Tailoring Cobalt‐Based Nanomaterials through Doping and Vacancy Engineering for Advanced Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER) Electrolysis

ABSTRACT

Electrolysis of water is an environmentally conscious technique for synthesizing extremely pristine hydrogen, indispensable to accommodating the power and renewable source requirements of modern‐day civilization. The real‐world implications of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) suffer from being restricted owing to their sluggish kinetics and dependence on catalysts incorporating noble metals (IrO₂, Pt, and RuO₂). Recently, cobalt‐based nanomaterials have garnered significant interest attributable to their distinguished electronic configuration and inexpensiveness, while demonstrating extensive potential applications in catalytic processes. Nonetheless, the poor conductance, inconsistent inherent catalytic activity, and constrained sites of action of cobalt‐based catalysts hinder their practical applicability. The review comprehensively examines design methodologies that strengthen the inherent catalytic activity of cobalt‐based catalysts, encompassing morphological and framework management, non‐metal heteroatom doping, metal heteroatom doping, anion vacancies and cation vacancies, oxygen/selenium vacancies, and interface engineering. A comprehensive evaluation is presented on diverse approaches to synthesizing heteroatom‐doped cobalt‐based electrocatalysts. Recent developments in cobalt‐based nanomaterials for the electrolysis of water are subsequently evaluated, emphasizing the structure property relationship. The primary objective is the manipulation of cobalt oxide electrocatalysts that contain non‐metal (anion) and metal (cation) components. Both the constraints and future implications of cobalt‐based electrocatalysts are highlighted.

Hydrothermal and solvothermal synthesis facilitate the precise production of catalysts with controlled morphology and composition. Ion exchange and electrodeposition provide additional methods of preparation for diverse catalyst architectures. Doping with metal and non‐metal heteroatoms modifies the electronic structure to enhance catalytic activity. Engineering of anion and cation vacancies optimizes active sites and improves overall water splitting efficiency.

1. Introduction

The prevailing energy infrastructure is inadequate to sustain the prerequisites for sustainable development due to population growth and global economic growth at an unprecedented pace [1, 2, 3, 4]. Currently, the energy dilemma and environmental contamination brought on by the overuse of petroleum and petroleum products have escalated into substantial concerns [5, 6, 7, 8]. To drastically decrease our reliance on fossil fuels, sustainable, efficient, and renewable energy alternatives must be established promptly [9, 10, 11, 12]. Hydrogen can be exploited to alleviate the inherent unpredictability of sources of renewable energy. High energy density, environmental stewardship, and minimal greenhouse gases are merely a few of the countless advantages of hydrogen‐based energy. Furthermore, it can be generated and preserved without consideration of geographical or territorial constraints [13, 14].

The overwhelming majority of hydrogen supplied for industrial purposes consequently originates from non‐renewable resources, including coal and natural gas, which emit an excessive amount of CO₂ throughout the extraction process and yield H₂ of inadequate purity [15, 16]. The main technique for producing pure H₂ is water electrolysis. Nevertheless, its advancement is impeded by sluggish reaction kinetics and a considerable overpotential, particularly given the complex four‐electron process of the oxygen evolution reaction (OER). Consequently, to mitigate the overpotential while boosting reaction efficiency, exceptionally active electrocatalysts must be designed [17, 18, 19]. Pt, RuO₂, and IrO₂, are examples of catalysts made from noble metals that have strong catalytic properties for the separation of water. Still, their large‐scale industrial applicability is limited by their scarcity and substantial price tag [20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30]. Designing and creating non‐precious metal catalysts that are extremely efficient, reasonably priced, and long‐lasting is therefore essential [31, 32]. Transition‐metal compounds and their derivatives have a prominent role in electrocatalysis owing to their high natural abundance and affordability. The aforementioned metals have unoccupied d‐orbitals and reactive d‐electrons, which assist in the optimum adsorption of oxygen intermediates and exhibit outstanding catalytic efficacy in the hydrogen evolution reaction (HER) and OER [33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43]. Non‐precious cobalt‐based compounds, characterized by copious reserves and low cost, have attained considerable interest owing to their straightforward manufacturing processes and variable oxidation states [44, 45, 46, 47, 48]. Cobalt's toxicity is relatively moderate; thus, minimal quantities are unlikely to pose potential hazards to people's health or negatively impact water sources and soil contamination [49, 50, 51].

The intrinsic electrochemical attributes of cobalt and the potential for diverse structural configurations of cobalt‐based derivatives offer substantial promise for efficient water splitting while substantially lowering costs [52]. Cobalt‐based catalysts are increasingly and intensively employed in fuel cell technology and water electrolysis for the generation of hydrogen. Cobalt‐based catalysts optimize the efficiency of water electrolysis by diminishing overpotential and promoting the energy conversion, consequently reducing H₂ production expenses. Cobalt‐based phosphides, sulfides, oxides, and metallic alloys exhibit excellent catalytic characteristics in alkaline water electrolysis techniques, hence accelerating the overall procedure [53, 54, 55, 56, 57, 58, 59]. As the worldwide electrical grid transforms and the global renewable energy sector expands, the use of cobalt‐based electrocatalysts will persistently increase [60, 61, 62]. Cobalt‐based nanomaterials present numerous advantages over alternative transition metals for water‐splitting applications, attributable to cobalt's favorable d‐electron configuration (d7), various accessible oxidation states (Co²⁺, Co³⁺, and Co⁴⁺), ample amounts, cost‐efficiency, superior intrinsic activity, and structural adaptability. The inclusion of heteroatoms markedly enhances electrocatalytic efficiency by altering electronic structure, generating additional active sites, introducing vacancies and lattice defects, improving electrical conductivity, optimizing intermediate adsorption energies, augmenting structural and electrochemical stability, and facilitating synergistic bifunctionality. Doping with more electronegative elements, such as phosphorus or sulfur, generates electron‐deficient centers that demonstrate a heightened propensity for binding process intermediates, whereas metal dopants like Fe or Ni produce synergistic electronic effects. Heteroatoms can serve as catalytic centers, shown by phosphorus in cobalt phosphide, which acts as a proton acceptor during the HER. Furthermore, co‐doping with multiple heteroatoms generates synergistic effects that facilitate the concurrent optimization of HER andOER activities. Cobalt‐based electrocatalysts, with their combined features, serve as feasible alternatives to noble metals for sustainable hydrogen production [63].

Consequently, the shortcomings of cobalt‐based catalysts, encompassing a restricted quantity of electroactive sites, inadequate electrical conductivity, and diminished intrinsic catalytic effectiveness, necessitate urgent attention. Advanced characterization approaches and computational models have facilitated the development of multiple strategies that boost intrinsic activity. A plethora of evaluations and study publications have delineated the advancements in cobalt‐based catalysts, emphasizing structural optimization and practical applications. Sun et al. proposed a modification technique for bifunctional electrocatalysts based on cobalt pertaining to microstructure and electronic structure [64]. He et al. evaluated techniques to augment the achievement of cobalt‐based catalysts for the OER with low‐dimensional structures and highlighted the advancements in research concerning cobalt‐based nanomaterials for the OER [65]. Fan et al. incorporated high‐valent zirconium into cobalt phosphide to strengthen the kinetics of water dissociation and improve the adsorption efficiency of reactant intermediates [66]. Meng et al. fabricated S‐modified Co₃O₄ with N/S co‐doped graphite to establish an effective bifunctional electrocatalyst for OER/ORR [67]. These academic works provide perspectives on cobalt‐based OER/ORR catalysts, emphasizing fundamental and electronic modification strategies.

In contrast, commercial utilization is constrained by their instability and high cost. Potential contenders in electrochemical water splitting were recognized through extensive research efforts [68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79]. Due to their extraordinary catalytic activity, minimal environmental impact, outstanding reliability, and abundant and non‐precious nature, cobalt (Co)‐based electrocatalysts are considered potential electrode materials for replacing noble metal catalysts [80, 81]. Cobalt‐based materials, such as cobalt selenides, cobalt phosphides, cobalt oxides, double‐layered hydroxides (Co‐LDHs), and cobalt sulfides, are prospective electrocatalysts for the HER owing to their moderate Gibbs free energy of hydrogen adsorption (ΔG_H∗) [82, 83, 84]. Additionally, they offer distinctive advantages for the HER. Numerous defects in cobalt oxides facilitate water molecule dissociation, which enhances HER performance. The wide range of cobalt phosphides, Co‐LDHs, cobalt sulfides, and cobalt selenides—in terms of composition, crystalline structure, and tunable morphology—enables the development of numerous electrocatalysts for water splitting applications [85, 86, 87].

Implementing electrocatalysis for water splitting is an effective and environmentally friendly strategy for producing high‐purity hydrogen fuel. The two distinct half‐reactions that constitute the process of water splitting are the oxidation of water (also referred to as the OER and the reduction of water (also designated as the HER [88]. The corresponding chemical reaction equations in an acidic environment are presented below.

$$\mathrm{Overall}\mathrm{reaction}:2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{O}}_2+2{\mathrm{H}}_2$$ (1)

$$\mathrm{Anode}:2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{O}}_2+4{\mathrm{e}}^{-}+4{\mathrm{H}}^{+}(\mathrm{OER})$$ (2)

$$\mathrm{Cathode}:4{\mathrm{e}}^{-}+4{\mathrm{H}}^{+}\to 2{\mathrm{H}}_2(\mathrm{HER})$$ (3)

The hydrogen evolution reaction at the cathode typically encompasses three potential reaction steps as outlined underneath [89, 90]:

(1) Volmer reaction (electrochemical hydrogen adsorption):

$${\mathrm{H}}_3{\mathrm{O}}^{+}+\mathrm{M}+{\mathrm{e}}^{-}\to \mathrm{M}-\mathrm{H}*+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{Acidic}\mathrm{environment}\right)$$ (4)

$${\mathrm{H}}_2\mathrm{O}+\mathrm{M}+{\mathrm{e}}^{-}\to \mathrm{M}-\mathrm{H}*+{\mathrm{OH}}^{-}\left(\mathrm{Alkaline}\mathrm{environment}\right)$$ (5)

(2) Heyrovsky reaction (electrochemical desorption):

$${\mathrm{H}}^{+}+{\mathrm{e}}^{-}+\mathrm{M}-\mathrm{H}*\to {\mathrm{H}}_2+\mathrm{M}\left(\mathrm{Acidic}\mathrm{environment}\right)$$ (6)

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}+\mathrm{M}-\mathrm{H}*\to {\mathrm{H}}_2+{\mathrm{OH}}^{-}+\mathrm{M}\left(\mathrm{Alkaline}\mathrm{environment}\right)$$ (7)

(3) Tafel reaction (desorption refers to the chemical):

$$2\mathrm{M}-\mathrm{H}*\to {\mathrm{H}}_2+2\mathrm{M}\left(\mathrm{together}\mathrm{acidic}\mathrm{and}\mathrm{alkaline}\mathrm{environment}\right).$$ (8)

The Tafel slope measurements enable the evaluation of the reaction mechanism on a given catalyst surface. The Tafel slopes of 30, 120, and 40 mV dec⁻¹ correspond to the rate‐determining steps that are the Volmer, Heyrovsky, and Tafel steps, respectively. The Gibbs free energy of hydrogen adsorption (ΔG_H) for H_ads is a crucial parameter for assessing the reaction [89, 91]. If ΔG_H is too positive, the H_ads will adhere tightly to the surface of the electrode, facilitating the preliminary Volmer step; nevertheless, the subsequent Heyrovsky or Tafel steps become challenging. If ΔG_H is too negative, H_ads has an inadequate affinity for the electrode's surface, culminating in a sluggish Volmer step that constrains the overall turnover frequency [92]. At the anode, the sluggish kinetics of the OER hinder the practical application of overall water splitting. This phenomenon arises from the four‐electron transfer process and the formation of the O─O bond, both of which require substantial energy to overcome the activation barrier [93, 94].

2. Electrochemical Mechanisms of Water Electrolysis

2.1. Summary of the Electrocatalytic Water Splitting Processes

The pioneering electrolysis cell for water splitting via electrocatalysis was originally proposed in 1789, with 3 components: (a) an aqueous electrolyte, (b) an anode, and (c) a cathode. The procedure encompasses a pair of half‐cell mechanisms at the anode and cathode, specifically the oxidation and reduction of water, commonly referred to as oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. In many electrolytes, the electrochemical reactions occurring at the electrodes usually incorporate acquaintances [95, 96, 97].

Comprehensive reaction:

$${\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2+1/2{\mathrm{O}}_2$$ (9)

In acidic electrolyte

$$\mathrm{Cathode}:2\mathrm{H}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2,{\eta }_{\mathrm{c}}=0\mathrm{V}$$ (10)

$$\mathrm{Anode}:{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2+1/2{\mathrm{O}}_2+2{\mathrm{e}}^{-},{\eta }_{\mathrm{a}}=1.23\mathrm{V}$$ (11)

In an alkaline or neutral electrolyte

$$\mathrm{Cathode}:2{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2+{\mathrm{OH}}^{-},{\eta }_{\mathrm{c}}=-0.83\mathrm{V}$$ (12)

$$\mathrm{Anode}:2{\mathrm{OH}}^{-}\to {\mathrm{H}}_2\mathrm{O}+1/2{\mathrm{O}}_2+2{\mathrm{e}}^{-},\eta \mathrm{a}=-0.40\mathrm{V}$$ (13)

1.23 V is the theoretical potential for water splitting at ambient temperature (25°C) under standard conditions, irrespective of the reaction medium used. In practical water splitting, the standard hydrogen electrode (SHE) can be implemented as a reference to determine the overpotential (η_a and η_c). An additional potential (η_Ω) is required across the electrodes in a water electrolyzer. Nonetheless, a higher potential than the conventional thermodynamic potential is consistently necessary to overcome the intrinsic activation energy barriers. Consequently, the overpotential for the overall water splitting process can be expressed as follows:

$$\mathrm{V}\mathrm{op}=1.23\mathrm{V}+{\eta }_{\mathrm{a}}+{\eta }_{\mathrm{c}}+{\eta }_{\mathrm{\Omega }}$$ (14)

In this instance, η_Ω represents the additional overpotential used to overcome the system resistance, including solution and contact resistance, while η_a and η_c denote the overpotentials required to overcome the activation barriers at the cathode and anode, respectively. The incorporation of extremely potent HER and OER catalysts can diminish the embedded activation hurdles. The primary research objective is to establish efficient water‐splitting electrocatalysts, wherein a and c are minimized, consequently enhancing water‐splitting performance through the utilization of cost‐effective substances and elements.

2.2. Fundamentals of the HER

The most appropriate catalysts for the HER based on Pt are limited in availability and expensive to produce, hindering the widespread implementation of water splitting systems. The development of earth‐abundant substitutes—both heterogeneous and homogeneous—for platinum is of considerable significance. The overpotential required to achieve a current density of 10 mA cm⁻², Faradaic efficiency for hydrogen production, and Tafel slope are all essential parameters for evaluating HER catalyst performance. To replace platinum, overpotentials close to the thermodynamic potential of the hydrogen evolution reaction and Faradaic efficiencies near 100% are achieved. The HER can proceed through three distinct pathways in an acidic environment. Numerous catalytic reactions have been considered and proposed. The following are the critical phases of the HER that transpire on the catalyst interface (Equations (15)‐(19))(7) [98]:

$$\mathrm{Acidic}\mathrm{media}:{\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{e}}^{-}+*\to \mathrm{H}*+{\mathrm{H}}_2\mathrm{O}\mathrm{Volmer}\mathrm{step}$$ (15)

$$\mathrm{Alkaline}\mathrm{medium}:{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}+*\to \mathrm{H}*+{\mathrm{OH}}^{-}$$ (16)

$$\mathrm{Acidic}\mathrm{medium}:\mathrm{H}*+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to {\mathrm{H}}_2\mathrm{Heyrovsky}\mathrm{step}$$ (17)

$$\mathrm{Alkaline}\mathrm{medium}:\mathrm{H}*+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}\to {\mathrm{H}}_2+{\mathrm{OH}}^{-}$$ (18)

$$\mathrm{On}\mathrm{the}\mathrm{Tafel}:\mathrm{H}\ast +\mathrm{H}\ast \to {\mathrm{H}}_2$$ (19)

The predominant mechanisms for the HER are the Volmer–Tafel and Volmer–Heyrovsky routes, depending on the relative contributions of each step. The HER catalysis process can be optimized by manipulating the binding energy of adsorbed intermediates, such as OH⁻ or H*, which are intrinsically related to desorption strength and chemical reactions by strategically choosing the electrocatalyst structure. A lower Tafel slope indicates a faster electrode reaction rate. Catalysts that quickly adapt to specific structural and morphological traits are essential for varying electrode impulses: (a) an extensive variety of sites that interact to encouraging transmission of mass, a well‐developed pore framework, and an extensive specific surface area; (b) increased electrical conductivity to facilitate instantaneous exchange of electrons; (c) significant hydrophilia for fostering interaction between reactants and the surface of the catalyst; (d) customized surface structure incorporating innovative applications, including oxygen‐vacancy sites; and (e) multifaceted engagement to enhance adsorption, liberate energy, and expedite the mechanism [99]. Developing highly active alkaline HER electrocatalysts that can work alongside OER catalysts for efficient overall water splitting is essential [96]. As illustrated in Figure 1a,d, one important metric for assessing HER activity is the Gibbs free energy of hydrogen adsorption (ΔG_H). A catalyst that exhibits balanced adsorption and desorption of hydrogen intermediates should have ΔG_H close to zero. While too positive ΔG_H limits the first Volmer step, too negative ΔG_H inhibits H₂ desorption. Being that the water‐splitting barrier disappears in acidic environments, HER activity tends to be two to three orders of magnitude higher than in alkaline ones [100, 101].

FIGURE 1.

Schematic depiction of the mechanistic pathways for hydrogen and oxygen evolution on electrocatalyst surfaces. (a) HER and (b) OER in acidic medium. (c,d) HER and OER in neutral and alkaline media. Reproduced with permission [102]. Copyright 2025, Elsevier.

2.3. Fundamentals of the OER

The entire effectiveness of the OER in entirely partitioning water is an important consideration in electrocatalytic fuel cell production. The intrinsically anodic is the OER method for splitting water that constitutes the most energy‐demanding phase of electrolysis because of the anode reaction's 4‐electron transmission [98, 103, 104, 105, 106]. Multiple teams of researchers have postulated the frameworks underlying the oxygen emissions interpreted at the anode in both alkaline and acidic environments. The electrolyte has an increased concentration of OH*, which might be promptly adsorbed onto the electrocatalyst, leading to the formation of an OH* transition [107]. The process that underlies the operation of the OER is delineated hereunder (computations (20–27)) [98]:

OER in a pH‐acid electrolyte:

$${\mathrm{H}}_2\mathrm{O}+*\to {\mathrm{HO}}^{-}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$ (20)

$$\mathrm{HO}*+\mathrm{O}*+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$ (21)

$$\mathrm{O}*+{\mathrm{H}}_2\mathrm{O}\to \mathrm{HOO}*+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$ (22)

$$\mathrm{HOO}*+*{\mathrm{O}}_2+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$ (23)

OER in an alkaline electrolyte:

$${\mathrm{OH}}^{-}+*\to \mathrm{HO}*+{\mathrm{e}}^{-}$$ (24)

$$\mathrm{HO}*+{\mathrm{OH}}^{-}\to \mathrm{O}*+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}$$ (25)

$$\mathrm{O}*+{\mathrm{OH}}^{-}\to \mathrm{HOO}*+{\mathrm{e}}^{-}$$ (26)

$$\mathrm{HOO}*\to \mathrm{O}*+{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}$$ (27)

Following their interaction with another OH^–, the OH* adsorbents yield H₂O and an appropriate O* intermediate, which proceeds to interact with OH^– resulting in HOO* by expelling O₂ gas. In contrast to the cathodic HER, the OER is a more complicated and lengthy process, especially because it requires the transfer of four electrons. Ir and Ru are the most commonly used precious metal‐based compounds in practical OER applications reported to date. Similar to Pt, their global availability is limited; therefore, suitable substitutes must be developed [108]. However, these catalysts are too costly and not abundant enough to be used on a large scale. Moreover, the OER has sluggish kinetics, and its overpotential is typically between 0.2 and 0.4 V [109]. Therefore, current investigations in this discipline might concentrate on developing high‐performance non‐noble metal catalysts [110]. The rate‐determining phase is associated with the most prevalent energy transition among the *OH, *O, and *OOH intermediates, explaining the normally slow kinetics of OER with overpotentials ranging from 0.2 to 0.4 V (Figure 1b,c) [111, 112]. Optimizing the adsorption energy of oxygen‐containing species is imperative to alleviate the energy barrier and expedite the reaction.

2.4. Assessment Criteria for Water‐Splitting Electrocatalysts: A Concise Summary

The optimum OER–HER achievement for comprehensive water electrolysis is characterized by cost‐effectiveness, productivity, and stability in an analogous electrolyte. The effectiveness of an electrocatalyst can be assessed through multiple kinetic parameters, including overpotential, Tafel slope, Faradaic efficiency, current density, stability, and frequency fluctuations.

2.4.1. Overpotential and Tafel Slope

The thermodynamic equilibrium possibilities for any electrolytes employed in the water splitting reaction are the reversible hydrogen electrode (RHE) for the HER and 0 V vs. 1.23 V vs. RHE for the OER. Surmounting the augmented energy theory of thermodynamics hurdle for the entire splitting of water necessitates an exceedingly elevated potential (η). The term “overpotential” pertains to these surplus potential vendors. The stimulation, concentration, and impedance overpotentials are three distinct subcategories of overpotential influencing the aggregate quantity. Various strategies are implemented to mitigate these overpotentials. The catalyst's activation overpotential can be considerably diminished by choosing the appropriate sacrificial substance. Agitating the remedy throughout an electrochemical assessment may minimize the extent of excess potential. The overpotential due to resistance can be determined from the initial measurements by increasing the current based on the amount of resistance measured, taking into account the slope of the Nyquist plot, or by applying IR compensation obtained from the electrochemical equipment [110].

A crucial parameter that determines the rate of water splitting (OER, HER) is the Tafel slope (b). The plot of Tafel corresponds to the speed of elevated overpotential (η) proportional to the associated current density, as computed pursuant to the equation of Tafel (Equation (28)).

$$\eta =\mathrm{b}\log \left|\mathrm{j}\right|+\mathrm{a}$$ (28)

In this context, η represents the overpotential; b indicates the importance of the Tafel slope; j refers to current density; and α is an independent variable [113, 114]. A diminished Tafel slope indicates a swift escalation in the density of current alongside raising excess potential, signifying rapid kinetics for the OER and HER of the electrocatalyst.

2.4.2. Turnover Frequency

The turnover frequency (TOF) is typically employed when assessing the catalysts' inherent function at particular catalytic locations. The inclusion of numerous catalyst atoms, each demonstrating fluctuating catalytic initiatives, renders this TOF computation handle inappropriate for yielding a precise turnover frequency significance. The TOF (s⁻¹) value can be determined employing the subsequent methodology:

$$\mathrm{TOF}=\left(\mathrm{jA}\right)/\left(\alpha \mathrm{Fn}\right)$$ (29)

In this context, j (mA cm⁻²) symbolizes the average density of current of the samples at a certain perspective throughout assessments with linear sweep voltammetry (LSV); A signifies the entire portion of the outermost of the working electrodes; and α demonstrates the proportion of electrons correlated with both the intended contents and the stimulant (electrons/mol). The concentration is n moles (mol) of metal elements on the electrode, and F is Faraday's constant (96485.3 C mol⁻¹), estimated by dividing m (g) by the molecular weight of the catalyst (M; gmol⁻¹) [114]. The turnover frequency indicates the inherent action associated with every catalyst's electrocatalytic vicinity; despite this, achieving an accurate value of TOF is complicated due to obstacles in identifying the electrocatalytic sites across the catalyst [105]. Nevertheless, assessing the catalytic efficacy of analogous water‐splitting electrocatalytic components continues to be considered an effective and substantial technique.

