Electrode comprising a composite coating and associated manufacturing process

Abstract

Electrode comprising a composite coating and associated manufacturing process. The invention relates to an electrode (1) and its manufacturing process, the electrode (1) comprising a body (10) covered at least partially by a composite coating (11) comprising a polypyrrole polymer (110) or its derivatives, the polymer (110) comprising pyrrole monomer units having a side chain comprising at least one carboxylate group, metal oxide particles (111) based on at least one metal, the amount of the at least one metal in the metal oxide being less than or equal to 7 mmol.cm-3 per carboxylate group of the pyrrole monomer units. The electrode (1) is improved, particularly in terms of resources, environment, and cost, while maintaining satisfactory electrocatalytic activity. Figure for the abstract: Fig. 1

Description

Title of the invention: Electrode comprising a composite coating and associated manufacturing method Technical field

[0001] The present invention relates to the field of electrodes whose surface is covered by a conductive composite polymer. It finds particularly advantageous application in the field of electrocatalysis of chemical reactions, and more particularly the electrocatalytic oxidation of water. STATE OF THE ART

[0002] For decades, inorganic micro- and nanoparticles have received considerable attention from researchers and industry due to improved catalytic performance compared to traditional heterogeneous catalysts and easier separation of target molecules compared to homogeneous catalysts.

[0003] There are many methods for manufacturing micro- and nanoparticles. These particles can be prepared by physical, chemical or mechanical means, although these methods often have a high manufacturing cost. Another limitation is their low stability during catalysis due to the aggregation phenomenon which leads to a reduction or even a loss of activity. Immobilizing these particles on the surface of an electrode in order to catalyze reactions electrochemically also represents an additional difficulty.

[0004] To overcome this, one approach is to immobilize these particles in a polymer-based coating. This approach facilitates the control of particle morphology, as well as their protection against degradation phenomena such as aggregation and corrosion. Many composite coatings exist today. (Shifrina, ZB et al., Role of polymer structures in catalysis by transition metal and metal oxide nanoparticle composites, Chemical Reviews, 2019, 120(2), 1350-1396).

[0005] It is known in particular from the document Boudissa, Nouredine, et al. "Synthesis of Manganese Dioxide by Mn2+ Complexation and Electro-Oxidation in Polypyrrole Benzoic Acid Films." Journal of Inorganic and Organometallic Polymers and Materials, 2020, pages 1-6, an electrode coated with a composite coating of polypyrrole of chemical formula [4-(pyrrol-1-yl methyl) benzoic acid] and metal oxide particles of formula MnO2. The monomer units bearing a carboxylic acid group make it possible to stabilize the metal ions of a solution by complexation, which makes it possible to make reliable the quantity of metal ions in- incorporated in the film for the formation of oxide particles by electro-oxidation.

[0006] In practice, existing solutions remain limited for obtaining high-performance electrodes while reconciling current economic, geopolitical and environmental issues. Indeed, the metals used in these metal oxide particles are rare and increasingly strategic resources. Their extraction is also expensive and generally polluting.

[0007] There is therefore a need to propose an improved electrode, particularly in terms of resources, environment and cost, while retaining satisfactory electrocatalytic activity.

[0008] Other objects, features and advantages of the present invention will become apparent from the following description and accompanying drawings. It is understood that other advantages may be incorporated. SUMMARY

[0009] To achieve this objective, according to a first aspect, an electrode is provided comprising a body covered at least in part by a composite coating. The composite coating comprises: - a polypyrrole polymer or its derivatives comprising pyrrole monomer units having a side chain, the side chain comprising at least one carboxylate group, - metal oxide particles based on at least one metal.

[0010] The coating comprises an amount of the at least one metal of the metal oxide less than or equal to 7 mmol / cm3 per carboxylate group of the pyrrole monomer units.

[0011] Thus, the quantity of metal in the coating is limited. The quantity of 7 mmol / cm3 is equivalent to a quantity of 245 nmol / cm2. During the development of the invention, it was surprisingly demonstrated that this quantity made it possible to obtain sufficient electrocatalytic activity. The electrode thus stands out from existing solutions using higher quantities of metal, to increase the performance of the electrodes.

[0012] As a comparative example, currently on an industrial scale, the quantity of metal catalyst used in proton exchange membrane electrolysers (commonly abbreviated as PEM, from the English Proton Exchange Membrane') in an acid medium is of the order of 5 to 10 mg / cm2. From an economic, environmental and geopolitical point of view, the objectives announced by IRENA (International Renewable Energy Agency) by 2050 are to move to 0.2 mg / cm2 (200 pg / cm2) or 1025 nmol / cm 2 of catalysts, while maintaining electrocatalytic activities making it possible to meet market expectations. The electrode in this respect exceeds these objectives with a quantity of metal of the order of pg / cm2 (and in particular between 3 and 10 pg / cm2, or between 50 and 180 nmol / cm2 depending on the nature of the metal). The electrode is therefore improved, particularly in terms of resources, environment and cost, while retaining satisfactory electrocatalytic activity.

[0013] According to a separable or combinable aspect, an electrode is provided comprising a body covered at least in part by a composite coating. The coating comprises: - a polypyrrole polymer or its derivatives comprising pyrrole monomer units having a side chain, the side chain comprising at least one carboxylate group, - metal oxide particles based on at least one metal.

[0014] The metal oxide particles have at least one dimension, preferably each dimension, less than or equal to 2 nm, preferably less than or equal to 1 nm.

[0015] A better distribution in the polymer film, as well as an increase in the exchange surface of the particles are obtained. This size of metal oxide particles therefore makes it possible to improve the electrocatalytic activity compared to existing solutions using micrometric-sized particles. This size is particularly advantageous for a limited quantity of metal in the coating, in order to improve the electrocatalytic activity of the electrode.

[0016] Another aspect relates to a method of manufacturing the electrode according to one of the two preceding aspects and comprising: - a supply of the body of the electrode, - an electropolymerization on the body of the electrode of a polymer comprising pyrrole monomer units having a side chain, the side chain comprising at least one carboxylic acid group, so as to form a coating covering at least part of the body, - bringing the coating into contact with a so-called “complexing” solution comprising at least one metal ion and a borate and / or phosphate buffer having a pH greater than or equal to 5.5, inducing complexation of the at least one metal ion by the at least carboxylate group, - after bringing the coating into contact with the complexing solution inducing the complexing of at least one metal ion, rinsing the coating, - after rinsing the coating, electro-oxidation of the complexed metal ions to form metal oxide particles.

[0017] During complexation, a pH greater than or equal to 5.5, allows on the one hand to quantitatively deprotonate the carboxylic groups into carboxylates in order to facilitate the complexation of the metal ions. The complexation of the metal ion is thus made more reproducible. Furthermore, the phosphate and / or borate ions introduced during complexation at this pH act as ligands at the periphery of the metal oxides generated during electro-oxidation. These ligands limit the aggregation phenomena during the formation of the oxide particles, promoting a small size, a limited quantity of metal as well as a homogeneous dispersion of the oxide particles in the coating. This further reduces the solubility of the oxide particles and improves the stability of the metal oxide particles formed.

[0018] Another aspect relates to a use of the electrode according to the first aspects or of the electrode manufactured by the method, the previous aspect for the electrocatalytic oxidation of water. BRIEF DESCRIPTION OF THE FIGURES

[0019] The aims, objects, as well as the characteristics and advantages of the invention will emerge more clearly from the detailed description of an embodiment thereof which is illustrated by the following accompanying drawings in which:

[0020] [Fig.l] [Fig.l] represents a diagram of an electrolyser comprising the electrode according to two exemplary embodiments.

[0021] [Fig.2] [Fig.2] represents an example of the manufacturing process of the electrode.

[0022] [Fig.3A][Fig.3B] Figures 3A and 3B each represent cyclovoltamperograms of the activity of the coating according to several embodiment examples, for the oxidation of water at pH 9.2 in a 0.1M borate buffer.

[0023] [Fig.4] [Fig.4] represents scanning electron microscopy images (SEM) of a polymer film and the coating according to several embodiment examples.

[0024] [Fig.5] [Fig.5] represents atomic force microscopy images and analyses of a polymer film and the coating according to several embodiment examples.

[0025] [Fig.6A][Fig.6B] Figures 6A and 6B represent transmission electron microscopy (TEM) images of a polymer film and the coating according to several exemplary embodiments.

