RU2799965C2 - Use of sulfide compositions - Google Patents

Abstract

FIELD: chemical industry.

SUBSTANCE: present invention relates to the use of a composition of formula I

where M denotes one or more elements characterized in the ionic state by an effective ionic radius in the range of 70-92 pm, and is a number in the range of 2.5≤a≤3.5, b is a number in the range 1.5≤b≤ 5.0, s is a number in the range 0.0≤c≤2.0, d is a number in the range 0.0≤d≤4.0 where the sum of a, b and c is in the range 5≤a+b+c≤8 and where ≥90 wt.% of the composition is in the pentlandite phase, for electrocatalytic water splitting. The invention also relates to an electrode.

EFFECT: use of the invention allows the use of a higher current density, and the composition also has better resistance to sulphur compounds and better start-stop properties compared to materials of the prior art.

9 cl, 25 dwg, 3 tbl, 14 ex

Description

The present invention relates to the use of a sulfide composition for electrocatalytic water splitting.

Background of the Invention

Hydrogen is mainly produced from natural gas by steam reforming of methane. Other fossil sources are other hydrocarbons and coal (see, for example, John A. Turner, Sustainable Hydrogen Production, Science 2004, Vol 305, 972-974). Ecological source materials for hydrogen production are biomass or water. With water as the starting material, various production processes are carried out, such as electrolysis, thermolysis, and photoelectrolysis. There are two different types of moderate temperature electrolyzers: alkaline and proton exchange membrane (PEM) electrolyzers (see, for example, Jamie D. Holladay, An overview of hydrogen production technologies, Catalysis Today 2009, 139, 244 - 260).

In commercial PEM electrolyzers, platinum is used as an electrocatalyst for the hydrogen evolution reaction (HER) (see, for example, Peter C.K. Vesborg, Recent Development in Hydrogen Evolution Reaction Catalysts and Their Practical Implementation, The Journal of Physical Chemistry Letters 2015, 6, 951-957).

For example, European patent document EP3222752 A1 discloses a method for generating hydrogen and oxygen from water, which includes providing a membrane electrode assembly containing an anode electrolyzer catalyst capable of catalyzing an oxygen evolution reaction at potentials exceeding +1.23 V relative to a reversible hydrogen electrode, while the anode electrolyzer catalyst contains nanostructured whiskers having several alternating layers containing Pt and Ir, respectively, in any order, membrane assembly leg electrode, in addition, contains a cathode; contacting the water with the anode catalyst of the cell; and providing an electrical potential with sufficient current through the membrane electrode assembly to convert at least a portion of the water to hydrogen and oxygen at the cathode and anode, respectively.

Efforts have already been made to provide platinum-free electrocatalysts for HER. In particular, metal sulfides, metal carbides, metal selenides, metal nitrides, and metal phosphides exhibit properties of interest for this reaction (see, for example, Xiaoxin Zou, Noble metal-free hydrogen evolution catalysts for water splitting, Chem. Soc. Rev. 2015, 44, 5148-5180).

International application WO 2017/062736 A1 discloses materials based on structured molybdenum disulfide for electrocatalytic applications.

WO2018/098451 A1 discloses catalysts comprising transition metal chalcogenide films for a hydrogen evolution reaction.

US Patent Document US2015/0259810 A1 claims a device containing a hydrogen evolution reaction catalyst. The catalyst includes at least one component selected from the group consisting of transition metal phosphides, first row transition metal sulfides and transition metal arsenides in the form of nanoparticles and, in particular, CoP.

Chinese Patent Document CN105132941 A claims a molybdenum diselenide/carbon black composite material for hydrogen evolution electrocatalysis and a method for producing the same.

WO2015/021019 A1 claims a catalyst for providing a hydrogen evolution reaction, the catalyst comprising a metal nitride having the formula M' _x M'' _y N _z , where M' is selected from the group consisting of Ag, Al, Ca, Co, Cr, Cu, Fe, Ga, In, Li, Mg, Mn, Na, Ni, Sc, Ti, V, Y, Zn, and mixtures thereof; where M'' is selected from the group consisting of Hf, Mo, Nb, Re, Ru, Ta, W, Zr and mixtures thereof; where x is a number from 0 to 1; where y is a number from 1 to 2; where z is a number greater than 1.8 and less than 2.2; and where the metal nitride contains a hexagonal lattice with a four-layer packing sequence, which includes two formula units of a mixed close-packed structure with alternating layers of metals M'' in trigonal prismatic coordination, and metals M' or M' and M'' in octahedral coordination.

European patent document EP2377971 A1 discloses electrocatalysts for the reduction of protons with the formation of H ₂ consisting of amorphous films of transition metal sulfides or solids with activity at all pH values. In addition, MoS ₂ and WS ₂ are mentioned as possible transition metal sulfides.

Iron-nickel pentlandite has proven to be an interesting composition for HER (see Konkena et al., "Pentlandite rocks as sustainable and stable efficient electrocatalysts for hydrogen generation", Nature Communication 2016, 7, Article number 12269).

Klein et al. ((Table A2) "Chemical composition of Pentlandite of ODP holes 209-1268A, 209-1270D, 209-1271 B and 209-1274A"; 2010), Schrocke et al. ("Mineralogie: Ein Lehrbuch Auf Systematischer Grundlage", page 137, 1981), in Mineral Data Publishing ("Pentlandite", 2001) and in Rajamani et al. ("Crystal Chemistry of Natural Pentlandites", Canadian Mineralogist, Vol. 12, page 178-187, 1973) discloses naturally occurring pentlandites.

Knop et al. ("Chalcogenides of the Transition Elements", Can. J. Chem., Vol. 39, 1961) synthetically prepared FeCoNiS pentlandites are disclosed.

Purpose and summary of the invention

The compositions for electrocatalytic water splitting disclosed in the prior art do not meet all the requirements for catalytic performance, resistance to sulfur compounds, stability of catalytic activity and/or start-stop properties. Thus, the aim of the present invention is to provide a novel application for an HER composition having higher current density, better resistance to sulfur compounds, and better start-stop properties compared to prior art materials.

According to one aspect, the present invention relates to the use of a composition of the formula

Where

M denotes one or more elements characterized in the ionic state by an effective ionic radius in the range of 70-92 pm,

a is a number in the range 2.5≤a≤3.5, more preferably 2.7≤a≤3.3,

b is a number in the range 1.5≤b≤5.0, more preferably 1.5≤b≤4.0, most preferably 2.5≤b≤3.5,

c is a number in the range 0.0≤c≤2.0, more preferably 0.0≤c≤1.0,

d is a number in the range 0.0≤d≤4.0, more preferably 0.0≤d≤1.0,

where the sum of a, b and c is in the range 5≤a+b+c≤8,

and where ≥90 wt. % of the composition is in the pentlandite phase, for electrocatalytic splitting of water, preferably for the hydrogen evolution reaction.

Brief description of graphic materials

In FIG. 1 shows linear potential sweep voltammetry charts obtained in the scan range from 0 to -0.65 V compared to RHE (from the English - reversible hydrogen electrode - reversible hydrogen electrode) in 0.5 M sulfuric acid at room temperature.

In FIG. 2 shows the results of voltage controlled coulometry measurements at -0.5 V compared to RHE at 20° C. and 60° C. for electrodes based on Pt (comparative example 6) and Fe ₃ Co ₃ Ni ₃ S ₈ (example 1) for 20 hours.

In FIG. 3 shows linear potential sweep voltammetry charts performed at 20°C (A) and 60°C (B) before and after a 20 hour electrolysis for platinum (comparative example 6) and Fe ₃ Co ₃ Ni ₃ S ₈ (example 1).

