Ceria-zirconia sol

Abstract

The invention provides a composition comprising nanoparticles of ceria-zirconia (CeZrOx) suspended in a liquid. The nanoparticles have a particle size smaller than 10 nm and comprise a solid solution

Description

FIELD OF THE INVENTION THE PRESENT INVENTION relates to a ceria-zirconia sol, a method of preparing it, and applications thereof. BACKGROUND TO THE INVENTION SOLS, which are classified as colloids and are also known as colloidal dispersions, are stable mixtures of nanosized particles dispersed within a liquid. Sols of metal oxides, such as cerium oxide (also known as ceria) find application in various industries, including as scavengers in fuel cells and electroysers, e.g. water electrolysers. Hydrogen peroxide is a by-product that may be produced by reactions that take place in fuel cells and electrolysers. It may have an adverse effect on the operation of such fuel cells and electrolysers, for example by inhibiting catalyst activity, interfering in electrochemical reactions, causing membrane degradation, and contaminating reactants. Cerium oxide is known to be an effective scavenger of hydrogen peroxide, including in the abovementioned applications. There is a continuing need to improve the efficacy of such scavengers. Cerium oxide is also known to form solid solutions with other metal oxides, which solid solutions, as nanosized particles thereof, may be rendered as sols. One such metal oxide, is zirconium oxide (also known is zirconia), with which cerium oxide may form a solid solution of cerium zirconium oxide. Cerium zirconium oxides also find wide industrial application, including as a store of oxygen in motor vehicle exhaust systems and as a carrier for active catalyst materials. CN104591275, CN104492418, CN101200371, JP2007031192, an CN1387943 each discloses aqueous cerium zirconium oxide sols, which comprise nanosized particles consisting of complete solid solutions of cerium zirconium oxide. Also disclosed are methods for the preparation of such sols. Methods for the preparation of the nanosized cerium zirconium oxide particles are also disclosed. Particle sizes range from about 2 to 8, and even up to about 100 nm. JP200906766 discloses a ceria-zirconia solid solution sol that finds application in catalysed conversion of automobile exhaust gases. Ceria-zirconia particles comprised by the sol have an average particle size of 100nm or less, typically between 5 and 100nm. The particles are produced by treating a slurry of a water insoluble solid zirconium compound and an aqueous solution of a water-soluble cerium compound with a base. K.Tanimoto et al, Bull, Chern. Soc. Jpn., (2013), 86, 1210-1215 discloses nanometric colloidal sols, including sols comprising ceria-zirconia particles of particle size 6-8nm agglomerated into aggregates larger than 40nm, also for application in catalytic automotive exhaust gas conversion. The present invention provides new cerium zirconium oxide sols and their preparation, and extends to applications of such sols. SUMMARY OF THE INVENTION ACCORDING TO A FIRST ASPECT OF THE INVENTION, there is provided a composition comprising nanoparticles of ceria-zirconia (CeZrOx), comprising Ce, Zr and O in a stoichiometric ratio (CeyZrz)Ox, suspended in a liquid, wherein -the nanoparticles have a particle size smaller than 10 nm; the nanoparticles comprise a solid solution of Ce, Zr and O in a solid solution crystal lattice; the mixing of Ce and Zr is incomplete such that not all of the Zr in the nanoparticles is present in the solid solution crystal lattice; and the composition additionally comprises stabiliser ions which are anions of a strong mineral acid or strong organic acid. The composition may be described as a colloid, specifically as a colloidal dispersion or sol, e.g., a CeZrOx sol. The term CeZrOx indicates that the sol comprises particles of ceriazirconia, i.e. comprising Ce, Zrand O. In the context of the present invention, these particles are individual nanoparticles comprising Ce, Zr and O in a solid solution crystal lattice in a stoichiometric ratio (CeyZrz)Ox, of a particle size smaller than 10 nm, and of which the mixing of Ce and Zr is incomplete such that not all of the Zr in the nanoparticles is present in the solid solution crystal lattice. A colloid, and more specifically a colloidal dispersion, is a mixture having a dispersed phase (the suspended nanoparticles) and a continuous phase (the liquid, which is acting as a medium for suspension of the particles). Without being bound by theory, the inventors propose that a composition, and specifically a composition in the form of a colloidal dispersion, of CeZrOx nanoparticles as characterised herein, i.e. comprising Ce, Zr and O in a stoichiometric ratio (CeyZrz)Ox of a particle size smaller than 10 nm and of which the mixing of Ce and Zr is incomplete, exhibits improved properties in acting as a scavenger than existing scavengers, particularly in fuel cells and electrolysers, most prominently in promoting decomposition of hydrogen peroxide and specifically when used as additive to fuel cell and electrolyser membranes. The inventors have found that existing scavengers, specifically similar compositions to that of the invention but comprising only ceria, have properties inferior to that of a colloidal CeZrOx nanoparticle composition according to the invention when used as scavenger additives to fuel cell and electrolyser membranes, including in respect of membrane durability. The composition of the invention may therefore have various useful applications. In particular, the composition of the invention may find application in the preparation of components for use in a fuel cell or electrolyser. Such components may, for example, include ion-conducting membranes, catalyst coated ion-conducting membranes (CCMs), membrane electrode assemblies (MEAs), etc. ACCORDING