Niobium (nb)-Based Atomically Distributed Electrocatalysts for Water Electrolysis

This application claims the benefit of U.S. provisional patent application 63/659,805, filed Jun. 13, 2025, to Zhaoyang Fan et al., titled “NIOBIUM (NB)-BASED ATOMICALLY DISTRIBUTED ELECTROCATALYSTS FOR WATER ELECTROLYSIS,” the entirety of the disclosure of which is hereby incorporated by this reference.

This invention was made with government support under 2129983 awarded by the National Science Foundation. The government has certain rights in the invention.

This document relates to niobium-based atomically distributed electrocatalysts for water electrolysis.

The increasing global demand for renewable and sustainable energy has driven significant interest in the development of hydrogen production technologies. Water electrolysis has emerged as a promising method for producing high-purity hydrogen without carbon emissions. This process involves splitting water into hydrogen and oxygen using electrical energy, which can be sourced from renewable technologies such as solar or wind power. However, widespread commercial adoption of water electrolysis faces significant challenges, primarily due to the high cost and limited availability of efficient electrocatalysts.

Conventional water electrolysis systems rely on platinum-group metals (PGMs) such as platinum (Pt) for the hydrogen evolution reaction (HER) and iridium oxide (IrO) or ruthenium oxide (RuO) for the oxygen evolution reaction (OER). While these materials offer excellent catalytic performance, they are scarce, expensive, and prone to degradation under operational conditions. This limits their scalability and economic viability, particularly for large-scale hydrogen production.

Efforts to develop alternative, cost-effective catalysts have led to investigations of transition metals such as iron, cobalt, nickel, and molybdenum, which are more abundant and less expensive than PGMs. However, these materials typically show limited performance and durability, especially in acidic environments used in proton exchange membrane (PEM) electrolyzers. Additionally, many non-PGM catalysts are not bifunctional, requiring separate materials for HER and OER, complicating system design and integration.

Single-atom catalysts (SACs) have recently emerged as a promising strategy for maximizing catalyst efficiency while minimizing material usage. By dispersing individual metal atoms onto a conductive support, SACs expose every atom as an active site, enhancing catalytic activity and reducing the amount of metal required. However, existing SACs rely on expensive or scarce metals, or are limited to specific electrochemical environments.

The present disclosure relates to a catalyst composition, comprising a plurality of niobium atoms that are atomically dispersed, and a doped carbon support, wherein each niobium atom of the plurality of niobium atoms is coordinated to heteroatoms within the doped carbon support.

Particular embodiments may comprise one or more of the following features. The heteroatoms within the doped carbon support may comprise at least one heteroatom selected from the group consisting of nitrogen, sulfur, boron, oxygen, and phosphorus. The plurality of niobium atoms may be atomically dispersed such that each niobium atom is isolated from other niobium atoms or is grouped with no more than two other niobium atoms. The plurality of niobium atoms may be atomically dispersed such that any cluster of niobium atoms of the plurality of niobium atoms has a maximum dimension of less than 0.5 nanometers. The doped carbon support may have a hierarchical porous structure. The doped carbon support may be graphitic. The catalyst composition may further comprise a plurality of atoms of a second metal that are atomically dispersed and coordinated to heteroatoms within the doped carbon support. The second metal may be molybdenum.

The present disclosure relates to a method for synthesizing a catalyst composition, the method comprising dissolving a carbon source in a first solution, dissolving a niobium source and a dopant source in a second solution, mixing the first solution with the second solution to form a mixture, drying the mixture to form a precursor material, and pyrolyzing the precursor material to form the catalyst composition, wherein the catalyst composition comprises niobium atoms that are atomically dispersed and coordinated to heteroatoms within a doped carbon support.

Particular embodiments may comprise one or more of the following features: The carbon source may be a carbohydrate, the niobium source may be niobium (V) chloride, and the dopant source may be hydroxylamine hydrochloride. The doped carbon support may be graphitic. Pyrolyzing may be conducted beginning at room temperature and ending at a temperature of at least 600° C. under an argon atmosphere. Pyrolyzing may be conducted with a temperature ramp rate of approximately 5° C./min. Drying may be conducted at 70° C. or less, for at least 12 hours.

