Dispersion containing a radical scavenger

Abstract

Dispersion According to the present invention there is provided a dispersion comprising: a solvent; and a radical scavenger dispersed in the solvent. The radical scavenger comprises particles and each particle comprises cerium, a metal (M) and oxygen, wherein the metal (M) is present in an oxidation state of +5. There is also provided an ion-conducting membrane, catalyst- coated ion-conducting membrane, membrane electrode assembly, electrochemical device, and associated methods of producing a dispersion.

Description

[0001] DISPERSION CONTAINING A RADICAL SCAVENGER

[0002] Field of the Invention

[0003] The present invention relates to a dispersion comprising a radical scavenger. The present invention also relates to associated dispersions for producing an ion-conducting membrane, which may be used in an electrochemical device, such as a fuel cell or water electrolyser. The present invention also relates to ion-conducting membranes (in particular proton exchange membranes), catalyst-coated membranes, membrane electrode assemblies, and electrochemical devices, such as fuel cells and electrolysers, produced using such dispersions.

[0004] Background of the Invention

[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] 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).

[0008] 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. In fuel cells and electrolysers, radicals can form during operation, for example from the breakdown of hydrogen peroxide which can be a biproduct 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 Company. It is desirable to improve the durability of ion-conducting membranes by stopping the action of radicals.

[0009] Summary of the Invention

[0010] The present invention seeks to address at least some of the above problems, desires and needs. In particular, an object of the present invention is to provide a dispersion exhibiting improved radical scavenging activity. A further object of the present invention is to provide a dispersion for manufacturing an improved ion-conducting membrane comprising an additive, which can better suppress radical degradation, and hence provide improved durability.

[0011] Accordingly, in a first aspect, there is provided a dispersion comprising: a solvent; and a radical scavenger dispersed in the solvent, wherein the radical scavenger comprises particles and each particle comprises cerium, a metal (M) and oxygen, wherein the metal (M) is present in an oxidation state of +5.

[0012] The dispersion can further comprise an ion-conducting polymer. Such dispersions can be used to prepare an ion-conducting layer, such as an ion-conducting membrane. Accordingly, in a further aspect there is provided a method of preparing an ion-conducting membrane comprising the steps of: providing a dispersion comprising a solvent, a radical scavenger dispersed in the solvent, and an ion-conducting polymer, wherein the radical scavenger comprises particles and each particle comprises cerium, a metal (M) and oxygen, wherein the metal (M) is present in an oxidation state of +5; depositing a layer of the dispersion, preferably onto a carrier sheet; and drying the layer of the dispersion to form an ion-conducting membrane. The ionconducting membrane comprises the ion-conducting polymer and the radical scavenger, wherein the radical scavenger comprises particles and each particle comprises cerium, a metal (M) and oxygen, wherein the metal (M) is present in an oxidation state of +5.

[0013] According to a further aspect there is provided a method of preparing a catalyst layer comprising the steps of: providing a dispersion comprising a solvent, a radical scavenger dispersed in the solvent, an ion-conducting polymer and an electrocatalyst, wherein the radical scavenger comprises particles and each particle comprises cerium, a metal (M) and oxygen, wherein the metal (M) is present in an oxidation state of +5; depositing a layer of the dispersion, preferably onto an ion-conducting membrane or carrier sheet; and drying the layer of the dispersion to form a catalyst layer. The catalyst layer comprises the ion-conducting polymer, the electrocatalyst and the radical scavenger, wherein the radical scavenger comprises particles and each particle comprises cerium, a metal (M) and oxygen, wherein the metal (M) is present in an oxidation state of +5.

[0014] According to a second aspect there is provided an ion-conducting (electrolyte) membrane for a fuel cell or an electrolyser (such as a water electrolyser), the ion-conducting membrane comprising an ion-conducting polymer and a radical scavenger comprising particles, wherein each particle comprises cerium, a metal (M) and oxygen, wherein the metal (M) is present in an oxidation state of +5.

[0015] Suitably, the ion-conducting (electrolyte) membrane for a fuel cell has a cross-sectional thickness of 20 pm or less, suitably 17 pm or less, suitably 10 pm or less. Suitably, the ionconducting (electrolyte) membrane for a fuel cell has a cross-sectional thickness of at least 5 pm. The ion-conducting (electrolyte) membrane for fuel cell can have a thickness in a range comprising any combination of the aforementioned limits. Suitably, the ion-conducting (electrolyte) membrane for an electrolyser has a cross-sectional thickness of at least 15 pm, preferably at least 20 pm, preferably at least 30 pm, preferably at least 40 pm or preferably at least 50 pm. Suitably, the ion-conducting (electrolyte) membrane for an electrolyser has a cross-sectional thickness of less than or equal to 150 pm, preferably 100 pm or less, 95 pm or less, 90 pm or less. The ion-conducting (electrolyte) membrane for an electrolyser can have a thickness in a range comprising any combination of the aforementioned limits. The cross- sectional thickness of an ion-conducting (electrolyte) membrane can be measured by scanning electron microscopy (SEM). SEM analysis can be carried out on cross-sections of the membrane and the membrane 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 membrane (suitably at 0% relative humidity), which is embedded in resin, ground and polished.

[0016] The ion-conducting membrane of the second aspect can be prepared using a dispersion of the first aspect which comprises an ion-conducting polymer.

[0017] According to a third aspect there is provided a catalyst-coated ion-conducting membrane comprising the ion-conducting membrane of the second aspect, wherein a first catalyst layer is applied to a first side of the ion-conducting membrane and a second catalyst layer is applied to a second side of the ion-conducting membrane. According to a fourth aspect, there is provided a membrane electrode assembly comprising the ion-conducting membrane of the second aspect or the catalyst-coated ionconducting membrane of the third aspect, and further comprising a gas diffusion layer and / or a porous transport layer. For example, the membrane electrode assembly can comprise an ion-conducting membrane of the second aspect, a first catalyst layer applied to a first side of the ion-conducting membrane, a second catalyst layer applied to a second side of the ionconducting membrane, a first gas diffusion layer (or a first porous transport layer) adjacent the first catalyst layer, and a second gas diffusion layer (or a second porous transport layer) adjacent the second catalyst layer.