In FIG. 4 shows start-stop potential controlled coulometry measured at 60° C. and -304 mV compared to RHE over 72 hours for Fe ₃ Co ₃ Ni ₃ S ₈ (example 1) and platinum (comparative example 6).

In FIG. 5 shows controlled potential coulometry with and without H ₂ S catalyst poison measured at 20° C. and -350 mV compared to RHE over 20 hours.

In FIG. 6 shows linear sweep voltammetry plots performed in the presence and absence of H ₂ S catalyst poison measured at 20° C. and -350 mV compared to RHE before and after 20 hours of electrolysis.

In FIG. 7 shows X-ray powder diffraction (diffraction patterns, (Cu-Kα radiation)) compositions used in accordance with the present invention and comparative examples.

In FIG. 8 shows a cross section of an electrochemical cell used for tests carried out with compositions used in accordance with the present invention and comparative examples.

In FIG. 9 shows start-stop controlled potential coulometry measured at 60° C. and -350 mV compared to RHE over 72 hours.

In FIG. 10 shows controlled potential coulometry with and without H ₂ S catalyst poison measured at 20° C. and -350 mV compared to RHE over 20 hours.

In FIG. 11 shows linear sweep voltammetry plots performed in the presence and absence of H ₂ S catalyst poison measured at 20° C. and -350 mV compared to RHE before and after a 20 hour electrolysis.

In FIG. 12 shows controlled potential coulometry with and without H ₂ S catalyst poison measured at 20° C. and -350 mV compared to RHE over 20 hours.

In FIG. 13 shows linear sweep voltammetry plots performed in the presence and absence of H ₂ S catalyst poison measured at 20° C. and -350 mV compared to RHE before and after a 20 hour electrolysis.

In FIG. 14 shows controlled potential coulometry with and without H ₂ S catalyst poison measured at 20° C. and -350 mV compared to RHE over 20 hours.

In FIG. 15 shows linear sweep voltammetry plots performed in the presence and absence of H ₂ S catalyst poison measured at 20° C. and -350 mV compared to RHE before and after 20 hours of electrolysis.

In FIG. 16 shows linear sweep voltammetry plots performed in the presence and absence of H ₂ S catalyst poison measured at 20° C. and -350 mV compared to RHE before and after 20 hours of electrolysis.

In FIG. 17 shows start-stop controlled potential coulometry measured at 60°C and -350 mV compared to RHE over 72 hours.

In FIG. 18 shows controlled potential coulometry with and without H ₂ S catalyst poison measured at 20° C. and -350 mV compared to RHE over 20 hours.

In FIG. 19 shows linear sweep voltammetry plots performed in the presence and absence of H ₂ S catalyst poison measured at 20° C. and -350 mV compared to RHE before and after 20 hours of electrolysis.

In FIG. 20 shows controlled potential coulometry with and without H ₂ S catalyst poison measured at 20° C. and -350 mV compared to RHE over 20 hours.

In FIG. 21 shows linear sweep voltammetry plots performed in the presence and absence of H ₂ S catalyst poison measured at 20° C. and -350 mV compared to RHE before and after 20 hours of electrolysis.

In FIG. 22 shows controlled potential coulometry with and without H ₂ S catalyst poison measured at 20° C. and -350 mV compared to RHE over 20 hours.

In FIG. 23 shows linear sweep voltammetry plots performed in the presence and absence of H ₂ S catalyst poison measured at 20° C. and -350 mV compared to RHE before and after 20 hours of electrolysis.

In FIG. 24 shows linear sweep voltammetry plots performed in the presence and absence of H ₂ S catalyst poison measured at 20° C. and -350 mV compared to RHE before and after 20 hours of electrolysis.

In FIG. 25 shows linear sweep voltammetry plots performed in the presence and absence of H ₂ S catalyst poison measured at 20° C. and -350 mV compared to RHE before and after 20 hours of electrolysis.

Detailed disclosure of the invention

Surprisingly, due to the incorporation of Co into the Fe,Ni-pentlandite, the compositions used in accordance with the present invention show a lower overvoltage for HER compared to prior art pentlandite or other non-platinum group metal (PGM) materials. It has also been found that additional metals having an effective ionic radius comparable to those of Fe ²⁺ , Ni ²⁺ or Co ²⁺ can be added to the composition. In addition, it was unexpectedly found that although Se ^2- has a different ionic radius compared to S ^2- (198 pm vs 184 pm), it is possible to replace up to 50% of S with Se without losing the integrity of the pentlandite phase molecular structure, electrocatalytic activity or stability.

The effective ionic radius of metal ions can be determined according to the method disclosed in Hollemann, Wieberg, Lehrbuch der Anorganischen Chemie, Verlag: De Gruyter; 102th edition (2007).

The compositions used in accordance with the present invention show higher current densities than platinum-based materials after several hours of electrolysis and do not lose significant activity during hydrogen production. In addition, the electrodes obtained from the compositions used in accordance with the present invention have significantly improved start-stop characteristics compared to platinum electrodes. Finally, the compositions used in accordance with the present invention are resistant to sulfur poisoning.

The crystalline phase of unmodified pentlandite is disclosed, for example, in A. Pearson, M. Buerger, Am. mineral. 1956, 41, 804-805. In the composition used in accordance with the present invention, ≥90 wt. % of the composition is in the pentlandite phase.

According to one embodiment of the present invention, M is not present in the formula I composition.

According to this embodiment, the composition may be selected from the group consisting of Fe ₃ Ni ₃ Co ₃ S ₈ , Fe ₁ Co ₅ Ni ₃ S ₈ , Fe ₂ Co ₄ Ni ₃ S ₈ , Fe ₄ Co ₂ Ni ₃ S ₈ , Fe ₃ Ni ₃ Co ₃ S ₄ Se ₄ , Fe ₃ Ni ₃ Co ₃ S ₇ Se and mixtures thereof.

According to another embodiment of the present invention, M is present in the composition of formula I.

M preferably has an effective ionic radius comparable to that of Fe ²⁺ , Ni ²⁺ or Co ²⁺ .

In particular, M may be selected from the group consisting of Nb, Cu, Mn or Cr and mixtures thereof, more preferably Mn, Cr and mixtures thereof, most preferably Cr.

In particular, the composition may be selected from the group consisting of CrFe _2.5 Co _2.5 Ni ₃ S ₈ , MnFe _2.5 Co _2.5 Ni ₃ S ₈ , Mn _0.25 Fe _2.875 Co _2.875 Ni ₃ S ₈ ,

NbFe ₂ , ₅ Co _2.5 Ni ₃ S ₈ and mixtures thereof.

The composition used in accordance with the present invention preferably has a hydrogen evolution reaction (HER) surge of 328 mV or less, preferably 300 mV or less, at a current density of -10 mA/cm ² , in particular in accordance with the test conditions detailed below.

According to a further aspect, there is provided an electrode containing a composition used in accordance with the present invention.

The electrode according to the present invention is characterized by a hydrogen evolution reaction (HER) surge of 328 mV or less, preferably 300 mV or less, at a current density of -10 mA/cm ² , in particular according to the test conditions detailed below.

According to the following aspect, the present invention relates to various processes for obtaining a composition used in accordance with the present invention. The first process (hereinafter - process No. 1) is characterized by the fact that the compositions are synthesized by thermal synthesis in the absence of oxygen in a vacuum. The second process (process No. 2) is characterized by the fact that the compositions are synthesized by thermal synthesis in the absence of oxygen in an inert gas. For selenium-free compositions used in accordance with the present invention, two additional processes are suitable. In one of these inert gas processes, compositions are synthesized using a co-precipitation process and a post-treatment step (Process #3). Another process (Process No. 4) is characterized in that the compositions are synthesized using a sol-gel method and an additional processing step.

