Membrane

Abstract

A polymer electrolyte membrane for an electrochemical device, such as a water electrolyser, is provided. The membrane is provided with a first and a second membrane layer, each comprising a recombination catalyst. The loading of recombination catalyst in the first membrane layer is 75 to 97% of the total recombination catalyst loading in the electrolyte membrane, and the loading of recombination catalyst in the second membrane layer is 3 to 25% of the total recombination catalyst loading in the electrolyte membrane.

Description

[0001] MEMBRANE

[0002] Field of the Invention

[0003] The present invention relates to polymer electrolyte membranes, and their use in electrochemical devices, such as water electrolysers, and includes catalyst-coated membranes (CCMs) incorporating such membranes, and methods of their manufacture.

[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 comprise an electrolyte membrane, such as a PEM or an 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 transport layers are added either side of a CCM to make an assembly, sometimes referred to as a membrane electrode assembly (MEA). 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] The durability of electrolyser components, such as polymer electrolyte membranes and catalyst-coated membranes, is a key aspect of successful deployment of electrolyser solutions and increasing durability can help to reduce cost and system reliability.

[0009] Hydrogen peroxide is a byproduct that may be produced by reactions that take place in electrolysers, and in particular through reaction of oxygen at the cathode. It may have an adverse effect on the operation of electrolysers, for example by inhibiting catalyst activity, interfering in electrochemical reactions, causing membrane degradation, and contaminating reactants.

[0010] It is known to incorporate certain additives, such as cerium oxide, into an electrolyte membrane in order to reduce the concentration of peroxide and other oxygen radicals, and therefore increase membrane durability. However, cerium and other components originating from such additives can migrate during cell operation leading to contamination of catalyst layers and loss of durability protection.

[0011] It is also known to incorporate recombination catalysts into electrolyte membranes. A recombination catalyst is a catalyst which catalyses the reaction between hydrogen gas and oxygen gas to form water. Typically, recombination catalysts are introduced into electrolyte membranes for electrolyser applications with the aim of reducing the problematic crossover of hydrogen through the membrane.

[0012] For example, W02024003540A2 (Johnson Matthey Hydrogen Technologies Limited) describes electrolyte membranes with a recombination catalyst layer offering low hydrogen crossover. It is described that it is beneficial for the recombination catalyst layer to be placed closer to the anode than to the cathode with regards to the reduction in hydrogen crossover.

[0013] WO2020148545A1 (Johnson Matthey Fuel Cells Limited) describes proton exchange membranes incorporating recombination catalysts supported on graphene. Figures 1A to 1 F show different membrane configurations incorporating layers containing supported recombination catalyst. There is no disclosure of the variation of the distribution of recombination catalyst between two or more of such layers.

[0014] There remains a need to further enhance and develop electrolyte membranes and catalyst- coated membranes for water electrolysis applications, which offer improved durability.

[0015] Summary of the invention

[0016] The present inventors have identified that the selective distribution of recombination catalyst between two layers in an electrolyte membrane can provide a substantial increase in membrane oxygen recombination efficiency without significantly impacting on membrane hydrogen recombination efficiency at a fixed recombination catalyst loading. Such an increase in membrane oxygen recombination efficiency offers a reduction in peroxide generation arising from oxygen reaching the cathode catalyst layer during operation, and an increase in electrolyte membrane durability.

[0017] Therefore, in a first aspect of the invention there is provided a polymer electrolyte membrane for an electrochemical device, the electrolyte membrane comprising a first major surface, a second major surface, a first membrane layer comprising a recombination catalyst and a second membrane layer comprising a recombination catalyst, wherein the membrane is configured such that the first membrane layer is positioned closer to the first major surface than the second membrane layer, and wherein the loading of recombination catalyst in the first membrane layer is 75 to 97% of the total recombination catalyst loading in the electrolyte membrane, and the loading of recombination catalyst in the second membrane layer is 3 to 25% of the total recombination catalyst loading in the electrolyte membrane.

[0018] In a second aspect of the invention, there is provided a method for the formation of a polymer electrolyte membrane for an electrochemical device, the polymer electrolyte membrane comprising a first major surface, a second major surface, a first membrane layer comprising a recombination catalyst, and a second membrane layer comprising a recombination catalyst, the method comprising the steps of:

[0019] (i) providing a first ink and a second ink, the first and the second inks each comprising an ion-conducting polymer and a recombination catalyst;

[0020] (ii) forming a first membrane layer from the first ink;

[0021] (iii) forming a second membrane layer from the second ink; wherein the loading of recombination catalyst in the first membrane layer is 75 to 97% of the total recombination catalyst loading in the polymer electrolyte membrane, and the loading of recombination catalyst in the second membrane layer is 3 to 25% of the total recombination catalyst loading in the polymer electrolyte membrane.

[0022] The polymer electrolyte membranes have particular utility as components of a catalyst coated membrane (CCM). Therefore, in a third aspect of the invention there is provided a CCM for an electrochemical device, the CCM comprising a polymer electrolyte membrane according to the first aspect or obtained using a method according to the second aspect.

[0023] Suitably, the CCM is for a water electrolyser, such as a PEM water electrolyser. In such cases the CCM comprises a cathode catalyst layer for catalysing a hydrogen evolution reaction and I or an anode catalyst layer for catalysing an oxygen evolution reaction. Typically, the cathode catalyst layer comprises platinum (such as platinum on carbon) and I or the anode catalyst layer comprises iridium and I or ruthenium. Preferably, the anode layer is provided on the first major surface of the membrane, and the cathode layer is provided on the second major surface of the membrane.

[0024] The CCM may also be for a fuel cell, such as a PEM fuel cell. In such cases the CCM comprises a cathode catalyst layer for catalysing an oxygen reduction reaction and I or an anode catalyst layer for catalysing a hydrogen oxidation reaction.

