Process and membrane

Abstract

The present invention provides a fuel cell catalyst-coated membrane comprising a catalyst layer and an ion-conducting membrane, wherein the ion-conducting membrane has a hydrogen permeation constant of less than 1.500*10-13 mol s-1 cm-1 kPa-1, wherein the ion- conducting membrane comprises dispersed platinum group metal-containing nanoparticles, a nanoparticle stabilising agent, and an ion-conducting polymer.

Description

[0001] PROCESS AND MEMBRANE

[0002] Field of the Invention

[0003] The present invention relates to fuel cell catalyst-coated membranes in which the ionconducting membrane contains nanoparticles, and to processes for their production.

[0004] Background

[0005] A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, e.g. hydrogen, an alcohol such as methanol or ethanol, or formic acid, is supplied to the anode and an oxidant, e.g. oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Electrocatalysts are used to promote the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of oxygen at the cathode.

[0006] Fuel cells are usually classified according to the nature of the electrolyte employed. Often the electrolyte is a solid polymeric membrane, in which the membrane is electronically insulating but ionically conducting. In the proton exchange membrane fuel cell (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.

[0007] Conventional ion-conducting membranes used in PEMFCs are generally formed from sulphonated fully-fluorinated polymeric materials (often generically referred to as perfluorinated sulphonic acid (PFSA) ionomers). As an alternative to PFSA type ionomers, it is possible to use ion-conducting membranes based on partially-fluorinated or non-fluorinated hydrocarbon sulphonated or phosphonated polymers.

[0008] In fuel cells, radicals can form in the ion-conducting membrane for example from the breakdown of hydrogen peroxide which can be a bi-product formed during operation. Such radicals can break down the ion-conducting membrane and reduce durability. Such breakdown of the ion-conducting membrane has been conventionally restricted by the inclusion of a radical scavenger, for example ceria-based materials as disclosed, for example, in W02007 / 120190 to 3M Innovative Properties Co. Ways of improving the durability of ionconducting membranes by stopping the action of radicals are desired, especially if metal cations or oxides are not required.

[0009] There is also a desire for ion-conducting membranes with high mechanical durability. It is desirable for ion-conducting membranes to be strong in the sense that they are resistant to deformation in real world use where the membranes can be subject to extremes of humidity and temperature which can cause deformation and ultimately failure of the membrane. It is desirable to reduce the thickness of such ion-conducting membranes to minimise electronic and ionic resistance. However, it is also important to minimise any hydrogen crossover through the membrane, to avoid hydrogen mixing with oxygen and associated safety concerns.

[0010] It is known to produce proton exchange membranes comprising recombination catalysts which catalyse the reaction between hydrogen and oxygen to form water and therefore reduce levels of hydrogen crossover. For example, it is described in W02020 / 148545 (Johnson Matthey Fuel Cells Ltd) that a catalyst comprising platinum on a graphene support may be introduced into a proton exchange membrane. In addition, CN114874475 (FOSHAN CLEANEST ENERGY TECH CO LTD) describes the use of platinum particles supported on hollow polydopamine microspheres in a proton exchange membrane. In such cases the platinum particles are localised within the membrane on the surface of the catalyst supports.

[0011] It is also known to incorporate platinum nanoparticles into ion-conducting membranes. For example, US10,476,094 (LG Chem Ltd) describes a reinforced membrane including a porous polymeric support. Platinum nanoparticles are provided on both surfaces of the porous polymer support and the surface in the pores. In order to prepare the membrane a porous support is immersed in a solution of a platinum precursor and the precursor subsequently reduced by adding a reducing agent. Such methodology has a number of manufacturing disadvantages. For example, immersion of the porous polymeric support in the platinum precursor solution can lead to swelling and I or deformation of the support during impregnation, and such immersion and drying methodology is hard to reproduce on a large manufacturing scale. Furthermore, the requirement to co-locate the reinforcement component and the platinum nanoparticles restricts the scope available for optimising performance through modifying membrane configuration and limits the ability to disperse the nanoparticles.

[0012] A similar approach involving the localised deposition of platinum nanoparticles onto the pores of a PTFE film is described in the paper ‘Reinforced and self-humidifying composite membrane for fuel cell applications’; Liu et al, Journal of Membrane Science, 330 (2009) 357-362. It is also described in US2015 / 0236354 (Solvicore GMBH & Co. KG) that colloidal dispersions comprising nano-sized precious metal particles and an ionomer component may be formed by dissolving a suitable precious metal precursor with a liquid acidic ionomer component followed by a reduction step. Such colloidal dispersions are free of other constituents.

[0013] The incorporation of platinum group metal-containing nanoparticles particles into ionconducting membrane layers remains challenging. It is difficult to reproducibly produce consistent inks containing both ion-conducting polymers and platinum group metal-containing nanoparticles which maintain their properties upon storage. Typically, this means that such inks have to be prepared shortly before manufacture therefore reducing manufacturing flexibility.

[0014] The inclusion of platinum group metal-containing nanoparticles also increases the cost of ionconducting membranes and therefore it is desirable to maximise the reduction of hydrogen crossover for a given amount of recombination catalyst.

[0015] There remains a need to further develop efficient and scalable processes for the production of ion-conducting membranes, and to enhance such membranes to enable efficient operation of thinner membranes with lower resistance to gas crossover.

[0016] Summary of the Invention

[0017] The present inventors have surprisingly found that stabilised dispersions of platinum group metal-containing nanoparticles may advantageously be combined with ion-conducting polymers to form inks suitable for ion-conducting membrane manufacture, and such inks demonstrate nanoparticle size stability over extended periods of time. The present inventors have also found that ion-conducting membranes may be produced using such inks which show excellent dispersion of platinum group metal-containing nanoparticles within a membrane layer. Additionally, the present inventors have found that the combination of such stabilisers with platinum group metal-containing nanoparticles has radical suppression properties. Moreover, the present inventors have found that the ion-conducting membranes containing a combination of such stabilisers with platinum group metal-containing nanoparticles exhibit an enhanced reduction of hydrogen crossover.

[0018] Therefore, in a first aspect there is provided a fuel cell catalyst-coated membrane comprising a catalyst layer and an ion-conducting membrane, wherein the ion-conducting membrane has a hydrogen permeation constant of less than 1.500*1 O'13mol s-1cm-1kPa-1, wherein the ionconducting membrane comprises dispersed platinum group metal-containing nanoparticles, a nanoparticle stabilising agent, and an ion-conducting polymer.

[0019] In a second aspect there is provided a process of producing an ion-conducting membrane as defined in the first aspect, the process comprising the steps of:

[0020] (i) providing a stabilised dispersion of platinum group metal-containing nanoparticles;

[0021] (ii) mixing the stabilised dispersion with an ion-conducting polymer to form an ink;

[0022] (iii) fabricating an ion-conducting membrane layer from the ink;

[0023] In a third aspect there is provided a process of producing a catalyst-coated membrane as defined in the first aspect, the process comprising the steps of:

[0024] (iv) producing an ion-conducting membrane by the process of the second aspect; (v) applying a catalyst layer to one or both faces of the ion-conducting membrane prepared in step (iv).