Thermal synthesis in the absence of oxygen to obtain compositions used in accordance with the present invention is a method similar to the thermal synthesis route disclosed in Piontek et al. ACS Catalysis 2018, 8, 987-996.

More specifically, thermal synthesis in the absence of oxygen in a vacuum (process #1) includes the following steps:

a) obtaining a homogeneous mixture of Fe powder, Ni powder, Co powder, S powder and optionally Se powder and/or M powder by mixing or grinding; while the average particle size of the final mixture is preferably below 100 microns;

b) transferring the mixture obtained in step a) into a closed vessel and evacuating the air to a vacuum in the range from 30 mbar to 10 ^-15 mbar, preferably from 1 mbar to 10 ^-7 mbar;

c) heating the closed vessel at a rate of 0.1-20 K min ^-1 in the temperature range of 600-1100°C and cooling for 1-120 hours, preferably 700-1100°C for 1-10 hours at a rate of 2-5 K min ^-1 , more preferably with two cooling temperatures - the first temperature of 600-800°C for 3-10 hours and the second temperature of 900-1100° C for 10-20 hours - at a rate of 2-5 K min ^-1 , most preferably with two cooling temperatures - the first temperature of 700°C for 3 hours and the second temperature of 1000°C for 10 hours.

More specifically, thermal synthesis in the absence of oxygen in an inert gas (process #2) includes the following steps:

a) obtaining a homogeneous mixture of Fe powder, Ni powder, Co powder, S powder and optionally Se powder and/or M powder by mixing or grinding; while the average particle size of the final mixture is preferably below 100 microns;

b) transferring the mixture obtained in step a) into a closed vessel and replacing the air with an inert gas; wherein the inert gas may be nitrogen, carbon dioxide, neon, argon, krypton, xenon or a mixture thereof, preferably nitrogen, carbon dioxide and argon, more preferably nitrogen and argon, most preferably nitrogen;

c) heating the closed vessel at a rate of 0.1-20 K min ^-1 in the temperature range of 600-1100°C and cooling for 1-120 hours, preferably 700-1100°C for 1-10 hours at a rate of 2-5 K min ^-1 , more preferably with two cooling temperatures - the first temperature of 600-800°C for 3-10 hours and the second temperature of 900-1100° C for 10-20 hours - at a rate of 2-5 K min ^-1 , most preferably with two cooling temperatures - the first temperature of 700°C for 3 hours and the second temperature of 1000°C for 10 hours.

More specifically, the co-deposition method with post-treatment step (Process No. 3) includes the following steps:

a) obtaining a solution of an iron salt, a nickel salt, a cobalt salt and optionally one or more salts of M defined in a compound of formula I and optionally an inorganic acid in water;

b) adding one or more sources of sulfide in water, optionally with an inorganic base, over 1 to 120 minutes, preferably 10 to 60 minutes;

c) filtering and washing the precipitate with water;

d) drying in the temperature range 60-120°C for 1-120 hours, preferably 80-100°C for 3-15 hours, or optional freeze-drying;

e) heating the precipitate in the temperature range of 200-500°C at a rate of 0.1-20 K minute ^-1 for 1-120 hours, preferably 250-400°C, for 2-10 hours at a rate of 2-5 K minute ^-1 , in an atmosphere of hydrogen and hydrogen sulfide;

f) and optionally heating the sample for 0.5-20 hours, preferably 2-5 hours, in an atmosphere of hydrogen, optionally diluted with an inert gas such as N ₂ or Ar.

In this process, a suitable salt of iron, nickel, cobalt and M includes, for example, nitrates, oxides, hydroxides, carbonates, sulfates, acetates, halides and mixtures thereof, preferably nitrates, sulfates, acetates and mixtures thereof, most preferably nitrates.

In this process, the inorganic acid in step a) includes the corresponding inorganic acids, for example nitric acid, sulfuric acid, hydrochloric acid and mixtures thereof, preferably nitric acid, sulfuric acid and mixtures thereof, most preferably nitric acid.

In this process, a suitable sulfide source includes sulfide salts such as sodium sulfide, sodium hydrosulfide, ammonium sulfide, ammonium hydrosulfide, potassium sulfide, potassium hydrosulfide and hydrogen sulfide, preferably sodium sulfide, hydrogen sulfide and ammonium sulfide, more preferably sodium sulfide and ammonium sulfide, most preferably sodium sulfide.

In this process, the inorganic base in step b) includes a suitable inorganic base such as sodium hydroxide, sodium oxide, ammonia, potassium oxide, potassium hydroxide, preferably sodium hydroxide, sodium oxide and ammonia, more preferably sodium hydroxide and ammonia, most preferably sodium hydroxide.

The mixture of hydrogen and hydrogen sulfide may have a ratio of 95:5 to 0:100, preferably 90:10 to 50:50, more preferably 90:10 to 80:20, eg 85:15. Such a mixture can be diluted with an inert gas such as argon, nitrogen.

The sol-gel method with an additional processing stage (process No. 4) includes the following stages:

a) obtaining a solution of an iron salt, a nickel salt, a cobalt salt and optionally one or more salts of M defined in a compound of formula I and one or more polymeric precursors and optionally one or more complexing agents in water;

b) optionally adding an inorganic acid to the solution obtained in step a);

c) heating the solution at 90°C for 1-120 hours until a viscous gel is formed;

d) calcining the gel in the temperature range of 300-700°C for 1-120 hours, preferably 400-500°C for 2-20 hours, in air;

e) heating the mixed oxide in the temperature range of 200-500°C for 1-120 hours, preferably 250-400°C for 2-10 hours in an atmosphere of hydrogen and hydrogen sulfide;

f) and optionally heating the sample for 0.5-20 hours, preferably 2-5 hours, in an atmosphere of hydrogen, optionally diluted with an inert gas such as N ₂ or Ar.

In this process, the complexing agent can also serve as a polymeric precursor.

In this process, suitable polymeric precursor includes polycarboxylic acids, hydroxycarboxylic acids, polyhydric alcohols and mixtures thereof, preferably polyhydric alcohols, polycarboxylic acids and mixtures thereof, more preferably polycarboxylic acids, most preferably citric acid.

In this process, a suitable complexing agent includes organic compounds such as organic acids, ketones, aldehydes, alcohols, amines and mixtures thereof, preferably polybasic carboxylic acids, more preferably citric acid and oxalic acid, and most preferably citric acid.

In this process, a suitable salt of iron, nickel, cobalt and M includes salts of these elements, for example, nitrates, oxides, hydroxides, carbonates, sulfates, acetates, halides and mixtures thereof, preferably nitrates, sulfates, acetates and mixtures thereof, most preferably nitrates.

In this process, the inorganic acid in step a) includes suitable inorganic acids, for example nitric acid, sulfuric acid, hydrochloric acid and mixtures thereof, preferably nitric acid, sulfuric acid and mixtures thereof, most preferably nitric acid.

Examples

Example 1 (according to process #1)

Fe ₃ Co3Ni ₃ S ₈

Iron powder (0.43 g, Sigma-Aldrich, ≥99 wt%), cobalt powder (0.46 g, ABCR, 99.8 wt%), nickel powder (0.45 g, ABCR, 99.9 wt%) and sulfur (0.66 g, Sigma-Aldrich, 99.5 wt% - 100.5 wt%) were mixed together for 1 0 minutes to obtain a visually homogeneous mixture of elements. A homogeneous mixture of elements was filled into a quartz ampoule (diameter 10 mm, total volume 15 ml), from which air was pumped out at a pressure below 4×10 ^-2 mbar for 16 hours, and then sealed in vacuum. The sample was placed in an oven and heated to 700° C. at a heating rate of 4.5 K min ^-1 . After keeping the temperature isotherm for 3 hours, so that the sulfur reacted with the mixture of metals without damaging the vessel, the temperature was raised to 1000°C (heating rate 3.33 K min ^-1 ) to enhance diffusion. The sample was then kept at this temperature for 10 hours. After that, the sample was cooled down to room temperature.

