WO2024256469A1 - A method of synthesis of transition metal nitrides and their use when they have two, three or four metals as catalysts for the oxygen evolution reaction - Google Patents

A method of synthesis of transition metal nitrides and their use when they have two, three or four metals as catalysts for the oxygen evolution reaction Download PDF

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WO2024256469A1
WO2024256469A1 PCT/EP2024/066249 EP2024066249W WO2024256469A1 WO 2024256469 A1 WO2024256469 A1 WO 2024256469A1 EP 2024066249 W EP2024066249 W EP 2024066249W WO 2024256469 A1 WO2024256469 A1 WO 2024256469A1
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transition metal
metals
gram
nitrides
synthesis
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Scott Jerald FOLKMAN
Ilario GELMETTI
Stefano GIANCOLA
Felipe Andrés GARCÉS-PINEDA
José Ramón GALÁN MASCARÓS
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Institut Catala D'investigacio Quimica Iciq
Institucio Catalana de Recerca i Estudis Avancats ICREA
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J27/00—Catalysts comprising the elements or compounds of halogens, sulfur, selenium, tellurium, phosphorus or nitrogen; Catalysts comprising carbon compounds
    • B01J27/24—Nitrogen compounds
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/08—Heat treatment
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B21/00—Nitrogen; Compounds thereof
    • C01B21/06—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron
    • C01B21/0602—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with two or more other elements chosen from metals, silicon or boron
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B21/00—Nitrogen; Compounds thereof
    • C01B21/06—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron
    • C01B21/0615—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with transition metals other than titanium, zirconium or hafnium
    • C01B21/062—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with transition metals other than titanium, zirconium or hafnium with chromium, molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B21/00—Nitrogen; Compounds thereof
    • C01B21/06—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron
    • C01B21/0615—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with transition metals other than titanium, zirconium or hafnium
    • C01B21/0622—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with transition metals other than titanium, zirconium or hafnium with iron, cobalt or nickel
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B21/00—Nitrogen; Compounds thereof
    • C01B21/06—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron
    • C01B21/0632—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with gallium, indium or thallium
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B21/00—Nitrogen; Compounds thereof
    • C01B21/06—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron
    • C01B21/0637—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with metals not specified in groups C01B21/0607 - C01B21/0635, other than aluminium, titanium, zirconium or hafnium
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B21/00—Nitrogen; Compounds thereof
    • C01B21/06—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron
    • C01B21/072—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with aluminium
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2002/00—Crystal-structural characteristics
    • C01P2002/70—Crystal-structural characteristics defined by measured X-ray, neutron or electron diffraction data
    • C01P2002/72—Crystal-structural characteristics defined by measured X-ray, neutron or electron diffraction data by d-values or two theta-values, e.g. as X-ray diagram
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/01—Particle morphology depicted by an image
    • C01P2004/03—Particle morphology depicted by an image obtained by SEM
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/40—Electric properties

Definitions

  • the present invention discloses a new up-scalable synthesis of heterogeneous transition metal nitrides (TMNs) using the carbonate salts as precursors.
  • TNNs transition metal nitrides
  • the synthesis leads to highly heterogeneous materials that are obtained in quantitative yields without the use of solvents or reagents other than anhydrous NH 3 .
  • the TMNs with between at least two and up to four metals shows catalytic behaviour in the oxygen evolution reaction (OER).
  • Transition metal nitrides are considered to be promising electrode materials for many devices, such as supercapacitors, due to their physical, chemical, and mechanical properties, low cost, excellent electronic conductivity, high stability at higher temperatures, and good chemical resistance. They exhibit high catalytic activity, distinctive electronic structures, and enhanced surface morphologies.
  • TMNs have been reported as highly efficient, stable and cost-effective electrocatalysts for oxygen evolution reaction (OER), which is critical for a range of renewable-energy technologies, including metal-air batteries, fuel cells and water-splitting reactions.
  • OER oxygen evolution reaction
  • TMNs can be prepared by maintaining a nitrogen rich atmosphere over metal- based precursors.
  • TMNs can be prepared from metal materials by direct calcination in the N 2 atmosphere at high temperatures above 1200 oC.
  • vanadium nitride can be prepared at 1200 °C from metal powder or film by passing N 2 gas.
  • the disadvantage of this direct synthetic route is the limitation to a few stable nitrides because the process is limited by large thermodynamic barriers which are due to the making and breaking of the triple bond in dinitrogen (945 kJ mol –1 ). Therefore, it is reported that this method is suitable only for highly thermally stable nitrides. Therefore, most nitrides are prepared by ammonothermal method at present. This method uses metal powder, oxides, hydroxide, halides, and sulfides, etc.
  • TMNs metal-based precursors in an ammonia atmosphere at certain temperatures enable the synthesis of corresponding TMNs.
  • nitrogen-based compounds like urea have been used as nitrogen sources for the synthesis of TMNs.
  • ethanol is usually used to dissolve metal salts. But in some cases, metal salts do not dissolve in ethanol. If urea is added to it, dissolution increases.
  • a problem to be solved is finding a synthetic method that allows the production of single and multi-transition metal nitrides in a single step without the use of solvents and allowing the upscaling of the production and at temperatures equal or lower than 1200 oC.
  • TBNs transition metal nitrides
  • Transition metal salts based on Ni, Co, Fe, Ga, Al, Ti, Zn and Mn can be mixed in specific desired stoichiometric ratios.
  • precursor powders can then be converted from the salt to the nitride in a tube furnace reactor with flowing of anhydrous ammonia at temperatures ranging from 350 to 1200 °C and times ranging from 1 to 24 hours.
  • the advantages of the direct nitridation of the carbonate salts disclosed herein, is the ease of synthesis in a single, high-yield (quantitative) step. Additionally, given that the carbonate salts of these metals are an inexpensive source of the transition metal, the cost of scaling this reaction to industrial levels is minimal compared to the synthesis procedure used for other, mixed metal oxide catalysts.