TO A SECOND ASPECT OF THE INVENTION, there is provided a process for producing a composition according to the first aspect of the invention, the process including - treating particulate CeZrOx, comprising nanoparticles of Ce, Zr and O in a stoichiometric ratio (CeyZrz)Ox of a particle size smaller than 10 nm in which the mixing of Ce and Zr is incomplete such that not all of the Zr in the nanoparticles is present in the solid solution crystal lattice, with an acid, wherein the acid comprises a strong mineral acid or strong organic acid. More particularly, treating the particulate CeZrOx with the nitrate acid may comprise peptising the particulate CeZrOx. The nanoparticles may typically subsist as agglomerates or clusters thereof in the particulate CeZrOx prior to the treatment with the acid, wherein treatment with the acid results in break-up of the agglomerates or clusters to provide a colloidal dispersion of individual nanoparticles. Treating I peptising the particulate CeZrOx may produce a CeZrOx sol, according to the first aspect of the invention, i.e. comprising CeZrOx nanoparticles having a particle size smaller than 10 nm. The method may include a prior step of producing the particulate CeZrOx, for example by precipitating the particulate CeZrOx using an aqueous solution of soluble salts thereof, such as an aqueous solution of nitrates thereof, and a suitable precipitant, typically a base, for example ammonia, as hereinafter described in more detail. The method may also include a subsequent step of combining the peptized CeZrOx with a dispersant. The dispersant would typically be water. Alternatively, or additionally, the dispersant may be an organic solvent, e.g. an alcohol such as methanol, ethanol or propanol. ACCORDING TO A THIRD ASPECT OF THE INVENTION, there is provided an ink comprising the composition of the first aspect of the invention. The ink may, additionally, comprise an ion-conducting polymer and / or an electrocatalyst. THE THIRD ASPECT OF THE INVENTION EXTENDS to a process of producing an ink, which comprises mixing the composition of the first aspect of the invention with an ionconducting polymer and / or an electrocatalyst. ACCORDING TO A FOURTH ASPECT OF THE INVENTION, there is provided a process for producing an ion-conducting membrane comprising CeZrOx, or more particularly a CeZrOx-containing membrane layer, the process comprising - applying the composition of the first aspect of the invention to an ion-conducting membrane; or fabricating a / the membrane layer using the ink of the third aspect of the invention, wherein the ink comprises an ion-conducting polymer. When using the ink of the third aspect of the invention, the process may include a prior step of producing the ink. Fabricating the membrane layer using the ink may comprise depositing the ink onto a substrate. Such a substrate may, for example, be a backing sheet, or an ion-conducting polymer layer or a catalyst layer on a backing sheet. It will be appreciated that, in accordance with the third aspect of the invention, the ink may optionally also comprise an electrocatalyst. It will also be appreciated that the membrane layer would typically be fabricated of the ink, such that it essentially consists of the ink. THE FOURTH ASPECT OF THE INVENTION EXTENDS to an ion-conducting membrane or membrane layer comprising CeZrOx, produced according to the method of the fourth aspect of the invention. The ion-conducting membrane may be a membrane for an electrochemical device, such as a fuel cell or a water electrolyser. ACCORDING TO A FIFTH ASPECT OF THE INVENTION, there is provided a process for producing a catalyst layer comprising CeZrOx, or more particularly a CeZrOx catalyst layer, the process comprising - fabricating a membrane layer using the ink of the third aspect of the invention, wherein the ink comprises an electrocatalyst. The process may include a prior step of producing the ink. Fabricating the catalyst layer using the ink may comprise depositing the ink onto a substrate. Such a substrate may, for example, be a backing sheet, or an ion-conducting polymer layer (to provide a catalyst-coated ion-conducting membrane), or a gas diffusion layer (to provide a gas diffusion electrode). It will be appreciated that, in accordance with the third aspect of the invention, the ink may optionally also comprise an ion-conducting polymer. It will also be appreciated that the catalyst layer would typically be fabricated of the ink, such that it essentially consists of the ink. THE FIFTH ASPECT OF THE INVENTION EXTENDS to a catalyst layer comprising CeZrOx, produced according to the method of the fifth aspect of the invention. The catalyst layer may be a catalyst layer for an electrochemical device, such as a fuel cell or a water electrolyser, more particularly of a membrane, such as a catalyst coated ionconducting membrane, of an electrochemical device. Alternatively, or additionally, the catalyst layer may be a catalyst layer of a gas diffusion electrode comprising, in addition to the catalyst layer, a gas diffusion layer, the gas diffusion electrode optionally being a gas diffusion electrode of a catalyst coated ion-conducting membrane. THE FIFTH ASPECT OF THE INVENTION ALSO EXTENDS, separately, to-a catalyst coated ion-conducting membrane, as hereinbefore described; and a gas diffusion electrode, as hereinbefore described. ACCORDING TO A SIXTH ASPECT OF THE INVENTION, there is provided a membrane electrode assembly comprising - an ion-conducting membrane; a catalyst layer; and a gas diffusion layer, wherein the ion-conducting membrane is an ion-conducting membrane according to the fourth aspect of the invention; and / or the catalyst layer is a catalyst layer according to the fifth aspect of the invention, applied to the ion-conducting membrane to provide a catalyst-coated ion-conducting