Transition metal-based electrocatalysts for alkaline overall water splitting: advancements, challenges, and perspectives

Abstract

Water electrolysis is a promising method for efficiently producing hydrogen and oxygen, crucial for renewable energy conversion and fuel cell technologies. The hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are two key electrocatalytic reactions occurring during water splitting, necessitating the development of active, stable, and low-cost electrocatalysts. Transition metal (TM)-based electrocatalysts, spanning noble metals and TM oxides, phosphides, nitrides, carbides, borides, chalcogenides, and dichalcogenides, have garnered significant attention due to their outstanding characteristics, including high electronic conductivity, tunable valence electron configuration, high stability, and cost-effectiveness. This timely review discusses developments in TM-based electrocatalysts for the HER and OER in alkaline media in the last 10 years, revealing that the exposure of more accessible surface-active sites, specific electronic effects, and string effects are essential for the development of efficient electrocatalysts towards electrochemical water splitting application. This comprehensive review serves as a guide for designing and constructing state-of-the-art, high-performance bifunctional electrocatalysts based on TMs, particularly for applications in water splitting.

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Muhammad Nazim Lakhan

Muhammad Nazim Lakhan received his MS degree in Chemical Engineering and Technology from Harbin Engineering University, China. Currently, he is a PhD student at RMIT University, Melbourne. His research focuses on nanomaterials, metal oxides, and 2D materials for electrochemical water-splitting reactions, including the HER and OER, in an alkaline medium.

Abdul Hanan

Abdul Hanan has recently completed his Master of Engineering in Materials Science and Engineering from Harbin Engineering University under the supervision of Prof. Dianxue Cao. Currently, he is doing a PhD at Sunway University, Malaysia. His current research focuses on energy materials and nanomaterials for energy conversion via electrochemical water-splitting systems.

Yuan Wang

Yuan Wang (Helena) is a Senior Lecturer, Group Leader of Renewable Resources & Sustainability (R^2S) and an ARC-DECRA Fellow at the Department of Chemical Engineering at the University of Melbourne. She completed her PhD in Chemical Engineering at the University of New South Wales (UNSW) in 2018. She held prestigious fellowships: the Alfred Deakin Research Fellowship-2022 at Deakin University and the International Hydrogen Research Fellowship-2023 at the National University of Singapore. She was also a DAAD Visiting Scholar at the Fritz Haber Institute of Max Planck-Berlin (2018). Her research focuses on green hydrogen production, carbon dioxide conversion and utilization, metal recovery and recycling, and circular economy.

Umair Aftab

Umair Aftab has completed his PhD in Metallurgy and Materials Engineering at the Mehran University of Engineering and Technology, Jamshoro. Currently, he is an Assistant Professor in the same department. His research mainly focusses on electrochemical water splitting systems: HER, OER, and ORR.

Hongyu Sun

Hongyu Sun received his PhD degree from the State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, in 2010. He then worked in the Department of Materials Science and Engineering (2010–2012), Beijing National Centre for Electron Microscopy (2012–2015), at Tsinghua University (with Prof. Jing Zhu), and Department of Micro- and Nanotechnology at the Technical University of Denmark (2015–2018, with Prof. Kristian Mølhave). Now, he is a senior application scientist at DENS solution B.V. He received the Robert P. Apkarian Award (Physical Sciences, 2018) from the Microscopy Society of America. His research interests include controllable synthesis of functional structures for energy storage and conversion, liquid cell transmission electron microscopy, and microfabrication.

Hamidreza Arandiyan

Hamidreza Arandiyan is a leader of Critical Minerals for Clean Energy Group and has an academic tenure in Applied Chemistry and Environmental Science at RMIT University. He completed his PhD from the School of Environment at Tsinghua University in 2014. He received a Vice-Chancellor's Research Fellowship from the University of New South Wales at the School of Chemical Engineering in 2015. He held a University of Sydney Senior VC Fellowship in the School of Chemistry in 2018. He is a Fellow of the Royal Society of Chemistry (FRSC). His research focuses on resource recovery for environmental remediation and energy applications.

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1. Introduction

As energy demand continues to grow, advances in efficient conversion systems have become the focus of the global research and development community.¹ In recent years, there has been a sharp rise in global energy demand, with projections indicating a 56% increase between 2010 and 2040.² The development of modern society needs the maximum use of fossil fuels, which produce harmful greenhouse gases such as carbon dioxide (CO₂) and carbon monoxide (CO) in the environment.³ Therefore, the search for alternative energy sources has attracted much attention from researchers.⁴ Renewable fuels are considered an attractive option for resolving the global environmental and energy crises due to their merits of renewability, greenhouse gas-free emission, and high energy density.^5–7 Among renewable energy conversion technologies, electrolysis is a cutting-edge technology that has gained popularity recently and is regarded as an effective method for splitting water into hydrogen and oxygen.⁸ The effectiveness of electrochemical water splitting (EWS) depends on the efficiency, stability, and cost of the electrocatalyst used in the process.^9,10

Along with stability and low cost, excellent charge transfer ability and many active sites are some of the pre-requisites for an effective electrocatalyst.¹¹ EWS comprises two half-cell reactions: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER).¹² As usual, the low electrochemical efficiency results from the slow kinetics of the HER and OER. Both reactions require more overpotential to break the inherent reaction energy barrier.¹³ An efficient electrocatalyst should have the features of better electronic conductivity, large active surface area, long-term stability, and strong catalytic activity.¹⁴ The best electrocatalysts for splitting water are based on noble metals, including Pt- and Ir/Ru-based compounds.¹⁵ However, the limited availability of resources and the high cost significantly restrict the scope of the potential applications of these materials.^16,17 Therefore, developing high-quality noble metal-free or noble metal doped transition metal (TM) electrocatalysts is crucial for commercialising large-scale water electrolysis.¹⁸ Recently, a lot of work has been done on developing several approaches to increase the stability and activity of noble metal-based materials using nanotechnology to catalyse water splitting. These techniques primarily target the structural design, which is accomplished by single-atom construction, interfacial structure engineering (strain effect), and heteroatom introduction (ligand effect). When heterogeneous materials are incorporated into compounds based on noble metals, the ligand effect happens, causing electrons to move between two distinct atom groups and altering the electronic structure.¹⁷ The adsorption/desorption characteristics of the intermediate species are associated with the electronic structures of catalysts, which are crucial for electrocatalytic efficiency. The d-band theory suggests that by altering the electronic structures of catalysts to provide the distinct electronic effect, the oxygen/hydrogen adsorption energy might be efficiently tuned. Heteroatom doping is one of the best methods for fine-tuning the electronic structures of catalysts. It is widely known that alloying noble metal NPs with TMs (Fe, Co, or Ni) that are readily available on Earth might improve the electrocatalytic activity of catalysts. The single atomic effect is a quickly developing field of study, which offers an efficient way to maximise atom utilisation efficiency and catalytic activity by shrinking catalysts to the size of a single atom.^16,17 We critically reviewed composites derived from TM-based materials for their effectiveness in the electrochemical HER and OER in alkaline environments, as depicted in Scheme 1.

Scheme 1 Schematic illustration of this review theme based on TM electrocatalysts for overall water splitting.

Recently, significant progress has been made in the design and synthesis of TM-based catalysts with outstanding performance.¹⁹ These catalysts include TM-based metal oxides (TMOs), phosphides (TMPs), nitrides (TMNs), carbides (TMCs), borides (TMBs), chalcogenides, and dichalcogenides (TMDs). These catalysts have been chosen because of their fascinating properties, which include high cyclability, ionic conductivity, and mechanical strength.^20,21 The advantages of transition metals (unique d electron configurations, low cost, synergistic effect of multi-metal atoms, and excellent stability) and their porous features (large surface areas, high pore volumes, and modulated pore structures) perfectly complement one another, which is especially advantageous for the development of next generation electrocatalysts.²²