  • the synthesis of the mixed metal oxides requires the use of the metal acetate salts, have yields in the 10-30% range (based on metals), and have aqueous waste containing unreacted metal precursors which must be processed for proper disposal.
  • the nitridation reaction described herein gives no liquid waste for disposal because the reaction is done using gaseous NH 3 with solid salts and solid TMN product.
  • the present invention discloses a novel method for the synthesis of transition metal nitrides characterized in that comprising the following steps: a) providing at least one precursor transition metal salts M x (A) z , wherein: M is a transition metal; A is CO 3 2 ⁇ or CH 3 CO 2 ⁇ ; wherein x is 1 or 2; and wherein z is the valence or mixed valences of the transition metal of the chosen precursor; with a ratio according to the expected stoichiometry of the metal or metals in the transition metal nitride; b) grinding the transition metal salts; and c) heating with a ramp temperature of between 8.75 oC/min and 20oC/min for a time of between 40 min and 70 min up to reach an annealing temperature of between 350 and 1200 °C the grinded transition metal salt of step (b) with NH 3 for nitridation at the annealing temperature for a period between 1 and 24 hours, and preferably for a period between
  • the method does not comprise the adding of tannin and/or the method does not comprise any further heating at a temperature over 40 oC.
  • the method consists of the step (a) to (c) previously mentioned.
  • tannin refers to a substance that is easily dissolved in water, whose aqueous solution is highly astringent, and which has the property of tanning leather, and is conventionally known by the generic name of tannin. Chemically, tannin is not a simple substance but an aggregation of complex organic polyphenolic compounds. Generally, tannins are extracted with warm water or hot water as an extraction agent is used as a base agent, and if required, tannin may be further purified with an organic solvent or modified with alkalis or the like.
  • Condensed tannins are found in virtually all families of plants and constitute more than 90% of the total world production of commercial tannins. They are known for their wide distribution in nature, in particular in wood and bark of various trees, and comprise up to 50% of the dry weight of leaves. Condensed tannins are polymerized to generate phlobaphene that is insoluble in water and reacts with aldehyde to become a polymer. Due to this property, condensed tannins, in particular tannin formaldehyde resins have been used as an adhesive or a binder for wood from the latter half of 1960s.
  • Hydrolyzable tannins are derivatives that are hydrolyzed by heating with a dilute acid to generate gallic acid (3,45-trihydroxyl benzoic acid).
  • a tannin is for example, without limit, gallic acid, tannic acid and Gallotannin.
  • the heating of step (c) takes place under a flow of NH 3 for nitridation with a flow rate between 50 and 1000 mL/min, and preferably between 50 and 100 mL/min, more preferably 100 mL/min. Lower flow is better because less ammonia would be used and up to 100 ml/L the management is without safety concerns.
  • the transition metal M is selected from Ni, Co, Fe, Ga, Al, Ti, Zn, Mn and any combination thereof, preferably from Fe, Ni, Co, Mn and any combination thereof.
  • the step (a) at least two precursors transition metal salts are provided.
  • the step (b) further comprising a mixing of the transition metal salts before the grinding. The use of a two or more transition metal salt precursor with a ratio according to the expected stoichiometry of the metals in the transition metal nitride give a multi transition metal nitride.
  • two different precursor transition metal salts Mx(A)z of step (a) are used with a ratio according to the expected stoichiometry of the metals in the transition metal nitride to form binary transition metal nitrides.
  • three or four different precursor transition metal salts Mx(A)z of step (a) are used with a ratio according to the expected stoichiometry of the metals in the transition metal nitride to form ternary or quaternary transition metal nitrides.
  • step (b) is by ball milling at a velocity of between 10 Hz and 30 Hz, preferably 20 Hz, for a time between 10 min and 50 min, preferably between 15 and 25 min.
  • a second aspect of the present invention is the transition metal nitrides obtained by the method described above wherein the transition metal nitride that has an amorphous phase of at least a 2% in weight in respect of the total composition.
  • the transition metal nitrides are selected from the list of table 1.
  • the multi transition metal nitrides comprising two or more transition metals.
  • the transition metals are selected from the list Ni, Co, Fe.
  • the nitride is Co 90 Fe 10 N x , Co 90 Ni 10 N x , Ni 90 Fe 10 N or Ni 90 Co 10 N x .
  • the term “N x or x as a subindex of nitrogen stoichiometry” as used herein refers to the amount of nitrogen in the mixed metal nitrides is variable, and not even quantifiable.
  • the nitrides have small, substoichiometric amounts of nitrogen that is doped into the otherwise metallic lattice.
  • the multi transition metal nitrides comprising three or more transition metals.
  • the transition metals are selected from Ni, Co, Fe, Mn and Zn.
  • the nitride is Co 45 Ni 45 Fe 10 N x , Ni 60 Co 30 Fe 10 N, Ni 33 Co 33 Fe 33 N, Ni 90 Co 5 Fe 5 N x , Co 90 Fe 5 Ni 5 N x , Co 57 Ni 14 Fe 29 N x , Ni 99 Co 0.3 Fe 0.3 Mn 0.3 N x , or Co 90 Ni 3.3 Fe 3.3 Mn 3.3 N x .
  • the multi transition metal nitride (multi TMN) is Co 45 Ni 45 Fe 10 N x .
  • the third aspect of the present invention is the use of the transition metal nitrides with two, three or four metals described above as a catalyst for the oxygen evolution reaction
  • the characterization of the materials was done by powder XRD (PXRD). Precise determination of the phases detected in the PXRD patterns for multi metal transition metal nitrides are difficult for several reasons: first, the peaks of the different phases are very close, second the peaks are often weak and very broad, making distinction of these phases difficult.
  • the used masses it is based on the percent by mass of metal in the carbonate salt precursor determined by digesting the carbonate salts in sulfuric acid and conducing ICP.
