomer (length of six atoms).

[0069] Ionomers were redispersed in EtOH-water solvent, cast onto 75 pm-thick Kapton (polyimide) substrate, and dried at 100°C to evaporate the solvent. The casting and solvent evaporation process was repeated twice to yield membranes with thickness of about 50 pm. The membranes were subsequently annealed for 12 min. Table 1 details the membrane layers that were prepared and measured for maximum burst strength.

[0070] For the glass transition temperature (Tg) values reported in Table 1 , dynamic mechanical analysis (DMA) data was obtained on a TA Instruments DMA850 with tensile film clamp geometry. Approximately 7 mm wide strips of membrane were cut and clamped into the film clamp using 3 inch-pounds of torque and with a gauge length of approximately 7 mm. The membranes were preconditioned in the DMA by applying a tensile force of 0.01 N while ramping the temperature from ambient to 80°C, holding at 80°C for 10 min, then cooling back to near ambient temperature (30 to 35°C). Then, the temperature was ramped from near ambient to 200°C at 2°C / min, while an oscillatory tensile displacement of 20 pm and 1 Hz was applied to the membrane. The storage and loss moduli of the membrane were recorded as a function of temperature. Tan 5 was calculated as the ratio of the loss modulus to the storage modulus. The Tgwas reported as the temperature corresponding to the maximum in tan 5.

[0071] Tab e 1

[0072] Blended ion-conducting polymer layer casting

[0073] Dispersions of 870 EWAguivion® and 1000 EW Nation™ in EtOH-water were combined and placed on a roller table for 24 hours at room temperature to facilitate mixing. Blended ion- conducting polymer layers having an 80 / 20 weight ratio 870 EW (short side chain) to 1000 EW (long side chain), were fabricated using the same dispersion-casting method described above. Table 2 details the ion-conducting polymer layer that was prepared.

[0074] Table 2

[0075] Conventional water electrolyser membranes Table 3 details comparative electrolyser membranes which were tested for maximum burst strength.

[0076] Table 3

[0077] Maximum Burst Strength

[0078] Maximum burst strength is normalised for thickness and is measure by the method detailed below. Maximum burst strength is an evaluation of the mechanical strength of a membrane, and a high maximum burst strength indicates a higher resistance to failure under pressure, for example water pressure during use of a water electrolyser cell.

[0079] Fig. 2 plots the maximum burst strength for the ion-conducting polymer layers of Examples 1 to 15, with a line provided to give the average maximum burst strength of conventional membranes 1 to 5. As can be seen in Fig. 2, the ion-conducting polymer layers of Examples 6, 9 and 14 to 16 have a higher maximum burst strength than the average for the conventional membranes. This is despite the fact that the Examples are dispersion-cast and do not contain a reinforcement, which will increase maximum burst strength. As can be seen in Table 3, extruded membranes have a higher maximum burst strength. However, extrusion casting is not favourable from a viewpoint of commercial manufacturing at scale due to difficulties and lack of customisation possible. The high maximum burst strengths of Examples 6, 9 and 14 to 16 correlate with an annealing temperature which is at least 65°C greater than the Tg of the ionomer. Example 17 is an illustration that, as shown in Fig. 3, for ionomer blends the properties of Tg and maximum burst strength follow the rule of mixtures. Accordingly, blending a high-TgPFSA such a lower-Tg PFSA can improve processability to achieve target properties by lowering the effective Tgof the membrane. Such ability to lower Tgwithout affecting other properties of the membrane could allow for manipulation of the maximum burst strength.

[0080] For rupture testing, a membrane rupture testing cell 1 as shown in Figs. 6a and b was used. The rupture test cell is made to heat, fully hydrate and pressurise membranes against a small hole. The rupture test cell 1 consists of an upper cell 3, lower housing 5, and locking collar 7. The lower housing 5 locates the rupture disc 6 (a disc with a small hole (<1 mm)) and also contains a leak path 9 for water after rupture of the membrane 11 . The membrane 11 sits on top of the rupture disc 7. The upper cell 3 seals against the membrane 11 with an o-ring, provides a route for pressurised water to hydrate the membrane 11 and houses heating elements, thermocouples and a water bleed valve. A brass threaded locking collar 13 secures the upper and lower sections of the cell together. All other rupture test cell components are manufactured from 316 austenitic stainless steel.

[0081] The cell allows heat and high-pressure water to be applied to the membrane, forcing it against the rupture disc. The temperature is controlled by the heating elements and via PID. The water pressure is supplied by a pneumatically operated high-pressure water pump and the pressure is maintained by a water pressure regulator capable of regulating a zero-flow system. A pressure transducer is installed between the water pressure regulator and the rupture cell water inlet. A data logger connected to the pressure transducer collects pressure measurements as a function of time. A schematic is shown in Fig. 6b.

