Use of a nickel-and ruthenium-based hetero-structured catalyst for electro-catalytic reduction of water

Abstract

The invention relates to a method for synthesising a nickel-ruthenium hetero-structured catalyst, comprising a single step of mixing, stirring and heating a nickel salt and a ruthenium salt in a solvent, the Ru / Ni molar ratio being between 0.5% and 10%, the hetero-structured catalyst comprising, at the end of the synthesis of the heterogeneous nanoparticles consisting of nickel nanoparticles aggregated together and decorated on the surface by ruthenium nanoparticles which are therefore in contact with the aggregated nickel nanoparticles, the ruthenium surface nanoparticles being smaller in size than the aggregated nickel nanoparticles, the metal percentage of ruthenium on the surface of the heterogeneous nanoparticles being greater than the total percentage of ruthenium in the heterogeneous nanoparticles.

Description

[0001] Use of a heterostructured nickel-ruthenium catalyst for the electrocatalytic reduction of water

[0002] The invention relates to the production of dihydrogen by alkaline electrolysis. Hydrogen is considered one of the major energy vectors of the future.

[0003] To be sustainable and meet current environmental challenges, the production of dihydrogen must, however, be carried out inexpensively and without greenhouse gas emissions.

[0004] In this perspective, water electrolysis is considered the main solution for producing hydrogen, given that the only by-product of the reaction is oxygen. Electrolyzers are devices for producing hydrogen from water, via an electrical energy supply. They are advantageous in the context of the energy transition for providing "green" hydrogen.

[0005] Three categories of techniques have been developed in the prior art for the electrolysis of water: alkaline electrolysis, which is the oldest technique, acid electrolysis (or PEM Proton Exchange Membrane electrolysis), which appeared commercially in the early 2000s, and high-temperature electrolysis (Solid Oxide Electrolysis), which is the most recently proposed technique.

[0006] A state of research concerning the production of hydrogen by water electrolysis was presented in 2022 by Kumar et al. (An overview of water electrolysis technologies for green hydrogen production, Energy reports, November 2022, pp. 13793-13813). Reference can also be made to the review carried out in 2020 by Grigoriev et al. (Current status, research trends, and challenges in water electrolysis science and technology, International Journal of Hydrogen Energy, October 2020, pp.26036-26058).

[0007] Acid electrolysis has several disadvantages. In particular, it is necessary to use precious materials such as platinum or iridium for the electrodes, and the membrane, most often made of Nation®, is expensive and its recycling is problematic due to the presence of fluorine.

[0008] High-temperature electrolysis has several disadvantages. In particular, solid oxide electrolysers (SOEC Solid Oxide Electrolyser Cells) operate at temperatures of several hundred degrees, which imposes significant mechanical stress on the materials.

[0009] In conventional alkaline electrolysers, the two electrodes are immersed in an aqueous solution of concentrated potassium hydroxide KOH, with a diaphragm separating the two electrodes. The diaphragm is, for example, made of ceramic material, potassium titanate or even polymer material. Reference may be made, for example, of document W02023280760 (Agfa Gevaert, 2023). Alkaline electrolysis carried out in the presence of a concentrated alkaline liquid electrolyte, typically a concentrated KOH solution (> 3M) is a mature and commercial technology, and currently allows the production of approximately 4% of the dihydrogen produced worldwide.

[0010] Alkaline electrolysers are marketed for example by the companies NEL Hydrogen, McPhy, Hydrogenics, Tianjin Mainland Hydrogen Equipement.

[0011] In another design of alkaline electrolysis, an anion exchange membrane (Anion Exchange Membrane AEM) is used. Reference can be made, for example, to document WO20212261 1 9 (Univ. Delaware, 2021). A state of the research was presented in 2021 by Li et al. The promise of hydrogen production from alkaline anion exchange membrane electrolyzers, Nano Energy, September 2021).

[0012] Among electrolysis technologies, electrolysis carried out in an alkaline medium has a considerable advantage compared to electrolysis carried out in an acidic medium: electrolysis in an alkaline medium allows the use of non-noble catalysts in the electrodes, anode and cathode, of the electrochemical device. Nickel deposited on steel or solid nickel have thus been widely used in industrial alkaline electrolysis systems.

[0013] In addition to the cost of the electrode materials, the overvoltages at the electrodes for the hydrogen evolution reaction and the water reduction reaction must be minimal, and the durability of the components must be maximized.