membrane; and / or the catalyst layer is a catalyst layer according to the fifth aspect of the invention, applied to the gas diffusion layer to provide a gas diffusion electrode. Nanoparticles The nanoparticles of Ce, Zr and O comprise a solid solution crystal lattice of Ce, Zr and O, and some Zr that is not present in the solid solution crystal lattice. Therefore, the mixing of Ce and Zr is such that less than 100% of the Zr in the nanoparticles is present in the Ce, Zr, O solid solution crystal lattice, typically as measured by lattice parameter determined using x-ray diffraction analysis. For example, less than 90%, or less than 80%, or less than 70%, e.g. up to a maximum of 62%, of the Zr in the nanoparticles is present in the Ce, Zr, O solid solution crystal lattice. In (CeyZrz)Ox, the ratio of y:z may typically be at most 9:1, 6:1, 3:1 or 2:1. The ratio of y:z may typically be at least 1:3, suitably 1:2. The ratio of y:z is preferably in the range of and including 3:1 to 1:3, for example 2:1 to 1:2. The ratio may be 1:1. The value of “x” may typically be about 2, e.g. x may be 2, which is relative to the total of the Ce and Zr. The nanoparticles are solid, meaning that, in the composition, they are suspended in the liquid, not dissolved in it. The particle size of the nanoparticles is smaller than 10 nm, suitably smaller than 9 nm, more suitably smaller than 8 nm, for example smaller than 6 nm. This means that greater than 85% of the particles have a modal particle size (diameter) smaller than the stated value. More specifically, the modal particle size (diameter) for particles in peak 1 is smaller than the stated value. Peak 1 is the peak of greatest area, typically representing at least 85% of the nanoparticles, as determined using Dynamic Light Scattering Particle Size Analysis (DLS), preferably using a Malvern Zetasizer, i.e. DLS peak 1. Liquid (suspension medium) Typically, the liquid would comprise water. Thus, the liquid, of the composition, may comprise an aqueous solution of the strong mineral or strong organic acid. The strong mineral or strong organic acid may be an acid with a pKa below 2, below 1, or below 0. The strong mineral acid may, for example, be a nitrate acid, preferably nitric acid (HNO3), as also discussed below. Thus, the stabiliser anions may be nitrate anions. In a case in which the acid is a nitrate acid, the molar ratio of nitrate anions to the sum of Ce and Zr, including in the composition, may be less than 1, more preferably in a range of from 0.4 to less than 1, e.g. about 0.6. The strong organic acid may, for example, be triflic acid (CF3SO3H) or trifluoroacetic acid (CF3CO2H). Alternatively, or in addition, the liquid may comprise 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. In one embodiment the liquid 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 liquid may comprise from 40 to 60wt% water and from 60 to 40wt% organic solvent. The composition of the invention (CeZrOx sol) The composition comprises CeZrOx nanoparticles (nanoparticles comprising or consisting of CeZrOx), as characterised herein, that are suspended, as individual nanoparticles, in a liquid together with the stabilizer ions (anions of a nitrate acid). The composition may have a total solids content of up to 700 g / L. The composition may have a pH of less than 2, or less than 1. Producing the composition of the invention The composition of the first aspect of the invention may be produced by treating particulate CeZrOx comprising nanoparticles of Ce, Zr and O in a stoichiometric ratio (CeyZrz)Ox in which the mixing of Ce and Zr is incomplete such that not all of the Zr in the nanoparticles is present in the solid solution crystal lattice, typically as a precipitate thereof, with a strong mineral or strong organic acid, for example a strong nitrate acid, preferably nitric acid (HNO3). More specifically, producing the composition of the first aspect of the invention may comprise peptising a CeZrOx wet cake, optionally a dried CeZrOx wet cake, using a strong mineral or strong organic acid, for example a strong nitrate acid, preferably HNO3. “Peptise” means to disperse a substance into a colloidal state. The CeZrOx wet cake may be prepared by co-precipitation from a solution, typically an aqueous solution, of Ce and Zr salts. Such co-precipitation may be performed by reacting the aqueous solution with a base. The Ce salt may be a Ce (III) or a Ce (IV) salt. Typically, the Ce salt would be a cerium nitrate, referring in this regard to the family of nitrates of cerium in the +3 or +4 oxidation state. The Ce salt may, for example, be ceric ammonium nitrate (NH4)2Ce(NO3)6 or cerium (III) nitrate (Ce(NO3)3). When using a Ce (III) salt, such as Ce(NO3)3, an oxidising agent may also be used when reacting it with the base, for Ce (III) to be oxidised to Ce (IV) to obtain CeOz. Such an oxidising agent may, for example, be hydrogen peroxide (H2O2). When using an oxidising agent, the oxidising agent may either be provided in admixture with the solution comprising the Ce salt or in admixture with the base. The Zr salt may be a zirconium nitrate. The Zr salt may, for example, be zirconyl nitrate (ZrO(NO3)z). The base may, for example, be ammonia (NH3), more specifically an aqueous solution of NH3 (NH3 (aq)), e.g. in a concentration in a range of from 1 to 4 M. The solution of Ce and Zr salts may be suitably prepared and added to the base, thereby providing a precipitate of CeZrOx. The addition of the solution of Ce and Zr salts to the base may be a slow, drop-wise addition, after which a period of precipitation may be observed. The precipitate may be suitably filtered and washed to remove any remaining salts. The filtered and washed precipitate, as a wet cake, may be used as is to produce the composition of the invention by subjecting it to the 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. The wet cake may have a solids content by weight of up to about 30 wt% or up to about 20 wt%. 