EP3887573A1

EP3887573A1 - Method for producing an active layer of an electrode for electrochemical reduction reactions

Info

Publication number: EP3887573A1
Application number: EP19806189.7A
Authority: EP
Inventors: Audrey BONDUELLE-SKRZYPCZAK; Sofiane BELAID; Philibert Leflaive; Gerhard Pirngruber
Original assignee: IFP Energies Nouvelles IFPEN
Current assignee: IFP Energies Nouvelles IFPEN
Priority date: 2018-11-30
Filing date: 2019-11-19
Publication date: 2021-10-06
Also published as: US20220010439A1; CN113366155A; FR3089133B1; WO2020109065A1; FR3089133A1

Abstract

A method for producing a catalytic material of an electrode for electrochemical reduction reactions, said material comprising an active phase based on at least one group VIB metal and an electrically conductive support, which method is carried out according to at least the following steps: - a step of bringing said support into contact with at least one solution containing at least one precursor of at least one group VIB metal; - a step of drying at a temperature below 250°C, without subsequent calcination step; - a step of sulfurization at a temperature between 100°C and 600°C.

Description

PROCESS FOR PREPARING AN ACTIVE ELECTRODE LAYER FOR ELECTROCHEMICAL REDUCTION REACTIONS

Technical area

The present invention relates to the field of electrodes capable of being used for electrochemical reduction reactions, in particular for the electrolysis of water in a liquid electrolytic medium in order to produce hydrogen.

State of the art

In recent decades, significant research and development efforts have been made to improve technologies enabling the direct conversion of incident solar radiation into electricity by the photovoltaic effect, the conversion of the energy of air masses into electricity. in motion thanks to wind turbines or the conversion into electricity of the potential energy of ocean water evaporated and condensed at altitude thanks to hydroelectric processes. Due to their intermittent nature, these renewable energies benefit from being valued by combining them with an energy storage system to compensate for their discontinuity. The possibilities considered are batteries, compressed air, reversible dam or energy carriers such as hydrogen. For the latter, the electrolysis of water is the most interesting way because it is a clean mode of production (no carbon emission when coupled with a renewable energy source) and provides hydrogen high purity.

In a water electrolysis cell, the hydrogen evolution reaction (HER) occurs at the cathode and the oxygen evolution reaction (OER) occurs at the anode. The overall reaction is:

H2O ^ H; + 1/2 O2

Catalysts are required for both reactions. Different metals have been studied as catalysts for the reaction to produce dihydrogen at the cathode. Today, platinum is the most used metal because it has a negligible overvoltage (voltage required to dissociate the water molecule) compared to other metals. However, the rarity and the cost (> 25 k € / kg) of this noble metal are obstacles to the economic development of the hydrogen sector in the long term. This is the reason why, for a number of years now, researchers have been turning to new catalysts, without platinum, but based on inexpensive metals which are abundant in nature. The production of hydrogen by electrolysis of water is well described in the work: "Hydrogen Production: Electrolysis", 2015, Ed Agata Godula Jopek. Water electrolysis is an electrolytic process which breaks down water into O ₂ and H ₂ gas with the help of an electric current. The electrolytic cell consists of two electrodes - usually made of inert metal (in the potential and pH zone considered) like platinum - immersed in an electrolyte (here water itself) and connected to the opposite poles of the source of direct current.

The electric current dissociates the water molecule (H ₂ 0) into hydroxide ions (HO) and hydrogen H ⁺ : in the electrolytic cell, the hydrogen ions accept electrons at the cathode in a redox reaction by forming gaseous dihydrogen (H ₂ ), depending on the reduction reaction:

The detail of the composition and the use of catalysts for the production of hydrogen by electrolysis of water is widely covered in the literature and we can quote a review article gathering the families of interesting materials under development since these last ten years: "Recent Development in Hydrogen Evolution Reaction Cataiysts and Their Practical Implémentation", 2015, PCK Vesborg et al. where the authors describe sulfides, carbides and phosphides as potential new electrocatalysts. Among the sulfide phases, dichalcogenides such as molybdenum sulfide MoS ₂ are very promising materials for the hydrogen evolution reaction (HER) due to their high activity, their excellent stability and their availability, the molybdenum and sulfur being abundant elements on earth and at low cost.

The materials based on MoS ₂ have a lamellar structure and can be promoted by Ni or Co in order to increase their electrocatalytic activity. The active phases can be used in mass form when the conduction of electrons from the cathode is sufficient or else in the supported state, then bringing into play a support of a different nature. In the latter case, the support must have specific properties:

- large specific surface to promote the dispersion of the active phase;

- very good electronic conductivity;

- chemical and electrochemical stability under the conditions of water electrolysis (acid medium and high potential).

Carbon is the most commonly used support in this application. The challenge lies in the preparation of this sulfurized phase on the conductive material. It is recognized that a catalyst having a high catalytic potential is characterized by an associated active phase perfectly dispersed on the surface of the support and having a high active phase content. It should also be noted that, ideally, the catalyst must have accessibility to the active sites with respect to the reactants, here water, while developing a high active surface, which can lead to specific constraints in terms of structure and texture, specific to the support constituting said catalysts.

The usual methods leading to the formation of the active phase of catalytic materials for the electrolysis of water consist of a deposit of precursor (s) comprising at least one metal from group VIB, and optionally at least one metal from group VIII, on a support by the technique known as "dry impregnation" or by the technique known as "excess impregnation", followed by at least one possible heat treatment to remove the water and by a final stage of generating sulphurization of the active phase, as mentioned above.

It appears interesting to find means of preparing the catalysts for the production of hydrogen by electrolysis of water, making it possible to obtain new catalysts with improved performance. The prior art shows that researchers have turned to several methods, including the deposition of Mo precursors in the form of salts or oxides or ammonium heptamolybdate followed by a sulfurization step in the gas phase or in the presence a chemical reducer

As an example, Chen et al. "Recent Development in Hydrogen Evolution Reaction Catalysts and Their Practical Implementation", 201 1, propose to synthesize a MOS ₂ catalyst by sulphurization of Mo0 ₃ at different temperatures under a mixture of H ₂ S / H ₂ gases with a ratio of 10/90. Kibsgaard et al. "Engineering the surface structure of MoS2 to preferentially expose active edge sites for electrocatalysis", 2012, propose to electrodeposit Mo on an Si support from a solution of peroxopolymolybdate then to carry out a step of sulfurization at 200 ° C under a gas mixture H ₂ S / H ₂ of ratio 10/90. Bonde et al. "Hydrogen evolution on nano-particulate metal transition sulfides", 2009, propose to impregnate a carbon support with an aqueous solution of ammonium heptamolybdate, to air dry it at 140 ° C then to carry out a sulfurization at 450 ° C under a mixture of H ₂ S / H ₂ gas with a 10/90 ratio for 4 hours. Benck et al. "Amorphous Molybdenum Sulfide Catalysts for Electrochemical Hydrogen Production: Insights into the Origin of their Catalytic Activity”, 2012, synthesized a MoS ₂ catalyst by mixing an aqueous solution of ammonium heptamolybdate with a sulfuric acid solution and then in a second time a sodium sulfide solution in order to form nanoparticles of MoS ₂ . Let us also point out a method of preparation consisting in the decomposition of a thiomolybdic salt using a reducing agent. Li et al. "MoS ₂ Nanoparticles Grown on Graphene: An Advanced Catalyst for Hydrogen Evolution Reaction", 201 1, thus synthesized a MoS ₂ catalyst on a graphene support from (NH ₄ ) ₂ MoS ₄ , a solution of DMF, d 'a solution of N ₂ H ₄ · H ₂ 0.

