WO2024047643A1

WO2024047643A1 - Mixed metal oxide aerogels for electrolyzers

Info

Publication number: WO2024047643A1
Application number: PCT/IL2023/050922
Authority: WO
Inventors: Lior ELBAZ; Wenjamin MOSCHKOWITSCH
Original assignee: Bar Ilan University
Priority date: 2022-08-31
Filing date: 2023-08-30
Publication date: 2024-03-07

Abstract

The technology generally concerns novel aerogels of mixed metal oxides and uses thereof as electrocatalysts.

Description

MIXED METAL OXIDE AEROGELS FOR ELECTROLYZERS

TECHNOLOGICAL FIELD

The invention generally contemplates novel electrocatalytic materials and electrochemical devices implementing the same.

BACKGROUND OF THE INVENTION

Increasing the production capacity of electrical energy to fulfill the continuously rising global demand, while reducing greenhouse gas emissions, is one of the main challenges of this era. What seems to be the biggest challenge on way to sustainable energy technologies is energy storage at large scales. One method is to rely on hydrogen for energy storage. Presently, most hydrogen is produced from fossil fuels via gasification and reforming. These so-called grey hydrogen production pathways still produce greenhouse gases. To achieve a true green hydrogen economy, it is necessary to produce hydrogen via emission-free processes, by using renewable energies in combination with electrochemical water electrolyzers.

Generally speaking, in water electrolysis two distinct chemical reactions take place: the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER). Both reactions require catalysts to execute at high rates. However, the HER is considered to be relatively facile and takes place at low overpotentials relative to the OER which requires higher overpotentials and precious metal catalyst loadings and is hence considered the bottleneck reaction.

The OER is a four-electron oxidation reaction per generated O₂ molecule, and proceeds in four distinct reaction steps. Since the use of a precious metal poses a significant challenge to the cost and sustainability of electrochemical water electrolysis, more and more effort has been dedicated to the development of suitable non-precious metal OER catalysts. Most notably is the family of mixed nickel-iron oxyhydroxides (NiFeOOH), which are relatively cheap, selective and efficient catalysts in alkaline media. Their performance has been increased by optimizing the Ni:Fe ratio or adding a third metal that either further increases the performance of the catalyst and/or its stability.

A challenge that still remains is to increase the electrochemically active site density (EASD).

REFERENCES

[1] Moschkowitsch, W.; Gonen, S.; Dhaka, K.; Zion, N.; Honig, H.; Tsur, Y.; Caspary-Toroker, M.; Elbaz, L., Bifunctional PGM-free metal organic framework-based electrocatalysts for alkaline electrolyzers: trends in the activity with different metal centers. Nanoscale 2021, (13), 4576-4584.

[2] Reier, T.; Oezaslan, M.; Strasser, P., Electrocatalytic Oxygen Evolution Reaction (OER) on Ru, Ir, and Pt Catalysts: A Comparative Study of Nanoparticles and Bulk Materials. ACS Catal 2012, 2, 1765-1772.

[3] Moschkowitsch, W.; Dhaka, K.; Gonen, S.; Attias, R.; Tsur, Y.; Caspary Toroker, M.; Elbaz, L., Ternary NiFeTiOOH Catalyst for the Oxygen Evolution Reaction: Study of the Effect of the Addition of Ti at Different Loadings. ACS Catalysis 2020, 4879-4887.

[4] Enman, L. J.; Burke, M. S.; Batchellor, A. S.; Boettcher, S. W., Effects of Intentionally Incorporated Metal Cations on the Oxygen Evolution Electrocatalytic Activity of Nickel (Oxy)hydroxide in Alkaline Media. ACS Catalysis 2016, 6 (4), 2416- 2423.

[5] Zaffran, J.; Stevens, M. B.; Trang, C. D. M.; Nagli, M.; Shehadeh, M.; Boettcher, S. W.; Caspary Toroker, M., Influence of Electrolyte Cations on Ni(Fe)OOH Catalyzed Oxygen Evolution Reaction. Chem. Mater. 2017, 29 (11), 4761-4767.

[6] Dionigi, F.; Strasser, P., NiFe-Based (Oxy)hydroxide Catalysts for Oxygen Evolution Reaction in Non-Acidic Electrolytes. Advanced Energy Materials 2016, 6 (23), 1600621.

[7] Zaffran, J.; Caspary Toroker, M., Benchmarking Density Functional Theory Based Methods To Model NiOOH Material Properties: Hubbard and van der Waals Corrections vs Hybrid Functionals. J Chem Theory Comput 2016, 12 (8), 3807-12.

[8] Klaus, S.; Cai, Y.; Louie, M. W .; Trotochaud, L.; Bell, A. T., Effects of Fe Electrolyte Impurities on Ni(OH)₂/NiOOH Structure and Oxygen Evolution Activity. The Journal of Physical Chemistry C 2015, 119 (13), 7243-7254. [9] Trotochaud, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W., Nickel-iron oxyhydroxide oxygen-evolution electrocatalysts: the role of intentional and incidental iron incorporation. J. Am. Chem. Soc. 2014, 136 (18), 6744-53.

[10] Katherine A. Pettigrew; Long, J. W.; Carpenter, E. E.; Baker, C. C.; Lytle, J. C.; Chervin, C. N.; Logan, M. S.; Stroud, R. M.; Rolison, D. R., Nickel Ferrite Aerogels with Monodisperse Nanoscale Building Blocks: The Importance of Processing Temperature and Atmosphere. ACS Nano 2008, 2 (4), 784-790.

GENERAL DESCRIPTION

To improve on the efficacy of mixed metal electrocatalysts, the inventors have developed and now disclose a class of novel mixed metal oxide aerogels and a novel synthetic method that allows for their production in high-yields and substantial cost- effectiveness. The aerogels differ from aerogels known in the art in their atomic composition and stoichiometry, which allow for achieving high surface areas. The substantial improvement in the materials surface areas led to a substantial increase in the number of catalytic sites and thus to an overall increase in the materail electrochemical properties, e.g., anodic oxygen evolution reaction (OER) mass activity compared to other similar materials.

Aerogels of the invention are porous 3D covalent frameworks (COF) characterized by a plurality of spatially distributed voids or pockets acting as catalytic sites that increase the overall effective surface of the material. The aerogels may be formed and used as self-standing materials that do not require any support; or may be formed into highly active powders that can be deposited on a support surface, making them denser, and amenable for a variety of different and specific applications.

In most general terms, the invention concerns an aerogel of a material of formula (I): NiFeMOn, wherein M may be present or may be absent and is an atom of a metal different from Fe and Ni, and wherein n is an integer between 1 and 4. The material is an “ oxide form " of the mixed metal NiFeM, which may be selected, for example, from an oxide of one or more of the atoms (Ni, Fe and/or M), an hydroxide (-OH) of the mixed metal material, or an oxyhydroxide (-OOH) of the mixed metal material. Thus, the material of formula (I) further encompasses a material represented by the formula NiFeMOnH. In a first aspect there is provided a material comprising or consisting an aerogel, wherein the aerogel consists a mixed metal oxide of iron (Fe) and nickel (Ni).

In some embodiments, the mixed metal oxide comprises a third different metal.

In some embodiments, the mixed metal oxide is a metal oxide, a metal hydroxide, a metal oxyhydroxide or a combination thereof.

In some embodiments, the aerogel is formed of the mixed metal oxide, or consists the mixed metal oxide.

Also provided is a material comprising or consisting an aerogel, wherein the aerogel consists a mixed metal oxide of iron (Fe) and nickel (Ni), wherein the amount of iron in the mixed metal oxide (relative to nickel) is not greater than 20 wt%.

In some embodiments, the mixed metal oxide is of the form NiFeMOn, wherein the molar ratio Ni:Fe is as defined herein, and wherein n is between 1 and 4.

In some embodiments, the aerogel comprises or consists a material of formula (I): NiFeMO_n, wherein each of Ni, Fe and O is nickel, iron and oxygen atoms, respectively; M is a metal, e.g., a transition metal, different from Ni and Fe; and n is an integer between 1 and 4, inclusive of 1 and 4.