[0018] According to a fifth aspect, there is provided fuel cell or water electrolyser comprising the ion-conducting membrane of the second aspect, the catalyst-coated ion-conducting membrane of the third aspect, or the membrane electrode assembly of the fourth aspect.

[0019] According to a sixth aspect, there is provided a method of producing a radical scavenger comprising the steps of: providing a solution comprising a cerium compound and metal (M) compound; effecting a precipitation reaction of the cerium compound and the metal (M) compound to form a precipitate comprising cerium, the metal and oxygen, wherein the metal is present in a 5+ oxidation state; and isolating the precipitate.

[0020] According to a seventh aspect, there is provided a method of producing a dispersion comprising the steps of: providing a radical scavenger produced by a method according to the sixth aspect; and dispersing the precipitate in a solvent.

[0021] According to an eighth aspect, there is provided a method of preparing an (ionomercontaining) dispersion comprising the steps of: combining the dispersion produced by the method of the seventh aspect and an ionconducting polymer.

[0022] According to a ninth aspect, there is provided a method of preparing an ion-conducting membrane comprising the steps of: providing a dispersion prepared by a method according to the eighth aspect; depositing a layer of the dispersion, preferably onto a carrier sheet; and heating the layer of the dispersion to form an ion-conducting membrane.

[0023] According to a tenth aspect, there is provided use of particles for preventing radical degradation of an ion-conducting membrane, wherein the particles comprise cerium, a metal (M) and oxygen, wherein the metal (M) is in an oxidation state of +5. Preferably, the particles comprise a composite material comprising cerium, the metal (M) and oxygen, wherein the metal (M) is in an oxidation state of +5. Brief Description of the Drawings

[0024] Figure 1 is a plot of cell open circuit voltage (OCV) as a function of OCV hold time (in hours) for some exemplary and comparative MEAs.

[0025] Detailed Description of the Invention

[0026] Preferred and / or optional features of the invention will now be set out. Any aspect of the invention may be combined with any other aspect of the invention, unless the context demands otherwise. Any of the preferred or optional features of any aspect may be combined, singly or in combination, with any aspect of the invention, unless the context demands otherwise.

[0027] The invention provides a dispersion comprising a solvent and a radical scavenger dispersed in the solvent. The dispersion can be referred to as a sol. The term “sol” is used here to mean a dispersion, such as a colloidal dispersion, of solid particles (e.g. nanoparticles) dispersed in a liquid phase.

[0028] The dispersion can comprise further components such as a dispersing agent, an ionconducting polymer and / or an electrocatalyst. In some preferred embodiments, the dispersion comprises an ion-conducting polymer and preferably both an ion-conducting polymer and a dispersing agent. A dispersion comprising the radical scavenger and an ion-conducting polymer (and suitably devoid of an electrocatalyst) may be described as an ion-conducting membrane dispersion. A dispersion comprising the radical scavenger, an ion-conducting polymer and an electrocatalyst may be described as a catalyst ink.

[0029] Radical scavenger

[0030] The radical scavenger comprises particles, preferably particles of a composite material (hereinafter composite particles). The particles comprise cerium, a metal (M) and oxygen, wherein the metal (M) is present in an oxidation state of +5. For example, the particles can comprise a composite material comprising cerium, the metal (M) and oxygen.

[0031] Oxygen is present as an oxide. Each (composite) particle of the radical scavenger can comprise a mixture of at least one oxide of cerium and an oxide of the metal (M). That is, the radical scavenger can comprise M(V) oxide and at least one of Ce(lll) oxide and Ce(IV) oxide as a mixture of oxides. For example, the mixture can comprise M(V) oxide, Ce(lll) oxide and Ce(IV) oxide. As an alternative to composite particles, each particle can comprise a mixed metal oxide of cerium and the metal (M). Preferably, the particles comprise a composite comprising Ce(IV) oxide, Ce(lll) oxide and M(V) oxide. The composite material does not comprise an intimate mixture of an oxide of cerium and the metal(V), i.e. the cerium and the metal (M) are not atomically mixed within a single lattice structure. In some embodiments, the radical scavenger can comprise a first phase comprising Ce (e.g. an oxide of Ce) and a second phase comprising the metal (M) (e.g. an M(V) oxide). In such embodiments, the Ce:M molar ratio of the first phase is different to the Ce:M molar ratio of the second phase. For example, the Ce:M molar ratio of the first phase can be in a range of 40:60 to 60:40, and the Ce:M molar ratio of the second phase can be less than 40:60 or greater than 60:40.

[0032] The radical scavenger can comprise an amorphous phase. The radical scavenger can comprise a crystalline phase. According to H. P. Klug and L. E. Alexander (“X-ray Diffraction Procedures - For Polycrystalline and Amorphous Materials”, 2ndEdition, 1974, John Wiley & Sons, page 791), crystalline materials are characterized by three-dimensional periodicity of the order of six unit translations. Accordingly, amorphous materials lack order on this scale. The amorphous phase can comprise a cerium oxide and / or a M(V) oxide. The degree of crystallinity of the radical scavenger is at most 50%, typically at most 20%. Suitably, the degree of crystallinity is at most 10%, suitably at most 5%, suitably at most 1%. The radical scavenger may consist essentially of, preferably consist of, the amorphous phase. The term “degree of crystallinity” refers to the percentage of crystalline phase, which is a representation of structural order in the composition. The degree of crystallinity can be determined by X-ray diffraction (XRD) or other known techniques. For example, in one method of determining the degree of crystallinity, the observed X-ray scattering from a sample can be fitted using a series of peak functions. Peaks which have a reflection broadening indicating a crystallite size of greater than 2 nm (e.g. sharp, well-defined peaks) are considered to come from crystalline material. The summed area from contributions from the crystalline phase is divided by the total area of all peaks used to fit the data to yield the degree of crystallinity, see TOPAS 5 Technical Reference (Oct. 18, 2014), page 127. The background can be fitted using suitable known techniques known in the art.