The Applicant has developed a new process for the preparation of a catalytic material making it possible to obtain an electrode usable in an electrolytic cell for carrying out an electrochemical reduction reaction, and more particularly making it possible to obtain a cathode usable in an electrolytic cell for the production of hydrogen by electrolysis of water. Indeed, the Applicant has discovered that the deposition of at least one metal of group VIB in the presence of an organic molecule on an electrically conductive support makes it possible to obtain catalytic performances at least as good, or even better, in particular when this the latter is used as the catalytic phase of an electrode for electrochemical reduction reactions, and this even more particularly when the catalytic material is used as the catalytic phase of a cathode for the production of hydrogen by electrolysis of water .

SUMMARY OF THE INVENTION

The present invention first relates to a process for the preparation of a catalytic material for an electrode for electrochemical reduction reactions, said material comprising an active phase based on at least one metal of group VIB and an electrically conductive support , which method is carried out according to at least the following steps:

a) a step of bringing said support into contact with at least one solution containing at least one precursor of at least one metal from group VIB;

b) optionally, a step of bringing the support into contact with an organic additive, it being understood that step b) is compulsory when said precursor of at least one metal of group VIB according to step a) is chosen from polyoxometallates corresponding to the formula (H _h X _x M _m O _y ) ^q in which H is hydrogen, X is an element chosen from phosphorus (P), silicon (Si), boron (B), nickel ( Ni) and cobalt (Co), M is one or more metal (s) chosen from molybdenum (Mo), tungsten (W), nickel (Ni), cobalt (Co) and iron ( Fe), O being oxygen, h being an integer between 0 and 12, x = 0, m being an integer equal to 5, 6, 7, 8, 9, 10, 11, 12 and 18, y being a integer between 17 and 72 and q being an integer between 1 and 20, it being understood that M is not a nickel atom, a cobalt atom, or an iron atom alone; steps a) and b), if both carried out, being carried out in an indifferent order or simultaneously;

c) a drying step at the end of step a), optionally from the sequence of steps a) and b) or b) and a), at a temperature below 250 ° C., without a subsequent calcination step ;

d) a step of sulfurization of the material obtained at the end of step c) at a temperature between 100 ° C and 600 ° C.

Preferably, said precursor of at least one metal from group VIB is chosen from polyoxometallates corresponding to the formula (H _h X _x M _m O _y ) ^q in which H is hydrogen, X is an element chosen from phosphorus (P), silicon (Si), boron (B), nickel (Ni) and cobalt (Co), said element being taken alone, M is one or more element (s) chosen from molybdenum (Mo), tungsten (W), nickel (Ni), cobalt (Co) and iron (Fe), O being oxygen, h being an integer between 0 and 12, x being an integer between 0 and 4, m being an integer equal to 5, 6, 7, 8, 9, 10, 11, 12 and 18, y being an integer between 17 and 72 and q being an integer between 1 and 20; salts of precursors of elements of group VIB, such as molybdates, thiomolybdates, tungstates or even thiotungstates; organic or inorganic precursors based on Mo or W, such as MoCI ₅ or WCI or WCI ₆ , and alkoxides of Mo or W. More preferably, said precursor is chosen from the polyoxometallates corresponding to the formula (H _h X _x M _m O _y ) ^q as formulated above.

Advantageously, the m atoms M are either only molybdenum atoms (Mo), or only tungsten atoms (W), or a mixture of molybdenum (Mo) and tungsten atoms (W), or a mixture of molybdenum (Mo) and cobalt (Co) atoms, either a mixture of molybdenum (Mo) and nickel (Ni) atoms, or a mixture of tungsten (W) and nickel (Ni) atoms.

Advantageously, the m atoms M are either a mixture of atoms of nickel (Ni), molybdenum (Mo) and tungsten (W), or a mixture of atoms of cobalt (Co), molybdenum (Mo) and tungsten (W).

Advantageously, said method comprises an additional step of introducing at least one promoter comprising at least one group VIII metal by a step of bringing said support into contact with at least one solution containing at least one precursor of at least one metal. of group VIII. Preferably, a maturation step is carried out after step a) and / or b), but before step c), at a temperature between 10 and 50 ° C. for a period of less than 48 hours.

Preferably, the drying step c) is carried out at a temperature below 180 ° C.

Preferably, when the precursor of the catalytic material comprises at least one metal from group VIB and at least one metal from group VIII, the sulfurization temperature in step d) is between 350 ° C. and 550 ° C.

Preferably, when the precursor of the catalytic material only comprises a group VIB metal, the sulfurization temperature in step d) is between 100 ° C and 250 ° C or between 400 ° C and 600 ° C.

Advantageously, the organic additive is chosen from:

- chelating agents, non-chelating agents, reducing agents, non-reducing agents;

- mono-, di- or polyalcohols, carboxylic acids, sugars, mono, di- or non-cyclic polysaccharides, esters, ethers, crown ethers, cyclodextrins and organic compounds containing sulfur or l 'nitrogen.

In one embodiment according to the invention, the support comprises at least one material chosen from carbonaceous structures of the carbon black type, graphite, carbon nanotubes or graphene.

In an embodiment according to the invention, the support comprises at least one material chosen from gold, copper, silver, titanium, silicon.

Another object according to the invention relates to an electrode characterized in that it is formulated by a preparation process comprising the following steps:

1) at least one ionic conductive polymeric binder is dissolved in a solvent or a mixture of solvent;

2) adding to the solution obtained in step 1) at least one catalytic material prepared according to the invention, in powder form, to obtain a mixture;

steps 1) and 2) being carried out in an indifferent order, or simultaneously;

3) the mixture obtained in step 2) is deposited on a metallic or metallic conductive support or collector.

Another object according to the invention relates to an electrolysis device comprising an anode, a cathode, an electrolyte, said device being characterized in that at least one of the anode or the cathode is an electrode according to the invention . Another object according to the invention relates to the use of the electrolysis device according to the invention in electrochemical reactions, and more particularly as:

- water electrolysis device for the production of a gaseous mixture of hydrogen and oxygen and / or the production of hydrogen alone;

- carbon dioxide electrolysis device for the production of formic acid,

- nitrogen electrolysis device for the production of ammonia;

- fuel cell device for the production of electricity from hydrogen and oxygen.

Detailed description of the invention

Definitions

In the following, groups of chemical elements are given according to CAS Classification (CRC Handbook of Chemistry and Physics, CRC press publisher, editor DR Lide, 81 ^th Edition, 2000-2001). For example, group VIII according to the CAS classification corresponds to the metals in columns 8, 9 and 10 according to the new IUPAC classification.

The term “BET surface” is understood to mean the specific surface determined by nitrogen adsorption in accordance with standard ASTM D 3663-78 established on the basis of the BRUNAUER - EMMET - TELLER method described in the periodical "The journal of the American Chemical Society", 60 , 309 (1938).