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%. Applications As indicated above, the nanoparticles of ceria and zirconia, comprised in the composition of the invention, may be employed to promote the decomposition of hydrogen peroxide (H2O2), typically in electrochemical systems such as fuel cells and electroysers. As also indicated above, the CeZrOx sol of the invention can be mixed with an ionconducting polymer and / or an electrocatalyst to form an ink that may be cast to form an ionconducting membrane / catalyst layer, typically for use in electrochemical systems as referenced above. Electroysers Electrolysers that employ a solid proton-conducting polymer electrolyte membrane, or proton exchange membrane (PEM), are known as proton exchange membrane water electrolysers (PEMWEs). Electrolysers that utilize a solid anion-conducting polymer electrolyte membrane, or anion exchange membrane (AEM), are known as anion exchange membrane water electrolysers (AEMWEs). The electrolysis of water to produce high purity hydrogen and oxygen can be carried out in both alkaline and acidic systems. Fuel cells A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. In fuel cells, 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 proton exchange membrane fuel cells (PEMFC) the membrane is proton conducting, and protons, produced at the anode, are transported across the membrane to the cathode, where they combine with oxygen to form water. Membrane electrode assemblies A principal component of the PEMFC or PEMWE is the membrane electrode assembly (MEA), which is essentially composed of five layers. The central layer of the MEA is the polymeric ion-conducting membrane. On either side of the ion-conducting membrane there is an electrocatalyst layer, containing an electrocatalyst material designed for the specific electrolytic reaction. Finally, adjacent to each electrocatalyst layer, there is a gas diffusion layer. The gas diffusion layer must allow the reactants to reach the electrocatalyst layer and must conduct the electric current that is generated by the electrochemical reactions. Therefore, the gas diffusion layer must be porous and electrically conducting. The electrocatalyst layers generally comprise an electrocatalyst material comprising a metal or metal alloy suitable for the fuel oxidation or oxygen reduction reaction, depending on whether the layer is to be used at the anode or cathode. The electrocatalyst is typically based on platinum or platinum alloyed with one or more other metals. The platinum or platinum alloy catalyst can be in the form of unsupported nanoparticles (such as metal blacks or other unsupported particulate metal powders) but more conventionally the platinum or platinum alloy is deposited as higher surface are nanoparticles onto a high surface area conducting carbon material, such as a carbon black or heat-treated versions thereof. The electrocatalyst layers also generally comprise a proton conducting material, such as a proton conducting polymer, to aid transfer of protons from the anode catalyst to the membrane and / or from the membrane to the cathode catalyst. Conventionally, the MEA can be constructed by a number of methods, as outlined below: (i) The electrocatalyst layer may be applied to the gas diffusion layer to form a gas diffusion electrode. A gas diffusion electrode is placed on each side of an ionconducting membrane and laminated together to form the five-layer MEA; (ii) The electrocatalyst layer may be applied to both faces of the ion-conducting membrane to form a catalyst coated ion-conducting membrane (CCM). Subsequently, a gas diffusion layer is applied to each face of the CCM. (iii) An MEA can be formed from an ion-conducting membrane coated on one side with an electrocatalyst layer, a gas diffusion layer adjacent to that electrocatalyst layer, and a gas diffusion electrode on the other side of the ion-conducting membrane. Typically, tens or hundreds of MEAs are required to provide enough power for most applications, so multiple MEAs are assembled to make up a fuel cell stack. Field flow plates are used to separate the MEAs. The plates perform several functions: supplying the reactants to the MEAs; removing products; providing electrical connections; and providing physical support. In accordance with the aspects of the invention characterised above, the CeZrOx sol of the invention may be employed in the preparation of an ion-conducting membrane; a catalyst coated membrane (CCM); a catalyst layer; a membrane electrode assembly (MEA); or a gas diffusion electrode. Ink In accordance with the invention, the CeZrOx sol may be combined with an ion-conducting polymer to form an ink and the ink may be cast to form 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. Examples of suitable protonconducting polymers include perfluorosulphonic acid ionomers (e.g. Nation® (E.l DuPont de Nemours and Co.), Aciplex® (Asahi Kasei), Aquivion™ (Solvay Speciality Polymers), Flemion® (Asahi Glass Co.), or ionomers based on a sulphonated hydrocarbon such as those available from Fu-MA-Tech GmbH as the fumapem® P, E or K series or products (JSR Corporation, Toyobo Corporation, and others). Examples of suitable anion- conducting polymers include A901 and A201 made by Tokuyama Corporation, Fumasep FAA from FuMA-Tech GmbH, and Ameion polymers from lonomr. The CeZrOx sol of the invention can be combined with an electrocatalyst to form an ink (a catalyst ink) and the ink can be cast to form a catalyst layer. In one embodiment, the catalyst ink comprises an electrocatalyst, CeZrOx nanoparticles as characterised herein, a dispersant, and an acid a defined herein. The catalyst ink may additionally comprise ion conducting polymer. The dispersant may be the same as or