Catalyst and method for producing hydrogen by proton reduction

Abstract

The invention relates to a catalyst comprising a nickel(II) complex comprising a bis(thiosemicarbazone) ligand derived from 2,2'-thenil, the nickel(II) complex having the general formula [Chem 6] wherein R1 and R2 each independently represent a phenyl group optionally having one or more identical or different substituents R3, R3 is selected from a halogen, a hydroxy group, a C1-C4 alkyl group, a C1-C4 alkoxy group, a C1-C4 thioalkyl group, a C1-C4 dialkylamino group, a cyano group, a CF3 group and an O-CF3 group.

Description

Description Title: Catalyst and process for the production of hydrogen by proton reduction. [Technical field.

[0001] The present invention relates to a catalyst comprising a nickel(ll) complex involving a bis(thiosemicarbazone) ligand derived from 2,2'-thenil and its use in a process for producing hydrogen by proton reduction. [2] The invention relates to the technical field of electrochemical production of hydrogen. State of the art. [3] Faced with the depletion of fossil fuels (oil, coal, natural gas, etc.), environmental pollution and climate change for which human activity (emissions of greenhouse gases such as carbon dioxide and methane) is largely responsible, the energy transition, which aims to achieve carbon neutrality and gradually move away from fossil fuels, is now becoming a major subject of collective concern. In this context, hydrogen (more precisely dihydrogen, H2) is expected to play a major role in the global transition towards an economy less dependent on fossil fuels, by contributing to the diversification of the energy mix and to achieving the objectives of reducing greenhouse gas emissions and protecting the climate. [4] Hydrogen is a gas whose chemical properties offer major energy benefits. It has the highest energy density of all fuels (three times more than oil and 4.5 times more than coal). The energy contained in hydrogen can be recovered in two ways: by burning it or by transforming it into electrical energy using a fuel cell. In both cases, the use of hydrogen generates only water, no greenhouse gases. Hydrogen also offers the possibility, after being produced, of being stored, transported and used. [5] Electrolysis of water is a common method of producing hydrogen. It is an electrolytic reaction that allows the water molecule to be decomposed into its chemical constituents, hydrogen (H2) and oxygen (O2), by means of an electric current, according to the overall reaction [Chem.1] below, and of course, without any CO2 emissions. [6] [Chem. 1] [7] Furthermore, if the electricity comes from renewable energy sources, then we are completely replacing fossil fuel sources. It should be remembered that water is also the end product of the recombination reaction of hydrogen (H2) and oxygen (O2), during which the energy from these fuels is released. [8] In an electrolysis system, the electrolytic reaction of water takes place via two half-reactions, one of which is a hydrogen (H2) evolution reaction occurring at a cathode and the other is an oxygen (O2) evolution reaction occurring at an anode. The hydrogen evolution and oxygen evolution reactions predominate in acidic media according to equations [Chem. 2] and [Chem. 3] below and in basic media according to equations [Chem. 4] and [Chem. 5] below: [9] [Chem. 2]

[0010] [Chem. 3]

[0011] [Chem. 4]

[0012] [Chem. 5] 4OH- O2+ 2H2O + 4e-

[0013] In the context of hydrogen production, water electrolysis requires the use of a catalyst to minimize the overvoltage needed to activate the hydrogen evolution reaction, also called the proton reduction reaction.

[0014] Various metals have been studied as catalysts for the hydrogen release reaction. To date, the most efficient catalysts are based on noble metals such as platinum. However, the global scarcity and high cost of noble metals make them unsuitable for industrial use, particularly for large-scale hydrogen production.

[0015] Recently, catalysts based on non-noble transition metals such as cobalt, copper, iron, manganese, and nickel have also been investigated as viable alternatives to activate the hydrogen evolution reaction, due to their natural abundance and low cost. In particular, many transition metal complexes with various ligand structures have been studied and tested as catalysts with varying degrees of efficiency.

[0016] Thiosemicarbazones represent a very attractive class of metal-chelating ligands due to their coordination versatility. From the article by Straistari T. et al., 2017, a nickel(ll) complex with, as ligand, a 1,2-dimethyl-1,2-ethanedione-1,2-di-[(4-methoxyphenyl)semicarbazone] (hereinafter referred to as the “reference NiTSC-OCH3 complex”) is known, which is capable of catalyzing the hydrogen evolution reaction and more precisely the proton reduction reaction (H + ) into hydrogen. However, it was found that this nickel complex (NiTSC-OCH3) suffers from low catalytic activity in the hydrogen evolution reaction (hereinafter referred to as "proton reduction reaction"). Therefore, an improvement in the catalytic activity of the reference NiTSC-OCHs complex is desirable for economical use on an industrial scale.

[0017] Therefore, there remains a need to find new catalysts for the proton reduction reaction that are efficient without the drawbacks mentioned above.

[0018] An object of the present invention is therefore to provide novel metal complexes suitable as catalysts for the hydrogen evolution reaction. Another object of the invention is to provide novel catalysts which exhibit good catalytic activities in the hydrogen evolution reaction. Another object of the invention is to provide such metal complexes and catalysts which are stable and which can be produced simply and inexpensively. Presentation of the invention.

[0019] Quite unexpectedly, following extensive research, the authors of the present invention found that at least one of the above-mentioned aims could be achieved by using a nickel(ll) complex involving a bis(thiosemicarbazone) ligand derived from 2,2'-thenil (also called bis(thiophen-2-yl)ethane-1,2-dione). Indeed, this nickel(ll) complex was found to have a very promising catalytic activity for the electrochemical reaction of proton reduction to hydrogen, with a low overpotential, a high faradaic efficiency and good performance stability. In addition, its synthesis is simple and inexpensive.

[0020] The nickel(ll) complexes proposed by the present invention therefore have significant potential.

[0021] Thus, according to a first aspect of the present invention, there is provided a catalyst comprising a nickel(ll) complex comprising a bis(thiosemicarbazone) ligand derived from 2,2'-thenil, said nickel(ll) complex corresponding to the following general formula [Chem. 6]:

[0022] [Chem. 6] in which, R 1 and R 2 each independently represent a phenyl group optionally having one or more substituents R 3 identical or different, R 3 is selected from halogen, C1-C4 alkyl, C1-C4 alkoxy, C1-C4 thioalkyl, C1-C4 alkylamino, C1-C4 dialkylamino, cyano, CF3 and O-CF3.

[0023] Preferably, when a phenyl group is substituted with one or more substituents R 3 , identical or different, this or these substituents R 3may be selected from a methyl group, a methoxy group, a methylthio group and a dimethylamino group.

[0024] In particular, said R groups 1 and R 2 are identical.

[0025] Preferably, said nickel complex is selected from the group consisting of nickel complexes corresponding to the formulas [Chem. 7] to [Chem. 11] below:

[0026] [Chem. 7]

[0027] [Chem. 8]

[0028] [Chem. 9]

[0029] [Chem. 10]

[0030] [Chem. 11]

[0031] Advantageously, the catalyst according to the present invention is suitable for catalyzing the reduction of protons to hydrogen.

[0032] According to another aspect, the present invention also relates to the use of the catalyst according to the invention for catalyzing the reduction of protons to hydrogen.