[0033] The particles of the radical scavenger are suitably amorphous. An amorphous material is one which lacks long-range atomic structure (e.g. a crystallite size of less than 2.0 nm). As such, an X-ray diffraction (XRD) pattern of the radical scavenger is suitably substantially devoid of sharp, well-defined diffraction peaks associated with crystalline phases, for example, of Ce(lll) oxide, Ce(IV) oxide and M(V) oxide.

[0034] The metal (M) is present in the particles (e.g. composite particles) in an oxidation state of +5. In some embodiments, the metal (M) may also be present (suitably in minor amounts) in an oxidation state other than +5, such as an oxidation state of +3 or +4. It is preferred that substantially all, and more preferably all, metal (M) in the particles is in a +5 oxidation state. Suitably, the metal (M) is selected from the group consisting of: niobium (Nb), tantalum (Ta), bismuth (Bi), antimony (Sb), and vanadium (V). Preferably, the metal (M) is niobium (Nb) or tantalum (Ta). Some elements, such as niobium and tantalum, are expected to exist in a 5+ oxidation state due to their electroreduction potential. The oxidation state of the metal (M) can be suitably determined using X-ray photoelectron spectroscopy (XPS) on a dry powder of the radical scavenger particles, i.e. the Ce-M-containing material, or X-ray absorption spectroscopy near edge (XANES) on a dry powder or suspension of the radical scavenger particles. The terms “metal(V)”, “M(V)” and “M5+” as used herein each refer to the metal (M) in a 5+ oxidation state, and are used interchangeably.

[0035] The molar ratio of total cerium to metal (M) can be 100:1 or less, 50:1 or less, suitably 36:1 or less, preferably 18:1 or less, 9:1 or less, or 6:1 or less. The molar ratio of total cerium to metal (M) can be at least 1 :6, preferably at least 1 :3, more preferably at least 1 :2, more preferably at least 1 :1 , and more preferably at least 2:1. The molar ratio of total cerium to metal (M) can be in a range comprising any combination of the aforementioned upper and lower limits. For example, the molar ratio of total cerium to metal (M) can be in a range of 100:1 to 1 :6, 50:1 to 1 :3, 36:1 to 1 :2, preferably 18:1 to 1 :1 , and more preferably 6:1 to 2:1.

[0036] Cerium is typically present as both Ce(lll) and Ce(IV). That is, the particles can comprise cerium in an oxidation state of +3 and cerium in an oxidation state of +4. Ce(lll) is paramagnetic, whereas Ce(IV) is diamagnetic. Therefore, for the Ce-based materials of the disclosure, magnetic susceptibility can be used as a proxy for the relative amounts of Ce(lll) and Ce(IV). Magnetic susceptibility can be determined using the methods described below.

[0037] Without wishing to be bound by any theory or conjecture, it is believed that incorporating a metal in a 5+ oxidation state in the Ce-containing composite material causes a greater proportion of the cerium to exist as Ce(lll), rather than Ce(IV), in order to balance the charges. As such, the (composite) particles of the radical scavenger typically have a magnetic susceptibility that is more positive than the magnetic susceptibility of CeC>2. Preferably, the radical scavenger is (or the (composite) particles thereof are) paramagnetic.

[0038] Solvent

[0039] The solvent suitably comprises water and / or an organic solvent. Examples of suitable organic solvents include ketones, such as acetone, methyl ethyl ketone, diethyl ketone, methyl isobutyl ketone, and cyclohexanone; esters such as methyl acetate, ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, t-butyl acetate, amyl acetate, methyl lactate, ethyl lactate, n-propyl lactate, isopropyl lactate, n-butyl lactate, and methoxypropyl acetate; alcohols such as methanol, ethanol, n-propanol, isopropanol, n-butanol, t-butanol, n-pentanol, and n- hexanol; glycols such as ethylene glycol, propylene glycol, glycerin, and diethylene glycol; glycol ethers such as methoxypropanol, ethylene glycol dimethyl ether, ethylene glycol diethylether, cellosolve, diethyleneglycol dimethylether, and diethylene glycol diethylether; and amides, as well as mixtures of two or more of the foregoing. Preferably, the solvent comprises water, methanol, ethanol, 1-propanol, iso-propanol, or a combination thereof. More preferably, the solvent comprises water and optionally an organic solvent selected from ethanol, 1-propanol and iso-propanol.

[0040] In one embodiment the solvent comprises water and an organic solvent. The ratio of water : organic solvent may be from 0.9 or less (<90wt% water : >10wt% organic solvent), 0.80 or less, 0.7 or less, 0.6 or less, 0.5 or less, 0.4 or less, 0.3 or less, or 0.2 or less; and / or the ratio of water: organic solvent may be 0.1 or more (> 10wt% water : < 90wt % organic solvent), 0.2 or more, 0.3 or more, 0.4 or more, 0.5 or more, 0.6 or more, 0.7 or more, 0.8 or more or 0.9 or more. The solvent may comprise from 40 to 60wt% water and from 60 to 40wt% organic solvent.

[0041] Dispersing agent

[0042] The dispersion can further comprise a dispersing agent. Suitable dispersing agents can comprise a strong mineral acid (such as a nitrate acid, preferably nitric acid) ora strong organic acid (such as triflic acid (CF3SO3H) or trifluoroacetic acid (CF3CO2H). The strong mineral or strong organic acid may be an acid with a pKa below 2, below 1 , or below 0. Alternatively, the dispersing agent can comprise an organic base, such as an amine. Preferably the dispersing agent is a tetraalkylammonium hydroxide, such as tetraethylammonium hydroxide. The choice of an acidic or basic dispersing agent can be selected depending on the isoelectric point of the radical scavenger in suspension.

