Cerium oxide / polyoxometalate particle dispersions and their use in the preparation of proton exchange membranes or electrocatalyst layers

Abstract

Dispersion comprising particles of cerium oxide with polyoxometalate species adsorbed thereon and processes to prepare such dispersion. The addition of said particles of cerium oxide with polyoxometalate species adsorbed thereon to ion exchange polymers increases their stability towards radical degradation when used in fuel cell applications or in electrolysis applications.

Description

DescriptionCERIUM OXIDE / POLYOXOMETALATE PARTICLE DISPERSIONS AND THEIR USE IN THE PREPARATION OF PROTON EXCHANGE MEMBRANES OR ELECTROCATALYST LAYERSTechnical Field

[0001] The present invention relates to aqueous dispersions comprising particles of cerium oxide with polyoxometalate species adsorbed thereon, a process for producing the same, their use in the preparation of compositions comprising the particles and ion exchange polymers as well as to the ion exchange membranes or electroactive layer binders obtained therefrom. The cerium oxide particles comprising the polyoxometalate species are capable of improving the resistance towards radical degradation of ion exchange polymers, used in the preparation of membranes used in certain electrochemical devices. The invention further relates to a process for preparing aqueous dispersions comprising cerium oxide particles comprising the polyoxometalate species.Background Art

[0002] Proton exchange membranes (PEMs) comprising ion exchange polymers are widely used in electrochemical cells, such as water electrolyzers, fuel cells, and flow batteries.

[0003] The membrane requires excellent ion conductivity, gas barrier properties (to avoid the direct mixing of hydrogen and oxygen), mechanical strength and chemical, electrochemical and thermal stability at the operating conditions of the cell. In particular, long-term stability of the membrane is a critical requirement. As an example, the lifetime goal for stationary fuel cell applications is up to 40,000 hours of operations, 20,000 hours of operation being the requirement for automotive fuel cell applications.

[0004] Similar properties are required for proton exchange membranes for use in water electrolysis applications in which water is introduced and oxidized to O2 and H+at the anode, H+being further reduced at the cathode to H2, which is then collected.

[0005] A problem encountered with proton exchange membranes is the migration of free radicals through the membrane. Such free radicals may be generated through reactions occurring at the electrode. It is generally believed that,among other mechanisms, hydrogen peroxide is formed as a result of the reaction between hydrogen and oxygen that permeate through the membrane. Hydrogen peroxide then decomposes to form peroxy and hydroperoxy radicals ('OH, ’OOH), see for instance SCHLICK, S., et al. Degradation of fuel cell membranes using ESR methods: ex situ and in situ experiments. Polymer Preprints. 2009, vol.50, no.2, p.745-746. Direct formation of the radicals is also believed to be possible.

[0006] These free radicals can reduce the effectiveness of the catalyst layer. They can also chemically degrade the membrane, which leads to thinning of the membrane and pinhole generation.

[0007] Methods for addressing the chemical degradation include blending free radical scavenger molecules in the polymer membrane or blending inorganic radical scavenging particles, generally metal oxide nanoparticles, in the polymer membrane.

[0008] Several attempts have been made to reduce radical degradation of proton exchange membranes, for instance by incorporation into the membrane of suitable metallic salts or oxides.

[0009] CN113410496A discloses the manufacture of an all-solid, low-temperature proton exchange membrane containing traces of water. In one embodiment, the membrane is prepared by casting of a precursor solution comprising a solvent such as water, a polymer (e.g. polyvinyl alcohol), nanoparticles of CeO2 as solid electrolyte additive and a proton donor such as phosphotungstic acid, wherein the mass ratio of CeO2 to the proton donor is from 5 to 15.

[0010] CN103887536A discloses a process for the manufacture of a polybenzoimidazole proton exchange membranes for fuel cell, which is hybridized with an inorganic material. The process comprises providing a dispersion of a heteropolyacid (e.g. a polymetalate such as phosphotungstic acid) and cerium oxide at a mass ratio of 1 :2 in a polar solvent other than water. The dispersion is then mixed with a polymer solution comprising polybenzoimidazole. Cerium oxide is used to quench free radicals during the operation of the fuel cell. The heteropolyacid serves to increase the proton conductivity of the membrane.Summary of invention

[0011] It has now been found that cerium oxide particles comprising polyoxometalate species increase the stability of proton exchange membranes towards radical degradation. The increase in stability is reflected in the longer life of service of the membrane.

[0012] Cerium oxide particles comprising polyoxometalate species can also contribute to minimize the deterioration of the electrodes even after long-term operation, when they are added to the catalyst composition of the electroactive layers.

[0013] Advantageously, the cerium oxide particles comprising polyoxometalate species of the invention can be prepared in the form of stable aqueous dispersions which allow very efficient mixing with ion exchange polymer dispersions. As a result it is possible to obtain compositions comprising ion exchange polymers in which the cerium oxide particles comprising polyoxometalate species are uniformly dispersed, which may then be used to prepare membranes or electrocatalyst layers also having a good dispersion of the inorganic particles in the polymeric matrix.

[0014] A first object of the present invention is thus an aqueous dispersion comprising particles of cerium oxide comprising polyoxometalate species adsorbed thereon (hereinafter referred to as “cerium oxide / POM particles”), wherein the pH of the dispersion ranges from 1 .0 to 9.0 and the hydrodynamic mean diameter Dh of the cerium oxide / POM particles, as measured by dynamic light scattering, ranges from 10 to 300 nm.

[0015] One remarkable advantage of the dispersion according to the present invention lies in its good colloidal stability. The dispersion of the invention can remain stable, in other words it exhibits no particles agglomeration, for as long as 30 days. This can be verified, for instance, by monitoring the evolution of the hydrodynamic mean diameter of the particles in the dispersion over a certain period of time, as this value will increase strongly in case of agglomeration.

[0016] Another advantage of the dispersion of the present invention lies in the good compatibility between the polyoxometalate species and the cerium oxide particles thanks to the strong interactions developed between the two. The adsorption of the polyoxometalate species onto the cerium oxide particles surface can notably be evidenced by infrared spectroscopy and / or quantifiedby inductively coupled plasma optical emission spectrometry (ICP-OES) measurements.

[0017] The cerium oxide and polyoxometalate particles in the dispersion of the present invention preferably comprise or consist of cerium oxide cores to which polyoxometalate species are ionically or covalently bonded.

[0018] The polyoxometalate species are generally uniformly distributed on the cerium oxide cores of the particles.

[0019] The dispersions of the invention have the further advantage of being efficiently and easily prepared, even at industrial scale.

[0020] A second object of the invention is a process for the preparation of the aqueous dispersion of the cerium oxide / POM particles which comprises at least the steps of:(a) providing a dispersion of cerium oxide particles in an aqueous medium, wherein the cerium oxide particles have an hydrodynamic mean diameter Dh ranging from 8 nm to 298 nm, as measured by dynamic light scattering, and the pH of the dispersion ranges from 1 .0 to 9.0;(b) providing a solution comprising a polyoxometalate salt in an aqueous medium, wherein the pH of the solution ranges from 1 .0 to 8.0;(c) contacting the dispersion provided in step (a) and the solution provided in step (b) so as to form a reaction medium in which polyoxometalate species are adsorbed onto the cerium oxide particles.

[0021] The process may comprise further steps, in particular steps performed at the end of step (c).

[0022] A third object of the present invention is a composition (C) comprising the cerium oxide / POM particles and an ion exchange polymer. The composition may be a liquid composition.

[0023] A further object of the present invention is an article, in particular a membrane or an electrocatalyst layer, comprising an ion exchange polymer and cerium oxide / POM particles.

[0024] Finally, another object is a fuel cell, an electrolysis cell or a redox flow battery comprising the article as defined above.

[0025] Further objects and characteristics of the present invention are as disclosed in the attached claims.

[0026] Cerium oxide particles comprising polyoxometalate species are known. For instance Z. Song et al., J. Taiwan Inst. Chem. Eng., 71 (2017) 277-284 discloses phosphotungstic acid supported on CeO2 obtained by impregnation of CeO2 with phosphotungstic acid (H3PW12O40) followed by calcination and its use in the selective catalytic reduction of NO with ammonia.

[0027] Similarly, Y. Geng et al., Ind. Eng. Chem. Res., 57 (2018), 856-866 discloses the selective catalytic reduction of nitrogen oxides with ammonia using CeO2 particles grafted with phosphotungstic acid (H3PW12O40). The particles were prepared by adding CeO2 to an aqueous solution of H3PW12O40 followed by separation of the particles and calcination.

[0028] Y. Guan et al., Biomaterials, 98 (2016), 92-102, discloses the preparation of aqueous dispersions containing ceria / polyoxometalates hybrid nanoparticles and their use as nanoenzymes in the treatment of neurotoxicity of amyloid-p- peptide. To render the dispersions biocompatible, they are buffered at physiological pH (i.e. pH = 7.4).

[0029] None of the documents disclose the ability of cerium oxide particles comprising polyoxometalate species to act as scavengers of free radicals, in particular radicals that are generated through reactions occurring at the electrode of a fuel cell or an electrolyser and which generally include peroxy and hydroperoxy radicals (-OH, -OOH). None of the documents disclose particles suitable for fine dispersion in a membrane (at sizes lower than 300 nm) or in a solution.Disclosure of invention

[0030] Definitions

[0031] In the present disclosure, the expression “comprising” should be understood as meaning “comprising at least one”. The expression “a” or “an” should be understood as meaning “at least one”.

[0032] An expression such as “Object P comprises at least the elements p1 , p2... pi” should also be understood as encompassing explicitly the embodiment wherein Object P consists essentially of the elements p1 , p2 ... pi.

[0033] “Essentially” in this context means that some impurities, undesired species, unintentional compounds or the like could be present in Object P without impacting its targeted function and effect in the framework of the present invention.

[0034] The expression “comprised between ... and ...” or “ranging from... to...” and the like should be understood as including the limits.

[0035] In the whole description, the term “cerium oxide” designates cerium oxide which has a purity degree of at least 95.0% by weight with respect to the dry weight of the oxide. Cerium oxide is generally crystalline ceric oxide (cerium (IV) oxide). Some impurities can be present in the oxide. The impurities may stem from the raw materials or starting materials used in the process of preparation of the oxide. The total amount of the impurities is generally lower than 5.0% by weight with respect to the oxide, even lower than 1 .0 wt%. Residual nitrates, carbonates and / or ammonium are not considered as impurities in the present description.

[0036] The expression “aqueous dispersion” denotes a system consisting of solid fine particles of submicronic dimensions, stably dispersed in a liquid aqueous medium. The particles may optionally contain residual amounts of bound or adsorbed ions such as, for example, nitrates, carbonates or ammonium ions, independently from the polyoxometalate species.

[0037] In the framework of the invention, the aqueous dispersion comprises particles of cerium oxide with polyoxometalate species adsorbed thereon. In other words, such particles comprise or consist, preferably consist, of cerium oxide core particles having polyoxometalate species ionically or covalently bonded to the cerium oxide core particles.

[0038] The expression “polyoxometalate” or “POM” has its usual meaning in the art. It designates a polyatomic anion, also referred to as “polyanionic cluster”, that consists of three or more metal atoms, linked together by shared oxygen atoms to form closed 3-dimensional frameworks. The metal atom of the POM, denoted M, may be selected from:- group 6 of the Periodic Table of Elements, particularly from Mo, W;- from group 5, particularly from V, Nb, Ta;- from transition metals, particularly Tc;- and combinations thereof.

[0039] The metal atom M may be more particularly selected from the metal of groups 5 and 6 as detailed above, even more particularly from W, Mo and V. The metal atom may be in its highest oxidation state.