Example 2-10

The following compositions were synthesized similar to the process disclosed in Example 1.

Example 11 (according to process #2)

Fe ₃ Co ₃ Ni ₃ S ₈

Iron powder (0.43 g, Sigma-Aldrich, ≥99 wt%), cobalt powder (0.46 g, ABCR, 99.8 wt%), nickel powder (0.45 g, ABCR, 99.9 wt%) and sulfur (0.66 g, Sigma-Aldrich, 99.5 wt% - 100.5 wt%) were mixed together for 1 0 minutes to obtain a visually homogeneous mixture of elements. A homogeneous mixture of elements was filled into a quartz ampoule (diameter 10 mm, total volume 15 ml), from which air was pumped out at a pressure below 4×10 ^-2 mbar, filled with argon, and then sealed in a static argon atmosphere. The sample was placed in an oven and heated to 700° C. at a heating rate of 4.5 K min ^-1 . After keeping the temperature isotherm for 3 hours, the temperature was raised to 1000° C. (heating rate 3.33 K min ^-1 ). The sample was then kept at this temperature for 10 hours. After that, the sample was cooled down to room temperature.

Example 12 (according to process #3)

Fe ₃ Co ₃ Ni ₃ S ₈

Iron(III) nitrate (Fe(NO ₃ ) ₃ ⋅9H ₂ O, 1.35 g), nickel(II) nitrate (Ni(NO ₃ ) ₂ ⋅6H ₂ O, 0.97 g) and cobalt(II) nitrate (Co(NO ₃ ) ₂ ⋅6H ₂ O, 0.97 g) were dissolved in 300 ml of water. In a separate container, sodium sulfide (Na ₂ S⋅9H ₂ O, 4.80 g) was dissolved in 200 ml of water. The sulfide solution was added to the nitrate solution over 30 minutes with vigorous agitation, which resulted in the formation of a black precipitate. The black precipitate was filtered off and washed with 200 ml of water. The precipitate was then freeze dried. Then, in a stream of N ₂ black powder was heated to 40°C (10 K minute ^-1 ) and kept at this temperature for 10 minutes. Thereafter, the temperature was raised to 300°C (10 K min ^-1 ) using a stream of hydrogen (85 vol.%)/hydrogen sulfide (15 vol.%). After 4 hours under these conditions, pure hydrogen was supplied to the sample for 4 consecutive hours. The reaction was quickly stopped by supplying a cold (25°C) flow of nitrogen gas. Throughout the experiment, the gas flow was adjusted to 40 ml min ^-1 and kept constant.

Example 13 (according to process #3)

Fe ₃ Co ₃ Ni ₃ S ₈

Iron(III) nitrate (Fe(NO ₃ ) ₃ ⋅9H ₂ O, 1.35 g), nickel(II) nitrate (Ni(NO ₃ ) ₂ ⋅6H ₂ O, 0.97 g) and cobalt(II) nitrate (Co(NO ₃ ) ₂ ⋅6H ₂ O, 0.97 g) were dissolved in 300 ml of water. In a separate container was dissolved ammonium sulfide ((NH ₄ ) ₂ S (20 wt.% in H ₂ O), 20 mmol, 6.81 g) in 200 ml of water. The sulfide solution was added to the nitrate solution over 30 minutes with vigorous agitation, which resulted in the formation of a black precipitate. The black precipitate was filtered off and washed with 200 ml of water. The precipitate was then freeze dried. Then, in a stream of N ₂ black powder was heated to 40°C (10 K minute ^-1 ) and kept at this temperature for 10 minutes. Thereafter, the temperature was raised to 300°C (10 K min ^-1 ) using a stream of hydrogen (85 vol.%)/hydrogen sulfide (15 vol.%). After 4 hours under these conditions, pure hydrogen was supplied to the sample for 4 consecutive hours. The reaction was quickly stopped by supplying a cold (25°C) flow of nitrogen gas. Throughout the experiment, the gas flow was adjusted to 40 ml min ^-1 and kept constant.

Example 14 (according to process #4)

Fe ₃ Co ₃ Ni ₃ S ₈

Iron(II) nitrate (Fe(NO ₃ ) ₃ ⋅9H ₂ O, 2.02 g), cobalt(II) nitrate (Co(NO ₃ ) ₂ ⋅6H ₂ O, 1.46 g) and nickel(II) nitrate (Ni(NO ₃ ) ₂ ⋅6H ₂ O, 1.45 g) were dissolved in 15 ml of water containing 2.88 g citric acid. The solution was heated to 90°C for 2-6 hours, which led to the formation of a viscous gel. The resulting gel was calcined at 500° C. for 18 hours, and the resulting black solid was filled into a quartz glass cuvette, which was placed in an oven. The oven was then purged with an inert gas (N ₂ ) for 10 minutes at 40°C. Then the gas flow was changed to a mixture of hydrogen (85 vol.%) and hydrogen sulfide (15 vol.%) and the temperature was raised to 300°C at a heating rate of 10 K minute ^-1 . After 15 minutes of purging the sample with hydrogen (85 vol.%) and hydrogen sulfide (15 vol.%) at 300°C, pure hydrogen was purged through the apparatus at 300°C for 4 hours. After that, the oven was turned off and the sample was rapidly cooled in a cold (25°C) flow of N ₂ . Throughout the experiment, the gas flow was adjusted to 40 ml min ^-1 and kept constant.

Comparative Example 1

Fe _4.5 Ni _4.5 S ₈ (Piontek et al. ACS Catalysis 2018, 8, 987-996)

Metallic iron powder (1.75 g, Sigma-Aldrich, ≥99 wt%), nickel (1.75 g, ABCR, 99.9 wt%) and sulfur (1.70 g, Sigma-Aldrich, 99.5 wt% -100.5 wt%) were mixed together for 10 minutes until a visually homogeneous mixture of elements was obtained. This mixture was placed in a 10 mm quartz tube. Thereafter, the quartz tube was sealed under static vacuum and heated to 700° C. at a rate of 5° C. minute ^-1 . After 3 hours of annealing at 700°C, the temperature was raised to 1100°C in 30 minutes. After 10 hours at 1100° C., the mixture was allowed to cool to room temperature.

Comparative Example 2

Co ₉ S ₈

Cobalt powder (1.35 g, ABCR, 99.8 wt.%) and sulfur (0.66 g, Sigma-Aldrich, 99.5 wt.% - 100.5 wt.%) were mixed together for 10 minutes until a visually homogeneous mixture of elements was obtained. The homogeneous mixture of elements was filled into a quartz ampoule (diameter 10 mm, total volume 15 ml), from which air was pumped out at a pressure below 4 x 10 ^-2 mbar for 16 hours, and then sealed in vacuum. The sample was placed in an oven and heated to 700° C. at a heating rate of 4.5 K min ^-1 . After keeping the temperature isotherm for 3 hours, so that the sulfur reacted with the mixture of metals without damaging the vessel, the temperature was raised to 1000°C (heating rate 3.33 K min ^-1 ) to enhance diffusion. The sample was then kept at this temperature for 10 hours. After that, the sample was cooled down to room temperature.

