Ion-conducting membrane

Abstract

An ion-conducting membrane comprises (a) an ion-conducting polymer; and (b) a hydrogen radical scavenger. Also, a method of preventing degradation of an ion-conducting membrane by hydrogen radicals c

Description

Field of the Invention This invention relates to ion-conducting membranes containing additives which reduce degradation by the action of hydrogen radicals. In particular, this invention relates to proton exchange membranes, and methods of manufacturing the same. The ion-conducting membranes can be suitable for use in electrochemical devices such as fuel cells and / or electrolysers. Background of the Invention 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. 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. An electrolyser is an electrochemical device for electrolysing water to produce high purity hydrogen and oxygen. Electrolysers can operate in both alkaline and acidic systems. Those electrolysers that employ a solid proton-conducting polymer electrolyte membrane, or proton exchange membrane (PEM), are known as proton exchange membrane water electrolysers (PEMWEs). Those electrolysers that utilise a solid anion-conducting polymer electrolyte membrane, or anion exchange membrane (AEM), are known as anion exchange membrane water electrolysers (AEMWEs). Conventional ion-conducting membranes used in PEMFCs or PEMWEs are generally formed from sulfonated 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 sulfonated or phosphonated polymers. There is a desire for ion-conducting membranes to have high chemical durability. In fuel cells and electrolysers, chemical degradation of the membrane is thought to occur through reactions with hydrogen peroxide, peroxyl (HOO), or hydroxyl (HO) radicals for example see Curton, D.E. et al. Journal of Power Sources, 2004, 131(1), 41-48 and F.D. etal. ECS Trans., 2008, 16(2), 235. Radical scavenging additives such as cerium or manganese ions or oxides have been shown to increase lifetimes through decomposing these reactive oxygen species, for example as disclosed in EP1946400. There remains a need for membranes which are chemically durable and withstand attack from reactive species generated during use. Summary of the Invention The present inventors have realised that chemical durability can be improved by using additives in the membrane which inhibit the action of hydrogen radicals to degrade ionconducting membranes. Accordingly, provided herein is an ion-conducting membrane comprising: (a) an ion-conducting polymer; and (b) a hydrogen radical scavenger. The hydrogen radical scavenger typically has a rate constant for the reaction with a hydrogen radical (H ) of greater than or equal to 1 x 107 M'1s'1. Also provided is a catalyst-coated membrane for a fuel cell or a water electrolyser comprising an ion-conducting membrane according to the disclosure, with a cathode catalyst layer applied to a first face of the membrane and I or an anode catalyst layer applied to a second face of the membrane. Also provided is a membrane-electrode assembly for a fuel cell or a water electrolyser comprising (i) an ion-conducting membrane according to the disclosure; or (ii) a catalyst-coated membrane according to the disclosure; and at least one of a gas diffusion layer or a porous transport layer. Also provided is a water electrolyser or a fuel cell comprising a catalyst-coated membrane according to the disclosure or a membrane-electrode assembly according to the disclosure. Also provided is use of a material having a rate constant for the reaction with a hydrogen radical (H ) of greater than or equal to 1 x 107 M~1s~1 for preventing degradation of an ionconducting membrane by hydrogen radicals. Also provided is a method of preventing degradation of an ion-conducting membrane by hydrogen radicals, the method comprising using a material having a rate constant for the reaction with hydrogen radicals of greater than or equal to 1x107 M'1s'1. It has become evident that a second type of reactive species can cause degradation resulting from the homolytic cleavage of hydrogen gas (H2) to form hydrogen radicals, i.e. hydrogen atoms ( H ), see for example Lin, L etal. Journal of Power Sources, 2013, 233, 98-106. These hydrogen atoms can react with the membrane resulting in chemical degradation and shortened lifetimes. The invention provides membranes with additives which selectively react with such radicals, without compromising the scavenging of other reactive species, such as peroxyl (HOO), or hydroxyl (HO) radicals, before they can react with the ion-conducting polymer in the membrane. Brief Description of the Drawings Figure 1a and b provided diagrams of catalyst-coated membranes incorporating ionconducting membranes as disclosed herein. Detailed Description of the Invention The ion-conducting membrane may be a fuel cell or water electrolyser ion-conducting membrane. The ion-conducting polymer can be a proton-conducting polymer or