[0040] The term “polyoxometalate species” as used herein encompasses any recombination species of the polyoxometalate specie. “Recombination species” denotes, in the context of the invention, any species resulting from the polyoxometalates speciation in the aqueous dispersion. The speciation denotes the evolution of the polyoxometalate species in the aqueous dispersion as a result of a variation of at least one of the following parameters: pH, temperature, concentration, ionic strength, counterions, aging time, ionic interactions and covalent bonding on surfaces.

[0041] In addition to the polyoxometalate species that are adsorbed onto the surface of the cerium oxide particles, the aqueous dispersion of the invention may comprise “free” polyoxometalate species, that is to say polyoxometalate species which are not bound to the cerium oxide particles.

[0042] The adsorption of the polyoxometalate species on the cerium oxide particles can notably be verified by infrared spectroscopy and / or quantified by inductively coupled plasma optical emission spectroscopy (ICP-OES). The protocols detailed in the experimental section can advantageously be used.

[0043] The following methodology can be followed for the quantification of elemental concentrations by ICP-OES in the dispersions of the invention.

[0044] Quantification of [Cel

[0045] An aliquot of the dispersion is taken and diluted with nitric acid acidified water (2% vol / vol) to achieve a concentration within the range of 0.1 to 5 mg / L; the concentration of elemental Ce therein is quantified (in g / L) by inductively coupled plasma optical emission spectroscopy (ICP-OES) using wavelengths at 413.765 nm and 418.660 nm. This concentration [Ce] is related to the total concentration of cerium oxide present in the dispersion.

[0046] Quantification of [Mltotai

[0047] An aliquot of the dispersion is diluted with nitric acid acidified water (2% vol / vol) to achieve a concentration within the range of 0.1 to 5 mg / L; the concentration of elemental metal M (in g / L) deriving from the polyoxometalate species is quantified by inductively coupled plasma optical emission spectroscopy (ICP-OES). This concentration [M]totai is related to the total concentration of the elemental metal M deriving from the polyoxometalate species present in the dispersion, adsorbed and not adsorbed.

[0048] Quantification of [Minot adsorbed

[0049] The dispersion is centrifuged at 10,000 rpm during 15 minutes. The top of supernatant is recovered, diluted with nitric acid acidified water (2% vol / vol) to achieve a concentration within the range of 0.1 to 5 mg / L; the concentration of elemental metal M deriving from the POM is quantified by ICP-OES. This concentration [M]not adsorbed is related to the concentration of the metal M deriving from the polyoxometalate species which are not adsorbed on the cerium particles.

[0050] Calculations

[0051] Based on the quantifications of elemental metal M deriving from the POM and elemental cerium in the dispersion, the following calculations can be made:Ratio of non-adsorbed elemental M relative to the total elemental M in the dispersion:wherein:[M]totai is the concentration of elemental M deriving from the POM quantified by ICP-OES (g / L) in the dispersion before centrifugation.[M]not adsorbed is the concentration of elemental M deriving from the POM quantified by ICP (g / L) in the supernatant retrieved after centrifugation of the dispersion.Ratio of adsorbed elemental M relative to the total elemental M in the dispersion:wherein:[M]totai and [M]not adsorbed are as defined above.Molar ratio of elemental Ce relatively to the total elemental M in the dispersion:wherein:[Ce] is the concentration of cerium oxide quantified by ICP-OES (g / L) in the dispersion (before centrifugation),Mce is the molar mass of cerium,[M]totai is the concentration of elemental M deriving from the POM quantified by ICP-OES (g / L) in the dispersion (before centrifugation),MM is the molar mass of metal(s) M deriving from the POM.Molar ratio of elemental Ce relative to the adsorbed elemental M in the dispersion:wherein:[Ce], [M]totai Mce, MM and Ratio Madsorbed are as defined above.Cerium oxide weight concentration in the dispersion: 100wherein: ddispersion denotes the density of the dispersion; it can be measured as described in ISO-758-1976 “Liquid chemical products for industrial use - Determination of density at 20°C”, Mce is the molar mass of cerium, Mce02 is the molar mass of cerium oxide.

[0052] In the calculations above, when more than one polyoxometalate species is adsorbed on cerium oxide particles or more than one metal M is contained in the polyoxometalate species, the content of each metal M is quantified using ICP-OES and [M]totai is the sum of the concentration of all metals M deriving from the POM.

[0053] In the framework of the present invention, the expression “dispersion with good colloidal stability” is used to refer to a dispersion of particles which are characterized by an initial value of the hydrodynamic mean diameter, Dhin, in the range of 10 nm to 300 nm and that after a time “t” are still characterized by a value of the hydrodynamic mean diameter, Dht, which is in the range of 10 nm to 300 nm. Time “t” is measured in days and it is at least 7 days.Advantageously, the dispersions of the inventions may remain stable up to 20 days, even up to 30 days.

[0054] The hydrodynamic mean diameter Dh may be determined using dynamic light scattering techniques, as known to the person skilled in the art. This technique allows measurement of the hydrodynamic mean diameter Dh of solid objects, the value of which is affected by the presence of aggregates of particles. Therefore, the measurement is usually performed on a dispersion of the particles in water. Dh can be for instance determined using the appliance Zetasizer Nano-ZS of Malvern following the guidelines of the constructor. Other equivalent instruments may be used. A suitable protocol for the determination of the hydrodynamic mean diameter Dh using dynamic light scattering is detailed in the experimental section of the present specification.

[0055] The zeta potential of particles in an aqueous dispersion may be determined using any suitable instrument known by the person skilled in the art using standar procedures. As an example, the zeta potential may be determined using a Zetameter DT-300 from Dispersion Technology or a Zetasizer Advance Pro Blue from Malvern Panalytical. The following protocol may be applied. A volume of 50 mL of the aqueous dispersion of particles, wherein the amount of particles has been adjusted at a value between 0.1 wt% and 5 wt% in weight, by addition of deionized water if needed, is poured into a container under magnetic stirring. The pHmeter and zetameter probes are calibrated following the constructor’s guidelines (referring to the ISO 13099- 3:2014 Colloidal systems - Methods for zeta potential determination - Part 3: Acoustic methods). After cleaning with deionized water and drying with a tissue the pH and zeta potential probes, the pH and zeta potential of the agitated aqueous dispersion are measured with the Zetameter.

[0056] Description of the invention

[0057] A first object of the present invention is an aqueous dispersion comprising particles of cerium oxide with polyoxometalate species adsorbed thereon, hereinafter referred to as the “cerium oxide / POM particles”, wherein the pH of the dispersion ranges from 1 .0 to 9.0, and the hydrodynamic mean diameter Dh of the cerium oxide / POM particles, as measured by dynamic light scattering, ranges from 10 nm to 300 nm.

[0058] A dispersion according to the invention has the advantage of exhibiting a good colloidal stability, in that the particles are well dispersed and remain stable over time. The stability of the dispersion is such that the hydrodynamic mean diameter Dh of the cerium oxide / POM particles typically remains in the range from 10 nm to 300 nm for a time “t” greater than 7 days, preferably greater than 14 days, even greater than 21 days.

[0059] The cerium oxide / POM particles are inorganic particles.

[0060] The cerium oxide / POM particles comprise or consist, preferably consist, of cerium oxide core particles to which polyoxometalate species are ionically or covalently bonded. This particle configuration enables strong interactions between the cerium oxide and the polyoxometalate species reducing the risk of leaching of the polyoxometalate species when the particles are used in proton exchange membranes or in electrocatalyst layers in electrochemical devices.

[0061] One type of polyoxometalate or a combination of different types of polyoxometalates can be used to prepare the cerium oxide / POM particles in the framework of the present invention.

[0062] Advantageously, at least 40%, in particular at least 50%, more particularly at least 60%, of the elemental metal(s) M present in the dispersion in the form of POM are adsorbed onto the cerium oxide particles. The methodology based on ICP-OES quantification described in the definition section can notably be used.

[0063] As defined above, the expression “polyoxometalate” or “POM” has its usual meaning in the art. It designates a polyatomic ion, typically a polyatomic anion, also referred to as “polyanionic cluster”, that consists of three or more metal atoms linked together by shared oxygen atoms to form closed 3- dimensional frameworks. The metal atom of the polyoxometalate species, denoted M, may be selected from: group 6 of the Periodic Table of the Elements, particularly from Mo, W; from group 5, particularly from V, Nb, Ta; from transition metals, particularly Tc; and combinations thereof.

[0064] The metal atom M may be more particularly selected from the metals of groups 5 and 6 as detailed above. Metal atom M may be preferably selected from the group consisting of W, Mo and V.

[0065] The metal atoms may be in their highest oxidation state.

[0066] The polyoxometalate species may be advantageously selected from the group consisting of the so-called Keggin, Wells-Dawson, Anderson and Lindqvist polyoxometalates.

[0067] The polyoxometalate species may be selected from the group consisting of: compounds of formula [HhXxMi204o]n’, compounds of formula [Y2M18O62]n’, compounds of formula [HhXxM6O24]n’, compounds of formula [MeO ]11’, mixtures of any of the above and recombination species thereof, wherein: h is 0 or 2; X is Si or P; x = 0 when h=2 and x=1 when h=0; Y is selected from Si or P; M is selected from W, Mo or V; and n is an integer different from 0 denoting the number of charges of the compound. The number n typically ranges from 2 to 12.

[0068] Recombination species in the meaning of the present invention are compounds resulting from the possible POM speciation in the aqueous dispersion and are such as defined earlier in the definitions section.

[0069] The polyoxometalate can be more particularly selected from compounds of formula [HhXxMi204o]n’, mixtures thereof and recombination species thereof, wherein h is 0 or 2, X is Si or P and x = 0 when h=2 and x=1 when h=0; Y is selected from Si or P. The number n typically ranges from 2 to 10.

[0070] The polyoxometalate can be even more particularly selected from compounds of formula [HhXxMi204o]n’, mixtures thereof and recombination species thereof, wherein: when M=W and h=2 and x=0, n is 6, 8 or 12; when M= W, X=P, h=0, x=1 and n=3; when M= W, X=Si, h=0, x=1 and n=4.

[0071] According to one particular embodiment, the polyoxometalate is a metatungstate selected from the group consisting of those of formula [H2W12O40]6', [H2W12O40]8' or [H2W12O40]12' or recombination species thereof.

[0072] According to another particular embodiment, the polyoxometalate is the phosphotungstate of formula [PWi204o]3’ or recombination species thereof.

[0073] According to another particular embodiment, the polyoxometalate is the silicotungstate of formula [SiWi204o]4-or recombination species thereof.

[0074] The dispersion may comprise cations selected from sodium, ammonium, potassium, phosphonium, H+and mixtures thereof, in particular selected from sodium and / or ammonium, and more particularly ammonium. The amount of such cations is generally minimized.

[0075] The hydrodynamic mean diameter Dh of the cerium oxide / POM particles ranges from 10 nm to 300 nm, in particular from 10 nm to 200 nm, in particular from 10 nm to 100 nm, and even from 10 nm to 65 nm, as measured by dynamic light scattering. This size range contributes to the good stability of the dispersion according to the invention. It is also advantageous for a usage of the dispersion in the preparation of thin membranes as well as binders for electrocatalyst layers as it allows an uniform distribution of the particles in the membranes or layers.

[0076] The ratio of adsorbed elemental M (g / L) deriving from the adsorbed polyoxometalate species relatively to the total elemental M (g / L) deriving from the polyoxometalate species in the dispersion may range from 0.40 to 1 .00, in particular from 0.50 to 1 .00, more particularly from 0.70 to 1 .00, for example from 0.75 to 0.95. This ratio may be noted Ratio Madsorbed in the present specification. It may be based on a determination of the content of elemental M by ICP-OES. It may be notably calculated as explained in the definition section.

