TRANSITION METAL-DOPED OXIDE NANOPARTICLES GROWN ON NICKEL FOAM FOR ELECTROCHEMICAL GENERATION OF HYDROGEN

A method of generating hydrogen using an electrocatalyst including NiMoxCo2-xO4 nanoparticles deposited on a nickel foam substrate, where x>0 and x≤0.06. A first portion of the NiMoxCo2-xO4 nanoparticles have a nano-needle morphology, where the nano-needles assemble to form a sphere in which the nano-needles project horizontally from the sphere, and the sphere has an average diameter of 1-5 micrometers (μm).

STATEMENT OF PRIOR DISCLOSURE BY INVENTOR

STATEMENT OF ACKNOWLEDGEMENT

Support provided by the King Fahd University of Petroleum and Minerals (KFUPM) is gratefully acknowledged.

BACKGROUND

Technical Field

The present disclosure is directed towards hydrogen generation, more particularly directed towards a method of generating hydrogen using transition metal doped oxide nanoparticles grown on nickel foam.

Description of Related Art

The “background” description provided herein is to present the context of the disclosure generally. 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 invention.

Developing renewable energy technology that can protect the environment and balance energy supply and demand is necessary to ensure socio-economic development. Oxygen (O2) gas is considered the precursor to clean combustion and is used in a plurality of industrial processes such as, but not limited to, combustion, oxidation, flame hardening, and flame cleaning. Subsequently, hydrogen (H2) has been viewed as a clean and high-density energy carrier for mitigating increasing environmental issues and global warming caused by the combustion of fossil fuels. Hydrogen is environmentally friendly, easy to convert into electricity or other forms of fuel with relatively high efficiencies and has convenient ways of storage. Currently, H2 can be generated from partial oxidation, gasification, and steam reforming of hydrocarbons. However, the main drawbacks to these methods of processing are high operational costs and high temperatures and pressures, in addition to the large consumption of fossil fuels such as natural gas, which results in CO2 emissions.

Thus, substantial efforts have been made to explore cleaner, more sustainable, and energy-efficient routes of hydrogen production. In such context, hydrogen may be obtained by the electrolysis of H2O in an electrolytic cell. In the electrolytic cell, hydrogen evolution reaction (HER) occurs at a cathode, and oxygen evolution reaction (OER) occurs at an anode. For both HER and OER, a suitable catalyst is vital for electrochemical water splitting. The catalyst must possess favorable qualities such as, but not limited to, low-cost, ease of preparation, high electrochemical activity, natural abundance, and long-term stability.

Presently, platinum (Pt), ruthenium (Ru), and iridium (Ir) based materials are extensively used for HER and OER, despite having high cost, low abundance, and poor stability under harsh conditions. Therefore, a need arises for designing a high-performance and low-cost electrocatalyst for HER and OER.

Spinel metal oxide (MO) nanostructures have been explored as potential catalysts due to their large surface area. Further, including additional metals to produce mixed transition MOs can alter HER electrode catalytic performance. Spinel oxides have a formula of AB2O4 (A=Mn, Cu, Co, Zn, Fe, Ni; B═Cr, Ni, Mn, Mo, Co) and can have normal, inverse, or complex structures determined by cation occupation of octahedral (Oh) or tetrahedral (Td) sites. Spinel-type oxides can improve catalytic performance, due to the structure, morphology, controllable composition, and valence of the spinel oxides.

Although several materials as OER/HER electrocatalysts have been developed in the past, there still exists a need to develop a method for water splitting that may circumvent the drawbacks of the prior art. It is one object of the present to make an efficient electrocatalyst for generating hydrogen in an environmentally friendly manner.

SUMMARY

In an exemplary embodiment, a method of generating hydrogen is described. The method includes applying a potential of 0.1 to 2 V to an electrochemical cell. The electrochemical cell is at least partially submerged in an aqueous solution; on applying the potential, the aqueous solution is reduced, forming the hydrogen. The electrochemical cell includes a counter electrode and an electrocatalyst. The electrocatalyst includes a nickel foam substrate and NiMoxCo2-xO4 nanoparticles as such, x>0 and x≤0.06. The NiMoxCo2-xO4 nanoparticles are distributed on the surface of the nickel foam substrate. A first portion of the NiMoxCo2-xO4 nanoparticles have a nano-needle morphology, where the nano-needles assemble to form a sphere in which the nano-needles project horizontally from the sphere, and the sphere has an average diameter of 1-5 micrometers (μm).

In some embodiments, the NiMoxCo2-xO4 nanoparticles have a cubic spinel oxide crystal structure.

In some embodiments, the NiMoxCo2-xO4 nanoparticles have a crystallite size of 12-18 nanometers (nm).

In some embodiments, Mo is only present at octahedral sites in the NiMoxCo2-xO4 nanoparticles.

In some embodiments, the nano-needles are uniformly spaced to form the sphere, and the spacing between the nano-needles forms a porous structure.

In some embodiments, the nano-needles have an average width of 10-30 nm.

In some embodiments, a second portion of the NiMoxCo2-xO4 nanoparticles have a morphology of spheres with an average diameter of 0.1-3 μm.

In some embodiments, the NiMoxCo2-xO4 nanoparticles include 1-20% of the first portion and 80-99% of the second portion, based on a total amount of the NiMoxCo2-xO4 nanoparticles.