In some embodiments, n is between 1 and 2, or between 2 and 3, or between 3 and 4. In some embodiments, n is 1 or 2.

In some embodiments, n is an integer between 1 and 4, being for example 1, 2, 3, or 4. In some embodiments, n is 1 or 2.

In some embodiments, the molar ratio Ni:Fe is such that the molar amount of Fe does exceed 20%. In some embodiments, the molar ratio Ni:Fe is between 4:1 and 1 ,000: 1. In some embodiments, the molar ratio Ni:Fe is between 4: 1 and 100: 1 or between 4:1 and 50:1, or between 4:1 and 25:1, or between 4:1 and 10:1. In some embodiments, the molar ratio Ni:Fe is 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, 11:1, 12:1, 13:1, 14:1, 15:1, 16:1, 17:1, 18:1, 19:1, 10:1, 21:1, 22:1, 23:1, 24:1, 25:1, 26:1, 27:1, 28:1, 29:1, 30:1, 31:1, 32:1, 33:1, 34:1, and so forth.

In some embodiments, aerogels in which a molar amount of Fe is greater than 20% are excluded.

In a mixed metal oxide, a third metal M may be optional. Such a metal or may be present or may be absent from the mixed metal oxide. In some embodiments, M is absent. In such embodiments, the aerogel comprises or consists a material of formula (II): NiFeOn, wherein n is as defined herein, and wherein the aerogel is for use as an electrocatalyst, or is for use in a method of electrocatalysis.

In some embodiments, M is absent and the aerogel consists NiFeOOH, NiFeOH or NiFeOn, wherein n is between 1 and 4 (inclusive), for use as an electrocatalyst.

The invention further provides an electrocatalyst in a form of an aerogel of NiFeOOH or NiFeOH or NiFeOn, wherein n is between 1 and 4 (inclusive). In some embodiments, the electrocatalyst consists of NiFeOOH or NiFeOH. In some embodiments, the electrocatalyst is for use in an electrochemical process.

In some embodiments, in an aerogel of the form NiFeOOH, NiFeOH or NiFeOn, the molar amount of Fe does not exceed 20%. In other words, in electrocatalysts of the form NiFeOOH, NiFeOH or NiFeOn, the molar amount of Fe, e.g., relative to that of Ni (Ni:Fe ratio), does not exceed 20%.

As demonstrated herein, the catalytic activity may depend on a variety of factors, amongst such the Ni:Fe ratio and the surface area of the material or aerogel. An aerogel with a Ni:Fe ratio wherein the molar amount of Fe did not exceed 20% (wherein the 20% is inclusive or exclusive), demonstrated unexpected superiority. For example, a Ni:Fe ratio of 94:6 (6 % Fe) showed a highly improved catalytic activity in terms of overpotential to reach 10 mA cm^-2, as well as current density at 1.8 V vs. RHE, when compared to a mixed metal oxide such as NiFeOOH with a Ni:Fe ratio of 75:25 (25% Fe), considered the state-of-the-art material for PGM free OER catalysts in alkaline media. The superiority allows construction of highly improved electrolyzer systems for green hydrogen production.

In some embodiments, in an aerogel of the invention, the material is of the form NiFeMOn, wherein M is present and is a metal different from Ni and Fe; and wherein n is between 1 and 4.

The invention further provides an aerogel of formula (I): NiFeMOn, being a mixed metal oxide selected from NiFeMOH and NiFeMOOH, wherein each of M and n is as defined herein.

In some embodiments, the Ni:Fe ratio in a material of formula (I) may be between 100:1 and 1:100. In some embodiments, the ratio may be between 100:1 and 1:1, or between 1:1 and 1:100. In other embodiments, the ratio may be between 50:1 and 1:50. In some embodiments, a nominal ratio may for example be 94:6, 91:9, 89:11, 80:20, 71:29, 61:39, 50:50, or 20:80.

In a mixed metal material, the metal M is a metal other than Ni and Fe. In some embodiments, M is a metal different from a transition metal, or a non-transition metal. The non-transition metal is any metal that is different from a transition metal. It may be selected from alkali metals, such as Li, Na, and K; alkaline earth metals, such as Be, Mg, Ca, Sr and Ba; rare earth metals; and others. Non-limiting examples of metals that are not transition metals include Al, Sn, K, Na and others.

In some embodiments, the metal M is a transition metal optionally selected from titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), cobalt (Co), copper (Cu), molybdenum (Mo), ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), tungsten (W), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), gold (Au), and other elements of d-block of the Periodic Table of the Elements.

In some embodiments, the transition metal is Mo, Ti and Co.

The aerogel of the invention may be formed into a powder form by mechanically reducing the aerogel in size. The powder may be used as is, namely as a powder electrocatalyst, or may be formed into a material film on a surface region of a substrate to thereby obtain an electrochemically active surface, e.g., an electrode. Thus, also provided is a powder aerogel of a material of the form NiFeMOn, as defined herein, in all aspects and embodiments above.

In some embodiments, the powder aerogel is the electrocatalyst NiFeOOH or NiFeOH or the electrocatalyst NiFeOn, wherein n is between 1 and 4. In some embodiments, n is 1 or 2.

In some embodiments, the powder aerogel is the electrocatalyst NiFeMOn, as defined herein.

In some embodiments, the powder aerogel is the electrocatalyst selected from NiFeOH, NiFeOOH, NiFeOn, NiFeOnH, NiFeMOn and NiFeMOnH.

The invention further provides a surface coated or associated with a film of an aerogel comprising or consisting a material of formula (I): NiFeMOn, as defined herein. In some embodiments, the material is of formula (II): NiFeOn, as defined herein.

In some embodiments, the aerogel is the electrocatalyst NiFeOOH, NiFeOH or NiFeOn, wherein n is between 1 and 4, or wherein n is 1 or 2. The “aerogeV of the invention is a solid object, formed into any shape, size and form, comprising a metallic or metallic oxide framework of interconnected solid structures, with a corresponding network of interconnected pores that is integrated within. The aerogel consists the metal oxide of formula (I) or (II), as defined herein. The aerogel contains a plurality of pore features, e.g., voids, cavities, pockets or channels. These pore features act as catalytic sites and permit transfer of reactive materials therethrough, thereby maximizing the electrocatalytic activity of the mixed metal material forming the aerogel. Typically, the pore features are variable and include nanometric-size pores or features and/or micrometric- size pores or features.

In some embodiments, the pore features, e.g., pores, have a variable size or a radius forming a hierarchical structure composed of, but not restricted to, micro-pores, meso-pores and macro-pores. The size of the pores may range from O.lnm to several microns or more. The hierarchical structure comprises pores of average size between 0.1 to 2nm, and/or pores of an average size between 2 and 20 nm, and/or pores of an average size greater than 20 nm.

In some embodiments, the aerogels of the invention have a porosity of about 60 percent or more and a specific surface area of at least 60 m²/g or more. In some embodiments, the specific surface area is above 100 m²/g.

The surface area was determined by multipoint Brunauer-Emmett-Teller (BET) surface area measurements, as acceptable and used in the art. The pore size distribution was calculated with the Density Functional Theory (DFT) method. Other means for determining the pore profile and surface area of the aerogels may be used.

An aerogel of the invention, e.g., of formula (I) or (II) is formed from a solution of a mixed metal material in a solvent. Upon mixing, a non-flowing gel forms (gel state) in which the mobile phase within the network of interconnected pores is primarily comprised of the solvent. The solvent is thereafter removed from the gel state in a manner that maintains the integrity of the aerogel or does not cause contraction of the gel. Solvent removal is achieved by solvent exchange and drying under freeze dry, or under supercritical conditions, e.g., drying may involve a liquid, such as carbon dioxide or other liquids, at supercritical conditions, and purging the solvent such that the release of the solvent does not apply capillary forces on the pore walls which may cause the pores to collapse. In some embodiments, a gelation material may be used, wherein the gelation material is a polyol.

Thus, a method for preparing an aerogel of the invention, the method comprises:

-forming a gel of at least one Ni salt/complex, at least one Fe salt/complex, optionally at least one salt/complex of a metal M, at least one precursor of a gelation material (such as propylene oxide) and a solvent, under conditions permitting formation of a network of interconnected pores, said pores comprising the solvent; and

-removing the solvent under drying conditions to form the aerogel without causing contraction of the interconnected pores.