different from the liquid employed to suspend the nanoparticles in the CeZrOx sol. The dispersant may, for example, comprise water and / or an organic solvent (e.g., an alcohol such as methanol, ethanol or propanol). The dispersant preferably comprises an organic solvent. Electrocatalyst In accordance with the invention, the CeZrOx sol may be combined with an electrocatalyst to form an ink and the ink may be cast to form a catalyst layer. The electrocatalyst may comprise any metal or metal alloy known to have activity for an electrochemical reaction, such as a hydrogen oxidation reaction, oxygen reduction reaction, oxygen evolution reaction etc. The metal or metal alloy comprises a primary metal suitably selected from: (i) The platinum group metals (platinum, palladium, rhodium, ruthenium, iridium and osmium), (ii) Gold or silver, (iii) A base metal (e.g., tin, lead, zinc, copper). The primary metal may be alloyed or mixed with one or more different metals selected from the above i.e. (i) the platinum group metals, (ii) gold or silver (iii) and a base metal or their oxides. A base metal is tin or a transition metal which is not a precious metal. A precious metal is a platinum group metal (platinum, palladium, rhodium, ruthenium, iridium or osmium) or gold or silver. Suitable base metals are copper, cobalt, nickel, zinc, iron, titanium, molybdenum, vanadium, manganese, niobium, tantalum, chromium and tin. Preferred base metals in the electrocatalyst include nickel, copper, cobalt, and chromium. More preferred base metals are nickel, cobalt and copper. The primary metal is preferably platinum, which may be alloyed with other precious metals, such as ruthenium, or one or more base metals. The loading of the primary metal in the electrocatalyst material is suitably 10-70 wt%, more suitably 20-50 wt%, more suitably 20-30 wt% based on the total weight of the electrocatalyst material (metal I metal alloy + support). The resulting membrane or catalyst layer may comprise nitrates of the nitrate acid as defined above, or traces thereof. The catalyst layer may be prepared by a number of methods known to those skilled in the art, for example by preparation of an ink and applying the ink to a membrane, gas diffusion layer or transfer substrate by standard methods such as gravure coating, slot die (slot, extrusion) coating (whereby the coating is squeezed out under pressure via a slot onto the substrate), screen printing, rotary screen printing, inkjet printing, spraying, painting, bar coating, pad coating, gap coating techniques such as knife or doctor blade over roll (whereby the coating is applied to the substrate then passes though a split between the knife and a support roller), and metering rod application such as with a Meyer bar. The thickness of the catalyst layer and loading of primary metal in the catalyst layer will depend on whether the catalyst layer is for use at the anode or cathode. If for use at the anode: the catalyst layer is suitably >1 pm; more suitably >2 pm in thickness; preferably >5 pm; the catalyst layer is suitably <15 pm; more suitably <10 pm in thickness; the loading of primary metal is suitably <0.3 mg / cm2; suitably <0.2mg / cm2; more suitably <0.15 mg / cm2; the loading of primary metal is suitably >0.02 mg / cm2. If for use at the cathode: the catalyst layer is suitably >2 pm; more suitably >5 pm in thickness; the catalyst layer is suitably <20 pm; more suitably <15 pm in thickness; the loading of primary metal in the catalyst layer is <0.4 mg / cm2; the loading of primary metal in the catalyst layer is >0.05 mg / cm2. The catalyst layer may also comprise additional components. Such components include, but are not limited to, a hydrophobic (a polymer such as PTFE or an inorganic solid with or without surface treatment) or a hydrophilic (a polymer or an inorganic solid, such as an oxide) additive to control water transport. In addition, the catalyst layer may also comprise a further catalytic material, which may or may not have the same function as the electrocatalyst material of the invention. Gas diffusion electrode The invention further provides a gas diffusion electrode comprising a gas diffusion layer (GDL) and a catalyst layer as described above. The catalyst layer is typically applied directly onto the surface of the GDL. The GDLs are suitably based on conventional non-woven carbon fibre gas diffusion substrates such as rigid sheet carbon fibre papers (e.g. the TPG-H series of carbon fibre papers available form Toray Industries Inc., Japan) or roll-good carbon fibre papers (e.g. the H2315 based series available from Freudenberg FCCT KG, Germany; the Sigracet® series available from SGL Technologies GmbH, Germany; the AvCarb® series available from Ballard Material Products, United States of America; or the NOS series available from CeTech Co., Ltd. Taiwan), or on woven carbon fibre cloth substrates (e.g. the SCCG series of carbon cloths available from the SAATI Group, S.p.A., Italy; or the WOS series available from CeTech Co., Ltd, Taiwan). For many PEMFC (including direct methanol fuel cell (DMFC)) applications the non-woven carbon fibre paper, or woven carbon fibre cloth substrates are typically modified with a hydrophobic polymer treatment and / or application of a microporous layer comprising particulate material either embedded within the substrate or coated onto the planar faces, or a combination of both to form the gas diffusion layer. The particular material is typically a mixture of carbon black and a polymer such as polytetrafluoroethylene (PTFE). Suitably the GDLs are between 100 and 400um thick. Preferably there is a layer of particulate material such as carbon black or PTFE on the face of the GDL that contacts the catalyst layer. Catalyst coated ion-conducting membrane The invention further provides a catalyst coated ion-conducting membrane for a