RED MUD-BASED CATALYSTS FOR WATER SPLITTING

Description

STATEMENT OF ACKNOWLEDGEMENT

Support provided by the Interdisciplinary Research Center, King Fahd University of Petroleum and Minerals, Saudi Arabia, through Project INRC2312 is gratefully acknowledged.

BACKGROUND

Technical Field

The present disclosure is directed towards electrocatalysts, and particularly, towards an electrocatalyst including red mud deposited on a nickel foam via laser annealing for water splitting reactions.

Description of Related Art

The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which 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.

Electrolysis plays a large role in advancing sustainability by providing a renewable source of energy that can help reduce reliance on fossil fuels. During electrolysis, an electric current is used to split water (H₂O) into hydrogen (H₂) and oxygen (O₂). Hydrogen production through electrolysis is a promising technology for sustainable hydrogen production, contributing to the transition to a cleaner energy system. This method emits only water as a byproduct and does not release greenhouse gases, making it an environmentally friendly alternative to traditional hydrogen production methods that rely on fossil fuels. Green hydrogen production, clean energy, sustainability, renewable energy, energy independence, and energy solutions for space research are some of the areas exploring water splitting.

Noble metals like platinum, ruthenium, and iridium are renowned for their catalytic activity, but their high cost and scarcity limit their widespread application. These metals have been effectively employed as catalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in water splitting processes. To address the challenge of noble metal use in water splitting, design and development of more affordable and environmentally friendly water-splitting catalysts is being explored.

The catalytic activity, stability, and overall effectiveness of non-noble metal catalysts in water splitting can be enhanced through various engineering strategies. Due to their availability and lower cost, transition metals and metal oxides, such as nickel, iron, cobalt, copper, and the like are frequently utilized as catalysts for both OER and HER. Surface modification techniques, such as doping with other elements or creating nanostructures, can improve the active surface area and electronic properties of catalysts.

Waste materials are being explored as affordable and effective catalysts for water splitting. Carbon-based resources like activated carbon, rusted iron oxide from corroded installations, and manganese oxide from discarded batteries are among waste elements considered. Optimizing the composition, structure, and surface characteristics of these materials may enhance their catalytic activity. Developing and improving these waste-derived catalysts may make water splitting more sustainable and efficient.

Although waste materials have been used as electrocatalysts in the past, there still exists a need to develop efficient, cost-effective electrocatalysts with abundant waste materials that contribute to sustainable and large-scale hydrogen production. Accordingly, an objective of the present disclosure is to develop a method of water splitting using a waste material, particularly red mud, that may overcome drawbacks of the current art.

SUMMARY

In an exemplary embodiment, a method of water splitting is described. The method includes applying a potential to an electrochemical cell that includes a working electrode, a reference electrode, and a counter electrode. The working electrode is a nickel foam substrate with a surface layer of annealed red mud, wherein the red mud consists of 10 to 30 wt. % iron, based on the total weight of the red mud. The electrochemical cell contains an aqueous basic solution. The method further includes generating hydrogen gas and oxygen gas at the working electrode.

In some embodiments, the working electrode has an overpotential of 120 to 140 millivolts (mV) at a current density of 10 mA/cm² while generating hydrogen at the working electrode.

In some embodiments, a method of making the working electrode is described. The method includes sonicating a red mud powder in an organic solvent to form a red mud suspension, drop-casting the red mud suspension on the nickel foam, and annealing the red mud suspension on the nickel foam with a laser to form the working electrode.

In some embodiments, a mass loading of the red mud suspension on the nickel foam is 0.5 to 12 mg/cm².

In some embodiments, the method includes annealing the red mud suspension at a wavelength of 10 to 11 μm.

In some embodiments, the method includes annealing the red mud suspension with the laser at 5 to 15% power.

In some embodiments, the method includes annealing the red mud suspension with the laser at a speed of 50 to 150 mm/s.

In some embodiments, the surface layer is discontinuous on the nickel foam and the annealed red mud is present as at least partially crystalline islands on the nickel foam.

In some embodiments, the partially crystalline islands have a diameter of 1 to 20 μm.

In some embodiments, the red mud further includes aluminum, silicon, sodium, titanium, manganese, vanadium, carbon, sulfur, calcium, chromium, zirconium, strontium, and magnesium.

In some embodiments, the red mud has a Brunauer-Emmett-Teller (BET) surface area of 30 to 60 m²/g.

In some embodiments, the red mud has a pore volume of 0.04 to 0.07 cm²/g.

In some embodiments, the reference electrode is a silver/silver chloride (Ag/AgCl) chloride.

In some embodiments, the counter electrode is a platinum rod.

In some embodiments, the aqueous basic solution is a potassium hydroxide electrolyte.

In some embodiments, the method includes applying a potential from −1.0 to −1.5 V vs.

RHE to generate hydrogen at the working electrode.

In some embodiments, the method includes applying a potential from 0 to 1.0 V vs. RHE to generate oxygen at the working electrode.

In some embodiments, the working electrode has a Tafel slope of 85 to 110 mV/dec while generating hydrogen at the working electrode.

