Patent Description:
Global warming, ocean acidification, extreme weather events, and low air quality require an urgent energy transition towards renewable energies (wind, solar, thermal, etc.). However, their intermittent nature implies their storage. Hydrogen in tandem with a fuel cell (FC) appears among the most promising solution for energy storage and recovery, and some are already on the market. Although hydrogen is the most abundant gas in the universe, it is found on earth almost only in combined form (CH<NUM>, H<NUM>O. Steam methane reforming in which CO<NUM> is emitted is currently the main industrial process for hydrogen manufacturing (> <NUM>%). If CO<NUM> is not sequestered, this hydrogen is called gray hydrogen, if captured upstream, it is then qualified as blue hydrogen. In the long term, its production must tend towards greenhouse gas-free process to give decarbonized hydrogen called green hydrogen. The electrolysis of water falls under this last appellation and represents, in a vision of sustainable development, the most attractive solution. However, the industrial development of electrolyzers is hampered by their low energy efficiency, related to the need of a significant overpotential to the anode as illustrated in <FIG>. Consequently, the cost of producing green hydrogen is four times higher than that of gray hydrogen.

According to the mechanisms proposed in the literature for the Oxygen Evolution Reaction (OER) reaction, the overpotential originates from the energy barriers of four intermediate reaction steps (Table <NUM>). The addition of a catalyst lowers these barriers and thus the overpotential (<FIG>). To date, the benchmark catalysts are iridium and ruthenium oxides, whether in acidic or basic medium. However, their prohibitive cost makes any industrial application difficult.

As an alternative to these benchmark oxides, new catalysts based on 3d elements have recently been proposed: ABO<NUM> perovskites, AB<NUM>O<NUM> spinels and layer structures of mono- or multimetallic oxyhydroxides MOOH (<NPL>). This latter family provides currently the best alternative, in alkaline medium, to precious metal oxides but not industrialized.

Mixed-metal oxyfluorides electrocatalysts and their use as electrodes in an electrolyzer have been described by Lemoine Kévin et al. (<NPL>; and <NPL>).

Despite the aforementioned progress, there is still a need to continually push the performance of earth-abundant water oxidation catalysts.

According to a first object, the present invention thus concerns a compound of formula (I):.

(M1<NUM>+xM2<NUM>+<NUM>-x)<NUM>-u-v(M1'<NUM>+yM2'<NUM>+<NUM>-y)u(M1"<NUM>+zM2"<NUM>+<NUM>-z)vO<NUM>-wF<NUM>+w     (I).

According to an embodiment, in formula (I) :.

As used herein, the transition metals of the d/f block include the following elements:.

Alkaline-earth elements refer to Be, Mg, Ca, Sr, Ba, Ra.

According to an embodiment, M1, M1' and M1" belong to d/f block or alkaline-earth elements in the periodic table and M2, M2' and M2" belong to d/f block of the periodic table.

In particular, M1 and M1', M1" are chosen from Mn, Fe, Co, Ni, Cu, Zn and M2 and, M2', M2" are chosen from Mn, Fe, Co, Ni, Ru, Rh.

According to an embodiment, either x = <NUM> and y = z = u = v = <NUM> or.

According to a particular embodiment, the compound of formula (I) is chosen from the following compounds:.

According to a further object, the present invention concerns a solid solution of the compound of formula (I) as defined above.

The term "solid solution" as used herein refers to a unique phase component made of the compound of formula (I) as defined above.

According to a still further object, the present invention concerns an electrode comprising the compound of the invention, as a catalyst.

As a catalyst, the compounds of formula (I) lower the kinetics barrier of the intermediates reactions involved in water electrolysis. Some illustrative reactions are set out in Table <NUM> below:.

The symbol * means the active site of the catalyst.

According to an embodiment, said electrode may be a working, a positive or a negative electrode.

Typically, the electrode comprises a support loaded with said catalyst.

Said support is generally chosen from stable, inert high-surface area substrates which are able to load the catalyst.

