BIMETALLIC IRON-VANADIUM OXIDE ELECTROCATALYST FOR OXYGEN EVOLUTION REACTION

Description

STATEMENT REGARDING PRIOR DISCLOSURE BY THE INVENTORS

Aspects of the present disclosure are described in Ehsan, M. A., et al., “Active Sites Engineered Bimetallic Iron-Vanadium Oxide (FeVO_x) Thin Film Electrocatalyst for Efficient and Sustainable Water Oxidation” published in Issue 8, Energy & Fuels, which is incorporated herein by reference in its entirety.

STATEMENT OF ACKNOWLEDGEMENT

Support provided by the Interdisciplinary Research Center for Hydrogen and Energy Storage, King Fahd University of Petroleum and Minerals, Saudi Arabia, is gratefully acknowledged.

BACKGROUND

Technical Field

The present disclosure is directed towards oxygen evolution reaction (OER), more particularly directed towards an electrode including a bimetallic iron-vanadium oxide electrocatalyst for OER.

Description of Related Art

The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. The work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.

As an energy source, clean hydrogen has potential to replace current fossil fuel energy sources. Natural gas is currently the largest source of hydrogen production; however, high CO₂ emissions limit further use of natural gas for hydrogen production until appropriate carbon capture and sequestration technology is coupled to bring the process closer to having net zero emissions. Hydrogen production through electrochemical water splitting is an alternative process that is gaining popularity as it can have net zero emissions and can produce hydrogen in large quantities without altering the climate.

The process of water electrolysis consists of two components commonly known as a hydrogen evolution reaction (HER) and an oxygen evolution reaction (OER). The slow reaction kinetics and large overpotential required to dissipate four electrons during the OER have become a bottleneck and reduce the overall performance of the water-splitting process. Numerous electrocatalysts have been developed and evaluated to accelerate OER catalysis, but the benchmark noble metals (IrO₂ and RuO₂) still dominate the field and exhibit elevated catalytic activity; however, noble metals are expensive, low in abundance, and are weakly stable in certain electrolytes, which limit them to commercial use. Development of highly active, stable, and economically viable OER catalysts remain an expanding area.

To meet this need, strategies such as tailoring catalyst composition and electronic structure to achieve enhanced catalytic performance have been widely practiced. First-row transition metal elements in the form of bi-multimetallic combinations have been studied to design geometric and electronic structures, resulting in a large class of OER catalysts with advanced OER performance. A variety of nanomaterials based on oxides, (oxy)hydroxides, chalcogenides, phosphides, etc., of the first transition metal series (Mn, Fe, Co, Ni, and Cu) have been tested for OER electrocatalysis in alkaline electrolytes. Ni—Fe-based bimetallic nanomaterials have shown promising OER activity and stability in alkaline electrolytes [Ehsan, M. A. et al., Rapid synthesis of nickel-iron-oxide (NiFeO_x) solid-solution nano-rods thin films on nickel foams as advanced electrocatalyst for sustained water oxidation, Materials Today Sustainability, 2023, 23, 100451]. In contrast, electrocatalysts containing early transition metals (Ti, V, Cr) have been studied less for water splitting.

Vanadium (V) containing electrocatalysts, including vanadium oxide and vanadium oxide derivative electrocatalysts, have received attention for OER catalysis due to the abundance, low cost, multiple oxidation states (V³⁺, V⁴⁺, and V⁵⁺), elevated energy capacity, and coordination chemistry of vanadium. Vanadium electrocatalyst are considered options to replace nickel and cobalt electrocatalysts for water splitting to produce oxygen. There are still challenges in increasing OER performances of V-based electrocatalysts due to vanadium oxide not being very conductive and requiring high overpotentials with low production efficiency. Appropriate strategies have been formulated to improve intrinsic activity and catalytic OER performances. For instance, incorporation of secondary metallic elements, including Ni, Fe, Co, etc., into vanadium oxide may promote a desired electronic structure modulation and may increase adsorption of reaction intermediates. An amorphous cobalt vanadium oxide (CoVO_x) electrocatalyst produced through hydrothermal route used a small overpotential (293 mV) to reach a current decade (10 mA cm⁻²) [Liardet, L. and Hu, X., Amorphous cobalt vanadium oxide as a highly active electrocatalyst for oxygen evolution, ACS catalysis, 2018, 8 (1), 644-650]. A hierarchical hollow FeV composite successfully executed OER with an overpotential of 390 mV at 10 mAcm⁻², a Tafel slope of 36.7 mV dec⁻¹, and durability.

A bimetallic vanadium iron metal-organic framework (VFe-MOF) nanoarray exhibited good durability and electrocatalytic activity, which was attributed to a synergistic cooperation and topographic control between the iron and vanadium metals [Han, L. et al., High-performance electrocatalyst of vanadium-iron bimetal organic framework arrays on nickel foam for overall water splitting, Chinese Chemical Letters, 2021, 32 (7), 2263-2268]. A scalloped shaped nickel/iron vanadium oxide coated vanadium dioxide electrocatalyst (denoted as VO₂@-NFVO) was fabricated by a urea-assisted chemical etching process in the chemical environment of Fe²⁺ and Ni²⁺ cations [Ma, Y. et al., Scalloped nickel/iron vanadium oxide-coated vanadium dioxides based on chemical etching-induced reconstruction strategy for efficient oxygen evolution, International Journal of Hydrogen Energy, 2022, 47 (78), 33352-33360]. The chemical etching process created numerous catalytic sites on the VO₂ surface where Ni²⁺/Fe²⁺ were coordinated to produce a new super-active phase with enhanced intrinsic activity of the resulting catalyst. Electrochemical study revealed that VO₂@NFVO exhibited an overpotential of 290 mV at 10 mA cm⁻² and stability in a 1 M KOH electrolyte. Iron vanadium sulfide electrocatalysts (Fe_1.94V_1.06S₄ and FeV₂S₄) on nickel foam were developed with large current densities of 500 and 1000 mA/cm² at low overpotentials of 231 and 248 mV, respectively, in an alkaline electrolyte [Wu, C. et al., Highly efficient and stable vanadium-based electrocatalysts: Stoichiometric iron vanadium sulfides for water-oxidation at large current densities, Chemical Engineering Journal, 2023, 146981].

Despite elemental selection and composition of the catalyst, the manufacturing process and the resulting morphology also influence the catalytic efficiency. Emphasis has been placed on developing binder-free thin film electrocatalysts instead of powder catalysts because the incorporation of a binder removes active sites of the catalyst and reduces conductivity. Thin film electrocatalysts fabricated via chemical vapor deposition (CVD) provide appreciable control over morphology, and a highly aligned nanostructure arranged directly on a surface substrate promotes charge transport during electrochemical reactions, thereby reducing overpotentials for OER. Thin-film electrocatalysts of VO_x and Fe₃O₄ were developed and supported on porous nickel foam using the aerosol-assisted chemical vapor deposition (AACVD) process with both catalysts showing good OER characteristics and prolonged durability in alkaline environments [Ehsan, M. A. at al, Aerosol-assisted chemical vapour deposited vanadium oxide thin films on nickel foam with auspicious electrochemical water oxidation properties, International Journal of Hydrogen Energy, 2023; and Ehsan, M. A. and Babar, N.-U.-A., Straightforward Preparation of Fe-Based Electrocatalytic Films at Various Substrates for IrO₂-like Water Oxidation Activity, Energy & Fuels 2023, 37 (5), 3934-3941]. Thin film electrocatalyst grown on 3D textured, porous, and conductive nickel foam provides a large surface area, greater distribution of catalysts, and abundant exposed active sites, which may enhance the rate of OER and facilitate mass transport and gas diffusion.

Although several materials have been developed for OER, there still exists a need to fabricate and explore bimetallic electrocatalysts for efficient OER. An object of the present disclosure is to provide a bimetallic iron-vanadium oxide electrocatalyst for OER that may circumvent drawbacks of the present art.

SUMMARY

In an exemplary embodiment, an electrocatalyst is described. The electrocatalyst includes a first layer including a porous nickel foam and a second layer including an iron-vanadium oxide (FeVO_x). The iron-vanadium oxide includes an iron oxide and a vanadium oxide. The iron-vanadium oxide contains 10 to 30 atomic percent (at. %) iron and 15 to 30 at. % vanadium based on a total number of atoms in the iron-vanadium oxide. The second layer includes iron-vanadium oxide particles having a longest dimension of 0.5 to 5 micrometers (μm).

In another exemplary embodiment, a method of making the electrocatalyst is described. The method includes dissolving an iron salt and a vanadium salt in a polar organic solvent to form a solution and depositing the solution as an aerosol mist via an aerosol-assisted chemical vapor deposition process on the porous nickel foam to form the electrocatalyst. The method of depositing the solution occurs at a temperature of 450 to 500 degrees Celsius (° C.) for 30 to 150 minutes (mins).

In some embodiments, a weight ratio of the iron salt to the vanadium salt is from 2:1 to 1:2.

In some embodiments, the particles of iron-vanadium oxide in the second layer include nanocrystallites having a longest dimension of 30 to 90 nanometers (nm).

In some embodiments, a plurality of the nanocrystallites is stacked.

In some embodiments, the iron oxide forms hematite (Fe₂O₃) crystals in a trigonal phase.

In some embodiments, the vanadium oxide is a vanadium(II) oxide (VO).

