Method for the preparation of an electrode comprising a metal sulfide compound

Abstract

The present invention relates to a process for the preparation of an electrode or a precursor thereof comprising sulfurizing a metal layer deposited on an electrode substrate, said metal layer comprising nickel, iron or a mixture of iron with nickel or cobalt. The invention also relates to the electrode or a precursor thereof obtainable by said process, the use thereof in electrocatalysis, for instance in alkaline water electrolysis, and to a device comprising said electrode.

Description

[0001] METHOD FOR THE PREPARATION OF AN ELECTRODE COMPRISING A METAL SULFIDE COMPOUND

[0002] FIELD OF THE INVENTION

[0003] [1] The present invention relates to a process for the preparation of an electrode or a precursor thereof comprising sulfurizing a metal layer deposited on an electrode substrate, said metal layer comprising nickel, iron or a mixture of iron with nickel or cobalt. The invention also relates to the electrode or a precursor thereof obtainable by said process, the use thereof in electrocatalysis, for instance in alkaline water electrolysis, and to a device comprising said electrode.

[0004] BACKGROUND

[0005] [2] Alkaline water electrolysers are the dominant type of systems for green hydrogen production in commercial operation today. The durability of these electrolysers is generally over 10 years, and they have the advantage of not degrading while resting at open circuit potential, which means they can operate under intermittent conditions, making them highly adaptable to renewable energy sources. Commercial systems can operate in the 1.8 kW-3.6 MW power range, having hydrogen production rates from 0.265 to 760 N m3h"1, and voltage efficiencies ranging between 47%-82%. One of the main issues that must be addressed to improve the performances of alkaline electrolysers is to reduce the energy losses associated with the efficiencies of electrode reactions and deactivation of the electrodes after long-time operation. Cathodic and anodic electrode reactions are the Hydrogen Evolution Reaction (HER) and the Oxygen Evolution Reaction (OER), respectively, being the bottleneck of the overall water splitting reaction the OER, as the Redox potential associated to said reaction is of 1.23 V vs. the reversible hydrogen electrode (RHE). The improvement of OER electrodes has been and still currently is the object of intensive research.

[0006] [3] The use of non-critical raw materials, such as earth abundant and readily available metals, e.g. metals of the iron group, is particularly attractive in the development of electrocatalysts for both the cathodic and anodic sites in alkaline water electrolysis, as current systems are typically based on precious metals, such as Pt and Ir. In this regard, numerous investigations have reported that materials based in the use of transition metal group VIII 3d oxides and oxyhydroxides (Fe, Co, Ni) and combinations between them and other metals have a high competitiveness for Oxygen Evolution Reaction, showing a high catalytic activity in alkaline media. For instance, among all the materials currently under investigation, iron and sulphur-based compounds, such as pyrite FeS2, represent one of the cheapest and most abundant material for the development of electrodes comprising electrocatalytically active compounds based on these species.

[0007] [4] Thus, pyrite containing electrodes are known and have been disclosed in the art. For instance, Barawi, M. et al disclose in Hydrogen Photoassisted Generation by Visible Light and an Earth Abundant Photocatalyst: Pyrite (FeS2) J. Phys. Chem. C 2016, 120, 18, 9547-9552, a photoelectrode consisting of pyrite deposited on a titanium disk support, which is useful as a photoanode in hydrogen production by water electrolysis. The electrocatalyst is formed according to a process whereby a titanium disk is contacted with thermally evaporated iron powder and later submitted to sulfurization for a period of time of 20 hours at a temperature of 350 °C under a pressure of sulphur of 0.2 bar. This document is however silent about the use of said electrocatalysts in the Oxygen Evolution Reaction of alkaline water electrolysis and about the incorporation or doping metals, such as nickel or cobalt in the electrocatalyst.

[0008] [5] Khalid, S. and co-workers disclose in Transition metal doped pyrite (FeS2) thin films: structural properties and evaluation of optical band gap energies - J. Mater. Chem. C, 2015, 3, 12068-12076 - a process for the preparation of pyrite materials doped with nickel, cobalt, copper or zinc. Said process involves chemical vapor deposition assisted by aerosol from dithiocarbamate coordination complexes of said metals. While studying the optical properties of the resulting materials, the authors are silent about their use in electrocatalytic processes such as the oxygen evolution reaction associated to alkaline water electrolysis.

[0009] [6] International patent application WO 2022 / 243441 A1 discloses a process for the preparation of electrodes for alkaline electrocalysis comprising the step of sulfiding a nickel foam substrate.

[0010] [7] Belgian patent application BE 883 785 A discloses an electrode obtained by the process of forming a layer of divided nickel on a conductive support for electrode using in particular a sacrificial element such as aluminium, followed by a sulfurization step. In said method, the sacrificial element is removed from the layer of the electrode prior to the sulfurization step.

[0011] [8] From what is disclosed in the art, it derives that there is still a need for providing improved electrodes and precursors thereof which are particularly useful for the Oxygen Evolution Reaction associated to alkaline water electrolysis, in particular because they are more efficient and more robust than the electrodes of the prior art. SUMMARY OF THE INVENTION

[0012] [9] After exhaustive research, the inventors have developed a process for the preparation of efficient electrodes for the oxygen evolution reaction of alkaline water electrolysis. In particular, the inventors have found that nickel or cobalt enriched pyrite and / or pyrrhotite materials constitute surprisingly efficient electrocatalysts for the oxygen evolution reaction in alkaline water electrolysis. The inventors have found a process allowing for preparing an electrode precursor that is particularly useful for the Oxygen Evolution Reaction in alkaline electrolysis. Said electrode precursor comprises an outer layer comprising a pyrite and / or pyrrhotite type material whereby at least a portion of the iron atoms may be substituted by Ni and / or Co. Said outer layer material is formed by sulfurization of a metal layer deposited on an electrode substrate or on a metal-based electrode substrate. The amount of sulphur employed in said sulfurization steps as well as the conditions employed for said steps determine which solid phase forms. The inventors have particularly found that the same active material forms from said electrode precursor, regardless of the crystalline phase of the active material (e.g. pyrite or pyrrhotite).

[0013]

[0010] More particularly, the electrode according to the invention comprises hydroxylated and / or oxyhydroxylated metals as active species in the reaction of oxygen evolution reaction in alkaline water electrolysis. Such electrodes are readily obtained by activation of electrode precursors in an alkaline medium, by replacement of sulphur atoms for OH and OOH groups in electrode precursors comprising metal sulphide species. The active electrode and the electrode precursor both form part of the invention. The electrode of the invention advantageously comprises readily available and abundant metals, such as iron, nickel and cobalt, thus precluding the use of critical materials such as ruthenium and iridium. The inventors have found that the electrodes obtained according to the process of the invention are surprisingly efficient in the Oxygen Evolution Reaction (OER) involved in alkaline water electrolysis, as they produce high current densities at low electrode potentials. The inventors have also found that the same active material for the OER forms from an electrode precursor comprising either pyrite materials or pyrrhotite materials. The electrodes according to the invention are also particularly more robust than electrodes known in the art.

[0014]

[0011] Thus, in a first aspect, the invention relates to a process for making a precursor of an electrode for alkaline water electrolysis, said electrode comprising a precursor of an electrocatalytically active material comprising a compound of formula M1XM2(1.X)S2 and / or a compound of formula M1X(i-y)M2(i-X)(i-y)S wherein y is higher than 0 and equal to or lower than 0.2; said process comprising the steps of:

[0015] (i) in a first alternative of step (i) providing a substrate for electrode comprising an outer layer which comprises a compound of formula M1XM2(1-X) wherein:

[0016] M1is Fe and M2is Co; x is higher than 0 and equal to or lower than 1 ; or, in a second alternative of step (i),

[0017] (a) providing a mixture of powders of M1and M2such that the molar ratio of M1to M2is x:(1-x) wherein M1is selected from the group consisting of Fe and Ni;

[0018] M2is selected from the group consisting of Ni and Co; when M1is Fe, x is higher than 0 and lower than 1 ; and when M1is Ni, x is 1 ;

[0019] (b) providing a substrate for electrode;

[0020] (c) evaporating the mixture of powders provided in step (a), thus producing a gaseous metal mixture,

[0021] (d) depositing the gaseous metal mixture produced in step (c) on the substrate provided in step (b); thereby producing a substrate for electrode comprising an outer layer which comprises a compound of formula M1XM2(1.X);

[0022] (ii) contacting the substrate for electrode comprising an outer layer which comprises a compound of formula M1XM2(1.X) provided in (i) with an atmosphere comprising gaseous sulphur in conditions allowing for sulfurizing at least partially one metal comprised in said coating.

[0023]

[0012] A second aspect of the invention relates to an electrode precursor obtainable by the method according to the first aspect of the invention. A process for the activation of said electrode precursor also forms part of the invention.

[0024]

[0013] Thus, a third aspect of the invention relates to a process of activating a precursor electrode as defined in the second aspect of the invention comprising the circulation of a current through said precursor electrode placed in a basic medium at an electrode potential equal to or higher than 1 .30 V vs RHE.

[0025]

[0014] A fourth aspect of the invention relates to an electrode obtainable by the method according to the third aspect of the invention.

[0026]

[0015] A fifth aspect of the invention relates to the use of the electrode precursor according to the second aspect of the invention or the electrode of the fourth aspect of the invention as electrode in an electrocatalytic process; preferably in alkaline water electrolysis; more preferably as anode in alkaline water electrolysis. The anode is the electrode whereby the oxygen evolution reaction takes place.

[0027]

[0016] A sixth aspect of the invention relates to a device, preferably an alkaline water electrolyzer, comprising one or more electrodes selected from the electrodes according to the second aspect of the invention and the electrode according to the fourth aspect of the invention.

[0028] BRIEF DESCRIPTION OF THE DRAWINGS

[0029]

[0017] Fig. 1 shows a schematic representation of a thermal evaporator apparatus as used in the process of Example 1 comprising (1) a crucible containing a mixture of metal powders, (2) a high voltage power supply for heating the crucible by means of an electric current, (3) a high vacuum chamber, (4) a substrate for an electrode, (5) means for monitoring the thickness of the deposited metal layer, (6) a sample holder for holding the substrates of electrodes.

