Water Electrolysis Catalysts

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application claims benefit and priority of U.S. provisional patent application Ser. No. 63/741,659 filed Jan. 3, 2025, and entitled “NICKEL/IRON-BASED (OXY) HYDROXIDE CATALYSTS FOR WATER ELECTROLYSIS AND METHODS FOR PRODUCING SAME,” which is hereby incorporated herein by reference in its entirety for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002]Not applicable.

TECHNICAL FIELD

[0003]The present disclosure relates generally to catalysts. More particularly, the present disclosure relates to water electrolysis catalysts. Still more particularly, the present disclosure relates to oxygen evolution reaction catalysts having enhanced catalytic activity and stability for use in water electrolysis and methods of making same.

BACKGROUND

[0004]In recent decades, the global energy economy based on hydrocarbons has faced serious constraints from both economic and environmental perspectives. The increasing demand for energy has led to a decline in fossil fuel production capacity, threatening global energy supply and putting immense pressure on the environment. Thus, finding alternatives to hydrocarbon fuels is crucial. Hydrogen, with its high energy density, pollution-free nature, and renewability, is considered a promising alternative to fossil fuels.

[0005]There are three main methods of hydrogen production leading to three kinds of hydrogen: gray hydrogen, blue hydrogen, and green hydrogen. Gray hydrogen, the most common method, is produced from fossil fuels like oil, natural gas, and coal. While it is a relatively simple process, it is associated with significant greenhouse gas emissions. Blue hydrogen production is like gray hydrogen but captures and stores CO₂ during the process. Although more environmentally friendly, it remains a transitional solution dependent on fossil fuel supply. In contrast, green hydrogen is produced through the electrolysis of water using renewable energy sources like wind, hydro, solar, and nuclear power, without emitting CO₂ or other greenhouse gases, making it the most environmentally friendly method.

[0006]Water electrolysis is a sustainable and clean route to produce hydrogen (H₂) fuel that involves two half-reactions: the hydrogen evolution reaction (HER) on the cathode and the oxygen evolution reaction (OER) on the anode. The OER process is crucial for green hydrogen production from water splitting. Compared with the HER process, the OER process is more sluggish because of the rigid 0-0 double bond and the multistep proton and electron transfer process, which hampers the overall efficiency of water electrolysis. Therefore, developing OER catalysts with fast reaction kinetics is of great importance for efficient alkaline-based water electrolysis.

[0007]NiFe-based (oxy) hydroxides have been demonstrated to be the most efficient noble-metal-based OER electrocatalysts in alkaline freshwater electrolytes. In the past several years, increasing numbers of NiFe-based (oxy) hydroxides, including NiFe layered double hydroxide (LDH), NiFe—CuCo LDH, WOi—intercalated NiFe LDH, Ni-doped FeOOH, S-doped (Ni, Fe) OOH, NiFeOH—NiS_x, FeOOH@NiCoOH, and FeOOH—NiFeO_xH_y, have been utilized as OER electrodes in seawater electrolytes with notable activity. These advancements in NiFe-based (oxy) hydroxides have significantly propelled the development of seawater electrolysis technology. Nevertheless, when operated at ampere-scaled current densities (≥1000 mA cm⁻²), these materials often experience diminished performance with reduced activity or limited lifetimes (less than 1000 h) compromised by insufficient mass transfer and Cl⁻ corrosion, which keeps the development of large-scale applications for NiFe-based (oxy) hydroxides in their infancy. In addition, the synthesis of NiFe-based (oxy) hydroxide catalysts involves tedious multistep procedures under high temperature that result in significant time and energy consumption. Currently, the most common methods to fabricate NiFe-based (oxy) hydroxide catalysts are electrodeposition and hydrothermal techniques-both of which require large energy inputs and/or elaborate equipment, which introduce a financial burden to large-scale industrial applications.

[0008]Accordingly, there remains a need in the art for more efficient, lower cost methods of making high performance NiFe-based (oxy) hydroxide OER catalysts for advancing the development of water electrolysis for large-scale hydrogen production.

BRIEF SUMMARY OF THE DISCLOSURE

[0009]Disclosed herein is a method for producing an oxygen evolution reaction (OER) catalyst comprising contacting at least two transition metal salt catalysts and an optional dopant with a substrate at room temperature for a time period of from about 15 minutes to about 24 hours.

[0010]Also disclosed herein is a method for producing a self-supported catalyst for water electrolysis, comprising dissolving an amount of sodium hydroxide (NaOH), an amount of ammonium persulfate ((NH₄)₂S₂O₈), and an amount of ammonium molybdate ((NH₄)₆Mo₇O₂₄) in a solvent at room temperature to form a solution; immersing a substrate into the solution for a time period sufficient to grow the self-supported catalyst thereon; and removing the substrate from the solution after the time period, wherein the substrate comprises the self-supported catalyst.

[0011]Also disclosed herein is a method for producing a self-supported catalyst for water electrolysis, comprising dissolving an amount of ferric nitrate (Fe(NO₃)3) and an amount of sodium thiosulfate (Na₂S₂O₃) in a solvent at room temperature to form a solution; immersing a substrate into the solution for a time period sufficient to grow the self-supported catalyst thereon; and removing the substrate from the solution after the time period, wherein the substrate comprises the self-supported catalyst.

[0012]Also disclosed herein is method for producing a self-supported catalyst for water electrolysis, comprising dissolving an amount of iron (II) sulfate (FeSO₄) and an amount of sodium thiosulfate (Na₂S₂O₃) in a solvent at room temperature to form a solution; immersing a substrate into the solution for a time period sufficient to grow the self-supported catalyst thereon; and removing the substrate from the solution after the time period, wherein the substrate comprises the self-supported catalyst.

[0013]Aspects described herein comprise a combination of features and characteristics intended to address various shortcomings associated with certain prior devices, systems, and methods. The foregoing has outlined rather broadly the features and technical characteristics of the disclosed aspects in order that the detailed description that follows may be better understood. The various characteristics and features described above, as well as others, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. It should be appreciated that the conception and the specific aspects disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes as the disclosed aspects. It should also be realized that such equivalent constructions do not depart from the spirit and scope of the principles disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]Further features and advantages can be ascertained from the following detailed description that is provided in connection with the drawings described below:

[0015]FIG. 1A is a first view of a scanning electron microscopy (SEM) images of the stainless-steel mat (SSM) at a first magnification.

[0016]FIG. 1B is a first view of a scanning electron microscopy (SEM) images of the stainless-steel mat (SSM) at a second magnification.

[0017]FIG. 1C is a first view of a scanning electron microscopy (SEM) images of the stainless-steel mat (SSM) at a third magnification.

[0018]FIG. 2 is an energy-dispersive X-ray spectroscopy (EDS) spectrum for the SSM.

[0019]FIG. 3 shows optical images of the SSM, NiFeOOH/SSM, and Mo—NiFeOOH/SSM electrodes.

[0020]FIG. 4 shows X-ray diffraction (XRD) patterns for the SSM, NiFeOOH/SSM, and Mo—NiFeOOH/SSM samples.

[0021]FIG. 5 shows Raman spectra for NiFeOOH/SSM and Mo—NiFeOOH/SSM.

[0022]FIG. 6A shows SEM images of NiFeOOH/SSM at low magnifications.

[0023]FIG. 6B shows SEM images of NiFeOOH/SSM at high magnifications.

[0024]FIG. 7A show SEM images of Mo—NiFeOOH/SSM at low magnifications.

[0025]FIG. 7B show SEM images of Mo—NiFeOOH/SSM high magnification.

[0026]FIG. 8A shows an SEM of Mo—NiFeOOH/SSM.

[0027]FIG. 8B shows an EDS elemental mapping image of nickel on Mo—NiFeOOH/SSM.

[0028]FIG. 8C shows an EDS elemental mapping image of iron on Mo—NiFeOOH/SSM.

[0029]FIG. 8D shows an EDS elemental mapping image of molybdenum on Mo—NiFeOOH/SSM.

[0030]FIG. 8E shows an EDS elemental mapping image of oxygen on Mo—NiFeOOH/SSM.

[0031]FIG. 9 shows a transmission electron microscopy (TEM) image of Mo—NiFeOOH/SSM.

[0032]FIG. 10 shows another TEM image of Mo—NiFeOOH/SSM.

[0033]FIG. 11 shows a high-resolution TEM (HRTEM) image of Mo—NiFeOOH/SSM.

