FR3162008A1 - Process for preparing a fluorinated MoS2-based electrode active layer for electrochemical reduction reactions - Google Patents

Abstract

A process for preparing a catalytic material for an electrode used in electrochemical reduction reactions, said catalytic material comprising an active phase based on at least one metal from Group VIB and fluorine. The process consists of contacting a solid material based on at least one sulfide metal from Group VIB with a gas containing at least fluorine, at a temperature between -50°C and 150°C, for a duration between 15 seconds and 120 minutes, at a fluorine concentration in the gas between 0.1 and 100% by volume relative to the total volume of said gas, at a pressure between 0.001 and 0.2 MPa, and at a PPH between 0.01 and 200 h⁻¹.

Description

Process for preparing a MoS2-based fluorinated electrode active layer for electrochemical reduction reactions

The present invention relates to the field of electrodes suitable for use in electrochemical reduction reactions, in particular for the electrolysis of water to produce hydrogen.

State of the art

To achieve the goals of the energy transition, manufacturers are now focusing on the production and use of low-carbon, renewable hydrogen (or dihydrogen _H2 ). Currently used primarily in the chemical and refining industries, this energy carrier could contribute to decarbonizing certain industrial sectors, ensuring electricity storage, or powering the transportation sector.

• le reformage du gaz naturel à la vapeur d'eau est la technique la plus répandue. Il s’agit de faire réagir du méthane avec de l’eau pour obtenir un mélange contenant de l’hydrogène et du CO₂.
• la gazéification permet de produire, par combustion, un mélange de CO et d’H₂à partir de charbon ou de biomasse. Le CO₂émis dans les deux cas précédents pourrait éventuellement être capté et stocké ou valorisé.
• l’hydrogène peut aussi être produit à partir d’eau et d’électricité, c’est l’électrolyse de l'eau. C’est la voie la plus intéressante car elle est un mode de production propre (pas d’émission carbone lorsqu’elle est couplée à une source d’énergie renouvelable) et fournit de l’hydrogène de pureté élevée, mais elle reste encore coûteuse (2 à 3 fois plus chère que le reformage du gaz naturel).

Various production techniques exist:

• Steam reforming of natural gas is the most widespread technique. It involves reacting methane with water to obtain a mixture containing hydrogen and _CO2 .
• Gasification allows the production, through combustion, of a mixture of CO and _H2 from coal or biomass. The _CO2 emitted in the two previous cases could potentially be captured and stored or utilized.
• Hydrogen can also be produced from water and electricity through water electrolysis. This is the most promising method because it is a clean production method (no carbon emissions when coupled with a renewable energy source) and provides high-purity hydrogen, but it remains expensive (2 to 3 times more expensive than natural gas reforming).

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

The overall reaction is:

[Chem 1]

_H₂O → _H₂ + 1/2 _O₂

Catalysts are necessary for both reactions. Various metals have been studied as catalysts for the dihydrogen production reaction at the cathode, and these differ depending on the technology and therefore the physicochemical conditions under which the catalysts operate. Today, platinum is the most widely used metal in membrane electrolyzers, particularly in acidic environments, in proton exchange membrane (PEM) electrolyzers. It exhibits a negligible overpotential (the voltage required to produce one _H₂ molecule from two ^H⁺ ions) compared to other metals. However, the scarcity, cost (> €25,000/kg), and competing uses (fuel cells, catalysis, etc.) of this noble metal are hindering the long-term economic development of the hydrogen sector. This is why, for a number of years now, researchers have been turning towards new catalysts, without platinum, but based on inexpensive metals that are abundant in nature.

The production of hydrogen by water electrolysis is well described in the book: " Hydrogen Production: Electrolysis ", 2015, Ed Agata Godula Jopek. Water electrolysis is an electrolytic process that decomposes water into gaseous _O₂ and _H₂ using an electric current. The electrolytic cell consists of two electrodes—usually made of an inert metal (within the relevant potential and pH range) such as platinum—immersed in an electrolyte and connected to opposite poles of the direct current source.

The electric current dissociates the water molecule at the anode by following the oxidation reaction:

[Chem 2]

H ₂ O(l) → 1/2 O ₂ (g) + 2 H+ (aq)+ 2e ⁻

This is also referred to as the OER (Oxygen Evolution Reaction).

Next, the H ⁺ protons migrate towards the cathode through the membrane under the effect of the electric field and the concentration gradient where they are reduced to molecular hydrogen (with electrons coming from the negative pole of the generator), hence the name of this electrolyzer technology "proton exchange membrane".

Therefore, at the cathode, the protons H ⁺ accept electrons, forming hydrogen gas ( _H2 ), according to the reduction reaction:

[Chem 3]

2 hours⁺+ 2 e^-→ H₂

This is also referred to as the HER (Hydrogen Evolution Reaction).

The composition and use of catalysts for hydrogen production by water electrolysis are extensively covered in the literature. A review article summarizing the families of interesting materials under development over the past decade is: PCK Vesborg et al., " Recent Development in Hydrogen Evolution Reaction Catalysts and Their Practical Implementation ," 2015, in which the authors describe sulfides, carbides, and phosphides as potential new electrocatalysts. Among the sulfide phases, dichalcogenides such as molybdenum sulfide ( _MoS₂) are very promising materials for the hydrogen evolution reaction (HER) due to their high activity, excellent stability, and availability, as molybdenum and sulfur are abundant and inexpensive elements on Earth. They are particularly well described in the review article on _MoS2 for an application in HER: “Improving intrinsic electrocatalytic activity of layered transition metal chalcogenides as electrocatalysts for water splitting” AP Tiwari, K. Kim and S. Jeon, in Current Opinion in Electrochemistry, 2022, 34,100982.

_MoS₂ -based materials have a lamellar structure. The active phases can be used in bulk form when electron conduction from the cathode is sufficient, or in a supported state, which then involves a support of a different nature. In the latter case, the support must have specific properties:

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

- very good electronic conductivity;

- chemical and electrochemical stability under water electrolysis conditions (acidic medium and high potential).

Carbon is the most commonly used support in this application. The key lies in the preparation of this sulfide phase on the conductive material.