[0025] In a fourth aspect there is provided use of dispersed platinum group metal-containing nanoparticles in combination with a nanoparticle stabilising agent for preventing the degradation of an ion-conducting membrane by radicals.

[0026] In a fifth aspect there is provided use of dispersed platinum group metal-containing nanoparticles in combination with a nanoparticle stabilising agent as a radical reducing additive in an ion-conducting membrane.

[0027] In a sixth aspect there is provided a method of preventing the degradation of an ion-conducting membrane using dispersed platinum group metal-containing nanoparticles in combination with a nanoparticle stabilising agent in the ion-conducting membrane.

[0028] In a seventh aspect there is provided a method of reducing radicals in an ion-conducting membrane using dispersed platinum group metal-containing nanoparticles in combination with a nanoparticle stabilising agent in the ion-conducting membrane.

[0029] In the fourth to sixth aspects, the ion-conducting membrane is suitably a fuel cell ionconducting membrane.

[0030] Brief Description of the Figures

[0031] Figures 1A and B show schematic representations of example arrangements of catalyst- coated membranes of the invention.

[0032] Figure 2 shows the results of stability testing of an ink comprising PVP-stabilised platinum nanoparticles and an ion-conducting polymer.

[0033] Figure 3 shows the results of Scanning Electron-Energy Dispersive X-Ray (SEM_EDX) analysis of a membrane including a platinum-containing membrane layer.

[0034] Figure 4 shows the results of hydrogen crossover testing of catalyst-coated membranes by way of a plot of average hydrogen crossover current density vs permeation number.

[0035] Figure 5 shows the results of hydrogen crossover testing of catalyst-coated membranes by way of a plot of hydrogen permeation constant v permeation number.

[0036] Figure 6 shows a plot of additive concentration vs Fenton recovery factor for PVP-stabilised platinum nanoparticles and other comparative additives. This plot is a representation of radical suppression properties. Detailed Description

[0037] Preferred and / or optional features will now be set out. Any aspect may be combined with any other aspect 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 other feature unless the context demands otherwise. For avoidance of doubt, any of the preferred and / or optional features described herein apply to the fourth to seventh aspects in addition to the first and second aspects.

[0038] Suitably, the ion-conducting membrane is a proton exchange membrane (PEM), such that the catalyst-coated membrane is a PEM fuel cell catalyst-coated membrane. It will however be understood by the skilled person that the ion-conducting may be an anion exchange membrane such that the catalyst-coated membrane is an anion exchange membrane (AEM) fuel cell catalyst-coated membrane.

[0039] 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 cross-section of the membrane by TEM and, from the resulting image, measuring the size of a population of (e.g. 100) particles by image analysis and then calculating the average (mean) value.

[0040] The platinum group metals are platinum, palladium, iridium, rhodium, ruthenium, and osmium. Suitably, the platinum group metal-containing nanoparticles comprise platinum group metal, typically platinum or palladium, or consists essentially of platinum group metal, typically platinum or palladium i.e. the nanoparticles are platinum group metal nanoparticles, typically platinum nanoparticles or palladium nanoparticles. Alternatively, the platinum group metalcontaining nanoparticles may be platinum group metal alloyed with one or more of the following elements: i) another platinum group metal; ii) gold; iii) base metals, such as iron, nickel, cobalt or chromium.

[0041] For example, a platinum-palladium alloy, a platinum-iridium alloy, a platinum cobalt alloy or a platinum-ruthenium alloy. The platinum group metal-containing nanoparticles are suitably recombination catalyst nanoparticles, i.e. they are a catalyst which catalyses the reaction between hydrogen gas and oxygen gas to form water.

[0042] The platinum group metal-containing nanoparticles are typically unsupported. The term unsupported will be readily understood by the skilled person. For example, it will be understood that the platinum group metal-containing nanoparticles are not bound or fixed to a solid catalyst support, such as a carbon support, by physical or chemical bonds, e.g. by way of ionic or covalent bonds, or non-specific interactions such as Van der Waals forces. The use of unsupported nanoparticles offers increased membrane stability during electrochemical operation, avoiding routes of degradation via corrosion or other chemical or electrochemical reactions of the support, and enables greater dispersion within the membrane.

[0043] The process described herein comprises the step of (i) providing a stabilised dispersion of nanoparticles. It will be understood by the skilled person that a stabilised nanoparticle dispersion comprises solid nanoparticles in a liquid phase comprising at least one nanoparticle stabilising agent which interacts with the nanoparticles to prevent nanoparticle agglomeration. Such agents additionally act as a capping agent during synthesis.

[0044] The process described herein involves a first step of forming a stabilised nanoparticle dispersion prior to a step of mixing the stabilised dispersion with an ion-conducting polymer in a subsequent ink forming step. It will therefore be understood by the skilled person that the or each stabilising agent used to form the stabilised nanoparticle dispersion is not the same as the ion-conducting polymer used in step (ii) to form the ink. This is also true in the ionconducting membrane of the catalyst-coated membrane.

[0045] Suitable platinum group metal-containing nanoparticle stabilising agents are selected from those agents which interact with the nanoparticle surface, preventing aggregation and coalescence of the nanoparticles, and enabling the formation of a nanoparticle dispersion. Such stabilisation of the nanoparticle surface is typically through interaction between the nanoparticle with a polar functional group of the stabilising agent. Typically, the stabilising agent comprises an amide, carboxylic acid, sulphonic acid, amine, alcohol, or ether functional group. The stabilising agent may comprise an amide or an ether functional group. The stabilising agent may suitably comprise a tertiary amide group. Suitably, the stabilising agent does not have acidic functional groups. Suitably, the stabilising agent does not have sulphonic acid functional groups. 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.

[0046] The stabilising agent may have a greater hydrophobicity and / or a lower water uptake value than the ion-conducting polymer used in step (ii).

[0047] Suitably, the stabilised dispersion comprises a polymeric nanoparticle stabilising agent. The stabilised dispersion may comprise a polymeric nanoparticle stabilising agent with a greater hydrophobicity and / or a lower water uptake value than the ion-conducting polymer used in step (ii). The polymeric nanoparticle stabilising agent may suitably have a lower weight average molecular weight than the ion-conducting polymer used in step (ii).

[0048] Suitably, the polymeric stabilising agent may comprise amide functional groups, such as tertiary amide functional groups, for example pyrrolidone functional groups (such as polyvinylpyrrolidone, or copolymers including vinylpyrrolidone as a first polymerisation unit).

[0049] 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.

[0050] 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.