an anion-conducting polymer, such as a hydroxyl anion-conducting polymer. Typically, a protonconducting polymer. The ion-conducting membrane is suitably a proton exchange membrane. Typically, the ion-conducting polymer comprises sulfonic acid groups. Typically, the ionconducting polymer is a perfluorinated sulfonic acid ionomer, or a partially-fluorinated or nonfluorinated hydrocarbon sulfonic acid ionomer. Such ion-conducting polymers optionally can contain partially or fully fluorinated vinyl ether. Such ion-conducting polymers can also optionally comprise bifunctional ion-conducting monomers. Examples of suitable protonconducting polymers include partially- or fully-fluorinated sulphonic acid polymers, such as perfluorosulphonic acid ionomers (e.g. Nation® (Chemours), Aciplex® (Asahi Kasei), Aquivion™ (Synesqo), Flemion® (Asahi Glass Co.); or ionomers based on a sulphonated hydrocarbon, otherwise known as hydrocarbon ion-conducting polymers, such as those available from FuMA-Tech GmbH as the fumapem® P, E or K series of products, JSR Corporation, Toyobo Corporation, and others including Pemion available from lonomr Innovations, Inc. Examples of suitable anion-conducting polymers include A901 made by Tokuyama Corporation and Fumasep FAA from FuMA-Tech GmbH. Typically, the ionconducting polymer has an equivalent weight of about 1100 or less, typically about 900 or less, or about 850 or less. Typically, the ion-conducting polymer has an equivalent weight of at least about 450. The equivalent weight of the ion-conducting polymer may be readily measured using an acid titration following a hydroxide exchange. For example, a membrane sample may be vacuum dried at about 110 °C for 16 hours to obtain about 2g of the dried film. The film may then be immersed in about 30 mL of a 0.1N NaOH solution to substitute sodium ions for protons in the membrane. Then titration by neutralisation is carried out, for example using 0.1N hydrochloric acid, to determine the number of exchangeable protons, and therefore the EW may be calculated. The hydrogen radical scavenger is a material which inhibits the degradation effects of hydrogen radicals on an ion-conducting membrane. The hydrogen radical scavenger suitably has a rate constant for the reaction with a hydrogen radical (H ) of greater than or equal to 1 x 107 M’1s~1 , typically greater than or equal to 1 x 108 M~1s~1 . Values which are greater than those for conventional oxygen based radical scavengers such as ceria. This rate constant can be determined using a pulse radiolysis method employing a referenced scavenger such as disclosed in Gogolev, A.V. et al, International Journal of Radiation Applications and Instrumentation. Part C. Radiation Physics and Chemistry, 1991, 37 (3), 531-535. In this method, an aqueous acid solution is subjected to an electron beam source to generate a hydrogen atom; e- + H+^H- [1] Since the reaction product of the hydrogen atom with the material of interest (such as a Ag(l), V(V), Ce(IV), etc.) is often difficult to measure directly, an easily detected competitive hydrogen atom scavenger with known rate constant is added. For example, the product of H-addition o-phenanthroline has a visible light absorption at 680nm. By varying the concentration ratio of the material of interest and the hydrogen atom scavenger, and measuring the absorption spectra, the ratio of two reaction rates can be determined and from that the reaction rate for the unknown material of interest. The reaction of a Ce(IV) ion and a hydrogen atom is determined using the following method. Ce4+ + H Ce3+ + H+ [2] A 15 mM solution of cerium(IV) sulphate hydrate (Ce2(SO4)3 x nH2O) is prepared in 0.35 M sulfuric acid. The solution is split into several aliquots and a competitive hydrogen radical scavenger, o-phenanthroline (Ph), is added in differing concentrations to make solutions with a [Ce(IV)]:[Ph] molar ratios from 2:1 to 12:1. The samples are deaerated by bubbling with argon gas then exposed to a pulsed electron beam with energy of 5MeV for a duration of 2.3 microseconds. The slope of the line from the plot of relative absorbance intensity at 680nm versus the [Ce(IV)]:[Ph] ratio provides the ratio of reaction rates. Since the reaction rate of the competitive hydrogen radical scavenger is known, the reaction rate of the Ce(IV) can be calculated. The hydrogen radical scavenger may, for example, take the form of ions, particles, or complexes. Such particles may be nanoparticles e.g. having a size of 500 nm or less. Such particles may be supported or unsupported, for example supported on an inorganic metal oxide, for example silica or zirconia oxides. Particle size is typically the Zave particle size measured by dynamic light scattering (typically in a Zetasizer from Malvern Panalytical) at an angle of 174 degrees. A solution with a weight percent in the range of and including 0.01 to 0.1 % by total weight of the solution may be used. About 1 mL of sample is placed