The loading may be achieved in particular by impregnating the support with an ink comprising the catalyst compound (I). Typically, the support may be soaked in the ink or spread with the ink.

Said ink may be a mixture of the catalyst compound (I) in an aqueous solution, optionally comprising a water soluble solvent (such as ethanol). Said ink may also comprise one or more binders, such as Nafion and multiwalled carbon nanotubes. The CNT helps electrically connect the catalyst and the Nafion helps with adhesion to the substrate.

Said support may be chosen in particular from carbon paper (such as Toray carbon paper), Ni foam, transparent conductive oxides (TCO) such as fluorinated tin oxide (Fluorine doped tin oxide, FTO), indium tin oxide (ITO) and antimony doped tin oxide (ATO).

The catalyst loaded support located on one side of the electrode contacts with the electrolyte whereas the other side of the electrode is connected with a current collector.

According to another object, the present invention also concerns a process of preparation of the solid solution of the invention. Said process comprises the following steps:.

The concentrated hydrofluoric acid (HF) solution is a solution of hydrogen fluoride in water, in a concentration comprised between <NUM> and <NUM> mol. L-<NUM>, typically <NUM> mol. L-<NUM> (HF<NUM>%).

The solution of dissolved metal salts refers to the concentrated hydrofluoric acid solution wherein salts of M1, M2, M1', M2', M1", M2" as well as their counter-ions are dissolved. Counter-ions may include anions such as chloride, nitrate, carbonate, phosphate, sulfate, acetate, carbonate, etc.. Such solution of dissolved metal salts can be achieved by dissolving under stirring salts of M1, M2, M1', M2', M1", M2"with their respective counter ions, or solvates therefrom in the concentrated hydrofluoric acid solution, in respective concentrations corresponding to the respective ratio of M1, M2, M1', M2', M1", M2" in formula (II).

The evaporation may be conducted by stirring the solution under heating (generally in an oil bath) at a temperature below or equal to the boiling temperature of the solution, typically at about <NUM> for a duration sufficient to achieve a precipitate.

Alternatively, the precipitate may be obtained by the precipitation of the solution of concentrated hydrofluoric acid (HF) of the dissolved metal salts, by addition of alcohol, typically ethanol. Said precipitation may be handled at room temperature.

According to both alternatives, the precipitate is then separated e.g. by filtration. Said precipitate is constituted by the hydrated fluorinated compound (II).

The step of thermal treatment may be carried out in a furnace at a temperature comprised between <NUM> and <NUM>, generally under ambient atmosphere, in a duration sufficient to achieve the weight loss corresponding of the loss of the six water and two HF molecules comprised in the hydrated fluorinated precursor of formula (II).

The compounds of formula (II) are novel and are another object of the invention:.

(M1<NUM>+xM2<NUM>+<NUM>-x)<NUM>-u-v(M1'<NUM>+yM2'<NUM>+<NUM>-y)u(M1"<NUM>+zM2"<NUM>+<NUM>-z)vF<NUM>(H<NUM>O)<NUM>     (II).

Where x, y, z, u, v, M1, M2, M1', M2', M1" and M2" are defined as above.

According to another object, the invention also concerns an electrochemical cell comprising at least one electrode of the invention.

Typically, said electrochemical cell may be a water electrolyzer and/or a fuel cell.

In the case of electrolysis, the electrode of the invention is typically a working electrode (i.e.) the electrode on which the reaction of interest (water electrolysis) occurs.

The working electrode is typically used in three electrode system, in conjunction with an auxiliary electrode (Pt or carbon counter electrode) and a reference electrode (such as Hg/HgO reference electrode).

The electrolyte may generally be chosen from strong acids such as sulfuric acid and strong bases such as potassium hydroxide and sodium hydroxide, due to their strong conducting abilities. A solid polymer electrolyte such as Nafion can also be used.

Fuel cells are made up of an anode and a cathode separated by an electrolyte.