PEMFC / PEMWE comprising a membrane and a catalyst layer as defined above. The membrane may be any membrane suitable for use in a PEMFC, for example the membrane may be based on a perfluorinated sulphonic acid material such as Nafion®(DuPont), Aquivion® (Solvay Plastics), Flemiun® (Asahi Glass) and Aciplex® (AsahiKasei); these membranes may be used unmodified, or may be modified to improve the high temperature performance, for example by incorporating an additive. Alternatively, the membrane may be based on a sulphonated hydrocarbon membrane such as those available from FuMA-Tech GmbH as the fumapem® P, E or K series of products, JSR Corporation, Toyobo Corporation, and others. The membrane may be a composite membrane, containing the proton-conducting material and other materials that confer properties such as mechanical strength. For example, the membrane may comprise an expanded PTFE substrate. Alternatively, the membrane may be based on polybenzimidazole doped with phosphoric acid and include membranes from developers. BRIEF DESCRIPTION OF THE DRAWINGS EMBODIMENTS OF THE PRESENT INVENTION will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a plot of k2 / K, which correlates with effectiveness as a radical scavenger, for a number of CeZrOx sols. Figure 2 shows a plot of the degree of mixing of Ce and Zr in the CeZrOx precipitates prepared according to the invention; and Figures 3 and 4 show high resolution transmission electron microscope images at <5 nm scale of CeZrOx nanoparticles according to the invention. DESCRIPTION OF EMBODIMENTS EIGHT COMPOSITIONS ACCORDING TO THE INVENTION were prepared and subjected to characterisation and performance analysis. Preparation of compositions Compositions according to the invention were prepared, broadly speaking, by coprecipitation of a CeZrOx wet cake using soluble Ce and Zr nitrates (1:1 as an aqueous salt solution) and aqueous ammonia. The wet cake was optionally subjected to drying to produce a dried wet cake. The wet cake, optionally having been subjected to drying, was then peptised using nitric acid (HNO3) at a molar ratio of NOa:(Zr+Cr) of less than 1, thereby to produce a composition according to the invention, i.e. a CeZrOx sol. More specifically, as soluble nitrates, cerium (III) nitrate (Ce(NO3)3), ceric ammonium nitrate ((NH4)2Ce(NO3)e), and zirconium oxynitrate (ZrO(NOs)2) were used. Aqueous ammonia (NH3) was used as the precipitant. When Ce(NOs)3 was used as the soluble nitrate of cerium, hydrogen peroxide (H2O2) was used as an oxidising agent, in separate cases being provided respectively in solution with the nitrates and with the aqueous NH3. In the case of each composition that was prepared, an aqueous solution of the Ce and Zr nitrates was prepared and was added to the aqueous NH3 at ambient temperature. This caused a precipitate for form, which was then filtered or decant washed to provide a wet cake. The wet cake was washed to <30 pS of the filter solution. The resulting washed cake was then divided between a portion that was retained and a portion that was ambiently dried for about 4 to 5 days. Thus, the filter cakes set out in table 1 were obtained: Table 1: Filter cake production Cake Salts (jointly in aqueous solution) Oxidiser Solids content C Ce(NO3)3 + ZrO(NO3)2 H2O2 in salt solution Wet cake 19.3 wt% D Dried wet cake 71.0 wt% E Ce(NO3)3 + ZrO(NO3)2 H2O2 in aqueous ammonia Wet cake 20.7 wt% F Dried wet cake 41.0 wt% G (NH4)2Ce(NO3)6 + ZrO(NO3)2 None Wet cake 27.5 wt% H Dried wet cake 49.0 wt% 1 Ce(NO3)3 + ZrO(NO3)2 None Wet cake 23.1 wt% J Dried wet cake 42.3 wt% In subsequently peptising the wet cakes and dried wet cakes, each wet cake and dried wet cake, respectively, was mixed with concentrated HNO3 at a molar ratio of NO3:(Zr+Cr) of 0.6 using a Speedymixer to produce a slurry. The slurry was heated to mild reflux with stirring for two hours and was then allowed to cool to produce a concentrated sol. The cooled slurry was then diluted to a solids concentration of about 285 g / l (or 10g oxide in 35 ml) to produce a diluted sol. The specific solids contents of the respective sols, as made from the indicated filter cakes, were as shown in Table 2. 5 Table 2: Sols and diluted sols solids concentration Filter cake used to produce sol C D E F G H I J Est. initial sol cone / g I’1 * 734 * 476 310 708 264 595 Final sol conc / g 1*1 221 262 231 245 266 263 260 238 * Volume of initial sol >35 ml To illustrate lattice parameter and degree of mixing of Ce and Zr (see Table 5), filter cakes, and sols based thereon, were produced using the same method as described above, as set 10 out in Table 3. Table 3: Comparative sols Ceria / Zirconia molar % Salts (jointly in aqueous solution) 160% Ceria Wet Cake CeO2 75% Ceria Wet Cake 75% Ceria Dried Wet Cake (NH4)2Ce(NO3)6 + ZrO(NO3)2 69% Ceria Wet Cake 69% Ceria Dried Wet Cake (NH4)2Ce(NO3)6 + ZrO(NO3)2 25% Ceria Wet Cake 25% Ceria Dried Wet Cake (NH4)2Ce(NO3)6 + ZrO(NO3)2 15 Characterisation of sols Each of the sols produced from filter cakes C to J were characterised by Dynamic Light Scattering Particle Size Analysis (DLS) using a Malvern Zetasizer. The DLS z-average peak distribution, and the modal particle size (diameter) of DLS peaks 1 and 2 are provided in Table 4, below. Table 3: Sol characterisation by DLS and MALDI analysis Sol-> C D E | F G H Illi1 J DLS z-avg / nm 54 6.3 11.3 8.8 5.2 15.7 8.7 8.8 DLS peak 1 (%) / nm 7.7 (97) 6.0 (88) 14 (99) 11 (100) 5.0 (86) 3.6 (96) 9.8 (100) 8.9 (100) DLS peak 2 (%) / nm 1800 (2) 108 (8) 5030 (2) 374 (10) 5700 (3) DLS peak 1 is most informative and representative of the sample, providing the size distribution of the majority of the particles in the sample (greater than 85% of the particles in the present case), and correlates with radical scavenging activity data which is discussed, below, with reference to Figure 1. Scavenging