[0014] High overvoltages lead to an increase in the amount of electricity required to produce hydrogen and therefore an increase in the cost of hydrogen. In electrolysis carried out in an alkaline medium, the electrochemical reactions and therefore the catalysts used to achieve the lowest overvoltages are identical, even if the electrode formulations may differ.

[0015] At the cathode, the electrochemical reaction is as follows:

[0016] In commercial liquid electrolysers, the cathode is usually solid nickel or nickel-plated steel combined with a nickel-based catalytic deposit.

[0017] To reduce overvoltages, it has been proposed to use platinum, palladium or gold electrodes, although the cost of these materials is very high.

[0018] In an attempt to overcome this problem, various proposals have been presented in the prior art, including the use of ruthenium (Ru), a transition metal from the platinum group whose price is lower than that of platinum or palladium.

[0019] For illustration, in 2022, the average price of palladium was USD 21.12 per ounce, the average price of platinum was USD 961 per ounce, and the average price of ruthenium was USD 620 per ounce.

[0020] US10637071 (Industrial Technology Research Institute, 2020) describes a catalyst for an alkaline electrochemical energy conversion reaction, the catalyst comprising a support of metallic material, carbonaceous material or metal oxide, and at least one Ru alloy x M y, M being chosen from the group comprising nickel (Ni), cobalt (Co), iron (Fe), manganese (Mn), chromium (Cr), vanadium (V), titanium (Ti), copper (Cu) or zinc (Zn), x being greater than or equal to y. In one example, the catalyst is an alloy of ruthenium Ru and nickel Ni, obtained by a process comprising mixing an aqueous solution of nickel chloride in hydrogen peroxide, the pH of this mixture being adjusted to form a solution A. The process further comprises adjusting the pH of an aqueous solution of ruthenium chloride with sodium carbonate Na2COs, a quantity of sodium hydrogen sulfite NaHSOs then being added, the resulting mixture being heated to 80°C for 30 minutes, carbon black being added to form a solution C. After dispersing solution C by ultrasound, solution A is added to solution C, and the pH of the mixture is adjusted with sodium hydroxide.After heating at 100°C for 8 hours, the powder obtained is centrifuged and dried in an oven, and then undergoes reduction at 500°C for two hours.

[0021] The process described in US1 0637071 has many disadvantages. In particular, this prior process involves a large number of steps, and the catalyst preparation time is very high. In the resulting alloy, ruthenium is in excess of metal such as nickel, with the ratio of ruthenium to nickel being up to 50, with the resulting cost consequences.

[0022] CN 1 14737216 (Hunan Univ. 2022) describes the electrolysis of water using a catalyst which is a ruthenium and nickel based alloy, in which ruthenium is in greater quantity than nickel. Catalyst nanoparticles are obtained by a method comprising a step of mixing in a solvent ruthenium acetylacetonate, nickel acetylacetonate with a surfactant (polyvinylpyrrolidone) and a reducing agent (resorcinol), followed by a reaction step at a temperature between 170°C and 190°C for 30 min to 120 min, this reaction step being followed by cooling and addition of solvent, centrifugation, and then washing. The product obtained then undergoes heat treatment in a reducing atmosphere, at a temperature below 600°C for less than 5 hours, resulting in a material with a dendritic structure.

[0023] The process described in document CN 1 1473721 6 has many disadvantages. Resorcinol is toxic to humans; its ingestion can lead to coma, and resorcinol can cause dermatitis or allergies. Nickel acetylacetonate is harmful if ingested or comes into contact with skin, is likely to cause genetic abnormalities, and can cause cancer.

[0024] Furthermore, the alloy obtained does not allow for correct reduction of hydrogen.

[0025] The invention aims to overcome the drawbacks of the prior art, by proposing a catalyst for the electrocatalytic reduction of water and the production of dihydrogen by alkaline electrolysis, the catalyst containing a minimal quantity of noble metal, the catalyst being obtained by a simple, rapid process, and presenting fewer safety risks than the previously known processes. Another object of the invention is to propose a catalyst having improved performance, reducing the overvoltage in the electrodes of the electrolysers necessary for the reduction of water for the formation of dihydrogen.

[0026] Another object of the invention is to provide a catalyst meeting at least in part the above objectives, and whose durability is identical to or superior to the catalysts of the prior art.

[0027] Another object of the invention is to propose a catalyst meeting at least in part the above objectives, and making it possible to reduce the quantity of electricity necessary for the production of dihydrogen and thus reduce the cost of production of dihydrogen.