[0025] In a fourth aspect of the invention there is provided an electrochemical device, such as a water electrolyser or a fuel cell, comprising a polymer electrolyte membrane according to the first aspect or obtained using a method of the second aspect, or comprising a catalyst coated membrane according to the third aspect. Brief description of the Figures

[0026] Figure 1 shows a schematic representation of an example of a polymer electrolyte membrane as described herein.

[0027] Figure 2 shows a schematic representation of another example of a polymer electrolyte membrane as described herein.

[0028] Figure 3 shows a schematic representation of further example of a polymer electrolyte membrane as described herein.

[0029] Figure 4 shows a schematic representation of an example of a catalyst-coated polymer electrolyte membrane as described herein.

[0030] Figure 5 shows a membrane configuration with two recombination-catalyst containing layers.

[0031] Figure 6 shows the results of modelling of oxygen and hydrogen cross-over based on the membrane configuration shown in Figure 5.

[0032] Figure 7 shows a membrane configuration with two recombination-catalyst containing layers.

[0033] Figure 8 shows the results of modelling of oxygen and hydrogen cross-over based on the membrane configuration shown in Figure 7.

[0034] Detailed Description

[0035] Preferred and / or optional features of the invention will now be set out. Any aspect of the invention may be combined with any other aspect of the invention unless the context demands otherwise. Any of the preferred and / or optional features of any aspect may be combined, either singly or in combination, with any aspect of the invention unless the context demands otherwise.

[0036] The present invention provides polymer electrolyte membranes for electrochemical devices, in particular for use in a water electrolyser. Preferably, the membrane is a proton exchange membrane (PEM), such as a PEM for a water electrolyser. It will however be understood by the skilled person that the membranes as described herein would have utility in other applications, such as PEMs for fuel cells, and anion exchange membranes (AEMs) for water electrolysers, fuel cells or other applications.

[0037] Such polymer electrolyte membranes are formed from ion-conducting polymers, such as proton-conducting polymers or anion-conducting polymers, for example hydroxyl-conducting polymers. Such materials are known to those skilled in the art.

[0038] Suitably, the polymer electrolyte membrane is a PEM and is formed from a proton-conducting polymer, typically comprising sulfonic acid groups. Suitably, the ion-conducting polymer is a perfluorinated sulfonic acid ionomer, or a partially-fluorinated or non-fluorinated hydrocarbon sulfonic acid ionomer. Examples of suitable proton-conducting polymers include partially- or fully-fluorinated sulfonic acid polymers, such as perfluorosulfonic acid ionomers (e.g. Nation® (Chemours), Aciplex® (Asahi Kasei), Aquivion™ (Solvay Speciality Polymers), Flemion® (Asahi Glass Co.); or ionomers based on a sulphonated hydrocarbon such as those available from FuMA-Tech GmbH as the fumapem® P, E or K series of products, Toyobo Corporation, and others. Suitably, the proton-conducting polymer has an equivalent weight of about 1100 or less, typically about 900 or less, suitably about 850 or less. Typically, the proton-conducting polymer has an equivalent weight of at least about 450.

[0039] Alternatively, the polymer electrolyte membrane is an anion-exchange membrane AEM and is formed from an anion (hydroxyl)-conducting polymer, typically comprising quaternary ammonium groups. Examples of suitable anion-conducting polymers include A901 and A201 made by Tokuyama Corporation, Fumasep FAA from FuMA-Tech GmbH, and Aemion polymers from lonomr Innovations

[0040] The polymer electrolyte membrane comprises a first membrane layer comprising a recombination catalyst and a second membrane layer comprising a recombination catalyst. Preferably, the recombination catalyst is in particulate form. More preferably, the recombination catalyst particles are dispersed in the first membrane layer and I or the second membrane layer. By dispersed in a membrane layer it is meant herein that the particles of recombination catalyst are distributed throughout the membrane layer, i.e. they are not located in a discrete layer or region of the membrane layer, or provided on or attached to a membrane component, such as a reinforcement component.

[0041] By recombination catalyst it is meant a catalyst which catalyses the reaction between hydrogen and oxygen to form water. Accordingly, the recombination catalyst used in the first and second membrane layers of the present invention may be any catalyst capable of catalysing the reaction between hydrogen and oxygen to form water, thus reducing or preventing the crossover of either hydrogen or oxygen, or both, through the membrane.

[0042] Suitably, the recombination catalyst comprises a platinum group metal. The platinum group metals are platinum, palladium, iridium, rhodium, ruthenium, and osmium. The recombination catalyst may comprise, or consists essentially of, one or more platinum group metals, and alloys or mixed oxides thereof. Suitably, the recombination catalyst comprises platinum group metal, typically platinum or palladium, or consists essentially of platinum group metal, typically platinum or palladium (e.g. the catalyst is in the form of particles of a platinum group metal, typically platinum particles or palladium particles). Alternatively, the recombination catalyst may be platinum group metal alloyed with one or more of the following elements: i) another platinum group metal; ii) gold; iii) base metals, such as iron, nickel, cobalt or chromium.

[0043] For example, a platinum-palladium alloy, a platinum-iridium alloy, a platinum cobalt alloy or a platinum-ruthenium alloy.

[0044] It may be particularly preferred that recombination catalyst consists of platinum.

[0045] Preferably, the recombination catalyst is in the form of nanoparticles. The term nanoparticle as used herein relates to a particle with a particle size in the range of and including 1 to 100 nm. The average particle size of nanoparticles in the ion-conducting membrane may be determined by transmission electron microscopy (TEM), for example analysing a crosssection of the membrane by TEM and, from the resulting image, measuring the size of a population of (e.g. 100) particles by image analysis and then calculating the average (mean) value. Preferably, the recombination catalyst is in the form of platinum-containing nanoparticles, such as platinum nanoparticles.