In some embodiments, the NiMoxCo2-xO4 nanoparticles include Ni(II), Ni(III), Co(II), and Co(III).

In some embodiments, the NiMoxCo2-xO4 nanoparticles are hydrothermally grown on the nickel foam substrate.

In some embodiments, the NiMoxCo2-xO4 nanoparticles form a continuous layer on the nickel foam substrate.

In some embodiments, the counter electrode includes at least one of graphite and platinum.

In some embodiments, the aqueous solution includes water and a base.

In some embodiments, the electrocatalyst has a Tafel slope of 60-115 millivolts/decade (mVdec−1).

In some embodiments, x=0.04, and the electrocatalyst has a Tafel slope of 60-65 mVdec−1.

In some embodiments, the electrocatalyst has an overpotential of 200-300 mV at 10 milliampere per square centimeters (mA/cm2).

In some embodiments, x=0.04, and the electrocatalyst has an overpotential of 220-230 millivolts (mV) at 10 mA/cm2.

In some embodiments, the electrocatalyst has an electrochemically active surface area of 12-22 centimeters squared (cm2).

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.

As used herein, “nanoparticles” are particles having a particle size of 1 nm to 500 nm in at least one aspect within the scope of the present invention. In the present disclosure, the NiMoxCo2-xO4 nanoparticles may be micron sized particles, however they may be made from needles with at least one nanosized dimension as will described later.

As used herein, “particle size” and “pore size” may be thought of as 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. in the present disclosure.

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

As used herein, the term “current density” refers to the amount of electric current traveling per unit cross-section area.

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

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

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

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 directly associated with a cell's voltage efficacy. In an electrolytic cell, the occurrence of overpotential implies that the cell needs more energy as compared to that 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. Isotopes of naturally occurring nickel 28Ni include 58Ni, 60Ni, 61Ni, 62Ni, and 64Ni. Isotopes of oxygen include 16O, 17O, and 18O and isotopes of cobalt (Co) are 56Co, 57Co, 58Co, and 60Co. 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 transition metal-doped oxide nanoparticles grown on nickel foam for electrochemical hydrogen production. The present disclosure uses low-metal-cost materials for efficient and durable hydrogen evolution reactions (HER) electrocatalysts.

A method of generating hydrogen is described. The method includes applying a potential of 0.1 to 2.0 volts (V), preferably 0.2-1.9 V, preferably 0.3-1.8 V, preferably 0.4-1.7 V, preferably 0.5-1.6 V, preferably 0.6-1.5 V, preferably 0.7-1.4 V, preferably 0.8-1.3 V, preferably 0.9-1.2 V, and preferably 1.0-1.1 V, to an electrochemical cell. The electrochemical cell includes a counter electrode and an electrocatalyst.

The electrocatalyst further includes a nickel foam substrate and transition metal-doped oxide nanoparticles. In some embodiments, the nickel foam substrate could be replaced with nickel in a form of a sheet or foil, Herein, the foam is used because metal foams with a three-dimensional open-pore structure have a high specific surface area and structural rigidity, and thus are suitable self-supported substrates on which active materials can be in situ grown or coated. In some embodiments, the nickel foam has an average pore size of 500 nm−1 μm, preferably 550-950 nm, preferably 600-900 nm, preferably 650-850, preferably 700-800 nm. In some embodiments, the pores have a circular, rectangular, or square shape.

In alternate embodiments, the substrate may be any metal foam selected from the group consisting of an aluminum foam, a nickel foam, a titanium foam, a titanium alloy foam, an aluminum alloy foam, a magnesium alloy foam, a nickel alloy foam, and a steel foam. The substrate may have a thickness in a range of about 10 micrometers (μm) to 140 μm, for example, ranging from about 20 μm to about 120 μm, from about 50 μm to about 100 μm, from about 70 μm to about 95 μm, or from about 85 μm to about 90 μm, including all ranges and sub-ranges therebetween.

In some embodiments, the transition metal-doped oxide nanoparticles are dispersed on the surface of the substrate using one of the techniques like the drop-casting method, spray coating, spin coating, dip coating, hydrothermal growth, or aerosol-assisted chemical vapor deposition (AACVD). In a preferred embodiment, the transition metal-doped oxide nanoparticles are hydrothermally grown on the nickel foam substrate. The transition metal-doped oxide nanoparticles are distributed on the surface of the nickel foam substrate. The particles cover at least 50%, preferably 55%, preferably 60%, preferably 65%, preferably 70%, preferably 75%, preferably 80%, preferably 85%, preferably 90%, and preferably >95% of the substrate. The transition metal-doped oxide nanoparticles form a continuous layer on the nickel foam substrate, in other words there are no islands formed of the transition metal-doped oxide nanoparticles.

In some embodiments, the transition metal-doped oxide nanoparticles are spinel oxides having a formula of AB2O4, where A is at least one of Mn, Cu, Co, Zn, Fe, Ni and B is at least one of Cr, Ni, Mn, Mo, Co. In a preferred embodiment, the transition metal-doped oxide nanoparticles have a formula of NiCo2O4 doped with at least one selected from the group consisting of Cr, Ni, Mn, and Mo. In a most preferred embodiment, the transition metal-doped oxide nanoparticles have a formula of NiMoxCo2-xO4, where x is greater than 0.00 and less than or equal to 0.06, preferably 0.02, preferably 0.04, and preferably 0.06.