In some embodiments, the iron-vanadium oxide does not contain V₂O₃ phases, V₂O₅ phases, and Fe₃O₄ phases.

In some embodiments, the second layer is a continuous particle layer formed of the iron-vanadium oxide particles with active sites on an exposed surface.

In some embodiments, the electrocatalyst has an onset potential of 1.4 to 1.6 volts (V) vs. reversible hydrogen electrode (RHE).

In some embodiments, the electrocatalyst has an overpotential of 270 to 280 mV at a current density of 10 milliamperes per centimeters square (mA/cm²).

In some embodiments, the electrocatalyst has a Tafel slope of 48 to 52 millivolt/decade (mV/dec).

In some embodiments, the electrocatalyst has a mass activity of 13,000 to 13,500 milliamperes per milligram (mA/mg) at an overpotential of 350 mV.

In some embodiments, the electrocatalyst has an electrochemically active surface area of 80 to 90 square centimeters (cm²).

In some embodiments, the electrocatalyst has a specific activity of 10 to 15 mA/cm².

In some embodiments, the electrocatalyst has a charge transfer resistance of 0.8 to 0.9 ohms (Q).

In some embodiments, the electrocatalyst has a turnover frequency of 1.5 to 2.5 s⁻¹ at an overpotential of 350 mV.

In some embodiments, the electrocatalyst has a stability of 90 to 110 hours (h) at a current density of 10 mA/cm² and 50 mA/cm².

In some embodiments, the electrocatalyst has 0.05 to 0.10 milligrams (mg) of iron-vanadium oxide in the second layer.

In yet another exemplary embodiment, a method of oxygen evolution is described. The method includes connecting a working electrode, a reference electrode, and a counter electrode with a potentiostat. The working electrode is the electrocatalyst, the reference electrode is a mercury/mercurous oxide (Hg/HgO) electrode, and the counter electrode is a platinum electrode. The method further includes contacting the working electrode, the reference electrode, and the counter electrode with an aqueous solution. The aqueous solution is an alkali metal salt and water. The method further includes applying a potential and generating oxygen at the working electrode.

These and other aspects of the non-limiting embodiments of the present disclosure will become apparent to those skilled in the art upon review of the following description of specific non-limiting embodiments of the disclosure in conjunction with the accompanying drawings. The foregoing general description of the illustrative present disclosure and the following detailed description thereof are merely exemplary aspects of the teachings of this disclosure and are not restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of this disclosure (including alternatives and/or variations thereof) and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1A is a method flowchart for making an electrocatalyst, according to certain embodiments.

FIG. 1B is a method flowchart for an oxygen evolution reaction (OER), according to certain embodiments.

FIG. 1C is a schematic representation of an aerosol-assisted chemical vapor deposition (AACVD) process to produce a bimetallic iron-vanadium oxide electrocatalyst (FeVO_x), according to certain embodiments.

FIG. 1D shows X-ray diffraction (XRD) patterns of FeVO_x grown on a non-crystalline glass substrate for 40 minutes (min), 80 min, and 120 min, according to certain embodiments.

FIG. 2A is a field emission scanning electron microscopy (FESEM) image of FeVO_x prepared for 40 min (FeVO_x-40) at a scale of 20 micrometers (μm), according to certain embodiments.

FIG. 2B is an FESEM image of FeVO_x-40 at a scale of 5 μm, according to certain embodiments.

FIG. 2C is an FESEM image of FeVO_x prepared for 80 min (FeVO_x-80) at a scale of 20 μm, according to certain embodiments.

FIG. 2D is an FESEM image of FeVO_x-80 at a scale of 5 μm, according to certain embodiments.

FIG. 2E is an FESEM image of FeVO_x prepared for 120 min (FeVO_x-120) at a scale of 20 μm, according to certain embodiments.

FIG. 2F is an FESEM image of FeVO_x-120 at a scale of 5 μm, according to certain embodiments.

FIG. 2G is an energy-dispersive X-ray (EDX) spectrum of FeVO_x-40, according to certain embodiments.

FIG. 2H is an EDX spectrum of FeVO_x-80, according to certain embodiments.

FIG. 2I is an EDX spectrum of FeVO_x-120, according to certain embodiments.

FIG. 2J shows EDX elemental mapping images for iron (Fe), vanadium (V), and oxygen (O) for FeVO_x-40, according to certain embodiments.

FIG. 2K shows EDX elemental mapping images for Fe, V, and O for FeVO_x-80, according to certain embodiments.

FIG. 2L shows EDX elemental mapping images for Fe, V, and O for FeVO_x-120, according to certain embodiments.

FIG. 3A is a transmission electron microscopy (TEM) image of FeVO_x-40 at a scale of 500 nm (low magnification), according to certain embodiments.

FIG. 3B is a TEM image of FeVO_x-40 at a scale of 100 nm (high magnification), according to certain embodiments.

FIG. 3C is an image showing a high-resolution transmission electron microscopy (HRTEM) image of FeVO_x-40 at a scale of 5 nm showing lattice fringes of Fe₂O₃ and VO phases, according to certain embodiments.

FIG. 3D is a selected area electron diffraction (SAED) pattern of FeVO_x-40 showing polycrystalline diffraction patterns of Fe₂O₃ and VO phases, according to certain embodiments.

FIG. 4A is an X-ray photoelectron spectroscopy (XPS) survey scan spectrum of FeVO_x-40, according to certain embodiments.

FIG. 4B is a high-resolution XPS spectrum of Fe 2p in FeVO_x-40, according to certain embodiments.

FIG. 4C is a high-resolution XPS spectrum of V 2p in FeVO_x-40, according to certain embodiments.

FIG. 4D is a high-resolution XPS spectrum of O is in FeVO_x-40, according to certain embodiments.

FIG. 5A is a cyclic voltammogram (CV) comparison of the 1^st and 40^th CVs for FeVO_x-40 at a scan speed of 50 mV s⁻¹ in 1.0 M KOH electrolyte, according to certain embodiments.

FIG. 5B is a CV comparison of the 1^st and 40^th CVs for FeVO_x-80 at a scan speed of 50 mV s⁻¹ in 1.0 M KOH electrolyte, according to certain embodiments.

FIG. 5C is a CV comparison of 1^st and 40^th CVs for FeVO_x-120 at a scan speed of 50 mV s⁻¹ in 1.0 M KOH electrolyte, according to certain embodiments.

FIG. 6A is a comparison of the 40^th CVs for FeVO_x-40, FeVO_x-80, FeVO_x-120, and bare nickel foam (NF) in 1.0 M KOH electrolyte, according to certain embodiments.

FIG. 6B is a magnified view of the 40^th CVs for FeVO_x-40, FeVO_x-80, FeVO_x-120, and bare NF in 1.0 M KOH electrolyte, according to certain embodiments.

FIG. 6C shows a comparison of overpotential for FeVO_x-40, FeVO_x-80, and FeVO_x-120 in 1.0 M KOH electrolyte, according to certain embodiments.

FIG. 6D shows a plot of Tafel slopes for FeVO_x-40, FeVO_x-80, and FeVO_x-120, according to certain embodiments.

FIG. 6E shows a plot of mass activity values for FeVO_x-40, FeVO_x-80, and FeVO_x-120, according to certain embodiments.

FIG. 6F shows a plot of electrochemical active surface area (ECSA) and specific activity for FeVO_x-40, FeVO_x-80, and FeVO_x-120, according to certain embodiments.

FIG. 6G shows CVs for FeVO_x-40 with varying scan rates obtained in a non-Faradaic region, according to certain embodiments.

FIG. 6H shows CVs for FeVO_x-80 with varying scan rates obtained in a non-Faradaic region, according to certain embodiments.

FIG. 6I shows CVs for FeVO_x-120 with varying scan rates obtained in a non-Faradaic region, according to certain embodiments.

FIG. 6J is a plot of ECSA measurements for FeVO_x-40 obtained in a non-Faradaic region, according to certain embodiments.

FIG. 6K is a plot of ECSA measurements for FeVO_x-80 obtained in a non-Faradaic region, according to certain embodiments.

FIG. 6L is a plot of ECSA measurements for FeVO_x-120 obtained in a non-Faradaic region, according to certain embodiments.

FIG. 7A is an electrochemical impedance spectroscopy (EIS) Nyquist plot for FeVO_x-40, FeVO_x-80, and FeVO_x-120, according to certain embodiments.

FIG. 7B is a plot of turnover frequency (TOF) for FeVO_x-40, FeVO_x-80, and FeVO_x-120, according to certain embodiments.

FIG. 7C is a chronopotentiometry (CP) response plot of FeVO_x-40 at 10 and 50 mA cm⁻² in 1.0 M KOH electrolyte solution, according to certain embodiments.

FIG. 7D shows polarization curves of FeVO_x-40 before and after CP test, according to certain embodiments.

FIG. 8A is a scanning electron microscopy (SEM) image of FeVO_x-40 at a scale of 20 μm after OER measurements, according to certain embodiments.

FIG. 8B is an SEM image of FeVO_x-40 at a scale of 10 μm after OER measurements, according to certain embodiments.

FIG. 8C is an SEM image of FeVO_x-40 at a scale of 5 μm after OER measurements, according to certain embodiments.

FIG. 8D is an EDX spectrum of FeVO_x-40 after OER measurements, according to certain embodiments.

FIG. 9A is a high-resolution XPS spectrum of V 2p in FeVO_x-40 after OER measurements, according to certain embodiments.