TMOs, TMPs, TMNs, TMCs, TMBs, and chalcogenides are potential catalysts because they can donate electrons to or accept electrons from a reagent, depending on the type of reaction. They are incredibly helpful in electrochemical transformations due to their capacity to access and switch between various oxidation states, form complexes with the reagents, and serve as good sources of electrons.²³ These materials have several appealing qualities, including abundance, fascinating physicochemical properties, and a high electrocatalytic potential. Furthermore, the d-electron density of TMs is enhanced by the doping of nitrogen, carbon, and boron atoms. This contraction of the d-band also results in the electronic structures of TMNs, TMCs, and TMBs approaching the Fermi level and resembling those of noble metals. In addition to having good corrosion resistance, TMNs, TMCs, and TMBs bind well during EWS under both acidic and alkaline conditions. Additionally, due to the alteration of the electron density distribution in the hybrid materials with great dispersity and more exposed active sites, there are synergistic effects between TMNs, TMCs, TMBs, and non-metal atoms that lead to good electrocatalytic activity and stability.²³ An important class of inorganic materials known as TMDs exhibit various catalytic, magnetic, optical, and electrical properties. Recently, the amazing characteristics of this family of materials have been studied by researchers.^24–26 Earth-abundant TMDs with the general formula MX₂ (M = transition metal; X = chalcogen) are currently being developed at a rapid rate for water splitting. These TMDs include two-dimensional (2D) layered materials such as MoS₂, WS₂, WSe₂, TaS₂, MoSe₂, and TiS₂ and pyrite phase structured TMDs (CoSe₂, CoS₂, NiS₂, and NiS₂).²⁷ These 2D layered MX₂ nanosheets exhibit remarkable physical, chemical, and electrical properties compared to their bulk counterparts, particularly due to their 2D shape and atomic thickness. Each layer typically consists of three atomic layers covalently joined in an X–M–X configuration. Weak van der Waals forces connect adjacent MX₂ layers to create bulk crystals.²⁸ TMDs have been investigated as HER and OER electrocatalysts in the last ten years and have demonstrated higher catalytic activities similar to noble metal-based catalysts. Atomically thin TMDs have a larger precise surface area with more active sites for the HER and OER than other Earth-abundant electrocatalysts.

This review discusses the TM-based composites for efficient electrochemical HER and OER in alkaline media. The Scopus database was used to observe the progress made in the last 10 years starting from 2014 to 2023 for the following keywords “Noble metals for water splitting”, “Transition metal oxides for water splitting”, “Transition metal chalcogenides for water splitting”, “Transition metal phosphides for water splitting”, “Transition metal dichalcogenides for water splitting”, and “Transition metal carbides, nitrides and borides for water splitting”. From data observation, we have seen that the number of publications for reported materials has increased yearly. In comparison, noble metals have been reported in many publications owing to their efficient water-splitting performance. TMOs stand second in terms of publication, followed by TMPs. It is also noted that the scientists have paid more attention to TM nitrides, carbides, borides, and chalcogenides, which has gradually increased in the last five years. The 10-year recent research progress is presented in Fig. 1. This timely review focuses on all classes of TMs for both water-splitting reactions in alkaline media and differs from previously reported reviews.^29–31 This is the first review in which all TM types as electrocatalysts are critically discussed with recent developments. The timeline of the development of TM-based electrocatalysts is shown in Table 1.

Fig. 1 10 years of recent progress in TM-based electrocatalysts; data were obtained through Scopus.

Table 1 Timeline showing the history of development of TM-based electrocatalysts^32–44

Year	Development history
1789	• Discovery of water electrolysis
1888	• Industrial alkaline water electrolysis
1890	• Asbestos membrane
1900–1966	• First pressurized electrolyzer
	• Nafion-based proton exchange membrane water electrolyzer
1974–1996	• Electronic configuration of tungsten carbide resembles that of platinum near the Fermi level
	• The chemisorption of hydrogen on the (111) and (110)-(1 × 2) surfaces of iridium and platinum
	• Hydrogen electrochemistry on a platinum low-index single-crystal surface in alkaline solution
2005	• Molecular chemistry of consequence to renewable energy
2007	• The edge structure of MoS2 as an active site toward the HER and OER revealed
2014–2016	• A Bi2Te3@CoNiMo composite as a high-performance bifunctional catalyst for hydrogen and oxygen evolution reactions
	• Nanocrystalline Mo2C as a bifunctional water-splitting electrocatalyst
	• Cobalt-oxide-based materials as water oxidation catalysts: recent progress and challenges
2017–2023	• Single-atom catalysts for electrocatalytic applications
	• Strategies for developing TMPs as heterogeneous electrocatalysts for water splitting
	• Transition metal (Fe, Co and Ni)–carbide–nitride (M–C–N) nanocatalysts: structure and electrocatalytic applications
	• Recent advances in 2D materials and their nanocomposites in sustainable energy conversion applications
	• Nanoarchitectonics for TMS-based electrocatalysts for water splitting
	• Designing TMB-based electrocatalysts for applications in EWS
	• Recent progress in transition metal selenide electrocatalysts for water splitting
	• Recent strategies to improve the catalytic activity of pristine MOFs and their derived catalysts in EWS
	• Phosphorus-doped MoS2 with sulfur vacancy defects for enhanced EWS

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2. Mechanisms of electrochemical water splitting reactions

Generally, overall water splitting (OWS) reactions involve two half-cell reactions, the HER and OER. In the HER reaction, water gets reduced to generate H₂ at the cathode, while it is oxidized at the anode in the OER reaction to generate O₂. The sluggish kinetic reactions of the HER and OER are the primary obstacles preventing the practical application of water splitting due to elevated overpotentials, especially in alkaline media. To generate H₂ and O₂ efficiently, highly efficient catalysts are required to reduce the potential for the HER and OER. The design of electrocatalysts depends upon the operational conditions of the water electrolysis cell. There are three main categories of electrolysis technology at the moment: (i) electrolysis using a proton exchange membrane (PEM), (ii) electrolysis in an alkaline solution, and (iii) solid oxide water electrolysis at high temperatures. Because the temperature is so high, solid oxide water electrolysis uses a lot of energy. The water splitting is carried out in an acidic environment and using a PEM in a PEM-based electrolysis cell. Other membranes, such as those with lower gas permeability and higher proton conductivity, do not have all of the advantages exhibited by PEMs.⁴⁵ The acidic medium is necessary because of its quick hydrogen production rate and good energy efficiency. However, cutting-edge noble metal and metal oxide-based catalysts can be used as the OER electrocatalysts. Water splitting occurs in an alkaline environment in an alkaline electrolysis cell. Compared to acidic cells, water splitting in alkaline cells broadens the options for electrocatalysts to include non-noble metals or metal oxides. As a result, superior electrocatalysts for electrolytic water splitting are inexpensive, have high catalytic activity, and are stable.⁴⁶

2.1. Mechanism of the HER

Hydrogen is considered as an efficient energy fuel to replace fossil fuels since it is the only byproduct of water. Electric current is used in the HER process to separate H₂ and O₂ from water. The HER is an electrochemical process that involves multiple steps, takes place at the cathode electrode, and makes use of acidic and alkaline solutions (Fig. 2a and b).⁴⁷ The Volmer, Tafel, and Heyrovsky reaction mechanisms can be used to explain the entire process of the HER. The HER takes place according to the following equations (eqn (1)–(3)) in an acidic solution:

Volmer reaction: M + H⁺ + e⁻ → M–H* (1)

Heyrovsky reaction: M–H* + H⁺ + e⁻ →M + H₂ (2)

Tafel reaction: 2M–H* → H₂ + 2M (3)

Fig. 2 HER mechanism route in (a) alkaline and (b) acidic media. OER mechanism route in (c) alkaline and (d) acidic media.⁴⁸ The corresponding images (c) and (d) have been reproduced with permission from Elsevier © Copyright.

In an alkaline solution, the HER reaction takes place according to the following equations (eqn (4)–(6)):

Volmer reaction: M + H₂O + e⁻ → M–H* + OH⁻ (4)

Heyrovsky reaction: M–H* + H₂O + e⁻ → M + OH⁻ + H₂ (5)

Tafel reaction: 2M–H* → H₂ + 2M (6)

The HER that occurs on the electrode surface is an adsorption–desorption process, as shown by eqn (1)–(6), and M stands for surface metal sites. First, the electrode surface adsorbs H atoms with the deduction of one electron, forming M–H*, which is equivalent to the Volmer process (eqn (1) and (4)). The Heyrovsky reaction and the Tafel reaction, which are both described in Fig. 2a and b, correspond to the formation of M–H*, which can subsequently mix with H⁺ in an acidic solution (H₂O in an alkaline solution) or another M–H* to create H₂ molecules. It is necessary to adsorb H atoms regardless of the HER reaction pathway, which is intuitively indicated by the free energy of hydrogen adsorption (GH*). In addition to generating strong bonds with the adsorbed H*, a good HER catalyst should also be weak enough to enable appropriate bond breaking, which aims to liberate the H₂ molecules.⁴⁹