[0026] [Fig.7] [Fig.7] represents a wide angle X-ray scattering graph in grazing incidence (commonly referred to in English by the acronym GiWAXS, for Grazing-Incidence Wide-Angle X-ray Seal ring) of a polymer film and the coating according to an exemplary embodiment.

[0027] [Fig.8A][Fig.8B][Fig.8C][Fig.8D][Fig.8E] Figures 8A to 8E represent several amperograms as a function of time and cyclovoltamperograms of the activity of the coating over time for the oxidation of water at pH 9.2, according to several exemplary embodiments.

[0028] [Fig.9A][Fig.9B] Figures 9A and 9B represent electron microscopy images electronic scanning electron microscope (SEM) with energy dispersive X-ray analysis (commonly referred to as EDX, from the acronym Energy Dispersive X-Ray Analysis) of a coating according to an exemplary embodiment, at Oh and 65h.

[0029] [Fig.lOA][Fig.lOB] Figures 10A and 10B represent scanning electron microscopy images with energy dispersive X-ray analysis (commonly referred to as EDX, from the acronym Energy Dispersive X-Ray Analysis) of a coating according to another exemplary embodiment, at Oh and 60h.

[0030] [Fig. 1 lA][Fig. 1 IB] Figures 1 IA and 1 IB respectively represent an amperogram as a function of time, and a cyclovoltamperogram of the activity over time, of the coating according to several embodiment examples.

[0031] [Fig.l2A][Fig.l2B] Figures 12A and 12B represent scanning electron microscopy images with EDX analysis of a coating according to an exemplary embodiment, at 0 and 90h.

[0032] The drawings are given as examples and are not limiting of the invention. They constitute schematic representations of principle intended to facilitate the understanding of the invention and are not necessarily on the scale of practical applications. In particular the relative dimensions between the polymer, the metal ions, the particles and the electrode are not representative of reality. DETAILED DESCRIPTION

[0033] Before beginning a detailed review of embodiments of the invention, optional features are set out below which may possibly be used in combination or alternatively.

[0034] According to one example, the metal oxide particles are based on at least two different metals. The catalytic activity of the electrode is thus improved. A bimetallic oxide in fact has significantly improved catalytic properties, compared to a single metal species. In addition, this also makes it possible to reduce manufacturing costs when the additional metal is more abundant and less expensive, such as iron, for example.

[0035] According to one example, the metal of the metal oxide is a metal capable of forming a divalent metal ion. According to one example, the at least one metal of the metal oxide is chosen from the group consisting of cobalt, nickel, iron. These metals are particularly suitable for the electrocatalytic oxidation of water. Iron is also an inexpensive metal while allowing good catalytic activity to be obtained.

[0036] According to one example, the side chain of the pyrrole monomer units is linear. The presence of a linear side chain between the pyrrole unit and the carboxylate group improves the flexibility of the polymer, which influences the structure of the polymer, its porosity, in particular compared to a polymer whose side chain of the units pyrroles are aromatic. The diffusion of species in the polymer (including metal ions, electrolyte, solvents and / or gases) is improved. This facilitates the formation of small oxide particles with homogeneous dispersion in the composite coating.

[0037] According to one example, the linear chain is unsaturated. According to one example, the side chain is saturated.

[0038] According to one example, the side chain comprises only a carboxylate group. This makes it possible to limit the size of the oxide particles formed, and in particular to obtain a size of less than 1 nm.

[0039] According to one example, the at least one carboxylate group has a pKa greater than or equal to 5.5, and preferably greater than or equal to 6. For comparison, the pKa of benzoic acid in a polypyrrole film is substantially 5.2. A pKa greater than 5.5 improves the kinetics of complexation of metals by the polymer and the stability of the complex obtained. This promotes the quantity of metal described, the generation of small oxide particles, and a homogeneous dispersion of these particles in the coating.

[0040] According to one example, the side chain comprises between 2 and 10 carbon atoms, preferably 5 carbon atoms. A short carbon chain between the pyrrole unit and the carboxylate group makes it possible to influence the particle size by promoting the production of very small particles. Indeed, this makes it possible to limit the steric and electronic constraints in the polymer. Conversely, a long chain increases the porosity of the film and promotes the mass transport of the reactants necessary for electro-oxidation (transport of ions, of the solvent for example) but also for the diffusion of electro-generated gases (O2 or H2). The range described allows a carbon chain length promoting both the generation of small oxide particles and allowing the transport of reactants and gases in the coating as well as the maintenance of metal oxides in the coating.

[0041] According to one example, the complexing solution comprises at least two different metal ions. Bimetallic oxide particles are thus formed in the coating.

[0042] According to one example, the complexing solution comprises a phosphate or borate buffer having a pH greater than or equal to 5.5, preferably greater than or equal to 6, preferably substantially equal to 7.2. During the development of the invention, it was demonstrated that these pH ranges further improve the stability of the metal oxide particles formed, to further promote a small size, a limited amount of metal as well as a homogeneous dispersion of the oxide particles in the coating. More particularly, when the buffer is a phosphate buffer, its pH is preferably greater than or equal to 6, preferably substantially equal to 7.2.

[0043] According to one example, the complexing solution comprises a borate ion buffer having a pH greater than or equal to 7, preferably substantially equal to 7.2. The borate ions act as ligands during the nucleation of the oxide particles during electro-oxidation. This thus limits their aggregation and leads to non-soluble and stable metal oxides at a pH of at least 7, and more particularly 9.2.

[0044] According to one example, when contacting with the complexing solution, the temperature of the solution is maintained at 30°C by a temperature control device. The complexing kinetics is temperature dependent. Having a temperature control system makes it possible to overcome variations in complexing kinetics and thus improve the reproducibility of the complexing.

[0045] According to one example, the coating is brought into contact with the complexing solution for at least 1 hour, preferably for at least 2 hours. This makes it possible to achieve saturation complexing of the carboxylate groups in the coating. Better repeatability of the process and reliability of the quantity of metal incorporated in the coating are thus obtained.

[0046] According to one example, the complexing solution has a concentration of at least one metal ion of between 1 and 20 mM, preferably between 1 and 16 mM, more preferably still equal to 4 mM.

[0047] According to one example, the contacting of the coating with the complexing solution is done under aerobic conditions, except when the at least one metal ion is an iron (II) ion for which the contacting of the coating with the solution comprising the at least one metal ion is done under anaerobic conditions in order to avoid oxidation of the iron (II) ion to iron (III).

[0048] According to one example, the electro-oxidation is an electro-oxidation by cyclic voltammetry, preferably by scanning between 0 and 1.2 V. By this oxidation scanning technique, the generation of metal oxide particles is done in a progressive and gentle manner by limiting the phenomena of diffusion of the metals and therefore of aggregation.

[0049] According to one example, the electro-oxidation of the metal ions is carried out in a solution having a pH greater than or equal to 7, preferably a pH greater than or equal to 9, preferably substantially equal to 9.2. A pH of the solution is greater than or equal to 7, and even more so for a pH greater than 9, in synergy with the borate or phosphate buffer of the complexing solution, makes it possible to further limit and preferably avoid dissolution of the metal oxides formed. Their stability is therefore improved. Indeed, the transformation of the carboxylate metal complexes in the form of oxides is favored under basic conditions (pH greater than or equal to 7). During the development of the invention, it was demonstrated that a pH greater than or equal to 9 promotes the formation by electro-oxidation of oxide particles which precipitate in the coating 11.

[0050] According to one example, the electro-oxidation of the metal ions is carried out in a solution comprising a phosphate buffer, preferably having a pH greater than or equal to 7, preferably substantially equal to 7.2. According to one example, the electro-oxidation of the metal ions is carried out in a solution comprising a borate buffer, preferably having a pH greater than or equal to 9, preferably substantially equal to 9.2. The borate buffer during the electro-oxidation makes it possible to further limit the aggregation of the metal oxide particles, which limits the quantity of metal incorporated as well as the size of the oxides.

[0051] An element “based on” a material A is understood to mean an element comprising this material A only or this material A and possibly other materials.

[0052] A parameter “substantially equal / greater / less than” a given value means that this parameter is equal / greater / less than the given value, to within plus or minus 10%, or even plus or minus 5%, of this value.