  • the precursor powder was then placed into the quartz tube reactor flushed with N 2 , then NH 3 was flowed at a rate of 100 mL/min using a mass flow controller.
  • the quartz tube was heated from room temperature to the final temperature (between 375 and 1200 °C) over one hour then maintained for 6 hours.
  • the reactor was allowed to cool in an NH 3 atmosphere before being flushed with N 2 prior to removal of the sample. While the crystallinity varied, the yields of the samples were always quantitative with respect to the metal.
  • Table 1 Indicative list of transition metal nitrides successfully prepared by the disclosed method. On the following examples, the synthesis procedure is shown for Ni 3 N, Fe 3 N, GaN, AlN, TiN, MnN x , CrN, CoN x , ZnN x , Co 90 Fe 10 N x , Ni 90 Fe 10 N x , Ni 90 Co 10 N x , Co 45 Ni 45 Fe 10 N x , Ni 60 Co 30 Fe 10 N x , Ni 33 Co 33 Fe 33 N x , Ni 90 Co 5 Fe 5 N x , Co 90 Fe 5 Ni 5 N x , Co 90 Ni 3.3 Fe 3.3 Mn 3.3 N x .
  • Ni 3 N Fe 3 N, CrN, GaN, AlN
  • a known phase-pure nitride Ni 3 N
  • Ni(OAc) 2 grinded 0.5 gram was placed in a ceramic holder . With a NH 3 flow rate of 100 mL/min the sample was heated to 420 °C over 1 h and the temperature was maintained for 2 h.
  • the material was characterized by PXRD in Fig.1 Fe 3 N To obtain this material, 1 gram of Fe(CO 3 ) ⁇ nH 2 O grinded and mixed was placed in a ceramic holder. With a NH 3 flow rate of 100 mL/min the sample was heated to 600 °C and maintained the temperature for 6 hours. The material was characterized by PXRD in Fig.2 GaN To obtain this material, 1 gram of Ga(OAc) 3 grinded and mixed was placed in a ceramic holder. With a NH 3 flow rate of 100 mL/min the sample was heated to 900 °C and maintained the temperature for 2 hours.
  • the material was characterized by PXRD in Fig.3 AlN To obtain this material, 1 gram of Al(OAc) 3 grinded and mixed was placed in a ceramic holder. With a NH 3 flow rate of 1000 mL/min the sample was heated to 1200 °C and maintained the temperature for 10 hours. Note that earlier attempts at lower time/temp led material that contained some AlN but the crystallinity was low and the peaks were broad.
  • the material was characterized by PXRD as in Fig.4 TiN x To obtain this material, 1 gram of Ti(OAc) 4 grinded and mixed was placed in a ceramic holder. With a NH 3 flow rate of 100 mL/min the sample was heated to 800 °C and maintained the temperature for 6 hours.
  • the resulting material has peaks consistent with TiN but also contains a large portion of peaks that are consistent with TiO 2 .
  • the material was characterized by PXRD as in Fig.5 MnN x To obtain this material, 0.5 gram of Mn(CO 3 ) ⁇ nH 2 O grinded and mixed was placed in a ceramic holder. With a NH 3 flow rate of 100 mL/min the sample was heated to 400 °C with a ramp temperature of 6.7 oC/min and maintained the temperature for 2 hours. CoN x To obtain this material, 1 gram of Co(CO 3 ) ⁇ nH 2 O grinded and mixed was placed in a ceramic holder.
  • Ni 90 Fe 10 N To obtain this material, 0.122 gram of Fe(CO 3 ) ⁇ nH 2 O and 0.878 gram of Ni(CO 3 ) ⁇ nH 2 O grinded and mixed was placed in a ceramic holder. With a NH 3 flow rate of 100 mL/min the sample was heated to 420 °C with a ramp temperature of 8 oc/min for 50 min and maintained at that temperature for 2 hours. The material was characterized by PXRD as in Fig.7.
  • Ni 33 Co 33 Fe 33 N To obtain this material, 0.327 gram of Ni(CO 3 ) ⁇ nH 2 O, 0.266 gram of Co(CO 3 ) ⁇ nH 2 O and 0.407 gram of Fe(CO 3 ) ⁇ nH 2 O grinded and mixed was placed in a ceramic holder. With a NH 3 flow rate of 100 mL/min the sample was heated to 425 °C with a ramp temperature of 6.7 oc/min for 60 min and maintained at that temperature for 6 hours.
  • Ni 90 Co 5 Fe 5 N x To obtain this material, 0.897 gram of Ni(CO 3 ) ⁇ nH 2 O, 0.04 gram of Co(CO 3 ) ⁇ nH 2 O and 0.062 gram of Fe(CO 3 ) ⁇ nH 2 O grinded and mixed was placed in a ceramic holder. With a NH 3 flow rate of 100 mL/min the sample was heated to 425 °C with a ramp temperature of 6.7 oc/min for 60 min and maintained at that temperature for 6 hours.
  • Ni 99 Co 0.3 Fe 0.3 Mn 0.3 N x To obtain this material, 1.981 gram of Ni(CO 3 ) ⁇ nH 2 O and 0.005 gram of Co(CO 3 ) ⁇ nH 2 O and 0.007 gram of Fe(CO 3 ) ⁇ nH 2 Oand 0.006 gram of Mn(CO 3 ) ⁇ nH 2 O grinded and mixed was placed in a ceramic holder. With a NH 3 flow rate of 100 mL/min the sample was heated to 420 °C with a ramp temperature of 6.6 oc/min for 60 min and maintained at that temperature for 6 hours.
  • Example 2 Example of characterization and use of nitrides as an OER catalyst.