The present disclosure relates to a method of catalyzing an electrochemical reaction, comprising loading at least one electrode of an electrochemical cell with a catalyst composition comprising niobium atoms that are atomically dispersed and coordinated to heteroatoms within a doped carbon support, and applying a potential across the electrochemical cell, wherein the catalyst composition catalyzes at least one of a hydrogen evolution reaction (HER) and an oxygen evolution reaction (OER).

Particular embodiments may comprise one or more of the following features: The electrochemical reaction may occur in both alkaline and acidic conditions. An overpotential required to achieve a current density of 100 mA/cmmay be less than 175 m V for HER and less than 325 mV for OER. The catalyst composition may exhibit stability under continuous operation at a current density of 100 mA/cmfor at least 35 hours, with a performance degradation of less than 2%. The catalyst composition may be bifunctional and may be loaded at a cathode and an anode within the electrochemical cell. The catalyst composition may catalyze both HER and OER.

The foregoing and other aspects, features, and advantages will be apparent from the DESCRIPTION and DRAWINGS, and from the CLAIMS if any are included.

Detailed aspects and applications of the disclosure are described below in the following drawings and detailed description of the technology. Unless specifically noted, it is intended that the words and phrases in the specification and the claims be given their plain, ordinary, and accustomed meaning to those of ordinary skill in the applicable arts.

In the following description, and for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the various aspects of the disclosure. It will be understood, however, by those skilled in the relevant arts, that embodiments of the technology disclosed herein may be practiced without these specific details. It should be noted that there are many different and alternative configurations, devices and technologies to which the disclosed technologies may be applied. The full scope of the technology disclosed herein is not limited to the examples that are described below.

The singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a step” includes reference to one or more of such steps.

The word “exemplary,” “example,” or various forms thereof are used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “exemplary” or as an “example” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Furthermore, examples are provided solely for purposes of clarity and understanding and are not meant to limit or restrict the disclosed subject matter or relevant portions of this disclosure in any manner. It is to be appreciated that a myriad of additional or alternate examples of varying scope could have been presented, but have been omitted for purposes of brevity.

Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, mean “including but not limited to”, and are not intended to (and do not) exclude other components.

As required, detailed embodiments of the present disclosure are included herein. It is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limits, but merely as a basis for teaching one skilled in the art to employ the present invention. The specific examples below will enable the disclosure to be better understood. However, they are given merely by way of guidance and do not imply any limitation.

The present disclosure may be understood more readily by reference to the following detailed description taken in connection with the accompanying figures and examples, which form a part of this disclosure. It is to be understood that this disclosure is not limited to the specific materials, devices, methods, applications, conditions, or parameters described and/or shown herein, and that the terminology used herein is for the purpose of describing particular embodiments by way of example only and is not intended to be limiting of the claimed inventions. The term “plurality”, as used herein, means more than one. When a range of values is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another embodiment. All ranges are inclusive and combinable.

More specifically, this disclosure, its aspects and embodiments, are not limited to the specific material types, components, methods, or other examples disclosed herein. Many additional material types, components, methods, and procedures known in the art are contemplated for use with particular implementations from this disclosure. Accordingly, for example, although particular implementations are disclosed, such implementations and implementing components may comprise any components, models, types, materials, versions, quantities, and/or the like as is known in the art for such systems and implementing components, consistent with the intended operation.

The present disclosure relates to catalyst compositions that comprise a plurality of niobium (Nb) atoms that are atomically dispersed and are coordinated to heteroatoms within a doped carbon support. The dispersion of Nb atoms into small clusters or single atoms, rather than large clusters, maximizes the exposure of catalytically active sites while minimizing material usage. These compositions are synthesized using a controlled approach, which may involve dissolution of a carbon source in a first solution and a niobium source and dopant source in a second solution, followed by mixing the two solutions, drying, and pyrolysis at elevated temperatures under an inert atmosphere to form the catalyst composition.

