WO2020109064A1

WO2020109064A1 - Method for preparing a catalytic material made from w or mo mononuclear precursor for electrochemical reduction reactions

Info

Publication number: WO2020109064A1
Application number: PCT/EP2019/081707
Authority: WO
Inventors: Audrey BONDUELLE-SKRZYPCZAK; Sofiane BELAID
Original assignee: IFP Energies Nouvelles
Priority date: 2018-11-30
Filing date: 2019-11-19
Publication date: 2020-06-04
Also published as: FR3089135A1

Abstract

Disclosed is a method for preparing a catalytic material of an electrode for electrochemical reduction reactions, the material comprising an active phase made from tungsten (W) and/or molybdenum (Mo), and an electrically conductive substrate, the method comprising: a) a step of impregnation by bringing the electrically conductive substrate into contact with a solution comprising an organic solvent A and a mononuclear precursor made from W or Mo, in its monomeric or dimeric form, having at least one X=O or X-OR bond or at least one X=S or X-SR bond (in which X = W or X = Mo); b) a step of drying the impregnated substrate at a temperature less than or equal to 200°C; c) a step of sulphiding in a mixture of H₂S/H₂ or H₂S/N₂ containing at least 5% by volume H₂S in the mixture or in pure H₂S at a temperature greater than or equal to ambient temperature.

Description

PROCESS FOR THE PREPARATION OF A CATALYTIC MATERIAL BASED ON A MONONUCLEAR PRECURSOR OF TYPE W OR MO FOR REACTIONS OF

ELECTROCHEMICAL REDUCTION

Field of the invention

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. The present invention describes a process for preparing a catalytic material for an electrode based on tungsten (W) and / or molybdenum (Mo) which is particularly effective in the reaction of evolution of hydrogen involved in the electrolysis process some water.

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:

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 generating sulphurization step 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's research work therefore led her to prepare catalytic materials for electrodes capable of being used in electrochemical reduction reactions from tungsten or molybdenum, and optionally from at least one element of group VIII, in particular nickel, by modifying the chemical and structural composition of the metallic species, precursors of the active phases, in order to modify the interactions between the electrically conductive support and these precursors and in order to better sulfurize tungsten or molybdenum, but also in order to modify the interactions between the support and the active sulphide phase of the catalyst to better disperse it. In particular, the work of the Applicant has led it to use mononuclear precursors based on molybdenum and / or tungsten, in their monomeric or dimeric form, having at least one X = 0 or X-OR bond or at least one link X = S or X-SR (with X = W or X = Mo) where [R = C _x H _y where x> 1 and (x-1) <y <(2x + 1) or R = Si (OR ') ₃ or R = If (R') ₃ where R '= C _x H _y · where x'> 1 and (x'-1) <y '<(2x' + 1)], as particular precursors of active phase of catalytic materials, especially for the production of hydrogen by electrolysis of water. Indeed, the Applicant has demonstrated that a catalytic (supported) material prepared from at least one mononuclear precursor based on W or Mo, in their monomeric or dimeric form, and having at least one bond X = 0 or W-OR or at least one link X = S or X-SR (with X = W or X = Mo) where [R = C _x H _y where x ³ 1 and (x-1) <y <(2x +1) or R = If (OR ') ₃ or R = If (R') ₃ where R '= C _x H _y · where x'> 1 and (x'-1) <y '<(2x' + 1)], has improved sulfurization and catalytic activity at least as good, or even improved, compared to catalytic materials prepared from standard precursors, such as polyoxometallates.