[0051] Typically, the stabilised dispersion is formed in an aqueous medium, such as water.

[0052] Suitably, the platinum group metal-containing nanoparticles are present in the dispersion in an amount in the range of any including 0.5 to 10 g L’1, for example in the case that the platinum group metal-containing nanoparticles are platinum particles such particles are typically present in an amount in the range of any including 0.5 to 10 gpt L’1. The nanoparticle concentration may be adjusted using techniques known to the skilled person, such as evaporation or cross-flow filtration.

[0053] Suitably, the dispersion formed in step (i) has a zeta potential more positive than + 25 mV or more negative than - 25 mV. The zeta potential may be measured using electrophoretic light scattering, for example using a Zetasizer Ultra (Malvern Panalytical).

[0054] The skilled person will be aware of methods for the production of suitable platinum group metal-containing nanoparticle dispersions. For example, dispersions may be produced by continuous flow hydrothermal synthesis. Suitably, such synthesis may be carried out in mixing reactors such as those described in WO2015075439A1 (The University of Nottingham) which is incorporated herein by reference.

[0055] Platinum group metal-containing nanoparticle dispersions may also be produced by mixing a suitable platinum group metal precursor with the stabilising agent in a solvent, such as water and then forming the nanoparticles in situ. For example, in the case of platinum nanoparticles, dispersions may be produced by mixing a platinum precursor, such as chloroplatinic acid (F^PtCle), platinum nitrate or Pt acetylacetonate, with the stabilising agent in a solvent, such as water, and then reducing the platinum precursor, for example using sodium borohydride or formaldehyde. An example of such a preparation is described in Du, Y.K., Journal of Applied Polymer Science, Vol. 99, 23-36 (2006) which is incorporated herein by reference.

[0056] The process comprises the step of (ii) mixing the stabilised dispersion with an ion-conducting polymer to form an ink. Typically, this is achieved by forming a dispersion of the ionconducting polymer and then mixing this dispersion with the stabilised dispersion of nanoparticles.

[0057] The skilled person will be aware of suitable ion-conducting polymers for ion-conducting membranes. The ion-conducting polymer can be a proton-conducting polymer or an anion- conducting polymer, such as a hydroxyl anion-conducting polymer. Examples of suitable proton-conducting polymers include PFSA ionomers (e.g. Nation® (Chemours Company), Aciplex® (Asahi Kasei), Aquivion™ (Synesqo), Flemion® (Asahi Glass Co.), or ionomers based on a sulphonated hydrocarbon, i.e. a sulphonated hydrocarbon ionomer, which as known in the art is an ionomer with a which does not contain fluoro- groups, such as those available from FuMA-Tech GmbH as the fumapem® P, E or K series of products (JSR Corporation, Toyobo Corporation, and others), or the Pemion™ range of sulphonated hydrocarbon ionomers from lonomr Innovations. 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.

[0058] In cases in which the ion-conducting membrane is a PEM ion-conducting membrane, the ionconducting polymer is suitably a proton conducting polymer, and in particular a partially- or fully-fluorinated sulphonic acid polymer. Examples of suitable proton-conducting polymers include PFSA polymers. Suitably, the ion-conducting polymer may be a PFSA polymer and has an equivalent weight (EW) greater than 700 EW, greater than 750 EW, greater than 760 EW, greater than 770 EW, or greater than 790 EW. The ion-conducting polymer may suitably be a PFSA polymer with an equivalent weight in the range of and including 700 EW to 1200 EW, such as in the range of and including 750 to 1200 EW, such as in the range of and including 770 to 1000 EW, or 800 to 900 EW. Suitably, the PFSA polymer may have an equivalent weight in the range of and including 700 EW to 900 EW, such as in the range of and including 750 EW to 850 EW. The EW of the PFSA polymer may, suitably, be less than 750 EW. The EW of the PFSA polymer may, suitably, be greater than or equal to 650 EW, or greater than or equal to 700 EW. The EW of the PFSA polymer may accordingly be at least 650 EW and less than 750 EW, or at least 700 EW and less than 750 EW. Alternatively, the ion-conducting polymer may be a sulphonated hydrocarbon ionomer which may suitably have an ion-exchange capacity (I EC) greater than 0.5 mmol / g, or greater than 1.0 mmol / g. The sulphonated hydrocarbon ionomer may suitably have an I EC of less than or equal to 3.5 mmol / g, or less than or equal to 3.0 mmol / g. The IEC may be in the range of and including 0.5 mmol / g to 3.5 mmol / g, or 1.0 mmol / g to 3.0 mmol / g.

[0059] 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.

[0060] The ink may also comprise a radical reducing additive (e.g., a peroxide radical reducing additive, such as ceria) which is an additional additive to, i.e. not including, the combination of the platinum group metal-containing nanoparticles and the nanoparticle stabilising agent. 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 ink once the stabilised dispersion is mixed with the ion-conducting polymer. It may be suitable that the ink does not contain such a radical reducing additive, for example ceria, for example because the combination of platinum group metal-containing nanoparticles and the nanoparticle stabilising agent acts as a radical reducing agent. Put another way, in that case the only additive with radical reducing properties is the combination of the platinum group metal-containing nanoparticles and the nanoparticle stabilising agent. As such, it may be suitable that the ink does not contain cerium (including ceria) or manganese. This reduces the amount of metal species that will be present in an ionconducting membrane, and in turn reduces the possible deleterious effects of these species on the performance of a catalyst-coated membrane.

[0061] The formed ink typically comprises, or consists essentially of :

[0062] (i) an ion-conducting polymer, such as a proton-conducting polymer, for example a PFSA ionomer or a sulphonated hydrocarbon ionomer. The ion-conducting polymer is typically provided in the ink at a weight percentage, with respect to the total weight of the ink components, in the range of and including 5 wt% to 25 wt%, such as in the range of and including 10 wt% to 20 wt%;

[0063] (ii) platinum group metal-containing nanoparticles, such as palladium or platinum nanoparticles. Typically the nanoparticles are present in the ink in an amount, with respect to the total weight of the ink components, in the range of and including 0.01 to 0.40 wt%, such as palladium or platinum nanoparticles, in the range of and including 0.01 to 0.40 wt% platinum; (iii) a nanoparticle stabilising agent, such as a polymeric nanoparticle stabilising agent, for example PVP. Typically, the nanoparticle stabilising agent is present in the ink in an amount, with respect to the total weight of the ink components, in the range of and including 0.05 to 2 wt % such as 0.05 to 0.50 wt %;

[0064] (iv) optionally, a radical reducing additive, such as ceria (CeC>2), typically in an amount at a weight percentage, with respect to the total weight of the ink components, in the range in the range of and including 0.15 wt% to 0.35 wt%. Alternatively, the ion-conducting membrane does not contain cerium (including ceria) or manganese. with components (i) to (iv) dispersed in a solvent, such as a mixture of an alcohol (e.g. ethanol or 1 -propanol) and water, for example in a volume ratio of alcohol: water: 95: 5 to 60: 40.