[0034]FIG. 12 shows a selected area electron diffraction (SAED) pattern of Mo—NiFeOOH/SSM.

[0035]FIG. 13A shows a scanning TEM (STEM) of Mo—NiFeOOH/SSM.

[0036]FIG. 13B shows an EDS elemental mapping image of nickel of Mo—NiFeOOH/SSM of FIG. 13A.

[0037]FIG. 13C shows an EDS elemental mapping image of iron of Mo—NiFeOOH/SSM of FIG. 13A.

[0038]FIG. 13D shows an EDS elemental mapping image of molybdenum of Mo—NiFeOOH/SSM of FIG. 13A.

[0039]FIG. 14 is a high-resolution X-ray photoelectron spectroscopy (XPS) spectra of Fe 2p for NiFeOOH/SSM and Mo—NiFeOOH/SSM.

[0040]FIG. 15 is a high-resolution XPS spectra of Ni 2p for NiFeOOH/SSM and Mo—NiFeOOH/SSM.

[0041]FIG. 16 is a high-resolution XPS spectra of Mo 3d for NiFeOOH/SSM and Mo—NiFeOOH/SSM.

[0042]FIG. 17 shows OER polarization curves in 1 M KOH for SSM, NiFeOOH/SSM, and Mo—NiFeOOH/SSM.

[0043]FIG. 18 is a graph showing a comparison of the overpotentials required to achieve current densities of 100, 500, and 1,000 mA cm⁻² for each of SSM, NiFeOOH/SSM, and Mo—NiFeOOH/SSM.

[0044]FIG. 19 shows OER polarization curves in 1 M KOH for Mo—NiFeOOH/SSM prepared with different amounts of the Mo source.

[0045]FIG. 20 shows Tafel plots derived from the OER polarization curves shown in FIG. 17 for SSM, NiFeOOH/SSM, and Mo—NiFeOOH/SSM.

[0046]FIG. 21 shows electrochemical impedance spectroscopy (EIS) Nyquist plots for SSM, NiFeOOH/SSM, and Mo—NiFeOOH/SSM.

[0047]FIG. 22A shows a digital image of a 1 M KOH electrolyte droplet placed on the surface of SSM.

[0048]FIG. 22B shows a digital image of a 1 M KOH electrolyte droplet placed on the surface of Mo—NiFeOOH/SSM.

[0049]FIG. 23 is a graph showing long-term stability testing of Mo—NiFeOOH/SSM at a constant current density of 500 mA cm⁻² in 1 M KOH.

[0050]FIG. 24 show OER polarization curves for the Mo—NiFeOOH/SSM electrode in different electrolytes.

[0051]FIG. 25 is a graph showing a comparison of the overpotentials required to achieve current densities of 100, 500, and 1,000 mA cm⁻² for the Mo—NiFeOOH/SSM electrode in different electrolytes.

[0052]FIG. 26 show OER polarization curves of Mo—NiFeOOH/SSM in 1 M KOH+Seawater and 6 M KOH+Seawater electrolytes.

[0053]FIG. 27 is a graph showing the calculation of Faradaic efficiency of Mo—NiFeOOH/SSM at a large current density of 1,000 mA cm⁻² in 6 M KOH+Seawater electrolyte.

[0054]FIG. 28 is a graph showing long-term stability tests at constant current densities of 500 and 1,000 mA cm⁻² for the Mo—NiFeOOH/SSM electrode in 1 M KOH+Seawater electrolyte.

[0055]FIG. 29 is a graph showing long-term stability tests at a constant current density of 1,000 mA cm⁻² for the Mo—NiFeOOH/SSM electrode in 6 M KOH+Seawater electrolyte.

[0056]FIG. 30A show high-resolution XPS spectra of Fe 2p Mo—NiFeOOH/SSM before and after more than 2,000 hours of OER stability testing under a large current density of 1,000 mA cm⁻² in 6 M KOH+Seawater.

[0057]FIG. 30B show high-resolution XPS spectra of Ni 2p for Mo—NiFeOOH/SSM before and after more than 2,000 hours of OER stability testing under a large current density of 1,000 mA cm⁻² in 6 M KOH+Seawater.

[0058]FIG. 30C show high-resolution XPS spectra Mo 3d for Mo—NiFeOOH/SSM before and after more than 2,000 hours of OER stability testing under a large current density of 1,000 mA cm⁻² in 6 M KOH+Seawater.

[0059]FIG. 31 is a schematic illustration of a two-electrode seawater electrolyzer using NiMON and Mo—NiFeOOH/SSM as the cathode and anode, respectively.

[0060]FIG. 32 shows polarization curves (with iR compensation) for the NiMoNIIMo—NiFeOOH/SSM electrolyzer in different seawater electrolytes at different temperatures.

[0061]FIG. 33 is a schematic illustration of an AEM water electrolyzer using NiMON and Mo—NiFeOOH/SSM as the cathode and anode, respectively.

[0062]FIG. 34 are images showing the synthesis of a large-size (−5 cm×5 cm) Mo—NiFeOOH/SSM electrode from commercial SSM for the AEM water electrolyzer measurements.

[0063]FIG. 35 shows polarization curves (without iR compensation) for the AEM water electrolyzer in different electrolytes at different temperatures.

[0064]FIG. 36 is a graph showing durability tests of the AEM water electrolyzer at a constant current of 12.5 A (current density of 500 mA cm⁻²) in 1 M KOH electrolyte at 60° C.

[0065]FIG. 37 is a graph showing durability tests of the AEM water electrolyzer at a constant current of 12.5 A (current density of 500 mA cm⁻²) in 1 M KOH+Seawater electrolyte at 60° C.

DETAILED DESCRIPTION

[0066]The following discussion is directed to various exemplary aspects. However, one skilled in the art will understand that the examples disclosed herein have broad application, and that the discussion of any aspect is meant only to be exemplary of that aspect, and not intended to suggest that the scope of the disclosure, including the claims, is limited to that aspect.

[0067]Certain terms are used throughout the following description and claims to refer to particular features or components. As one skilled in the art will appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name but not function. The drawing figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in interest of clarity and conciseness.

[0068]Unless the context dictates the contrary, all ranges set forth herein should be interpreted as being inclusive of their endpoints, and open-ended ranges should be interpreted to include only commercially practical values. In addition, with respect to all ranges disclosed herein, such ranges are intended to include any combination of the mentioned upper and lower limits even if the particular combination is not specifically listed. All lists of values should be considered as inclusive of intermediate values unless the context indicates the contrary. Where numerical ranges or limitations are expressly stated, such express ranges or limitations should be understood to include iterative ranges or limitations of like magnitude falling within the expressly stated ranges or limitations (e.g., from about 1 to about 10 includes, 2, 3, 4, etc.; greater than 0.10 includes 0.11, 0.12, 0.13, etc.).

[0069]In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” As used herein, the phrases “consist(s) of” and “consisting of” are used to refer to exclusive components of a composition, meaning only those expressly recited components are included in the composition; whereas the phrases “consist(s) essentially of” and “consisting essentially of” are used to refer to the primary components of a composition, meaning that only small or trace amounts of components other than the expressly recited components (e.g., impurities, byproducts, etc.) may be included in the composition. For example, a composition consisting of X and Y refers to a composition that only includes X and Y, and thus, does not include any other components; and a composition consisting essentially of X and Y refers to a composition that primarily comprises X and Y, but may include small or trace amounts of components other than X and Y. In aspects described herein any such small or trace amounts of components other than those expressly recited following the phrase “consist(s) essentially of” or “consisting essentially of” preferably represent less than 5.0 wt. % of the composition, more preferably less than 4.0 wt. % of the composition even more preferably less than 3.0 wt. % of the composition, and still more preferably less than 1.0 wt. % of the composition. Use of broader terms such as comprises, includes, having, etc. should be understood to provide support for narrower terms such as consisting of, consisting essentially of, comprised substantially of, etc. Use of the term “optionally” with respect to any element of a claim is intended to mean that the subject element is required, or additionally or alternatively, is not required. Both alternatives are intended to be within the scope of the claim. As used herein, the terms “approximately,” “about,” “substantially,” and the like mean within 10% (i.e., plus or minus 10%) of the recited value. Thus, for example, a recited angle of “about 80 degrees” refers to an angle ranging from 72 degrees to 88 degrees.