[0053] It is specified that in the context of the present invention, the thickness of a layer or coating is measured in a direction perpendicular to the surface along which this layer or coating has its maximum extension. The thickness is thus taken in a direction perpendicular to the main faces of the body on which the different layers and / or the coating rest. In the case of a three-dimensional element, for example a pattern of the body, the thickness of a layer extending on a flank of this element can be measured perpendicular to this flank.

[0054] When an element is said to be "homogeneous" in another element or volume, the quantity of the element per unit of volume is substantially identical in any portion of the same determined size of the other element or volume.

[0055] A metal ion generally refers to an ion of a metallic element in the periodic table of chemical elements.

[0056] Generally speaking, in the field of chemistry, a catalyst is understood to mean a compound that increases the speed of a chemical reaction by participating in the reaction without being part of the reactants and products. A catalyst makes it possible, in particular, to introduce new reaction pathways; for example, it does not participate directly in the reaction but its presence facilitates the breaking of bonds, or, for example, it participates in it and is regenerated during the reaction.

[0057] The term "monomer unit" means a molecular structure repeating in a polymer, formed from a monomer. Polymers formed from a single monomer unit are called homopolymers. A copolymer is used when at least two monomer units, with different molecular structures, constitute the polymer.

[0058] In the context of the present invention, the average molar masses of polymer are given in mass.

[0059] The electrode 1 and its manufacturing method 2 are now described in more detail according to several exemplary embodiments.

[0060] The electrode 1 is firstly described with reference to figures 1 and 2. The electrode 1 comprises a body 10 on which a composite coating 11 is deposited. The composite coating 11 covers at least part of the body 10, preferably on each of its faces. The coating 11 comprises a polypyrrole polymer 110 or its derivatives, detailed later, on which are absorbed metal oxide particles 111, which can also be designated by “clusters” of metal oxide. For this, the polypyrrole polymer 110 is anionic and forms a matrix stabilizing the particles 111. The pyrrole monomer units more particularly have a side chain comprising at least one carboxylate group, of chemical formula -COO. As will be described in more detail later, the carboxylate groups of polymer 110 allow the complexation of metal ions 220a, which will then be transformed into metal oxide particles 111.

[0061] The metal oxide particles 111 are intended to catalyze redox reactions. More particularly, the electrode 1 may be included in an electrolyzer. This electrolyzer may typically comprise two electrodes connected to a power source and making it possible to impose a potential between the two electrodes. When the electrode 1 and the counter-electrode are immersed in a reaction medium and subjected to a potential, the electrode 1 can catalyze redox reactions. By way of example, several reactions that can be catalyzed by the electrode 1, depending on the nature of the metals and alloys, are described below: - oxidation of water into oxygen, - oxidation of chlorides into hypochlorite or dichlorine, - oxidation of methanol, - reduction of water into hydrogen, - reduction of carbon dioxide to hydrogen, methane, formic acid, carbon monoxide, ethylene, propylene.

[0062] The coating 11 preferably comprises a quantity of metal in the metal oxide less than or equal to 7 mmol / cm3 (i.e. 245 nmol / cm2) per carboxylate group present on the monomer units. This quantity corresponds here to the total quantity of metal in the coating 11. When the coating 11 comprises multi-metal oxide particles, this quantity corresponds to the sum of the quantities of each metal in the coating 11.

[0063] As detailed later with reference to the examples, this quantity makes it possible to obtain sufficient electrocatalytic activity, while limiting the quantity of metal in the coating 11. This electrode 1 is therefore entirely in line with the economic and ecological objectives announced today aimed at limiting the amount of catalysts to mg / cm2 (200 pg / cm2) while maintaining electrocatalytic activities to meet market expectations. Indeed, depending on the nature of the metal, this volume quantity of 7 mmol / cm3 corresponds to a surface mass quantity of around 13 pg / cm2 and molar quantity of around 245 nmol / cm2).

[0064] Furthermore, the metal oxide particles 111 preferably have at least one dimension, and preferably each dimension, less than or equal to 2 nm, and more preferably 1 nm. The metal oxide particles 111 thus have a nanometric, and preferably sub-nanometric, size. The very small size of the metal oxide particles 111 acting as catalyst, dispersed and stabilized in the polymer matrix 110, makes it possible to access a large active surface area for catalysis. The electrocatalytic performance of the electrode 1 is therefore increased.

[0065] According to one example, the metal of the metal oxide is a metal capable of forming a divalent metal ion. According to one example, the metal oxide of the particles 111 comprises a transition metal or an alkaline earth metal. The nature of this metal can be chosen according to the redox reaction to be catalyzed. Preferably, the metal is chosen from the group consisting of cobalt, nickel, iron. These metals are particularly suitable for the electrocatalytic oxidation of water. Iron is also an inexpensive metal while making it possible to obtain good catalytic activity, which makes it possible to minimize the cost of the electrode 1.

[0066] According to example, the metal oxide of the particles 111 comprises a single metal, as illustrated in [Fig.l] with the oxide particle 111. According to an alternative example, the metal oxide of the particles 111 comprises several metals, and in particular at least two metals as illustrated in [Fig.l] with the bimetallic oxide particle 111'. During the development of the invention, and as described later with reference to the particular examples, it has been demonstrated that a bimetallic oxide has significantly improved catalytic properties, compared to a single metal species. When the oxide comprises several metals, one metal may for example be iron. This also makes it possible to reduce the manufacturing costs of the electrode 1, iron being more abundant and less expensive than other transition or alkaline earth metals.

[0067] The polypyrrole polymer 110 or its derivatives is now described. By polypyrrole polymer 110 or its derivatives is meant a polymer comprising at least in part pyrrole monomer units carrying a side chain with at least one carboxylate group per chain. The polymer 110 may be formed solely from these pyrrole monomer units. Alternatively, the polymer 110 may be a copolymer comprising these pyrrole monomer units and dif monomer units The polymer 110 may, alternatively or additionally, comprise different pyrrole monomer units carrying a side chain with at least one carboxylate group per chain.

[0068] The use of a polypyrrole polymer 110 bearing carboxylate groups to form the coating 11 makes it possible to form a matrix maintaining the structure of the metal oxide particles 111 during the catalysis of the chemical reactions. In addition, the carboxylates promote self-repair mechanisms of the particles 111. This synergistically makes it possible to improve the stability of the coating 11 during its use.

[0069] The side chain of the pyrrole monomer unit is more particularly attached to the pyrrole unit by the nitrogen atom. For example, the side chain of the pyrrole monomer unit is linear. The side chain may be saturated or have unsaturations. The presence of a linear side chain between the pyrrole unit and the carboxylate group improves the flexibility of the polymer. The diffusion of species in the polymer (and in particular metal ions, the electrolyte, solvents and / or gases) is further improved. It may be provided that the side chain is non-linear. For example, the side chain may comprise a cycle, and more particularly an aromatic cycle such as benzoic acid.

[0070] Preferably, the carboxylate group of the side chain has a pKa greater than or equal to 5.5, and preferably greater than 6. During the development of the invention, it was demonstrated that a pKa in these ranges improved the complexation kinetics and the stability of the complex obtained. A carboxylate group of a linear side chain has in particular a pKa greater than or equal to 5.5. In addition to the flexibility of the polymer, a linear side chain therefore improves the complexation of metal ions.

[0071] According to example, the side chain comprises between 2 and 10 carbon atoms. This range of side chain length makes it possible to obtain a good compromise between the generation of small metal oxide particles 111 and good transport of reactants and gases in the coating 11. Preferably, the side chain comprises 5 carbon atoms.

[0072] The side chain may furthermore have one or more carboxylate groups. For example, the side chain comprises a single carboxylate group. This makes it possible to limit the quantity of metal ions adsorbed in the coating 11 and therefore to limit the quantity of metal in the coating 11 obtained. According to one example, the side chain may comprise two carboxylate groups. This makes it possible to absorb more metal ions in the coating 11 and to form particles 111 of metal oxide of a slightly larger size, for example a particle size 111 of between 1 nm and 2 nm, for at least one dimension of the particle 111.