2.2. Mechanism of the OER

The OER, or water oxidation, is a key electrochemical process in water splitting and rechargeable metal–air batteries.^50,51 The OER involves the generation of oxygen molecules on the catalyst surface through four discrete electron transfer steps.⁵²Fig. 2c and d shows the OER mechanism under alkaline and acidic conditions. The Sabatier principle is the foundational hypothesis underpinning our understanding of the OER activity of electrocatalysts. The surface metal cations are the primary OER active sites (M). Many M–O-bonded reaction intermediates will develop during the process. According to the concept of Sabatier, a high level of catalytic activity can be observed on a surface if the adsorbed species adhere to it neither too strongly nor poorly. Furthermore, it has been found that the OER is very pH sensitive, and the reaction kinetics in alkaline and acidic media are different. As shown in Fig. 2d, hydroxyl groups (OH) in alkaline environments converted into O₂ and H₂O molecules, but under acidic and neutral conditions, two H₂O molecules converted into four protons (H) and O₂. Typically, the OER process has four steps: (i) H₂O/OH⁻ adsorption on the catalyst surface; (ii) the formation of reactive intermediates; (iii) O₂ molecule release; the formation of OH adsorbed species as a result of OH⁻ anion adsorption and discharge at the anode surface indicates that, mainly on the metal site, the OER activity in the alkaline solution begins. The OH⁻ ion and this adsorbed OH species then react, releasing an electron and creating H₂O and adsorbed atomic O. Following this, an OH⁻ anion reacts with an adsorbate O atom to form adsorbate OOH species; and (iv) combination of this adsorbed OOH species with additional OH⁻ anions, releasing an electron. This reaction also results in the production of adsorbed O₂ and H₂O. The desorption of O₂ occurs last. The production of adsorbed OOH species is considered the rate-restraining step in this series of events. The mechanism can be visually illustrated as shown in Fig. 2c. The hypothesized process differs in specific ways from that in the alkaline medium and is analogous to those under acidic conditions. The reactions involved in the OER reaction in acidic media are described in the following equations (eqn (7)–(11)), and in an alkaline solution, the OER reaction takes place according to the following equations (eqn (12)–(16)):

H₂O + M → M–OH* + H⁺ + e⁻ (7)

M–OH* → M–O* + H⁺ + e⁻ (8)

2M–O → 2M + O₂ (9)

M–O* + H₂O → M–OOH* + H⁺ + e⁻ (10)

M–OOH* → M + O₂ + H⁺ + e⁻ (11)

OH⁻ + M → M–OH* + e⁻ (12)

M–OH* + OH⁻ → M–O* + H₂O + e⁻ (13)

2M–O* → 2M + O₂ (14)

M–O* + OH⁻ → M–OOH* + e⁻ (15)

M–OOH* + OH⁻ → M + O₂ + H₂O + e⁻ (16)

Moreover, two primary reaction pathways are also involved in the OER: firstly, the adsorbate evolution mechanism (AEM), where a metal serves as the redox centre, and the electronic state around the Fermi level has metallic traits.⁵³ Secondly, the lattice-oxygen mediated mechanism (LOM) involves oxygen serving as the redox centre, and the electronic state close to the Fermi level displays features like oxygen. In the conventional AEM, the OER involves different reaction intermediates such as *OH, *O, *OOH, and *O₂, which are adsorbed on active metal centres.⁵⁴ The minimal theoretical overpotential for the explored catalysts using the AEM is estimated to be nearly 0.37 V, determined by a linear scaling relationship between the adsorption energies of *OH and *OOH intermediates (ΔG_*OOH = ΔG_*OH + 3.2 eV).⁵⁵ Nevertheless, this scaling relationship fails to account for some catalysts that have been shown to have lower overpotentials. The existence of a scaling relationship restricts the advancement of catalysts aimed at reaching the optimal state of the OER. Nevertheless, this scaling relationship also provides certain benefits.⁵⁶ The scaling relationship-based volcano plot may be used as a universal tool to forecast catalyst activity based on the Sabatier principle, leading to a decrease in experimental and computational costs. Electrocatalysts using the AEM, distinguished by its reaction pathway, showed notable stability.⁵⁷

3. Significant parameters for HER and OER electrocatalysts

The electrocatalytic activity can be determined using the following fundamental parameters, such as electrolyte, electrode, potential (onset), Tafel slope (b), exchange current density (j_o), overpotential (η), stability, Faraday efficiency (FE), and turnover frequency (TOF).⁵⁸ The rate of HER and OER electrocatalysts for electrochemical performance depends on different structures, degree of wettability, conductivity, and catalyst entry into an electrolyte. Electrodes are divided into two types: flat surface electrodes and 3D electrodes. In the literature, glassy carbon (GC) electrodes include flat surface electrodes (FSEs), which are frequently employed to evaluate the efficiency of electrocatalysts. FSEs provide only a single channel for electrolytes to penetrate, limiting catalysis to the surface of catalysts. Although GC electrodes are straightforward, they only provide a modest quantity of catalyst loading. A binder is needed in GC electrodes to stabilize the catalysts (such as Nafion). However, overusing binders makes the catalyst surface more resistant and blocks the active sites. Therefore, scientists are greatly focused on generating catalyst inks without a binder and directly building catalysts onto 3D conducting electrodes such as carbon cloth (CC), Ni foam (NF), and carbon paper (CP). 3D electrodes are very conductive owing to the various channels for electrolyte diffusion from all sides of catalysts.⁵⁹ The pH of the electrolytes strongly influences the effectiveness of HER and OER electrocatalysts. Regarding HER and OER electrocatalysis, the alkaline medium is preferable to a neutral or acidic one. Alkaline solutions have mainly been used as electrolytes in recent investigations. This is of utmost importance when preparing electrocatalysts that can operate at any pH. When comparing the catalytic behaviour of different electrocatalysts, the onset potential is the parameter that is utilized most frequently. The point at which a sudden increase in current is noticed is referred to as the onset potential. It is challenging to determine the precise magnitude of the onset potential.⁶⁰ As a result, the overpotential value of 10 mA cm⁻² is thought to be more reliable and frequently employed. For the OER, a rotating ring disc electrode (RRDE) can be used to monitor the precise onset potential. The overpotential is one of the most crucial factors to consider when assessing the HER and OER performances of electrocatalysts. In an ideal situation, the applied potential required to perform the specific reaction would be identical to the optimal value. However, in the real world, to break the energy barrier, an applied potential that is significantly higher than the equilibrium potential is required.⁶¹ Because of this, the overpotential is typically defined as the possible distinction between the potential that is applied and the potential that exists when conditions are in equilibrium. Typically, it is expressed in volts (V) or millivolts (mV). Tafel analysis is used to determine the HER and OER mechanisms to compare the catalytic efficiency of several catalysts and reaction kinetics. For water splitting, it is possible to achieve anodic or cathodic overpotential. The concerned parameters including overpotential, Tafel slope value, and reversible hydrogen electrode (RHE) conversion can be determined using the following expressions:

The Nernst equation can be applied to calculate the reference potential (for example, Ag/AgCl) for the conversion into the RHE potential:

where is 0.2412 and overpotential (η) is obtained by subtracting an onset thermodynamic potential of 1.23 V (for the OER) and 0 V (for the HER) of the water splitting system.

Overpotential (η) = Onset potential (E_RHE) V − 1.23 V (for the OER)

Moreover, the Tafel slope can be calculated by utilizing the Tafel equation:⁶²

η = b log j + a

where η, b, and j indicate the overpotential, Tafel slope, and current density, respectively.⁶³ Another crucial quantity that may be calculated using the Tafel equation is the exchange current density when it equals zero. It reveals the material's inherent activity in the electrolyte at the reversible potential and shows the rate at which electrons migrate from the electrode to the electrolyte. A higher exchange current density indicates improved electrocatalytic capability. An ideal HER and OER electrocatalyst should have a significant exchange current density and a moderate Tafel slope. FE is the proportion of real products to theoretical products and is also frequently utilized in fuel cells and water-splitting reactions. For water electrolysis, FE was equal to the percentage of produced H₂ and theoretical H₂, which was another significant parameter for analysing the reaction process of water electrolysis. In addition to the previously specified parameters, stability is crucial for practical application.⁶⁴ In general, a material's stability can decide whether it can be utilized to produce hydrogen and oxygen by water electrolysis in a practical manner or not.⁶⁵ As a result, assessing a material's electrochemical stability is crucial. Generally, the electrochemical stability of an HER and OER catalyst can be easily determined by long-term chronoamperometry (CA), chronopotentiometry (CP) and cyclic voltammetry (CV). Furthermore, the scientists have also used TOF to measure the catalytic interaction of every active site by calculating the number of reactants that transformed into desirable products by each active site in unit time.⁶⁶ Therefore, the TOF value is a significant parameter for evaluating the electrocatalytic performance of the catalyst.