[0028] For these purposes, there is proposed, according to a first aspect, a process for the synthesis in an alkaline medium of a hetero-structured Nickel-Ruthenium catalyst, comprising a single step of mixing, stirring and heating a nickel salt and a ruthenium salt in a solvent, process in which the stirring of the nickel and ruthenium salts is followed by a progressive heating of the nickel and ruthenium salts and carried out with a temperature ramp, the molar ratio Ru / (Ni + Ru), measured by energy dispersive X-ray spectroscopy being between 0.05 and 0.1, the heterostructured catalyst comprising, at the end of the synthesis, heterogeneous nanoparticles having a nickel content greater than 90% and composed of: nickel nanoparticles, and ruthenium nanoparticles, which are on the surface of each nickel nanoparticle, the size of the ruthenium nanoparticles, which is measured by transmission electron microscopy (TEM), being smaller than the size of the nickel nanoparticles which is measured by transmission electron microscopy (TEM), the metallic percentage of ruthenium on the surface of the heterogeneous nanoparticles, measured by X-ray photoelectron spectroscopy (XPS), being greater than the total metallic percentage of ruthenium in the heterogeneous nanoparticles, measured by energy dispersive X-ray spectroscopy (EDX).

[0029] By heterostructured, we mean here the fact that the catalyst is composed of several materials. By nanoparticles we mean here objects whose three dimensions are on the nanometric scale, that is to say whose nominal diameter is less than approximately 100 nm. By nanoparticles we also mean here nanostructured particles, that is to say particles having at least one dimension less than approximately 100 nm, these nanostructured particles being able in particular to agglomerate in the form of an agglomerate or an aggregate of nanoparticles or remaining in individual form.

[0030] By nickel nanoparticles decorated on the surface with ruthenium particles, we mean here the fact that the nickel nanoparticles, for example from 20 nm to 50 nm, and which can be aggregated in the form of aggregates from 50 nm to 300 nm, carry on their surface ruthenium nanoparticles, for example of a size less than 5 nm.

[0031] These heterogeneous nanoparticles have a nickel content greater than 90%, advantageously greater than 95%, even more advantageously greater than 98%.

[0032] The catalyst is advantageously synthesized by a one-step method allowing the direct formation of nickel particles decorated with ruthenium particles. This one-step method is particularly effective in ensuring intimate contact between the nickel and ruthenium particles and ensuring good dispersion of the ruthenium particles on the surface of the nickel particles.

[0033] According to various implementations, the method has the following characteristics, possibly combined.

[0034] The mixture obtained is stirred at room temperature before heating, preferably in the presence of sodium hydroxide.

[0035] The stirring of the nickel and ruthenium salts is followed by heating of the nickel and ruthenium salts progressively and carried out with a temperature ramp, for example from room temperature to a maximum temperature Tmax, for example a temperature ramp between 1 °C / min and 10 °C / min is applied from room temperature to a maximum temperature Tmax below 200 °C.

[0036] The mixture is made with nickel acetate as the nickel salt, and ruthenium chloride as the ruthenium salt.

[0037] Nickel acetate here refers to a product sometimes called nickel diacetate (CAS 373-02-4), of formula Ni(CH3CO2)2. Ruthenium chloride here refers to a product in anhydrous form (CAS 1 0049-08-8), of formula RuCh.

[0038] The solvent in which the mixture of nickel salts and ruthenium salts is prepared is chosen from the group comprising 1,2-butanediol (CAS 584-03-2, C4H10O2), 1,2-propanediol (CAS 57-55-6, C3H8O2), 1,3-butanediol (CAS 107-88-0), 1,4-butanediol (CAS 110-63-4), ethylene glycol (CAS 107-21-1).

[0039] The catalyst is synthesized in one step from nickel acetate in the presence of sodium hydroxide and ruthenium chloride in 1,2-butanediol, the medium is stirred at room temperature for 30 minutes, then a temperature ramp between 1°C / min and 10°C / min is applied, and left at a temperature Tmax between 150°C and 200°C, advantageously 170°C, for a period of one hour to three hours.

[0040] Heating is carried out at a temperature Tmax below 200°C, for example 170°C, for a duration of less than 3 hours, for example 1.5 h.