2.4.3. Electrochemical Surface Area

The electrochemical surface area (ECSA) is an essential component that significantly influences the efficiency of electrocatalytic reactions [115]. Electrochemical C_dl, a parameter that is directly consistent with ECSA, has gained acceptance as an appropriate metric to evaluate the ECSA of electrocatalysts. The calculation of the electrode ECSA has implications for assessing the catalyst's accomplishment. A common method for estimating the ECSA involves measuring the cyclic voltammetry (CV) across non‐Faradaic regions at various scanning rates [116]. The non‐faradaic zone consistently encompasses an achievable range of 0.1 V positioned approximately on the system's perspective for an open circuit. A graph of the charging current vs. the frequency of scans exhibits a linear relationship, with the slope corresponding to the electrochemical C_dl. For the ECSA‐estimated LSV bends, a specified double‐layer capacitance (Cs) of 40 µF cm⁻² is routinely deployed for an uninterrupted, seamless metal substrate.

$$\mathrm{ECSA}={\mathrm{C}}_{\mathrm{dl}}/{\mathrm{C}}_{\mathrm{s}}$$ (30)

In this instance, Cs symbolizes the specific capacitance (SC). A typical value of 40 µF cm⁻² is the standard deviation for transition metal‐based in 1 M KOH. The C_dl has been assessed by implementing EIS in the identical non‐Faradaic domain [117, 118, 119, 120].

2.4.4. Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) is an efficient technique for assessing the electrochemical properties of catalysts, interfaces, textures, surface coatings, and various materials. EIS measurement in multiple energy domains investigates charge, potential, and current to determine performance. In the electrocatalytic water splitting sector, which aims to produce high‐purity hydrogen, EIS is employed to analyze performance by assessing charge transfer resistance (R_ct) [121]. Additionally, EIS is utilized to characterize energy storage and conversion components across various frequencies to investigate the conductivity and electrochemical kinetics of different catalysts [122, 123]. The charge transfer resistance (R_ct), calculated from the high‐frequency region, is determined by the radius size. A smaller radius corresponds to a faster reaction rate [124]. Consequently, EIS is a highly versatile method applicable for examining metal oxide electrodes across numerous energy‐related fields. An important aspect of this assessment is how the frequency dependence of key circuit components affects current flow through the metal oxide interface. Modeling EIS data with equivalent circuits is necessary for acquiring physically appropriate knowledge [125].

2.4.5. Faradaic Efficiency Test

Faradaic efficiency is the effectiveness of electron transfer facilitated by an external circuit to accelerate the electrochemical reaction, wherein the charged particles involved in the chain reaction (i.e., OER or HER) serve a purpose. This ratio compares the total amount of gas collected throughout the investigation to the total amount predicted by theoretical assumptions. Water displacement or gas chromatography (GC) can be employed to precisely evaluate the production of the target gas of interest (i.e., O₂ or H₂). Faradaic performance in the hydrogen evolution reaction implies the effectiveness of how well the electrons delivered by an external circuit are utilized for facilitating HER. Faradaic losses may occur due to the emission of thermal radiation or residuals that remain during contact between the electrodes. Practical and theoretical metrics of hydrogen generation need to be established for assessing Faradaic efficiency. Encompassing methods can be used to estimate the theoretical generation of H₂ from potentiostatic or galvanostatic electrolysis. Simultaneously, the water–gas shift and GC technique may be adopted to ascertain the practical hydrogen generation efficiency [103].

2.4.6. Stability

A vital attribute for catalysts is stability. Electrolysis or cyclic voltammetry (CV) is an approach for evaluating catalytic stability via galvanostatic or potentiostatic electrolysis. CV may also be used to accelerate catalytic degradation for examination, usually performed for HER across multiple cycles and for OER over dozens of cycles [126]. A change in the initial overpotential indicates catalyst stability. In a stable system, such fluctuations will be barely noticeable. Stability testing via galvanostatic or potentiostatic electrolysis is typically performed for an adequate duration of 12 h. The duration may range from several to many hours, with a longer period corresponding to enhanced stability [99, 110]. Atomic doping and vacancy engineering address durability challenges through various synergistic mechanisms. Doping with heteroatoms such as Fe, Ni, or rare‐earth elements alters the electronic composition of cobalt, enhancing its resistance to over‐oxidation and preventing cobalt dissolution, which enables catalysts to function for extended periods while avoiding metal leaching. Dopants like phosphorus, sulfur, or boron impede nanoparticle agglomeration by serving as atomic‐scale separators. Consequently, cobalt‐based nanomaterials that initially degrade after 10–20 h can be stabilized to operate for over 200–500 h with less than 20% activity loss.

2.4.7. Stability of Cobalt‐Based Catalysts under Acidic Conditions

Although cobalt‐based catalysts display exceptional stability in alkaline solutions, proton exchange membrane (PEM) electrolyzers continue to encounter substantial challenges when it comes to sustaining their endurance in moderately acidic conditions (pH 0–3) [127]. Cobalt corrodes rapidly in acidic conditions, dissolving as Co²⁺ ions and triggering catalyst deterioration and system breakdown. Covalent bonding with heteroatoms such as phosphorus or sulfur, carbon coating or encapsulation, metal alloying with noble metals like ruthenium or iridium, and the inclusion of oxygen vacancies are several strategies previously developed to enhance acid stability. Despite these advances, commercial PEM electrolyzers require stability exceeding 5000–10 000 h, yet most cobalt‐based catalysts still break apart within 20–50 h [128]. The development of cobalt‐based catalysts that can withstand severe acidic conditions must be the primary objective of future research.

2.4.8. Integration of Membrane Electrode Assemblies (MEAs) for Industrial Applications

A significant portion of research utilizes three‐electrode systems, featuring rotating disk electrodes (RDE), in liquid electrolytes to evaluate cobalt‐based electrocatalysts. However, integration into membrane electrode assemblies (MEAs) is necessary for industrial applications. When fabricating MEAs, several crucial parameters must be considered, including the choice of membrane material, electrode thickness, and arrangement of catalyst layers. Initially, catalyst ink development requires optimization of the ionomer type, ionomer‐to‐catalyst ratio, and solvent composition. Second, coating techniques such as spray coating, doctor blade coating, or electrodeposition must be carefully chosen. Third, membrane selection depends on operational pH: Nafion is used for acidic PEM systems, whereas anion exchange membranes are employed for alkaline systems. Fourth, hot‐pressing parameters such as temperature, pressure, and duration require optimization. Cobalt‐based catalysts that show strong performance in RDE testing often exhibit significantly poorer performance in MEAs, primarily due to mass transport limitations [129]. Future research must address the disparity between laboratory‐scale testing and device‐level integration.

2.4.9. Cost and Environmental Impact of Cobalt‐Based Catalysts

Despite being more abundant and affordable than noble metals, cobalt carries associated costs and environmental impacts that require recognition. The price of cobalt is extremely variable owing to supply chain interruptions and increasing demand from the battery sector. Approximately 60–70% of worldwide cobalt extraction occurs in the Democratic Republic of Congo, where significant ethical problems, including child labor and environmental destruction, have been documented. Cobalt extraction produces hazardous waste and carbon dioxide emissions. Several approaches can alleviate these problems. First, reducing cobalt loading through atomic dispersion or nanostructuring decreases cobalt consumption. Second, developing recycling methods for spent catalysts can recover valuable cobalt. Third, investigating cobalt‐free alternatives like iron, nickel, or manganese‐based catalysts presents more sustainable solutions, as these materials can reduce reliance on cobalt and potentially lower environmental impacts associated with cobalt extraction and processing. Fourth, life‐cycle assessment (LCA) studies are necessary to determine the actual environmental impacts of cobalt‐based catalysts.

To provide a quantitative comparison of the electrocatalytic performance of representative cobalt‐based materials, Table 1 summarizes recently reported catalysts, their modulation strategies, HER and OER overpotentials, and overall water‐splitting voltages measured under comparable conditions.

TABLE 1.

Comparative performance of cobalt‐based electrocatalysts.

Catalysts	Modulation strategies	ηHER (mV) at 10 mV/cm2	ηOER (mV) at 10 mV/cm2	Overall voltage (V)	Refs.
Co/CoMoN/NF	Interface engineering	73	147	1.48	[130]
CuCo2O4	Bimetallic heterostructure	115	290	1.64	[131]
Co–O–C/CPs	Interface & heterostructure	115	240	1.60	[132]
NiCoO2/NiCo	Interface & support	61.9	329	−	[133]
CoSe2–MoSe2	Heterostructure (bifunctional)	90	218	1.63	[134]
V‐Co2P@HE	Heteroatom doping	33	227	1.53	[135]
CoO/Co4S3	Interface engineering	61	−	1.50	[136]
Cr‐Cu/CoOx	Heteroatom doping	21	252	1.51	[137]
CoxPy@CNT‐CC	Structural and morphology control	94	280	1.55	[138]
Co3–xO4	Defect engineering	−	268	−	[139]
VOB‐Co3O4	Heteroatom doping and defect engineering	111	280	1.67	[140]
MnCoPi/NF	Support effect	102	225	1.55	[22]
CoO/Co4S3	Interface engineering	81	190	1.48	[136]
NiFeP/CoP	Morphology and interface engineering	213	274	1.72	[141]
CoMo‐P/NF	Support effect	40	278	1.53	[142]
Mo‐Co3O4@CC	Heteroatom doping and defect engineering	−	276	1.54	[143]
NiO@Co3O4/CF	Morphology and interface engineering	104	−	1.53	[144]
S‐Co2P@Ni2P	Heteroatom doping and interface engineering	43	−	1.52	[145]
S‐CoO/Co3O4	Heteroatom doping and interface engineering	181	275	1.60	[146]
RuO2/Co3O4	Interface engineering	57	231	1.50	[147]
Co2P‐Fe2P/NF	Defect engineering and interface engineering	96	220	1.56	[148]
CoSe2/MoSe2	Alloying/Porous support	−	202	1.29	[149]
CoPx(CoP/Co2P)	Defect engineering and interface engineering	190	300	1.75	[150]
P, Cu‐Co0.85Se/NF	Heteroatom doping	97	−	1.57	[151]
Ni‐doped Co3O4	Hydrothermal	62	64.2	1.63	[133]
Co@CNB‐N4	Hydrothermal method	45	250	1.59	[152]
Co/Co3O4	Hydrothermal treatment	252	316	1.76	[153]
CoSe/Co(OH)2	In situ selenization/Acid etching	126	91	1.65	[154]
Co‐BM‐C	Facile mechanochemical‐induced self‐sustaining reaction	126	240	1.60	[155]
CoTe2/CoP	Hydrothermal method	80	260	1.55	[156]
Co‐MOF@CoP	In situ phosphorization	93	275	1.58	[157]
Ir‐Co3O4@NC	Ion exchange method	188	89	16.2	[158]
Co0.95Cu0.05	Sol‐gel method	160	285	1.58	[159]

3. Synthesis of Heteroatom‐Doped Cobalt Electrocatalysts

The inclusion methodologies in cobalt‐based electrocatalysts are predominantly categorized into metal incorporation (including Ni, Fe, Mo, Mn, and W) and non‐metallic component incorporation (which encompasses C, N, S, P, Se, B, and Te). The incorporation of heteroatoms throughout cobalt‐based materials substantially impacts their catalytic efficiency by disrupting electronic structure, improving surface traits, and assisting with enhanced charge transmission. Synthetic techniques are crucial for regulating the phase composition, appearance, and particle dimension of the synthesized nanostructures, which directly influence their catalytic properties and persistence. Furthermore, pursuant to the expanding demand for environmentally conscious approaches in nanotechnology, numerous eco‐friendly, economically viable, and scalable manufacturing objectives were initially pitched for the synthesis of heteroatom‐doped cobalt‐based electrocatalysts. The approaches encompass hydrothermal, solvothermal, thermal treatment, electrodeposition, co‐precipitation, and ion exchange techniques. Additionally, methods such as high‐temperature air/vacuum calcination, spray pyrolysis, and plasma treatment are commonly employed to produce heteroatom‐doped cobalt‐based components that are becoming increasingly recognized as exceptionally efficient electrocatalysts for water splitting. The following paragraphs outline the most common methods for synthesizing cobalt‐based electrocatalysts that are doped with metals and non‐metals. This information aims to guide novice researchers in selecting the most suitable and straightforward doping synthesis techniques for their specific applications.

3.1. Hydrothermal Method

Currently, transition metal‐based molecules (TMXs, where X represents C, N, S, P, O, B, etc.) are under consideration and extensively explored for water‐splitting applications [160, 161]. Phosphides made from transition metals have garnered significant curiosity due to their remarkable catalytic efficiency and long‐term reliability. For example, Mo─P, Ni─P, Co─P, and Fe─P, as representative electrocatalysts for water splitting, have undergone extensive research [162, 163, 164, 165]. Co─P has been widely explored; yet, its capacity to function persists inadequately relative to that of precious materials [166, 167, 168, 169]. Heteroatom doping regimens are implied to substantially improve the desorption and adsorption of intermediates on sites of activity [170, 171, 172]. Ruthenium is the most economical metal in the category of platinum and demonstrates Gibbs free energy coefficients for the adhesion of Ru–H associates with hydrogen that are equivalent to the values of Pt‐based compounds [173, 174, 175] Consequently, the incorporation of Co–P with Ru might produce enough electrocatalytic effectiveness.

Low‐temperature phosphorization, impregnation, and hydrothermal processes have been successfully employed by Zhao et al. [176] to design CoP_x enriched with Ru on nickel foam (Figure 2a). Through modulating the local positioning of electrons encircling the CoP, the incorporation of Ru makes the electrocatalyst interface a better candidate for HER. Studies have shown that Ru‐doped CoP_x/NF achieves excellent hydrogen evolution activity (overpotential as low as 41 mV at 10 mA cm^{− 2}), while the undoped CoP_x/NF sample indicates outstanding OER performance. Integrating Ru‐CoP_x/NF as the cathode and CoP_x/NF as the anode enables a comprehensive water‐splitting framework. Regarding dual‐functioning electrocatalysts, Mn–W–CoP/NF dual‐metal co‐doping CoP nanoflower complexes on nickel foam have been fabricated by Shang et al. [177], as shown in (Figure 2b). The distinctive nanoflower‐like structure facilitates an extensive solid‐liquid interaction zone and expedites desorption of reaction gases. In alkaline medium, the Mn–W–CoP/NF catalyst reveals minimal elevated potentials for OER and HER (95.8 and 229.2 mV at 10 mA cm⁻²), and a two‐electrode electrolyzer requires only 1.57 V to achieve 10 mA cm⁻². The catalyst also demonstrates outstanding resilience over 48 h. The catalytic efficiency of Ru‐CoP_x/NF is modulated by the percentage of ruthenium trichloride, and the impermeability interval also impacts the efficiency of the synthesized electrocatalyst. As depicted in Figure 2c, the onset excessive potential of the Ru‐CoP_x/NF catalyst is analogous to that of commercial Pt/C/NF. Comparative analysis reveals that Ru‐CoP_x/NF necessitates an excess potential of merely 41 mV to accomplish 10 mA cm⁻², which is only 6 mV greater than Pt/C/NF and significantly lower than CoP_x/NF, Co(OH)F/NF, and naked NF. Furthermore, Figure 2d demonstrates that Ru‐CoP_x/NF exhibits the minimal overpotential required for attaining 10 and 50 mA cm⁻², with the exception of Pt/C/NF, when compared to the reference specimens. The Tafel descent of 68 mV dec⁻¹ indicates that Ru‐CoP_x/NF demonstrates accelerated reaction fundamentals, surpassing Pt/C/NF (70 mV dec⁻¹), Co(OH)F/NF (88 mV dec⁻¹), CoP_x/NF (80 mV dec⁻¹), and bare NF (97 mV dec⁻¹) (Figure 2e). The C_dl of Ru‐CoP_x/NF is 32 mF cm⁻², exceeding NF (1 mF cm⁻²), Co(OH)F/NF (12 mF cm⁻²), and CoP_x/NF (27 mF cm⁻²), confirming its superior active locations for HER (Figure 2f). The results confirm that proper Ru incorporation boosts the proportion of engaged sites, thus enhancing electrocatalytic functioning. In addition to activity, sustainability is a crucial component of effective electrocatalysts.

FIGURE 2.

(a) Diagrammatic scenario of the fabrication procedure for Ru‐CoP_x/NF. Reproduced with permission [176]. copyright 2024, Elsevier. (b) Illustration of the synthesis procedure for Mn–W–CoP/NF. (c) LSV curves of synthetic specimens in 1 M KOH. (d) Comparative analysis of the overpotentials assigned to accomplish 10 and 50 mA cm⁻². (e) Tafel slopes. (f) The ECSA (C_dl). Reproduced with permission [177]. Copyright 2024, Elsevier.

In the present research by Pang et al. [178], Figure 3a, self‐sustainable NiCo₂O₄ nanoneedle arrangements are hydrothermally manufactured on CoO nanofilms laminated by ALD on nickel foam (NF). The CoO layer strengthens the connection between the substrate and nanoneedle arrays, minimizing resistance while sustaining the specific area. The electrode NiCo₂O₄@C₁₂NF with the CoO barrier exhibits exceptional HER performance. At 10 mA cm⁻², the Tafel slope was 50.6 mV dec⁻¹ and a maximum potential of 96 mV vs. RHE in 1 M KOH electrolyte. Atomic layer accumulation deposits CoO nanofilms on NF, followed by hydrothermal manufacturing of NiCo₂O₄ nanoneedles. The composite exhibits a low interaction barrier, multiple active sites, and a permeable nanoarray structure. A hierarchy‐based nanostructure catalyst with expansive surface dispersion on a self‐sustainable electrode is mandatory for HER and OER. Jiang et al. [179] achieved this by synthesizing CoMoO₄ nanosheets on NF using hydrothermal and calcination strategies, then depositing NiSe nanoparticles to form NF/CoMoO₄@NiSe electrodes (Figure 3b). MXene has gathered attention due to its layered architecture, favorable hydrophilicity, metallic conductivity, and varied superficial chemistry [180]. Ti₃C₂ MXene is frequently utilized as a catalyst foundation [181]. Transition metal‐based materials are important water electrolysis catalysts due to their affordability and their ability to facilitate efficient hydrogen production, which is crucial for sustainable energy applications [182]. Interface and surface engineering boost HER and OER activity [183, 184, 185]. Transition metal chalcogenides have captured attention due to their exceptional conductive properties [186, 187]. However, their catalytic ability needs augmentation due to a restricted number of active locations [188].

FIGURE 3.

(a) Illustrative figure depicting the synthesis of NiCo₂O₄ nanowires on NF coated with cobalt oxide nanofilms. Reproduced with permission [178]. Copyright 2023, Elsevier. (b) Synthetic pathway of the NF/CoMoO₄@NiSe Composite. Reproduced with permission [179]. Copyright 2024, Elsevier. (c) Schematic of the synthesis method for Ce‐CoSe₂/MXene on NF electrode. (1,2) Ce‐CoSe₂/MXene electrocatalyst SEM images. (3,4) TEM pictures. Reproduced with permission [189]. Copyright 2025, Elsevier. (d) Synthesis schematic of Fe‐Co₂RuO₄/RuO₂. Reproduced with permission [191]. Copyright 2025, Elsevier.

A simple hydrothermal and selenium procedure fabricated the Ce‐CoSe₂/MXene heterostructure as a dual‐functional electrocatalyst by Zhou et al. [189]. Ce‐doping CoSe₂ nanoneedle arrangements with MXene boost substrate hydrophilic characteristics and quicken reaction dynamics. The CoSe₂/MXene catalyst displays remarkable HER and OER properties, with excessive potentials of 34 and 279 mV. A two‐electrode water‐splitting structure requires 1.45 V to achieve 10 mA cm⁻². The catalyst's activity is preserved for 200 h at 100 mA cm⁻². The hierarchical arrangement of Ce‐CoSe₂/MXene became apparent after hydrothermal and selenium modifications (Figure 3c). Undergoing the selenium procedure, Ce‐CoSe₂/MXene sustained the nanoneedle array shape (Figure 3c(1,2)). The nanoneedle shape stimulates H⁺ ion aggregation and optimizes HER results [190]. The inclusion of nickel foam as a base material facilitates close communication between the NF skeleton and the catalyst, alleviating the need for extra binders and enhancing mechanical durability. TEM pictures further confirm Ce‐CoSe₂/MXene's needle‐like configuration (Figure 3c(3,4)). A novel Fe‐Co₂RuO₄/RuO₂ heterostructure on NF was generated by Qi et al. [191] using hydrothermal synthesis, ion exchange, and calcination techniques (Figure 3d). The distinctive nanosheet‐on‐nanosheet structure provides an immense surface area, boosting the connection region between electrolyte and catalyst. The interface linking and synergistic effect between RuO₂ and Fe‐doped Co₂RuO₄ optimize electronic design and charge delivery capacity. The Fe‐Co₂RuO₄/RuO₂ displays remarkable OER catalytic properties with a restricted excess potential of 253 mV at 50 mA cm⁻². This research delivers an innovative concept for water splitting in industries by demonstrating how the enhanced catalytic properties of Fe‐Co₂RuO₄/RuO₂ can improve efficiency and reduce energy costs in industrial applications.

These heterostructures' greater accomplishments come from the components' synergistic effects. Ce‐doping minimizes reaction barriers for both HER and OER in Ce‐CoSe₂/MXene by introducing oxygen vacancies, while MXene improves conductivity and hydrophilicity. The shape of the nanoneedle further encourages gas desorption and H⁺ aggregation. The RuO₂/Fe‐doped Co₂RuO₄ interface maximizes electron transfer and stability for Fe‐Co₂RuO₄/RuO₂, while the nanosheet‐on‐nanosheet design maximizes active site exposure. When building high‐performing bifunctional electrocatalysts, these design concepts provide advantageous direction.

3.2. Electrodeposition Method

Considering their extraordinary conductive properties and site activities, transition metal‐based phosphates have captured sufficient curiosity among the transition metal‐based materials previously reported for hydrogen generation by water electrolysis [192, 193, 194]. Catalytic compounds deliver a slightly negatively charged P center that constitutes the connection point, whereas a positively charged metal core functions as the attachment spot for protons by transferring some electrons from metal atoms to P molecules. The two components collaborate to boost the metal site's capacity for absorbing and eliminating hydrogen atoms, which raises HER catalytic activity. Transition metal phosphide, a bifunctional electrocatalyst, has the shortcoming of partial oxidation throughout water electrolysis operation, particularly during the OER procedure [163]; this problem complicates the maintenance of sustainability for the entire catalytic process and electrode. A CoP/CoCr₂O₄ heterostructural material, for illustration, was designed by Saad et al. and exploited as a dual‐functional electrochemical water catalyst [195]. The CoP/CoCr₂O₄ heterostructure catalyst displayed greater resilience and catalytic performance against both OER and HER when juxtaposed with the independent CoP and CoCr₂O₄ catalysts. Furthermore, the vast majority of transition metal‐containing phosphides originate through temperature‐programmed reduction, gas‐solid interaction, and solution‐phase reaction [196].