Obtaining the electrode according to example 1-10, as well as according to comparative example 1-2

A specially made Teflon casing with a brass rod was used as a contact for electrodes (3 mm in diameter). The appropriate material used to make the electrode (50 mg) was ground to a fine powder material. The crushed powder was loaded into a pressing tool (3 mm in diameter), and the crushed powder was pressed with a maximum weight force of 800 kg/cm ² . A two-component epoxy adhesive with silver was applied to the brass rod in the cavity of the Teflon casing to connect the brass substrate to the electrode material. The pellet was then pressed into the Teflon jacket and any contaminants were removed from the Teflon jacket. The contact between the brass rod and the pellet was tested with a voltmeter to ensure proper conduction. Thereafter, the electrode was kept at 60° C. for 12 hours to allow the two-component adhesive to cure. After cooling to room temperature, the electrode was polished with sandpaper (grit 20 µm, 14 µm, 3 µm and 1 µm) to obtain a shiny smooth flat surface inside the Teflon case. After cleaning the surface with deionized water and drying at ambient conditions, the electrode could be used without additional treatment.

Comparative Example 3

Nanosheets MoS ₂

Bulk MoS ₂ crystals were synthesized by the chemical vapor transfer method. In conventional synthesis, powders of Mo and S elementary particles were mixed in stoichiometric proportions (1:1) and placed in a quartz tube. The quartz tube was evacuated to ~10 ^-6 mbar and sealed. The sealed quartz tube was placed in a tube furnace at 800° C. for 2 weeks to allow crystals to form. The quartz tube was cooled to room temperature and opened to collect the formed crystals.

These MoS ² crystals were exfoliated by dispersing 5 mg/mL of the crystals in a solution of the surfactant cetyltrimethylammonium bromide CTAB (2 mg/mL) in water, followed by sonication for 10 hours in a 100 W ultrasonic bath. After sonication, the dispersions were subjected to differential centrifugation to narrow the size distribution. In a typical method, the dispersions were centrifuged at 1000 rpm for 1 hour. The supernatant was separated and subjected to successive centrifugation at 2000 and 4000 rpm for periods of 2 hours each. At this stage, the process was stopped (at 4000 rpm). The precipitate was collected and redispersed in water under sonication. After sonication, the dispersion remained stable for 3 months without any flocculation and was used for further studies. To obtain an electrode, an aqueous suspension was prepared at a concentration of 5 g MoS ₂ per liter, followed by sonication for 30 minutes. A volume of 5 ml of this suspension was dropped onto a polished glassy carbon electrode with a geometric area of 0.126 cm ² and dried in air at room temperature. The modified electrodes were subjected to continuous potential cycling in a potential window from -0.5 to 0.5 V relative to Ag/AgCl/3 M KCl until reproducible voltammetry diagrams were obtained.

Comparative Example 4

NiS ₂ nanosheets

NiS ₂ nanosheets were synthesized by a one-stage hydrothermal method. In a typical method, 4 mmol nickel chloride hexahydrate (NiCl ₂ -6H ₂ O) and 4 mmol Na ₂ S ₂ O ₃ .5H ₂ O were mixed in a beaker containing 30 ml Milli-Q quality water and agitated for 1 hour. The mixed solution was transferred to a 60 ml Teflon-coated stainless steel autoclave and heated for 24 hours at 180°C. The precipitate was collected by centrifugation and washed several times with a mixture of ethanol and water (1:2) and then dried. To obtain an electrode, an aqueous suspension was prepared at a concentration of 5 g MoS ₂ per liter, followed by sonication for 30 minutes. A volume of 5 ml of this suspension was dropped onto a polished glassy carbon electrode with a geometric area of 0.126 cm ² and dried in air at room temperature. The modified electrodes were subjected to continuous potential cycling in a potential window from -0.5 to 0.5 V relative to Ag/AgCl/3 M KCl until reproducible voltammetry diagrams were obtained.

Comparative Example 5

Nanosheets FeS ₂

FeS ₂ nanosheets were synthesized by a one-stage hydrothermal method. In a typical method, 4 mmol of ferric chloride tetrahydrate (FeCl ₂ ⋅4H ₂ O) and 4 mmol of Na ₂ S ₂ O ₃ ⋅5H ₂ O were mixed in a beaker containing 30 ml of Milli-Q quality water and agitated for 1 hour. The mixed solution was transferred to a 60 ml Teflon-coated stainless steel autoclave and heated for 24 hours at 180°C. The precipitate was collected by centrifugation and washed several times with a mixture of ethanol and water (1:2) and then dried. To obtain an electrode, an aqueous suspension was prepared at a concentration of 5 g MoS ₂ per liter, followed by sonication for 30 minutes. A volume of 5 ml of this suspension was dropped onto a polished glassy carbon electrode with a geometric area of 0.126 cm ² and dried in air at room temperature. The modified electrodes were subjected to continuous potential cycling in a potential window from -0.5 to 0.5 V relative to Ag/AgCl/3 M KCl until reproducible voltammetry diagrams were obtained.

Comparative Example 6

Pt

A specially made Teflon casing with a brass rod was used as a contact for electrodes (3 mm in diameter). 200 mg of fine mesh platinum mesh (99.99% by weight) was ground to obtain a finely divided powder material. The fine powder was loaded into a pressing tool (diameter 3 mm) and the powder was pressed with a maximum weight force of 800 kg/cm ² . A two-component epoxy adhesive with silver was applied to the brass rod in the cavity of the Teflon casing to connect the brass substrate to the electrode material. The pellet was then pressed into the Teflon jacket and any contaminants were removed from the Teflon jacket. The contact between the brass rod and the pellet was tested with a voltmeter to ensure proper conduction. Thereafter, the electrode was kept at 60° C. for 12 hours to allow the two-component adhesive to cure. After cooling to room temperature, the electrode was polished with sandpaper (grit 20 µm, 14 µm, 3 µm and 1 µm) to obtain a shiny smooth flat surface inside the Teflon case. The mesh was cleaned in the first step with concentrated hydrochloric acid and then with deionized water and dried at ambient conditions. The electrode could be used without additional processing.

Conditions for electrochemical testing

For the electrochemical study of catalyst materials, the corresponding catalyst was pressed into granules and inserted into specially made electrodes with the following steps:

a) the catalyst material (50 mg) was ground into a fine powder material, loaded into a specially designed pressing tool (diameter 3 mm), and then pressed with a maximum weight force of 800 kg/cm ² ;

b) contacting a specially designed Teflon sheath with a brass rod as a contact for the electrodes (3 mm diameter); a silver epoxy adhesive (Polytec EC 151 L) was applied to the brass rod in the cavity of the Teflon jacket to bond the brass substrate to the electrode material;

c) the pellet was pressed into the Teflon casing and any contaminants were removed from the Teflon casing;

d) the contact between the brass rod and the pellet was tested with a voltmeter (VOLTCRAFT VC840) to ensure proper conductivity;

e) the electrode was held at 60° C. for 12 hours to allow the two-component adhesive to cure;

f) the electrode was cooled to room temperature and polished with sandpaper (grit 20 µm, 14 µm, 3 µm and 1 µm, 3M Lapping Film Bogen 261X/262X) to obtain a shiny smooth flat surface inside the Teflon housing; after cleaning the surface with deionized water (Millipore Milli-Q Academic Water Purification System) and drying at ambient conditions, the electrode was usable without further treatment.

The experiments were carried out on a standard three-electrode setup using the obtained electrode (A _geom. = 0.071 cm ² ) as a working electrode, an Ag/AgCl electrode (saturated solution of KCl or 3 M KCl) as a reference electrode, and a Pt wire (diameter 1 mm) or Pt grid (A _geom. = 1.25 cm ² ) as a counter electrode. Then, a specially designed H-type gas-tight cell was equipped with a stirring rod and filled with an electrolyte consisting of 0.5 M H ₂ SO ₄ for all electrochemical experiments. The electrolyte was not replaced during the electrochemical test of the electrode. All potentials refer to E _RHE (RHE = reversible hydrogen electrode) according to E _RHE = E _Ag / _AgCl + X + 0.059 pH, where X = 0.197 V (saturated KCl solution), or X = 0.210 V (3 M KCl), unless otherwise indicated. A Gamry Reference 600+ device was used as a potentiostat.