Objects of the invention

The present invention firstly 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 tungsten (W) and / or molybdenum (Mo), and an electrically conductive support, said method comprising at least the following steps:

a) an impregnation step by bringing said electroconductive support into contact with at least one solution comprising an organic solvent A and at least one mononuclear precursor based on W or Mo, in its monomeric or dimeric form, having at least an X = 0 or X-OR bond or at least one X = S or X-SR bond (with X = W or X = Mo) where [R = CxHy where x> 1 and (x-1) <y <( 2x + 1) or R = Si (OR ') 3 or R = Si (R') 3 where R '= Cx'Hy' where x '> 1 and (x'-1) <y' <(2x '+ 1)];

b) a step of drying said impregnated support at a temperature less than or equal to 200 ° C;

c) a sulfurization step under an H ₂ S / H ₂ or H ₂ S / N ₂ mixture containing at least 5% by volume H ₂ S in the mixture or under pure H ₂ S at a temperature equal to or greater than room temperature .

Preferably, the catalytic material also comprises at least one active phase based on group VIII metal chosen from cobalt, iron or nickel.

Preferably, the group VIII metal is nickel.

Preferably, the tungsten or molybdenum precursor is a mononuclear precursor based on W or Mo, used in its monomeric or dimeric form, of formula X (= 0) _n (= S) _{n '} (0R) _a (SR ') _b (L1) _c (L2) _d (L3) _e (L4) _f (L5) _g (with X = W or X = Mo) where R = C _x H _y where x> 1 and (x-1) <y <(2x + 1) or R = Si (OR ") ₃ or R = Si (R") ₃ where R "= C _x H _y ·· where [x "> 1 and (x" -1) <y "<(2x"+1)];

where 0 <n + n '<2 and 0 <n <2 and 0 <n' <2;

where, if n = n '= 0, then (a¹0 or b¹0) and [(a + b + c + d + e + f + g = 6 and 0 £ a <6, 0 <b <6, 0 <c <6 , 0 <d <6, 0 <e <6, 0 £ f £ 6, 0 <g <6, or (a + b + c + d + e + f + g = 5 and 0 £ a <5, 0 <b <5, 0 <c <5, 0 <d <5, 0 <e <5, 0 <f <5, 0 <g <5), or (a + b + c + d + e + f + g = 4 and 0 £ a <4, 0 <b <4, 0 <c <4, 0 <d <4, 0 <e <4, 0 <f <4, 0 £ g £ 4)];

where, if [(n = 1 and n '= 0) or (h' = 1 and n = 0)], then [(a + b + c + d + e + f + g = 4 and 0 £ a <4, 0 <b <4, 0 <c <4, 0 <d <4, 0 <e <4, 0 <f <4, 0 <g £ 4)] or [(a + b + c + d + e + f + g = 3 and 0 £ a <3, 0 <b <3, 0 <c <3, 0 <d <3, 0 <e <3, 0 <f <3, 0 £ g £ 3)];

where, if [n + n '= 2 and 0 <n <2 and 0 <n' <2], then (a + b + c + d + e + f + g = 2 and 0 £ a <2, 0 <b <2, 0 <c <2, 0 <d <2, 0 <e <2, 0 <f <2, 0 <g <2);

with (L1), (L2), (L3), (L4) and (L5) _, ligands of the THF type, dimethyl ether, dimethylsulfide, P (CH ₃ ) ₃ , allyl, aryl, halogenated, amine, acetate, acetylacetonate , halide, hydroxide, -SH.

Preferably, said precursor is chosen from X (OEt) ₅ , X (OEt) ₆ , X (= 0) (OEt) ₄ , X (= S) (OEt) ₄ , X (= S) (SEt) ₄ , X (= 0) ₂ (0Et) ₂ , X (OC ₆ H ₅ ) ₆ , X (SEt) ₅ , X (SEt) ₆ , X (OEt) ₃ (SEt) ₂ , X (OEt) ₄ (SEt) , X (= 0) (OEt) ₃ (acac) with X = W or X = Mo, with Et = CH ₂ CH ₃ and with acac = (CH ₃ COCHCOCH ₃ ) IN their monomeric or dimeric form.

In an embodiment according to the invention, the sulfurization temperature in step c) is between 50 ° C and 200 ° C.