Liu et al. [197] propose a straightforward one‐step electrodeposition approach to synthesize a pore‐filled Co–P/Fe₃O₄ composite material layer on NF (Co–P/Fe₃O₄@NF). The composite layer consists of Fe₃O₄ nanoparticles and cobalt phosphide (Co–P) microspheres. The inclusion of Fe₃O₄ considerably constricts the Co–P microspheres, revealing additional active sites. Concurrently, Fe₃O₄ is distributed uniformly within the Co–P film, forming an unconventional interaction that stimulates the catalytic reaction. Furthermore, three‐dimensional nickel foam (NF) is extensively coated with a porous Co–P/Fe₃O₄ composite sheet, facilitating swift electron transfer and guaranteeing excellent durability during the HER and OER processes. The Co–P/Fe₃O₄@NF reveals outstanding efficiency. When deployed as a self‐standing electrode for HER and OER, η20 (overpotential at 20 mA cm⁻²) is only 132.8 mV for HER and 343.0 mV for OER. Furthermore, an alkaline electrolyzer constructed using these dual‐functional electrodes requires a cell voltage of 1.686 V to reach 10 mA cm⁻². The dual‐functional electrodes demonstrate outstanding durability and approximately 100% Faradaic efficiency. Figure 4a highlights the design and preparation procedure using one‐step electrodeposition.

FIGURE 4.

(a) An illustration of the Co–P/Fe₃O₄@NF electrode preparation technique. Reproduced with permission [197]. Copyright 2024, Elsevier. (b) Diagrammatic representation of the NiCo(PO₄)_x synthesis on nickel foam. Reproduced with permission [200]. Copyright 2024, Elsevier. (c) Visual depiction of the generation procedure. Reproduced with permission [202]. Copyright 2024, Elsevier. (d) Synthesis of CoP@CoNi LDH and CoP@CoNi LDH‐Pt. Reproduced with permission [201]. Copyright 2025, Elsevier.

Untreated electrocatalysts have undesirable adsorption strengths for the intermediate compounds, so their fundamental function is inadequate, leading to reduced efficiency in catalyzing reactions compared to treated alternatives. Through efficient manipulation of the electrical configurations, the intentional addition of precise quantities of oxygen and phosphorus to yield bimetallic phosphates significantly boosts the functioning and acceptance of electrocatalysts [198, 199]. P‐O's outstanding coordinating capability renders the production of water‐soluble compounds with numerous metal ions possible. In this instance, Liu et al. [200] propose designing a precise bimetallic phosphate catalytic framework and subsequently applying it as an itinerary for fabricating a NiCo(PO₄)_x/NF nanocomposite utilizing a universal electrochemical deposition technique (Figure 4b). Thorough characterizations and theoretical computations confirm that a substantial quantity of intrinsic HER activity can be stimulated by the NiCo(PO₄)_x with a customized electronic configuration. In particular, the site‐protophilic properties and advantageous covalent metal‐oxygen (M‐O) interaction of NiCo(PO₄)_x/NF are vital for encouraging efficient proton transfer and H─OH bond cleavage via HER. The NiCo(PO₄)_x/NF's aerophobic attributes suggestively accelerate the dispersion of H₂ bubbles from the catalyst interface. When these cutting‐edge properties are brought together, NiCo(PO₄)_x/NF reveals exceptional catalytic activity for HER, as evidenced by its exceptionally low overpotential of 68 mV in 1 M KOH at 10 mA cm⁻².

Researchers fabricated a unified amorphous cobalt phosphoselenide (a‐CoPSe) with a customizable charging concentration to substantially improve the bulk activity of non‐noble metal catalysts. Incorporating phosphorus and selenium atoms with multiple electronegativities into the one‐step electrodeposition procedure assists in inducing disordered structures and regulates the catalyst's electrical configuration. Flaws and superficial bonds exist in the amorphous structure, making it more desirable for catalysis. For alkaline water splitting, a‐CoPSe laid onto NF reveals exceptional catalytic behavior, far surpassing commercial catalysts and the vast majority of renowned materials. To achieve an output of 500 mA cm⁻² for HER and OER, excess potentials of 285 and 244 mV are needed. The total mass operations of a‐CoPSe reach up to 890.4 A g⁻¹ Co for HER at an excess potential of 300 mV and 677.4 A g⁻¹ Co for OER at 250 mV. To facilitate optimal alkaline overall water splitting, Shi et al. [200] construct an incorporated amorphous CoPSe using a straightforward one‐step electrodeposition technique (Figure 4c). The flexible regulation of the electrical configuration, achieved through phosphorus and selenium atoms with distinct electronegativities, strengthens the chemical activity of the integrated catalysts.

Here, Li et al. [201] recommend an in situ anchored Pt cluster strategy for creating CoP@CoNi LDH‐Pt electrodes for HER and a two‐step electrodeposition technique for creating CoP@CoNi LDH electrodes for OER (Figure 4d). The objective is to boost the durability and efficacy of water electrolysis at elevated current densities. Ni foil was chosen as the supporting material to ensure strong conductivity. Strong interfacial electrical interactions were utilized to achieve optimal electrocatalytic kinetics and elevate intrinsic activity by constructing heterostructure electrodes. To boost mass transfer at elevated current density, superhydrophilic electrodes were designed. To enhance overall durability, electrodeposition and in situ oxidation techniques were implemented to reinforce the bond between the catalyst and substrate, firmly establish active elements beneath the catalyst, and restrict degradation during water electrolysis. This research presents a new approach for producing highly durable water splitting catalysts on a large scale and at high current densities.

3.3. Ion Exchange Method

Cobalt phosphides are one example of a transition metal phosphide (TMP) that has garnered a considerable amount of curiosity from researchers owing to their affordability, customizable structure, and exceptional catalytic performance. Nonetheless, their fundamental activity and stability are still inadequate [203, 204]. The limited natural activity of CoP is mainly due to its sluggish reaction speed and unfavorable hydrogen adsorption Gibbs free energy (ΔG_H*), which greatly limits its ability to produce hydrogen in both alkaline and acidic conditions. Numerous methods, including defect engineering, heteroatom doping, heterostructure formation, and morphological control, can enhance the fundamental activity of catalysts, according to prior research. As a supplementary dopant, sulfur (S) can only partially replace phosphorus (P), which alters the electronic configuration and increases the variety of available active sites by enhancing the electrochemical surface area (ECSA) of the components, potentially helping to reduce negative effects.

Motivated by this concept, Pan et al. [205] propose an intuitive ion exchange method for synthesizing multiple types of S‐ and Cu‐Co‐doped CoP catalysts, all of which exhibit an impressive rise in the electrocatalytic activity of HER as opposed to single‐doped CoP (Figure 5a). The optimized S_0.5‐Cu‐CoP catalyst exhibits excellent HER performance, achieving low overpotentials of 65 mV in acid and 139 mV in alkali at 10 mA cm⁻², with corresponding Tafel slopes of 38.13 and 91.65 mV dec⁻¹. It continues to demonstrate outstanding HER efficiency and catalytic stability even in a neutral entity system. Concurrently, in the general water separation test, with an output of 10 mA cm⁻², the electrolytic cell comprised of S_0.5‐Cu‐CoP and commercial nickel‐Fe foam necessitates a barely modest cell voltage of 1.55 V. Li et al. [206] established a manganese‐incorporated cobalt boride (Co_xMn_yB) nanoarray on a carbon fiber substrate adopting an MOF precursor, resulting in it being reinforced by nitrogen‐doped porous carbon (NC), leading to superior HER and OER performance (Figure 5b). In an attempt to boost Co_xMn_yB's conductance while facilitating charge/ion exchange, NC acts as a conducting reinforcement structure. Co_xMn_yB with porous carbon has a substantial number of successful catalytic locations. As envisioned, Co_xMn_yB displays dual‐functioning electrocatalytic functions for OER and HER with low overpotentials and modest Tafel slopes, which is considerably better than the results of preceding CoB specimens. Co_xMn_yB catalysts exhibit exceptional long‐term electrocatalytic functions and can be implemented as dual‐functioning electrocatalysts for the entirety of water breaking down.

FIGURE 5.

(a) Schematic diagram of the synthesis of S_x‐Cu‐CoP. Reproduced with permission [205]. Copyright 2024, Elsevier. (b) An illustration of the cobalt boride production process. Reproduced with permission [206]. Copyright 2024, Elsevier. (c) Diagrammatic illustration of the NiCoP/FeNiCoP hollow ellipsoids production. Reproduced with permission [207]. Copyright 2024, Elsevier. (d) Methodology of Co‐NC@NC production. (1) SEM pictures of Zn/Co‐ZIF (2) SEM pictures of Zn/Co‐ZIF@ZIF‐8. (3) SEM pictures of Co‐NC@NC. Reproduced with permission [208]. Copyright 2024, Elsevier.

Gao et al. [207] by using a regulated NaH₂PO₂‐assisted phosphorization method, consistently cylindrical NiCoP/FeNiCoP heterostructured ellipsoids with abundant P vacancies were meticulously manufactured via an intuitive ion‐exchange technique (Figure 5c). The distinctive ellipsoid hollow structure offers a broad electrochemical contact area, revealing additional active regions while facilitating gas and electron transmission. Furthermore, the external N₂‐doped carbon layer created during phosphating in a nitrogen‐rich atmosphere boosts conductivity and durability. The NiCoP/FeNiCoP heterostructure and P vacancies collaborate to accelerate HER and OER kinetics. Consequently, the as‐designed NiCoP/FeNiCoP approached Pt/C and outperformed RuO₂ by demanding only insignificant overpotentials of 45 mV (_η10) for HER and 266 mV (_η50) for OER at current densities of 10 and 50 mA cm⁻², respectively. This investigation provides an innovative strategy for constructing a highly productive and economically viable water‐splitting electrocatalyst. Additionally, Co‐MOF was adopted as a precursor for developing the NC‐supported Co_xMn_yB nanoarray. The Co_xMn_yB catalyst exhibits superior OER performance, a minimal Tafel slope, and excellent HER activity, making it a long‐term, sustainable, dual‐functional electrocatalyst for overall water splitting.

Xi et al. [208] designed a core‐shell composite called Co‐NC@NC based on cobalt nanomaterials using pyrolysis, ion exchange, and epitaxial growth from Zn/Co‐ZIF@ZIF‐8 crystals. The core Zn/Co‐ZIF prevents cobalt nanoparticle aggregation by promoting uniform distribution through zinc integration. The microporous N‐doped carbon from ZIF‐8 acts as an additional barrier, preventing electrolyte disintegration of interior cobalt nanoparticles. The Co‐NC@NC catalyst exhibits a stratified porous material with an exceptionally high surface area. It outperforms Pt/C in 0.1 M KOH, showing superior stability, excellent 4‐electron selectivity, and an E_1/2 up to 0.886 V, with adequate activity and resilience in acidic conditions. Figure 5d depicts an illustrative representation of cobalt particulate matter wrapped within nitrogen‐doped carbon coatings (Co‐NC@NC) [209]. Zn/Co‐ZIF nanocrystals resembling sodalite were reported to be competently produced with methanol, considering the equivalent ionic radius and coordinating arrangement of Co²⁺ and Zn²⁺. Zn ions subsequently triggered homogeneous epitaxial development of ZIF‐8 using Zn/Co‐ZIF as the seed for the crystal structure, leading foremost to the emergence of a core‐shell heterostructure designated Zn/Co‐ZIF@ZIF‐8 [210]. Zn/Co‐ZIF has a rhomboidal dodecahedral geometry, with angular edges and a smooth outer surface (Figure 5d(1)). Zn/Co‐ZIF‐8 has a more substantial particle dimension but has comparable dimensions owing to ZIF‐8's consistent growth (Figure 5d(2)). The SEM pictures of Co‐NC@NC in (Figure 5d(3)) render it readily obvious that the structure is rough to the touch, deviating from the initial dodecahedral morphology and unambiguously showcasing detectable minuscule metallic nanoparticles. The comparatively thin ZIF‐8 layer of Zn/Co‐ZIF@ZIF‐8 is considered devoid of a substantial carbon supply to sustain the dodecahedral structure throughout the heating degradation procedure [211].

3.4. Design 0D Co‐Based Catalysts

Regarding interfacial technology, heterostructured materials composed of 0D and 2D concepts are particularly fascinating. Intimate contact between the two phases establishes an exceptionally strong interaction in the 0D/2D heterostructure. The bulk phase and the interface possess extremely different physical and chemical traits. The adsorption energy, conductivity, and reaction kinetics of the intermediates are all influenced by an intense interface interaction, which induces a rearrangement of electrons at the heterogeneous interface. For example, Kim et al. constructed a 0D@2D Ni₂P@NSG heterostructure via intuitive ex situ sonication and a robust solution phase [212]. Subsequently, in a slightly alkaline electrolyte, Ni₂P@NSG electrocatalysts showcase robust catalytic abilities regarding HER and OER. A Ni₂P@NPC heterostructure with substantial interaction implications is proposed by Zhao et al. The electrode material has excellent energy preservation properties due to the interconnected stability of monodispersed TMP nanoparticles and carbon miniature sheets (NSs) [213]. Liu et al. [214] present an intuitive method for anchoring TMPs QDs (<10 nm) on permeable carbon NSs, yielding a unique 0D/2D heterostructure (Figure 6a). Using a gradient temperature calcination procedure, uniformly distributed QDs mounted on 2D NSs are generated from MOF precursors. After thermal phosphating, the resulting NiCoFe‐P/C exhibits exceptional electrochemical capabilities for supercapacitors and water splitting (OER and HER). The NiCoFe‐P/C electrode demonstrates outstanding catalytic activity with an overpotential of 87 mV (HER at 10 mA cm⁻²) and 257 mV (OER at 100 mA cm⁻²) over a 34 h operating interval. The NiCoFe‐P/C//NiCoFe‐P/C electrolyzer requires only 1.55 V to achieve 10 mA cm⁻². This research suggests an intriguing manufacturing strategy for 0D/2D heterostructured multimetallic phosphides.

FIGURE 6.

(a) A representation illustrating how NiCoFe‐ZIF, NiCoFe‐C, and NiCoFe‐P/C are synthesized. Reproduced with permission [214]. Copyright 2022, Elsevier. (b) Diagrammatic representation of the MoS₂ QD, CoP_x, and MoS₂@CoP_x composite synthesis process. Reproduced with permission [244]. Copyright 2025, Elsevier. (c) Diagrammatic representation of the CoS/P preparation process. (1) SEM picture of precursor Co(OH)₂/Co(OH)F. (2) Sulphide CoS. (3) Phosphosulfide CoS/P. (d,e) Crystal structures. (f–i) Density of states, and the d‐band center of Co₉S₈ and CoP. Reproduced with permission [236]. Copyright 2024, Elsevier.

Embedding 0D quantum dots on 2D TMP nanosheets boosts electrocatalyst functionality due to the substantial interfacial interaction region, robust charge transfer, and excellent conductivity of 2D semiconductors. MoS₂ QDs attached to 2D nanosheets facilitate charge transfer at the heterojunction interface [215]. CoP nanosheets transfer electrons to the MoS₂ QD surface, boosting charge separation and HER kinetics [216]. Yu et al. described a 0D/2D MoS₂ QDs/CoSe₂ NSs hybrid heterostructure with homogeneous distributions of ultra‐small MoS₂ QDs, where the synergistic effect boosted efficiency and stability for overall water splitting [217]. In this investigation, Liu et al. [214] effectively implemented a hydrothermal approach for generating enormous amounts of evenly distributed 0D MoS₂ QDs mounted on 2D CoP_x heterojunction substrates (Figure 6b). In juxtaposition with sophisticated MoS₂‐based catalysts, in 0.5 M H₂SO₄, the MoS₂ QDs/CoP_x heterostructure exhibits outstanding functionality of HER, requiring only a −14 mV overpotential for the 0D/2D MoS₂ QDs/CoP_x heterostructure to achieve a current density of 10 mA cm⁻². With the goal of effectively maximizing electron transfer for elevated HER on MoS₂ QDs/CoP_x, theoretical calculation indicates that tuning of electronic structures occurs and electrons are transferred at the heterojunction interface after MoS₂ QDs couple with CoP_x. The researchers assume that the conductive properties, stability, and synergistic effects of 0D and 2D materials may facilitate substantially higher efficiency. The development of novel electrocatalysts with renewable resources, affordable cost, outstanding performance, and stability is significantly supported by this investigation. The synergistic combination of 0D and 2D materials is responsible for the enhanced productivity of the MoS₂ QDs/CoP_x heterostructure. Optimal electron transport is rendered achievable by the interaction of MoS₂ quantum dots with CoP_x, triggering charge dispersion and electronic structural alterations at the heterojunction interface. While the ideal mass ratio guarantees maximum synergistic effects, the metallization of 2H‐MoS₂ at the interaction junction reveals more active edge sites. This study points out that building effective, affordable, and stable electrocatalysts involves meticulous design of heterojunction materials.

Electrocatalytic water splitting has facilitated the use of transition metal‐based oxides that augment oxygen vacancies and establish permeable materials [218, 219]. Nitrogen‐doped carbon compounds containing transition metals are currently considered an exceptional class of bifunctional catalysts [220, 221, 222]. The water‐splitting catalysis technique additionally incorporates a variety of transition metal‐based hydroxides, [222, 223, 224] phosphides, [225, 226] sulfides, [227] and alloy compounds [228]. Furthermore, transition‐metal phosphosulfides, notably FePS₃, [229] NiPS₃, [230] NiFeSP, [231] ZnCoSP, [232] and CoSP, [233] have demonstrated exceptional qualities, including outstanding stability in electrocatalytic water splitting. Nonetheless, hollow nanostructured materials typically arise as an effect of the phosphorization procedure [232]. Because the active sites are more extensively exposed, three‐dimensional and hierarchical substances have been successfully utilized as electrode materials for chemical reactions in recent years [234, 235]. For water splitting, it is consequently crucial to construct transition metal phosphosulfides with hierarchical structures. Researchers proposed a three‐dimensional hierarchical Co(OH)₂/Co(OH)F precursor‐template methodology to fabricate CoS/P materials using phosphorization and sulfidation. The hexagonal core is wrapped by six branches, forming hierarchical structures. Hollow CoS/P nanoparticles were discovered on the lateral edges after phosphorization and sulfidation. CoS/P reveals excellent water‐splitting activity with an overpotential of 470 mV at 10 mA cm⁻². Theoretical calculations confirmed the beneficial impact of CoP compared to CoS. This work highlights a hierarchical 3D CoS/P material with hollow nanoparticles to enhance catalytic activity. Liang et al. [236] illustrate that the Co‐based hydroxide precursor experiences phosphorization and sulfidation in sequence in a schematic representation of the CoS/P preparation method, and Figure 6c illustrates the crystalline structures of these substances. The dimension of Co(OH)₂/Co(OH)F is approximately 5 µm, and the orientation among each dendritic array is 60°, pursuant to an SEM image in (Figure 6c (1)). Co‐based sulfide's morphology persists as six‐branched, evident within the SEM picture in (Figure 6c (2)). The hexagonal shape of CoS/P remains clearly visible within the SEM image (Figure 6c (3)), and the nanoparticles that are doped onto the branches become abrasive. The Co₉S₈ and CoP were chosen as hypothetical structures to execute the computational analysis with the aim to better comprehend the variations in electronic configuration between CoS and CoS/P Figure 6c–e. Both of these, CoP and Co₉S₈, reveal transition metal characteristics with electron energies adjacent to the Fermi level, which is apparent by the total density of states graph of their crystal arrangements in Figure 6c,f,g). However, in spite of its modest bandgap, the CoP is exceptionally conductive, which guarantees rapid exchange of electrons over the electrocatalysis phase. In contrast to Co₉S₈ (εd = −1.497 eV), the d‐band position of CoP (εd = −1.989 eV) oscillates inward in contrast to the Fermi level at the precise same time, Figure 6c,h,i. According to recent studies, the more impaired the binding of intermediates carrying oxygen, the lower the εd energy [237, 238]. The HER and OER chemical reactions were enhanced as well as an effect of the d‐band center's downward displacement diminishing the interaction between Co and OH*, which assisted in the dissociation of hydroxyl (OH*) and hydrogen (H*) adsorbates [239, 240]. Sulfide/phosphide and oxygen may boost the adsorption capacities of O precursors on the site of activation during the operation of OER [241]. Furthermore, the d‐band center value for CoP is −1.989 eV, which puts it incredibly adjacent to the Pt/C value of −1.92 eV [242] Phosphorus atoms' electron‐rich characteristic for CoS/P might considerably strengthen hydrogen desorption and adsorption over the HER cycle [243]. Consequently, when compared to CoS, CoP performed better in both OER and HER. The hierarchical 3D CoS/P structure's distinct multidimensional architecture with hollow nanoparticles on the branches is precisely what grants it its higher catalytic performance. The reaction rate is increased and more active sites are exposed by these hollow nanoparticles. According to theoretical estimates, the phosphorization procedure further improves conductivity and catalytic efficacy when compared to CoS alone. Large surface area and effective mass transfer are provided by the hierarchical six‐branched hexagram structure. This study shows that precursor‐template approaches present a viable path toward the development of sophisticated 3D electrocatalysts for water splitting.

A distinct pattern can be observed in all of the synthesis approaches discussed in this section: hydrothermal and solvothermal methods are the most favored for laboratory‐scale research due to their ability to provide flexible control over dopant distribution, shape, and defect concentration. Electrodeposition enables superior direct growth on conductive substrates, reducing contact resistance and eliminating the need for binders, both of which enhance device integration. Ion exchange techniques are particularly effective for producing unique heterostructures and transforming pre‐existing materials into doped phases while preserving their original shape. Hydrothermal synthesis remains the best choice for real‐world applications that require scalable production, thanks to its ease of use, affordability, and repeatability. While high‐temperature calcination and spray pyrolysis offer less precise control over nanostructure morphology, they are more suitable for large‐scale industrial production. The structural characteristics and ultimate catalytic performance of cobalt‐based electrocatalysts are significantly influenced by the choice of synthesis technique, with each method presenting distinct trade‐offs in terms of cost, scalability, and controllability.

4. Heteroatom Doping Strategies for Cobalt‐Based Electrocatalysts

Heteroatom doping involves introducing foreign atoms into the cobalt lattice to modify its electronic structure, create active sites, and enhance conductivity. This approach effectively addresses the limitations of pristine cobalt‐based nanoparticles, such as insufficient electrical conductivity and a lack of active sites. By introducing heteroatoms, the d‐band center of cobalt can be adjusted, optimizing the adsorption energy of reaction intermediates for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Additionally, heteroatom doping generates lattice defects and vacancies that serve as extra catalytic sites, thereby improving overall water splitting efficiency. This section discusses both metal (cation) doping and non‐metal (anion) doping as complementary strategies for enhancing water splitting efficiency.