[0033] According to yet another aspect, the present invention also provides a method of producing hydrogen comprising contacting a source of protons with a catalyst according to the invention in an electrochemical cell, and applying a potential to the electrochemical cell, thereby reducing the protons to produce hydrogen.

[0034] According to yet another aspect, the present invention also relates to the nickel(ll) complex corresponding to the general formula [Chem. 6] mentioned above. Brief description of the figures.

[0035] Other advantages and characteristics of the invention will appear more clearly on reading the description of the preferred embodiments which follows, with reference to the appended drawings, produced as indicative and non-limiting examples and in which: [Fig. 1] shows a general scheme for the synthesis of a bis(thiosemicarbazone) ligand derived from 2,2'-thenil and whose structure is asymmetric (R 1 + R 2 ). [Fig. 2] shows a general scheme for the synthesis of a bis(thiosemicarbazone) ligand derived from 2,2'-thenil and whose structure is symmetrical. [Fig. 3] shows a general scheme for the synthesis of a nickel(ll) complex according to the present invention (with R 1 = R 2 or R 1 + R 2 ). [Fig. 4] shows a graphical overview of the UV-Visible spectrum of the nickel(ll) complex prepared in Example 1a (hereinafter, Ni(thenil)TSC-OCH3 complex), in solution in DMF with a concentration of 0.05 M. [Fig. 5] represents an ORTEP diagram of the nickel(ll) complex prepared in Example 1a, the ORTEP diagram being based on the results of X-ray crystallographic analysis. Ellipsoids are shown with a probability of 50%. Hydrogen atoms have been removed for clarity. [Fig. 6] is a graph illustrating cyclic voltammograms of the Ni(thenil)TSC-OCH3 complex (1.0 mmol.L -1 ) recorded in a solution of n-Bu4NPFe (0.1 mol.L -1 ) in DMF on a stationary glassy carbon electrode of 1 mm diameter, at different potential scanning rates from 0.1 Vs-1 to 2 Vs -1 . [Fig. 7] is a graph illustrating cyclic voltammograms of the Ni(thenil)TSC-OCH3 complex (1.0 mmol.L -1 ) in the presence of trifluoroacetic acid {(A) 0 mmol.L -1 ; (B) 20 mmol.L -1 ; (C) 40 mmol.L -1 ; (D) 60 mmol.L -1 ; (E) 80 mmol.L -1 ; (F) 100 mmol.L -1} or trifluoroacetic acid alone ((G)100.0 mmol.L -1 ), recorded in a solution of n-Bu4NPFe (0.1 mol.L -1 ) in DMF on a stationary 1 mm diameter glassy carbon electrode, at 0.5 Vs -1 . [Fig. 8] is a graph illustrating cyclic voltammograms of the first reduction wave obtained with the Ni(thenil)TSC-OCH3 complex (1.0 mmol.L -1 ) in the presence of trifluoroacetic acid {(A) 0 mmol.L -1 ; (B) 20 mmol.L -1 ; (C) 40 mmol.L -1 ; (D) 60 mmol.L -1 ; (E) 80 mmol.L -1 ; (F) 100 mmol.L -1}, recorded in a solution of n-Bu4NPFe (0.1 mol.L -1 ) in DMF on a glassy carbon electrode, at 0.5 Vs -1 . [Fig. 9] is a graph illustrating cyclic voltammograms recorded on a glassy carbon electrode at 0.5 Vs -1 , after washing the electrode of glassy carbon used in cyclic volammetry measurements with the Ni(thenil)TSC-OCH3 complex with a fresh solution of n-Bu4NPFe (0.1 mol.L -1 ) in DMF, then immersion in said fresh solution, in the presence of trifluoroacetic acid {(A) 0 mmol.L -1 ; (B) 100 mmol.L -1}. [Fig. 10] is a graph illustrating coulometry curves of the (thenil)TSC-OCH3 complex (1.0 mmol.L -1 ) in the presence and absence of trifluoroacetic acid {(A) 100 mmol.L -1 ; (B) 0 mmol.L -1}, recorded in a solution of n-Bu4NPFe (0.1 mol.L -1) in 8 ml of DMF on a mercury electrode at -1.70 Vs -1 . [Fig. 11] represents a graph of the UV-Visible spectrum of the complex (thenil)TSC-OCH3 (0.05 mmol L -1 ) in solution in DMF, in the absence and presence of trifluoroacetic acid {(A) 0 mmol.L -1 ; (B) 2.5 mmol.L -1 ; (C) 5 mmol.L -1}. Detailed description.

[0036] This description is given without limitation, each characteristic disclosed only in one embodiment being able to be generalized to the other embodiments presented here or in the summary of the invention presented previously. Similarly, one or more characteristics disclosed only in one embodiment can be combined with one or more other characteristics disclosed only in another embodiment. It should be noted from now on that the appended figures are very simplified, the elements represented therein are therefore not necessarily to scale with respect to each other or from one figure to another.

[0037] The present invention is based on the unexpected discovery that nickel(ll) complexes involving a bis(thiosemicarbazone) ligand derived from 2,2'-thenil exhibit increased electrocatalytic activity for the reduction of protons to hydrogen.

[0038] Indeed, nickel(ll) complexes are capable of catalyzing the reduction of protons (H + ) into hydrogen (H2) with notably an improved overvoltage (q), renewal frequency (TOF) and faradaic efficiency (FY). They are also very stable under acidic conditions.

[0039] The present invention therefore relates firstly to a catalyst comprising a particular nickel (II) complex involving a bis(thiosemicarbazone) ligand derived from 2,2'-thenil. This nickel (II) complex corresponds to the general formula [Chem.6]:

[0040] [Chem. 6] in which, R 1 and R 2 each independently represent a phenyl group optionally having one or more substituents R 3 identical or different, R 3is selected from halogen, hydroxy group, C1-C4 alkyl group, C1-C4 alkoxy group, C1-C4 thioalkyl group, C1-C4 dialkylamino group, cyano group, CF3 group and O-CF3 group.

[0041] Throughout the description, the expressions: “nickel complex”, “nickel(ll) complex”, “Ni(ll) complex” and “complex according to the invention” are used interchangeably and equivalently.

[0042] The term “halogen” designates, within the meaning of the invention, an atom chosen from fluorine, chlorine, bromine and iodine atoms.

[0043] The term “hydroxy” designates, within the meaning of the invention, the -OH group.

[0044] The term "C1-C4 alkyl" designates, within the meaning of the invention, a linear or branched saturated aliphatic substituent, comprising from 1 to 4 carbon atoms. Examples that may be mentioned are methyl, ethyl, propyl, isopropyl, butyl, isobutyl and tertbutyl groups.

[0045] The term "C1-C4 alkoxy" denotes, within the meaning of the invention, a linear or branched saturated alkyl radical having 1 to 4 carbon atoms (as mentioned above) which is linked via an oxygen atom, i.e., for example, any of the radicals methoxy, ethoxy, n-propoxy, iso-propoxy (or 1- methylethoxy), n-butoxy, iso-butoxy (or 1-methylpropoxy), tert-butoxy (or 2-methylpropoxy or 1,1-dimethylethoxy).