FIG. 9B is a high-resolution XPS spectrum of Fe 2p in FeVO_x-40 after OER measurements, according to certain embodiments.

FIG. 9C is a high-resolution XPS spectrum of O is in FeVO_x-40 after OER measurements, according to certain embodiments.

DETAILED DESCRIPTION

When describing the present disclosure, the terms used are to be construed in accordance with the following definitions, unless a context dictates otherwise.

Embodiments of the present invention will now be described more fully hereinafter with reference to the accompanying drawings wherever applicable, in that some, but not all embodiments of the disclosure are shown.

In the following description, it is understood that other embodiments may be utilized, and structural and operational changes may be made without departure from the scope of the present embodiments disclosed herein.

Reference will now be made to specific embodiments or features, examples of which are illustrated in the accompanying drawings. In the drawings, whenever possible, corresponding or like reference numerals will be used to designate identical or corresponding parts throughout the several views. Moreover, references to various elements described herein are made collectively or individually when there may be more than one element of the same type. However, such references are merely exemplary in nature. It may be noted that any reference to elements in the singular may also be constructed to relate to the plural and vice-versa without limiting the scope of the disclosure to the exact number or type of such elements unless set forth explicitly in the appended claims. Further, as used herein, the words “a,” “an,” and the like generally carry a meaning of “one or more,” unless stated otherwise.

Furthermore, the terms “approximately,” “approximate,” “about,” and similar terms generally refer to ranges that include the identified value within a margin of 20%, 10%, or preferably 5%, and any values therebetween.

The use of the terms “include,” “includes,” “including,” “have,” “has,” or “having” should be generally understood as open-ended and non-limiting unless specifically stated otherwise.

As used herein, “nanoparticles” are particles having a particle size of 1 nm to 500 nm, 1 nm to 200 nm, or 1 nm to 100 nm in diameter.

As used herein, “particle size” and “pore size” refers to the lengths or longest dimensions of a particle and of a pore opening, respectively.

As used herein, the term “room temperature” refers to a temperature range of 25° C.±3° C.

As used herein, the term “electrode” refers to an electrical conductor used to make contact with a non-metallic part of a circuit, such as a semiconductor, an electrolyte, a vacuum, or air.

As used herein, “working electrode” refers to the electrode in an electrochemical cell/device/sensor on which the electrochemical reaction of interest is occurring.

As used herein, “counter electrode” also known as an auxiliary electrode refers to an electrode used in an electrochemical cell for voltametric analysis or other reactions in which an electric current is expected to flow. The counter electrode, along with the working electrode, allows the circuit for which current is applied and/or measured to be complete.

As used herein, the term “current density” refers to the total amount of electric current flowing through one unit value of a cross-sectional area.

As used herein, the term “Tafel slope” refers to the relationship between the overpotential and the logarithmic current density of an electrochemical reaction.

As used herein, the term “electrochemical cell” refers to a device capable of either generating electrical energy from chemical reactions occurring in it or using electrical energy to facilitate chemical reactions in it.

As used herein, the term “aerosolizing” refers to a process of converting a solution or colloidal suspension into a spray.

As used herein, the term “aerosol” refers to small solid particles or liquid droplets suspended in the atmosphere, air, or a gas.

As used herein, the term “water splitting” refers to the chemical reaction in which water is broken down into oxygen and hydrogen, 2H₂O→2H₂+O₂.

As used herein, the term “overpotential” refers to the difference in potential that exists between a thermodynamically determined reduction potential of a half-reaction and the potential at which the redox event is experimentally observed. The term is associated with an electrochemical cell's voltage efficacy. In an electrochemical cell, the occurrence of an overpotential implies that the cell needs more energy as compared to that which is thermodynamically expected to drive a reaction. The quantity of overpotential is specific to each cell design and varies across cells and operational conditions, even for the same reaction. Overpotential is experimentally measured by determining the potential at which a given current density is reached.

The present disclosure is intended to include all hydration states of a given compound or formula, unless otherwise noted or when heating a material.

In addition, the present disclosure is intended to include all isotopes of atoms occurring in the present compounds and complexes. Isotopes include those atoms having the same atomic number but different mass numbers. By way of general example, and without limitation, isotopes of hydrogen include deuterium and tritium. Isotopically labeled compounds of the disclosure may generally be prepared by conventional techniques known to those skilled in the art or by processes analogous to those described herein, using an appropriate isotopically labeled reagent in place of the non-labeled reagent otherwise employed.

Aspects of the present disclosure are directed to a bimetallic electrocatalyst, including an iron-vanadium composite oxide, Fe₂O₃—VO_x (FeVO_x) deposited on a nickel foam (NF) substrate via aerosol-assisted chemical vapor deposition (AACVD) for robust and efficient catalysis of oxygen evolution reaction (OER).

An electrocatalyst is described. The electrocatalyst includes a first layer, including a porous nickel foam, and a second layer, including iron and vanadium (FeVO_x) oxide. The iron-vanadium oxide includes an iron oxide and a vanadium oxide. In some embodiments, the vanadium oxide is a vanadium(II) oxide (VO). In some embodiments, the iron oxide forms hematite (Fe₂O₃) crystals in a trigonal phase. In some embodiments, the iron-vanadium oxide does not contain V₂O₃ phases, V₂O₅ phases, and Fe₃O₄ phases.

In some embodiments, the iron-vanadium oxide contains 10-30 atomic percent (at. %) iron, preferably 11-29 at. % iron, preferably 12-28 at. % iron, preferably 13-27 at. % iron, preferably 14-26 at. % iron, preferably 15-25 at. % iron, preferably 16-24 at. % iron, preferably 17-23 at. % iron, preferably 18-22 at. % iron, and preferably 19-21 at. % iron based on a total number of atoms in the iron-vanadium oxide. In some embodiments, the iron-vanadium oxide contains 15-30 at. % vanadium, preferably 16-29 at. % vanadium, preferably 17-28 at. % vanadium, preferably 18-27 at. % vanadium, preferably 19-26 at. % vanadium, preferably 20-25 at. % vanadium, preferably 21-24 at. % vanadium, and preferably 22-23 at. % vanadium based on the total number of atoms in the iron-vanadium oxide.

In some embodiments, the second layer is a continuous particle layer formed of iron-vanadium oxide particles with active sites on an exposed surface. The second layer includes iron-vanadium oxide particles having the longest dimension of 0.5-5 μm, preferably 0.5-4.5 μm, preferably 1-4 μm, preferably 1.5-3.5 μm, and preferably 2-3 μm. In some embodiments, the particles of iron-vanadium oxide in the second layer include nanocrystallites having the longest dimension of 30-90 nm, preferably 35-85 nm, preferably 40-80 nm, preferably 45-75 nm, preferably 50-70 nm, and preferably 55-65 nm. In some embodiments, the nanocrystallites are stacked. In other embodiments, the nanocrystallites are agglomerated. In some embodiments, the electrocatalyst has 0.05-0.10 mg of iron-vanadium oxide in the second layer, preferably 0.06-0.09 mg of iron-vanadium oxide in the second layer, and preferably 0.07-0.08 mg of iron-vanadium oxide in the second layer. In some embodiments, the iron-vanadium oxide particles may be in the shape of spheres, cubes, rectangles, oblong spheres, pyramids, rhombohedrons, rods, and any shape known in the art.

FIG. 1A illustrates a flow chart of a method 50 of making the electrocatalyst. The order in which the method 50 is described is not intended to be construed as a limitation, and any number of the described method steps can be combined in any order to implement the method 50. Additionally, individual steps may be removed or skipped from the method 50 without departing from the spirit and scope of the present disclosure.

At step 52, the method 50 includes dissolving an iron salt and a vanadium salt in a polar organic solvent to form a solution. In some embodiments, the weight ratio of the iron salt to the vanadium salt is from 2:1-1:2, preferably 1.5:1-1:1.5, and more preferably about 1:1. In a preferred embodiment, the weight ratio of the iron salt to the vanadium salt is 1:1. Suitable examples of iron salts include, but are not limited to, iron bromide, iron chloride, iron phosphate hydrate, iron phosphate tetrahydrate, iron chloride hydrate, iron chloride tetrahydrate, iron fluoride, ammonium iron sulfate hexahydrate, iron citrate tribasic monohydrate, iron gluconate dehydrate, iron pyrophosphate, iron phthalocyanine, iron phthalocyanine chloride, ammonium iron citrate, ammonium iron sulfate, ammonium iron sulfate, ammonium iron sulfate dodecahydrate, iron chloride, iron bromide, iron chloride hexahydrate, ferric citrate, iron fluoride, iron nitrate nonahydrate, iron oxide, iron phosphate, iron sulfate hydrate, iron gluconate hydrate, iron iodide, iron lactate hydrate, iron oxalate dehydrate, ferrous sulfate heptahydrate, iron sulfide, iron acetate, iron fluoride tetrahydrate, iron iodide tetrahydrate, iron perchlorate hydrate, iron acetylacetonate, iron acetylacetonate, iron ascorbate or its hydrate, or mixtures thereof, and the like. In a preferred embodiment, the iron salt is iron(III) acetylacetonate (Fe(acac)₃).

Suitable examples of vanadium salts include, but are not limited to, vanadyl oxalate, vanadium oxide, vanadium pentoxide, vanadium monoethanolamine, vanadium chloride, vanadium trichloride, vanadyl sulfate, vanadium antimonite, vanadium antimonate, vanadium oxyacetylacetonate, or hydrate, or mixtures thereof, and the like. In a preferred embodiment, the vanadium salt is vanadium(IV) oxyacetylacetonate (VO(acac))₂.