The electrolyte can be chosen from potassium hydroxide, salt carbonates, and phosphoric acid or can be a polymer membrane in particular.

According to another object, the present invention also concerns a process of preparation of hydrogen comprising the electrolysis of water with an electrochemical cell of the invention.

Water electrolysis (also called water splitting) refers to the process of using electricity to decompose water into oxygen and hydrogen gas, requiring a minimum potential difference of <NUM> volts.

Hydrogen gas formed in this way can be used as hydrogen fuel in a fuel cell combined with said water electrolyzer.

In the fuel cell, hydrogen is consumed, water is created, and an electric current is generated, which can be used to power electrical devices.

The following reactions are involved (Table <NUM>):.

Electrocatalytic measurements are performed in a standard <NUM>-electrode setup in a custom-built glass single compartment reaction cell. Hg/HgO reference and Pt or carbon counter electrodes are used for all measurements. For a working electrode substrate, Toray Carbon Paper is used as a model high-surface area support (catalyst loading of <NUM>. Alternatively, a rotating disk setup with a glassy carbon surface is used (catalyst loading of <NUM>. To generate a catalyst-coated working electrode, a catalyst ink is generated by sonicating <NUM>µl ethanol, <NUM>µl de-ionized water, <NUM>µl of <NUM>% Nafion™ solution, <NUM> catalyst and <NUM> multiwalled carbon nanotubes (<NUM>-<NUM> diameter). The ink is spread onto a working electrode surface and allowed to dry under ambient conditions. <NUM> KOH is used as the electrolyte in all measurements at room temperature (approximately <NUM>) and <NUM>% compensation of the solution resistance is used correct correction. The solution resistance is measured at open circuit at <NUM> frequency before each measurement. Turnover frequencies (TOFs) are calculated by dividing the reaction rate (extracted from the current density, assuming <NUM> electrons extracted from each water molecule) by the redox-active cobalt species, calculated through integrating the area under the Co(II/III) redox peak at <NUM> V vs. RHE (Reversible Hydrogen Electrode).

The Co<NUM>+<NUM>Fe<NUM>+<NUM>O<NUM>F<NUM> catalyst-loaded carbon paper electrode exhibits <NUM> mV overpotential for a current density of <NUM> mA. cm-<NUM> (<FIG>). The Tafel slope, measured in a current density range of <NUM>-<NUM> mA. cm-<NUM>, is calculated to be <NUM> mV. decade-<NUM> (<FIG>). At <NUM> mV overpotential, the mass activity of Co<NUM>+<NUM>Fe<NUM>+<NUM>O<NUM>F<NUM> reaches <NUM> A. g-<NUM>, with a TOF of <NUM>-<NUM> per electroactive site (or <NUM>-<NUM> using the total mass of cobalt deposited) (<FIG>). The high activity is also evident through the low activation energy of <NUM> kJ. mol-<NUM>, calculated from the Arrhenius plot (<FIG>). The activity is stable for more than <NUM>, as measured through chronopotentiometric tests at current densities of <NUM>, <NUM>, <NUM> and <NUM> mA. cm-<NUM> (<FIG>).

Overall, the electrocatalytic performance of Co<NUM>+<NUM>Fe<NUM>+<NUM>O<NUM>F<NUM> is arguably the best reported for a cobalt-based material under the above-mentioned standard testing conditions in terms of a combination of overpotential, mass activity, turnover frequency and stability. This conclusion is attained after a careful comparison of quantitative performance metrics of notable catalysts reported over the last several years (Table <NUM>).

Table <NUM> gathers their electrochemical properties in comparison with precious metals oxides and with the best reported oxohydroxides. It should be noted that in order to properly compare catalytic performances of anodic materials, it is necessary to take into account several electrochemical characteristics as well as the measuring conditions:.