[0041] According to a second aspect, there is proposed a hetero-structured catalyst based on nickel and ruthenium for the electro-catalytic reduction of water to dihydrogen, at the cathode of an electrolyser, in an alkaline medium, this catalyst being produced by heterogeneous nucleation, with a synthesis method defined above, and comprising a single step of mixing, stirring and heating a nickel salt and a ruthenium salt in a solvent, the hetero-structured catalyst comprising, at the end of the synthesis method defined above, a Ru / (Ni + Ru) molar ratio of between 0.005 and 0.1 , measured by energy dispersive X-ray spectroscopy (EDX), and being formed of heterogeneous nanoparticles having a nickel content greater than 90%, and composed of nickel nanoparticles, ruthenium nanoparticles which are on the surface and in contact with each nickel nanoparticle, the size of the ruthenium nanoparticles, which is measured by transmission electron microscopy (TEM), being smaller than that of the nickel nanoparticles, which is measured by transmission electron microscopy (TEM), the metallic percentage of ruthenium on the surface of the heterogeneous nanoparticles, measured by X-ray photoelectron spectroscopy (XPS), being greater than the total metallic percentage of ruthenium in the heterogeneous nanoparticles, measured by energy dispersive X-ray spectroscopy.

[0042] According to various implementations, the catalyst has the following characteristics, possibly combined.

[0043] The molar ratio Ru / (Ni + Ru) in the catalyst is between 0.005 and 0.05.

[0044] The metallic percentage of ruthenium on the surface of heterogeneous nanoparticles compared to the total metallic percentage of ruthenium and nickel on the surface of heterogeneous nanoparticles and equal to Ru / (Ni + Ru) is between 0.05 and 0.4 according to analysis by X-ray photoelectron spectroscopy.

[0045] The catalyst comprises individual nickel nanoparticles ranging in size from 20 nm to 150 nm, surface decorated with ruthenium surface nanoparticles smaller than 10 nm.

[0046] The catalyst has individual nickel nanoparticles of 20 to 50 nm, decorated on the surface by ruthenium nanoparticles of 2 to 5 nm.

[0047] The catalyst comprises nickel nanoparticles which are aggregated together, for example in the form of clusters of size between 40 nm and up to a micron, these aggregates being decorated on the surface by ruthenium nanoparticles.

[0048] In certain implementations, the heterogeneous nanoparticles have a nickel content greater than 95%, advantageously greater than 98%. According to a third aspect, the use of the hetero-structured catalyst based on nickel and ruthenium is proposed for the electro-catalytic reduction of water to dihydrogen, as presented above, at the cathode of an electrolyzer, in an alkaline medium.

[0049] According to various implementations, the use has the following characteristics, possibly combined.

[0050] Advantageously, the molar ratio between the quantity of ruthenium (Ru) and the quantity of ruthenium and nickel (Ru + Ni) in the catalyst, measured by energy dispersive X-ray spectroscopy (EDX) is between 0.005 and 0.05. The catalyst thus comprises a small quantity of noble metal, and its manufacture is therefore economical.

[0051] The amounts of nickel acetate and ruthenium chloride required for catalyst production are also reduced. The Ru / (Ni + Ru) ratio in the catalyst is between 0.025 and 0.035.

[0052] The metallic percentage of ruthenium on the surface of heterogeneous nanoparticles compared to the total metallic percentage of ruthenium and nickel on the surface of heterogeneous nanoparticles is equal to Ru / (Ni + Ru) and is between 0.05 and 0.40 according to the analysis by X-ray photoelectron spectroscopy.

[0053] The specific surface area of ​​the catalyst is between 3 and 50 m2 / g.

[0054] This specific surface area is, for example, measured by nitrogen physisorption, according to the BET method (Brunauer, Emmett and Teller).

[0055] The catalyst is used at the cathode of the electrolyser, in the presence of a polymer material.

[0056] According to various implementations, the electrolyser comprises an alkaline liquid electrolyte or an anionic or cationic conductive polymer electrolyte.

[0057] Alkaline electrolysis technology using an anion-conducting polymer membrane as the electrolyte eliminates the need for a liquid electrolyte. Advantageously, the hydrogen is produced directly under partial pressure, and the resulting hydrogen is of higher purity than hydrogen produced in devices operating with a liquid electrolyte.

[0058] Advantageously, the catalyst is used at the cathode of the electrolyser, in the presence of a polymer material chosen from fluoropolymers, preferably a copolymer based on sulphonated tetrafluoroethylene such as for example marketed under the brand Nation®, advantageously a copolymer based on sulphonated polytetrafluoroethylene.