In some embodiments, the NiMoxCo2-xO4 nanoparticles have a cubic spinel oxide crystal structure. In some embodiments, the crystal structure includes cation occupation of octahedral (Oh) and/or tetrahedral (Td) sites. In some embodiments, the NiMoxCo2-xO4 nanoparticles include Ni(II), Ni(III), Co (II), and Co(III). In some embodiments, 50-80% of the Co and Ni ions occupy octahedral sites, preferably 55-75%, or 60-70%, with the remainder of the Co and Ni occupying tetrahedral sites. In some embodiments, doping with Mo replaces the Co ions at the octahedral sites. In some embodiments, Mo is only present at octahedral sites in the NiMoxCo2-xO4 nanoparticles. In some embodiments, the NiMoxCo2-xO4 nanoparticles have a crystallite size of 12-18 nm, preferably 13-17 nm, and preferably 14-16 nm.

In some embodiments, a first portion of the NiMoxCo2-xO4 nanoparticles have a nano-needle morphology. In some embodiments, the nano-needles, also referred to as the needles, have an average width of 10-30 nm, preferably 15-25 nm, or about 20 nm. In some embodiments, the needles have an average length of 0.5-2.5 μm, preferably 1.0-2.0 μm, or about 1.5 μm. In some embodiments, the needles assemble to form a spherical shape similar to that of a chestnut or a sea urchin, where the needles project horizontally from the sphere. The sphere may have an overall diameter of 1-5 μm, preferably 1.5-4.5 μm, 2.0-4.0 μm, 2.5-3.5 μm, or about 3 μm. In a preferred embodiment, the needles are uniformly spaced to form the sphere and the spacing between the needles forms a porous structure. In some embodiments, the needles are spaced 10-100 nm, preferably 20-90 nm, 30-80 nm, 40-70 nm, or about 50-60 nm apart on an outermost surface of the sphere.

In some embodiments, the NiMoxCo2-xO4 nanoparticles include the first portion having a chestnut shape and a second portion having a different morphology. In some embodiments, the first portion and the second portion are randomly distributed and do not form agglomerations with themselves. In some embodiments, a second portion of the NiMoxCo2-xO4 nanoparticles have a morphology of spheres with an average diameter of 0.1-3 μm, preferably 0.2-2.5 μm, 0.3-2.0 μm, 0.4-1.5 μm, 0.5-1.0 μm, or about 0.6-0.8 μm. In some embodiments, a third portion of the NiMoxCo2-xO4 nanoparticles of randomly distributed needles that do not form a sphere. In some embodiments, the third portion includes needles having a width of 10-100 nm, preferably 20-90 nm, 30-80 nm, 40-70 nm, or about 50-60 nm and a length of 1-5 μm, preferably 1.5-4.5 μm, 2.0-4.0 μm, 2.5-3.5 μm, or about 3 μm.

The NiMoxCo2-xO4 nanoparticles include 1-100%, preferably 5-95%, preferably 10-90%, preferably 15-85%, preferably 20-80%, preferably 25-75%, preferably 30-70%, preferably 35-65%, preferably 40-60%, and preferably 45-55% of the first portion based on a total amount of the NiMoxCo2-xO4 nanoparticles. In some embodiments, the NiMoxCo2-xO4 nanoparticles include 1-99%, preferably 5-95%, preferably 10-90%, preferably 15-85%, preferably 20-80%, preferably 25-75%, preferably 30-70%, preferably 35-65%, preferably 40-60%, and preferably 45-55% of the second portion based on a total amount of the NiMoxCo2-xO4 nanoparticles. In some embodiments, NiMoxCo2-xO4 nanoparticles include 1-10% of the third portion, preferably 2-8%, 3-7%, or 4-6%, based on a total amount of the NiMoxCo2-xO4 nanoparticles.

In some embodiments, the counter electrode includes at least one of graphite and platinum. In alternate embodiments, the counter electrode is made from a material selected from the group consisting of platinum, gold, and carbon. In alternate 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, 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 needs to supply sufficient current to the electrolyte solution to support the current required for the electrochemical reaction of interest. The counter electrode material should thus be sufficiently inert to withstand the chemical conditions in the electrolyte solution, such as acidic or basic pH values, without substantially 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 graphite.

In one embodiment, the electrochemical cell further includes a reference electrode in contact with the electrolyte solution. A reference electrode is an electrode that has a stable and well-known electrode potential. The high stability of the electrode potential is usually 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 (which herein is the electrocatalyst) or the counter electrode. 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 an Ag/AgCl electrode. However, in some embodiments, the electrochemical cell does not include a third electrode.

