Porous composite catalyst consisting of an fe-n-c catalytic structure and a tin metal phase, and method for synthesizing same

Abstract

The invention relates to a porous composite catalyst consisting of an Fe-N-C catalytic structure and a tin metal phase, wherein the Sn / Fe atomic ratio of tin to iron is greater than 2 and less than 50, said porous composite catalyst containing micropores and mesopores, wherein the micropore volume is greater than 0.02 cm3 g-1, the mesopore volume is greater than 0.20 cm3 g-1, the micropore volume is the total volume of the micropores, and the mesopore volume is the total volume of the mesopores with a width of between 2 and 30 nm. The invention also relates to the method for synthesizing the catalyst, an ink comprising the catalyst and an ionomer, an electrode containing the catalyst, and a membrane electrode assembly, in particular for a fuel cell.

Description

[0001] Description

[0002] Title: Porous composite catalyst consisting of an Fe-NC catalytic structure and a tin metallic phase and its synthesis process.

[0003] The field of the present invention is that of non-precious metal-based catalysts intended for use in the oxygen reduction reaction, particularly in ion-exchange membrane fuel cells.

[0004] A fuel cell is a series assembly of cells, each comprising two electrodes, an anode and a cathode, and an electrolytic layer separating them. The cathode is the site of the oxygen reduction reaction. The catalysts commonly used in the oxygen reduction reaction in ion-exchange membrane fuel cells are platinum-based. Due to platinum's relative scarcity and cost, extensive research has been conducted over several decades to find alternatives to platinum catalysts. This research has focused on catalysts based on non-precious metals, such as transition metals. Fe-NC catalysts are among the most promising alternatives.

[0005] According to studies cited in the journal Adv. Sci. 2021, 8, 2102209, 1-25, the catalytic sites of Fe-NC catalysts are formed from iron atoms forming a coordination sphere with nitrogen atoms. These catalytic sites have the general formula FeN x , such as FeN? or FeN4, are described as being localized in a nitrogen-doped carbonaceous microporous structure.

[0006] According to the article entitled "Single-Atom Catalysts: A Perspective toward Application in Electrochemical Energy Conversion," JACS Au 2021, 1, 1086-1100, numerous variations in the synthesis of these catalysts have been proposed. They differ from one another, for example, in the choice of precursors for the microporous structure, the metal, and the choice of processes. One of the most promising and studied variations is based on the pyrolysis of a mixture comprising a crystalline porous hybrid solid (MOF, from "Metal Organic Framework"), such as ZIF-8, and an iron salt. The syntheses and catalysts described in patent applications WO 2017042564 and WO2012107838 illustrate this variation.

[0007] The performance of a fuel cell operating at high current density is dictated not only by the catalytic properties of the cathode, but also by its mass transport properties (oxygen, electrons, and ions, for the oxygen reduction reaction), and in particular by the diffusion of oxygen molecules to the catalytic sites. In the case of Fe-NC catalysts, the FeN catalytic sites xare primarily located in the micropores, making them relatively inaccessible to dioxygen molecules. In general, and for Fe-NC catalysts in particular, it is recognized that the presence of mesopores facilitates the diffusion of dioxygen molecules to the micropore inlets, thereby increasing cathode performance during high current density operation in a fuel cell. Therefore, there is a significant advantage to increasing the mesopore volume of Fe-NC catalysts while maintaining a sufficient micropore volume to accommodate the FeN sites. x .

[0008] Furthermore, since the catalytic properties of Fe-NC catalysts with respect to the oxygen reduction reaction remain inferior to those of platinum-based catalysts, there are

[0009] 2024PAT00083WO also a need to improve them, the main properties to improve being the catalytic efficiency for the oxygen reduction reaction, or catalytic activity, and the stability of the catalyst in operation, properties evaluated by electrochemical measurements which can be carried out either by controlling the current and measuring the electrochemical potential, or by controlling the electrochemical potential and measuring the current produced.

[0010] The inventors have discovered a new porous composite catalyst for the oxygen reduction reaction, and its preparation process; this catalyst is a doped catalyst containing FeN sites. xand having a significantly increased mesoporous volume compared to known Fe-NC catalysts. The inventors discovered that catalysts according to preferred embodiments of the invention also exhibit improved electrochemical catalytic activity (measured at low current density), and also show improved performance in fuel cells operating at high current density.

[0011] Thus, a first object of the invention is a porous composite catalyst consisting of an Fe-NC catalytic structure and a tin metallic phase, the Sn / Fe atomic ratio between tin and iron being greater than 2 and less than 50, which porous composite catalyst contains micropores and mesopores, the microporous volume being greater than 0.02 cm³ 3 g' 1 the mesoporous volume greater than 0.20 cm 3 g' 1, the microporous volume being the total volume of micropores and the mesoporous volume being the total volume of mesopores with a width between 2 and 30 nm.

[0012] Another object of the invention is a process for synthesizing a porous composite catalyst comprising the following steps a), b), c), d), and e): a) Mixing an iron salt and a crystalline porous hybrid solid (MOF), with or without a secondary nitrogen source, to form a mixture; b) Adding to the mixture obtained in step a) tin oxide powder in a mass percentage greater than or equal to 3% and less than or equal to 30%, calculated relative to the total mass of the mixture obtained in step a) and the tin oxide powder, the tin oxide being optionally doped with tantalum; c) Mixing the tin oxide powder and the mixture obtained in step a); d) Performing pyrolysis, preferably flash pyrolysis, of the mixture obtained in step c) at a temperature greater than or equal to 800°C and less than 1200°C under an inert atmosphere; e) Recovering the composite catalyst porous after pyrolysis.

[0013] Yet another object of the invention is an ink comprising a catalyst and an ionomer dispersed in a liquid phase, the catalyst being a porous composite catalyst according to the invention, which catalyst is capable of being prepared according to the process according to the invention.

[0014] The invention also relates to an electrode comprising a porous composite catalyst according to the invention, which catalyst is capable of being prepared according to the process according to the invention.

[0015] The invention also relates to a Membrane Electrode Assembly, MEA, which includes an electrode according to the invention.

[0016] 2024PAT00083WO The invention also relates to a fuel cell which includes an AME according to the invention.

[0017] Detailed description of the invention:

[0018] The catalyst according to the invention is a porous composite catalyst consisting of an Fe-NC catalytic structure (or Fe-NC type phase) and a tin metallic phase.

[0019] Like conventional Fe-NC catalysts, the porous composite catalyst according to the invention has as its constituent element catalytic sites formed of iron atoms coordinated with nitrogen atoms in a nitrogen-doped, porous carbon structure. The catalytic sites are typically of the formula FeN x , x an integer from 1 to 4, such as FeN? or FeN4. As is known to those skilled in the art, the formula FeN xThis designates a catalytic site in which one iron atom is coordinated with x nitrogen atoms. For example, the formula FeN4 designates a catalytic site in which one iron atom is coordinated with four nitrogen atoms. Preferably, all or some of the iron atoms are coordinated with four nitrogen atoms each. More preferably, all the iron atoms are coordinated with four nitrogen atoms each, in which case the catalytic sites have the formula Fe

[0020] In the porous composite catalyst according to the invention, the tin metallic phase consists of metallic tin, in other words, tin in oxidation state 0, Sn(0). The tin metallic phase is an essential constituent of the porous composite catalyst according to the invention. It is responsible for the porous structure of the catalyst, which comprises both mesopores and micropores. Furthermore, the tin metallic phase has a catalytic support function that also contributes to performance by significantly improving the electronic and thermal conductivity of the porous composite catalyst compared to a prior art Fe-NC catalyst.

[0021] The Sn / Fe ratio can be determined by the relative amounts of tin and iron used in the catalyst preparation. The ratio can also be determined by X-ray fluorescence spectrometry (XRF) performed on the catalyst. Preferably, the Sn / Fe atomic ratio between tin and iron in the catalyst according to the invention is greater than 2 and less than 40. More preferably, the Sn / Fe atomic ratio between tin and iron in the catalyst according to the invention is greater than 3 and less than 30. Even more preferably, the Sn / Fe atomic ratio between tin and iron in the catalyst according to the invention is greater than 3 and less than 20, advantageously greater than 3 and less than 15. Better still, the Sn / Fe atomic ratio between tin and iron in the catalyst according to the invention is greater than 5 and less than 15.According to any one of the embodiments of the invention, the Sn / Fe atomic ratio between tin and iron in the catalyst according to the invention is very advantageously less than 14, particularly greater than 3 and less than 14, more particularly greater than 5 and less than 14.