[0046] The term "C1-C4 alkylamino" denotes, within the meaning of the invention, an amino group (-NH2) where the nitrogen atom is substituted by a C1-C4 alkyl group. The term "C1-C4 alkyl" is as mentioned above. Examples of C1-C4 alkylamino groups include methylamino, ethylamino, n-propylamino, iso-propylamino, n-butylamino or tert-butylamino radicals.

[0047] The term "C1-C4 dialkylamino" denotes, within the meaning of the invention, an amino group (-NH2) whose nitrogen is substituted by two C1-C4 alkyl groups, identical or different. The term "C1-C4 alkyl" is as mentioned above. Examples of C1-C4 dialkylamino groups include dimethylamino, diethylamino, ethylmethylamino, methyl(n-propyl)amino, ethyl(n-propyl)amino, di(n-propyl)amino, methyl(n-butyl)amino, ethyl(n-butyl)amino, di(n-butyl)amino or methyl(tert-butyl)amino.

[0048] The term "C1-C4 alkylthio" denotes, within the meaning of the invention, a linear or branched saturated alkyl radical having 1 to 4 carbon atoms (as mentioned above) which is linked via a sulfur atom, i.e., for example, any of the radicals methylthio, ethylthio, n-propylthio, iso-propylthio (or 1-methylethylthio), n-butylthio, iso-butylthio (or 1-methylpropylthio), tert-butylthio (or 2-methylpropylthio or 1,1-dimethylethylthio).

[0049] In particular, the optional substituent(s) R 3 phenyl groups of R 1 and R 2 are selected, indifferently, from a hydroxy group, a C1-C4 alkyl group, a C1-C4 alkoxy group, a C1-C4 alkylthio group, a C1-C4 dialkylamino group, a CF3 group and an O-CF3 group.

[0050] As specific examples of R groups 1 and R 2the following groups may be mentioned: phenyl, 4-methylphenyl, 4-(trifluoromethyl)phenyl, 2-methylphenyl, 2,4-dimethylphenyl, 3,4-dimethylphenyl, 4-ethylphenyl, 4-isopropylphenyl, 4-tert-butylphenyl, 4-hydroxyphenyl, 4-methoxyphenyl, 2,4-dimethoxyphenyl, 3,4-dimethoxyphenyl, 4-(trifluoromethoxy)phenyl, 4-chlorophenyl, 4-fluororophenyl, 4-(methylthio)phenyl, 4-(N,N-dimethylamino)phenyl, 4-cyanophenyl.

[0051] Preferably, the optional substituent(s) R 3 phenyl groups of R 1 and R 2 are selected, indifferently, from a hydroxy group, a C1-C4 alkyl group, a C1-C4 alkoxy group, a C1-C4 alkylthio group, and a C1-C4 dialkylamino group, in particular, from a methyl group, a methoxy group, a methylthio group and a dimethylamino group.

[0052] Preferably, R groups 1 and R 2 complexes corresponding to the formula [Chem. 6] are identical.

[0053] Among the nickel (II) complexes of formula I in which the R groups 1 and R 2 are identical, we can notably cite the complexes corresponding to the formulas [Chem. 7] to [Chem.11] below:

[0054] [Chem. 7]

[0055] [Chem. 8]

[0056] [Chem. 9]

[0057] [Chem. 10]

[0059] Synthesis processes:

[0060] The present invention also relates to the preparation of nickel(ll) complexes of formula [Chem 6] and the corresponding ligands.

[0061] The nickel complexes of the invention and the corresponding ligands may be synthesized using any suitable method, including, but not limited to, those described herein.

[0062] An example of a process for the synthesis of ligands of nickel complexes of formula [Chem. 6] where R1 and R 2 are distinct is shown in [Fig. 1],

[0063] Another example of a method for synthesizing ligands of nickel complexes of formula [Chem. 6] where R 1 and R 2 are identical is shown in [Fig. 2],

[0064] An example of a process for the synthesis of nickel complexes of formula [Chem. 6] is shown in [Fig. 3]:

[0065] With reference to [Fig. 1], the synthesis of the ligands of the complexes according to the invention where R 1 and R 2 are distinct comprises the steps of: i) reacting 2,2'-thenil (1) with one molar equivalent of a thiosemicarbazide of formula (5) in which R 1 is as defined under the general formula [Chem. 6], to give the intermediate product of formula (2) in which R 1 is as defined under the general formula [Chem. 6]; ii) reacting the product of formula (2) with one molar equivalent of a thiosemicarbazide of formula (5) in which R 2 is as defined under the general formula [Chem. 6], to give the ligand of formula (3) in which R1 and R 2 are as defined under the general formula [Chem. 6] (with R1 + R 2 ).

[0066] The intermediate product of formula (2) and the ligand of formula (3) obtained, respectively, in steps (i) and (ii) can be purified according to any conventional purification method known to those skilled in the art, and for example by recrystallization in one or more suitable solvents or by chromatography, in particular by chromatography on a silica column. They can also be used immediately after the respective steps (i) and (ii), without purification.

[0067] With reference to [Fig. 2], the synthesis of ligands of complexes of formula [Chem. 6] where R 1 and R2 are identical, comprises a step consisting of: i') reacting 2,2'-theni I (1) with at least two molar equivalents of a thiosemicarbazide of formula (5) in which R 1 is as defined under the formula [Chem. 6], to give the ligand of formula (4) in which R 1 is as defined under the formula [Chem. 6],

[0068] Similarly, ligand 4 obtained in step (i') above can be purified according to any conventional purification method known to those skilled in the art, and for example by recrystallization in one or more suitable solvents or by chromatography, in particular by chromatography on a silica column. It can also be used immediately after this step (i') without purification.

[0069] The reactions of the processes illustrated in [Fig.1] (steps (i) and (ii)) and in [Fig.2] (step (i')) are generally carried out using organic solvents such as, for example, ethanol, methanol or toluene, at temperatures usually varying between 15°C and 150°C, for example at reflux of ethanol.

[0070] Such reactions may, where appropriate, be carried out in the presence of an acid catalyst such as hydrochloric acid, acetic acid, trifluoroacetic acid or other suitable acid catalyst.

[0071] The thiosemicarbazides of formulae (5) and (6) used are generally commercially available, for example, from Sigma-Aldrich or Merck, or can be prepared, for example, by reaction between hydrazine (NH2-NH2) and an isothiocyanate of formula R 1 -N=C=S or R 2 -N=C=S corresponding, where R 1 and R 2are as defined under the general formula [Chem. 6], The isothiocyanate reagents of formula R 1 -N=C=S (or R 2 - N=C=S) are commercially available or described in the literature, or can be prepared according to methods described therein or known to those skilled in the art (see for example the article by Srivastava K., 2020).

[0072] With reference to [Fig.3], the synthesis of the nickel complexes according to the invention comprises a step consisting of: iii) reacting the ligand of formula (3) or (4) obtained previously with a nickel salt to give the nickel complex of general formula [Chem.6] (R 1 + R 2 or R 1 = R 2 , respectively).