These catalyst compositions demonstrate exceptional performance in electrochemical reactions, particularly in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in both acidic and alkaline media. They exhibit low overpotentials and high current densities at industrially relevant current densities (e.g., 100 mA/cm) with superior stability, enabling their application in scalable, cost-effective, and energy-efficient water electrolyzers. This technology also has potential applications beyond water electrolysis, such as COreduction to fuels, metal-air batteries, and other processes involving OER half-reactions.

The present disclosure relates to a catalyst composition, an embodiment of which is shown in. In some embodiments, the catalyst compositioncomprises a plurality of niobium atomsthat are atomically dispersed and a doped carbon support. As used herein, “atomically dispersed” refers to metal species that exist as isolated single atoms or as small clusters of no more than three metal atoms or that have a maximum dimension of no more than 0.5 nanometers. Thus, in some embodiments, these small clusters of atomically dispersed niobium atomshave no more than three metal atoms, which means that each niobium atom is isolated from other niobium atoms or is grouped with no more than two other niobium atoms. In some embodiments, these small clusters of atomically dispersed niobium atomshave a maximum dimension of less than 0.5 nanometers. The absence of large niobium clusters and nanoparticles may be confirmed via backscattering electron scanning electron microscopy (BSE-SEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and energy dispersive spectroscopy (EDS. In certain embodiments, the atomic dispersion may be further validated using X-ray photoelectron spectroscopy (XPS), which reveals characteristic binding energies for Nb—N interactions.

Each niobium atom of the plurality of niobium atomsis coordinated to heteroatomswithin the doped carbon support. In some embodiments, in particular those with small clusters of atomically dispersed niobium atoms, metal-metal bonds within the small cluster may exist, but the cluster remains immobilized through the coordination to the heteroatomswithin the doped carbon support. The metal atoms, in addition to their bond with the doped carbon support, may also have a bond with a single oxygen atom axially at the apex. In some embodiments, each niobium atom is individually anchored to the doped carbon support. In some embodiments, the doped carbon supportcomprises at least one heteroatomthat is selected from the group consisting of nitrogen, sulfur, boron, oxygen, and phosphorus. In particular embodiments, the doped carbon supportcomprises nitrogen atoms that act as primary anchoring sites for niobium. While nitrogen is exemplified below as the primary heteroatomfor anchoring niobium atoms, other heteroatomssuch as sulfur, boron, oxygen, or phosphorus may also contribute anchoring functionality or electronic modulation of the local environment, enhancing catalytic performance. The selection of heteroatomsand their incorporation into the doped carbon supportmay be tailored to optimize both the stability and activity of the catalyst.

In some embodiments, the doped carbon supporthas a hierarchical porous structure. This structure provides a high surface area and interconnected pore network that enhances electrolyte accessibility and facilitates mass transport during electrochemical reactions. The hierarchical porous structure is typically achieved through the generation of micro-, meso-, and macropores within the doped carbon supportduring the pyrolysis process, and may be influenced by factors such as the selection of carbon source, dopant source, and pyrolysis conditions. The porous architecture not only maximizes the dispersion of niobium atoms but also reduces resistance to ion diffusion and mitigates gas bubble formation on electrode surfaces during electrolysis.

In some embodiments, the doped carbon supportis graphitic. Graphitic carbon offers a highly conductive and chemically stable matrix that promotes efficient electron transfer and durable anchoring of the plurality of niobium atoms. This structure not only enhances the catalytic activity of the system but also contributes to the hierarchical porous architecture that facilitates mass transport and electrolyte accessibility. The degree of graphitization can be tailored by adjusting pyrolysis conditions, including temperature and atmosphere, to optimize conductivity and porosity.