Measurement of catalytic performance

Electrocatalytic experiments were performed with the following steps:

a) electrolyte (0.5 M H ₂ SO ₄ , 25 ml) was added to the electrochemical cell and the electrodes were adjusted to ensure that the electrodes were completely immersed in the solution;

b) turn on the magnetic stirrer (IKA Topolino);

c) a cyclic voltammetry (CV) experiment was performed to obtain a quick overview of the electrochemical processes that could be observed;

d) The CV experiment was repeated to ensure fast electrochemical cleaning of the surface in the potential range from 0.2 to -0.2 V. The experiments were carried out with a scan rate of 100 mV/s (non-catalytic potential region) and the number of cycles was set to 20. Before starting the experiment, the iR compensation value for the electrochemical setup was determined using a GAMRY Reference 600 potentiostat and the installed software system program. The potential range for linear sweep voltammetry (LSV) experiments was set from -0.2 to -0.8 V compared to RHE, and the scanning speed was 5 mV/s, taking into account the drop in iR in the experiment. Linear sweep experiments were repeated at least three times to ensure reproducibility.

Stability test and gas analysis

To evaluate the stability of the materials according to the examples, the electrodes were kept at a constant potential (experiments with CPC) for a long period of time (at least 12 hours). The electrodes were evaluated at 0.5 M H ₂ SO ₄ at a constant potential of -0.71 V compared to RHE. In addition, gas samples were taken from free space at the same time. For this, a gas chromatograph (GC; Agilent system with JAS injection system) equipped with a thermal conductivity detector was connected to the electrochemical cell. The GC loop dispenser was continuously supplied with gas from the headspace of the cell. Argon (10-20 ml min ^-1 ) was used as carrier gas to purge the reactor and the GC dosing loop. Thus, the amount of hydrogen gas generated during electrolysis was determined, and the Faraday efficiency was calculated by correlating the measured amount obtained from the flowing charge during the CPC experiments with the maximum theoretically possible amount.

Start-stop characteristics

When working with renewable energy sources, the ability to start and stop an electrochemical process is of paramount importance. Thus, the start-stop properties of pentlandite electrodes were investigated. For this purpose, the long-term experiments described above (see stability testing) with reduced overvoltage (-350 mV compared to RHE or -304 mV compared to RHE) were performed intermittently (Table 2). Then the measurements were automatically resumed under identical conditions. Measurements on pentlandite electrodes were standardly compared with a platinum electrode of the same diameter.

Poison experiments

Because poisoning with small molecules (eg H ₂ S) renders many catalysts unusable or requires extensive purification of the starting materials, the inventors of the present invention investigated the utility of pentlandites as stable electrocatalysts also in the presence of conventional catalyst poisons. Thus, long-term experiments (see above experimental procedure "Stability test and gas analysis") were carried out in the presence of H ₂ S. For this, the electrolyte was constantly purged with H ₂ S, and the total pressure was maintained at 1.5 bar.

Characteristic

The compositions used in accordance with the present invention have been partially characterized in relation to XRD (from English - X-ray diffraction - X-ray diffraction), DSC (from English - differential scanning calorimetry - differential scanning calorimetry), ICP-OES (from English - inductively coupled plasma optical emission spectrometry - optical emission spectrometry with inductively coupled plasma) and SEM- EDX (from English - scanning electron microscopy-electron dispersive X-ray spectroscopy - scanning electron microscopy with electron dispersive X-ray spectroscopy).

Scanning electron microscope (SEM (from English - scanning electron microscope)) LEO (Zeiss) 1530 Gemini FESEM operated at a voltage of 20 kV (SEM) and 4.4 kV (electron dispersive X-ray spectroscopy, EDX (from English - electron dispersive X-ray spectroscopy)).

Powder X-ray diffraction (PXRD) was performed using a diffractometer from HUBER with Mo-Kα radiation (0.709 A) and a Bruker Advance D8 with Cu-Kα radiation (0.154 A) while scanning in the angle range of 3-50° with a step size of 0.037 s. All reflection positions were converted from Mo radiation to Cu radiation according to the Bragg law.

The composition of Fe and Ni for all different samples was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) with a Thermo Scientific iCAP 6500 Duo equipped with a CETAC ASX 520 autosampler. Data collection was performed on iTEVA. Calibration curves were built on double deionized water (ddw (from English - double deionized water)) with 3 vol.% nitric acid in the range of 5000-100 parts per billion (6 points). Standards and samples were prepared fresh in ddw with 3 vol% metal-free nitric acid. Readings were recorded in gasless mode. Samples (1 mg) were diluted with 3 ml of metal-free aqua regia, placed in 55 ml TFM (modified polytetrafluoroethylene) vials, and digested in a CEM Mars Xpress microwave oven (160° C., change rate 15 minutes, exposure 15 minutes). The digestible mixtures were then diluted to 10 ml by adding doubly deionized water.

The materials were examined by differential scanning calorimetry (DSC) using a NETZSCH STA 449 F3 Jupiter. Approximately 50 mg of the sample was placed in a closed corundum crucible and treated at room temperature to 1000° C. and vice versa at 10 K min ^-1 in a continuous flow of gaseous N ₂ .

Electrochemical cell

Most parts of the cell are custom made, unless otherwise noted, and are explained below with reference to Figure 8, which shows a cross section of the cell. The specially made electrochemical cell 1 consists of two glass compartments 2, 3. The working electrode 32 and the reference electrode (36) are located in the right compartment 3. The left compartment 2 houses the counter electrode 24. The cell 1 is equipped with a specially designed aluminum membrane holder 4, optionally wrapped with PTFE tape (conventional sealing tape) connecting the parts. The membrane holder 4 fixes the membrane 5 (Fumatech FUMASEP F 10100) separating the cathode from the anode space with two rubber rings 6 (18×5 mm). Membrane holder 4 is placed between the glass flanges of cells 2, 3, and an aluminum retainer (NeoLab KF16 three-point lock) locks everything in place. Compartments 2, 3 of the electrochemical cell 1 can be thermally treated with a liquid recirculating cooler or heater. Specially made inserts 21, 31 of the electrode holder for the cell compartments are made of PEEK polymer and have drilled holes to accommodate the electrodes 24, 32, 36 and the temperature sensor (optional) in the electrolyte. Specially made PEEK screws 23, 33 and compression rings (not shown) serve to lock the electrodes 24, 32, 36 and optional temperature sensors in place. Additional drilled holes are provided for the optional gas purge devices 22, 34. Lid inserts are locked in place with 25, 35 GL45 SCHOTT threaded lids (Duran connection systems).

Results Overvoltage

Table 3 briefly characterizes the potentials measured with the compositions used in accordance with the present invention and comparative examples. The negative values are the result of the potential being measured relative to a reversible hydrogen electrode.

Table 3 shows that the compositions used in accordance with the present invention show a higher overvoltage compared to platinum-based catalysts, but, surprisingly, they predominantly show a lower overvoltage for HER compared to another prior art catalyst (Comparative Example 1).

Figures 1A-1E show linear swept voltammetry plots obtained over a scan range of 0 to -0.65 V compared to RHE in 0.5 M sulfuric acid at room temperature. The X-axis shows the potential in V and the Y-axis shows the current density in mA cm ^-2 .

Fig. 1A: Example 1 (solid line), Comparative Example 1 (dashed line), Comparative Example 2 (dashed line), Comparative Example 3 (dash-dotted line), Comparative Example 4 (dash-double-dotted line), Comparative Example 5 (short-dotted line), and Comparative Example 6 (short-dotted line).

Fig. 1B: Example 1 (solid line), Example 3 (dashed line), Example 4 (dash-dotted line), and Comparative Example 6 (short-dotted line).