In an embodiment according to the invention, the sulfurization temperature in step c) is between 300 ° C and 600 ° C.

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. Advantageously, said active phase also comprises at least one metal from group VIII chosen from Ni, Co and Fe. Preferably, said metal from group VIII is introduced in solution either:

i) in the impregnation step a) in co-impregnation with the precursor of the mononuclear active phase based on tungsten or molybdenum;

ii) after the drying step b), in a so-called post-impregnation step a2) using a solution using an organic solvent B;

iii) after step c), in a post-impregnation step a3) using an aqueous solution or an organic solution.

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 one in the month of the anode or of the cathode is an electrode according to the invention .

Another object according to the invention relates to the use of the electrolysis device in electrochemical reactions, and more particularly as according to the invention 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, the groups of chemical elements are given according to the CAS classification (CRC Handbook of Chemistry and Physics, CRC press editor, editor-in-chief D.R. 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.

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).

Preparation process

The process for preparing a catalytic material for an electrode for electrochemical reduction reactions, said material comprising an active phase based on tungsten (W) and / or molybdenum (Mo), and an electrically conductive support, comprises at least the following steps:

a) an impregnation step by bringing said electroconductive support into contact with at least one solution comprising an organic solvent A and at least one precursor of the mononuclear active phase based on W or Mo, optionally at least one precursor of the active phase based on at least one group VIII metal, in its monomeric or dimeric form, having at least one X = 0 or X-OR bond or at least one X = S or X-SR bond (with X = W or X = Mo) where [R = C _x H _y where x> 1 and (x-1) <y <(2x + 1) or R = Si (OR ') ₃ or R = Si (R') ₃ where R '= C _x H _y · where x'> 1 and (x'-1) <y '<(2x' + 1)];

Steps a) to c) of the process are described in detail below. In accordance with step a) of the preparation process according to the invention, an impregnation step is carried out by bringing said electrically conductive support into contact with at least one solution comprising an organic solvent A and at least one mononuclear precursor based on of W or Mo, in its monomeric or dimeric form, having at least one X = 0 or X-OR bond or at least one X = S or X-SR bond (with X = W or X = Mo) where [R = C _x H _y where x> 1 and (x-1) <y <(2x + 1) or R = Si (OR ') ₃ or R = Si (R') ₃ where R '= C _x H _y · where x '> 1 and (c'- 1) <y'<(2x'+1)];

Step a), called bringing the solution into contact with the support, is an impregnation. The impregnations are well known to those skilled in the art. The impregnation method according to the invention is chosen from dry or excess impregnation 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 of the active phase based on Mo and / or W (and optionally at least one precursor of the active phase based on at least one metal from group VIII), the volume of the solution of which is between 0.25 and 1.5 times the pore volume of the support to be impregnated.

The organic solvent A used in step a) is generally an alkane, an alcohol, an ether, a ketone, a chlorinated solvent or an aromatic compound. Cyclohexane and n-hexane are preferably used.

One of the advantages of the present invention therefore lies in a preparation of catalytic electrode material suitable for use for electrochemical reduction reactions, and in particular for the production of hydrogen by electrolysis of water, said preparation process allowing a better dispersion of the active phase by grafting the precursors on the surface of the electrically conductive support. In addition, depending on the sulfurization temperature, two kinds of active phase can be generated: an oxysulfide type phase or a sulfide type phase. Each of them has good activity, which is more important than that of conventional catalytic materials encountered in the literature.