[0070]Disclosed herein are compositions and methods for producing robust oxygen evolution reaction (OER) catalysts that exhibit high-performance water electrolysis at industrially required multiple amperes per square centimeter current densities. In one or more aspects, the compositions and methods disclosed herein produce OER catalysts that are characterized as being (i) robust; (ii) cost-effective; (iii) industrially compatible; (iv) provide multiple amperes per square centimeter current densities or (v) any combination of (i) through (iv).

[0071]In one or more aspects, the synthetic methods disclosed herein can be carried out in a single step. For example, the synthesis methods described herein can be used to produce, in a single step, OER catalysts such as highly porous NiFe-based (oxy) hydroxide on different metal substrates for water/seawater electrolysis with outstanding catalytic activity and long-term durability under industrially required multiple amperes per square centimeter current density.

[0072]In one aspect, the present disclosure provides methods of making a catalyst for water electrolysis. For example, the present disclosure provides methods of making catalysts for an OER as an anode in water splitting. In some aspects, the catalysts of the present disclosure are self-supported. The term, “self-supported,” as used herein, refers to a catalyst in which the catalytically active material is directly grown on a conductive substrate to provide an electrode for use in an electrochemical system for catalyzing an electrochemical reaction.

[0073]In some aspects, the catalyst produced in accordance with the present disclosure is a nickel/iron (NiFe)-based (oxy) hydroxide catalyst. For example, the catalyst may be a NiFe layered double hydroxide catalyst. In another aspect, the catalyst may be a NiFe (oxy) hydroxide catalyst incorporated (or doped) with a metal. Doping these materials with a third element (ternary doping) is a primary strategy to enhance their activity and stability by modulating electronic structures, surface intermediates, and reaction pathways. Nonlimiting examples of metals for ternary doping of the NiFe (oxy) hydroxide catalyst include molybdenum (Mo), cobalt (Co), chromium (Cr), copper (Cu), tungsten (W), aluminum (Al), zinc (Zn), cerium (Ce), vanadium (V), manganese (Mn), ruthenium (Ru), iridium (Ir), and combinations thereof. In still another aspect, the catalyst may be a Fe-based (oxy) hydroxide, such as a cobalt/iron (CoFe) layered double hydroxide catalyst or an iron (oxy) hydroxide (FeOOH) catalyst. While the methods are described herein with regard to a NiFe-based (oxy) hydroxide catalyst, those skilled in the art will appreciate that other cation sources can also be utilized to form a variety of catalysts via the methods of the present disclosure. In one or more aspect, a ternary dopant of the type disclosed herein may be present in amounts of from about 0.04 g to about 0.5 g; additionally or alternatively from about 0.08 g to about 0.45 g; additionally or alternatively from about 0.1 g to about 0.4 g.

[0074]The methods of the present disclosure include dissolving one or more transition metal ion sources, such as a nickel ion source, an iron ion source, and/or other metal ion source (e.g., ternary dopant), and one or more bases in a solvent to form a solution. In one aspect, the solvent may be deionized (DI) water that has been treated to remove all ions. In another aspect, the solvent may be an organic solvent. For example, the organic solvent may be ethanol, isopropanol, or dimethylformamide (DMF). In still another aspect, the solvent may be a mixture of DI water and any of the organic solvents described above.

[0075]In one aspect, the method includes dissolving an amount of sodium hydroxide (NaOH), an amount of ammonium persulfate ((NH₄)₂S₂O₈), and an amount of ammonium molybdate ((NH₄)₆Mo₇O₂₄) in deionized water to form a solution. Without being bound by any particular theory, it is believed that the introduced Mo contributes to the generation of MoO₄²⁻ anions in situ during OER, and these adsorb near the catalyst surface to repel and inhibit the adsorption of Cl⁻ through electrostatic repulsion, thereby enhancing the catalyst's corrosion resistance and suppressing chlorine evolution reaction (CER) side reaction.

[0076]In one aspect, the method includes dissolving about 0.75 g to 1.50 g NaOH, about 0.15 g to 0.50 g (NH₄)₂S₂O₈, and about 0.05 g to 0.40 g (NH₄)₆Mo₇O₂₄ in about 5 mL to about 20 mL deionized water to form a solution. In another aspect, the method includes dissolving about 0.90 g to 1.10 g NaOH, about 0.25 g to 0.40 g (NH₄)₂S₂O₈, and about 0.10 g to 0.30 g (NH₄)₆Mo₇O₂₄ in about 5 mL to about 15 mL deionized water to form a solution. In still another aspect, the method includes dissolving about 1.05 g to 1.10 g NaOH, about 0.30 g to 0.35 g (NH₄)₂S₂O₈, and about 0.18 g to 0.22 g (NH₄)₆Mo₇O₂₄ in about 8 mL to about 12 mL deionized water to form a solution. In a specific aspect, the method includes dissolving about 1.08 g sodium hydroxide (NaOH), about 0.32 g ammonium persulfate ((NH₄)₂S₂O₈), and about 0.32 g ammonium molybdate ((NH₄)₆Mo₇O₂₄) in 10 mL deionized water to form a solution.

[0077]In another aspect, the method includes dissolving an amount of ferric nitrate (Fe(NO₃)₃) and an amount of sodium thiosulfate (Na₂S₂O₃) in deionized water to form a solution. For instance, in one aspect, the method includes dissolving about 0.20 g to 0.50 g Fe(NO₃)₃ and about 0.01 g to 0.10 g Na₂S₂O₃ in about 5 mL to about 20 mL deionized water to form a solution. In another aspect, the method includes dissolving about 0.25 g to 0.45 g Fe(NO₃)₃ and about 0.02 g to 0.08 g Na₂S₂O₃ in about 5 mL to about 15 mL deionized water to form a solution. In still another aspect, the method includes dissolving about 0.32 g to 0.38 g Fe(NO₃)₃ and about 0.03 g to 0.07 g Na₂S₂O₃ in about 8 mL to about 12 mL deionized water to form a solution. In a specific aspect, the method includes dissolving about 0.35 g ferric nitrate (Fe(NO₃)₃) and about 0.05 g sodium thiosulfate (Na₂S₂O₃) in 10 mL deionized water to form a solution.

[0078]In still another aspect, the method includes dissolving an amount of iron (II) sulfate (FeSO₄) and an amount of sodium thiosulfate (Na₂S₂O₃) in deionized water to form a solution. For example, in one aspect, the method includes dissolving about 0.25 g to 0.75 g FeSO₄ and 0.01 g to 0.10 g Na₂S₂O₃ in deionized water to form a solution. In another aspect, the method includes dissolving about 0.35 g to 0.65 g FeSO₄ and 0.02 g to 0.08 g Na₂S₂O₃ in deionized water to form a solution. In still another aspect, the method includes dissolving about 0.45 g to 0.55 g FeSO₄ and 0.03 g to 0.07 g Na₂S₂O₃ in deionized water to form a solution. In a specific aspect, the method includes dissolving about 0.50 g iron (II) sulfate (FeSO₄) and 0.05 g sodium thiosulfate (Na₂S₂O₃) in deionized water to form a solution. The relative amounts of the reagents and solvents in each of the methods disclosed herein can be scaled upwards as needed while maintaining the approximate ratios as disclosed herein. Scaling up the process can provide for an improved and larger scale production process in some aspects.

[0079]In one or more aspects, the molar ratio of nickel:iron:ternary dopant may range from about X1:Y1:Z1 to X2:Y2:Z2 and the amount of base may be present in a molar amount of about 1:9:0.5 to about 2:18:1.0; additionally or alternatively from about 1:10:0.5 to about 1:15:5.0; additionally or alternatively from about_6:54:0.5 to about 6:54:5.

[0080]The dissolving step may take place at ambient temperature. “Ambient temperature,” as used herein, refers to the temperature of the air in a given environment. In some aspects, the dissolving step takes place at room temperature, for example, at a temperature between 20° C. (68° F.) to 25° C. (77° F.). This makes the synthesis method energy efficient.

[0081]After preparation of the above-described solution, the methods of the present disclosure include immersing a substrate into the solution to grow the catalyst thereon. Any suitable material may be utilized as a substrate for the catalyst growth. Nonlimiting examples of substrates include porous substrates, nickel foam, carbon cloth, carbon paper, glassy carbon, fluorine-doped tin oxide and metal foils. In one or more aspects, the substrate is a stainless-steel mat.