4. Transition metals as electrocatalysts for the HER and OER

The stable electrocatalyst can effectively reduce the reaction energy and accelerate the reaction rate. Exploring highly efficient, lower-cost, and stable electrocatalysts is a feasible strategy for energy conversion.⁶⁷ As discussed before, the most efficient catalysts for water electrolysis to date are Pt, IrO₂, and RuO₂ for the HER and OER, but their scarcity and high price have severely limited their use in large-scale industrial applications. Some parameters should be considered for selection of materials, which can help find efficient electrocatalysts, such as high cyclability, high ionic conductance, mechanical and thermal stability, and higher surface area.⁶⁸ In this regard, TM catalysts, such as copper (Cu), nickel (Ni), cobalt (Co), iron (Fe), and molybdenum (Mo), are known to be active electrocatalytic materials for the HER and OER.^69,70 These TMs have unfilled d orbital electrons and unpaired electrons in the d orbital, which are conducive to the adsorption and desorption of reactive groups.⁷¹ These materials have been reported to be highly active towards the electrocatalytic HER or OER with both high electrocatalytic activity and stability.^72,73 Moreover, some of them have also been revealed to possess bifunctional properties, which could simultaneously accelerate the HER and OER, thus enabling them to be promising electrocatalysts for commercial utilization. In the literature, there are many studies on Ni toward OWS reactions, and Ni is an Earth-abundant metal with exceptional catalytic activity because of its electronic structure. It has high electrical and thermal conductivities due to its remarkable electronic structure.⁷⁴ In contrast to other TMs like Cu, Co, Fe, and Mo, which are known to be effective catalysts for the HER and OER, use of the Ni dopant with other TM materials is a very active approach toward the HER and OER.^75,76 It should also be pointed out that the Ni-based catalyst can be further improved through surface structure and morphology tunning.⁷³ In contrast to Ni-based materials, Cu-based materials are extensively utilized in electrochemical catalysis and other fields because of their cheap cost and high performance. The synergistic activity of Cu and Ni, porous structure, enhancement in surface area and the number of active sites led to a boost-up in intrinsic activity.⁷⁷ These advanced hybrids take advantage of the intrinsic activity of Ni and Cu as well as hybrid TM characteristics such as strong conductivity, large surface area, easy surface functionalization, and high stability. Due to its superior electrical conductivity and stability under alkaline conditions, Co, one of the numerous non-noble metal elements on Earth, has garnered more attention as a prospective HER and OER catalyst. Co-based metal–organic framework (MOF) compounds and their derivatives, Co oxides, Co chalcogenides, Co phosphides, Co-based layered double hydroxides, and so forth are examples of cobalt-based catalysts.⁷⁸ Their distinct electrochemical characteristics and distinctive structural diversity make them a hotspot for research and development. Fe is an inexpensive metal widely used in industrial practice. At the same time, it exhibits relatively high HER and OER activities. Pure Fe is occasionally used as a reference material that can show diverse results concerning various preparation methods, morphology or surface area.⁷⁹ Regrettably, very little data on pure Fe as a cathode material are available. One possible reason is that Fe alone exhibits worse stability into the elevated alkaline environment. Fe-based catalysts have also attracted much attention because of their abundance and relatively low cost. They have been extensively used as dopants with other TMs to enhance the HER and OER performance.⁸⁰ Mn, Mo, and Se-based electrocatalysts have also been extensively studied in artificial water-splitting systems. Doping TMs can be an effective process for enhancing catalytic behaviour because doping can change the electronic properties of the host metals by adjusting the local charge redistribution without altering the desired intrinsic properties of the host elements.⁸¹

Inspired by conventional electrodeposition technology, Niu et al.⁸² reported a rapid self-etching electrodeposition method for constructing a copper-incorporated three-dimensional Ni–Cu coating on copper sheets as a superior bifunctional catalyst (Fig. 3a). According to this study, the optimal overpotentials for the HER and OER are 76 mV and 290 mV at 10 mA cm⁻², respectively. The calculation also shows that Ni–Cu has a higher theoretical hydrogen desorption energy and OER overpotential than pure Ni and Cu. The as-prepared Ni–Cu electrocatalyst showed exceptional stability, particularly in the HER reaction process stability test up to 50 hours. He et al.⁸³ developed a new bifunctional Fe–Cu–N co-doped carbon catalyst toward the HER and OER in alkaline media using a simple hydrothermal annealing method (Fig. 3b). Experimental results showed that high temperatures caused nitrogen to move from degraded g-C₃N₄ to carbon spheres, shattering the inert atmosphere of the surface of the graphite carbon and enhancing the active surface area. However, the capacity of as-prepared samples to split water was improved by controlling the ratio of Fe and Cu. According to electrochemical measurements, the catalyst with co-doped Fe–Cu had improved electron conductivity and a larger ESCA. Fe–Cu@CN₃ showed good catalytic water splitting performance, with overpotentials of 91 mV and 362 mV at 10 mA cm⁻² for catalysing the HER and OER, respectively. Qiu et al.⁸⁴ created an effective, long-lasting, bifunctional Ni–Fe/NiMoN_x electrocatalyst on nickel foam (Fig. 3c–e). This electrocatalytic system's HER and OER performances are notable, with 49 and 260 mV overpotentials at 20 mA cm⁻², respectively. Ni–Fe/NiMoN_x electrodes only need 1.54 V for 10 mA cm⁻² to split water overall (Fig. 3f and g). This improved electrocatalytic activity for the HER and OER results from the interaction of many active components created following NH₃ treatment. Badarnezhad et al.⁸⁵ used one-step electrodeposition to create Ni–Mo–Fe as a bifunctional electrocatalyst. Minimal overpotentials of 65 and 161 mV for delivering 10 and 100 mA cm⁻² for the HER in alkaline media were accomplished using this electrocatalyst with appropriate composition and current density. For the OER, the as-fabricated electrode demonstrated 344 and 408 mV for supplying 10 and 100 mA cm⁻². Furthermore, in long-term stability experiments in an alkaline medium, the as-prepared electrocatalyst exhibited excellent stability and low degradation in overpotential for the HER and OER. The superb performance of the produced Ni–Mo–Fe material may be due to the synergistic effects of Ni, Mo, and Fe components and the non-binding structure.⁸⁵

Fig. 3 (a) Schematic illustration of the self-etching electrodeposition process.⁸² (b) Schematic of the fabrication process of Fe–Cu@CN and CN.⁸³ (c) Illustration of synthesis and SEM characterization of the Ni–Fe/NiMoN_x catalyst. (d) TEM and (e) HRTEM images of Ni–Fe/NiMoN_x. (f) Overall water splitting polarization curves of Ni–Fe/NiMoN_x. (g) Stability test at 100 mA cm⁻² for 40 h.⁸⁴ The corresponding images have been reproduced with permission from Elsevier © Copyright.

For the preparation of bi-metallic Ni–Mo alloy catalysts for the HER and OER, Sun et al.⁸⁶ used a high throughput technique involving a straightforward co-sputtering approach. The inherent catalytic performance of Ni–Mo alloy electrocatalysts is increased due to the synergistic action between Ni and Mo. This results in their excellent HER and OER performance. The Ni₄₀Mo₆₀ electrocatalyst shows an overpotential of 258 mV at 10 mA cm⁻² in the OER. In contrast, the Ni₉₀Mo₁₀ electrocatalyst exhibits the highest HER performance with an incredibly lower overpotential value of 58 mV at 10 mA cm⁻². Cheng et al.⁸⁷ used an easy and affordable electrodeposition technique to create three-dimensional freestanding porous Cu foam in situ shielded CoNi alloy nanosheet arrays with customizable compositions. These freestanding Co₈₁Ni₁₉ alloy nanosheet arrays display exceptional electrocatalytic characteristics with overpotentials of 132 and 240 mV for the HER and OER to produce a catalytic current density of 10 mA cm⁻², respectively. Tian et al.⁸⁸ used a one-step electrodeposition method to synthesize quaternary Ni–Co–S–P nanoparticles (NPs) directly on carbon fibre fabric. Furthermore, the Ni–Co–S–P NPs demonstrate exceptional stability and longevity for both the HER and OER owing to their super hydrophilic and superhydrophobic features.⁸⁸ Recently, great attention has been paid to TM oxides (TMOs), phosphides (TMPs), chalcogenides, dichalcogenides (TMDs), nitrides, carbides, and borides due to their Earth-abundance, low-cost and superior electrical and optical properties.^30,89–91 These catalysts have proven to be highly active, stable, and promising for the EWS scaling-up process. Thus, it is vital to further understand the reaction pathways and basic concepts of the structure-related performance relationship of HER and OER processes. Then, the design and production of an effective catalyst for EWS can be considered.