Precursors of the active phase based on Mo and / or W

The mononuclear precursor based on W or on Mo, used in its monomeric or dimeric form, advantageously has the following formula:

X (= 0) _n (= S) _{n '} (0R) _a (SR') _b (L1) _c (L2) _d (L3) _e (L4) _f (L5) _g (with X = W or X = Mo ); where R = C _x H _y where x> 1 and (x-1) <y <(2x + 1) or R = Si (OR ") ₃ or R = Si (R") ₃ where R "= C _x H _y ·· where [x "> 1 and (x" -1) <y "<(2x"+1)];

where 0 <n + n '<2 and 0 <n <2 and 0 <n' <2;

where, if n = n '= 0, then (a¹0 or b¹0) and [(a + b + c + d + e + f + g = 6 and 0 £ a <6, 0 <b <6, 0 <c <6 , 0 <d <6, 0 <e <6, 0 £ f £ 6, 0 <g <6, or (a + b + c + d + e + f + g = 5 and 0 £ a <5, 0 <b <5, 0 <c <5, 0 <d <5, 0 <e <5, 0 <f <5, 0 <g <5), or (a + b + c + d + e + f + g = 4 and 0 £ a <4, 0 <b <4, 0 <c <4, 0 <d <4, 0 <e <4, 0 <f <4, 0 <g <4)];

where, if [(n = 1 and n '= 0) or (h' = 1 and n = 0)], then [(a + b + c + d + e + f + g = 4 and 0 £ a <4, 0 <b <4, 0 <c <4, 0 <d <4, 0 <e <4, 0 <f <4, 0 <g £ 4)] or [(a + b + c + d + e + f + g = 3 and 0 £ a <3, 0 <b <3, 0 <c <3, 0 <d <3, 0 <e <3, 0 <f <3, 0 <g <3)];

with (L1), (L2), (L3), (L4) and (L5) of the THF type, dimethyl ether, dimethylsulfide, P (CH ₃ ) ₃ , allyl, aryl, halogenated (chosen from fluorinated, chlorinated, bromines), amine, acetate, acetylacetonate, halide, hydroxide, -SH.

Preferably, the ligands (L1), (L2), (L3), (L4) and (L5) are chosen from acetylacetonate, THF and dimethyl ether.

Preferably, the precursors according to the invention do not contain a ligand (L1), (L2), (L3), (L4) and (L5).

Preferably, the precursors according to the invention are X (OEt) ₅ , X (OEt) ₆ , X (= 0) (OEt) ₄ , X (= S) (OEt) ₄ , X (= S) (SEt ) ₄ , X (= 0) ₂ (0Et) ₂ , X (OC ₆ H ₅ ) ₆ , X (SEt) ₅ , X (SEt) ₆ , X (OEt) ₄ (SEt), X (OEt) ₃ ( SEt) ₂ , X (OEt) ₂ (SEt) ₃ , X (OEt) (SEt) ₄ , X (= 0) (OEt) ₃ (acac) with Et = CH ₂ CH ₃ (ethyl group) and acac = ( CH ₃ COCHCOCH ₃ ) (acetylacetonate), in their monomeric or dimeric form (with X = W or X = Mo)

Very preferably, the precursors according to the invention are W (OEt) ₅ or Mo (OEt) ₅ or W (OEt) ₆ or Mo (OEt) ₆

Advantageously, after step a), but before step b), a maturation step is carried out intended to allow the species to diffuse through the support. It is advantageously carried out under an anhydrous atmosphere (without water), preferably between 30 minutes and 24 hours at room temperature. The atmosphere must preferably be anhydrous so as not to polycondense the precursors previously impregnated.

Group VIII metal-based active phase precursors

Advantageously, the active phase of the catalytic material comprises at least one group VIII metal chosen from Ni, Co and Fe. Preferably, the group VIII element is nickel or cobalt.

Said group VIII metal is advantageously present in the catalytic material at a content of between 0.1 and 16% by weight, preferably between 0.5 and 14% by weight relative to the weight of the final catalytic material obtained after the last preparation step. catalytic material (ie after the last sulfurization step).

The group VIII metal is advantageously introduced in the form of salts, chelating compounds, alkoxides or glycoxides, and preferably in the form of acetylacetonate or acetate.