[0030]

[0018] Fig. 2 shows the energy dispersive x-ray (EDX) spectrum as measured in a Scanning Electron Microscope (SEM) of a metal layer deposited on glassy carbon substrate using mixture 3 of Example 1.

[0031]

[0019] Fig. 3 shows SEM pictures of metal layer deposited on glassy carbon substrate using (a) mixture 2, (b) mixture 3, (c) mixture 4 of Example 1.

[0032]

[0020] Fig. 4 shows (left) photographs of samples of 15 mm diameters of sulfurized metal layers prepared from mixture 2 according to Example 1 and (right) SEM images of the corresponding samples when sulfurization is carried out at (a) and (b) 200 °C for 4 h, (c) and (d) 300 °C for 4 hours, (e) and (f) 300 °C for 1 hour.

[0033]

[0021] Fig. 5 shows powder X-ray diffraction patterns of (a) sulfurization product of a Fe layer on glassy carbon prepared as disclosed above having a thickness of 100 nm carried out at 200 °C for 2 h in the presence of 100 ig S; (b) sulfurization product of a Fe layer on glassy carbon prepared as disclosed above having a thickness of 100 nm carried out at 200 °C for 4 h in the presence of 100 pg S; (c) sulfurization product of a Fe layer on glassy carbon prepared as disclosed above having a thickness of 60 nm carried out at 200 °C for 4 h in the presence of 130 pg S; (d) sulfurization product of a Fe layer on glassy carbon prepared as disclosed above having a thickness of 60 nm carried out at 200 °C for 7.5 h in the presence of 180 pg S; (e) sulfurization product of a commercial Fe disk carried out at 200 °C for 4 h in the presence of 30 mg S; (f) sulfurization product of a commercial Fe disk carried out at 200 °C for 4 h in the presence of 120 pg S.

[0034]

[0022] Fig. 6 shows Linear Sweep Voltammetry curves recorded in 0.1 M KOH electrolyte for the sulfurization product comprising pyrite of a commercial Fe disk carried out at 200 °C for 4 h in the presence of 30 mg S; and the sulfurization product comprising pyrrhotite of a commercial Fe disk carried out at 200 °C for 4 h in the presence of 120 pg S.

[0035]

[0023] Fig. 7 shows (a) 20 cycles of cyclic voltammetry experiments carried out with the sample c-Fe5sNi45-S, (b) SEM pictures of said material before (top) and after electrocatalysis (bottom) and (c) Raman spectra of / -Fe-S as measured before (upper curve) and after (lower curve) electrocatalysis (EC).

[0036]

[0024] Fig. 8 shows chronoamperometric measurements carried out at a potential of 1.75 V vs RHE for (a) / -Fe and / -Fe-S electrodes and (b) a / -Fe5sNi45-S, b / -FessNiis-S, c c-Fe5sNi45-S and d / -Ni-onFe-S. X-axis represent time, expressed in hours in both (a) and (b).

[0037]

[0025] Fig. 9 shows for two different samples a and b of / -Fe5sNi45-S (a) EDX composition in % atom, b) SEM images of the material (top) and (bottom) photographs of the electrodes (15 mm diameter), c) l / V curves, d) measured overpotentials at 1 mA per cm2current density and e) chronoamperometric measurements carried out at a potential of 1.75 V vs RHE.

[0038]

[0026] Fig. 10 shows chronoamperometric measurements carried out at a potential of - 0.5 V vs RHE of a) c-Fe5sNi45 b) c-Fe5sNi45-S c) / -Ni-onFe d) / -Ni-onFe-S, e) c-Ni.

[0039]

[0027] Fig. 11 shows a SEM picture of / -Fe-S.

[0040]

[0028] Fig. 12 shows a high resolution SEM picture of / -Fe5sNi45-S.

[0041]

[0029] Fig. 13 shows chronoamperometric measurements carried out at a potential of 1 .7 V vs RHE of electrode c-Fe-S-1 of Example 6.

[0042]

[0030] Fig. 14 shows polarization curves for Ni foam, c-Fe-S-1 and c-Fe-S-2 electrodes of Example 6 measured in a two electrodes zero-gap electrochemical cell as described in Example 6.

[0043] DETAILED DESCRIPTION

[0044]

[0031] All terms as used herein in this application, unless otherwise stated, shall be understood in their ordinary meaning as known in the art. Other more specific definitions for certain terms as used in the present application are as set forth below and are intended to apply uniformly throughout the specification and claims unless an otherwise expressly set out definition provides a broader definition.

[0045]

[0032] For the purposes of the invention, any ranges given include both the lower and the upper end-points of the range unless otherwise indicated. Ranges given, such as temperatures, times, molar ratio and the like, should be considered approximate (i.e. with a 5% margin of variation around indicated point), unless specifically stated. Similarly, any value disclosed in a manner that it is preceded by the term “about” should be considered approximate (i.e. with a 5% margin of variation around indicated point), unless specifically stated.

[0046]

[0033] In the context of the invention, the term “alkaline water electrolysis” refers to an electrochemical process allowing for the conversion of aqueous solutions of alkaline hydroxide salts into hydrogen and oxygen according to the following electrochemical half-reactions:

[0047] 2 OH' (I) - 1 O2 (g) + 2 e- + H2O (I) (1) also called Oxygen Evolution Reaction (OER)

[0048] 2 H2O (I) + 2 e- -> H2(g) + 2 OH' (I) (2) also called Hydrogen Evolution Reaction (HER) In an alkaline water electrolyzer, the OER takes place at the anode, while the HER takes place at the cathode.

[0049]

[0034] The efficiency of the electrodes and the performance of the cell can thus be measured by measuring the current densities (expressed in mA / cm2) at each electrode; since, assuming no faradaic loss, said current is directly proportional to the amount of gases produced at each electrode according to equations (1) and (2) above.

[0050]

[0035] In the context of the invention, the term “electrode” refers to a body comprising an electron conductive section, said body being used to close an electrical circuit through a medium, such as a solid or an ionic solution, separating two electrodes. An electrode suitable for electrocatalysis is an electrode comprising an electrocatalytically active material that can be used as a catalyst in an electrochemical reaction, such as reduction or oxidation reactions.

[0051]

[0036] In the context of the invention the term “electrocatalytically active material” refers to a material suitable for promoting chemical reactions taking place at one or more sites of the material, said sites being in contact with an electrode. In particular embodiments, the electrocatalytically active material of the electrode of the invention comprises hydroxy and / or oxyhydroxy salts of nickel or of mixtures of iron with Ni or Co.

[0052]

[0037] The term “electrolyser” is known in the art and refers to an electrochemical device able to convert electrical energy into chemical energy. A water electrolyser typically splits water into oxygen and hydrogen.

[0053]

[0038] The term “overpotential”, when related to OER, is known in the art and refers to the difference between the potential that needs to be applied to an anode in a water electrolyser in order to achieve a certain degree of performance of OER, expressed as anodic current density, and the standard potential for water splitting (1 .23 V respect to the Reversible Hydrogen Electrode is the thermodynamic value). The anodic current density is directly correlated to the yield of production of oxygen. The overpotential required to reach a current density of 1 mA per cm2, also abbreviated r|1, is frequently used in the art as a parameter of performance of an electrocatalytically active material suitable for OER.

[0054]

[0039] The term “overpotential”, when related to HER, is known in the art and refers to the absolute value of the difference between the potential that needs to be applied to a cathode in a water electrolyser in order to achieve a certain degree of performance of HER, expressed as cathodic current density, and the standard potential for water splitting (0 V respect to the Reversible Hydrogen Electrode is the thermodynamic value). The cathodic current density is directly correlated to the yield of production of hydrogen. The overpotential required to reach a current density of -1 mA per cm2, also abbreviated r|1, is frequently used in the art as a parameter of performance of an electrocatalytically active material suitable for HER.

[0055]

[0040] In the context of the invention, the term “pyrite” refers to a crystalline phase of iron sulphide of formula FeS2 having a cubic crystal structure. In the materials of the invention, at least a part of the iron of pyrite is replaced by nickel atoms or cobalt atoms in the crystal structure. Thus, the term “pyrite” also refers to a compound of formula M1xM2(i-x)S2 as defined in the first aspect of the invention.

[0056]

[0041] In the context of the invention, the term “pyrrhotite” refers to a crystalline phase of iron sulphide of formula Fe(i.y)S (y = 0 to 0.2). In the materials of the invention, at least a part of the iron of pyrrhotite is replaced by nickel atoms or cobalt atoms in the crystal structure. Thus, the term “pyrrhotite” also refers to a compound of formula M1x(i-y)M2(i-X)(i- y)S as defined in the first aspect of the invention.

[0057]

[0042] The term “at least partially” refers to “partially” and encompasses the embodiment “totally”. For instance, in the context of the process of the invention which allows producing different phases of materials, such as pyrite and pyrrhotite, the term “sulfurizing at least partially one metal”, refers to the fact that said metal may be partially sulfurized, as is the case for instance of pyrrhotite phase, or totally sulfurized, as in the case for instance of the pyrite phase. As mentioned above, pyrite and pyrrhotite phases may be such that a portion of iron atoms is replaced by Ni or Co atoms.

[0058]

[0043] Unless otherwise specified, all electrode potential values are herein provided taking as reference the potential of the Reversible Hydrogen Electrode (RHE).