[0022] The porous composite catalysts according to the invention, for which the Sn / Fe atomic ratio between tin and iron is greater than 3 and less than 15, particularly less than 14, preferably greater than 5 and less than 15, particularly less than 14, also exhibit electrocatalytic activity, measured at low current density, which is higher than that of prior art Fe-NC catalysts. They also retain their advantage at high current density, the regime in which a fuel cell typically operates.

[0023] 2024PAT00083WO The porous composite catalyst according to the invention has the essential characteristic of containing micropores and mesopores. In the present invention, the terms micropores and mesopores are understood according to the definitions given by the International Union of Pure and Applied Chemistry (IUPAC). IUPAC defines a micropore as a pore with a width of less than 2 nm, and a mesopore as a pore with a width between 2 and 50 nm. The simultaneous presence of micropores and mesopores in the porous composite catalyst constituting an electrode facilitates the accessibility of the electrode's catalytic sites to oxygen molecules during its operation in an electrochemical device such as a fuel cell.

[0024] The simultaneous existence of micropores and mesopores in the porous composite catalyst is attributed to the dual structure of the porous composite catalyst, the tin metallic phase and the Fe-NC phase, and to the formation conditions of the tin metallic phase and the Fe-NC phase during the synthesis of the porous composite catalyst according to the invention. The microporous volume is greater than 0.02 cm³ 3 g' 1 and the mesoporous volume is greater than 0.20 cm 3 g' 1The microporous volume is the total volume of micropores, and the mesoporous volume is the total volume of mesopores with a width between 2 and 30 nm. The pore distribution between mesopores with a width between 2 and 30 nm and micropores is defined as the ratio between the total volume of mesopores with a width between 2 and 30 nm and the total volume of micropores, which ratio is preferably greater than 2 and more preferably greater than 3. The higher the ratio, the more the limiting factor in fuel cells related to mass transport is minimized. The pore width, as well as the microporous and mesoporous volumes, are determined in a known manner from nitrogen adsorption isotherms using the "Carbon-N2-77, NLDFT, Standard Slit" model. The porous composite catalyst according to the invention is preferably doped with tantalum.Given the amount of tantalum introduced relative to tin in the preparation of the catalyst when tantalum-doped, the tantalum / tin atomic ratio in the doped porous composite catalyst typically ranges from 0.005 to 0.015.

[0025] The porous composite catalyst according to the invention can be prepared by a process, another object of the invention, which comprises the following steps a), b), c), d), and e): a) Mixing an iron salt and a crystalline porous hybrid solid, MOF, with or without a secondary nitrogen source; b) Adding to the mixture obtained in step a) tin oxide powder in a mass percentage greater than or equal to 3% and less than or equal to 30%, the mass percentage being calculated relative to the total mass of the mixture obtained in step a) and the tin oxide powder, the tin oxide being optionally doped with tantalum; c) Mixing the tin oxide powder and the mixture obtained in step a); d) Performing pyrolysis, preferably flash pyrolysis, of the mixture obtained in step c) at a temperature greater than or equal to 800°C and less than 1200°C under an inert atmosphere; e) Recovering the catalyst porous composite after pyrolysis.

[0026] Crystalline porous hybrid solids (MOFs) are well known to those skilled in the art, particularly in the synthesis of Fe-NC type catalysts. They are porous crystalline solids composed of an assembly of organic and inorganic entities. The organic entities are ligands, while the inorganic entities can be cations of a metal, M, isolated or in the form of clusters, with or without oxo or hydroxo bridges, or a

[0027] 2024PAT00083WO infinite chain of M-oxo / hydroxo polyhedra. The organic entity of the MOF useful for the purposes of the invention is preferably a divalent cation azolate, more preferably an imidazolate such as 2-methylimidazolate, 2-ethylimidazolate, and even more preferably 2-methylimidazolate. The MOF is advantageously a structure belonging to the subcategory of zeolitic imidazolate MOFs well known by the acronym ZIF (acronym from the English "Zeolitic Imidazolate Framework"). The inorganic entity of the MOF useful for the purposes of the invention is preferably zinc, which has the advantage of being eliminated as volatile products during step d) of pyrolysis. Advantageously, the crystalline porous hybrid solid is an imidazolate network, ZIF, containing the metal ion Zn 2+Advantageously, the crystalline porous hybrid solid is ZIF-8, a structure well known to those skilled in the art and readily available commercially, the ligand being 2-methylimidazolate, the divalent cation of zinc, Zn 2+ .

[0028] The iron salt required for the purposes of the invention, which constitutes the iron source in the synthesis of the Fe-NC catalytic structure of the porous composite catalyst, can be an Fe(II) salt such as Fe(II) acetate, Fe(II) chloride, Fe(II) nitrate, Fe(II) oxalate, or Fe(II) sulfate, or alternatively, an Fe(III) salt. Preferably, the organic iron salt is anhydrous. The iron salt is advantageously Fe(II) acetate, and more advantageously, anhydrous Fe(II) acetate.

[0029] The secondary nitrogen source is typically a ligand containing at least two nitrogen atoms, an organic compound capable of forming coordinate bonds with a cation. Suitable ligands include 1,10-phenanthroline, bipyridine, tripyridyltriazine, pyrazine, imidazole, purine, pyrimidine, pyrazole, or their derivatives. The preferred secondary nitrogen source is 1,10-phenanthroline.

[0030] Step a) consists of preparing a precursor for the Fe-NC phase by mixing the iron salt and the MOF, or alternatively, by mixing the iron salt, the MOF, and the ligand used as a secondary nitrogen source. The iron salt, MOF, and ligand are all generally in powder form. Typically, the mass percentage of the chemical element iron, Fe, in step a) is greater than 0.1% and less than 2%, calculated relative to the mass of the Fe-NC phase precursor, i.e., the total mass of the iron salt, MOF, and secondary nitrogen source. Preferably, it is greater than or equal to 0.3% and less than or equal to 1%, calculated relative to the total mass of the iron salt, MOF, and secondary nitrogen source. Typically, the mass ratio between the secondary nitrogen source and the MOF in step a) is greater than or equal to 0 and less than or equal to 1. It is equal to 0 when the precursor consists of the mixture of the iron salt and the MOF.Preferably, it is greater than or equal to 0.125 and less than or equal to 0.375. The mixture is preferably prepared by bulk, advantageously using a ball mill comprising a grinding chamber, in particular a planetary ball mill, into which the iron salt and the MOF are introduced, and where applicable, the ligand used as a secondary nitrogen source. The constituent materials of the balls and the internal surface of the grinding chamber are chosen by those skilled in the art, generally for their hardness, so as not to contaminate the compounds used in the preparation of the porous composite catalyst. The balls and the internal surface of the grinding chamber are preferably made of the same hard material. The balls are preferably made of zirconium oxide. They generally have a diameter of about 5 mm. The ball mill preferably has a grinding chamber whose internal surface is made of zirconium oxide.

[0031] 2024PAT00083WO temperature inside the grinding tank is generally within a range of 25°C to 50°C.

[0032] Step b) of the process according to the invention consists of preparing a mixture of tin oxide powder, optionally doped with tantalum, and the precursor of the Fe-NC phase, the precursor also being in powder form.