The process for preparing a catalytic material for an electrode for electrochemical reduction reactions, said material comprising an active phase based on at least one metal from group VIB and an electrically conductive support, comprises at least the following steps :

b) optionally, a step of bringing the support into contact with an organic additive, it being understood that step b) is compulsory when said precursor of at least one metal of group VIB according to step a) is chosen from polyoxometallates corresponding to the formula (H _h X _x M _m O _y ) ^q in which H is hydrogen, X is an element chosen from phosphorus (P), silicon (Si), boron (B), nickel ( Ni) and cobalt (Co), M is one or more metal (s) chosen from molybdenum (Mo), tungsten (W), nickel (Ni), cobalt (Co) and iron ( Fe), O being oxygen, h being an integer between 0 and 12, x = 0, m being an integer equal to 5, 6, 7, 8, 9, 10, 11, 12 and 18, y being an integer between 17 and 72 and q being an integer between 1 and 20, it being understood that M is not a nickel atom, a cobalt atom, or an iron atom alone;

steps a) and b), if both carried out, being carried out in an indifferent order or simultaneously;

Step a)

In accordance with step a) of the preparation process according to the invention, at least one step of bringing the support into contact with at least one solution containing at least one precursor of the active phase comprising at least one metal from group VI B is carried out. Advantageously, the step of bringing the support into contact with at least one precursor of the active phase comprising at least one metal from group VIB (and optionally at least one metal from group VIII) with the support, in accordance with the implementation of step a) can be carried out by impregnation, dry or in excess, or else by deposition - precipitation, according to methods well known to those skilled in the art. Preferably, said step a) is carried out by dry impregnation, which consists in bringing the support into contact with a solution, containing at least one precursor comprising a VIB group (and optionally of group VIII), the volume of the solution is between 0.25 and 1.5 times the pore volume of the support to be impregnated.

Precursors comprising at least one metal from group VIB

The precursors comprising at least one metal from group VIB can be chosen from all the precursors of elements from group VIB known to those skilled in the art. They can be chosen from polyoxometallates (POM) or salts of precursors of elements of group VIB, such as molybdates, thiomolybdates, tungstates or even thiotungstates. They can be chosen from organic or inorganic precursors, such as MoCI ₅ or WCI ₄ or WCI ₆ or alkoxides of Mo or W, for example Mo (OEt) ₅ or W (OEt) ₅ . In the context of the present invention, the term polyoxometallates (POM) is understood to be the compounds corresponding to the formula (H _h X _x M _m O _y ) ^q in which H is hydrogen, X is an element chosen from phosphorus (P), silicon (Si), boron (B), nickel (Ni) and cobalt (Co), said element being taken alone, M is one or more element (s) chosen from molybdenum (Mo), tungsten (W), nickel (Ni), cobalt (Co) and iron (Fe), O being oxygen, h being an integer between 0 and 12, x being an integer between 0 and 4, m being an integer equal to 5, 6, 7, 8, 9, 10, 11, 12 and 18, y being an integer between 17 and 72 and q being an integer between 1 and 20.

Preferably, the element M cannot be a nickel atom, a cobalt atom, or an iron atom alone.

The polyoxometallates defined according to the invention include two families of compounds, isopolyanions and heteropolyanions. These two families of compounds are defined in the article Heteropoly and Isopoly Oxometallates, Pope, Ed Springer-Verlag, 1983.

The isopolyanions which can be used in the present invention are polyoxometallates of general formula (H _h X _x M _m O _y ) ^q in which x = 0, the other elements having the abovementioned meaning.

Preferably, the m atoms M of said isopolyanions are either only molybdenum atoms, or only tungsten atoms, or a mixture of molydene and tungsten atoms, or a mixture of molybdenum and cobalt atoms, or a mixture of molybdenum and nickel atoms, either a mixture of tungsten and cobalt atoms, or a mixture of tungsten and nickel atoms.

The m atoms M of said isopolyanions can also be either a mixture of nickel, molybdenum and tungsten atoms or a mixture of cobalt, molybdenum and tungsten atoms.

Preferably, in the case where the element M is Molybdenum (Mo), m is equal to 7. Likewise, preferably, in the case where the element M is tungsten (W), m is equal to 12 .

The isopolyanons Mo ₇ 0 ₂₄ ⁶ and H ₂ W ₁₂ 0 ₄ o ⁶ are advantageously used as active phase precursors in the context of the invention.

The heteropolyanions which can be used in the present invention are polyoxometallates of formula (H _h X _x M _m O _y ) ^q in which x = 1, 2, 3 or 4, the other elements having the abovementioned meaning.

Heteropolyanions generally have a structure in which the element X is the "central" atom and the element M is a metallic atom practically systematically in octahedral coordination with X different from M. Preferably, the m atoms M are either only molybdenum atoms, or only tungsten atoms, or a mixture of molybdenum and cobalt atoms, or a mixture of molybdenum and nickel, or a mixture of atoms tungsten and molybdenum, either a mixture of tungsten and cobalt atoms, or a mixture of tungsten and nickel atoms. Preferably, the m atoms M are either solely molybdenum atoms, or a mixture of molybdenum and cobalt atoms, or a mixture of molybdenum and nickel. Preferably, the m atoms M cannot be only nickel atoms, nor only cobalt atoms.

Preferably, the element X is at least one phosphorus atom or one Si atom.

Heteropolyanions are negatively charged polyoxometallate species. To compensate for these negative charges, it is necessary to introduce counterions and more particularly cations. These cations can advantageously be H ⁺ protons, or any other cation of NH ₄ ⁺ type or metal cations and in particular metal cations of Group VIII metals.

In the case where the counterions are protons, the molecular structure comprising the heteropolyanion and at least one proton constitutes a heteropolyacid. The heteropolyacids which can be used as active phase precursors in the present invention can be, for example, phosphomolybdic acid (3H ⁺ , PM012O40 ³ ) or also phosphotungstic acid (3H ⁺ , PW ₁₂ O ₄₀ ³ )

In the case where the counterions are not protons, we then speak of a heteropolyanion salt to designate this molecular structure. One can then advantageously take advantage of the association within the same molecular structure, via the use of a heteropolyanion salt, of the metal M and of its promoter, ie of the element cobalt and / or of the nickel element which can either be in position X within the structure of the heteropolyanion, or in partial substitution of at least one atom M of molybdenum and / or tungsten within the structure of the heteropolyanion , or in the counter-ion position.

Preferably, the polyoxometallates used according to the invention are the compounds corresponding to the formula (H _h X _x M _m O _y ) ^q in which H is hydrogen, X is an element chosen from phosphorus (P), silicon (Si), boron (B), nickel (Ni) and cobalt (Co), said element being taken alone, M is one or more element (s) chosen from molybdenum (Mo), tungsten (W), nickel (Ni), cobalt (Co) and iron (Fe), O being oxygen, h being an integer between 0 and 6, x being an integer which can be equal to 0, 1 or 2, m being an integer equal to 5, 6, 7, 9, 10, 1 1 and 12, y being an integer between 17 and 48 and q being an integer between 3 and 12. More preferably, the polyoxometallates used according to the invention are the compounds corresponding to the formula (H _h XxM _m Oy) ^q in which h being an integer equal to 0, 1, 4 or 6, x being an integer equal to 0, 1 or 2, m being an integer equal to 5, 6, 10 or 12, y being an integer equal to 23, 24, 38, or 40 and q being an integer equal to 3, 4, 6 and 7, H , X, M and O having the above meaning.

The preferred polyoxometallates used according to the invention are advantageously chosen from the polyoxometallates of formula PM012O40 ³ , HPCoMonCUo ⁶ , HPNiMon0 ₄ o ⁶ , P2M05O23 ⁶ , CO ₂ MO ₁₀ O38H ₄ ⁶ , COMO ₆ 0 ₂₄ H ₆ ⁴ taken alone or as a mixture .

Preferred polyoxometallates which can advantageously be used in the process according to the invention are the so-called Anderson heteropolyanions of general formula XM ₆ 0 ₂₄ ^q for which the ratio m / x is equal to 6 and in which the elements X and M and the charge q have the abovementioned meaning. The elements X is therefore an element chosen from phosphorus (P), silicon (Si), boron (B), nickel (Ni) and cobalt (Co), said element being taken alone, M is one or several element (s) chosen from molybdenum (Mo), tungsten (W), nickel (Ni) and cobalt (Co), and q is an integer between 1 and 20 and preferably between 3 and 12.