The synthesis of oxyfluorinated solid solutions M<NUM>+xM'<NUM>+<NUM>-xO<NUM>-xF<NUM>+x is carried out in two steps: the first one is the synthesis of a hydrated fluorinated precursor M<NUM>+M'<NUM>+F<NUM>(H<NUM>O)<NUM>, frequently written MM'F<NUM>. <NUM><NUM>O, or M<NUM>+xM'<NUM>+<NUM>-xF<NUM>(H<NUM>O)<NUM> in concentrated hydrofluoric acid (HF) and the second one consists of a moderate thermal treatment in ambient atmosphere to obtain the solid solutions M<NUM>+xM'<NUM>+<NUM>-xO<NUM>-xF<NUM>+x.

For the synthesis of MM'F<NUM>·<NUM><NUM>O: The metal salts (chlorides, nitrates, carbonates, phosphates, sulfates, acetates. ) are dissolved in a concentrated HF solution. Then the solution is evaporated at a temperature below the boiling point of the solution until the beginning of the precipitation. The solution is cooled at ambient air and the as-synthesized solid is filtered out, washed and dried leading to MM'F<NUM>(H<NUM>O)<NUM> with a yield around <NUM>%. These synthesis methods can be extending to three (or more) metals by multi-substitution as well as on +II and/or +III metal cations such as M<NUM>+<NUM>-xM"<NUM>+xM'<NUM>+F<NUM>(H<NUM>O)<NUM> or M<NUM>+M'<NUM>+<NUM>-xM"<NUM>+xF<NUM>(H<NUM>O)<NUM>. As alternative synthesis, after dissolution of the metal salts, the precipitation is triggered by an addition of an alcohol such as ethanol at room temperature. The solid is recovered by the same aforementioned protocol.

For the synthesis of M<NUM>+xM'<NUM>+<NUM>-xF<NUM>(H<NUM>O)<NUM>. the metal salts (chlorides, nitrates, carbonates, phosphates, sulfates, acetates. ) are dissolved in alcohol (ethanol, isopropanol,. The solvent should be degassed prior to the reaction by bubbling it with argon for <NUM>. After the degassed concentrated HF (bubbling with argon for <NUM>) is added allowing the hydrated fluoride to precipitate. The as-synthesized solid is filtered out, washed and dried leading to MM'F<NUM>(H<NUM>O)<NUM> with a yield around <NUM>%. These synthesis methods can be extending to three (or more) metals by multi-substitution such as M<NUM>+<NUM>-xM"<NUM>+xM'<NUM>+F<NUM>(H<NUM>O)<NUM>.

<NUM><NUM>O (<NUM>) and Fe(NO<NUM>)<NUM>·<NUM><NUM>O (<NUM>) are dissolved into <NUM> of a concentrated hydrofluoric acid solution (<NUM> mol. L-<NUM>, HF<NUM>%). The reaction mixture is placed in a Teflon Becher and stirred for <NUM> at <NUM> in an oil bath until the formation of a precipitate. After cooling, the mixture is filtered, washed with technical ethanol and dried at room temperature giving pink powder.

Ni(NO<NUM>)<NUM>·<NUM><NUM>O (<NUM>) and Fe(NO<NUM>)<NUM>·<NUM><NUM>O (<NUM>) are dissolved into <NUM> a concentrated hydrofluoric acid solution (<NUM> mol. L-<NUM>, HF<NUM>%). The reaction mixture is placed in a Teflon Becher and stirred for <NUM> at <NUM> in an oil bath until the formation of a precipitate. After cooling, the mixture is filtered, washed with technical ethanol and dried at room temperature giving green powder.

CoCl<NUM>·<NUM><NUM>O (<NUM>), Ni(NO<NUM>)<NUM>·<NUM><NUM>O (<NUM>) and Fe(NO<NUM>)<NUM>·<NUM><NUM>O (<NUM>), are dissolved into <NUM> a concentrated hydrofluoric acid solution (<NUM> mol. L-<NUM>, HF<NUM>%). The reaction mixture is placed in a Teflon Becher and stirred for <NUM> at <NUM> in an oil bath until the formation of a precipitate. After cooling, the mixture is filtered, washed with technical ethanol and dried at room temperature giving brownish powder.