[0059] Other objects and advantages of the invention will appear in the light of the description of embodiments, given below with reference to the appended drawings in which: Figure 1 is an X-ray diffractogram of a catalyst according to an implementation of the invention, Figure 2 are two scanning electron microscopy images of catalyst particles according to an implementation of the invention, Figure 3 are two transmission electron microscopy images showing the presence of ruthenium surface nanoparticles on the surface of nickel plates, Figure 4 is a nitrogen adsorption isotherm at 298K, for a catalyst obtained according to an implementation of the invention, Figure 5 represents the current-voltage variation curves, measured at 10 mV / s in potassium hydroxide, and a diagram showing the mass activity relative to the mass of noble metal deposited on an electrode, and measured at -0.3V vs.RHE in 0.1 M potassium hydroxide, Figure 6 are plots of measured overpotentials vs. RHE at 10 mV / s in 0.1 M potassium hydroxide KOH at -10 mA / cm. 2 and -40 mV / cm 2 ' for catalyst particles prepared with different ruthenium contents, Figure 7 are measured overpotential vs. RHE diagrams at 10 mV / s in 0.1 M potassium hydroxide KOH at - 10 mA / cm 2 and -40 mV / cm 2 for materials prepared with 2.5 atomic% of a noble metal: ruthenium Ru, palladium Pd, iridium Ir or platinum Pt, or without noble metal, Figure 8 are measured overpotential diagrams vs. RHE at 10 mV / s in 0.1 M potassium hydroxide KOH at - 10 mA / cm 2 and -40 mV / cm 2for materials prepared in different solvents: 1,2-butanediol (1,2-BD), 1,2-propanediol (1,2-PD), 1,3-butanediol (1,3-BD) and ethylene glycol (EG), the concentration of metals and sodium hydroxide NaOH being identical to the reference synthesis, Figure 9 are measured overpotential diagrams vs. RHE at 10 mV / s in 0.1 M potassium hydroxide KOH at - 10 mA / cm 2 and -40 mV / cm 2 for materials prepared in one step (1-step) or two steps (2-steps), Figure 10 describes the percentage of ruthenium in the metallic fraction of the nanoparticles according to the invention, in the case of progressive heating of the reaction mixture, and in the case of instantaneous heating.

[0060] Catalyst manufacturing

[0061] An example of the manufacture of the catalytic material is now described. The synthesis is advantageously carried out in one step, in 1,2-butanediol with a Ru / (Ru + Ni) ratio of 0.025.

[0062] Specifically, the material is synthesized in one step from nickel acetate in the presence of sodium hydroxide and ruthenium chloride (2.5 at.% relative to nickel) in 1,2-butanediol. For example, nickel acetate at a concentration of 0.08 M, sodium hydroxide at a concentration of 0.08 M and ruthenium chloride trihydrate (molar ratio Ru / Ni: 0.025) are added in powder form to 50 ml of 1,2-butanediol in a flask.

[0063] The medium is stirred using a mechanical stirrer, at room temperature, for 30 minutes, then heated to 170°C with a temperature ramp of 8°C / min, and left at 170°C for 1.5 hours.

[0064] After cooling the reaction medium, the product can be recovered by centrifugation (for example 20,000 rpm for 1.5 min), the powders are washed with ethanol and acetone.

[0065] Properties of the obtained catalyst

[0066] The material obtained is in the form of a black powder. Analysis of the powder by X-ray diffraction reveals a crystalline phase of nickel, see figure 1.

[0067] Powder diffractogram analysis by Rietveld analysis allows determining a lite crystal size of about 20-30 nm.

[0068] A Rietveld refinement of the diffractograms is for example carried out using the MAUD software (Material Analysis Using Diffraction).

[0069] Rietveld refinement is presented for example by Stephens Uniting electron crystallography and powder diffraction, 2012, 10. 1007 / 978-94- 007-5580-2 2) . Rietveld refinement is a method for global simulation of X-ray diffraction patterns on polycrystalline samples, this method being implemented in different software such as FullProf, Jana2006 and MAU D. The MAUD software was developed by Luca Lutterotti { L. Lutterotti et al. MAUD: a friendly Java program for material analysis using diffraction. lUCr: Newsletter of the CPD, 21: 14- 15, 1999).

[0070] Characterization of the material by scanning electron microscopy (SEM) reveals particles of approximately 20 to 50 nm aggregated in the form of particles of size varying from 80 nm to 300 nm, see figure 2.

[0071] Transmission electron microscopy (TEM) coupled with EDX (Energy-Dispersive X-ray spectroscopy) analysis reveals the presence of ruthenium nanoparticles deposited on the surface of nickel wafers, see figures 2 and 3. The material obtained is thus made up of nickel-ruthenium nanoparticles of 20 to 50 nm aggregated into aggregates of size between 80 and 300 nm. Ruthenium surface nanoparticles of 2 to 5 nm are deposited on the surface of the nickel nanoparticles.