[0059]

[0044] As mentioned above, a first aspect of the invention relates to a process for making a precursor of an electrode for alkaline water electrolysis, said electrode comprising a precursor of an electrocatalytically active material comprising a compound of formula M1xM2(i-x)S2 and / or a compound of formula M1x(i-y)M2(i-X)(i-y)S wherein y is higher than 0 and equal to or lower than 0.2; said process comprising the steps of:

[0060] (i) in a first alternative of step (i) providing a substrate for electrode comprising an outer layer which comprises a compound of formula M1XM2(1-X) wherein:

[0061] M1is Fe and

[0062] M2is Co; x is higher than 0 and equal to or lower than 1 ; or, in a second alternative of step (i),

[0063] (a) providing a mixture of powders of M1and M2such that the molar ratio of M1to M2is x:(1-x) wherein M1is selected from the group consisting of Fe and Ni;

[0064] M2is selected from the group consisting of Ni and Co; when M1is Fe, x is higher than 0 and lower than 1 ; and when M1is Ni, x is 1 ;

[0065] (b) providing a substrate for electrode;

[0066] (c) evaporating the mixture of powders provided in step (a), thus producing a gaseous metal mixture,

[0067] (d) depositing the gaseous metal mixture produced in step (c) on the substrate provided in step (b); thereby producing a substrate for electrode comprising an outer layer which comprises a compound of formula M1XM2(1-X);

[0068] (ii) contacting the substrate provided in (i) with an atmosphere comprising gaseous sulphur in conditions allowing for sulfurizing at least partially one metal comprised in said coating.

[0069]

[0045] In preferred embodiments, the first aspect of the invention relates to a process for making a precursor of an electrode for alkaline water electrolysis, said electrode comprising a precursor of an electrocatalytically active material comprising a compound of formula M1XM2(1.X)S2 and / or a compound of formula M1X(i-y)M2(i-X)(i-y)S wherein y is higher than 0 and equal to or lower than 0.2; said process comprising the steps of:

[0070] (i) in a first alternative of step (i) providing a substrate for electrode comprising an outer layer which comprises a compound of formula M1XM2(1.X) wherein:

[0071] M1is Fe and M2is Co; x is higher than 0 and lower than 1 ; or, in a second alternative of step (i),

[0072] (a) providing a mixture of powders of M1and M2such that the molar ratio of M1to M2is x:(1-x) wherein M1is selected from the group consisting of Fe and Ni;

[0073] M2is selected from the group consisting of Ni and Co; when M1is Fe, x is higher than 0 and lower than 1 ; and when M1is Ni, x is 1 ;

[0074] (b) providing a substrate for electrode;

[0075] (c) evaporating the mixture of powders provided in step (a), thus producing a gaseous metal mixture,

[0076] (d) depositing the gaseous metal mixture produced in step (c) on the substrate provided in step (b); thereby producing a substrate for electrode comprising an outer layer which comprises a compound of formula M1XM2(1.X); (ii) contacting the substrate provided in (i) with an atmosphere comprising gaseous sulphur in conditions allowing for sulfurizing at least partially one metal comprised in said coating.

[0077]

[0046] In the first alternative of step (i), the substrate for electrode comprising an outer layer which comprises a compound of formula M1XM2(1-X) is a commercially available substrate, such as a disk, a foil, a felt, a mesh, a foam, or a plate consisting of said compound of formula M1XM2(1.X). In preferred embodiments of this first alternative, the substrate for electrode comprising an outer layer which comprises a compound of formula M1XM2(1.X) that is or consists essentially of Fe (i.e. x is 1).

[0078]

[0047] In preferred embodiments of this first alternative, the substrate for electrode comprising an outer layer which comprises a compound of formula M1XM2(1.X) is substantially free of nickel. In particular, the compound of formula M1XM2(1.X) is not a iron nickel alloy.

[0079]

[0048] In preferred embodiments of this first alternative, the substrate for electrode comprising an outer layer which comprises a compound of formula M1XM2(1.X) is selected from the group consisting of iron disk, iron foil, iron foam, iron felt and iron mesh.. The use of high surface substrates, such as iron foam, iron mesh or iron felt, is particularly privileged.

[0080]

[0049] In the second alternative of step (i), step (a) of the process of the first aspect of the invention may be one wherein the substrate for electrode comprising an outer layer which comprises a compound of formula M1XM2(1.X) is prepared by thermal evaporation of metals and deposition onto the surface of a substrate for an electrode of a layer of said metals. When M1is Fe, x is higher than 0 and or lower than 1.

[0081]

[0050] Thus, in preferred embodiments, the second alternative of the first aspect of the invention relates to a process for making a precursor of an electrode for alkaline water electrolysis, said electrode comprising a precursor of an electrocatalytically active material comprising a compound of formula M1X(i-y)M2(i-X)(i-y)S and / or a compound of formula M1XM2(1.X)S2 as defined above wherein M1is selected from the group consisting of Fe and Ni and;

[0082] M2is selected from the group consisting of Ni and Co; when M1is Fe, x is higher than 0 and lower than 1 ; when M1is Ni, x is 1 ; and wherein step (i) comprises

[0083] (a) providing a mixture of powders of M1and M2such that the molar ratio of M1to M2is x:(1-x);

[0084] (b) providing a substrate for electrode;

[0085] (c) evaporating the mixture of powders provided in step (a), thus producing a gaseous metal mixture, (d) depositing the gaseous metal mixture produced in step (c) on the substrate provided in step (b); thereby producing a substrate coated with an outer layer comprising M1and, optionally, M2.

[0086]

[0051] As defined above, step (a) of the process of the first aspect of the invention comprises providing a mixture of powders of M1and M2wherein:

[0087] M1is selected from the group consisting of Fe and Ni and;

[0088] M2is selected from the group consisting of Ni and Co; when M1is Fe, x is higher than 0 and lower than 1 ; when M1is Ni, x is 1.

[0089]

[0052] In a preferred embodiment, step (a) of the process of the first aspect of the invention comprises providing a mixture of powders of M1and M2wherein M1is Fe and x is lower than 1 or equal to or lower than 0.99, 0.98, 0.97, 0.96, 0.95, 0.94, 0.93, 0.92, 0.91 , 0.90, 0.85, 0.80, 0.75, 0.70, 0.65, 0.60 or 0.55.

[0090]

[0053] In another preferred embodiment, step (a) of the process of the first aspect of the invention comprises providing a mixture of powders of M1and M2wherein M1is Fe and x is higher than 0 or equal to or higher than 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80 or 0.85.

[0091]

[0054] The lower-end and higher-end values of x described above when M1is Fe may be combined to form ranges representing particular embodiments of the value of x when M1is Fe.

[0092]

[0055] Thus, in another preferred embodiment, step (a) of the process of the first aspect of the invention comprises providing a mixture of powders of M1and M2wherein M1is Fe and x is comprised between 0.05 and 0.95; preferably x is comprised from 0.25 to 0.90; more preferably x is comprised from 0.55 to 0.85. In further preferred embodiments, when M1is Fe, x is 0.55 or 0.85. In other embodiments, when M1is Fe, x is equal to or higher than 0.50 and lower than 1 .

[0093]

[0056] In further preferred embodiments, step (a) of the process of the first aspect of the invention comprises providing a mixture of powders of M1and M2wherein M1is Fe and M2is nickel. In such embodiments, x is preferably comprised between 0.05 and 0.95; preferably x is comprised from 0.25 to 0.90; more preferably x is comprised from 0.55 to 0.85. In other embodiments, when M1is Fe, x is equal to or higher than 0.50 and lower than 1 and M2is nickel. In further preferred embodiments, when M1is Fe, x is 0.55 or 0.85 and M2is nickel.

[0094]

[0057] In other preferred embodiments, step (a) of the process of the first aspect of the invention comprises providing a mixture of powders of M1and M2wherein M1is Fe and M2is cobalt. In such embodiments, x is preferably comprised between 0.05 and 0.95; preferably x is comprised from 0.25 to 0.90; more preferably x is comprised from 0.55 to 0.85. In other embodiments, when M1is Fe, x is equal to or higher than 0.50 and lower than 1 and M2is cobalt. In further preferred embodiments, when M1is Fe, x is 0.55 and M2is cobalt.

[0095]

[0058] Thus, in preferred embodiments, step (a) of the process of the first aspect of the invention comprises providing a mixture of powders of M1and M2selected from the group consisting of:

[0096] Ni powder; a mixture of Fe powder and Ni powder whereby the molar ratio of Fe to Ni is 55:45; a mixture of Fe powder and Ni powder whereby the molar ratio of Fe to Ni is 85:15; and a mixture of Fe powder and Co powder whereby the molar ratio of Fe to Co is 55:45.

[0097]

[0059] In particular embodiments, step (b) of the process of the first aspect of the invention comprises providing a substrate for an electrode that is selected from the group consisting of metal mesh, metal foam, metal foil, metal felt, metal disk, carbon paper, carbon felt, transparent conducting oxides, glassy carbon and carbon cloth. Thus, the substrate for an electrode of step (b) may be selected from the group consisting of copper mesh, iron mesh, nickel mesh, titanium mesh, platinum mesh, copper felt, iron felt, nickel felt, titanium felt, platinum felt, iron foam, aluminium foam, titanium foam, copper foam, nickel foam, steel foam, nickel-iron foam, nickel-iron disk, iron disk, aluminium foil, nickel foil, copper foil, iron foil, titanium foil, platinum foil, carbon paper, carbon felt, glassy carbon, carbon cloth, indium tin oxide (ITO) and fluoride doped tin oxide (FTO).

[0098]

[0060] In more particular embodiments, step (b) of the process of the first aspect of the invention comprises providing a substrate for an electrode that is selected from the group consisting of glassy carbon, steel, nickel-iron disk and iron disk. More preferably, the substrate for an electrode is selected from the group consisting of glassy carbon, and iron disk.

[0099]

[0061] In even more particular embodiments, when M1is Fe in the mixture provided in step (a), the substrate for an electrode is glassy carbon. The inventors have found that the presence of M2in the powder mixture provided in step (a), in particular when M2is nickel, confers improved adhesion of the electrocatalytically active material to glassy carbon than when iron alone is used.

[0100]

[0062] In other more particular embodiments, when M1is Ni in the mixture provided in step (a), the substrate for an electrode is iron disk.