[0073] The reaction of step (iii) above is typically carried out in an organic solvent such as, for example, ethanol, methanol or toluene, at a temperature between 15°C and 150°C (for example, at reflux of ethanol). As nickel salt, nickel chloride hexahydrate, nickel nitrate hexahydrate or nickel acetate tetrahydrate may in particular be used.

[0074] The nickel complexes obtained in step (iii) above can be purified according to any conventional purification method known to those skilled in the art, and for example by recrystallization in one or more suitable solvents or by chromatography, in particular by chromatography on a neutral alumina (AI2O3) column.

[0075] In summary, the present invention relates to a process for preparing a nickel (II) complex according to the formula [Chem 6] comprising the following steps: (a) providing a ligand of formula (3) or (4), (b) reacting said ligand of formula (3) or (4) with a nickel salt in order to obtain the desired nickel (II) complex (R 1 + R 2 or R 1 = R 2 , respectively).

[0076] Catalytic applications:

[0077] The nickel complexes according to the present invention demonstrate remarkable electrocatalytic efficiency when it comes to catalyzing the reduction of protons. This fully justifies their use as catalysts, more specifically as electrocatalysts in the electrochemical reaction of reduction of protons to hydrogen (molecular hydrogen (H2)).

[0078] Also, the present invention also relates to the use of a nickel complex, as described above, as a catalyst intended to promote the electrochemical reaction of proton reduction. This catalyst can be either homogeneous or heterogeneous.

[0079] The term "homogeneous catalyst" means, for the purposes of the invention, a catalyst which is present in the same liquid phase (or electrolyte solution) as the proton source during the electrochemical proton reduction reaction. In particular, a homogeneous catalyst is a catalyst which is at least partly soluble in the solvent (or electrolyte solution) in which the electrochemical proton reduction reaction takes place.

[0080] The term "heterogeneous catalyst" means, within the meaning of the invention, a catalyst which is in a phase different from that of the proton source during the electrochemical proton reduction reaction. In particular, a heterogeneous catalyst is a catalyst deposited, adsorbed or printed on the surface of an electrode or adsorbed on a substrate (polymer material (for example Nation®), metal or ceramic) deposited on the surface of the electrode.

[0081] The term "electrode" refers to an electrical current conductor used to make contact with a non-metallic part of a circuit (e.g., an electrolyte solution). An electrode may also refer to an anode (or negative electrode) or a cathode (or positive electrode). An electrical current conductor may be made of a metallic material (e.g., aluminum, copper, titanium, and the like), a carbon material (e.g., one or more allotropes of carbon such as: glassy carbon, graphene, graphite, amorphous carbon, activated carbon, carbon nanotubes and fibers, and their similar), (semi)conducting polymers (e.g. polyacetylenes, polyphenylenees and polypyrroles) and combinations thereof.

[0082] Conventional methods of manufacturing modified electrodes by deposition, adsorption or printing (e.g. printing or depositing an ink based on a carbon material such as graphite and a catalyst on the electrode) of a catalyst can be used (see for example documents: W02009098403A2; W02005088657A2).

[0083] The advantages of immobilizing (or deposition, printing, adsorption, etc.) the catalyst on the surface of an electrode are numerous, including: - Optimized use of the catalyst, thus reducing costs and minimizing catalyst losses; - Improved exposure of the catalytic site: by being directly bonded to the electrode, the catalyst is optimally exposed, thus promoting the desired chemical reaction, - Improved diffusion kinetics: immobilizing the catalyst on the electrode surface facilitates the diffusion of reactants such as protons to the catalytic site, which accelerates the course of the electrochemical reaction, - Circumvention of solubility challenges: organic catalysts are poorly or not soluble in water, the heterogeneous system allows these catalysts to circumvent these challenges, allowing them to function in proton-rich aqueous solutions, - Easier determination of catalyst life: by monitoring the performance of the immobilized catalyst over time, it is easier to assess its longevity, - Good reproducibility of electrode characteristics: the electrodes incorporating the catalyst have reproducible characteristics, which ensures consistency in the experimental results

[0084] The present invention also relates to a method for producing hydrogen (H2) comprising contacting a source of protons (H + ) with a catalyst according to the present invention, in an electrochemical cell, and applying a potential to the electrochemical cell, thereby reducing the protons (H + ) into hydrogen (H2).

[0085] Typically, an electrochemical cell comprises a working electrode, a counter electrode, and, if applicable, a reference electrode, all immersed in an electrolytic solution allowing the transport of ions from the working electrode to the counter electrode.

[0086] An electrochemical cell can be designed in two main ways: either it is undivided, or it is divided into at least two separate compartments. In the latter case, one of these compartments is equipped with a working electrode, while the other is equipped with a counter electrode. These compartments are separated by a separator, which can, for example, take the form of a membrane permeable to the ions of the electrolytic support. In general, the use of an undivided electrochemical cell is preferable. Electrochemical cells, as well as methods for their manufacture, are known to those skilled in the art.

[0087] The term "working electrode" designates, within the meaning of the invention, the electrode of an electrochemical cell on which the electrochemical reaction of interest occurs, such as the reduction of protons into hydrogen.

[0088] The term "counter electrode" refers to a secondary electrode in an electrochemical cell that is used to complete the electrochemical circuit so that current can flow through the electrochemical cell. Typically, a potential is applied between the working electrode and the counter electrode to allow the electrochemical reaction of interest (e.g., reduction of protons to hydrogen) to occur.

[0089] The term "electrolytic solution" designates, within the meaning of the invention, a solution comprising an electrolyte and a solvent. Electrolytes are ions (for example: H + , OH", Li + , (C4H9)4N +, OH", BF4, PF6-, CF3-SO3; (CF3-SO2)2N-, etc.) which promote the passage of current within the electrolytic solution. In particular, the term "electrolytes" applies to ions which actively participate in the transport of current. An electrolytic solution is therefore electrically conductive. Electrolytes can be obtained, for example, by dissolving a salt corresponding to a combination of cations and anions in the solvent of the electrolytic solution.

[0090] Generally, the nickel(II) complexes of the present invention have good solubility (> 0.5 M) in various organic solvents and water-organic solvent mixtures at room temperature (about 25°C). Suitable organic solvents for the electrochemical proton reduction reaction include aprotic solvents, which may be selected from amides, carbonates, ketones, aromatic compounds, esters, ethers, nitriles, halogenated solvents, sulfoxides, and mixtures thereof. Examples of amides include, but are not limited to, N,N-dimethylformamide (DMF) and N-methyl-2-pyrrolidone (NMP). Examples of suitable carbonates include, but are not limited to, propylene carbonate, ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, and dimethyl carbonate.Examples of ketones include, but are not limited to, acetone, methyl ethyl ketone, diethyl ketone, and cyclohexanone. Examples of aromatic compounds include, but are not limited to, benzene, toluene, xylene, chlorobenzene, and nitrobenzene. Examples of esters include, but are not limited to, ethyl acetate and methyl acetate. Examples of suitable ethers include, but are not limited to, diethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, and 1,4-dioxane. Examples of suitable nitriles include, but are not limited to, acetonitrile and prioprionitrile. Examples of halogenated solvents include, but are not limited to, dichloromethane, trichloromethane, and tetrachloromethane. Examples of sulfoxides include, but are not limited to, dimethyl sulfoxide (DMSO).