4.1. Metal‐Doped Cobalt Metal Oxide Electrocatalysts

Cation doping corresponds to the addition or substitution of cobalt with complementary ions of metallic elements (e.g., Ni²⁺, Fe³⁺, Mn^{2+ +}, Cu²⁺) within the oxide crystalline framework. This enrichment changes the electronic characteristics, composition, and catalytic functionality of cobalt oxides. Metal doping modulates the oxidation states of cobalt, leading to improvements in electron transfer capability. Cation dopants accelerate charge transfer by modulating the electronic structure, contributing to greater conductivity channels. Doping introduces supplementary active sites and stabilizes reaction intermediates, thereby minimizing overpotentials and enhancing reaction rates. Dopant metals like Ni, Fe, and Mn markedly diminish OER overpotential by boosting the adsorption strength of oxygenated intermediates while encouraging multi‐electron transfer mechanisms. Cation doping can enhance hydrogen adsorption energies, thereby improving HER functionality [245, 246]. The introduction of metal heteroatoms into cobalt‐based nanomaterials alters the coordination environment and electronic structure of cobalt active sites by adjusting the d‐band center position, facilitating charge transfer, changing coordination number, generating oxygen vacancies, modifying spin states, and producing synergistic electronic effects. The incorporation of a foreign metal atom alters the local electron density around cobalt, shifting the d‐band center to achieve optimal binding energies for both HER and OER. Metal heteroatoms typically substitute cobalt atoms in the crystal lattice, leading to lattice strain, diminished coordination number, and establishment of heterometallic bonds such as Co─Fe or Co─Ni. Oxygen vacancies arise when metal heteroatoms with valence states differing from cobalt create charge imbalances, acting as Lewis's acid sites that bind and activate water molecules, lowering OER overpotential. Co‐doping with two distinct metal heteroatoms (e.g., Fe and Ni into Co₃O₄) generates a synergistic electronic environment, resulting in catalytic activity that surpasses single‐dopant systems. The distribution of metal dopants is vital for catalyst stability. Uniformly dispersed dopants integrate seamlessly into the cobalt lattice, maintaining structural integrity and enabling consistent performance for over 100 h. In contrast, segregated dopant distribution induces sluggish interfaces and localized galvanic cells, resulting in rapid degradation after 20 to 50 h [247, 248].

The incorporation of rare‐earth elements (Ce, La, Pr, Nd) with distinctive 4f electronic arrangements, static oxidation states, and big ionic radii delivers substantial possibilities for influencing the electronic framework and catalytic efficacy of Co₃O₄ [249, 250, 251]. The inclusion of rare‐earth elements boosts electrocatalytic ability through regulation of band structure, defect commencement, and mediator adsorption capacities [252, 253]. La doping fine‐tunes the chemical composition of Co₃O₄ and raises electrical conductivity by implementing oxygen vacancies and altering the Co valence state [254]. Nd‐doped Co₃O₄ operates continuously for 24 h in a PEM flow cell, with an OER overpotential of roughly 304 mV at 10 mA cm^{− 2}, analogous to noble‐metal catalysts comprising Ru. Nd inclusion considerably elevates OER catalytic output via boosting electrical conductivity and minimizing the energy hurdle for intermediate generations, as established by DFT modeling techniques. Zhang et al. [255] Nd‐doped Co₃O₄ catalyst's manufacturing procedure is displayed in Figure 7a. Following that, a straightforward hydrothermal technique was implemented to immediately produce aligned, vertical predecessors of Nd‐doped nanorods on a carbon cloth (CC) base material with the aid of urea and NH₄F. The final Nd‐Co₃O₄ composite catalyst was fabricated through incorporating an additional thermal calcination operation to the gathered precursors. Computations using DFT were employed to continue investigating the enhancement in Nd doping‐induced OER functionality of spinel catalysts, with particular emphasis on the projected density of states (PDOS) and the liberated energies of reaction precursors. The electrical configuration of the Co₃O₄ catalyst has been substantially altered by the inclusion of Nd, according to DFT calculations. Furthermore, Nd‐ Co₃O₄ indicated metallic properties with a considerably greater DOS adjacent to the Fermi level, consistent with the total density of states (DOS) inquiry in Figure 7b [256]. The evidence presented implies that Nd doping successfully elevated the concentration of carriers and promoted the catalyst's conductive properties. Additionally, the simulated PDOS findings suggested that the Nd‐Co₃O₄ d‐band center dropped downstream and was more distant from the Fermi level (Figure 7c,d). By diminishing the antibonding state energy, an upshift in the d‐band center to −1.26 eV for Nd‐Co₃O₄ versus −1.63 eV for Co₃O₄ mitigates the ability to absorb the intensity of the OH* intermediate and optimizes OER functionality. The process of switching from OH* to O* was the potential determining step (PDS) responsible for the OER on Nd‐Co₃O₄, judging by the outcomes. Nd doping substantially decreased the free energy generated by the Gibbs barrier for the generation of O* precursors (Figure 7e,f). Additionally, the Nd atoms' immersion energy in Co₃O₄ is −4.41 electron volts, validating extremely excellent thermodynamic durability amid the dispersion procedure. The aforementioned further illustrates the neodymium‐doped sites' structural stability amid forced catalysis parameters. Consequently, such analytical findings indicate that Nd doping conveniently minimizes the energy barriers of essential OER phases, thereby accelerating the formation of oxygen while optimizing the energy consumed during the adsorption of intermediary substances. This study successfully developed a cost‐efficient and high‐performance OER catalyst by incorporating the rare‐earth element Nd into Co₃O₄ in acidic circumstances. The Nd‐doped Co₃O₄ demonstrated an OER overpotential of merely 304 mV at 10 mA cm⁻² and functioned continuously for 24 h in a PEM flow cell, exceeding the performance of most documented noble‐metal‐free catalysts. Density functional theory (DFT) simulations verified that Nd inclusion enhances electrical conductivity, generates oxygen vacancies, alters cobalt valence states, and diminishes the energy barrier for intermediate generation. This study presents a feasible strategy for the development of efficient and stable acidic OER electrocatalysts. The results provide considerable theoretical and practical significance for the advancement of proton‐exchange membrane water electrolysis devices.

FIGURE 7.

(a) Diagrammatic representation of Nd‐Co₃O₄ Synthesis. (b) Density of states. (c,d) Predicted Co site d‐band sites for Nd‐Co₃O₄ and Co₃O₄ simulations. (e,f) The 4‐electron oxygen evolution reaction route of Nd‐Co₃O₄. Computed free energy graph for oxygen adsorption. Reproduced with permission [255]. Copyright 2025, Elsevier.

Song et al. [257] deploy a solid‐state heat alteration technique to generate cobalt phosphide/heteroatom‐doped CNT composites (Co₂P/doped‐CNTs). This approach enables sequential phosphorization and carbonization by applying phytic acid during thermal processing, eliminating conventional post‐treatment phosphating. The methodology facilitates assembly of multifunctional active sites, exploiting N‐doped carbon species for ORR and Co₂P for OER. Formation of Co₂P alongside Co during carbonization boosts defect density, assisting emergence of spare ORR‐active nitrogen species. Electrochemical tests demonstrate bifunctional operation with a potential divergence (ΔE) of 0.814 V between E_1/2 (ORR) and Ej₁₀ (OER) under alkaline conditions, along with exceptional cycling stability. The synthesis procedure is demonstrated in Figure 8a. Carbon nanotubes (CNTs) operate as platforms for single‐atom catalysts (SACs) and are blended with transition metals including Co, Ni, and Fe to synthesize nanomaterials with exceptional electrocatalytic functionality for ORR and HER [258, 259]. The hollow carbon substrate offers advantages including enormous surface area, boosted conductivity, and admirable stability [260, 261]. Implanting CNTs with nonmetallic heteroatoms (P, N, S) improves connections between the carbon matrix and metal, leading to plenty of anchoring sites [262, 263]. Feng et al. [264] created Co‐Zn SAs immobilized on Co‐functionalized nanotubes (Co‐Zn SAs@Co‐CNTs). Leveraging synergistic interaction within Co‐Zn bimetallic atoms and nitrogen (Co‐Zn‐N₆) active sites, the catalyst required minimal overpotentials of 142 and 89 mV for HER in acidic and alkaline electrolytes at 10 mA cm⁻², along with an ORR E_1/2 of 0.92 V (Figure 8b). The scanning electron microscope (SEM) results shown in Figure 8c indicate that Co,Zn SAs@Co‐CNTs display consistent necklace‐like porous nanotube architectures with dimensions ranging from 20 to 50 nm. The transmission electron microscopy (TEM) image in Figure 8d confirms that cobalt nanoparticles are uniformly encased within the carbon nanotubes, thereby preventing the agglomeration of metal particles. Figure 8e presents the XRD patterns for Co‐CNTs, Co single atoms supported on Co‐CNTs, and Co and Zn single atoms supported on Co‐CNTs. The diffraction peaks observed at 44.3° and 55.3° correspond to the (111) and (200) planes of metallic cobalt. Additionally, a peak around 26.2° confirms the presence of graphitic carbon in all samples. Notably, Co,Zn SAs@Co‐CNTs exhibit a positive peak shift compared to the other samples, likely due to strong Zn–Co interactions. The Raman spectra shown in Figure 8f reveal D and G bands at 1359.3 and 1591.5 cm⁻¹, respectively. The increased ID/IG ratio of Co,Zn SAs@Co‐CNTs (0.992) relative to Co SAs@Co‐CNTs (0.886) and Co@CNTs (0.816) suggests a higher density of lattice defects in SP² carbon, attributed to the atomic dispersion of Co and Zn within the N‐doped carbon matrix. The inherent electrocatalytic efficiency gained from the distinctive interaction arrangement of Co and Zn single atoms on Co‐embedded CNTs was evaluated by exploring the HER dynamics of Co single atoms on Co‐CNTs and Pt/C (25 wt.%) as a basis for comparison, using a conventional three‐electrode system in multiple media types under similar conditions. Originally, the LSV curves of each specimen were collected in N₂‐saturated 1 M KOH at a scan rate of 3 mV/s with iR compensation, as depicted in Figure 8g. Co‐Zn single atoms on Co‐embedded CNTs displayed the greatest level of current density throughout an entire potential span, necessitating an excess potential of merely 89 mV (η10) to accomplish a present density of 10 mA cm⁻², which was considerably less compared to the values of Co SAs@Co‐CNTs (η10 = 252 mV) and Co‐CNTs (η10 = 474 mV). The Co, Zn SAs@Co‐CNTs endured remarkable HER achievement, as is apparent in Figure 8h. For instance, the Co, Zn SAs@Co‐CNTs catalyst requires an overpotential of only 115 mV to achieve 10 mA cm^{− 2}, substantially lower than Co SAs@Co‐CNTs (221 mV) and Co‐CNTs (319 mV). Additionally, computational methods are employed for investigating the processes of HER amplification. It is widely accepted that the Gibbs liberated hydrogen binding energy (ΔG_H*) is a reliable indicator of the HER function of electrocatalysts; a desirable hydrogen adsorption strength is indicated by ΔG_H* being consumed or adjacent to negligible [265]. The HER reaction energy barrier related to potential active sites in the Co,Zn SAs@Co‐CNTs catalyst is shown in Figure 8i. H* exhibited greater insensitivity to both desorption and adsorption at the Co center of Co‐Zn‐N₆ throughout the progression of the HER, as indicated by the ΔG_H* value for the Co site in the Co‐Zn‐N₆ active center, which was only 0.0654 eV, compared to Co‐N₄ (−0.111 eV), Zn‐N₄ (1.257 eV), and the Co‐Zn‐N₆‐Zn site (0.4713 eV). Thus, in Co‐Zn SAs@Co‐CNTs catalysts, the electrostatic connection among Co and Zn bimetallic atoms considerably improved the electronic arrangements of the Co center, minimizing the reactive potential hurdle via the water‐mediated dissociation phase. In light of this, Figure 8j suggests a plausible HER reaction route for Co and Zn single atoms on the Co‐doped CNT catalyst. Co‐Zn bimetallic atoms and nitrogen (Co‐Zn‐N₆) active sites interact synergistically to deliver the outstanding bifunctional performance of Co,Zn SAs@Co‐CNTs. By activating adjacent nitrogen atoms, this diatomic coordination boosts the number of electrocatalytic sites and adjusts the energy level to produce reaction intermediates with favorable kinetic energy. Atomic scattering of both metals with a controlled ensemble size is deemed feasible by the self‐decomposition technique. The Co‐Zn‐N₆ coordination minimizes the energy barrier and accelerates the reaction rate for both HER and ORR, according to experimental and theoretical findings. A novel approach for creating inexpensive, high‐performing dual‐atom electrocatalysts is presented in this work.

FIGURE 8.

(a) The Synthesis procedure for Co₂P/Doped‐CNTs and Co/N‐CNTs. Reproduced with permission [257]. Copyright 2022, Elsevier. (b) Diagram of the synthetic process for Co, Zn SAs@Co‐CNTs. Structural and morphologic characterizations of Co,Zn SAs@Co‐CNTs. (c) SEM image. (d) TEM image. (e) XRD pattern. (f) Raman spectra of Co‐CNTs, Co SAs@Co‐CNTs and Co,Zn SAs@Co‐CNTs. (g) LSV curves with a 3‐mV s⁻¹ scan interval. (h) LSV polarization curves with a 3‐mV s⁻¹ scan rate. (i) The Gibbs free energy of the HER reaction at potential sites. (j) The potential HER pathway of Co and Zn single‐atom catalysts on carbon nanotubes in both alkaline and acidic environments is illustrated. Reproduced with permission [264]. Copyright 2024, Elsevier.

To accomplish an intricate conceptual knowledge of the successful ability to catalyze Co, Zn SACs supported on co‐embedded CNTs and to inspect the objective of the Co‐Zn dual center across the oxygen reduction response procedure. In accordance with thermodynamic principles, all five essential processes appeared exothermic, indicating that all catalysts can autonomously accelerate the ORR reaction at conventional potential (U = 0 V) (Figure 9a). At a starting prospect of 0 V, the minimal ΔG revealed the rate‐limiting procedure in the oxygen reduction manipulation, resulting in the 4th step (OH* to O²⁻) regulating the entire rate of reaction [266]. At the optimum potential U = 1.23 V, the transformations from OOH to O* and OH* to O* seemed kinetically favorable for Co‐N₄ and Zn‐N₄, while Co‐Zn‐N₆ had an upward trajectory, illustrating that the reactive sites of Co‐Zn‐N₆ are susceptible to unforeseen reactions during the oxygen reduction process (Figure 9a). The constrained efficacy of Co‐Zn‐N₆ for the ORR is estimated to be 1.04 V, transcending the single‐atom sites of Co‐N₄ (0.92 V) and Zn‐N₄ (0.43 V) (Figure 9b). At the ORR procedure's restricted limit of 1.04 V, the alteration of O₂* to OOH* at nanometallic Co‐N₄ and Zn‐N₄ spots, alongside the subsequent change of OH* to * for Co‐N₄, was thermodynamically appreciated. In contrast, every stage for Co‐Zn‐N₆ exhibited a thermodynamic collapse, suggesting the hypothesis that bimetallic Co‐Zn‐N₆ is the raised site of action all over the complete ORR procedure. Pursuant to the vital response ΔG associated with each activity core in Figure 9c, the Co‐Zn‐N₆ center revealed the lowest △G₄ (1.04 V), which is right next to the 1.23 V equilibrium potential, suggesting that the Co‐Zn‐N₆ center surmounted a minimal potential hurdle over the reactivity stage. Figure 9d,e illustrates the substantial inverse energy shifting at the Co‐Zn‐N₆ activated core rather than the single‐atomic Co‐N₄ and Zn‐N₄ configurations. The εd (d‐band center) concept presumes that an inferior ε_d signifies that electrons migrate from the metal center's 3d orbitals to the 2p orbital of oxygen, triggering electron accumulation that hastens the ORR and HER [267]. The thermodynamic adsorption‐desorption energy barrier of the bimetallic Co‐Zn‐N₆ sites in the Co, Zn SACs@Co‐CNTs catalyst has emerged as increasingly compatible with the resultant precursors. To reinforce the electrostatic collaboration of valence charge density difference (VCDD) and bimetallic atoms, patterns were employed to investigate the fundamental charging intensities of Co and Zn atoms immediately prior to and following adherence [268]. Figure 9f illustrates the proposed mechanism for ORR of Co and Zn single‐atom catalysts (SACs) on cobalt‐carbon nanotubes (Co‐CNTs). The electrostatic interactions between dual bimetallic Co and Zn atoms were substantially stronger than those of SACs, and the resulting synergistic effect facilitated electron transfer throughout the electrochemical process, consequently decreasing the energy barrier. At the Fermi level, the density of states (DOS) of Co‐N₄, Zn‐N₄, and Co‐Zn‐N₆ configurations were analyzed to determine the binding strength between the metal center and reactive intermediates. According to the d‐band center (εd) hypothesis, a reduced εd value denotes electron transport from the metal center's 3d orbitals to the oxygen's 2p orbital, causing electron buildup that speeds up the HER/ORR reaction. The bimetallic Co‐Zn‐N₄ active sites in Co,Zn SACs@Co‐CNTs offer a favorable thermodynamic adsorption–desorption energy barrier for chemical intermediates. To assess the electronic interaction between the bimetallic atoms, the atomic charge distributions of the Co and Zn atoms were analyzed before and after binding using valence charge density difference (VCDD) maps.

FIGURE 9.

(a,b) Diagrams of the energy barriers for ORR at varying voltages. (c) Energy potential comparison graph for each step. (d,e) The Co 3d and Zn 3d density of states (DOS) diagrams are presented. (f) Potential hydrogen evolution reaction pathway of Co and Zn single‐atom catalysts on carbon nanotubes in either alkaline or acidic conditions. Reproduced with permission [264]. Copyright 2024, Elsevier.

4.1.1. Fe‐Doped Cobalt as an Efficient Electrocatalyst

The electronic configuration of the electrode components is intrinsically linked to their proficiency in electrochemical reactions. The inclusion of a second metal cation is considered an appropriate strategy for manipulating the physicochemical properties and electrostatic charge distribution of transition metal phosphides (TMPs) to boost their effectiveness as catalysts [269, 270]. Xu et al. [271] implemented a straightforward strategy to synthesize Fe‐doped CoP electrocatalysts, which rendered appropriate defect‐rich sites for HER and OER in both acidic and alkaline environments. Li et al. [272] synthesized Cr‐doped CoP nanoarchitecture, and Cr‐CoP/CP showed greater effectiveness for HER and OER attributable to the electronically controlled manipulation phenomenon. Tang et al. [273] discovered that CoP's hydrogen evolution is maximized when Co atoms are replaced with Fe, resulting in significantly enhanced HER activity. The aforementioned investigation corroborated the prospective utilization of metal‐doped TMPs as promising electrocatalysts for water electrolysis. In accordance with multiphase structure, heteroatom doping is an exceptionally successful strategy for regulating heterogeneous interfaces and modulating interfacial electronic structure effects [274]. Iron (Fe) has particular capabilities in influencing the electronic configuration of cobalt (Co)‐based electrocatalysts owing to its deviations; the 3d orbital electron contributes along with an analogous ionic dimension. Furthermore, minimal amounts of iron indicate a low spin state, permitting its d‐band energy level to shift closer to the Fermi level and contribute as an electron donor, thus boosting electron transfer efficiency [274]. Consequently, integrating iron into Co‐based TMP catalysts is a considerably economically viable strategy. Encouraged by these factors, Fe‐doped allotropic Co₂P/CoP heterostructures with a nitrogen‐ and phosphorus‐co‐doped carbon matrix were synthesized from the ZIF‐67 precursor for adequate water dispersion. The collaborative effect of allotropic heterojunctions stimulates additional active sites and disrupts the electronic structure, contributing to elevated ECSA and enhanced HER/OER performance. Fe doping constitutes an essential factor in influencing the electronic configuration of Co₂P/CoP nanoparticles, making it a promising bifunctional catalyst for overall water splitting.

Yu et al. [275] illustrate the synthesis procedure of Fe‐Co₂P/CoP, which incorporates pyrolysis treatment and ion exchange processes in a hydrogen‐enriched atmosphere, in Figure 10a. The layout of Fe‐doped phosphide included changeable Co/P ratios, utilizing ZIF‐67 as the cobalt starting point and structural arrangement, K₃[Fe(CN)₆] as the elemental iron base, and (NH₄)₂HPO₄ as the phosphorus base. The structural characteristics and morphological principles of Co_xP and Fe‐Co_xP were examined employing SEM. SEM image (Figure 10b) shows that Fe‐Co₂P/CoP consists of asymmetrical nanoparticles. The catalyst reveals a more textured and permeable interface with an intumescent look compared to the ZIF‐67 precursor, due to decomposition of organic ligands at high temperatures. Fe doping has little influence on catalyst microstructures, as Fe‐Co_xP exhibits a compressed nanoparticle surface compared to Co_xP. In addition, TEM analyses have been conducted to investigate the internal organization of the permeable polyhedron in detail. Figure 10c illustrates that Fe‐Co₂P/CoP showcases an urchin‐like irregular nanoparticle shape, consistent with the SEM data. Furthermore, the magnified TEM picture in Figure 10d illustrates that the innermost CoP NS's are enveloped by a nitrogen‐and phosphorus‐doped amorphous carbon framework.

FIGURE 10.

(a) Schematic of the synthesis of Fe‐doped Co₂P, CoP, and Co₂P/CoP heterojunctions. (b) SEM picture. (c) TEM picture. (d) The corresponding magnified TEM image. (e) HER LSV curves of various catalysts in 1.0 M KOH. (f) Overpotential at 10 mA cm⁻². (g) Corresponding Tafel slopes. (h) EIS spectra for each specimen for HER. Reproduced with permission [275]. Copyright 2024, Elsevier.

Besides its outstanding OER performance, Fe‐Co₂P/CoP exhibited remarkable functionality of HER catalysis in 1.0 M potassium hydroxide. In Figure 10e,f, the LSV contours regarding each catalyst are illustrated. Subsequently, it was confirmed that Fe‐Co₂P/CoP demonstrates exceptional HER performance, necessitating an inadequate overpotential of 174 mV to accomplish a current density of 10 mA cm⁻². This is substantially less than the reported values of Co₂P/CoP (247 mV), CoP (252 mV), Fe‐CoP (204 mV), Fe‐Co₂P (227 mV), and Co₂P (281 mV). The gradient of Tafel for Fe‐Co₂P/CoP is 74 mV dec⁻¹, significantly lower than any of the other opposition catalysts in the current investigation, corresponding to rapid dynamics amid the creation of hydrogen (Figure 10g). EIS was additionally employed to promote evaluation of interactions between the electron transmission impedance and the electrocatalytic dynamics, as illustrated in Figure 10h. It implies that Fe‐Co₂P/CoP is equipped with the most minimal charge transfer resistance across all catalysts, suggesting an increasingly rapid interstitial charge exchange at the electrode material/electrolyte interface. It is predicted that the construction of heterojunctions and appropriate doping of Fe will boost both the fundamental activity and charge transmission rates of the catalyst, consequently substantially boosting the entire water‐decomposing catalytic function.

Fe doping and allotropic heterojunctions collaborate to enhance the catalytic performance of Fe‐Co₂P/CoP. The formation of heterointerfaces between Co₂P and CoP leads to an increased electrochemically active surface area, which modifies the electronic structure and generates a greater number of active sites. Additionally, Fe doping improves charge transfer and mass transport kinetics by further adjusting the surface electron density. As a result, the Fe‐Co₂P/CoP catalyst demonstrates superior performance compared to its undoped counterpart, achieving low overpotentials of 174 mV for HER and 286 mV for OER at a current density of 10 mA cm⁻². The benefits of heterointerface and dopant engineering are illustrated by the dual‐electrode electrolytic cell, which requires only 1.711 V to achieve the same current density. This work presents a valuable approach for developing ion‐doped transition metal heterojunction catalysts for energy‐related applications.

4.1.2. Mn‐Doped Cobalt as an Efficient Electrocatalyst

The latest innovations in transition metal‐based electrocatalysts, particularly those incorporating Mn and Co, highlight their future prospects as economical alternatives with configurable electronic structures [276, 277]. Mn‐Co composites, including mixed oxides, layered double hydroxides, and spinels, showcase exceptional OER performance due to their integrated effects and elevated resilience in alkaline environments [278, 279]. Mn incorporation into the Co‐based lattice transforms electrostatic conditions and optimizes catalyst efficiency [280, 281, 282]. Nonetheless, constraints endure in augmenting activity, reducing synthesis costs, boosting durability, and clarifying fundamental principles. This research evaluates current achievements, identifies constraints, and presents the conceptualization of an effective and environmentally conscious OER catalyst with a specific architecture and simple manufacturing technique.