In some embodiments, the catalyst compositioncomprises a plurality of atoms of a second metal. The plurality of atoms of the second metalmay be atomically dispersed and coordinated to heteroatomswithin the doped carbon support. In particular embodiments, the second metalis molybdenum. Other metals may also be implemented, such as iron, cobalt, and nickel. The inclusion of such transition metals can introduce synergistic effects that enhance catalytic activity and stability, particularly in bifunctional electrochemical applications involving both HER and OER. The selection of the second metal, its concentration, and its distribution within the support can be tailored to optimize specific performance parameters such as overpotential, reaction kinetics, and durability.

As mentioned above, the present disclosure is also related to a method for synthesizing the catalyst composition described above. In some embodiments, the catalyst composition is synthesized through a controlled process involving dissolution, mixing, drying, and carbonization, illustrated in. A carbon sourcemay be dissolved in a first solutionand a niobium sourceand a dopant sourcemay be dissolved in a second solution. The first solutionmay be mixed with the second solutionto form a mixture. The mixturemay be dried to form a precursor material. The precursor materialmay then be pyrolyzed to form the catalyst compositionwhich comprises niobium atomsthat are atomically dispersed and coordinated to heteroatomswithin a doped carbon support, as described above.

As used herein, a dopant sourceincludes nitrogen sources such as hydroxylamine hydrochloride, as well as other heteroatom donors capable of incorporating elements like sulfur, boron, or phosphorus into the carbon structure. The choice of dopant source, its concentration, and the reaction conditions can significantly affect the incorporation of heteroatomsinto the doped carbon supportand their coordination with niobium atoms. The carbon sourceincludes, but is not limited to, carbohydrates such as glucose or sucrose, polymers, or other organic compounds that yield carbon upon pyrolysis. In certain embodiments, the solution selection (e.g., ethanol, deionized water) and drying parameters (e.g., temperature, duration) are optimized to control the morphology and uniformity of the precursor material, which directly impacts the dispersion of the plurality of niobium atomsduring pyrolysis.

In some embodiments, the carbon sourceis a carbohydrate, the niobium sourceis niobium (V) chloride, and/or the dopant sourceis hydroxylamine hydrochloride. Specifically, glucose or sucrose may be used as the carbon source, providing a readily pyrolyzable framework for the doped carbon support. Niobium (V) chloride (NbCl) serves as a soluble and reactive niobium source, facilitating homogeneous mixing with the carbon sourceand the dopant source. The dopant sourcemay be hydroxylamine hydrochloride ((NHOH)Cl), delivering nitrogen atoms that serve as anchoring sites for niobium atoms upon pyrolysis. The combination of these specific sources under optimized conditions supports the formation of a nitrogen-doped carbon support with atomically dispersed niobium, exhibiting desirable hierarchical porosity and catalytic properties.

In addition to the glucose/NbCls/hydroxylamine route describe above, alternative approaches may include using different carbon sources(e.g., sucrose, cellulose, starch), niobium sources(e.g., Nb ethoxide, Nb nitrate), and dopant sources(e.g., melamine, urea, ammonia). Solution variations (ethanol, water, methanol) and drying techniques (spray drying, freeze drying) can be optimized. Pyrolysis may occur at temperatures between 400-900° C. under N, Ar, or NH, with ramp rates from 1-10° C./min.

As mentioned above, in some embodiments, the doped carbon supportis graphitic carbon. Graphitic carbon provides a highly conductive and chemically stable framework that facilitates efficient electron transfer and robust anchoring of the plurality of niobium atoms. The graphitic structure also contributes to the formation of a hierarchical porous architecture, enhancing mass transport and exposure of active sites during electrochemical reactions.

In some embodiments, the evolution of ammonia during pyrolysis contributes to formation of a hierarchical porous structure in the doped carbon support. Specifically, the thermal decomposition of dopant sources, such as hydroxylamine hydrochloride, generates ammonia gas (NH) under pyrolysis conditions. This evolving ammonia etches the carbon matrix, creating a network of micro-, meso-, and macropores that significantly increase the surface area and electrolyte accessibility of the carbon support. The controlled generation of ammonia and its interaction with the carbon framework can be tuned by adjusting the dopant source concentration, pyrolysis temperature, and ramp rate, thereby tailoring the porosity and optimizing the catalyst's performance.