[0043] Ion-conducting polymer

[0044] The dispersion can further comprise an ion-conducting polymer. Such dispersions may be suitable for forming an 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. Preferably a proton-conducting polymer. Typically, the ion-conducting polymer comprises sulfonic acid groups. Suitably, the ion-conducting polymer is a perfluorinated sulfonic acid ionomer, or a partially-fluorinated or non-fluorinated hydrocarbon sulfonic acid ionomer. Examples of suitable proton-conducting polymers include partially- or fully-fluorinated sulphonic acid polymers, such as perfluorosulphonic acid ionomers (e.g. Nation® (Chemours), Aciplex® (Asahi Kasei), Aguivion™ (Solvay Speciality Polymers), Flemion® (Asahi Glass Co.); or ionomers based on a sulphonated hydrocarbon such as those available from FuMA-Tech GmbH as the fumapem® P, E or K series of products, and others. Examples of suitable anion-conducting polymers include A901 made by Tokuyama Corporation and Fumasep FAA from FuMA-Tech GmbH. Typically, the ion-conducting polymer has an eguivalent weight of about 1100 or less, typically about 900 or less, suitably about 850 or less. Typically, the ion-conducting polymer has an eguivalent weight of at least about 450. The eguivalent 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 0.1 N NaOH solution to substitute sodium ions for protons in the membrane. Then titration by neutralisation is carried out, for example using 0.1 N hydrochloric acid, to determine the number of exchangeable protons, and therefore the EW may be calculated.

[0045] The radical scavenger can be present in the dispersion in an amount in a range of from 0.01 wt.% to 20 wt.% based on the total weight of the ion-conducting polymer.

[0046] The ion-conducting polymer can comprise ion exchange groups (preferably cation exchange groups), and the proportion of the radical scavenger to the number (100 mol%) of the ion exchange groups contained in the ion-conducting polymer can be from 0.3 to 50 mol%.

[0047] Electrocatalyst

[0048] The dispersion can further comprise an electrocatalyst. The electrocatalyst suitably comprises metal particles optionally supported on an electrically conductive support. That is, the electrocatalyst can be unsupported metal particles (e.g. finely divided unsupported metal powder) or may be a supported electrocatalyst wherein metal particles (e.g. nanoparticles) are dispersed on an electrically conductive support, such as an electrically conducting particulate carbon support. The metal particles of the electrocatalyst are suitably selected from:

[0049] (i) the platinum group metals (i.e. platinum, palladium, rhodium, ruthenium, iridium, and osmium),

[0050] (ii) gold or silver,

[0051] (iii) a base metal, or

[0052] (iv) an alloy or mixture comprising one or more of these metals or their oxides.

[0053] Preferably, the metal in the metal particles of the electrocatalyst is a platinum group metal or an alloy of a platinum group metal. A preferred electrocatalyst metal is platinum, which may be alloyed with other precious metals or base metals. Another preferred electrocatalyst metal is iridium, 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), silver or gold. Suitable base metals include copper, cobalt, nickel, zinc, iron, titanium, molybdenum, vanadium, manganese, niobium, tantalum, chromium and tin. Preferred base metals are nickel, copper, cobalt, and chromium. More preferred base metals are nickel, cobalt and copper.

[0054] If the electrocatalyst is a supported catalyst, the loading of metal particles on the electrically conductive support material is suitably in the range 10 wt% to 90 wt% or 20 wt% to 80wt%, based on the weight of the electrocatalyst. The loading of the metal particles can be determined using inductively coupled plasma mass spectrometry (ICPMS).

[0055] Preferably, the electrocatalyst comprises an electrically conductive support and metal particles supported on the electrically conductive support. The term “supported” will be readily understood by a skilled person. For example, it will be understood that the term “supported” includes the metal particles of the electrocatalyst being dispersed on (and / or in the pores of) the support material and bound or fixed to the support material by physical or chemical bonds. For instance, the catalyst may be bound or fixed to the support material by way of ionic or covalent bonds, or non-specific interactions such as van der Waals forces.

[0056] The electrically conductive support may be an electrically conductive carbon support material. Suitably, the electrically conductive carbon support material is a carbon powder which may be, for example, a carbon black or graphitised carbon black for example a commercially available carbon black (such as from Cabot Corp. (Vulcan® XC72R) or Akzo Nobel (the Ketjen® black series)). Another suitable carbon support material is an acetylene black (e.g. those available from Chevron Phillips (Shawinigan Black®) or Denka). The electrically conductive carbon support can be prepared by the method disclosed in WO20 13 / 045894. Alternatively, the electrically conductive support can be a metal oxide or a mixed oxide, in particular a conductive mixed oxide such as niobia-doped titania, phosphorus- doped tin oxide and mixed platinum group metal oxides or mixed metal oxides (as disclosed in WO2012 / 080726), a carbide (e.g. tungsten carbide, molybdenum carbide or titanium carbide, suitably tungsten carbide or titanium carbide), a nitride, in particular a conductive nitride (e.g. titanium nitride or titanium aluminium nitride).

[0057] Methods of preparation

[0058] To prepare a radical scavenger, a solution comprising a cerium compound and a metal (M) compound is provided. Suitable cerium compounds include cerium salts, such as cerium(lll) salts (e.g. CeC ) and cerium(IV) salts (e.g. (NH4)2Ce(NOs)6). Suitably, the metal compound is a compound comprising the metal in a 5+ oxidation state. Suitable metal compounds include a metal oxychloride or a metal peroxynitrate, which may be formed in situ. Preferably, at least one of the cerium and / or metal (M) compounds comprises oxygen. Preferably, the solution is an aqueous solution. Preferably, the cerium and metal compounds are soluble in water.