The metal “salt/complex” may be a precursor of the metal that is any one or more salt form of the metal, and/or any one or more complex form of the metal. Thus, a “Ni salt/complex” encompasses any precursor of Ni that is any one or more salt form of Ni, and/or any one or more complex form of Ni. Same is applicable to iron or any M metal used.

In some embodiments, the drying conditions are supercritical conditions.

In some embodiments, M is a transition metal.

The “ conditions permiting formation of a network of interconnected pores'¹'¹ are typically mild stirring conditions at a temperature permitting formation of the network. These mild stirring conditions include room temperature (15-33 °C) stirring of a solution of the salts/complexes forming the mixed metal material, i.e., at least one Ni metal salt/complex, at least one Fe metal salt/complex, and at least one salt/complex of a metal M, with at least one gelation precursor. The stirring may proceed for a period of several minutes to several hours.

In some embodiments, the conditions include:

-stirring the solution of salt/complexes in the solvent for a period of time; and -allowing the solution to form a gel (by stopping the stirring).

In some embodiments, the conditions may include thermal treatment of the solution to a temperature between room temperature (15-33°C) and 80°C, and heat treatment of the aerogel after its formation at temperatures ranging from room temperature (15-33 °C) to 900°C from several minutes to several hours.

In some embodiments, the process comprises forming a solution of the salt/complexes forming the mixed metal material, i.e., at least one Ni metal salt/complex, at least one Fe metal salt/complex, and at least one salt/complex of a metal M, with at least one precursor of a gelation material in a solvent.

In some embodiments, the solvent is typically a low temperature solvent (e.g., having a boiling temperature below that of water), such as an alcohol, an ether, an ester or any other water-miscible solvent, as known in the art.

In some embodiments, the solvent is an alcohol such as methanol, ethanol, propanol, iso propanol, butanol and others.

In some embodiments, the metal salt/complex is a salt/complex form of a metal making up the mixed metal material. In other words, the metal salt/complex is one or more of a Ni salt/complex, a Fe salt/complex and a salt/complex of a metal M, e.g., a transition metal. For a mixed metal material of formula (I) or (II), as defined herein, the salt/complex of each of Ni, Fe and M, independent, may be selected from (in the following “Mr” designates any of the metals making the mixed metal material):

-chlorides, e.g., selected from MrCl, MrCl₂, MrCl₃, MrCl₄, MCl₅, and MrCl₆;

-chlorides hydrates, e.g., selected from MrCl·xH₂O, MrCl₂·xH₂O, MrCh xH₂O, MrCl₄·xH₂O, MrCl₅·xH₂O, and MrCl₆·xH₂O, wherein x varies based on the nature of the metal atom;

-bromides, e.g., selected from MrBr, MrBr₂, MrBr₃, MrBr₄, MBr₅, and MrBr₆;

-bromide hydrates, e.g., selected from MrBr·xH₂O, MrBr₂·xH₂O, MrBrs xH₂O, MrBr₄·xH₂O, MrBr₅·xH₂O, and MrBr₆·xH₂O, wherein x varies based on the nature of the metal atom;

-iodides, e.g., selected from Mrl, MrF, Mrl₃, Mrl₄, Ml₅, and Mrl₆;

-iodide hydrates, e.g., selected from Mrl·xH₂O, Mrh·xH₂O, Mrh xH₂O, Mrl₄ xH₂O, Ml₅·x H₂O, and Mrl₆·xH₂O, wherein x varies based on the nature of the metal atom;

-fluorides, e.g., selected from MrF, MrF2, MrF₃, MrF4, MF5, and MrF₆;

-fluoride hydrates, e.g., selected from MrF xH₂O, MrF2·xH₂O, MrF₃ xH₂O, MrF4·xH₂O, MrF₅xH₂O , and MrF6·xH₂O, wherein x varies based on the nature of the metal atom;

-hypochlorites/chlorites/chlorates/cerchlorates (abbreviated ClOn-, n=l, 2, 3, 4), e.g., selected from MrClOn, Mr(C10n)₂, Mr(C10n)₃, Mr(C10n)4, Mr(C10n)₅, and Mr(C10n)₆; -hypochlorites/chlorites/chlorates/perchlorates hydrates, e.g., selected from MrC10n·xH₂0, Mr(C10n)₂·xH₂0, Mr(C10n)₃·xH₂0, Mr(C10n)₄·xH₂0,

Mr(C10n)₅·xH₂0, and Mr(C10n)6·xH₂0, wherein x varies based on the nature of the metal atom, and n=l, 2, 3, 4;

-carbonates, e.g., selected from Mr₂CO₃, MrCO₃, Mr₂(CO₃)₃, Mr(CO₃)₂, Mr₂(CO₃)₂, Mr(CO₃)₃, Mr₃(CO₃)₄, Mr(CO₃)₅, Mr₂(CO₃)₇;

-carbonate hydrates, e.g., selected from Mr₂CO₃·xH₂O, MrCO₃·xH₂O, Mr₂(CO₃)₃ xH₂O, Mr(CO₃)₂ xH₂O, Mr₂(CO₃)₂ xH₂O, Mr(CO₃)₃ xH₂O, Mr₃(CO₃)₄·xH₂O, Mr(CO₃)₅·xH₂O, and Mr₂(CO₃)₇·xH₂O, wherein x varies based on the nature of the metal atom;

-carboxylates (abbreviated RCO₂“, and including acetates), e.g., selected from MrRCO₂, Mr(RCO₂)₂, Mr(RCO₂)₃, Mr(RCO₂)₄, Mr(RCO₂)₅, and Mr(RCO₂)₆;

-carboxylates hydrates (abbreviated RCO² ), e.g., selected from MrRCO₂·xH₂O, Mr(RCO₂)₂·xH₂O, Mr(RCO₂)₃·xH₂O, Mr(RCO₂)₄·xH₂O, Mr(RCO₂)₅·xH₂O, and

Mr(RCO₂)6·xH₂O, wherein x varies based on the nature of the metal atom;

-carboxylate (the group RCOO-, R is aliphatic chain, which may be saturated or unsaturated), e.g., selected from CH₃CH=CHCOOMr (metal crotonate),

CH₃(CH₂)₃CH=CH(CH₂)₇COOMr (metal myristoleate),

CH₃(CH₂)₅CH=CH(CH₂)₇COOMr (metal palmitoleate),

CH₃(CH₂)₈CH=CH(CH₂)₄COOMr (metal sapienate), CH₃(CH₂)₇CH=CH(CH₂)₇COOMr (metal oleate), CH₃(CH₂)₇CH=CH(CH₂)₇COOMr (metal elaidate),

CH₃(CH₂)₅CH=CH(CH₂)₉COOMr (metal vaccinate),

CH₃(CH₂)₇CH=CH(CH₂)nCOOMr (metal erucate), Ci₇H₃₅COOMr (metal stearate);

-oxides, e.g., selected from Mr₂O, MrO, Mr₂O₃, MrO₂, Mr₂O₂, MrO₃, Mr₃O₄, MrOs, and Mr₂O₇;

-acetates, e.g., (the group CH₃COO-, abbreviated AcO-) selected from AcOMr, AcO₂Mr, AcO₃Mr, and AcO₄Mr;

-acetates hydrates, (the group CH₃COO-, abbreviated AcO-), e.g., selected from AcOMr- xH₂O, AcO₂Mr·xH₂O, AcO₃Mr·xH₂O, and AcO₄Mr·xH₂O, wherein x varies based on the nature of the metal atom;

-acetylacetonates (the group C₂H₇CO₂-, abbreviated AcAc-), e.g., selected from AcAcMr, AcAc₂Mr, AcAc₃Mr, and AcAc₄Mr; -acetylacetonate hydrates (the group C2H7CO₂-, abbreviated AcAc-), e.g., selected from AcAcMr x H₂O, AcAc2Mr xH₂O, AcAc₃Mr xH₂O, and AcAc4Mr xH₂O, wherein x varies based on the nature of metal atom;