[0091] Thus, when using the nickel(II) complex of the present invention as a homogeneous catalyst, one or more of the previously mentioned aprotic solvent alternatives may be used as solvents in the electrolytic solution.

[0092] As electrolytes, the use of salts that are at least partially soluble in the electrolytic solution, particularly in the solvent thereof, is preferred. Non-limiting examples of electrolytes include tetrabutylammonium hexafluorophosphate (n-Bu4NPFe or NBU4PF6), tetrabutylammonium tetrafluoroborate (n-Bu4NBF4), lithium hexafluorophosphate (LiPFe) and lithium bis(trifluoromethanesulfonyl)imide. In practice, the concentration of the electrolyte in the electrolytic solution ranges from 0.01 to 5.00 mol / L, preferably from 0.05 to 3.00 mol / L, in particular from 0.10 to 1.00 mol / L.

[0093] The method of the present invention comprises contacting a source of protons (H + ) with a catalyst (homogeneous or heterogeneous) according to the present invention. The term "proton source" designates, within the meaning of the invention, a compound, composition or material which is capable of giving at least one proton (H +) during the reaction of reduction of protons to hydrogen.Suitable proton sources include organic acids, including carboxylic acids, such as acetic acid, propionic acid, trifluoroacetic acid (TFA), and citric acid; organic sulfonic acids, such as methanesulfonic acid, ethanesulfonic acid, p-toluenesulfonic acid; organic phosphonic acids, such as methanecylphosphonic acid, ethanephosphonic acid, benzenephosphonic acid, nitrobenzenephosphonic acid, chlorobenzenephosphonic acid; bis(sulfony)imides, such as bis(trifluoromethanesulfonyl)imide; organic ammonium salts, e.g., triethylammonium tetrafluoroborate, benzyldiethylammonium tetrafluoroborate, benzyldiethylammonium chloride, cetyldimethylammonium chloride; phenols such as phenol, inorganic acids such as sulfuric acid and hydrochloric acid; and a combination of at least two of these proton sources.

[0094] In practice, the proton source (H + ) with the catalyst by adding the proton source to the electrolytic solution. The concentration of the proton source in the electrolytic solution can be between 10' 2 mol / L and 5 mol / L, for example between 10 -2 mol / L and 2 mol / L, more particularly between 10 -2 mol / L and 10' 1 mol / L.

[0095] In one embodiment according to the invention, the nickel(II) complex according to the present invention is used as a homogeneous catalyst. In this case, it is at least partially, preferably completely dissolved in the electrolytic solution. Its concentration in the electrolytic solution can be between 10 -4 mol / L and 2 mol / L, for example between 10 -4 mol / L and 1 mol / L, more particularly between 10 -4 mol / L and 1 O' 1 mol / L.

[0096] The reaction of reduction of protons to hydrogen is carried out by applying a voltage across the terminals of the working electrode and the counter electrode of the electrochemical cell so as to reduce the protons (H + ) into hydrogen (H2).

[0097] Conventionally, the electrochemical cell implemented is composed of several elements, including the working electrode (modified or not), the counter-electrode, and a reference electrode.

[0098] By "reference electrode" is meant an electrode that provides a stable and constant potential regardless of the type or concentration of species present in the electrolytic solution in which the reference electrode is placed. Thus, significant changes in the potential of a working electrode associated with a reference electrode can be detected. These significant changes can be detected by comparing the changes in the paired working electrode to the constant potential of the reference electrode. Non-limiting examples of reference electrodes are the silver electrode involving the silver / silver chloride pair (denoted Ag / AgCl hereinafter), the standard hydrogen electrode (SHE), the standard calomel electrode (SCE) and the reversible hydrogen electrode (RHE).

[0099] The electrochemical cell can also be equipped with a potentiostat type device (for example an Origaflex OGF01A potentiostat), making it possible to apply a voltage between the working electrode and the counter-electrode of the electrochemical cell and to measure the current produced.

[0100] Potentials are usually measured (or applied) by the potentiostat type device with respect to the reference electrode. In practice, ferrocene or a similar internal standard such as cobaltocene is used to calibrate the potential of the reference electrode such as the Ag / AgCl electrode. The use of internal standards such as the ferrocenium / ferrocene redox couple (Fc+ / Fc°) or (Fc + / 0 ) is known to be suitable for electrolysis experiments in non-aqueous media. The potential of the redox couple Fc + / 0 is 0.5 ± 0.2 V relative to the Ag / Ag electrode + .

[0101] Advantageously, the potential applied to the working electrode is less than 0 V (for example relative to Fc + / 0 ), especially less than -0.5 V (for example compared to Fc + / 0 ), in particular less than -1.0 V (e.g., relative to Fc + / 0 ) and, more particularly, the applied potential is between -2.5 and -1.0 V (for example with respect to Fc + / 0 ), and even more particularly between -2.0 V and -1.5 V (for example with respect to Fc + / 0 ).

[0102] The hydrogen production process according to the present invention can be implemented in a wide range of operating conditions of temperature or pressure; it is particularly preferred to operate at a temperature between ambient temperature and 100°C and at a pressure between atmospheric pressure and 30 bars.

[0103] The reduction of protons according to the present invention can be carried out for a period of time greater than 1 min, in particular between 1 min and 30 h and in particular between 2 h and 20 h.

[0104] The present invention has a wide range of industrial applications, including in the production of hydrogen for clean energy production, chemical manufacturing, and other fields requiring a source of hydrogen.

[0105] In the following description and examples, value ranges labeled as "between ... and ..." include the specified lower and upper bounds.

[0106] Furthermore, in the claims, any reference sign in parentheses cannot be interpreted as a limitation of the claim. Furthermore, the use of the verb "comprise", "comprise" or "include" and its conjugated forms does not exclude the presence of other elements or other steps than those set out in a claim.

[0107] Other features and advantages of the invention will become more apparent from the following examples, given by way of illustration and not limitation. The scope of the invention should not be limited in any way by these examples. Examples:

[0108] Example 1: Preparation, characterization and study of the electrochemical behavior of a Nickel (II) complex according to the invention.

[0109] The following description is made essentially with reference to the preparation and implementation of a Nickel (II) complex which is "Ni(thenil)TSC-OCH3", namely the 2,3-di(4-(methoxyphenyl-thiosemicarbazone)-2,2'- (buta-1,3-diene-2,3-diyl)dithiophene nickel (II) complex, but it is quite obvious that this description can be applied to all Nickel (II) complexes of formula (I) according to the invention, subject to the necessary adaptations of the processes and means of implementation which can be easily carried out by those skilled in the art in this field of the technique.

[0110] Example 1a: Synthesis of the 2,3-di(4-(methoxyphenyl-thiosemicarbazone)-2,2'-(buta-1,3-diene-2,3-diyl)dithiophene nickel (II) complex (Ni(thenil)TSC-OCH3)

[0111] The Ni(thenil)TSC-OCH3 complex was synthesized according to the method of [Fig. 4] combined with that of [Fig. 5], the intermediate bisthiosemicarbazone ligand not having been isolated.