The particular structure of said heteropolyanions called Anderson is described in the article Nature, 1937, 150, 850. The structure of said heteropolyanions called Anderson comprises 7 octahedra located in the same plane and connected together by the edges: of the 7 octahedra, 6 octahedra surround the central octahedron containing the element X.

Anderson heteropolyanions containing within their structure cobalt and molybdenum or nickel and molybdenum are preferred. Anderson heteropolyanions of the formula CoMo ₆ 0 ₂₄ H ₆ ³ and NiMo ₆ 0 ₂₄ H ₆ ⁴ are particularly preferred. According to the formula, in these Anderson heteropolyanions, the cobalt and nickel atoms are respectively the X heteroelements of the structure.

In the case where the Anderson heteropolyanion contains within its structure cobalt and molybdenum, a mixture of the two monomeric forms of formula CoMo ₆ 0 ₂₄ H ₆ ³ and dimeric of formula Co2Mo ₁₀ O3 ₈ H ₄ ^{6 of} said heteropolyanion , the two forms being in balance, can advantageously be used. In the case where the Anderson heteropolyanion contains within its structure cobalt and molybdenum, said Anderson heteropolyanion is preferably dimeric of formula CO ₂ MOI _O 0 ₃₈ H ₄ ⁶ .

In the case where the Anderson heteropolyanion contains within its structure nickel and molybdenum, a mixture of the two monomeric forms of formula NiMo ₆ 0 ₂₄ H ₆ ⁴ and dimeric of formula Ni ₂ Moio03 ₈ H ₄ ^{8 of} said heteropolyanion , the two forms being in balance, can advantageously be used. In the case where the Anderson heteropolyanion contains within its structure nickel and molybdenum, said Anderson heteropolyanion is preferably monomeric with the formula NiMo ₆ 0 ₂₄ H ₆ ⁴ .

Anderson heteropolyanion salts can also be advantageously used as active phase precursors according to the invention. Said Anderson heteropolyanion salts are advantageously chosen from the cobalt or nickel salts of the monomeric ion 6-molybdocobaltate respectively of formula CoMo ₆ 0 ₂₄ H ₆ ³ , 3/2 Co ²⁺ or COMO ₆ 0 ₂₄ H ₆ ³ , 3/2 Ni ²⁺ having an atomic ratio of said promoter (Co and / or Ni) / Mo of 0.41, the cobalt or nickel salts of the dimeric decamolybdocobaltate ion of formula CO ₂ MO ₁₀ O ₃₈ H ₄ ⁶ , 3 Co ²⁺ or CO ₂ MO ₁₀ O ₃₈ H ₄ ^{6 ·} _, 3 Ni ²⁺ having an atomic ratio of said promoter (Co and / or Ni) / Mo of 0.5, the cobalt or nickel of the 6- molybdonickellate ion of formula NiMo ₆ 0 ₂₄ H ₆ ⁴ , 2 Co ²⁺ or NiMo ₆ 0 ₂₄ H ₆ ⁴ , 2 Ni ²⁺ having an atomic ratio of said promoter (Co and / or Ni) / Mo 0.5, and the cobalt or nickel salts of the dimeric decamolybdonickellate ion with the formula Ni ₂ Moio0 ₃₈ H ₄ ⁸ , 4 Co ²⁺ or Nί ₂ Mqio0 ₃₈ H ₄ ⁸ , 4 Ni ²⁺ having an atomic ratio of said promoter (Co and / or Ni) / Mo of 0.6.

The highly preferred Anderson heteropolyanion salts used in the invention are chosen from the dimeric heteropolyanion salts containing cobalt and molybdenum within their structure of formula C0 ₂ M0 ₁₀ O ₃₈ H ₄ ⁶ , 3 Co ^{2 +} and C0 ₂ M0 ₁₀ O ₃₈ H ₄ ⁶ _, 3 Ni ²⁺ . An even more preferred Anderson heteropolyanion salt is the dimeric Anderson heteropolyanion salt of formula C0 ₂ M0 ₁₀ O ₃₈ H ₄ ⁶ , 3 Co ²⁺

Other preferred polyoxometallates which can advantageously be used in the process according to the invention are the so-called Keggin heteropolyanions of general formula XM ₁₂ 0 ₄ o ^q for which the m / x ratio is equal to 12 and the so-called incomplete Keggin heteropolyanions of general formula XMn0 ₃₉ ^q for which the ratio m / x is equal to 11 and in which the elements X and M and the charge q have the abovementioned meaning. X is therefore an element chosen from phosphorus (P), silicon (Si), boron (B), nickel (Ni) and cobalt (Co), said element being taken alone, M is one or more element ( s) chosen from molybdenum (Mo), tungsten (W), nickel (Ni) and cobalt (Co), and q being an integer between 1 and 20 and preferably between 3 and 12.

Said Keggin species are advantageously obtained for variable pH ranges according to the methods of production described in the publication by A. Griboval, P. Blanchard, E. Payen, M. Fournier, JL Dubois, Chem. Lett., 1997, 12, 1259. A preferred Keggin heteropolyanion, advantageously used according to the invention, is the heteropolyanion of formula RMqi ₂ 0 ₄ o ³ or PW ₁₂ 0 ₄ o ³ or SiMoi ₂ O ₄₀ ⁴ or SiW ₁₂ O ₄₀ ⁴ .

The preferred Keggin heteropolyanion can also be advantageously used in the invention in its heteropolyacid form of formula PMOI ₂ O ₀ ³ , 3H ⁺ or PW ₁₂ O ₀ ³ , 3H ⁺ or SiMoi ₂ O ₄₀ ⁴ , 4H ⁺ or SiW ₁₂ O ₄₀ ⁴ , 4H ⁺ _.

Salts of heteropolyanions of the Keggin or Keggin lacunary type can also be advantageously used according to the invention. Preferred salts of heteropolyanions or heteropolyacids of the Keggin and Keggin type are advantageously chosen from cobalt or nickel salts of phosphomolybdic, silicomolybdic, phosphotungstic or silicitungstic acids. Said salts of heteropolyanions or heteropolyacids of the Keggin or Keggin lacunary type are described in US Pat. No. 2,547,380. Preferably, a Keggin type heteropolyanion salt is the nickel phosphotungstate of formula 3 / 2Ni ²⁺ _, PW ₁₂ O ₀ ³ having an atomic ratio of the metal of group VIB to the metal of group VIII, that is to say Ni / W of 0.125.

Another preferred polyoxometallate which can advantageously be used as a precursor used in the process according to the invention is the Strandberg heteropolyanion of formula H _h P ₂ Mo ₅ 0 ₂₃ ^{(6 h)} , h being equal to 0, 1 or 2 and for which the ratio m / x is equal to 5/2.

The preparation of said Strandberg heteropolyanions and in particular of said heteropolyanion of formula H _h P ₂ Mo ₅ 0 ₂₃ ^{(6 h)} is described in the article by WC. Cheng, NP Luthra, J. Catal., 1988, 109, 163.

Thus, thanks to various preparation methods, many polyoxometallates and their associated salts are available. In general, all these polyoxometallates and their associated salts can be advantageously used during the electrolysis implemented in the process according to the invention. The above list is not exhaustive, however, and other combinations can be envisaged.