The element or elements of group VIII, if any, hereinafter indicated as being the promoter (s), can be (are) introduced into solution either:

i) in the impregnation step a) in co-impregnation with the precursor of the mononuclear active phase based on tungsten or molybdenum,

ii) after the drying step b), in a so-called post-impregnation step a2) using a solution using an organic solvent B. In this case, it is necessary to add a new drying step b2) under the same conditions as described above;

iii) after step c), in a post-impregnation step a3) using an aqueous solution or an organic solution. In this case, it is necessary to add a new drying step b3) and a new sulfurization step c2) before using the electrode catalytic material in the process of use.

If the promoter is introduced as described in the invention in i) and in ii), the compounds containing the element of group VIII are preferably the sulfur compounds, the oxygenated compounds, the chelating compounds, the alkoxides and the glycoxides. Preferably, it is introduced in the form of acetylacetonate or acetate.

If the promoter is introduced as described in the invention in iii), the compounds containing the element of group VIII can be introduced in the form of salts, sulfur-containing compounds, oxygenated compounds, chelating compounds, alkoxides and of glycoxides. Preferably, it is introduced in the form of acetylacetonate or acetate.

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.

The organic solvent B used when the promoter is introduced after step c), in a so-called post-impregnation step, is generally an alkane, an alcohol, an ether, a ketone, a chlorinated compound or an aromatic compound. Toluene, benzene, dichloromethane, tetrahydrofuran, cyclohexane, n-hexane, ethanol, methanol and acetone are preferably used.

The solvent used for the impregnation of the promoter (element of group VIII) in the case of step iii) corresponds either to organic solvent B in the case of the use of non-saline precursors and of water or an alcohol when the precursors are saline.

The drying carried out during step b) is intended to remove the impregnating solvent A. The atmosphere must preferably be anhydrous (without water) so as not to polycondense said precursors previously impregnated. Advantageously, the temperature should not exceed 200 ° C in order to keep intact said precursors grafted or deposited on the surface of the support. Preferably, the temperature will not exceed 120 ° C. Very preferably, the drying is carried out under vacuum at ambient temperature. This step can be carried out alternately by passing an inert gas flow.

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.

Step c)

Step c) of sulfurization is carried out using a H ₂ S / H ₂ or H ₂ S / N ₂ gas mixture containing at least 5% by volume of H ₂ S in the mixture or using of pure H ₂ S at a temperature equal to or greater than ambient temperature, advantageously under a total pressure equal to or greater than 0.1 MPa for at least 2 hours. Preferably, the sulfurization temperature is between 50 ° C and 600 ° C. When one seeks to generate an oxysulfide phase, then very preferably, the sulfurization temperature is between 50 ° C. and 200 ° C.

When one seeks to generate a sulphide phase, then very preferably, the sulphurization temperature is between 300 ° C and 600 ° C.

Catalytic material

The active function of the catalyst according to the invention is ensured by at least one element from group VI B which is tungsten W and / or molybdenum Mo. Advantageously, the active function is chosen from the group formed by the combinations of tungsten elements or molybdenum or tungsten-molybdenum, nickel-tungsten or nickel-tungsten-molybdenum or nickel-molybdenum or cobalt-molybdenum or cobalt-tungsten or cobalt-molybdenum-tungsten or nickel-cobalt-molybdenum or nickel-cobalt-tungsten.

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.

Support

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 capacity to be deposited in the form of a layer of variable thickness and for its capacity for ionic conduction in an aqueous medium and for 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 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 blend having anionic conductor properties; either conductor of cationic groups allowing the conduction of protons and is chosen from the group comprising in particular: polymers stable in aqueous medium, which can be perfluorinated, partially fluorinated or non-fluorinated and having anionic groups allowing the conduction of protons;

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 Nafion®, 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 Nation®. 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 the 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 a 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:

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.