[0101]

[0063] As defined above, step (c) of the process of the first aspect of the invention comprises evaporating the mixture of powders provided in step (a). Methods and apparatus therefor for reducing this step to practice are known in the art and will become apparent to the skilled person upon reduction to practice of the invention.

[0064] Such methods include for instance thermal evaporation and electron beam evaporation. A preferred method is thermal evaporation, in particular where the composition to be evaporated is heated by means of an electric current. Thermal evaporators are known in the art. An example of such apparatus is described in Fig. 1. Thus, in preferred embodiments, step (c) of the process of the first aspect of the invention comprises heating said mixture of powders by means of an electric current.

[0102]

[0065] Once evaporated, in step (d) of the process of the invention, the mixture of metals is deposited on the surface of the substrate for an electrode provided in step (b). Preferably, step (d) of the process of the first aspect of the invention comprises depositing the gaseous mixture produced in step (c) on the surface of the substrate; thereby producing a substrate coated with a layer comprising M1and further comprising M2when x is not 1 . When a thermal evaporator is used, the substrate is typically located at an area within reach of the free movement of the metal gas molecules, so as to facilitate the condensation of the gas molecules on the surface of the substrate.

[0103]

[0066] In preferred embodiments, step (d) of the process of the first aspect of the invention allows producing a substrate coated with a layer comprising M1and further comprising M2when x is not 1 , said layer having a thickness of between 50 nm and 10 .m; more preferably of between 50 nm and 500 nm; even more preferably of 100 nm.

[0104]

[0067] It is further preferred that steps (c) and (d) of the first aspect of the invention be carried out under vacuum. This allows minimizing the energy input required to evaporate the powder mixtures provided in step (a) while avoiding the deposition of further impurities concomitantly with the deposition of M1and, optionally M2, on the surface of the substrate. Preferably, a pressure of equal to or less than 10'4mbar is used. Even more preferably, a pressure of 10'5mbar is used.

[0105]

[0068] In further preferred embodiments, steps (c) and (d) of the process of the first aspect of the invention are carried out in a thermal evaporator placed under vacuum; preferably at a pressure of 10'5mbar.

[0106]

[0069] As defined above, step (ii) of the process of the first aspect of the invention may comprise contacting the product of step (d) of the second alternative of step (i) or a substrate as provided in the first alternative of step (i) with an atmosphere comprising gaseous sulphur in conditions allowing for sulfurizing at least partially one metal comprised in said coating.

[0107]

[0070] In preferred embodiments, step (ii) of the process of the first aspect of the invention comprises contacting the product of step (d) of the second alternative of step (i) or a substrate as provided in the first alternative of step (i) with an atmosphere consisting essentially of gaseous sulphur in conditions allowing for sulfurizing at least partially one metal comprised in said coating.

[0071] Said gaseous sulphur is preferably obtained by sublimation of solid sulphur, for instance by heating solid sulphur under vacuum, preferably at a pressure prior to sulphur vaporization of between 10'5and 10'6mbar.

[0108]

[0072] In preferred embodiments, step (ii) of the process of the first aspect of the invention comprises heating solid sulphur at a temperature of between 150 °C and 350 °C; more preferably of between 180 °C and 300 °C. In preferred embodiments, step (ii) of the process of the first aspect of the invention comprises heating solid sulphur at a temperature of between 200 °C and 300 °C. Even more preferably, step (ii) of the process of the first aspect of the invention comprises heating solid sulphur at a temperature of 200 °C. This is particularly the case when the pressure prior to sulphur vaporization is of between 10'5and 10'6mbar. These conditions are particularly suitable for producing a gaseous atmosphere of sulphur.

[0109]

[0073] In preferred embodiments, step (ii) of the process of the first aspect of the invention is carried out for a period of time of between 1 and 20 hours, preferably of between 1 and 8 hours; even more preferably of 4 hours.

[0110]

[0074] In more preferred embodiments, step (ii) of the process of the first aspect of the invention is carried out at a temperature of 200 °C and at a pressure prior to sulphur vaporization of between 10'5and 10'6mbar.

[0111]

[0075] In more preferred embodiments, step (ii) of the process of the first aspect of the invention is carried out at a temperature of 200 °C for a period of time of 4 hours.

[0112]

[0076] In more preferred embodiments, step (ii) of the process of the first aspect of the invention is carried out at a pressure prior to sulphur vaporization of between 10'5and 10'6mbar and for a period of time of 4 hours.

[0113]

[0077] In more preferred embodiments, step (ii) of the process of the first aspect of the invention is carried out at a temperature of 200 °C, at a pressure prior to sulphur vaporization of between 10'5and 10'6mbar and for a period of time of 4 hours.

[0114]

[0078] In more preferred embodiments, step (ii) of the process of the first aspect of the invention is carried out in the presence of excess sulphur, preferably such that the mol number of elemental sulphur is at least twice the mol number of metals deposited in step (d) of the second alternative of step (i) on the surface of the substrate or the mol number of metals comprised in the substrate provided in the first alternative of step (i).

[0115]

[0079] In more preferred embodiments, step (ii) of the process of the first aspect of the invention is carried out in a closed vessel, such as a glass ampoule.

[0116]

[0080] The inventors have found that the amount of sulphur employed in step (ii), the temperature, the reaction time and the thickness of the metal layer deposited in step (d) of the second alternative of step (i) or provided in the first alternative of step (i) have an impact on the sulfurization step (ii), such that crystalline phases of the pyrite and / or pyrrhotite type are produced. As further detailed below, it was further found that the same electrocatalytically active material for OER forms from either crystalline phase, such that there is advantageously no need for controlling which phase(s) form(s) during step (ii).

[0117]

[0081] In other particular embodiments, the process of the first aspect of the invention comprises:

[0118] (a) providing a mixture of powders of M1and M2wherein M1is Fe and x is equal to or higher than 0.50 and lower than 1 ; or, alternatively, M1is nickel and x is 1 ; preferably, when M1is Fe, x is 0.55 or 0.85 and M2is nickel; or, alternatively, when M1is Fe, x is 0.55 and M2is cobalt;

[0119] (b) providing a substrate for an electrode that is selected from the group consisting of glassy carbon, steel, nickel-iron disk and iron disk; preferably, said substrate is iron disk when M1is nickel; or, alternatively, said substrate is glassy carbon when M1is iron;

[0120] (c) evaporating the mixture of powders provided in step (a) in a thermal evaporator by heating said mixture of powders by means of an electric current under a pressure of equal to or less than 10'4mbar;

[0121] (d) depositing the gaseous metal mixture produced in step (c) on the substrate provided in step (b); thereby producing a substrate coated with a layer comprising M1and, optionally, M2; preferably, said step is carried out in the thermal evaporator of step (c); and

[0122]

[0082] (ii) contacting the coated substrate produced in (i) with an atmosphere comprising, preferably consisting essentially of, gaseous sulphur in conditions allowing for sulfurizing at least partially one metal comprised in said coating, said conditions being preferably such that step (ii) is carried out in a closed vessel, such as a glass ampoule, at a temperature of 200 °C, at a pressure prior to sulphur vaporization of between 10'5and 10'6mbar and for a period of time of 4 hours.

[0123]

[0083] In other particular embodiments, the process of the first aspect of the invention comprises: a first step (i) which comprises providing a substrate for electrode comprising an outer layer which comprises a compound of formula M1XM2(1.X) wherein M1is Fe, M2is Co and x is higher than 0 and equal to or lower than 1 ; and

[0124] (ii) contacting the substrate provided in (i) with an atmosphere comprising, preferably consisting essentially of, gaseous sulphur in conditions allowing for sulfurizing at least partially one metal comprised in said coating, said conditions being preferably such that step (ii) is carried out in a closed vessel, such as a glass ampoule, at a temperature of about 200 °C, at a pressure prior to sulphur vaporization of between 10'5and 10'6mbar and for a period of time of about 4 hours.

[0084] In other particular embodiments, the process of the first aspect of the invention comprises: a first step (i) which comprises providing a substrate for electrode comprising an outer layer which comprises a compound of formula M1XM2(1-X) consisting essentially of iron; and

[0125] (ii) contacting the substrate provided in (i) with an atmosphere comprising, preferably consisting essentially of, gaseous sulphur in conditions allowing for sulfurizing at least partially one metal comprised in said coating, said conditions being preferably such that step (ii) is carried out in a closed vessel, such as a glass ampoule, at a temperature of 200 °C, at a pressure prior to sulphur vaporization of between 10'5and 10'6mbar and for a period of time of 4 hours.

[0126]

[0085] In other particular embodiments, the process of the first aspect of the invention comprises: a first step (i) which comprises providing a substrate for electrode selected from the group consisting of iron disk, iron foam, iron mesh, iron disk and iron felt; particularly it is iron foam or iron disk; and

[0127] (ii) contacting the substrate provided in (i) with an atmosphere comprising, preferably consisting essentially of, gaseous sulphur in conditions allowing for sulfurizing at least partially one metal comprised in said coating, said conditions being preferably such that step (ii) is carried out in a closed vessel, such as a glass ampoule, at a temperature of 200 °C, at a pressure prior to sulphur vaporization of between 10'5and 10'6mbar and for a period of time of 4 hours.

[0128]

[0086] As further defined above, the second aspect of the invention relates to an electrode precursor obtainable by the method according to the first aspect of the invention.

[0129]

[0087] Preferred embodiments of the second aspect of the invention relate to an electrode precursor obtainable by the method as defined in any of the single and combined embodiments defined above for the first aspect of the invention.

[0130]

[0088] The electrode precursor of the second aspect of the invention comprises a partially sulfurized coating material deposited on a substrate for an electrode.

[0131]

[0089] In preferred embodiments, the partially sulfurized coating material of the electrode precursor of the second aspect of the invention does not essentially comprise particles or flakes of said material. This is particularly the case when M1is Fe, as shown for instance in Figure 11. This is also particularly the case when M1is Fe and M2is Nickel, as shown for instance in Figure 12. More particularly, when M1is Fe, it does not essentially comprise particles or flakes of said material having a size of 250 nm or more. In more particular embodiments, it does not essentially comprise particles or flakes of said material having a size of 50 nm or more. The size of a particle or a flake refers herein to the greatest dimension of a tridimensional shape.