[0046] Preferably, the recombination catalyst is unsupported. The term unsupported will be readily understood by the skilled person. For example, it will be understood that the catalyst is not bound or fixed to a catalyst support, such as a carbon support, by physical or chemical bonds, e.g. by way of ionic or covalent bonds, or non-specific interactions such as an der Waals forces. It has been found that the use of an unsupported recombination catalyst facilitates ink processing prior to membrane formation, and offers increased membrane stability during electrochemical operation, avoiding routes of degradation via corrosion of the catalyst support.

[0047] Alternatively, the recombination catalyst may be supported on a catalyst support, for example a carbon support, such as a recombination catalyst comprising a platinum group metal (such as platinum) on a carbon support.

[0048] In some embodiments, the recombination catalyst in the first membrane layer is unsupported and the recombination catalyst in the second membrane layer is supported. Such a configuration may offer benefits relating to catalyst particle stability whilst minimising electrochemical degradation by placement of the supported catalyst near the cathode during electrolyser operation.

[0049] Typically, the polymer electrolyte membrane has a thickness of less than or equal to 100 .m. It may be preferred that the membrane has a thickness of less than or equal to 95 .m, 90 .m, or 85 .m. It may be preferred that the membrane has a thickness of at least 10 .m, such as at least 15 .m, at least 20 .m, at least 25 .m, at least 30 .m or at least 40 .m. It may be further preferred that the membrane has a thickness in the range of and including 10 to 100 .m, such as 15 to 100 .m, 20 to 100 .m, 30 to 100 .m, 30 to 90 .m, or 40 to 90 .m. The thickness of the polymer electrolyte membrane 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 membrane 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 polymer electrolyte (ion-conducting) membrane (prior to adding catalyst layers) at a temperature of 20 °C ± 3 °C, and relative humidity (RH) of 30-50%.

[0050] The polymer electrolyte membrane has a first major surface and a second, opposed, major surface. The membrane is configured such that the first membrane layer is positioned closer to the first major surface than the second major surface. Correspondingly, the membrane is configured such that the second membrane layer is positioned closer to the second major surface than to the first major surface.

[0051] In some embodiments, the first membrane layer is positioned adjacent to the first major surface. In some embodiments, the membrane is configured such that a recombination catalyst-free membrane layer is positioned between the first membrane layer and the first major surface.

[0052] In some embodiments, the second membrane layer is positioned adjacent to the second major surface. In some embodiments, the membrane is configured such that a recombination catalyst-free membrane layer is positioned between the second membrane layer and the second major surface.

[0053] In some embodiments, the membrane is configured such that a recombination catalyst-free membrane layer is positioned between first membrane layer and the second membrane layer.

[0054] It may be preferred that the membrane is configured such that the polymer electrolyte membrane comprises, or consists essentially of, the following layers in order:

[0055] (i) a first recombination catalyst-free membrane layer;

[0056] (ii) a first membrane layer comprising recombination catalyst;

[0057] (iii) a second recombination catalyst-free membrane layer;

[0058] (iv) a second membrane layer comprising recombination catalyst;

[0059] (v) a third recombination catalyst-free membrane layer.

[0060] It may be preferred that the membrane is configured such that the polymer electrolyte membrane comprises, or consists essentially of, the following layers in order:

[0061] (i) a first membrane layer comprising recombination catalyst;

[0062] (ii) a first recombination catalyst-free membrane layer; (iii) a second membrane layer comprising recombination catalyst.

[0063] In some embodiments, the membrane is configured such that the first membrane layer and the second membrane layer are adjacent. It may be preferred that the membrane is configured such that the polymer electrolyte membrane comprises, or consists essentially of, the following layers in order:

[0064] (i) optionally, a first recombination catalyst-free membrane layer;

[0065] (ii) a first membrane layer comprising recombination catalyst;

[0066] (iii) a second membrane layer comprising recombination catalyst;

[0067] (iv) optionally, a second recombination catalyst-free membrane layer.

[0068] Suitably, the thickness of the first membrane layer comprising recombination catalyst is in the range of and including 5 to 50 % of the total thickness of the polymer electrolyte membrane, such as in the range of and including 10 to 40 %, or 10 to 35 % of the total thickness of the polymer electrolyte membrane.

[0069] The thickness of the membrane layers may be adjusted, for example, by varying the number of deposition passes of ion conducting polymer during manufacture of the membrane, or by variation in the pump speed during deposition of ion-conducting polymer.

[0070] Suitably, the thickness of the second membrane layer comprising recombination catalyst is in the range of and including 5 to 60 % of the total thickness of the polymer electrolyte membrane, such as in the range of and including 10 to 40 %, or 10 to 35 % of the total thickness of the polymer electrolyte membrane.

[0071] Preferably, the thickness of the first membrane layer is less than the thickness of the second membrane layer. Such as configuration is considered by the present inventors to provide optimal conditions for reduction in gas crossover for a given catalyst loading.

[0072] The thickness of the membrane layers as a proportion of total membrane thickness may be determined by scanning electron microscopy (SEM). SEM analysis is carried out on cross sections of the ion-conducting membrane and the membrane and I or layer thickness measured at multiple (for example 10) points. The thickness values are then determined by calculating the arithmetic mean of the measured values. Typically, the SEM measurement is carried out on a cross section of the ion-conducting membrane which is prepared for SEM analysis by embedding in resin, grinding and polishing.

[0073] Suitably, the polymer electrolyte membrane comprises a polymeric reinforcement component. Such reinforcement components comprise a porous polymer material, and the membrane is configured such that the membrane ion-conducting polymer is impregnated within the porous polymer material. The reinforcing component can confer mechanical strength to the membrane. The reinforcing component is typically planar. The porous polymer material may be a fluoropolymer. The porous polymer material may be selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyethersulfone (PES), poly(vinylidene fluoride-co- hexafluoropropylene) (PVDF-HFP), polyimides (PI), polyetherimide (PEI), poly(aryl ether ketone) (PAEK), poly(aryl ether sulfone), poly(phenylene sulfide) (PPS) and polyvinylpyrrolidone (PVP). The porous polymer material may be expanded polytetrafluoroethylene (ePTFE). The porous polymer material may also comprise a polymer backbone based on a nitrogen-containing heterocycle, such as polybenzimidazole.