[0077] The content of polyoxometalate species in the dispersion may be expressed as a molar ratio Ce / Mtotai, wherein Ce denotes the total molar amount of cerium present in the dispersion, Mtotai denotes the total molar amount of metal deriving from the POM in the dispersion, said metal being preferably selected from W, Mo and V. The molar ratio Ce / Mtotai may range from 2 to 250, in particular from 3 to 200, more preferably from 5 to 150. This content can be confirmed by using ICP-OES analysis by quantification of the elemental Ce and elemental metal M deriving from the POM present in the dispersion. The methodology explained in the definition section or the example section can be used for the determination of this parameter.

[0078] The content of adsorbed polyoxometalate species in the dispersion may be expressed as a molar ratio Ce / Madsorbed wherein Ce denotes the total molar amount of cerium present in the dispersion and Madsorbed denotes the molar amount of metal deriving from the adsorbed polyoxometalate species present in the dispersion, said metal being preferably selected from W, Mo and V. The molar ratio Ce / Madsorbed may range from 2 to 625, in particular from 3 to 500, more preferably from 5 to 375. This content can be confirmed by using ICP-OES analysis by quantification of the elemental Ce and elemental metal M deriving from the adsorbed POM present in the dispersion. The methodology explained in the definition section or the example section can be used for the determination of this parameter.

[0079] The pH of the dispersion of the invention ranges from 1 .0 to 9.0. It may range from 1 .0 to 7.0, in particular from 1 .0 to 6.0, from 2.0 to 6.0, more particularly from 2.0 to 5.0. This pH range contributes to the good stability of the dispersion.

[0080] The concentration of the cerium oxide in the dispersion of the invention may range from 0.5 wt% to 40.0 wt%, preferably from 2.0 wt% to 35.0 wt%. This content can be confirmed by using ICP-OES analysis performed on the cerium oxide / POM dispersion. Inductively coupled plasma optical emission spectroscopy (ICP-OES) may be performed according to the methodology detailed in the “Definitions” section above.

[0081] The zeta potential of the cerium oxide and polyoxometalate particles in the dispersion is advantageously negative, as measured on the aqueous dispersion thereof at a concentration of said particles ranging from 0.1 wt % to 5.0 wt% in terms of cerium oxide. The protocol for measuring the zeta potential described in the definitions section may notably be implemented. The zeta potential of the cerium oxide and polyoxometalate particles in the dispersion may be advantageously below -10 mV, in particular below than -20 mV, more particularly below -25 mV, even below -30 mV. The cerium oxide and polyoxometalate particles may notably exhibit a zeta potential ranging from -60 to -10 mV, as measured on the aqueous dispersion thereof at a concentration of particles ranging from 0.1 wt % to 5.0 wt% in terms of cerium oxide. This negative zeta potential participates to the colloidal stability of the dispersion.

[0082] The zeta potential of the cerium oxide and polyoxometalate particles in the dispersion may also be positive, as measured on the aqueous dispersion thereof at a concentration of said particles ranging from 0.1 wt % to 5.0 wt% in terms of cerium oxide. The zeta potential of the cerium oxide and polyoxometalate particles in the dispersion may be advantageously higher than +10 mV, in particular higher than +20 mV, more particularly higher than +25 mV, even higher than +30 mV. The cerium oxide and polyoxometalate particles may notably exhibit a zeta potential ranging from +10 to +60 mV, as measured on the aqueous dispersion thereof at a concentration of particles ranging from 0.1 wt % to 5.0 wt% in terms of cerium oxide. This positive zeta potential participates to the colloidal stability of the dispersion.

[0083] As previously mentioned, the term “aqueous” when referring to the “aqueous dispersion” in accordance with the invention means that the particles are dispersed in a liquid aqueous medium. The aqueous medium may be water or a mixture of water and a water-miscible organic liquid. Mixtures of water and a water-miscible organic liquid typically contain 50 wt% or more of water, even 80 wt% or more, still 99 wt% or more. The water-miscible organic liquid is preferably selected among those that do not make the particles precipitate or agglomerate. The water-miscible organic liquid may for instance be an alcohol. Notable non-limiting examples of suitable alcohols are methanol, ethanol, 1 -propanol, 2-propanol, 1 -hexanol.

[0084] In some other embodiments, the dispersion may further comprise polar aprotic organic solvents such as ketones, like acetone, methylethylketone; esters, like dimethyl carbonate, diethyl carbonate, ethyl formate, propyl formate, ethyl acetate, methyl acetate, methyl lactate, butyl lactate, ethyl lactate; nitriles, like acetonitrile; sulfoxides, like dimethylsulfoxide (DMSO); sulfones like dimethylsulfone (DMSO2); amides, like N,N-dimethylformamide, N,N-dimethylacetamide; pyrrolidones, like N-methylpyrrolidone, N- ethylpyrrolidone; and mixtures thereof.

[0085] In some embodiments, the aqueous dispersion comprises water or a water / alcoholic mixture. Good results were obtained with aqueous dispersions comprising cerium oxide / POM particles dispersed in water / alcoholic mixtures in which the alcohol is selected from ethanol, 1-propanol, 2-propanol. According to one particular embodiment, the liquid medium in the dispersion consists of water.

[0086] The cerium oxide / POM particles used in the framework of the present invention may further exhibit one or several of the following features: an average particle size, as measured by TEM ranging from 1 to 200 nm; Said average size of n (>100) particles may be measured using a photograph of dispersions thereof obtained by transmission electron microscopy (TEM); when measuring the size of a particle on a photograph (by TEM or any other microscopy technique), the largest dimension thereof on the photograph is taken. Average size is hence determined as arithmetic mean of the size of n particles.The standard deviation of the value of said average particles size may be of at most 20-30%; The standard deviation may also be determined from the TEM method. It has its usual mathematical meaning. It is the square root of the variance and is expressed by the formula:n being the number of particles taken into account in the measurement, (>100);Xi being the size of a particle i; x being the average value of the size of the particles (1 In £ixi); a specific surface area, determined by BET, comprised between 9 m2 / g to 835 m2 / g. The specific surface area may be determined on a powder by adsorption of nitrogen by the Brunauer-Emmett-Teller method (BET method). The method is disclosed in standard ASTM D 3663-03 (reapproved 2015). The method is also described in the periodical “The Journal of the American Chemical Society, 60, 309 (1938)”. The specific surface area may be determined automatically with an appliance TriStar 3000 of Micromeritics according to the guidelines of the constructor. Prior to the measurement, the samples in the form of powders shall be degassed under static air by heating at a temperature of at most 210°C to remove the adsorbed species;an average crystallite size, as determined by XRD, comprised between 3nm to 80nm. The average crystallite size measured by XRD can be calculated from the FWHM of diffraction peaks range from 2Theta = 50° and 2Theta = 140° by applying the Scherrer model with a Scherrer constant equal to 0.9.

[0087] A second object of the invention is a process for the preparation of the aqueous dispersion of the cerium oxide / POM particles which comprises at least the steps of:(a) providing a dispersion of cerium oxide particles in an aqueous medium, wherein the cerium oxide particles have a hydrodynamic mean diameter Dh ranging from 8 nm to 298 nm, as measured by dynamic light scattering, and the pH of the dispersion ranges from 1 .0 to 9.0;(b) providing a solution comprising a polyoxometalate salt in an aqueous medium, wherein the pH of the solution ranges from 1 .0 to 8.0;(c) contacting the dispersion provided in step (a) and the solution provided in step (b) to form a reaction medium in which polyoxometalate species are adsorbed onto the cerium oxide particles.

[0088] The process may further comprise at least one of the following optional steps:(d) performing a solid / liquid separation to separate the cerium oxide particles having polyoxometalate species adsorbed thereon from the liquid medium and dispersing the particles in an aqueous medium, wherein the liquid medium is either the reaction medium obtained at the end of step (c) or the liquid medium obtained after step (e); and(e) washing and / or acidifying the dispersion.

[0089] The dispersion of the cerium oxide / POM particles, therefore, can be used as obtained at the end of the step (c). Alternatively, the cerium oxide / POM particles can be separated from the reaction medium, optionally washed, and subsequently redispersed in an aqueous medium before being used. If needed, the pH of the dispersion as obtained at the end of the step (c) or after redispersion may be adjusted to the required value.

[0090] A dispersion in accordance with the invention may be obtained at the end of step (c), (d) or (e).

[0091] The process is based on the combination of two dispersions of cerium oxide particles and polyoxometalate species that have been separately prepared. Mixing two separate solutions allows a better control of the absorption of the POM particles on the CeO2 surface, for example by adjusting the pH and / or the order of the addition of one dispersion into the other. Compared to a one- pot reaction in which both cerium oxide and POM are mixed at the same time, the process herein described allows to prevent CeO2 particles aggregation and thus to finally obtain a suspension containing well-dispersed particles. Moreover, undesired reactions of the POM with further components possibly present in the reaction medium are avoided, which may unpredictably affect the absorption of the POM species on the CeO2 surface.

[0092] Dispersion of cerium oxide particles provided in step (a)

[0093] According to one embodiment, the dispersion of cerium oxide particles provided in step (a) (starting dispersion) can be prepared by dispersing, in an aqueous medium, a powder of cerium oxide particles having a hydrodynamic mean diameter Dh comprised between 8 nm to 298 nm and by adjusting the pH of the obtained dispersion to the required value. Nitric acid or aqueous ammonia can be used to adjust the pH.

[0094] The cerium oxide powder used to prepare the starting dispersion has a fineness suitable for achieving a hydrodynamic mean diameter Dh comprised between 8 nm to 298 nm. Notably, a cerium oxide powder comprising agglomerated particles having an average particle size higher than ca. 300 nm (e.g. 1 to 100 micrometres), the agglomerated particles being formed by loosely aggregated nanoparticles, may be used insofar the agglomerated particles are redispersible in water forming particles having a hydrodynamic mean diameter Dh comprised between 8 nm to 298 nm.

[0095] According to another embodiment, dispersions of cerium oxide particles having the required hydrodynamic mean diameter Dh can be used, e.g. commercially available dispersions, and the pH thereof can be adjusted if necessary to the required value. If applicable, these dispersions may be concentrated or diluted and / or transferred from their original organic phase to an aqueous medium in order to implement step (a), by methods known per se.

[0096] According to another embodiment, the dispersion of cerium oxide particles provided in step (a) may be prepared by one of the processes which are described in WO 2008 / 043703, WO 2010 / 020466 and WO 2015 / 091495, which are incorporated herein by reference.

[0097] The cerium oxide particles of the dispersions may exhibit one or more of the following: an average particles size measured by TEM (as detailed before for cerium oxide / POM dispersions) of at most 250 nm, in particular at most 200 nm, more particularly at most 170 nm; an average particles size measured by TEM of at least 10 nm, in particular at least 30 nm, more particularly at least 50 nm. The standard deviation of the value of said average particles size may be of at most 30%, particularly of at most 20%, more particularly of at most 15%; and / or an average particle size calculated from BET surface measurement of at most 120 nm, particularly of at most 110 nm; an average particles size calculated from BET surface measurement of at least 5 nm, in particular of at least 19 nm, in particular of at least 30 nm, in particular of at least 40 nm; and / or a median diameter D50, determined from a distribution obtained by laser diffraction, comprised between 60 nm and 170 nm, particularly between 70 nm and 160 nm, more particularly between 80 nm and 150 nm, even more particularly between 90 nm and 150 nm; and / or a dispersion index, determined from a distribution obtained by laser diffraction, of at most 0.5, particularly of at most 0.4, more particularly of at most 0.3. The “dispersion index” is defined by the following formula o / m = (D90-D10) / 2D50. D10 is the diameter determined from a distribution obtained by laser diffraction for which 10% of the particles have a diameter of less than D10. D50 is the diameter determined from a distribution obtained by laser diffraction for which 50% of the particles have a diameter of less than D50; D50 is, as specified above, the median diameter determined from a distribution obtained by laser diffraction. D90 is the diameter determined from a distribution obtained by laser diffraction for which 90% of the particles have a diameter of less than D90.