  • Catalyst inks were prepared by grinding 60 mg of the desired catalyst with 270 ⁇ L of KOH- activated FAA-3 ionomer (5% w:w in EtOH) in a mortar and pestle (resulting in dried inks that contained 18% wt:wt FAA-3). Next, the catalyst/ionomer mixture was added to a scintillation vial with 6 mL of a 3:1 isopropanol/water solution and sonicated for 1 hour. The catalyst ink was then sprayed using an airbrush onto three pre-weighed Ni foil (purchased form Goodfellow), which were masked using kapton tape to expose 1 cm2 surface area on a hotplate set to 80 °C.
  • Ni foil purchasedd form Goodfellow
  • the electrodes were then weighed after cooling to room temperature to determine the actual catalyst mass loading (0.9-1.1 mg/cm 2 ).
  • electrochemical impedance spectroscopy was conducted and the uncompensated series resistance (R u ) of the electrochemical system was determined using the high frequency point that intercepts the real axis of the Nyquist plot (typically 0.8-1.2 ⁇ ).
  • Cyclic voltammetry was then conducted from the open circuit potential of the electrode to 1.0 V vs Hg/HgO at a scan rate of 10 mV/s, back to 0.0 V vs Hg/HgO.
  • a set of membrane electrode assemblies (MEAs) were constructed using IrO x , NiCoFeO x and Co 45 Ni 45 Fe 10 N x as OER catalysts and 0.5 mg/cm 2 PtC 60% on carbon cloth as hydrogen evolution reaction (HER) catalyst with a FAA-3-PK-75 membrane.
  • Figure 13 and 14 shows the linear sweep voltammetry curves, and chronoamperometry, respectively, of the three membrane electrode assemblies (MEAs) in alkaline anion exchange membrane electrolysis.
  • IrO x and NiCoFeO x have a current density of approximately 400 mA/cm 2 at an applied cell voltage of 2.0V, similar to reported MEAs made with NiCoFeO x albeit under somewhat different conditions (nanopure water, 50 °C, with the catalyst sprayed onto the GDE). Meanwhile, the novel Co 45 Ni 45 Fe 10 N x shows approximately 900 mA/cm 2 at the same applied potential, more than double the current density.

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Abstract

The present invention discloses a new up-scalable synthesis of heterogeneous transition metal nitrides (TMNs) using the carbonate salts as precursors. The synthesis leads to highly heterogeneous materials that are obtained in quantitative yields without the use of solvents or reagents other than anhydrous NH3. In addition, the TMNs with between at least two and up to four metals shows catalytic behaviour in the oxygen evolution reaction (OER).

Description

A METHOD OF SYNTHESIS OF TRANSITION METAL NITRIDES AND THEIR USE WHEN THEY HAVE TWO, THREE OR FOUR METALS AS CATALYSTS FOR THE OXYGEN EVOLUTION REACTION. The present invention discloses a new up-scalable synthesis of heterogeneous transition metal nitrides (TMNs) using the carbonate salts as precursors. The synthesis leads to highly heterogeneous materials that are obtained in quantitative yields without the use of solvents or reagents other than anhydrous NH3. In addition, the TMNs with between at least two and up to four metals shows catalytic behaviour in the oxygen evolution reaction (OER). BACKGROUND ART Transition metal nitrides (TMNs) are considered to be promising electrode materials for many devices, such as supercapacitors, due to their physical, chemical, and mechanical properties, low cost, excellent electronic conductivity, high stability at higher temperatures, and good chemical resistance. They exhibit high catalytic activity, distinctive electronic structures, and enhanced surface morphologies. Specifically, several ternary TMNs have been reported as highly efficient, stable and cost-effective electrocatalysts for oxygen evolution reaction (OER), which is critical for a range of renewable-energy technologies, including metal-air batteries, fuel cells and water-splitting reactions. In general, TMNs can be prepared by maintaining a nitrogen rich atmosphere over metal- based precursors. TMNs can be prepared from metal materials by direct calcination in the N2 atmosphere at high temperatures above 1200 ºC. As an example, vanadium nitride can be prepared at 1200 °C from metal powder or film by passing N2 gas. The disadvantage of this direct synthetic route is the limitation to a few stable nitrides because the process is limited by large thermodynamic barriers which are due to the making and breaking of the triple bond in dinitrogen (945 kJ mol–1). Therefore, it is reported that this method is suitable only for highly thermally stable nitrides. Therefore, most nitrides are prepared by ammonothermal method at present. This method uses metal powder, oxides, hydroxide, halides, and sulfides, etc. as metal-based precursors in an ammonia atmosphere at certain temperatures enable the synthesis of corresponding TMNs. In addition to nitrogen or ammonia, nitrogen-based compounds like urea have been used as nitrogen sources for the synthesis of TMNs. When urea is used as the nitrogen source, ethanol is usually used to dissolve metal salts. But in some cases, metal salts do not dissolve in ethanol. If urea is added to it, dissolution increases. [Cheng et al, Adv. Funct. Mater.2021, 2100553] Thus, a problem to be solved is finding a synthetic method that allows the production of single and multi-transition metal nitrides in a single step without the use of solvents and allowing the upscaling of the production and at temperatures equal or lower than 1200 ºC. SUMMARY OF THE INVENTION The process of the present invention described herein is a method for synthesizing multimetallic transition metal nitrides (TMNs) in a single step using the metal salt precursors. Transition metal salts based on Ni, Co, Fe, Ga, Al, Ti, Zn and Mn can be mixed in specific desired stoichiometric ratios. These precursor powders can then be converted from the salt to the nitride in a