In some embodiments, pyrolyzing is conducted beginning at room temperature and ending at a temperature between 400° C. and 900° C. This temperature range facilitates the carbonization of the precursor material, formation of the doped carbon support, and stabilization of atomically dispersed niobium species. In some embodiments, the ending temperature is at least 600° C. In some embodiments, the temperature ramp rate is approximately 5° C./min, which ensures a gradual and controlled heating profile to minimize structural collapse or sintering of the carbon framework and to optimize the incorporation of heteroatomsinto the doped carbon support. The selection of the pyrolysis temperature and ramp rate can be tailored based on the choice of sources and desired material properties, such as porosity, conductivity, and surface area.

In some embodiments, the first solutionis ethanol and the second solutionis deionized water. Ethanol may serve as a polar organic solvent to effectively dissolve the carbon source. Deionized water may be used as the second solutionto dissolve the dopant source(e.g., hydroxylamine hydrochloride) and promote homogeneous mixing with the niobium source, ensuring a controlled and uniform distribution of heteroatomsin the precursor material. The choice of solutions facilitates miscibility, improves precursor interactions, and contributes to the formation of a uniform and stable precursor material for subsequent pyrolysis.

In some embodiments, drying is conducted at 70° C. or less for at least 12 hours. Other drying temperatures and durations may be implemented. This step facilitates the gradual removal of solvents from the mixture, yielding a stable and uniform solid precursor material. The drying temperature and duration may be optimized to prevent rapid solvent evaporation, which could cause aggregation or phase separation of components. Controlled drying conditions contribute to the preservation of homogeneity and the formation of a precursor materialwith desirable morphology and uniform distribution of niobiumand heteroatomsfor subsequent pyrolysis.

The presently disclosed catalyst compositioncan be useful in producing hydrogen via water electrolysis and/or reducing COinto CO or hydrocarbons to fuels. Specifically, the catalyst compositioncatalyzes both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) with high efficiency and stability, enabling its integration into water-splitting systems such as Alkaline Electrolyzers (AEL), Proton Exchange Membrane (PEM) Electrolyzers, and Anion Exchange Membrane (AEM) Electrolyzers. Furthermore, the catalyst composition's ability to enhance the OER under various conditions extends its applicability to COelectroreduction, where it promotes the conversion of carbon dioxide into valuable products such as carbon monoxide or hydrocarbons for fuel synthesis. The combination of scalability, cost-effectiveness, and multifunctionality underscores its potential impact in renewable energy technologies.

The present disclosure is related to a method of catalyzing an electrochemical reaction using the catalyst composition, as shown in. At least one electrodeof an electrochemical cellmay be loaded with the catalyst compositiondescribed above, where the niobium atomsare atomically dispersed and coordinated to heteroatoms within a doped carbon support, and a potentialmay be applied across the electrochemical cell. The catalyst compositioncatalyzes at least one of a hydrogen evolution reaction (HER) and an oxygen evolution reaction (OER). The catalyst composition's high surface area, atomically dispersed niobium atoms, and hierarchical porosity contribute to enhanced electron transfer, reaction kinetics, and stability during the electrochemical process. This method can be employed in both acidic and alkaline environments, with the catalyst compositionoperating efficiently under industrially relevant conditions to support scalable hydrogen production and COreduction applications.

In some embodiments, the electrochemical reaction occurs in both alkaline and acidic conditions. This demonstrates the versatility and robustness of the catalyst composition, which maintains high catalytic activity and stability across a broad pH range. The catalyst composition's ability to operate efficiently in diverse electrochemical environments makes it suitable for integration into various electrolyzer systems, including Alkaline Electrolyzers (AEL), Proton Exchange Membrane (PEM) Electrolyzers, and Anion Exchange Membrane (AEM) Electrolyzers. This compatibility is critical for scalable hydrogen production and other electrochemical applications where operating conditions may vary.