An organic solvent is a carbon-based substance employed for the dissolution of other substance(s). Suitable examples of organic solvents include, but are not limited to, methanol, ethanol, acetone, dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide, isopropanol, benzene, hexane, carbon tetrachloride, toluene, diethyl ether, chloroform, mixtures thereof, and the like. In a preferred embodiment, the polar organic solvent is methanol. In a preferred embodiment, the dissolution may be carried out manually, via stirring, via sonication, or any other methods known in the art. The mixing is carried out until the particles are dissolved in the solvent and a homogenous solution is obtained.

At step 54, the method 50 includes depositing the solution as an aerosol mist via an aerosol-assisted chemical vapor deposition (AACVD) process on the porous nickel foam to form the electrocatalyst. The solution is deposited on the nickel foam in the form of an aerosol mist. The aerosol mist is obtained by aerosolizing the solution using the AACVD process, which is carried out using methods known in the art. In some embodiments, the solution may be deposited on other substrates such as, but not limited to, nickel foam, titanium foam, titanium alloy foam, aluminum alloy foam, magnesium alloy foam, nickel alloy foam, steel foam, mixtures thereof, and the like. In some embodiments, the deposition occurs at a temperature of 450-500° C., preferably 455-495° C., preferably 460-490° C., more preferably 465-485° C., and yet more preferably 470-480° C. In a preferred embodiment, the deposition occurs at a temperature of about 475° C. In some embodiments, the deposition occurs for 30-150 minutes (min), preferably 40-140 min, preferably 50-130 min, preferably 60-120 min, preferably 70-110 min, and preferably 80-100 min. In some embodiments, the deposition occurs for 40 minutes, 80 minutes, and 120 minutes.

FIG. 1B illustrates a flow chart of a method 70 of oxygen evolution. The order in which the method 70 is described is not intended to be construed as a limitation, and any number of the described method steps can be combined in any order to implement the method 70. Additionally, individual steps may be removed or skipped from the method 70 without departing from the spirit and scope of the present disclosure.

At step 72, the method 70 includes connecting a working electrode, a reference electrode, and a counter electrode with a potentiostat. The working electrode is the bimetallic iron-vanadium oxide (FeVO_x) electrocatalyst. A reference electrode is an electrode that has a stable and well-known electrode potential. The stability of an electrode potential in a reference electrode may be reached by employing a redox system with constant (buffered or saturated) concentrations of each relevant species of the redox reaction. A reference electrode may enable a potentiostat to deliver a stable voltage to the working electrode or the counter electrode. In some embodiments, the reference electrode may be a standard hydrogen electrode (SHE), a normal hydrogen electrode (NHE), a reversible hydrogen electrode (RHE), a saturated calomel electrode (SCE), a copper-copper(II) sulfate electrode (CSE), a silver chloride electrode (Ag/AgCl), a pH-electrode, a palladium-hydrogen electrode, a dynamic hydrogen electrode (DHE), a mercury-mercurous sulfate electrode, mercury/mercuric oxide (Hg/HgO) electrode, or some other type of electrode. In a preferred embodiment, a reference electrode is Hg/HgO electrode. In some embodiments, the electrochemical cell does not include a reference electrode.

In some embodiments, the counter electrode is made from a material selected from the group consisting of platinum, gold, and carbon. In some embodiments, the counter electrode may contain an electrically-conductive material such as platinum, platinum-iridium alloy, iridium, titanium, titanium alloy, stainless steel, gold, cobalt alloy, and/or some other electrically-conductive material, where an “electrically-conductive material” as defined here is a substance with an electrical resistivity of at most 10⁻⁶ Ω·m, preferably at most 10⁻⁷ Ω·m, and more preferably at most 10⁻⁸ Ω·m at a temperature of 20-25° C. The form of the counter electrode may be generally relevant only in that it supplies sufficient current to the electrolyte solution to support the current for the electrochemical reaction of interest. The counter electrode material should thus be sufficiently inert to withstand chemical conditions in the electrolyte solution, such as acidic or basic pH values, without degrading during the electrochemical reaction. The counter electrode should preferably not leach out any chemical substance that interferes with the electrochemical reaction or might lead to undesirable electrode contamination. In a preferred embodiment, the counter electrode is platinum.

At step 74, the method 70 includes contacting the working electrode, the reference electrode, and the counter electrode with an aqueous solution. In some embodiments, the aqueous solution is an alkali metal salt and water. In some embodiments, the alkali metal salt in the aqueous solution is at a concentration of 0.05-3 M, preferably 0.1-2.5 M, preferably 0.2-2 M, more preferably 0.5-1.5 M, and yet more preferably 0.8-1.2 M. In a preferred embodiment, the alkali metal salt in the aqueous solution has a concentration of 1 M. In some embodiments, the alkali metal salt is at least one selected from the group consisting of lithium hydroxide (LiOH), sodium hydroxide (NaOH), potassium hydroxide (KOH) and rubidium hydroxide (RbOH), and cesium hydroxide (CsOH). In a preferred embodiment, the alkali metal salt is KOH. In some embodiments, the water may be tap water, distilled water, bidistilled water, deionized water, deionized distilled water, reverse osmosis water, and/or some other water. In a preferred embodiment, the water is distilled water. Preferably, to maintain uniform concentrations and/or temperatures of the electrolyte solution, the electrolyte solution may be stirred or agitated during the step of the subjecting. The stirring or agitating may be done intermittently or continuously. This stirring or agitating may be done by a magnetic stir bar, a stirring rod, an impeller, a shaking platform, a pump, a sonicator, a gas bubbler, or some other device and/or method known in the art. Preferably, the stirring is done by an impeller or a magnetic stir bar.

At step 76, the method 70 includes applying a potential. In some embodiments, the potential is in a range between 0-2 V vs. RHE, preferably 0.5-1.9 V vs RHE, preferably 0.8-1.8 V vs. RHE, preferably 0.9-1.7 V vs. RHE, and preferably 1-1.6 V vs. RHE. In some embodiments, the electrocatalyst has an onset potential of 1.4-1.6 V vs. RHE, preferably 1.45-1.55 V vs. RHE, preferably 1.48-1.52 V vs. RHE. In some embodiments, the electrocatalyst has an onset potential of 1.49 V vs. RHE, 1.52 V vs. RHE, and 1.55 V vs. RHE. In some embodiments, the electrocatalyst has an overpotential of 270-280 mV, preferably 271-279 mV, preferably 272-278 mV, more preferably 273-277 mV, and yet more preferably 274-276 mV at a current density of 10 milliamperes per centimeter square (mA/cm²). In a preferred embodiment, the electrocatalyst has an overpotential of about 274 mV at a current density of 10 mA/cm². In some embodiments, the electrocatalyst has an overpotential of 280-290 mV, preferably 282-290 mV, preferably 284-290 mV, more preferably 285-289 mV, and yet more preferably 286-288 mV at a current density of 100 mA/cm². In a preferred embodiment, the electrocatalyst has an overpotential of about 287 mV at a current density of 100 mA/cm².

At step 78, the method 70 includes generating oxygen at the working electrode. The oxygen may be generated by decomposing water into H₂ and O₂. In one embodiment, the space above each electrode may be confined to a vessel to receive or store the evolved gases from one or both electrodes. The collected gas may be further processed, filtered, or compressed. Preferably, the H₂-enriched gas is collected above the cathode, and the O₂-enriched gas is collected above the anode. The electrolytic cell, or an attachment, may be shaped so that the headspace above the working electrode is kept separate from the headspace above the reference electrode. In one embodiment, the H₂-enriched gas and the O₂-enriched gas are not 100 vol. % H₂ and 100 vol. % O₂, respectively. For example, the enriched gases may also include N₂ from the air, water vapor, and/or other dissolved gases from the electrolyte solution. The H₂-enriched gas may also include O₂ from the air. The H₂-enriched gas may include greater than 20 vol. % H₂, preferably greater than 40 vol. % H₂, more preferably greater than 60 vol. % H₂, and even more preferably greater than 80 vol. % H₂, relative to a total volume of the receptacle collecting the evolved H₂ gas. The O₂-enriched gas may include greater than 20 vol. % O₂, preferably greater than 40 vol. % O₂, more preferably greater than 60 vol. % O₂, and even more preferably greater than 80 vol. % O₂, relative to a total volume of the receptacle collecting the evolved O₂ gas. In some embodiments, the evolved gases may be bubbled into a vessel, including water or some other liquid, and higher concentrations of O₂ or H₂ may be collected. In one embodiment, evolved O₂ and H₂, or H₂-enriched gas and O₂-enriched gas, may be collected in the same vessel.

In some embodiments, the electrocatalyst has a Tafel slope of 48-52 millivolt/decade (mV/dec), and preferably 49-51 mV/dec. In a preferred embodiment, the electrocatalyst has a Tafel slope of 49.7 mV/dec. In some embodiments, the electrocatalyst has a mass activity of 13,000-13,500 milliamperes per milligram (mA/mg), preferably 13,100-13,400 mA/mg, and more preferably 13,200-13,300 mA/mg at an overpotential of 350 mV. In a preferred embodiment, the electrocatalyst has a mass activity of about 13,228.75 mA/mg at an overpotential of 350 mV.