-nitrates, e.g., selected from MrNO₃, Mr(NO₃)₂, Mr(NO₃)₃, Mr(NO₃)4, Mr(NO₃)5, and Mr(NO₃)6;

-nitrates hydrates, e.g., selected from MrNCh xH₂O, Mr(NO₃)₂·xH₂O, Mr(NO₃)₃·xH₂O, Mr(NO₃)4 xH₂O, Mr(NO₃)₅·xH₂O, and Mr(NO₃)6 xH₂O, wherein x varies based on the nature of the metal atom;

-nitrites, e.g., selected from MrNO₂, Mr(NO₂)₂, Mr(NO₂)₃, Mr(NO₂)4, Mr(NO₂)₅, and Mr(NO₂)e;

-nitrites hydrates, e.g., selected from MrNO₂·xH₂O, Mr(NO₂)₂·xH₂O, Mr(NO₂)₃·xH₂O, Mr(NO₂)4·xH₂O, Mr(NO₂)₅·xH₂O, and Mr(NO₂)6·xH₂O, wherein x varies based on the nature of the metal atom;

-cyanates, e.g., selected from MrCN, Mr(CN)₂, Mr(CN)₃, Mr(CN)4, Mr(CN)₅, Mr(CN)₆;

-cyanates hydrates, e.g., selected from MrCN·xH₂O, Mr(CN)₂·xH₂O, Mr(CN)₃ xH₂O, Mr(CN)₄ xH₂O, Mr(CN)₅ xH₂O, and Mr(CN)₆ xH₂O, wherein x varies based on the nature of the metal atom;

-sulfides, e.g., selected from Mr₂S, MrS, Mr₂Ss, MrS2, Mr2S2, MrSs, Mr₃S4, MrSs, and M2S7;

-sulfides hydrates, e.g., selected from Mr₂S xH₂O, MrS·xH₂O, Mr₂Ss·x H₂O, MrS₂ xH₂O, Mr₂S2 xH₂O, MrS₃ xH₂O, Mr₃S4 xH₂O, MrS₅ xH₂O, and Mr₂S7 xH₂O, wherein x varies based on the nature of metal atom;

-sulfites, e.g., selected from Mr₂SO₃, MrSO₃, Mr2(SO₃)₃, Mr(SO₃)₂, Mr₂(SO₃)₂, Mr(SO₃)₃, Mr₃(SO₃)4, Mr(SO₃)₅, and Mr₂(SO₃)7;

-sulfites hydrates selected from Mr₂SO₃·xH₂O, MrSO₃·x H₂º, Mr₂(SO₃)₃·xH₂O, Mr(SO₃)₂ xH₂O, Mr₂(SO₃)₂ xH₂O, Mr(SO₃)₃ xH₂O, Mr₃(SO₃)4 xH₂O, Mr(SO₃)5 xH₂O, and Mr2(SO₃)7·xH₂O, wherein x varies based on the nature of metal atom;

-hyposulfite, e.g., selected from Mr₂SO₂, MrSO₂, Mr2(SO₂)₃, Mr(SO₂)₂, Mr₂(SO₂)₂, Mr(SO₂)₃, Mr₃(SO₂)4, Mr(SO₂)₅, and Mr₂(SO₂)7;

-hyposulfite hydrates, e.g., selected from Mr₂SO₂·xH₂O, MrSO₂·xH₂O, Mr₂(SO₂)₃ xH₂O, Mr(SO₂)₂ xH₂O, Mr₂(SO₂)₂ xH₂O, Mr(SO₂)₃ xH₂O, Mr₃(SO₂)4 xH₂O, Mr(SO₂)5·xH₂O, and Mr2(SO₂)₇ xH₂O, wherein x varies based on the nature of the metal atom;

-sulfate, e.g., selected from Mr₂SCE, MrSO₃, Mr2(SO₃)₃, Mr(SO₃)₂, Mr₂(SO₃)h, Mr(SO₃)₃, Mr₃(SO₃)₄, Mr(SO₃)₅, and Mr₂(SO₃)₇;

-sulfate hydrates, e.g., selected from Mr₂SOyxH₂O, MrSO₃·xH₂O, Mr₂(SO₃)₃ xH₂O, Mr(SO₃)₂ xH₂O, Mr₂(SO₃)₂ xH₂O, Mr(SO₃)₃ xH₂O, Mr₃(SO₃)₄ xH₂O, Mr(SO₃)₅·xH₂O, and Mr2(SO₃)₇ xH₂O, wherein x varies based on the nature of the metal atom;

-thiosulfate, e.g., selected from Mr2S2O₃, MrS2O₃, Mr2(S2O₃)₃, Mr(S2O₃)₂, Mr2(S2O₃)₂, Mr(S2O₃)₃, Mr₃(S2O₃)₄, Mr(S2O₃)₅, and Mr2(S2O₃)₇;

-thioulfate hydrates, e.g., selected from M2S2O₃ xH₂O, MrS2O₃ xH₂O, Mr2(S₂O₃)₃ xH₂O, Mr(S₂O₃)₂ xH₂O, Mr₂(S2O₃)₂ xH₂O, Mr(S₂O₃)₃ xH₂O, Mr₃(S2O₃)₄·xH₂O, Mr(S2O₃)₅·xH₂O, and Mr2(S2O₃)₇ xH₂O, wherein x varies based on the nature of the metal atom;

-dithionites, e.g., selected from Mr2S₃O₄, MrS₃O₄, Mr2(S2O₄)₃, Mr(S2O₄)₂, Mr₂(S₂O₄)₂, Mr(S₂O₄)₃, Mr₃(S₂O₄)₄, Mr(S₂O₄)₅, and Mr₂(S₂O₄)₇;

-dithionites hydrates, e.g., selected from Mr2S2O₄ xH₂O, MrS2O₄ xH₂O, Mr2(S₂O₄)₃ xH₂O, Mr(S₂O₄)₂ xH₂O, Mr₂(S2O₄)₂ xH₂O, Mr(S₂O₄)₃ xH₂O, Mr₃(S2O₄)₄·xH₂O, Mr(S2O₄)₅·xH₂O, and Mr2(S2O₄)₇ xH₂O, wherein x varies based on the nature of the metal atom;

-phosphates, e.g., selected from Mr₃PO₄, Mr₃(PO₄)₂, MrPO₄, and Mr₄(PO₄)₃;

-phosphates hydrates, e.g., selected from Mr₃PO₄ xH₂O, Mr₃(PO₄)₂·xH₂O, MrPO₄·xH₂O, and Mr₄(PO₄)₃ xH₂O, wherein x varies based on the nature of the metal atom;

-Metal alkyls;

-Metal alkoxides;

-Metal amines;

-Metal phosphines;

-Metal thiolates;

-Combined cation-anion single source precursors, i.e., molecules that include both cation and anion atoms, for example of the formula Mr(E2CNR2)₂ (Mr = is a metal, E = is for example a chalcogenide, and R = alkyl, amine alkyl, silyl alkyl, phosphoryl alkyl, phosphyl alkyl). In some embodiments, the metal salt or complex (of Ni, Fe, or M) is a metal halide or a metal oxide.

In some embodiments, the metal salt or complex is a hydrate form of the salt or complex. In some embodiments, the metal salt or complex is a metal halide hydrate or a metal oxide hydrate.

In some embodiments, the solution forming the gel comprises salt/complex is one or more of a Ni halide or oxide, a Fe halide or oxide and a halide or oxide for of the transition metal.

In some embodiments, the metal halide is a metal chloride or a metal bromide.

In some embodiments, the metal halide is a metal chloride selected from NiCl₂, FeCl₂, and a halide of a transition metal.

In some embodiments, the metal halide is a hydrate of a metal chloride, such as a metal halide- x H₂O, wherein x is between 1 and 6.

In some embodiments, the hydrate of the metal halide is selected from NiCl₂-bH₂O, FeC12·4H₂O and a hydrate halide of a transition metal.