It is generally accepted that a catalyst with high catalytic potential is characterized by an associated active phase that is perfectly dispersed across the surface of the support and has a high active phase content. Ideally, the catalyst should also offer accessibility of its active sites to the reactants, in this case protons, while simultaneously developing a large active surface area. This can lead to specific constraints in terms of structure and texture, inherent to the support constituting these catalysts.

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

It appears interesting to find ways of preparing catalysts for the production of hydrogen by electrolysis of water, making it possible to obtain new catalysts with improved performance.

The improvement of the electrocatalytic performance of electrocatalysts composed of _MoS2 can be described by the two principles proposed by Li Xin Chen et al. “Design of Dual-Modified MoS2 _with Nanoporous Ni and Graphene as Efficient Catalysts for the Hydrogen Evolution Reaction” , 2018, namely the increase in the number of active sites or the intrinsic activity of the latter.

Regarding the increase in the number of active sites, we can cite nanoporous structures, heterostructures, and 3D, 2D, or 1D morphologies such as nanoflowers, nanosheets, or nanotubes (see article by D. Vory et al., " Conducting MoS2 Nanosheets as Catalysts for Hydrogen Evolution," 2013). The creation of new active sites through doping and/or the creation of sulfur vacancies, potentially filled by Mo atoms, particularly on the basal plane, as described in the articles by N. Abidi et al ., "Revisiting the Active Sites at the MoS2 _/ H2O _Interface via Grand-Canonical DFT: The Role of Water Dissociation," 2020 , and " How to dope the basal plane of 2H-MoS2 _to boost the hydrogen evolution reaction?" , 2023.

Regarding increasing the intrinsic activity of active sites, numerous strategies are also conceivable, including:

- The integration of conductive nanomaterials such as graphene or nanoporous metals, described in the article by J Joyner "Graphene Supported MoS ₂ Structures with High Defect Density for an Efficient HER Electrocatalysts" , 2020.

- Reinforcement of the intrinsic activity of the edges via doping with nanoporous nickel for example as described in the article by LX Chenet al., "Design of Dual-Modified MoS ₂ with Nanoporous Ni and Graphene as Efficient Catalysts for the Hydrogen Evolution Reaction" , 2018.

Fluoridation is a chemical process that dopes a material with fluorine atoms. Fluorine is a unique element in the periodic table because it has the highest electronegativity, a small size, and significant stability. However, fluoriation processes are still not widely used to modify layered structures such as _MoS₂ , particularly for applications in water electrolysis.

However, since the fluorine atom is very electronegative, it can induce a significant change in the electronic structure of _MoS2 . Moreover, since the substitution energy of sulfur by fluorine is negative, it is therefore easy to replace sulfur atoms with fluorine atoms within the _MoS2 structure (see article by R. Zhang, et al., "Creating Fluorine-Doped MoS2 _Edge Electrodes with Enhanced Hydrogen Evolution Activity" , 2021).

The reaction of fluorine atoms with the material can also lead to the creation of a new material containing fluorine (fluorides, oxyfluorides, etc.) and/or the conversion of the material into a new one with vacancies, as some atoms recombine with fluorine atoms to generate volatile compounds. In the case of _MoS₂ , fluorination can consume Mo and S atoms to form _MoF₆ (g) and _SF₆ (g), which can create porosities and thus increase its specific surface area. Fluorination is therefore both a doping method that allows the creation of new active sites on the material's edges and a means of modifying its morphology.

Several types of fluorination processes for fluorinating catalytic materials are found in the prior art. Copetti Gabreila and EH Nunes, " Tuning MoS2 _reactivity toward halogenation" , 2019, disclose a fluorination by exposure to a fluorinating agent of the _XeF2 type which releases fluorine by decomposition.

G. Venkateswarlu, et al. “ Electroanalytical characterization of F-doped MoS ₂ cathode material for rechargeable magnesium battery” , 2019, D. Gao, et al. “Tunable Ferromagnetic Ordering in MoS ₂ nanosheets with Fluorine Adsorption” 2015, and Y. Wang, et al . “Fluorine- and Nitrogen-Codoped MoS ₂ with a Catalytically Active Basal Plane” , 2017, disclose a hydrothermal fluorination consisting of dissolving MoS ₂ in a solution of aqueous fluorinated species such as HBF ₄ (aq) or NH ₄ F(aq) and then carrying out a heat treatment in a hydrothermal bomb.

Finally, S.-S. Chee et al. “Substitutional Fluorine Doping of Large-Area Molybdenum Disulfide Monolayer Films for Flexible Inverter Device Arrays” , 2020 and R. Zhang et al. “Creating Fluorine-Doped MoS2 _Edge Electrodes with Enhanced Hydrogen Evolution Activity” , 2021 disclose a fluorination process by plasma exposure in the presence of a fluorinated species such as _SF6 or _CHF3 .

The prior art cited above clearly demonstrates a beneficial effect of fluorination on the electrocatalytic performance of _MoS₂ in acidic HER. However, the prior art does not disclose a process for preparing _MoS₂ material by direct fluorination with gaseous difluorine _F₂ (also referred to here interchangeably as difluor, _F₂ , or _F₂ (g)). Yet, the use of difluorine offers high fluorination efficiency given the very high reactivity of _F₂ (g) (the dissociation energy of molecular fluorine is relatively low (153 ^kJ.mol⁻¹ ), which gives it exceptional reactivity), and the large quantities of potentially committable fluorine that will modify the morphology and structure of _MoS₂ .

Direct fluorination is defined as a controlled diffusion phenomenon where only the outermost surface of the material is affected. It allows for the modification of a material's surface properties without altering its intrinsic properties, provided the fluorination is perfectly controlled.

The thickness and chemical nature of the fluorinated layer formed are of paramount importance to the treatment's efficiency in relation to its properties. These characteristics depend on the nature of the material used, its chemical composition and crystallinity, as well as the synthesis conditions applied. However, limitations arise during fluorination under overly harsh conditions. A large excess of fluorine in the reactor can lead to degradation of the catalytic material. Therefore, controlling the operating conditions of the direct fluorination process with _F₂ (g) is essential.