[0098] The aqueous medium in the dispersion of cerium oxide particles provided in step (a) can be water or a water-miscible organic liquid. If a water-miscible organic liquid is chosen, it is preferably selected among the list described earlier in connection with the final dispersion of cerium oxide / POM particles. According to one preferred embodiment, the aqueous medium is water.

[0099] The pH of the dispersion of cerium oxide particles provided in step (a) is to be set at a value comprised between 1 .0 and 9.0, preferably comprised between 1 .0 and 8.0, preferably between 1 .0 and 6.0, preferably between 2.0 and 6.0, more preferably between 3.0 and 5.0.

[0100] A basic or acidic pH adjuster can be used to this end. As suitable acid, mention can be made of nitric acid, hydrochloric acid, sulfonic acid, carbonic acid, picolinic acid, propionic acid, and mixtures thereof, being preferably nitric acid. As suitable base, mention can be made of alkali metal and alkaline earth metal hydroxides and aqueous ammonia. Secondary, tertiary or quaternary amines can also be used. Aqueous ammonia is preferred.

[0101] The dispersion of cerium oxide particles provided in step (a) may comprise from 0.5 wt% to 40.0 wt%, preferably from 1 .0 wt% to 35.0 wt%. of cerium oxide particles relative to the total weight of the dispersion.

[0102] The hydrodynamic mean diameter Dh of the cerium oxide particles ranges from 8 to 298 nm, in particular from 8 nm to 198 nm, as measured by dynamic light scattering. The hydrodynamic mean diameter Dh of the cerium oxide particles may be in the range from 8 nm to 150 nm, as measured by dynamic light scattering.

[0103] Solution comprising a polyoxometalate salt provided in step (b)

[0104] A solution comprising a polyoxometalate salt is provided in step (b) of the process.

[0105] For the avoidance of doubt the expression “polyoxometalate salt” refers to compounds comprising a polyoxometalate species as defined above and suitable counterions to achieve charge neutrality.

[0106] The counterion in the polyoxometalate salt can be selected from the group consisting of alkali metal ions, such as sodium and potassium, ammonium, phosphonium, H+; the counterion is preferably selected from the group consisting of sodium and ammonium, more preferably ammonium ions.

[0107] The polyoxometalate can be selected from the compounds detailed earlier in connection with the final cerium oxide / POM particles.

[0108] The aqueous medium can be water or a water-miscible organic liquid. If a water-miscible organic liquid is chosen, it is preferably selected among the members of the list described earlier in connection with the dispersion of cerium oxide / POM particles. According to one preferred embodiment, the aqueous medium is water.

[0109] The pH of the solution comprising the polyoxometalate salt provided in step (a) is to be set up at a value comprised between 1 .0 and 8.0, in particular it can be comprised between 1.0 and 7.0, more particularly between 1.0 and 6.0, even more particularly between 1 .0 and 5.0. A basic or acidic pH adjuster can be used to this end, notably one selected among the compounds detailed earlier in connection with the cerium oxide dispersion.

[0110] The solution comprising a polyoxometalate salt provided in step (b) preferably comprises a suitable amount of polyoxometalate salt to achieve a molar ratio Ce / Mtotai as defined hereafter.

[0111] Contacting and adsorption step (c)

[0112] Step (c) consists in contacting the dispersion provided in step (a) and the solution provided in step (b) so as to form a reaction medium in which polyoxometalate species deriving from the polyoxometalate salt are adsorbed onto the cerium oxide particles.

[0113] According to one embodiment, the dispersion of cerium oxide is added to the solution of polyoxometalate salt.

[0114] The contacting step can be conducted over a period of time ranging from 5 minutes to 5 hours, in particular from 15 minutes to 1 hour. Stirring means can be used. The contacting step can be advantageously conducted at room temperature, being typically in the range 20°C to 25°C. The pressure can advantageously be atmospheric pressure, being typically about 1013,25 hPa. The use of an inert atmosphere is not generally required.

[0115] The molar ratio Ce / Mtotai preferably ranges from 2 to 250, in particular from 3 to 200, more preferably from 5 to 150; Ce denotes the total molar amount of cerium introduced, Mtotai denotes the total molar amount of the metal coming from the POM introduced, M is preferably selected from W, Mo and V.

[0116] Optional steps (d) and (e)

[0117] In step (d), the mixture obtained at the end of step (c) may be subjected to a solid / liquid separation step to separate the cerium oxide particles having polyoxometalate species adsorbed thereon from the reaction medium.

[0118] Filtration can be performed using e.g. a Buchner funnel at lab scale. At larger scale, any system of filtration well known in the art for dewatering, such as filter presses can be used.

[0119] After separation the solid may be washed and the particles may then be dispersed again in an aqueous medium providing a dispersion which is still a dispersion according to the invention.

[0120] Alternatively or additionally, the mixture obtained at the end of step (c) may be acidified and / or washed. Washing may be performed with water, preferably deionized water. This step may be carried out by filtering the solid from the medium and redispersing the solid in water. Filtration and redispersion may be performed several times if necessary.

[0121] Acidification may be performed by using a suitable acid such as nitric acid, picolinic acid, propionic acid, hydrochloric acid, sulfonic acid, carbonic acid, and mixtures thereof, preferably nitric acid. The reaction mixture may be acidified to a pH ranging from 1 .0 to 9.0, in particular 1 .0 to 8.0, more particularly 1 .0 to 7.0, notably 2.0 to 6.0.

[0122] When both acidification and washing steps are performed, they may be performed in any order.

[0123] The solid obtained after the optional filtration step (d) may be dried at a temperature of 70°C to 170°C. The particles thus obtained comprise or consist of cerium oxide / POM particles.

[0124] The particles may be used in the form of powders. Alternatively they can be redispersed in a liquid medium different from water. In an advantageous embodiment, the solvent may be an alcohol. Particularly advantageous are dispersions of the cerium oxide / POM particles in 1 -propanol or 2-propanol. Accordingly, an object of the invention is a dispersion of cerium oxide / POM particles having an hydrodynamic mean diameter Dh ranging from 10 nm to 300 nm in 1 -propanol or 2-propanol.

[0125] Once dispersed in a liquid medium, such as water or alcohol, for instance by means of sonication, cerium oxide / POM particles may be advantageously characterized by a hydrodynamic mean diameter Dh ranging from 10 to 300nm, in particular from 10 nm to 200 nm, in particular from 10 nm to 100 nm as measured by dynamic light scattering.

[0126] A further object of the invention is a composition (C) comprising an ion exchange polymer and cerium oxide / POM particles as detailed above. The cerium oxide / POM particles comprise particles of cerium oxide with polyoxometalate species adsorbed thereon.

[0127] The polyoxometalate species may be selected from the group consisting of: compounds of formula [HhXxMi204o]n’, compounds of formula [Y2M18O62]n; compounds of formula [HhXxM6O24]n’, compounds of formula [MeOi9]n’ mixtures of any of the above and recombination species thereof, wherein h is 0 or 2, X is Si or P and x = 0 when h=2 and x=1 when h=0; Y is selected from Si or P; M is selected from W, Mo or V and n is an integer different from 0 denoting the number of charges of the compound. The number n typically ranges from 2 to 12.

[0128] The content of adsorbed polyoxometalate species in the particles may be expressed as a molar ratio Ce / Madsorbed wherein Ce denotes the total molar amount of cerium and Madsorbed denotes the molar amount of metal deriving from the adsorbed polyoxometalate species present in the dispersion, said metal being preferably selected from W, Mo and V. The molar ratio Ce / Madsorbed may range from 2 to 625, in particular from 3 to 500, more preferably from 5 to 375. This content can be confirmed by using ICP-OES analysis by quantification of the elemental Ce and elemental metal M deriving from the adsorbed POM.

[0129] Within the context of the present invention the expression “an ion exchange polymer” is intended to denote one or more than one ion exchange polymer. Mixtures of ion exchange polymers can be advantageously used for the purposes of the invention.

[0130] The term “ion exchange polymer ” is used herein to refer to a polymer comprising recurring units which comprise ionic or ionizable groups, or both. Examples of ion exchange groups include cation exchange groups such as - SO3H, -COOH, -PO(OH)2, -POH(OH), -SO2NHSO2- and -Ph(OH) (Phdenotes a phenyl group). Examples of ionizable groups include groups such as -SO2X’ wherein X’ is selected from the group consisting of F, Cl, Br, I, or - COOR, wherein R is an alkyl group.

[0131] Composition (C) comprises ion exchange polymers which are suitable for use in the preparation of ion exchange membranes or catalyst binders in fuel cells, electrolyzers and redox flow batteries.

[0132] The ion exchange polymer may advantageously comprise -SO3H, -COOH, - PO(OH)2, -POH(OH), -SO2NHSO2- ion exchange groups.

[0133] The ion exchange polymer is typically a polymer comprising -SO2X functional groups, wherein X is selected from X’ or from OZ and wherein X’ is selected from the group consisting of F, Cl, Br, I and Z is selected from the group consisting of H, alkaline metals, NH4.

[0134] The composition (C) may comprise an ion exchange polymer in the neutral form, wherein the expression “neutral form” indicates that in the -SO2X functional groups X is X’ and X' is selected from the group consisting of F, Cl, Br, I. Preferably X’ is selected from F or Cl. More preferably X’ is F.

[0135] Alternatively, the composition (C) may comprise an ion exchange polymer in the ionic (acid or salified) form, wherein the expression “ionic form” indicates that in the -SO2X functional groups X is OZ and Z is selected from the group consisting of H, alkaline metals, NH4.

[0136] For the avoidance of doubt, the term "alkali metal" is hereby intended to denote the following metals: Li, Na, K, Rb, Cs. Preferably the alkali metal is selected from Li, Na, K.

[0137] The ion exchange polymer may be a fluorinated ion exchange polymer. The expression “fluorinated” is used herein to refer to compounds (e.g. compounds, polymers, monomers etc.) that are either totally or partially fluorinated, i.e. wherein all or only a part of the hydrogen atoms have been replaced by fluorine atoms. Preferably, the term “fluorinated” refers to compounds that contain a higher proportion of fluorine atoms than hydrogen atoms, more preferably the term refers to compounds that are totally free of hydrogen atoms, i.e. wherein all the hydrogen atoms have been replaced by fluorine atoms.

[0138] Fluorinated ion exchange polymers comprising -SO3Z functional groups (i.e., comprising -SO2X functional groups wherein X=OZ) are typically preparedfrom fluorinated polymers comprising -SO2X’ functional groups, preferably - SO2F functional groups, by methods known in the art.

[0139] The fluorinated ion exchange polymer can be obtained in its salified form, i.e. wherein Z is a cation selected from the group consisting of NH4 and alkali metals, by treatment of the corresponding polymer comprising - SO2X’ functional groups, typically -SO2F functional groups, with a strong base (e.g. NaOH, KOH).

[0140] The fluorinated ion exchange polymer can be obtained in its acid form, i.e. wherein Z is H, by treatment of the corresponding salified form of the polymer with a concentrated acid solution.

[0141] Suitable fluorinated ion exchange polymers comprising -SO2X’ functional groups are those polymers comprising recurring units deriving from at least one ethylenically unsaturated fluorinated monomer containing at least one - SO2X’ functional group (monomer (A) as hereinafter defined) and recurring units deriving from at least one ethylenically unsaturated fluorinated monomer (monomer (B) as hereinafter defined).

[0142] The phrase “at least one monomer” is used herein with reference to monomers of both type (A) and (B) to indicate that one or more than one monomer of each type can be present in the polymer. Hereinafter the term monomer will be used to refer to both one and more than one monomer of a given type.