Tang et al. [283] fabricated a Mn‐doped Mn_0.10Co_0.90‐CoCo₂O₄/NF heterostructure catalyst with a high percentage of oxygen vacancies using a straightforward hydrothermal method followed by NaBH₄ reduction. Mn doping minimizes the potential for forming active CoOOH, while the 2D/2D heterostructure promotes integration of nanowire arrays and nanosheets, significantly elevating OER productivity [284]. The epitaxial growth phase diminishes the energetic interaction of the nanocomposite and stimulates the development of highly crystalline alloy laminated materials. Thus, multi‐metal alloy systems with accessible topologies and extremely thin morphological characteristics, epitaxially created on substrates, demonstrate enhanced catalytic efficiency and long‐term reliability. The fabrication procedure for the Mn_0.10Co_0.90‐CoCo₂O₄/NF catalyst is depicted in Figure 11a. The MnO(OH)‐CoCo₂O₄/NF precursor had previously been achieved through hydrothermally synthesizing Mn(OH)₂‐CoCo₂O₄/NF nanowire arrays on a pretreated NF substrate, followed by annealing, as shown in Figure 11b,c. The configuration of the catalyst transformed from tiny wires to an increased, elongated form throughout fabrication. The Mn(OH)₂‐CoCo₂O₄/NF was subsequently administered with a NaBH₄ solution by employing a reductive impregnation technique, culminating in the Mn_0.10C_0.90‐CoCo₂O₄/NF catalyst. An additional solvothermal method response subsequently adorned the tiny wire structure with consistent nanosheets (Figure 11d). SEM photos demonstrate that ultrathin, undulating nanosheets engulf the nanowire arrays, yielding an interconnected 2D/3D heterostructure with interconnected 3D permeability networks. The catalyst's crystal structure was analyzed using X‐ray diffraction (XRD). The primary hydrothermal reaction produced MnO(OH)‐CoCo₂O₄/NF, which then converted to Mn(OH)₂‐CoCo₂O₄/NF after undergoing high‐temperature calcination. This material was subsequently reduced to Mn_0.10Co_0.90 (JCPDS: 78–1991) and CoCo₂O₄ (JCPDS: 80–1544) using NaBH₄ as a reducing agent, as shown in Figure 11e. In comparison, precursor molecules containing only Co or Mn on NF generated Co(OH)₂‐CoCo₂O₄/NF and Mn₂O₃‐Mn(OH)₂/NF, respectively. Raman spectroscopy was employed to investigate the bonding characteristics of the catalysts. The Raman spectrum of Mn_0.10Co_0.90‐CoCo₂O₄/NF (Figure 11f) shows prominent peaks at 177, 312, and 362 cm⁻¹, which are linked to Co─O bonds. A peak near 587 cm⁻¹ is attributed to the CoxMnyO index, confirming the presence of both Mn and Co. Additional peaks at approximately 191 and 486 cm⁻¹ is associated with Mn─O bending vibrations, while the peak around 660 cm⁻¹ corresponds to Mn‐O stretching vibrations. The OER performance of Mn_0.10Co_0.90‐CoCo₂O₄/NF was evaluated in an industrial alkaline electrolyzer, in conjunction with a commercial Pt/C cathode for overall water splitting. The polarization curves (Figure 11g) show that Mn_0.10Co_0.90‐CoCo₂O₄/NF⁽⁺⁾||Pt/C⁽⁻⁾ outperforms the commercial RuO₂ ⁽⁺⁾||Pt/C⁽⁻⁾, especially at higher current densities. Figure 11h demonstrates that this electrolyzer requires only 1.70, 1.75, 1.82, and 1.89 V to achieve current densities of 100, 200, 500, and 1000 mA cm⁻², respectively. In contrast, the commercial RuO₂ ⁽⁺⁾||Pt/C⁽⁻⁾ electrolyzer needs higher voltages to reach equivalent current densities. Figure 11i demonstrates that the Mn_0.10Co_0.90‐CoCo₂O₄/NF⁽⁺⁾||Pt/C⁽⁻⁾ electrocatalyst surpasses the performance of most previously reported noble metal‐free catalysts for overall water splitting in alkaline media, particularly at a current density of 10 mA cm⁻².

FIGURE 11.

(a) Graphical representation of the manufacturing of Mn_0.10Co_0.90‐CoCo₂O₄/NF. (b) SEM pictures of MnO(OH)‐CoCo₂O₄/NF, (c) Mn(OH)₂‐CoCo₂O₄/NF, and (d) Mn_0.10Co_0.90‐CoCo₂O₄/NF. (e) XRD pattern of Mn_0.10Co_0.90‐CoCo₂O₄/NF. (f) Raman spectra. (g) LSV curves of overall water splitting for Mn_0.10Co_0.90‐CoCo₂O₄/NF⁽⁺⁾||Pt/C⁽⁻⁾ and RuO₂ ⁽⁺⁾||Pt/C⁽⁻⁾ in 1.0 M KOH. (h) Comparison of the required voltages at different current densities. (i) Comparison of the cell voltage for Mn_0.10Co_0.90‐CoCo₂O₄/NF at 10 mA cm⁻² with recently reported catalysts. (j) LSV polarization curve. (k) Tafel slopes. (l) Comparison of excess potential (at 10 and 50 mA cm⁻²) and gradients of Tafel of analogous catalysts in 1.0 M KOH. (m) C_dl Values. (n) Overview of the electrochemical surface area (ECSA) of manufactured catalysts. Reproduced with permission [283]. Copyright 2025, Elsevier.

The successful application of the manufactured catalysts for the OER was methodically assessed in a solution of 1.0 M KOH, adopting a conventional three‐electrode configuration. All prospective one has been amended for 100% iR adjustment and validated utilizing a reversible hydrogen electrode. In consideration of the synthesis technique's viability, the fundamental sample Mn_0.10Co_0.90‐CoCo₂O₄/NF has been generated over three sections, and the efficiency has been evaluated to figure out the appropriate settings. The statistical evaluation and inaccuracy rate evaluations were performed on the composite material for ensuring outcome consistency [285]. Figure 11j,k presents the linear sweep voltammetry (LSV) curves in tandem with the accompanying Tafel slopes, as well. The Mn_0.10Co_0.90‐CoCo₂O₄/NF catalyst exhibited excess potentials of 178 and 233 mV, leading to current capacities of 10 and 50 mA cm⁻², as well as an exceptionally minimal Tafel slope of 70.49 mV dec⁻¹, transcending virtually all other evaluated electrocatalysts (Figure 11l). The outcomes illustrate that Mn_0.10Co_0.90‐CoCo₂O₄/NF highlights superior reaction acceleration as opposed to the additional catalysts [286]. The electrochemical double‐layer capacitance (C_dl), obtained from cyclic voltammetry (CV) curves in the non‐Faradaic region (Figure 11m), was used to calculate the ECSA (Figure 11n).

Mn doping, oxygen vacancies, and a hierarchical 2D/2D heterostructure engage in conjunction to yield the exceptional OER performance of Mn_0.10Co_0.90‐CoCo₂O₄/NF. While oxygen vacancies boost electrical conductivity and active site exposure, Mn doping inhibits the probability of producing highly active CoOOH species and promotes charge redistribution. Mass transfer is facilitated, and reaction kinetics are accelerated, by the special nanowire array wrapped with ultrathin nanosheets. The conversion of CoO₄ to CoOOH as the centers of action under operating circumstances is verified by in situ Raman spectroscopy. The catalyst outperforms the RuO₂ benchmark and exhibits remarkable stability for nearly 280 h. Mn‐doped heterostructures are highlighted in this paper as an affordable substitute for commercial wastewater electrolysis.

4.1.3. Mo‐Doped Cobalt as an Efficient Electrocatalyst

The architecture and fabrication of heterostructure catalysts represent a significant approach for molecular‐level manipulation [287]. Heterostructures provide researchers with a more convenient platform for the adhesion and activation of reactive species, while additionally enabling electron transfer across various components, thus significantly enhancing the activity of electrocatalytic materials [288]. To date, numerous heterostructure electrocatalysts have demonstrated enhanced electrocatalytic activity for outstanding performance in water splitting. For instance, the precise synthesis of defect‐rich heterogeneous MoS₂/NiS₂ nanosheets on carbon fabric was conducted by Lin et al., which has been explored as bifunctional electrocatalysts for overall water splitting [289]. A novel fundamental double‐shell heterostructure bifunctional electrocatalyst, Co₉S₈@Co₉S₈@MoS₂, was designed by Li in recent years [290]. This metal‐organic framework‐based heterostructure illustrates outstanding performance for HER and OER by successfully manipulating the electronic structure of all components while maintaining intricate communication across all phases.

Furthermore, as apparent in Figure 12a, the Co₃O₄@Mo‐Co₃S₄‐Ni₃S₂/NF hybrid structure by Wu et al. [291] is generated by accumulating Co₃O₄ nanowires@ZIF‐67 on nickel foam followed by experiencing a sulfurization reaction. The open architecture of the nanoflower form facilitates mass distribution and delivers plentiful interface spots. Additionally, electronic configuration could be tightly regulated due to the rich heterogeneous interface and inclusion of non‐3d Mo atoms. OER and HER have demonstrated moderate excess potentials of 116 mV at 10 mA cm⁻² and 295 mV at 50 mA cm⁻². This catalyst‐based alkaline electrolyzer has a voltage rating of 1.62 V at 10 mA cm⁻². The Co₃O₄@Mo‐Co₃O₄‐Ni₃S₂/NF‐0.15 electrode exhibits substantially better conductivity to electricity compared to competing specimens, as displayed in Figure 12b, and significantly accelerates the electrocatalytic dynamics. It further demonstrates a substantially reduced charge‐transfer resistance. The decreased resistance could potentially be a consequence of Co₃O₄ nanorod arrays produced in situ, enabling optimized electrode transfer kinetics through strengthening the interaction between Co₃O₄ and Ni₃S₂. The existence of more substantial effective surface area on Co₃O₄@Mo‐Co₃S₄‐Ni₃S₂/NF‐0.15 is demonstrated by Figure 12c, which demonstrates that the Co₃O₄@Mo‐Co₃S₄‐Ni₃S₂/NF‐0.15 has a higher C_dl value (21 mF cm⁻²) than the comparison samples. The Co₃O₄@Ni₃S₂‐Co₃S₄/NF exhibits comparatively minimal HER performance due to its inadequate electrical conductivity and operational positions. It is undisputed that supplementing with non‐3d high‐valence metal Mo atoms promote HER electrocatalytic effectiveness and the H bond interaction strength. The excess potentials of HER and OER of the Co₃O₄@Mo‐Co₃O₄‐Ni₃S₂/NF‐0.15 sample observed in the present investigation were contrasted against the values from additional specimens, as illustrated in Figure 12d. Co₃O₄@Mo‐Co₃S₄‐Ni₃S₂/NF's superior water‐splitting performance results from its non‐3D high valence metal doping and 3D multistage structure. Mo doping influences the chemisorption of H⁺ and OH^– intermediates at response sites by altering the electronic structure. Diffusion routes are condensed, rapid‐carrying ion channels are included, and active material aggregation is effectively prevented by the special heterogeneous interface. Achieving 1.62 V at 10 mA cm⁻², each component lends its own benefits to enhance total water splitting efficacy. This work introduces a novel approach to the construction of inexpensive, effective heterostructure electrocatalysts.

FIGURE 12.

(a) Diagrammatic representation of the Co₃O₄@Mo‐Co₃S₄‐Ni₃S₂/NF synthesis processes. (b) Nyquist plots. (c) Computed C_dl across numerous specimens. (d) Correlation of produced specimens' OER and HER excess potential at 50 mA cm⁻² and 10 mA cm⁻² current density levels. Reproduced with permission [291]. Copyright 2021, Elsevier.

4.1.4. Cu‐Doped Cobalt as an Efficient Electrocatalyst

In recent decades, significant efforts have been made to substitute valuable metals with affordable alternatives. Transition metal (Co, Ni, or Fe) phosphides (TMPs) were previously considered among the most appropriate options, given their possession of exceptional stability and resilience [292, 293]. For instance, materials such as CoP, FeP, Cu₃P, and Ni₂P have been specifically designed for electrocatalytic water splitting [294]. Cu₃P distinguishes itself especially among TMPs due to its exceptional electrical conductivity and synergistic interactions with multiple active sites, consequently boosting catalytic performance [295]. For example, self‐supported Cu₃P/NF electrodes were fabricated with a two‐step methodology by Guo et al. The Cu₃P/NF demonstrated outstanding stability and activity for OER and the HER simultaneously [296]. Because of their relatively inexpensive cost, customizable composition, and natural abundance, transition metal complexes have garnered a great deal of interest [297, 298]. Nitrides, [299] phosphides, [300] sulfides, [299] borides, [301] selenides, [302] and oxides [303] are all members of this group of substances. These catalysts are highly effective owing to their plentiful active sites, excellent electrical conductivity, rapid charge/mass transfer, and synergistic operation, which cumulatively facilitate the electrolysis of water. Transition metal phosphoselenides represent some of the most intriguing materials for a variety of applications. They feature a tunable, layer‐dependent bandgap; rapid reaction kinetics; high charge carrier mobility; high specific surface area; and distinctive electrocatalytic characteristics [304, 305]. Such attributes render them superior nonprecious metal catalysts for water‐splitting. With a stronger electronic conductivity than sulfur, selenium in particular may have the potential to propel the creation of a novel class of beneficial substance [306, 307].

By implementing a one‐stage phosphoselenide methodology to effectively manufacture FePSe₃ nanoparticles, Liu et al. were able to demonstrate HER function across an extensive pH range [308]. These results imply that the variety of active sites for electrocatalytic activities may be augmented by the introduction of P and Se atoms together. The increase in efficiency may be the consequence of alterations to the catalysts' electronic structures, which provide intermediates with favorable energy requirements for adsorption. Nevertheless, it remains difficult to fully stimulate the intrinsic electrocatalytic activity of bimetal‐based catalysts, regardless of whether P and Se are included in the lattice. This phenomenon prompts an inquiry into whether HER/OER electrocatalytic activity might be substantially enhanced by including P and Se in bimetallic catalysts. Maniyazagan et al. [309] implement S/CuCoPSe@C, a sulfur‐doped bimetallic electrocatalyst based on a metal organic framework (MOF) fabricated from 2,2‐thioacetic acid (S/CuCo‐MOF), which is the conceptual framework for CuCo‐MOF (Figure 13a). Initially, a hydrothermal method was used to synthesize the S/CuCo‐MOF nanostructure composed of two different metals. Then, employing a single‐step pyrolysis procedure, P and Se atoms are included in S/CuCoO_x@C to successfully generate S/CuCoPSe@C nanoparticles. By extending the electrolyte–catalyst contact area, the MOF‐derived nanoparticles accelerate the transmission of charge throughout the catalytic response. Additionally, the smooth crystalline exterior of S/CuCoPSe@C nanoparticles actively accelerates the timing of reactions and minimizes HER/OER overpotentials, whereas the nanoparticles render an extensive range of electrocatalytically activated locations for OER/HER. The S/CuCoPSe@C catalyst outperforms commercial catalysts with regard to resilience and catalytic activity, as anticipated. Subsequently, MOF‐derived S/CuCoPSe@C nanostructures exhibit the inherent HER/OER processes enabling water splitting in its entirety, exposing novel domains of activity.

FIGURE 13.

(a) Schematic representation of S/CuCoPSe@C nanoparticle synthesis. (b) HER polarization curves. (c) Comparable excessive potential (at 10 and 50 mA cm⁻²). (d) Tafel slopes. (e) EIS spectrum. Reproduced with permission [309]. Copyright 2025, Elsevier.

The electrocatalytic HER was employed using linear LSV in 1.0 M KOH at the scan rate of 5 mA s⁻¹; the performance of S/CuCo‐MOF, S/CuCoO_x@C, S/CuCoP@C, S/CuCoPSe@C, and commercial 20% Pt/C was evaluated in Figure 13b. The LSV data illustrate that S/CuCoPSe@C demonstrated exceptional excess potentials (η) of 35 and 179 mV at current densities of 10 and 50 mA cm⁻², as well. The overpotential for S/CuCoPSe@C experienced is minimized relative to that of S/CuCo‐MOF (104 mV), S/CuCo_x@C (100 mV), and S/CuCoP@C (92 mV) at 10 mA cm⁻² (Figure 13c). Furthermore, the excessive potential of S/CuCoPSe@C was equivalent to the value of the standard catalyst Pt/C (22 mV), highlighting that this compound is an acceptable substitute for platinum for economic water‐splitting activities [310]. The results indicate that Se and phosphorus alter the electronic characteristics. Nevertheless, it remains difficult to fully stimulate the intrinsic electrocatalytic activity of bimetal‐based catalysts, even when Se and P atoms are included in the lattice. This prompts the investigation into whether adding P and Se atoms to a bimetallic catalyst could significantly enhance HER/OER activity. The Tafel slope of S/CuCoPSe@C was 140 mV dec⁻¹, comparable to Pt/C (115 mV dec⁻¹) but considerably lower than S/CuCo‐MOF (261 mV dec⁻¹), S/CuCo_x@C (157 mV dec⁻¹), and S/CuCoP@C (170 mV dec⁻¹) (Figure 13d). The HER kinetic reaction gets triggered by the selenium process, indicated by the relatively small level of the Tafel slope of S/CuCoPSe@C. Additionally, the low slope value suggests there might be numerous unoccupied areas on the S/CuCoPSe@C surface that are suitable for the Volmer–Heyrovsky procedure for absorbing hydrogen molecules and atomic particles (H₂) [311]. Electrochemical impedance spectroscopy was implemented for evaluating manufactured materials' resistance attributes (Figure 13e). Two distinct areas may be detected in the spectra: a low‐frequency region featuring the response being linear and a vicinity with a semicircle sensitivity at high frequencies. The resistance, or solution resistance (R_s), across the solution and the electrode interface, is symbolized by the high‐energy region. In this instance, the charge transfer resistance (R_ct) is shown by the semicircle diameter. S/CuCo‐MOF, S/CuCoO_x@C, S/CuCoP@C and S/CuCoPSe@C, had respective R_s values of 2.37, 1.39, 1.24, and 0.82 Ω. In contrast to the remaining catalysts, S/CuCoPSe@C was also displayed. Sulfur doping, phosphoselenide production, and bimetallic CuCo sites work together to strengthen the bimetallic behavior of S/CuCoPSe@C. The MOF‐derived nanostructure delivers multiple active sites for both OER and HER and boosts the electrolyte‐catalyst contact area. The addition of P and Se atoms optimizes reaction kinetics, minimizes charge transfer resistance, and optimizes charge density distribution. With a water‐splitting voltage of 1.58 V, the catalyst attains overpotentials of 360 mV (OER) and 35 mV (HER) at 10 mA cm⁻². This study demonstrates the potential of bimetallic phosphoselenides as highly successful electrocatalysts for practical water electrolysis.

A recurring pattern emerges among the metal dopants discussed in the following paragraphs: Mo and Mn demonstrate significant improvements in HER performance, while Ni and Fe exhibit the most substantial enhancements in OER activity. In acidic environments, rare‐earth elements such as Nd and La are particularly effective in enhancing OER stability, achieving performance levels comparable to those of noble metals. The catalytic performance is primarily influenced by the electronic configuration of the dopant, especially its ability to adjust the d‐band center and create oxygen vacancies. Overall, due to synergistic electronic effects, multi‐metal doping strategies (like Fe‐Ni co‐doping) often outperform single‐metal systems.

4.2. Non‐Metal Doping

Cobalt‐based metal oxides (e.g., CoO, Co₃O₄) are extensively researched materials for electrocatalysis, especially for the hydrogen evolution reaction (HER, oxygen evolution reaction (OER), and oxygen reduction reaction (ORR)—all vital processes in energy conversion technologies, such as metal‐air batteries, fuel cells, and water splitting. Cobalt oxides have gained popularity because of their affordability, adjustable oxidation states (Co^{2 +}/Co^{3 +}/Co^{4 +}), and adequate resilience in alkaline conditions. Nevertheless, pristine cobalt oxides frequently demonstrate poor intrinsic conductivity, constrained active sites, and inadequate catalytic efficiency, thereby hampering their wide‐ranging implementation potential. To address these constraints, anion doping—the deliberate incorporation of non‐metal atoms (e.g., S, N, B, P, F, Se, etc.) into the lattice of cobalt oxides—has consistently been shown to be a promising approach to improving electrocatalytic activity. This doping generally substitutes oxygen atoms in the lattice, resulting in a modified electronic structure that boosts electrical conductivity and charge transfer. This process optimizes the binding energies of key intermediates (OH*, OOH*, and O*), which constitutes a prerequisite for efficient OER/ORR operations. Among these anions, the co‐doping of phosphorus and selenium to form transition metal phosphoselenides has garnered significant interest, as it can induce synergistic electronic modulation and enhanced structural stability. The integration of non‐metal anions into cobalt oxide lattices is an effective strategy to augment electrocatalytic activity through multiple mechanisms. Examining this domain reinforces the current understanding, emphasizes fundamental design concepts, and delineates prospective strategies for developing productive, economically viable, and durable electrocatalysts for sustainable energy technologies.

4.2.1. P‐Doped Cobalt as an Efficient Electrocatalyst (Heterojunction Electrocatalysts)

In particular, a calcination‐nitridation strategy was suggested for synthesizing nanoarray heterojunction electrocatalysts based on P‐doped cobalt containing numerous heterointerfaces believed to focus on POMOFs generated on nickel foam (NF) via an in situ procedure. The nanosheet arrangements of P‐CoN/CWO/Co₃O₄/NF (where CWO = CoWO₄) and P‐CoN/CMO/Co₃O₄/NF (where CMO = CoMoO₄), which exhibit considerable catalytic activity, are produced using the PX₁₂@Co‐ZIF/NF precursors (with X = W or Mo). The electrolytic cell's anode and cathode, as‐synthesized P‐CoN/CWO/Co₃O₄/NF and P‐CoN/CMO/Co₃O₄/NF, showcase optimized wholehearted separation of water effectiveness when exposed to 1.0 molar potassium hydroxide. The subsequent five considerations are recognized for the P‐CoN/CWO/Co₃O₄/NF (+)||P‐CoN/CMO/Co₃O₄/NF (−) couple's substantial OWS endeavor. (i) The OER/HER characteristics are considerably improved by the prominent heterointerfaces, successfully boosting electron mobility and ion transmission speed. (ii) Doping non‐metallic element P yields a substantial number of active locations by fine‐tuning the metal center's electrical structure. (iii) Appropriate communication between electrolyte and electrode is facilitated by the innovative nanoarray heterojunction. (iv) A self‐supported nanoarray promptly emerges on the nickel foam substrate to raise electronic communication efficiency and diminish electrical contact impedance. (v) P‐CoN/CWO/Co₃O₄/NF and P‐CoN/CMO/Co₃O₄/NF are complementary electrocatalysts that collaborate to significantly improve water disintegration dynamics. In summary, high‐efficiency water separation heavily relies on the thoughtful selection of electrocatalysts possessing the aforementioned characteristics. The production procedure of the P‐CoN/CWO/Co₃O₄/NF and P‐CoN/CMO/Co₃O₄/NF electrocatalysts by Ran et al. [312] is illustrated concisely in Figure 14a. In particular, in situ‐grown PX₁₂@Co‐ZIF (X = W, Mo) on NF was initially generated at ambient temperature through a straightforward self‐assembled form.