in a glass cuvette and measurements run to give the Zave particle size for 5 measurements. Suitable hydrogen radical scavenger materials include metal ions such as silver (Ag+), palladium (Pd2+), vanadium (V5+), americium (Am6+), dioxoamericium (AmO22+), bismuth (Bi3+), dichromate (Cr2O72’), neptunium (Np3+). Suitable hydrogen radical scavenger materials include also include metal complexes, such as complexes of cobalt such as of the Co(ll) ion, or metal complexes of nickel such as of the Ni(ll) ion, e.g. N- meso-5,7,7,12,14,14-hexamethyl-, 1,4,8,11-tetraazacyclotetradeca-, or 4,11-diene complexes, or complexes of ruthenium such as of the Ru(ll) ion, e.g. 1,4,8,11-tetraazacyclotetradecane or tris-2, 2-bypyrazine complexes. Typical materials are silver ions (Ag+) and palladium ions (Pd2+), suitably silver ions (Ag+). The hydrogen radical scavenger may be present in an amount of about 5 percent by weight or less, suitably about 1 percent by weight or less, relative to the total weight of the dry ionconducting membrane. The hydrogen radical scavenger may be present in an amount of at least about 0.05 percent by weight, suitably at least about 0.1 percent by weight, relative to the total weight of the dry ion-conducting membrane. The loading may alternatively be quantified in terms of moles per unit area and as such the hydrogen radical scavenger may be present in an amount of greater than or equal to about 0.001 micromoles / cm2. The hydrogen radical scavenger may be present in an amount of less than or equal to 5 micromoles / cm2, typically less than or equal to about 2.5 micromoles / cm2. The hydrogen radical scavenger may be uniformly distributed across the thickness of the ion-conducting membrane. By “uniform” it is meant that the amount of particles typically does not vary by more than ±50%, suitably by more than ±20%. The material may also not be uniformly distributed. For example, there may be a variation in distribution in the z-direction, with a preference for the higher relative percentage closer to the hydrogen electrode i.e. closer the anode in a proton exchange membrane fuel cell of closer to the cathode in a proton exchange membrane electrolyser. One way to achieve this is to have a sub-layer of the ion-conducting membrane which contains the hydrogen radical scavenger and is positioned closer to the anode than the cathode. The sub-layer can be directly adjacent to the hydrogen electrode or closer to the hydrogen electrode side but not directly adjacent to the hydrogen electrode, e.g. with at least one further sub-layer between the hydrogen electrode and the sub-layer containing the hydrogen radical scavenger. The ion-conducting membrane may also comprise a peroxide radical reducing additive, such as ceria. It will be noted that peroxide can decompose to form a range of radicals (O, OH, OOH) and the radical reducing additive may reduce the amount of one, more, or all of these radicals. If present, such a peroxide radical reducing additive is typically a separate and distinct entity from the hydrogen radical scavenger. The ion-conducting membrane may also comprise a recombination catalyst. A recombination catalyst is a catalyst which catalyses the reaction between hydrogen gas and oxygen gas to form water, thus minimise any hydrogen crossover through the membrane, to avoid hydrogen mixing with oxygen and associated safety concerns. The recombination catalyst may comprise platinum or palladium, or consists essentially of platinum or palladium (i.e. the nanoparticles are platinum nanoparticles or palladium nanoparticles). Alternatively, the recombination catalyst may be platinum alloyed with another element, for example a platinum-palladium alloy, a platinum-iridium alloy, a platinum cobalt alloy or a platinum-ruthenium alloy. The ion-conducting membrane may further comprise: (c) a reinforcing layer which may be a porous polymer material, wherein the ion-conducting polymer is impregnated within the porous polymer material. The reinforcing layer 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), polyvinylpyrrolidone (PVP) and polyether ether ketone (PEEK). 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. The nitrogen-containing heterocycle may comprise basic functional groups. The nitrogen-containing basic functional groups can be nitrogen with a lone pair. The polymer backbone can be derived from polybenzimidazoles, poly(pyridine)s, poly(pyrimidine)s, polybenzthiazoles, polyoxadiazoles, polyquinolines, polyquinoxalines, polythiadiazoles, polytriazoles, polyoxazoles, polybenzoxazoles, polythiazoles, polypyrazoles, and derivatives thereof. The polymer backbone may be derived from a functionalised polyazole or a zwitterionic polyazole, such as a polybenzimidazole, polytriazole, polythiazole and polydithiazole and their derivatives; most suitably a polybenzimidazole. It will be understood by the skilled person that the polymer backbone may comprises more than one type of nitrogen-containing