Object of the invention

The Applicant has developed a new process for preparing a catalytic material that can be used in an electrolytic cell for electrochemical reduction reactions, and more specifically, for obtaining a cathode that can be used in an electrolytic cell for hydrogen production by acidic water electrolysis. Indeed, the Applicant has discovered that the direct fluorination with difluorine of a material based on at least one metal from group VIB sulfide yields catalytic performance that is at least as good, or even better, particularly when the material is used as the catalytic phase of an electrode for electrochemical reduction reactions, and even more so when the catalytic material is used as the catalytic phase of a cathode for hydrogen production by acidic water electrolysis.

This specific fluorination process for a material based on at least one metal from group VIB sulfide relies on direct fluorination using difluor gas under specific and controlled operating conditions.

The present invention relates first to a method for preparing a catalytic material for an electrode for electrochemical reduction reactions, said catalytic material comprising an active phase based on at least one metal from group VIB and fluorine, said method comprising at least the following steps:

a) a solid material is supplied based on at least one metal from group VIB sulfide;

b) a fluorination step is carried out on the solid material supplied in step a) in the presence of a gas comprising at least difluorine, at a temperature between -50°C and 150°C, for a duration between 15 seconds and 120 minutes, with a concentration of difluorine in the gas between 0.1 and 100% by volume relative to the total volume of said gas, a pressure between 0.001 and 0.2 MPa, and a PPH between 0.01 and 200 h ^-1 , to obtain said catalytic material.

Preferably the concentration of difluor in the gas used in step b) is between 1% and 50% by volume relative to the total volume of said fluorinating gas.

Advantageously, the duration of step b) is between 1 minute and 60 minutes.

Advantageously, the duration of step b) is between 1 minute and 20 minutes.

Preferably, the gas used in step b) comprises an inert gas selected from dinitrogen (N ₂ ), argon (Ar), helium (He).

Preferably, in step a) said metal of group VIB is chosen from molybdenum or tungsten.

Advantageously, which of the VIB group metals is molybdenum.

Preferably, the solid material based on at least one metal from group VIB sulfide supplied in step a) is in bulk form or supported on an electro-conductive support.

Advantageously, the solid material based on at least one metal from group VIB sulfide supplied in step a) is supported on an electro-conductive support.

Advantageously, the electro-conductive support is carbon.

Another object of the invention relates to a method for preparing an electrode comprising the following steps:

1) We prepare an ink by carrying out the following sub-steps:

1a) at least one ionically conductive polymer binder is dissolved in a solvent or a mixture of solvents;

1b) at least one catalytic material prepared according to the invention in powder form is added to the solution obtained in step 1a);

steps 1a), 1b) being carried out in any order, or simultaneously;

2) the ink obtained in step 1) is deposited onto a membrane, or an electronically conductive collector.

Preferably, an electro-conductive support is added to the ink in step 1).

Another object of 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 cathode is an electrode prepared according to the invention.

Another object of the invention relates to the use of the electrolysis device according to the invention in electrochemical reactions.

Preferably, said device is used as:

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

- nitrogen electrolysis device for the production of ammonia;

- carbon dioxide electrolysis device for the production of formic acid, CO and ethylene;

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

Detailed description of the invention Definitions

In what follows, the groups of chemical elements are given according to the CAS classification (CRC Handbook of Chemistry and Physics, publisher CRC Press, editor-in-chief DR Lide, ^81st 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.

BET surface refers to the specific surface area determined by nitrogen adsorption in accordance with ASTM D 3663-78, established from the BRUNAUER – EMMET – TELLER method described in the periodical "The Journal of the American Chemical Society", 60, 309 (1938).

Electrode

An electrode is the layer containing the catalytic material that is the site of electrochemical reactions.

Current collector

A current collector is a layer that allows the transport of electrons to or from the electrodes. It can be dense, like glassy carbon in a three-electrode setup, or porous, like the PTL (Porous Transport Layer) or the bipolar plate used for fluid transport in an electrolyzer.

Ink

An ink is a mixture of powdered active material (in this case, based on at least one metal from group VIB and fluorine), a conductive agent, a binding agent, and a solvent, which forms part of the electrode formulation. This "ink" is then deposited onto the current collector or the cathode side of the membrane and dried. After assembly, this process yields the electrode, which is positioned between the current collector and the membrane.

P.P.H.

By hourly mass velocity (P.P.H), we mean the mass flow rate of the gaseous feed into the reactor in kg/h divided by the mass of catalyst in kg contained in the reactor.

The metal content of group VIB is measured by X-ray fluorescence and is given as a percentage by weight relative to the total mass of the catalytic material.

The Fluorine/VIB group element atomic ratios are determined by Energy-dispersive X-ray Spectroscopy (EDX) and by Photoelectron Spectrometry (XPS).

In this application, the concentrations of difluor in the gas phase are given as a percentage (volume % of difluor relative to the total volume of gas).

Process for preparing the catalytic material

1. on approvisionne un matériau solide à base d’au moins un métal du groupe VIB sulfuré ;
2. on réalise une étape de fluoration du matériau solide approvisionné à l’étape a) en présence d’un gaz comprenant au moins du difluor, à une température comprise entre -50°C et 150°C, une durée comprise entre 15 secondes et 120 minutes, une concentration en difluor dans le gaz comprise entre 0,1 et 100% volume par rapport au volume total dudit gaz, une pression comprise 0,001 et 0,2 MPa, une P.P.H comprise entre 0,01 et 200 h^-1, pour obtenir ledit matériau catalytique.

The invention relates to a method for preparing a catalytic material for an electrode for electrochemical reduction reactions, said catalytic material comprising an active phase based on at least one metal from group VIB and fluorine, said method comprising at least the following steps:

1. we supply a solid material based on at least one metal from the VIB group sulfide;
2. a fluorination step is carried out on the solid material supplied in step a) in the presence of a gas comprising at least difluorine, at a temperature between -50°C and 150°C, for a duration between 15 seconds and 120 minutes, with a concentration of difluorine in the gas between 0.1 and 100% by volume relative to the total volume of said gas, a pressure between 0.001 and 0.2 MPa, and a PPH between 0.01 and 200 h ^-1 , to obtain said catalytic material.

Steps a) and b) are described in detail below.