[0074] In some other embodiments the 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)

[0075] Preferably, the first membrane layer and I or the second membrane layer do not contain a polymeric reinforcement component.

[0076] It may be preferred that the membrane is configured such that a recombination catalyst-free membrane layer is positioned between first membrane layer and the second membrane layer, and that the recombination catalyst-free membrane layer comprises a polymeric reinforcement component. It may be preferred that the recombination catalyst-free membrane layer comprises two polymeric reinforcement component or a polymeric woven fabric.

[0077] The ion-conducting membrane may suitably be formed by methods that do not require lamination steps to form the membrane, for example by depositing multiple layers of ion- conductive polymer on top of each other via a liquid phase deposition process such as printing, spraying, or coating.

[0078] The polymer electrolyte membrane may suitably be a single coherent polymer film comprising one or more, as defined above, ion-conducting polymer layers. The term ‘coherent’ as used herein means that the membrane is free from internal lamination interfaces.

[0079] Lamination of polymer electrolyte membranes comprises pressing and / or bonding at least two solid membranes together, such membranes optionally being coated with a catalyst layer. A lamination interface is formed between the two membranes where solid surfaces of the individual membranes are pressed and / or bonded together. Lamination interfaces comprise physical defects. Furthermore, the structural and / or chemical nature of a lamination interface also differs from that of the bulk polymer material. This is because when a solid membrane is formed, the outer surfaces of the solid membrane have surface features which are distinct from those in the bulk material. For example, a hydrophobic skin forms on a surface of a membrane at an air interface. Raman spectroscopy can detect this difference. As such, when two solid membranes are pressed together, the lamination interface formed by the two solid surfaces is distinctive in chemical and / or structural form compared to the bulk of the ion- conductive polymer material. Microscopy and spectroscopy techniques can thus distinguish between lamination interfaces between layers of ion-conductive polymer and interfaces which have been formed via a liquid phase deposition process such as printing, spraying, or coating of layers to build up a multi-layer structure. That is, a non-laminated interface is structurally and / or chemically distinct from a laminated interface and is not just a feature of the manufacturing method. Furthermore, a non-laminated interface can be identified as being non-laminated in a membrane without prior knowledge of the manufacturing method. Examples of analysis techniques for detecting a laminated interface include cross-section SEM. Variations of crystallinity at interfaces can be detected using cross-section TEM. Other techniques for detecting laminated interfaces include 13C / 1 H / 19F solid state NMR, neutron diffraction, and / or a combination of two or more of the aforementioned techniques.

[0080] Due to physical defects and / or chemical variations at lamination interfaces between ionconducting polymer membranes, such interfaces can increase the resistance of a multi-layer ion-conducting membrane. As such, it has been found to be advantageous to fabricate a multilayer ion conductive 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 / mem branes of ion conductive polymer.

[0081] The loading of recombination catalyst in the first membrane layer is 75 to 97% of the total recombination catalyst loading in the electrolyte membrane, and the loading of recombination catalyst in the second membrane layer is 3 to 25% of the total recombination catalyst loading in the electrolyte membrane. The selective distribution the recombination catalyst between the first and second membrane layers offers a reduction in peroxide formation occurring through oxygen crossover during operation, without significantly reducing hydrogen crossover mitigation for a given recombination catalyst loading. By “loading” it is meant herein the amount of catalyst per unit area of membrane. The skilled person will understand that amount of catalyst refers to the amount of catalytic metal present (and not the amount of catalytic metal and catalyst support in the case of supported catalysts). Therefore, in the case of platinum as the recombination catalyst, the loading of platinum in the first membrane layer is 75 to 97% of the total platinum loading in the electrolyte membrane, and the loading of platinum in the second membrane layer is 3 to 25% of the total platinum loading in the electrolyte membrane. As an example, for a total recombination catalyst (e.g. platinum) loading of 100 .g / cm2, the first membrane layer contains 75 to 97 .g / cm2of recombination catalyst (e.g. platinum) and the second membrane layer contains 3 to 25 .g / cm2of recombination catalyst (e.g. platinum). The distribution of the recombination catalyst in the membrane may be determined by scanning electron microscopy-energy dispersive x-ray (SEM-EDX) analysis carried out on cross sections of the membrane at multiple (for example 10) points.

[0082] Selective distribution of the recombination catalyst between two layers offers a significant reduction in the levels of oxygen reaching the catalyst layer through the membrane, whilst having minimal impact on the hydrogen crossover levels (at a fixed catalyst loading).

[0083] Preferably, that the loading of recombination catalyst in the first membrane layer is 75 to 95% of the total recombination catalyst loading in the electrolyte membrane. It may be further preferred that the loading of recombination catalyst in the first membrane layer is 80 to 95%, or 85 to 95%, of the total recombination catalyst loading in the electrolyte membrane.

[0084] Preferably, the loading of recombination catalyst in the first membrane layer is 5 to 25% of the total recombination catalyst loading in the electrolyte membrane. It may be further preferred that the loading of recombination catalyst in the second membrane layer is 5 to 20 or 5 to 15% of the total recombination catalyst loading in the electrolyte membrane.

[0085] Suitably, the total loading of recombination catalyst in the electrolyte membrane is in the range of and including 2 to 150 .g / cm2. It has been found that this range of catalyst loading provides a suitable balance between reducing the level of hydrogen crossover during use and the cost associated with the inclusion of catalyst in the membrane. The catalyst loading may be determined by inductively coupled plasma mass spectrometry (ICP-MS). It may be preferred, that the total loading of recombination catalyst in the electrolyte membrane is in the range of and including 5 to 100 .g / cm2, or 20 to 100 .g / cm2.