In some embodiments, the electrocatalyst has an electrochemically active surface area (ECSA) of 80-90 square centimeters (cm²), preferably 81-89 cm², preferably 82-88 cm², more preferably 83-87 cm², and yet more preferably 84-86 cm². In a preferred embodiment, the electrocatalyst has ECSA of about 85.2 cm². In some embodiments, the electrocatalyst has a specific activity of 10-15 mA/cm², preferably 11-14 mA/cm², and more preferably 12-13 mA/cm².

In a preferred embodiment, the electrocatalyst has a specific activity of about 12.42 mA/cm². In some embodiments, the electrocatalyst has a charge transfer resistance of 0.8-0.9 ohms (Ω), preferably 0.81-0.89Ω, preferably 0.82-0.88Ω, more preferably 0.83-0.87Ω, and yet more preferably 0.84-0.86Ω. In a preferred embodiment, the electrocatalyst has a charge transfer resistance of about 0.851Ω.

In some embodiments, the electrocatalyst has a turnover frequency of 1.5-2.5 s⁻¹, preferably 1.75-2.25 s⁻¹, more preferably 2-2.1 s⁻¹, and yet more preferably about 2.04 s⁻¹ at an overpotential of 350 mV. In some embodiments, the electrocatalyst has a stability of 90-110 h, preferably 91-109 h, preferably 92-108 h, preferably 93-107 h, preferably 94-106 h, preferably 95-105 h, preferably 96-104 h, preferably 97-103 h, preferably 98-102 h, more preferably 99-101 h, and yet more preferably about 100 h at a current density of 10 mA/cm² and 50 mA/cm².

EXAMPLES

The following examples describe and demonstrate an electrode including a bimetallic iron-vanadium oxide electrocatalyst for oxygen evolution reaction (OER). The examples are provided solely for illustration and are not to be construed as limitations of the present disclosure, as many variations thereof are possible without departing from the spirit and scope of the present disclosure.

Example 1: Chemicals

Vanadium(IV) oxyacetylacetonate (VO(acac)₂) and Fe(III) acetylacetonate (Fe(acac)₃) were obtained from Sigma-Aldrich. Nickel foam (1.6 mm thick and 95% porous) manufactured by Good-fellow Cambridge Ltd were utilized. Analytical grade chemicals and materials were used as received and no further post-treatment was made prior to use.

Example 2: Preparation of Fe₂O₃—VO_x Composite Oxide Electrocatalyst

Before catalyst growth, nickel foam was ultrasonically cleaned by treatment with different solvents, such as diluted HCl, acetone, and ethanol. The cleaned nickel foam was rinsed with deionized water and dried under a flow of inert gas (high-purity nitrogen). Fabrication of the catalyst was achieved using a house-designed single-step aerosol-assisted chemical vapor deposition (AACVD) process. The deposition process and behavior of AACVD are documented in previous works [Ehsan, M. A.; Shah, S. S.; Basha, S. I.; Hakeem, A. S.; Aziz, M. A., Recent Advances in Processing and Applications of Heterobimetallic Oxide Thin Films by Aerosol-assisted Chemical Vapor Deposition. The Chemical Record 2021, 22, e202100278, which is incorporated herein by reference in its entirety]. A clear and transparent reddish-green solution was prepared by dissolving the precursor Fe(acac)₃ (100 mg, 284 mmol) and VO(acac)₂ (100 mg, 377 mmol) in methanol (30 mL). The Fe—V oxide catalyst was grown on porous and rough nickel foam (NF) by varying the growth times from 40 to 120 minutes.

A general method for developing catalytic thin films using AACVD uses a homogeneous and clear precursor solution of the desired metals (Fe and V) in a low-density methanol solvent. An aerosol mist is created from a clear precursor solution using an ultrasonic humidifier. The precursor aerosol mist is then transported to a reactor tube, installed in a plane tube furnace kept at a temperature of 475° C. Industrial N₂ (99.99% purity) is used as a carrier gas. The NF in the reactor tube is arranged to collect the precursor stream on its exposed surface, where the gaseous precursor is decomposed, and a series of CVD reactions are subsequently carried out to yield the Fe—V oxide catalyst. The mass loading of the Fe—V catalyst can be controlled by allowing the deposition process to occur for desired periods of 40, 80, and 120 minutes. The aerosol supply can be stopped at any desired time, and the furnace is cooled under the nitrogen gas stream until room temperature is reached. The prepared thin film electrocatalyst grown after 40, 80, and 120 minutes of AACVD processing are generally represented as FVO_x-40, FVO_x-80, and FVO_x-120, respectively. A schematic representation of the AACVD process to produce the FeVO_x catalyst is shown in FIG. 1C.

Example 3: Structural Characterization

Field emission scanning electron microscopy, FESEM, (TESCAN MIRA3) was used to characterize the microstructural patterns of FeVO_x catalysts. Elemental compositions were measured with an energy-dispersive X-ray (EDX) spectrometer (INCA Energy 200, Oxford Instruments, UK). The crystalline structure of catalytic films was recorded using X-ray diffraction, XRD, (Rigaku MiniFlex X-ray diffractometer). Transmission electron microscopy (TEM) was carried out (JEOL-JEM2100F, Japan) at an accelerating voltage of 200 kV. Chemical composition and valence states were investigated by X-ray photoelectron spectroscopy XPS, (Thermo Scientific EscaLab 250Xi, USA) using an Al Kα source (1486.6 eV).

Example 4: Electrochemical Investigations

Oxygen evolution reaction (OER) performance of the synthesized catalysts, FeVO_x, was measured with a Gamry potentiostat instrument (model No: INTERFACE 1010 E). A three-electrode cell was set up using a calibrated Hg/HgO, platinum (Pt) mesh, and FeVO_x/NF as reference electrode, counter electrode, and working electrode, respectively. A range of electrochemical characterizations, including cyclic voltammetry (CV), linear scan voltammetry (LSV), electrochemically active surface areas (ECSAs), electrochemical impedance spectroscopy (EIS), and chronopotentiometry (CP), were conducted in a 1.0 M KOH electrolyte. The working electrode, comprised of FeVO_x/NF, had an active geometric area of 1×1 cm². All potential measurements are expressed versus the standard reversible hydrogen electrode (RHE) using the Nernst equation: V_RHE=V_Hg/Hg0+E_Hg/HgO+0.059 pH, where V_RHE and V_Hg/HgO are the efficient applied potentials against the RHE and Hg/HgO reference electrode, respectively, and E_Hg/HgO (0.098 V) is the reference electrode potential vs. the standard hydrogen electrode. Initially, working electrodes were employed for continuous CV between 1.0 and 1.8 V_RHE at a scan rate of 50 mV s⁻¹ until a stable and reproducible CV signal was obtained. LSV was performed at a scan rate of 2 mV s⁻¹ to determine the onset potential, overpotential, and Tafel slope values. The relation “η=b log j+a” was used to measure the value of the Tafel slope, where, η represents the overpotential value, b is the Tafel slope, and a is constant. The mass activity of all electrocatalysts was calculated using the following relationship [Joya, K. S.; Ehsan, M. A.; Sohail, M.; Yamani, Z. H., Nanoscale palladium as a new benchmark electrocatalyst for water oxidation at low overpotential. Journal of Materials Chemistry A 2019, 7 (15), 9137-9144, which is incorporated herein by reference in its entirety]:

MA = J η @ 350 mass ⁢ of ⁢ catalyst

The exchange current density of all electrocatalysts was calculated considering the charge transfer resistance at the electrode-electrolyte interface [Ehsan, M. A.; Khan, A.; Suliman, M. H.; Javid, M., Facile deposition of FeNi/Ni hybrid nanoflower electrocatalysts for effective and sustained water oxidation. Nanoscale Advances 2023, 5 (18), 5122-5130, which is incorporated herein by reference in its entirety]:

J exc = RT nAF

Where n is the number of electrons in the oxygen evolution reaction, F is the Faraday constant, and A is the geometric area of the working electrodes (1 cm²).

Electrochemical impedance spectroscopy (EIS) was studied at 1.53 V in the frequency range of 0.01 Hz to 100 kHz. The stability of catalysts was measured with a chronopotentiometric test conducted at applied current densities of 10 and 50 mA cm⁻². ECSA was calculated using the following relationship [McCrory, C. C.; Jung, S.; Peters, J. C.; Jaramillo, T. F., Benchmarking heterogeneous electrocatalysts for the oxygen evolution reaction. Journal of the American Chemical Society 2013, 135 (45), 16977-16987, which is incorporated herein by reference in its entirety]:

ECSA = C dl C sp

The value for double-layer capacitance (C_dl) was obtained by running consecutive CVs in the non-Faradic region of 1.06-1.26 V @10-60 mV sec⁻¹. C_sp represents the specific capacitance of the metal electrodes (0.04 cm² for alkaline electrolytes). The C_dl is determined from the cathodic and anodic slopes of the average current versus scan rate (v) curve as the straight-line relation v=C_dlj.