Step a)

We supply a solid material based on at least one metal from group VIB sulfide. Preferably, the solid material is based on molybdenum and/or tungsten sulfide, and preferably, the solid material is based on molybdenum.

According to one or more embodiments, said solid material based on at least one metal from group VIB sulfide is doped with a doping element selected from boron, phosphorus and silicon, nickel, cobalt or iron, titanium, zirconium, or hafnium or added with organic compounds as described in patent application WO2020/109065.

The solid material based on at least one metal from group VIB sulfide can be used in bulk form when electron conduction from the cathode is sufficient, or in a supported state, which then involves a support of a different nature. Preferably, the solid material based on at least one metal from group VIB sulfide is supported on a substrate.

The said support comprises at least one electrically conductive material.

In a particular embodiment of the invention, said support comprises at least one electrically conductive material selected from carbon structures such as carbon black, graphite, carbon nanotubes or graphene.

In a particular embodiment of the invention, said support comprises at least one electroconductive material selected from gold, copper, silver, titanium, silicon, fluorine-doped tin dioxide, and indium tin oxide.

A porous and non-electrically conductive material can be made electrically conductive by depositing an electrically conductive material on its surface; for example, a refractory oxide, such as alumina, within which graphitic carbon is deposited.

The catalytic material support advantageously has a specific surface area BET (SS) greater than 75 ^m² /g, preferably greater than 100 ^m² /g, most preferably greater than 130 ^m² /g.

The material support can be added during the preparation of the catalytic material and/or during the preparation of the electrode.

The solid material based on at least one metal from group VIB sulfide may come from a commercial source or be prepared by processes known from the prior art.

When the solid material based on at least one metal from group VIB sulfide is supported on a support, the deposition of said solid material based on at least one metal from group VIB onto a support is carried out according to the following substeps:

a1) 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;

a2) optionally, a step of bringing the support into contact with at least one solution containing at least one precursor of a dopant;

steps a1) and a2), if both carried out, being carried out in any order or simultaneously and allow obtaining the impregnated solid material supported wet;

a3) a heat treatment step of the wet-supported impregnated solid material obtained at the end of steps a1) and optionally a2) to obtain a dried solid material,

a4) a step of sulfidation of the dried solid material obtained at the end of step a3) to obtain said solid material based on at least one supported sulfided metal from group VIB.

Advantageously, the step of contacting the substrate with at least one precursor of at least one metal from group VIB, in accordance with the implementation of step a1), can be carried out by impregnation, either dry or in excess, or by deposition-precipitation, according to methods well known to those skilled in the art. Preferably, said step a1) is carried out by dry impregnation, which consists of contacting the substrate with a solution containing at least one precursor of at least one metal from group VIB, the volume of which is between 0.25 and 1.5 times the porosity volume of the substrate to be impregnated.

According to one or more embodiments, said precursor of at least one metal from group VIB, added in step a1), is chosen from:

-polyoxometalates 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), said element being taken alone, M is one or more element(s) chosen from molybdenum (Mo), tungsten (W), 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 or 18, y being an integer between 17 and 72 and q being an integer between 1 and 20;

-precursor salts of elements in group VIB; and

-organic or inorganic precursors based on Mo or W.

The precursor of a dopant element used in the optional step a2) is chosen from boron, phosphorus and silicon, nickel, cobalt or iron, titanium, zirconium, or hafnium.

Advantageously, organic additives are introduced into the impregnation solution during step a1) or a2) as described in application WO2020/109065.

Heat treatment step a3) is carried out at a temperature below 250°C, preferably below 180°C, and more preferably below 120°C. Most preferably, drying is carried out under 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 stage 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 a4) of sulfidation is intended to sulfide at least partially the metal of group VIB. It can advantageously be carried out using a gaseous mixture _H2S / _H2 or _H2S / _N2 containing at least 5% vol of _H2S in the mixture or under a flow of pure _H2S 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.

The solid material based on at least one metal from group VIB sulfide can also be prepared, as described in patent FR3089134.

Step b)

Step b) of fluorinating the solid material supplied in step a) consists of bringing said solid material into contact with a gas comprising at least difluorine, at a temperature between -50°C and 150°C, for a duration between 15 seconds and 120 minutes, a concentration of difluorine in the gas between 0.1% and 100% volume relative to the total volume of said gas, a pressure between 0.001 MPa and 0.2 MPa, a VVH between 0.01 and 200 h ^-1 , to obtain said catalytic material.

According to one or more embodiments, the fluoridation temperature is between 0°C and 40°C, and preferably between 0°C and 25°C.

According to one or more embodiments, the fluoridation time is between 1 minute and 60 minutes, and preferably between 1 minute and 20 minutes.

According to one or more embodiments, the concentration of difluor in the gas is between 1% and 50%, preferably between 1% and 10%.

According to one or more embodiments, difluorine is diluted in an inert gas chosen from dinitrogen, helium, or argon.

In one or more embodiments, the contact between the solid material and the gas containing at least difluorine is carried out according to methods known to those skilled in the art. These methods include, but are not limited to, flow-through-bed reactors with upward or downward flow, licked-bed reactors, and fluidized-bed reactors. Those skilled in the art shall ensure that the atmosphere within the reactor is purged with an inert gas, either before the reaction to remove traces of residual moisture and oxygen, or afterward to remove unreacted molecular fluorine molecules. The purging step prior to each fluorination is important as it eliminates all potential traces of moisture. Indeed, molecular fluorine (F₂ ₎ readily combines with residual moisture within the reactor to form hydrofluoric acid (HF). Controlling the fluorination conditions is crucial to avoid randomly increasing the reaction kinetics of the sample and to prevent inhomogeneity on the surface of the treated materials when the fluorinated gas mixture is introduced into the reactor. Indeed, the hydrofluoric acid generated can catalyze the reaction and induce inhomogeneities on the surface of the treated materials.

Catalytic material

The material obtained has F/Mo atomic ratios between 0.01 and 200. Preferably, the F/Mo ratio is between 0.05 and 1 and very preferably between 0.1 and 1.

In a particular embodiment of the invention, the catalytic material is in bulk. In this embodiment, the content of metal from group VIB in the catalytic material obtained after fluoridation step b) is 100% by weight of the metallic element relative to the weight of the catalytic material.