4.1. Noble metal-based electrocatalysts

Noble metals such as Pt, Pd, Rh, Ru, and Ir are still the most efficient catalysts for the HER and OER. However, their high price and scarcity prevent them from being used widely. Thus, it is crucial to create noble-metal-based catalysts with low noble metal loading, high activity, and superior tolerance for energy conversion systems.⁹² Noble metal-based catalysts are inherently flawed, mostly due to their low stability and restricted reserves. To solve these problems, scientists have created several strategies, for example: (i) modifying the shape of noble metal catalysts; (ii) adding new elements; (iii) building single-atom anchors for noble metals on sophisticated support materials; (iv) shrinking the size of noble metal-enhanced nanocatalysts to the single-atom scale; and (v) designing the interface between noble metal and transition metal compounds.⁹³ Pt is a highly favoured active HER catalyst for the generation of H₂ because it has the highest HER activity. Noble metal oxides are regarded as cutting-edge materials because of their superior stability and catalytic activity in both basic and acidic environments. It is commonly recognised that IrO₂ and RuO₂ are thought to be the most advanced OER electrocatalysts, with IrO₂ being more stable under harsh acidic and alkaline conditions. Other noble metals such as silver and palladium have also been used for the HER and OER.⁹⁴ For instance, a simple technique for creating Pt single atoms in N-doped mesoporous carbon was presented by Lou et al.⁹⁵ According to experimental results, when compared to Pt/C, the resulting Pt single atoms on carbon can exhibit a 25-fold increase in mass activity for the HER. Pt single atoms can contribute to the exceptional HER activity by increasing the density of electronic states and creating a favourable charge density distribution. A simple wet-chemical technique was used by Huang et al.⁹⁶ to create Mn-doped Ru nanosheet branches, which have an ultrathin thickness of 1.09 nm and are made up of many nanoscale branches with plenty of edges. Taking advantage of their electrical and structural properties that are inherent, when it comes to the HER and OER, the as-prepared RuMn nanosheets can demonstrate exceptional catalytic performance. The Ir–Ni₃N heterogeneous interface was formed on the surface of the NF substrate and noble metal Ir NPs were directly implanted into the Ni₃N nanosheet structure by the one-step ammonia nitridation procedure reported by Liu et al.⁹⁷ XPS demonstrated that the promotion of the HER and OER was significantly aided by the heterogeneous interfaces between IrO_x and NiOOH that developed during surface self-reconstruction. To further enhance the electrocatalytic performance, Manjunatha et al.⁹⁸ produced decorated Pd nanoclusters and a nitrogen-doped carbon nanotube-supported Co–Fe nanocomposite. The Pd nanocluster-coated Co–FeNCNTs demonstrated higher trifunctional electrocatalytic activity for the ORR, OER, and HER.

4.2. Transition metal oxide-based electrocatalysts

TMO electrocatalysts have garnered considerable attention in the energy conversion and storage field due to their affordability, abundance in the Earth's crust, excellent corrosion resistance, and enhanced electrochemical activity.^99,100 In the past, pure TMOs showed better OER potentials but low reactivity towards the HER due to their unattractive hydrogen desorption ability.¹⁰¹ Therefore, significant efforts have been undertaken to modify TMOs for HER activity in alkaline environments, making the modified TMOs excellent candidates for OWS.¹⁰² Hybridisation of TMOs with other electroactive substances (such as TMOs, TMPs, TMs, and TM alloys) can enhance the catalytic performance.¹⁰³ As a result of the mixed-valence states of Co^2+/3+/4+, the electrocatalysis of the HER and OER on cobalt oxide electrodes has long been a subject of intense interest in electrochemistry.¹⁰⁴ Min et al.¹⁰⁵ created Co- and Ce-based bimetallic organic frameworks that served as self-templates for the production of CeO₂-decorated Co₄N (Co₄N/CeO₂) nanostructures with highly linked heterointerfaces as active electrocatalysts for the OER in alkaline media. The corresponding illustration can be seen in Fig. 4a. Xu et al.¹⁰⁶ effectively created a porous cobalt oxide material with oxygen vacancies using a ligand-assisted polyol reduction process. Such technology gives a chance to successfully create oxygen vacancies on the surface with a regulated concentration while successfully converting Co(OH)₂ nanoplates into CoO_x with well-maintained morphology. OER performance benefits greatly from the vast surface area created by the 2D porous structure and the abundance of active sites caused by oxygen vacancies, producing overpotential values as low as 306 mV at 10 mA cm⁻².¹⁰⁶ Cai et al.¹⁰⁷ meticulously developed 3D porous NiFe LDH nanosheets electrodeposited on Ni nanochains. SEM showed the core–shell nanostructure, which is advantageous for increasing the catalyst's surface area and exposing more active sites. The excellent conductivity of the Ni@NiFe LDH is also reflected in its low charge transfer resistance (R_ct).Zhang et al.¹⁰⁸ hydrothermally created a three-dimensional (3D) self-supporting nickel sponge (SN) with nano-Ni synapses and then deposited 2D CoFeLDH nanosheets on the nickel sponge using the electrodeposition approach. CoFeLDH was also deposited on a NF substrate for comparison using the same deposition procedure. In a 1 M KOH alkaline solution, the as-prepared CoFeLDH/SN showed superior electrocatalytic capabilities with 208 mV at 10 mA cm⁻² for the OER and 63 mV at 10 mA cm⁻² for the HER. Hanan et al.¹⁰⁹ created PdO decorated CoSe₂ through a combination of wet-chemical and ultraviolet irradiation methods to get the desired doping of PdO (as a promoting agent) as an active electrocatalyst for the OER in 1.0 M KOH. However, the corresponding XRD patterns of CoSe₂ and PdO-based composites can be seen in Fig. 4b. It can be observed that the crystallinity of CoSe₂ is disturbed due to the addition of PdO, and a bit of shifting of the peak can be noticed for the structure. Elakkiya and colleagues created a nanoporous NiCo₂O₄ material with a floral morphology, which served as a bi-functional electrocatalyst for water splitting in a 1.0 M KOH electrolyte. This material demonstrated exceptional performance, with high mass activity, low overpotential (360 mV at 10 mA cm⁻² for the OER and HER, respectively) and modest Tafel slopes (150 mV dec⁻¹ and 123 mV dec⁻¹ for the OER and HER, respectively). These extraordinary capabilities were ascribed to the material's unique surface and electrical band structure, including an abundance of active sites and porosity, electrochemical active surface area, quick electron transfer, and structural stability.¹¹⁰ Moreover, the Co₂FeO₄@rGO composite has been prepared by Hanan et al.¹¹¹ through the hydrothermal method. The prepared material exhibited activity as a trifunctional electrocatalyst toward the HER, OER, and ORR in an alkaline medium having 320 and 240 mV overpotentials. The structure of composite materials was nanosheets of rGO combined with Co₂FeO₄, as shown in Fig. 4c and d. Srinivas et al.¹¹² demonstrated a carbon nanotube (CNT) network with remarkable water-splitting performance that was produced from a MOF and inserted in a CoFe₂O₄/CoO heterostructure.¹¹³ Because of the collective advantages of MOF-derived porous carbon with abundant oxygen vacancies, the CoFe₂O₄/CoO heterostructure embedded in the CNT network, the hybrid catalyst (CoFe/C-650), shows great performance for the OER, HER, and overall water splitting with several exposed catalytic sites and synergistic interactions. The catalyst requires minimum overpotentials of 246 mV (for the OER) and 164 mV (for the HER) at 10 mA cm⁻² and has low Tafel slopes of 45.27 and 86.38 mV dec⁻¹, respectively. It is interesting to note that the overpotential needed for complete water-splitting catalysis is 1.614 V (η₁₀), which is close to the overpotential needed for the standard Pt/C (cathode) and RuO₂ (anode) combination.¹¹² Additionally, Aftab et al.¹¹⁴ reported a mixed CoS₂@Co₃O₄ composite material through a hydrothermal method as a promising electrocatalyst for the HER in alkaline media. The proposed composite is a flower-like structure embedded with Co₃O₄ NPs. Synergistic effects between metallic oxides and metallic sulfides provided the composite with an overpotential value 320 mV at 10 mA cm⁻² and a Tafel slope value of 42 mV dec⁻¹. In their study, Xin et al.¹¹⁵ described a very effective 3D bulk catalyst made of core–shell nanostructures. This catalyst has a one-of-a-kind design in which flower-shaped CuCoMoO_x nanosheets are decorated with CoCu alloy NPs. These nanosheets were wrapped uniformly around the copper nanowire (Cu NW) cores, which were held together by a CF substrate. Meanwhile, the nanosheets connected with Cu NWs demonstrated excellent HER and OER activities and amazing overall water splitting properties in the alkaline medium due to their notable conductivity, leading to quick electron transfer. The electrocatalyst performed well in both the HER and the OER, with overpotentials of 75 mV and 315 mV at 100 mA cm⁻², respectively.¹¹⁵ Simultaneously, Solangi et al.¹¹⁶ introduced TMO (Co₃O₄) with doping of palladium (Pd) oxide as a promoting agent through an aqueous chemical and ultraviolet-irradiation method (for PdO doping). The composite had a nanoneedle-like structure embedded with NPs of Pd. Fig. 4e and f shows the XPS spectra of Co₃O₄ and its composite with Pd, including Co 2p and O 1s. The as-prepared material successfully catalyzed the electrochemical water splitting reaction in alkaline media with an overpotential value of 250 mV at 20 mA cm⁻², a Tafel slope of 58 mV dec⁻¹, and a charge transfer resistance of 48.5 Ω, leading to fast transfer kinetics for the OER.