Precursors comprising at least one group VIII metal:

The preferred group VIII elements are non-noble elements: they are chosen from Ni, Co and Fe. Preferably, the group VIII elements are Co and Ni. The group VIII metal can be introduced in the form of salts, chelating compounds, alkoxides or glycoxides. The sources of group VIII elements which can advantageously be used in the form of salts, are well known to those skilled in the art. They are chosen from nitrates, sulfates, hydroxides, phosphates, carbonates, halides chosen from chlorides, bromides and fluorides. Said precursor comprising at least one metal from group VIII is partially soluble in the aqueous phase or in the organic phase. The solvents used are generally water, an alkane, an alcohol, an ether, a ketone, a chlorine compound or an aromatic compound. Acidified water, toluene, benzene, dichloromethane, tetrahydrofuran, cyclohexane, n-hexane, ethanol, methanol and acetone are preferably used.

The group VIII metal is preferably introduced in the form of acetylacetonate or acetate when an organic solvent is used, in the form of nitrate when the solvent is water and in the form of hydroxides or carbonates or hydroxycarbonates when the solvent is water at an acidic pH, ie less than 7, advantageously less than 2.

Preferably, said precursor comprising at least one group VIII metal is introduced either:

i) before the contacting steps a) and optionally b), in a so-called pre-impregnation step a1) using a solution comprising at least one precursor comprising at least one group VIII metal;

ii) during the contacting step a), in co-contact with said solution comprising at least one precursor comprising at least one metal from group VIB;

iii) after the drying step c), in a so-called post-impregnation step c1), using a solution containing at least one precursor comprising at least one group VIII metal. In this particular embodiment, an optional maturation step, and a drying step at a temperature below 250 ° C, preferably below 180 ° C, can be carried out under the same conditions as the conditions described above;

iv) after step d) of sulfurization, in a step called post-impregnation d1) using a solution comprising at least one precursor comprising at least one group VIII metal. In this particular embodiment, a new maturation step can optionally be carried out, a new drying step at a temperature below 250 ° C., preferably below 180 ° C., and optionally a new sulfurization step, in the same operating conditions as described above. Other promoters

The solutions used in the various impregnation or successive impregnation steps may optionally contain at least one precursor of a doping element chosen from boron, phosphorus and silicon. The precursors of a doping element chosen from boron, phosphorus and silicon can also advantageously be added in impregnation solutions not containing the precursors of at least one metal chosen from the group formed by the metals of groups VIII and Group VI B metals, taken alone or as a mixture.

Said precursors of group VIII metals and of metals of group VIB, the precursors of doping elements and organic compounds are advantageously introduced into the impregnation solution (s) in an amount such as the contents of element of group VIII, VIB, doping element and organic additives on the final catalyst are as defined below.

According to one embodiment according to the invention, when the precursor of at least one metal of group VIB according to step a) is chosen from the polyoxometallates corresponding to the formula (H _h X _x M _m O _y ) ^q - with x = 0, then an additional step of bringing said electroconductive support into contact with at least one solution containing at least one organic compound, in accordance with the implementation of step b), can be carried out by any method well known to the skilled person. In particular, said step b) can be carried out by impregnation, dry or in excess according to methods well known to those skilled in the art. Preferably, said step b) is carried out by dry impregnation, which consists in bringing the support into contact with a volume of said solution of between 0.25 and 1.5 times the pore volume of the support to be impregnated.

Said organic compound can be chosen from all the organic compounds known to those skilled in the art, and is selected in particular from chelating agents, non-chelating agents, reducing agents, non-reducing agents. It can also be chosen from optionally etherified mono-, di- or polyalcohols, carboxylic acids, sugars, mono, di- or non-cyclic polysaccharides such as glucose, fructose, maltose, lactose or sucrose, esters, ethers, crown ethers, cyclodextrins and compounds containing sulfur or nitrogen such as nitriloacetic acid, ethylenediaminetetraacetic acid, or diethylenetriamine alone or as a mixture. Implementation of steps a) and b)

When the process according to the invention comprises the implementation of steps a) and b, then the process for preparing the catalytic material includes several modes of implementation. They are distinguished in particular by the order of introduction of the organic compound and the metal precursor of the active phase, the contacting of the organic compound with the support can be carried out either after the contact of the metal precursor of the active phase with the support, either before contacting the metal precursor of the active phase with the support, or simultaneously.

A first mode of implementation consists in carrying out step b) before step a) (pre-impregnation).

A second mode of implementation consists in carrying out step b) after step a) (post-impregnation).

A third mode of implementation consists in carrying out steps a) and b) simultaneously (co-impregnation).

Each step a) and b) of bringing the support into contact with the metal precursor (step a), and of bringing the support into contact with at least one solution containing at least one organic compound (step b), is carried out at least one times and can advantageously be carried out several times, all the possible combinations of implementation of steps a) and b) are included within the scope of the invention.

Each contacting step can preferably be followed by an intermediate drying step. The intermediate drying step is carried out at a temperature below 250 ° C, preferably between 15 and 250 ° C, more preferably between 30 and 220 ° C, even more preferably between 50 and 200 ° C, and again more preferable between 70 and 180 ° C.

Advantageously, after each contacting step, whether it is a contacting step of the metal precursor of the active phase or a contacting step of the organic compound, the impregnated support can be allowed to mature, possibly before a contacting step. intermediate drying. The maturation allows the solution to distribute itself evenly within the support. When a maturation step is carried out, said step is advantageously carried out at atmospheric pressure, under an inert atmosphere or under an atmosphere containing oxygen or under an atmosphere containing water or the impregnating solvent, and at a temperature between 10 ° C and 50 ° C, and preferably at room temperature. Generally a maturation period of less than 48 hours and preferably between 5 minutes and 12 hours is sufficient. Step c) of drying

The drying step is carried out at a temperature below 250 ° C, preferably below 180 ° C, more preferably below 120 ° C. Very preferably, the drying is carried out at reduced pressure at a temperature not exceeding 80 ° C. The drying time is between 30 minutes and 24 hours, preferably between 30 minutes and 16 hours. Preferably, the drying time does not exceed 4 hours.

The drying step can be carried out by any technique known to those skilled in the art. It is advantageously carried out under an inert atmosphere or under an atmosphere containing oxygen. It is advantageously carried out at atmospheric pressure or at reduced pressure.

Step d) of sulfurization

The sulfurization carried out during step d) is intended to at least partially sulfurize the metal of group VIB, and optionally at least partially the metal of group VIII.

Step d) of sulfurization can advantageously be carried out using a gas mixture H ₂ S / H ₂ OR H ₂ S / N ₂ containing at least 5% by volume of H ₂ S in the mixture or under flow pure H ₂ S at a temperature between 100 ° C and 600 ° C, under a total pressure equal to or greater than 0.1 MPa for at least 2 hours.

Preferably, when the precursor of the catalytic material comprises the metals of groups VIB and VIII, the sulfurization temperature is between 350 ° C. and 550 ° C.

Preferably, when the precursor of the catalytic material only comprises the metals of group VIB, the sulfurization temperature is between 100 ° C and 250 ° C or between 400 ° C and 600 ° C.

Catalyst material

The activity of the catalytic material for the production of hydrogen by electrolysis of water is ensured by an element of group VIB and optionally by at least one element of group VIII. Advantageously, the active phase is chosen from the group formed by the combinations of the elements nickel-molybdenum or cobalt-molybdenum or nickel-cobalt-molybdenum or nickel-tungsten or nickel-molybdenum-tungsten.

When the group VIB metal is molybdenum, the molybdenum content (Mo) is between 4 and 60% by weight of Mo element relative to the weight of the final catalytic material, and preferably between 7 and 50% by weight relative to the weight of the final catalytic material obtained after the last preparation step, ie the sulfurization. When the metal of group VI B is tungsten, the tungsten content (W) is between 7 and 70% by weight of element W relative to the weight of the final catalytic material, and preferably between 12 and 60% by weight relative the weight of the final catalytic material obtained after the last preparation step, ie the sulfurization.