In some embodiments, the working electrode has at least 95% of an initial activity after 18 to 22 hours at a current density of 10 mA/cm² while generating hydrogen at the working electrode.

In some embodiments, the working electrode has an overpotential of 290 to 310 mV at a current density of 10 mA/cm² while generating oxygen at the working electrode.

The foregoing general description of the illustrative embodiments 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 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 flowchart depicting a method of water splitting, according to certain embodiments.

FIG. 1B is a flowchart depicting a method of making a working electrode, according to certain embodiments.

FIG. 2 depicts X-ray diffraction (XRD) patterns of red mud samples (RM1 and RM2), according to certain embodiments.

FIG. 3 depicts XRD profiles of laser-irradiated red mud samples before deposition on nickel foams, according to certain embodiments.

FIG. 4 depicts an XRD pattern of laser-annealed red mud samples after deposition on nickel foam (NF) (designated as RM1/NF and RM2/NF), according to certain embodiments.

FIG. 5 depicts scanning electron microscopy (SEM) images and elemental mapping of laser-annealed red mud samples after depositing on the NF, (RM1/NF and RM2/NF) along with their used samples, according to certain embodiments.

FIG. 6A depicts comparative linear sweep voltammograms of laser-annealed red mud electrodes (RM1/NF and RM2/NF) at a scan rate of 5 mV s⁻¹, according to certain embodiments.

FIG. 6B depicts a zoomed-in plot of FIG. 6A, according to certain embodiments.

FIG. 6C is a bar graph depicting overpotentials of the laser-annealed red mud electrodes, according to various embodiments.

FIG. 6D depicts Tafel plots of the laser-annealed red mud electrodes, according to certain embodiments.

FIG. 6E depicts electrochemical impedance spectroscopy (EIS) analysis of the laser-annealed red mud electrodes, according to certain embodiments.

FIG. 6F depicts a plot of current density vs. time for the laser-annealed red mud electrode, RM2/NF, according to certain embodiments.

FIG. 7A depicts linear sweep voltammograms of laser-annealed red mud electrodes at a scan rate of 5 mV s⁻¹, according to certain embodiments.

FIG. 7B depicts Tafel plots of the laser-annealed red mud electrodes, according to certain embodiments.

FIG. 7C depicts EIS analysis of the laser-annealed red mud electrodes, according to certain embodiments.

FIG. 7D depicts a plot of current density vs. time for the laser-annealed red mud electrode, RM2/NF, according to certain embodiments.

DETAILED DESCRIPTION

In the drawings, like reference numerals designate identical or corresponding parts throughout the several views. 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.

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

As used herein, the words “about,” “approximately,” or “substantially similar” may be used when describing magnitude and or position to indicate that the value and/or position described is within a reasonable expected range of values and/or positions. For example, a numeric value may have a value that is +/−0.1% of the stated value (or range of values), +/−1% of the slated value (or range of values), +/−2% of the stated value (or range of values), +/−5% of the slated value (or range of values), +/−10% of the staled value (or range of values), +/−15% of the stated value (or range of values), or +/−20% of the stated value (or range of values). Within the description of this disclosure, where a numerical limit or range is stated, the endpoints are included unless stated otherwise. Also, all values and subranges within a numerical limit or range are specifically included as if explicitly written out.

The present disclosure is intended to include all hydration states of a given compound or formula, unless otherwise noted or when heating a material. For example, Ni(NO₃)₂ includes anhydrous Ni(NO₃)₂, Ni(NO₃)₂·6H₂O, and any other hydrated forms or mixtures.

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.

As used herein, the term “electrolytic cell” refers to an electrochemical cell that utilizes an external source of electrical energy to force a chemical reaction that would otherwise not occur. It is a device that facilitates a chemical reaction by applying an external electric current. The current drives a non-spontaneous reaction that would not occur spontaneously under standard conditions.

As used herein, the term “electrochemical cell” refers to a device that can generate electrical energy from the chemical reactions occurring in it, or use the electrical energy supplied to it to facilitate chemical reactions in it. These devices are capable of converting chemical energy into electrical energy and vice versa.

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 “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 overpotential and logarithmic current density.

Red mud is a reddish-brown, highly alkaline byproduct generated during the extraction of alumina from bauxite ore using the Bayer process. It results from the chemical treatment of bauxite with sodium hydroxide (NaOH) under high temperature and pressure. Red mud primarily consists of iron oxides (e.g., hematite), aluminum oxides, silica (SiO₂), other metal oxides (e.g., TiO₂, CaO, and the like), and alkali compounds (e.g., NaOH, Na₂CO₃, and the like).

Annealing is a controlled heating and cooling process applied to materials, such as metals, alloys, or glass, to relieve internal stresses, improve mechanical properties, and enhance workability. This process involves heating the material to a specific temperature, holding it at that temperature for a defined period, and then cooling it at a controlled rate.