For the purposes 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 comprising including soaking, printing, induction, pressing, coating, spinning (or "spin-coating" according to English terminology), filtration, vacuum deposition, deposition by spraying, casting, extruding or rolling. Said support or said collector may be solid or perforated. As an example of support, mention may be made of 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 which are easily accessible and which 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 (ionic conductive medium). The anode is the seat of water oxidation. The cathode is the seat of proton reduction and the formation of hydrogen. 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 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:

- 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 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 + SH * + 6th- - 2 N Ha

The electrolyte can be:

- Or 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 which 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);

- or a ceramic membrane that conducts ions 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.

Example 1: Catalytic material based on Mo SUPPC on carbon, with a surface density of 8 Mo / nm ² sulfurized at 100 ° C (C1 - compliant)

The molybdenum is dry impregnated in a strictly non-aqueous medium on a support of the commercial carbon type (ketjenblack, 1400 m ² / g). The support is previously heat treated under N ₂ at 300 ° C for 6 hours at atmospheric pressure. It is then heated to 300 ° C for 14 hours under secondary vacuum (10 ⁹ MPa) before being stored in an inert medium, in a glove box. The molybdenum precursor is molybdenum pentaethoxide Mo (OC ₂ H ₅ ) ₅ . Dry degassed cyclohexane is used as the solvent. The impregnation solution is prepared from 14 g of precursor to which are added 2 ml of cyclohexane, then is impregnated on 5.21 g of dry support. The amount of molybdenum is adjusted so as to obtain 8 Mo / nm ² . After 15 hours of maturation, the extrudates are dried under vacuum (10 ⁹ MPa) for 2 hours at room temperature. The extrudates are then sulfurized under pure H ₂ S at 100 ° C for 2 hours.

This catalytic material C1 is in accordance with the invention. EXAMPLE 2 Catalytic Material Based on Mo suooi

on carbon, with a surface density of 8 Mo / nm ² sulfurized at 450 ° C (C2 - conform)

The molybdenum is dry impregnated in a strictly non-aqueous medium on a support of the commercial carbon type (ketjenblack, 1400 m ² / g). The support is previously heat treated under N2 at 300 ° C for 6 hours at atmospheric pressure. It is then heated to 300 ° C for 14 hours under secondary vacuum (10 ⁹ MPa) before being stored in an inert medium, in a glove box. The molybdenum precursor is molybdenum pentaethoxide Mo (OC ₂ H ₅ ) ₅ . Dry degassed cyclohexane is used as the solvent. The impregnation solution is prepared from 14 g of precursor to which are added 2 ml of cyclohexane, then is impregnated on 5.21 g of dry support. The amount of tungsten is molybdenum so as to obtain 8 W / nm ² . After 15 hours of maturation, the extrudates are dried under vacuum (10 ⁹ MPa) for 2 hours at room temperature. The extrudates are then sulfurized under pure H ₂ S at 450 ° C for 2 hours.

This catalytic material C2 is in accordance with the invention.

Example 3: Description of the commercial catalyst at Pt (Catalyst material)

the material C3 comes from Alfa Aesar®: it comprising platinum particles of S _B ET = 27 m ² / g.

The catalytic activity of the catalysts is characterized 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 and C3) 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, C3) . 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 pL 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 -180 and -190 mV vs RHE, the catalytic materials C1 and C2 have performances relatively close to that of platinum vis-à-vis according to the prior art. This result demonstrates the indisputable interest of these materials for the development of the hydrogen sector by electrolysis of water.

Claims

1. Method for preparing a catalytic material for an electrode for electrochemical reduction reactions, said material comprising an active phase based on tungsten (W) and / or molybdenum (Mo), and an electrically conductive support, said method comprising at least the following steps:

2. The method of claim 1, wherein said catalytic material further comprises at least one active phase based on group VIII metal chosen from cobalt, iron or nickel.