FIGURE 14.

(a) Conceptual design of the fabrication pathway for the P‐CoN/CWO/Co₃O₄/NF nanowire array and P‐CoN/CMO/Co₃O₄/NF nanosheet matrix. Evaluation of the OER electrocatalytic activity in 1.0 M KOH electrolyte. (b) LSV curves. (c) Tafel slope. (d) Contrast between the excess potential (η₁₀) and Tafel coefficients for several electrocatalysts. (e) EIS Nyquist charts (inset: an optimized conventional circuit design). (f) C_dl data. (g) Comparative analysis of C_dl and ECSA values for various electrocatalysts. Reproduced with permission [312]. Copyright 2024, Elsevier.

The entirety of the electrocatalysts' linear sweep voltammetry (LSV) slopes was computed employing a 3‐electrode arrangement in a solution of 1.0 M KOH using an anodic assessment at 0.005 V s⁻¹ alongside no iR adjustment. PX₁₂@Co‐ZIF/NF (328 mV), P‐CWO/Co₃O₄/NF (204 mV), RuO₂/NF (290 mV), Co‐ZIF/NF (374 mV), and Naked NF (453 mV) exhibit superior OER functionality to the P‐CoN/CWO/Co₃O₄/NF electrocatalyst, which requires an ultra‐low η₁₀ of 175 mV, as revealed in Figure 14b–d. Additionally, the LSV curve is used to calculate the Tafel slope, which can be used to explore the OER kinetics [313, 314]. The excellent OER dynamics of the P‐CoN/CWO/Co₃O₄/NF electrocatalyst are demonstrated by its minimized elevation of 98.9 mV dec⁻¹, which is considerably slighter than the minimal slope of PX₁₂@Co‐ZIF/NF (196.3 mV dec⁻¹), P‐CWO/Co₃O₄/NF (169.6 mV dec⁻¹), Co‐ZIF/NF (220.2 mV dec⁻¹), and bare NF (256.5 mV dec⁻¹), respectively. Additionally, P‐CoN/CWO/Co₃O₄/NF exhibits superior oxygen evolution capability compared to the remaining electrode components generated in this investigation.

Nyquist plots by employing electrochemical impedance spectroscopy (EIS) are considered a valuable approach for confirming the feasibility of electron transmission; the outcomes are interpreted and observed in Figure 14e. P‐CoN/CWO/Co₃O₄/NF, P‐CWO/Co₃O₄/NF, PW₁₂@Co‐ZIF/NF, Co‐ZIF/NF, and bare NF experienced charge‐transfer resistances (R_ct) of 1.91, 2.48, 5.71, 7.65, and 8.99 Ω, respectively. With the goal of substantially strengthening the electron migration potential of P–CoN/CWO/Co₃O₄/NF, prosperous heterointerfaces and P doping can unquestionably stimulate electronic transmission and establish an electron intensity variation, thereby creating an inherent electrostatic field that delivers inadequate Lorentz forces [315]. Additionally, the density of exchange currents (j₀), an indispensable internal consideration influencing the extent of polarization between electrodes (overpotential), can be computed by implementing the R_ct statistic [316]. The double layer capacitance (C_dl) can be employed to calculate the electrochemical active surface area (ECSA), which is an additional essential metric for estimating the activity of electrocatalysts [317]. By integrating and measuring the CV curves at multiple scan rates, the Cdl value can be obtained. The ECSA and C_dl value observations for electrocatalysts are calculated and laid out in Figure 14f,g. PW‐₁₂@Co‐ZIF/NF (20.8 mF cm⁻², 520 cm²), P‐CWO/Co₃O₄/NF (27.6 mF cm⁻², 690 cm²), Co‐ZIF/NF (14.9 mF cm⁻², 372.5 cm²), and unadorned NF (2.9 mF, 72.5 cm²) are unambiguously inferior to the P‐CoN/CWO/Co₃O₄/NF electrocatalyst, resulting in the unambiguously strongest C_dl and ECSA (31.7 mF cm⁻², 792.5 cm²). The large specific surface area of the nanowire array structure, particularly the immense number of chemically active sites it exposes, is the main contributor to the high ECSA value of P‐CoN/CWO/Co₃O₄/NF. Sulfur doping, phosphoselenide synthesis, and bimetallic CuCo sites work together to enhance the bimetallic behavior of S/CuCoPSe@C. The MOF‐derived nanostructure delivers multiple active sites for both OER and HER and increases the electrolyte‐catalyst contact area. The addition of P and Se atoms optimizes reaction kinetics, minimizes charge transfer resistance, and improves charge density distribution. With a water‐splitting voltage of 1.58 V, the catalyst achieves overpotentials of 360 mV (OER) and 35 mV (HER) at 10 mA cm⁻². The potential of bimetallic phosphoselenides as highly successful electrocatalysts for practical water electrolysis is demonstrated by this study.

Phosphorus doping enhances cobalt‐based heterojunction catalysts through three key processes. First, phosphorus withdraws electron density from cobalt due to its higher electronegativity, creating an electron‐deficient environment that increases the binding affinity for intermediates involved in the HER and OER. X‐ray photoelectron spectroscopy (XPS) confirms the formation of Co‐P species, which lowers hydrogen adsorption energy and improves the kinetics of the hydrogen evolution reaction. Second, phosphorus atoms act as proton carriers, facilitating the Volmer process (H⁺ + e⁻ → H*), while the structural instability induced by phosphorus generates coordinatively unsaturated sites. Additionally, the crystalline‐amorphous interfaces in P_v‐CoP/NF enhance conductivity and provide defect sites. Phosphorus enhances metallic characteristics and charge transfer efficiency at heterojunction interfaces. The P_v‐CoP/NF achieves overpotentials of 60 mV for the HER and 198 mV for the OER at a current density of 10 mA cm⁻², requiring only 1.49 V for water splitting. Similarly, Co₂P/Co₃O₄/NF reaches 10 mA cm⁻² at 1.51 V, where electron transfer increases the number of oxygen vacancies and Co‐P species, thereby improving OER kinetics [318].

4.2.2. B‐Doped Cobalt as an Efficient Electrocatalyst (Solvothermal Method)

In their study on the facile synthesis of a CoMoO₄/CoMoB/boron‐doped carbon nanocomposite as a highly durable bifunctional electrocatalyst for water splitting, Yaseen et al. [319] present the design and synthesis of a well‐organized boron‐doped carbon‐based CoMoO₄/CoMoB (CoMo@BC) nanocomposite for overall water splitting via a straightforward solvothermal method. To the best of our knowledge, no researcher has reported using sodium boron tetrafluoroborate (NaBF₄) as a reducing agent, boron source, or structural template for the CoMo@BC nanocomposite, which functions as a dual‐purpose OER/HER electrode material. The as‐prepared CoMo@BC nanocomposite illustrates an assortment of notable advantages: (1) The diminutive nanoparticle size shortens the ion diffusion channel and boosts the available surface, thereby accelerating electron‐transport accessibility. (2) The effective incorporation of boron atoms into the carbon lattice enhances electrical conductivity and optimizes electron transport. (3) The porous structure and interconnected voids encourage the electrolyte to propagate into the interior part of the electrode, diminishing diffusion resistance. (4) The prolonged performance at elevated current densities demonstrates exceptional structural integrity, improving catalytic durability. (5) The use of B‐doped carbon nanosheet scaffolds facilitates high surface area, promotes strong catalyst adhesion, prevents material detachment, and safeguards catalytic performance. The as‐prepared CoMo@BC nanocomposite illustrates optimized electrocatalytic performance with outstanding stability in 1.0 M KOH for up to 200 h at 100 mA cm⁻². A straightforward one‐step solvothermal technique was implemented to synthesize the covalently connected CoMo@BC nanocomposite, depicted in Figure 15a. Ultrasmall CoMoO₄ nanoparticles (NPs) embellish the surface of three‐dimensional (3D) CoMoB octahedrons, which augment the extent of the surface and facilitate water adsorption and disintegration, as well as offering abundant active sites for dissociating water via electrocatalysis. Additionally, the oxygen vacancies and B‐doped carbon reveal extensive electronic structural alterations, optimizing the ability to transmit electricity, which enhances the overall efficiency of the electrocatalytic process in water splitting applications.

FIGURE 15.

(a) Methodology for the synthesis of CoMo@BC electrocatalyst. (b) HER LSV was measured in 1.0 M KOH. (c) Tafel slopes. (d) Nyquist plot (inset: appropriate circuit model). (e) CV of the CoMo@BC sample was recorded at different scan rates. (f) The as‐synthesized samples' C_dI value. Reproduced with permission [319]. Copyright 2024, Elsevier.

The as‐synthesized catalyst's electrocatalytic HER efficiency has been assessed in an alkaline KOH (1.0 M) condition utilizing a standard three‐electrode configuration. High‐porosity activated carbon cloth was employed as the electrochemical measurement's working electrode. Furthermore, under comparable anticipation circumstances, 10 weight percent Pt/C was utilized as the conventional HER electrode. Studies revealed that the CoMo@BC electrocatalyst has a low Tafel slope, a large electrochemically active surface area, and substantial HER activity (Figure 15b–f). As illustrated in Figure 15b, LSV curves were recorded at a scan rate of 0.005 V s⁻1. To assess the reaction kinetics and the rate‐determining step of HER, the LSV data were then converted to determine the Tafel slope. As indicated by the results, the Tafel coefficient of 98.7 mV dec⁻¹ for the CoMo@BC sample indicates that the Volmer‐Heyrovsky pathway is the reaction's rate‐limiting step (Figure 15c). Conversely, the Tafel slope of CoMoB (134.0 mV dec⁻¹) and CoMo@C (149.1 mV dec⁻¹) was indicated by the remaining catalysts previously synthesized. Pt/C (10 wt. %) displayed a minimal Tafel gradient of 65.1 mV dec⁻¹ (Figure 15c), as hypothesized. The HER mechanism traditionally encompasses the preliminary dissociation of H₂O compounds, yielding the Volmer step's reaction intermediates (OH⁻ and H_ads) (H₂O + C + e⁻ → C‐H_ad + OH⁻), subsequently being followed by a conventional decomposition phase designated as the Heyrovsky step (H₂O + C‐H_ad + e⁻→ C + H₂ + OH⁻) or a recombination frequently referred to as the Tafel step (2C‐H_ad → 2C + H₂) [320]. The outcomes illustrate that the incorporation of CoMoO₄ NPs with B‐doped carbon nanosheets might decrease the energy obstacles of the Volmer step, consequently accelerating H₂O separation. Along with access to the Tafel slope, the EIS investigation garnered assistance with estimating the as‐prepared materials' interfacial responses. The catalysts' as‐prepared charge‐transfer resistance (R_ct) was estimated by adopting the appropriate electrical circuit model of R (C (R (QR))(CR), as displayed in the inset of Figure 15d, and Nyquist plots of all as‐prepared catalysts were recorded at a potential of ‐0.10 V (vs. RHE), as shown in Figure 15d. Increased conductance corresponds to a smaller semicircle diameter, indicating lower charge transfer resistance (inset Figure 15d). Generally, a large ECSA leads to enhanced electrocatalytic efficiency. The electrochemical double‐layer capacitance (C_dl) and ECSA are strongly correlated [321]. As apparent in Figure 15e,f, the C_dl was estimated by accumulating CV curves in the non‐Faradaic region. The computed C_dl of CoMo@BC is 42.5 mF cm⁻², as illustrated in Figure 15f. This is substantially in excess of the value for CoMoB (31.6 mF cm⁻²) and CoMo@C (13.2 mF cm⁻²). HER's catalytic effectiveness is substantially enhanced by an especially significant ECSA, indicating that the connection between the generation of H₂ absorption/desorption on the electrode surface throughout the separation of water is of exceptional permeability [322, 323].

A solvothermal technique using inexpensive, earth‐abundant intermediates synthesizes a deliberately constructed bifunctional CoMo@BC electrocatalyst. The findings indicate that the interfacial effect between B‐doped carbon and the CoMoO₄ NP heterostructure, ultrasmall bimetal‐oxide NPs, nanoporous structure, high electrical conductivity, and strong adsorption of water‐splitting intermediates provide broad active regions, efficient electron/mass transport pathways, and enhanced catalyst efficiency. Additionally, more active sites may be enabled to establish efficient nanocomposites for HER and OER due to the 3D octahedron decorated with CoMoO₄ NPs.

4.2.3. N‐Doped Cobalt as an Efficient Electrocatalyst

An alternative version of nanospheres comprising N‐doped CoMo MOF‐derived carbon with Co/Mo‐N linkages was developed to boost HER efficiency. Initially, CoMo‐MOFs (CoMo‐M) were produced as precursors via a solvothermal technique using cobalt, molybdenum salt, and homobenzene tricarboxylic acid ligands. The optimum conditions for synthesizing the CoMo‐M catalyst were evaluated by examining how the Co/Mo molar ratio, hydrothermal duration, and temperature affect electron mobility at the electrode‐electrolyte interface [161]. The synthesized CoMo‐M precursors were further processed with urea via temperature‐programmed polymerization in a combustion chamber, leading to the formation of N‐doped CoMo MOF‐derived carbon nanospheres, designated as N‐CoM‐M. The exploration of temperature, duration, and nitrogen content revealed the mechanisms underlying the MOF‐derived framework formation, driven by nitrogen atoms, enhanced electron transmission, and improved electrocatalytic performance. Additionally, investigations including SEM, XRD, TEM, Raman spectroscopy, N₂ adsorption‐desorption, XPS, and electrochemistry were conducted to understand the collaborative effect of Co/Mo and N atoms on superior electrical conductivity, with Co/Mo─N bonds proposed as the preferred electron transfer channel. N–CoMo‐M carbon nanospheres possess extraordinary HER activity and stability, achieving a low overpotential of 100 mV to deliver 10 mA cm⁻² in alkaline electrolyte, with a Tafel slope of 106.8 mV dec⁻¹. Zhao et al. [324] illustrate the synthesis methods for Co‐M, CoMo‐M and N–CoMo‐M, in Figure 16a. The structural characteristics and chemical constitution of the fabricated nanospheres were investigated using SEM and TEM. The scanning electron microscope photograph in Figure 16b,c illustrates that the outermost portion of N–CoMo‐M nanostructures reveal micrometer‐sized bulks with an abrasive spheroidal shape. The HRTEM pictures in Figure 16d of N–CoMO‐M corroborate the apparent existence of CoO and CoMoO₃, and additionally demonstrate the formation of CoMo‐induced carbon‐based substances experiencing distinctive lattice fringes that correspond to the (111) and (004) planes at 0.205 and 0.247 nm, respectively [325, 326]. In addition to their abrasive and permeable surface shape, which exposing active regions during catalysis, N‐doped CoMo‐MOF‐derived carbon nanospheres exhibit improved HER functionality. At the electrode/electrolyte interface, the emergence of Co/Mo─N bonds conveniently control electron transmission, encouraging water dissociation and quickening HER kinetics. At 10 mA cm⁻², this contributes to a low excess current of 112 mV. N‐doped CoMo‐MOFs are positioned as equally intriguing catalysts for conventional water‐splitting applications by this investigation.

FIGURE 16.

(a) Diagram of the synthesis procedure for CoMo‐M and N–CoMo‐M. (b,c) SEM images. (d) HRTEM image. Reproduced with permission [324]. Copyright 2024, Elsevier.

In summary, carbon nanospheres derived from N‐doped CoMo‐MOFs serve as an outstanding and durable electrocatalyst for HER. The synthesized electrocatalyst is equipped with a perforated and decorated external framework, which may enhance the availability of active sites during the catalytic process. Furthermore, the Co/Mo–N interactions established in the nanospheres can efficiently manipulate electron transfer at the electrode/electrolyte interface, consequently encouraging water dissociation while enhancing HER kinetics.

4.2.4. Se‐Doped Cobalt as an Efficient Electrocatalyst

CoMoO₄ is a particularly desirable catalyst attributable to its embedded anion dispersion mechanism, encouraging lightning‐fast and extensive structural reconfiguration for obtaining CoOOH in environments with alkaline pH [327, 328]. Significantly, γ‐CoOOH, characterized by an expanded interlayer distance and heightened conventional oxidation level of cobalt atoms, has superior performance for OER contrasted with β‐CoOOH [329, 330]. The comparatively lenient conjunction configuration of γ‐CoOOH diminishes the interaction area, causing Co³⁺ to gravitate toward a high‐spin configuration. Nonetheless, intrinsic CoMoO₄ typically generates β‐CoOOH contrary to γ‐CoOOH prior to the course of evolutionary processes. Zhang et al. synthesized in situ CoMoO₄ on a laminate of carbon fiber while confirming the dissociation of MoO₄ ²⁻ and the resulting manufacture of β‐CoOOH employing in situ Raman spectroscopy [331]. Wang et al. similarly produced P‐doped CoMoO₄, which ultimately yielded β‐CoOOH succeeding the oxygen evolution reaction (OER) [332]. In spite of the truly outstanding catalytic characteristics of γ‐CoOOH, the transformation of pre‐catalysts into γ‐CoOOH species within the dynamic self‐reconstruction procedure for OER aimed at improving functionality is currently relatively rarely explored. Hence, with the intent to considerably improve the activities, it is vital to formulate a methodology that can speedily alter the pre‐catalyst into the γ‐CoOOH type. Appropriate heteroatomic treatment might cause minuscule matrix deformation, triggering intrinsic strain or compressive tension in the interior of the lattice [333]. The interior strain might affect the electronic arrangement, consequently influencing the emergence strategy of the pre‐catalyst to generate authentic species of activity with a desirable crystalline state [334]. Nevertheless, a comprehensive investigation on the strategies used for fostering advantageous phase shifts has probably to be accomplished. The present investigation streamlined the phase transformation from CoMoO₄ to γ‐CoOOH with elevated activities, as opposed to the typical β‐CoOOH, by interstitially doping Se atoms into CoMoO₄ to influence its electronic and atomic attributes. Assembled on Ni foam, the CoMoO₄ and Se‐CoMoO₄ nanosheets by Shen et al. [335] were produced using a two‐step sequential technique encompassing a procedure known as hydrothermal followed by calcination with or without the powdered selenium (Figure 17a). The SEM images show that the nanosheets are homogeneously distributed on the Ni foam substrate, creating an overlapping vertical structure (Figure 17b,c).

FIGURE 17.

(a) Schematic of the Se‐CoMoO₄/NF and CoMoO₄/NF synthesis pathway. (b,c) SEM pictures of Se‐CoMoO₄/NF. (d) The designed atomic arrangement simulations of CoMoO₄ and Se‐CoMoO₄/NF exhibit varying bond strengths. (e) Schematic representation of Co─O bond dissociation in CoMoO₄ with selenium doping. (f) β‐CoOOH system and (g) γ‐CoOOH system. (h) Free energy profiles of the OER process on β‐CoOOH and γ‐CoOOH at an applied overpotential of 0 V. (i) Gibbs free energy values of the rate‐determining steps (RDS) in the OER process. Reproduced with permission [335]. Copyright 2025, Elsevier.

Density functional theory simulations were subsequently conducted for evaluating the implications of selenium doping on CoMoO₄ at the level of the atomic structure. Their frameworks were established using the optimization of structural components (Figure 17d). The deformation index and metal‐oxygen bonding length are investigated for confirmation of the structure's deformation of CoMoO₄ adhering to the Se atoms’ incorporation. The computed configurations of CoMoO₄ and Se‐CoMoO₄ imply that concerning Se incorporation, the Co─O bonds encounter substantial elongation and rupture (Figure 17d,e), triggering a spike of 0.827% in the typical Co─O bond duration from 1.934 to 1.950 Å. Structural deformation leading to perturbations to the Co─O bond is an important component in the emergence of high spin of Co²⁺ [336]. The alteration in the Mo─O bond induced by interstitial Se is minimal, demonstrating that this conceptual conclusion is consistent with actual outcomes, which reveal insignificant differences in Mo's XAS and XPS. Calculations of the OER Gibbs free energy were performed on two distinct planes: the (012) plane of β‐CoOOH and the (101) plane of γ‐CoOOH. Both materials feature CoO₂ layers with an octahedral cobalt environment, and the Co sites on the (101) plane of γ‐CoOOH exhibit the same coordination as those on the (012) plane of β‐CoOOH. Geometric representations of β‐CoOOH, γ‐CoOOH, and their intermediates are shown in Figure 17f,g. The energy profiles of the reaction pathway (Figure 17h) suggest that the transformation from O to OOH is the rate‐determining step (RDS) for both catalysts. With an overpotential of 0.280 V, γ‐CoOOH outperforms β‐CoOOH (0.649 V), indicating that a‐Se‐CoMoO4/NF demonstrates improved OER catalytic activity compared to Se‐CoMoO₄/NF (Figure 17i).

The presence of intact MoO₄ tetrahedrons ensures their continued smooth leaching. This study offers an innovative way of doping Se into CoMoO₄ to preferentially produce γ‐CoOOH, a highly active OER phase. Tensile strain and lattice distortion induced by interstitial Se atoms produce high‐spin Co²⁺ species that steer the phase shift away from the less active β polymorph toward γ‐CoOOH. With an overpotential of 332 mV at 1000 mA cm⁻² and reliable performance lasting over 200 h, the reconstructed catalyst demonstrates exceptional OER performance. This work suggests that advantageous phase reconstruction can be influenced by tuning atomic and electronic frameworks through strain engineering. By employing specific phase transition pathways, this approach offers novel opportunities for creating high‐performance electrocatalysts, enhancing their stability and efficiency in OER for improved energy conversion in renewable energy applications.

4.2.5. Phosphorus and Sulfur Co‐Doped as an Efficient Electrocatalyst

Nowadays, most acknowledge that doping reaction engineering is an achievable method for boosting catalyst functionality [249, 337, 338]. Anionic materials can be integrated into catalysts to enhance catalytic activity, increase the proportion of free electrons in the conduction region, improve charge transfer efficiency, and act as active sites [339, 340, 341, 342]. Prior research has demonstrated that the doped element P can function as a proton carrier, encouraging the synthesis of H during HER and accelerating HER kinetics. The presence of phosphorus may improve OER performance by facilitating the emergence of high‐valent metal reactive species (e.g., Ni³⁺) [343, 344, 345]. Sulfur, with its metallic properties and strong electronegativity, can enhance electrical conductivity and chemisorption capabilities, thus boosting OER behavior by encouraging the reemergence of metal compounds and their cooperation with oxygen species [346]. The deliberate introduction of apertures (e.g., S, O, P) significantly impacts catalytic performance. Simultaneous co‐doping of phosphorus and sulfur triggers the production of oxygen‐rich vacancies (P, S‐NiMoO₄) in nickel molybdate prismatic nanorods on nickel foam, creating a highly potent dual‐functional electrocatalyst for water separation. Integrating highly electronegative phosphorus and sulfur into NiMoO₄ dramatically improves electrical conductivity, augments catalytically active sites, and generates abundant oxygen vacancies. P, S‐NiMoO₄ serves as a highly efficient bifunctional electrocatalyst, requiring only 31 and 206 mV to achieve 10 mA cm⁻² for OER and HER in 1 M KOH. Additionally, the electrolyzer comprising P, S‐NiMoO₄ delivers 50 mA cm⁻² at 1.55 V with outstanding durability exceeding 110 h.