In some embodiments, the electrochemical cell is at least partially submerged in an electrolyte, preferably 50%, preferably 60%, or more preferably at least 70%. In some embodiments, the aqueous solution includes water and a base. The water may be tap water, distilled water, bidistilled water, deionized water, deionized distilled water, reverse osmosis water, and/or some other water. In some embodiments, the electrolyte includes the aqueous solution of the base at a concentration of 0.05-0.5 M, preferably 0.1-0.45 M, preferably 0.15-0.4 M, preferably 0.2-0.35 M, and preferably 0.25-0.3 M. In a preferred embodiment, the electrolyte includes the aqueous solution of a base at a concentration of 1 M. In some embodiments, the base is at least one selected from the group consisting of sodium hydroxide (NaOH), potassium hydroxide (KOH), lithium hydroxide (LiOH), barium hydroxide (Ba(OH)2), calcium hydroxide (Ca(OH)2). In an alternative embodiment, an organic base may be used, such as sodium acetate and potassium acetate. In a preferred embodiment, the base is KOH. 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. Preferably, the stirring is done by an impeller or a magnetic stir bar.

Preferably, the electrocatalyst functions as the cathode, receiving a negative potential to reduce H2O into H2 gas and OH−, while the counter electrode functions as the anode, receiving a positive potential to oxidize OH− into O2 gas. This is summarized by the following reactions:

In some embodiments, the working electrode and the counter-electrode are connected to each other by way of electrical interconnects that allow for the passage of current between the electrodes, when a potential is applied between them. In one embodiment, the potential may be applied to the electrodes by a battery, such as a battery including one or more electrochemical cells of alkaline, lithium, lithium-ion, nickel-cadmium, nickel metal hydride, zinc-air, silver oxide, and/or carbon-zinc. In another embodiment, the potential may be applied through a potentiostat or some other source of direct current, such as a photovoltaic cell. In one embodiment, a potentiostat may be powered by an AC adaptor, which is plugged into a standard building or home electric utility line. In one embodiment, the potentiostat may connect with a reference electrode in the electrolyte solution. Preferably the potentiostat is able to supply a relatively stable voltage or potential. For example, in one embodiment, the electrochemical cell is subjected to a voltage that does not vary by more than 5%, preferably by no more than 3%, preferably by no more than 1.5% of an average value throughout the subjecting. In another embodiment, the voltage may be modulated, such as being increased or decreased linearly, being applied as pulses, or being applied with an alternating current.

In one embodiment, the method further comprises the step of separately collecting H2-enriched gas and O2-enriched gas. In one embodiment, the space above each electrode may be confined to a vessel in order 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 electrocatalyst 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 comprise N2 from the air, water vapor, and other dissolved gases from the electrolyte solution. The H2-enriched gas may also comprise O2 from the air. The H2-enriched gas may comprise 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 comprising 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 60-115 millivolt/decade (mVdec−1), preferably 65-110 mVdec−1, preferably 70-105 mVdec−1, preferably 75-100 mVdec−1, preferably 80-95 mVdec−1, and preferably 85-90 mVdec−1. In some embodiments, for NiMoxCo2-xO4 (x=0.04), the electrocatalyst has a Tafel slope of 60-65 mVdec−1, preferably 61-64 mVdec−1, and preferably 62-63 mVdec−1. In a preferred embodiment, the electrocatalyst has a Tafel slope of 61.9 mVdec−1 for NiMoxCo2-xO4 (x=0.04).

While not wishing to be bound to a single theory, it is thought that the introduction of Mo altered the electron distribution and impeded the surface oxidation of the NiCo2O4, leading to an increase in the availability of active sites and ultimately promoting the efficacy of the HER process. However, if there is an overabundance of Mo doping in NiCo2O4, the Mo hinders the active sites and functions as a recombination center due to alterations in structure and composition.

EXAMPLES

The following examples demonstrate a method of generating hydrogen. 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: Materials

The chemicals used were analytical grade and utilized with no additional purification. Nickel nitrate (Ni(NO3)2·6H2O, 98%), ammonium molybdate ((NH4)6Mo7O24·4H2O, 99%), cobalt (II) nitrate (Co(NO3)2·6H2O, 98%), urea (CO(NH2)2) were obtained from Sigma-Aldrich and the nickel foam (NF).

Referring to FIG. 1C, a schematic illustration of the synthesis of three dimensional (3D) NiMoxCo2-xO4 (x≤0.06) CNSPs grown on NF. The 3D NiMoxCo2-xO4 (x≤0.06) CNSPs grown on NF were synthesized hydrothermally. Samples were synthesized for the NiMoxCo2-xO4, where x=0.00, 0.02, 0.04, and 0.06. The amounts of the components in the synthetic method were varied based on the desired composition.

Initially, the NF sheet was segregated into pieces of size 5×5 cm2 and cleaned in an ultrasonic bath for 10 minutes (min) using 3 molar (M) hydrochloric acid (HCl). Subsequently, ethanol and deionized (DI) water were used to separate the nickel oxide from the NF surface. For growing CNSPs on NF, nickel nitrate (98%), cobalt (II) nitrate (98%), ammonium molybdate (99%) were mixed with 3 milli-molar (mM) urea, 40 milliliters (mL) DI water and stirred for 20 min. Further, the solution was transferred to a 50 mL Teflon-lined stainless-steel autoclave. Concurrently, a piece of NF was directly immersed into the mixture in the Teflon and heated at 120° C. for 8 hours. In addition, the autoclave was chilled at room temperature (RT). The NF was cleaned with ethanol, assembled with residual powder, washed with DI water, collected, and dried at 50° C. overnight. Furthermore, the powder was calcinated at 350° C. in a furnace for 3 hours to produce the 3D NiMoxCo2-xO4 (x≤0.06) CNSPs grown on Ni foam.