[0082] Membrane samples are loaded in the cell by placing a sample on top of the rupture disc in the lower housing. The upper cell is then placed on top of the membrane and lower housing and secured with the locking collar. Next, the cell is bled by flowing water through the water bleed valve until no bubbles are observed. The bleed valve is subsequently shut and heating elements turned on.

[0083] Membranes were hydrated and heated by the cell to 80°C for two hours before testing. Then, water pressure was used to pressurise membranes against a disc with a 0.8 mm diameter hole, resulting in deformation and creep of the membrane into the hole. The temperature and water pressure were maintained for the duration of the test, ensuring the membranes did not dry out. To measure maximum burst strength, the water pressure was increased at about 1 .5 bar per second until pressure loss associated with rupture of the membrane occurred. The maximum pressure was recorded and converted to burst strength using the following equation: o: burst strength

[0084] Bo: coefficient in Hencky’s solution ~ 1.777

[0085] E: membrane Young’s modulus * p: pressure a: hole radius h: membrane thickness

[0086] * The Young’s modulus of commercially available membranes is around 35 MPa. This value of 35 MPa was used to convert rupture pressure into burst strength for all commercially available membranes tested. The Young’s modulus of the dispersion-cast membranes described herein was measured as described below.

[0087] Young’s Modulus of Dispersion-Cast Membranes in 80°C Water

[0088] The Young’s modulus of dispersion-cast membranes was determined using a TA Instruments DMA850 with submersion tank and tensile film clamp geometry. This geometry enables evaluation of the tensile mechanical properties of membranes immersed in water. Approximately 7 mm wide strips of membrane were cut and soaked in 80°C water for 30 min. Then, the wet membrane strips were clamped into the film clamp in the submersion tank using 2 inch-pounds of torque. The tank containing the membrane strip and film clamp was filled with water and heated to 80°C. The membrane strip was strained at 2.54 mm / min while immersed in 80°C water to obtain tensile stress-strain curves. The Young’s modulus was taken as the slope of the stress-strain curve between 0 and 7.5% strain.

[0089] Swelling to Measure Membrane Dimensional Change in Water

[0090] Fig. 4 is a plot of annealing temperature (Tann) - Tgvs in-plane swelling for Examples 7 to 9, 14 to 16 and 17. It is evident that the high performing, high maximum burst strength membranes of Examples 9 and 14 to 16 also have the advantage of low in-plane swelling. This advantage is beneficial during real-world use because swelling is a mode of failure in particular in water electrolyser membranes. The value for Example 17 is further evidence that for ionomer blends the properties follow the rule of mixtures. Accordingly, blending a high-TgPFSA with a lower-TgPFSA can improve processability to achieve target properties by lowering the effective Tgof the membrane.

[0091] To measure in-plane swelling, at least three 3 x 3 cm squares were cut from each membrane. The squares were immersed in 80°C water for two hours, then removed to measure their length, width and thickness. The wet mass of the membrane squares was recorded after any water droplets on the surface of the squares was gently blotted using lint-free wipes. The squares were then dried at 80°C under vacuum overnight to remove all water. The length, width, thickness and mass of the squares were then recorded. In-plane swelling was reported as the average difference in in-plane swelling between wet and vacuum dried samples.

[0092] Long-Term Mechanical Durability

[0093] Fig. 5 is a plot of time to rupture at a particular nominal membrane strength for Examples 7, 8 and 9 as well as Comparative Example 1. It is evident that the high performing, high maximum burst strength, membranes such as Example 9 have a long time to rupture, which is indicative of high durability in real-world use. Much longer, for example, than dispersion cast Comparative Example 1 .

[0094] To measure time to rupture, the water pressure was increased at about 1.5 bar per second until a target pressure below the maximum burst pressure of the membrane was reached using the equipment describes above. This target water pressure was maintained until pressure loss associated with rupture of the membrane occurred. The time and pressure during the test were recorded by an electronic data logger. The pressure was converted to the nominal stress in the membrane at rupture using the equation shown above and plotted as a function of time to rupture of the membrane. Converting pressure to nominal stress in the membrane normalises for membrane thickness, allowing comparison between membranes of different thicknesses.

Claims

Claims:

1. A dispersion-cast ion-conducting membrane for a proton exchange membrane water electrolyser, the membrane comprising a dispersion cast ion-conducting polymer layer which layer comprises an ion-conducting polymer, the ion-conducting polymer layer having a maximum burst strength of greater than or equal to 10.5 MPa.