[0112] 2,2'-Thenil (100 mg, 0.450 mmol) and N-(4-methoxyphenyl)-hydrazinecarbothioamide (222 mg, 1.124 mmol) and a few drops of acetic acid were added to 10 ml of ethanol. Then, the mixture was stirred under reflux of ethanol for 6 hours. After this time, the mixture was allowed to cool to room temperature to give a solution containing the intermediate bisthiosemicarbazone ligand. Nickel acetate tetrahydrate “Ni(CH3COO)2,4H2O” (168 mg, 0.675 mmol) was added to this solution. Then, the solution was stirred under reflux of ethanol for 4 hours. After this After this time, the solution was allowed to cool to room temperature. Triethylamine (1 mL) and water (80 mL) were added to the cooled solution. Then, the solution was filtered to give a crude solid containing the Ni(thenil)TSC-OCH3 complex. This crude solid was washed with water and ethanol, and filtered. The resulting solid was solubilized in THF, filtered, and concentrated under reduced pressure to give a solid. The latter was purified by column chromatography on neutral alumina (Al2O3) (dichloromethane / cyclohexane 7 / 3) to give the Ni(thenil)TSC-OCH3 complex as a dark brown solid (47 mg, 16%). NMR 1 H (500 MHz, DMSO-d6, TMS): 5: 10.23 (s, 2H), 7.89 (dd, Ji= 1.08, J2= 5.03 Hz, 2H), 7.49 (d, J= 8.40 Hz, 4H), 7.04 (t, J= 3.85 Hz, 2H), 6.86-6.84 (m, 6H), 3.73 (s, 6H) ppm. NMR 13C (125 MHz, DMSO-c / 6, TMS): 5: 173.28 (N=CS), 156.68 (OC=C), 133.91 (NC=C), 132.90 (CC=N), 132.60 (SC=C) 131.33 (C=CC), 127.08 (C=CC), 123.10 (oCC=C), 120.80 (CC=C), 114.59 (mC=CC), 56.15 (COC) ppm. HRMS ESI m / z: [MH]' calculated for C26H2iN6O2S4Nr: 634.9968, found: 634.9967.

[0113] UV-visible:

[0114] The electronic structure of the Ni(thenil)TSC-OCH3 complex was characterized by UV-Visible spectroscopy.

[0115] The UV-Visible spectrum of the Ni(thenil)TSC-OCH3 complex of Example 1a was recorded in DMF with a concentration of 0.05 M, at room temperature. It is presented in [FIG.4], The UV-Visible spectrum obtained indicates three electronic absorption bands at: 273 nm (s = 41963 M -1 cm -1 ), 353 nm (s = 16592 M“ 1 cm“ 1 ) and 516 nm (s = 16600 M^cm' 1 ).

[0116] Crystal structure:

[0117] Crystals of the Ni(thenil)TSC-OCH3 complex of Example 1a were obtained by slow evaporation of a solution of this complex in a 50 / 50 (V / V) mixture of dichloromethane and cyclohexane.

[0118] The crystal structure was determined by X-ray diffraction.

[0119] [FIG. 5] shows an ORTEP representation of the formed complex with atom numbering. Hydrogen atoms have been removed for clarity. Ellipsoids are represented with a probability of 50%.

[0120] The crystallographic data, the conditions of the collection of the diffracted intensities and their treatments of the Ni(thenil)TSC-OCH3 complex of Example 1a are recorded in [Table 1] below: [Table 1]

[0121]

[0122] [Table 1] highlights that the Ni(thenil)TSC-OCH3 complex crystallizes in the monoclinic system (space group P2i).

[0123] The ORTEP representation ([FIG. 5]) shows that the nickel ion adopts a distorted square-planar coordination geometry often encountered for neutral mononuclear nickel(ll) complexes.

[0124] The important geometric parameters determined from the crystal structure of the Ni(thenil)TSC-OCH3 complex of Example 1a are shown in [Table 2] below. [Table 2]

[0125]

[0126] Electrochemical studies:

[0127] The electrochemical properties of the Ni(thenil)TSC-OCH3 complex were studied using cyclic voltammetry (CV) and controlled-potential coulometry methods, also called controlled-potential (bulk) electrolysis.

[0128] Cyclic voltammetry:

[0129] The electrochemical behavior of the Ni(thenil)TSC-OCH3 complex (1 mM) was explored in DMF in the presence of 0.1 M tetrabutylammonium hexafluorophosphate (NBU4PF6) as a supporting electrolyte, and, where appropriate, trifluoroacetic acid (TFA) as a proton donor.

[0130] Cyclic voltammetry (CV) measurements were performed using an Origaflex OGF01 A potentiostat. They were made under a nitrogen atmosphere, using degassed solutions, at room temperature.

[0131] A conventional three-electrode system (electrochemical cell) comprising a stationary glassy carbon electrode (1 mm in diameter) as the working electrode, a counter electrode formed by a platinum wire and a silver chloride (AgCl) reference electrode involving the silver / silver chloride (Ag / AgCl) couple, was used for CV measurements. The capacity of the electrochemical cell of the said system is approximately 10 mL.

[0132] In the present electrochemical studies, the redox couple Fc + / 0 (Abbreviation for ferrocenium / ferrocene) was used as an internal standard to calibrate the potentials recorded at the glass carbon (GC) working electrode in DMF. As a reminder, E°(Fc + / 0 ) = 0.52 V relative to Ag / AgCl. All measured potential values ​​will now be quoted relative to this redox couple.

[0133] The inventors carried out a first series of CV measurements of the Ni(thenil)TSC-OCH3 complex (1 mM) in DMF in the presence of 0.1 M of NBU4PF6, at different potential scanning rates (0.1 to 2 V.s' 1 ). The cyclic voltammograms recorded at the GC electrode are shown in [FIG. 6]. These cyclic voltammograms reveal that the Ni(thenil)TSC-OCH3 complex exhibits quasi-irreversible electrochemical behaviors when scanning the cathodic region from 0 to -2.65 V. As shown in [FIG. 6], two quasi-reversible peak-like waves are observed for each potential scan rate, with a first cathodic peak at about -2.0 V and a second cathodic peak at about -1.34 V. The two quasi-reversible waves observed can be attributed to the formation of, respectively, [Ni^theniOTSC-OCHs]'- and [Ni'(thenil)TSC- OCHs] 2These attributions are confirmed by quantum chemical modeling and are consistent with observations relating to the reference NiTSC-OCHs complex (See the article by Straistari, T. et al, 2017).

[0134] Furthermore, the cyclic voltammogram shown in [FIG. 6] also exhibits a first series of anodic peaks at approximately -1.9 V and a second series of anodic peaks at approximately -1.3 V.

[0135] The results of this first series of CV measurements thus showed that the products [Ni^theniOTSC-OCHs]'- and [Ni'(thenil)TSC-OCH3] 2 ' generated during reductions are stable on the time scale of cyclic voltammetry. This demonstrates the ability of the bis(thiosemicarbazone) ligand of the Ni(thenil)TSC-OCH3 complex to avoid demetallation during reduction of the Ni(ll) ion.