In some embodiments, the method for preparing an aerogel of the invention comprises:

-forming a gel of NiCl₂-bH₂O, FeCl₂ ·4H₂O, a hydrate halide of a metal, e.g., a transition metal, at least one precursor of a matrix material and ethanol under conditions permitting formation of a network of interconnected pores, said pores comprising the solvent; and

-removing the solvent under drying, e.g., supercritical drying conditions to form the aerogel without causing contraction of the interconnected pores.

At least one precursor of the gelation material is a monomer, oligomer or polymer capable of forming a continuous matrix material with the metals, as disclosed herein. In some embodiments, the matrix material is polymer selected from polyimides, polyols and others. The precursor material may be any known precursor material or a selected gelation material. In some embodiments, the gelation material is a polyol and the precursor material is an epoxide such as propylene oxide.

-forming a gel of NiCl₂- 6H₂O, FeCl₂-4H₂O, a hydrate halide of a metal (optional), e.g., a transition metal, propylene oxide and ethanol under conditions permitting formation of a network of interconnected pores, said pores comprising the water-miscible solvent; and

-removing the solvent under supercritical CO₂ drying conditions to form the aerogel without causing contraction of the interconnected pores.

The drying conditions used may be freeze dry conditions as known and practiced in the art. In some cases, supercritical conditions are utilized. The “supercritical drying conditions are selected to dry the aerogel from solvent following the displacement of the reaction solvent, e.g., ethanol, by water. The drying step is optionally performed in an autoclave. Supercritical carbon dioxide is used to achieve low temperature drying and is possible due to the material low critical temperature. In the process, the wet aerogel is periodically or continuously flushed with liquid CO₂ until all other solvent/s and other residuals are removed.

In some embodiments, the drying step is carried out in a critical point dryer such as a Tousimis 931GL unit.

In some embodiments, the dried aerogel may be thermally treated to provide an anhydrous solid material. The thermal treatment may comprise heating the aerogel at a temperature between room temperature and 150°C. In some embodiments, the temperature is between room temperature and 60°C, between 30°C and 60°C, between 30°C and 90°C, between 30°C and 120°C, between 60°C and 90°C, between 60°C and 120°C, between 60°C and 120°C, or between 120°C and 150°C. In some embodiments, the temperature is raised from 30°C to 150°C over a period of time.

Aerogels of the invention may be synthesized with different metal ratios, opening the door for manufacturing of a variety of different active materials or catalysts. The aerogel may be used as is or may be mechanically treated, e.g., by diminution (such as by crushing) to afford an active powder which may be used to form active surfaces such as electrodes and catalytic surfaces. As mentioned hereinabove, the aerogel of the invention may be used as an electrochemical active material for forming an electrode or may be used as the electrode material itself. As such, the aerogel may, for example, be used in an anodic oxygen evolution reaction (OER). Aerogels manufactured according to the invention were tested for OER activity and were determined to be the first aerogel material that propagates OER, rather than being used as a support material for an OER catalyst. Thus, the invention further contemplates an electrode material, e.g., an anode material consisting or comprising an aerogel of the invention.

Also provided is an electrode for use in an electrochemical device or an electrolyzer.

Also provided is an electrochemical device or an electrolyzer implementing an electrode assembly (at least anode and a cathode) comprising an electrode, e.g., an anode, formed of an aerogel of the invention.

In another aspect the present invention discloses an electrochemical device comprising an electrode comprising the electrocatalyst aerogel of the invention.

An electrocatalytic device is also provided which comprises a porous electrode according to the invention, wherein the device is configured and operable to perform an electrocatalytic process. The electrolytic process may be water splitting, generation of hydrogen from water and/or generation of oxygen from water.

In some embodiments, the device implements an electrocatalyst in an aerogel form which consists NiFeOOH, NiFeO_n, NiFeOH, NiFeCoOOH, NiFeCoOH, NiFeTiOOH, NiFeTiOH, NiFeMOH and NiFeMOOH, wherein M is a metal different from Ni, Fe, Co and Ti, and wherein n is between 1 and 4 or is 1 or 2.

The invention also provided an electrocatalyst in an aerogel form comprising or consisting NiFeOOH, NiFeOn, NiFeOH, NiFeCoOOH, NiFeCoOH, NiFeTiOOH, NiFeTiOH, NiFeMOH and NiFeMOOH, wherein M is a metal different from Ni, Fe, Co and Ti, and wherein n is 1 or 2. The electrocatalyst of the invention may be used for generating oxygen.

Further provided is an electrocatalytic process comprising use of an electrocatalyst or an aerogel according to any of the aspects and embodiemnts of the invention.

The electrochemical process” using an electrocatalyst or aerogel of the invention refers to a process wherein a chemical reaction takes place at the interface of an electron conductor (an electrode) and an ionic conductor (an electrolyte) and involves transfer of a charged species between the electrode and the electrolyte. The process may be or may involve hydrogen oxidation reaction (HOR), hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), or oxygen evolution reaction (OER).

In some embodiments, the process is OER. BRIEF DESCRIPTION OF THE DRAWINGS

In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Figs. 1A-B: TGA and MS for NiFeO_x AG1. (A) Mass loss vs. water release. (B) Temperature vs. water release.

Fig. 2: X-ray diffractions (XRD) of NiFeO_x AG1 -4, NiO_x AG, and FeO_x AG.

Figs. 3A-B: (A) Pore size distribution and (B) Cumulative pore volume for NiFeO_x AG1-4, NiO_x AG, and FeO_x AG.

Figs. 4A-D: HAADF-STEM images of NiFeO_x AG1 showing (A) interconnected structure exhibiting macropores as well as (B) meso- and micropores. (C, D) Regions extracted from (B) showing layered hydroxide structure.

Figs. 5A-B: Atomic-resolution HAADF-STEM images showing crystalline domains which were formed as a result of electron beam exposure.

Figs. 6A-B: Deconvoluted XPS spectra of NiFeO_x AGE A) Ni, B) Fe.

Figs. 7A-B: Deconvoluted XPS spectra of NiFeO_x AG2. A) Ni, B) Fe.

Figs. 8A-B: Deconvoluted XPS spectra of NiFeO_x AG3. A) Ni, B) Fe.

Figs. 9A-B: Deconvoluted XPS spectra of NiFeO_x AG4. A) Ni, B) Fe.

Fig. 10: Deconvoluted Ni-XPS spectrum ofNiO_xAG.

Figs. 11A-B: Cyclic voltammograms of (A) NiFeO_x AG1-4, NiO_x AG, and FeO_x AG and (B) heat treated NiFeO_x AGHT1-4, NiO_x AGHT, and FeO_x AGHT in deaerated 1 M KOH solution in DI water (scan rate 50 mV/s).

Figs. 12A-B: Cyclic voltammograms of NiFeO_x AG1, NiFeO_x AGHT1, and NiFeOOH in deaerated 1 M KOH solution in DI water (scan rate 50 mV/s). (A) potential window of 1.2-1.6 V (vs. RHE) and (B) potential window of 1.2-1.8 V (vs. RHE).

Fig. 13: Cyclic voltammogram of NiFeOOH in deaerated 1 M KOH solution in DI water (scan rate 50 mV/s)

Fig. 14: Cyclic voltammogram of heat treated aerogels and NiFeOOH powder in deaerated 1 M KOH solution in DI water (scan rate 50 mV/s). The current density is normalized to BET surface area, that was measured. The current density was normalized to the BET surface area by dividing the measured current by the BET surface area and the catalyst loading (0.015 mg).

DETAILED DESCRIPTION OF EMBODIMENTS

Experimental

Aerogel Synthesis: NiO_x aerogel (AG) was formed by dissolving NiCl₂ 6 H₂O (93 mg) in ethanol (0.625 mL). Propylene oxide (0.3 g, ~ 0.3 mL) was added slowly drop- wise to the solution. Afterwards, the stirring was stopped, and a green gel formed (same color as NiO) after about 30 min. For FeO_x AG the same procedure was used, but FeCl₂ •4 H₂O (78 mg) was used as precursor. A rust colored (same color as FeO) gel formed after about 60 min.