Example 4: Structural Analysis

The crystalline structure of the FeVO_x thin films was evaluated in the XRD patterns, as shown in FIG. 1D. Due to the highly crystalline peaks of Ni from the nickel foam substrate potentially masking the XRD peaks from the FeVO_x films, FeVO_x films were grown on a non-crystalline glass surface for XRD measurements. In the XRD pattern, the FeVO_x has characteristic peaks that represent two distinct species, namely Fe₂O₃ and VO. The diffraction peaks indexed with “*” at 20 values of 38.3°, 44.5°, 64.7°, and 77.7° correspond to planes (111), (200), (220), and (311) of VO, respectively. These crystalline planes match well with cubic phased vanadium(II) oxide (VO) JCPDS card #071-6420. Another set of crystalline peaks signed with “+” at 20 values of 24.2°, 33.3°, 35.7°, 41°, 49.7°, 62.3°, 64.2°, 69.8°, 80.9°, and 89° correspond to planes (012), (104), (110), (113), (024), (214), (300), (208), (312), and (226) of Fe₂O₃. These XRD peaks agree well with JCPDS card #01-084-0311, indicating the formation of hematite (Fe₂O₃) crystals in the trigonal phase. It is observed that increasing the deposition time from 40 to 120 minutes improves the crystallinity of the deposited material (Fe₂O₃—VO), as shown by the growth of the peak at 20 of 33.3° (FIG. 1D). Overall, the composition of the crystalline product Fe₂O₃—VO remains unchanged with increasing deposition time from 40 to 120 minutes, as supported by the overlaid XRD fingerprints in FIG. 1D. During the XRD analysis, no crystalline peaks of other vanadium oxide phases (V₂O₃, V₂O₅, etc.) nor iron oxide (Fe₃O₄) were observed, showing the synthesis of a pure phase product containing only Fe₂O₃ and VO.

The morphology of the FeVO_x composite catalyst grown directly on the nickel foam was examined using FESEM. FIG. 2A, FIG. 2C, and FIG. 2E show FESEM images of FeVO_x-40, FeVO_x-80, and FeVO_x-120 analyzed at low magnification. Low-resolution images show the good coverage of the composite oxide material over the nickel foam surface. FIG. 2B, FIG. 2D, and FIG. 2F show FESEM images of FeVO_x-40, FeVO_x-80, and FeVO_x-120 analyzed at high magnification and depict size, shape, and texture of morphological features of the composite oxide material over the nickel foam. The FeVO_x grown for 40 minutes showed an intertwined nanoparticle morphology (FIG. 2B). The morphology of the film becomes a denser and more compact nanostructure with an increase in film growth time to 80 minutes (FIG. 2D). The nanoparticles attained more fused and intact geometry due to a longer sintering time. A single-step growth of the FeVO_x composite film for 120 minutes resulted in a transformation of the film where the nanoparticle morphology was distorted and small and large-sized lumps on the nickel foam surface were developed (FIG. 2E).

The FESEM results show the influence of sintering time on developing the nanostructure of the film. A well-defined nanoparticle morphology transforms into a broken and distorted structure, which could lead to a change in the catalytic active sites and the OER performance of the FeVO_x catalyst. With the AACVD process, the catalyst morphology can be tuned, and a variety of nanostructures can be generated.

The elemental composition of FeVO_x catalysts was assessed with energy dispersive X-ray (EDX) analysis. FIG. 2G-2I shows the EDX spectra and atomic percents of Fe and V for the FeVO_x-40 catalyst, FeVO_x-80 catalyst, and FeVO_x-120 catalyst. The Fe—V composite oxide grown for 40 minutes includes 13.6 at. % Fe and 16.7 at. % V. The sample grown for 80 minutes includes 22.8% Fe and 25.2% V. the sample prepared for 120 minutes contains 25.5% Fe and 23.3% V. Overall, the empirical elemental ratio between Fe/V remains about 1 to 1. Nickel atom contributions are from the underlying NF substrate. The EDX elemental map was analyzed to observe homogenous distribution of Fe and V atoms in all composite oxide samples prepared for different time periods. FIGS. 2J-2L show the EDX spectra of the FeVO_x-40 catalyst, the FeVO_x-80 catalyst and the FeVO_x-120 catalyst, respectively.

To further investigate the crystalline structure and morphology of the composite oxide catalyst, FeVO_x-40 was examined by TEM analysis. FIGS. 3A-3D show the TEM analysis of the FeVO_x catalysts. The low-resolution TEM images (FIGS. 3A-3B) show that several nanocrystallites are stacked together, and their size may vary from 40 to 80 nm. Multiple lattice fringes appeared in the high-resolution (HR)-TEM image (FIG. 3C), further supporting the crystalline nature of the FeVO_x catalyst. The d-spacing or inter-planar distance measured from an individual nanocrystallite was found to be 0.204 nm and 0.1573 nm. This shows agreement with the d-spacing value of 0.205 nm related to (200) plane of VO, and 0.1584 nm corresponding to (018) plane of Fe₂O₃. These results are consistent with the XRD pattern. The polycrystalline structure of the Fe₂O₃—VO was further confirmed by the selected area electron diffraction (SAED) pattern, as shown in FIG. 3D. The rings marked 1, 4, and 5 correspond to the planes (104), (300), and (128) of the Fe₂O₃ phase, while the diffraction rings labeled 2 and 3 indicate the crystal planes (111) and (200) of VO phase, respectively. Thus, the HRTEM analysis supports the presence of crystalline phases of Fe₂O₃ and VO as perceived from XRD analysis.

The FeVO_x catalyst grown for 40 min was investigated by X-ray photo spectroscopy (XPS) to determine the chemical oxidation states of the constituent elements. The scanning spectrum of the XPS survey (FIG. 4A) detects Fe, V, and O atoms on the surface of the composite catalyst, supporting the EDX results. The Fe 2p spectrum (FIG. 4B) contains doublet peaks referring to bands for Fe 2p_3/2 and Fe 2p_1/2. The binding energy peaks found at 710.6 eV and 724.5 eV correspond to 2p_3/2 and 2p_1/2 of Fe²⁺ ions, respectively. The spectral bands at 713.0 eV and 725.4 eV are assigned to the 2p_3/2 and 2p_1/2 of Fe³⁺ ions, respectively. The weak satellite peaks around 720.0 eV and 732.5 eV, implying the hematite (Fe₂O₃) phase. The deconvoluted spectrum of V 2p (FIG. 4C) indicates peaks at a binding energy of 516.8 eV and 524.6 eV, related to V 2p_3/2 and V 2p_1/2, respectively, suggesting an electronic configuration of V⁴⁺. The 0 is spectrum (FIG. 4D) is divided into two peaks, which appear at binding energies of ˜530 eV and 531.5 eV, respectively, and are assigned to the oxygen directly coordinated with metal (M-O) and hydroxide (O—H) species adsorbed on the catalyst surface.

Electrocatalytic water oxidation performance of FeVO_x/NF electrodes was determined through a series of electrochemical experiments conducted in a 1.0 M KOH electrolyte. First, cyclic voltammetry was used to determine the stability and electrical response of the functioning FeVO_x/NF electrodes in a given electrochemical environment. Concurrent forty (40) cyclic voltammograms (CVs) were obtained for each electrocatalyst, and their 1^st and 40^th CVs are compared in FIGS. 5A-5C. The CVs in FIG. 5A and FIG. 5B of FeVO_x-40 and FeVO_x-80 are almost identical, indicating the stable and reproducible behavior of these electrodes. FeVO_x-120 experienced a slight shift in the CV while running from the 1^st to the 40^th cycle, and the overpotential and peak current values are shifted toward the lower-potential region, which may be due to the activation of the catalyst surface under the influence of concurrent CVs. The FeVO_x electrodes grown for 40 and 80 minutes show well-defined nanoparticle morphology (FIG. 2B and FIG. 2D), with active sites fully available for oxidation reactions. No further activation was used for the catalysts; however, the FeVO_x electrode grown for 120 minutes lacked distinct morphology and uniformity (FIG. 2F), and surfaces of large lumps needed conditioning under cyclic voltammetry to perform better in OER conditions. The FeVO_x-40 electrode involves a high redox peak (inset of FIG. 5A), which decreases in the case of FeVO_x-80 and FeVO_x-120 (inset of FIGS. 5B and 5C). This may be due to catalyst mass growing with extension of the deposition time from 40 to 120 minutes and heavily covering NF, and thus the redox contribution of NF is minimized.

Polarization curves (LSVs) of all FeVO_x catalysts were measured at a scan rate of 2 mV s⁻¹ to analyze OER performance indicators, such as oxidation onset potential and overpotential to achieve current decade and peak current density. FIG. 6A depicts the FeVO_x-40 electrode starting the oxidation process at a lower potential of 1.49 V vs. RHE (η_onset=260 mV), followed by FeVO_x-80 (1.52 V vs. RHE, η_onset=290 mV), and FeVO_x-120 (1.55 V vs. RHE, η_onset=320 mV) electrodes. The trend remains the same in the approach characteristics of the current decade (10 mA cm⁻²). A magnified view of LSV curves is presented in FIG. 6B further supports this observation. FeVO_x-40 exhibits a lower overpotential (η₁₀) of 274 mV than that of FeVO_x-80 (310 mV) and FeVO_x-120 (389 mV). FIG. 6C further compares the overpotential intake to gain a current density of 1000 mA cm⁻². FeVO_x-40, within the overpotential limit of 310 mV, outperforms the other two variants, FeVO_x-80 (370 mV) and FeVO_x-120 (475 mV). The better OER performance of the FeVO_x-40 catalyst may be explained based on the thin film nanostructure presented in FIGS. 2A-2F. The well-defined and uniformly oriented nanoparticles of FeVO_x-40 may have more active sites available than the morphological features displayed by the other two FeVO_x electrodes, FeVO_x-80 and FeVO_x-120.