[0033] The tin oxide powder added in step b) is preferably obtained by calcining nanofibers formed by electrospinning. The use of electrospinning is advantageous because it results in nanofibers that are easily ground into powder after calcination, for example, without the use of a planetary mill. The nanofibers are preferably prepared by electrospinning a solution containing an organic polymer and an inorganic tin salt. The solution is generally prepared by mixing a solution of the organic polymer with a solution of the inorganic tin salt. The concentration of the electrospinning solution is adjusted by those skilled in the art, particularly according to the weight-average molar mass of the organic polymer and the nature of the solvent, to ensure its suitability for use in an electrospinning device for forming nanofibers.Typically, the mass ratio between the organic polymer and the inorganic tin salt is greater than 0.3 and less than 1.8. The organic polymer is preferentially poly(vinylpyrrolidone), and the inorganic tin salt is preferentially a tin halide, more preferably a tin chloride or a tin chloride hydrate such as tin(II) chloride, tin(IV) chloride, their hydrates, and even more preferably SnCh or SnChFLO. The calcination of the nanofibers is preferentially carried out at a temperature above 500°C and below 700°C to eliminate volatile compounds resulting from thermal degradation, such as organic derivatives, and to oxidize the tin.Preferably, the tin oxide powder is obtained from nanofibers obtained by electrospinning a solution containing an organic polymer and an inorganic tin salt, then calcined, the organic polymer and the inorganic tin salt being advantageously poly(vinylpyrrolidone) and a tin chloride respectively.

[0034] When the tin oxide added in step b) is doped with a doping metal, tantalum, making the doped tin oxide electronically conductive, a porous composite catalyst doped with tantalum is obtained at the end of step e). Typically, tantalum is introduced during the preparation of the tin oxide. It is generally added as an inorganic salt in a solution containing an inorganic tin salt, the precursor of the tin oxide. For example, in the preparation of tin oxide powder by calcination of electrospin-formed nanofibers, tantalum is typically added as an inorganic salt in the solution containing the organic polymer, preferably polyvinylpyrrolidone, and the inorganic tin salt, preferably tin chloride. The inorganic tantalum salt is preferably tantalum halide, more preferably tantalum chloride, TaCh.The amount of inorganic tantalum salt added is adjusted by a person skilled in the art to obtain the desired atomic percentage of tantalum in the tin oxide. Preferably, doping the tin oxide means introducing more than 0.1% but less than 10% atomic percentage of tantalum into the tin oxide, the percentage being expressed relative to the mass of tin Sn introduced in step b).

[0035] Tin oxide powder is added in step b) in an amount greater than or equal to 3% and less than or equal to 30%, mass percentage calculated relative to the total mass of the

[0036] 2024PAT00083WO mixture obtained in step a) and tin oxide powder. If the amount of tin oxide powder is greater than 30%, the catalytic activity of the porous composite catalyst is much lower than that obtained with an Fe-NC catalyst that differs from it by the absence of tin oxide powder in the catalyst preparation. If it is less than 3%, the amount of tin oxide is insufficient to observe an improvement in the catalytic activity of the porous composite catalyst compared to an Fe-NC catalyst that differs from it by the absence of tin oxide powder in the catalyst preparation. Preferably, the amount of tin oxide powder added in step c) is greater than or equal to 5% and less than 20%, mass percentage calculated relative to the total mass of the mixture obtained in step a) and the tin oxide powder.Preferably, the amount of tin oxide powder added in step b) is greater than 5% and less than 20%, a mass percentage calculated relative to the total mass of the mixture obtained in step a) and the tin oxide powder. Within this preferred range, the porous composite catalyst exhibits the best catalytic activity (highest electrochemical potential at low current) and also the best performance (highest potential) at high current density.

[0037] The tin oxide powder is mixed with the mixture resulting from step a), which is also generally in powder form, to form, at the end of step c), a second mixture, also generally in powder form, which is then pyrolyzed in step d). The pyrolysis in step d) is carried out under an inert atmosphere, preferably nitrogen or argon, at a temperature of 800°C or higher but less than 1200°C. The second mixture is heated to the pyrolysis temperature for a period typically between 5 and 120 minutes. Preferably, the pyrolysis is flash pyrolysis. Flash pyrolysis refers to high-temperature pyrolysis characterized by a very rapid temperature rise of the compound to reach the pyrolysis temperature, typically in less than 5 minutes, preferably in less than 1 minute.During step d), a liquid tin metallic phase in oxidation state 0 and a nitrogen-doped porous carbon structure containing iron atoms coordinated with nitrogen atoms are formed, constituting the Fe-NC catalytic structure, also called the Fe-NC phase. The tin metallic phase Sn(0), the Fe-NC phase, and optionally tantalum Ta, are the constituent elements of the composite catalyst according to the invention.

[0038] According to a particular embodiment of the invention, the porous composite catalyst is coated with a continuous or discontinuous passivation layer made of tin oxide. The continuous or discontinuous passivation layer typically forms when the porous composite catalyst is exposed to the ambient atmosphere (air).

[0039] The porous composite catalyst according to the invention, which can be obtained by the process according to the invention, can be used in any electrochemical device that carries out an oxygen reduction reaction. It constitutes all or part of an electrode, another object of the invention. In particular, the electrode, consisting all or part of the porous composite catalyst, can be used in an electrode-membrane assembly (EMI), for example, in the form of a catalytic layer applied to a membrane to form a catalyst-coated membrane, known as a CCM (catalyst-coated membrane), or in the form of a catalytic layer applied to a gas diffusion layer to form an electrode structure known as a CCS (catalyst-coated membrane).

[0040] 2024PAT00083WO substrate). The porous composite catalyst according to the invention is intended more particularly for use in an AME assembly for a fuel cell.

[0041] Electrolytic membranes (EMs) are well-known building blocks, for example, in fuel cells. An EM typically comprises five layers: an electrolytic layer, two catalytic layers, and two gas diffusion layers. The electrolytic layer, which can be an ion-exchange polymer membrane, forms the core of the EM. On either side of the electrolytic layer is a catalytic layer; the layer adjacent to the catalytic layer is a gas diffusion layer. The entire assembly of gas diffusion layers, catalytic layers, and the electrolytic layer is held together by two bipolar plates. One of the two catalytic layers forms the anode, and the other the cathode.

[0042] The porous composite catalyst according to the invention is typically used in the form of a catalytic layer that constitutes all or part of the cathode, the site of the oxygen reduction reaction. In the preparation of a catalytic layer, the porous composite catalyst according to the invention is typically applied in the form of an ink onto a substrate. In a first embodiment, the substrate is an ion, proton, or anion exchange membrane, and the membrane coated with the catalytic layer constitutes a CTM (Crystal Layer Mold). In a second embodiment, the substrate is a gas diffusion layer, and the gas diffusion layer coated with the catalytic layer constitutes a GDE (Gas Deposition Environment).

[0043] The ink, another object of the invention, comprises a catalyst and an ionomer dispersed in a liquid phase, the catalyst being a porous composite catalyst according to the invention or obtainable by the process according to the invention. The liquid phase in which the porous composite catalyst and the ionomer are dispersed is preferably a dispersion consisting of water and an alcohol such as methanol, ethanol, 1-propanol, isopropanol, preferably 1-propanol, the proportion of alcohol by volume in the dispersion being typically greater than that of water. Preferably, the ink has a solids concentration ranging from 0.1% to 20% by mass. If the solids concentration is less than 0.1%, the ink must be applied in several stages to obtain the desired thickness of the catalytic layer, making the use of the ink inefficient.If the solids concentration is 20%, the ink viscosity may be too high, leading to the formation of an uneven layer. The ink preferably contains the porous composite catalyst and the ionomer in an ionomer / porous composite catalyst mass ratio ranging from 0.5 to 2, preferably from 0.8 to 1.5. The ionomer in the ink composition can be a cation- or anion-conducting polymer, depending on whether the membrane used in the AME assembly is a cation-exchange or anion-exchange membrane. Suitable cation-conducting polymers include, for example, polymers bearing sulfonic, carboxylic, and phosphonic groups and their salts, preferably sulfonic. Suitable anion-conducting polymers include, for example, polymers bearing ammonium and imidazolium groups.