[0143] Non limiting examples of suitable monomers (A) are:- sulfonyl halide fluoroolefins of formula: CF2=CF(CF2)PSO2X’ wherein p is an integer between 0 and 10, preferably between 1 and 6, more preferably p is equal to 2 or 3, and wherein preferably X’=F;- sulfonyl halide fluorovinylethers of formula: CF2=CF-O-(CF2)mSO2X’ wherein m is an integer between 1 and 10, preferably between 1 and 6, more preferably between 2 and 4, even more preferably m equals 2, and wherein preferably X’=F;- sulfonyl halide fluoroallylethers of formula: CF2=CF-CF2-O-(CF2)qSO2X’ wherein q is an integer from 1 to 10, preferably from 1 to 6, more preferably from 2 to 4, and wherein preferably X’=F;- sulfonyl halide fluoroalkoxyvinylethers of formula:CF2=CF-(OCF2CF(RFi))w-O-CF2(CF(RF2))ySO2X’ wherein w is an integer between 0 and 2, RFI and RF2, equal or different from each other, are independently F, Cl or a C1-C10 fluoroalkyl group, optionally substituted with one or more ether oxygens, y is an integer from 0 to 6; preferably w is 1 , RFI is -CF3, y is 1 and RF2 is F, and wherein preferably X’=F;- sulfonyl halide aromatic fluoroolefins of formula CF2=CF-Ar-SO2X’ wherein Ar is a C5-C15 aromatic or heteroaromatic substituent, and wherein preferably X’=F.

[0144] Preferably monomer (A) is selected from the group of the sulfonyl fluorides, i.e. wherein X’=F.

[0145] More preferably monomer (A) is selected from the group of the fluorovinylethers of formula CF2=CF-O-(CF2)m-SO2F, wherein m is an integer from 1 to 6, preferably from 2 to 4.

[0146] Even more preferably monomer (A) is CF2=CFOCF2CF2-SO2F (perfluoro-5- sulfonylfluoride-3-oxa-1 -pentene).

[0147] Non limiting examples of suitable ethylenically unsaturated fluorinated monomers of type (B) are:- C2-C8 fluoroolefins, such as tetrafluoroethylene, pentafluoropropylene, hexafluoropropylene, and hexafluoroisobutylene;- vinylidene fluoride and vinyl fluoride;- C2-C8 chloro- and / or bromo- and / or iodo-fluoroolefins, such as chlorotrifluoroethylene and bromotrifluoroethylene;- fluoroalkylvinylethers of formula CF2=CFORfi, wherein Rn is a Ci-Ce fluoroalkyl, e.g. -CF3, -C2F5, -C3F7;- fluoro-oxyalkylvinylethers of formula CF2=CFOROI , wherein R01 is a Ci- 012 fluoro-oxyalkyl having one or more ether groups, for example perfluoro-2- propoxy-propyl;- fluoroalkyl-methoxy-vinylethers of formula CF2=CFOCF2ORf2 in which Rt2 is a Ci-Ce fluoroalkyl, e.g. -CF3, -C2F5, -C3F7 or a Ci-Ce fluorooxyalkyl having one or more ether groups, like -C2F5-O-CF3;fluorodioxoles, of formula:wherein each of Rf3, Rf4, Rfs, Rf6, equal or different each other, is independently a fluorine atom, a Ci-Ce fluoroalkyl, optionally comprising one or more oxygen atom, e.g. -CF3, -C2F5, -C3F7, -OCF3, -OCF2CF2OCF3.

[0148] Preferably monomer (B) is selected among:- C3-C8 fluoroolefins, preferably tetrafluoroethylene and / or hexafluoropropylene ;- chloro- and / or bromo- and / or iodo-C2-Ce fluoroolefins, like chlorotrifluoroethylene and / or bromotrifluoroethylene;- fluoroalkylvinylethers of formula CF2=CFORfi in which Rn is a Ci-Ce fluoroalkyl, e.g. -CF3, -C2F5, -C3F7 ;- fluoro-oxyalkylvinylethers of formula CF2=CFOROI , in which R01 is a Ci- 012 fluorooxyalkyl having one or more ether groups, like perfluoro-2-propoxy- propyl.

[0149] More preferably monomer (B) is tetrafluoroethylene.

[0150] The fluorinated polymer comprising -SO2X’ functional groups may be prepared by any polymerization process known in the art. Suitable processes for the preparation of such polymers are for instance those described in EP 1323751 A (SOLVAY SOLEXIS SPA) 02 / 07 / 2003, EP 1172382 A (SOLVAY SOLEXIS SPA) 16 / 11 / 2002, W02018167190A1 and WO2023165912A1 .

[0151] Alternatively, the ion exchange polymer may be a non-fluorinated ion exchange polymer. The ion exchange polymer may be an aromatic polymer comprising ionic or ionizable groups, hereinafter referred to as “aromatic ion exchange polymer”. The aromatic ion exchange polymer may be a polymeric compound having an aromatic ring in a main chain and an ion exchange group in a side chain and / or a main chain. An aromatic ion exchange polymer soluble in a solvent is typically used.

[0152] Examples of ion exchange groups include cation exchange groups such as - SO3H, -COOH, -PO(OH)2, -POH(OH), -SO2NHSO2- and -Ph(OH) (Ph denotes a phenyl group). These ion exchange groups may be directly bound with an aromatic ring composing a polymeric main chain, introduced into a substituent in an aromatic ring and an aliphatic chain composing a main chain as well as a side chain, or a combination thereof.

[0153] Among these, a sulfonic group or a phosphonic group is preferable and most preferably a sulfonic group.

[0154] Typical examples of an aromatic ion exchange polymer suitable for composition (C) include a polymer in which a sulfonic group and / or a phosphonic group are introduced into a polymer having an aromatic ring in a main chain.

[0155] A polymer having an aromatic ring in a main chain may be, for example, such that a main chain is discontinued by a hetero atom such as an oxygen atom. Examples of such an aromatic ion exchange polymer include polymers in which ion exchange groups such as sulfonic groups and phosphonic groups are introduced into any of the following polymers: polyether ketone, polyether ether ketone, polysulfone, polyether sulfone, polyether ether sulfone, poly(arylene ether), polyimide, polyphenylene, poly((4-phenoxybenzoyl)-1 ,4- phenylene), polyphenylene sulfide and, sulfoarylated polybenzimidazole, sulfoalkylated polybenzimidazole, phosphoalkylated polybenzimidazole, and phosphonated poly(phenylene ether).

[0156] Examples of such an aromatic ion exchange polymer include an alternating copolymer and a random copolymer having a repeating unit into which an ion exchange group is introduced and a repeating unit into which an ion exchange group is not introduced. Alternatively, the aromatic ion exchange polymer may be a block copolymer having a segment into which an ion exchange group is introduced and a segment into which an ion exchange group is not substantially introduced by one or more with respect to each of them. Among these, a block copolymer having a segment into which an ion exchange group is introduced and a segment into which an ion exchange group is not introduced is preferable, and an ion exchange group thereof is preferably a sulfonic group and / or a phosphonic group, particularly preferably a sulfonic group.

[0157] The ion exchange capacity of a ion exchange polymer is preferably 0.2 to 4.0 meq / g; the lower limit is more preferably 0.5 or more, particularly preferably 0.8 or more, and the upper limit is more preferably 3.0 or less, particularly preferably 2.5 or less. An ion exchange capacity of less than 0.2 meq / g causes output as a fuel cell to be lowered by reason of lowering ionic conductance, while an ion exchange capacity of more than 4.0 meq / g causes water resistance of an obtained membrane to be lowered, either of which case is not preferable.

[0158] The cerium oxide / POM particles according to the invention are present in composition (C) in any amount sufficient to reduce the degree of radical degradation of the ion exchange polymer.

[0159] The amount of Ce element with respect to the total weight of the ion exchange polymer in composition (C) comprising the cerium oxide / POM particles according to the invention is generally of at least 0.1 wt %, preferably of at least 0.2 wt %, more preferably of at least 0.5 wt %.

[0160] The amount of Ce element with respect to the total weight of the ion exchange polymer in composition (C) comprising the cerium oxide / POM particles according to the invention generally does not exceed 20.0 wt %, preferably it does not exceed 15.0 wt %, more preferably it does not exceed 10.0 wt %.

[0161] Composition (C) may be prepared using conventional methods.

[0162] When both the ion exchange polymer and the cerium oxide / POM particles are provided in solid form, for instance in the form of powder, pellets or granules, composition (C) may be prepared using techniques such as dry blending, melt blending, or extrusion.

[0163] An object of the present invention is thus, a process for the preparation of a composition (C) comprising blending cerium oxide / POM particles detailed above and an ion exchange polymer in solid form.

[0164] Alternatively, another object of the present invention is a process for the preparation of a liquid composition (LC) comprising blending the dispersion of cerium oxide / POM particles which is the object of the present invention and an ion exchange polymer.

[0165] This process is advantageous for the preparation of compositions (C) wherein the ion exchange polymer functional groups are in the acid or salt form, and in particular -SO3H functional groups.

[0166] Liquid composition (LC) may be prepared according to a process comprising a step in which a liquid composition comprising the ion exchange polymer dispersed or dissolved in a liquid medium (Lp) is prepared by a dissolution process under suitable temperature conditions, for instance as described in WO9816581A1.

[0167] Generally, the liquid medium comprises water, alcohol or a water / alcoholic mixture, and optionally comprises additional ingredients and / or additives.

[0168] Suitable alcohols which can be used, in particular as water / alcoholic mixture, are notably methanol, ethanol, propyl alcohols (i.e. 1 -propanol, 2-propanol), ethylene glycol, diethylene glycol.

[0169] The liquid medium may further comprise a solvent selected from polar aprotic organic solvents such as ketones, like acetone, methylethylketone, esters, like methylacetate, dimethylcarbonate, diethylcarbonate, ethylacetate, nitriles, like acetonitrile, sulfoxides, like dimethylsulfoxide (DMSO), sulfones like dimethylsulfone (DMSO2), amides, like N,N-dimethylformamide, N,N- dimethylacetamide, pyrrolidones, like N-methylpyrrolidone, N- ethylpyrrolidone and mixtures thereof.

[0170] Good results have been obtained when the liquid medium is water or a mixture of water and alcohol, preferably of water and propyl alcohol(s).

[0171] Good results have been also obtained when the liquid medium is a mixture of water, propyl alcohol(s) and DMSO2.

[0172] The cerium oxide / POM particles may be added to the liquid composition comprising the ion exchange polymer in the solid form or, preferably, as the previously described dispersion according to the invention to give a liquid composition (LC) comprising an ion exchange polymer and cerium oxide / POM particles dispersed in a liquid medium (L).

[0173] A further object of the invention is a liquid composition (LC) comprising an ion exchange polymer comprising -SO2X functional groups, advantageously a fluorinated ion exchange polymer, and cerium oxide / POM particles. Liquid composition (LC) comprises the ion exchange polymer comprising -SO2X functional groups and the cerium oxide / POM particles dispersed or dissolvedin a liquid medium (L). Typically the liquid medium (L) comprises water or a mixture of water and alcohol.

[0174] In some embodiments, the liquid medium (L) is water or a mixture of water and propanol(s), preferably 1 -propanol.

[0175] In some other embodiments, the liquid medium (L) is a mixture of water, propanol(s), preferably 1 -propanol, and DMSO2.

[0176] Preferably the ion exchange polymer, preferably the fluorinated ion exchange polymer, in the liquid composition (LC) is in its ionic form, i.e. it comprises - SO3Z functional groups, wherein Z is as defined above, and in particular - SO3H functional groups.

[0177] The liquid composition (LC) comprising a fluorinated ion exchange polymer and cerium oxide / POM particles may optionally comprise additional ingredients.

[0178] An object of the present invention is thus a process for the preparation of a liquid composition (LC) comprising blending an aqueous dispersion comprising the cerium oxide / POM particles and an ion exchange polymer comprising -SO2X functional groups in a liquid medium (L). The ion exchange polymer is typically a fluorinated ion exchange polymer.