Tafel slope measurements are used to comprehend OER kinetics of the catalysts. The Tafel curves extracted from corresponding LSVs of the FeVO_x electrocatalysts are shown in FIG. 6D. The Tafel slope value for the FeVO_x-40 electrode is lowest at 49.7 mV dec⁻¹, suggesting a quicker reaction rate of OER with fast charge transfer at the electrode-electrolyte interface. FeVO_x-80 shows a Tafel slope value of 52.5 mV dec 1, and FeVO_x-120 shows a value of 69.5 mV dec⁻¹. Catalytic activity among different catalysts can be found by comparing mass activity, which provides information about the specific amount of mass required to produce maximum current density. The mass activity was estimated at an overpotential of 350 mV (calculations are given below), and a comparison bar chart is shown in FIG. 6E.

Calculation of Overpotential

The over-potential calculation of FeVO_x/NF electrocatalyst for OER was performed utilizing the following equation:

η = E RHE - 1.23 V

where η accounts for overpotential.

Mass ⁢ activity [ MA ⁢ ( mA ⁢ mg - 1 ) ]

Mass activity is calculated by using the following equation:

MA = J m FeVO x ⁢ ‐ ⁢ 40 ⁢ catalyst = 1058.3 mA / 0.08 mg = 13228.76 mA / mg FeVO x ⁢ ‐ ⁢ 80 ⁢ catalyst = 516.8 mA / 0.2 mg = 2584 ⁢ mA / mg FeVO x ⁢ ‐ ⁢ 120 ⁢ catalyst = 179.9 mA / 0.38 mg = 473.42 mA / mg

FeVO_x-40 showed the highest mass activity of 13,228.75 mA mg⁻¹, which reflects the highest available active sites for this catalyst. FeVO_x-80 and FeVO_x-120 displayed smaller mass activity values of 2584 mA mg⁻¹ and 473.42 mA mg⁻¹, respectively, and lower OER performances accordingly. The electrochemical active surface area (ECSA) measurement provides insight into the surface area of the catalyst that is electrochemically participating in catalyzing the OER process. The comparison of ECSA and the specific activity of the FeVO_x catalysts is shown in FIG. 6F. The ECSA values were estimated from double-layer capacitance (C_dl) that could be obtained by recording CVs in a non-faradic region (FIGS. 6G-6L), where the charge transfer is due to the formation of an electric double layer over the electrocatalytic active sites. FIGS. 6G-6I depict CVs for FeVO_x-40, FeVO_x-80, and FeVO_x-120 obtained in the non-Faradaic region. FIGS. 6J-6L depict plots for ECSA measurements for FeVO_x-40, FeVO_x-80, and FeVO_x-120 obtained in the non-Faradaic region. As seen in FIGS. 6G-6L, as deposition time is increased, the ECSA and specific activity decreases. This data supports that FeVO_x-40, with a thin and homogenized layer of the catalyst over NF, is the best performing catalyst among the prepared catalysts in the current disclosure.

Specific Activity (mA/Cm²) Calculations:

FeVO x ⁢ ‐ ⁢ 40 ⁢ catalyst = 1058.3 mA / 85.2 = 12.42 mA / cm 2 FeVO x ⁢ ‐ ⁢ 80 ⁢ catalyst = 516.8 mA / 69.5 = 7.43 mA / cm 2 FeVO x ⁢ ‐ ⁢ 120 ⁢ catalyst = 179.9 mA / 65.6 = 2.74 mA / cm 2

Electrochemical impedance spectroscopy (EIS) gives information about resistive losses due to external factors (R_s) and charge transfer resistance (R_ct) that may affect the electrocatalytic activity of a material. EIS data of the catalysts was recorded, and the Nyquist plot is shown in FIG. 7A. It is established that the diameter of a semicircle of the Nyquist plot relates to the R_ct value. FeVO_x-40 showed the least charge transfer resistance with the smallest diameter of 0.851Ω. FeVO_x-80 and FeVO_x-120 exhibit larger charge transfer resistances of 1.16Ω and 2.60Ω, respectively, than FeVO_x-40. These results are in accordance with the structural and electrochemical characterization discussed above.

Turnover frequency (TOF) is another metric to compare the intrinsic catalytic activity of the prepared catalysts. It is assumed that all metallic elemental (Fe and V) sites on the catalyst surface take part in OER for TOF computations. The following mathematical relation is used to determine the TOF of FeVO_x catalysts:

TOF = J × A 4 × F × m

where A is the NF substrate's surface area (1 cm⁻²), m is the number of moles of catalyst put onto the NF substrate, and J is the current density (A cm⁻²) measured at different values of potential. The mass of FeVO_x obtained on NF substrate as the result of 40, 80, and 120 minute growth periods was measured to be 0.08 mg, 0.2 mg, and 0.38 mg, respectively. The procedure for calculating the TOF values is given below.

Determination of TOF for FeVO_x-40 Catalyst:

TOF ⁢ at 1.49 V = [ ( 0.00076 A ) × ( 1 ⁢ cm 2 ) ] / 
 [ ( 4 ) × ( 3.5289 × 10 - 7 mol ) × ( 96485 ⁢ C ⁢ mol - 1 ) ] = 0.00558 s - 1 TOF ⁢ at 1.52 V = [ ( 0.02938 A ) × ( 1 ⁢ cm 2 ) ] / 
 [ ( 4 ) × ( 3.5289 × 10 - 7 mol ) × ( 96485 ⁢ C ⁢ mol - 1 ) ] = 0.215721 s - 1 TOF ⁢ at 1.52 V = [ ( 0.10522 A ) × ( 1 ⁢ cm 2 ) ] / 
 [ ( 4 ) × ( 3.5289 × 10 - 7 mol ) × ( 96485 ⁢ C ⁢ mol - 1 ) ] = 0.772572 s - 1 TOF ⁢ at 1.58 V = [ ( 0.2779 A ) × ( 1 ⁢ cm 2 ) ] / 
 [ ( 4 ) × ( 3.5289 × 10 - 7 mol ) × ( 96485 ⁢ C ⁢ mol - 1 ) ] = 2.040466 s - 1 TOF ⁢ at 1.61 V = [ ( 0.639 A ) × ( 1 ⁢ cm 2 ) ] / 
 [ ( 4 ) × ( 3.5289 × 10 - 7 mol ) × ( 96485 ⁢ C ⁢ mol - 1 ) ] = 4.691824 s - 1 TOF ⁢ at 1.64 V = [ ( 1.5 A ) × ( 1 ⁢ cm 2 ) ] / 
 [ ( 4 ) × ( 3.5289 × 10 - 7 mol ) × ( 96485 ⁢ C ⁢ mol - 1 ) ] = 11.01367 s - 1 For ⁢ FeVO x ⁢ ‐ ⁢ 80 ⁢ catalys t: TOF ⁢ at 1.49 V = [ ( 0.00076 A ) × ( 1 ⁢ cm 2 ) ] / 
 [ ( 4 ) × ( 8.832 × 10 - 7 mol ) × ( 96485 ⁢ C ⁢ mol - 1 ) ] = 0.002232 s - 1 TOF ⁢ at 1.52 V = [ ( 0.01121 A ) × ( 1 ⁢ cm 2 ) ] / 
 [ ( 4 ) × ( 8.832 × 10 - 7 mol ) × ( 96485 ⁢ C ⁢ mol - 1 ) ] = 0.032924 s - 1 TOF ⁢ at 1.55 V = [ ( 0.04556 A ) × ( 1 ⁢ cm 2 ) ] / 
 [ ( 4 ) × ( 8.832 × 10 - 7 mol ) × ( 96485 ⁢ C ⁢ mol - 1 ) ] = 0.13381 s - 1 TOF ⁢ at 1.58 V = [ ( 0.0856 A ) × ( 1 ⁢ cm 2 ) ] / 
 [ ( 4 ) × ( 8.832 × 10 - 7 mol ) × ( 96485 ⁢ C ⁢ mol - 1 ) ] = 0.251407 s - 1 TOF ⁢ at 1.61 V = [ ( 0.2837 A ) × ( 1 ⁢ cm 2 ) ] / 
 [ ( 4 ) × ( 8.832 × 10 - 7 mol ) × ( 96485 ⁢ C ⁢ mol - 1 ) ] = 0.833226 s - 1 TOF ⁢ at 1.64 V = [ ( 0.653 A ) × ( 1 ⁢ cm 2 ) ] / 
 [ ( 4 ) × ( 8.832 × 10 - 7 mol ) × ( 96485 ⁢ C ⁢ mol - 1 ) ] = 1.917858 s - 1 For ⁢ FeVO x ⁢ ‐ ⁢ 120 ⁢ catalys t: TOF ⁢ at 1.49 V = [ ( 0.00076 A ) × ( 1 ⁢ cm 2 ) ] / 
 [ ( 4 ) × ( 0.000001676 mol ) × ( 96485 ⁢ C ⁢ mol - 1 ) ] = 0.001175 s - 1 TOF ⁢ at 1.52 V = [ ( 0.0069 A ) × ( 1 ⁢ cm 2 ) ] / 
 [ ( 4 ) × ( 0.000001676 mol ) × ( 96485 ⁢ C ⁢ mol - 1 ) ] = 0.0106673 s - 1 TOF ⁢ at 1.55 V = [ ( 0.02266 A ) × ( 1 ⁢ cm 2 ) ] / 
 [ ( 4 ) × ( 0.000001676 mol ) × ( 96485 ⁢ C ⁢ mol - 1 ) ] = 0.0350321 s - 1 TOF ⁢ at 1.58 V = [ ( 0.05844 A ) × ( 1 ⁢ cm 2 ) ] / 
 [ ( 4 ) × ( 0.000001676 mol ) × ( 96485 ⁢ C ⁢ mol - 1 ) ] = 0.0903476 s - 1 TOF ⁢ at 1.61 V = [ ( 0.12856 A ) × ( 1 ⁢ cm 2 ) ] / 
 [ ( 4 ) × ( 0.000001676 mol ) × ( 96485 ⁢ C ⁢ mol - 1 ) ] = 0.1987523 s - 1 TOF ⁢ at 1.64 V = [ ( 0.2621 A ) × ( 1 ⁢ cm 2 ) ] / 
 [ ( 4 ) × ( 0.000001676 mol ) × ( 96485 ⁢ C ⁢ mol - 1 ) ] = 0.4050489 s - 1