Example 3: Characterization Techniques

Lattice structural information of NiMoxCo2-xO4 (x≤0.06) CNSPs phase was investigated using X-ray diffraction (XRD) with a Rigaku X-ray diffractometer, at a wavelength (λ) of about 1.54059 angstrom (Å). Morphology, including shape and particle size of NiMoxCo2-xO4 CNSPs, was examined by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). Chemical compositions and oxidation states of the elements in NiMoxCo2-xO4 CNSPs were studied using X-ray photoelectron spectroscopy (XPS) instruments. First-principles calculations were made using density functional theory (DFT)-based Quantum ATK software.

Example 4: Electrochemical Procedure

Electrocatalytic activity of NiMoxCo2-xO4 (x≤0.06) CNSPs electrodes for hydrogen evolution reaction (HER) was evaluated using a computer-controlled potentiostat (Metrohm AutoLab PGSTAT302N) in a conventional three-electrode chemical cell containing 1M potassium hydroxide (KOH) electrolyte with a pH of 13.6. The NF electrodes, having a size of about 1×1 cm2, were subjected to modification for the working electrode. In addition, the chemical cell employed Ag/AgCl 3.5 M potassium chloride (KCl) solution as the reference electrode, and graphite as the counter electrode. Before conducting the electrochemical measurements, the electrolytes were bubbled with nitrogen gas for a minimum duration of 30 min.

The catalytic efficacy of the electrodes was assessed through linear sweep voltammetry (LSV) at a scan rate of 10 millivolts per second mV/s in a 1 M KOH solution. The voltage that was measured, underwent an adaptation process to conform to the reversible hydrogen electrode (RHE) through the utilization of a formula as follows:

Further, Tafel slopes were computed utilizing the Tafel equation:

E
   ⁢
   C
   ⁢
   S
   ⁢
   A
  
  =
  
   
    C
    dl
   
   /
   
    C
    S

RF
  =
  
   E
   ⁢
   C
   ⁢
   S
   ⁢
   A
   /
   
    A
    geomatric

The calculation of the specific activity (SA in mA/cm2) was performed through the utilization of ECSA, employing the equation SA=j/ECSA, where j represents the cathodic current at an overpotential of 300 mV. The surface-active sites (n) of the electrodes depicted were assessed by conducting CV at a scan rate of 40 mV/s in 1M KOH electrolyte solution. To determine the surface charge density (Qs), the integrated charge of each electrode was divided in half, under an assumption of a one-electron redox process. The computation of the value of n was calculated using the following equation:

n
  =
  
   Qs
   /
   F

Chronopotentiometry (CP) analyses were conducted to evaluate its stability and durability, utilizing current densities of 10 mA/cm2.

Example 5: Structural Characterization

To analyze the phase formation of 3D NiMoxCo2-xO4 (x≤0.06) CNSPs grown on Ni foam, an X-ray powder pattern was employed, as shown in FIG. 2. As can be seen from FIG. 2, the phase formation exhibited the formation of pure cubic spinel oxide structure without undesired phases corresponding to JCPDS card no. 20-0781. The XRD displayed the structure of the NF substrate according to JCPDS card no. 04-0850. Further, the spectrum showed a broadened peak representing a small crystal size. The lattice parameters such as a=d(h2+k2+l2)1/2), cell volume (V), crystal size (DXRD) (DXRD=Kλ/β cos θ), weighted-profile R-factor (Rwp), expected R-factor (Rexp), chi-squared values (χ2=(Rwp/Rexp)2) and RBrag, indicates goodness of fit between theoretical calculation and the experimental data. All parameters were computed using the Rietveld refinement method via Match3!linked with full-proof software, the results are cataloged in Table 1. Furthermore, structural parameters and cell volume increased with the increase in the amount of Mo due to the expansion of spinel structure caused by the difference of ionic radii between the dopant (Mo) and the host ion (Co). As can be seen from Table 1, the crystal size is within a range of 12 nm to 18 nm. Moreover, FIG. 2 depicted the XRD pattern of modified NF that showed the peaks around 440 and 52.5°, while the peaks around 37°, 43°, and 640 are relevant to planes 311, 400, and 440, respectively.

The cation distribution in the NiMoxCo2-xO4 CNSPs system was analyzed using the Bertaut method. The method involved using intensity ratios from specific crystallographic planes in the XRD pattern, specifically I220/I440 and I422/I400, due to their sensitivity to cation placement. The detailed cation distribution for the NiMoxCo2-xO4 CNSPs system is depicted in Table 1. This analysis indicated that Co and Ni ions exist in both divalent and trivalent states, contributing to the stoichiometric balance of the system. Predominantly, Co and Ni ions occupied octahedral sites, with about 60% of Ni ions located at the octahedral B sites and the remaining 40% at tetrahedral A sites. The introduction of Mo ions decreased the proportion of Co ions at the octahedral B sites. As rare-earth elements with larger ionic radii, Mo ions were exclusively found at the octahedral B-sites.