[0136] The inventors then evaluated the performance of the Ni(thenil)TSC-OCH3 complex of Example 1a as an electrocatalyst to catalyze the hydrogen evolution reaction (HER). They recorded a series of cyclic voltammograms of the Ni(thenil)TSC-OCH3 complex (1 mM) in DMF in the presence of 0.1 M NBu4PFe using TFA (from 0 to 100 mM) as a proton source, at a scan rate of 0.5 Vs -1 As a reminder, TFA has a pKa of 6.00 ± 0.03 in DMF.

[0137] The recorded cyclic voltammograms are shown in [FIG. 7] and [FIG. 8],

[0138] Cyclic voltammograms ([FIG. 7]) show that the addition of TFA into the solutions of the Ni(thenil)TSC-OCH3 complex generates a catalytic current with the appearance of two proton-dependent peak-like waves when scanning in the cathodic direction, with a first peak at about -1.60 V and a second peak at about -1.95 V relative to Fc + / 0. This second peak slowly shifts towards more negative potential values ​​upon successive addition of TFA ([FIG. 7]). The inventors further observed that the catalytic current response of such peak-shaped waves is directly proportional to the proton concentration in the Ni(thenil)TSC-OCH3 complex solution.

[0139] Furthermore, the inventors studied the process occurring at the first wave whose potential is about -1.60 V relative to Fc + / 0 They observed that the potential value remained practically unchanged upon gradual addition of TFA ([FIG. 8]).

[0140] The inventors then recorded the cyclic voltammogram of TFA (100 mM) in DMF in the presence of 0.1 M NBu4PFe but in the absence of the Ni(thenil)TSC-OCH3 complex. The result shown in [FIG. 7] (dotted curve G) proves that the observed improvement in catalytic current is indeed due to the presence of the Ni(thenil)TSC-OCH3 complex.

[0141] In order to verify that the observed catalytic current is not related to species absorbed on the surface of the working electrode, the inventors performed soaking and rinsing tests. After the CV measurements of the Ni(thenil)TSC-OCH3 complex, the working electrode was removed from the solution and then immersed in a new DMF solution containing NBU4PF6 (0.1 M). As can be seen in [FIG. 9] (curve A), only the background current was recorded during the potential sweep between 0 and -2.75. When 100 mM TFA was added, the potential sweep only provided the electrochemical response of TFA (Curve B, [FIG. 9]). These soaking and rinsing tests confirm that no species are absorbed on the glassy carbon working electrode during the CV measurements performed. They also proved that the current catalytic obtained is indeed due to the dissolved Ni(thenil)TSC-OCH3 complex and also to confirm that the electrolytic system studied indeed corresponds to a homogeneous catalyst.

[0142] Studies by controlled potential coulometry:

[0143] In order to confirm that the recorded electrochemical responses shown in [FIG. 7] and [FIG. 8] correspond to hydrogen production, the inventors conducted controlled potential coulometry (CPC) experiments.

[0144] Potential-controlled coulometry, or electrolysis, allows the measurement of the charge (often expressed in Coulombs (C)) which passes through the electrode used for the measurement for a duration t and at a fixed potential.

[0145] The coulometry experiments were coupled with gas detection and quantification by gas chromatography (GC) itself coupled with a flame ionization detector (FID) which makes it possible to ensure that the only gas produced is hydrogen and to quantify its production as the experiment progresses.

[0146] The coulometry experiments were combined with gas detection and quantification using gas chromatography (GC). This approach ensures that the only gas generated is hydrogen, while allowing us to quantify its production throughout electrocatalysis.

[0147] Experiments were conducted in 8 ml DMF solutions containing 1 mM Ni(thenil)TSC-OCH3 complex, 0.1 M NBU4PF6 and 100 mM TFA, at a fixed potential of -1.7 V vs. Fc + / 0and for approximately 960 min (16 hours) by monitoring the electrocatalytic reaction of hydrogen (H2) production by continuous online (GPC) analysis of the produced gases. A mercury-based working electrode was used to ensure that all the produced nanoparticles would be absorbed into the mercury, thus preserving the homogeneous nature of the electrolytic system studied.

[0148] These experiments provided access to the following performance parameters: - The number of moles of hydrogen produced, - The total charge (Q; expressed in Coulombs “C”) passed during the electrolysis experiment, - Faradaic efficiency or Faradaic Yield (FY, abbreviation for Faradaic Yield). FY gives the percentage of electrons intended for the reduction of protons. A Faradaic efficiency of 100% makes it possible to estimate that all the electrons supplied to the catalyst are used for the production of hydrogen. When this yield is not quantitative, this means that part of the electrons is intended for other electrochemical reactions (for example because of the solvent, TFA or other electroactive species present in the reaction medium) or quite simply that the catalyst degrades during electrolysis, - The TON number (abbreviation for Turnover Number) which is the number of catalytic cycles that the catalyst is capable of carrying out. The TON number is calculated by the ratio between the number of moles of hydrogen produced and the number of moles of the complex studied, which has the value nH2measured = 3.36 10'4 moles - The TOF number (abbreviation for Turnover Frequency) which is the number of cycles that the catalyst completes in a given period of time. In practice, the TOF number is expressed in s -1 The higher the TOF number, the more products are formed per catalyst per unit of time (s -1 ), - The overpotential (r|) also called overvoltage. It can be expressed in V or in mV.

[0149] The above parameters are well understood by those skilled in the art of electrochemistry.

[0150] GC (or Gas Chromatography) detection showed that the gas produced in the coulometry experiment is indeed hydrogen. The number of moles of hydrogen produced over a period of 960 minutes (16 hours) and quantified by GC is 3.36 x 10' 4 moles.

[0151] The graph of the evolution of the total charge (Q in coulombs) as a function of time (t in min) is represented in [FIG. 10]. This graph shows that over a period of 960 minutes (16 hours), the total charge which has crossed the working electrode is negligible for solutions containing only TFA (curve b, [FIG. 10]). In contrast, for solutions containing both TFA and the Ni(thenil)TSC-OCH3 complex, a relatively high total charge (Q) was recorded (curve A, [FIG. 10]), indicating increased catalytic activity. For the Ni(thenil)TSC-OCH3 complex, a total charge (Q) of 65.53 C crossed the working electrode, which corresponds to a theoretical number (nth) of 3.40 x 10' 4 moles of hydrogen produced and a TON number of 42.

[0152] A comparison of the catalytic activities of the Ni(thenil)TSC-OCH3 complex of example 1a and the reference NiTSC-OCHs complex (prior art) is given in table [Table 3] below:

[0153] [Table 3]

[0154]

[0155] [Table 3] clearly shows that the Ni(thenil)TSC-OCH3 complex of Example 1a exhibits increased catalytic activity with a quasi-quantitative faradic efficiency (= 99%) compared to 80% for the reference complex, and a TON index (= 42) which is twice the TON index (= 21) obtained for the reference complex.

[0156] The TOF value can be calculated using the data obtained during bulk electrolysis experiments, following the protocol previously described in the literature (see the following articles: Roy, S. et al., 2017 and Queyriaux, N. et al., 2020.