[0065] The process comprises the step of (iii) fabricating an ion-conducting membrane layer from the ink. The ion-conducting membrane layer is typically formed by depositing the ink onto a substrate to form the layer.

[0066] The coating 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), knifecoating, bar coating, inkjet printing, curtain coating, spray coating, or casting processes. Suitably, the coating composition can be deposited using slot-die coating, bar coating, or inkjet printing. Deposition using slot-die coating may be preferred.

[0067] The coating composition is deposited onto a substrate to form an ion-conducting membrane layer. In some cases, the ion-conducting membrane is formed from a single membrane layer. Alternatively, the ion-conducting membrane may be formed from two or more layers, such as two or three layers, typically two. The number of layers will be determined, for example, by the thickness of the desired ion-conducting membrane, and the degree of variation in desired composition across the membrane.

[0068] Typically, the substrate is a backing sheet, an ion-conducting 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 ion-conducting membrane.

[0069] In the case that the ion-conducting membrane is formed of a single membrane layer, or at the start of production of a multi-layer membrane, the substrate is typically a backing layer. The backing layer provides support for the 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, typically be compatible with the ink, typically be impermeable to the ink, be able to withstand the process conditions involved in producing the ion-conducting 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).

[0070] When 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 coating composition may then be deposited onto the catalyst layer such that the catalyst layer is disposed between the backing layer and the ion-conducting membrane layer formed by depositing the ink.

[0071] In some cases, the substrate is a previously formed ion-conducting membrane 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, an ink containing an ion-conducting polymer may be deposited onto a backing layer to form a first ion-conducting polymer layer, which is then dried. In a second pass, an ink is deposited onto the first ion-conducting polymer layer to form a second ion-conducting polymer layer. The second ion-conducting polymer layer is then dried. This sequence of application and drying is continued to produce further ion-conducting polymer layers as required to form the desired membrane structure. It will be understood by the skilled person that the platinum group metal-containing nanoparticles ink as described hereinbefore is used in one or more of the coating passes as required by the final membrane structure.

[0072] The ion-conducting membranes formed by the method as described herein may be used in the production of catalyst-coated membranes. In such cases, the method comprises the step of forming a catalyst layer on the first and I or the second face of the ion-conducting membrane to form an anode and / or a cathode. The skilled person will understand that the specific type of catalysts for the cathode and anode are chosen depending on, for example, whether the membrane is for a PEM or AEM fuel cell as described previously. 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 inks may be directly printed onto the membrane.

[0073] In the fuel cell catalyst-coated membrane, the ion-conducting membrane has a hydrogen permeation constant of less than 1.500*1 O'13mol s'1cm-1kPa-1, suitably less than 1.300*1 O'13mol s'1cm-1kPa-1, typically less than 1.200*1 O'13mol s-1cm-1kPa-1. The hydrogen permeation constant can be measured by the procedure set out in the Examples section. This permeation constant may typically be the value calculated after permeation 1 according to the procedure set out in the Examples section. I.e., it corresponds with the inherent level of hydrogen crossover observed for the ion-conducting membrane. However, the ionconducting membrane will typically have such a hydrogen permeation constant after permeation 2, suitably after permeation 4, for example after permeation 6. I.e., the ionconducting membrane will maintain its inherent level of hydrogen crossover after numerous cycles of harsh running conditions. With reference to the detailed procedure set out in the Examples section, to calculate hydrogen permeation constant from measurements of hydrogen crossover current density, a catalyst-coated membrane is exposed to an initial conditioning step, which is routine in the art. Hydrogen crossover current density is measured at permeation 1 by carrying out a linear sweep voltammogram in which the anode is supplied with hydrogen and the cathode is supplied with nitrogen. The measurement is repeated three times. Hydrogen crossover current density is found by the averaging the current density between 0.3 and 0.35V. A lower hydrogen crossover current density means less hydrogen crossover.

[0074] Before a second hydrogen crossover current density measurement, the catalyst-coated membrane is exposed to an open circuit voltage hold for 25 hours. Then, the second hydrogen crossover current density measurement is carried out to give the crossover current density for permeation 2. This cycle of open circuit voltage hold followed by crossover current density measurement is repeated to give permeations 3, 4, 5 etc until the hydrogen crossover current density exceeds 20 mA cm-2.

[0075] Hydrogen permeation constant at a particular permeation number is calculated using the formula set out in the Examples section.

[0076] Typically, the ion-conducting membrane may have a thickness of less than or equal to 50 .m. Suitably, the ion-conducting membrane has a thickness of less than or equal to 40 .m, 30 .m, 20 .m, 15 .m, or less than 10 .m. The ion-conducting membrane may suitably have a thickness of at least 1 .m, such as at least 5 .m. The ion-conducting membrane may suitably have a thickness in the range of and including 1 to 50 .m, such as 1 to 40 .m, 1 to 30 .m, 1 to 20 .m, 1 to 15 .m, or, for example, greater than or equal to 1 .m and less than 10 .m. The ion-conducting membrane may suitably have a thickness in the range of and including 5 to 50 .m, such as 5 to 40 .m, 5 to 30 .m, 5 to 20 .m, 5 to 15 .m, or, for example, greater than or equal to 5 .m and less than 10 pm. It is a benefit of the low level of hydrogen crossover exhibited by the ion-conducting membranes described herein that very thin membranes can be produced which facilitate higher performance in a CCM, without the deleterious effects of hydrogen crossover being exacerbated.

[0077] The ion-conducting membrane thickness (and the thickness of layers of the membranes) may be measured by scanning electron microscopy (SEM) at 0% relative humidity. 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 embedded in resin, ground and polished.

[0078] The platinum group metal-containing nanoparticles are dispersed in the ion-conducting membrane layer. The membrane layer also comprises an ion-conducting polymer and nanoparticle stabilising agent, each suitably as described hereinbefore with reference to the process. By dispersed in the ion-conducting membrane layer it is meant herein that the nanoparticles are distributed evenly through some or all of the membrane. The skilled person will understand that the term ‘dispersed’ does not preclude clustering of the nanoparticles, although in such cases the clusters are themselves distributed throughout the membrane layer. The platinum group metal-containing nanoparticles and nanoparticle stabilising agent may be dispersed throughout a plane of the ion-conducting membrane which is adjacent to one catalyst layer, typically the cathode. This means that part of the thickness of the membrane which is adjacent to one catalyst layer, typically the cathode, comprises the platinum group metal-containing nanoparticles and nanoparticle stabilising agent dispersed therein, and the remaining thickness of the ion-conducting membrane, which is adjacent to the other catalyst layer, typically the anode, does not. In this case, when the membrane is a two- or three-layer membrane, one sub-layer, the sub-layer which is adjacent to, i.e. in contact with, the desired catalyst layer, typically the cathode, will contain the platinum group metalcontaining nanoparticles and nanoparticle stabilising agent dispersed therein.