heterocycle, or a mixture of a nitrogencontaining heterocycles and other aliphatic or aromatic groups. The porous polymer structure may comprise a porous mat of nanofibers. The porous mat may be formed from entangled nanofibres. Typically, the nanofibres are ionically non-conductive. For example, the nanofibres are devoid of sulphonic acid groups and / or phosphoric acid groups. The nanofibres may comprise discrete nanofibres that are entwined. For example, the nanofibres can cross each other or be twisted with other nanofibres or itself. The porous mat of nanofibres can be in the form of a non-woven fabric material. The nanofibres may have a substantially random orientation in the plane of the reinforced ion-conducting membrane (i.e. the xy plane). The nanofibres may have a diameter of 50-700 nm, suitably 200-600 nm or 250-550 nm. The length of the nanofibres is not material to the disclosure, but each nanofibre should be sufficiently long (for example several millimetres or centimetres) to be entangled, either with one or more other nanofibres or with itself. The nanofibres may be spun nanofibres, i.e. the nanofibres are formed using a spinning technique. Examples of suitable spinning techniques include, but are not limited to, electrospinning and force spinning. The reinforcing layer may also be a woven fabric, for example a woven PEEK fabric. The reinforcing layer may have a maximum thickness of 100 % of the thickness of the reinforced ion-conducting membrane such as a maximum thickness of 90 %, 80 %, 70 %, 60 %, or 50 % of the thickness of the ion-conducting membrane. The porous polymer material may have a minimum thickness of 5 % of the thickness of the ion-conducting membrane, such as a minimum thickness of 10 %, 15 %, 20 %, 25 % or 30 % of the thickness of the ionconducting membrane. The porous polymer material in the ion-conducting membrane may have a thickness in the range of and including 5 to 95 % of the thickness of the reinforced ionconducting membrane, such as a thickness in the range of and including 10 to 90 % or 20 to 80 % of the thickness of the reinforced ion-conducting membrane. The ion-conducting membrane may contain more than one, for example two, reinforcing layer(s) each having ion-conducting polymer impregnated in at least a region thereof. It will be understood that, in the case that the reinforced ion-conducting membrane has more than one reinforcing layer the maximum and / or minimum thickness is the sum of the thickness of each porous polymer structure. The thickness of the or each porous polymer structure, as a proportion of the reinforced ion-conducting membrane may be determined, for example, from a scanning electron microscope (SEM) image of a cross section of the reinforced ionconducting membrane. The thickness of the ion-conducting membrane will depend on its intended use. For example, an ion-conducting membrane for a water electrolyser will typically be thicker than for a fuel cell but that may not always be the case. The ion-conducting membrane may have a thickness of at least about 5 micrometres. The ion-conducting membrane may have a thickness of at least about 6 micrometres, about 7 micrometres, about 8 micrometres, about 9 micrometres or at least about 10 micrometres. Typically, the thickness of the ion-conducting membrane at is less than or equal to about 200 micrometres, such as less than or equal to about 150 micrometres, less than or equal to about 100 micrometres, less than or equal to about 50 micrometres, less than or equal to about 30 micrometres, less than or equal to about 25 micrometres, or less than or equal to about 20 micrometres. The thickness of the ionconducting 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 thickness. A motorised spindle is used to take measurement readings with a measuring force of 0.01 N. At least three readings are taken from different locations on the ion-conducting membrane or polymer layer (prior to adding catalyst layers) at a temperature of 20 °C ± 3 °C, and relative humidity (RH) of 30-50%. The ion-conducting membrane may be prepared by a process comprising the steps of: (a) mixing the hydrogen radical scavenger with the ion-conducting polymer and a solvent; (b) forming the ion-conducting membrane from the mixture produced in step (a). Typically, in step (a) the ion-conducting polymer is in a suspension or solution in a solvent and step (b) comprises forming the ion-conducting membrane from the suspension or solution. When the hydrogen radical scavenger is an ionic species, a skilled person will appreciate that it will be added to step (a) in the form of a salt. A skilled person can select an appropriate salt which is compatible with the process. Any suitable method of formation of the ion-conducting membrane may be used including, for example spraying, electro-spraying, screen printing, rotary screen printing, inkjet printing, brush coating, painting, immersion or dipping, bar coating, pad