[0086] In some embodiments, the recombination catalyst is in the form of nanoparticles, and the total loading of recombination catalyst in the electrolyte membrane is in the range of and including 2 to 50 .g / cm2. It has been found that the use of nanoparticles can enable a low level of hydrogen crossover at lower overall catalyst loadings.

[0087] It has been found that dispersions of recombination catalyst (e.g. platinum group metalcontaining) nanoparticles stabilised with a nanoparticle stabilising agent may advantageously be combined with ion-conducting polymers to form inks, and that ion-conducting membranes may be produced using such inks which show excellent dispersion of recombination catalyst (e.g. platinum group metal-containing) nanoparticles within a membrane layer. Moreover, it has been found that the ion-conducting membranes containing a combination of such stabilisers with recombination catalyst (e.g. platinum group metal-containing) nanoparticles exhibit an enhanced reduction of hydrogen crossover. Therefore, preferably the first and I or the second membrane layer comprises recombination catalyst nanoparticles and a nanoparticle stabilising agent. Suitable, nanoparticle stabilising agents are selected from those agents which interact with the nanoparticle surface, preventing aggregation and coalescence of the nanoparticles, and enabling the formation of a nanoparticle dispersion. Such stabilisation of the nanoparticle surface is typically through interaction between the nanoparticle with a polar functional group of the stabilising agent. Typically, the stabilising agent comprises an amide, carboxylic acid, sulphonic acid, amine, alcohol, or ether functional group. The stabilising agent may comprise an amide or an ether functional group. The stabilising agent may suitably comprise a tertiary amide group. Suitably, the stabilising agent does not have acidic functional groups. Suitably, the stabilising agent does not have sulphonic acid functional groups. The nanoparticle stabilising agent may be water-soluble, such as a water-soluble polymer. The nanoparticle stabilising agent may have a water solubility at 25 °C of at least 1 mg / mL, typically at least 10 mg / mL, more typically at least 100 mg / mL. The stabilising agent may have a greater hydrophobicity and / or a lower water uptake value than the ion-conducting polymer used in the membrane layer. Suitably, the polymeric stabilising agent may comprise amide functional groups, such as tertiary amide functional groups, for example pyrrolidone functional groups (such as polyvinylpyrrolidone, or copolymers including vinylpyrrolidone as a polymerisation unit).

[0088] Suitably, the stabilising agent is polyvinylpyrrolidone (PVP). The use of PVP has been found to provide excellent dispersion stability in the presence of perfluorosulphonic (PFSA) acid polymers, and greater nanoparticle dispersion stability than nanoparticles with PFSA alone. Suitably, the stabilising agent is PVP with a weight average molecular weight in the range of and including 5,000 to 50,000. Such a range is considered to provide a suitable balance between dispersion stability and ease of polymer processability. The polymeric stabilising agent may be PVP with a weight average molecular weight in the range of and including 8,000 to 45,000.

[0089] In some embodiments, the membrane comprises a radical reducing additive (e.g. peroxide radical reducing additive). 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 recombination catalyst layer. Suitable radical reducing additives are known to the skilled person, and include salts and oxides of cerium and manganese, for example cerium oxide.

[0090] It is however advantageous in certain circumstances to avoid the use of radical reducing additives, for example additives which are cerium-containing compounds or manganese- containing compounds, as the elements in such additives can migrate during use, leading to contamination of components and a reduction in membrane durability over time. The membrane structures as set out herein offer a reduction in oxygen crossover and therefore peroxide generation. Therefore, preferably, the polymer electrolyte membranes do not include a radical reducing additive. More preferably, the polymer electrolyte membranes do not contain a cerium-containing compound and / or a manganese-containing compound.

[0091] The membranes as described herein may suitably be used as part of a catalyst coated membrane (CCMs). Such CCMs have an anode catalyst layer and / or a cathode catalyst layer applied to a face of the membrane.

[0092] In the case of a CCM 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. It may be preferred that the cathode catalyst layer comprises platinum, for example a platinum- on-carbon catalyst.

[0093] In the case of a CCM 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 CCM is for a PEMWE, it may be preferred that the anode catalyst layer comprises iridium or ruthenium, such as iridium oxide or mixed oxides of iridium and another metal or metals, such as an oxide of iridium and ruthenium.

[0094] The catalyst materials for the anode catalyst layer and the cathode catalyst layers can be formulated into an ink, suitably with an ion conducting polymer, printed ex-situ onto a PTFE sheet, and transferred onto the membrane by hot pressing. Alternatively, the ink can be directly coated onto the membrane.

[0095] The CCMs for water electrolysers are configured such that the anode layer is provided on the first major surface of the membrane (closer to the first membrane layer than the second membrane layer) and the cathode layer is provided on the second major surface.

[0096] 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 (ion-conducting) 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.

[0097] In a water electrolyser, additional transport layers are positioned each side of a membrane to facilitate reagent and product transfer to and from the catalyst layers, and to provide electrical contact. The catalyst-coated membrane and transport layer(s) are referred to together as a membrane electrode assembly (MEA). These additional transport layers may be known as porous transport layers or gas diffusion layers. These layers may or may not be directly attached to the CCM. Other components of a water electrolyser may include bipolar plates and current collector plates. Stacks of such assemblies make up an electrolyser system including power and control systems.

[0098] Suitable transport layers at the anode side of the CCM are known to the skilled person and are typically formed from a metal-based porous structure. Such transport layers must be sufficiently conducting and in a form that is compatible with positioning adjacent to the CCM (without, for example, sharp edges or protrusions that would damage the membrane during use). Such metal-based porous structures may be in the form of, for example, felts or nonwoven cloths, mesh, foams and sintered compacts of metal-containing particles. For PEMWE applications, suitable porous transport layers comprise titanium. For AEMWE applications, suitable porous transport layers comprise nickel or stainless steel.