[0079] When there are two sub-layers in the ion-conducting membrane, one of the ion-conducting polymer sub-layers may suitably be thicker than the other. The ratio of the thicker ionconducting polymer sub-layer to the thinner ion-conducting polymer sub-layer may suitably be greater than 1 :1. The ratio of the thicker ion-conducting polymer sub-layer to the thinner ionconducting polymer sub-layer may suitably be no more than 10:1 , or no more than 5:1 suitably no more than 3:1 , for example no more than 2:1. The thickness of the ion-conducting membrane layer may be determined by SEM analysis of a cross-section of the membrane as hereinbefore described.

[0080] Suitably, the platinum group metal-containing nanoparticles have an average particle size less than 50 nm, such as in the range of and including 1 to 50 nm. Suitably, the nanoparticles have an average particle size in the range of and including 1 to 40 nm, 1 to 30 nm, 1 to 20 nm, or 1 to 10 nm. The average particle size of platinum group metal-containing nanoparticles in the ion-conducting membrane may be determined by transmission electron microscopy (TEM), for example analysing a cross-section of the membrane by TEM and, from the resulting image, measuring the size of a population of (e.g. 100) particles by image analysis and then calculating the average (mean) value.

[0081] Typically, the platinum group metal-containing nanoparticles are substantially in the form of clusters of discrete nanoparticles. In such cases the nanoparticles are present in the form of individual nanoparticles which are co-located in clusters, and not in the form of nanoparticle aggregates or agglomerates in which the nanoparticles are bonded together through nanoparticle surface-nanoparticle surface interactions. Without being bound by theory, it is proposed that such as arrangement of nanoparticles may offer benefits associated with greater accessibility for hydrogen to reach catalytic sites. Typically, the clusters have an average size in the range of and including 100 to 500 nm. The average particle size of the clusters of platinum group metal-containing nanoparticles in the ion-conducting membrane may be determined by transmission electron microscopy (TEM), for example analysing a cross-section of the membrane by TEM and, from the resulting image, measuring the size of a population of (e.g. 100) clusters by image analysis and then calculating the average (mean) value.

[0082] The ion-conducting membrane comprises a nanoparticle stabilising agent as hereinbefore described, such as a polymeric nanoparticle stabilising agent, for example PVP. Typically, the platinum group metal-containing nanoparticles are at least partially coated with the nanoparticle stabilising agent.

[0083] As described hereinbefore, the nanoparticle stabilising agent may have a greater hydrophobicity and / or a lower water uptake value than the ion-conducting polymer used in the ion-conducting membrane. Without being bound by theory, the use of a nanoparticle stabilising agent with a higher hydrophobicity and I or lower water uptake value than the ionconducting polymer has the potential to improve the efficiency of the platinum group metalcontaining nanoparticles as a recombination catalyst by increasing the rate of hydrogen gas access to the surface of the platinum group metal-containing nanoparticles and facilitating removal of formed water from the catalyst surface. This offers increased rates of recombination of hydrogen and oxygen and therefore enhanced protection from hydrogen crossover for a given catalyst loading in the ion-conducting membrane. The water uptake value of the ion-conducting polymer and the nanoparticle stabilising agent may be determined by drying a sample of material and then measuring the weight of the sample before and after immersion in water (e.g. at 23°C for 24 hours). For example the water uptake value may be measured by (i) drying a sample in an oven until the weight stabilises; (ii) cooling the sample in a desiccator; (iii) weighing the sample; (iv) immersing the sample in water (e.g. at 23°C for 24 hours); (iv) removing the sample and patting dry with a lint free cloth; and (v) re-weighing the samples. The ion-conducting membrane may suitably have a platinum group metal-containing nanoparticle (e.g. platinum or palladium) loading of at least 1 p,g / cm2, suitably at least 5 .g / cm2. The ion-conducting membrane may suitably have a platinum group metal-containing nanoparticle (e.g. platinum or palladium) loading of at most 30 .g / cm2, suitably at most 25 .g / cm2, suitably at most 15 .g / cm2, for example at most 10 .g / cm2. It has been found that this range of nanoparticle loading provides a suitable balance between reducing the level of hydrogen crossover during use and the cost associated with the inclusion of the nanoparticles in the ion-conducting membrane. It is also of benefit that hydrogen crossover can be effectively reduced with such a low amount of platinum group metal-containing nanoparticle. Such a low amount has benefits in terms of, for example, lower cost and lower risk of side effects. The loading may be determined by inductively coupled plasma mass spectrometry (ICP-MS). Suitably, the molar ratio of nanoparticle stabilising agent to platinum group metal-containing nanoparticle will be at most 60: 1 , suitably at most 50: 1 , suitably at most 40: 1 , typically at most 30:1. Suitably, the molar ratio of nanoparticle stabilising agent to platinum group metalcontaining nanoparticle will be at least 10:1.

[0084] 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.

[0085] The ion-conducting 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.

[0086] Lamination of ion-conductive membranes comprises pressing and / or bonding at least two solid ion-conductive membranes together, such membranes optionally being coated with a catalyst layer. A lamination interface is formed between the two membranes where solid surfaces of the individual membranes are pressed and / or bonded together. Lamination interfaces comprise physical defects. Furthermore, the structural and / or chemical nature of a lamination interface also differs from that of the bulk polymer material. This is because when a solid membrane is formed, the outer surfaces of the solid membrane have surface features which are distinct from those in the bulk material. For example, a hydrophobic skin forms on a surface of a membrane at an air interface. Raman spectroscopy can detect this difference. As such, when two solid membranes are pressed together, the lamination interface formed by the two solid surfaces is distinctive in chemical and / or structural form compared to the bulk of the ion-conductive polymer material. Microscopy and spectroscopy techniques can thus distinguish between lamination interfaces between layers of ion-conductive polymer and interfaces which have been formed via a liquid phase deposition process such as printing, spraying, or coating of layers to build up a multi-layer structure. That is, a non-laminated interface is structurally and / or chemically distinct from a laminated interface and is not just a feature of the manufacturing method. Furthermore, a non-laminated interface can be identified as being non-laminated in a membrane without prior knowledge of the manufacturing method. Examples of analysis techniques for detecting a laminated interface include cross-section SEM. Variations of crystallinity at interfaces can be detected using cross-section TEM. Other techniques for detecting laminated interfaces include 13C / 1 H / 19F solid state NMR, neutron diffraction, and / or a combination of two or more of the aforementioned techniques.

[0087] 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.