[0044] In summary, the invention can be implemented according to any one of embodiments 1 to 33:

[0045] Mode 1: Porous composite catalyst consisting of an Fe-NC catalytic structure and a tin metallic phase, the Sn / Fe atomic ratio between tin and iron being greater than 2 and

[0046] 2024PAT00083WO less than 50, which porous composite catalyst contains micropores and mesopores, the microporous volume being greater than 0.02 cm³ 3 g' 1 the mesoporous volume greater than 0.20 cm 3 g' 1 , the microporous volume being the total volume of micropores and the mesoporous volume being the total volume of mesopores with a width between 2 and 30 nm.

[0047] Mode 2: Porous composite catalyst according to mode 1 in which the ratio between the total volume of mesopores with a width between 2 and 30 nm and the total volume of micropores is greater than 2.

[0048] Mode 3: Porous composite catalyst according to mode 1 or 2 in which the ratio between the total volume of mesopores with a width between 2 and 30 nm and the total volume of micropores is greater than 3.

[0049] Mode 4: Porous composite catalyst according to any one of modes 1 to 3 in which all or part of the iron atoms are coordinated each with 4 nitrogen atoms, preferably all the iron atoms are coordinated each with 4 nitrogen atoms.

[0050] Mode 5: Porous composite catalyst according to any one of modes 1 to 4 in which the Sn / Fe atomic ratio between tin and iron is greater than 2 and less than 40.

[0051] Mode 6: Porous composite catalyst according to any one of modes 1 to 5 in which the Sn / Fe atomic ratio between tin and iron is greater than 3 and less than 30.

[0052] Mode 7: Porous composite catalyst according to any one of modes 1 to 6 in which the Sn / Fe atomic ratio between tin and iron is greater than 3 and less than 20.

[0053] Mode 8: Porous composite catalyst according to any one of modes 1 to 7 in which the Sn / Fe atomic ratio between tin and iron is greater than 3 and less than 15.

[0054] Mode 9: Porous composite catalyst according to any one of modes 1 to 7 in which the Sn / Fe atomic ratio between tin and iron is greater than 5 and less than 15.

[0055] Mode 10: Porous composite catalyst according to any one of modes 1 to 9 in which the Sn / Fe atomic ratio between tin and iron is less than 14.

[0056] Mode 11: Porous composite catalyst according to any one of modes 1 to 10, which catalyst is tantalum-doped.

[0057] Mode 12: Porous composite catalyst according to any one of modes 1 to 11, which catalyst is covered with a continuous or discontinuous passivation layer of tin oxide.

[0058] Method 13: Process for synthesizing a porous composite catalyst comprising the following steps a), b), c), d), and e): a) Mixing an iron salt and a crystalline porous hybrid solid, MOF, with or without a secondary nitrogen source; b) Adding to the mixture obtained in step a) tin oxide powder in a mass percentage greater than or equal to 3% and less than or equal to 30%, calculated relative to the total mass of the mixture obtained in step a) and the tin oxide powder, the tin oxide being optionally doped with tantalum; c) Mixing the tin oxide powder and the mixture obtained in step a); d) Carrying out the reaction at a temperature greater than or equal to 800°C and less than 1200°C under

[0059] 2024PAT00083WO inert atmosphere pyrolysis, preferably flash pyrolysis, of the mixture obtained in step c), e) Recover the doped porous composite catalyst after pyrolysis.

[0060] Mode 14: Process according to mode 13 in which the iron salt is a salt of Fe(II), preferably Fe(II) acetate.

[0061] Mode 15: A process according to mode 13 or 14 in which the crystalline porous hybrid solid is an imidazolate network, ZIF, containing the metal ion Zn 2+ .

[0062] Mode 16: Process according to any one of modes 13 to 15 in which the crystalline porous hybrid solid is ZIF-8.

[0063] Mode 17: A process according to any one of modes 13 to 16 in which the secondary nitrogen source is 1,10-phenanthroline.

[0064] Method 18: A process according to any one of methods 13 to 17 in which the quantity of tin oxide powder added in step b) is greater than or equal to 5% and less than 20%, mass percentage calculated with respect to the total mass of the mixture obtained in step a) and of the tin oxide powder.

[0065] Method 19: A process according to any one of methods 13 to 18 in which the quantity of tin oxide powder added in step b) is greater than 5% and less than 20%, mass percentage calculated with respect to the total mass of the mixture obtained in step a) and of the tin oxide powder.

[0066] Mode 20: A process according to any one of modes 13 to 19 in which the tin oxide powder is obtained by calcining nanofibers formed by electrospinning.

[0067] Mode 21: A process according to mode 20 in which the nanofibers are obtained by electrospinning a solution containing an organic polymer, an inorganic tin salt and optionally an inorganic tantalum salt.

[0068] Mode 22: Process according to mode 21 in which the organic polymer is poly(vinylpyrrolidone), the inorganic tin salt is tin chloride or its hydrate and the inorganic tantalum salt is tantalum chloride, TaCF.

[0069] Mode 23: Process according to any one of modes 20 to 22 in which the calcination of the nanofibers is carried out at a temperature above 500°C and below 700°C.

[0070] Mode 24: Process according to any one of modes 13 to 23 in which the tantalum in the tin oxide represents from 0.1% to less than 10% atomic, percentage expressed in relation to the mass of tin Sn introduced in step b).

[0071] Mode 25: A process according to any one of modes 13 to 24 in which the mass percentage of the chemical element iron, Fe, at step a) is greater than 0.1% and less than 2%, the percentage calculated with respect to the total mass of iron salt, MOF and secondary nitrogen source.

[0072] Mode 26: A process according to any one of modes 13 to 25 in which the mass percentage of the chemical element iron, Fe, in step a) is greater than or equal to 0.3% and less than or equal to 1%, percentage calculated with respect to the total mass of iron salt, MOF and secondary nitrogen source.

[0073] 2024PAT00083WO Mode 27: Process according to any one of modes 13 to 26 in which the mass ratio between the secondary nitrogen source and the MOF in step a) is greater than or equal to 0 and less than or equal to 1.

[0074] Mode 28: A process according to any one of modes 12 to 27 in which the mass ratio between the secondary nitrogen source and the MOF in step a) is greater than or equal to 0.125 and less than or equal to 0.375.

[0075] Mode 29: A process according to any one of modes 12 to 28 in which pyrolysis is flash pyrolysis at a temperature greater than or equal to 800°C and less than 1200°C.

[0076] Mode 30: Ink comprising a catalyst and an ionomer dispersed in a liquid phase, the catalyst being a porous composite catalyst defined in any one of modes 1 to 12 or obtained by a process defined in any one of modes 13 to 29.

[0077] Mode 31: Electrode comprising a porous composite catalyst defined in any one of modes 1 to 12 or a porous composite catalyst obtained by the process defined in any one of modes 13 to 29.

[0078] Mode 32: Membrane Electrode Assembly, MEA, which includes an electrode defined in mode 31.

[0079] Mode 33: Fuel cell that includes an AME defined in mode 32.

[0080] The aforementioned features of the present invention, as well as others, will be better understood upon reading the following description of several examples of embodiments of the invention, given by way of illustration.

[0081] Examples

[0082] Preparation of catalysts according to the invention:

[0083] Preparation of tantalum-doped tin oxide powder, hereinafter referred to as TTO: TTO is prepared according to the following procedure, implementing the synthesis process described by I. Jimenez-Morales et al., ACS Catalysis 10 (2020) 10399, DOI: 10.1021 / acscatal.0c02220. SnChEhO₂ salt (882 mg) is dissolved in ethanol (6.4 mL) and stirred for 30 minutes. Anhydrous TaCh₂ salt (14 mg) is added, and the solution is stirred for 3 hours. A second solution is prepared by dissolving polyvinylpyrrolidone polymer (PVP, 0.9 g) in N,N-dimethylformamide (DMF, 5 mL). The two solutions are then mixed and stirred for 2 days. A viscous solution is obtained. The viscous solution is then electrospun using a "LINARI" brand device. A 5 mL syringe is used, which is filled with the solution to be electrospun (when the syringe becomes empty, it is refilled until all the viscous solution prepared above has been used).Appropriate electrospinning parameters (needle-to-drum distance of 80 mm, translation speed of 10 mm / s, rotation speed of 300 rpm, and a spinning time of approximately 16 hours (20 hours maximum, limited by solution stability), solution spinning speed of 0.3 mL / h, and an electrical potential difference of 15 kV between the syringe and the drum) are used under conditions of 24–40% relative humidity to obtain regular fibers with an average diameter of 140 nm. The drum, which collects the fibers as a fiber mat, is covered with an antistatic conductive polymer film that facilitates mat removal.