[0132]

[0090] In further preferred embodiments, the partially sulfurized coating material of the electrode precursor of the second aspect of the invention comprises a crystalline phase of the pyrite type. This is particularly the case when the sulfurization step is carried out in the presence of an amount of sulphur of at least twice the amount of M1and M2deposited on the substrate.

[0133]

[0091] In further preferred embodiments, the partially sulfurized coating material of the electrode precursor of the second aspect of the invention comprises a crystalline phase of the pyrrhotite type.

[0134]

[0092] In further preferred embodiments, the partially sulfurized coating material of the electrode precursor of the second aspect of the invention comprises a crystalline phase of the pyrite and a crystalline phase of the pyrrhotite type.

[0135]

[0093] Said electrocatalytically active material for OER is obtained by activation of the electrode precursor of the second aspect of the invention.

[0136]

[0094] Thus, a third aspect of the invention relates to a process of activating a precursor electrode as defined in the second aspect of the invention comprising the circulation of an anodic current through said precursor electrode placed in a basic medium at an electrode potential equal to or higher thanl .30 V vs RHE.

[0137]

[0095] In preferred embodiments, the process of the third aspect of the invention comprises the circulation of an anodic current through said precursor electrode placed in a basic medium at an electrode potential equal to or higher than 1.30 V vs RHE, being said electrode precursor as defined in any of the embodiments disclosed above for the second aspect of the invention.

[0138]

[0096] In other preferred embodiments, the process of the third aspect of the invention comprises the circulation of a current through said precursor electrode placed in a basic medium at an electrode potential equal to or higher than 1 .30 V vs RHE, being said basic medium an aqueous solution of alkaline hydroxide salt, such as potassium hydroxide or sodium hydroxide, with a preferred concentration equal to or higher than 0.1 M.

[0139]

[0097] In other preferred embodiments, the process of the third aspect of the invention is one wherein the electrode precursor is comprised in a closed electric circuit.

[0140]

[0098] In other preferred embodiments, the process of the third aspect of the invention comprises the circulation of a current through said electrode precursor at an electrode potential equal to or higher than 1 .30 V vs RHE in a basic medium for a period of time.

[0141]

[0099] In particular embodiments, the process of the third aspect of the invention provides an electrode comprising an electrocatalytically active material obtainable by the at least partial replacement of S atoms in the partially sulfurized coating material of the electrode precursor of the second aspect of the invention by hydroxy and oxyhydroxy groups.

[0142]

[0100] As mentioned above, the fourth aspect of the invention relates to an electrode obtainable by the method according to the third aspect of the invention.

[0143]

[0101] In preferred embodiments, the fourth aspect of the invention relates to an electrode obtainable by the method according to any particular and preferred embodiment of the third aspect of the invention.

[0144]

[0102] Such electrode comprises an electrocatalytically active material obtainable by the at least partial replacement of S atoms in the partially sulfurized coating material of the electrode precursor of the second aspect of the invention by hydroxy and oxyhydroxy groups. Thus, when M1is Fe, the electrocatalytically active material of the electrode is one having a Raman absorption peak at about 705 cm-1, accounting for the Fe-OOH bond.

[0145]

[0103] In other embodiments, the fourth aspect of the invention relates to an electrode comprising an electrocatalytically active material comprising a compound of formula M1x(i-y)M2(i-x)(i-y)Znand / or a compound of formula M1xM2(i-x)Znwherein:

[0146] M1is selected from the group consisting of Fe and Ni and;

[0147] M2is selected from the group consisting of Ni and Co; when M1is Fe, x is higher than 0 and equal to or lower than 1 ; when M1is Ni, x is 1 ; y is higher than 0 and equal to or lower than 0.2;

[0148] Z is OH and / or OOH; and n is 1 or 2.

[0149]

[0104] In other embodiments, the fourth aspect of the invention relates to an electrode comprising an amorphous phase.

[0150]

[0105] In other embodiments, the fourth aspect of the invention relates to an electrode characterized in that its overpotential to reach a current density of 1 mA per cm2for the OER is lower than 0.5 V; preferably it is lower than 0.45 V or lower than 0.4 V. More preferably, it is lower than 0.35 V. The above overpotential values are provided when the electrode is placed in a basic medium that is preferably an aqueous solution of alkaline hydroxide salt, such as potassium hydroxide or sodium hydroxide, with a preferred concentration equal to or higher than 0.1 M.

[0151]

[0106] In other embodiments, the fourth aspect of the invention relates to an electrode comprising an electrocatalytically active material comprising an amorphous phase of a compound of formula M1X(i-y)M2(i-x)(i-y)Znand / or a compound of formula M1xM2(i-x)Znwherein: M1is selected from the group consisting of Fe and Ni and;

[0152] M2is selected from the group consisting of Ni and Co; when M1is Fe, x is higher than 0 and equal to or lower than 1 ; when M1is Ni, x is 1 ; y is higher than 0 and equal to or lower than 0.2;

[0153] Z is OH and / or OOH; and n is 1 or 2.

[0154]

[0107] In other embodiments, the fourth aspect of the invention relates to an electrode comprising an electrocatalytically active material comprising a compound of formula M1x(i-y)M2(i-x)(i-y)Znand / or a compound of formula M1xM2(i-x)Znwherein:

[0155] M1is selected from the group consisting of Fe and Ni and;

[0156] M2is selected from the group consisting of Ni and Co; when M1is Fe, x is higher than 0 and equal to or lower than 1 ; when M1is Ni, x is 1 ; y is higher than 0 and equal to or lower than 0.2;

[0157] Z is OH and / or OOH; and n is 1 or 2; said electrode being characterized in that its overpotential to reach a current density of 1 mA per cm2for the OER is lower than 0.5 V; preferably it is lower than 0.45

[0158] V or lower than 0.4 V; more preferably, it is lower than 0.35 V, when said electrode is placed in a basic medium that is preferably an aqueous solution of alkaline hydroxide salt, such as potassium hydroxide or sodium hydroxide, with a preferred concentration equal to or higher than 0.1 M.

[0159]

[0108] In other embodiments, the fourth aspect of the invention relates to an electrode comprising an electrocatalytically active material comprising an amorphous phase of a compound of formula M1x(i-y)M2(i-x)(i-y)Znand / or a compound of formula M1xM2(i-x)Znwherein:

[0160] M1is selected from the group consisting of Fe and Ni and;

[0161] M2is selected from the group consisting of Ni and Co; when M1is Fe, x is higher than 0 and equal to or lower than 1 ; when M1is Ni, x is 1 ; y is higher than 0 and equal to or lower than 0.2;

[0162] Z is OH and / or OOH; and n is 1 or 2; said electrode being characterized in that its overpotential to reach a current density of 1 mA per cm2for the OER is lower than 0.5 V; preferably it is lower than 0.45

[0163] V or lower than 0.4 V; more preferably, it is lower than 0.35 V, when said electrode is placed in a basic medium that is preferably an aqueous solution of alkaline hydroxide salt, such as potassium hydroxide or sodium hydroxide, with a preferred concentration equal to or higher than 0.1 M.

[0164]

[0109] In other embodiments, the fourth aspect of the invention relates to an electrode characterized in that it produces a current density of at least 4 mA per cm2, preferably at least 5 mA per cm2; more preferably, at least 6 mA per cm2, when submitted to a potential of +1.75 V vs RHE and placed in a basic medium that is preferably an aqueous solution of alkaline hydroxide salt, such as potassium hydroxide or sodium hydroxide, with a preferred concentration equal to or higher than 0.1 M.

[0165]

[0110] In other embodiments, the fourth aspect of the invention relates to an electrode comprising an electrocatalytically active material comprising a compound of formula M1x(i-y)M2(i-x)(i-y)Znand / or a compound of formula M1xM2(i-x)Znwherein:

[0166] M1is selected from the group consisting of Fe and Ni and;

[0167] M2is selected from the group consisting of Ni and Co; when M1is Fe, x is higher than 0 and equal to or lower than 1 ; when M1is Ni, x is 1 ; y is higher than 0 and equal to or lower than 0.2;

[0168] Z is OH and / or OOH; and n is 1 or 2; said electrode being characterized in that it produces a current density of at least 4 mA per cm2, preferably at least 5 mA per cm2; more preferably, at least 6 mA per cm2, when submitted to a potential of +1 .75 V vs RHE and placed in a basic medium that is preferably an aqueous solution of alkaline hydroxide salt, such as potassium hydroxide or sodium hydroxide, with a preferred concentration equal to or higher than 0.1 M.

[0169]

[0111] In other embodiments, the fourth aspect of the invention relates to an electrode comprising an electrocatalytically active material comprising an amorphous phase of a compound of formula M1x(i-y)M2(i-x)(i-y)Znand / or a compound of formula M1xM2(i-x)Znwherein:

[0170] M1is selected from the group consisting of Fe and Ni and;

[0171] M2is selected from the group consisting of Ni and Co; when M1is Fe, x is higher than 0 and equal to or lower than 1 ; when M1is Ni, x is 1 ; y is higher than 0 and equal to or lower than 0.2;

[0172] Z is OH and / or OOH; and n is 1 or 2; said electrode being characterized in that it produces a current density of at least 4 mA per cm2, preferably at least 5 mA per cm2; more preferably, at least 6 mA per cm2, when submitted to a potential of +1.75 V vs RHE and further characterized in that its overpotential to reach a current density of 1 mA per cm2for the OER is lower than 0.5 V, preferably it is lower than 0.45 V or lower than 0.4 V; more preferably, it is lower than 0.35 V, when said electrode is placed in a basic medium that is preferably an aqueous solution of alkaline hydroxide salt, such as potassium hydroxide or sodium hydroxide, with a preferred concentration equal to or higher than 0.1 M.