[0099] Suitable transport layers at the cathode side of the CCM are known to the skilled person and are typically non-woven papers or webs comprising a network of carbon fibres and a thermoset resin binder (e.g. the TGP-H series of carbon fibre paper available from Toray Industries Inc., Japan or the H2315 series available from Freudenberg FCCT KG, Germany, or the Sigracet® series available from SGL Technologies GmbH, Germany or AvCarb® series from AvCarb Material Solutions), or woven carbon cloths. The carbon paper, web or cloth may be provided with a further treatment prior to being incorporated into as MEA either to make it more wettable (hydrophilic) or more wet-proofed (hydrophobic). The nature of any treatments will depend on the type of electrochemical device and the operating conditions that will be used.

[0100] An example of a polymer electrolyte membrane (10) as described herein is shown in Figure 1. The membrane has a first major surface (11) and a second major surface (12). The membrane is provided with a first membrane layer comprising dispersed recombination catalyst particles (13) and a second membrane layer comprising dispersed recombination catalyst particles (14). The first membrane layer (13) is positioned closer to the first major surface (11) than the second membrane layer (14). Recombination-catalyst free membrane layers (15) are provided between the first membrane layer (13) and the first major surface (11), the second membrane layer (14) and the second major surface (12), and between the first membrane layer (13) and the second membrane layer (14). The central recombinationcatalyst free membrane layer is provided with an embedded reinforcement component (16). The recombination catalyst is distributed such that loading of recombination catalyst in the first membrane layer (13) is 75 to 97% of the total recombination catalyst loading in the polymer electrolyte membrane (10). A further example of a polymer electrolyte membrane (20) as described herein is shown in Figure 2. The membrane has a first major surface (11) and a second major surface (12). The membrane is provided with a first membrane layer comprising dispersed recombination catalyst particles (13) and a second membrane layer comprising dispersed recombination catalyst particles (14). The first membrane layer (13) is positioned closer to the first major surface (11) than the second membrane layer (14). A recombination-catalyst free membrane layer (15) is provided between the first membrane layer (13) and the second membrane layer (14). The central recombination-catalyst free membrane layer is provided with an embedded reinforcement component (16). The recombination catalyst is distributed such that loading of recombination catalyst in the first membrane layer (13) is 75 to 97% of the total recombination catalyst loading in the electrolyte membrane (20).

[0101] A further example of a polymer electrolyte membrane (25) as described herein is shown in Figure 3. The membrane has a first major surface (11) and a second major surface (12). The membrane is provided with a first membrane layer comprising dispersed recombination catalyst particles (13) and a second membrane layer comprising dispersed recombination catalyst particles (14). The first membrane layer (13) is positioned closer to the first major surface (11) than the second membrane layer (14). Recombination-catalyst free membrane layers (15) are provided between the first membrane layer (13) and the first major surface

[0102] (11), the second membrane layer (14) and the second major surface (12). The recombination catalyst is distributed such that loading of recombination catalyst in the first membrane layer (13) is 75 to 97% of the total recombination catalyst loading in the electrolyte membrane (20). The first membrane layer (13) has a thickness less than the second membrane layer (14).

[0103] An example of a catalyst-coated polymer electrolyte membrane (30) for a water electrolyser as described herein is shown in Figure 4. An anode catalyst layer (17) is provided on the first major surface (11). A cathode catalyst layer (18) is provided on the second major surface

[0104] (12).

[0105] The polymer electrolyte membranes as described herein may be formed by sequential deposition of inks comprising ion-conducting polymer and, in the case of the first and second membrane layers, recombination catalyst particles.

[0106] The inks typically comprise the ion conducting polymer dispersed in a solvent. The solvent may be 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: at least 60: 40, 70: 30, or 75: 25; no more than 95: 5; 90: 10, or 85: 15; or within a range defined by any combination of the aforementioned lower and upper limits. The solvent is formulated for achieving the desired dispersion, coating, and drying characteristics.

[0107] The ion conducting polymer may be provided in the ink at a weight percentage with respect to the total weight of recombination catalyst and ion conducting polymer: at least 7 wt%, 10 wt%, 14 wt%, or 16 wt%; no more than 22 wt%, 20 wt%, or 18 wt%; or within a range defined by any combination of the aforementioned lower and upper limits. The ion conducting polymer content is selected for achieving the desired dispersion, coating, and drying characteristics.

[0108] The ink may also comprise a radical reducing additive (e.g., a peroxide radical reducing additive such as ceria). For example, the radical reducing additive may be provided in the dispersion at a weight percentage, relative to the weight of ion conducting polymer, of : at least 0.15 wt%, 0.20 wt%, or 0.23 wt%; no more than 0.35 wt%, 0.30 wt%, or 0.28 wt%; or within a range defined by any combination of the aforementioned lower and upper limits.

[0109] The inks used to form the layers are applied sequentially. The application of the inks may be carried out by any suitable method, for example by casting or printing the ink to form the layer. Preferably, formation of the layers is carried out by slot-die coating. Typically, each layer is dried, or partially dried, to remove solvent prior to subsequent layer formation. Suitable methods of ink and membrane formation may be found in W02024003540A2 (Johnson Matthey Hydrogen Technologies Limited) and PCT / GB2023 / 052510 (Johnson Matthey Hydrogen Technologies Limited) which are both incorporated herein by reference.

[0110] 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.

[0111] Examples

[0112] The inks formed in accordance with following methods may be used to form membranes incorporating layers with dispersed platinum.

[0113] Example 1 - Formation of a Pt-black containing ink

[0114] A mixture of an ion conducting polymer (PFSA ionomer, 3M Advanced Materials) and ethanokwater (80:20) was prepared. Platinum black catalyst (Johnson Matthey pic) was added to the mixture with the target amount of platinum black of 8.8 wt% based on a total weight of platinum, ionomer, ethanol and water.