activity Radical scavenging activity of the CeZrOx nanoparticles comprised by the CeZrOx sol of the invention was assessed by performing an ex situ additive screening “Fenton” test compared to ceria, the results of which are shown in Figure 1 in which C to I correlate with samples C to I referenced in tables 1 and 3, above. For the Fenton test 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 etal: 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: FeSO4 2H2O2---> HO ■ +H00 ■ +H2O 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, Agilent’s Cary 5000 UV-vis-NIR was used utilising 1 cm QS cuvettes. The abs peak at 554 nm is followed. To mimic the low pH environment created in a fuel cell from the PFSA ionomer, our tests were done in 1M H2SO4. Each test was conducted at the following concentrations of each chemical: • 14 ppm RhB • 35 ppm Fe2+ • 10 microlitres of 3% H2O2 were added and left to mix for 10 minutes before measuring the UV-vis spectra During this test there are two competing reactions, the oxidation of rhodamine B (RhD), and the free-radical scavenging of the additive (FRS), with rate constants ki and fe respectively: (1) RhB +■ 0H-*oxRhB fc2 (2) FRS+- OH^oxFRS In the absence of any additive, only reaction 1 occurs, and the addition of H2O2 results in a drop Ab in the UV-vis peak height at 554 nm. In the presence of a radical scavenging additive, both reactions 1 and 2 occur, resulting in a smaller drop La in the UV-vis peak height at 554 nm. A ‘recovery’ factor, R can be calculated via equation 3 for each additive at a given additive concentration: (3) R = 1 In order to compare different additives this test was repeated at >3 additive concentrations in order to plot 1 / R vs 1 / [FRS]. Following from the derivation found in [J. Agri. Food Chem., 2008, 56, 730-735], where the assumption is made that the experimental parameters of time, temperature, rate constant ki, and concentration of rhodamine B and H2O2 are constants incorporated into a constant K, via equation 4: (4) - = —— + 1 v ’ R k2 [FRS] The normalised rate constant kz / K of different additives is found from 1 / gradient of the 1 / R vs 1 / [FRS] plot, i.e. for an effective radical scavenger, kz » ki which is seen in the UV-vis when La « Lb. The larger the value of k2 / K, the more effective the radical scavenging ability of the additive. As will be seen, sols G and H show the highest response in the screening test, compared to ceria, and have the lowest particle size, i.e. the modal particle size (diameter) of DLS peak 1, representing the bulk (i.e. greater than 85%) of the particles - see Table 3. Sols F (and therefore E) and I (and therefore J) show poorer activity than ceria. In the context of the CeZrOx nanoparticles of the invention, scavenging activity therefore correlates with particle size and a particle size less than about 9nm, i.e. the modal particle size (diameter) of DLS peak 1, representing the bulk (i.e. greater than 85%) of the particles, correlates with best activity. The optimal activity correlates with a particle size smaller than about 6 nm. Ce and Zr mixing The CeZrOx particles of the invention, of dried wet cakes as described hereinbefore, were subjected to X-ray diffraction (XRD) measurement to determine the degree of crystallinity and the degree of Ce and Zr mixing using a determination of lattice parameter. The results are shown in Table 4, below. Table 4: CeZrOx particle characterisation by XRD Sample Norn. Ce / % Calc Ce / % Cryst Size / nm 100% ceria (see Table 3) 75% ceria dried wet cake (see Table 3) 100 5.4214(9) 102 <2 75 5.3790(12) 87 <2 69% ceria dried wet cake (see Table 3) 69 5.3606(11) 81 <2 H 50 5.3276(18) 69 <2 J 50 5.3701(6) 84 2.5 D 50 5.3491(15) 76 <2 22AC3738B * 50 5.3778(8) 87 <2 25% ceria dried wet cake (see Table 3) 25 - - < 1 * This sample has a different Ce : Zr ratio (not 1:1) and was prepared in the same manner as Sample F as a comparative example. The samples were all found to be, generally, near-amorphous, with crystallite sizes <2 nm. Lattice parameters were obtainable and %Ce content calculated using Vegard’s law. A lattice parameter “a” of less than 5.4214 implies there is some Zr in the ceria lattice i.e. there is incomplete mixing of ceria and zirconia to form a solid solution. With reference also to Figure 2, the lattice parameter is not small enough to constitute complete Ce and Zr mixing in a solid solution, based on the reference lattice parameters for Ce / Zr extrapolated using Vegard's law. It follows that the data in Table 4 simply demonstrates incomplete mixing. With reference to Figure 2, the degree of incorporation of Zr in the Ce, Zr, O solid solution lattice was assessed using the XRD lattice parameters of the dried precipitates and comparing them to ideal mixing parameters determined using reference values (from XRD Powder Diffraction File (PDF) reference values) and extrapolation using Vegard's law. Figure 2 shows the lattice parameter values for solid solutions of Ce, Zr and O as a linear plot with Ce concentration (blue dotted line correlation, using Vegard’s law) and the lattice parameters for the dried precipitates prepared according to the present method. It is clear from the fact that the lattice parameters are larger for a particular Ce concentration that there is not complete incorporation of Zr in the Ce, Zr, O solid solution crystal lattice. With reference to Figures 3 and 4, Ce-rich areas of the particles tested can clearly be seen, providing further evidence of incomplete mixing of Ce and Zr. Discussion 5 The invention as described has been shown, with reference to the examples, to provide a CeZrOx sol comprising CeZrOx nanoparticles of scavenging activity improved over that of ceria alone, which particles have a particle size smaller than 9 nm, exhibit incomplete mixing of Ce and Zr, and are stabilised by nitrate acid. 10