In another particular embodiment of the invention, the catalytic material is supported on an electrically conductive support. In this embodiment, the content of group VIB metals in the catalytic material obtained after fluorination step b) is between 4 and 60% by weight of the metal element relative to the weight of the final catalytic material, and preferably between 7 and 50% by weight relative to the weight of the catalytic material. Furthermore, the surface density, which corresponds to the quantity of group VIB metal atoms deposited per unit area of support, is advantageously between 0.5 and 20 atoms of the metal element per square nanometer of support, and preferably between 2 and 15 atoms of the metal element per square nanometer of support.

The catalytic material that can be obtained by the preparation process according to the invention can be used as a catalytic electrode material suitable for use in electrochemical reactions, and in particular for the electrolysis of water in a membrane electrolyzer (PEM "proton exchange membrane" or AEM "anion exchange membrane").

Advantageously, the electrode comprises a catalytic material obtained by the preparation process according to the invention, an electro-conductive support, and a binder. This mixture is obtained after deposition of the "ink" and drying.

The electrode preparation process is described below.

Electrode preparation process

The electrode can be prepared using techniques well known to those skilled in the art. More specifically, the electrode is formulated by a preparation process comprising the following steps:

1) We prepare an ink by carrying out the following sub-steps;

1a) at least one ionically conductive polymer binder is dissolved in a solvent or a mixture of solvents;

1b) at least one catalytic material prepared according to the invention, in powder form, is added to the solution obtained in step 1a);

steps 1a), 1b) being carried out in any order, or simultaneously;

2) the ink obtained in step 1) is deposited onto a membrane, or an electronically conductive collector.

The electrode obtained following steps 1) and 2) is then dried either in open air or under a controlled atmosphere (typically under nitrogen or argon) at a temperature between 5°C and 100°C, preferably between 15°C and 60°C and at a pressure between 0.0001MPa and 0.1MPa.

Step 1) Ink preparation

The binder is preferably a polymer binder chosen for its ability to be deposited as a layer of variable thickness and for its ionic conductivity in aqueous media and its diffusion capacity for dissolved gases. The layer of variable thickness, advantageously between 1 and 500 µm, particularly on the order of 10 to 100 µm, can be, in particular, a gel or a film.

Advantageously, the ionically conductive polymer binder is:

*either a conductor of anionic groups, particularly hydroxyl groups, and is chosen from the group including, in particular:

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

- non-grafted polybenzimidazole;

- chitosan; and

- polymer mixtures comprising at least one of the different polymers mentioned above, said mixture having anionic conductive properties;

* either a conductor of cationic groups enabling the conduction of protons and is chosen from the group including in particular:

- polymers stable in aqueous media, which may be perfluorinated, partially fluorinated or non-fluorinated and which have anionic groups allowing the conduction of protons;

- grafted polybenzimidazole;

- chitosan; and

- polymer mixtures comprising at least one of the different polymers mentioned above, said mixture having cationic conductivity properties.

Among the polymers that are stable in aqueous media and have cationic groups that allow the conduction of anions, we can notably mention perfluorinated polymer chains such as polytetrafluoroethylene (PTFE), partially fluorinated polymer chains such as polyvinylidene fluoride (PVDF) or non-fluorinated polymer chains such as polyethylene, which will be grafted with anionically conductive molecular groups.

Among the polymers stable in aqueous media and possessing anionic groups allowing proton conduction, one can consider any polymer chain stable in aqueous media containing groups such as _-SO3- ^{, -COO-} , _-PO32- , ^-PO3H- _, ^{-C6H4O- .} Notable examples include Nafion®, sulfonated phosphon polybenzimidazole (PBI), _and sulfonated or _phosphon polyetheretherketone (PEEK).

According to the present invention, any mixture comprising at least two polymers, at least one of which is selected from the groups of polymers mentioned above, may be used, provided that the final mixture is ionically conductive in aqueous media. For example, a mixture comprising a polymer stable in alkaline media and having cationic groups enabling the conduction of hydroxide anions with polyethylene not grafted by anionically conductive molecular groups may be used, provided that this final mixture is anionically conductive in alkaline media. Another example is a mixture of a polymer stable in acidic or alkaline media and having anionic or cationic groups enabling the conduction of protons or hydroxides with polybenzimidazole, whether grafted or not.

Advantageously, polybenzimidazole (PBI) is used in the present invention as a binder. While not inherently a good ionic conductor, in alkaline or acidic media it proves to be an excellent polyelectrolyte with very good anionic and cationic conductivity, respectively. PBI is a polymer commonly used, in grafted form, in the manufacture of proton-conducting membranes for fuel cells, in membrane-electrode assemblies, and in PEM-type electrolyzers, as an alternative to Nafion®. In these applications, PBI is generally functionalized/grafted, for example by sulfonation, to make it proton-conducting. The role of PBI in this type of system is therefore different from its role in the manufacture of electrodes according to the present invention, where it serves only as a binder and has no direct role in the electrochemical reaction.

Although its long-term stability in concentrated acidic media is limited, chitosan, which can also be used as an anionic or cationic conducting polymer, is a polysaccharide with ionic conduction properties in basic media that 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 solvent removal step, either concurrently with or following the drying step. Solvent removal can be achieved by any technique known to those skilled in the art, in particular by evaporation or phase inversion.

In the case of evaporation, the solvent is an organic or inorganic solvent whose evaporation temperature is lower than the decomposition temperature of the polymer binder used. Examples include dimethyl sulfoxide (DMSO) and acetic acid. A person skilled in the art is capable of selecting the appropriate organic or inorganic solvent for the polymer or polymer blend used as a binder and capable of evaporating.

According to a preferred embodiment of the invention, the electrode is suitable for use for the electrolysis of water in an AEM electrolyzer and the polymer binder is then an anionic conductor in an alkaline electrolyte medium, in particular a conductor of hydroxides.

For the purposes of the present invention, an alkaline electrolyte medium is understood to be a medium whose pH is greater than 7, advantageously greater than 10.