For mixed metal materials both precursor salts were dissolved together in ethanol (2.5 mL): NiCl₂ 6 H₂O (290 mg, 296.6 mg, 187.9 mg, 74.6 mg), and FeCl₂ 4 H₂O (30 mg, 62.3 mg, 155.4 mg, 248.9 mg) for NiFeO_x AG1-4, respectively. Propylene oxide (1.0 g, ~ 1.2 mL) was added slowly drop- wise. Green, orange, and red gels formed, respectively. The higher the Fe content, the longer it took for the gelation process time (between 30-60 min).

The gels were covered with ethanol for 24 h. To wash the gels from excess/unreacted precursors, the solvent was exchanged once a day for three consecutive days. They were then dried under supercritical conditions with CO₂ with a Tousimis 931GL. Different colored aerogels formed (Table 1).

Table 1: Summary of synthesized NiFeO_x AG1-4, NiO_x AG, and FeO_x AG.

It is important to note that hydrated materials were used as the hydration layer may be necessary for the reaction to take place.

Heat treatment: The aerogels were heated to 30°C, 60°C, 90°C, 120°C, and 150°C successively. They were held at each temperature for 30 minutes and at 150°C for 3 h.

Characterization: X-ray diffraction (XRD) was performed on a Bruker-AXS D8- Advanced Diffractometer with CuKa (1.54 A) radiation source. X-ray photoelectron spectra (XPS) were measured with a Thermo Scientific Nexsa spectrometer, and X-ray fluorescence (XRF) spectra with a Horiba XGT 7200 V instrument. BET surface area was measured with a Quantachrome Autosorb iQ instrument, thermogravimetric analysis (TGA) with a Setaram Setsys TGA-DTA/DSC, and mass spectrometry (MS) with a Hiden analytical HPR-20 QIC, respectively. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were taken in a JEOL NEOARM operated at 80kV.

Electrochemical measurements: All of the electrochemical measurements were carried out in a standard three-electrode Teflon cell (Pine) with a GC rod as counter electrode and reversible hydrogen electrode (RHE) as reference electrode. The catalyst was dispersed in Ao-propanol (1 mg/ mL) and 3 x 5 pL were drop casted on a glassy carbon (GC) working electrode, which previously was successively polished with 0.3 pm and 0.05 pm Al slurry, and dried in air. 1 M KOH was used as the electrolyte, when dissolved in deionized water. The electrolyte solution was degassed with Ar for 30 min prior to measurements and Ar was bubbled at a slow rate throughout the measurements. The electrochemical measurements were conducted using a Bio-Logic VMP 300 bi- potentiostat. Cyclic voltammetry (CV) experiments were conducted at a scan rate of 50 mV/s and a rotation speed of 900 rpm. The rotation was necessary to get rid of oxygen bubbles formed on the working electrode. The catalysts were activated by cycling the potential between 1.2 and 1.55 V (vs. RHE) for 20 cycles, prior to measuring between 1.2 and 1.8 V (vs. RHE).

Results and Discussion

NiO_x aerogel was synthesized as described in the experimental section, while FeO_x was synthesized with Fe(II) chloride for the first time (previous syntheses used Fe(III) chlorides or other Fe salts). Four different NiFeO_x aerogels, with different Ni:Fe ratio, were also synthesized, and designated as NiFeO_x AG1-4. The nominal Ni:Fe ratios were 89:11, 80:20, 50:50, and 20:80, respectively. XRF measurements showed the actual ratios are 94:6 91:9, 71:29, and 61:39, respectively. In all samples, the Fe content was lower than expected. One reason could be that the gelation of NiO_x is faster than the gelation of FeO_x. The theoretical and measured Ni:Fe ratios are summarized in Table 1.

The color of the gels was dependent on the Ni:Fe ratio and varied widely. When they are heat-treated up to 150°C, their color further changes. NiO_x AG had the exact same green color and changed color to beige. FeO_x AG had a rust red color as is expected from Fe oxides. It changed color to a dark rust-brown. NiFeO_x AG1 was green that looks almost exactly like NiO_x AG but changed color to a yellow- brown. NiFeO_x AG2 was yellow-green and resembled NiO. It changed color to orange-brown. NiFeO_x AG3 changed color from orange to brown and NiFeO_x AG4 from red-brown to rust brown. The higher the Ni content, the more the materials resembled the pure Ni material, and the higher the Fe content is, the more they resembled the Fe-only material. After the gels were left exposed to air for several weeks the color changed back to that of the material before heat treatment. These color changes are summarized in Table 1.

The effects of the heat-treatment process were studied using TGA, coupled with a MS. Fig. 1 shows the mass loss for NiFeO_x AG1, which adds up to about 50% of the sample’s starting weight. The TGA spectra for the other gels look similar. Water was the only molecule identified leaving during the process. Since the color change reverts after a couple of weeks, it can be assumed that the materials are hygroscopic and water hydrates the aerogels.

The XRD-diffraction for NiO_x AG corresponds to PXRD measurements for NiO aerogel (Fig. 2). There, three major very broad peaks were observed at around 15°, 35°, and 60°, as well as relatively small peaks at 21°, 40°, and 70°. FeO_x AG shows very weak peaks at around 26°, 35°, 39°, 46°, 56°, and 64°, which corresponds very well to previous work, where it was identified as P-FeOOH. The XRD diffraction of the mixed NiFeO_x AGs shows peaks at around 11°, 21°, 34°, and 60°. There are also much smaller peaks at 39° and 70°. These differences in the peak positions are miniscule. They correspond very closely to those of NiFeOOH, which are at 11°, 23°, 35°, 38°, and 62° and are attributed to (003), (006), (101) + (009) + (012) and (110) planes, respectively. This suggests that NiFeO_x AGs are very similar to conventional NiFeOOH, which is a layered double hydroxide (LDH). This could also explain the formation of the hydration layer in the aerogels which could be bound in its hydroxide or oxyhydroxide form or be intercalated in NiFe LDH. The broadness of the XRD peaks is expected from an aerogel, and indicative of the fact that it is indeed consisting of nanometric crystallites, with an average diameter of 2.8 nm (calculated using the Scherrer equation, with the peaks at 40° and 60°). The intensity of each peak, however, gets smaller with the increase in the Fe content, and the drop in the peaks at 21°, 39°, and 70° is attributed to the decrease of the NiO phase in NiFeO_x AGs.

Multipoint Brunauer-Emmett-Teller (BET) surface area measurements were conducted in order to measure the surface area of the newly synthesized aerogels. NiO_x AG has a surface area of 143 m² g ¹, smaller than that of the FeO_x AG (294 m² g ¹). In the mixed Ni:Fe aerogels the BET surface area increases with the increase in Fe content: it was measured as 164, 244, 241, and 617 m² g^-1 for NiFeO_x AG1-4, respectively. The surface area of aerogels is usually high, higher than in other materials, including many 2D materials. The pore size distribution was calculated with the DFT method (Fig. 3). It shows that all of the aerogels prepared in this work have nanometric-size pores, most of them with a radius of 1-8 nm. It is not quite clear where the difference in surface area comes from, but it correlates well with Ni:Fe ratio. NiFeO_x AG4 has the largest volume of small pores and NiO_x AG has the smallest amount of small pores, which also correlates with their total surface area. HAADF-STEM images of NiFeO_x AG1 shown in Fig. 4 confirm the porous structure of the aerogels in the nano- and micro-scales. Fig. 4A shows the macro-porosity of aerogels, which supports previous observations of the high surface area. It shows an intricate 3D coral-like covalent framework, as was expected from XRD observations. The higher magnification image in Fig. 4B shows the presence of mesopores within the COF network, as well as the microporosity that exists within the interconnected NiFeOOH particles which are similar in size and shape to previous mono-metallic oxide aerogels. Eayered oxyhydroxide structures, analogous to the EDH planes that are known from NiFeOOH powders, with an interlayer spacing of roughly 0.47 nm are visible throughout the image, as also shown in the cropped images in Figs. 4C,D. Attempts to perform higher resolution imaging on these features led to electron beam-induced reduction and subsequent crystallization of the material (Fig. 5).