[0084] 2024PAT00083WO of electrospun PVP polymer fibers at the end of this step. The PVP fiber mat (containing inorganic Sn and Ta salts) obtained by electrospinning is then calcined under ambient air in a muffle furnace at a temperature of 600°C for 4 h, with a heating rate of 5°C / min and a cooling rate of 5°C / min. The calcination step removes the PVP polymer primarily as CO2 gas and induces the formation of tantalum-doped SnCl crystalline fibers. The TTO fibers thus produced have a Ta / Sn atomic ratio of 1%.

[0085] Example 1 according to the invention: FeNL TTO-1000-60:

[0086] A first example of a catalyst according to the invention is prepared by mixing 1.6 g of ZIF-8 (“Basolite” Z1200, Sigma Aldrich, product reference 691348), 0.4 g of 1,10-phenanthroline, and 31.2 mg of anhydrous iron(II) acetate in a planetary mill (step a). These compounds are solid powders, and no solvents or liquids are added. The planetary mill (a “Fritsch” brand micro-mill, Pulverisette 7 premium line) is equipped with two 45 mL grinding bowls or crucibles. The masses of ZIF-8, 1,10-phenanthroline, and anhydrous iron(II) acetate indicated above are weighed and introduced into the first grinding bowl (or crucible). The same masses are weighed and introduced into the second crucible. One hundred ZrCL beads with a diameter of 5 mm are introduced into each crucible, the internal surface of which is also made of ZrCL.The planetary mill (Fritsch Pulverisette 7 premium line micro-mill) is programmed to perform four 30-minute grinding cycles at 400 rpm (main plate rotation speed). Both crucibles are installed in the mill simultaneously. A 5-minute pause is programmed between each cycle, and the direction of rotation is alternated between cycles. Both crucibles, filled with the same ZIF-8 / phenanthroline / iron acetate mixture, are rotated according to the program described above.

[0087] The mixtures thus obtained are recovered from the two crucibles, combined to form a single mixture, the total mass of which is measured. To this mixture of known mass, TTO fibers are added at a rate of 5% by mass of TTO fibers relative to the total mass (ZIF-8, phenanthroline, iron acetate and TTO fibers) (step b), and the whole is mixed manually using a mortar and pestle (step c).

[0088] A portion of the mixture obtained in step c) is poured into a quartz pod, which is then placed inside a quartz tube. Typically, 300 mg of the mixture is placed in the quartz pod. A Thermcraft split-tube furnace is used. The quartz tube is installed in the furnace in the usual way, but the pod containing the mixture is positioned so that it is initially outside the heating zone. Next to the quartz pod, a quartz rod with an attached magnet is placed inside the quartz tube; this is referred to as the quartz rod + magnet. The inlet and outlet of the quartz tube are connected to argon gas, and the air is purged for at least 15 minutes under argon flow before the furnace is heated. The tube furnace is then preheated to 1000°C under argon flow.To obtain a uniform and precise furnace temperature, a 2-hour waiting period is required from the moment the thermocouple regulating the furnace reaches 1000°C, always under argon flow. The quartz capsule containing the poured mixture is then introduced into the furnace's heating zone, using an external magnet to push the quartz bar and magnet, which in turn pushes the capsule. This is typically accomplished in approximately 30-40 seconds. The quartz bar and magnet are then removed, again using the external magnet, leaving the capsule and the mixture in the center of the heating zone. From the moment the mixture is introduced...

[0089] 2024PAT00083WO In the center of the furnace heating zone, the mixture is left at 1000 °C for 60 minutes, still under an argon flow. Pyrolysis is stopped by opening the split tube furnace, removing the quartz tube and its contents from the furnace, and then allowing the tube and its contents to cool naturally in ambient air, still under an argon flow. When the assembly has cooled sufficiently (to 80 °C or less), the tube is opened by disconnecting the inlet or outlet tube from the argon flow and allowing air to gradually enter the quartz tube by diffusion. The resulting powder is collected without further processing and constitutes the final catalyst. The catalyst has the acronym FeNCLs / TTO-1000-60. The number 95 represents the mass percentage of ZIF-8, phenanthroline and iron salt in the mixture before pyrolysis, 1000 indicates the pyrolysis temperature in degrees Celsius, and 60 the pyrolysis time in minutes.The Sn / Fe ratio is 3.9.

[0090] Example 2 according to the invention: FeNCss / TTO-1000-60

[0091] A second example of a catalyst according to the invention is prepared in the same way, but by adding in step b) the amount of TTO necessary to achieve a mass percentage of 15% of TTO fibers relative to the total mass (ZIF-8, phenanthroline, iron acetate, and TTO fibers), and the mixture is then manually blended using a mortar and pestle (step c). Steps d) (pyrolysis) and e) (catalyst recovery) are identical to those described above. This catalyst has the acronym FeNCss / TTO-1000-000. The Sn / Fe ratio is 13.1.

[0092] Example 3 in accordance with the invention: FeNCss / TTO-lOOO-lS

[0093] A third example of a catalyst according to the invention is prepared in the same way as in example 2, except that the pyrolysis time is 15 minutes. The Sn / Fe ratio is 13.1.

[0094] Example 4 in accordance with the invention: FeNC?o / TTO-1000-60

[0095] A fourth example of a catalyst according to the invention is prepared in the same way as the catalyst of Example 2, but by adding in step b) the amount of TTO necessary to achieve a mass percentage of 30% of TTO fibers relative to the total mass (ZIF-8, phenanthroline, iron acetate, and TTO fibers), and the mixture is then manually blended using a mortar and pestle (step c)). Steps d) (pyrolysis) and e) (catalyst recovery) are identical to those described above. This catalyst has the acronym FeNC7O / TTO-1000-60. The Sn / Fe ratio is 31.9.

[0096] Example 5 according to the invention: FeNCLo / TTO-1000-60

[0097] A fifth example of a catalyst according to the invention is prepared in the same way as the catalyst of Example 2, but by adding in step b) the amount of TTO necessary to achieve a mass percentage of 10% of TTO fibers relative to the total mass (ZIF-8, phenanthroline, iron acetate, and TTO fibers), and the mixture is then manually blended using a mortar and pestle (step c)). Steps d) (pyrolysis) and e) (catalyst recovery) are identical to those described above. This catalyst has the acronym FeNC TTO-1000-000. The Sn / Fe ratio is 8.3.

[0098] Example 6 according to the invention: FeNCsî / TTO- 1000-60

[0099] A sixth example of a catalyst according to the invention is prepared in the same way as the catalyst of Example 2, but by adding in step b) the amount of TTO necessary to achieve a mass percentage of 13% of TTO fibers relative to the total mass (ZIF-8, phenanthroline, iron acetate, and TTO fibers), and the mixture is blended

[0100] 2024PAT00083WO manually using a mortar and pestle (step c)). Steps d) (pyrolysis) and e) (catalyst recovery) are identical to those described above. This catalyst has the acronym FeNC 87 / TTO-1000-60. The Sn / Fe ratio is 11.1.

[0101] Example 7 in accordance with the invention: FeNC TTO-1000-60

[0102] A seventh example of a catalyst according to the invention is prepared in the same way as the catalyst of Example 2, but by adding in step b) the amount of TTO necessary to achieve a mass percentage of 17% of TTO fibers relative to the total mass (ZIF-8, phenanthroline, iron acetate, and TTO fibers), and the mixture is then manually blended using a mortar and pestle (step c)). Steps d) (pyrolysis) and e) (catalyst recovery) are identical to those described above. This catalyst has the acronym FeNCss / TTO-1000-000. The Sn / Fe ratio is 15.2.