[0072] The deposition of ruthenium Ru surface nanoparticles on the surface of aggregated nickel particles occurs directly during the synthesis of the material, carried out in a single step.

[0073] Chemical analyses by energy dispersive X-ray spectroscopy (EDX) allow the determination of an atomic ratio of Ru / (Ru + Ni) of 0.025, comparable to that initially introduced into the solution. The final Ru / Ni ratio in the particles thus corresponds to that initially introduced into the reaction medium.

[0074] The specific surface area of ​​the materials obtained, calculated by the BET method (Brunauer Emmett and Teller) from nitrogen physisorption measurements, is between 5 and 15 m 2 / g, see figure 4.

[0075] Arranging ruthenium nanoparticles on nickel nanoparticles with different sizes provides very close contact between the ruthenium and nickel atoms.

[0076] Thus, the heterogeneous nanoparticles according to the invention, through this heterostructuring which creates an increase in the interface between the surface ruthenium nanoparticles and the aggregated nickel nanoparticles, make it possible to observe a very efficient reaction of the reduction of water in an alkaline medium as described below.

[0077] Catalyst activity

[0078] The activity of the catalyst is now described.

[0079] The obtained catalyst is tested for water reduction in an alkaline medium, in a three-electrode electrochemical cell, in an alkaline medium (0.1 M potassium hydroxide KOH solution).

[0080] The activity of this material and of a reference catalyst measured under these conditions is shown in Figure 5.

[0081] The material was deposited on a glassy carbon electrode in the presence of Vulcan XC72 carbon and Nation®. The product marketed under the Vulcan brand, with the reference XC72, is a carbon black in the form of spherical nanoparticles with a diameter of approximately 50 nm and a specific surface area of ​​approximately 250 m 2 / g. The electrochemical performances of the electrode thus prepared, as well as those of an electrode prepared according to the same protocol with commercial catalyst Pt / C 60 wt.% are presented in Figure 5.

[0082] In both cases, the same amount of catalyst was used. Figure 5 shows the calculated specific activity versus the amount of noble metal introduced into the electrode measured at an overvoltage of - 300 mV vs. RHE Reversible Hydrogen Electrode.

[0083] The syntheses of the materials were carried out and repeated at least three times.

[0084] The performance of a material according to the invention is now described in various implementations.

[0085] The following synthesis parameters were varied and the performances of the prepared materials were tested, under the conditions described above.

[0086] Effect of the Ru / (Ni + Ru) ratio

[0087] Tests were carried out with a Ru / (Ni + Ru) ratio varying between 0.005 and 0.5.

[0088] Measured overvoltages vs. RHE at -10 mA / cm 2 and -40 mA / cm 2 are shown in Figure 6.

[0089] The lower the overvoltage for a given current density, the more active the material.

[0090] In the range of atomic percentage of ruthenium Ru initially introduced during synthesis (0.5% to 5%), the most active material is that prepared with 2.5% Ru.

[0091] Effect of the metal used

[0092] Different metals were tested (ruthenium Ru, platinum Pt, iridium Ir, palladium Pd, no noble metal).

[0093] Measured overvoltages vs. RHE at -10 mA / cm 2 and -40 mA / cm 2 on materials prepared with 2.5 atomic% of different noble metals, or without noble metal, are reported in Figure 7.

[0094] The lower the overvoltage for a given current density, the more active the material.

[0095] The most active material is the one prepared with 2.5% ruthenium Ru. Effect of solvent

[0096] Tests were carried out with different solvents: 1,2-butanediol, 1,3-butanediol, propanediol, ethylene glycol.

[0097] Measured overvoltages vs. RHE at -10 mA / cm 2 and -40 mA / cm 2on materials prepared in different solvents: 1,2-butanediol (1,2-BD), 1,2-propanediol (1,2-PD), 1,3-butanediol (1,3-BD) and ethylene glycol (EG), and for concentrations identical to those of the reference synthesis are reported in Figure 8.

[0098] The lowest overpotentials are obtained for materials synthesized in 1,2-butanediol.

[0099] Effect of the one-step process

[0100] Two-step material formation was compared to one-step formation.

[0101] In one-step synthesis, all reagents are introduced simultaneously in the cold, before being heated to 170°C for 1.5 hours.

[0102] In the two-step synthesis, the ruthenium salt is first introduced and brought to 170 °C, then the solution is cooled to room temperature, the nickel salt is added and then the solution is heated again to 170 °C.