2. A membrane according to claim 1 , wherein the membrane has a thickness of less than 100 microns.

3. A membrane according to claim 1 or claim 2, wherein the membrane is a single coherent polymer film comprising a plurality of the ion-conducting polymer layers.

4. A membrane according to any preceding claim, wherein the membrane comprises one or more reinforcing layer(s).

5. A membrane according to any preceding claim, wherein the ion-conducting polymer is a partially- or fully-fluorinated sulphonic acid polymer, preferably a perfluorosulphonic acid polymer.

6. A membrane according to claim 5, wherein the ion-conducting polymer has an equivalent weight in the range of and including 500 to 1200 g / mol.

7. A membrane according to claim 5 or claim 6, wherein the ion-conducting polymer is a long side chain perfluorosulphonic acid polymer.

8. A membrane according to claim 5 or claim 6, wherein the ion-conducting polymer is a short side chain perfluorosulphonic acid polymer.

9. A membrane according to claim 8, wherein the ion-conducting polymer has an equivalent weight in the range of and including 750 to 900 g / mol.

10. A membrane according to claim 5, wherein the ion-conducting polymer layer comprises a blend of two or more, preferably two, partially- or fully-fluorinated sulphonic acid polymers.

11. A membrane according to claim 10, wherein the ion-conducting polymer layer is a blend of two partially- or fully-fluorinated sulphonic acid polymers, wherein one polymer is a long side chain perfluorosulphonic acid polymer and the other polymer is a short side chain perfluorosulphonic acid polymer.

12. A catalyst-coated membrane for a water electrolyser, the catalyst-coated membrane comprising an ion-conducting membrane according to any preceding claim and a catalyst layer.

13. A proton exchange membrane water electrolyser comprising an ion-conducting membrane according to any of claims 1 to 11 , or comprising a catalyst-coated membrane according to claim 12.

14. A process of preparing a dispersion-cast ion-conducting membrane for a proton exchange membrane water electrolyser, the membrane comprising a dispersion cast ionconducting polymer layer which layer comprises an ion-conducting polymer, the process comprising the steps of: i) providing a dispersion of the ion-conducting polymer and a solvent; ii) casting an ion-conducting polymer layer from the dispersion; and iii) annealing the ion-conducting polymer layer; wherein the step of annealing comprises heating the ion-conducting polymer layer casted in step ii) at a temperature which is at least about 65°C greater than the glass transition temperature, Tg, of the ion-conducting polymer.

15. A process according to claim 14, wherein the membrane is a single coherent polymer film comprising a plurality of the ion-conducting polymer layers, wherein each ion-conducting polymer layer is prepared by the recited process.

16. A process according to claim 15, wherein the step iii) of annealing is performed after all of the required ion-conducting polymer layers have been casted.

17. A process according to any of claims 14 to 16, wherein a reinforcing layer is applied to an ion-conducting polymer layer prior to step iii) being carried out.

18. A process according to claim 17, wherein a reinforcing layer is applied to more than one ion-conducting polymer layer.

19. A process according to any of claims 14 to 18, wherein the ion-conducting polymer is a partially- or fully-fluorinated sulphonic acid polymer, preferably a perfluorosulphonic acid polymer.

20. A process according to claim 19, wherein the ion-conducting polymer has an equivalent weight in the range of and including 500 to 1200 g / mol.21 . A process according to claim 19 or claim 20, wherein the ion-conducting polymer is a short side chain perfluorosulphonic acid polymer.

22. A process according to claim 21 , wherein the ion-conducting polymer has an equivalent weight in the range of and including 750 to 900 g / mol.

23. A process according to claim 19, wherein the ion-conducting polymer layer comprises a blend of two or more, preferably two, partially- or fully-fluorinated sulphonic acid polymers, and step ii) comprises providing a dispersion of the two ion-conducting polymers and a solvent.

24. A process according to claim 23, wherein the ion-conducting polymer layer is a blend of two partially- or fully-fluorinated sulphonic acid polymers, wherein one polymer is a long side chain perfluorosulphonic acid polymer and the other polymer is a short side chain perfluorosulphonic acid polymer.

25. A dispersion-cast ion-conducting membrane for a proton exchange membrane water electrolyser, the membrane obtainable by a process as defined in any of claims 14 to 24.

26. A catalyst-coated membrane for a water electrolyser, the catalyst-coated membrane comprising an ion-conducting membrane and a catalyst layer, wherein the catalyst-coated membrane has a maximum burst strength of greater than or equal to 10.5 MPa, suitably greater than or equal to 11 .0 MPa.

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