Fig. 4 (a) Schematic illustration of the synthesis of Co and Ce-based bimetallic organic frameworks.¹⁰⁵ (b) XRD spectra of pristine CoSe₂, PdO, and their composites for the OER.¹⁰⁹ TEM and HRTEM images, FFT patterns, and line profiles of (c) GO and (d) Co₂FeO₄@rGO.¹¹¹ XPS analysis of Co₃O₄ and Co₃O₄–Pd and (e) Co 2p and (f) O 1s.¹¹⁶ All the corresponding images have been reproduced with permission from Elsevier and Royal Society of Chemistry © Copyright.

Digraskar et al.¹¹⁷ conducted a study where they created a composite material consisting of oleylamine-functionalized graphene oxide and Cu₂ZnSnS₄ (OAm-GO/CZTS), with the morphology as shown in Fig. 5a–c. They examined its potential as a highly efficient electrocatalyst for the HER and OER (Fig. 5d). The OAm-GO/CZTS composite exhibited exceptional electrocatalytic performance and long-lasting durability for generating hydrogen and oxygen, even in acidic and basic environments. At a current density of 10 mA cm⁻², the composite displayed overpotentials of 47 mV for the HER and 1.36 V for the OER (Fig. 5e and f). Additionally, it exhibited Tafel slopes of 64 and 91 mV dec⁻¹ for the HER and OER, respectively. These results surpass those achieved by TMCs and are on par with the performance of commercially available precious metal catalysts. Li et al.¹¹⁸ successfully used a rapid two-step electrodeposition approach to create a heterostructure comprising CoNiP and NiFe-layered double hydroxides (CoNiP@NiFe LDHs). CoNiP's electron density may be increased, further optimising the Gibbs free energy for hydrogen adsorption via the electron rearrangement facilitated by the discrepancy in the distinct work functions of CoNiP and NiFe LDHs. To achieve 10 mA cm⁻², CoNiP@NiFe LDHs require a low overpotential of 68 mV. Furthermore, the OER performance of the CoNiP@NiFe LDHs is significantly improved. At 50 mA cm⁻², they show a drastically lower overpotential of 255 mV. This enhancement may be linked to valence band energy level (EVB) changes and the effective collection of holes in the NiFe LDH layer's space charge region. These CoNiP@NiFe LDHs are used as electrodes in the electrolyser, resulting in an astonishingly low cell voltage of 1.59 V at 50 mA cm⁻².¹¹⁸ In contrast, Hanan et al.¹¹⁹ recently reported the PdO@Co₂FeO₄ nanostructures as a bifunctional electrocatalyst toward the HER and OER in an alkaline medium of 1.0 M KOH. The PdO@Co₂FeO₄ nanostructure material was prepared through a hydrothermal method under ultraviolet irradiation. This composite material has nanoparticle-like morphology with irregular structure (due to PdO addition). The material showed small crystal structure changes due to the addition of PdO within the crystal of Co₂FeO₄. Interestingly, the electrocatalyst showed promising bifunctional behaviour with overpotential values of 269 and 259 mV at 10 and 20 mA cm⁻² current densities, with Tafel slope values of 49 and 59 mV dec⁻¹ for the HER and OER, respectively. Moreover, Laghari et al.¹²⁰ produced a magnesium (Mg) supported Co₃O₄ composite material through the wet chemical growth method. XRD analysis of the proposed composite material revealed a nanoneedle-like structure and the cubic crystal structure of Co₃O₄. The MgO-based Co₃O₄ composite material demonstrated a robust electrochemical behaviour, with an overpotential value of 273 mV to reach a current density of 10 mA cm⁻², an ECSA of 12.8 cm², a low charge transfer resistance of 45.96 Ω, and a Tafel slope value of 64 mV dec⁻¹. On the other hand, a composite material based on iron oxide (Fe₃O₄) and reduced-graphene-oxide (rGO) as a potential candidate for the HER in 1.0 M KOH has been introduced by Hanan et al.¹²¹ They have produced the rGO@Fe₃O₄ nanostructure through a hydrothermal method using hydrazine hydrate (N₂H₄) as the reducing agent. They introduced graphene nanosheets decorated with Fe₃O₄ NPs. The addition of 2D reduced graphene-oxide sheets improved the conductivity of Fe₃O₄, facilitating the HER with an overpotential value of 300 mV at 10 mA cm⁻², a Tafel slope value of 80 mV dec⁻¹, and an R_ct of 95 Ω. Faid et al.¹²² used in situ Raman to investigate the reaction mechanism of the alkaline HER on Ni/NiO nanosheets (Fig. 5g and h). They revealed that the active phase was β-Ni(OH)₂, which remained unchanged during the HER reaction (Fig. 5i), contributing to the good activity and stability. Overall, numerous TMOs have been thoroughly researched and showed improved HER and OER performances in alkaline media, including binary, ternary, and quaternary compositions. Crystal structure, elemental content, surface morphology, and electrical characteristics all impact the performance of these oxides. Doping, surface changes, and nano-structuring have all been used to improve their electrocatalytic activity and efficiency.¹²³ Understanding structure–function correlations of TMOs and developing better synthesis techniques are critical areas of study to further enhance their electrocatalytic efficiency. These materials have a lot of potential for enhancing energy conversion and storage technologies, thus contributing to a more sustainable and clean energy future.^124,125

Fig. 5 SEM images of (a) GO, (b) pure CZTS NPs, and (c) OAm-GO/CZTS. (d) Diagram of the OAm-GO/CZTS electrocatalyst on a GCE for the total water-splitting reaction (HER and OER). Polarization curves for (e) HER and (f) OER of OAm-GO/CZTS and related samples.¹¹⁷ (g) Scheme of in situ Raman. (h) Digital photo of in situ Raman. (i) in situ Raman spectra of Ni/NiO after 4000 seconds of chronoamperometry experiments at −0.4 V vs. RHE in 1 M KOH.¹²² All the corresponding images have been reproduced with permission from Elsevier and ACS © Copyright.