The binder is advantageously conductive of hydroxides in alkaline media. 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, the sites of the redox reactions that produce _H2 and _O2 gases. Thus, a surface that would not be in direct contact with the electrolyte is still involved in the electrolysis reaction, a key factor in the system's efficiency. The chosen binder and the electrode's shape do not impede the diffusion of the gases formed and limit their adsorption, thereby allowing their release. According to another preferred embodiment of the invention, the electrode is suitable for use in the electrolysis of water in a PEM electrolyzer, and the polymer binder is a cationic conductor in acidic electrolyte media, particularly a proton conductor.

For the purposes of the present invention, an acidic medium is understood to be a medium whose pH is less than 7, advantageously less than 2.

A person skilled in the art, drawing on their general knowledge, will be able to determine the quantities of each component of the electrode. The density of catalytic material particles must be sufficient to reach their electrical percolation threshold.

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

For the purposes of this invention, catalytic material powder means a powder consisting of particles of micron, sub-micron, or nanometer size. The powders can be prepared using techniques known to those skilled in the art.

In a particular embodiment of the invention, an electroconductive support is added to the ink in step 1). For the purposes of this invention, an electroconductive support is understood to be a conductive agent that ensures electronic contact between the particles of the catalytic material and the current collector and/or the membrane. Most often, this consists of nanoparticles with a very large surface area. The electroconductive support comprises at least one material selected from carbon-based structures such as carbon black, graphite, carbon nanotubes, or graphene. A porous, non-electroconductive material can be made electroconductive by depositing an electroconductive material on its surface; for example, a refractory oxide, such as alumina, within which graphitic carbon is deposited.

In a particular embodiment of the invention, the support for the catalytic material comprises at least one material selected from gold, copper, silver, titanium, silicon.

The catalytic material support advantageously has a specific surface area BET(SS) greater than 75 ^m² /g, preferably greater than 100 ^m² /g, and most preferably greater than 130 ^m² /g. The electroconductive support can be added during the preparation of the catalytic material and/or during the preparation of the electrode.

Step 2) Deposition of ink onto a membrane, or an electronically conductive collector.

In one embodiment, the ink obtained in step 1) is deposited on a proton exchange or anion exchange membrane.

For the purposes of this invention, a proton exchange membrane is defined as a semi-permeable, extremely thin membrane (20-300 micrometers) made from ionomers that allows proton conduction while being impermeable to gases such as oxygen or hydrogen: protons pass through while gases are blocked. This characteristic is exploited in the Membrane-Electrode Assemblies (MEAs) of PEM electrolyzers. PEMs are made from pure polymer membranes or composite membranes where the materials form a polymer matrix. They are almost exclusively composed of acid perfluorosulfonated membranes (PFSAs), the best-known of which is Nafion® from DuPont de Nemours. Nafion® is an ionomer composed of a hydrophobic poly(tetrafluoroethylene) (PTFE) backbone onto which are grafted perfluoroethylene (perfluorovinyl ether) pendant chains terminated by sulfonic groups.

An anion exchange membrane is defined as a semi-permeable, extremely thin membrane (between 20 and 300 micrometers) made from ionomers that allows the conduction of hydroxyl (or hydroxide) anions while remaining impermeable to gases such as oxygen or hydrogen: ^OH- ions pass through the membrane while gases are blocked. This characteristic is exploited in the Membrane-Electrode Assemblies (MEAs) of electrolyzers.

In another embodiment, the ink obtained in step 1) is deposited onto a current collector. For the purposes of this invention, a metallic-type support or collector is understood to mean any conductive material having the same conductive properties as metals, for example, graphite or certain conductive polymers such as polyaniline and polythiophene. This support can have any shape that allows the resulting mixture (between the binder and the catalytic material) to be deposited by a method selected from the group including, in particular, dipping, printing, induction, pressing, coating, spin-coating , filtration, vacuum deposition, spray deposition, casting, extrusion, or lamination. The support or collector can be solid or perforated. Examples of supports include a grid (openwork support), a plate or sheet of stainless steel (304L or 316L for example) (solid supports).

The advantage of the ink according to the invention is that it can be deposited on a solid or perforated collector, by usual deposition techniques easily accessible and allowing deposition in the form of layers of variable thicknesses ideally on the order of 10 to 100 µm.

According to the invention, the mixture can be prepared by any technique known to those skilled in the art, in particular by mixing the binder and at least one catalytic material in powder form with a suitable solvent or a mixture of suitable solvents to obtain a mixture with rheological properties that allow the electrode materials to be deposited as a film of controlled thickness onto an electronically conductive substrate. The use of the catalytic material in powder form maximizes the surface area of the electrodes and enhances their associated performance. Those skilled in the art will be able to select the various formulation parameters based on their general knowledge and the physicochemical characteristics of the mixtures.

Usage methods

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. The device is 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 (an ionically conductive medium). The anode is the site of water oxidation. The cathode is the site of hydrogen formation.

The electrolyte is a polymer membrane that exchanges protons H ⁺ or anions ^OH- which ensures the transfer of protons H ⁺ or anions ^OH- from the anode to the cathode or from the cathode to the anode respectively and allows the separation of the anodic and cathodic compartments, which avoids reoxidizing at the anode the species reduced at the cathode and vice versa;

The minimum water supply for an electrolysis unit is 0.8 L/ ^Nm³ of hydrogen. In practice, the actual value is closer to 1 L/ ^Nm³ . The water used must be as pure as possible because impurities remain in the equipment and accumulate during electrolysis, ultimately disrupting the electrolytic reactions by:

- the formation of sludge; 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 μS/cm).

There are many suppliers offering highly diversified technologies, particularly in terms of the nature of the electrolyte and associated technology, ranging from possible upstream coupling with a renewable power supply (photovoltaic or wind), to the direct final supply of pressurized hydrogen.

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 unit consists of two electrodes (an anode and a cathode, both electronic conductors) connected to a direct current generator and separated by an electrolyte (an ionically conductive medium). The anode is where water is oxidized. The cathode is where nitrogen is reduced and ammonia is formed. Nitrogen is continuously injected into the cathode compartment.