[0179] The composition (C) or the liquid composition (LC) of the invention is particularly suitable for the preparation of proton exchange membranes and electrocatalytic layers for use in fuel cell applications. It is also particularly suitable for the preparation of ion exchange membranes and electrocatalytic layers for use in water electrolysis applications. It is also suitable for the preparation of ion exchange membranes for use in redox flow batteries.

[0180] Indeed, the presence of the cerium oxide / POM particles of the invention has shown to improve the resistance of ion exchange polymers, in particular fluorinated ion exchange polymers comprising -SO2X functional groups towards radical degradation. This effect is shown by the longer lifetime of ion exchange membranes obtained therefrom at the conditions of use.

[0181] A further object of the present invention is an article comprising at least one ion exchange polymer and cerium oxide / POM particles as defined above. The cerium oxide / POM particles preferably comprise a polyoxometalate which is selected from compounds of formula [HhXxMi204o]n’, mixtures thereof and recombinations thereof, wherein:when M=W and h=2 and x=0, n is 6, 8 or 12; when M= W, X=P, h=0, x=1 and n=3; when M= W, X=Si, h=0, x=1 and n=4.

[0182] Advantageously the article comprises at least one fluorinated ion exchange polymer comprising -SO2X functional groups and cerium oxide / POM particles as defined above. Preferably, the article comprises a fluorinated ion exchange polymer comprising -SO2X functional groups and cerium oxide / POM particles comprising a polyoxometalate which is selected from compounds of formula [HhXxMi204o]n’, mixtures thereof and recombinations thereof, wherein: when M=W and h=2 and x=0, n is 6, 8 or 12; when M= W, X=P, h=0, x=1 and n=3; when M= W, X=Si, h=0, x=1 and n=4.

[0183] In a first embodiment the article is a proton exchange membrane for a fuel cell, an electrolysis or a redox flow battery device , herein referred to also as a “membrane”.

[0184] Compositions (C) comprising an ion exchange polymer and the cerium oxide / POM particles in solid form may advantageously be converted into membranes by conventional extrusion techniques.

[0185] When the ion exchange polymer is a fluorinated polymer it is typically extruded with the functional groups in the -SO2F form. The extruded films can subsequently be converted into ion conducting membranes by hydrolysis, i.e. conversion of the -SO2X’ functional groups into the corresponding -SO3H functional groups, as discussed above.

[0186] Membranes can be obtained from liquid compositions (LC) according to the invention comprising an ion exchange polymer, typically comprising -SO3Z functional groups, preferably -SO3H functional groups, and the cerium oxide / POM particles using techniques known in the art. Known, useful techniques are for instance impregnation, casting, coating, e.g. roller coating, gravure coating, reverse roll coating, dip coating, spray coating.

[0187] The membranes may optionally be reinforced, for instance by lamination of the extruded membrane to a suitable reinforcing support or by impregnation of the liquid composition (LC) onto a porous support. Suitable supports may be made from a wide variety of components. The porous supports may bemade from hydrocarbon polymers such as woven or non-woven polyolefin membranes, e.g. polyethylene or polypropylene, or polyesters, e.g. polyethylene terephthalate). Porous supports of fluorinated polymers are generally preferred for use in fuel cell applications because of their high chemical inertia. Biaxially expanded PTFE porous supports (otherwise known as ePTFE membranes) are among preferred supports. These supports are notably commercially available under trade names GORE-TEX®, TETRATEX®.

[0188] In a second embodiment the article is an electrocatalytic layer.

[0189] Electrocatalytic layers may advantageously be prepared starting from a liquid composition (LC) according to the invention comprising catalyst particles in addition to an ion exchange polymer, typically comprising -SO3Z functional groups, preferably -SO3H functional groups, and the cerium oxide / POM particles. Said liquid compositions are generally referred to as “catalytic inks”. Typical catalyst particles comprise an active compound selected among metals like iron, manganese, cobalt, nickel, platinum, ruthenium, gold, palladium, rhodium, indium; their electro conductive oxides and alloys. The active compound is generally supported on a suitable material, herein called “carrier”, which is preferably electrically conductive. The carrier is advantageously chosen from carbon powder, for instance carbon black.

[0190] The amount of catalyst particles (including the carrier, if any) in the catalytic ink is generally of at least 1 wt% based on the total weight of the catalytic ink. Preferably, it is of at least 3 wt% and more preferably of at least 5wt %. The amount of catalyst particles (including the carrier, if any) in the catalytic ink is advantageously of at most 50 wt% based on the total weight of the catalytic ink, preferably of at most 40 wt% and more preferably of at most 30 wt%.

[0191] The electrocatalytic layers may for instance be prepared by screen printing or solution coating the catalyst ink on the surface of a proton exchange membrane. The ion exchange membrane may comprise cerium oxide / POM particles, having the same or different composition as the cerium oxide / POM particles present in the catalytic ink, or it may be free of the cerium oxide / POM particles.

[0192] In a third embodiment the article is a membrane electrode assembly. The membrane electrode assembly comprises a membrane having first andsecond surface, a first electrocatalytic layer adhered to said first surface and a second electrocatalytic layer adhered to said second surface, wherein at least one of said membrane, said first or second electrocatalytic layers comprises an ion exchange polymer comprising -SO2X functional groups and cerium oxide / POM particles as defined above. When the cerium oxide / POM particles are present in more than one component of the membrane electrode assembly the cerium oxide / POM particles may be the same or different.

[0193] Finally an object of the present invention is a fuel cell, an electrolysis cell or a redox flow battery comprising the article as above defined.

[0194] All definitions and preferences defined previously within the context of cerium oxide / POM particles or of the process for their preparation apply to the compositions comprising the cerium oxide / POM particles and an ion exchange polymer composition as well as to any article containing said compositions.

[0195] The invention will be now described in more detail with reference to the following examples, whose purpose is merely illustrative and not intended to limit the scope of the invention.

[0196] EXAMPLES

[0197] The invention will now be further described in the following examples without intending to limit it.

[0198] The aqueous dispersion of cerium oxide used as starting material in the following examples is:

[0199] Dispersion A1 : a dispersion containing cerium oxide particles prepared by a precipitation method as described in Example 1 , wherein the hydrodynamic mean diameter of the cerium oxide particles is Dh=39 nm, as measured by dynamic light scattering.

[0200] Dispersion A2: a commercially available dispersion of cerium oxide particles provided by Solvay SA of the Zenus® product range wherein the hydrodynamic mean diameter of the cerium oxide particles is Dh=87 nm, as measured by dynamic light scattering.

[0201] Dispersion A3: a commercially available dispersion of cerium oxide particles marketed by Merck Sigma Aldrich® (ref. 643009) wherein the hydrodynamicmean diameter of the cerium oxide particles is Dh=143 nm, as measured by dynamic light scattering.[002021 Dispersion A4: a commercially available dispersion of cerium oxide particles marketed by Nyacol® (ref. NYACOL® CeO2(NO3)) wherein the hydrodynamic mean diameter of the cerium oxide particles is Dh=31 nm, as measured by dynamic light scattering.

[0203] All the other materials used were commercially available materials supplied by Merck Sigma Aldrich:- Ammonium metatungstate hydrate (NH4)eH2Wi204o.xH20 - reference 463922;- Phosphotungstic acid hydrate H3PW12O40.XH2O - reference 455970;- Tungstosilicic acid hydrate H4SiWi204o.xH20 - reference: T2786.

[0204] Characterization

[0205] Particles size measurement

[0206] Dynamic light scattering (DLS) measurements of the particles contained in the dispersions prepared in the following examples were made using a Zetasizer Nano-ZS Malvern apparatus following the guidelines of the constructor. Samples of the dispersions to be analyzed were prepared in the following manner. A volume of 100 mL of the dispersion was magnetically stirred at 300 rpm. A droplet of the previously homogenized dispersion was introduced in 4mL of MilliQ water and re-homogenized using a pipette.

[0207] Infrared (IR) Spectroscopy

[0208] The presence of adsorbed polyoxometalate species on cerium oxide particles was detected using infrared spectroscopy on the solid particles obtained at the end of step (c) of the process. The dispersion obtained at the end of step (c) was centrifuged (9600 rpm 15 min - Sigma 6 16KS - Refrigerated bench top centrifuge, rotor Sigma 12269) and the resulting product was flash dried for 1 hour at 160°C with a Halogene HC-103 dessicant from Mettler Toledo. The infrared spectrum of the sample (CeO2 with adsorbed POM) was analyzed in Total Attenuated Reflection mode, on a Broker Tensor 27 spectrometer equipped with the ATR Diamant accessory, with the following parameters:- Measurement range: 650 to 4000 cm’1- Resolution: 4 cm’1- No. of scans: 35

[0209] Zeta potentialThe zeta potential measurements were made using a Zetameter DT-300 from Dispersion Technology. A volume of 50 mL of each of the dispersions prepared in the examples was poured into a container under magnetic stirring. The pHmeter and zetameter probes were calibrated following the constructor’s guidelines (referring to the ISO 13099-3:2014 Colloidal systems - Methods for zeta potential determination - Part 3: Acoustic methods). After cleaning with deionized water and drying with a tissue the pH and zeta potential probes, the pH and zeta potential of the agitated dispersions were measured with the Zetameter.

[0210] Quantification of the elemental concentration of metals by ICP-OES

[0211] Samples of 100 grams of the dispersions containing the cerium and polyoxometalate particles prepared in the following examples were taken.

[0212] Quantification of [Ce]

[0213] An aliquot of each sample was taken and diluted with nitric acid acidified water (2% vol / vol) to achieve a concentration within the range of 0.1 to 5 mg / L (e.g., a dilution of 1 / 15000 (wt / wt); the concentration of elemental Ce therein was quantified (in g / L) by inductively coupled plasma optical emission spectroscopy (ICP-OES) using wavelengths at 413.765 nm and 418.660nm. This concentration [Ce] was related to the total concentration of cerium oxide present in the dispersion.

[0214] Quantification of [Wltotai

[0215] An aliquot of each sample was taken and diluted to with nitric acid acidified water (2% vol / vol) to achieve a concentration within the range of 0.1 to 5 mg / L (e.g., a dilution of 1 / 15000 (wt / wt); the concentration of elemental W therein was quantified (in g / L) by inductively coupled plasma optical emission spectroscopy (ICP-OES) using wavelengths at 207.911 nm and 239.709 nm. This concentration [W]totai was related to the total concentration of the polyoxometalate species present in the dispersion, adsorbed and not adsorbed.

[0216] Quantification of [Wlnot adsorbed

[0217] The samples were centrifuged at 10,000 rpm during 15 minutes, using a Sigma 6 16KS - Refrigerated bench top centrifuge, rotor Sigma 12269. The top ofsupernatant was then carefully recovered, diluted with nitric acid acidified water (2% vol / vol) to achieve a concentration within the range of 0.1 to 5 mg / L (e.g., a dilution of 1 / 15000 (wt / wt); the concentration of elemental metal W therein was quantified (in g / L) by ICP-OES using wavelengths at 207.911 and 239.709. This concentration [W]not adsorbed was related to the concentration of the polyoxometalate species which were not adsorbed on the cerium particles.

[0218] Calculations

[0219] Based on the quantifications of elemental tungsten and elemental cerium in the samples, the following calculations were made:Ratio of not adsorbed elemental W relative to the total elemental W in the dispersion sample:wherein:[W]totai is the concentration of elemental W quantified by ICP-OES (g / L) in the whole sample before centrifugation.[W] not adsorbed is the concentration of elemental W quantified by ICP (g / L) in the supernatant retrieved after centrifugation of the sample.Ratio of adsorbed elemental W relative to the total elemental W in the dispersion sample:wherein:[W]totai and [W]not adsorbed are as defined above.Molar ratio of elemental Ce relative to the total elemental W in the dispersion sample:wherein:[Ce] is the concentration of cerium oxide quantified by ICP-OES (g / L) in the whole sample (before centrifugation),Mce is the molar mass of cerium,[W]totai is the concentration of elemental W quantified by ICP-OES (g / L) in the whole sample (before centrifugation),Mw is the molar mass of tungsten.Molar ratio of elemental Ce relative to the adsorbed elemental W in the dispersion sample:wherein:[Ce], [W]totai Mce, Mw and Wadsorbed are as defined above.Cerium oxide weight concentration in the dispersion sample: 100wherein: dsampie denotes the density of the sample, measured as described in ISO-758- 1976 “Liquid chemical products for industrial use - Determination of density at 20°C”Mce is the molar mass of cerium,Mce02 is the molar mass of cerium oxide.