[0157] For heterolytic processes, the formulas [Math 1], [Math 2] and [Math 3] below were used:

[0158] [Math 1]

[0159] In this formula [Math 1]: F is the Faraday constant (F = 96500 C.mol' 1 ), A is the surface area of ​​the mercury-based working electrode (A ~ 0.47 cm 2 ), Dcat is the diffusion coefficient (expressed in cm 2 .s' 1 ), [Cat] is the catalyst concentration ([Cat] = 1 mM), Eappi is the potential applied during electrolysis (E appi =-1.70 V relative to an Fc electrode + / 0 ), EI / 2 is the half-wave potential for the catalytic wave, and iei represents the current density produced during bulk electrolysis.

[0160] The current density (i e i) can be calculated using the formula [Math 2] below.

[0161] [Math 2]

[0162] In this formula [Math 2]: Qei is the charge passed during electrolysis, FY is the faradic efficiency (or effectiveness), t is the electrolysis time.

[0163] The maximum turnover frequencies (TOFmax) of the nickel(ll) complexes were then evaluated using the formula [Math 3] below.

[0164] [Math 3]

[0165] [Table 4] below is a comparative table of different parameters determined for the Ni(thenil)TSC-OCH3 complex of Example 1a compared to the reference NiTSC-OCH3 complex.

[0166] [Table 4],

[0167]

[0168] It can be deduced from [Table 4] that with the Ni(thenil)TSC-OCH3 complex of Example 1a, the catalytic performances are better. This major improvement in catalytic performances is obtained thanks to the presence of thiophenyl groups in the 2,2' positions of the bis(thiosemicarbazone) ligand.

[0169] The overpotential value (r|) of the Ni(thenil)TSC-OCH3 complex of example 1 was determined according to the protocol previously reported in the literature (see for example: the article by Fourmond, V. et al., 2010). This value was calculated by the following formula [Math 4]:

[0170] [Math 4]

[0171] In this formula [Math 4]: E cat i / 2 is the half-wave catalytic potential, and E T I / 2 is the theoretical half-wave potential.

[0172] [Table 5] below is a comparative table of the overpotential determined for the Ni(thenil)TSC-OCH3 complex of Example 1a compared to the reference NiTSC-OCH3 complex.

[0173]

[0174] [Table 5]

[0175]

[0176] As shown in [Table 5] the Ni(thenil)TSC-OCH3 complex of Example 1 has a significantly lower overpotential value than that obtained for the reference NiTSC-OCH3 complex. From these results it can be deduced that the introduction of two thiopenyl groups in the 2,2' positions of the bisthiosemicarbazone ligand leads to a reduction of the overpotential of the proton reduction reaction (HER reaction).

[0177] Finally, to study the sequence of events leading to hydrogen production with the Ni(thenil)TSC-OCH3 complex of Example 1a, the present inventors recorded the UV-Visible spectra in the absence and presence of TFA. The results are presented in [FIG. 11 ]. This clearly shows that upon addition of up to 100 equivalents of TFA, no shift occurred in the main absorption bands of the spectra. These experiments exclude a chemical event (protonation) as the first step in the catalytic process.

[0178] Références bibliographiques : Fourmond, V., Jacques, P. -A., Fontecave, M. & Artero, V. H2 Evolution and Molecular Electrocatalysts: Determination of Overpotentials and Effect of Homoconjugation. Inorg. Chem. 49, 10338-10347 (2010). Queyriaux N., Sun D., Fize J., Pécaut J., Field M.J., Chavarot-Kerlidou M., Artero V., Electrocatalytic Hydrogen Evolution with a Cobalt Complex Bearing Pendant Proton Relays: Acid Strength and Applied Potential Govern Mechanism and Stability. J. Am. Chem. Soc. 142, 274-282 (2020). Roy S., Sharma B, Pécaut J., Simon P., Fontecave M., Tran P.D., Derat E., Artero V., Molecular Cobalt Complexes with Pendant Amines for Selective Electrocatalytic Reduction of Carbon Dioxide to Formic Acid. J. Am. Chem. Soc. 139, 3685-3696 (2017). Srivastava K., Bhatt A., Singh N., Khare R., Shukla R., Chaturvedi D., Kant R. “Synthesis of Isothiocyanates: A Review” Chemistry & Biology Interface, 2020, 10, 2, 34-50. Straistari T., Fize J., Shova S., Réglier M., Artero V., Orio M., A Thiosemicarbazone-Nickel(ll) Complex as Efficient Electrocatalyst for Hydrogen Evolution. ChemCatChem 9, 2262-2268 (2017).

Claims

Claims

1. [A catalyst comprising a nickel(ll) complex comprising a bis(thiosemicarbazone) ligand derived from 2,2'-thenil, said nickel(ll) complex corresponding to the following general formula [Chem. 6]: [Chem. 6] in which, R 1 and R 2 each independently represent a phenyl group optionally having one or more substituents R 3 identical or different, R 3 is selected from halogen, hydroxy group, C1-C4 alkyl group, C1-C4 alkoxy group, C1-C4 thioalkyl group, C1-C4 dialkylamino group, cyano group, CF3 group and O-CF3 group.

2. Catalyst according to claim 1, characterized in that when a phenyl group is substituted by one or more substituents R 3 , identical or different, this or these substituents R 3may be selected from a methyl group, a methoxy group, a methylthio group and a dimethylamino group.

3. Catalyst according to claim 1 or 2, characterized in that the groups R 1 and R 2 are identical.

4. Catalyst according to one of claims 1 to 3, characterized in that the nickel complex is selected from the group consisting of nickel complexes corresponding to the following formulas [Chem. 7] to [Chem. 11]: [Chem. 7]

5. Use of the catalyst according to one of claims 1 to 3, for catalyzing the reduction of protons to hydrogen (H2).

6. A method of producing hydrogen (H2) comprising contacting a source of protons (H +) with a catalyst according to one of claims 1 to 3, in an electrochemical cell, and applying a potential to the electrochemical cell, thereby reducing the protons (H + ) into hydrogen (H2).

7. A method according to claim 6, wherein: - the electrochemical cell comprises a working electrode, a counter-electrode, and where appropriate, a reference electrode, immersed in an electrolytic solution comprising an electrolyte, a solvent and said catalyst; - contacting the proton source (H + ) with the catalyst includes the addition of the proton source (H + ) to the non-aqueous solution; and - the application of a potential to the electrochemical cell includes the application of a voltage across the working electrode and the counter electrode so as to reduce the protons (H + ) into hydrogen (H2).

8. A method for producing hydrogen (H2) according to claim 6 or 7, wherein the solvent of the electrolytic solution is an aprotic solvent selected from amides, carbonates, ketones, aromatic compounds, esters, ethers, nitriles, halogenated solvents, sulfoxides and mixtures thereof.

9. Nickel(ll) complex comprising a bis(thiosemicarbazone) ligand derived from 2,2'-thenil, said nickel(ll) complex corresponding to the following general formula [Chem. 6]: [Chem. 6] in which, R 1 and R 2 each independently represent a phenyl group optionally having one or more substituents R 3 identical or different, R 3is selected from halogen, hydroxy, C1-C4 alkyl, C1-C4 alkoxy, C1-C4 thioalkyl, C1-C4 dialkylamino, cyano, CF3, and O-CF3.

Images