tube furnace reactor with flowing of anhydrous ammonia at temperatures ranging from 350 to 1200 °C and times ranging from 1 to 24 hours. The advantages of the direct nitridation of the carbonate salts disclosed herein, is the ease of synthesis in a single, high-yield (quantitative) step. Additionally, given that the carbonate salts of these metals are an inexpensive source of the transition metal, the cost of scaling this reaction to industrial levels is minimal compared to the synthesis procedure used for other, mixed metal oxide catalysts. For example, the synthesis of the mixed metal oxides requires the use of the metal acetate salts, have yields in the 10-30% range (based on metals), and have aqueous waste containing unreacted metal precursors which must be processed for proper disposal. On the other hand, the nitridation reaction described herein gives no liquid waste for disposal because the reaction is done using gaseous NH3 with solid salts and solid TMN product. The present invention discloses a novel method for the synthesis of transition metal nitrides characterized in that comprising the following steps: a) providing at least one precursor transition metal salts Mx(A)z, wherein: M is a transition metal; A is CO3 2− or CH3CO2 −; wherein x is 1 or 2; and wherein z is the valence or mixed valences of the transition metal of the chosen precursor; with a ratio according to the expected stoichiometry of the metal or metals in the transition metal nitride; b) grinding the transition metal salts; and c) heating with a ramp temperature of between 8.75 ºC/min and 20ºC/min for a time of between 40 min and 70 min up to reach an annealing temperature of between 350 and 1200 °C the grinded transition metal salt of step (b) with NH3 for nitridation at the annealing temperature for a period between 1 and 24 hours, and preferably for a period between 2 and 10 hours. In another embodiment the method does not comprise the adding of tannin and/or the method does not comprise any further heating at a temperature over 40 ºC. In another embodiment the method consists of the step (a) to (c) previously mentioned. In the present invention the term "tannin" as used herein refers to a substance that is easily dissolved in water, whose aqueous solution is highly astringent, and which has the property of tanning leather, and is conventionally known by the generic name of tannin. Chemically, tannin is not a simple substance but an aggregation of complex organic polyphenolic compounds. Generally, tannins are extracted with warm water or hot water as an extraction agent is used as a base agent, and if required, tannin may be further purified with an organic solvent or modified with alkalis or the like. There are two main classes of tannins: hydrolyzable tannins and condensed tannins. Condensed tannins are found in virtually all families of plants and constitute more than 90% of the total world production of commercial tannins. They are known for their wide distribution in nature, in particular in wood and bark of various trees, and comprise up to 50% of the dry weight of leaves. Condensed tannins are polymerized to generate phlobaphene that is insoluble in water and reacts with aldehyde to become a polymer. Due to this property, condensed tannins, in particular tannin formaldehyde resins have been used as an adhesive or a binder for wood from the latter half of 1960s. Hydrolyzable tannins, on the other hand, are derivatives that are hydrolyzed by heating with a dilute acid to generate gallic acid (3,45-trihydroxyl benzoic acid). In the present invention a tannin is for example, without limit, gallic acid, tannic acid and Gallotannin. In one embodiment, the heating of step (c) takes place under a flow of NH3 for nitridation with a flow rate between 50 and 1000 mL/min, and preferably between 50 and 100 mL/min, more preferably 100 mL/min. Lower flow is better because less ammonia would be used and up to 100 ml/L the management is without safety concerns. In a further embodiment, the transition metal M is selected from Ni, Co, Fe, Ga, Al, Ti, Zn, Mn and any combination thereof, preferably from Fe, Ni, Co, Mn and any combination thereof. In a further embodiment in the step (a) at least two precursors transition metal salts are provided. In a further embodiment, the step (b) further comprising a mixing of the transition metal salts before the grinding. The use of a two or more transition metal salt precursor with a ratio according to the expected stoichiometry of the metals in the transition metal nitride give a multi transition metal nitride. In a preferred embodiment of the method two different precursor transition metal salts Mx(A)z of step (a) are used with a ratio according to the expected stoichiometry of the metals in the transition metal nitride to form binary transition metal nitrides. In another preferred embodiment three or four different precursor transition metal salts Mx(A)z of step (a) are used with a ratio according to the expected stoichiometry of the metals in the transition metal nitride to form ternary or quaternary transition metal nitrides. In another preferred embodiment up to 7 different precursor transition metal salts Mx(A)z are used with a ratio according to the expected stoichiometry of the metals in the transition metal nitride to form a transition metal nitrides with a combination of the seven metals and nitride phases. In another preferred embodiment the grinding of step (b) is by ball milling at a velocity of between 10 Hz and 30 Hz, preferably 20 Hz, for a time between 10 min and 50 min, preferably between 15 and 25 min. A second aspect of the present invention is the transition metal nitrides obtained by the method described above wherein the transition metal nitride that has an amorphous phase of at least a 2% in weight in respect of the