[0103] Measured overvoltages vs. RHE at -10 mA / cm 2 and -40 mA / cm 2 on materials prepared in two stages are higher, and therefore the material performs less well, than those of materials prepared in a single stage.

[0104] Effect of heating

[0105] This effect of heating is illustrated in Figure 10.

[0106] Advantageously, for heating, a temperature ramp of 10°C / min is applied from room temperature in a heating mantle to reach a temperature of 170°C, the medium being kept stirring.

[0107] This heating method is compared to that of an embodiment in which the flask containing the reaction medium is directly immersed in an oil bath previously heated to 170°C.

[0108] The percentages of ruthenium in the metallic fraction of the NPs (ratio of the numbers of moles Ru / (Ru +Ni)*100) are evaluated by two methods. The initial ratio of Ru, as introduced at the beginning of the synthesis, is 2.5%.

[0109] According to a first method, these percentages are evaluated by XPS, only the surface of the NPs being probed (approx. 10 nm maximum). A value of 25 is obtained when heating is carried out with a temperature ramp, compared to approximately 7.5 when heating is carried out according to the comparative method of the state of the art.

[0110] According to a second method, the percentages of ruthenium in the metallic fraction of the NPs are evaluated by EDX in an electron microscope, which is rather a measurement of the bulk, namely that the entire material is analyzed. A value of 5 is obtained when heating is carried out with a temperature ramp, compared to approximately 1 when heating is carried out according to the comparative method of the state of the art.

[0111] Heating by temperature ramp leads to much higher ruthenium content values ​​at the surface, compared to instantaneous direct heating in an oil bath at 170°C.

Claims

AMENDED CLAIMS received by the International Bureau on November 13, 2023 (11 / 13 / 2024) 1. Process for the synthesis in an alkaline medium of a heterostructured nickel-ruthenium catalyst, comprising a single mixing step, followed by a stirring step and a heating step of a nickel salt and a ruthenium salt in a solvent, process in which the mixture obtained is stirred at room temperature in the presence of sodium hydroxide and the stirring of the nickel and ruthenium salts is followed by progressive heating of the nickel and ruthenium salts and carried out with a temperature ramp, the molar ratio Ru / (Ni + Ru), measured by energy dispersive X-ray spectroscopy, being between 0.05 and 0.1, the heterostructured catalyst comprising, at the end of the synthesis, heterogeneous nanoparticles having a nickel content greater than 90% and composed of: nickel nanoparticles, and ruthenium nanoparticles which are on the surface of each nickel nanoparticle, the size of the ruthenium nanoparticles, which is measured by transmission electron microscopy (TEM), being smaller than the size of the nickel nanoparticles, which is measured by transmission electron microscopy (TEM), the metallic percentage of ruthenium on the surface of the heterogeneous nanoparticles, measured by X-ray photoelectron spectroscopy (XPS), being greater than the total metallic percentage of ruthenium in the heterogeneous nanoparticles, measured by energy dispersive X-ray spectroscopy (EDX), the nanoparticles having at least one dimension less than 100 nm.

2. Synthesis process according to claim 1, in which the progressive heating of the nickel and ruthenium salts is carried out with a temperature ramp, from room temperature to a maximum temperature Tmax, with a temperature ramp between 1°C / min and 10°C / min applied from room temperature to a maximum temperature Tmax of less than 200°C.

3. Synthesis process according to any one of claims 1 to 2, in which the mixture is produced with nickel acetate as nickel salt, and ruthenium chloride as ruthenium salt. AMENDED SHEET (ARTICLE 19) 4. Synthesis process according to any one of claims 1 to 3, in which the solvent in which the mixture of nickel salts and ruthenium salts is produced is chosen from the group comprising 1,2-butanediol, 1,2-propanediol, 1,3-butanediol, 1,4-butanediol, ethylene glycol.

5. Synthesis process according to any one of claims 1 to 4, in which the catalyst is synthesized in one step from nickel acetate in the presence of sodium hydroxide and ruthenium chloride in 1,2-butanediol, the medium is stirred at room temperature for 30 minutes, then a temperature ramp between 1°C / min and 10°C / min is applied, and left at a temperature Tmax between 150°C and 200°C, advantageously 170°C, for a period of one hour to three hours.

6. Synthesis process according to any one of claims 1 to 4, in which the heating is carried out at a temperature Tmax of less than 200°C, for example 170°C, for a duration of less than 3 hours, for example 1.5 h.