The XPS spectra of NiFeO_x AG1-4 are shown in Figs. 6-9, for NiO_x AG in Fig. 10 and for FeO_x AG in Fig. 11. For all materials, Ni shows a doublet with two peaks at 855 and 872 eV, as well as two satellites, in agreement with the literature for NiO and confirms that Ni is in the oxidation state +2. The case is different for Fe, however. For NiFeO_x AG1, the Fe spectrum shows one doublet with peaks at 711 eV and 725 eV. It shows that Fe is in the oxidation state +3. The spectrum for NiFeO_x AG2 looks very similar to NiFeO_x AG1, and for NiFeO_x AG3 a relatively small peak that is attributed to Fe(III) is visible. For NiFeO_x AG4 and FeO_x AG the peaks have satellites and are shifted towards 709 eV, implying that there is a mix between Fe(II) and Fe(III). In NiFe mixed metal oxide compounds, like NiFeOOH, the Ni is usually in the oxidation state +2 and Fe in the oxidations state +3. The similarities here prove that the aerogels are indeed NiFe oxide materials. It is intersting to note, however, that the materials with mixed Fe states show rust color, while the materials with Fe(III) are green.

However, it can be seen that the higher the iron content, the higher the mix in the Fe oxidation, i.e. at +2 and +3 states. Fe needs to be in high oxidation states for the OER to work well, and thus these differences may result in different catalytic activity of the aerogels. It is often thought of as Fe(IV) to easier facilitate the electron transfer between Ni and O or to be the active site, as was discussed in previous works. It is easier to get to Fe(IV) if Fe starts out as Fe(III) and does not need to be oxidized from Fe(II). This suggests that aerogels consisting primarily of Fe(III), like NiFeO_x AG1, are expected to perform better as OER catalysts when compared to the other aerogels with higher Fe loadings but lower oxidation state.

Cyclic voltammetry (CV) was used to study the OER catalytic activity of the newly synthesized aerogels. Fig. 12 shows the first 10 CV cycles of NiFeO_x AG1-4 before (Fig. 12A) and after (Fig. 12B) heat-treatment in 1 M KOH at a potential window of 1.2- 1.8 V vs. RHE. The overpotential to reach 10 mA cm^-2 is 400 mV for as synthesized NiFeO_x AG1, 510 mV for NiFeO_x AG3, and 520 mV for NiO_x AG, respectively. The rest of the aerogels did not reach this current density in this potential window. The highest current density at 1.8 V vs. RHE is 67.9, 9.7, 17.7, and 7.6 mA cm^-2 for NiFeO_x AG1-4, respectively, 16.1 mA cm^-2 for NiO_x AG and 0.94 mA cm^-2 for FeO_x. The heat-treated materials show improved catalytic activity, with lower overpotentials to reach 10 mA cm' ² (380, 540, and 490 mV for NiFeO_x AGHT1-3, respectively). NiO_x AG reaches the required current density at an overpotential of 380 mV as well, but is outperformed at higher overpotentials by NiFeO_x AGHT1. NiFeO_x AGHT4 falls a little short of reaching this current density in the measured potential window. The maximal current densities measured with the aerogels at 1.8 V vs. RHE are 83.4, 13.0, 20.9, and 9.5 mA cm'² for NiFeO_x AGHT1-4, respectively, and 58.1 mA cm'² for NiO_x AGHT. FeO_x AGHT reaches only 0.63 mA cm'². The current density is higher for all heat-treated aerogels than non- heat-treated materials, with the exception of FeO_x, which is not active at all.

Except for the Ni:Fe ratio, surface area and oxidation state of Fe, only miniscule differences were observed between the aerogels. This means that these factors are chiefly responsible for the difference in activity.

The ratio of Ni:Fe plays a large role in the catalytic activity of known oxide- and oxyhydroxide-derived materials like NiFeOOH, as was shown in previous studies, and the best Fe content was found to be around 15-25 at.%. A similar trend was observed for these metal oxide aerogels, although peak performance was achieved with an Fe content of only 6 at.%. The performance at this Fe content was also higher than that of the NiO_x aerogel, confirming iron plays an important role in increasing the OER catalytic activity. The highest activity materials (NiFeO_x AG1) also contained the highest percentage of in the +3 oxidation state. This indicates catalytic activity is most heavily influenced by the Fe oxidation state, which in this case was found in the material with the lowest Fe content which is very different from the known optimum in other NiFeOOH materials. This may be the explanation for the high activity of this aerogel and the activity trend in general. NiFeO_x aerogels were studied for OER because of their high surface area, which should lead to a high utilization of the active sites. The reason for the increase in surface area with Fe content is not quite clear but in our previous work, we showed that very small differences in metal ratios can have a significant influence on the catalyst surface area.

The pore size and pore size distribution seem to play a role in the OER catalysis as well. When the surface area gets too large it is possible that mass transfer phenomena play a larger role during the catalytic process and not enough reactants reach the active sites. It is also possible that through the larger amount of nanometric pores the created O₂ molecules will be released slower and therefore the reaction will be kinetically hindered.

When compared to NiFeOOH powder, that has been considered the state-of-the- art PGM-free OER catalyst in alkaline media, NiFeO_x AG1 shows higher activity, the heat treated NiFeO_x AGHT1 is even better. The reason the heat-treatment makes the catalysts better is the absence of water in the structure, which might block catalytic sites by being bound too tightly to the metals or shielding them from the reactants. The heat- treated aerogels are pure oxides, however, and when they are immersed in the electrolyte solution they transform to hydroxides, which are easier to oxidize further to the catalytically active oxyhydroxide form than the hydrated materials. This was confirmed by the Ni redox peak before the OER onset. It is much larger in the heat-treated materials and shifted to lower potentials, as can be seen in Fig. 12A. The oxidation peak shifts from 1.46 to 1.44 V (vs. RHE) and the reduction peak from 1.35 to 1.33 V (vs. RHE). In comparison, in NiFeOOH the oxidation peak is at 1.45 V (vs. RHE) and the reduction peak is at 1.35 V (vs. RHE). This peak is much smaller, though, and not very pronounced compared to the aerogels. A magnification is shown in Fig. 13. It is much smaller because it has a much lower surface area than the porous aerogel material, and thus lower EASD. Also, it is already in the oxyhydroxide form and does not need to be oxidized further.

The comparison of all heat treated aerogels to NiFeOOH powder with the current normalized against the measured BET surface area is shown in Fig. 14. NiFeOOH shows a very high intrinsic activity as does NiFeO_x AGHT1 and NiO_x AGHT. The other materials show much lower intrinsic activity. It is not surprising that the activity of the best measured catalysts is very similar and the differences appear very small. However, NiFeO_x AG1 and NiFeO_x AGHT1 show better catalytic activity than Nio.75Feo.2sOOH in terms of overpotential and current density at high potentials (Fig. 12B) on the geometric surface area. The higher surface area and special characteristics of aerogel materials led to improved catalysts, due to a larger number of accessible active sites. Compared to commercial IrO₂ (Fig. 12B) the aerogels with the best Ni:Fe ratio also show higher catalytic activity and the heat treated NiFeO_x AGHT1 a slightly better over potential. This class of aerogels can be considered for future electrolyzer applications.

Conclusions

Mixed metal NiFeO_x aerogels and NiFeMO_x aerogels were synthesized with different metal ratios for the first time using the epoxide route synthesis and heat-treated to give anhydrous materials with a large surface area. They were tested for OER activity and are, to the best of our knowledge, the first aerogel materials that propagate OER themselves, rather than being used merely as support material for OER catalysts. The catalytic activity depends largely on the Ni:Fe ratio and not the surface area, which can lead to mass transport limitations when too high. The material with the Ni:Fe ratio 94:6 shows a highly improved catalytic activity in terms of overpotential to reach 10 mA cm' ², as well as current density at 1.8 V vs. RHE when compared to NiFeOOH with a ratio of 75:25, which is considered to be the state-of-the-art material for PGM free OER catalysts in alkaline media. This new class of OER active aerogels can open new research efforts and highly improve electrolyzer systems for green hydrogen production.

Claims

CLAIMS:

1. An electrocatalyst in a form of an aerogel consisting NiFeMOn, wherein M is optionally present and is a metal different from Fe and Ni; and wherein n is between 1 and 4.