CoCl<NUM>·<NUM><NUM>O (<NUM>), and FeCl<NUM>·<NUM><NUM>O (<NUM>), are dissolved into <NUM> of isopropanol. The latter has been, prior to the synthesis, carefully degassed by bubbling it using argon for <NUM>. The reaction mixture is placed in a round bottom flask previously flushed with argon. Once the solubilisation is completed, <NUM> of degassed concentrated hydrofluoric acid solution (<NUM> mol. L-<NUM>, HF<NUM>%) (<NUM> minute by bubbling it using argon) is added to the solution and the solution is allowed to stir for <NUM> under argon atmosphere until precipitation is complete. Finally, the mixture is filtered, washed with technical ethanol and dried at room temperature giving a pink powder.

CoCl<NUM>·<NUM><NUM>O (<NUM>), Ni(NO<NUM>)<NUM>·<NUM><NUM>O (<NUM>) and FeCl<NUM>·<NUM><NUM>O (<NUM>), are dissolved into <NUM> of isopropanol. The latter has been, prior to the synthesis, carefully degassed by bubbling it using argon for <NUM>. The reaction mixture is placed in a round bottom flask previously flushed with argon. Once the solubilisation is completed, <NUM> of degassed concentrated hydrofluoric acid solution (<NUM> mol. L-<NUM>, HF<NUM>%) (<NUM> minute by bubbling it using argon) is added to the solution and the solution is allowed to stir for <NUM> under argon atmosphere until precipitation is complete. Finally, the mixture is filtered, washed with technical ethanol and dried at room temperature giving a brownish powder.

A thermal treatment at moderated temperature (+/- <NUM>) in atmospheric conditions of hydrated fluorides MM'F<NUM>(H<NUM>O)<NUM> leads to the formation of solid solutions M<NUM>M'<NUM>O<NUM>F<NUM> along the general reaction:.

MM'F<NUM>(H<NUM>O)<NUM> → <NUM><NUM>M'<NUM>O<NUM>F<NUM> + 2HF + <NUM><NUM>O.

Co<NUM>+<NUM>Fe<NUM>+<NUM>O<NUM>F<NUM> is obtained by thermal treatment of Co<NUM>+Fe<NUM>+F<NUM>(H<NUM>O)<NUM> under ambient atmosphere at <NUM> for <NUM> in a furnace corresponding to an experimental weight loss of <NUM>% close to the theoretical value <NUM>% (mbefore = <NUM>, mafter = <NUM>).

Ni<NUM>+<NUM>Fe<NUM>+<NUM>O<NUM>F<NUM> is obtained by thermal treatment of Ni<NUM>+Fe<NUM>+F<NUM>(H<NUM>O)<NUM> under ambient atmosphere at <NUM> for 1h30 min in a furnace corresponding to an experimental weight loss of <NUM>% close to the theoretical value <NUM>% (mbefore = <NUM>, mafter = <NUM>).

Co<NUM>+<NUM>Ni<NUM>+<NUM>Fe<NUM>+<NUM>O<NUM>F<NUM> is obtained by thermal treatment of Co<NUM>+<NUM>Ni<NUM>+<NUM>Fe<NUM>+F<NUM>(H<NUM>O)<NUM> under ambient atmosphere at <NUM> for <NUM> in a furnace corresponding to an experimental weight loss of <NUM>% close to the theoretical value <NUM>% (mbefore = <NUM>, mafter = <NUM>).

Co<NUM>+<NUM>Fe<NUM>+<NUM>O<NUM>F<NUM> is obtained by thermal treatment of Co<NUM>+<NUM>Fe<NUM>+<NUM>F<NUM>(H<NUM>O)<NUM> under dry synthetic air at <NUM> for <NUM> in a furnace previously flushed for <NUM> under dry synthetic air, corresponding to an experimental weight loss of <NUM>% close to the theoretical value <NUM>% (mbefore = <NUM>, mafter = <NUM>).