4.3. Transition metal phosphide-based electrocatalysts

TMPs are the most prominent candidates for the alkaline HER and OER due to their superior catalytic efficiency and low cost. The phosphorus (P) atoms in TMPs can accept electrons from neighbouring TMs due to their high electronegativity, which is the primary cause of their exceptional catalytic activity.¹²⁶ The negatively charged P atoms can trap the positively charged proton as a base. As a result, the moderate bonding between the reaction intermediates and the presence of P atoms lead to product formation on the catalyst surface. Therefore, TMPs often have a higher electrocatalytic activity than other TM-based composites.^127–129 Different techniques have been developed to improve the intrinsic catalytic activity and structural stability of TMP materials. These methods include: (i) doping with heteroatoms, a practical approach for changing the electronic structure and creating lattice defects, resulting in optimized adsorption and desorption of intermediates at active sites;¹³⁰ (ii) constructing composites based on TM–P to create heterogeneous catalysts. To increase the intrinsic catalytic activity, more catalytically active sites must be exposed; (iii) depositing the grafted TM–P active phases on well-conductive supports, and (iv) creation of a core–shell structure with a strong contact between the core material and the shell layer.¹³¹ This improves the conductivity and endurance of Co–P-based electrocatalysts and increases the number of active sites. The most often reported TMPs are Co_xPy, Ni_xPy, Fe_xPy, Mo_xPy, Cu_xPy, and W_xPy, and they have demonstrated outstanding performances to date.¹³²

Based on the DFT calculations, the highly conductive 2D TMP monolayers and their corresponding oxidized counterparts are anticipated to be effective HER and OER catalysts. Mo₂P-2H and Fe₂P-2H show the best electrochemical performance among a group of predicted TMPs, further supported by an experiment showing that bifunctional catalysts exhibit good performance. Consequently, the advanced DFT simulations are excellent tools for finding novel bifunctional electrocatalysts. Bimetallic phosphides possess a better activity for the alkaline HER and OER than single TM-based phosphides by utilizing the synergistic effects of bimetallic sites with modified electrical configurations and structural flexibility. This section will address the application aspects and practical methods for increasing the catalytic activity of TMPs in the alkaline HER and OER. For instance, Zhou et al.¹³³ have developed a workable topological conversion technique for preparing O-doped CoP nanosheets through a moderate phosphidation process on well-defined Co(OH)₂ nanosheets. O-CoP nanosheets with moderate O incorporation showed excellent electrocatalytic activity toward the HER and OER, with overpotentials of 98 and 310 mV at a current density of 10 mA cm⁻², respectively. This has been revealed by experimental and theoretical studies, which also showed that the O doping level in the O-CoP nanosheets played an important role in determining their electrocatalytic efficiency. According to mechanistic research, the right O dopant considerably increased the electrical conductivity and controlled CoP's electronic structure, which optimized the adsorption energy of intermediates and resulted in exceptional electrocatalytic performance. Chen et al.¹³⁴ have published a study describing the development of a highly efficient catalyst Ru–MnFeP/NF, which originated from a Prussian blue analogue (Fig. 6a). This catalyst, consisting of an ultralow concentration of ruthenium (1.08 wt%) supported on NF (Fig. 6b and c), demonstrates exceptional performance in both the OER and HER when tested in 1 M KOH (Fig. 6d–i). Remarkably, the Ru–MnFeP/NF electrode achieves current densities of 20 mA cm⁻² for the OER and 10 mA cm⁻² for the HER, with only minimal overpotentials of 191 mV and 35 mV, respectively (Fig. 6j).

Fig. 6 (a) Schematic illustration of the synthesis of an Ru–MnFeP/NF catalyst. XPS profiles of (b) Ru 3p and Fe 2p of Ru–MnFeP/NF. Digital photos of the (d) Ru–MnFeP/NF electrode, (e) water splitting cell, and (f) gas collection device. (g) The volume of H₂ and O₂ collected during electrolysis. Digital photos of gas collection of (h) O₂ and (i) H₂. (j) Polarization curves of Ru–MnFeP/NF and control samples for the OER and HER in 1 M KOH.¹³⁴ All the corresponding images have been reproduced with permission from Wiley © Copyright.

Hao et al.¹³⁵ presented the methodical design and discovery of a tri-functional electrocatalyst Co/CoP-HNC by embedding Co/CoP within a nitrogen-doped carbon polyhedral structure derived from MOFs. The catalyst exhibited a large electrochemical surface area and high electrical conductivity, leading to notable performance in the HER, OER, and ORR.¹³⁶ A recent study conducted by Ding et al.¹⁰¹ investigated the potential of three different Prussian blue analogues (PBA) with varying iron-based components as precursors for synthesizing metal phosphides through a simple heat treatment process. By adjusting the temperature of phosphidation, they successfully obtained a series of metal phosphides, namely FeCoP, FeNiP, and FeMnP. Among these, FeCoP exhibited the most pronounced porous structure and the widest range of pore sizes when phosphidated at an optimal temperature of 400 °C. This unique porous structure of FeCoP facilitated efficient mass transfer and oxygen release during the OER.

Furthermore, introducing Co into the metal phosphides resulted in a significant decrease in R_ct compared to Ni and Mn. This decrease in R_ct enhanced the electron transport during the OER process. Because of its exceptional porous geometric structure and distinctive electronic properties, FeCoP-400 demonstrated outstanding stability and catalytic performance for the OER in 1.0 M KOH solution. At a current density of 10 mA cm⁻², FeCoP-400 demonstrated the overpotential of 261 mV, surpassing both commercial RuO₂ and most other OER electrocatalysts. Yun et al.¹³⁷ recently presented a novel approach to synthesize a self-supporting catalyst electrode, P-NiFe@CF, for facilitating the urea oxidation reaction (UOR) and HER. A Ni–Fe alloy was electroplated over a carbon fiber substrate to create the electrode, which was subsequently phosphidated. In the HER process, the P-NiFe@CF electrode displayed exceptional performance with a remarkably low overpotential of 0.023 V (versus RHE) at the current density value of 10 mA cm⁻². The analysis encompassed the Fe₂p, Ni–Fe@CF, Ni₂p, P₂p, and XPS peak profiles of P-NiFe@CF, providing valuable insights into the composition and properties of the synthesized catalyst electrode. Liang et al.¹³⁸ produced NiCoP by converting Ni–Co hydroxide by plasma treatment as a bifunctional catalyst for water splitting. As expected, the NiCoP electrocatalyst on a Ni foam support performed better for the HER in an alkaline solution. The catalyst showed an incredibly lower overpotential of 32 mV at a current density of 10 mA cm⁻² with an exchange current density value of 1.36 mA cm⁻² and a Tafel slope of 37 mV dec⁻¹. In the case of the OER, the active sites were surface oxides originating from NiCoP that were created during anode scanning, resulting in an overpotential of 280 mV and a Tafel slope of 87 mV dec⁻¹ at a current density of 10 mA cm⁻². A two-electrode water electrolysis cell with a cell potential of 1.58 V at 10 mA cm⁻² was also demonstrated. NiCoP was used as the bifunctional catalyst in this cell.¹³⁸ Salvo et al.¹³¹ successfully synthesized mixed metal phosphides with various metal ratios, specifically Ni/Co and Ni/Fe phosphides. These phosphides were obtained through the phosphidization process of high-surface-area hydroxides grown on CC via a hydrothermal method. The HER performance of the synthesized materials reached its peak when synergistic interactions were established among the different metal centres. Remarkably, the materials containing the highest proportion of ternary compounds, namely NiCoP and NiFeP, exhibited outstanding performance with the overpotential value of only 50 mV needed to achieve a current density of 10 mA cm⁻². The as-prepared catalysts displayed a needle-like structure. To create a bifunctional iron cobalt phosphide (FeCoP/3C) electrocatalyst, carbon black Vulcan XC-72, carbon nanotubes, and graphene oxide (referred to as 3C) were used. The catalytic performance toward the HER and the OER in 1.0 M KOH electrolyte was examined. The FeCoP/3C electrocatalyst exhibits 120 and 220 mV overpotentials at a current density of 10 mA cm⁻² with Tafel slopes of 129 and 73 mV dec⁻¹.¹³⁹ In a groundbreaking achievement, crystalline Cu₃P phosphide nanosheets were successfully grown on conductive NF, forming a Cu₃P@NF composite material. This composite material exhibits exceptional electrocatalytic properties for water splitting, in terms of overall water splitting by visible light irradiation and electrocatalysis. For OER catalysis, a current density of 10 mA cm⁻² can be achieved with an impressively low overpotential of approximately 320 mV and a Tafel plot slope of only 54 mV dec⁻¹ in a KOH solution with a concentration of 1.0 M. While, in HER catalysis, the Cu₃P@NF electrode displays excellent performance, requiring a mere overpotential of approximately 105 mV to attain a current density of 10 mA cm⁻². Moreover, the Cu₃P@NF electrode demonstrates its capability for overall water splitting in a water electrolyser. At an applied voltage of approximately 1.67 V, the electrode exhibits a catalytic current density of 10 mA cm⁻², enabling the efficient conversion of water into hydrogen and oxygen. Wang et al.¹⁴⁰ devised an innovative microwave-induced plasma synthesis approach for the fabrication of an ordered porous structure containing cobalt phosphate and cobalt oxides, as illustrated in Fig. 7a. In this method, an ordered porous Co₃O₄ framework with a pore size of approximately 200 nm was initially grown in situ on a conductive graphite plate. Subsequently, a plasma treatment was conducted in the presence of red phosphorus, transforming a portion of Co₃O₄ into Co₃(PO₄)₂ and CoO (Fig. 7b–j). The plasma treatment not only resulted in the formation of cobalt phosphate but also induced a substantial density of oxygen vacancy defects, which served as active sites for both the HER and OER as shown in Fig. 7(k). This unique combination of features allowed for the achievement of overall water splitting at a low cell voltage of 1.65 V (Fig. 7l).