total composition. Preferably the transition metal nitrides are selected from the list of table 1. In a preferred embodiment the multi transition metal nitrides comprising two or more transition metals. In a more preferred embodiment the transition metals are selected from the list Ni, Co, Fe. In an even more preferred embodiment, the nitride is Co90Fe10Nx, Co90Ni10Nx, Ni90Fe10N or Ni90Co10Nx. The term “Nx or x as a subindex of nitrogen stoichiometry” as used herein refers to the amount of nitrogen in the mixed metal nitrides is variable, and not even quantifiable. Indeed, some of the nitrides have small, substoichiometric amounts of nitrogen that is doped into the otherwise metallic lattice. In a preferred embodiment the multi transition metal nitrides comprising three or more transition metals. In a more preferred embodiment, the transition metals are selected from Ni, Co, Fe, Mn and Zn. In an even more preferred embodiment, the nitride is Co45Ni45Fe10Nx, Ni60Co30Fe10N, Ni33Co33Fe33N, Ni90Co5Fe5Nx, Co90Fe5Ni5Nx, Co57Ni14Fe29Nx, Ni99Co0.3Fe0.3Mn0.3Nx, or Co90Ni3.3Fe3.3Mn3.3Nx. In an even more preferred embodiment, the multi transition metal nitride (multi TMN) is Co45Ni45Fe10Nx. The compound shows more than double the current density The third aspect of the present invention is the use of the transition metal nitrides with two, three or four metals described above as a catalyst for the oxygen evolution reaction The characterization of the materials was done by powder XRD (PXRD). Precise determination of the phases detected in the PXRD patterns for multi metal transition metal nitrides are difficult for several reasons: first, the peaks of the different phases are very close, second the peaks are often weak and very broad, making distinction of these phases difficult. The majority of the samples had at least 2% of amorphous material in weight in respect of the total composition and often included more than one nitride phase indicating the heterogeneity of this synthesis method and the unique materials exhibiting advantageous properties for OER catalysts due to this heterogeneity. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skilled in the art to which this invention belongs. Methods and materials similar or equivalent to those described herein can be used in the practice of the present invention. Throughout the description and claims the word "comprise" and its variations are not intended to exclude other technical features, additives, components, or steps. Additional objects, advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention. The following examples, drawings and sequence listing are provided by way of illustration and are not intended to be limiting of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1. Powder X Ray Diffraction (PXRD) of Ni3N Fig.2. PXRD of Fe3N Fig.3. PXRD of GaN Fig.4. PXRD of AlN Fig.5. PXRD of TiNx Fig.6. PXRD of Co90Fe10Nx Fig.7. PXRD of Ni90Fe10N Fig.8. PXRD of Co45Ni45Fe10Nx Fig.9. HRTEM/EELS of Co45Ni45Fe10Nx Fig.10. PXRD of Co57Ni14Fe29Nx Fig.11. PXRD of Co9,0Ni3.3Fe3.3Mn3.3N Fig. 12. Cyclic voltammogram of CoNx, NiNx, FeNx, Co45Ni45Fe10Nx, Co57Ni14Fe29Nx, in comparison with state-of-the-art catalysts NiCoFeOx and commercially available IrOx Fig. 13. Polarization curve conducted at 1 mV/s conducted in 0.1 M KOH, and unless otherwise stated, 65 °C of Co45Ni45Fe10Nx in comparison with state-of-the-art catalysts NiCoFeOx and commercially available IrOx Fig.14. Chronoamperometry at 1.8 V of Co45Ni45Fe10Nx in comparison with state-of-the-art catalysts NiCoFeOx and commercially available IrOx EXAMPLES Example 1: General synthetic route For the synthesis of TMNs from the volatile salt precursors, the pure precursors M(A) (where M= Ni, Co, Fe, Ga, Al, Ti, Zn, Mn, and A= CO3 2− or CH3CO2 −), or a chosen stoichiometric mixture (0.5-2 gram total) was ground in a mortar and pestle, or ball milled, before being placed in a ceramic sample holder. In order to know the used masses it is based on the percent by mass of metal in the carbonate salt precursor determined by digesting the carbonate salts in sulfuric acid and conducing ICP. The precursor powder was then placed into the quartz tube reactor flushed with N2, then NH3 was flowed at a rate of 100 mL/min using a mass flow controller. The quartz tube was heated from room temperature to the final temperature (between 375 and 1200 °C) over one hour then maintained for 6 hours. The reactor was allowed to cool in an NH3 atmosphere before being flushed with N2 prior to removal of the sample. While the crystallinity varied, the yields of the samples were always quantitative with respect to the metal. In the table below is a summary of the mixed metal nitrides synthesized, characterized by PXRD results. Table 1: Indicative list of transition metal nitrides successfully prepared by the disclosed method.
Figure imgf000008_0001
Figure imgf000009_0001
On the following examples, the synthesis procedure is shown for Ni3N, Fe3N, GaN, AlN, TiN, MnNx, CrN, CoNx, ZnNx, Co90Fe10Nx, Ni90Fe10Nx, Ni90Co10Nx, Co45Ni45Fe10Nx, Ni60Co30Fe10Nx, Ni33Co33Fe33Nx, Ni90Co5Fe5Nx, Co90Fe5Ni5Nx, Co90Ni3.3Fe3.3Mn3.3Nx. In the cases of Ni3N, Fe3N, CrN, GaN, AlN, has been obtained a known phase-pure nitride. Note that when it is said phase pure, that does not mean that we observed zero amorphous, it has an amorphous of at least 2%, but it means that we had no mixed phases and did not observe any precursor or any other oxide or nitride phases. Ni3N To obtain this material, 0.5 gram of Ni(OAc)2 grinded was placed in a ceramic holder . With a NH3 flow rate of 100 mL/min the sample was heated to 420 °C over 1 h and the temperature was maintained for 2 h. The material was characterized by PXRD in Fig.1 Fe3N To obtain this material, 1 gram of Fe(CO3)∙nH2O grinded and mixed was placed in a ceramic