Co<NUM>+<NUM>Ni<NUM>+<NUM>Fe<NUM>+<NUM>O<NUM>F<NUM> is obtained by thermal treatment of Co<NUM>+<NUM>Ni<NUM>+<NUM>Fe<NUM>+<NUM>F<NUM>(H<NUM>O)<NUM> under dry synthetic air at <NUM> for <NUM> in a furnace previously flushed for <NUM> under dry synthetic air, corresponding to an experimental weight loss of <NUM>% close to the theoretical value <NUM>% (mbefore = <NUM>, mafter = <NUM>).

Experimental XRD patterns match with the known XRD pattern of the CoFeF<NUM>(H<NUM>O)<NUM> (PDF card number <NUM>-<NUM>-<NUM>) (<FIG>). Small shifts of diffraction peaks related to different metal ionic radii are observed. The metal ratio M'<NUM>+/M<NUM>+ close to one, obtained by SEM-EDX, is in good agreement with M<NUM>+M'<NUM>+F<NUM>(H<NUM>O)<NUM> formulations (<FIG>).

TXRD: The monitoring of the structural evolution of Co<NUM>Fe<NUM>O<NUM>F<NUM> as function of the temperature exhibits four domains (<FIG>):.

TGA: The formulation of the intermediate phase Co<NUM>Fe<NUM>O<NUM>F<NUM> observed by TDRX is confirmed by TGA under ambient air (<FIG>). CoFeF<NUM>(H<NUM>O)<NUM> undergoes the following decompositions upon thermal treatment, the experimental weight loss values are in good agreement with theoretical values:.

XRD: XRD patterns of Co<NUM>Fe<NUM>O<NUM>F<NUM> and Ni<NUM>Fe<NUM>O<NUM>F<NUM> match with the PDF card (<NUM>-<NUM>-<NUM>) of FeOF (rutile structure type) and show line profiles which are characteristic of low crystalline compounds (<FIG>).

FTIR: Water removal of hydrated fluorides CoFeF<NUM>(H<NUM>O)<NUM> and CoFeF<NUM>(H<NUM>O)<NUM> after thermal treatment is confirmed by FTIR (<FIG>). The FTIR spectra of CoFeF<NUM>(H<NUM>O)<NUM> and CoFeF<NUM>(H<NUM>O)<NUM> present a broad signal between <NUM> and <NUM>-<NUM> and a sharp peak centered at <NUM>-<NUM> attributed to the stretching (νO-H) and bending (δH-O-H) modes respectively. After thermal treatment, these signals are no longer present, confirming H<NUM>O removal.

The thermal treatment induces a significant increase of the specific surface area (SBET) between the hydrated fluorides and the oxyfluorides: CoFeF<NUM>(H<NUM>O)<NUM> (<NUM><NUM>. g-<NUM>), NiFeF<NUM>(H<NUM>O)<NUM> (<NUM><NUM>. g-<NUM>), Co<NUM>Fe<NUM>O<NUM>F<NUM> (<NUM><NUM>. g-<NUM>), and Ni<NUM>Fe<NUM>O<NUM>F<NUM> (<NUM><NUM>. g-<NUM>) (<FIG>).

Claim 1:
A compound of formula (I):

        (M1<NUM>+xM2<NUM>+<NUM>-x)<NUM>-u-v(M1'<NUM>+yM2'<NUM>+<NUM>-y)u(M1"<NUM>+zM2"<NUM>+<NUM>-z)vO<NUM>-wF<NUM>+w     (I)

Wherein
M1, M2, M1', M2', M1", M2" are transition metals of the d/f block or alkaline-earth elements in the periodic table;
u, v, x, y and z are identical or different and are such that
u+v < <NUM>;
<NUM> < x,y,z < <NUM>;
w = x+u(y-x)+v(z-x).