[0088] The ion-conducting membrane may suitably comprise a planar reinforcing component, e.g. porous reinforcement polymer sheet, which is impregnated with ion-conducting polymer. As typical reinforcement polymer materials are not conductive to ions, or not sufficiently conductive to ions, the reinforcement layer is thus formed using a porous reinforcement polymer 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. The ion-conducting membrane may suitably comprise a reinforcement polymer, such as expanded polytetrafluoroethylene (ePTFE) or polybenzimidazole (PBI). The PBI is typically in the form of PBI nanofibres. The reinforcing component may suitably occupy at least 5% of the volume fraction of the ion-conducting membrane, typically at least 10%. The reinforcing component may suitably occupy at most 50% of the volume fraction of the ion-conducting membrane, typically at most 40%, more typically at most 30%, for example at most 20%. For example, the reinforcing component may suitably occupy between 5 and 50% of the volume fraction of the ion-conducting membrane, typically between 10 and 40%, more typically between 10 and 30%, for example between 10 and 20% of the volume fraction. The thickness of the reinforcing component may suitably be distributed across at least 50% of the thickness of the ionconducting membrane, typically at least 60%, for example at least 70%.

[0089] The ion-conducting membrane may comprise a radical reducing additive (e.g. peroxide radical reducing additive, such as ceria) which is an additional additive to, i.e. not including, the combination of the platinum group metal-containing nanoparticles and the nanoparticle stabilising agent. 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 membrane. It may be suitable that the ion-conducting membrane does not contain such a radical reducing additive, for example ceria, for example because the combination of platinum group metalcontaining nanoparticles and the nanoparticle stabilising agent acts as a radical reducing agent. Put another way, in that case the only additive with radical reducing properties is the combination of the platinum group metal-containing nanoparticles and the nanoparticle stabilising agent. As such, it may be suitable that the ion-conducting membrane does not contain cerium (including ceria) or manganese. This reduces the amount of metal species that will be present in an ion-conducting membrane, and in turn reduces the possible deleterious effects of these species on the performance of a catalyst-coated membrane.

[0090] Figure 1 , A and B configurations show fuel cell catalyst-coated membranes 1 in which the platinum group metal-containing nanoparticles and the nanoparticle stabilising agent are dispersed throughout the entire ion-conducting membrane (A), and throughout one sub-layer of a two sub-layer ion-conducting membrane (B). With reference to Figure 1A, the ionconducting membrane 4 is disposed between a cathode catalyst layer 2 and an anode catalyst layer 3. Platinum group metal-containing nanoparticles are distributed throughout the entire ion-conducting membrane, and a planar reinforcing component 5 is distributed across at least 50 % of the thickness of the ion-conducting membrane. With reference to Figure 1 B, there are two ion-conducting membrane sub-layers 6 and 7, with sub-layer 6 being thicker than sublayer 7. Sub-layer 6 contains the platinum group metal-containing nanoparticles and is positioned adjacent to the cathode catalyst layer 2. A planar reinforcing component akin to that shown in Figure 1A, and which bridges the two sub-layers, may be present but is not shown here.

[0091] The catalyst-coated membranes described herein have an anode catalyst layer and / or a cathode catalyst layer applied to a face of the ion-conducting membrane. Accordingly, provided herein is a process of producing a catalyst-coated membrane as defined herein, the process comprising the steps of:

[0092] (iv) producing an ion-conducting membrane by the process described herein;

[0093] (v) applying a catalyst layer to one or both faces of the ion-conducting membrane prepared in step (iv).

[0094] In a fuel call catalyst-coated membrane, a cathode catalyst layer may be applied to a surface of the ion-conducting membrane comprising a catalyst for catalysing the oxygen reduction reaction. In a PEM fuel cell, it may be that the cathode catalyst layer comprises a platinum group metal catalyst, e.g. platinum, either as reduced metal or in an alloy, e.g. with another platinum group metal or a base metal, such as iron, nickel, cobalt or chromium, for example a platinum- or platinum alloy-on-carbon catalyst. The catalyst material can be formulated into an ink, 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] In a fuel cell catalyst-coated membrane, an anode catalyst layer may be applied to a surface of the ion-conducting membrane comprising a catalyst for catalysing the hydrogen oxidation reaction. In a PEM fuel cell, it may be that the anode catalyst layer comprises platinum, such as a platinum-on-carbon catalyst.

[0096] The anode material can be formulated into an ink, suitably in an ion-conducting polymer, printed ex-situ onto a PTFE sheet, and transferred onto the ion-conducting membrane by hot pressing. Alternatively, the ink can be directly coated onto the ion-conducting membrane.

[0097] 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.

[0098] Examples

[0099] Example 1 - Formation of a stabilised dispersion of platinum nanoparticles using PVP

[0100] A PVP-stabilised dispersion of platinum nanoparticles in water was prepared from aqueous platinum nitrate solution and a 2 w / v% aqueous PVP (MW 10,000) solution using a continuous flow hydrothermal reactor operating at elevated temperature and pressure. The platinum loading in the dispersion measured using ICP was 3.93 g / L.

[0101] Analysis by dynamic light scattering (DLS) indicated that the z-average value of PVP-Pt clusters in the dispersion is 280 nm with small-angle X-ray scattering (SAXS) analysis indicating that the size of platinum nanoparticles is in the range 1 to 10 nm.

[0102] Example 2 - Formation of an ink comprising PVP-stabilised platinum nanoparticles and an ion-conducting polymer

[0103] The aqueous dispersion prepared in Example 1 was mixed with additional water and EtOH to create an ethanol in water mix (wt% ratio 4:1). Dry PFSA ionomer (3M Corporation, EW- 800) is added to create a ~17 wt% ionomer and 0.08 wt% platinum ink. The ink was mixed using a roller mixer.

[0104] Analysis by dynamic light scattering (DLS) indicated that the z-average diameter of the Pt- PVP clusters in the dispersion is 1200 nm. The stability of the ink during storage was assessed by measuring the z-average diameter after 1 day and after 3 months. The results are shown in Figure 2 which show no change in the DLS analysis profile indicating high stability and resistance to agglomeration.

[0105] Example 3 - Preparation of a membrane including a platinum-containing membrane layer

[0106] An ion-conducting membrane containing a platinum-containing membrane layer was prepared by knife coating a layer of an ink prepared in accordance with the method of Example 2 onto a 15 .m thick PFSA membrane and drying the formed layer at room temperature.

[0107] A cross-section of the formed membrane was analysed by Scanning Electron Microscopy with Energy Dispersive X-Ray Analysis (SEM-EDX). This showed that the platinum- containing membrane layer had a thickness of around 30 .m. An expanded section of an SEM-EDX image of the platinum-containing membrane layer is shown in Figure 3. This indicates that nanoparticles of platinum were distributed throughout the membrane layer.

[0108] Example 4 - Preparation of a membrane including a platinum-containing membrane layer and analysis by cryo-TEM

[0109] An ink prepared in accordance with Example 2, was frozen using liguid nitrogen in the cryomicrotome and cut using a diamond knife in sections. One section is placed on the copper grid and thawed, forming a <100nm thick film.