[0082]A stainless-steel mat (also known as a stainless-steel fiber felt or mesh) is a porous, three-dimensional (3D) material made of interwoven stainless-steel fibers, typically sintered at high temperatures to create a stable, bonded structure. Its characteristics are a combination of the intrinsic properties of the stainless-steel alloy (e.g., Grade 304 or 316L) and the unique advantages conferred by its fibrous, porous architecture. In one or more aspects, the stainless steel has an open, reticulated (net-like) structure with high porosity ranging from about 70% equal to or greater than about 90%. In one or more aspects, the SSM has a surface area of from about 0.01 m²/g to about 0.1 m²/g, additionally or alternatively about 0.025 m²/g to about 0.1 m²/g, additionally or alternatively about 0.05 m²/g to about 0.1 m²/g. The primary characteristic of stainless steel is its resistance to corrosion, due to a passive, self-healing chromium oxide layer on its surface. This makes the mat suitable for use in harsh chemical environments (acidic, alkaline, or chloride-containing, depending on the specific grade, e.g., 316L with molybdenum for better pitting resistance). In one or more aspects, the SSM is characterized by its mechanical integrity and corrosion resistance at elevated temperatures such as from about 600° C. to about 800° C. (1112° F.-1472° F.).

[0083]The substrate may include any suitable conductive scaffold on which the catalyst can be directly grown or that can serve as a source of metal for growth of the catalyst. Examples of suitable substrates include, but are not limited to, stainless steel mat, carbon felt paper (CFP), carbon cloth (CC), graphite plates, metallic foams, metal wire meshes, metal foils (such as titanium plates and copper plates), and conductive glass (such as FTO and ITO). In one aspect, the substrate is a metal substrate. For example, the substrate may be a stainless-steel mat. Stainless steel is a ubiquitous and inexpensive material with high conductivity and erosion resistance that can be used as a good substrate for seawater electrolysis. Stainless steel also includes many OER-active elements, such as Fe, Ni, Cr, and Mo, making it suitable to be manufactured into reliable OER catalysts. Additionally, the stainless-steel mat serves both as the corrosion-resistant substrate and as the Ni/Fe source that directly reacts in the surface-corrosion process, which enables intimate and strong contact between the substrate and the active material, thus contributing to a robust integrated catalyst with high mechanical stability to resist the aggressive seawater and intense gas bubbles. Moreover, the corrosion-engineering method described herein effectively transforms the surface of the stainless-steel mat from hydrophobic to hydrophilic, which benefits electrolyte diffusion and gas bubble release for efficient mass transfer at ampere-scale current densities.

[0084]In another aspect, the substrate is a metal wire mesh. In this aspect, the metal wire mesh may be a stainless-steel mesh, nickel (Ni) mesh, copper (Cu) mesh, iron (Fe) mesh, cobalt (Co) mesh, titanium (Ti) mesh, or a combination thereof. In still another aspect, the substrate is a metallic foam. For example, the metallic foam may include nickel (Ni) foam, copper (Cu) foam, iron (Fe) foam, cobalt (Co) foam, titanium (Ti) foam, or a combination thereof. In one aspect, the substrate is nickel (Ni) foam. The foam of the substrate can have any suitable thickness. For example, in some aspects, the foam has a thickness in the range of from about 1 mm to about 3 mm. In another aspect, the foam may have a thickness of about 1 mm to about 2 mm. In further aspects, the foam may have a purity of at least 99 percent. In still further aspects, the foam can have a porosity of greater than or equal to about 95, 96, or 97 percent, or in the range of from about 95 to about 97 percent, and may include from about 80 to about 110 pores per inch, and/or average pore diameters in the range of from about 0.2 to about 0.6 mm.

[0085]In some aspects, the method may further include treating the substrate to improve (i.e., increase) a hydrophilicity thereof prior to placing the substrate into the solution. Any suitable method may be used to increase the hydrophilicity of the substrate surface. For example, the method may include placing the substrate in acid to improve the hydrophilicity of the substrate prior to immersing the substrate into the solution. Nonlimiting examples of acids suitable for use in the present disclosure include hydrochloric acid, sulfuric acid, nitric acid, hydrobromic acid, chloric acid, acetic acid and combinations thereof. In one or more aspects, the substrate hydrophilicity as assessed by contact angle is decreased to values between 0 degrees and 20 degrees, additionally or alternatively from about 1 degree to about 20 degrees, additionally or alternatively from about 5 degrees to about 20 degrees.

[0086]The methods of the present disclosure involve leaving the substrate in the solution at ambient temperature for a certain time duration to form a treated substrate having the catalyst grown thereon. During the time duration, the catalyst is grown on the substrate; that is, at the end of the time duration, the catalyst has grown on the substrate. The catalyst does not necessarily grow for the entire time duration, depending on the time duration length and the substrate satiation. In one aspect, the substrate may be immersed in the solution for a time period sufficient to form a self-supported catalyst. Longer time periods generally lead to stronger corrosion with a greater amount of active material on the surface. For example, the substrate may be immersed in the solution for a time period ranging from about 6 hours to about 48 hours. In another aspect, the substrate may be immersed in the solution for a time period of about 5 minutes to about 2 hours. In still another aspect, the substrate may be immersed in the solution for a time period of about 15 minutes to about 24 hours. For example, in one aspect, the substrate may be immersed in the solution for about 24 hours.

[0087]After the time duration, the treated substrate is removed from the solution. The treated substrate includes the catalyst grown thereon and may be used as an OER catalyst. The methods described herein effectively engineer the surface of the substrate into hydrophilic NiFe-based (oxy) hydroxide layers, which results in a self-supported catalyst exhibiting multiple levels of porosity with a large surface area, sufficient mass transfer, and numerous active sites. In fact, unlike traditional electrodeposition or hydrothermal methods that result in weak contact between the catalyst and the substrate, the substrates in the disclosed synthesis methods are directly reacted with the solution and quickly etched to produce the NiFe-based (oxy) hydroxide layer, which produces highly robust contact and strong bonds and contributes to rapid electron transfer and very good stability. The disclosed synthesis operation is fast and is conducted at ambient temperature, which makes the synthesis both time-efficient and energy-efficient. Moreover, the described synthesis operation is scalable and, thus, is suitable for large-scale applications. Hereinafter the OER catalysts disclosed herein are termed improved OER catalysts and designated iOERs.

[0088]The iOERs produced by the methods of the present disclosure demonstrate excellent catalytic activity as oxygen evolution reaction (OER) catalysts at high current densities. For example, in some aspects, in 6 M KOH natural seawater at room temperature (20° C.), the iOERs only require an overpotential of about 274 mV to deliver a current density of 500 mA cm⁻². In another aspect, in 6 M KOH natural seawater at room temperature (20° C.), the iOERs only require an overpotential of about 296 mV to deliver a current density of 1,000 mA cm⁻². In still another aspect, in 6 M KOH natural seawater at room temperature (20° C.), the iOERs only require an overpotential of about 316 mV to deliver a current density of 2,000 mA cm⁻². In one or more aspects, in 6 M KOH natural seawater at room temperature (20° C.) the iOERs require an overpotential of from about 260 mV to about 380 mV; additionally or alternatively from about 275 mV to about 380 mV; additionally or alternatively from about 275 mV to about 380 mV to deliver a current 750 mA cm⁻² to about 2,000 mA cm⁻², additionally or alternatively from about 1000 mA cm⁻² to about 2,000 mA cm⁻².

[0089]The catalysts of the present disclosure also demonstrate enhanced stability and durability. In one aspect, the catalysts maintain stability and durability at a current density of 1,000 mA cm⁻² for over 1,000 hours in 6 M KOH seawater. In another aspect, the catalysts maintain stability and durability at a current density of 1,000 mA cm⁻² for over 1,500 hours in 6 M KOH seawater. In still another aspect, the catalysts maintain stability and durability at a current density of 1,000 mA cm⁻² for over 1,750 hours in 6 M KOH seawater. In yet another aspect, the catalysts of the present disclosure maintain stability and durability at a current density of 1,000 mA cm⁻² for over 2,000 hours in 6 M KOH seawater.

[0090]The catalysts formed via the methods of the present disclosure may be used in electrocatalytic systems. For example, the iOERs can be used as an oxygen evolution reaction electrode in the water electrolysis process. In this aspect, iOERs can be used in a two-electrode electrolyzer for alkaline seawater electrolysis. The iOERs can be directly used as the OER electrode and be paired with a hydrogen evolution electrode (HER) in an electrolyte, as described in more detail later herein.