[0103] Preparation of catalysts not conforming to the invention:

[0104] Example 8 not in accordance with the invention: FeNCis / TTO-1000-60:

[0105] A first example of a catalyst not conforming to the invention is prepared in a manner similar to the first example of a catalyst conforming to the invention (Example 1), but by adding in step b) the amount of TTO necessary to achieve a mass percentage of 85% TTO fibers relative to the total mass (ZIF-8, phenanthroline, iron acetate, and TTO fibers), and the mixture is then manually blended using a mortar and pestle (step c). Steps d) (pyrolysis) and e) (catalyst recovery) are identical to those in Example 1. This catalyst has the acronym FeNCis / TTO-1000-60. The Sn / Fe ratio is 421.9.

[0106] Example 9 not in accordance with the invention: FeNCso / TTO-1000-60

[0107] A second example of a catalyst not according to the invention is prepared in a similar manner to the first example of a catalyst according to the invention (Example 1), but by adding in step b) the amount of TTO necessary to achieve a mass percentage of 50% TTO fibers relative to the total mass (ZIF-8, phenanthroline, iron acetate, and TTO fibers), and the mixture is then manually blended using a mortar and pestle (step c). Steps d) (pyrolysis) and e) (catalyst recovery) are identical to those in Example 1. This catalyst has the acronym FeNCso / TTO-1000-60. The Sn / Fe ratio is 74.5.

[0108] Example 10 not in accordance with the invention: FeNCso / TTO-900-60

[0109] A third example of a catalyst not according to the invention is prepared in a similar manner to the second example of a catalyst not according to the invention (Example 9), but by carrying out flash pyrolysis at a temperature of 900 °C. This catalyst has the acronym FeNCso / TTO-900-60. The Sn / Fe ratio is 74.5.

[0110] Example 11 not in accordance with the invention: FeNC:

[0111] The FeNC catalyst is prepared following the procedure of example 1, omitting step b) of adding the TTO powder. The Sn / Fe ratio is equal to 0.

[0112] Results :

[0113] Figure 1 shows the N2 adsorption isotherms of the FeNC, FeNCso / TTO-900-60, and FeNC catalysts. 70 / TTO- 1000-60, FeNC 83 / TTO- 1000-60, FeNC 85 / TTO- 1000-15, FeNC 87 / TTO- 1000-60, FeNC 95 / TTO- 1000-60.

[0114] Figure 2 (2024PAT00083WO) represents, for the catalysts in Figure 1, the cumulative pore volume as a function of pore size. Values ​​derived from the modeling of N2 adsorption isotherms using the "Carbon-N2-77, NLDFT, Standard Slit" model.

[0115] Figure 3 represents, for each of the catalysts FeNCis / TTO-1000-60, FeNCso / TTO-1000-60, FeNC5o / TTO-900-60, FeNC 70 TTO-1000-60, FeNC 83 / TTO- 1000-60, FeNC 85 / TTO- 1000-60, FeNC TTO- 1000-15, FeNC 87 / TTO- 1000-60, FeNC 95 / TTO-1000-60 and FeNC, polarization curves for the electrochemical reduction of CL measured with a rotating disc electrode device. Catalyst loading of 200 pg / cm² 2 , rotation speed 900 rpm, 0.5 M sulfuric acid electrolyte saturated with O2, electrochemical potential sweep speed of 2 mV / s.

[0116] Figure 3B represents a histogram of current density values ​​at 0.8 V vs. ERH, Reversible Hydrogen Electrode, values ​​extracted from the polarization curves of Figure 3.

[0117] Figure 4 shows the powder X-ray diffractograms of: a) TTO fibers, b) FeNCso / TTO-1000-60, c) FeNC 85 / TTO- 1000-60, d) FeNC 95 / TTO- 1000-60.

[0118] Figure 5 is a SEM (scanning electron microscopy, moderate resolution, with a Hitachi S-4800 instrument) image of the FeNC8s / TTO-1000-60 catalyst.

[0119] Figure 6 is a SEM (scanning electron microscopy, moderate resolution, with a Hitachi S-4800 instrument) image of the FeNC TTO-1000-60 catalyst.

[0120] Figure 7 is a SEM (scanning electron microscopy, moderate resolution, with a Hitachi S-4800 instrument) image of the TTO fibers.

[0121] Figure 8 is a SEM (scanning electron microscopy, higher resolution, with a Hitachi S-4800 instrument) image of the FeNCss / TTO-1000-60 catalyst.

[0122] Figure 9 represents the polarization curves in a PEMFC cell, a proton exchange membrane fuel cell.

[0123] Textural property analysis by nitrogen sorption

[0124] The analysis of the textural properties of the solids, pore size and volume, was performed by determining the nitrogen adsorption-desorption isotherm at 77.36 K. These isotherms were recorded using the "3Flex" model marketed by Micromeritics. The instrument used in this study is unique in that it is equipped with three "Micropore" analysis ports; that is, in addition to the 10 and 1000 ton pressure transducers, it also features a 0.1 torr precision transducer with a reading accuracy of 0.15%.

[0125] Prior to analysis, the catalysts were conditioned using Micromeritics' "Smart VacPrep" device. This pretreatment combines vacuum and heat to remove atmospheric contaminants, including water and gases adsorbed on the surface and within the pores of the solid. The samples are subjected to vacuum at room temperature for 1 h, then to 150 °C for 12 h until a pressure of 0.001 torr is reached.

[0126] Within the relative pressure range P / Po [0 - 0.1], isotherms are measured with an AP / Po interval of 0.005 between each point on the relative pressure axis, with an equilibration time of 30 s after each dose of nitrogen added. For relative pressures

[0127] 2024PAT00083WO higher the pressure increment is gradually increased up to an AP / Po increment of 0.1 and the equilibration time is reduced to 10 s.

[0128] After recording the adsorption curve, nitrogen desorption is carried out by applying decreasing relative pressure setpoints with an increment of relative pressure and equilibration time adapted to the different regions of the isotherm (slowly in the relative pressure range 0.995-0.98 and more rapidly up to a final relative pressure value of 0.2).

[0129] The total specific surface area of ​​the catalysts is obtained from the experimental adsorption isotherm by applying the Brunauer Emmett Teller (BET) model to a chosen P / Po interval for which the BET model is applicable, i.e. a P / Po interval in which the model can reproduce the experimental data with a linear regression coefficient of 0.999 or more, and a value of the constant C of the BET model that is positive.

[0130] Furthermore, the experimental adsorption curve over a wider range of P / Po values ​​can be modeled to quantify the pore size distribution. For this, we used the "SAIEUS" software developed by Micromeritics. The chosen model is "Carbon-N2-77, NLDFT, Standard Slit". For this analysis, the model allows the option of not considering pores smaller than a theoretical minimum size, due to the hydrodynamic radius of the adsorbate. For nitrogen as the adsorbate, we selected a minimum pore size of 5 Å for the analysis using the "Carbon-N2-77, NLDFT, Standard Slit" model. This more advanced analysis allows, for example, the quantification of the cumulative pore volume as a function of pore size.

[0131] The nitrogen adsorption isotherms shown in Figure 1 indicate that the FeNC catalyst is mainly microporous (pore size between 0 and 2 nm), reflected by the large volume adsorbed at very low P / Po values, and contains few mesopores (relatively flat isotherm between P / Po values ​​of 0.1 and 0.8), whereas the catalyst examples of the invention, FeNCss / TTO-1000-1S and FeNCos / TTO-1000-60, are microporous (but less so than FeNC) and also mesoporous (pore size from 2 nm to 30 nm) with a continuous and large increase in the porous volume adsorbed at all P / Po values.

[0132] Figure 2, representing the cumulative adsorbed pore volume as a function of pore size (derived from the analysis using the "Carbon-N2-77, NLDFT, Standard Slit" model), allows for a more precise quantification of the microporous and mesoporous volumes of these materials (Table 1). The presence of micropores in these materials is necessary to host the iron catalytic sites, but good accessibility of the iron catalytic sites by O2 during the electrochemical reaction requires mesopores. The catalysts of the invention exhibit a significant microporous volume (though less than that of the reference FeNC) and a high mesoporous / microporous volume ratio.