P, S‐NiMoO₄ was synthesized by Wang et al. [347] using a hydrothermal process, thereafter undergoing co‐doping with phosphorus and sulfur (Figure 18a). First, Ni²⁺ and MoO₄ ^2– were assimilated on the surface of NF electrostatically, resulting in the development of perpendicular NiMoO₄ prismatic nanoarrays underneath hydrothermal circumstances. In the subsequent simultaneous co‐doping process of phosphorus and sulfur, H₂S and PH₃, originating from NaH₂PO₂·H₂O and CH₄N₂S respectively, react with NiMoO₄, contributing to surface deterioration and the concurrent incorporation of phosphorus and sulfur elements. Throughout the doping process, some O atoms are either removed or substituted by P and S elements, attributable to the higher electronegativity of P and S compared to O, triggering the emergence of many oxygen vacancies (O_V) in NiMoO₄. SEM investigations indicate that the NiMoO₄ precursors, as well as P‐NiMoO₄, S‐NiMoO₄, and P, S‐NiMoO₄, maintain consistent morphological characteristics, characterized by regular and evenly dispersed prismatic morphology. The unique 3‐dimensional columnar architecture facilitates adequate collaboration at the catalyst‐electrolyte interface, the optimal utilization of the active substrates, and accelerated transmission of mass kinetics (Figure 18b–e). The enhanced performance of P, S‐NiMoO₄ corresponds to three parameters. The concurrent co‐doping of phosphorus and sulfur engenders a synergistic electrical effect. Phosphorus retrieves electron density from Ni and Mo centers, whereas sulfur introduces excess oxygen vacancies, optimizing the d‐band center for OER and HER. Second, the prismatic nanoarray structure offers an elevated level of density of active sites and promotes fast gas sphere discharge. Third, in comparison to various transition metal molybdates, nickel‐based P, S‐NiMoO₄ demonstrates boosted conductivity and resilience in alkaline environments. The integration of electronic engineering and morphological changes in design renders P, S‐NiMoO₄ a viable multifunctional catalyst for water splitting.

FIGURE 18.

(a) Schematic of P, S‐NiMoO₄ Synthesis. (b,c) NiMoO₄ precursor SEM picture. (d,e) SEM images of P, S‐NiMoO₄ at low and high resolution. (f) The specimens' LSV graph at 5 mV s⁻¹. (g) Tafel slopes. (h) The Tafel slopes and overpotentials at 10 and 100 mA cm^{− 2} is compared for each specimen. (i) Nyquist plots of the specimens. (j) C_dl findings for every single specimen. Reproduced with permission [347]. Copyright 2023, Elsevier.

The catalytic performance of P, S‐NiMoO₄ and other reference samples was assessed by measuring LSV curves in 1 M KOH with 90% iR compensation. Figure 18f demonstrates the exceptional HER activity of P, S‐NiMoO₄, with a low overpotential of 31 mV at 10 mA cm⁻², performing on par with commercial Pt/C (25 mV). At 100 mA cm⁻², P, S‐NiMoO₄ requires 104 mV, outperforming Pt/C (115 mV), indicating superior HER efficiency at higher current densities. P‐ NiMoO₄, S‐NiMoO₄, NiMoO₄, and NF exhibited overpotentials of 51/161 mV, 176/283 mV, 183/302 mV, and 185/324 mV at 10/100 mA cm⁻², respectively. P‐and/or S‐doped NiMoO₄ catalysts showed enhanced HER performance compared to pristine NiMoO₄, implying that P, S, and Ov promote HER catalysis. The Tafel slope of P, S‐NiMoO₄ is 34.46 mV dec⁻¹, considerably lower than other samples and close to Pt/C (30.84 mV dec⁻¹) (Figure 18g). Irrespective of all the created materials, the P, S co‐doped P, S‐NiMoO₄ reveals the fastest kinetic characteristics and the most effective HER catalytic efficiency (Figure 18h). EIS and C_dl statistics obtained from the non‐Faradaic region of CV curves (Figure 18i,j) were used to evaluate the charge transfer rate and ECSA of the catalysts. P, S‐NiMoO₄ exhibits the lowest charge transfer resistance (R_ct) of 1.7 Ω among all samples, while P‐NiMoO₄ (6.1 Ω) and S‐NiMoO₄ (14.3 Ω) show a considerable reduction compared to NiMoO₄ (195 Ω). It is apparent that introducing S, P, and O_v significantly increases the pristine NiMoO₄’s mass transfer dynamics and electrical conductivity, which boosts the HER potential for P and S‐NiMoO₄. Furthermore, P, S‐NiMoO₄ has the most significant value of C_dl of each specimen (64.74 mF cm⁻²); therefore, it predicts that it will possess the largest ECSA. Considering these findings, elemental doping and O_v help establish more electrochemically attainable sites of activity, therefore dramatically improving the functionality of the HER.

Phosphorus (P) and sulfur (S) doping consistently enhance HER activity by optimizing hydrogen adsorption kinetics. In contrast, nitrogen (N) and boron (B) doping show greater effectiveness for the OER due to improved hydroxyl binding. This trend is observable across all non‐metal dopants discussed in this section. For applications requiring bifunctionality, selenium (Se) doping offers a balanced improvement for both HER and OER. The modulation of electronic structure and catalytic effectiveness is significantly impacted by the dopant's electronegativity and atomic size. Importantly, co‐doping techniques, such as the combination of P and S, produce synergistic effects that surpass the performance of single‐element doping. This highlights the importance of multi‐anion engineering in maximizing overall water splitting efficiency (Tables 2 and 3).

TABLE 2.

Comparative in‐depth analysis of cobalt‐based electrocatalyst systems.

Catalyst	Doping type	Modification	Performance (HER)	Performance (OER)	Stability	Key finding	Refs.
Nd‐Co3O4	Rare‐earth (Nd)	Nanorod	Not reported	304 mV@10 mA cm−2	24 h	Enables acidic OER stability; comparable to Ru catalysts	[253]
Cu‐F‐Co3O4	Metal (Cu, F)	Nanosheet	Not reported	290 mV@10 mA cm−2	>100 h	Surface reconstruction forms CuCo2O4‐yFy/CuO heterojunction	[348]
Mn‐W‐CoP/NF	Metal (Mn, W)	Nanoflower	95.8 mV@10 mA cm−2	229.2 mV@10 mA cm−2	48 h	Dual‐metal co‐doping improves both HER and OER	[177]
Fe‐Co2RuO4/RuO2	Metal (Fe, Ru)	Nanosheet‐on‐nanosheet	Not reported	253 mV@50 mA cm−2	Not reported	Interface linking optimizes electronic design	[189]
P, S‐NiMoO4/NF	Non‐metal (P, S)	Prismatic nanorod	31 mV@10 mA cm−2	206 mV@10 mA cm−2	110 h	Co‐doping creates oxygen‐rich vacancies	[347]
CoP/NF	Non‐metal (P)	Nanoarray	41 mV@10 mA cm−2	Not reported	48 h	Ru doping enhances HER	[194]
Ce‐CoSe2/MXene	Non‐metal (Se) + MXene	Nanoneedle	34 mV@10 mA cm−2	279 mV@10 mA cm−2	200 h	MXene enhances conductivity and hydrophilicity	[177]
P‐CoN/CWO/Co3O4/NF	Non‐metal (P)	Nanowire	Not reported	175 mV@10 mA cm−2	80 h	Heterointerfaces boost electron mobility	[312]
P‐CoN/CMO/Co3O4/NF	Non‐metal (P)	Nanosheet	109 mV@10 mA cm−2	Not reported	110 h	Complementary OER/HER catalysts for OWS	[312]

TABLE 3.

Summary of key patterns and findings.

Observation	Pattern/Key finding
Best HER performance	P, S‐NiMoO4 (31 mV) and Ce‐CoSe2/MXene (34 mV) due to oxygen‐rich vacancies and MXene synergy
Best OER performance	P‐CoN/CWO/Co3O4/NF (175 mV) and P, S‐NiMoO4 (206 mV) due to heterointerfaces and co‐doping
Best long‐term stability	P, S‐NiMoO4 (110 h) and Ce‐CoSe2/MXene (200 h)
Most balanced HER/OER	Mn‐W‐CoP/NF (95.8 mV HER, 229.2 mV OER) and P, S‐NiMoO4 (31 mV HER, 206 mV OER)
Role of rare‐earth doping	Nd‐Co3O4 and Cu‐F‐Co3O4 enable acidic OER stability (>24 h) comparable to noble metals
Role of MXene	Ce‐CoSe2/MXene shows that MXene enhances conductivity, hydrophilicity, and stability
Synergistic effect	Co‐doping (P, S or Mn,W) outperforms single‐element doping in both HER and OER
Heterojunction advantage	P‐CoN/CWO/Co3O4/NF and P‐CoN/CMO/Co3O4/NF show that complementary catalysts enable efficient overall water splitting

5. Anion Vacancies

This section discusses anion vacancies, the most fundamental defect type in cobalt‐based nanomaterials. Cobalt oxide can replace conventional precious metals, and researchers have accelerated efforts to enhance its manufacturing capability by manipulating its chemical composition and electronic configurations. The combination of cobalt oxide nanocrystals and graphene substantially accelerates oxygen evolution reaction (OER) performance due to the enhanced conductivity of the carbon support and the increased active sites from small dimensions. Since the electronic configuration of Co atoms is the most important indicator of OER efficiency in Co₃O₄, numerous techniques are used to alter surface oxidation states, particularly by manipulating accessible facets or introducing oxygen vacancies [348, 349]. When oxygen vacancies arise, the Co²⁺/Co³⁺ ratio on the Co₃O₄ surface consistently rises, making the surface highly conductive and OER‐active. Researchers have established a straightforward laser irradiation method for altering the morphology and Co²⁺/Co³⁺ ratio associated with OER‐active aspects of Co₃O₄. Metallic oxide films typically undergo surface modification using laser irradiation, an intriguing way to induce imperfections during rapid light stimulation. On epitaxial NiO film, laser thermal annealing can yield an n‐type NiO layer alongside the p‐type NiO layer, with a greater proportion of defect sites. Laser annealing optimizes TiO₂ photocatalytic capabilities by increasing oxygen vacancy concentration and titanium interstitials. The percentage of sulfur atoms is affected by laser‐induced stimulation in MoS₂ nanosheets [350]. This study uses laser illumination to texture synthetic Co₃O₄ nanosheets, leading to the formation of numerous nanoholes. The abundance of vacancies enhances water molecule adsorption, as confirmed by density functional theory (DFT) simulations. The synergistic effect of these actions contributes to substantial improvement in OER catalytic performance. Vacancy engineering enables precise control of vacancy concentration, distribution, and type, typically ranging from 5% to 15%, thus enhancing lattice flexibility and fracture resistance. The deliberate introduction of anion vacancies, particularly oxygen and selenium vacancies, improves catalytic activity while also providing structural reinforcement, inhibiting unwanted phase transformations and preserving the high‐surface‐area structure essential for sustained efficiency.

The Co(OH)₂@nickel foam hybrid by Lu et al. [351] was subjected to a muffle furnace, producing black pure Co₃O₄ nanosheets, referred to as P‐Co₃O₄ NSs. The Co₃O₄ nanosheets were then bombarded with laser beams at predetermined exposure times and energy levels (designated as L‐ Co₃O₄ NSs), as illustrated in Figure 19a. DFT calculations were performed to determine the band structure of bulk Co₃O₄ and ultrathin Co₃O₄ nanosheets on the (111) plane. DFT simulations revealed a relatively ultrathin Co₃O₄ slab of 1.14 nm, showing an elevated conduction band (CB) edge and a bandgap of 0.60 eV, considerably lower than the 1.68 eV bandgap of the standard crystalline form (Figure 19b,c). To investigate prospective imperfections, specifically vacant positions generated during laser stimulation, high‐angle annular dark‐field scanning electron microscopy (HAADF‐STEM) was implemented; this atomic imaging methodology allows for immediate assessment of atomic configurations and interstitial point imperfections following the illumination of an electron channel. The imaging contrast is established by the atomic number (Z), owing to the localized strength of the signal being inversely correlated to (Z²). The variation in brightness is frequently referred to as Z‐contrast. Corresponding to these opposite circumstances, the bright points discernible in Figure 19d,e, which originate from the facet of {1 1 1} L‐Co₃O₄ NSs, can be attributed to the cobalt cations. The elevated luminosity of particular Co atoms corresponding to certain others is attributable to the augmented proportion of columnar Co atoms across the < 1 1 1 > direction. As an illustration, multiple Co vacancies are displayed in Figure 19d, particularly a pair of these symbolized by an arrow in red. The aforementioned additional evidence is provided by the diminished atomic signals at the respective positions of these 2 distinct atomic particles contrasted with the alternative atomic sites in the peak intensity curve (Figure 19e) acquired across the extended highlighted line depicted in Figure 19d. This conclusion is adequately compatible alongside the simulations included in the evaluation depicted in Figure 19d,e. A perspective image (perpendicular to the [1–1] direction) and an upward perspective (parallel to the [1–1] direction) of an atomic model of Co₃O₄ were employed for the purpose of computing Figure 19h. A Co vacant position, apparent in Figure 19h, emerges whenever a Co atom gets eliminated from its lattice structure orientation. Figure 19f demonstrates the computer‐simulated outcome, highlighting the Co vacancy (depicted by the red arrowhead), resulting in it appearing dimmer in contrast to the flawless lattice of Co₃O₄ displayed in Figure 19g. Oxygen, residing in an inadequate number of atomic particles, is significantly lighter in weight than cobalt and virtually imperceptible in HAADF STEM pictures. The continued existence of oxygen atom vacancies may trigger the relocation of Co atoms to alternative interstitial lattice destinations. Consequently, the straightforward visualization of the malignant configuration of Co atoms should facilitate the identification of oxygen‐vacant positions. Figure 19i is an HAADF‐STEM depiction that displays several oxygen vacancies, highlighted by the rectangular red shapes. Figure 19i inset illustrates the possibility of a solitary oxygen vacancy that triggers the Co atom to steer away from its original setting. The present research illustrates how to trigger lattice deformation and multi‐vacancies in crystalline Co₃O₄ crystals utilizing a conventional laser. In comparison to pristine CoO₄ and RuO₂, the synergistic effect of these vacancies, alongside permeable morphology and elevated conductivity, leads to superior OER performance. DFT calculations show that numerous vacancies minimize the bandgap and yield a more uniformly distributed conduction spectrum edge, thereby promoting electron excitation. Additionally, the vacancies weaken O─H bonds and function as active sites for water adsorption, therefore lowering the OER activation energy. Through targeted defect engineering, this laser‐mediated technique introduces a feasible approach to improve the catalytic functionality of different electrocatalysts.

FIGURE 19.

(a) Diagrammatic representation of the synthesis of Co₃O₄ nanosheets with multiple defects. (b) Computed PDOS for massive Co₃O₄ on the (1 1 1) substrate. (c) Computed PDOS for 1.14 nm Co₃O₄ on the (1 1 1) substrate. (d) HAADF‐STEM illustration of L‐Co₃O₄ nanosheets. The red circles highlight zones of diminished intensity attributed to cobalt vacancy deficits. The red dashed box indicates the line pattern area in Fig (e). The red arrows highlight two vacancy shortcomings. (d) Features an inset that illustrates a mapping for low‐contrast vacancies. (e) The level of intensity pattern graph of the dotted red rectangle in the overlay of Figure (d). The red and green spheres represent Co³⁺ and Co²⁺ species, respectively. The red circles highlight lacking cobalt atoms. (h) The opposed and peak perspectives of the ideal Co₃O₄ crystal model across the < 1 1 1 > direction overlay. The structure elucidates the organization of Co²⁺ and Co³⁺ cations, corresponding to the configuration of the most vibrant positions. The green and red spheres denote the Co²⁺ and Co³⁺ cations, respectively. The blue circle represents O^2– anions. (f,g) The computations of the Co₃O₄ slab (58.74 Å) feature cobalt deficiencies and an unaltered crystalline structure. The red arrows highlight the locations of the vacant positions. The rectangle indicates the degree of intensity pattern area. (i) Complex HAADF STEM picture of L‐Co₃O₄ nanosheets. The red squares depict the chaotic configuration that ensues from vacant oxygen spaces. The inset delivers a magnified depiction of atomic instability. Reproduced with permission [351]. Copyright 2021, Elsevier.

Lu et al. [351] introduce a laser‐assisted approach to incorporate regulated oxygen and cobalt flaws into CoO nanosheets. Considering a low high overpotential of 290 mV and a Tafel slope of 76 mV dec⁻¹, the coexistence of both vacancy types supports favorable oxidation kinetics, resulting in exceptional OER activity that outperforms both commercial RuO₂ and pure CoO₄. According to theoretical calculations, the produced multi‐vacancies enhance water molecule interaction and effectiveness of responses by generating intermediate electronic states inside the bandgap. Compared to conventional methods, this laser‐based methodology permits more precise and consistent flaw engineering. The outcomes establish new perspectives for the tunable multi‐vacancy design of more advanced electrocatalysts.

5.1. Oxygen Vacancies

Theoretical computations were conducted to strengthen the comprehension of the interaction between surface reassembly and the inherent catalytic activity of spinel oxide Co₃O₄ electrocatalysts doped with Cu and F. The intrinsic oxygen vacancy formation in spinel oxides is an important consideration influencing outer layer reconstruction, owing to lower development energy levels contributing to more advantageous kinetics of structural reconfiguration. Oxygen vacancies, which are anionic deficiencies arising from the elimination of oxygen atoms from the cobalt oxide lattice, significantly impact the electrical structure and catalytic performance of cobalt oxides in water splitting. From an electrical structural point of view, oxygen vacancies create localized defect states along the bandgap of cobalt oxide. These states act as electron donors and raise the Fermi level near the conduction band. This modification enhances the metallic characteristics of the material and expedites charge transfer kinetics, leading to decreased charge transfer resistance. The eradication of oxygen atoms triggers coordinatively unsaturated cobalt sites that are electron‐rich and demonstrate elevated reactivity. These unsaturated sites have a strong attraction to water molecules and reaction intermediates, like H* (for HER) and OH*, O*, and OOH* (for OER). This lowers the activation energy barriers for both processes. Moreover, oxygen vacancies facilitate water dissociation in alkaline conditions, usually serving as the rate‐limiting factor for the HER, while for the OER, they enhance the binding affinity of oxygen‐containing species, preventing both inadequate and excessive adsorption that could hinder the reaction. The cumulative effects provide oxygen vacancies a powerful mechanism for enhancing the bifunctional catalytic efficiency of cobalt oxides.

Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) were used to examine the morphology of the catalysts. The pristine Co₃O₄ exhibited needle‐like clusters that formed a chrysanthemum‐like structure measuring approximately 4 µm (Figure 20a. After doping with Cu and F, the Cu‐F‐Co₃O₄‐0.7 showed increased roughness and compactness, taking on a spherical cauliflower appearance (Figure 20b). TEM analysis (Figure 20c) revealed that the needle structures were composed of nanoparticles approximately 30 nm in size. As a result, the catalyst exhibited a three‐dimensional cauliflower‐like morphology, which was assembled from one‐dimensional nanowires made up of zero‐dimensional nanoparticles. The surface oxygen vacancy formation enthalpy in Cu‐F‐Co₃O₄‐0.7, Cu‐Co₃O₄‐0.7, F‐Co₃O₄‐0.7, and pristine Co₃O₄ was assessed by Li et al. [352] based on the customized adjustments illustrated in Figure 20d, and the resulting findings are shown in Figure 20e. The approximate surface oxygen‐vacancy establishment energies for Cu‐F‐Co₃O₄‐0.7 is 1.43 eV, considerably inferior to those of Cu‐Co₃O₄‐0.7 (1.49 eV), F‐Co₃O₄‐0.7 (1.74 eV), and untreated Co₃O₄ (1.88 eV). These observations indicate that compared to mono‐doped Cu or F specimens and pristine Co₃O₄, the co‐doped Cu‐F‐Co₃O₄‐0.7 possesses a higher propensity for oxygen vacancies, consequently encouraging structural reconstruction and potentially improving catalyst effectiveness. This aligns with the XPS findings, which reveal that Cu‐F‐Co₃O₄‐0.7 possesses the highest quantity of oxygen‐deficient surfaces, advantageous for outer layer mass transfer. The rate‐determining step (RDS), the slowest step of the process, influences the overall reaction speed in OER. According to the energy diagrams of various catalysts in Figure 20f, the predominant step (*OH formation) for pure Co₃O₄ has the highest potential in the RDS, with an overall energy of 1.37 eV. The rate‐determining step (OOH formation) potentials are 1.03 eV for Cu‐Co₃O₄‐0.7 and 1.26 eV for F‐Co₃O₄‐0.7, which are substantially higher than the potential for Cu‐F‐Co₃O₄‐0.7 (0.72 eV). This implies that Cu‐F‐Co₃O₄‐0.7 exhibits a low overpotential during OER, consistent with the electrochemical findings. Notably, for the *O formation step, the reassembled CuCo₂O₄‐yFy/CuO heterostructure reveals a minimum potential of 0.70 eV. According to these results, the reconstructed CuCo₂O₄‐yFy/CuO heterojunction is the primary active component responsible for enhancing the catalytic effectiveness of Cu‐F‐Co₃O₄‐0.7 [353].

FIGURE 20.

(a) SEM images of Co₃O₄ and (b) Cu‐F‐Co₃O₄‐0.7. (c) TEM. (d) Oxygen vacancy surface architecture of CuCo₂O₄‐yFy/CuO, Cu‐F‐Co₃O₄‐0.7, Cu‐Co₃O₄‐0.7, F‐Co₃O₄‐0.7, and Co₃O₄. (e) Oxygen vacancy generation probabilities in Cu‐F‐ Co₃O₄‐0.7, Cu‐ Co₃O₄‐0.7, F‐ Co₃O₄‐0.7, and Co₃O₄. (f) OER Gibbs free energy diagrams and rate‐determining steps. (g) The density of states (PDOS) plot. (h) The density of states (TDOS) diagram for CuCo₂O₄‐yFy/CuO, Cu‐F‐Co₃O₄‐0.7, Cu‐Co₃O₄‐0.7, F‐Co₃O₄‐0.7, and Co₃O₄. Reproduced with permission [352]. Copyright 2025, Elsevier.

For further understanding, Li et al. conducted partial density of states (PDOS) calculations on the CuCo₂O₄‐yFy/CuO heterojunction, Cu‐F‐Co₃O₄‐0.7, F‐Co₃O₄‐0.7, Cu‐Co₃O₄‐0.7, and Co₃O₄. The results indicate that the d‐band centers of Cu‐F‐Co₃O₄‐0.7 (−1.36 eV) and CuCo₂O₄‐yFy/CuO (−1.11 eV) are closest to the Fermi level compared to F‐Co₃O₄‐0.7 (−1.37 eV), Cu‐Co₃O₄‐0.7 (−1.39 eV), and Co₃O₄ (−1.45 eV) (Figure 20g). This demonstrates that CuCo₂O₄‐yFy/CuO, which undergoes reconstruction during OER, exhibits superior adsorption ability for reaction intermediates. The simultaneous doping of Cu and F in Co₃O₄ shifts the d‐band center closer to the Fermi level, thereby enhancing its OER catalytic activity. This observation aligns with experimental findings, highlighting the beneficial effects of co‐doping Cu and F for tuning the electronic structure to boost OER efficiency [354]. Additionally, as illustrated in Figure 20h, assessment of the total density of states (TDOS) highlights that the CuCo₂O₄‐yFy/CuO heterojunction exhibits a higher value at the Fermi level compared to Cu‐F‐Co₃O₄‐0.7, F‐Co₃O₄‐0.7, Cu‐Co₃O₄‐0.7, and Co₃O₄. The higher value at the Fermi level indicates that the CuCo₂O₄‐yFy/CuO heterojunction possesses enhanced charge transfer speed and improved conductivity. Co‐doping of Cu and F substantially boosts the electrical conductivity of Co₃O₄, consequently accelerating the OER. This enhancement in conductivity supports the previous discussion and further confirms the complementary effect of co‐doping Cu and F in strengthening the electrocatalytic capacity for OER. Cu‐F‐Co₃O₄, a highly successful OER electrocatalyst created by quenching, is exhibited in this study. Both the experimental and theoretical results verify that adding Cu and F causes surface reconstruction into a CuCo₂O_{4 −}yFy/CuO heterojunction, creates a significant amount of oxygen vacancies, and lowers the d‐band center nearer the Fermi level. The ideal catalyst, Cu‐F‐Co₃O₄‐0.7, outperforms pristine Co₃O₄ and RuO₂, attaining a low overpotential of 290 mV at 10 mA·cm⁻² with exceptional stability exceeding 100 h. Cu and F doping additionally reduces the rate‐determining energy barrier, improves charge transfer kinetics, and lowers the oxygen vacancy formation energy, according to theoretical estimates. The multidimensional layout promotes mass and electron transit while disclosing more active areas. This work provides a rational approach for dual anion‐cation defect engineering to activate surface regeneration in spinel oxides for oxidizing the water.