Aspects of the present disclosure are directed to a method of water splitting using red mud as an electrocatalyst. The red mud is deposited and annealed onto a nickel foam substrate using a laser-based annealing method to form an electrode. The electrode was evaluated for its performance in water-splitting reactions, and the results indicate that the electrode comprising red mud is efficient and cost-effective for water-splitting applications.

FIG. 1A illustrates a flowchart of a method 50 of water splitting. 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 applying a potential to an electrochemical cell, comprising a working electrode, a reference electrode, and a counter electrode. “Working electrode” refers to the electrode in an electrochemical cell/device/sensor on which the electrochemical reaction of interest is occurring. The working electrode is a nickel foam (NF) substrate with a surface layer of red mud annealed on the nickel foam. In an embodiment, the NF may optionally include metals in addition to nickel, such as nickel, aluminum, or alloys thereof. The NF substrate is a porous material. In an embodiment, the average pore size, or largest diameter, of the NF substrate is about 50 to 500 micrometers (m), preferably 100 to 400 μm, or preferably 200 to 300 μm. In an embodiment, the substrate has a thickness of 0.1 to 10 mm, preferably 0.5 to 8 mm, preferably 1 to 5 mm, or preferably 2-3 mm. In an embodiment, the pores of the NF substrate have a shape such as cubical, conical, cuboidal, pyramidical, or cylindrical. In a preferred embodiment, the pores of the NF substrate have a spherical shape.

In some embodiments, the red mud further comprises aluminum, iron, silicon, sodium, titanium, manganese, vanadium, carbon, sulfur, calcium, chromium, strontium, zirconium, and magnesium. The red mud comprises 10 to 30 wt. %, preferably 12 to 28 wt. %, preferably 14 to 25 wt. %, preferably 16 to 24 wt. %, preferably 20 to 23.5 wt. %, or preferably about 21 to 22 wt. % iron based on a total weight of the red mud. In some embodiments, the red mud comprises 1-10 wt. %, preferably 2-9 wt. %, preferably 3-8 wt. %, preferably 4-6 wt. %, or preferably about 5 wt. % aluminum based on a total weight of the red mud. In some embodiments, the red mud comprises 5-10 wt. %, preferably 6-9 wt. %, preferably 7-9 wt. %, or preferably about 8-9 wt. % silicon based on a total weight of the red mud. In some embodiments, the red mud comprises 1-10 wt. %, preferably 2-9 wt. %, preferably 3-8 wt. %, preferably 5-7 wt. %, or preferably 5.5-6.5 wt. % sodium based on a total weight of the red mud. In some embodiments, the red mud comprises 0.1 to 5 wt. %, preferably 0.3-4 wt. %, preferably 0.5-3 wt. %, or preferably 0.8-2.1 wt. % titanium based on a total weight of the red mud. In some embodiments, the red mud comprises 0.1 to 1 wt. %, preferably 0.2 to 0.9 wt. %, preferably 0.3 to 0.8 wt. %, preferably 0.4 to 0.75 wt. %, or preferably 0.7 to 0.75 wt. % manganese based on a total weight of the red mud. In some embodiments, the red mud comprises 0.1 to 1 wt. %, preferably 0.2 to 0.9 wt. %, preferably 0.2 to 0.5 wt. %, or preferably 0.21 to 0.26 wt. % vanadium based on a total weight of the red mud. In some embodiments, the red mud comprises 1-5 wt. %, preferably 1.5-3 wt. %, preferably 1.9-2.6 wt. %, or preferably 1.98-2.57 wt. % carbon based on a total weight of the red mud. In some embodiments, the red mud comprises 0.1-0.5 wt. %, preferably 0.1-0.3 wt. %, preferably 0.1-0.2 wt. %, or preferably 0.12-0.14 wt. % sulfur based on a total weight of the red mud. In some embodiments, the red mud comprises 0.1 to 1 wt. %, preferably 0.1 to 0.5 wt. %, preferably 0.5 to 3 wt. %, preferably 0.8 to 2.5 wt. %, or preferably 0.82 to 2.47 wt. % calcium based on a total weight of the red mud. In some embodiments, the red mud comprises 0 to 1 wt. % or preferably 0 to 0.84 wt. % chromium based on a total weight of the red mud. In some embodiments, the red mud comprises 0-0.5 wt. % or preferably 0-0.26 wt. % zirconium based on a total weight of the red mud. In some embodiments, the red mud comprises 0.1 to 0.3 wt. %, preferably 0.15 to 0.25 wt. %, or preferably 0.19 to 0.22 wt. % strontium based on a total weight of the red mud. In some embodiments, the red mud comprises 0.01 to 0.05 wt. % or preferably 0.02 to 0.05 wt. % of magnesium based on a total weight of the red mud. The elemental composition of red mud may vary slightly beyond the ranges described depending on the source/location. In some embodiments, the red mud does not include phosphorus.