Structural parameters and cation distribution of

Example 6: Morphological Characterization

FESEM and scanning electron microscopy (SEM) were utilized to analyze the surface morphology of prepared electrocatalysts. FIGS. 3A-3I are FESEM images of NiMoxCo2-xO4 CNSPs grown on NF at different magnifications 3D NiMoxCo2-xO4 (x≤0.06) CNSPs grown on NF and as-synthesized microsphere particles.

FIG. 3A provides different magnification images for the microspheres grown on the NF. The images show the microsphere particles covering the entire NF substrate. FIGS. 3B-3E show SEM images of NiMoxCo2-xO4 (x=0.00, 0.02, 0.04, 0.06) CNSPs grown on NF, respectively. The microsphere particles are composed of lengthy, thin, and steep tips nanoneedles with a diameter of around 20 nm. The nanoneedles grow horizontally from the center, which positively affects the electrode performances. Furthermore, FIGS. 3F-3I illustrate the SEM images of the of NiMoxCo2-xO4 (x=0.00, 0.02, 0.04, 0.06) CNSPs microspheres obtained after washing the NF. The samples include groups of microsphere particles interspersed with some chestnut-like particles.

Moreover, TEM analysis of NiMoxCo2-xO4 (x=0.04) CNSPs was performed. FIG. 4A-4B are TEM images of NiMoxCo2-xO4 (x=0.04) CNSPs grown on NF at 200 nm and 20 nm magnification. The TEM illustrates that the nano-needles shape includes uniform distribution of nano-needles, which leads to the creation of porous chestnut-like structures. The HR-TEM image in FIG. 4C revealed lattice spacing that was calculated via GATAN software confirming the spinel oxide structure.

Example 7: XPS Analysis

The chemical composition and ionic state of constituent elements in the sample NiMoxCo2-xO4 (x=0.04) CNSPs were studied using XPS, as shown in FIGS. 5A-5E. All elements in the sample are identified over a broad energy spectrum and indexed appropriately in the survey scan, as shown in FIG. 5A. FIG. 5B shows the core spectra of Co 2p spin-orbit doublets were detected at 779.5 eV for 2p3/2 and 794.5 eV for 2p1/2, along with two satellite peaks at 785.6 eV and 802.9 eV, respectively. The deconvolution of Co 2p3/2 and Co 2p1/2 spectrum was performed to get two peaks in each case that were assigned to Co2+ and Co3+ states. The peaks positioned at 779.2 eV and 794.2 eV were referred to as the Co2+ state, whereas the peaks situated at 781.1 eV and 796.1 eV were referred to as the Co3+ state. The slight deviation of Co 2p3/2 and Co 2p1/2 peak intensities from 2:1 was attributed to the overlapping of auger peak Co L3M45M45 caused by using Al Kα X-ray source.

Further, FIG. 5C illustrates the core spectra of Ni 2p with spin-orbit splitting of 17.9 eV, along with two satellite peaks at 860.9 eV and 879.5 eV. The deconvoluted Ni 2p3/2 spectrum introduced two characteristic peaks at 853.6 eV and 855.1 eV, assigned to Ni2+ and Ni3+ states, respectively. Similarly, two characteristic peaks were obtained through the deconvolution of the Ni 2p1/2 spectrum at 870.7 eV and 872.2 eV, corresponding to Ni2+ and Ni3+ states. A broad peak of the 0 is spectrum, as shown in FIG. 5D, was detected at 529.0 eV, which was fitted using three peaks. The peaks obtained at 529.0 eV, 530.6 eV, and 532.1 eV were assigned to covalently bonded oxygen in cobaltite (metal-O), oxygen vacancy/defects (vacancy-O), and atmospheric oxygen in the form of organic contaminations (C═O), respectively. Mo 3d spectra were detected with well-defined splitting of 3d5/2 and 3d3/2 components having ΔBE=3.1 eV, as shown in FIG. 5E. The core level peaks observed at 231.8 eV and 234.9 eV were attributed to the Mo 3d5/2 and Mo 3d3/2 spin-orbit doublet states indicated to Mo6+, which indicated that Mo substituted into the NiCo2O4 successfully.

Example 8: Electrochemical HER Performance

The electrochemical efficacy of NiMoxCo2-xO4 (x≤0.06) CNSPs electrocatalyst, modified using representative NF conducting substrates, was assessed through LSV and CP using a three-electrode system in N2-saturated 1M KOH solution with a pH of 13.6, as shown in FIGS. 6A-6F and FIGS. 7A-7F. The catalytic activity of the NF electrode alone was high. The LSV curves were acquired utilizing a 10 mV/s scan rate, as illustrated in FIG. 6A. However, upon modification of the NF electrode with NiCo2O4 (x=0.00), the current density was further improved and attained a value of 0.318 V vs RHE when subjected to an overpotential at current density (−10 mA/cm2). Further, the catalytic efficacy experienced a notable enhancement after introducing Mo into the Co sites within NiCo2O4, leading to the formation of NiMoxCo2-xO4 CNSPs. Introducing Mo6+ into the NiCo2O4 framework modulated the electronic configuration of Co within the framework, thereby augmenting the HER efficacy through its function as a catalytic center and amplifying the collective catalytic active sites. The NiMoxCo2-xO4 (where x=0.04) CNSPs display superior catalytic activity for the HER process, requiring a low overpotential of 0.224 V to achieve a current density of −10 mA/cm2, as illustrated in FIG. 6B.