In some embodiments, the overpotential required to achieve a current density of 100 mA/cmis less than 175 mV for HER. In some embodiments, the overpotential required to achieve a current density of 100 mA/cmis less than 325 mV for OER. These performance metrics demonstrate the catalyst composition's exceptional electrocatalytic activity, offering lower energy barriers compared to conventional noble metal-based catalysts such as platinum and iridium oxide. Operating at these overpotentials at industrially relevant current densities enhances the overall efficiency of water electrolysis, reducing the energy input required for hydrogen production. These values are derived from electrochemical measurements conducted in both acidic and alkaline environments, further highlighting the catalyst composition's versatility and suitability for scalable applications.

In some embodiments, the catalyst compositionexhibits stability under continuous operation at a current density of 100 mA/cmfor at least 35 hours, with a performance degradation of less than 2%. This durability under sustained high-current-density operation demonstrates the catalyst composition's resilience against degradation mechanisms such as active site leaching, carbon support corrosion, and aggregation of niobium atoms. The minimal performance loss over extended testing periods ensures the long-term reliability of the catalyst compositionin practical electrochemical systems, making it suitable for industrial water electrolysis and related applications. The stability was confirmed through repeated electrochemical measurements, including polarization curves and monitoring of overpotential shifts.

In some embodiments, the catalyst compositionis bifunctional and is loaded at a cathodeand an anodewithin the electrochemical cellto perform overall water electrolysis. This bifunctional capability allows the catalyst compositionto simultaneously catalyze both the oxygen evolution reaction (OER)at the anodeand the hydrogen evolution reaction (HER)at the cathode, simplifying the electrolyzer design and reducing the need for separate catalysts. The use of a single, highly active material for both electrodes improves system integration, reduces material costs, and streamlines manufacturing processes, as discussed in the method of use outlined above. This approach has demonstrated efficient overall water splitting performance with industrially relevant current densities and voltage profiles.

The utility of the catalyst composition, its method of synthesis, and its method of use was verified through a series of tests that are discussed below. 288 mg of glucose (CHO), which serves as the carbon source, was dissolved in a first solutionof 80 ml of ethanol. Simultaneously, 10 mg of Niobium (V) chloride (NbCl), which serves as the niobium source, was ultrasonically dissolved with 1.38 g of hydroxylamine hydrochloride ((NHOH) Cl), which serves as the dopant source, in the second solution, 80 ml of deionized water. The first solutioncontaining the carbon sourceand the second solutioncontaining the niobium sourceand the dopant sourcewere combined and mixed to form a mixture. The mixturewas dried in an oven at 70° C. for 12 hours to remove solvents and facilitate the formation of a stable precursor material. Following drying, the precursor materialwas transferred to a crucible and subjected to pyrolysis. The temperature was gradually increased from room temperature to 600° C. at a rate of 5° C./min under an argon (Ar) atmosphere. The precursor materialwas maintained at 600° C. for 4 hours to complete the carbonization process. The obtained Niobium atoms anchored on N-doped graphitic carbon (NC) are denoted as Nb-SA/NC. The bare reference sample NC was prepared using the same procedure without adding NbCls salt. Alternative carbon sources (e.g., sucrose), nitrogen sources (e.g., urea), pyrolysis temperatures (400-900° C.), and atmospheres (N, NH) may be used to tune properties. Dual-metal SACs (e.g., Nb—Mo) may be synthesized similarly.