The surface density which corresponds to the quantity of molybdenum atoms Mo deposited per surface unit of support will advantageously be between 0.5 and 20 atoms of Mo per square nanometers of support and preferably between 2 and 15 atoms of Mo per nanometers support square.

When the catalytic material comprises at least one group VIII metal, the group VIII metal content is advantageously between 0.1 and 15% by weight of group VIII element, preferably between 0.5 and 10% by weight relative to the total weight of the final catalytic material obtained after the last preparation step, ie the sulfurization.

The support for the catalytic material is a support comprising at least one electrically conductive material.

In an embodiment according to the invention, the support for the catalytic material comprises at least one material chosen from carbonaceous structures of the carbon black type, graphite, carbon nanotubes or graphene.

In an embodiment according to the invention, the support for the catalytic material comprises at least one material chosen from gold, copper, silver, titanium, silicon.

A porous and non-electrically conductive material can be made electrically conductive by depositing on the surface thereof an electrically conductive material; let us quote for example a refractory oxide, such as an alumina, within which graphitic carbon is deposited. The support for the catalytic material advantageously has a BET specific surface area (SS) greater than 75 m ² / g, preferably greater than 100 m ² / g, very preferably greater than 130 m ² / g.

Electrode

The catalytic material capable of being obtained by the preparation process according to the invention can be used as an electrode catalytic material capable of being used for electrochemical reactions, and in particular for the electrolysis of water in the medium liquid electrolytic.

Advantageously, the electrode comprises a catalytic material obtained by the preparation process according to the invention and a binder. The binder is preferably a polymeric binder chosen for its capacities to be deposited in the form of a layer of variable thickness and for its capacities of ionic conduction in aqueous medium and diffusion of dissolved gases. The layer of variable thickness, advantageously between 1 and 500 μm, in particular of the order of 10 to 100 μm, may in particular be a gel or a film.

Advantageously, the ionic conductive polymer binder is:

^* or conductor of anionic groups, in particular of hydroxy group and is chosen from the group comprising in particular:

- polymers stable in an aqueous medium, which can be perfluorinated, partially fluorinated or non-fluorinated and having cationic groups allowing the conduction of hydroxide anions, said cationic groups being of quaternary ammonium, guanidinium, imidazolium, phosphonium, pyridium or sulfide type;

- ungrafted polybenzimidazole;

- chitosan; and

- mixtures of polymers comprising at least one of the various polymers mentioned above, said mixture having anionic conductor properties;

^* is a conductor of cationic groups allowing the conduction of protons and is chosen from the group comprising in particular:

- polymers stable in an aqueous medium, which can be perfluorinated, partially fluorinated or non-fluorinated and having anionic groups allowing the conduction of protons;

- the grafted polybenzimidazole;

- chitosan; and

- mixtures of polymers comprising at least one of the various polymers mentioned above, said mixture having properties of cationic conductor.

Among the polymers stable in an aqueous medium and having cationic groups allowing the conduction of anions, mention may in particular be made of polymer chains of perfluorinated type such as for example polytetrafluoroethylene (PTFE), of partially fluorinated type, such as for example polyvinylidene fluoride (PVDF) or non-fluorinated type such as polyethylene, which will be grafted with anionic conductive molecular groups.

Among the polymers stable in aqueous medium and having anionic groups allowing the conduction of the protons, one can consider any polymeric chain stable in aqueous medium containing groups such as -S03, -COO, -P0 ₃ ² , -P0 ₃ H, - C ₆ H ₄ 0. Mention may in particular be made of Nation®, phosphonated sulfonated polybenzimidazole (PBI), sulfonated or phosphonated polyetheretherketone (PEEK).

In accordance with the present invention, it is possible to use any mixture comprising at least two polymers, at least one of which is chosen from the groups of polymers mentioned above, provided that the final mixture is ionic conductor in an aqueous medium. Thus, by way of example, mention may be made of a mixture comprising a polymer stable in an alkaline medium and having cationic groups allowing the conduction of hydroxide anions with a polyethylene which is not grafted with anionic conductive molecular groups, provided that this final mixture is anionic conductor in medium alkaline. Mention may also be made, for example, of a mixture of a polymer which is stable in an acid or alkaline medium and which has anionic or cationic groups allowing the conduction of protons or hydroxides and of grafted or non-grafted polybenzimidazole.

Advantageously, polybenzimidazole (PBI) is used in the present invention as a binder. It is intrinsically not a good ionic conductor, but in an alkaline or acidic medium, it turns out to be an excellent polyelectrolyte with respectively very good anionic or cationic conduction properties. PBI is a polymer generally used, in grafted form, in the manufacture of proton-conducting membranes for fuel cells, in membrane-electrode assemblies and in PEM-type electrolysers, as an alternative to Nafion®. In these applications, the PBI is generally functionalized / grafted, for example by sulfonation, in order to make it proton conductive. The role of PBI in this type of system is then different from that which it has in the manufacture of electrodes according to the present invention where it only serves as a binder and has no direct role in the electrochemical reaction.

Even if its long-term stability in concentrated acid medium is limited, chitosan, also usable as an anionic or cationic conductive polymer, is a polysaccharide having ionic conduction properties in basic medium which are similar to those of PBI (G. Couture, A. Alaaeddine, F. Boschet, B. Ameduri, Progress in Polymer Science 36 (2011) 1521-1557).

Advantageously, the electrode according to the invention is formulated by a process which further comprises a step of removing the solvent at the same time or after step 3). The removal of the solvent can be carried out by any technique known to a person skilled in the art, in particular by evaporation or phase inversion. In the event of evaporation, the solvent is an organic or inorganic solvent whose evaporation temperature is lower than the decomposition temperature of the polymeric binder used. Examples that may be mentioned are dimethylsulfoxide (DMSO) or acetic acid. Those skilled in the art are able to choose the organic or inorganic solvent suitable for the polymer or the polymer mixture used as binder and capable of being evaporated.

According to a preferred embodiment of the invention, the electrode is suitable for being used for the electrolysis of water in an alkaline liquid electrolyte medium and the polymeric binder is then an anionic conductor in an alkaline liquid electrolyte medium, in particular conductor d 'hydroxides.

For the purposes of the present invention, the term “alkaline liquid electrolyte medium” means a medium whose pH is greater than 7, advantageously greater than 10.

The binder is advantageously conductive of hydroxides in an alkaline medium. It is chemically stable in electrolysis baths and has the capacity to diffuse and / or transport the OH ions involved in the electrochemical reaction to the surface of the particles, seats of the redox reactions for the production of H ₂ and O ₂ gases. . Thus, a surface which is not in direct contact with the electrolyte is nevertheless involved in the electrolysis reaction, a key point in the efficiency of the system. The chosen binder and the shaping of the electrode do not impede the diffusion of the gases formed and limit their adsorption, thus allowing their evacuation. According to another preferred embodiment of the invention, the electrode is suitable for being used for the electrolysis of water in an acidic liquid electrolyte medium and the polymeric binder is a cationic conductor in an acidic liquid electrolyte medium, in particular conductor of protons.

For the purposes of the present invention, the term “acid medium” is intended to mean a medium whose pH is less than 7, advantageously less than 2.

Those skilled in the art, in the light of their general knowledge, will be able to define the quantities of each component of the electrode. The density of particles of catalytic material must be sufficient to reach their threshold of electrical percolation.

According to a preferred embodiment of the invention, the mass ratio of polymer binder / catalytic material is between 5/95 and 95/5, preferably between 10/90 and 90/10, and more preferably between 10/90 and 40/60.