In one embodiment, the red mud comprises 9.5 wt. % aluminum, 14.5 wt. % iron, 8.9 wt. % silicon, 5.55 wt. % sodium, 0.88 wt. % titanium, 0.72 wt. % manganese, 0.21 wt. % vanadium, 1.98 wt. % carbon, 0.14 wt. % sulfur, 0.82 wt. % calcium, 0.84 wt. % chromium, 0.19 wt. % strontium, and 0.02 wt. % manganese based on a total weight of the red mud. In another embodiment, the red mud comprises 5 wt. % aluminum, 23.3 wt. % iron, 8.3 wt. % silicon, 6.35 wt. % sodium, 2.09 wt. % titanium, 0.73 wt. % manganese, 0.26 wt. % vanadium, 2.57 wt. % carbon, 0.12 wt. % sulfur, 2.47 wt. % calcium, 0.261 wt. % zirconium, 0.22 wt. % strontium, and 0.05 wt. % manganese based on a total weight of the red mud.

In some embodiments, the red mud has a Brunauer-Emmett-Teller (BET) surface area in the range of 20-70 m²/g, preferably 25-65 m²/g, more preferably 30-60 m²/g, and yet more preferably about 32-58 m²/g, and a pore volume of 0.01 to 0.1 cm³/g, preferably 0.02 to 0.08 cm²/g, more preferably 0.04 to 0.07 cm³/g, and yet more preferably about 0.044 to 0.067 cm³/g. In a specific embodiment, the red mud has a BET surface area of 25-35 m²/g, preferably 28-34 m²/g, more preferably 31-33 m²/g, and yet more preferably about 32.6 m²/g, and a pore volume of 0.01 to 0.05 cm³/g, preferably 0.03 to 0.045 cm³/g, and more preferably about 0.044 cm³/g. In another specific embodiment, the red mud has a BET surface area of 50-65 m²/g, preferably 52-63 m²/g, more preferably 55-60 m²/g, and yet more preferably about 58 m²/g, and a pore volume of 0.01 to 0.1 cm³/g, preferably 0.04 to 0.08 cm³/g, more preferably 0.06 to 0.07 cm³/g, and yet more preferably about 0.067 cm³/g. In some embodiments, the red mud is amorphous or crystalline in nature. In a preferred embodiment, the red mud is crystalline.

As used herein, the term “reference electrode” refers to an electrode with a stable and well-known electrode potential. In some embodiments, the reference electrode is a silver/silver chloride (Ag/AgCl) electrode. In some embodiments, the reference electrode may be, but is not limited to, a standard hydrogen electrode (SHE), a calomel electrode (saturated calomel electrode, SCE), a copper/copper sulfate electrode (Cu/CuSO₄), a standard calomel electrode (SCE), and a Luggin capillary. As used herein, the term “counter 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. An outer surface of the counter electrode may include an inert, electrically conducting chemical substance, such as platinum, gold, or carbon. The carbon may be in the form of graphite or glassy carbon. In some embodiments, the counter electrode is a platinum rod. In one embodiment, the counter electrode may be a wire, a rod, a cylinder, a tube, a scroll, a sheet, a piece of foil, a woven mesh, a perforated sheet, a brush, and the like. 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. In addition, 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 some embodiments, the reference electrode and the counter electrode may be connected through electrical interconnects that allow for the passage of current between the electrodes when a potential is applied between them. In an embodiment, the reference electrode and the counter-electrode can have the same or different dimensions. The reference electrode and the counter electrode may be arranged as known to a person of ordinary skill in the art.

The electrochemical cell contains an aqueous basic solution. The aqueous basic solution includes water and an inorganic base. In some embodiments, the base is selected from an alkaline earth metal hydroxide, such as beryllium hydroxide (Be(OH)₂), magnesium hydroxide (Mg(OH)₂), strontium hydroxide (Sr(OH)₂), and calcium hydroxide (Ca(OH)₂), and an alkali metal hydroxide, such as lithium hydroxide (LiOH), sodium hydroxide (NaOH), potassium hydroxide (KOH) and rubidium hydroxide (RbOH), and cesium hydroxide (CsOH). In other embodiments, the base may be any base known in the art. In a preferred embodiment, the aqueous basic solution includes potassium hydroxide as an electrolyte. In some embodiments, the molar concentration of KOH is in the range of 0.1 to 3 M, preferably 0.3 to 2 M, more preferably 0.5 to 1.5 M, and yet more preferably about 1 M.

At step 54, the method 50 includes generating hydrogen gas and oxygen gas at the working electrode. The hydrogen gas and the oxygen gas are generated by applying a potential at the working electrode. In an embodiment, the applied potential ranges from −2.0 to 1.5 V versus RHE.

In other embodiments, a potential may be applied in any range known in the art to generate hydrogen gas and oxygen gas. In a specific embodiment, a potential of −1.0 to −1.5 V versus RHE is applied for the generation of hydrogen. In another embodiment, a potential of 0 to 1.0 V versus RHE is applied for oxygen evolution.