The secondary electron (SE) imaging of Nb-SA/NC in Scanning Electron Microscopy (SEM) may be used to verify the microstructure and surface morphology of the catalyst. When tested, the nitrogen-doped carbon for hosting the atomically dispersed atoms exhibited a hierarchical porous architecture, which is beneficial for the exposure of abundant SACs active sites as well as electrolyte mass transfer. This intricate porosity is a product of carbon matrix etching, a process facilitated by the ammonia (NH) generated through the decomposition of the hydroxylamine hydrochloride precursor during the pyrolysis process. A backscattering electron (BSE) SEM image may be used to penetrate deeper through the sample and give a higher material contrast depending on element atomic number. For Nb-SA/NC sample, no obvious bright metallic bright spots were observed in BSE SEM images, indicating that there are no visible Nb clusters. EDS analysis confirms the presence of Nb despite the lack of large particles, as shown in. Atomic scale microscopic imaging (HAADF-STEM) demonstrated atomic dispersion of Nb. XPS N 1s shows Nb—N bonding (397.5 eV), as shown in, and Nb 2p shows +5 oxidation state (207.5, 210.3 eV), as shown in.

The Oxygen Evolution Reaction (OER) and Hydrogen Evolution Reaction (HER) were measured using a three-electrode system in alkaline solution (1 M KOH) and acidic solution (0.5 M HSO), respectively. A working electrode was synthesized. A catalyst ink comprising 10 mg catalyst powder, 490 μL of deionized water, 490 μL of isopropanol and 20 μL of Nafion was thoroughly mixed by sonication for a duration of 30 min. Subsequently, 20 μL of the treated ink was deposited onto the glassy carbon electrode and left to dry out at ambient temperature, which serves as the working electrode. Linear sweep voltammetry curves (LSVs) were performed at a scan rate of 5 mV s. All potentials were referenced to an Ag/AgCl reference electrode, and a platinum rod was used as the counter electrode in OER/HER measurements. All potentials were calibrated with respect to a reversible hydrogen electrode (RHE): Evs. RHE=Evs. Ag/AgCl+0.197+0.059×pH.

A two-electrode system was utilized to test overall water splitting performance. Two identical freestanding carbon cloth sheets were used as the working electrode substrates to immobilize catalyst powder. As benchmark catalysts, commercial Pt nanoparticle anchored carbon (Pt/C) was loaded on the cathode side for HER, and IrO/C was loaded on the anode side for OER. Meanwhile, Nb-SA/NC served as a bifunctional catalyst loaded at both the cathode and anode. All the catalyst loading (including the carbon) was 1 mg/cm. For the following measurements, if the temperature of the electrolyte is not specified, it is room temperature.

The performance of the Nb-SA/NC catalyst was first evaluated in OER and HER using a three-electrode system in a 1 M KOH solution, shown in. Fe single atom catalysts are usually reported as an effective catalyst in OER, thus an Fe-SA/NC sample was also synthesized by carbonizing Fe-doped ZIF-8 for comparison purpose. In the OER polarization curve (), at a given potential, the Nb-SA/NC catalyst exhibits significantly higher current density than Fe-SA/NC, commercial IrO/C catalyst, and blank counterparts (NC). Notably, Nb-SA/NC demonstrates the lowest overpotential of 289 mV at 100 mA cm 2, outperforming other materials. The Tafel slope values depicted infurther confirm this trend, with Nb-SA/NC showing a lower value (26.9 mV dec) compared to Fe-SA/NC (31.2 mV dec), IrO/C (33.6 mV dec) and NC (45.5 mV dec). These results highlight that Nb-SA/NC, featuring an atomically dispersed Nb structure, exhibits faster reaction kinetics in OER in alkaline solution than other catalysts tested.

In addition to exceptional catalytic performance in the OER, Nb-SA/NC also demonstrates outstanding performance in the HER within the same electrolyte solution. The LSV curve results () show that the overpotential of Nb-SA/NC was merely 143 mV at 100 mA cm, surpassing the performance of the commercially available noble metal catalyst Pt/C. The evaluation of reaction kinetics in the HER process based on Tafel slope results () indicate that Nb-SA/NC had a Tafel slope of 83.25 mV dec, signifying a higher rate of reaction kinetics compared to Pt/C (55.12 mV dec) and NC (83.25 mV dec).