[0173]

[0112] In other embodiments, the fourth aspect of the invention relates to an electrode comprising an electrocatalytically active material comprising a compound of formula M1x(i-y)M2(i-x)(i-y)Znand / or a compound of formula M1xM2(i-x)Znwherein:

[0174] M1is selected from the group consisting of Fe and Ni and;

[0175] M2is selected from the group consisting of Ni and Co; when M1is Fe, x is higher than 0 and equal to or lower than 1 ; when M1is Ni, x is 1 ; y is higher than 0 and equal to or lower than 0.2;

[0176] Z is OH and / or OOH; and n is 1 or 2; said electrode being characterized in that it produces a current density of at least 4 mA per cm2, preferably at least 5 mA per cm2; more preferably, at least 6 mA per cm2, when submitted to a potential of +1 .75 V vs RHE and placed in a basic medium that is preferably an aqueous solution of alkaline hydroxide salt, such as potassium hydroxide or sodium hydroxide, with a preferred concentration equal to or higher than 0.1 M.

[0177]

[0113] In other embodiments, the fourth aspect of the invention relates to an electrode comprising an electrocatalytically active material comprising an amorphous phase of a compound of formula M1x(i-y)M2(i-x)(i-y)Znand / or a compound of formula M1xM2(i-x)Znwherein:

[0178] M1is selected from the group consisting of Fe and Ni and;

[0179] M2is selected from the group consisting of Ni and Co; when M1is Fe, x is higher than 0 and equal to or lower than 1 ; when M1is Ni, x is 1 ; y is higher than 0 and equal to or lower than 0.2;

[0180] Z is OH and / or OOH; and n is 1 or 2; said electrode being characterized in that it produces a current density of at least 4 mA per cm2, preferably at least 5 mA per cm2; more preferably, at least 6 mA per cm2, when submitted to a potential of +1.75 V vs RHE and further characterized in that its overpotential to reach a current density of 1 mA per cm2for the OER is lower than 0.5 V, preferably it is lower than 0.45 V or lower than 0.4 V; more preferably, it is lower than 0.35 V, when said electrode is placed in a basic medium that is preferably an aqueous solution of alkaline hydroxide salt, such as potassium hydroxide or sodium hydroxide, with a preferred concentration equal to or higher than 0.1 M.

[0114] Thus, the electrode of the fourth aspect is particularly useful as electrode for the OER associated to alkaline water electrolysis.

[0181]

[0115] The fifth aspect of the invention relates to the use of the electrode precursor according to the second aspect of the invention or the electrode of the fourth aspect of the invention as electrode in an electrocatalytic process; preferably in alkaline water electrolysis; more preferably as anode in alkaline water electrolysis. The anode is the electrode whereby the oxygen evolution reaction takes place.

[0182]

[0116] A method for the production of hydrogen using the electrode precursor according to the second aspect of the invention or the electrode of the fourth aspect of the invention as anode also forms part of the invention. Said method comprises anodically polarizing an electrode precursor according to the second aspect of the invention or the electrode of the fourth aspect of the invention with hydroxide ions, said potential being sufficiently high to produce oxygen gas on the surface of said electrode by oxidation of hydroxide ions.

[0183]

[0117] As mentioned above, the sixth aspect of the invention relates to a device, preferably an alkaline water electrolyser, comprising one or more electrodes selected from the electrodes according to the second aspect of the invention and the electrode according to the fourth aspect of the invention.

[0184]

[0118] In preferred embodiments, the device of the sixth aspect of the invention comprises one or more electrodes selected from the electrodes as defined in any of the particular and preferred embodiments of the second aspect of the invention and the electrode electrodes as defined in any of the particular and preferred embodiments of the fourth aspect of the invention.

[0185]

[0119] Throughout the description and claims the word “comprises" and variations of the word, are not intended to exclude other technical features, additives, components or steps. Furthermore, the word “comprise” encompasses the cases of “consist of” and “consists essentially of”. Additional objects, advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention. The following examples are provided by way of illustration, and they are not intended to be limiting of the present invention.

[0186] EXAMPLES

[0187] The powder x-ray diffraction (PXRD) measurements have been carried out with a Panalytical X’pert Pro X-ray diffractometer. The diffractograms were acquired at a fixed glancing angle of incidence (1.7°), with an angular increment A0=O.O2° maintained for 2 s, and with Cu-Ka radiation beam of mean wavelength A=1.5418 A (Ka1 =1.5406 A, Ka2 =1.5445 A). SEM images have been acquired in a Hitachi S-3000N Scanning Electron Microscope. This equipment is coupled with an EDX analyser BRUKER Quantax.

[0188] Raman spectra have been obtained with a WiTec ALPHA 300AR instrument, using a confocal microscope with different lenses (20x and 100x). The laser excitation wavelength was 532.3 nm with a power of 0.2-2 mW.

[0189] The electrochemical measurements of Examples 3 to 5 have been done using a cell with a 3-electrode configuration, comprising:

[0190] (i) a working electrode (WE). This electrode is the material to investigate; (ii) a counter electrode (CE) consisting of a platinum foil and (iii) a reference electrode. The reference potential of this electrode is well-defined and stable, so it is easy to refer our measurements to the NHE and RHE scales. Those three electrodes are immersed in an electrolyte and connected to a potentiostat AUTOLAB® with a Software Nova® that enables to make the measurements. An Argon flow of 20 seem circulates inside the cell to purge the electrolyte. The electrolytes used in the experiments have been aqueous solutions of KOH, having concentrations of 0.1 M and 1.0 M.

[0191] Example 1 : Preparation of electrode precursors following a process according to the second alternative of the process of the first aspect of the invention

[0192]

[0120] General procedure: The electrodes according to the invention can be prepared according to the following experimental procedure:

[0193] Step (a): providing a mixture of powders of M1and M2such that the molar ratio of M1to M2is x:(1-x)

[0194]

[0121] Iron powder Fe was purchased from Goodfellow 99.0% LS125500 NL. Nickel powder 99.5% was purchased from Goodfellow metals. Cobalt powder was purchased from Johnson Matthey Chemicals LTD 100 MESH 99.9%. From these starting materials, the following mixture of powders of M1and M2wherein the molar ratio of M1to M2is x:(1- x) were prepared by mixing said powders:

[0195] Table 1 Step (b): providing a substrate for electrode;

[0196]

[0122] Different substrates for electrodes have been employed in the preparation of electrodes:

[0197] - glassy carbon, provided by Micro to Nano (High Purity Vitreous Carbon Disc).

[0198] - Iron disk Fe-disk (Thickness: 1 .0 mm or 0.025 mm. Diameter: 15mm.), purchased from Goodfellow Fe- 99.5%.

[0199] Step (c): evaporating the mixture of powders provided in step (a), thus producing a gaseous metal mixture

[0200]

[0123] Thermal evaporation using an Edwards 306 evaporator has been employed to prepare the metallic films. Such device is shown in Figure 1. In this method, an electric current passing through a crucible containing one of the mixtures provided in step (a) heats the material, so that it evaporates. The evaporator was under vacuum (10-5mbar) to avoid the presence of contaminants during thermal evaporation, such as water or oxygen.

[0201] Step (d): depositing the gaseous metal mixture produced in step (c) on the substrate provided in step (b); thereby producing a substrate coated with a layer comprising M1and, optionally, M2;

[0202]

[0124] The thermal evaporation device employed in step (c) also comprises a holder for the substrate of the electrode provided in step (b), whereby the evaporated metal powder condensates to form a thin layer of a metal or a mixture of metals. A layer of 100 nm thick of the metal mixture was grown on the surface of the substrate.

[0203]

[0125] The following mixtures of Table 1 were used to grow a metal layer of 100 nm thick on the surface of glassy carbon: mixtures 1 , 2, 3, 4 and 5. Mixture 5 was also used to grow a metal layer of 100 nm thick on the surface of an iron disk.

[0204]

[0126] Figure 2 shows the EDX spectrum of a metal layer deposited on glassy carbon using Mixture 3 from Table 1 according to this procedure. Table 2 below reports the atomic content of metal layers deposited on the electrode substrate according to this method as measured by EDX spectroscopy using the mixtures of metals of Table 1 above - percentage compositions measured by EDX in the present application are provided with a + / - 3% error margin around the disclosed value:

[0205] Table 2

[0206]

[0127] The results of Table 2 show that the composition of the metal layer deposited during step (d) is substantially the same as the composition of the mixture of powders provided in step (a).

[0207]

[0128] Figure 3 shows SEM images of metal layers deposited on glassy carbon susbtrates using (a) Mixture 2, (b) Mixture 3, (c) Mixture 4 of Table 1.

[0208] Step (ii) contacting the coated substrate produced in (d) or provided in (i) with an atmosphere comprising gaseous sulphur in conditions allowing for sulfurizing at least partially one metal comprised in said coating.

[0209]

[0129] Sulfurization of the metal layers prepared in step (ii) was carried out by thermal treatment in a controlled atmosphere of sulphur. The sample to be sulfurized and the sulphur powder (Merck, 99.99% purity) were placed in a Pyrex® ampoule, which was sealed under vacuum (10'5-1 O'6mbar). The ampoule was placed inside a furnace whose temperature was fixed by a Eurotherm 818P controller. Two K-type thermocouples were placed at both ends of the ampoule to monitor the temperatures. When heating, a temperature gradient was created inside the ampoule. The highest temperature was obtained in the centre of the furnace, where the sample is placed, and the lowest temperature is obtained on the other side of the ampoule. The temperatures used to sulfurize the samples ranged from 180 to 300 °C, creating inside the ampoule a temperature gradient not higher than 40 °C.

[0210]

[0130] During the heat treatment, sulphur melts and evaporates, and a liquid- vapour equilibrium is established in the ampoule. Sulphur vapour pressure is controlled by the cooler temperature of the ampoule, where liquid sulphur condensates. In all cases, the amount of sulphur placed is sufficient to not be the limiting step of the reaction. Therefore, since there is an excess of sulphur in the ampoule, the sulfurization process is controlled by the liquid-gas equilibrium pressure of sulphur (saturated vapour pressure).