[0091]The two-electrode electrolyzer may include an electrochemical cell configured to contain a fluid including an electrochemical reactant (for example, a species to be oxidized to form an oxidation product, a species to be reduced to form a reduction product, or both); a cathode in contact with the fluid; and an anode, where the anode includes any of the disclosed catalysts supported thereon. For example, the anode may be a NiFe-based (oxy) hydroxide catalyst, such as a NiFe-layered double hydroxide (LDH)-based catalyst, or a Fe-based (oxy) hydroxide catalyst, produced in accordance with the present disclosure. The cathode may be an efficient hydrogen evolution reaction catalyst. For instance, in one aspect, the cathode may be a NiMON catalyst (e.g., made of nanowire arrays supported on a metallic foam).

[0092]The selection of fluid depends upon the particular electrochemical reaction to be catalyzed. In one aspect, the fluid may be an electrolyte solution (for instance, a solution of water and a water-soluble electrolyte). In one aspect, the electrolyte solution is an alkaline medium. For instance, the electrolyte may be alkaline water, such as alkaline fresh water or alkaline natural seawater. In further aspects, the electrolyte can be a freshwater or a seawater electrolyte, or a combination of both freshwater and seawater.

[0093]The electrodes, for instance, the cathode and anode, may be immersed in the fluid and may be in electrical communication with one another. The electrocatalytic system may further include a power source in electrical communication with the electrodes, the power source configured to apply an electrical potential across the electrodes. Other components may be included in the systems of the present disclosure, for example, a membrane separating the electrodes, a collection cell configured to collect the oxidation/reduction product(s) from the electrochemical cell, etc.

[0094]Methods of using the disclosed catalysts (or electrodes or electrocatalytic systems including the catalysts) to catalyze an electrochemical reaction are also provided. In some aspects, the catalysts are used to produce oxygen from alkaline water, such as alkaline fresh water or alkaline seawater. By applying a certain voltage across the anode and cathode of the electrolysis cell, water undergoes an oxidation reaction at the anode to produce oxygen, while protons combine with electrons at the cathode to undergo a reduction reaction to produce hydrogen. In this aspect, the methods of the present disclosure include exposing any of the disclosed catalysts (which are supported on the anode) to a fluid including an electrochemical reactant. The exposure results in the oxidation of water molecules to produce oxygen gas (O₂) and release protons (H⁺) as a byproduct.

[0095]In some aspects, when combined with an efficient hydrogen evolution reaction catalyst, such as NiMON, the iOER catalysts produced in accordance with the present disclosure achieve an industrially required current density of 1,000 cm⁻² at a voltage as low as about 1.52 V to about 1.54 V. For example, when combined with an efficient hydrogen evolution reaction catalyst, such as NiMON, the iOERs achieve an industrially required current density of 1,000 cm⁻² at a voltage as low as about 1.532 V for two-electrode seawater splitting in 6 M KOH natural seawater at 60° C. In another aspect, when combined with an efficient hydrogen evolution reaction catalyst, such as NiMON, the iOER catalysts produced in accordance with the present disclosure achieve an industrially required current density of 2,000 mA cm⁻² at a voltage as low as about 1.57 V to about 1.59 V. For instance, when combined with an efficient hydrogen evolution reaction catalyst, such as NiMON, the OER catalysts produced in accordance with the present disclosure achieve an industrially required current density of 2,000 mA cm⁻² at a voltage as low as about 1.582 V.

[0096]The methods of the present disclosure are more energy efficient and time-saving than existing technologies, indicating that large-size samples can be made with lower energy consumption, which is crucial for commercial applications. In addition, due to the sufficient mass transfer and robust contact between the active materials and metal substrates during synthesis, OER electrodes fabricated in accordance with the present disclosure work actively and stably at industrially required multiple amperes per square centimeter current densities. The catalysts of the present disclosure are easy to prepare using a low-cost and scalable method that is compatible with commercial electrode production. These breakthroughs pave the way for more efficient and economically viable hydrogen production from seawater, advancing the potential for a clean energy future.

Additional Disclosure

[0097]The following are nonlimiting aspects of the present disclosure

[0098]A first aspect which is a method for producing an oxygen evolution reaction (OER) catalyst comprising contacting at least two transition metal salt catalysts and an optional dopant with a substrate at room temperature for a time period of from about 15 minutes to about 24 hours.

[0099]A second aspect which is the method of the first aspect wherein the at least two transition metal salt catalysts comprise an Fe-based (oxy) hydroxide.

[0100]A third aspect which is the method of any of the first through second aspects wherein the at least two transition metal salt catalysts comprise nickel, iron or a combination thereof.

[0101]A fourth aspect which is the method of any of the first through third aspects wherein the optional dopant comprises molybdenum (Mo), cobalt (Co), chromium (Cr), copper (Cu), tungsten (W), aluminum (Al), zinc (Zn), cerium (Ce), vanadium (V), manganese (Mn), ruthenium (Ru), iridium (Ir), or combinations thereof.

[0102]A fifth aspect which is the method of any of the first through fourth aspects wherein the at least two transition metal salt catalysts comprise nickel and iron and the optional dopant comprises molybdenum.

[0103]A sixth aspect which is the method of the fifth aspect wherein the molar ratio of nickel:iron:molybdenum is from about 1:9:0.5 to about 1:9:5.

[0104]A seventh aspect which is a method for producing a self-supported catalyst for water electrolysis, comprising dissolving an amount of sodium hydroxide (NaOH), an amount of ammonium persulfate ((NH₄)₂S₂O₈), and an amount of ammonium molybdate ((NH₄)₆Mo₇O₂₄) in a solvent at room temperature to form a solution; immersing a substrate into the solution for a time period sufficient to grow the self-supported catalyst thereon; and removing the substrate from the solution after the time period, wherein the substrate comprises the self-supported catalyst.

[0105]An eight aspect which is a method for producing a self-supported catalyst for water electrolysis, comprising dissolving an amount of ferric nitrate (Fe(NO₃)₃) and an amount of sodium thiosulfate (Na₂S₂O₃) in a solvent at room temperature to form a solution; immersing a substrate into the solution for a time period sufficient to grow the self-supported catalyst thereon; and removing the substrate from the solution after the time period, wherein the substrate comprises the self-supported catalyst.

[0106]A ninth aspect which is a method for producing a self-supported catalyst for water electrolysis, comprising dissolving an amount of iron (II) sulfate (FeSO₄) and an amount of sodium thiosulfate (Na₂S₂O₃) in a solvent at room temperature to form a solution; immersing a substrate into the solution for a time period sufficient to grow the self-supported catalyst thereon; and removing the substrate from the solution after the time period, wherein the substrate comprises the self-supported catalyst.

[0107]A tenth aspect which is the method of the ninth aspect wherein the dissolving step further comprises dissolving about 0.90 g to 1.10 g NaOH, about 0.25 g to 0.40 g (NH₄)₂S₂O₈, and about 0.10 g to 0.30 g (NH₄)₆Mo₇O₂₄ in the solvent to form the solution.

[0108]An eleventh aspect which is the method of the tenth aspect wherein the dissolving step further comprises dissolving about 0.25 g to 0.45 g Fe(NO₃)₃ and about 0.02 g to 0.08 g Na₂S₂O₃ in the solvent to form the solution.

[0109]A twelfth aspect which is the method of the ninth aspect wherein the dissolving step further comprises dissolving about 0.35 g to 0.65 g FeSO₄ and 0.02 g to 0.08 g Na₂S₂O₃ in the solvent to form the solution.

[0110]A thirteenth aspect which is the method of any of the ninth through twelfth aspects wherein the catalyst is a nickel/iron (NiFe)-based (oxy) hydroxide catalyst.

[0111]A fourteenth aspect which is the method of any of the ninth through thirteenth aspects wherein the at least two transition metal salt catalysts is NiMoN.

[0112]A fifteenth aspect which is the method of any of the ninth through fourteenth aspects wherein the substrate is a metal substrate comprising a stainless-steel mat, a metal wire mesh, or a metallic foam.

[0113]A sixteenth aspect which is the method of any of the ninth through fifteenth aspects wherein the solvent is deionized water.

[0114]A seventeenth aspect which is the method of any of the ninth through sixteenth aspects wherein the time period ranges from about 15 minutes to 24 hours.