[0220] Preparation of Dispersion A1

[0212] A dilute cerium nitrate solution was prepared by adding 13.6 kg of a 2.9M trivalent cerium nitrate solution, 2.1 kg of 68wt% HNO3 solution (density d=1 .7), 0.48 kg of deionized water and cerium nitrate (IV) corresponding to a molar ratio Ce IV I Ce total = 1 / 100. This solution was loaded into a semiclosed 20 L vessel and then degassed with agitation and with nitrogen bubbling.

[0213] A dilute aqueous ammonia solution was prepared by mixing 79 kg of deionized water and a solution of 10.1 kg of 25wt % aqueous ammonia. This solution was loaded into a semi-closed 100 L jacketed reactor and then subjected to agitation and nitrogen bubbling.

[0214] The diluted cerium nitrate solution was then added in approximately 30 min, at ambient temperature, to the dilute aqueous ammonia solution, with the same agitation and under nitrogen sweeping. The temperature of the reaction mixture was then increased to 80°C and then maintained at this temperature for 1 hour. At the end of this heat treatment, the reaction mixture was left to cool and was acidified to pH 2 by adding 68 wt% HNO3.

[0215] The reaction mixture was filtrated and washed with deionized water. The washing was repeated until the conductivity of washing solution was less 0.04 mS / cm. The amount of water of the dispersion was adjusted so as to reach a CeO2 content of 25 wt% relative to the total weight of the aqueous dispersion (Dispersion A1 ).

[0216] The hydrodynamic mean diameter of the cerium oxide particles was of 39 nm, as measured by dynamic light scattering.

[0217] Preparation of the dispersions of cerium and polyoxometalate particles

[0218] Example 1

[0219] 32 grams of Dispersion A1 of cerium oxide particles (CeO2 content was 25 wt%) was diluted with 160 grams of water. The pH was 4.0. 1 .56 grams of a salt of ammonium metatungstate (NH4)eH2Wi204o.xH20 was dissolved in 40 grams of water. The pH was 4.2.

[0220] The diluted cerium oxide suspension was added over the course of 30 minutes under stirring to the solution of ammonium metatungstate. After addition, the mixture was stirred for a period of 30 minutes. The pH was 4.0.

[0221] The molar ratio Ce / W in the dispersion was 7.2.

[0222] The hydrodynamic mean diameter of the cerium oxide / POM particles obtained was 110 nm as measured by DLS. The zeta potential was -32 mV.

[0223] The concentration by weight of CeO2 in the dispersion was of 3.43 wt% relative to the total weight of the dispersion.

[0224] The presence of adsorbed polyoxometalate species on the cerium oxide particles was evidenced by infra-red spectroscopy in total attenuated reflection mode as a band assigned to W-0 was observed at 958 cm’1.

[0225] Using the quantification method by ICP-OES described above, it was established that the Ratio Wadsorbed was 0.75 meaning that 75% of the elemental W present in the dispersion in the form of POM were adsorbed onto the surface of cerium oxide particles. The molar ratio Ce / Wadsorbed was: 7.2 / 0.75 = 9.6.

[0226] Example 2

[0227] An amount of 32 grams of Dispersion A1 (CeO2 content was 25 wt%) was diluted with 160 grams of water. The pH was 4.0.

[0228] 1 .55 grams of a salt of tungstosilicic acid H4SiWi204o.xH20 was dissolved in 40 grams of water. The pH was 1 .6.

[0229] The diluted cerium oxide suspension was added over the course of 30 minutes under stirring to the solution of tungstosilicic acid. After addition, the mixture was stirred for 30 minutes. The pH was 2.1 .

[0230] The molar ratio Ce / W in the dispersion was 7.2.

[0231] The hydrodynamic mean diameter of the cerium oxide / POM particles obtained was 190 nm as measured by DLS. The concentration by weight of CeO2 in the dispersion was 3.43 wt% relative to the total weight of the dispersion. The zeta potential was -21 mV.

[0232] Using the quantification method by ICP-OES described above, it was established that the Ratio Wadsorbed was 0.75 meaning that 75% of the elemental W present in the dispersion in the form of POM were adsorbed on the surface of cerium oxide particles. The molar ratio Ce / Wadsorbed was of 9.5.

[0233] Example 3

[0234] An amount of 32 grams of Dispersion A1 (CeO2 content was 25 wt%) was diluted with 160 grams of water. The pH was 4.0.

[0235] 1 .54 grams of a salt of phosphotungstic acid H3PW12O40.XH2O was dissolved in 40 grams of water. The pH was 1 .7.

[0236] The diluted cerium oxide suspension was added over the course of 30 minutes under stirring to the solution of phosphotungstic acid. After addition, the mixture was stirred for 30 minutes. The pH was 2.1 .

[0237] The molar ratio Ce / W in the dispersion was 7.2.

[0238] The hydrodynamic mean diameter of the cerium oxide / POM particles was 50 nm as measured by DLS and the zeta potential was -35 mV.

[0239] Using the quantification method by ICP-OES described above, it was established that 95% of the elemental W present in the dispersion in the form of POM were adsorbed on the surface of cerium oxide particles. The molar ratio Ce / Wadsorbed was 7.6.

[0240] Example 4

[0241] An amount of 228 grams of Dispersion A3 (with CeO2 content = 10 wt%) was prepared. The pH was 4.3.

[0242] 1 .44 grams of a salt of tungstosilicic acid H4SiWi204o.xH20 was dissolved in 34.5 grams of water. The pH was 1 .5.

[0243] The 10 wt% cerium oxide suspension was added over the course of 30 minutes under stirring to the solution of tungstosilicic acid. After addition, the mixture was stirred for 30 minutes. The pH was 2.0.

[0244] The molar ratio Ce / W in the dispersion was 7.8.

[0245] The hydrodynamic mean diameter of the cerium oxide / POM particles was 137 nm as measured by DLS and the zeta potential was -24 mV.

[0246] Using the quantification method by ICP-OES described above, it was established that 48% of the elemental W present in the dispersion in the form of POM were adsorbed on the surface of cerium oxide particles. The molar ratio Ce / Wadsorbed was 16.4.

[0247] Example 5

[0248] An amount of 24.2 grams of Dispersion A4 (CeO2 content was 20 wt%) was diluted with 125.8 grams of water. The pH was 1.8.

[0249] 0.46 grams of a salt of tungstosilicic acid H4SiWi204o.xH20 was dissolved in 11 .2 grams of water. The pH was 1 .5.

[0250] The diluted cerium oxide suspension was added over the course of 30 minutes under stirring to the solution of tungstosilicic acid. After addition, the mixture was stirred for 30 minutes. The pH was 2.2.

[0251] The molar ratio Ce / W in the dispersion was 18.

[0252] The hydrodynamic mean diameter of the cerium oxide / POM particles was 115 nm as measured by DLS and the zeta potential was +35 mV.

[0253] Using the quantification method by ICP-OES described above, it was established that 72% of the elemental W present in the dispersion in the form of POM were adsorbed on the surface of cerium oxide particles. The molar ratio Ce / Wadsorbed was 24.9.

[0254] Example 6

[0255] An amount of 60.5 grams of Dispersion A4 (CeO2 content was 20 wt%) was diluted with 314.5 grams of water. The pH was 1.8.

[0256] 1 .2 grams of a salt of ammonium metatungstate (NH4)eH2Wi204o.xH20 was dissolved in 20.8 grams of water. The pH was 4.1.

[0257] The diluted cerium oxide suspension was added over the course of 30 minutes under stirring to the solution of tungstosilicic acid. After addition, the mixture was stirred for 30 minutes. The pH was 2.4.

[0258] The molar ratio Ce / W in the dispersion was 17.9.

[0259] The hydrodynamic mean diameter of the cerium oxide / POM particles was 32 nm as measured by DLS and the zeta potential was +21 mV.

[0260] Using the quantification method by ICP-OES described above, it was established that 97% of the elemental W present in the dispersion in the form of POM were adsorbed on the surface of cerium oxide particles. The molar ratio Ce / Wadsorbed was 18.5.

[0261] Tables 1 to 4 summarize the main parameters and features of the examples.Table 1 : Main features of the raw materials used for Examples 1 to 3Table 2: Main features of the aqueous dispersions of cerium oxide and polyoxometalate particles of Ex. 1 to 3Table 3: Main features of the raw materials used for Examples 4 to 6Table 4: Main features of the aqueous dispersions of cerium oxide and polyoxometalate particles of Ex. 4 to 6

[0262] Evaluation of the colloidal stability

[0263] The suspension of cerium oxide and polyoxometalate particles prepared in Examples 1 , 4 and 5 were tested for colloidal stability over time at room temperature (25°C). On day 0, 52 mL of the suspension was taken from the original batch for testing. The sample was centrifuged and redispersed in deionized water to a concentration of CeO2 / POM particles of 1 wt% in water. An aliquot was taken for DLS measurement. The aliquot diluted in water (miliQ grade), homogenized in ultrasound bath and characterized using Malvern Panalytical Zetasizer Advance Pro Blue. The protocol was repeated after a number of days as reported in the following Table 5.

[0264] For Example 1 the value of the hydrodynamic mean diameter Dh showed a slight increase after 14 and 21 days, however it remained well below the threshold of 300 nm. Similar results were observed for Example 4 and 5.

[0265] Table 5: Dh measured at different time (days)

[0266] Chemical stability test

[0267] The dispersion of Examples 4 as obtained at the end of step (c) was centrifuged (9600 rpm 15 min - Sigma 6 16KS - Refrigerated benchtop centrifuge, rotor Sigma 12269) and the resulting product was dried (120°C, overnight) generating a solid powder (powder 1 ) and a supernatant (supernatant 1 ).1 g of the powder 1 was dispersed in approx. 99 g of 1 M H2SO4 and the dispersion was heated at 80°C for 24 hours under stirring (500 rpm) to carry out a first leaching cycle (cycle 1 ).After 24 hours the dispersion was cooled down to room temperature (25°C) and centrifuged (9600 rpm 15 min - Sigma 6 16KS - Refrigerated benchtop centrifuge, rotor Sigma 12269).The liquid (supernatant 2) and the wet solid (powder 2) fractions were separated.1 g of the wet powder 2 was then dispersed in approx. 99 g of 1 M H2SO4. The dispersion was heated at 80°C for 24 hours under stirring (500 rpm) to carry out a second leaching cycle (cycle 2).After 24 hours the dispersion was cooled down to room temperature and centrifuged (9600 rpm 15 min - Sigma 6 16KS - Refrigerated benchtop centrifuge, rotor Sigma 12269) generating a wet powder (powder 3) and a supernatant (supernatant 3).The supernatant 2 obtained at the end of cycle 1 and the supernatant 3 obtained at the end of cycle 2 were analysed by ICP-OES to determine the mass of cerium leached by sulphuric acid.The above procedure was carried out also on the dispersions of Example 5 and 6 and, for comparative purposes, on the Dispersions A3 and A4.

[0268] The results are shown in Table 6.Table 6: Chemical stability test* After cycle 1 the amount of powder recovered by centrifugation was insufficient to carry out cycle 2.Note: percentages are expressed as wt% of Ce leached referred to the weight of Ce present in the initial dispersion

[0269] In each case the total amount of cerium leached by the cerium oxide / POM particles was lower than that of the of the cerium oxide particles in the comparative starting dispersion thus indicating that the cerium oxide / POM particles are more stable than cerium oxide particles under harsh conditions.