[0110] The formed ion-conducting membrane was analysed by cryo-transmission electron microscopy (cryo-TEM). This analysis indicates that the platinum nano-particles in the membrane have an average particle size in the range of 1 to 10 nm and are in the form of clusters of discrete nano-particles of platinum.

[0111] An algorithm was used to measure the average nearest neighbour distance between platinum particles from the cyro-TEM images. This data was compared to an analysis of a comparative ion-conducting membrane incorporating a platinum-containing membrane layer with the same platinum loading but prepared from an ink containing platinum particles from a platinum black source without any stabilising agent. This indicated a significant reduction in interparticle distance when the membrane is formed from the stabilised nanoparticle dispersion and therefore increased platinum dispersion in the membrane layer.

[0112] Example 5 - Formation of CCMs incorporating membranes with a platinum-containing membrane layer and hydrogen crossover testing.

[0113] An ion-conducting membrane of thickness ~25 .m was prepared by casting a 10 .m layer containing 13pg / cm2platinum (measured using XRF) and PVP (using the method described in Example 3) onto a 15 .m, ePTFE reinforced PFSA membrane. CCMs were obtained by laminating a Pt / C cathode catalyst layer (with a platinum loading of 0.4 mg cm-2of platinum) and a Pt / C anode catalyst layer (with a platinum loading of 0.08 mg cm-2) on either side of the composite membrane. The platinum-containing membrane sub-layer was adjacent to the cathode catalyst layer. Lamination was achieved by hot-pressing at 80°C and 800 PSI for 2 mins.

[0114] A blank CCM having a thickness of ~25 .m was also made for comparison using the exact same method, but the dispersion cast on to 15 .m, ePTFE reinforced PFSA membrane did not contain platinum.

[0115] Hydrogen crossover and hydrogen permeation constant

[0116] The CCMs prepared in Example 5 were exposed to an initial conditioning step in which they were run under the following conditions: Anode: H2, Cathode: air, relative humidity (RH) 100%, 80C, 100kPag pressure, 500mA cm-2current density, 54 hour hold.

[0117] To determine hydrogen crossover current density, a linear sweep voltammogram is carried out in which the anode is supplied with hydrogen and the cathode is supplied with nitrogen. A lower hydrogen crossover current density means less hydrogen crossover. Specifically, a linear sweep voltammogram was carried out under the following conditions: Anode: H2, Cathode: N2, 100% RH, 80°C, 13.8kPag pressure, sweep from 0.1V to 0.5V RHE at 1 mV / s. Hydrogen crossover current density is found by averaging the current density between 0.3 and 0.35V. The test is repeated three times. This is permeation 1. The average crossover current density is plotted against permeation number in Figure 4.

[0118] Before a second hydrogen crossover current density measurement, the CCM is exposed to harsh open circuit voltage (OCV) conditions. Specifically, an OCV hold is carried out under the following conditions: Anode: H2, Cathode: air, RH 30%, 90°C, 50kPag pressure, 0mA cm-2current density, 25 hour hold. Then, the second hydrogen crossover current density measurement is carried out to give permeation 2, which is plotted in Figure 4. This combination of measurements is repeated to give permeations 3, 4, 5 etc until the hydrogen crossover current density exceeds 20 mA cm-2. Accordingly, at each permeation number, the CCM has been exposed to additional harsh conditions.

[0119] As can be seen in Figure 4, the CCMs with a platinum and PVP-containing sub-layer have a lower initial hydrogen crossover current density, and maintain a lower hydrogen crossover current density for more permeations, which means that they are resistant to hydrogen crossover for a larger number of OCV cycles. Accordingly, in real-world use the CCMs would be inherently more resistant to degradation caused by hydrogen crossover pathways, and would maintain that degradation resistance for longer. The relationship between crossover current density (A cm-2) and hydrogen permeation constant (P) is shown below1'1:

[0120] A cm'2= P(mol crrr1s'1cm'2kPa'1) PH2(kPa) F(C mol'1) -2e t(cm'1)

[0121] Upon rearranging, the equation becomes:

[0122] P(mol cm'1s'1cm'2kPa'1) = A crrr2PH2(kPa)'1F(C mot1)'1-2e'1t(cm)

[0123] Where F is Faraday’s Constant, ‘e’ is the number of electrons in the hydrogen oxidation reaction (two), and the partial pressure of hydrogen is determined by subtraction of the partial pressure of water vapor1"1from the measured cell pressure. Membrane thickness (t) was measured using a micrometer. Values used in these calculations are shown below.

[0124] [ilS. S. Kocha, J. D. Yang, J. S. Yi, AIChE J. 2006, 52, 1916.

[0125] [ii]Free Professional Online HUMIDITY CALCULATOR (humcal.com) - Hardy (ITS-90).

[0126] Figure 5 shows a plot of hydrogen permeation constant vs permeation number, calculated using the crossover current density data shown in Figure 4.

[0127] Radical suppression properties

[0128] A dispersion as prepared in Example 1 which had a solids content of 0.1 wt% was used to make three dilutions containing 10, 20 and 30 ppm solids in H2SO4 1 M. These three dilutions were used to measure free radical suppression ability by the method defined below.

[0129] Comparative aqueous dispersions containing platinum but which did not contain PVP were also prepared in three dilutions 5, 20 and 30 ppm solids in H2SO4 1 M. These three dilutions were also used to measure free radical suppression ability by the method defined below.

[0130] Comparative aqueous dispersions containing ceria and which did not contain PVP were also prepared in three dilutions 20, 25 and 45 ppm solids in H2SO4 1 M. These three dilutions were also used to measure free radical suppression ability by the method defined below.

[0131] A dispersion containing PVP alone was also prepared at a dilution of 20 ppm solids in H2SO4 1 M. This dispersion was also used to measure free radical suppression ability by the method defined below. The experimental procedure for measuring free radical suppression ability follows that of Fei et al: Fei Yu, Da Xu, Rong Lei, Na Li, and Ke’an Li, Journal of Agricultural and Food Chemistry, 2008, 56, 730-735; and Mei-Fang et al: Mei-Fang Hou, Lin Liao, Wei-De Zhang, Xiao-Yan Tang, Hong-Fu Wan, and Guang-Cai Yin, Chemosphere, 2011, 83, 9, 1279-1283 where the Fenton reaction: fci

[0132] RhB +■ OH -> oxRhB (reaction 1) fc2

[0133] FRS +■ OH -> oxFRS (reaction 2) is utilised to create radical species which degrade rhodamine B (RhB) dye. This degradation is followed using UV-vis spectroscopy to ascertain if the presence of different additives can prevent this degradation from happening by means of a radical suppression mechanism. For UV-vis spectroscopy, Agilent’s Cary 5000 UV-vis-NIR was used utilising 1 cm QS cuvettes. The abs peak at 554 nm is followed. To mimic the low pH environment created in a fuel cell from the PFSA ionomer, our tests were done in 1M H2SO4. Each test was conducted at the following concentrations of each chemical:

[0134] • 14 ppm RhB

[0135] • 35 ppm Fe2+

[0136] • 10 microlitres of 3% H2O2 were added and left to mix for 10 minutes before measuring the UV-vis spectra

[0137] In the absence of any additive, only reaction 1 occurs, and the addition of H2O2 results in a drop Ab in the UV-vis peak height at 554 nm. In the presence of a radical reducing additive, both reactions 1 and 2 occur, resulting in a smaller drop Aa in the UV-vis peak height at 554 nm. A ‘recovery’ factor, R can be calculated via the following equation for each additive at a given additive concentration:

[0138] Figure 6 shows the recovery factor (R) for each additive The higher the value of R, the more radical suppression ability is demonstrated.