A graphical presentation of TOF values determined at different overpotentials of the FeVO_x catalysts is shown in FIG. 7B. At an overpotential of 350 mV, the TOF values for FeVO_x-40, FeVO_x-80, and FeVO_x-120 are 2.04 s⁻¹, 0.25 s⁻¹, and 0.09 s⁻¹, respectively. The larger TOF of FeVO_x-40 indicates its ability to transfer electrons at a faster rate and make it a catalyst with improved OER kinetics over FeVO_x-80 and FeVO_x-120.

FIG. 7C shows the long-time chronopotentiometric test (η vs t) carried out on the FeVO_x-40 electrode at a fixed applied current density of 10 and 50 mA cm⁻², respectively. The FeVO_x reveals good operational ability for 100 hours without decay or losses during this testing time. After the long-term stability test, a polarization curve (LSV) of FeVO_x-40 was measured and compared with data before the long-term stability test. FIG. 7D indicates the overlapped trend of LSVs, showing durability of FeVO_x material after 100 hours of OER performance. This data indicates that thin film electrocatalysts have active sites that keep emerging as the OER reaction proceeds, and the catalyst retains its activity for a longer time period.

After long-term OER testing, the surface of FeVO_x-40 was characterized again by SEM and EDX analysis. The footprints of intertwined nanoparticles can still be observed in SEM images, as shown in FIGS. 8A-8C. EDX analysis, as shown in FIG. 8D shows the presence of Fe and V in an elemental ratio of 1 to 1, as observed before OER experiments. The appearance of potassium (K) peak is due to the KOH electrolyte used for OER studies. XPS analysis spectra of FeVO_x-40 after OER measurements are shown in FIGS. 9A-9C, for V, Fe, and O, respectively.

Tables 1A and 1B compare the OER results of the catalysts in the current disclosure, concluding that FeVO_x-40 performs the best. Table 2 compares the OER results of FeVO_x-40 with those of other reported similar materials, supporting that the as-fabricated catalyst FeVO_x-40 demonstrates good catalytic activity and stability.

TABLE 1A
Performance comparison of all fabricated materials

	Overpotential	Overpotential		Specific
	@ 10 mA	@ 100 mA	Onset	activity
Catalysts	cm2 (mV)	cm−2 (mV)	Potential	(mA/cm2)

FeVOx-40	274	287	1.49 V vs. RHE	12.42
FeVOx-80	310	338	1.52 V vs. RHE	179.9
FeVOx-120	389	420	1.55 V vs. RHE	2.74

TABLE 1B
Performance comparison of all fabricated materials

			Mass
	CSA	Tafel slope	Activity	jexc	Roughness
Catalysts	(cm2)	(mv dec−1)	(mA/mg)	(mA/cm2)	factor

FeVOx-40	85.2	49.7	13228.76	7.6	85.2
FeVOx-80	69.5	52.5	2584	5.4	65.6
FeVOx-120	65.6	69.5	473.42	2.48	69.5

Calculation of Roughness Factor:

FeVO x ⁢ ‐ ⁢ 40 ⁢ catalyst = 85.2 cm 2 / 1 ⁢ cm 2 = 85.2 FeVO x ⁢ ‐ ⁢ 80 ⁢ catalyst = 65.6 cm 2 / 1 ⁢ cm 2 = 65.6 FeVO x ⁢ ‐ ⁢ 120 ⁢ catalyst = 69.5 cm 2 / 1 ⁢ cm 2 = 69.5

The OER activity of FeVO_x-40 may be attributed to the following factors: (i) the bimetallic Fe—V interaction tunes the electronic structure and conductive properties of the catalyst, which promotes balanced binding energy sites with reaction intermediates and makes the adsorption/desorption process easier to increase the rate of OER processes; (ii) the nanoparticle morphology and uniform dispersion achieved in 40 minutes of AACVD process provided abundant exposed active sites to promote charge transfer and release of O₂ during OER processes; and/or (iii) adopting a suitable thin film deposition protocol in the form of AACVD produces a mechanically stable and adhesive electrode without the need of a binding reagent and preserves the original charge transfer efficiency and stability of the catalyst material during long-term ER operations.

TABLE 2
OER parameters of vanadium-based catalysts measured in alkaline electrolytes

		Synthetic	η [mV]@10	Stability	Tafel slope
Catalyst/System	Support	method	mA/cm2	(h)	[mV dec−1]	Ref.

FeVOx-40 catalyst	NF	AACVD	274	100	49.7	This
						work
Scalloped	GCE	Chemical	290	12	53	1
VO2@NiFeVOx		etching-
		reconstruction
m-CoVOx	NF	Hydrothermal	293	15	49	2
Hollow Fe0.5V0.5	NF	Hydrothermal	390	10	36.7	3
NiCo2O4/VN800	GCE	Thermal	385	7	75.7	4
		treatment with
		ammonia
VOx/NiS/NF	NF	Hydrothermal	300	N/A	121	5
VOx/Ni3S2	NF	Hydrothermal	292	10	68	6
Magnetite (Fe3O4)	NF	AACVD	300	24	65	7
thin film
VOx	NF	AACVD	310	48	70	8

• [1] Ma, Y. et al, Scalloped nickel/iron vanadium oxide-coated vanadium dioxides based on chemical etching-induced reconstruction strategy for efficient oxygen evolution, International Journal of Hydrogen Energy, 2022, 47 (78), 33352-33360; [2] Liardet, L. and Hu, X., Amorphous cobalt vanadium oxide as a highly active electrocatalyst for oxygen evolution, ACS catalysis, 2018, 8 (1), 644-650; [3] Fan, K. et al., Hollow Iron-vanadium composite spheres: a highly efficient iron-based water oxidation electrocatalyst without the need for nickel or cobalt, Angewandte Chemie International Edition, 2017, 56 (12), 3289-3293; [4] Zheng, Z. et al., Efficient and stable NiCo₂O₄/VN nanoparticle catalyst for electrochemical water oxidation, ACS Sustainable Chemistry & Engineering, 2018, 6 (9), 11473-11479; [5] Chai, Y.-M. et al., Three-dimensional VO_x/NiS/NF nanosheets as efficient electrocatalyst for oxygen evolution reaction, International Journal of Hydrogen Energy, 2019, 44 (21), 10156-10162; [6] Niu, Y. et al., Amorphous nickel sulfide nanosheets with embedded vanadium oxide nanocrystals on nickel foam for efficient electrochemical water oxidation, Journal of Materials Chemistry A, 2019, 7 (17), 10534-10542; [7] Ehsan, M. A. and Babar, N.-U.-A., Straightforward Preparation of Fe-Based Electrocatalytic Films at Various Substrates for IrO₂-like Water Oxidation Activity, Energy & Fuels, 2023, 37 (5), 3934-3941; and [8] Ehsan, M. A. et al., Aerosol-assisted chemical vapour deposited vanadium oxide thin films on nickel foam with auspicious electrochemical water oxidation properties, International Journal of Hydrogen Energy, 2023, which are incorporated herein by references in their entirety.

A bimetallic iron-vanadium oxide (FeVO_x) thin film electrocatalyst deposited on a nickel foam (NF) support is prepared using an AACVD process. The catalyst morphology may be transformed from well-defined nanoparticles to larger lumps within the time of 40 to 120 minutes, which affects the efficiency of OER processes in a 1.0 M KOH electrolyte. Nanoparticle morphology obtained after 40 minutes of AACVD process showed good OER activity by achieving a current density of 10 mA cm⁻² at an overpotential (f) of 270 mV and further leading to a high current density beyond 1000 mA cm-2 at a minimum voltage of 1.54 V vs. RHE (I=310 mV). Detailed electrochemical properties, including the Tafel slope, ECSA, TOF, and EIS Nyquist plot, reflected the enhanced intrinsic catalytic activity of the FeVO_x-40 electrocatalyst. Additionally, the catalyst showed good stability by maintaining current densities of 10 mA cm-2 and 50 mA cm⁻² for almost 100 hours without changing the cell voltage. This catalyst exhibited good performance in water oxidation compared to other known catalysts made from similar materials. The thin film electrocatalyst fabricated using the AACVD approach opens a new gateway for producing highly active, efficient, and state-of-the-art water electrolysis powered by intermittent energy resources.

Numerous modifications and variations of the present disclosure are possible in light of the above teachings. Therefore, it is to be understood that within the scope of the appended claims, the disclosure may be practiced other than as specifically described herein.