The oxygen may be generated by splitting 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. In one embodiment, an H₂-enriched gas is collected above the working electrode. In another embodiment, an O₂-enriched gas is collected above the working electrode. The electrochemical cell, or an attachment, may be shaped to keep the headspace above the working electrode separate from the headspace above the reference electrode. In one embodiment, the H₂-enriched gas and the O₂-enriched gas are not 100% by volume H₂ and 100% by volume 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. In some embodiments, the H₂-enriched gas may include greater than 20% by volume H₂, preferably greater than 40% by volume H₂, more preferably greater than 60% by volume H₂, and even more preferably greater than 80% by volume H₂, relative to a total volume of the receptacle collecting the evolved H₂ gas. In some embodiments, the O₂-enriched gas may include greater than 20% by volume O₂, preferably greater than 40% by volume O₂, more preferably greater than 60% by volume O₂, and even more preferably greater than 80% by volume 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. Several parameters for the method for splitting water may be modified to lead to different reaction rates, yields, and other outcomes. These parameters include, but are not limited to, electrolyte type and concentration, pH, pressure, solution temperature, current, voltage, stirring rate, electrode surface area, size of manganese oxide particles, porosity, and exposure time. A variable DC current may be applied at a fixed voltage, or a fixed DC current may be applied at a variable voltage. In some instances, AC current or pulsed current may be used.

FIG. 1B illustrates a schematic flow chart of a method 70 of making the working electrode. 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 sonicating a red mud powder in an organic solvent to form a red mud suspension. The sonication is carried out to break down the particles of red mud into smaller particles and to allow for its dissolution in the organic solvent. In some embodiments, other modes of agitation known to those of ordinary skill in the art, for example, via stirring, swirling, mixing, or a combination thereof, may be employed to form the red mud suspension. In some embodiments, the organic solvent may include, but is not limited to, tetrahydrofuran, ethyl acetate, dimethylformamide, acetonitrile, acetone, dichloromethane, toluene, dimethyl sulfoxide, nitromethane, propylene carbonate, ethanol, formic acid, n-butanol, methanol, and a combination thereof. In a preferred embodiment, the organic solvent is ethanol. The sonication is carried out for 10 to 60 minutes, preferably 20 to 50 minutes, and more preferably about 30 minutes. At step 74, the method 70 includes drop-casting the red mud suspension on a nickel foam.

In some embodiments, the red mud suspension may be dispersed on the surface of the nickel foam using a technique such as physical vapor deposition (PVD), chemical vapor deposition (CVD), spin coating, dip coating, electrophoretic deposition (EPD), Langmuir Blodgett (LB) technique, drop casting, sol-gel process, layer-by-layer (LbL) assembly, inkjet printing, spray coating, ultrasonic spray deposition, a combination thereof, and the like. In some embodiments, a substrate other than nickel foam may be used as a support for the red mud. The amount of red mud suspension loaded on the nickel foam influences the OER and HER reaction. It is preferred that the mass loading of the red mud suspension on the nickel foam is in the range of 0.5 to 12 mg/cm², preferably 1 to 11 mg/cm², preferably 2 to 10 mg/cm², preferably 3 to 9 mg/cm², preferably 4 to 8 mg/cm², or preferably 5 to 7 mg/cm². In a preferred embodiment, the mass loading of the red mud suspension on the nickel foam is about 10 mg/cm². In another embodiment, preferred the mass loading of the red mud suspension on the nickel foam is about 1 mg/cm².

In one embodiment, the red mud suspension may substantially cover the substrate, whereby the % surface area coverage of the substrate that is covered with the metallic material is at least 75%, preferably at least 80%, preferably at least 85%, preferably at least 90%, preferably at least 95%, preferably at least 96%, preferably at least 97%, more preferably at least 98%, and yet more preferably at least 99%. In another embodiment, the substrate may incompletely cover, or only cover a portion or portions of the substrate, whereby the % surface area coverage of the substrate that is covered with the metallic material is less than 75%, preferably less than 65%, preferably less than 60%, preferably less than 55%, preferably less than 50%, preferably less than 45%, preferably less than 40%, preferably less than 35%, preferably less than 30%, preferably less than 25%, preferably less than 20%, preferably less than 15%, or preferably less than 10%. In another embodiment, the entire substrate and, therefore, the surface of the substrate (preferably 100%) is covered with the red mud suspension.

At step 76, the method 70 includes annealing the red mud suspension on the nickel foam with a laser to form the working electrode. During the laser annealing process, a focused laser beam irradiates the nickel foam at a high temperature, which is then accompanied by rapid cooling after the irradiation. Types of lasers that can be used for ablating the nickel foam deposited with the red mud suspension include, but are not limited to, helium-neon lasers, argon lasers, krypton lasers, xenon ion lasers, nitrogen lasers, carbon dioxide (CO₂) lasers, carbon monoxide lasers, excimer lasers, hydrogen fluoride lasers, deuterium fluoride lasers, chemical oxygen-iodine lasers, all gas-phase iodine lasers, dye lasers, ruby laser, yttrium-aluminum-garnet (YAG) lasers (e.g., YAG and any of Nd, Cr, Er, Y, Ca, glass, Th, Yb, Ho, and the like), and the like, so long as the laser can be used in conjunction with a N₂ assist gas. In a preferred embodiment, the nickel foam is ablated by directing a laser beam produced by a CO₂ laser onto the metallic surface. In one embodiment, the CO₂ laser produces a laser beam of infrared light having an operation wavelength of 8.0-12.0 μm, preferably 8.5-11.5 μm, preferably 9.0-11.0 μm, preferably 10-11 μm, more preferably 10.4-10.8 μm, and yet more preferably about 10.6 μm. In one embodiment, the CO₂ laser is powered by a traverse pump (high power). In an alternative embodiment, the CO₂ laser is powered by a longitudinal electrical discharge pump (low power). The CO₂ laser may also have an efficiency rating, as defined by the ratio of output power to pump power, of up to 25%, preferably up to 22%, preferably up to 20%, preferably up to 18%, preferably up to 15%, and more preferably between 5 and 15%. In a preferred embodiment, the metallic surface of the nickel foam is ablated with a laser beam having a power in the range of 10-50 W, preferably 15-45 W, preferably 20-40 W, more preferably 25-35 W, and yet more preferably about 30 W at a speed of 50-150 mm/s, preferably 75-125 mm/s, and more preferably about 100 mm/s.