[0115] The concentrated ink was then diluted again using a mixture of ionomer and ethanol: water (80:20) in order to achieve ~10 micrograms of Pt per cm2in the formed membrane. The platinum loading in the membrane layer may be varied by altering the quantity of ionomer added during the dilution step. Example 2A - Formation of a stabilised dispersion of platinum nanoparticles using PVP and formaldehyde.

[0116] Pt(NOs)4 (equivalent to 1g of Pt) was added to water (500mL) and stirred. PVP10 (average molecular 10,000, 8.5g) was added followed by the addition of formaldehyde (37% in water, 20.8 g). The mixture was heated to 68 °C and then allowed to cool to room temperature and stirred overnight to form a dispersion.

[0117] Example 2B - Formation of an ion-conducting polymer ink containing PVP-stabilised nanoparticles

[0118] A stabilised aqueous dispersion of Pt nanoparticles (formed according to a method analogous to Example 2A) was mixed with ethanol and water to create a mixture with an ethanol: water weight ratio of 80:20. Dry PFSA ionomer (3M Corp) is added to the mixture to create a dispersion, where the ionomer solids is ~ 17 %wt. The platinum loading in the membrane layer may be varied by altering the quantity of nanoparticle dispersion mixed with ethanol and water.

[0119] Membrane formation

[0120] Membranes incorporating first and second membrane layers may be formed using slot-die coating using a series (e.g. 5) printing I coating passes onto a backing substrate (for example PET with one side release layer) with drying to remove solvent in between each coating pass. An example coating order would be:

[0121] Pass one - coating using a Pt-free ink comprising perfluorosulfonic acid (PFSA) ionomer in ethanokwater (80:20).

[0122] Pass two - coating using a Pt-containing ink, such as an ink formed in accordance with Example 1 or Example 2B.

[0123] Pass three - coating using a Pt-free ink comprising perfluorosulfonic acid (PFSA) ionomer in ethanokwater (80:20). A reinforcement, such as an ePFTE reinforcement, may be incorporated during coating and prior to drying the layer.

[0124] Pass four - coating using a Pt-containing ink, such as an ink formed in accordance with Example 1 or Example 2B.

[0125] Pass five - coating using a Pt-free ink comprising perfluorosulfonic acid (PFSA) ionomer in ethanokwater (80:20).

[0126] The membrane may be dried, such as a temperature between 100 and 160 °C, and then annealed, for example at 160 °C.

[0127] Membrane analysis and characterisation The total platinum loading in the membrane may be measured by inductively coupled plasma mass spectrometry (ICP-MS).

[0128] The thickness of membrane layers may be measured using scanning electron microscopy (SEM). A cross section of membrane is embedded in resin, ground, polished and carbon coated for SEM. The samples may be analysed using a Zeiss Ultra 55 Field emission electron microscope. The boundary between the recombination catalyst layer and another layer is identified by where the dispersion of recombination catalyst terminates.

[0129] Modelling of oxygen cross-over

[0130] A computational model was developed which uses numerical methods to solve the convectiondiffusion eguation and calculate the transport of hydrogen and oxygen species through a membrane. The model incorporated the operating conditions of the electrode, such as partial pressures in the cathode and anode sides, current density, and operating temperature.

[0131] The transport of both species is linked to the local recombination reaction at the surface of a catalyst. A kinetic model is used to calculate the local reaction rate, which is proportional to the local available surface area of the catalyst within the membrane and the local concentration of oxygen and hydrogen. The model also uses input of the catalyst loading, distribution within the membrane, and specific surface area.

[0132] The model uses minimisation algorithms to predict the optimal catalyst distribution within the membrane to minimise the crossover amount.

[0133] Modelling of a membrane with two Pt-containing layers with varying distribution of Pt.

[0134] Modelling was carried out of a membrane structure with two layers as shown in Figure 5. The overall Pt loading was set at 10 .g / cm2with a pressure at the cathode of 30 bar and a pressure at the anode of 2 bar.

[0135] The results of modelling are shown in Figure 6 (catalyst mass fraction refers to the mass % of Pt in the Pt-containing layer closest to the anode). The modelling indicates that additional of a second Pt-containing layer with a small proportion of the total Pt in the membrane can lead to a significant increase in membrane O2 recombination efficiency. Distribution of the Pt between two layers reduces H2 recombination efficiency for a given Pt loading. A loading of 3-25% in the second layer produces maximum effect on O2 recombination efficiency whilst producing only a minimum reduction in H2 recombination efficiency.

[0136] Modelling was also carried out of a membrane structure with two layers as shown in Figure 7. The overall Pt loading was set at 20 .g / cm2with a pressure at the cathode of 30 bar and a pressure at the anode of 2 bar. The results of modelling are shown in Figure 8. The modelling indicates that at a higher loading of recombination catalyst the hydrogen recombination efficiency is increased but that distribution of a small proportion (e.g. up to 20%) of the catalyst into a second layer can provide significant mitigation of oxygen crossover.

[0137] Further modelling was carried out with a catalyst loading of 5 .g / cm2. As expected, a reduction in recombination catalyst efficiency was observed. The model showed an optimal distribution between the two layers to be around 90% of the catalyst in the first catalyst layer and 10% in the second catalyst layer.

[0138] Further modelling was carried out across a range of pressure differentials across the membrane (cathode pressure - anode pressure (bar) = 10, 20, 30, 40, 50, and 60). This modelling also showed the benefit of recombination catalyst distribution as disclosed herein across the pressure differential range, and that for a number of pressure differentials the optimal layer thickness of the first catalyst layer is less than the thickness of the second membrane layer.