[0115]An eighteenth aspect which is the method of any of the ninth through seventeenth aspects wherein the substrate is a stainless-steel mat.

[0116]A nineteenth aspect which is the method of any of the ninth through eighteenth aspects further comprising using the self-supported catalyst directly as an oxygen evolution reaction (OER) electrode.

[0117]A twentieth aspect which is a catalyst for water electrolysis produced by the method of the ninth aspect.

[0118]A twenty-first aspect which is a water electrolyzer, comprising: an anode comprising the catalyst of the twentieth aspect and a cathode.

EXAMPLES

[0119]The following non-limiting examples demonstrate the synthesis methods of the present disclosure as well as the catalysts formed therefrom. The examples are merely illustrative of aspects of the present disclosure and are not to be construed as limiting the disclosure, the scope of which is defined by the appended claims.

Example 1

Room-Temperature-Fabricated Large-Area Mo-Modified NiFe(Oxy) Hydroxide Catalyst for Ampere-Scale Seawater Oxidation

[0120]An iOER catalyst for alkaline seawater electrolysis including Mo-modified NiFe oxyhydroxide (NiFeOOH) nanosheet arrays converted from stainless-steel mat (denoted as Mo—NiFeOOH/SSM) was formed using a one-pot surface-corrosion method at room temperature. The iOER, Mo—NiFeOOH/SSM catalyst, showed high OER performance for seawater electrolysis, delivering industrially-required current densities of 500, 1,000, and 2,000 A cm⁻¹. at overpotentials of 274, 296, and 316 mV, respectively, in a harsh 6 M KOH seawater electrolyte, accompanied by high selectivity (Faradaic efficiency of 97% at 1,000 mA cm⁻²) and superior stability (increased overpotential of 76 mV after more than 2,000 h electrolysis at 1,000 mA cm⁻².

[0121]Additionally, using a large-size Mo—NiFeOOH/SSM catalyst (5 cm×5 cm) as the anode paired with a good cathode catalyst, NiMON, in an anion exchange membrane seawater electrolyzer, it was found that the electrolyzer required voltages of 1.856 and 2.205 V to deliver current densities of 500 and 1,000 mA cm⁻² at absolute currents of 12.5 and 25 A, respectively. As demonstrated herein, the disclosed methods provide a low-cost and scalable manufacturing process to convert commercial stainless-steel mat into high-performance OER electrodes that work efficiently at ampere-scale current densities and thus are good candidates for industrial seawater electrolysis.

Materials and Methods

[0122]Chemicals. The following chemicals were used as received: sodium hydroxide (NaOH, AR, Macron), ammonium persulfate ((NH₄)₂S₂O₈, 98%, Sigma-Aldrich), ethanol (C₂H₅OH, Decon Labs, Inc.), ammonium molybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O, 98%, Sigma-Aldrich), nickel (II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O, 98%, Sigma-Aldrich), sodium chloride (NaCl, Fisher Chemical), and potassium hydroxide (KOH, 50% w/v, Alfa Aesar). The stainless-steel mat was provided by Dioxide Materials Inc. Deionized (DI) water (resistivity: 18.3 MΩ·cm) was utilized for all aqueous solution preparations. Natural seawater (pH−7.2) was collected from Galveston Bay near Houston, Texas, USA.

[0123]Synthesis of Mo—NiFeOOH/SSM catalyst on SSM. The Mo—NiFeOOH/SSM catalyst was synthesized by directly etching the surface of a piece of stainless-steel mat (SSM). A solution was prepared by dissolving 1.08 g NaOH, 0.32 g (NH₄)₂S₂O₈, and 0.2 g (NH₄)₆Mo₇O₂₄·4H₂O dissolved in 10 mL deionized water in a small glass bottle. The SSM was immersed in the solution for 24 hours at room temperature. The sample was then removed from the solution, washed with deionized water, and dried in air before use. To study the effect of the amount of Mo on the oxygen evolution reaction (OER) activity, additional samples were synthesized in the same way except that the solution was prepared with 0.1 or 0.3 g (NH₄)₆Mo₇O₂₄·4H₂O. For comparison, a pure NiFe (oxy) hydroxide catalyst (denoted as NiFeOOH/SSM) was also prepared in the same way other than adding (NH₄)₆Mo₇O₂₄·4H₂O to the solution.

[0124]Synthesis of NiMON catalyst on Ni foam. Initially, NiMoO₄ was synthesized on Ni foam using a hydrothermal method. Subsequently, the as-prepared NiMoO₄ was converted into NiMoN through a high-temperature nitridation step.

[0125]Materials characterization. The morphology and nanostructure of each catalyst were investigated using scanning electron microscopy (SEM, JEOL JSM-6330F) and transmission electron microscopy (TEM, JEOL 2010F) coupled with energy-dispersive X-ray spectroscopy (EDS). The phase composition was analyzed via X-ray diffraction (XRD) using a Rigaku diffractometer with a Cu Ka radiation source. X-ray photoelectron spectroscopy (XPS) was performed using a PHI Quantera SXM Scanning X-ray Microprobe. Raman spectroscopy measurements were conducted using a custom-built Raman microscope equipped with a 532 nm light source operating at 10 mW power and with an exposure time of 120 s.

[0126]Electrochemical tests. All electrochemical tests were conducted using an electrochemical station (Gamry, Reference 600). For OER testing, a standard three-electrode cell was employed, with a prepared sample, a graphite rod, and a standard Hg/HgO electrode serving as the working, counter, and reference electrodes, respectively. Five different electrolytes were used: 1 M KOH, 1 M KOH+0.5 M NaCl, 1 M KOH+1.5 M NaCl, 1 M KOH+Seawater, and 6 M KOH+Seawater. OER polarization curves were recorded at a scan rate of 2 m V s⁻¹ and stability tests were performed under constant current densities of 500 and 1,000 mA cm⁻². For the two-electrode water electrolyzer, the polarization curves were also obtained at a scan rate of 2 m V s⁻¹. The generated gaseous products were detected using a displacement method in a homemade setup at a fixed current density of 1,000 mA cm⁻², and the Faradaic efficiency was calculated by comparing the experimentally measured amount to the theoretically calculated one. Electrochemical impedance spectroscopy (EIS) was conducted at an overpotential of 300 m V from 0.1 Hz to 100 KHz with an amplitude of 10 m V. All of the measured potentials vs. Hg/HgO were converted to the reversible hydrogen electrode (RHE) by the Nernst equation (E_RHE=E_Hg/Hgo+0.0591×pH+0.098). All data were reported with iR compensation, which was automatically applied using a current-interrupt method.

[0127]AEM electrolyzer fabrication and testing. The fabrication of the anion exchange membrane (AEM) water electrolyzer is schematically illustrated in FIG. 33. A 5 cm×5 cm piece of NiMON and Mo—NiFeOOH/SSM were used as the cathode and anode, respectively, along with a Sustainion® X37-50 Grade RT AEM (Dioxide Materials). Pieces of cleaned Ni foam served as the gas diffusion layers, while Teflon polytetrafluoroethylene (PTFE) films were employed as the gaskets. Polarization curves and stability tests were performed in 1 M KOH and 1 M KOH+Seawater under different temperatures. A VOLTEQ HY7530EX power source in constant-current mode was employed as the power supply, with voltages recorded using a Keithley 2400-C SourceMeter.

Results

[0128]The scanning electron microscopy (SEM) images in FIG. 1 showed that the SSM substrate has a highly porous microwire structure with a smooth surface, and its pores (10 to 50 μm) are much smaller than those of the commonly used Ni foam (50 to 500 μm). The corresponding energy-dispersive X-ray spectroscopy (EDS) further demonstrates the presence of Ni, Fe, Cr, and Mo metal elements in the SSM (FIG. 2). Color changes can be clearly seen following the one-step solution treatment, with uniform light- and dark-brown films generated on the SSM surface for the NiFeOOH/SSM and Mo—NiFeOOH/SSM catalysts, respectively (FIG. 3).