[0211]

[0131] After the sulfurization time has elapsed, during the cooling stage, gaseous sulphur condenses on the cold side of the ampoule, which advantageously prevents contamination of the samples with elemental sulphur.

[0212] Example 2: Preparation of electrode precursors following a process according to the first alternative of the process of the first aspect of the invention General procedure: The electrodes according to the invention can be prepared according to the following experimental procedure:

[0213] Step (i): Iron disk Fe-disk(Thickness: 1.0 mm or 0.025 mm. Diameter: 15mm) was purchased from Goodfellow Fe- 99.5% and used as support.

[0214] Step (ii) was carried out as defined above in Example 1 . Optimal sulfurization conditions for iron disk as commercial support were found when a temperature of 2000C was used and a reaction time of 4 hours was employed. This provided the samples with most homogeneity and morphological uniformity. EDX analyses of the resulting samples further confirmed the homogeneity of the elemental composition of the resulting surfaces.

[0215] Comparative Example 1

[0216]

[0132] Iron-Nickel disk Fe (55%)- Ni (45%) disk (Thickness: 0.100 mm. Diameter: 15mm) was purchased from Goodfellow and used as support. Nickel foil Ni-99% (0.5 mm thickness, cur in circular shapes of 15 mm diameters) was purchased from Goodfellow and used as support.

[0217]

[0133] Step (ii) was carried out as defined above in Example 1 .

[0218]

[0134] Various conditions with different parameters (temperature, time of sulfurization, amount of sulphur, thickness of metal layer) have been investigated. Fig. 4 shows:

[0219] (a) photograph of a 15 mm sample of Fe Ni disk sulfurized at 200 °C for 4 h;

[0220] (b) SEM image of a sample of Fe Ni disk sulfurized at 200 °C for 4 h;

[0221] (c) photograph of a 15 mm sample of Fe Ni disk sulfurized at 300 °C for 4 h;

[0222] (d) SEM image of a sample of Fe- Ni disk sulfurized at 300 °C for 4 h;

[0223] (e) photograph of a 15 mm sample of Fe Ni disk sulfurized at 300 °C for 1 h;

[0224] (f) SEM image of a sample of Fe- Ni disk sulfurized at 300 °C for 1 h.

[0225]

[0135] Optimal sulfurization conditions for iron-nickel disk as commercial support were found when a temperature of 2000C was used and a reaction time of 4 hours was employed. This provided the samples with most homogeneity and morphological uniformity. EDX analyses of the resulting samples further confirmed the homogeneity of the elemental composition of the resulting surfaces.

[0226]

[0136] Table 3 below shows the results of EDX analyses measured at three different points of a sample resulting from said sulfurization step when carried out in different conditions for the sulfurization of commercial Fe-Ni disk.

[0227] Table 3

[0228]

[0137] Fig. 5 shows powder X-ray diffraction patterns of sulfurization products of a commercial Fe disk (Example 2) or a deposited layer of Fe on glassy carbon (example

[0229] 1) in varying conditions:

[0230] (a) sulfurization product of a Fe layer on glassy carbon prepared as disclosed above in Example 1 of a thickness of 100 nm carried out at 200 °C for 2 h in the presence of 100 F9 S;

[0231] (b) sulfurization product of a Fe layer on glassy carbon prepared as disclosed above in Example 1of a thickness of 100 nm carried out at 200 °C for 4 h in the presence of 100 F9 S;

[0232] (c) sulfurization product of a Fe layer on glassy carbon prepared as disclosed above in Example 1 of a thickness of 60 nm carried out at 200 °C for 4 h in the presence of 130 F9 S;

[0233] (d) sulfurization product of a Fe layer on glassy carbon prepared as disclosed above in Example 1of a thickness of 60 nm carried out at 200 °C for 7.5 h in the presence of 180 F9 S;

[0234] (e) sulfurization product of a commercial Fe disk carried out at 200 °C for 4 h (Example

[0235] 2) in the presence of 30 mg S;

[0236] (f) sulfurization product of a commercial Fe disk carried out at 200 °C for 4 h in the presence of 120 .g S (Example 2).

[0237] The results of Figure 5 show that, depending on the conditions of sulfurization employed, crystalline phases of pyrite and pyrrhotite form in varying amounts.

[0238]

[0138] Linear Sweep Voltammetry curves for the samples of Fig. 5 (e) (pyrrhotite + Fe) and Fig. 5 (f) (pyrite+Fe) are shown in Figure 6 and reveal that both phases have the same electrocatalytic behaviour, regardless the formed crystalline phase. It is advantageous as there is no need to favour the formation of a specific chalcogenide crystalline phase over the other one. Summary of prepared electrodes

[0239]

[0139] The following electrodes were prepared according to the general procedure described above, using an optional sulfurization step carried out at 200 °C for 4 hours according to the procedure described above for step (ii):

[0240] Table 4

[0241] Electrodes prepared with no sulfurization step Fe Ni disk or Ni foil as electrode support, are provided as comparative Examples.

[0242] Example 3: Activation of electrode precursors

[0243]

[0140] Electrochemical experiments were carried out in a three-electrode cell consisting of: a working electrode (WE), which comprises the material to be tested; a counter electrode (CE), in which a Pt foil of 9 cm2has been placed; and a reference electrode

[0244] (RE) which permits controlling the potential applied to the WE with respect to the RE. In this case the RE selected has been Ag / AgCI (1.0M KNO3), which has an electrode potential of +0.484 V vs N HE. These three electrodes were connected to a potentiostat AUTOLAB ® with a module FRA II and a Software Nova that enables to perform the measurements. The electrodes were immersed in a 0.1 M KOH aqueous electrolyte, and an Ar flow of 20 seem was bubbled through the electrolyte to purge it during the measurements. To perform the OER, positive potentials over 1.23 V vs RHE need to be applied, which means that the materials placed on the WE function as anodes. On the other hand, when performing the HER, potentials below 0.0 V vs RHE are applied, and the WE behaves as the cathode.

[0245]

[0141] Cyclic voltammetry sweeps were carried out to displace S atoms in the electrode precursors and replace with hydroxy and / or oxyhydroxy groups. Figure 7 (a) shows said cyclic voltammetry experiments when carried with the sample c-Fe5sNi45-S, and performed at a scan rate of 5 mV / s. A similar behaviour was obtained with the other electrodes prepared by metal layer deposition. As shown, an equilibrium is reached after the third CV cycle. Figure 7 (b) shows SEM pictures of said material before (top) and after electrocatalysis (bottom). Table 5 below shows the results of EDX analyses measured before and after electrocatalysis and showing a decrease in the content of S together with an increase in the content of O, which clearly suggests that the displacement of S by OH and / or OOH group has occurred:

[0246] Table 5

[0247] Figure 7 (c) shows Raman spectra of / -Fe-S as measured before and after electrocatalysis. Before the EC tests, two clear bands centred around 340 cm-1and 380 cm-1were observed, typical in pyrite and pyrrhotite compounds. After EC, there is a clear shift to a Fe-OOH phase, as revealed by the appearance of two broad bands centred at 390 cm-1and 705 cm-1.

[0248] Example 4: Use of the electrodes in Oxygen Evolution Reaction

[0249]

[0142] The overpotentials required to reach a current density of 1 mA / cm2as well as the value of current density after one hour of operation at a potential of 1 .75 V vs RHE for each of the prepared electrodes was measured by Linear Sweep Voltammetry. Table 6 shows the results of said measurements. Table 6

[0250]

[0143] The results of T able 6 show that the electrodes prepared according to the process of the invention surprisingly exhibit lower overpotentials and higher current densities than electrodes having similar metal compositions but have not been submitted to a sulfurization step.

[0251]

[0144] These results indeed show that sulfurized samples present a better performance than their equivalent material without sulfurising, as the measured current densities are higher. This effect outstands the good electrocatalytic properties of the oxyhydroxide phases formed from the sulfides. Furthermore, the addition of Ni or Co to Fe based electrodes has a positive effect as the current density significantly increases 3-folds in the case of / -FeS upon addition of 15% (mol) nickel. In addition, the comparison of the measured current densities for / -Fe5sNi45-S and / -FessNhs-S suggests that there exists a value of amount of nickel in the material, above which the current density does not increase significantly despite increasing the Ni content. It is advantageous as low quantities of Ni in Fe-Ni mixtures are efficient catalysts with a reduced manufacturing cost, as Ni is typically more expensive than Fe. The above results also show that sulfurized mixtures of Fe-Ni show better results (e.g. a higher current density) than sulfurized Ni. The results of Table 6 also show that electrodes prepared according to the process of the invention are more efficient electrodes than electrodes prepared by sulfurization of commercial electrode substrates, such as Fe Ni disk, Ni foam or Ni foil.

[0252]

[0145] The long-term electrochemical activity of the electrodes was then investigated by performing chronoamperometric measurements at a potential of 1.75 V vs RHE. Figure 8 (a) shows long-term chronoamperometry tests for / -Fe and / -Fe-S, where it can be seen that the current starts to decrease after 1.5-3 h to values close to the glassy carbon current density values (0.5 mA / cm2, see Table 6) due to electrode degradation by detachment of the metal layer from the GC substrate, as was observed by SEM. Surprisingly, curves a and b of Fig. 8 (b) shows that electrodes prepared using mixtures 2 or 3 (mixtures of Fe and Ni) present long-term stability with no fall of the current density to the level of glassy carbon even after a period of 50 h, which suggests that the mixture of Ni and Fe favours the adhesion of the metal layer to the glassy carbon substrate. It was further observed that the electrochemical performance of the electrode was not affected by exposure to air of the electrodes.

[0253]

[0146] Figure 9 shows that two electrodes of the kind / -Fe5sNi45-S prepared in different batches of the method of the invention (i) have a similar composition (Fig. 9 (a)) and provide similar electrochemical behaviour towards Oxygen Evolution Reaction (Fig. 9, (c), (d) and (e)).