The kinetics of hydrogen evolution on electrodes composed of bare and Mo-doped NiCo2O4 CNSPs were further analyzed using Tafel slopes. The Tafel slopes were quantified through utilization of a linear regression model applied to the Tafel plot, which plots the overpotential against the logarithm of current density. The Tafel slope of the 4% Mo-doped NiCo2O4 CNSPs electrode is 61.9 mV/dec, as depicted in FIG. 6C. The Tafel slope increased in the NiCo2O4 electrodes doped with 6% Mo. This may be due to the inhibition of active sites, which may agree with the LSV curves depicted in FIG. 6A. Table 2 provides a comparison of the electrocatalyst as described in the present disclosure and other electrocatalysts in alkaline environments.

Comparison of electrochemical HER performance over some reported

(mV at 10
Slope
Stability

Further, an analysis was conducted on the ECSA to examine the inherent HER performance of both unmodified and Mo-doped NiCo2O4 CNSPs composite electrodes. The calculation of the ECSA involved the evaluation of the Cdl, which was determined by measuring the CV at varying scan rates within a non-faradic region, as depicted in FIG. 6D. Furthermore, as shown in FIG. 6E, the electrode denoted as NiMoxCo2-xO4 (x=0.04) CNSPs demonstrated the highest Cdl value of 852.0 μF/cm2 compared to other electrodes of the same type. The observed Cdl value exhibits an increase of nearly 2.7 times compared to the unmodified NiCo2O4 electrode, thus indicating the influence of Mo doping on the NiCo2O4 CNSPs framework on Ni foam.

The NiMoxCo2-xO4 (x=0.04) CNSPs electrode displayed a notably greater ECSA of 21.3 cm2 in comparison to both unmodified (8.0 cm2) and alternative Mo-doped NiCo2O4 electrodes. The ECSA histogram is depicted in FIG. 6F and indicates that introducing Mo doping in the NiMoxCo2-xO4 CNSPs electrode results in a considerable increase in ECSA compared to the unmodified NiCo2O4 electrode. The introduction of Mo doping altered the electron distribution and impeded the surface oxidation of NiCo2O4, leading to an increase in the availability of active sites and ultimately promoting the efficacy of the HER process. Moreover, an abundance of Mo doping in NiCo2O4 CNSPs hindered the active sites and functioned as a recombination center due to alterations in structure and composition. The results presented align with the LSV outcomes, as depicted in FIG. 6A.

The findings indicated that the NiMoxCo2-xO4 (x=0.04) CNSPs electrocatalyst had a higher roughness factor, specifically 21.3, than alternative Mo-doped NiCo2O4 CNSPs electrocatalysts. In addition, the roughness factor of the NiMoxCo2-xO4 CNSPs electrocatalyst (x=0.04) was approximately 2.7 times higher than that of the unmodified NiCo2O4 electrode, as depicted in FIG. 7A.

The number of represented electrodes (N) was determined through surface charge density (QS) analysis to mitigate the influence of the catalyst's active sites. The results indicated that the NiMoxCo2-xO4 (x=0.04) CNSPs electrocatalyst had a higher number of calculated active sites, at 6.0×10−5 mol/cm2, compared to both the bare and other Mo-doped NiCo2O4 CNSPs electrocatalysts, as depicted in FIG. 7B. The findings indicated that the NiMoxCo2-xO4 CNSPs electrocatalyst was more effective in enhancing the HER process. This finding aligns with the LSV, Tafel plot, and ECSA analyses conducted on the electrodes selected as representatives. The specific activity (SA) was measured after the normalization of the ECSA to the current density at 300 mV in a 1M KOH solution. The NiMoxCo2-xO4 (x=0.04) CNSPs electrocatalyst demonstrated greater specific activity than the unmodified and other Mo-doped NiCo2O4 CNSPs electrodes, as depicted in FIG. 7C. Hence, the NiMoxCo2-xO4 (x=0.04) CNSPs electrocatalyst exhibits a significant increase in the HER activity.

Additional measurements utilizing EIS were conducted. The charge transfer kinetics at the semiconductor electrolyte interface (SEI) was evaluated by measuring the Nyquist plot of bare and Mo-doped NiCo2O4 CNSPs electrocatalyst at a potential of −0.3V RHE. The observed semicircles for bare and Mo-doped NiCo2O4 electrocatalysts are depicted in FIG. 7D. The NiMoxCo2-xO4 (x=0.04) CNSPs electrocatalyst demonstrated a reduced charge transfer resistance compared to the unmodified NiCo2O4 electrocatalyst. The SEI facilitated a more rapid charge transfer rate, enhancing the HER process. The results exhibited high concordance with the HER efficacy.

Extended CP evaluations on the NiMoxCo2-xO4 (x=0.04) CNSPs electrocatalyst were performed at a steady current density of 10 mA/cm2, as depicted in FIG. 7E. Following a 36 h uninterrupted cathodic protection experiment, the electrochemical potential experienced a reduction of 12 mV, corresponding to a suppression factor of approximately 4%. The CP measurements revealed a minor range fluctuation attributable to the persistent emergence of bubbles that were subsequently eliminated from the electrocatalyst surface. The electrocatalyst NiMoxCo2-xO4 (x=0.04) CNSPs electrocatalyst demonstrated exceptional catalytic performance, maintaining its potential for 36 h.