[0073] For example, the monomer unit may have one of the chemical formulas below:

[0074] [Chem.l]

[0077] The mass molar mass of the polymer 110 may be substantially between 100 g.mol1 and 250 g.mol *, preferably between 135 g.mol1 and 215 g.mol *. The surface molar quantity of polymer may be substantially between 100 nmol / cm2 and 200 nmol / cm2, preferably between 125 and 160 nmol / cm2. The following values ​​for the polymer 110 were observed for the monomer units (I) to (III) presented above. [Tables 1] Monomer unit (I) (II) (HD Molar mass in mass (in g.mol ') 139.15 181.24 211.22 Molar surface quantity (in nmol / cm2) 125 140 160 Mass surface quantity (in pg / cm2) 17.4 25.4 33.8

[0078] The body 10 of the electrode 1 may be made from numerous electrical materials. trically conductive. Several materials have been tested. For example, the body 10 of the electrode 1 may be based on glassy carbon, indium-tin oxide (commonly abbreviated ITO), or carbon paper, for example a carbon paper marketed by Toray Carbon Fibers®, designated C toray in the specific examples described later. The body 10 may have a more or less extensive external surface depending on the uses. By way of example, the following surfaces S have been used: - glassy carbon electrode with a diameter of 3 mm: S = 0.071 cm2, - ITO electrode: S = 0.8 cm2 (l cm x 0.8 cm), - carbon paper electrode: S = 2.4 cm2 (1.5 cm x 0.8 cm x two sides).

[0079] The manufacturing method 2 is now described in more detail with reference to [Fig. 2]. The method 2 firstly comprises providing the body 10 of the electrode 1. On the body 10, the coating 11 is formed by electropolymerization of pyrrole monomers having a side chain with a carboxylic acid group, to form the polypyrrole polymer 110. After the electropolymerization of the polymer 110, the coating 11 is immersed in a complexing solution 220, so as to form complexes 22 between at least one metal ion 220a and the carboxylate groups of the polymer 110. The carboxylate groups are derived from the carboxylic acid groups after deprotonation.

[0080] The electrode 1 is then rinsed 23 in order to remove the excess of uncomplexed metal ions 220a. After rinsing 23, the metal oxide particles 110 are formed by electro-oxidation in order to obtain the coating 11 described above. This composite coating 11 is therefore prepared under mild conditions by a simple and versatile electrochemical method. The electro-oxidation of these complexes gives an in situ formation of the catalysts in the form of metal oxides, in particular for applications in electrocatalysis.

[0081] The method further allows the generation of metal oxide particles 111 of varied chemical composition, for example based on noble metals (such as Ir, Pt, Ru, etc.), non-noble metals (such as Ni, Co, Fe, Mn, Cu, etc.) or multi-metal oxides of controlled proportion (NixFebx, NixCoi.x, CoxFc,x, FcxCu|X, MnxCabx, RuxFei x etc.).

[0082] Process 2 thus provides access to a large number of different metal oxides, each of which can have different electrocatalytic applications. It has furthermore been shown that the process is easily adaptable to electrodes comprising high surface area bodies. The transition from electrode surface area of ​​0.071 cm2 to more than 2.5 cm2 has been made, i.e. a factor of 30, while retaining the same electrocatalytic properties.

[0083] Each step is now described in more detail.

[0084] Electropolymerization 21 of polypyrrole 110

[0085] To electropolymerize 21 the polymer 110, the pyrrole monomer carrying the linear chain with the carboxylic group may be provided or synthesized in a process step prior to electropolymerization. For example, for the three monomer units described above, the monomer (I) with two carbon atoms may be commercially available. The bicarboxylic monomer (III) may be synthesized as described in the publication J.-C. Moutet, A. Zouaoui, Electrochimica Acta 2001, 46, 4035-4041. The monomer (II) may be synthesized as described below.

[0086] [Chem.4] H « 67%

[0087] The electropolymerization 21 of the polypyrrole 110 can be carried out electrochemically (and in particular by chronoamperometry), preferably in an anaerobic environment. The electropolymerization 21 is for example carried out in an acetonitrile solution comprising the pyrrole monomer to be polymerized, in which the body 10 of the electrode 1 is immersed.

[0088] For example, electropolymerization 21 is carried out at a controlled potential of +0.85V vs Ag / AgNO3 for a charge of 0.056 C / cm2 in an acetonitrile solution (CH3CN + 0.05 M of [Bu4N]C104 in electrolyte) containing 4 mM of pyrrole monomer.

[0089] Complexation 22 of metal ions 220a

[0090] Once the body 10 of the electrode 1 is at least partly coated with the polymer 110, the body 10 is soaked in a complexing solution 220 containing metal cations 220a, preferably of the M2+ form, in order to allow their complexing by the carboxylate functions of the polymer 110. This solution 220 is preferably an aqueous solution. This contacting of the coating 11 with the complexing solution 220 induces the complexing of the metal ions 220a. Equivalently, this step is referred to as the complexing 22 of the metal ions.

[0091] The carboxylate functions have a good affinity for many metal cations, even alkaline earth cations. Their presence in the coating 11 of polypyrrole 110 electropolymerized on the surface of the body 10 will allow the incorporation and stabilization of a wide variety of metal ions 220a (for example Ni, Co, Fe, Cu, Mn, etc.) but also the simultaneous incorporation of several cations in controlled proportions (for example NixFei_x, CoxFc,x, NixCoix, ...) by simple complexation between the metal ion and the carboxylate.

[0092] In order to improve the repeatability of the process and make the quantity of complexed metal ions 220a and the subsequent formation of metal oxide particles more reliable 110, the complexation conditions are carried out in a buffer medium at controlled pH to allow concomitant deprotonation of the carboxylate functions carried by the polymer film and the solubility of the metal salts used.

[0093] For this, the complexing solution 220 comprises a borate and / or phosphate buffer having a pH greater than or equal to 5.5. Preferably, the pH of the buffer is greater than or equal to the pKa of the carboxylic acid group of the monomer unit, in order to deprotonate it into carboxylate and induce the complexing of the metal ions 220a. The phosphate and / or borate ions introduced during the complexation at this pH also act as ligands at the periphery of the metal oxides generated during the electro-oxidation 24. These ligands limit the aggregation phenomena during the formation of the oxide particles 111, promoting a small size, a limited quantity of metal as well as a homogeneous dispersion of the oxide particles 111 in the coating 11. This also reduces the solubility of the oxide particles 111 which would otherwise be soluble at pH values ​​above 5.5, and improves their stability.

[0094] According to one example, the pH of the complexing solution 220 is greater than 6, for example substantially equal to 7.2. This pH range allows quantitative deprotonation of the carboxylate groups and facilitates the complexing of the metal ions 220a.

[0095] According to a preferred example, the complexing solution comprises a borate buffer, more preferably still having a pH greater than or equal to 6, preferably substantially equal to 7.2. Note that the pH of the complexing and / or electro-oxidation solution can furthermore be adapted according to the nature of the metal salts so that the salts are soluble under the pH conditions. During the development of the invention, it was demonstrated that a borate buffer at this pH was particularly suitable as a ligand at the periphery of the metal oxides generated during the electro-oxidation.

[0096] According to one example, the complexing solution 220 comprises at least two different metal ions 220a. Multi-metal oxide particles 111 are thus formed in the coating 11, as described previously. The metal ions in the complexing solution are chosen according to the oxide to be obtained. According to an alternative example, the complexing solution 220 may comprise a single type of metal ion 220a, to obtain oxide particles 111 of a single metal.

[0097] According to one example, the complexing solution 220 has a total concentration of metal ions of between 1 and 20 mM (1 mM = 103 mol / L), preferably between 1 and 16 mM, and more preferably still equal to 4 mM. During the development of the invention, a total concentration of 4 mM gave the best results in terms of electrocatalytic performance of the electrode. When the complexing solution 220 comprises several metal ions, the sum of their respective concentration gives the total concentration specified above. This total concentration allows saturation complexation of the metal ions 220a by the carboxylate groups of the polymer 110, while avoiding overloading the complexation solution 220 with metal ions 220a. The cost and environmental impact of process 2 are thus limited.

[0098] When the complexing solution 220 comprises several metal ions, the respective concentration of each metal ion can be adjusted so as to vary the relative proportions of metal in the oxide formed, and in particular for a total concentration of 4 mM.

[0099] Preferably, during complexation 22, the experimental conditions are controlled so as to promote reproducible complexation of the metal ions 220a.

[0100] For this, the temperature of the complexing solution 220a can be on the one hand controlled at a fixed temperature. In particular, the temperature of the complexing solution 220 is preferably maintained at a temperature between 20 and 35°C, and preferably substantially equal to 30°C, by a temperature regulation device 221 as for example illustrated by [Fig.2]. This temperature range, and in particular the value of 30°C, makes it possible to improve the complexing kinetics while making it possible to preserve the integrity of the compounds of the coating 11. Note that the complexing solution 220 can be at room temperature, i.e. 20°C.