[0254]

[0147] The above experiments show that the electrodes prepared according to the invention are particularly suitable for Oxygen Evolution Reaction as they exhibit low overpotential combined with robustness, which makes them suitable as efficient anodes based on earth abundant metals for alkaline water electrolysis.

[0255] Example 5: Use of the electrodes in Hydrogen Evolution Reaction

[0256]

[0148] The three-electrode cell of Example 2 was used applying negative potentials, such that the working electrode works as a cathode. Figure 10 shows the results of chronoamperometric measurements carried out at a potential of - 0.5 V vs RHE when the following electrodes as prepared according to Example 1 were used: c-Fe5sNi45, c- Fe5sNi45-S, / -Ni-onFe, / -Ni-onFe-S, c-Ni.

[0257]

[0149] In addition, the overpotential of these samples at -1 mA / cm2ranges between 0.2 V and 0.3 V, being around 0.20V for c-Ni, / -Ni-onFe, and / -Ni-onFe-S, 0.23 V for c- Fe5sNi45-S and 0.28 V for c-Fe5sNi45. These overpotential values indicate that the Ni- based electrodes (c-Ni, l-Ni-onFe, and l-Ni-onFe-S) show lower overpotentials for the HER than those based on Fe-Ni alloys. Example 6: Preparation of electrode precursors following a process according to the first alternative of the process of the first aspect of the invention using foam substrates.

[0258]

[0150] Foam-like electrodes were prepared by using commercial Ni foam (Recemat ref. Ni-5763.014, porosity: 96%, pore size: 57-63 ppi), and Fe foams from two different suppliers: c-Fe-S-1 samples were prepared by sulfurization of Zopin ZP-IF foams (pore size: 0.4 mm, porosity: 95%), whereas c-Fe-S-2 were obtained by sulfurizing Fe foams from Standford Advanced Materials (porosity: 85-95%, pore size: 80 ppi). Sulfurization of Ni and Fe foams was done in Pyrex ampoules sealed under vacuum after placing elemental sulphur (Merck, 99.99% purity) and the metal foams inside them (according to the procedure of step (ii) of Example 1). Then, the ampoules were heated in a tube furnace at 200 °C for 4 h.

[0259]

[0151] Different characterizations of the electrochemical performances of foam-like electrodes were done. First, laboratory experiments were accomplished in a three- electrodes electrochemical cell. The reference electrode was an Ag / AgCI filled with 1 M KNO3 from Methrom, the counter electrode was a Ni foam (Recemat ref. Ni-5763.014) with apparent areas 2-3 times higher than those of the corresponding working electrodes. Different working electrodes were tested (pure and sulfurized Ni foam as well as c-Fe-S- 1 and c-Fe-S-2 foams). The electrolyte was 6M KOH and the temperature 60 °C. Ar was bubbled through the electrolyte during the experiments. Chronoamperometry curves of the different electrodes were recorded at a constant electrode potential of 1.7 V (vs. RHE) during more than 20h. A chronoamperometric curve corresponding to an anode comprising c-Fe-S-1 is shown in Figure 13. In that curve, original raw data (absolute electrochemical currents) are shown. Values recorded at 6h and 20h were compared for different samples, by considering the apparent areas (geometric areas) of the electrodes immersed in the electrolytes, as shown in Table 7.

[0260] Table 7: Apparent current density (A / cm2) in function of time during chronoamperometry measurements

[0261]

[0152] It can be concluded from Table 7 that sulfurized Fe foams presented a better electrochemical current density and a better stability than commercial and sulfurized Ni foams. It must be noticed that pure Fe foams were also tested, but results are not comparable, since strong corrosion was observed in that case and intense Fe dissolution in the electrolyte was observed, as determined by Inductively Coupled Plasma (ICP) analyses. However, sulfurized Fe foams were very stable and presented very weak Fe dissolution in the electrolyte, as confirmed by ICP analyses.

[0153] Next, electrochemical characterizations of the metallic foam anodes were also done in a two electrodes zero-gap electrochemical cell. In that case, Ni foams (Recemat ref. Ni-5763.014) were used as cathodes and a Zirfon membrane was placed between the two electrodes, which were in close contact with it. The apparent area of the electrodes was 9.8 cm2. The electrolyte used was 6M KOH at a temperature of 70 °C and it was continuously circulating through the cell using a pump. Polarization curves were recorded by measuring the cell voltage at different current densities after an initial activation consisting in circulating through the cell an electrochemical current of 1 .6 A (about 160 mA / cm2of current density) during 22h. The results obtained for the Ni foam and the two sulfurized Fe foams are shown in Figure 14. Again, it can be clearly seen that Fe-S foams present better electrochemical performances than commercial Ni foam when used as anodes in alkaline electrochemical cells.

Claims

CLAIMS1. Process for making a precursor of an electrode for alkaline water electrolysis, said electrode comprising a precursor of an electrocatalytically active material comprising a compound of formula M1x(i-y)M2(i-X)(i-y)S and / or a compound of formula M1XM2(1-X)S2 wherein y is higher than 0 and equal to or lower than 0.2; said process comprising the steps of:(i) in a first alternative of step (i) providing a substrate for electrode comprising an outer layer which comprises a compound of formula M1XM2(1.X) wherein:M1is Fe and M2is Co; x is higher than 0 and equal to or lower than 1 ; or, in a second alternative of step (i),(a) providing a mixture of powders of M1and M2such that the molar ratio of M1to M2is x:(1-x) wherein M1is selected from the group consisting of Fe and Ni;M2is selected from the group consisting of Ni and Co; when M1is Fe, x is higher than 0 and lower than 1 ; and when M1is Ni, x is 1 ;(b) providing a substrate for electrode;(c) evaporating the mixture of powders provided in step (a), thus producing a gaseous metal mixture,(d) depositing the gaseous metal mixture produced in step (c) on the substrate provided in step (b); thereby producing a substrate for electrode comprising an outer layer which comprises a compound of formula M1XM2(1.X);(ii) contacting the substrate provided in (i) with an atmosphere comprising gaseous sulphur in conditions allowing for sulfurizing at least partially one metal comprised in said coating.

2. The process according to claim 1 wherein the first alternative of step (i) comprises providing a substrate for electrode comprising an outer layer which comprises a compound of formula M1XM2(1.X) that is substantially free of nickel.

3. The process according to any one of claims 1 to 2 wherein the first alternative of step (i) comprises providing a substrate for electrode comprising an outer layer which comprises a compound of formula M1XM2(1.X) wherein M1is Fe and x is 1 ; preferably said substrate is iron foam.

4. The process according to claim 1 wherein M1is selected from the group consisting of Fe and Ni and;M2is selected from the group consisting of Ni and Co; when M1is Fe, x is higher than 0 and lower than 1 ; when M1is Ni, x is 1 ; y is higher than 0 and equal to or lower than 0.2; and wherein step (i) comprises:(a) providing a mixture of powders of M1and M2such that the molar ratio of M1to M2is x:(1-x);(b) providing a substrate for electrode;(c) evaporating the mixture of powders provided in step (a), thus producing a gaseous metal mixture,(d) depositing the gaseous metal mixture produced in step (c) on the substrate provided in step (b).

5. The process according to claim 4 wherein x is equal to or higher than 0.50 and lower than 1 when M1is Fe; preferably x is selected from 0.55 and 0.85 when M1is Fe.

6. The process according to any one of claims 4 to 5 wherein the mixture of powders provided in step (a) is selected from the group consisting of:Ni powder; a mixture of Fe powder and Ni powder whereby the molar ratio of Fe to Ni is 55:45; a mixture of Fe powder and Ni powder whereby the molar ratio of Fe to Ni is 85:15; and a mixture of Fe powder and Co powder whereby the molar ratio of Fe to Co is 55:45.

7. The process according to any one of claims 4 to 6 wherein the substrate for electrode of step (b) is selected from the group consisting of metal mesh, metal foam, metal foil, metal felt, metal disk, carbon paper, carbon felt, transparent conducting oxides, glassy carbon and carbon cloth; preferably, the substrate for electrode is selected from the group consisting of glassy carbon, steel and iron disk; more preferably, the substrate for electrode is selected from the group consisting of glassy carbon and iron disk.

8. The process according to any one of claims 4 to 7 wherein step (c) comprises heating said mixture of powders by means of an electric current.

9. The process according to any one of claims 4 to 8 wherein step (d) comprises condensing the gaseous mixture produced in step (c) on the surface of the substrate; thereby producing a substrate coated with an outer layer comprising M1and further comprising M2when x is not 1 , said layer having preferably a thickness of between 50 nm and 10 .m; more preferably of 100 nm.

10. The process according to any one of claims 8 to 9 wherein steps (c) and (d) are carried out under vacuum; preferably at a pressure of equal to or less than 10'4mbar.

11. The process according to any one of claims 1 to 10 wherein step (ii) comprises heating at a temperature of between 180 °C and 300 °C for a period of time of between 1 hour and 20 hours and under vacuum the substrate provided in (i) in the presence of solid sulphur; preferably at a temperature of 200 °C and / or at a pressure prior to sulphur vaporization of between 10'5and 10'6mbar and / or for a period of time of 4 hours.

12. Electrode precursor obtainable by the method according to any of claims 1 to 11.

13. Electrode precursor according to claim 12 wherein the at least partially sulfurized coating material does not essentially comprise particles or flakes of said material.

14. Process of activating an electrode precursor as defined in claim 12 comprising the circulation of a current through said precursor electrode placed in a basic medium at an electrode potential equal to or higher than 1.30 V vs RHE; said basic medium being preferably an aqueous solution of alkaline hydroxide salt, such as potassium hydroxide, with a preferred concentration equal to or higher than 0.1 M.

15. Electrode obtainable by the method of claim 14.

16. Use of the electrode precursor according to any one of claims 12 to 13 or the electrode of claim 15 as electrode in an electrocatalytic process; preferably alkaline water electrolysis.

17. Device, such as an alkaline water electrolyser, comprising one or more electrodes selected from the electrodes according to any one of claims 12 to 13 and the electrode of claim 15.

Images