5.2. Selenium Vacancies

Selenium vacancies in cobalt‐based nanomaterials have attracted considerable attention for their potential to enhance the electronic, electrochemical, and catalytic properties of these materials. Cobalt selenide (CoSe₂) and other cobalt‐selenium compounds are particularly promising in applications such as energy storage, electrocatalysis, and sensors. The introduction of selenium vacancies can create localized active sites that facilitate key reactions, such as OER or HER, making these materials more efficient as catalysts in renewable energy processes. Additionally, selenium vacancies can modify the electronic structure of cobalt‐based nanomaterials, potentially improving their conductivity and stability. However, the creation and stabilization of such vacancies remain challenging, as they can affect the overall structural integrity and material performance under certain conditions, such as high temperatures or reactive environments. Therefore, a fundamental investigation of selenium vacancies—their controlled generation, stability, and structure‐property relationships—is essential. Understanding the formation mechanisms of selenium vacancies and their impact on material properties is crucial for advancing high‐performance cobalt‐based nanomaterials in next‐generation energy systems and catalysis applications. Selenium vacancies in cobalt‐based nanomaterials, namely in cobalt selenide (CoSe₂), modify conductivity and electronic structure by methods that are distinct from those of oxygen vacancies. Unlike oxygen vacancies that may intermittently trap charge carriers, selenium vacancies produce delocalized electron states around the Fermi level, significantly enhancing the density of free charge carriers and elevating metallic conductivity. This enhancement expedites electron passage from the electrode to the active sites, reducing ohmic losses and facilitating higher current densities during electrolysis. The removal of selenium atoms alters the local coordination environment of cobalt, causing an elevation of the d‐band center toward the Fermi level. M‐Se compounds (M‐Se_x, M = Co, Ni, Fe, etc.) are considered outstanding candidates among non‐noble metal‐based nanomaterials due to their structural positions and appropriate d‐electron arrangements [355]. The Se atom sites at the outer boundaries of M‐Se_x, for instance, are HER‐active sites, as determined by DFT calculations [356]. It is widely acknowledged that modifying with multiple metals and creating lattice vacancies to increase the number of edge‐active centers can boost the electrocatalytic performance of selenides [357]. Furthermore, complementary properties from multiple metal cations enhance catalyst activity when selenides are doped with different metallic cations [358]. Indeed, incorporating metal cations into selenides can increase accessible active sites and alter atomic spins due to lattice strain [359].

DFT calculations were conducted by Feng et al. [360] to explore the formation of *OOH intermediates and the contrasting behavior of catalytic processes in CoSe₂ and Fe‐CoSe‐HT catalysts during the ORR. Figure 21a,b highlight the structure and computed density of states (DOS) of four distinct models with Fe doping or Se vacancies. The highest density of states (15.12 eV) at the Fermi level is observed in Fe‐CoSe‐V(Se) with an Fe‐induced Se vacancy, which significantly contributes to electron transfer. In the ORR process, oxygen (O₂) must first be adsorbed on the catalyst surface to form the OOH intermediate. Adsorbed *OOH on Fe‐CoSe‐V(Se) indicates more substantial discernible mobility for electrons, according to the charge‐density difference between activated and *OOH (Figure 21c,d). As an outcome, the *OOH Gibbs free energy across multiple structural models was computed. The analogous OOH adsorption of Fe‐CoSe‐V(Se) in side‐on bonding is evidenced by the free energy variations of +0.60, −0.17, +0.33, and 0.04 eV among the CoSe, CoSe‐V(Se), and Fe‐CoSe‐V(Se) samples (Figure 21e). In addition, Figure 21f simulates the Gibbs energy transform in the conceivable 2e‐ORR and 4e‐ORR channels at 0 eV (vs. RHE). CoSe and Fe‐CoSe‐V(Se) have *OOH intermediates adsorbed on them at +0.6 and −1.37 eV, as well. An inferior adsorption intensity can dramatically accelerate the procedure of oxygen reduction (ORR). The amount of energy necessary for transferring Fe‐CoSe‐V(Se) to H₂O is positive (+0.84 eV) amid the OH diminution and decomposition stage of 4e‐ORR. A significant energy barrier significantly impedes the 4e‐ORR route, which in turn enhances the 2e‐ORR performance. Using ΔGOOH as a descriptor, free‐energy illustrations and activity volcano plots for the 2e‐OOR route were established with the objective of evaluating the ORR capabilities of these CoSe specimens (Figure 21g). The Fe‐CoSe‐V(Se) has a significantly greater UL value (0.61 V) for the *OOH intermediate at 4.18 eV in comparison to the immaculate CoSe model; the 2e‐ORR prefers the procedure versus the 4e‐ORR approach. Figure 21h provides a comprehensive overview of the catalytic mechanism for CoSe and Fe‐CoSe‐V(Se) via 2e⁻‐ORR and 4e⁻‐ORR. To form the OOH intermediate, O₂ is initially adsorbed on the catalyst surface through electronic interaction during the activation process. In CoSe, the O─O bond in *OOH may break, releasing O species for 4e⁻‐ORR. After the conversion of O to OH, the four‐electron process ultimately ends with the formation of H₂O. However, during the 2e⁻‐ORR process, the adsorbed OOH on Fe‐CoSe‐V(Se) is directly converted into H₂O₂ [361]. Through Se's multivalent redox states (Se₄/Se²⁻/Se⁴⁺), where Se─O bond emergence and cleavage drive ·O²⁻ dismutation, selenium‐based nanozymes mimic SOD function. Se vacancies (VSe) enhance catalytic efficiency and stimulate electron redistribution. With a free energy of +0.04 eV and an adsorption energy of −1.37 eV, Fe‐CoSe‐V(Se) exhibits the highest Fermi level state density (15.12 eV) and achieves excellent *OOH adsorption. This arrangement enables the efficient conversion of ·O₂ ⁻ to H₂O₂. Therefore, an appropriate approach to fabricate high‐performance SOD‐mimicking nanozymes is Se vacancy‐induced electronic modulation.

FIGURE 21.

(a) DOS. (b) The four fundamental structural frameworks are as follows: CoSe, CoSe‐V(Se), Fe‐CoSe₂, and Fe‐CoSe‐V(Se). (c,d) Charge density variation of *OOH. (e) Adsorptive *OOH intermediate generation energy at 0.7 V. (f) Comparative analysis of 2‐electron and 4‐electron oxygen reduction reaction pathways. (g) Volcano diagram of 2e‐ORR activity. (h) Limitations of catalytic processes for different designs. Reproduced with permission [360]. Copyright 2025, Springer.

Throughout different investigations, oxygen vacancies are frequently identified to stimulate OER performance through boosting hydroxyl adsorption and encouraging the production of activated oxyhydroxide species. In contrast, selenium vacancies offer better possibilities for HER owing to their potential to form delocalized electronic states that exceed the Fermi level, increasing hydrogen adsorption kinetics. A significant characteristic is that moderate vacancy concentrations (usually 5–15%) deliver optimum catalytic results, while excessive vacancy levels lead to structural failure. The amalgamation of both vacancy patterns in a single material constitutes an insufficiently investigated yet intriguing area for prospective studies.

6. Cation Vacancies

The development of transition‐metal‐based bifunctional catalysts, comprising metallic oxides, [362] metallic nitrides, [363] metallic hydroxides, [364] metallic selenides, [365] metallic phosphides, [366] metal chalcogenides, [367] and metallic sulfides, [368] has been the focus of deep investigation for decades. Nonetheless, the aforementioned dual‐functional electrocatalysts are often designed to boost catalytic efficiency for one specific reaction while performing substantially less effectively in the other [369]. Additional challenges, including the blocking of active sites and nanoparticle agglomeration caused by modification methods, are undesirable in catalyst synthesis and lead to poor ion diffusion and chemical integration, ultimately hindering overall efficiency and effectiveness in practical applications [359]. In this context, integrating foreign cations into selenides is expected to significantly enhance electronic interactions and lattice structure, thereby substantially improving catalytic performance [149]. Unfortunately, however, relatively few studies elucidate the electronic modulation strategy during overall water splitting.

In addition to conventional oxygen vacancies that modulate the Co²⁺/Co³⁺ ratio, cobalt vacancies have recently been recognized alongside oxygen deficits, speculated to be induced by laser radiation. Such cobalt vacancies are exemplified in the Fe‐coordinated and Co‐bridged Fe@Co/Se₂ nanorod catalyst, where they prevent aggregation and promote efficient water splitting through robust electromagnetic interfacial interactions. This tightly interconnected Fe‐coordinated and Co‐bridged Fe@Co/Se₂ nanorod catalyst prevents aggregation and promotes efficient water splitting due to its robust electromagnetic interfacial interactions. For example, the synthesized Fe@Co/Se₂ nanorods (Fe@Co/Se₂‐NRs) demonstrate outstanding catalytic performance for HER (78 mV at 10 mA cm⁻²) and OER (200 mV at 10 mA cm⁻²). The Fe@Co/Se₂‐NRs catalyst requires a cell voltage of only 1.51 V to achieve 10 mA cm⁻² during overall water splitting. Fe@Co/Se₂ nanorods (NRs), by Ibraheem et al. [370], a selenide doped with Fe and Co dual cations, were designed by employing a carefully monitored single‐stage hydrothermal processing process implementing the in situ coordinative self‐templated method, as apparent in Figure 22a. To better understand the HER/OER catalytic mechanisms of Fe@Co/Se₂‐NRs, they performed DFT simulations using first‐principles calculations to investigate the electronic structures and reaction pathways of the Fe²⁺/³⁺ and Co²⁺ co‐doped selenide architectures. To determine the catalytic HER/OER efficacy of these materials, all potential doping configurations (i.e., Co‐Se₂ and Fe‐Se₂) were considered to elucidate the effects of metal doping sites. A significant fluctuation in the density of states (DOS) near the Fermi level of Fe@Co/Se₂ NRs (Figure 22b,c) was observed compared to Fe‐Se₂ (Figure 22d) and Co‐Se₂ (Figure 22e), which contributed to a substantial increase in the electrical conductivity of Fe^2+/3+ and Co²⁺ co‐doped selenides relative to their conventional counterparts. Additionally, regarding HER assessment, three states have been identified: (i) the H⁺ state, (ii) the intermediate H state, and (iii) the catalyst‐_1/2H₂ stage. The Gibbs free energy for adsorbed hydrogen (ΔG_H*) is a critical parameter for HER, and the ΔG_H* between steps (ii) and (iii) should be minimized, ideally approaching zero, for an effective catalyst. The ΔG_H* values for Fe‐Se₂, Co‐Se₂, and Fe@Co/Se₂‐NRs were estimated to be 0.49, 0.39, and 0.32 eV at the Fe, Co, and Co sites, respectively (Figure 22f). These observations suggest that HER is enhanced by Co atoms in Fe@Co/Se₂‐NRs following Fe incorporation into Co‐Se₂, with dual metal doping interactions substantially improving intrinsic HER activity. The DFT calculations for OER determined that Co‐Se₂ and Fe‐Se₂ components showcase lessened electrical resistance, with bandgaps corresponding to 0.41 and 0.95 eV, as well. Conversely, the Fe@Co‐Se₂‐NRs samples demonstrate boosted electromagnetic conductance, minimizing the bandgap from 0.41 to 0.29 eV. This demonstrates that Fe^2+/3+ and Co²⁺ co‐doped selenides might encourage rapid transmission of electrons (Figure 22g). The disparity regarding charge concentration dispersion was intended to contribute to accelerated passage of electrons from Co to Se, adhering to the incorporation of Fe in Co‐Se₂, resulting in substantially authenticating the Fe‐coordinated cobalt‐bridged selenide bond in Fe@Co/Se₂‐NRs. Comprehensive DFT calculations were performed for each specimen by optimizing every O‐intermediate in each OER scheme. Investigations revealed that, in all cases, the formation of OOH intermediates from the O state required considerable free energy, making this step the rate‐determining step of the OER process [371]. Fe@Co/Se₂‐NRs exhibited a lower free energy of 1.51 eV compared to Fe‐Se₂ (1.78 eV) and Co‐Se₂ (1.64 eV), demonstrating efficient O₂ desorption from the active site. The consequent improvement in OER performance of Co‐Se₂ following Fe incorporation reduced the energy barrier for the rate‐determining step, thereby accelerating OER kinetics in Fe@Co/Se₂‐NRs. Considering the influence of Fe doping on the electrocatalytic performance of Fe‐Co/Se₂‐NRs, with Co as the active site, they performed additional calculations assuming Fe as the active site. The results indicated that Fe@Co‐Se₂‐NRs exhibited remarkable HER and OER performance compared to both FeSe₂ and CoSe₂, demonstrating that Fe‐Co/Se₂‐NRs is a highly efficient catalyst, with cobalt serving as the key active component for overall water splitting. The enhanced OER/HER catalytic performance of Fe‐Co/Se₂‐NRs can be attributed to favorable adsorption‐desorption dynamics and accelerated kinetics associated with Fe doping into CoSe₂.

FIGURE 22.

(a) Diagrammatic illustration of the fabrication procedure of the Fe‐Co/Se₂‐NRs catalyst. (b) The superior and lateral perspective of the Fe‐Co/Se₂‐NRs interface. (c) The anticipated density of states for pure Fe@Co/Se₂‐NRs. (d) Fe‐Se₂ (e), Co‐Se₂. (f) Presented are Gibbs free energy diagrams for Fe‐Se₂, Co‐Se₂, and Fe@Co/Se₂‐NRs, along with an embedded illustration of the molecular structure showing Fe‐coordinated Co‐Se₂ active sites of Fe@Co‐Se₂‐NRs. (g) OER Gibbs free energy diagrams for the four‐step processes of Fe‐Se₂, Co‐Se₂, and Fe@Co/Se₂‐NRs samples (the inset illustrates the molecular structure with O binding at the Co site of Co‐Se₂). Reproduced with permission [370]. Copyright 2022, Elsevier.

In this study, Fe@Co/Se₂ nanorods were fabricated as highly active electrocatalysts for both OER and HER using a simple one‐step hydrothermal method. A unique Fe‐Co bridging connection is formed when Fe²⁺/³⁺ and Co²⁺ species are incorporated into selenides, creating an efficient electron transfer channel. To achieve low overpotentials of 200 mV (OER) and 78 mV (HER) at 10 mA cm⁻² with 20 h stability, the porous nanorod structure provides abundant active sites and interconnected conductive networks. Using this catalyst, a complete water‐splitting device requires only 1.51 V to reach 10 mA cm⁻². According to DFT calculations, multi‐cation doping into selenides is necessary to produce outstanding catalytic activity for both OER and HER. Cation vacancies are comparatively less examined than anion vacancies in cobalt‐based electrocatalysts. Emerging data indicates that they represent a complementary function in changing electronic structure by modifying the coordination surroundings of the residual metal sites. A discernible pattern indicates that cation vacancies are most efficacious when paired with anion vacancies, forming a dual‐defect system that optimizes electronic modulation and promotes active site accessibility. Subsequent study must concentrate on the meticulous regulation of cation vacancy concentration and propagation to optimally exploit their catalytic characteristics.

7. Conclusion

The global enthusiasm for producing hydrogen through electrochemical water splitting has increased as an outcome of the global search for sustainable alternative energy sources. Cobalt‐based nanomaterials have established themselves as successful electrocatalysts within this paradigm due to their affordability, configurable physicochemical characteristics, and outstanding adaptability in incorporating different modification techniques. Notwithstanding the aforementioned benefits, their fundamental constraints—such as insufficient electrical conductivity, inappropriate active sites, and slow reaction kinetics—require morphological and compositional modifications to attain operational catalytic efficiency akin to noble metal standards. This review has thoroughly analyzed the conceptual strategies, synthesizing approaches, and synergistic adjustments that improve the operational efficiency of cobalt‐based electrocatalysts through doping and vacancy engineering. The improvement of cobalt‐based catalysts via heteroatom doping and vacancy manipulation is explained by their capacity to manipulate the electronic structure, augment the adsorption energies of intermediates, and stimulate charge transmission at the catalyst–electrolyte interface. Strategies for doping with metals and non‐metals have proven crucial in manipulating the d‐band center of cobalt, consequently influencing its binding affinity for reactant intermediates throughout the HER and OER processes.

The integration of metal heteroatoms like Cu, Fe, Mn, and Mo triggers novel electronic configurations and synergistic effects that strengthen conductivity while encouraging multi‐site catalysis. Simultaneously, non‐metal dopants like N, P, B, and Se have demonstrated a tendency to facilitate charge redistribution, establish active defect sites, and strengthen surface hydrophilicity, collectively contributing to diminished overpotentials and improved Tafel slopes. Furthermore, vacancy engineering—specifically the creation of oxygen and selenium vacancies—has arisen as an extremely effective method for manipulating catalytic activity. The introduction of vacancies modulates local coordination environments and escalates the concentration of interfacial defects, ultimately resulting in enhanced ion transport and catalytic turnover. The simultaneous application of vacancy engineering and atomic doping delivers a synergistic impact that surpasses the advantages achievable by each alteration independently. The structural architecture of cobalt‐based catalysts, including 2D nanosheets, 3D porous structures, and 0D nanoparticles, is essential for maximizing active site exposure and enhancing mass and electron transport efficiency. Hydrothermal, solvothermal, electrodeposition, and ion‐exchange synthesis methods provide versatile approaches for achieving precise control over shape, composition, and defect concentration. In assessing catalytic performance, critical electrochemical parameters—including overpotential, Tafel slope, turnover frequency, electrochemical impedance, and Faradaic efficiency—collectively delineate the efficiency and stability inherent in these catalysts. The literature consensus indicates that rationally tailored cobalt‐based materials demonstrate significantly lower Tafel slopes (<60 mV dec⁻¹) and overpotentials (approximately 200–250 mV for OER and 100–150 mV for HER at 10 mA cm⁻²), suggesting advantageous kinetics and successful charge transmission.

Notwithstanding these advancements, considerable challenges endure. The accurate regulation of dopant distribution, concentration, and interaction with host lattices remains an important hurdle. Furthermore, comprehension of the long‐term structural evolution of doped and defect‐laden catalysts under operational conditions remains constrained. The integration of atomic doping and vacancy engineering is exceptionally significant for the development of cobalt‐based nanomaterials that facilitate efficient water electrolysis. By integrating structural design, compositional regulation, and interface optimization, researchers might address the persistent trade‐offs among activity, stability, and cost, ultimately leading to enhanced performance of cobalt‐based nanomaterials in water electrolysis applications.

8. Perspectives

Cobalt‐based electrocatalysts are essential for advancing a green hydrogen economy by addressing the two primary challenges to widespread adoption: high costs and reliance on scarce noble metals. Cobalt is considerably more ubiquitous and over 1000 times more plentiful than platinum in the Earth's crust, yet it is priced at only a fraction of these noble metals. The substitution of noble metal catalysts with cobalt‐based replacements can substantially reduce the initial investment cost of electrolyzers, potentially by 50%–80%. The bifunctional properties of many cobalt‐based catalysts, which demonstrate activity for both HER and OER, simplify electrolyzer design, reduce system complexity, and lower maintenance expenses. The combination of doping and vacancy engineering has revealed considerable potential in addressing the inherent limitations of cobalt‐based materials. Based on this review, we propose the following specific and actionable research directions:

• Develop acid‐stable cobalt‐based catalysts; most cobalt catalysts degrade in acidic PEM electrolyzers within 20–50 h. Future research should focus on achieving stability exceeding 1000 h through carbon encapsulation or high‐entropy alloying strategies.

• Reduce cobalt loading without compromising activity. Cobalt loading should be reduced to below 1 wt.% through atomic dispersion while maintaining HER and OER performance comparable to noble metals.

• Scale up synthesis from laboratory to industrial scale; current synthesis methods produce milligram quantities. Future work must develop continuous flow or spray pyrolysis techniques capable of producing kilogram quantities.

• Demonstrate catalysts in membrane electrode assemblies (MEAs); industrial applications require integration with MEAs. Future research should study catalyst ink formulation, coating methods, membrane selection, and hot‐pressing conditions to bridge the gap between RDE and MEA performance.

• Perform a life‐cycle assessment (LCA) of cobalt‐based catalysts; the environmental impact of cobalt mining, purification, synthesis, operation, and disposal must be quantified to evaluate true sustainability.

• Establish a publicly accessible database for machine learning; a comprehensive database of dopant combinations, vacancy concentrations, and corresponding HER/OER performance metrics would enable machine learning‐guided discovery of optimal catalyst formulations.

In addition to performance enhancement, economic and environmental considerations will influence future research. Eco‐friendly synthesis methods employing low‐temperature or green‐chemical procedures will assume significance in minimizing the carbon footprint of catalyst production. Strategies for recycling and regeneration of cobalt‐based electrodes should be investigated to mitigate material loss and environmental impacts. Given the price fluctuations of cobalt, partially substituting it with abundant elements like nickel, iron, or manganese may offer a viable approach to environmentally conscious catalyst design. Further investigation ought to incorporate in situ and operando spectroscopic methods to track the dynamic reconstruction of active sites under real electrochemical conditions. Computational modeling and machine learning‐enhanced design will also be crucial in expediting discoveries. DFT simulations have provided significant insights into the influence of dopants and vacancies on the d‐band center and reaction energetics of cobalt oxides. A key consideration is sustainability and scalability. Although many cobalt‐based nanocatalysts show exceptional activity in research, their application in practical electrolyzers remains limited. Challenges such as inadequate catalyst‐substrate adhesion, mechanical instability, and disintegration in alkaline or acidic solutions must be addressed. Future research should focus on binder‐free electrodes, self‐supporting nanostructures, and corrosion‐resistant coatings. Ultimately, device‐level engineering is needed for practical hydrogen production. By integrating atomic doping, vacancy engineering, advanced characterization, and computational intelligence within a sustainable framework, the next generation of cobalt‐based nanomaterials might achieve enhanced electrocatalytic performance alongside the scalability and stability necessary for global hydrogen energy implementation.

Author Contributions

Muzamil Ahmad: Writing – review & editing, Writing – original draft, Resources, Data curation. Kaili Wu: Conceptualization, Supervision. Ikram Ullah and Muhammad Adnan: Validation. Abdul Raouf: Visualization. Adeel Ahmed: Formal Analysis. Raza Ullah: Methodology. Nasim Ullah: Investigation. Hailin Cong and Bing Yu: Project administration, Funding acquisition.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.