The electrode can be prepared according to techniques well known to those skilled in the art. More particularly, the electrode is formulated by a preparation process comprising the following steps: 1) at least one ionic conductive polymeric binder is dissolved in a solvent or a mixture of solvent;

steps 1) and 2) being carried out in an indifferent order, or simultaneously;

Within the meaning of the invention, the term “catalytic material powder” means a powder consisting of particles of micron, sub-micron or nanometric size. The powders can be prepared by techniques known to those skilled in the art.

Within the meaning of the invention, the expression “support or collector of metallic type” means any conductive material having the same conduction properties as metals, for example graphite or certain conductive polymers such as polyaniline and polythiophene. This support can see any shape allowing the deposition of the mixture obtained (between the binder and the catalytic material) by a method chosen from the group notably comprising soaking, printing, induction, pressing, coating , spinning (or “spin-coating” according to English terminology), filtration, vacuum deposition, spray deposition, casting, extrusion or rolling. Said support or said collector may be solid or perforated. As an example of support, there may be mentioned a grid (perforated support), a plate or a sheet of stainless steel (304L or 316L for example) (solid supports).

The advantage of the mixture according to the invention is that it can be deposited on a solid or perforated collector, by usual deposition techniques that are easily accessible and allow deposition in the form of layers of variable thicknesses ideally of the order of 10 at 100 pm. According to the invention, the mixture can be prepared by any technique known to a person skilled in the art, in particular by mixing the binder and the at least one catalytic material in powder form in a suitable solvent or a mixture of solvents suitable for 'obtaining a mixture with rheological properties allowing the deposition of electrode materials in the form of a film of controlled thickness on an electronic conductive substrate. The use of the catalytic material in powder form allows a maximization of the surface developed by the electrodes and an enhancement of the associated performances. Those skilled in the art will be able to make the choices of the different formulation parameters in the light of their general knowledge and the physico-chemical characteristics of said mixtures. Operating procedures

Another object according to the invention relates to an electrolysis device comprising an anode, a cathode, an electrolyte, in which at least one of the anode or the cathode is an electrode according to the invention.

The electrolysis device can be used as a water electrolysis device for the production of a gaseous mixture of hydrogen and oxygen and / or the production of hydrogen alone comprising an anode, a cathode and an electrolyte, said device being characterized in that at least one of the cathode or the anode is an electrode according to the invention, preferably the cathode. The electrolysis device consists of two electrodes (an anode and a cathode, electronic conductors) connected to a direct current generator, and separated by an electrolyte (ionically conductive medium). The anode is the seat of water oxidation. The cathode is the seat of proton reduction and hydrogen formation.

The electrolyte can be:

either an acidic aqueous solution (H ₂ S0 ₄ or HCl, ...) or basic (KOH);

either a polymeric proton exchange membrane which ensures the transfer of protons from the anode to the cathode and allows the separation of the anode and cathode compartments, which avoids reoxidizing at the anode the reduced species at the cathode and vice versa;

or an ion-conducting ceramic membrane 0 ₂ . We then speak of a solid oxide electrolysis (SOEC or 'Solid Oxide Electrolyser Cell' according to English terminology).

The minimum water supply for an electrolysis device is 0.8 l / Nm ³ of hydrogen. In practice, the real value is close to 1 l / Nm ³ . The water introduced must be as pure as possible because the impurities remain in the equipment and accumulate over the electrolysis, ultimately disrupting the electrolytic reactions by:

sludge formation; and by

the action of chlorides on the electrodes.

An important specification for water relates to its ionic conductivity (which must be less than a few pS / cm).

There are many suppliers offering very diverse technologies, particularly in terms of the nature of the electrolyte and associated technology, ranging from a possible upstream coupling with a renewable electrical supply (photovoltaic or wind), to the direct final supply of hydrogen under pressure. The reaction has a standard potential of -1.23 V, which means that it ideally requires a potential difference between the anode and the cathode of 1.23 V. A standard cell generally operates under a potential difference of 1 , 5 V and at room temperature. Some systems may operate at higher temperatures. Indeed, it has been shown that high temperature electrolysis (HTE) is more efficient than electrolysis of water at room temperature, on the one hand, because part of the energy required for the reaction can be provided by heat (cheaper than electricity) and, on the other hand, because the activation of the reaction is more effective at high temperature. HTE systems generally operate between 100 ° C and 850 ° C.

The electrolysis device can be used as a nitrogen electrolysis device for the production of ammonia, comprising an anode, a cathode and an electrolyte, said device being characterized in that at least one of the cathode or anode is an electrode according to the invention, preferably the cathode.

The electrolysis device consists of two electrodes (an anode and a cathode, electronic conductors) connected to a direct current generator, and separated by an electrolyte (ionically conductive medium). The anode is the seat of water oxidation. The cathode is the seat of nitrogen reduction and the formation of ammonia. Nitrogen is continuously injected into the cathode compartment.

The nitrogen reduction reaction is:

N2 + 6H * + 6th- - »2 N Ha

The electrolyte can be:

either an aqueous solution (Na ₂ S0 ₄ or HCL), preferably saturated with nitrogen;

or a polymeric proton exchange membrane which ensures the transfer of protons from the anode to the cathode and allows the separation of the anode and cathode compartments, which avoids reoxidizing at the anode the reduced species at the cathode and vice versa.

The electrolysis device can be used as a carbon dioxide electrolysis device for the production of formic acid, comprising an anode, a cathode and an electrolyte, said device being characterized in that at least one of the cathode or anode is an electrode according to the invention. An example of anode and electrolyte that can be used in such a device is described in detail in the document FR3007427. The electrolysis device can be used as a fuel cell device for the production of electricity from hydrogen and oxygen comprising an anode, a cathode and an electrolyte (liquid or solid), said device being characterized in that at least one of the cathode or the anode is an electrode according to the invention.

The fuel cell device consists of two electrodes (an anode and a cathode, electronic conductors) connected to a load C to deliver the electric current produced, and separated by an electrolyte (ionic conductive medium). The anode is the seat of hydrogen oxidation. The cathode is the seat of oxygen reduction.

The electrolyte can be:

- either an acidic aqueous solution (H ₂ S0 ₄ or HCl, ...) or basic (KOH);

- either a proton-exchange polymer membrane which ensures the transfer of protons from the anode to the cathode and allows the separation of the anode and cathode compartments, which avoids reoxidizing at the anode the reduced species at the cathode and vice versa;

- either an ion-conducting ceramic membrane 0 ₂ . We then speak of a solid oxide fuel cell (SOFC or 'Solid Oxide Fuel Ce //' according to English terminology).

The following examples illustrate the present invention without, however, limiting its scope. The examples below relate to the electrolysis of water in a liquid electrolytic medium for the production of hydrogen.

Examples

Example 1 Preparation of a catalytic material C1 (according to the invention ^') from and citric acid.

The catalytic material C1 (compliant) is prepared by dry impregnation of 10 g of support of commercial carbon type (ketjenblack®, 1400 m2 / g) with 26 ml of solution. The solution is obtained by solubilization in water of H ₃ RMo ₁₂ O ₄₀ at a concentration of 2.6 mol / l, of Ni (OH) ₂ such that the ratio Ni / Mo = 0.2 and of citric acid such that the citric acid / Mo ratio = 0.5. The preparation of the catalyst continues with a maturation step where the impregnated solid is kept in a closed enclosure, the atmosphere of which is saturated with water for 12 hours before undergoing a drying step under an inert atmosphere and at reduced pressure (by drawing under vacuum) at 60 ° C (oil bath). The precatalyst is sulfurized under pure H ₂ S at a temperature of 400 ° C for 2 hours under 0.1 MPa of pressure. On the final catalyst, the quantity of Mo corresponds to a surface density of 7 atoms per nm ² and the ratios of Ni and P are respectively: Ni / Mo = 0.2 and P / Mo = 0.08.