Claims

Claims1. A polymer electrolyte membrane for an electrochemical device, the polymer electrolyte membrane comprising a first major surface, a second major surface, a first membrane layer comprising a recombination catalyst and a second membrane layer comprising a recombination catalyst, wherein the membrane is configured such that the first membrane layer is positioned closer to the first major surface than the second membrane layer, and wherein the loading of recombination catalyst in the first membrane layer is 75 to 97% of the total recombination catalyst loading in the electrolyte membrane, and the loading of recombination catalyst in the second membrane layer is 3 to 25% of the total recombination catalyst loading in the electrolyte membrane.

2. A polymer electrolyte membrane according to claim 1 , wherein the recombination catalyst comprises a platinum group metal, such as platinum.

3. A polymer electrolyte membrane according to claim 1 or claim 2, wherein the first membrane layer and I or the second membrane layer comprises dispersed recombination catalyst particles.

4. A polymer electrolyte membrane according to any one of claims 1 to 3, wherein the recombination catalyst in the first membrane layer and I or the second membrane layer is in the form of nanoparticles.

5. A polymer electrolyte membrane according to claim 4, wherein the first membrane layer and I or the second membrane layer comprises a nanoparticle stabilising agent.

6. A polymer electrolyte membrane according to claim 5, wherein the nanoparticle stabilising agent is a polymer, such as polyvinylpyrrolidone or copolymers including vinylpyrrolidone as a polymerisation unit.

7. A polymer electrolyte membrane according to any one of claims 1 to 6, wherein the thickness of the first membrane layer is less than the thickness of the second membrane layer.

8. A polymer electrolyte membrane according to any one of claims 1 to 7, wherein the recombination catalyst is unsupported.

9. A polymer electrolyte membrane according to any one of the preceding claims, wherein the membrane is configured such that a recombination catalyst-free membrane layer is positioned between the first membrane layer and the first major surface and / or a recombination catalyst-free membrane layer is positioned between the second membrane layer and the second major surface and / or a recombination catalyst-free membrane layer is positioned between the first and the second membrane layers.

10. A polymer electrolyte membrane according to any one of the preceding claims, wherein the thickness of the polymer electrolyte membrane is in the range of and including 10 to 100 .m.

11. A polymer electrolyte membrane according to any one of the preceding claims, wherein the total loading of recombination catalyst in the electrolyte membrane is in the range of and including 2 to 150 .g / cm2, such as in the range of and including 5 to 100 .g / cm2.

12. A polymer electrolyte membrane according to any one of the preceding claims, wherein the electrolyte membrane comprises at least one polymeric reinforcement component, such as expanded polytetrafluoroethylene (ePTFE), polybenzimidazole (PBI) or a polymeric woven fabric.

13. A polymer electrolyte membrane according to any one of the preceding claims, wherein the first membrane layer and I or the second membrane layer do not contain a polymeric reinforcement component such as expanded polytetrafluoroethylene (ePTFE), polybenzimidazole (PBI) or a polymeric woven fabric.

14. A polymer electrolyte membrane according to any one of the preceding claims, wherein the electrolyte membrane does not contain a cerium-containing compound and / or a manganese-containing compound.

15. A polymer electrolyte membrane according to any one of the preceding claims, wherein the electrolyte membrane comprises, or consists essentially of, the following layers in order:(i) a first recombination catalyst-free membrane layer;(iii) a first membrane layer comprising recombination catalyst particles;(iv) a second recombination catalyst-free membrane layer;(v) a second membrane layer comprising recombination catalyst particles;(vi) a third recombination catalyst-free membrane layer.

16. A polymer electrolyte membrane according to claim 15, wherein the second recombination catalyst-free membrane layer comprises at least one polymeric reinforcement component, such as two polymeric reinforcement components or a polymeric woven fabric.

17. A polymer electrolyte membrane according to any one of the preceding claims, wherein the electrolyte membrane comprises, or consists essentially of, the following layers in order:(i) a first membrane layer comprising recombination catalyst particles;(ii) a first recombination catalyst-free membrane layer;(iii) a second membrane layer comprising recombination catalyst particles.

18. A polymer electrolyte membrane according to claim 17, wherein the first recombination catalyst-free membrane layer comprises at least one polymeric reinforcement component, such as two polymeric reinforcement components or a polymeric woven fabric.

19. A polymer electrolyte membrane according to any one of claims 1 to 14, wherein the polymer electrolyte membrane comprises, or consists essentially of, the following layers in order:(i) optionally, a first recombination catalyst-free membrane layer;(ii) a first membrane layer comprising recombination catalyst particles;(iii) a second membrane layer comprising recombination catalyst particles;(iv) optionally, a second recombination catalyst-free membrane layer.

20. A method for the formation of a polymer electrolyte membrane for an electrochemical device, the polymer electrolyte membrane comprising a first major surface, a second major surface, a first membrane layer comprising a recombination catalyst and a second membrane layer comprising a recombination catalyst, the method comprising the steps of:(i) providing a first ink and a second ink, the first and the second inks each comprising an ion-conducting polymer and a recombination catalyst;(ii) forming a first membrane layer from the first ink;(iii) forming a second membrane layer from the second ink; wherein the loading of recombination catalyst in the first membrane layer is 75 to 97% of the total recombination catalyst loading in the polymer electrolyte membrane, and the loading of recombination catalyst in the second membrane layer is 3 to 25% of the total recombination catalyst loading in the polymer electrolyte membrane.

21. A catalyst-coated membrane for an electrochemical device, the catalyst-coated membrane comprising an electrolyte membrane according to any one of claims 1 to 19 or obtained using a method according to claim 20.

22. A catalyst-coated membrane according to claim 21, wherein the electrochemical device is a water electrolyser, and wherein an anode catalyst layer is provided on the first major surface of the polymer electrolyte membrane.

23. A water electrolyser comprising a polymer electrolyte membrane according to any one of claims 1 to 19 or obtained using a method according to claim 20, or a catalyst coated membrane according to claim 21 or claim 22.

Images