[0059] A precipitation reaction of the cerium compound and the metal (M) compound is effected to form a precipitate. The precipitation reaction can be effected by changing the pH of the solution. The precipitation reaction is suitably a co-preci pitation reaction. The precipitate comprises cerium, the metal (M) and oxygen, wherein the metal (M) is present in a 5+ oxidation state. The precipitate can be isolated, for example by filtration or other known separation techniques.

[0060] The filtered and washed precipitate, as a wet cake, may be used as is to produce a dispersion of this disclosure by subjecting it to a peptising step, or may be dried to remove at least some water and provide a dried wet cake which may then be subjected to the peptising step. For example, the precipitate can be dispersed (or “peptised”) in a solvent to form a dispersion comprising the radical scavenger dispersed in the solvent. Such a dispersion can be facilitated using a dispersing agent. The term “peptise” is used to mean disperse a substance into a colloidal state.

[0061] The wet cake may have a solids content by weight of up to about 30 wt% or up to about 20 wt%.

[0062] The dried wet cake may have a solids content by weight of more than 30 wt%, e.g. up to about 80 wt%, up to about 70 wt%, up to about 60 wt%, or up to about 50 wt%.

[0063] After dispersing the radical scavenger in a solvent, the resulting dispersion can be combined with an ion-conducting polymer (preferably as an ion-conducting polymer dispersion / solution) to form an ion-conducting membrane dispersion. Optionally, an electrocatalyst can also be combined to form a catalyst ink.

[0064] Applications

[0065] The radical scavengers formed can be employed to prevent radical degradation within an electrochemical device. For example, the radical scavengers can be used for preventing radical degradation of an ion-conducting membrane (e.g. a polymer electrolyte membrane), wherein each particle comprises cerium, a metal (M) and oxygen, and wherein the metal (M) is in an oxidation state of +5.

[0066] Ion-conducting layers

[0067] Dispersions of the present disclosure comprising a radical scavenger and an ionconducting polymer can be suitable for preparing an ion-conducting layer, such as an ionconducting membrane (e.g. an electrolyte membrane) or an ion-conducting catalyst layer, for an electrochemical device such as a fuel cell or water electrolyser. The ion-conducting layer can comprise an ion-conducting polymer and a radical scavenger comprising particles, wherein each particle comprises cerium, a metal (M) and oxygen, wherein the metal (M) is present in an oxidation state of +5.

[0068] When used in an ion-conducting layer, the particles of the radical scavenger suitably have a mean average particle size (e.g. diameter) of less than 40 pm, preferably less than 20 pm, and more preferably less than 10 pm. Preferably, the particles of the radical scavenger have a mean average particle size of <10 pm, <8 pm, <6 pm, <4 pm, <3 pm (when measured in an ion-conducting layer). The lower limit of the mean average particle size is not particularly limited. For example, the particles of the radical scavenger can have a mean average particle size of at least 1 nm, at least 10 nm, at least 20 nm, at least 50 nm, at least 100 nm, at least 500 nm, at least 1 pm, or at least 2 pm (when measured in an ion-conducting layer). The particles of the radical scavenger can have a mean average particle size in a range comprising any combination of the aforementioned upper and lower limits. For example, the particles of the radical scavenger can have a mean average particle size in a range of from 1 nm to 40 pm, 10 nm to 20 pm, 20 nm to 10 pm, 50 nm to 10 pm, 100 nm to 8 pm, 500 nm to 6 pm, 1 pm to 4 pm, or 2 pm to 3 pm (when measured in an ion-conducting layer). Particles of the radical scavenger having a smaller mean average particle size can further help prolong membrane durability. The mean average particle size of the radical scavenger can be determined using scanning electron microscopy (SEM) and directly measuring particle sizes across a representative proportion of an ion-conducting layer. The particles of the radical scavenger suitably have an average particle size that is less than the cross-sectional (through-plane) thickness of the ion-conducting layer.

[0069] Preferably, the ion-conducting layer is an ion-conducting membrane, such as a polymer electrolyte membrane. A dispersion suitable for preparing an ion-conducting membrane may be described as an ion-conducting membrane dispersion. In such embodiments, catalyst layers can be applied to one or both sides of the ion-conducting membrane to provide a catalyst-coated ion-conducting membrane. For example, a first catalyst layer can be applied to a first side of the ion-conducting membrane, and a second catalyst layer can be applied to a second side of the ion-conducting membrane. Such a catalyst-coated ion-conducting membrane can form part of a membrane electrode assembly (MEA), which in turn can form part of an electrochemical device, such as a fuel cell or water electrolyser.

[0070] The ion-conducting membrane may further comprise a reinforcing layer comprising 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) and polyvinylpyrrolidone (PVP). The porous polymer material may be expanded polytetrafluoroethylene (ePTFE). The porous polymer material may also comprise a polymer backbone based on a nitrogen-containing heterocycle. 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 suitably derived from polybenzimidazoles, poly(pyridine)s, poly(pyrimidine)s, polybenzthiazoles, polyoxadiazoles, polyquinolines, polyquinoxalines, polythiadiazoles, polytriazoles, polyoxazoles, polybenzoxazoles, polythiazoles, polypyrazoles, and derivatives thereof. Suitably, the polymer backbone is 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 comprise more than one type of nitrogen-containing heterocycle, or a mixture of a nitrogen-containing heterocycles and other aliphatic or aromatic groups.

[0071] Suitably, the porous polymer structure comprises a porous mat of nanofibers. The porous mat is suitably formed from entangled nanofibres. Typically, the nanofibres are ionically non-conductive. For example, the nanofibres are suitably 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. Suitably, the nanofibres have a substantially random orientation in the plane of the reinforced ion-conducting membrane (i.e. the xy plane). The nanofibres suitably have a diameter of 50-700 nm, suitably 200-600 nm and preferably 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 are suitably 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.

[0072] Suitably, the reinforcing layer has 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 suitably has 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. It may be preferred that the porous polymer material in the ionconducting membrane may have a thickness in the range of and including 5 to 95 % of the thickness of the reinforced ion-conducting 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.