[0101] Furthermore, the coating 11 may be immersed in the complexing solution 220 for at least 1 hour, preferably for at least 2 hours, in order to achieve saturation complexing of the metal ions 220a by the carboxylate groups of the polymer 110.

[0102] According to one example, the complexation 22 is carried out under aerobic conditions. This makes it possible to simplify this step of the method 2. When the complexation solution 220 comprises an iron (II) ion, the complexation 22 is preferably carried out under anaerobic conditions in order to avoid oxidation of the iron (II) ion to iron (III).

[0103] For example, the complexing solution 220 is an aqueous solution comprising 0.1 mol / L of boric acid (H3BO3) at pH 7.2 with a concentration of cationic metal ion M2+ substantially equal to 4 mM (and in particular for Ni2+, Co2+, Fe2+).

[0104] Rinse 23

[0105] After the complexation 22, the electrode 1, and more particularly the coating 11, is rinsed in order to remove the uncomplexed metal ions 220a. This makes it possible to limit and make more reliable the quantity of metal incorporated in the coating 11.

[0106] For this, the electrode 1 can be rinsed with water, and more particularly with water, preferably in an aqueous solution having a pH greater than or equal to 5.5, in order to avoid protonation of the carboxylates. According to one example, the pH of the solution used during rinsing 23 is greater than or equal to 6, preferably substantially equal to 7.2. The pH may be identical to the pH of the complexing solution 220. The aqueous rinsing solution may comprise a borate or phosphate buffer, for example the same buffer as that used in the complexing solution 220.

[0107] At this stage, the modified electrodes 1 can be stored for several weeks in a humid atmosphere, which is a real advantage in terms of the manufacturing process.

[0108] Electro-oxidation 24 and formation of metal oxide particles 111

[0109] After rinsing 21, the electrode 1 is subjected to electro-oxidation 24 so as to form the metal oxide particles 111 from the complexed metal ions 220a.

[0110] For this, the electrode 1 is at least partly immersed in an electrically conductive aqueous solution 240, so as to immerse the coating 11. The solution 240 may have a pH greater than or equal to 5.5. Preferably, the solution 240 has a pH greater than or equal to 7, that is to say a basic pH. Thus, protonation of the carboxylates is avoided. Preferably, the solution 240 has a pH greater than 9, and preferably substantially equal to 9.2. A pH greater than or equal to 9 makes it possible to promote the kinetics of formation of the metal oxide particles 111, compared to a lower pH for which a portion of the metal ions 220a could remain in ionic form.

[0111] The solution 240 is preferably free of metal ions, so as not to add ions to the coating and thus limit the quantity of metal incorporated as well as the size of the particles 111.

[0112] The solution 240 may comprise a borate buffer, preferably with a pH substantially equal to 9.2. Thus, the borate ions stabilize the metal oxide particles 111 by acting as a ligand, and prevent their dissolution. Note that phosphate buffers (preferably at pH 7) or acetate buffers (preferably at pH 5.5) may alternatively be used, in particular depending on the nature of the metal ions 220a. For example, an aqueous borate solution at 0.1 mol / L H3BO3, 0.1 mol / L Na2SO4 at pH 9.2 and free of metal salt is used.

[0113] The electro-oxidation is preferably carried out by cyclic voltammetry (commonly abbreviated CV). By this technique, the generation of metal oxide particles is done in a progressive and “gentle” manner by limiting the diffusion of the metal ions 220a. The fact that the metal ions 220a diffuse less in the polymer 110 during their oxidation greatly limits their aggregation and therefore the size of the particles 111 obtained. For this, potentials of 0 to 1.2V (vs Ag / AgCl) can be scanned. Potentials of 1 to 1.2V (vs Ag / AgCl) can be scanned cyclically. Scanning beyond 1V allows the electro-generation of oxide particles in the polymer film, particularly for the metals Co, Ni and Fe. Note that this potential range is particularly suitable for a solution pH of 9.2. This scanning range can however vary depending on the pH conditions.

[0114] It will be noted that, in the following, the electro-oxidations are carried out by scanning over a range of 0 to 1.2 V in order to be able to follow the evolution of the redox signature of the Niu / Niui and CoII / Co111 couples for potentials lower than IV.

[0115] According to one example, between 30 and 200 scanning cycles were used for electro-generation of oxide particles 111. Note that this number can vary depending on several parameters such as the nature of the metal, the electrode support, the pH, for example. Electro-oxidation can be carried out over a sufficient number of cycles so that there is no longer any variation in the intensity of both the oxide signal (if present) and the catalytic current.

[0116] Particular examples of implementation

[0117] Particular examples and their characterization are now described with reference to Figures 3A to 12B.

[0118] The table below describes examples of metals of the metal oxide of the particles 111 as well as the molar volume quantity incorporated in the coating 11. [Tables 2] Metal Quantity of metals (TM, in mmol.cm3.COOH) Ratio of quantity of metals to the quantity of carboxylate (Fm / Fcooh) Nickel 1.26 0.28 Cobalt 2.08 0.46 Nickel-cobalt 2.07 0.46 Iron 3.83 0.85 Nickel-iron 2.27 0.50 Cobalt-iron 2.14 0.47

[0119] In the tests carried out, a maximum quantity of metal of 190 nmol / cm2 in total in the coating 11, and for a mass of polypyrrole of 25.4 pg / cm2 (140 nmol / cm2) was incorporated, with a single carboxylate group per monomer unit.

[0120] Electrodes 1 were manufactured according to method 2 and tables 2 and 3 below summarize the composition of the complexing solution 220 and their measured characteristics. Any deviations are due to experimental variation. All the measurements are carried out in the same way on coatings manufactured under the same conditions (except for the parameters varying as indicated from one example to another).

[0121] To manufacture these electrodes, the following experimental conditions were used: - electropolymerization of a monomer according to the formula (II) presented previously, hereinafter abbreviated PpyC5CO2H and PpyC5CO2-M after complexation of the metal M, carried out at a controlled potential of +0.85V vs Ag / AgNO 3 for a charge of 0.056 C / cm2 in an acetonitrile solution (CH3CN + 0.05 M of [Bu4N]C1O4 in electrolyte) containing 4 mM of pyrrole monomer, on a carbon paper hereinafter designated Ctoray. Note that glassy carbon and ITO electrodes were also tested, as described previously. - complexation in the complexation solution 220 indicated in the tables below under the conditions indicated, - rinsing of electrode 1 in aqueous solution of 0.1 mol / L borate buffer at pH 7.2, - electro-oxidation by CV in aqueous borate solution at 0.1 mol / L H3BO3, 0.1 mol / L Na2SO4 at pH 9.2 and free of metal salts, by scanning between 0 and +1.2 V vs Ag / AgCl.

[0122] The quantities of metal incorporated in the coating 11 were measured by inductively coupled plasma atomic emission spectrometry (commonly referred to as ICP-AES, from the English Inductively Coupled Plasma-Atomic Emission Spectroscopy). Note that other measurement methods would make it possible to achieve the same result.

[0123] The ICP-AES assay is described below according to an example. The coating 11 is first dissolved in an aqueous nitric acid solution (HNO3, 1 mol / L). For this, the Ctoray electrodes modified by the coatings are placed in 2 mL of the 1 mol / L aqueous nitric acid solution, contained in metal-free plastic analysis tubes. The metal-free plastic analysis tubes make it possible not to distort the quantity of metal measured compared to tubes made of other materials, for example glass, which could lead to an overestimation of the quantity of metal. These tubes are preferably ICP analysis tubes. The dissolution of the coating 11 is carried out after at least 4 hours of soaking in these analysis tubes, interspersed with a passage in an ultrasonic bath every 2 hours for 1 min.

[0124] After dissolving the coating, a volume of 4 mL of ultrapure water is added to each analysis tube to obtain a solution with a total volume of 6 mL. The undissolved Ctoray electrode support is removed from the analysis tube to allow the automated dosage by ICP-AES.

[0125] The ICP-AES device is then calibrated. The equipment used is a Varian 720ES ICP-AES spectrometer with a detection limit of 1-20 ppb (ppb abbreviated from part-per-billion translated as part per billion, equivalent to pg.L ') and a quantification limit of the order of 1-50 ppb.