Fig. 1C: Example 1 (solid line), Example 5 (dash-doubly dashed line), Example 6 (short dashed line), and Comparative Example 6 (short dashed line).

Fig. 1D: Example 1 (solid line), Example 11 (dashed line), Example 12/13 (dashed line), Example 14 (dash-dotted line) and Comparative example 6 (short-dotted line).

Fig. 1E: Example 1 (solid line), Example 7 (dashed line), Comparative Example 1 (short dashed line), and Comparative Example 6 (short dashed line).

Current densities after several hours of electrolysis

The compositions used in accordance with the present invention show a higher overvoltage for HER compared to platinum based catalysts, but surprisingly they show higher current densities than platinum based materials after several hours of electrolysis. At higher current densities, more hydrogen can be produced at Faraday's quantitative efficiency. Therefore, industrial production of hydrogen using these compositions is more efficient and cheaper than with commercial catalysts.

Figure 2 shows the results of voltage controlled coulometry measurements at -0.5 V compared to RHE at 20°C and 60°C for electrodes based on Pt (comparative example 6) and Fe ₃ Co ₃ Ni ₃ S ₈ (example 1) for 20 hours. Shown are the curves of example 1 at 20°C (checker) and at 60°C (square), as well as comparative example 6 at 20°C (triangle) and 60°C (circle).

The X-axis shows the time in hours and the Y-axis shows the current density in mA cm ^-2 .

The level of activity in the production of hydrogen

Prior art materials lose significant potency during use compared to their fresh state. Surprisingly, the electrodes obtained from the compositions used in accordance with the present invention do not lose significant activity during the production of hydrogen, as shown in figure 3.

Figure 3 shows linear sweep voltammetry charts performed at 20°C (A) and 60°C (B) before and after a 20 hour electrolysis for platinum (comparative example 6) and Fe ₃ Co ₃ Ni ₃ S ₈ (example 1).

Figure 3A: example 1 before (solid line) and after (dash-dotted line) and comparative example 6 before (dashed line) and after (dotted line) electrolysis at 20°C.

Figure 3B: example 1 before (solid line) and after (dash-dotted line) and comparative example 6 before (dashed line) and after (dotted line) electrolysis at 60°C.

The X-axis in each case shows the potential in V, and the Y-axis shows the current density in mA cm ^-2 .

Start-stop characteristic

Start-stop characteristic - compared with comparative example 6

Surprisingly, the electrodes made from the compositions used in accordance with the present invention have significantly better start-stop characteristics than platinum electrodes, as evidenced by figures 4 and 9. While the current density of platinum is reduced to less than 20% of its initial current density, the compositions used in accordance with the present invention can maintain a significantly higher value of their original current densities. Thanks to these improved start-stop characteristics, it is possible to take full advantage of the peaks of renewable energy sources (eg wind or photovoltaic) for sustainable hydrogen production.

Figure 4 shows start-stop potential controlled coulometry measured at 60° C. and -304 mV compared to RHE over 72 hours for Fe ₃ Co ₃ Ni ₃ S ₈ (example 1, square) and platinum (comparative example 6, checker).

The X-axis shows the time in hours and the Y-axis shows the current density in mA cm ^-2 .

Figure 9A also shows start-stop controlled potential coulometry for Fe ₃ Co ₃ Ni ₃ S ₈ (example 1, square) and platinum (comparative example 6, checker), however measured at 60° C. and -350 mV compared to RHE over 72 hours. The current densities in Example 1 and Comparative Example 6 in Figure 4 are lower than those obtained with the same catalysts in Figure 9A. These differences arise from the lower potential applied in the measurement in Figure 4 (-304 mV) compared to Figures 9A - 9E (-350 mV). However, both figures (FIGS. 4 and 9A) show that the start-stop response in Example 1 is superior to that of Comparative Example 6.

Figures 9B - 9E show start-stop controlled potential coulometry measured at 60° C. and -350 mV compared to RHE over 72 hours. When measured with the same set of parameters, the results shown in figures 9A and 9B-9E can be compared with each other.

Figure 9B: example 2 (square) and comparative example 6 (checker).

Figure 9C: example 4 (square) and comparative example 6 (checker).

Figure 9D: Example 6 (square) and Comparative Example 6 (checker).

Figure 9E: example 7 (square) and comparative example 6 (checker).

The X-axis shows the time in hours and the Y-axis shows the current density in mA cm ^-2 .

Start-stop characteristic - compared with comparative example 1

Surprisingly, the electrodes obtained from the compositions used in accordance with the present invention also have similar or even better start-stop characteristics than electrodes from other prior art compositions, namely Fe _4.5 Ni _4.5 S ₈ (comparative example 1), as shown in figure 17. The compositions used in accordance with the present invention can maintain the same or even greater value of their initial current densities compared to the composition of comparative example 1, especially after several start/stop cycles. Thanks to these improved start-stop characteristics, it is possible to take full advantage of the peaks of renewable energy sources (eg wind or photovoltaic) for sustainable hydrogen production.

Figures 17A-17D show start-stop controlled potential coulometry measured at 60° C. and -350 mV compared to RHE over 72 hours.

Figure 17A: Example 1 (square) and Comparative Example 1 (checker).

Figure 17B: example 2 (square) and comparative example 1 (checker).

Figure 17C: example 4 (square) and comparative example 1 (checker).

Figure 17D: Example 7 (square) and Comparative Example 1 (checker).

The X-axis shows the time in hours and the Y-axis shows the current density in mA cm ^-2 .

Stability against sulfur poisoning

Stability against sulfur poisoning - compared with comparative example 6

A further advantage of the compositions used in accordance with the present invention is the stability against sulfur poisoning (figures 5, 6 and 10-16). While platinum is very sensitive to sulfur compounds and loses activity, the compositions used in accordance with the present invention show almost the same or even higher (eg, Fig. 10 and 12) activity when treated with sulfur compounds. A further effect of the sulfur compounds is that the poisoning of the platinum electrodes is irreversible, while the sulfur compounds have a reversible effect on the compositions used in accordance with the present invention. Therefore, very pure water is needed as a feedstock for PEM electrolyzers with platinum as an electrocatalyst for HER, while different sources of water can be used with the compositions used in accordance with the present invention.

Figures 5, 10, 12 and 14 show controlled potential coulometry in the presence and absence of H ₂ S catalyst poison measured at 20° C. and -350 mV compared to RHE over 20 hours.

Figure 5: Curves of Example 1 without (square) and with (checker) H ₂ S, and Comparative Example 6 without (circle) and with (triangle) H ₂ S.

Figure 10: Curves of Example 2 without (square) and with (checker) H ₂ S, and Comparative Example 6 without (circle) and with (triangle) H ₂ S.

Figure 12: Curves of Example 4 without (square) and with (checker) H ₂ S, and Comparative Example 6 without (circle) and with (triangle) H ₂ S.

Figure 14: Curves of Example 6 without (square) and with (checker) H ₂ S, and Comparative Example 6 without (circle) and with (triangle) H ₂ S.

The X-axis shows the time in hours and the Y-axis shows the current density in mA cm ^-2 .

Figures 6, 11, 13, 15 and 16 show linear sweep voltammetry charts performed in the presence and absence of H ₂ S catalyst poison measured at 20° C. and -350 mV compared to RHE before and after 20 hours of electrolysis.

Figure 6A: curves of example 1 in the absence of H ₂ S before (solid line) and after (dashed line) electrolysis next to the curves of example 1 in the presence of H ₂ S before (dashed line) and after (dashed line) electrolysis.

Figure 6B: curves of comparative example 6 in the absence of H ₂ S before (solid line) and after (dashed line) electrolysis next to the curves of comparative example 6 in the presence of H ₂ S before (dashed line) and after (dash-dotted line) electrolysis.