coating, gravure; gap coating techniques such as knife or doctor blade over roll (whereby the coating is applied to the substrate then passes through a split between the knife and a support roller); slot die (slot, extrusion) coating (whereby the coating is squeezed out by gravity or under pressure via a slot onto the substrate); metering rod application such as with a Meyer bar and gravure coating. In step (b), the ion-conducting membrane may be formed on a substrate, for example a decal backing layer. Alternatively, the ion-conducting membrane may be formed on a fuel cell or electrolyser catalyst layer, typically as part of an additive layer manufacturing approach to manufacturing a fuel cell or electrolyser catalyst-coated membrane. Also provided is a catalyst-coated membrane comprising an ion-conducting membrane of the disclosure, with a cathode catalyst layer applied to a first face of the membrane and / or an anode catalyst layer applied to a second face of the membrane. The catalyst layer comprises one or more electrocatalysts. The one or more electrocatalysts may be independently a finely divided unsupported metal powder, or a supported catalyst wherein small nanoparticles are dispersed on a catalyst support, such as electrically conducting particulate carbon supports or a metal oxide such as titania. The electrocatalyst metal may be selected from; (i) the platinum group metals (platinum, palladium, rhodium, ruthenium, iridium and osmium), (ii) gold or silver, (iii) a base metal, or an alloy or mixture comprising one or more of these metals or their oxides. A typical electrocatalyst metal is platinum, which may be alloyed with other precious metals or base metals. A base metal is tin or a transition metal which is not a noble metal. A noble metal is a platinum group metal (platinum, palladium, rhodium, ruthenium, iridium or osmium), gold or silver. Suitable base metals include copper, cobalt, nickel, zinc, iron, titanium, molybdenum, vanadium, manganese, niobium, tantalum, chromium and tin. Suitable base metals are nickel, copper, cobalt, and chromium. More suitable base metals are nickel, cobalt and copper. If the electrocatalyst is a supported catalyst, the loading of metal particles on the support material may be in the range 10-90 wt%, or 15-75 wt% of the weight of resulting electrocatalyst. The exact electrocatalyst used will depend on the reaction it is intended to catalyse and its selection is within the capability of the skilled person. In the case of a catalyst-coated membrane for a water electrolyser, a cathode catalyst layer may be applied to a surface of the membrane comprising a catalyst for catalysing the hydrogen evolution reaction. Typically, the cathode catalyst layer comprises platinum, for example a platinum-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. In the case of a catalyst-coated membrane for a water electrolyser, an anode catalyst layer may be applied to a surface of the membrane comprising a catalyst for catalysing the oxygen evolution reaction. In the case that the catalyst-coated membrane is for a proton exchange membrane water electrolyser, typically the anode catalyst layer comprises iridium, such as iridium oxide or mixed oxides of iridium and another metal or metals. 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 membrane by hot pressing. Alternatively, the ink can be directly coated onto the membrane. In the case of a catalyst-coated membrane for a fuel cell, a cathode catalyst layer may be applied to a surface of the membrane comprising a catalyst for catalysing the oxygen reduction reaction. Typically, that the cathode catalyst layer comprises platinum, for example a platinum-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. In the case of a catalyst-coated membrane for a fuel cell, an anode catalyst layer may be applied to a surface of the membrane comprising a catalyst for catalysing the hydrogen evolution reaction. Typically, the anode catalyst layer comprises platinum, for example a platinum-on-carbon catalyst. 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 membrane by hot pressing. Alternatively, the ink can be directly coated onto the membrane. The catalyst layer may further comprise additional components. Examples of such catalysts and any other additives suitable for inclusion in the catalyst layer will be known to those skilled in the art. For example, a hydrogen peroxide decomposition catalyst. For example in a fuel cell anode, such additional components include, but are not limited to, a catalyst which facilitates oxygen evolution and therefore will be of benefit in cell reversal situations and high potential excursions. Figs 1, A and B show fuel cell catalyst-coated membranes 1 in which the hydrogen radical scavenger agent is 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 ion-conducting membrane 4 is disposed between a cathode catalyst layer 2 and an anode catalyst layer 3. Hydrogen radical scavenger is 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 1B, 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 hydrogen radical scavenger and is positioned adjacent to the anode catalyst layer 2. Alternatively, the sub-layer containing the hydrogen radical scavenger is positioned closer to the anode than the cathode, but is not directly adjacent to the anode with another sub-layer between the anode and the sub-layer containing the hydrogen radical scavenger. 