[0129]To determine the phase composition of each of these films, X-ray diffraction (XRD) was conducted for the different samples. As shown in FIG. 4, except for three peaks originating from the SSM substrate, no other diffraction peaks were detected due to the low crystallinity and thinness of the films. Therefore, Raman measurements were performed on the NiFeOOH/SSM and Mo—NiFeOOH/SSM catalysts. As shown in FIG. 5, the Raman spectra for both catalysts exhibited four broad peaks at around 366, 489, 669, and 1060 cm⁻¹, which are typical vibration modes of FeOOH and Ni OOH. Notably, in the spectrum for the Mo—NiFeOOH/SSM sample, an additional peak appeared at about 816 cm⁻¹ and was ascribed to the Mo—O vibrations of MoO₄²⁻ species. Consequently, the Raman spectra proved that the light- and dark-brown films grown on the SSM surface were mainly composed of Ni/FeOOH and Mooi-modified Ni/FeOOH, respectively.

[0130]FIG. 6 displays SEM images of the NiFeOOH catalyst, from which it can be observed that the smooth SSM surface was etched into a rough layer of nanosheet arrays following the solution treatment. After introducing MoO₄²⁻, the nanosheet morphology was well maintained without any change, as shown in FIGS. 7A and 7B. These fish scale-like nanosheets are rooted in situ on the SSM surface with robust contact, which offers good mechanical stability with high capacity to resist the driving force from the gas bubbles during large-current-density electrolysis. FIG. 8 shows the SEM and corresponding EDS elemental mapping images of Mo—NiFeOOH/SSM, revealing that the SSM surface was fully covered by numerous vertically aligned nanosheet arrays with uniform dispersion of elemental Ni, Fe, Mo, and O.

[0131]Transmission electron microscopy (TEM) was utilized to further explore the nanostructure and phase details for the Mo—NiFeOOH/SSM catalyst. As the TEM image in FIG. 9 shows, all the nanosheets were vertically interconnected with many edge active sites exposed, and their transparent appearance indicated their ultrathin nature. FIG. 10 further revealed that the ultrathin nanosheets had many nanopores on their surfaces, which may be beneficial for electrolyte penetration and gaseous product release, thereby promoting mass transfer at the gas-liquid-solid interface. In FIG. 11, the high-resolution TEM (HR TEM) image showed clear lattice fringes with interplanar spacings of 0.25 nm, which were assigned to the (031) plane of FeOOH. The selected area electron diffraction (SAED) pattern in FIG. 12 further showed well-defined diffraction rings that belong to the (031), (220), and (122) planes of γ-FeOOH (PDF #76-2301). FIG. 13 shows the scanning TEM (STEM) and corresponding elemental mapping images of Mo—NiFeOOH/SSM, confirming the existence and homogenous distribution of Ni, Fe, Mo, and O in the nanosheets.

[0132]X-ray photoelectron spectroscopy (XPS) measurements were then performed to investigate the chemical states and electronic structures of the NiFeOOH/SSM and Mo—NiFeOOH/SSM catalysts. As the high-resolution XPS spectra of Fe 2p in FIG. 14 shows, both catalysts exhibited two typical peaks for Fe 2p_3/2 and Fe 2p_1/2, accompanied by two satellite peaks (each denoted as “Sat.”), which is indicative of the Fe³⁺ oxidation state. The two main peaks each can be fitted into a large peak and a small peak corresponding to Fe—O and Fe—OH of FeOOH, respectively. In comparison to the spectrum for NiFeOOH/SSM, that for Mo—NiFeOOH/SSM showed that both large Fe—O peaks shifted positively to higher binding energy, which indicates charge migration and electronic structure modulation caused by the additional MoO₄²⁻. In the Ni 2p XPS spectra (FIG. 15), the two spin-orbit peaks located at 855.7 and 873.5 eV were ascribed to Ni 2p_3/2 and Ni 2p_1/2, respectively, and two satellite peaks were detected at 861.7 and 879.2 eV. In FIG. 16, a visible Mo signal was detected in the Mo 3d XPS spectrum for the Mo—NiFeOOH/SSM sample, in which the peak at −232.9 eV was indexed to Mo⁶⁺ from the MoO₄²⁻.

[0133]A Pourbaix diagram previously computed for an artificial seawater model indicated that the onset potential difference between OER and CER peaks at approximately 490 mV when the electrolyte pH exceeds 7.5. This suggests that an alkaline environment is more conducive to selective OER in seawater electrolysis. To provide a direct comparison, the OER performance of the different catalysts in a 1 M KOH freshwater electrolyte was evaluated. As shown by the polarization curves in FIG. 17, both the NiFeOOH/SSM and Mo—NiFeOOH/SSM electrodes demonstrated significantly enhanced OER activity after surface treatment, outperforming the bare SSM electrode. Notably, the Mo—NiFeOOH/SSM electrode, with modification by MoO₄²⁻, exhibited substantial improvements in OER activity. It required overpotentials of 291, 331, and 349 mV to reach current densities of 100, 500, and 1,000 mA cm⁻², respectively, which are much lower than the corresponding values for the NiFeOOH/SSM and SSM electrodes, as shown in FIG. 18.

[0134]To optimize the Mo content in the Mo—NiFeOOH/SSM catalyst, additional samples with varying amounts of Mo were prepared and their OER activity was assessed. The polarization curves in FIG. 19 revealed that the catalyst synthesized with 0.2 g (NH₄)₆Mo₇O₂₄·4H₂O exhibited the highest OER activity. This quantity was employed to synthesize Mo—NiF eOOH/SSM catalysts for further analyses. In FIG. 20, the Mo—NiFeOOH/SSM electrode displayed a remarkably low Tafel slope of 16.6 mV dec⁻¹, in contrast to 23.4 mV dec⁻¹ for NiFeOOH/SSM and 39.9 mV dec⁻¹ for SSM, indicating favorable and rapid OER kinetics.

[0135]Electrochemical impedance spectroscopy (EIS) was also employed to explore the catalysts' charge-transfer kinetics. The Nyquist plots in FIG. 21 illustrate that the Mo—NiFeOOH/SSM electrode exhibited a reduced charge-transfer resistance (Rct) of 1.42Ω, compared to 1.98Ω for NiFeOOH/SSM and 42.6Ω for SSM, which signifies more efficient charge transfer at the electrode/electrolyte interface that is attributed to the Mo accelerator and the integrated structure. As shown in FIGS. 22A and 22B, it was further observed that the SSM surface was steered from hydrophobic to hydrophilic in the Mo—NiFeOOH/SSM electrode through the solution-corrosion treatment, which not only provides intimate contact between the electrolyte and the catalyst surface for more electrolyte diffusion, but also benefits gas bubble release under large current densities. Notably, in addition to the high OER activity, the Mo—NiFeOOH/SSM electrode also exhibited excellent stability in 1 M KOH. As shown in FIG. 23, at a constant current density of 500 mA cm⁻², the overpotential remained relatively stable over 200 h of electrolysis, highlighting its superior OER stability in alkaline freshwater electrolyte.

[0136]After verifying the enhanced OER activity of the Mo—NiFeOOH/SSM electrode in alkaline freshwater electrolyte, its OER performance in alkaline seawater electrolytes was assessed, including in alkaline simulated seawater (1 M KOH+0.5 M NaCl and 1 M KOH+1.5 M NaCl) and in alkaline natural seawater (1 M KOH+Seawater). As presented in FIGS. 24 and 25, the Mo—NiFeOOH/SSM electrode maintained excellent OER activity with minimal decline in alkaline simulated seawater, indicating that the presence of high concentrations of chloride ions had little effect on its catalytic performance, which in turn is a positive indication of its potential for achieving high OER selectivity. Remarkably, in the alkaline natural seawater electrolyte (1 M KOH+Seawater), negligible retrogression in the electrode's activity was observed. As shown in FIG. 25, the Mo—NiFeOOH/SSM electrode continued to deliver current densities of 100, 500, and 1,000 mA cm⁻² at low overpotentials of 291, 333, and 357 mV, respectively, which are significantly below the 490 m V threshold needed to initiate CER. To further investigate its practical application, the electrode's OER performance was also evaluated in a harsh environment of 6 M KOH+Seawater. As depicted in FIG. 26, its OER activity was further enhanced, achieving industrially relevant current densities of 500, 1,000, and 2,000 mA cm⁻² at exceptionally low overpotentials of 274, 296, and 316 mV, respectively. Additionally, the Mo—NiFeOOH/SSM electrode demonstrated an impressive Faradaic efficiency (FE) of 97% for OER at a large current density of 1,000 mA cm⁻² in the harsh alkaline seawater electrolyte, as shown in FIG. 27, indicating that the electron transfer during seawater electrolysis is primarily governed by the desired OER rather than by CER.