The Tafel slope was employed to investigate rate-determining steps in the HER mechanism process for enhanced comprehension. Overall, Volmer, Heyrovsky, and Tafel reactions are generally employed to convert hydrogen protons into molecular hydrogen. Based on the underlying mechanisms, the amalgamation of the Volmer pathway alongside either the Heyrovsky or Tafel pathways produce molecular hydrogen. Typically, when the slope is around 120 mV/dec, the Volmer step mechanism is a limiting factor for the rate of reaction. The Heyrovsky or Tafel step mechanism is considered the rate-determining step when the slope is around 40 or 30 mV/dec, respectively. In accordance with the present disclosure, the Tafel slopes for NiMoxCo2-xO4 (x=0.04) CNSPs electrocatalyst were determined to be 61.9 mV/dec, as shown in FIG. 6C. The Volmer-Heyrovsky step mechanism determine the rate of reaction. Furthermore, FIG. 7F depicts a proposed schematic illustration of the Volmer-Heyrovsky step mechanism of HER in an alkaline environment.

Example 9: DFT Calculations

DFT was employed to predict the activity of catalysts for the HER. DFT provides a location of the specific sites where hydrogen evolution may occur. In general, DFT involves designing transition metal-based catalyst materials and optimizing the electronic structure and surface properties thereof to enhance HER activity. In accordance with the present disclosure, first-principles calculations based on DFT were employed to investigate the impact of Mo dopants on catalytic and HER activity. DFT-based Quantum ATK software was utilized for designing and assessing a plurality of atomic configurations. The plurality of atomic configurations includes the adsorption of hydrogen (H) and water (W) molecules on both pristine CNSPs and Mo-doped CNSPs (MCNSPs). To accomplish this, the following atomic arrangements are utilized:

Additionally, the DFT analysis considered the adsorption of these molecules around the edge of the unit cell with a 3.6% Mo content to investigate the associated effects, denoted as MECNSP—H and MECNSP—W, respectively. The structure of CNSP was established using a unit cell including 56 atoms, specifically 8 Ni, 16 Co, and 32 O atoms. The associated lattice parameters were defined as follows: A=11.53 Å, B=5.76 Å, C=18.15 Å. A periodic-slab configuration was employed for the infinite (100) surface of CNSPs, which was oriented perpendicular to the z-axis (C-axis). This introduced non-periodicity in the C-direction due to the inclusion of a vacuum layer (around 13 Å thick) positioned directly above the surface to prevent interactions. FIG. 8 is a schematic illustration of unit cells for adsorption of hydrogen on molybdenum doped CNSP (MIICNSP—H) and adsorption of water on molybdenum doped CNSP (MICNSP—W), respectively. The crystal structures were optimized to a stable configuration. To refine the atomic distances within modified unit cells, a force tolerance of 0.01 eV/Å was set. The spin-resolved generalized gradient approximation (SGGA) with Perdew-Burke-Ernzerhof (PBE) functional (SGGA.PBE) was selected for the exchange and correlation potential. SGGA was employed due to its effectiveness in describing the distribution of ions in the corresponding nanoparticles. The atomic cores were represented using Pseudo Dojo pseudopotentials. The electronic structure of valence electrons such as Ni 4s23d8, Co 4s23d7, Mo 5s14d5, and O 2s22p4 was determined using high-basis sets of local numerical orbitals. A k-point sampling of was selected, along with a mesh cutoff energy of 125 Hartree, ensuring reasonable total energy convergence.

The chemical reactivity of the catalyst surfaces and their performance in the HER involve two key processes. Firstly, the adsorption and separation of water molecules, and secondly, the adsorption and release of hydrogen atoms. Introducing substitutional dopants enhances both reactivity and HER performance. To assess catalytic activity, the water molecules that adhere to the surface of the catalyst were examined. The adsorption energy (EA) of the above-mentioned water molecules was calculated using the following formula:

E
   A
   W
  
  =
  
   
    E
    system
   
   -
   
    E
    slab
   
   -
   
    E
    W

In accordance with the present disclosure, the water adsorption energies (EAW) were computed for four different catalysts, including CNSP—W, MICNSP—W, MIICNSP—W, and MECNSP—W. The values are listed in Table 3, including −2.138 eV (−49.31 kcal/mol), −2.224 eV (−51.28 kcal/mol), −2.331 eV (−53.75 kcal/mol), and −2.332 eV (−53.78 kcal/mol), respectively. Further, on comparing the water adsorption energies, it was revealed that MECNSP—W exhibits a slightly more negative value than MIICNSP—W. This indicates that MECNSP—W was relatively more stable and reactive in terms of water adsorption. The introduction of Mo dopants, which replace Co ions in both MECNSP—W and MIICNSP—W, was attributed to the enhanced catalytic activity observed. The dissociation of water molecules was directly linked to the strength of their interaction with the surface atoms in the slab of the catalyst.

EAH(W) values for the CNSP-H(W), MICNSP-H(W),

MIICNSP-H(W) and MECNSP-H(W). Gibbs free energies of hydrogen

adsorption (ΔGH) are also listed.

System
H
W
ΔGH (eV)