[0133] Table 1: 2024PAT00083WO

[0134] The catalysts according to the invention have a very greatly increased mesoporous volume compared to the FeNC catalyst, since their mesoporous volume is, at a minimum, more than 5 times greater than the mesoporous volume of the FeNC catalyst.

[0135] Electrochemical characterization using a rotating disk electrode

[0136] The electrochemical characterization results for the electroreduction of CL using a rotating disk electrode in aqueous electrolyte (0.5 M H2SO4) are shown in Figure 3. The rotating disk electrode device, equipped with a tip that screws onto the rotation axis, is a commercial device from Pine Instruments (model "MSR Rotator" or "Wave Vortex 10 Rotator"). The tip that screws onto the rotation axis is made of Teflon or polyetheretherketone (PEEK) and encloses a glassy carbon disk (5 mm in diameter) onto which a desired amount of catalytic ink is deposited. This ink consists of a catalyst that forms the working electrode. During the electrochemical measurement, the tip with the catalyst deposit is immersed in the electrolyte (0.5 M H2SO4 saturated with O2) while the rotation axis rotates at a speed of 900 rpm.The polarization curve for the electroreduction of Û2 in the catalyst is then measured by sweeping the electrochemical potential from 0.05 V to 0.9 V relative to a hydrogen reference electrode (HRE) at a sweep speed of 2 mV / s, while simultaneously measuring the current. The counter electrode is made of graphite. These electrochemical measurements are performed using a potentiostat from the "Biologie" brand.

[0137] The catalytic ink is prepared by weighing 5 mg of catalyst powder, which is dispersed in 636 µL of 1-propanol, 500 µL of deionized water, and 57 µL of commercial Nafion solution (5% by weight of Nafion in an alcohol-water mixture; Nafion is an ionomer with an equivalent weight of 1100 (1100 g per mole of proton-conducting SO3H group)). The solution is sonicated for at least 1 hour in an ice bath. A volume of 9.4 µL of ink is then pipetted onto the glassy carbon disk. The solvents are evaporated at room temperature by rotating the electrode to ensure a homogeneous deposit of the catalyst.

[0138] The resulting polarization curves exhibit an identical shape, with negative current values ​​conventionally indicating a reduction reaction current, which in this case corresponds to the electroreduction of CL. The electrocatalytic activity trend for the reduction of CL between different catalysts can be evaluated by comparing the current values ​​(normalized by the geometric surface area of ​​the glassy carbon disk) at a fixed electrochemical potential of 0.8 V. At this potential, the current values ​​are low and controlled.

[0139] 2024PAT00083WO by the kinetics of the electroreduction reaction of Ch, which is the main information sought by a rotating disc electrode device.

[0140] Figure 3 therefore shows an increase in activity for FeNC catalysts 85 / TTO- 1000-60, FeNC 85 / TTO- 1000-15 and FeNC 87 / TTO- 1000-60 compared to the FeNC catalyst taken as a reference. Figure 3 also shows the lowest activity for the FeNCis / TTO- 1000-60 (inactive), FeNCso / TTO- 1000-60 and FeNCso / TTO-900-60 catalysts compared to the reference.

[0141] The electrocatalytic activities are shown even more clearly and quantitatively in Figure 3B, which relates the current density at an electrochemical potential of 0.8 V for the different catalysts in Figure 3. The FeNCis / TTO-1000-60 catalyst is inactive at this potential, while FeNCso / TTO-1000-60 is about ten times less active than FeNC. Conversely, FeNC8s / TTO-1000-60 and FeNC85 / TTO-1000-15 are about 25% more active than FeNC.

[0142] X-ray diffraction

[0143] The crystalline phases present in the catalysts were studied by powder X-ray diffraction (Cu K aFigure 4 shows the diffractograms recorded for the FeNCso / TTO-1000-60 and FeNC catalysts. 85 / TTO- 1000-60, FeNC 95 / TTO-1000-60, and for the TTO fibers used in the preparation of these catalysts. Powder X-ray diffraction (XRD) patterns were recorded in the Bragg-Brentano configuration using a Malvern Panalytical X'pert diffractometer, operating with Cu Kα radiation (wavelength = 1.541 Å). Measurements were performed at 20 °C in the angle range of 20 to 80° with a step size of 0.1°.

[0144] The diffractogram of the TTO fibers shows a series of peaks between 25 and 80 °C, all of whose positions correspond to the expected positions for the tetragonal crystal structure SnÛ2, for which a JCPDS (Joint Committee on Powder Diffraction Standards) file is number 00-041-1445. In contrast, the diffractograms of the three FeNC / TTO catalysts shown in Figure 4 indicate the absence of the SnÛ2 phase, and the appearance of a series of new diffraction peaks between 30 and 80 °C, all of whose positions correspond to the expected positions for the tetragonal crystal structure of metallic tin Sn (P-Sn), for which a JCPDS file is number 00-004-0673. The position of the diffraction peaks due to the P-Sn phase is the same in the three FeNC / TTO catalysts shown in Figure 4, but their intensity decreases in the order FeNCso / TTO- 1000-60 > FeNC8s / TTO- 1000-60 > FeNC9s / TTO- 1000-60, which indicates a decreasing relative amount of P-Sn in this order.This is related to the initial amount of SnO7 used for the preparation of the catalyst precursor, which is 50% by weight for FeNCso / TTO- 1000-60, 15% by weight for FeNC. 85 / TTO- 1000-60 and 5% by weight for FeNC 95 / TTO- 1000-60.

[0145] The XRD results therefore indicate the complete chemical reduction of TTO fibers in the P-Sn metallic phase during pyrolysis carried out during the preparation of FeNC / TTO catalysts at 1000°C under argon flow.

[0146] Regardless of the exact mechanism of SnÛ2 reduction to P-Sn during pyrolysis, the XRD results imply the formation of metallic P-Sn from TTO fibers during pyrolysis. Since the melting point of metallic P-Sn is 231.9 °C, it follows that the P-Sn phase formed during pyrolysis flashes at 1000 °C of the FeNCso / TTO-1000- catalysts.

[0147] 2024PAT00083WO 60, FeNCss / TTO- 1000-60 and FeNCgs / TTO- 1000-60 is in liquid form as soon as it forms, and remains in liquid form during the pyrolysis and until the sample temperature drops below this value of 231.9 °C, during cooling at the end of pyrolysis.

[0148] Morphological and structural analysis by scanning electron microscopy

[0149] A morphological analysis was performed by scanning electron microscopy (SEM) at moderate resolution using a Hitachi S-4800 instrument. Figures 5 and 6 each show a representative SEM image of the FeNCss / TTO-1000-60 and FeNCgs / TTO-1000-60 catalysts, respectively. Figure 7 shows an SEM image of the TTO fibers used to prepare these catalysts. A first observation is that the fibrous morphology of the TTO obtained after electrospinning and calcination, observed in Figure 7, is no longer present in the catalysts of the invention (Figures 5-6). This implies that the structural change of the SnU2 phase (tantalum-doped SnU2 fibers, TTO) to metallic P-Sn during pyrolysis also induced a change in shape. This is explained by the liquid state of P-Sn at the pyrolysis temperature, as described above.Figures 5 and 6 allow us to distinguish, by contrast due to atomic mass, the P-Sn phase and the Fe-NC phase (primarily composed of carbon), with the P-Sn phase appearing brighter. In particular, we can see the presence of numerous small, bright, spherical objects, which can be attributed to metallic P-Sn particles that were liquid during pyrolysis and solidified in this form upon cooling at the end of the pyrolysis process. A higher-resolution SEM image allows us to better distinguish these relatively spherical objects (Figure 8).