The nitrogen reduction reaction is:

[Chem 4]

_N₂ + 6H⁺ ^{+ 6e⁻} → _2NH₃

The electrolyte can be:

- either an _aqueous solution ( _Na2SO4 or HCl), preferably saturated with nitrogen;

- either a proton exchange polymer membrane which ensures the transfer of protons from the anode to the cathode and allows the separation of the anodic and cathodic compartments, which avoids re-oxidizing at the anode the species reduced 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 the anode is an electrode according to the invention. An example of an anode and electrolyte that can be used in such a device is described in detail in 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 anode is an electrode according to the invention.

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

The electrolyte can be:

- either an _acidic aqueous solution ( _H2SO4 or HCl) or a basic one (KOH);

- either a polymer membrane exchanging protons or OH- anions which ensures the transfer of protons or ^OH- anions from the anode to the cathode or from the cathode to the anode respectively and allows the separation of the anodic and cathodic compartments, which avoids re-oxidizing at the anode the species reduced at the cathode and vice versa.

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: Supply of a solid material S1 based on molybdenum sulfide in bulk form: Commercial bulk _MoS2

A first solid material S1 based on commercial molybdenum sulfide (supplier Sigma Aldrich; CAS number: 1317-33-5), in which the particles have an average size of 90 nm, is supplied in bulk form. The Mo content of the solid sulfide material is 100% by weight relative to the total weight of said solid material.

Example 2: Supply of a solid S2 material based on molybdenum in sulfide and supported form: preparation of the _MoS2 material supported on carbon black

A second solid material S2, based on molybdenum sulfide supported on commercial Ketjenblack ec-660jd carbon black (supplier Nouryon; CAS number: 1333-86-4) with a specific surface area of 1380 ^m² /g _, is prepared. To do this, 10.39 g of _{H₃PMo₂O₄O · 28H₂O} are dissolved in 18.5 mL of water. The resulting solution is used to impregnate 3 g of carbon black. The resulting wet material is then dried in an oven at 120°C for 2 h. The dry material is then sulfided under pure _H₂S at 300°C for 2 h. The Mo content of the sulfided solid material is 44 wt% relative to the total weight of the solid material. The surface density of molybdenum is 8 molybdenum atoms per ^nm² of support.

Example 3: Preparation of a catalytic material C1: fluorination of S1 (according to the invention)

The solid material S1 is brought into contact with a gas comprising at least difluorine at a temperature of 20°C, for a duration of 20 minutes, a difluorine concentration in the gas of 20% volume relative to the total volume of said gas, a pressure of 0.1 MPa, a PPH of 0.2 h ^-1 , to obtain a catalytic material C1. The dilution gas is dinitrogen.

Example 4: Preparation of a catalytic material C2: fluorination of S2 (according to the invention)

The solid material S2 is brought into contact with a gas containing at least difluorine at a temperature of 20°C, for a duration of 20 minutes, with a difluorine concentration in the gas of 20% by volume relative to the total volume of said gas, a pressure of 0.1 MPa, and a PPH of 0.2 ^h⁻¹ , to obtain a catalytic material C2. The dilution gas is dinitrogen.

Example 5: Preparation of a working electrode E1 based on a catalytic material C1 (compliant)

To deposit the catalytic material C1 onto an electrode, an ink is formulated. For this purpose, 50 mg of catalytic material C1, to which 5 mg of acetylene black have been added to ensure electronic conductivity and 5 mg of polyvinylene fluoride (PVDF) to ensure mechanical adhesion of the ink deposit to the working electrode surface, are dispersed in 10 mL of a 4:1 water/ethanol mixture and then heated ultrasonically for 30 minutes to form the ink. A 10 µL deposit of the ink is made on the glassy carbon tip of the rotating working electrode. After the ink has been deposited on the working electrode, the electrode is dried to evaporate the solvent at 20°C in open air to obtain the working electrode E1.

The working electrode consists of a 5 mm diameter disc of vitreous carbon set in a Teflon tip (rotating disc electrode).

Example 6: Preparation of a working electrode E2 based on a catalytic material C2 (compliant)

To deposit the C2 catalytic material onto an electrode, an ink is formulated. For this purpose, 19 mg of C2 catalytic material were added to 80 µL of Nafion® (a 20:80 _H₂O /Propanol solution with 5 wt% Nafion®), which ensures the mechanical adhesion of the deposit to the electrode surface. The mixture was dispersed in 523 µL of isopropanol and 2.8 mL of ultrapure water. Two mg of commercial Ketjenblack carbon (supplier Nouryon; CAS number: 1333-86-4) were added to the mixture to ensure electronic conductivity. The mixture was then ultrasonically heated for 30 minutes. A 10 µL deposit of ink was applied to the vitreous carbon tip of the rotating working electrode. After the ink has been deposited on the working electrode, the electrode is dried to evaporate the solvent at a temperature of 20°C and in open air to obtain the working electrode E2.

The working electrode consists of a 5 mm diameter disc of vitreous carbon set in a Teflon tip (rotating disc electrode).

Example 7: Electrochemical tests of working electrodes

The catalytic activity of catalytic materials is characterized in a 3-electrode cell. This cell consists of a working electrode, a platinum counter electrode, and an Ag/ _AgCl reference electrode. The electrolyte is a 0.5 mol/L aqueous solution of sulfuric acid ( _H₂SO₄ ). This medium is deoxygenated by bubbling with nitrogen, and measurements are performed under an inert atmosphere (nitrogen deaeration).

Vitreous carbon has the advantage of having no catalytic activity and being a very good electrical conductor.

The performance of the working electrodes E1, E2 prepared in examples 5 and 6 is tested.

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

- Linear voltammetry: This involves applying a time-varying potential signal to the working electrode, from 0 to -0.5 V vs. ENH (Normal Hydrogen Electrode) at a rate of 2 mV/s, and measuring the faradaic response current, i.e., the current due to the redox reaction occurring at the working electrode. This method is ideal for determining the catalytic power of a material for a given reaction. Among other things, it allows for the determination of the overpotential required for the reduction of protons to _H₂ .

It also allows us to determine the value of the Tafel slope, which provides information on the kinetic aspect of the electrochemical reaction. The Tafel equation is an electrochemical kinetic equation that relates the overpotential to the rate of the electrochemical reaction, itself represented by the current density:

The sign is positive for an anodic reaction and negative for a cathodic reaction.