2. The electrocatalyst according to claim 1, for use in an electrochemical process.

3. The electrocatalyst accoridng to claim 1 or 2, wherein M is absent, and the aerogel consists NiFeOn.

4. The electrocatalyst according to claim 1, wherein M is absent and the aerogel consists of NiFeOn, NiFeOOH, or NiFeOH, wherein the molar amount of Fe relative to Ni is not greater than 20%, and wherein n is between 1 and 4.

5. An electrocatalyst in a form of an aerogel consisting NiFeOn, NiFeOOH, or NiFeOH, wherein the molar amount of Fe relative to Ni is not greater than 20%, and wherein n is 1 or 2.

6. The electrocatalyst according to claim 1, wherein M is present.

7. The electrocatalyst according to claim 1, wherein M is present and the aerogel consists NiFeMOH, NiFeMOn or NiFeMOOH, wherein M is a metal and wherein n is 1 or 2.

8. The electrocatalyst according to any one of claims 1 to 7, wherein M is a metal selected amongst transition metals and non-transition metals.

9. The electrocatalyst according to claim 8, wherein the metal is selected from titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), cobalt (Co), copper (Cu), molybdenum (Mo), ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), tungsten (W), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), and gold (Au).

10. The electrocatalyst according to claim 8 or 9, wherein M is Mo, Ti or Co.

11. The electrocatalyst according to claim 1, wherein the aerogel consists of NiFeMOH, NiFeMOn or NiFeMOOH, wherein M is Ti, Mo or Co, and wherein n is 1 or

2.

12. The electrocatalyst according to claim 1, wherein the aerogel consists NiFeCoOH, or NiFeCoOOH, NiFeTiOH, or NiFeTiOOH.

13. The electrocatalyst according to any one of the preceding claims provided as a powder.

14. The electrocatalyst according to claim 13, being a surface coated with the powder.

15. A powder electrocatalyst aerogel comprising or consisting of NiFeMOOH, NiFeMOn or NiFeMOH, wherein M is a metal optionally present, and wherein n is between 1 and 4.

16. The powder according to claim 15, wherein M is absent and the aerogel is NiFeOOH, NiFeOn or NiFeOH, wherein n is 1 or 2.

17. The powder according to claim 15 or 16 for use in forming an electrocatalyst surface or film.

18. An electrocatalyst film of an aerogel comprising or consisting a material of the form NiFeMOH, NiFeMOn or NiFeMOOH, wherein M is a metal optionally present, and wherein n is 1 or 2.

19. The film according to claim 18, wherein the aerogel comprises or consists NiFeOOH, NiFeOn or NiFeOH, wherein n is 1 or 2.

20. The film according to claim 18, wherein the aerogel comprises or consists NiFeMOOH or NiFeMOH, wherein M is Mo, Ti or Co.

21. An aerogel consisting NiFeMOn, wherein M is a metal different from Fe and Ni; and wherein n is between 1 and 4.

22. The aerogel according to claim 21, consisting NiFeMOOH, NiFeMOn or NiFeMOH, wherein M is a transition metal selected from titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), cobalt (Co), copper (Cu), molybdenum (Mo), ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), tungsten (W), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), and gold (Au), and wherein n is 1 or 2.

23. The aerogel according to claim 21, wherein the aerogel consists NiFeCoOOH, NiFeTiOOH, NiFeCoOH or NiFeTiOH.

24. The aerogel according to any one of claims 21 to 23, being a hierarchical structure composed of micro-pores, meso-pores and macro-pores.

25. The aerogel according to any one of claims 21 to 24, having a porosity of about 60 percent or more and a specific surface area of at least 60 m²/g or more.

26. The aerogel according to any one of claims 21 to 25, formed by a method comprising:

-forming or obtaining a gel of at least one Ni salt/complex, at least one Fe salt/complex, optionally at least one salt/complex of a metal M, at least one precursor of a gelation material and a solvent under conditions permitting formation of a network of interconnected pores, said pores comprising the solvent; and -removing the solvent under drying conditions to form the aerogel without causing contraction of the interconnected pores.

27. The aerogel according to claim 26, wherein the drying conditions are supercritical conditions.

28. The aerogel according to claim 26, wherein each of the at least one Ni salt/complex and the at least one Fe salt/complex is provided in an amount sufficient to provide an aerogel wherein the molar amount of Fe relative to Ni is not greater than 20%.

29. The aerogel accoridng to cairn 26, wherein the conditions permitting formation of a network of interconnected pores comprises stirring at room temperature (15-33°C).

30. The aerogel according to claim 26, wherein the method comprises forming a solution of the at least one Ni salt/complex, the at least one Fe salt/complex, optionally the at least one salt/complex of a metal M, the at least one precursor of a gelation material and the solvent.

31. The aerogel according to claim 26, wherein the method comprises thermal treatment of the solution to a temperature between room temperature and 80°C, and heat treatment of the aerogel after its formation at a temperature ranging from room temperature and 900°C.

32. The aerogel according to any one of claims 26 to 31, wherein the metal salt or complex is a metal halide or a metal oxide.

33. The aerogel according to claim 32, wherein the metal salt or complex is a hydrate form of the salt or complex.

34. The aerogel according to claim 32 or 33, wherein salt or complex is one or more of a Ni halide or Ni oxide, a Fe halide or Fe oxide and a halide or oxide of the metal M.

35. The aerogel according to claim 34, wherein the Ni halide is NiCl₂, and the Fe halide is FeCl₂.

36. The aerogel according to any one of claims 26 to 35, the method comprising:

-forming a gel of NiCl₂ dH₂O, FcCF' H₂O, a hydrate halide of a transition metal, at least one precursor of a matrix material and ethanol under conditions permitting formation of a network of interconnected pores, said pores comprising the solvent; and

-removing the solvent under supercritical drying conditions to form the aerogel without causing contraction of the interconnected pores.

37. The aerogel according to any one of claims 26 to 36, wherein the at least one precursor of the gelation material is a monomer, an oligomer or a prepolymer of a polyol.

38. The aerogel according to claim 37, wherein the precursor material is propylene oxide.

39. The aerogel according to any one of claims 21 to 38, or an electrocatalyst according to any one of claims 1 to 14 for forming an electrode for electrocatalytic reactions.

40. The aerogel according to claim 39, for use as an anodic oxygen evolution reaction (OER).

41. An electrode material comprising or consisting an aerogel according to any one of claims 21 to 40.

42. An electrolyzer implementing an electrode of an electrode material according to claim 41.

43. An electrocatalytic device comprising a porous electrode material according to claim 41, wherein the device performs an electrocatalytic process.

44. The electrocatalytic device according to claim 43, wherein the process is water splitting.

45. The electrocatalytic device according to claim 43, wherein the process is generation of hydrogen and/or oxygen from water.

46. The electrocatalytic device according to claim 43, comprising an electrocatalyst in an aerogel form consisting NiFeOOH, NiFeOn, NiFeOH, NiFeCoOOH, NiFeCoOH, NiFeTiOOH, NiFeTiOH, NiFeMOH and NiFeMOOH, wherein M is a metal different from Ni, Fe, Co and Ti and wherein n is 1 or 2.

47. An electrocatalyst in an aerogel form comprising or consisting NiFeOOH, NiFeOn, NiFeOH, NiFeCoOOH, NiFeCoOH, NiFeTiOOH, NiFeTiOH, NiFeMOH and NiFeMOOH, wherein M is a metal different from Ni, Fe, Co and Ti and wherein n is 1 or

2.

48. An electrocatalyst for generating oxygen, the electrocatalyst being according to claim 47.

49. The electrocatalyst according to claim 47 or 48, wherein the molar amount of Fe relative to Ni is no greater than 20%.

50. A porous material comprising an aerogel consisting a mixed metal oxide of iron and nickel, wherein the amount of iron relative to nickel in the mixed metal oxide is not greater than 20 wt%

51. An electrocatalytic process comprising use of an electrocatalyst according to any one of claims 1 to 15 or an aerogel accoridng to any one of claims 21 to 40.

52. The electrocatalytic process according to claim 51 , wherein the process comprises a hydrogen oxidation reaction (HOR), a hydrogen evolution reaction (HER), an oxygen reduction reaction (ORR), or an oxygen evolution reaction (OER).