holder. With a NH3 flow rate of 100 mL/min the sample was heated to 600 °C and maintained the temperature for 6 hours. The material was characterized by PXRD in Fig.2 GaN To obtain this material, 1 gram of Ga(OAc)3 grinded and mixed was placed in a ceramic holder. With a NH3 flow rate of 100 mL/min the sample was heated to 900 °C and maintained the temperature for 2 hours. The material was characterized by PXRD in Fig.3 AlN To obtain this material, 1 gram of Al(OAc)3 grinded and mixed was placed in a ceramic holder. With a NH3 flow rate of 1000 mL/min the sample was heated to 1200 °C and maintained the temperature for 10 hours. Note that earlier attempts at lower time/temp led material that contained some AlN but the crystallinity was low and the peaks were broad. The material was characterized by PXRD as in Fig.4 TiNx To obtain this material, 1 gram of Ti(OAc)4 grinded and mixed was placed in a ceramic holder. With a NH3 flow rate of 100 mL/min the sample was heated to 800 °C and maintained the temperature for 6 hours. The resulting material has peaks consistent with TiN but also contains a large portion of peaks that are consistent with TiO2. The material was characterized by PXRD as in Fig.5 MnNx To obtain this material, 0.5 gram of Mn(CO3)∙nH2O grinded and mixed was placed in a ceramic holder. With a NH3 flow rate of 100 mL/min the sample was heated to 400 °C with a ramp temperature of 6.7 ºC/min and maintained the temperature for 2 hours. CoNx To obtain this material, 1 gram of Co(CO3)∙nH2O grinded and mixed was placed in a ceramic holder. With a NH3 flow rate of 100 mL/min the sample was heated to 900 °C with a ramp temperature of 6.7 ºC/min and maintained temperature for 6 hours. CrN To obtain this material, 1 gram of Cr(OAc)2 grinded and mixed was placed in a ceramic holder. With a NH3 flow rate of 500 mL/min the sample was heated to 425 °C with a ramp temperature of 6.7 ºC/min and maintained temperature for 6 hours. ZnNx To obtain this material, 1 gram of Zn(CO3)∙nH2O grinded and mixed was placed in a ceramic holder. With a NH3 flow rate of 100 mL/min the sample was heated to 425 °C with a ramp temperature of 6.7 ºC/min and maintained temperature for 6 hours. Co90Fe10Nx To obtain this material, 0.146 gram of Fe(CO3)∙nH2O and 0.854 gram of Co(CO3)∙nH2O grinded and mixed was placed in a ceramic holder. With a NH3 flow rate of 100 mL/min the sample was heated to 450 °C with a ramp temperature of 8.5 ºc/min for 50 min and maintained at that temperature for 6 hours. The material was characterized by PXRD as in Fig. 6. Its advantageous properties for OER catalysts when compared with homometallic nitrides as shown in Fig.6. Ni90Fe10N To obtain this material, 0.122 gram of Fe(CO3)∙nH2O and 0.878 gram of Ni(CO3)∙nH2O grinded and mixed was placed in a ceramic holder. With a NH3 flow rate of 100 mL/min the sample was heated to 420 °C with a ramp temperature of 8 ºc/min for 50 min and maintained at that temperature for 2 hours. The material was characterized by PXRD as in Fig.7. Co90Ni10Nx To obtain this material, 0.146 gram of Ni(CO3)∙nH2O and 0.854 gram of Co(CO3)∙nH2O grinded and mixed was placed in a ceramic holder. With a NH3 flow rate of 100 mL/min the sample was heated to 420 °C with a ramp temperature of 8 ºc/min for 50 min and maintained at that temperature for 2 hours. Ni90Co10Nx To obtain this material, 0.878 gram of Ni(CO3)∙nH2O and 0.122 gram of Co(CO3)∙nH2O grinded and mixed was placed in a ceramic holder. With a NH3 flow rate of 100 mL/min the sample was heated to 420 °C with a ramp temperature of 8 ºc/min for 50 min and maintained at that temperature for 2 hours. Ni60Co30Fe10N To obtain this material, 0.620 gram of Ni(CO3)∙nH2O, 0.252 gram of Co(CO3)∙nH2O and 0.129 gram of Fe(CO3)∙nH2O grinded and mixed was placed in a ceramic holder. With a NH3 flow rate of 100 mL/min the sample was heated to 425 °C with a ramp temperature of 6.7 ºc/min for 60 min and maintained at that temperature for 6 hours. Ni33Co33Fe33N To obtain this material, 0.327 gram of Ni(CO3)∙nH2O, 0.266 gram of Co(CO3)∙nH2O and 0.407 gram of Fe(CO3)∙nH2O grinded and mixed was placed in a ceramic holder. With a NH3 flow rate of 100 mL/min the sample was heated to 425 °C with a ramp temperature of 6.7 ºc/min for 60 min and maintained at that temperature for 6 hours. Ni90Co5Fe5Nx To obtain this material, 0.897 gram of Ni(CO3)∙nH2O, 0.04 gram of Co(CO3)∙nH2O and 0.062 gram of Fe(CO3)∙nH2O grinded and mixed was placed in a ceramic holder. With a NH3 flow rate of 100 mL/min the sample was heated to 425 °C with a ramp temperature of 6.7 ºc/min for 60 min and maintained at that temperature for 6 hours.
Figure imgf000012_0001
0.133 gram of Fe(CO3)∙nH2O, 0.479 gram of Ni(CO3)∙nH2O and 0.389 gram of Co(CO3)∙nH2O grinded and mixed was placed in a ceramic holder. With a NH3 flow rate of 100 mL/min the sample was heated to 425 °C with a ramp temperature of 6.7 ºc/min for 60 min and maintained at that temperature for 6 hours. The material was characterized by PXRD as in Fig.8 and HRTEM/EELS Fig 9. Co57Ni14Fe29Nx To obtain this material, 0.929 gram of Fe(CO3)∙nH2O and 0.371 gram of Ni(CO3)∙nH2O and 1.200gram of Co(CO3)∙nH2O grinded and mixed was placed in a ceramic holder. With a NH3 flow rate of 100 mL/min the sample was heated to 425 °C with a ramp temperature of 6.7 ºc/min for 60 min and maintained at that temperature for 6 hours. The material was characterized by PXRD as in Fig.10. Co90Fe5Ni5Nx To obtain this material, 0.867 gram of Co(CO3)∙nH2O and 0.074 gram of Fe(CO3)∙nH2O and 0.059 gram of Ni(CO3)∙nH2O grinded and mixed was placed in a ceramic holder. With a NH3 flow rate of 100 mL/min the sample was heated to 425 °C with a ramp temperature of 6.7 ºc/min for 60 min and maintained at that temperature for 6 hours. Co90Ni3.3Fe3.3Mn3.3Nx To obtain this material, 0.049 gram of Fe(CO3)∙nH2O