With :

A: the slope of Tafel in volts per decade (V/dec)

: the overvoltage in Volts (V)

i: current density in Amperes (A)

i₀: the exchange current density in Amperes per cm²of electrode. The term i₀translated the rate of electron transfer between the electrode and the electrolyte. For example, in the case of HER at 1 mol/kg H₂₂SO₄, i₀is equal to - log₁₀3.1. For tungsten, i₀is - log₁₀5.9 for tungsten. For palladium, which has the highest velocity, i₀is equal to - log₁₀3.03.

Note that the lower the absolute value of the Tafel slope, the faster the reaction kinetics. Recall that in the case of platinum, the overpotential is -10 mA.cm^-2is between -15 and -40 mV depending on the type of electrocatalyst used (depending on the Pt/C ratio) and the Tafel slope is approximately 23 mV/dec vs 95 mV/dec for a MoS₂massive.

- Chronopotentiometry: This involves applying a current or current density for a specific time and measuring the resulting potential. This study allows us to determine the catalytic activity at constant current as well as the stability of the system over time. It is performed with a current density of -10 mA/ ^cm² and for a given duration.

The electrochemical performances of the working electrodes E1, E2 are summarized in Table 1 below. They are expressed as overpotential at a current density of -10 mA/ ^cm² and as a Tafel slope (mV/dec).

These results are also compared to performance results of catalytic materials disclosed in the prior art:

- Massive _MoS2 (non-fluorinated) disclosed by LX Chen, et al. “Design of Dual-Modified _MoS2 with Nanoporous Ni and Graphene as Efficient Catalysts for the Hydrogen Evolution Reaction” 2018.

- Hydrothermally fluorinated _MoS2 which consists of dissolving _MoS2 in a solution of aqueous fluorinated species _NH4F (aq) and then carrying out a heat treatment with a hydrothermal bomb disclosed by Y. Wang, et al . “Fluorine- and Nitrogen-Codoped MoS2 _with a Catalytically Active Basal Plane” , 2017.

-Fluorine-Doped _MoS2 by exposure under plasma in the presence of _CHF3 disclosed by R. Zhang, et al. “ Creating Fluorine-Doped MoS2 _Edge Electrodes with Enhanced Hydrogen Evolution Activity ”, 2021.

Catalytic Materials Overvoltage at -10 mA/ ^cm² [(mV) vs ENH] Tafel slope (mV/dec) E1 (in accordance with the invention) 90 50 E2 (in accordance with the invention) 85 50 massive _MoS2 (non-fluorinated)
(LX Chen, et al., 2018) 282 151 _MoS2 treated with _NH4F (aq)
(Y. Wang, et al, 2017) 110 57 _MoS2 treated with CHF ₃ plasma
(R. Zhang, et al. 2021) 160 136

With an overvoltage of only 90 mV vs ENH, and 85 mV vs ENH, the electrodes E1 and E2 prepared from bulk _MoS2 or from supported and fluorinated _MoS2 under _F2 (g) under controlled conditions according to the invention exhibit better performance compared to non-fluorinated materials or materials fluorinated by other prior art methods.

This result demonstrates the undeniable interest of fluorination by _F2 (gas) under controlled conditions for the development of the hydrogen sector by water electrolysis.

Claims (15)

A process for preparing a catalytic material for an electrode for electrochemical reduction reactions, said catalytic material comprising an active phase based on at least one metal from group VIB and fluorine, said process comprising at least the following steps:
a) a solid material is supplied based on at least one metal from group VIB sulfide;
b) a fluorination step is carried out on the solid material supplied in step a) in the presence of a gas comprising at least difluorine, at a temperature between -50°C and 150°C, for a duration between 15 seconds and 120 minutes, with a concentration of difluorine in the gas between 0.1 and 100% by volume relative to the total volume of said gas, a pressure between 0.001 and 0.2 MPa, and a PPH between 0.01 and 200 h ^-1 , to obtain said catalytic material. A process according to claim 1, wherein the concentration of difluor in the gas used in step b) is between 1% and 50% by volume relative to the total volume of said fluorinating gas. A method according to claim 1 or 2, wherein the duration of step b) is between 1 minute and 60 minutes. A method according to any one of claims 1 to 3, wherein the duration of step b) is between 1 minute and 20 minutes. A process according to any one of the preceding claims, wherein the gas used in step b) comprises an inert gas selected from dinitrogen ( _N2 ), argon (Ar), helium (He). A method according to any one of the preceding claims, wherein in step a) said metal of group VIB is selected from molybdenum or tungsten. A method according to the preceding claim, wherein said metal of group VIB is molybdenum. A process according to any one of the preceding claims, wherein the solid material based on at least one metal from group VIB sulfide supplied in step a) is in bulk form or supported on an electro-conductive support. A method according to the preceding claim, wherein the solid material based on at least one metal from group VIB sulfide supplied in step a) is supported on an electro-conductive support. Method according to claim 9, wherein the electro-conductive support is carbon. A process for preparing an electrode comprising the following steps:
1) We prepare an ink by carrying out the following sub-steps:
1a) at least one ionically conductive polymer binder is dissolved in a solvent or a mixture of solvents;
1b) at least one catalytic material prepared according to any one of claims 1 to 10, in powder form, is added to the solution obtained in step 1a);
steps 1a), 1b) being carried out in any order, or simultaneously;
2) the ink obtained in step 1) is deposited onto a membrane, or an electronically conductive collector. A method according to claim 11, wherein in step 1) an electro-conductive support is added to the ink. Electrolysis device comprising an anode, a cathode, an electrolyte, said device being characterized in that at least one of the anode or cathode is an electrode prepared according to one of claims 11 or 12. Use of the electrolysis device according to claim 13 in electrochemical reactions. Use according to claim 14, wherein said device is used as:
- water electrolysis device for the production of a gaseous mixture of hydrogen and oxygen and/or the production of hydrogen alone;
- nitrogen electrolysis device for the production of ammonia;
- carbon dioxide electrolysis device for the production of formic acid, CO and ethylene;
- fuel cell device for the production of electricity from hydrogen and oxygen.