Treating the substrate using the method of the present disclosure may alter the morphology of the substrate. In an embodiment, the surface layer is discontinuous on the nickel foam, and the annealed red mud is present as at least partially crystalline islands. In some embodiments, the partially crystalline islands have a diameter of 1 to 20 μm, preferably 5 to 19 μm, preferably 10 to 18 μm, more preferably 15 to 17 μm, and yet more preferably about 16 μm.

One parameter used to evaluate the kinetics of the reaction is a Tafel slope. The slope of a Tafel curve represents the Tafel slope, which is related to the activation energy of the reaction. Therefore, the slope indicates the reaction rate. The steeper the slope, the higher the activation energy required for the reaction to occur and the slower the reaction rate. In some embodiments, the working electrode has a Tafel slope of 40 to 110 millivolt/decade (mV/dec), preferably 85 to 110 mV/dec, or preferably 50 to 100 mV/dec. In a specific embodiment, the working electrode has a Tafel slope of about 50 mV/dec. The lower Tafel slope value (lower than that obtained with commercially used electrodes: 122 mV/dec for bare substrate (nickel foam) and 85 mV/dec for iridium oxide on nickel foam) indicates a faster kinetic process, indicating that the catalyst can achieve a current at a lower overpotential. Overpotential in electrolysis refers to the extra energy needed than thermodynamically expected to drive a reaction. To make a process commercially viable, it is beneficial to reduce overpotential during the electrolysis of water and improve upon the exchange current density, which measures the reaction rate at equilibrium potential. The electrocatalyst of the present disclosure produces oxygen from water efficiently at a considerably low overpotential and a high exchange current density. In an embodiment, the working electrode has an overpotential of 120 to 140 mV, preferably 121 to 136 mV, preferably 124 to 132, or preferably 127 to 130 mV at a current density of 10 mA/cm² while generating hydrogen at the working electrode. In some embodiments, the working electrode has an overpotential of 290 to 310 mV, preferably 293 to 307 mV, preferably 295 to 305, or preferably 297 to 302 mV at a current density of 10 mA/cm² while generating oxygen at the working electrode. In some embodiments, the working electrode has at least 95%, preferably at least 96%, preferably at least 97%, more preferably at least 98%, and yet more preferably about 99% of its initial activity after 18 to 22 hours at a current density of 10 mA/cm² while generating hydrogen at the working electrode, indicating long-term stability.

In some embodiments, the electrode of the present disclosure may be used in the field of batteries, fuel cells, photochemical cells, water splitting cells, electronics, water purification, hydrogen sensors, semiconductors (such as field effect transistors), magnetic semiconductors, capacitors, data storage devices, biosensors (such as redox protein sensors), photovoltaics, liquid crystal screens, plasma screens, touch screens, OLEDs, antistatic deposits, optical coatings, reflective coverings, anti-reflection coatings, reaction catalysis, a combination thereof, and the like.

EXAMPLES

The following examples describe and demonstrate the electrocatalytic performance of two different red mud samples deposited on nickel forms and annealed by laser irradiation for water-splitting OER and HER activities. 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: Details about Red Mud, Elemental Composition, and Characterization

Two different lots of red mud materials were collected from two different geographical locations and are named RM1 and RM2. For elemental analysis, these raw materials are separately extracted to an aqueous solution using acid mixtures and quantitatively analyzed using inductive coupled plasma-optical emission spectrometry (ICP-OES). The structural properties, reducibility/H₂ consumption, and surface area/porosity of the red mud samples by were analyzed by the respective techniques before deposition and annealing on the nickel substrates.

Crystallographic information of the alloy samples was acquired by X-ray diffraction (XRD, Rigaku MiniFlex X-ray diffractometer (Japan)) using Cu Kα radiation. The morphology of the resulting bimetallic alloy thin films was characterized using the scanning electron microscope, (SEM) JEOL JSM-6610LV (Japan). Surface areas and pore volumes of the samples were analyzed using the N₂ adsorption method and are calculated using the Brunauer-Emmett-Teller (BET) and Barrett, Joyner, and Halenda (BJH) equation. The samples were degassed at 300° C. for 12 h, and the analysis was performed in a liquid N₂ bath at −195.8° C. The metals in the red mud samples and their concentrations are listed in Table 1, along with the ICP results.