[0139] It is evident that the dispersion containing platinum stabilised with PVP shows exceptional free radical suppression activity (higher R value) compared with platinum alone, and especially with regard to the incumbent ceria-based membrane free radical reducing additive. Therefore, surprisingly, the Pt stabilised with PVP will be useful as a radical reducing agent in an ion-conducting membrane. The recovery factor for each additive, including PVP alone, at the dilution of 20 ppm solids in H2SO4 1 M is provided in Table 1. It is evident that PVP alone does not have radical suppression properties and that the combination of platinum and PVP has stronger radical suppression properties than platinum alone.

[0140] Table 1

[0141] Example 6 - Formation of a stabilised dispersion of platinum nanoparticles using PVP and formaldehyde, by a simple wet synthesis technique.

[0142] Pt(NOs)4 (equivalent to 1g of platinum) 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.

[0143] Example 7 - Formation of an ion-conducting polymer inks containing PVP-stabilised nanoparticles

[0144] A stabilised aqueous dispersion of platinum nanoparticles (formed according to a method analogous to Example 6) was mixed with ethanol and water to create a mixture with a ethanol: water weight ratio of 80:20. Dry ionomer (3M Corp, 800 EW) is added to the mixture to create a dispersion, where the ionomer solids is ~ 17 %wt.

Claims

Claims1 . A fuel cell catalyst-coated membrane comprising a catalyst layer and an ionconducting membrane, wherein the ion-conducting membrane has a hydrogen permeation constant of less than 1.500*1 O'13mol s-1cm-1kPa-1, wherein the ion-conducting membrane comprises dispersed platinum group metal-containing nanoparticles, a nanoparticle stabilising agent, and an ion-conducting polymer.

2. A catalyst-coated membrane according to claim 1 , wherein the ion-conducting membrane has a thickness of less than or equal to 50 pm.

3. A catalyst-coated membrane according to claim 1 or claim 2, wherein the ionconducting polymer has an EW in the range of and including 700 to 1200.

4. A catalyst-coated membrane according to any preceding claim, where the ionconducting membrane comprising a planar reinforcing component.

5. A catalyst-coated membrane according to 4, wherein the reinforcing component comprises expanded polytetrafluoroethylene (ePTFE) or polybenzimidazole (PBI).

6. A catalyst-coated membrane according to claim 4 or claim 5, wherein the reinforcing component occupies between 5 and 50% of the volume fraction of the ion-conducting membrane.

7. A catalyst-coated membrane according to any of claims 4 to 6, wherein the thickness of the reinforcing component is distributed across at least 50% of the thickness of the ionconducting membrane.

8. A catalyst-coated membrane according to any preceding claim, wherein the ionconducting membrane does not contain cerium or manganese.

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

10. A catalyst-coated membrane according to any preceding claim, wherein the platinum group metal-containing nanoparticles are in the form of clusters of discrete nanoparticles.

11. A catalyst-coated membrane according to any preceding claim, wherein the nanoparticle stabilising agent is a polymer, such as polyvinylpyrrolidone.

12. A catalyst-coated membrane according to any preceding claim, wherein the platinum group metal-containing nanoparticles are at least partially coated with the nanoparticle stabilising agent.

13. A catalyst-coated membrane according to any preceding claim, wherein the average particle size of the nanoparticles is less than 50 nm.

14. A catalyst-coated membrane according to any preceding claim, wherein the ionconducting membrane is a single coherent polymer film comprising two ion-conducting polymer sub-layers.

15. A catalyst-coated membrane according to claim 14, wherein one of the ion-conducting polymer sub-layers is thicker than the other.

16. A catalyst-coated membrane according to claim 15, wherein the thickness ratio of the thicker ion-conducting polymer sub-layer to the thinner ion-conducting polymer sub-layer is greater than 1 :1 and no more than 10:1.

17. A catalyst-coated membrane according to any of claims 14 to 16, wherein the platinum group metal-containing nanoparticles and stabilising agent are present in one sub-layer of the ion-conducting membrane.

18. A catalyst-coated membrane according to any preceding claim, wherein the platinum group metal-containing nanoparticles are present adjacent to a cathode catalyst layer of the catalyst-coated membrane.

19. A fuel cell membrane electrode assembly comprising a catalyst-coated membrane according to any preceding claim, and a gas diffusion layer in contact with the catalyst layer.

20. A process of producing an ion-conducting membrane as defined in any of claims 1 to 18, the process comprising the steps of:(i) providing a stabilised dispersion of platinum group metal-containing nanoparticles;(ii) mixing the stabilised dispersion with an ion-conducting polymer to form an ink;(iii) fabricating an ion-conducting membrane layer from the ink.21 . An ion-conducting membrane produced by the process according to claim 20.

22. A process of producing a catalyst-coated membrane according to any of claims 1 to 18, the process comprising the steps of:(iv) producing an ion-conducting membrane by the process of claim 20;(v) applying a catalyst layer to one or both faces of the ion-conducting membrane prepared in step (iv).

23. Use of dispersed platinum group metal-containing nanoparticles in combination with a nanoparticle stabilising agent for preventing the degradation of an ion-conducting membrane by radicals.

24. Use of dispersed platinum group metal-containing nanoparticles in combination with a nanoparticle stabilising agent as a radical reducing additive in an ion-conducting membrane.

25. Use according to claim 23 or claim 24, wherein the ion-conducting membrane is a fuel cell ion-conducting membrane26. A method of preventing the degradation of an ion-conducting membrane by radicals using dispersed platinum group metal-containing nanoparticles in combination with a nanoparticle stabilising agent in the ion-conducting membrane.

27. A method of reducing radicals in an ion-conducting membrane using dispersed platinum group metal-containing nanoparticles in combination with a nanoparticle stabilising agent in the ion-conducting membrane.

28. A method according to claim 26 or claim 27, wherein the ion-conducting membrane is a fuel cell ion-conducting membrane

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