and 0.039 gram of Ni(CO3)∙nH2O and 0.87 gram of Co(CO3)∙nH2Oand 0.042 gram of Mn(CO3)∙nH2O grinded and mixed was placed in a ceramic holder. With a NH3 flow rate of 100 mL/min the sample was heated to 420 °C with a ramp temperature of 6.6 ºc/min for 60 min and maintained at that temperature for 6 hours. The material was characterized by PXRD as in Fig.11. Ni99Co0.3Fe0.3Mn0.3Nx To obtain this material, 1.981 gram of Ni(CO3)∙nH2O and 0.005 gram of Co(CO3)∙nH2O and 0.007 gram of Fe(CO3)∙nH2Oand 0.006 gram of Mn(CO3)∙nH2O grinded and mixed was placed in a ceramic holder. With a NH3 flow rate of 100 mL/min the sample was heated to 420 °C with a ramp temperature of 6.6 ºc/min for 60 min and maintained at that temperature for 6 hours. Example 2: Example of characterization and use of nitrides as an OER catalyst. Catalyst inks were prepared by grinding 60 mg of the desired catalyst with 270 µL of KOH- activated FAA-3 ionomer (5% w:w in EtOH) in a mortar and pestle (resulting in dried inks that contained 18% wt:wt FAA-3). Next, the catalyst/ionomer mixture was added to a scintillation vial with 6 mL of a 3:1 isopropanol/water solution and sonicated for 1 hour. The catalyst ink was then sprayed using an airbrush onto three pre-weighed Ni foil (purchased form Goodfellow), which were masked using kapton tape to expose 1 cm2 surface area on a hotplate set to 80 °C. The electrodes were then weighed after cooling to room temperature to determine the actual catalyst mass loading (0.9-1.1 mg/cm2). For each electrode, electrochemical impedance spectroscopy was conducted and the uncompensated series resistance (Ru) of the electrochemical system was determined using the high frequency point that intercepts the real axis of the Nyquist plot (typically 0.8-1.2 Ω). Cyclic voltammetry was then conducted from the open circuit potential of the electrode to 1.0 V vs Hg/HgO at a scan rate of 10 mV/s, back to 0.0 V vs Hg/HgO. The iRu-corrected cyclic voltammogram of CoNx, NiNx, FeNx, Co45Ni45Fe10Nx, Co57Ni14Fe29Nx, obtained as described in example 1, in comparison with state-of-the-art catalysts NiCoFeOx and commercially available IrOx is presented in Figure 11. Notably, the mixed metal nitrides Co45Ni45Fe10Nx, Co57Ni14Fe29Nx of example 1 are significantly outperform the pure metal nitrides, state-of-the-art NiCoFeOx, and are comparable/better than IrOx. Example 3 Membrane electrode assemblies (MEAs) of Co45Ni45Fe10Nx of example 1. A set of membrane electrode assemblies (MEAs) were constructed using IrOx, NiCoFeOx and Co45Ni45Fe10Nx as OER catalysts and 0.5 mg/cm2 PtC 60% on carbon cloth as hydrogen evolution reaction (HER) catalyst with a FAA-3-PK-75 membrane. Figure 13 and 14 shows the linear sweep voltammetry curves, and chronoamperometry, respectively, of the three membrane electrode assemblies (MEAs) in alkaline anion exchange membrane electrolysis. IrOx and NiCoFeOx have a current density of approximately 400 mA/cm2 at an applied cell voltage of 2.0V, similar to reported MEAs made with NiCoFeOx albeit under somewhat different conditions (nanopure water, 50 °C, with the catalyst sprayed onto the GDE). Meanwhile, the novel Co45Ni45Fe10Nx shows approximately 900 mA/cm2 at the same applied potential, more than double the current density.

Claims

CLAIMS 1. A method of synthesis of transition metal nitrides characterized in that comprising the following steps: a) providing at least one precursor transition metal salts Mx(A)z, wherein: the transition metal M is selected from Ni, Co, Fe, Cr, Ga, Al, Ti, Zn, Mn and any combination thereof, preferably from Fe, Ni, Co, Mn and any combination thereof; A is CO3 2− or CH3CO2 −; wherein x is 1 or 2; and wherein z is the valence or mixed valences of the transition metal of the chosen precursor; with a ratio according to the expected stoichiometry of the metal or metals in the transition metal nitride; b) grinding the transition metal salts; and c) heating with a ramp temperature of between 8.75 ºC/min and 20ºC/min for a time of between 40 min and 70 min up to reach an annealing temperature of between 350 and 1200 °C the grinded transition metal salt of step (b) with NH3 for nitridation at the annealing temperature for a period between 1 and 24 hours, and preferably for a period between 2 and 10 hours.
2. The method according to claim 1 wherein the heating of step (c) takes place under a flow of NH3 for nitridation with a flow rate between 50 and 1000 mL/min, and preferably between 50 and 100 mL/min.
3. The method according to any of the claims 1 to 2, wherein in the step (a) at least two different precursors transition metal salts are provided.
4. The method according to claim 3, wherein the step (b) further comprising a pre- mixing of the selected salts before the griding.
5. The method according to any of the claims 1 to 4 wherein two different precursor transition metal salts M(A) of step (a) are used with a ratio according to the expected stoichiometry of the metals in the transition metal nitride to form binary transition metal nitrides.
6. A method of synthesis of transition metal nitrides according to claim 1 to 4 wherein three or four different precursor transition metal salts M(A) of step (a) are used with a ratio according to the expected stoichiometry of the metals in the transition metal nitride to form ternary or quaternary transition metal nitrides.
7. Transition metal nitrides obtained by the method according to any of claims 1 to 6 wherein the nitride is Co45Ni45Fe10Nx, Ni60Co30Fe10Nx, Ni33Co33Fe33Nx, Ni90Co5Fe5Nx, Co90Fe5Ni5Nx, Co57Ni14Fe29Nx, Ni99Co0.3Fe0.3Mn0.3Nx, or Co90Ni3.3Fe3.3Mn3.3Nx, preferably multi transition metal nitride is Co45Ni45Fe10Nx.
8. The use of the transition metal nitrides according to claim 7 as a catalyst for the oxygen evolution reaction.
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