TABLE 1
Elemental composition of different red mud samples

Sample name

	Elements	RM1	RM2

	Al	9.5	5.0
	Fe	14.5	23.3
	Si	8.9	8.3
	Na	5.55	6.35
	Ti	0.88	2.09
	Mn	0.72	0.73
	P	ND	ND
	V	0.21	0.26
	C	1.98	2.57
	S	0.14	0.12
	Ca	0.82	2.47
	Cr	0.84	ND
	Zr	ND	0.261
	Sr	0.19	0.22
	Mg	0.02	0.05

Example 2: Discussion of Results Obtained for Prepared Samples

Major elements present are Fe, Al, Si, and Na, whereas metals like V, C, and Mg were detected at varying low concentrations in each sample. The X-ray diffraction patterns of the red mud samples are given in FIG. 2. The results show that the samples are crystalline in nature, with peaks identified for major components like iron oxide, aluminum oxide, and silica. BET/BJH results from the N₂ adsorption analysis are given in Table 2.

TABLE 2
Surface area, pore volume, and H2-temperature
programmed reduction (TPR) results

	Sample	BET Surface Area (m2/g)	Pore volume (cm3/g)

	RM1	32.6	0.044
	RM2	58.0	0.067

Example 3: Preparation of the Electrode

The preparation of electrodes is explained for thin film deposition. For thin film preparation, the nickel foam (NF) or substrate pieces (1 cm×2 cm) were cleaned in an ultrasonic bath for 10 minutes each with diluted HCl, acetone, and ethanol. Then, the NF was rinsed with deionized water and dried with high-purity N₂ gas. The red mud powder was deposited on both sides of the NF by drop-casting a suspension of red mud in ethanol, which was previously ultrasonicated for 30 minutes. The mass loading of the red mud was 10 mg/cm². The red mud on the NF was then subjected to laser annealing using a CO₂ laser machine with a wavelength of 10.6 μm (Universal Laser System Inc. USA, Model 2.30 DT and Power 30 W, a maximum speed of 1780 mm s⁻¹). The electrodes were prepared by laser annealing using 10% power and a speed of 100 mm s⁻¹ under ambient conditions and were denoted as RM1/NF and RM2/NF. The RM1/NF films were then cleaned by ultrasonication in ethanol to remove any residual untreated particles and dried in a vacuum oven. The mass loading of the red mud on the substrate was 1 mg cm⁻².

FIG. 3 shows the XRD profiles of laser-irradiated RM1 and RM2 powders (without deposition on nickel foams) to observe changes to the RM1 and RM2 powders upon laser irradiation. It is observed that despite the crystallinity of the materials being slightly enhanced after irradiation, there are no changes in their profiles. FIG. 4 displays XRD profiles of red mud samples on nickel foam, with no observable diffraction peaks due to low content. SEM images and elemental mapping of the red mud samples after deposition on the nickel forms by laser annealing are shown in FIG. 5, along with the SEM images of the bare nickel foam as blank for the sake of comparison. For both RM1/NF and RM2/NF, the SEM images of the electrodes after testing, denoted as “used” samples, are also given. The elemental mapping of all the prepared electrodes is also included to observe the distribution of metallic components after deposition and after use. It was observed that potassium content is increased in the SEM-EDX results for both used samples. The increase in potassium content is due to the absorption of potassium from the KOH solution used as an electrolyte.

Example 4: Water Splitting Tests of Red Mud Samples

The electrocatalytic water oxidation was studied via a computer-controlled Autolab potentiostat workstation using the Nova 2.1.6 interface. A three-electrode cell consisting of Ag/AgCl, platinum (Pt) rod, and CoNi or FeCoNi film electrodes deposited on nickel foam and immersed in 1.0 M KOH electrolyte were used as the reference, counter, and working electrodes, respectively. Cyclic voltammetry (CV) analysis was conducted to activate the catalyst's surface for water oxidation. For the hydrogen evolution reaction (HER), the measurements were performed in a potential range of −1.0 to −1.5 V versus RHE at a scan rate of 5 mV/s. For the oxygen evolution reaction (OER), the measurements were carried out in a potential range of 0 to 1.0 V versus RHE at a scan rate of 5 mV/s. The potentials were converted to the RHE scale, and the data obtained were reported after iR corrections. Electrochemical impedance spectroscopy (EIS) was investigated at a potential at −1.12 and 1.50 V in the frequency range from 0.01 Hz to 100 kHz for HER and OER, respectively. The chronopotentiometry (CP) method was used to evaluate the catalytic durability of the prepared samples at the current densities of 10 mA cm⁻². All measured potentials were converted to reversible hydrogen electrode (RHE) scale.