[0073] 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 I 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.

[0074] Dispersions of the present disclosure comprising a radical scavenger, an ion-conducting polymer and an electrocatalyst can be suitable for preparing a catalyst layer. Such dispersions can be described as catalyst inks. The ion-conducting catalyst layer can be suitable for an anode or a cathode of an electrochemical device such as a fuel cell or water electrolyser. The ion-conducting catalyst layer can be applied to or formed on a gas diffusion layer or porous transport layer to form a gas diffusion electrode or porous transport electrode respectively. Alternatively, the ion-conducting catalyst layer can be applied to or formed on an ionconducting membrane to form a (half or full) catalyst-coated ion-conducting membrane. As a further alternative, the ion-conducting catalyst layer can be applied to or formed on a carrier sheet.

[0075] Examples

[0076] General method of preparing a Ce-M-containing precipitate from Ce(lll) salt

[0077] To a solution of a Ce3+salt (e.g. CeC ) in H2O (1.3 mol. dm-3) was added a solution of a metal(V) (M5+) salt (e.g. NbCU or TaCU) in concentrated HCI (2 M) to give the desired Ce:M molar ratio. To this was added 30 vol% H2O2 (H2O2:(Ce+M) = 1 :1). This was added with stirring to 4 M agueous NH3 solution (NH3:(Ce+M) = 8:1) subsurface via peristaltic pump. The resulting precipitate was then filtered and washed with demineralised H2O to a low conductivity (ca. < 50 pS). The washed precipitate was kept damp (typically ~25 wt.% solids).

[0078] General method of preparing a Ce-M-containing precipitate from Ce(IV) salt

[0079] A metal peroxynitrate solution (e.g. Nb peroxynitrate solution) was prepared as follows. To a 2 M HNO3 solution was added 30 vol% H2O2 to give a molar ratio of H2O2:M of 1 :1. To this was added a metal oxalate (e.g. Nb oxalate hydrate or Ta oxalate solution) and heated until complete dissolution. The solution was heated further to 60 °C with gas evolution for 2 hr. On cooling, solid (NH4)2Ce(NOs)6 was added in portions to achieve the desired Ce:M molar ratio. The resulting solution was added with stirring to 4 M agueous NH3 solution (NH3:(Ce+M) = 10:1) subsurface via peristaltic pump. The resulting precipitate was then filtered and washed with demineralised H2O to a low conductivity (ca. < 50 pS). The washed precipitate was kept damp (typically ~25 wt.% solids).

[0080] General method of preparing a Ce-M-containing sol

[0081] The washed precipitates, prepared by the general methods of preparing a Ce-M(V)- containing precipitate from a Ce3+or Ce4+salt described above, were peptized using a peptising agent (also referred to as a dispersing agent) (e.g. a mineral acid, such as HNO3, or an organic base such as tetraethylammonium hydroxide (TENOH)) to produce a dispersion (or sol). The peptising agent was added to the damp precipitate to give a 30 mL slurry. The molar ratio of peptizing agent to the total metal (Ce+M) (e.g. NC>3':(Ce+M) or Et4N+:(Ce+M)) was varied between 0.5 and 1 with more dispersing agent required for higher M contents. The slurry was processed using an ultrasonic probe for up to about 10 min, which de-aggregated the precipitate and formed a stable sol.

[0082] Comparative Example 1 - CeZr-containinq sol (Ce:Zr = 1 :1)

[0083] To a 2 M HNO3 solution was added a zirconium(IV) nitrate and heated until complete dissolution. The solution was heated further to 60 °C with gas evolution for 2 hr. On cooling, solid (NH4)2Ce(NO3)ewas added in portions to achieve a Ce:Zr molar ratio of 1 :1. The resulting solution was added with stirring to 4 M aqueous NH3 solution (NH3:(Ce+Zr) = 10:1) subsurface via peristaltic pump. The resulting precipitate was then filtered and washed with demineralised H2O to a low conductivity (ca. < 50 pS). The washed precipitate was kept damp (typically ~25 wt.% solids).

[0084] A Ce-Zr-containing sol was then prepared using the Ce-Zr wet precipitate and following the general method of preparing a Ce-M sol described above.

[0085] Comparative Example 2 - CeZr-containinq sol (Ce:Zr = 3:1)

[0086] A Ce-Zr-containing wet precipitate was prepared in the same way as Comparative Example 1 except that the Ce:Zr molar ratio was 3:1.

[0087] A Ce-Zr-containing sol was then prepared using the Ce-Zr wet precipitate and following the general method of preparing a Ce-M sol described above.

[0088] Comparative Example 3 - CeC>2 sol

[0089] A commercially available colloidal CeC>2 (available from Nyacol®) was used as Comparative Example 3.

[0090] Comparative Example 4 - Nb20s sol

[0091] A commercially available Nb20s powder (from Alfa Aesar) was dispersed in water and used as Comparative Example 4.

[0092] Magnetic susceptibility

[0093] Magnetic susceptibility measurements were performed on packed powder samples of dried sols at room temperature using a Sherwood Magnetic Susceptibility Balance (MSB) Mk 1 instrument available from Sherwood Scientific Limited, UK. Magnetic susceptibility values stated herein are mean averages of at least two separate measurements.

[0094] Ce(lll) is paramagnetic, whereas Ce(IV) is diamagnetic. Therefore, for Ce-based composite materials, magnetic susceptibility can be used as a proxy for the relative amounts of Ce(lll) and Ce(IV) in the composite material.

[0095] Free radical scavenging activity test

[0096] The experimental procedure 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:

[0097] FeSO4

[0098] 2H2O2- > HO ■ +H00 ■ +H20 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 inhibition mechanism. For UV-vis spectroscopy, a Cary 5000 UV-vis-NIR spectrophotometer (from Agilent Technologies Inc.) was used utilising 1 cm QS guartz glass cuvettes.