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. Also provided is a membrane electrode assembly comprising an ion-conducting membrane of the disclosure and a gas diffusion electrode and / or a porous transport layer on a first and / or second face of the ion-conducting membrane. Also provided is a membrane electrode assembly comprising a catalyst-coated ion-conducting membrane and a gas diffusion layer or porous transport layer present on the at least one of the catalyst layers. The anode and cathode gas diffusion layers may be based on conventional gas diffusion substrates. Typical substrates include 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 Ballard Power Systems Inc.), or woven carbon cloths. The carbon paper, web or cloth may be provided with a further treatment prior to being incorporated into a MEA either to make it more wettable (hydrophilic) or more wet-proofed (hydrophobic). The nature of any treatments will depend on the type of fuel cell and the operating conditions that will be used. The substrate can be made more wettable by incorporation of materials such as amorphous carbon blacks via impregnation from liquid suspensions, or can be made more hydrophobic by impregnating the pore structure of the substrate with a colloidal suspension of a polymer such as PTFE or polyfluoroethylenepropylene (FEP), followed by drying and heating above the melting point of the polymer. For applications such as the proton exchange membrane fuel cell, a microporous layer may also be applied to the gas diffusion substrate on the face that will contact the catalyst layer. The microporous layer typically comprises a mixture of a carbon black and a polymer such as polytetrafluoroethylene (PTFE). The porous transport layer may be based on conventional porous transport substrates, such as a titanium mesh. Also provided is an electrochemical device comprising an ion-conducting membrane, a catalyst-coated membrane, or a membrane-electrode assembly of the disclosure. The electrochemical device can be a fuel cell, such as a proton exchange membrane fuel cell. The electrochemical device can be an electrolyser, such as a proton exchange membrane water electrolyser. Also provided is use of a material, as defined herein, having a rate constant for the reaction with a hydrogen radical (H ) of greater than or equal to 1 x 107 M'1s-1, typically greater than or equal to 1 x 108 M'1s'1, for preventing degradation of an ion-conducting membrane by hydrogen radicals. The material may be the hydrogen radical scavenger as defined herein. The ionconducting membrane is as defined herein and can be used and incorporated into components as described herein. Also provided is a method of preventing degradation of an ion-conducting membrane by hydrogen radicals, the method comprising using a material, as defined herein, having a rate constant for the reaction with a hydrogen radical (H ) of greater than or equal to 1 x 107 M’1s~ 1, typically greater than or equal to 1 x 10s M1 s~1 .The particles prevent degradation of the ionconducting membrane by hydrogen radicals. The material may be the hydrogen radical scavenger as defined herein. The ion-conducting membrane is as defined herein and can be used and incorporated into components as described herein. An ion-conducting membrane containing a silver ion hydrogen radical scavenger is prepared by initially preparing separate additive and ion-conducting polymer (ionomer) dispersions. For the additive, a 3 wt% AgNO3 solution is prepared by dissolving AgNO3 in milli-Q H2O; for the ionomer, a 25 wt% PFSA (EW725) solution is prepared by dissolving the ionomer in a mix of EtOH and milli-Q H2O (EtOH:H2O 80:20 by weight). Then, based on the additive loading targeted, required amounts of AgNO3 solution are further diluted in a mix of EtOH and milli-Q H2O (EtOH:H2O 80:20 by weight). The new AgNO3 dissolution is then added dropwise to the PFSA solution initially prepared. The additiveionomer dispersion is stirred with a magnetic stirrer for 10 to 15 min before using it for casting. Casting of the additive-ionomer solution is performed on a coated PET substrate using a casting knife vacuum film applicator at room temperature. Dry thickness around 20 microns is achieved by performing two coats of 200 and 150 microns (wet thickness), allowing the film to dry after each coat. Once the film is dry, the sample is annealed. For subsequent evaluations and membrane-electrode assemblies, the membrane film is carefully removed from the substrate.