7. Hetero-structured catalyst based on nickel and ruthenium for the electro-catalytic reduction of water to dihydrogen, at the cathode of an electrolyser, in an alkaline medium, this catalyst being produced by heterogeneous nucleation, with a synthesis process defined in any one of claims 1 to 6, the hetero-structured catalyst comprising at the end of the synthesis process a Ru / (Ni + Ru) molar ratio of between 0.005 and 0.1 , measured by energy dispersive X-ray spectroscopy (EDX), and being formed of heterogeneous nanoparticles having a nickel content greater than 90% and composed of nickel nanoparticles, ruthenium nanoparticles which are on the surface and in contact with each nickel nanoparticle, the size of the ruthenium nanoparticles, which is measured by transmission electron microscopy (TEM), being smaller than that of the nickel nanoparticles, which is measured by transmission electron microscopy (TEM), the metallic percentage of ruthenium on the surface of the heterogeneous nanoparticles, measured by X-ray photoelectron spectroscopy (XPS), being greater than the total metallic percentage of ruthenium in the heterogeneous nanoparticles, measured by energy dispersive X-ray spectroscopy, the nanoparticles having at least one dimension less than 100 nm. AMENDED SHEET (ARTICLE 19) 8. Heterostructured catalyst according to claim 7, wherein the molar ratio Ru / (Ni + Ru) in the catalyst is between 0.005 and 0.

05.

9. Hetero-structured catalyst according to one of claims 7 or 8, in which the metallic percentage of ruthenium on the surface of the heterogeneous nanoparticles relative to the total metallic percentage of ruthenium and nickel on the surface of the heterogeneous nanoparticles and equal to Ru / (Ni + Ru) is between 0.05 and 0.4, according to analysis by X-ray photoelectron spectroscopy.

10. Hetero-structured catalyst according to any one of claims 7 to 9, in which the catalyst comprises individual nickel nanoparticles of size between 20 nm and 150 nm, decorated on the surface by ruthenium nanoparticles of size less than 10 nm. 1 1. Heterostructured catalyst according to claim 1 0, in which the catalyst has individual nickel nanoparticles of 20 to 50 nm, decorated on the surface by ruthenium nanoparticles of 2 to 5 nm.

12. Hetero-structured catalyst according to any one of claims 7 to 9, in which the catalyst comprises nickel nanoparticles which are aggregated together, for example in the form of clusters of size between 40 nm and up to one micron, these aggregates being decorated on the surface by ruthenium nanoparticles.

13. Heterostructured catalyst according to any one of claims 7 to 12, in which the heterogeneous nanoparticles have a nickel content greater than 95%, advantageously greater than 98%.

14. Use of the hetero-structured catalyst based on nickel and ruthenium for the electro-catalytic reduction of water into dihydrogen, defined according to any one of claims 7 to 13, at the cathode of an electrolyser, in an alkaline medium.

15. Use according to claim 14, wherein the Ru / (Ni + Ru) molar ratio in the catalyst, measured by energy dispersive X-ray spectroscopy (EDX), is between 0.005 and 0.

05. AMENDED SHEET (ARTICLE 19) 16. Use according to claim 14 or 15, wherein the molar ratio Ru / (Ni + Ru) in the catalyst, measured by energy dispersive X-ray spectroscopy (EDX), is between 0.025 and 0.

035.

17. Use according to any one of claims 14 to 16, in which the metallic percentage of ruthenium on the surface of the heterogeneous nanoparticles relative to the total metallic percentage of ruthenium and nickel on the surface of the heterogeneous nanoparticles is equal to Ru / (Ni + Ru) and is between 0.05 and 0.40, according to analysis by X-ray photoelectron spectroscopy.

18. Use according to any one of claims 14 to 17, in which the specific surface area of ​​the catalyst measured by nitrogen physisorption, according to the BET method (Brunauer, Emmett and Teller) is between 3 and 50 m 2 / g .

19. Use according to any one of claims 14 to 18, in which the catalyst is used at the cathode of the electrolyser, in the presence of a polymer material.

20. Use according to any one of claims 14 to 19, wherein the electrolyser comprises an alkaline liquid electrolyte or an anionic or cationic conductive polymer electrolyte.

21. Use according to any one of claims 14 to 20, in which the catalyst is used at the cathode of the electrolyser, in the presence of a polymer material chosen from fluoropolymers, advantageously a copolymer based on sulfonated polytetrafluoroethylene. AMENDED SHEET (ARTICLE 19)

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