[0150] Results in proton conductive membrane fuel cell

[0151] For a mass ratio of "Nafion" ionomer to catalyst of 1.4 and a cathode catalyst loading of 4 mg per cm 2The following ink was used to prepare the cathode: 20 mg of catalyst were dispersed in 652 pL of commercial Nafion solution (5 wt% Nafion in an alcohol-water mixture (20 wt% water and 75 wt% alcohol), Nafion being an ionomer with an equivalent weight of 1100 (1100 g per mole of proton-conducting SO3H group)), 326 pL of 1-propanol, and 272 pL of deionized water. The catalytic ink was homogenized ultrasonically for at least 1 h in an ice bath and then pipetted onto a carbon fiber diffusion layer (Sigracet 28-BC, from BalticFuelCells) with a geometric surface area of ​​4.84 cm². 2A 200 pL volume of ink is deposited at a time, and the solvents are evaporated in an oven at 60 °C at each step until the deposit is almost dry. A second 200 pL deposit is then made, and so on until all the ink has been used. After the last deposit, the cathode is completely dried in an oven overnight. A commercial Pt electrode was used as the anode (0.3 mg Pt / cm² loading). 2 on a carbon fiber diffusion layer, "Sigracet 28BC", from the company balticFuelCells GmbH), with a geometric surface area of ​​4.84 cm 2 . A membrane electrode assembly (MEA) is then prepared by hot-pressing an anode and a cathode on either side of a "Nafion 211" membrane for 2 minutes at 2 tons at a temperature of 135 °C.

[0152] The resulting AMEs are then tested using a fuel cell equipped with a serpentine gas flow channel (from Fuel Cell Technologies). A fuel cell test station (model 850E, from Scribner Associates) and

[0153] 2024PAT00083WO is a potentiostat from Biologie, coupled to a 20 A load, also from Biologie. Operating conditions are a fuel cell temperature of 80 °C, pure H2 at the anode and pure O2 at the cathode, which are humidified to 100% relative humidity at 80 °C just before entering the fuel cell. A flow rate of 150 mL / min and a back pressure of 1 bar are applied to both the H2 and O2 sides. EC-Lab software is used to control the potentiostat and collect electrochemical data.

[0154] Figure 9 shows the polarization curves obtained in a PEMFC cell for different cathode catalysts, for an identical cathode catalyst loading of 4 mg cm⁻¹ 2 At low current (cell voltage of 0.75 V or more), the curve is mainly dictated by the electrocatalysis of CL, and the major trends observed here are the same as with a rotating disc electrode. At a voltage of 0.75 V, the current density with the FeNC10 / TTO-1000-60 catalyst is low (about ten times lower than with the reference FeNC), then increases for lower quantities of TTO (FeNC10s / TTO-1000-00, FeNC10s / TTO-1000-00, FeNC2ss / TTO-1000-60), reaching an optimum with FeNC2ss / TTO-1000-00, before decreasing slightly (FeNC10 TTO-1000-60, FeNC2 TTO-1000-60) and then sharply (FeNC20). 95 / TTO-1000-60). At 0.75 V, FeNC 85 / TTO-1000-60 has a current density comparable to FeNC, which is slightly different from the results in rotating disk electrode and can be attributed to the different environments (acidic aqueous electrolyte in rotating disk electrode and proton-conducting solid ionomer in fuel cell).

[0155] However, FeNC catalysts 95 / TTO- 1000-60, FeNC 90 / TTO- 1000-60, FeNC 85 / TTO-1000-60, for which the Sn / Fe atomic ratio is greater than 3 and less than 15, demonstrate their advantage at high current density, a regime in which the polarization curve is dictated not only by the catalytic properties of the cathode but also by its so-called mass transport properties, and in particular the diffusion of CL to the catalytic sites located in the micropores. Indeed, at 600 mA cm' 2 or more, FeNC catalysts 85 / TTO- 1000-60, FeNC 90 / TTO- 1000-60 and FeNC 95 / TTO- 1000-60 have improved performances compared to FeNC. For example at the voltage of 0.5 V, the FeNCss / TTO- 1000-60 and FeNC9o / TTO- 1000-60 catalysts produce 20% and 13% more current than FeNC.

[0156] 2024PAT00083WO

Claims

Demands 1. A porous composite catalyst consisting of an Fe-NC catalytic structure and a tin metallic phase, the Sn / Fe atomic ratio of tin to iron being greater than 2 and less than 50, which porous composite catalyst contains micropores and mesopores, the micropore volume being greater than 0.02 cm³ 3 g' 1 the mesoporous volume greater than 0.20 cm 3 g' 1 , the microporous volume being the total volume of micropores and the mesoporous volume being the total volume of mesopores with a width between 2 and 30 nm.

2. Porous composite catalyst according to claim 1 wherein the ratio between the total volume of mesopores with a width between 2 and 30 nm and the total volume of micropores is greater than 2.

3. Porous composite catalyst according to claim 1 or 2 wherein the ratio between the total volume of mesopores with a width between 2 and 30 nm and the total volume of micropores is greater than 3.

4. Porous composite catalyst according to any one of claims 1 to 3 wherein all or part of the iron atoms are coordinated each with 4 nitrogen atoms, preferably all of the iron atoms are coordinated each with 4 nitrogen atoms.

5. Porous composite catalyst according to any one of claims 1 to 4 wherein the Sn / Fe atomic ratio between tin and iron is greater than 2 and less than 40.

6. Porous composite catalyst according to any one of claims 1 to 5 wherein the Sn / Fe atomic ratio between tin and iron is greater than 3 and less than 30, preferably less than 20.

7. Porous composite catalyst according to any one of claims 1 to 6 wherein the Sn / Fe atomic ratio between tin and iron is greater than 3 and less than 15.

8. Porous composite catalyst according to any one of claims 1 to 7 wherein the Sn / Fe atomic ratio between tin and iron is greater than 5 and less than 15.

9. Porous composite catalyst according to any one of claims 1 to 8, wherein catalyst is covered with a continuous or discontinuous passivation layer formed of tin oxide.

10. Porous composite catalyst according to any one of claims 1 to 9, wherein porous composite catalyst is tantalum-doped.

11. Process for the synthesis of a porous composite catalyst comprising the following steps a), b), c), d) and e): a) Mixing an iron salt and a crystalline porous hybrid solid, MOF, with or without a secondary nitrogen source, b) Adding to the mixture obtained in step a) tin oxide powder in a mass quantity greater than or equal to 3% and less than or equal to 30%, mass percentage calculated with respect to the total mass of the mixture obtained in step a) and the tin oxide powder, the tin oxide being optionally doped with tantalum, c) Mixing the tin oxide powder and the mixture obtained in step a), d) Carrying out pyrolysis, preferably flash pyrolysis, of the mixture obtained in step c) at a temperature greater than or equal to 800°C and less than 1200°C under an inert atmosphere, e) Recovering the porous composite catalyst after pyrolysis. 2024PAT00083WO 12. A process according to claim 11 wherein the iron salt is a salt of Fe(II), preferably Fe(II) acetate.

13. A process according to claim 11 or 12 wherein the crystalline porous hybrid solid is an imidazolate network, ZIF, containing the metal ion Zn 2+ .

14. A method according to any one of claims 11 to 13 wherein the crystalline porous hybrid solid is ZIF-8.

15. A method according to any one of claims 11 to 14 wherein the secondary nitrogen source is 1,10-phenanthroline.

16. A process according to any one of claims 11 to 15 wherein the quantity of tin oxide powder added in step b) is greater than or equal to 5% and less than 20%, mass percentage calculated with respect to the total mass of the mixture obtained in step a) and of the tin oxide powder.

17. A process according to any one of claims 11 to 16 wherein the quantity of tin oxide powder added in step b) is greater than 5% and less than 20%, mass percentage calculated with respect to the total mass of the mixture obtained in step a) and of the tin oxide powder.

18. A process according to any one of claims 11 to 17 wherein the tin oxide powder is obtained by calcining nanofibers formed by electrospinning.

19. Ink comprising a catalyst and an ionomer dispersed in a liquid phase, the catalyst being a porous composite catalyst as defined in any one of claims 1 to 10 or obtained by a process as defined in any one of claims 11 to 18.

20. Electrode comprising a porous composite catalyst as defined in any one of claims 1 to 10 or a porous composite catalyst obtained by the process defined in any one of claims 11 to 18.

21. Membrane Electrode Assembly, MEA, which includes an electrode defined in claim 20.

22. Fuel cell which includes an AME as defined in claim 21. 2024PAT00083WO

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