Claims

:

1. An ion-conducting membrane comprising:(a) an ion-conducting polymer; and(b) a hydrogen radical scavenger;5 wherein the hydrogen radical scavenger has a rate constant for the reaction with ahydrogen radical (H ) of greater than or equal to 1 x 107 M~1s~1.

2. An ion-conducting membrane according to claim 1, wherein the ion-conducting polymer is a proton-conducting polymer.

103. An ion-conducting membrane according to claim 2, wherein the proton-conducting polymer is a perfluorinated sulfonic acid ionomer.

4. An ion-conducting membrane according to claim 2, wherein the proton conducting 15 polymer is a hydrocarbon ion-conducting polymer.

5. An ion-conducting membrane according to any of the preceding claims, further comprising a reinforcing layer.20 6. An ion-conducting membrane according to claim 5, wherein the reinforcing layer is aporous polymer material, wherein the ion-conducting polymer is impregnated within the porous polymer material.

7. An ion-conducting membrane according to claim 6, wherein the reinforcing layer25 comprises a porous mat of nanofibers.

8. An ion-conducting membrane according to claim 5, wherein the reinforcing layer is a woven fabric.30 9. An ion-conducting membrane according to any of claims 5 to 8, wherein the reinforcinglayer comprises a planar porous polymer material which is a fluoropolymer.

10. A catalyst-coated membrane for a fuel cell or a water electrolyser comprising an ionconducting membrane according to any of claims 1 to 9, with a cathode catalyst layer applied 35 to a first face of the membrane and I or an anode catalyst layer applied to a second face of the membrane.

11. A membrane-electrode assembly for a fuel cell or a water electrolyser comprising (i) an ion-conducting membrane according to any of claims 1 to 9; or (ii) a catalyst-coated membrane according to claim 10; and at least one of a gas diffusion layer or a porous transport layer.

512. A water electrolyser or a fuel cell comprising a catalyst-coated membrane according to claim 10 or a membrane-electrode assembly according to claim 11.

13. Use of a material having a rate constant for the reaction with a hydrogen radical (H ) 10 of greater than or equal to 1 x 107 M'1s'1 for preventing degradation of an ion-conducting membrane by hydrogen radicals, wherein the ion-conducting membrane comprises an ionconducting polymer.

14. A method of preventing degradation of an ion-conducting membrane by hydrogen 15 radicals, the method comprising using a material having a rate constant for the reaction with a hydrogen radical (H ) of greater than or equal to 1 x 107 M'1s_1, wherein the ion-conducting membrane comprises an ion-conducting polymer.27 11 24