The OER performance of red mud samples was evaluated in 1 M KOH using linear sweep voltammetry (LSV) and cyclic voltammetry (CV). IrO₂ was coated on nickel foam by drop casting to compare the OER activities off the red mud electrodes, and its OER performance was also tested. FIG. 6A displays the LSVs of red mud samples on NF (RM1/NF and RM2/NF), IrO₂ on NF, and bare NF. The bare NF exhibited the lowest OER activity. The laser-annealed red mud samples demonstrated good OER activity, as seen in FIG. 6B (a zoomed image of the LSV plots shown in FIG. 6A). All samples exhibited a pre-oxidation peak before the onset of OER, which is observed in Ni-based electrocatalysts with Ni²⁺ and is attributed to the oxidation of Ni(II) to Ni(III). OER electrocatalysts are usually benchmarked by comparing their overpotential, f, needed to achieve a current density of 10 mA cm⁻². The overpotential for O₂ evolution was measured at 1.54 V vs. RHE for both IrO₂ and RM2/NF samples. The RM1/NF sample showed a slightly higher potential of 1.55 V vs. RHE. The overpotential was further compared at higher current densities of 50 and 100 mA cm⁻² (FIG. 6C). It was observed that at these current densities, the RM2/NF sample showed the lowest overpotential followed by the RM1/NF sample.

OER kinetics were investigated by analyzing the Tafel plots, as shown in FIG. 6D. The Tafel slopes were obtained from the LSV values. The Tafel values were calculated as 122, 85, 98, and 50 mV dec⁻¹ for bare NF, IrO₂/NF, RM1/NF, and RM2/NF electrodes, respectively.

Electrochemical impedance spectroscopy (EIS) was used to further understand electronic conduction and the results are shown in FIG. 6E. The radius of the semicircular arc of RM2/NF samples was the smallest, indicating that it has a low charge transfer resistance and fast reaction rate. The long-term stability for the RM2/NF sample was checked by performing chronoamperometric measurements by applying a potential of 1.54 V vs RHE. In the oxidizing potential, the electrode loses its activity, and about a 40% loss in activity was recorded in 4 hours of testing (FIG. 6F).

The HER catalytic activity of the laser-annealed red mud electrodes in 1 M KOH was evaluated by various electrochemical techniques such as CV, LSV, EIS, and chronoamperometry. The LSVs were recorded in the same three-electrode system, as mentioned above. The performance of the red mud samples was benchmarked against 20% Pt/C, a known catalyst for HER. The LSV curves of both laser-annealed red mud electrodes (RM1/NF and RM2/NF) showed good HER catalytic activity, as depicted in FIG. 7A. The RM1/NF electrode has an overpotential of 122 mV and the RM2/NF electrode has an overpotential of 135 mV at a current density of 10 mA/cm². At higher current densities (>20 mA), both electrodes exhibited similar overpotentials, with RM2/NF showing a slight advantage over RM1/NF. The overpotential and the maximum current density values for the two red mud electrodes are summarized in Table 3. The Tafel slopes, which indicate the rate-determining step of the HER process, were derived from the LSV plots and are presented in FIG. 7B. The Tafel values were calculated as 114, 45, 91, and 105 mV dec⁻¹ for bare NF, Pt/C, RM1/NF, and RM2/NF electrodes, respectively. The EIS measurement (FIG. 7C) revealed that RM2/NF had better charge transfer kinetics than RM1/NF, as evidenced by the smaller semicircle arc. The stability of the catalyst for the continuous HER process in 1 M KOH was assessed by chronoamperometric measurements, as shown in FIG. 7D The current vs. time plot demonstrated that the RM2/NF electrode maintained a high stability for HER activities for prolonged periods of time (20 hours).

TABLE 3
Electrochemical parameters for laser-annealed red mud samples

	Electrodes/	Max current	Overpotential
	catalysts	density (mA/cm2)	(mV) @ 10 mA/cm2

	RM1/NF	−1.20 A	122
	RM2/NF	−0.85 A	135

Electrocatalytic performance of two different red mud samples deposited on nickel forms and annealed by laser irradiation was evaluated for water splitting OER and HER activities. It was observed that at current densities of 10, 50, and 100 mA cm², the RM2/NF sample showed the lowest overpotential, followed by the RM1/NF sample for the OER experiments. The LSV curves of both laser-annealed red mud electrodes (RM1/NF and RM2/NF) showed good HER catalytic activity compared to the bench marked 20% Pt/C electrode. The EIS measurements revealed that RM2/NF had better charge transfer kinetics than RM1/NF. The current vs. time plot demonstrated that the RM2/NF electrode maintained a high stability for HER activities for prolonged periods of time. The presence of metals like Fe, Na, Ti, and Ca in the red mud samples may be responsible for their increased performance, and RM2 has these metals in relatively higher concentrations than RM1. The surface area and pore volume of RM2 is higher than RM1. The results indicate that the red mud samples can be applied as bi-functional electrocatalysts for OER and HER reactions to produce green hydrogen by water splitting. RM2/NF samples are stable even after use, whereas the RM1/NF sample appears to show agglomeration of foreign material on the foam, as observed from the “used” samples of RM1/NF and RM2/NF.

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