[0099] FeSCU and rhodamine B (RhB) were added to 1 M agueous sulfuric acid to form a first solution. The concentration of Fe2+was 35 ppm and the concentration of RhB was 14 ppm. A first UV-vis absorbance spectrum was measured using a spectrophotometer. 10 pL of 3% hydrogen peroxide was added to an 8 mL aliguot of the first solution. The solution was mixed for 5 minutes and a second UV-vis absorbance spectrum was measured. The difference in the peak absorbance (at a wavelength of 550-570 nm) between the first and second UV-vis spectra was recorded as Ab.

[0100] Separately, FeSCU and rhodamine blue were added to 1 M agueous sulfuric acid in the same concentrations as described above. To this, a radical scavenger (e.g. Ce-M composite) was added to form a second solution. 10 pL of 3% hydrogen peroxide was added to an 8 mL aliguot of the second solution. The solution was mixed for 5 minutes and a further UV-vis absorbance spectrum was measured. The difference in the peak absorbance (at a wavelength of 550-570 nm) between the first and the further UV-vis spectra was recorded as Aa.

[0101] The recovery factor (R) is a measure of radical scavenging activity. The recovery factor (R) can be calculated as: R = 1 - (Aa / Ab). A higher recovery factor corresponds to a higher radical scavenging activity. When employing a radical scavenger to help mitigate against radical degradation of an ion-conducting membrane during fuel cell and electrolyser operation, it is generally desirable for the radical scavenger to have a higher radical scavenging activity. General method of preparing an ion-conducting membrane

[0102] A sol (e.g. prepared as described above) was added to a dispersion of an ion-conducting polymer (800EW from 3M). The mixture was stirred for 10 minutes at high shear to form a well-dispersed ion-conducting membrane dispersion. The particles forming the sol were added in an amount of up to about 0.35 wt.% based on the total weight of the ion-conducting polymer.

[0103] The ion-conducting membrane dispersion was deposited onto a skived PTFE sheet using a k-bar coating process to form a substantially uniform wet membrane layer. The wet membrane layer was dried and annealed to form an ion-conducting membrane with a thickness of about 20 pm.

[0104] The loading of cerium used in the ion-conducting membranes to measure the mean average particle size of the radical scavenger was about 1 wt.% with respect to the total weight of the ion-conducting polymer. The mean average particle size was measured using scanning electron microscopy (SEM) by directly measuring the largest particle diameter of at least 200 particles and calculating the arithmetic mean average.

[0105] General method of preparing membrane electrode assembly

[0106] Anode (20 wt.% Pt supported on carbon black with Pt loading of 0.08mgpt / cm2) and cathode (50 wt% Pt / C with a Pt loading of 0.4 mgpt / cm2) catalyst layers were applied to opposite sides of the ion-conducting membrane using a decal transfer process by lamination under heat and pressure to form a catalyst-coated ion-conducting membrane.

[0107] A gas diffusion layer (Sigracet 22BB) was applied to each face of the catalyst-coated ion-conducting membrane to form a complete membrane electrode assembly. The gas diffusion layer used was a carbon fibre paper with a hydrophobic microporous layer containing carbon and PTFE applied to the face in contact with the catalyst coated ion-conducting membrane.

[0108] Cell testing protocol

[0109] Cell testing was conducted on a test stand available from Hydrogenics in a 50 cm2screener cell. The cells were initially conditioned under H2 and air at 80°C, 100% RH and a current density of 500 mA / cm2 for 54 hours prior to cell testing. Then multiple cycles consisting of a linear sweep voltammetry between +0.1 V and +0.5 V (with respect to the anode potential), and OCV holds were performed. Each OCV hold was performed at 90°C and 30% RH for a duration of 25 hours. The [linear sweep voltammetry - OCV hold] cycles were repeated until the voltage was below a certain threshold value. The open circuit voltage (OCV) was measured. Results and discussions

[0110] Table 1 shows properties of some Ce-containing sols prepared using the general method described above.

[0111] Table 1

[0112] The general methods above can be used to make CeMvOx composites at different Ce:M molar ratios. TENOH and HNO3are both suitable as peptising agents for CeMvOx composites. However, TENOH is preferred because, in general, it leads to more stable sols with a lower Z-average particle size when dispersed in water and ethanol. The Z-average particle size is the intensity weighted mean hydrodynamic size of the ensemble collection of particles measured by dynamic light scattering (DLS). The Z-average is derived from a cumulants analysis of the measured correlation curve, wherein a single particle size is assumed and a single exponential fit is applied to the auto-correlation function. The particle size measurements necessary to obtain Z-average particle size of the Ce-containing solids can be obtained by Dynamic Light Scattering Particle Size Analysis using a Malvern Zetasizer Nano available from Malvern Panalytical, UK. On the other hand, the CeZrOx sol formed using TENOH as peptising agent (Comparative Example 1) had a larger Z-average particle size, broad particle distribution, and sedimented over time.

[0113] Table 2 shows properties and the calculated recovery factor (R) at a real Ce content of 20 ppm and 40 ppm for Examples 3-8 and Comparative Examples 2 to 4. Table 2

[0114] Comparative Example 4 demonstrates that the Nb-containing sol without cerium does not exhibit any radical scavenging properties. The CeNbOx composites of Examples 3-8 exhibited a higher recovery factor at real Ce concentrations of 20 ppm and 40 ppm (in the second solution of the Recovery Factor test) compared to the commercially available CeC>2 material, i.e. Comparative Example 3. Moreover, Examples 3-8 also showed comparable or improved recovery factor compared to Comparative Example 2. The composites of Examples 4-8 had a more positive magnetic susceptibility than each of the Comparative Examples. Ce(lll) is paramagnetic and Ce(IV) is diamagnetic. Therefore, magnetic susceptibility measurements can be used as a proxy for amount of Ce(lll) present in the Ce-containing composite materials. Where the Ce-containing composite material includes a metal in a 5+ oxidation state, the magnetic susceptibility is more positive, which is indicative of a greater proportion of the cerium being in the form of Ce(lll) rather than Ce(IV). Without wishing to be bound by any theory or conjecture, it is believed that incorporating a metal in a 5+ oxidation state in the Ce-containing composite material causes a greater proportion of the cerium to exist as Ce(lll), rather than Ce(IV), in order to balance the charges. Again without wishing to be bound by any theory, it is believed that Ce(lll) is more active than Ce(IV) at scavenging hydroxyl radicals, and therefore by increasing the Ce(lll):Ce(IV) ratio in the Ce-containing composite, the radical scavenging activity may be improved (i.e. recovery factor increased).