[0270] Example 7- Preparation of fluorinated ion exchange polymer (P1) comprising -SOsH functional groups

[0271] In a 22 L autoclave the following reagents were charged:11 .5 L of demineralised water;980 g of the monomer with formula: CF2=CF-O-CF2CF2-SO2F3100 g of a 5% weight solution of CF2CIO(CF2CF(CF3)O)n(CF2O)mCF2COOK in water (average molecular weight = 521 , ratio n / m=10).

[0272] The autoclave, stirred at 470 rpm, was heated at 60°C. A water-based solution with 6 g / L of potassium persulfate was added in a quantity of 150mL. The pressure was maintained at a value of 12 bar (abs) by feeding tetrafluoroethylene.

[0273] After adding 1200 g of tetrafluoroethylene in the reactor, 220 g of the monomer CF2=CF-O-CF2CF2-SO2F were added every 200 g of tetrafluoroethylene fed to the autoclave.

[0274] The reaction was stopped after 280 min by stopping the stirring, cooling the autoclave and reducing the internal pressure by venting the tetrafluoroethylene; a total of 4000 g of tetrafluoroethylene were fed.

[0275] The latex was then coagulated by freezing and thawing and the recovered polymer was washed with water and dried at 150°C for 24 hours. The polymer was then treated with fluorine gas in a metallic vessel for 8 hours at 80°C, then purged several hours with nitrogen to remove any residual unstable end-groups.

[0276] The polymer thus obtained was immersed in a KOH solution (10% by weight) at 80°C for 8 hours, followed by washing in demineralised water at room temperature. Immersion in a HNO3 solution (20% by weight) at room temperature for 2 hours, followed by washing in demineralised water at room temperature converted all functional groups into -SO3H functional groups.

[0277] The resulting fluorinated polymer in -SO3H form (P1 ) was then dried in a vacuum oven at 80°C. The equivalent weight of the polymer (EW) was determined (by IR analysis on the precursor polymer) to be 720 g / eq.

[0278] Example 8 - Membrane Preparation - General procedure

[0279] The suspension comprising polymer P1 alone (Membrane M1 ), polymer P1 and the cerium oxide / POM particles obtained in Examples 1 and 2 (Membranes M2 and M3) or polymer P1 and Dispersion A2 (Membrane M4) were used to prepare membranes according to the following procedure. The same procedure was used to prepare membranes starting from a suspension comprising polymer P1 and the cerium oxide / POM particles obtained in Examples 5 and 6 (Membranes M6 and M7) or comprising polymer P1 and Dispersion A4 (Membrane M5). The dispersion containing the cerium oxide particles (when present) was added to 20 g of a water dispersion of P1 (30 wt.%) further comprising 1 -propanol (6.6 g), DMSO2 (1.7g) and H2O (3.8g). This mixture was stirred at room temperature to homogenize the dispersion.

[0280] The compositions thus obtained were cast on a glass support using the doctor-blade technique with a wet thickness of 500 pm and then dried in a ventilated oven at a temperature of 60°C for 1 hour, from 60°C to 90°C in 1 hour, and then from 90°C to 190°C in 1 hour. The thickness of the resulting membrane was 60±5 pm.

[0281] The amount of Ce element measured by ICP-OES in membrane M2 was 0.74 % ±0.1 % w / wt % with regards to the total weight of the membrane (M2).

[0282] The amount of Ce element measured by ICP-OES in membrane M3 was 0.64 % ± 0.1 % w / wt % with regards to the total weight of the membrane (M3).

[0283] The amount of Ce element measured by ICP-OES in membrane M4 was 0.53 % ± 0.1 % w / wt % with regards to the total weight of the membrane (M4).

[0284] The amount of Ce element measured by ICP-OES in membrane M5 was 0.70 % ± 0.1 % w / wt % with regards to the total weight of the membrane (M5).

[0285] The amount of Ce element measured by ICP-OES in membrane M6 was 0.59 % ± 0.1 % w / wt % with regards to the total weight of the membrane (M6).

[0286] The amount of Ce element measured by ICP-OES in membrane M7 was 0.65 % ± 0.1 % w / wt % with regards to the total weight of the membrane (M7).

[0287] The preparation of the samples for ICP-OES analysis can be carried out according to the techniques known to the person skilled in the art, for example by subjecting a piece of membrane to digestion in an acid mixture capable of dissolving the element to be detected possibly followed by dilution with water to an appropriate concentration (e.g., 0.1 - 5 mg / L).

[0288] Example 9 - Fuel cell characterization of membranes prepared in Example 8

[0289] Membranes obtained as described in Example 8 were assembled in a custom-designed stack fixture with an active area of 50 cm2and tested on a Greenlight Innovation G100 test stand. The membranes were assembled with self-made catalyst-coated gas diffusion layers (~0.35 mg / cm2Pt).

[0290] The membranes M1 to M4 were tested at the following operating conditions: Anode side flow: 250 nccm pure H2, 61 °C dew point, 1 bar (abs); Cathode side flow: 250 nccm pure O2, 61 °C dew point, 1 bar (abs); Cell temperature: 90°C.The membranes M5 to M7 were tested under the same conditions used for M1 to M4, except for: air was used instead of pure O2; the pressure for H2 and O2 gases was set to 1 ,5 bar (abs); RH 30%.

[0291] The voltage was monitored during the test. The end of the test was set at a voltage below 0.7 V, which is typically assumed to indicate the formation of pinholes in the membrane. The results are reported in Table 7.Table 7* comparative

[0292] With respect to a membrane comprising fluorinated polymer (P1 ) alone (reference membrane (M1 )), the membranes (M2) and (M3) comprising the cerium oxide / POM particles of the invention show a significant increase in stability under fuel cell operating conditions. Indeed, the time to reach a voltage below 0.7 V is 560 hours in the case of the membrane (M2) and 778 hours in the case of the membrane (M3).

[0293] Moreover, the membranes (M2) and (M3) comprising the cerium oxide / POM particles of the invention shows a significant increase in stability under fuel cell operating conditions with respect to the membrane (M4) comprisingcerium oxide particles without any POM adsorbed on their surface. Indeed, the time to reach a voltage below 0.7 V is 560 hours in the case of the membrane (M2) and 778 hours in the case of membrane (M3) and only 118 hours in the case of the membrane (M4).

[0294] Similar results were observed for the membranes (M6) and (M7) according to the invention compared to membrane (M5) comprising cerium oxide particles without any POM adsorbed thereon.

[0295] Thus, the use of the cerium oxide / POM particles according to the invention is advantageous over the use of CeO2 particles alone.

Claims

Claims1 . An aqueous dispersion comprising particles of cerium oxide with polyoxometalate species adsorbed thereon wherein the pH of the dispersion ranges from 1.0 to 9.0, the hydrodynamic mean diameter Dh of the particles of cerium oxide with polyoxometalate species adsorbed thereon, as measured by dynamic light scattering, ranges from 10 nm to 300 nm.

2. The dispersion of claim 1 wherein the polyoxometalate species are selected from the group consisting of: compounds of formula [HhXxMi204o]n’, compounds of formula [Y2M18O62]n’, compounds of formula [HhXxM6O24]n’, compounds of formula [MeO ]11’, mixtures of any of the above and recombination species thereof, wherein h is 0 or 2, X is Si or P; x = 0 when h=2 and x=1 when h=0; Y is selected from Si or P; M is selected from W, Mo or V and n is an integer different from 0 denoting the number of charges of the compound; preferably n ranges from 2 to 12.

3. The dispersion of claim 1 or 2 exhibiting a negative zeta potential, in particular a zeta potential ranging from -60 to -10 mV, as measured on the dispersion at a concentration of the particles of cerium oxide with polyoxometalate species adsorbed thereon comprised between 0.1 wt% and 5 wt% by weight of cerium oxide.

4. The dispersion of any one of claims 1 to 3, exhibiting a positive zeta potential, in particular a zeta potential ranging from +10 to +60 mV, as measured on the dispersion at a concentration of the particles of cerium oxide with polyoxometalate species adsorbed thereon comprised between 0.1 wt% and 5 wt% by weight of cerium oxide.

5. The dispersion of any one of claims 1 to 4, wherein the pH thereof ranges from 1 .0 to 7.0, in particular from 1 .0 to 6.0, more particularly from 2.0 to 6.0.

6. The dispersion of any one of claims 1 to 5, wherein the molar ratio Ce / Madsorbed ranges from 2 to 625, in particular from 3 to 500, more particularly from 5 to 375, wherein Ce denotes the total molar amount of cerium present in the dispersion and Madsorbed the molar amount of metal M deriving from the adsorbed polyoxometalate species.

7. The dispersion of any one of claims 1 to 6, wherein the ratio of adsorbed elemental M (g / L) deriving from the adsorbed polyoxometalate species relative to the total elemental M (g / L) deriving from all the polyoxometalate species in the dispersion ranges from 0.40 to 1.00, in particular from 0.50 to 1.00, more particularly from 0.70 to 1 .00, for example from 0.75 to 0.95.

8. The dispersion of any one of claims 1 to 7, wherein the molar ratio Ce / Mtotai ranges from 2 to 250, in particular from 3 to 200, more preferably from 5 to 150, wherein Ce denotes the total molar amount of cerium present in the dispersion and Mtotai denotes the total molar amount of metal M deriving from the polyoxometalate species present in the dispersion.

9. A process for the preparation of the aqueous dispersion comprising particles of cerium oxide with polyoxometalate species adsorbed thereon of any one of claims 1 to 8 which comprises the following steps:(a) providing a dispersion of cerium oxide particles in an aqueous medium, wherein the cerium oxide particles have an hydrodynamic mean diameter Dh ranging from 8 nm to 298 nm, as measured by dynamic light scattering, and the pH of the dispersion ranges from 1 .0 to 9.0;(b) providing a solution comprising a polyoxometalate salt in an aqueous medium, wherein the pH of the solution ranges from 1 .0 to 8.0;(c) contacting the dispersion provided in step (a) and the solution provided in step (b) to form a reaction medium in which polyoxometalate species are adsorbed onto the cerium oxide particles.

10. The process according to claim 9 which further comprises at least one of the following steps:(d) performing a solid / liquid separation to separate the cerium oxide particles having polyoxometalate species adsorbed thereon from the liquid medium and dispersing the particles in an aqueous medium, wherein the liquid medium is either the reaction medium at the end of step (c) or the liquid medium obtained after step (e); and(e) washing and / or acidifying the dispersion.11 . A composition comprising a dispersion of any one of claims 1 to 8 and an ion exchange polymer, wherein the ion exchange polymer is selected from: a fluorinated polymer comprising -SO2X functional groups, wherein X is selected from X’ or from OZ and wherein X’ is selected from the groupconsisting of F, Cl, Br, I and Z is selected from the group consisting of H, alkaline metals, NF ; or a non-fluorinated polymer having an aromatic ring in a main chain and at least one ion exchange group in a side chain and / or a main chain, the ion exchange group being preferably selected from: SO3H, -COOH, -PO(OH)2, -POH(OH), - SO2NHSO2’ and -Ph(OH), wherein Ph denotes a phenyl group.

12. The composition according to claim 11 , wherein the particles of cerium oxide with polyoxometalate species adsorbed thereon are present in an amount of at least 0.1 wt % and not exceeding 20.0 wt % with regard to the weight of the ion exchange polymer.

13. A method for producing an article, in particular a membrane or an electrocatalyst layer, comprising:- depositing a liquid film comprising a composition according to any one of claims 11 or 12 on a surface,- drying the liquid film to form the article.

14. An article, in particular a membrane or an electrocatalyst layer, as obtainable from the method of claim 13.

15. A fuel cell, an electrolysis cell or a redox flow battery comprising at least one article as defined in claim 14.

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