[0126] The concentrations (in mg / L) of the different metals present in the analysis solution were obtained using an external calibration curve made from aqueous solutions mixing Ni(NO3)2, Co(NO3)2, Fe(NO3)3 and Ca(CO3) (ICP Standard samples, Roth, 99.995%) over a concentration window ranging from 0.001 mg / L to 1 mg / L. Each metallic element has characteristic emission lines allowing, by integration, their dosages. In our studies we chose the following emission lines: (XNi = 216 nm, XCo = 230 nm, XFe = 234 nm, XCa = 317 nm).

[0127] All the metallic elements originating from a dissolved coating are measured simultaneously by integration on their respective wavelengths. 3 replicates are carried out for each metal at their respective wavelengths, making it possible to define, using the calibration curve, an average concentration value (in mg / L) in the solution originating from the dissolution of the coating of an electrode.

[0128] The same procedure is carried out for each solution resulting from the dissolution of the coating of a different electrode (results in the table in mg / L).

[0129] -The quantities in nmol / cm2 and pg / cm2 are calculated from the surface of the Ctoray electrode (Sgeometric, in these examples equal to 2.4 cm2). The current densities j, the mass activities, the TOF (Turnover Frequency, translated by renewal frequency) and the incorporation rates (Fm / FCooh) are calculated from the currents measured at a potential of 1.2 V vs Ag / AgCl, i.e. an overvoltage of 0.71 V by cyclic voltammetry in an aqueous solution of 0.1M borate, pH 9.2 at 30°C. [Tables 3] Ctoray electrode (digital ref.) Metal quantity in mg / L Metal quantity in nmol.cm2 Metal quantity in pg.cm2 Proportions %Co / %N i Current density J (mA.c m2) at 0.71 V Specific activity (A.mg) at 0.71 V TOF s4 0.71 V Tm / TcOOH Complexation solution (0.1M H3BO3, pH 7.2, 30°C, 2h) with 4 mM NiSO4 c / '—'loray7 PPyC5CO2 -NiOx (41) 1.31 55.65 3.28 11.6 3.54 0.54 0.31 C / '“-'toray' PPyC5CO2 -NiOx (42) 0.92 39.18 2.31 11.91 5.16 0.79 0.22 C / '—-loray7 PPyC5CO2 -NiOx (43) 1.33 56.44 3.33 12.53 3.77 0.58 0.31 Average 1.189 50.42 2.97 12.01 4.15 0.63 0.28 Standard Deviation 0.230 9.75 0.57 0.47 0.88 0.13 0.05 Complexation solution 0.1M H3BO3, pH 7.2, 30°C, 2h) with 4 mM CoSO4 C / '--'toray' PPyC5CO2 -CoOx (44) 1.90 80.41 4.74 15.26 3.22 0.49 0.45 C / '-■-toray' PPyC5CO2 -CoOx (45) 2.03 86.20 5.08 15.06 2.96 0.45 0.48 Average 1.964 83.31 4.91 15.16 3.09 0.47 0.46 Standard Deviation 0.097 4.09 0.24 0.14 0.18 0.03 0.02 Complexation solution (0.1M H3BO3, pH 7.2, 30°C, 2h) with 2 mM NiSO4 and 2 mM CoSO4 Ctoray electrode (numerical ref.) Metal quantity in mg / L (Co / Ni) Metal quantity in nmol.cm2 (Co / Ni and (Co+Ni)) Metal quantity in pg.cm2 (Co / Ni and (Co+Ni) ) Proportions %Co / %N i Current density J (mA.c m2) at 0.71 V Specific activity (A.mg) at 0.71 V TOF s1 0.71 V rM / TcOOH C / '—'toray7 PPyC5CO2 -Coo,7Nio,3 Ox(46) 1.32 / 0.6 8 55.80 / 28.9 1 (84.71) 3.29 / 1.7 0 (4.99) 66 / 34 17.61 3.53 0.54 0.47 C / '—'toray7 PPyC5CO2 -Coo.4Nio.5 Ox(47) 1.30 / 0.5 9 55.04 / 24.9 2 (79.96) 3.24 / 1.4 7 (4.71) 69 / 31 15.37 3.26 0.50 0.44 C / '—'toray7 PPyC5CO2 -Coo.5Nio.4 Ox(48) 1.35 / 0.6 3 57.07 / 26.7 0 (83.77) 3.36 / 1.5 7 (4.94) 68 / 32 17.5 3.54 0.54 0.47 Mean 1.32 / 0.6 3 55.97 / 26.8 5 (82.81) 3.30 / 1.5 8 (4.88) 68 / 32 16.83 3.44 0.53 0.46 Standard Deviation 0.02 / 0.0 5 1.03 / 2.00 (2.52) 0.06 / 0.1 2 (0.15) 1.26 0.16 0.02 0.01 [Tables 4] Otoray electrode (digital ref.) Metal quantity in mg / L Metal quantity in nmol.cm2 Metal quantity in pg.cm2 Proportions %Co / % Ni / %Fe Current density J (mA.c m2) at 0.71 V Specific activity (A.mg) at 0.71 V TOF s1 0.71 V rM / Fcoou Complexation solution (0.1M H3BO3, pH 7.2, 30°C, 2h) with 4 mM FeSO4 Ctoray / PPyC 5CO2-FeOx (51) 4.02 179.97 10.05 12.11 1.20 0.17 1.00 Ctoray / PPyC 5CO2-FeOx (52) 3.65 163.27 9.12 11.87 1.30 0.19 0.91 Ctoray / PPyC 5CO2-FeOx (53) 2.60 116.33 6.50 12.42 1.91 0.28 0.65 Mean 3.422 153.19 8.56 12.13 1.47 0.21 0.85 Standard Deviation 0.737 32.99 1.84 0.28 0.38 0.06 0.18 Complexation solution (0.1M H3BO3, pH 7.2, 30°C, 2h) with 3 mM NiSO4 ImM FeSO4 Electrode Quantity Quantity Quantity Proportion Density Activity TOF rM / Ctoray (ref. metal in metal in metal in ons of mass s1 FcOOH numerical mg / L nmol.cm2 pg.cm2 %Ni / %F current e 0.71 ) (Ni / Fe) (Ni / Fe and (Ni+Fe)) (Ni / Fe and (Ni+Fe)) e J (mA.c m2) at 0.71 V (A.mg) à 0.71 VV Ctoray / PPyC 5CO2-Ni0.64 Fe0>36 Ox (54) 1.40 / 0.79 59.23 / 35.21 (94.43) 3.49 / 1.9 7 (5.46) 64 / 36 25.37 4.93 0.78 0.52 Ctoray / PPyC sCO2-Nio,7i Fe0j2gOx 1.42 / 0.59 60.29 / 26.34 (86.63) 3.55 / 1.4 7 (5.02) 71 / 29 27.63 5.50 0.83 0.48 . (55) Ctoray / PPyC sC02-Nioj67 Fe0j33Ox (56) 1.41 / 0.7 1 59.72 / 31.65 (91.37) 3.52 / 1.7 7 (5.29) 67 / 33 25.74 4.87 0.73 0.51 Mean 1.41 / 0.69 59.74 / 31.07 (90.81) 3.52 / 1.7 4 (5.26) 67 / 33 26.25 5.10 0.78 0.50 Standard Deviation 0.01 / 0.10 0.53 / 4.46 (3.93) 0.03 / 0.2 5 (0.22) 1.21 0.35 0.05 0.02 Complexation solution (0.1M H3BO3, pH 7.2, 30°C, 2h) with 3 mM CoSO4 and 1 mM FeSO4 Electrode Ctoray (numerical ref.) Metal quantity in mg / L (Co / Fe) Metal quantity in nmol.cm2 (Co / Fe and (Co+Fe)) Metal quantity in pg.cm2 (Co / Fe and (Co+Fe)) Proportions %Co / % Fe Current density J (mA.c m2) at 0.71 V Specific activity (A.mg) at 0.71 V TOF s1 0.71 V rM / Fcoou Ctoray / PPyC 5CO2-CO071 Fe0j29Ox (57) 1.23 / 0.4 9 52.33 / 22.16 (74.43) 3.08 / 1.2 3 (4.32) 71 / 29 22.26 5.15 0.77 0.41 Ctoray / PPyC 5CO2-CO072 Fe0j2sOx (58) 1.60 / 0.6 2 68.02 / 27.75 (96.38) 4.04 / 1.5 5 (5.59) 72 / 28 24.8 4.43 0.67 0.54 Average 1.43 / 0.5 6 60.48 / 24.93 (85.40) 3.56 / 1.3 9 (4.96) 72 / 28 23.53 4.79 0.72 0.47 Standard Deviation 0.27 / 0.0 9 11.52 / 4.00 (15.52) 0.68 / 0.2 2 (0.90) 1.80 0.51 0.08 0.09 .