Claims

1. A composition comprising nanoparticles of ceria-zirconia (CeZrOx),comprising Ce, Zr and O in a stoichiometric ratio (CeyZrz)Ox, suspended in a liquid, whereinthe nanoparticles have a particle size smaller than 10 nm;the nanoparticles comprise a solid solution of Ce, Zr and O in a solid solution crystal lattice;the mixing of Ce and Zr is incomplete such that not all of the Zr in the nanoparticles is present in the solid solution crystal lattice; andthe composition additionally comprises stabiliser ions which are anions of a strong mineral acid or strong organic acid.

2. The composition of claim 1, wherein, in (CeyZrz)Ox, the ratio of y:z is at most9:1,6:1, 3:1 or 2:1 and at least 1:3 or 1:2, more preferably is in the range of 3:1 to 1:3, morepreferably is in the range of 2:1 to 1:2, more preferably is 1:

13. The composition of claim 1 or claim 2, wherein the mixing of Ce and Zr issuch that less than 100% of the Zr in the nanoparticles is present in the Ce, Zr, and O solidsolution crystal lattice, as measured by lattice parameter, more preferably less than 90%, or less than 80%, or less than 70%, e.g. up to a maximum of 62%.

4. The composition of any one of claims 1 to 3, wherein the acid is a strongmineral acid, the strong mineral acid is a nitrate acid, preferably being nitric acid HNO3, and the anions are anions of a nitrate acid, preferably being nitrate anions.

5. The composition of claim 4, wherein the anions are nitrate anions and themolar ratio of nitrate anions to the sum of Ce and Zr is less than 1, more preferably in a range of from 0.4 to less than 1, more preferably being 0.6.

6. The composition of any one of claims 1 to 5, wherein the liquid is water,comprising the anions of the strong mineral acid or strong organic acid.

7. The composition of any one of claims 1 to 6, having a total solids content ofup to 700 g / L.

8. The composition of any one of claims 1 to 7, having a pH of less than 2, orless than 1.

9. The composition of any one of claims 1 to 8, additionally comprising (i) anion-conducting polymer; and / or (ii) an electrocatalyst10. A process for producing the composition of any one of claims 1 to 9, theprocess comprising -treating particulate CeZrOx, comprising nanoparticles of Ce, Zr and O in a stoichiometric ratio (CeyZrz)Ox of a particle size smaller than 10 nm in which the mixing of Ce and Zr is incomplete such that not all of the Zr in the nanoparticles is present in the solid solution crystal lattice, with an acid, wherein the acid comprises a strong mineral acid or strong organic acid, thereby forming a sol comprising the nanoparticles.

11. The process of claim 10, wherein treating the particulate CeZrOx with thestrong mineral acid or strong organic acid, thereby forming the sol, comprises peptising the particulate CeZrOx using the strong mineral acid or strong organic acid.

12. The process of claim 10 or claim 11, wherein the acid is a strong mineralacid which is nitric acid.

13. The process of any one of claims 10 to 12, wherein the particulate CeZrOxis a CeZrOx precipitate.

14. The process of claim 13, wherein the CeZrOx precipitate is selected from -a CeZrOx wet cake, having a solids content by weight of up to about 30 wt% or up to about 20 wt%; anda dried CeZrOx wet cake, having a solids content by weight of more than 30 wt%, up to about 80 wt%, up to about 70 wt%, up to about 60 wt%, or up to about 50 wt%.

15. The process of claim 13 or claim 14, which includes a prior step of producingparticulate CeZrOx as a precipitate, using an aqueous solution of soluble salts thereof and a precipitant, wherein the salts are nitrates of Ce and Zr and the precipitant is ammonia.

16. The process of any one of claims 10 to 15, which includes a subsequent stepof combining the peptized CeZrOx with a dispersant, wherein the dispersant is selected from water, an organic solvent, or a combination thereof.

17. A process for producing an ion-conducting membrane comprising a CeZrOx-containing membrane layer, the process comprising -mixing the composition of any one or claims 1 to 9 with an ion-conducting polymer to form an ink; andfabricating the membrane layer using the ink, optionally wherein fabricating the membrane layer using the ink comprises depositing the ink onto a substrate, wherein the substrate is optionally selected from a backing sheet, or an ion-conducting polymer layer or a catalyst layer on a backing sheet.

18. An ion-conducting membrane for an electrochemical device, such as a fuelcell or electrolyser, wherein the ion-conducting membrane is produced by the process of claim 17 or using the composition of claim 9.

19. A process for producing a catalyst layer comprising a CeZrOx catalyst layer,the process comprising -mixing the composition of any one or claims 1 to 9 with an electrocatalyst to form an ink; andfabricating the catalyst layer using the ink, optionally wherein fabricating the catalyst layer using the ink comprises depositing the ink onto a substrate, wherein the substrate is optionally selected from a backing sheet, or an ion-conducting polymer layer or a catalyst layer on a backing sheet.

20. A catalyst layer for an electrochemical device, such as a fuel cell orelectrolyser, wherein the catalyst layer is produced by the process of claim 19 or using the composition of claim 9.

21. A catalyst coated ion-conducting membrane, comprising an ion-conductingmembrane and a catalyst layer according to claim 20.

22. A gas diffusion electrode comprising -a gas diffusion layer; andan ion-conducting membrane of claim 18, and / or a catalyst layer of claim 20, and / or a catalyst coated ion-conducting membrane according claim 21.

23. A membrane electrode assembly comprising -5 an ion-conducting membrane;a catalyst layer; anda gas diffusion layer,whereinthe ion-conducting membrane is an ion-conducting membrane according to claim10 18; and / orthe catalyst layer is a catalyst layer of a catalyst-coated ion-conducting membrane according to claim 21; and / orthe catalyst layer is a catalyst layer according to claim 20, and is applied to the gas diffusion layer to provide a gas diffusion electrode.30

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