Figure 11: curves of example 2 in the absence of H ₂ S before (solid line) and after (dashed line) electrolysis next to the curves of example 2 in the presence of H ₂ S before (dashed line) and after (dash-dotted line) electrolysis.

Figure 13: curves of example 4 in the absence of H ₂ S before (solid line) and after (dashed line) electrolysis next to the curves of example 4 in the presence of H ₂ S before (dashed line) and after (dash-dotted line) electrolysis.

Figure 15: curves of example 6 in the absence of H ₂ S before (solid line) and after (dashed line) electrolysis next to the curves of example 6 in the presence of H ₂ S before (dashed line) and after (dash-dotted line) electrolysis.

Figure 16: curves of example 7 in the absence of H ₂ S before (solid line) and after (dashed line) electrolysis next to the curves of example 7 in the presence of H ₂ S before (dashed line) and after (dash-dotted line) electrolysis.

The X-axis in each case shows the potential in V, and the Y-axis shows the current density in mA cm ^-2 .

Stability against sulfur poisoning - compared with comparative example 1

The stability against sulfur poisoning of the compositions used in accordance with the present invention is further compared with another prior art electrode composition, namely Fe ₄₅ Ni _4.5 S ₈ (Comparative Example 1) (Figures 18-25). The compositions used in accordance with the present invention show comparable or even better activity when treated with sulfur compounds compared to comparative example 1.

Figures 18, 20 and 22 show controlled potential coulometry with and without H ₂ S catalyst poison measured at 20° C. and -350 mV compared to RHE over 20 hours.

Figure 18: Curves of Example 1 without (square) and with (checker) H ₂ S, and Comparative Example 1 without (circle) and with (triangle) H ₂ S.

Figure 20: Curves of Example 2 without (square) and with (checker) H ₂ S, and Comparative Example 1 without (circle) and with (triangle) H ₂ S.

Figure 22: Curves of Example 4 without (square) and with (checker) H ₂ S, and Comparative Example 1 without (circle) and with (triangle) H ₂ S.

The x-axis shows the time in hours and the y-axis shows the current density in mA cm ^-2 .

Figures 19, 21, 23, 24 and 25 show linear swept voltammetry charts performed in the presence and absence of H ₂ S catalyst poison measured at 20° C. and -350 mV compared to RHE before and after a 20 hour electrolysis.

Figure 19A: curves of example 1 in the absence of H ₂ S before (solid line) and after (dashed line) electrolysis next to the curves of example 1 in the presence of H ₂ S before (dashed line) and after (dashed line) electrolysis.

Figure 19B: Comparative example 1 curves in the absence of H ₂ S before (solid line) and after (dashed line) electrolysis next to curves of comparative example 1 in the presence of H ₂ S before (dashed line) and after (dash-dotted line) electrolysis.

Figure 21: curves of example 2 in the absence of H ₂ S before (solid line) and after (dashed line) electrolysis next to the curves of example 2 in the presence of H ₂ S before (dashed line) and after (dashed line) electrolysis.

Figure 23: curves of example 4 in the absence of H ₂ S before (solid line) and after (dashed line) electrolysis next to the curves of example 4 in the presence of H ₂ S before (dashed line) and after (dash-dotted line) electrolysis.

Figure 24: curves of example 6 in the absence of H ₂ S before (solid line) and after (dashed line) electrolysis next to the curves of example 6 in the presence of H ₂ S before (dashed line) and after (dash-dotted line) electrolysis.

Figure 25: curves of example 7 in the absence of H ₂ S before (solid line) and after (dashed line) electrolysis next to the curves of example 7 in the presence of H ₂ S before (dashed line) and after (dashed line) electrolysis.

The X-axis in each case shows the potential in V, and the Y-axis shows the current density in mA cm ^-2 .

Powder X-ray (diffraction patterns)

Figure 7 shows powder x-ray (diffraction patterns, (Cu-Kα radiation)) compositions used in accordance with the present invention and comparative examples.

Fig. 7A: Patterns of Example 1 (square), Comparative Example 1 (triangle), Comparative Example 2 (circle), Comparative Example 3 (checker), Comparative Example 4 (open square), and Comparative Example 5 (open circle).

Figure 7B: example 1 (square), example 3 (circle) and example 4 (checker).

Figure 7C: example 1 (square), example 5 (triangle) and example 6 (circle).

Figure 7D: example 1 (square), example 11 (triangle), examples 12/13 (circle) and example 14 (checker).

Figure 7E: example 1 (square) and example 7 (triangle).

The x-axes in each case show the angles in °, and along the Y axes - the intensity in dimensionless quantities.

From FIG. 7A shows that the compositions used in accordance with the present invention (eg in example 1, square) exhibit an XRD pattern very similar to that of unmodified pentlandite (comparative example 1, triangle).

Surprisingly, the compositions used in accordance with the present invention operate over a wide pH range of 0 to 14 when applied at elevated potentials, while other iron, cobalt, or nickel sulfide compositions are not stable over such a wide pH range. Pentlandite as a crystalline phase seems to be the reason for such high stability.

The compositions used in accordance with the present invention are useful alone or in combination with an electrically conductive substrate material (eg, graphene) as an electrocatalyst for the hydrogen evolution reaction in a PEM electrolyzer.

The composition used in accordance with the present invention may also be applicable to the production of hydrogen by electrolysis in alkaline water or with high pressure electrolyzers.

Claims (30)

1. Application of formula composition Where M denotes one or more elements characterized in the ionic state by an effective ionic radius in the range from 70 to 92 pm, a is a number in the range 2.5≤a≤3.5, more preferably 2.7≤a≤3.3, b is a number in the range 1.5≤b≤5.0, more preferably 1.5≤b≤4.0, most preferably 2.5≤b≤3.5, c is a number in the range 0.0≤c≤2.0, more preferably 0.0≤c≤1.0, d is a number in the range 0.0≤d≤4.0, more preferably 0.0≤d≤1.0, where the sum of a, b and c is in the range 5≤a+b+c≤8, and where ≥90 wt. % of the composition is in the pentlandite phase, for electrocatalytic splitting of water, preferably for the hydrogen evolution reaction. 2. Use according to claim 1, where M is not present. 3. Use according to claim 2, where the composition is selected from the group consisting of Fe ₃ Ni ₃ Co ₃ S ₈ , Fe ₁ Co ₅ Ni ₃ S ₈ , Fe ₂ Co ₄ Ni ₃ S ₈ , Fe ₄ Co ₂ Ni ₃ S ₈ , Fe ₃ Ni ₃ Co ₃ S ₄ Se ₄ , Fe ₃ Ni ₃ Co ₃ S ₇ Se and their mixtures. 4. Use according to claim 1, where M is present. 5. Use according to claim 4 wherein M is selected from the group consisting of Nb, Cu, Mn or Cr and mixtures thereof, more preferably Mn, Cr and mixtures thereof, most preferably Cr. 6. Use according to claim 5, where the composition is selected from the group consisting of CrFe _2.5 Co _2.5 Ni ₃ S ₈ , MnFe _2.5 Co _2.5 Ni ₃ S ₈ , Mn _0.25 Fe _2.875 Co _2.875 Ni ₃ S ₈ , NbFe _2.5 Co _2.5 Ni ₃ S ₈ and their mixtures. 7. Use according to any one of the preceding claims, characterized in that the composition is characterized by an overvoltage for the hydrogen evolution reaction (HER) of 328 mV or less, preferably 300 mV or less, at a current density of -10 mA/cm ² . 8. An electrode containing a composition of formula I as defined in any of the preceding paragraphs. 9. An electrode according to claim 8, characterized in that it is characterized by an overvoltage for the hydrogen evolution reaction (HER) of 328 mV or less at a current density of -10 mA/cm ² .