[0115] Figure 1 shows how the cell potential changed after a series of potential holds at open circuit potential for MEAs made using the materials of Example 5 (MEA 1), Example 6 (MEA 2), and Comparative Example 3 (Comparative MEA 3). As a further comparison, a comparative MEA was made in the same way as described above except that the ionconducting membrane consisted of the ionomer (800EW, 3M) without any radical scavenger additive (i.e. without any Ce-containing additive) (Comparative MEA 4). The results are also summarised in Table 3 below.

[0116] Table 3

[0117] The MEAs prepared using the sol of Examples 5 and 6 (i.e. MEAs 1 and 2) exhibited improved durability compared to the commercially available CeC>2 additive (Comparative MEA 3) and where no additive was used (Comparative MEA 4). Moreover, the MEA prepared using the sols of Examples 5 and 6 (MEA 1 and MEA 2 respectively) had a lower radical scavenger loading compared to Comparative MEA 3 prepared using the commercially available CeC>2 additive (0.10 wt.% and 0.17 wt.% respectively).

Claims

Claims1. A dispersion comprising: a solvent; and a radical scavenger dispersed in the solvent, wherein the radical scavenger comprises particles and each particle comprises cerium, a metal (M) and oxygen, wherein the metal (M) is present in an oxidation state of +5.

2. A dispersion according to claim 1, wherein each particle of the radical scavenger comprises a mixture of an oxide of cerium and an oxide of the metal (M).

3. A dispersion according to claim 1 or 2, wherein the particles of the radical scavenger are amorphous.

4. A dispersion according to any previous claim, wherein the metal (M) is selected from the group consisting of niobium (Nb), tantalum (Ta), bismuth (Bi), antimony (Sb), and vanadium (V).

5. A dispersion according to claim 4, wherein the metal (M) is niobium (Nb) or tantalum (Ta).

6. A dispersion according to any previous claim, wherein the molar ratio of total cerium to metal (M) is in a range of 100:1 to 1 :6.

7. A dispersion according to any previous claim, wherein the radical scavenger has a magnetic susceptibility that is more positive than the magnetic susceptibility of CeC>2.

8. A dispersion according to claim 7, wherein the radical scavenger is paramagnetic.

9. A dispersion according to any previous claim, wherein the solvent comprises water, methanol, ethanol, 1-propanol, iso-propanol, or a combination thereof.

10. A dispersion according to any previous claim further comprising a dispersing agent.

11. A dispersion according to claim 10, wherein the dispersing agent comprises an organic base, preferably the organic base comprises an amine, and more preferably the amine is a tetraalkylammonium hydroxide.

12. A dispersion according to any previous claim further comprising an ion-conducting polymer.

13. A dispersion according to claim 11 , wherein the ion-conducting polymer is a proton conducting polymer.

14. A dispersion according to claim 12 or 13, wherein the radical scavenger is present in an amount in a range from 0.01 wt.% to 20 wt.% based on the total weight of the ionconducting polymer.

15. An ion-conducting membrane for a fuel cell or a water electrolyser, the ionconducting membrane comprising an ion-conducting polymer and a radical scavenger comprising particles, wherein each particle comprises cerium, a metal (M) and oxygen, wherein the metal (M) is present in an oxidation state of +5.

16. An ion-conducting membrane according to claim 15, wherein the particles of the radical scavenger have a mean average particle size of less than 40 pm.

17. A catalyst-coated ion-conducting membrane comprising the ion-conducting membrane of claim 15 or 16, wherein a first catalyst layer is applied to a first side of the ionconducting membrane and a second catalyst layer is applied to a second side of the ionconducting membrane.

18. A membrane electrode assembly comprising the ion-conducting membrane of claim 15 or 16, or the catalyst-coated ion-conducting membrane of claim 17, and further comprising a gas diffusion layer and / or a porous transport layer.

19. A fuel cell or water electrolyser comprising the ion-conducting membrane of claim 15 or 16, the catalyst-coated ion-conducting membrane of claim 17, or the membrane electrode assembly of claim 18.

20. A method of producing a radical scavenger comprising the steps of: providing a solution comprising a cerium compound and metal (M) compound; effecting a precipitation reaction of the cerium compound and the metal (M) compound to form a precipitate comprising cerium, the metal and oxygen, wherein the metal is present in a 5+ oxidation state; andisolating the precipitate.21 . A method of producing a dispersion comprising the steps of: providing a radical scavenger produced by the method according to claim 20; and dispersing the radical scavenger in a solvent.

22. A method according to claim 21 , wherein the step of dispersing the precipitate in the solvent uses a dispersing agent comprising an organic base, preferably the organic base comprises an amine, and more preferably the amine is a tetraalkylammonium hydroxide.

23. A method according to claim 21 or 22 comprising the step of providing an ionconducting polymer so that the dispersion comprises the solvent, the radical scavenger dispersed in the solvent and the ion-conducting polymer.

24. A method of preparing an ion-conducting membrane comprising the steps of: providing a dispersion according to claim 12 or prepared by a method according to claim23; depositing a layer of the dispersion; and heating the layer of the dispersion to form an ion-conducting membrane.

25. Use of particles for preventing radical degradation of an ion-conducting membrane, wherein each particle comprises cerium, a metal (M) and oxygen, wherein the metal (M) is in an oxidation state of +5.

Images