[0130] During electropolymerization, the formation of nickel oxide particles 111 NiOx was observed by the progressive appearance of an irreversible system, characteristic of the electroactivity of NiOx (redox couple Niu(0H)2 / NiIU(0)(0H) with an anodic peak and a cathodic peak located respectively at +0.78 and +0.58 V vs Ag / AgCl. The appearance of NiOx is associated with a catalytic current above +0.95 V whose intensity increases progressively as a function of the scanning number. This process is attributed to the catalytic oxidation of water to O2 by NiOx, concomitant with the formation of the NiOx species under two preferential phases |3-NiIU(0)(0H) and / or y-NiIV(0)(0H).

[0131] After about thirty or even fifty cycles, there is no longer any significant increase in the Niu(0H)2 / NiUI(0)(0H) redox process or in the associated catalytic current, suggesting that all the Ni11 complexes initially present in the film have been transformed into NiOx.

[0132] Note that the oxidation and overoxidation of the polypyrrole 110 are observable during the first scan via two large anodic processes around +0.6 and +1.0 V respectively. Consequently, after the first scan, the conductivity of the polypyrrole 110 is reduced, or even destroyed, due to its overoxidation. Thus, after electropolymerization, the polymer 110 mainly plays the role of matrix and the conductivity of the composite coating 11 is ensured by the dispersed metal oxide particles 111.

[0133] The formation of clusters of cobalt oxides, CoOx, iron oxides, FeOx, as well as that of bimetallic clusters NiyCobyOx, CoyFei yOx, NiyCoi yOx was carried out according to the same procedure.

[0134] The formation of CoOx particles was more difficult to observe than in the case of NiOx because the redox process Co^OHVCo^OXOH) is of a lower intensity, with an anodic peak and a cathodic peak located respectively at +0.58 and +0.54 V. The modified glassy carbon (Cvit) electrodes Cvit / PPyC5CO2-CoOx present superior performances in terms of current density which reaches 16 mA.cm 2 at 1.2V, after about thirty cycles from 0 to 1.2V.

[0135] Unlike nickel oxide, iron oxide does not exhibit a redox system; only a catalytic current is observed.

[0136] For monometallic oxides, the optimal concentration of metal salts in the immersion solution observed was 4 mM regardless of the metal cation, Ni2+, Co2+ or Fe2+. Note that in each case, concentrations of 1, 2, 4 and 16 mM were tested and the corresponding ICP-AES measurements made it possible to define incorporated values ​​for 4 mM, considered as optimal conditions of respectively 2.97, 4.91 and 8.56 pg / cm2.

[0137] Figures 3A and 3B illustrate the electrocatalytic activity of the coatings 11 for the oxidation of water at pH 9.2 (the numerical reference for each curve is indicated in Tables 2 and 3 above, 40 and 50 designating a carbon paper electrode designated Ctoray, uncoated). The presence of oxidation and reduction peaks is characteristic of metal oxides depending on the nature of the metal.

[0138] In Tables 2 and 3 as well as in Figures 3A and 3B, the current density j (mA.cm2) defines the pure performance of the coating with respect to the water oxidation reaction at an overvoltage of 0.71V (equivalent to a potential of 1.2V vs Ag / AgCl in the figures). The mass activity (A / mg) defines the performance as a function of the quantity of metal oxide catalysts and the applied overvoltage. The ratio rM / TCooH allows the quantity of metal to be defined as a function of the quantity of carboxylate group.

[0139] As regards iron and mixed metal oxides, in [Fig.3B], the shape of the signal, corresponding to the electroactivity of the oxide, varies according to the nature of the metals and their respective proportions. Even higher catalytic current densities have been obtained with bimetallic oxides reaching 20-28 mA / cm2 at 1.2V.

[0140] Electrochemical characterizations were also carried out to demonstrate the effectiveness of these materials at different temperatures, not illustrated here. It was observed that the intrinsic catalytic activity (i.e. the reaction kinetics) of these materials is almost not influenced by the temperature although the increase in temperature still makes it possible to reach increasingly high currents (up to 40 mA.cm2 in the case of mixed oxides at 50°C.

[0141] As illustrated in [Fig.4], the thickness of a coating without oxide particles (PPyC 5CO2 ) equivalent to a quantity of carboxylate of 140 nmol / cm2 could be determined by carrying out a deposition on a flat surface of ITO 10. The measured thickness value is 350 nm (0.35 pm). After incorporation of the particles 111 of metal oxides, no particles are observable either on the coating 11 or at the interface with the body 10 of electrode 1. The fact of not observing an increase in the thickness of the coating after generation of the particles 111 goes in the direction of small particles 111 less than one nm not influencing the physical characteristics of the coating 11.

[0142] By atomic force microscopy (AFM), as illustrated in [Fig. 5], after incorporation of the metal oxide particles 111, no particles are observable on the coating 11. The fact of observing average surface roughnesses (RMS in the figure, Rough Mean Square) of the same order of magnitude for a PPyC5CO2H coating without or with metal oxide particles 111 also goes in the direction of small particles not influencing the physical characteristics of the coating 11.

[0143] As illustrated by Figures 6A and 6B, the coatings 11 were observed by transmission electron microscopy. After incorporation of the oxide particles 111 metallic, here again no particles are observed in the coating 11 on the observed scales. The fact of not observing any 111 particle by transmission electron microscopy despite the presence of metals confirmed by electrochemistry and ICP-AES measurement goes in the direction of particles of size less than 1 nm outside the detection limits of the techniques.

[0144] The measurements carried out indicate that the in situ electro-generation of oxides from carboxylate complexes makes it possible to avoid aggregation phenomena and thus to achieve nanoparticle sizes close to those of a cluster (< 1 nm) composed of a small number of metallic entities in accordance with a “bottom-up” nucleation method.

[0145] To support this, additional X-ray diffraction measurements by GiWAXS were carried out to seek to know the nature of these clusters (crystalline or amorphous) as well as their spatial distributions. The results obtained for the ITO / PPyC5CO2-NiOx composite coating 11 are summarized in [Fig.7] which is a wide-angle X-ray scattering graph at grazing incidence representing the intensity 60 as a function of the scattering vector. It is possible to observe the presence of a diffraction peak 610 at around 2 Â ', attributed to the spatial organization of the polypyrrole chains in the case of a coating comprising the polymer alone (ITO / PPyC5CO2, curve 61).

[0146] After generation of the NiOx particles 111 (curve 62), the decrease 620 of this signal indicates that there is a disorganization of the structure of the polymer 110. Nevertheless, it was expected in the case of the electro-generation of crystalline nanoparticles greater than 1 nm distributed in a structured manner to obtain a signal around 4 Â 1 (621), which is not observed. This is in line with a polypyrrole composite material comprising sub-nanometric particles 111 of amorphous metal oxides distributed in a homogeneous but random manner without apparent structure. Residual signals 611, 622 of the ITO are also observable.

[0147] The fact of having very small clusters with a high electroactive surface makes it possible to explain, among other things, the extremely high mass activity values ​​measured by CV.

[0148] Generally, sub-nanometric architectures such as oxide particles may suffer from a lack of strength and stability under electrocatalytic conditions. This was not observed for the coatings 11 tested, as described below.

[0149] The stability of the coatings 11 is studied by the variation of the current density as a function of time at a given potential (1.2V vs Ag / AgCl or an overvoltage of 0.71V) with respect to the oxidation of water at pH 9.2, as illustrated in figures 8A (time t in hours) and 8B (time t in days), for: - 70: an uncoated Ctoray electrode, - 71: a Ctoray / PPyC5CO2-NiOx electrode, - 72: a Ctoray / PPyC5CO2-CoOx electrode, - 73: a Ctoray / PPyC5C02-Nioj3Cooj70x electrode.