[0137]Moreover, the operational durability of Mo—NiFeOOH/SSM was evaluated by conducting long-term OER electrolysis at various current densities in different alkaline seawater electrolytes. FIG. 28 shows that, in 1 M KOH+Seawater electrolyte, the Mo—NiFeOOH/SSM electrode functioned quite stably with no degradation over 100 h of electrolysis at 500 mA cm⁻². The operation current density was then increased to 1,000 mA cm⁻² and the performance remained highly stable over an additional 4 days. More impressively, the Mo—NiFeOOH/SSM electrode also displayed excellent durability in the harsh alkaline seawater electrolyte (6 M KOH+Seawater). As shown in FIG. 29, the overpotential was found to increase by only 76 mV over 2,000 h operation at a large current density of 1,000 mA cm⁻², with a decay rate of 38 μV h⁻¹, demonstrating the high durability of this anode material and thus its promise for practical application.

[0138]After the long-term electrocatalysis under the harsh condition, the chemical states of the metals on the surface of the Mo—NiFeOOH/SSM catalyst were investigated. XPS results presented in FIG. 30 indicate that the surface chemical states of Fe and Ni remained unchanged following the stability testing. Notably, no Mo signal was detected in the Mo 3d XPS spectrum for the post-OER sample, suggesting the occurrence of Mo leaching during the OER process. This phenomenon appears to be inevitable in Mo-containing OER catalysts, particularly under large-current-density operation. Nevertheless, the leaching of Mo does not affect the chemical states of Fe and Ni, but rather results in the generation of MoO₄²⁻ anions in situ, and these preferentially adsorb within a few nanometers of the anode surface due to electrostatic forces. The accumulation of these MoO₄²⁻ anions near the anode effectively repels and inhibits the adsorption of Cl⁻ through electrostatic repulsion, which provides a protective effect, reducing Cl⁻ adsorption and penetration into the active layer and thereby enhancing the corrosion resistance and suppressing the CER side reaction. The catalyst's remarkable stability is also attributed to the robust corrosion resistance of the SSM skeleton and the inseparable contact between the active materials and the SSM due to the in situ corrosion growth. Table 1 below presents a comprehensive comparison between the Mo—NiFeOOH/SSM catalyst and other recently reported alkaline seawater OER electrocatalysts. In terms of activity, selectivity, and stability, the iOER Mo—NiFeOOH/SSM electrode stands out as one of the most efficient OER electrocatalysts for alkaline seawater electrolysis reported to date.

[0139]To demonstrate hydrogen generation from seawater electrolysis, a two-electrode seawater electrolyzer for overall seawater splitting was set up by pairing an iOER (Mo—NiFeOOH/SSM) catalyst with a reported hydrogen evolution reaction (HER) catalyst, NiMON, as shown in FIG. 31. Strikingly, the integrated seawater electrolyzer exhibited outstanding activity in alkaline natural-seawater electrolytes. As shown in FIG. 32, the required voltages for delivering current densities of 500 and 1,000 mA cm⁻² were only 1.661 and 1.753 V, respectively, in 1 M KOH+Seawater at room temperature (20° C.). In the 6 M KOH+Seawater electrolyte, the performance was further promoted. To produce current densities of 500 and 1,000 mA cm⁻², the required voltages were decreased to 1.602 and 1.671 V, respectively (FIG. 32). To deliver an extremely large current density of 2,000 mA cm⁻², the corresponding cell voltage was as low as 1.769 V in the 6 M KOH+Seawater electrolyte. Notably, the corresponding voltages for current densities of 500, 1,000, and 2,000 mA cm⁻² were further lowered to 1.495, 1.532, and 1.582 V, respectively, when the electrolyte was heated to 60° C.

[0140]To further demonstrate the practical applicability of Mo—NiFeOOH/SSM for industrial use, a large-size electrode was synthesized, and its performance was assessed in an anion exchange membrane (AEM) water electrolyzer, as schematically illustrated in FIG. 33. Due to the accessible one-step synthesis method used, a uniform 5 cm×5 cm Mo—NiFeOOH/SSM electrode was made from commercial SSM (FIG. 34) and was directly utilized as the anode in the AEM water electrolyzer without the use of any binder. From the polarization curves in FIG. 35, it was observed that the AEM water electrolyzer delivered similar performance in both 1 M KOH and 1 M KOH+Seawater electrolytes, indicating that the iOER electrodes can resist the corrosive seawater without obvious corrosion. Specifically, the AEM water electrolyzer required voltages of 2.193 and 2.768 V to attain absolute currents of 12.5 and 25 A, respectively, corresponding to respective current densities of 500 and 1,000 mA cm⁻² in 1 M KOH+Seawater at room temperature (20° C.). When operated in a heated seawater electrolyte (60° C.), the performance was significantly improved with lower voltages of 1.856 and 2.205 Vat 12.5 and 25 A, respectively, proving the good performance of these electrodes in an AEM water electrolyzer. More importantly, this AEM water electrolyzer also displayed very good durability in both alkaline freshwater and seawater electrolytes. As shown in FIG. 36, under a constant current of 12.5 A (current density of 500 mA cm⁻²), the detected voltage remained highly stable over 4 days of electrolysis in 1 M KOH heated to 60° C. In an alkaline seawater electrolyte (1 M KOH+Seawater) at 60° C., the AEM water electrolyzer also operated stably up to 4 days with only small fluctuation, as shown in FIG. 37. Consequently, the AEM water electrolyzer based on the efficient iOER catalyst exhibited very high activity and superior stability, demonstrating great potential for hydrogen generation from seawater electrolysis.

CONCLUSIONS

[0141]A facile room-temperature surface-corrosion strategy to convert commercial SSM into robust OER electrodes through highly scalable and low-cost manufacturing was developed. The MoO₄²⁻ anions formed in situ from Mo leaching during OER effectively repel Cl⁻ away from the Mo—NiFeOOH/SSM catalyst surface, which, together with an integrated porous structure and a hydrophilic surface, endows this catalyst with high corrosion resistance to Cl⁻— and efficient mass transfer, making it an ideal anode material for seawater electrolysis at ampere-scale current densities. The iOER catalyst (Mo—NiFeOOH/SSM) achieved low overpotentials of 274, 296, and 316 mV at industrially relevant current densities of 500, 1,000, and 2,000 mA cm⁻², respectively, in a 6 M KOH seawater electrolyte. Additionally, the iOER catalyst demonstrated high OER selectivity and durability, maintaining stable performance over 2,000 hours at a current density of 1,000 mA cm⁻². A large-size AEM electrolyzer incorporating the Mo—NiFeOOH/SSM catalyst (25 cm²) achieved high seawater electrolysis performance, delivering current densities of 500 and 1,000 mA cm⁻² at voltages of 1.856 and 2.205 V, respectively. The high OER performance of the inventive Mo—NiFeOOH/SSM catalyst at ampere-scale current densities, along with its simple and scalable synthesis method, represent a significant advancement in the development of non-noble-metal-based materials for seawater electrolysis and other energy conversion applications.

[0142]While various aspects have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the disclosure. The aspects described herein are exemplary only, and are not intended to be limiting. Many variations and modifications of the aspects disclosed herein are possible and are within the scope of the disclosure. Where numerical ranges or limitations are expressly stated, such express ranges or limitations should be understood to include iterative ranges or limitations of like magnitude falling within the expressly stated ranges or limitations (e.g., from about 1 to about 10 includes, 2, 3, 4, etc.; greater than 0.10 includes 0.11, 0.12, 0.13, etc.). Use of the term “optionally” with respect to any element of a claim is intended to mean that the subject element is required, or alternatively, is not required. Both alternatives are intended to be within the scope of the claim. Use of broader terms such as comprises, includes, having, etc. should be understood to provide support for narrower terms such as consisting of, consisting essentially of, comprised substantially of, etc.

[0143]Accordingly, the scope of protection is not limited by the description set out above but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims. Each and every claim is incorporated into the specification as an aspect of the present disclosure. Thus, the claims are a further description and are an addition to the aspects disclosed herein. The discussion of a reference herein is not an admission that it is prior art to the present disclosure, especially any reference that may have a publication date after the priority date of this application. The disclosures of all patents, patent applications, and publications cited herein are hereby incorporated by reference, to the extent that they provide exemplary, procedural or other details supplementary to those set forth herein.