Example 2 Description of the Pt commercial catalyst (Catalyst C2)

The material C2 comes from Alfa Aesar®: it comprising platinum particles of SBET = 27 m ² / g.

Example 3: Catalyst test

The characterization of the catalytic activity of catalytic materials is carried out in a cell with 3 electrodes. This cell is composed of a working electrode, a platinum counter electrode and an Ag / AgCI reference electrode. The electrolyte is an aqueous solution of sulfuric acid (H ₂ S0 ₄ ) at 0.5 mol / L. This medium is deoxygenated by bubbling with nitrogen and the measurements are made under an inert atmosphere (deaeration with nitrogen).

The working electrode consists of a 5 mm diameter disc of vitreous carbon set in a Teflon tip (rotating disc electrode). Glassy carbon has the advantage of having no catalytic activity and of being a very good electrical conductor. In order to deposit the catalysts (C1, C2) on the electrode, a catalytic ink is formulated. This ink consists of a binder in the form of a solution of 10 μL of Nafion® 15% by mass, of a solvent (1 ml of 2-propanol) and of 5 mg of catalyst (C1, C2). The role of the binder is to ensure the cohesion of the particles of the supported catalyst and the adhesion to vitreous carbon. This ink is then placed in an ultrasonic bath for 30 to 60 minutes in order to homogenize the mixture. 12 µL of the prepared ink is deposited on the working electrode (described above). The ink is then deposited on the working electrode and then dried in order to evaporate the solvent.

Different electrochemical methods are used to determine the performance of the catalysts:

- linear voltammetry: it consists in applying a potential signal to the working electrode which varies over time, ie from 0 to - 0.5 V vs RHE at a speed of 2 mV / s, and measuring the current faradaic response, that is to say the current due to the oxidation-reduction reaction taking place at the working electrode. This method is ideal for determining the catalytic power of a material for a given reaction. It allows, among other things, to determine the overvoltage necessary for the reduction of protons to H ₂ . - chronopotentiometry: it consists of applying a current or a current density for a determined time and measuring the resulting potential. This study makes it possible to determine the catalytic activity at constant current but also the stability of the system over time. It is carried out with a current density of - 10 mA / cm ² and for a given time.

The catalytic performances are collated in Table 1, below. They are expressed in overvoltage at a current density of -10 mA / cm ² .

Table 1

With an overvoltage of only - 190 mV vs RHE, the catalytic material C1 has performances relatively close to that of platinum vis-à-vis according to the prior art. This result demonstrates the indisputable interest of this material for the development of the hydrogen sector by electrolysis of water.

Claims

1. Process for the preparation of a catalytic material for an electrode for electrochemical reduction reactions, said material comprising an active phase based on at least one metal from group VI B and an electrically conductive support, which process is carried out according to at least the following steps:

b) optionally, a step of bringing the support into contact with an organic additive, it being understood that step b) is compulsory when said precursor of at least one metal of group VIB according to step a) is chosen from polyoxometallates corresponding to the formula (H _h X _x M _m O _y ) ^q in which H is hydrogen, X is an element chosen from phosphorus (P), silicon (Si), boron (B), nickel ( Ni) and cobalt (Co), M is one or more metal (s) chosen from molybdenum (Mo), tungsten (W), nickel (Ni), cobalt (Co) and iron ( Fe), O being oxygen, h being an integer between 0 and 12, x = 0, m being an integer equal to 5, 6, 7, 8, 9, 10, 11, 12 and 18, y being a integer between 17 and 72 and q being an integer between 1 and 20, it being understood that M is not a nickel atom, a cobalt atom, or an iron atom alone;

2. Method according to claim 1, in which said precursor of at least one metal from group VIB is chosen from polyoxometallates corresponding to the formula (H _h X _x M _m O _y ) ^q in which H is hydrogen, X is an element chosen from phosphorus (P), silicon (Si), boron (B), nickel (Ni) and cobalt (Co), said element being taken alone, M is one or more element (s) chosen from molybdenum (Mo), tungsten (W), nickel (Ni), cobalt (Co) and iron (Fe), O being oxygen, h being an integer between 0 and 12 , x being an integer between 0 and 4, m being an integer equal to 5, 6, 7, 8, 9, 10, 1 1, 12 and 18, y being an integer between 17 and 72 and q being an integer between 1 and 20; salts of precursors of elements of group VIB, such as molybdates, thiomolybdates, tungstates or even thiotungstates; organic or inorganic precursors based on Mo or W, such as M0CI ₅ or WCI ₄ or WCI ₆ , and alkoxides of Mo or W.

3. Method according to one of claims 1 or 2, wherein the m atoms M are either only molybdenum atoms (Mo), or only tungsten atoms (W), or a mixture of molybdenum atoms (Mo ) and tungsten (W), either a mixture of molybdenum (Mo) and cobalt (Co) atoms, or a mixture of molybdenum (Mo) and nickel (Ni) atoms, or a mixture of atoms tungsten (W) and nickel (Ni).

4. Method according to one of claims 1 or 2, wherein the m atoms M are either a mixture of nickel (Ni), molybdenum (Mo) and tungsten (W) atoms, or a mixture of atoms cobalt (Co), molybdenum (Mo) and tungsten (W).

5. Method according to any one of claims 1 to 4, comprising an additional step of introducing at least one promoter comprising at least one group VIII metal by a step of bringing said support into contact with at least one solution containing at least one precursor of at least one group VIII metal.

6. Method according to any one of claims 1 to 5, in which a maturation step is carried out after step a) and / or b), but before step c), at a temperature between 10 and 50 ° C for a period of less than 48 hours.

7. Method according to any one of claims 1 to 6, in which the drying step c) is carried out at a temperature below 180 ° C.

8. Method according to any one of claims 1 to 7, in which when the precursor of the catalytic material comprises at least one metal from group VI B and at least one metal from group VIII, the sulfurization temperature in step d) is between 350 ° C and 550 ° C.

9. Method according to any one of claims 1 to 8, in which when the precursor of the catalytic material comprises only a metal from group VIB, the sulfurization temperature in step d) is between 100 ° C and 250 ° C or between 400 ° C and 600 ° C.

10. Method according to any one of claims 1 to 9, in which the organic additive is chosen from:

- chelating agents, non-chelating agents, reducing agents, non-reducing agents;

11. Method according to any one of claims 1 to 10, in which the support comprises at least one material chosen from carbonaceous structures of the carbon black type, graphite, carbon nanotubes or graphene.

12. Method according to any one of claims 1 to 10, wherein the support comprises at least one material chosen from gold, copper, silver, titanium, silicon.

13. Electrode characterized in that it is formulated by a preparation process comprising the following steps:

2) adding to the solution obtained in step 1) at least one catalytic material prepared according to any one of claims 1 to 12, in powder form, to obtain a mixture;

steps 1) and 2) being carried out in an indifferent order, or simultaneously;

14. An electrolysis device comprising an anode, a cathode, an electrolyte, said device being characterized in that at least one of the anode or the cathode is an electrode according to claim 13.

15. Use of the electrolysis device according to claim 14 in electrochemical reactions.

16. Use according to claim 15, in which said device is used as:

- carbon dioxide electrolysis device for the production of formic acid;

- nitrogen electrolysis device for the production of ammonia;

- fuel cell device for the production of electricity from hydrogen and oxygen.