[0150] The electrocatalytic activity of the coatings 11 for the oxidation of water at pH 9.2 was also measured over time, as illustrated in Figures 8C to 8E, for: - 74: a Ctoray / PPyC5CO2-NiOx electrode, at t = Oh, - 75: the Ctoray / PPyC5CO2-NiOx electrode, at t = 65h, with a quantity of charge electric of Q = 9500 C, - 76: a Ctoray / PPyC5CO2-CoOx electrode, at t = Oh, - 77: the Ctoray / PPyC5CO2-CoOx electrode at t = 48h, with a quantity of charge electric of Q = 7800 C, - 78a: a Ctoray / PPyC5C02-Nioj3Cooj70x electrode, at t = Oh, - 78b: the Ctoray / PPyC5C02-NioJ3Cooj70x electrode, at t = 1.8 days, with a quantity of electric charge of Q = 7290 C, - 78c: the Ctoray / PPyC5C02-NioJ3Cooj70x electrode, at t = 4.7 days, with a quantity of electric charge of Q = 18,795 C, - 78d: the Ctoray / PPyC5C02-NioJ3Cooj70x electrode, at t = 8.2 days, with a quantity of electric charge of Q = 32,871 C, - 78e: the Ctoray / PPyC5C02-NioJ3Cooj70x electrode, at t = 10.2 days, with a quantity of electric charge of Q = 40 323 C, - 78f: the electrode C^^y / PPyQCOo-NiojCoo^CL at t = 14.2 days, with a quantity of electric charge of Q = 55 530 C.

[0151] No significant loss of electrocatalytic activity was observed over a period of several days. The coating 11 therefore has very good stability over time, at least for the oxidation of water at pH 9.2. The variations observed are due to acidification and a reduction in the volume of the solution in which the electrodes 1 are studied. A change of solution makes it possible to recover the initial activities. Observing the characteristic signals of the particles 111 throughout the electrolysis confirms their presence as the electrodes are used.

[0152] Figures 9A, 10A, and 12A are SEM images making it possible to visualize the presence of the coating before and after electrolysis and therefore to confirm its mechanical stability.

[0153] Figures 9B, 10B, and 12B show EDX spectra allowing the characteristic emission lines of each element detected in the window (white square) of the images of Figures 9A and 10A respectively to be visualized. This confirms that the metallic elements constituting the particles are indeed found in the coatings. oxide before and after electrolysis. The ability of the coating to stabilize the catalysts and prevent their release into solution during catalysis is therefore validated.

[0154] [Fig. 1 IA] illustrates the stability of the coatings 11, characterized by the study of the variation of the current density as a function of time t (in h) at a given potential (1.2V vs Ag / AgCl or an overvoltage of 0.71V) with respect to the oxidation of water at pH 9.2, for: - 90: a coated Ctoraynon electrode, - 91: a Ctoray / PpyC5CO2-FeOx coated electrode, - 92: a Ctoray / PpyC5C02-Nioj67Feoj330x coated electrode, - 93: a Ctoray / PpyC5C02-Cooj78Feoj280x coated electrode,

[0155] The electrocatalytic activity of the coatings 11 for the oxidation of water at pH 9.2 was also measured over time for: - 94a: a Ctoray / PpyC5C02-Nioj67Feoj330x coated electrode, at t = Oh - 94b: the Ctoray / PpyC5C02-Nio,67Fe0j330x coated electrode, at t = 65h - 94c: the Ctoray / PpyC5C02-Nio,67Fe0j330x coated electrode, at t = 89h.

[0156] No significant loss of activity was observed over a period of several days. The variations observed in the case of iron-based alloys are due to acidification and a reduction in the volume of the solution in which the electrodes are studied. A change of solution again makes it possible to recover the initial activities.

[0157] The rapid decrease in activity of electrodes with iron-based coatings is attributed to the formation of less effective particles with high oxidation states. Scanning at potentials between 0 and 1.2V is sufficient to restore high activity to our coatings. Again, observing the characteristic signals of the metal oxide particles 111 throughout the electrolysis confirms their presence.

[0158] All of these observations are consistent with the presence of nanometric-sized metal oxide particles 111, and more particularly smaller than 1 nm, and with the fact that the polymer-carboxylate matrix maintains their integrity during catalysis by preventing their corrosion and aggregation and by promoting self-repair, i.e. the reassembly of the particles 111 with compounds trapped in the polymer. If metal ions are released during electrocatalysis, they would then be complexed by the carboxylate groups of the polymer 110 to be transformed again into corresponding oxides. The voltam-grams recorded after electrolysis are very close to the initial ones, confirming the excellent stability of the electrode 1 and its coating 11, with the metal oxide particles 111 being maintained in the coating 11.

[0159] These results show that these composite coatings are very promising for control the formation, size of metal oxide catalysts and positively influence their catalytic performance as well as their stabilities.

[0160] In view of the above description, it appears clearly that the invention proposes an improved electrode, particularly in terms of resources, environment and cost, while retaining satisfactory electrocatalytic activity.

[0161] The invention is not limited to the embodiments previously described and extends to all embodiments covered by the invention. The present invention is not limited to the examples previously described. Many other variant embodiments are possible, for example by combining previously described characteristics, without departing from the scope of the invention. Furthermore, the characteristics described in relation to one aspect of the invention may be combined with another aspect of the invention. For example, the electrode may have any characteristic resulting from the implementation of the method and the method may have any step configured to result in a characteristic of the electrode.

Claims

Claims

1. Electrode (1) comprising a body (10) covered at least in part by a composite coating (11) comprising: • a polypyrrole polymer (110) or its derivatives comprising pyrrole monomer units having a side chain comprising at least one carboxylate group, • particles (111) of metal oxide based on at least one metal, Characterized in that the coating (11) comprises a quantity of the at least one metal of the metal oxide less than or equal to 7 mmol / cm3 per carboxylate group of the pyrrole monomer units.

2. Electrode (1) according to the preceding claim, in which the particles (111) of metal oxide have at least one dimension (d) less than or equal to 2 nm, preferably less than or equal to 1 nm.

3. Electrode (1) according to any one of the preceding claims, wherein the metal oxide particles (111) are based on at least two different metals.

4. Electrode (1) according to any one of the preceding claims, wherein the at least one metal of the metal oxide is selected from the group consisting of cobalt, nickel, iron.

5. Electrode (1) according to any one of the preceding claims, wherein the side chain of the pyrrole monomer units is linear.

6. Electrode (1) according to any one of the preceding claims, wherein the side chain comprises between 2 and 10 carbon atoms, preferably 5 carbon atoms.

7. Method (2) for manufacturing the electrode (1) according to any one of the preceding claims comprising: • a supply (20) of the body (10) of the electrode (1), • an electropolymerization (21) on the body (10) of the electrode (1) of a polymer (110) comprising pyrrole monomer units having a side chain comprising at least one carboxylic acid group, so as to form a coating (11) at least partially covering the body (10), • a contacting of the coating (11) with a so-called solution "complexing" (220) comprising at least one metal ion (220a) and a borate and / or phosphate buffer having a pH greater than or equal to 5.5, inducing a complexation (22) of the at least one metal ion (220a) by at least one carboxylate group, • after bringing the coating (11) into contact with the complexing solution (220) inducing the complexation (22) of the at least one metal ion (220a), a rinsing (23) of the coating (H), • after rinsing (23) of the coating (11), an electro-oxidation (24) of the complexed metal ions (220a) so as to form metal oxide particles (111).

8. Method (2) according to the preceding claim, in which the complexing solution (220) comprises at least two different metal ions (220a).

9. Method (2) according to any one of the two preceding claims, in which the complexing solution (220) comprises a borate buffer having a pH greater than or equal to 6, preferably substantially equal to 7.

2.

10. Method (2) according to any one of the three preceding claims, wherein when contacting with the complexing solution (220), the temperature of the complexing solution (220) is maintained at 30°C by a temperature regulating device (221).

11. A method according to any one of the four preceding claims, wherein the coating (11) is contacted with the complexing solution (220) for at least 1 hour, preferably for at least 2 hours.

12. Method according to any one of the five preceding claims, in which the electro-oxidation (24) of the metal ions (220a) is carried out in a solution (240) having a pH greater than or equal to 7, preferably a pH greater than or equal to 9.

13. Method (2) according to any one of the six preceding claims, wherein the electro-oxidation (24) of the metal ions (220a) is carried out in a solution (24) comprising a borate buffer.

14. Use of the electrode (1) according to claims 1 to 6 or the electrode (1) manufactured by the method (2) according to any one of claims 7 to 13 for the electrocatalytic oxidation of water.

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