3. The method of claim 2, wherein the group VIII metal is nickel.

4. Method according to any one of claims 1 to 3, wherein the tungsten or molybdenum precursor is a mononuclear precursor based on W or Mo, used in its monomeric or dimeric form, of formula X (= 0) _n (= S) _{n '} (0R) _a (SR') _b (L1) _c (L2) _d (L3) _e (L4) _f (L5) _g (with X = W or X = Mo)

where R = C _x H _y where x> 1 and (x-1) <y <(2x + 1) or R = Si (OR ") ₃ or R = Si (R") ₃ where R "= C _x - H _y ·· where [x "> 1 and (x" -1) <y "<(2x"+1)];

where 0 <n + n '<2 and 0 <n <2 and 0 <n'<2; where, if n = h '= 0, then (a¹0 or b¹0) and [(a + b + c + d + e + f + g = 6 and 0 £ a <6, 0 <b <6, 0 <c <6, 0 <d <6, 0 <e <6, 0 £ f £ 6, 0 <g <6, or (a + b + c + d + e + f + g = 5 and 0 £ a <5 , 0 <b <5, 0 <c <5, 0 <d <5, 0 <e <5, 0 <f <5, 0 <g <5), or (a + b + c + d + e + f + g = 4 and 0 <a <4, 0 <b <4, 0 <c <4, 0 <d <4, 0 <e <4, 0 <f <4, 0 £ g £ 4)];

where, if [(n = 1 and n '= 0) or (h' = 1 and n = 0)], then [(a + b + c + d + e + f + g = 4 and 0 £ a < 4, 0 <b <4, 0 <c <4, 0 <d <4, 0 <e <4, 0 £ f £ 4, 0 <g £ 4)] or [(a + b + c + d + e + f + g = 3 and 0 <a <3, 0 <b <3, 0 <c <3, 0 <d <3, 0 <e <3, 0 <f <3, 0 £ g £ 3) ];

where, if [n + n '= 2 and 0 <n <2 and 0 <n' <2], then (a + b + c + d + e + f + g = 2 and 0 <a <2, 0 <b <

2, 0 <c <2, 0 <d <2, 0 <e <2, 0 <f <2, 0 <g <2);

5. Method according to claim 4, in which said precursor is chosen from X (OEt) ₅ , X (OEt) ₆ , X (= 0) (OEt) ₄ , X (= S) (OEt) ₄ , X (= S) (SEt) ₄ , X (= 0) ₂ (0Et) ₂ , X (OC ₆ H ₅ ) ₆ , X (SEt) ₅ , X (SEt) ₆ , X (OEt) ₃ (SEt) ₂ , X (OEt) ₄ (SEt), X (= 0) (OEt) ₃ (acac) with X = W or X = Mo, with Et = CH ₂ CH ₃ and with acac = (CH ₃ COCHCOCH ₃ ) in their monomeric form or dimeric.

6. Method according to any one of claims 1 to 5, wherein the sulfurization temperature in step c) is between 50 ° C and 200 ° C.

7. Method according to any one of claims 1 to 5, wherein the sulfurization temperature in step c) is between 300 ° C and 600 ° C.

8. Method according to any one of claims 1 to 7, wherein the support comprises at least one material chosen from carbonaceous structures of the carbon black type, graphite, carbon nanotubes or graphene.

9. Method according to any one of claims 1 to 7, wherein the support comprises at least one material chosen from gold, copper, silver, titanium, silicon.

10. A method of preparing a catalyst according to one of claims 1 to 9, wherein said active phase further comprises at least one group VIII metal selected from Ni, Co and Fe.

11. The method of claim 10, wherein said group VIII metal is introduced in solution either:

12. 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 11, in powder form, to obtain a mixture;

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

13. An electrolysis device comprising an anode, a cathode, an electrolyte, said device being characterized in that one in the month of the anode or of the cathode is an electrode according to claim 12.

14. Use of the electrolysis device according to claim 13 in electrochemical reactions.

15. Use according to claim 14, in which 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;

- 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.