Fig. 7 (a) Schematic illustration of the synthetic strategy of cobalt phosphate and cobalt oxides on a carbon plate via a microwave-induced plasma method. Digital photos of (b) PMMA/CP (a polymethyl methacrylate template on a carbon plate), (c) 3D Co₃O₄/CP, (d) 3D Co:Pi/CoO_x/CP (a 3D cobalt phosphate and cobalt oxide mixture on a carbon plate); the microwave-induced plasma treatment of the carbon plate sample inside a vacuumed round bottom flask. SEM images of (e) and (h) PMMA/CP, (f) and (i) 3D Co₃O₄/CP, and (g) and (j) 3D Co:Pi/CoO_x/CP. (k) Digital photo of a water electrolysis cell comprising 3D Co:Pi/CoO_x/CP as both the anode and cathode in 1.0 M KOH. (l) LSV curve and inset of chronopotentiometry responses of 3D CoPi/CoO_x/CP at a constant current density of 10 mA cm⁻² for 100 h.¹⁴⁰ All the corresponding images have been reproduced with permission from ACS © Copyright.

Kumaravel et al.¹⁴¹ and his team proposed that TMPs exhibit remarkable performance in the OER and HER under different pH conditions. Additionally, the various phases of phosphides, such as phosphorus-rich or phosphorus-deficient metal composites like Ni/Co–P, present significant potential for applications involving total water splitting. In this study, a low-crystallinity and micro-spherical CoFe-P/NF catalyst that was produced via potentiostat electrodeposition on an NF substrate demonstrated excellent performance in terms of HER, OER, and water splitting in a 1 M solution of KOH. To create a current density of 10 mA cm⁻², the CoFe-P/NF material required overpotentials of 45 mV for the HER and 287 mV for the OER. In addition, it was found that the Tafel slopes for the HER and OER were 35.4 and 43.2 mV dec⁻¹, respectively.¹⁴² Wu et al.¹⁴³ created cobalt phosphide NP modified N-doped porous carbon spheres (CoP@NPCSs) using an innovative and effective coordination–polymerization–pyrolysis (CPP) technique, which served as a potent catalyst for the OWS (Fig. 8a–c). To evaluate its HER performance, LSV polarization curves were obtained from recent reports for various metal phosphide coatings, including CN, Co@NPCSs, CoP NPs, CoP@NPCSs, untreated metal foil and a Pt mesh electrode, in a 1.0 M KOH electrolyte.¹⁴⁴

Fig. 8 (a) Schematic illustration of the synthesis method for CoP@NPCSs. (b) XRD profile (up) and Raman spectrum (down), (c) SEM image of CoP@NPCSs. (d) Overall water splitting of CoP@NPCSs.¹⁴³ (e) and (f) TEM images and FFT pattern of Fe–Co–P. (g) Schematic illustration of synthesis of Fe–Co–P.¹⁴⁵ (h) and (i) TEM images of Ru–NiCoP/NF. (j) Polarization curves of the HER and OER of Ru–NiCoP/NF. (k) Overall water splitting using Ru–NiCoP/NF||Ru–NiCoP/NF. (l) Stability of Ru–NiCoP/NF||Ru–NiCoP/NF.¹⁴⁶ All the corresponding images have been reproduced with permission from Elsevier, ACS © Copyright.

Tafel plots in an alkaline electrolyte were generated for CN, Co@NPCSs, CoP NPs, CoP@NPCSs, and Pt/C to analyze their electrochemical behaviour during the HER. The current–time (I–t) curves were obtained for CoP@NPCSs in both acidic (upward) and alkaline (downward) electrolytes to assess their performance in the HER at fixed overpotentials of 112 mV and 115 mV, respectively (Fig. 8d). Wang et al.¹⁴⁷ used a simple technique to strategically manufacture an iron-nickel phosphide (Fe₂P/Ni₂P) heterostructure on nickel foam using Fe₂P nanoparticles placed on Ni₂P nanosheets. The Fe₂P/Ni₂P heterostructure functions as a bifunctional electrocatalyst, delivering very low overpotentials of 64 and 185 mV to achieve a current density of 10 mA cm⁻² for the HER and OER. The permeable 3D heterostructure also helps expose abundant active sites, enhance ion diffusion, and enable gas bubble release, improving catalytic performance.¹⁴⁷ In a recent study conducted by Liu et al.,¹⁴⁴ the experiments were carried out using a rotating disk electrode in a KOH solution with a concentration of 1 M. The HRTEM images presented in Fig. 8e and f exhibit d-spacings of 2.44 and 2.77 Å. These values can be correlated with the fast Fourier transform (FFT) pattern and the interplanar spacings of (111) and (011) of the Fe–Co–P alloy. The d-spacing values fell between those of FeP and CoP, indicating the production of a nanostructured Fe–Co–P alloy. They created hollow and conductive iron-cobalt phosphide (Fe–Co–P) alloy nanostructures utilising an Fe–Co metal organic complex in a simple and scalable manner. The Fe–Co–P alloy catalyst was subjected to chronopotentiometric and chronoamperometric measurements (Fig. 8g–i). These measurements were conducted at a steady current density of 10 mA cm⁻² (V–t) and an overpotential of 252 mV (i–t). Polarization curves were obtained for various catalysts,^145,146 such as Fe–Co–P, Ru–NiCoP/NF, NiCoP/NF, Ni₂P/NF, and commercial RuO₂/NF, in a KOH solution with a concentration of 1 M (Fig. 8j–l). Lin et al.,¹⁴⁸ using a straightforward technique, developed a Mo-doped NiCoP nanosheet array through hydrothermal and phosphation processes for the HER and OER. Notably, Mo doping may successfully modify the electronic structure of NiCoP, increasing the number of electroactive sites and improving each site's intrinsic activity. This highly effective dual-functional Mo-doped NiCoP catalyst has shown relatively low overpotentials and long-term stability. In an alkaline environment, the overpotentials for the HER and OER to actuate a current density of 10 mA cm⁻² were 76 and 269 mV in 1.0 M KOH, respectively.¹⁴⁸ In conclusion, TMPs show extraordinary stability and catalytic activity for both the HER and OER because of their distinct electrical and structural characteristics. They have a bunch of new sites and have great electrical conductivity, which makes electrochemical processes possible. To improve the electrocatalytic efficiency of different TMPs, researchers have investigated binary, ternary, and quaternary compositions. We can open the door to more effective and ecologically sustainable energy storage and conversion technologies by using the properties of TMPs as bifunctional electrocatalysts.

4.4. Transition metal nitride-, carbide- and boride-based electrocatalysts

TMCs, TMNs and TMBs are also regarded as promising electrocatalysts for general water splitting owing to their unique physical, chemical, and electrical characteristics.^149,150 The electronic structure of TMNs is comparable to that of Pd and Pt noble metals. It is up to the Fermi level because of the nitrogen atoms in TMNs, which could boost d-electron density and cause the d-band to constrict. In addition to their strong metallic conductivity, TMNs have high corrosion resistance, which makes them desirable for water splitting.¹⁵¹ Nevertheless, the intrinsically low conductivity, stability, and water-splitting activity of bifunctional electrocatalysts constantly put a cap on their performance. Additionally, with the introduction of interstitial N with a modest radius, most TMNs maintain their close-packed metallic range, leading to high electronic conductivity. These intriguing characteristics of TMNs make them potential replacements for noble metal compounds as catalysts.¹⁵² This section focuses on new research initiatives to create enhanced TMNs with high activity for water splitting and emphasizes the impact of structure engineering on electrochemical performance. TMNs exhibit the same catalytic capabilities due to their structural similarities, where either C or N exists in the interstitial sites.

Panda et al.¹⁵³ showed a new easy way to make copper nitride (Cu₃N), which acts as a bifunctional electrocatalyst for the OER, HER, and overall water electrolysis in alkaline media, as shown in Fig. 9a. Interestingly, the electrochemically deposited Cu₃N on NF had exceptionally low overpotentials for both the OER and HER, and the overall water splitting cell potential was just 1.62 V, with an incredible durability of more than 10 days (Fig. 9b). Most significantly, to drive effective catalysis, the coordinatively unsaturated Cu in Cu₃N changed in situ under reducing and oxidative conditions into a copper-rich shell that serves as the active site over an equally significant electrically conductive Cu₃N core. Fang et al.¹⁵⁴ discovered a suitable technique to produce cobalt nitride nanosheets encapsulated in graphdiyne (CoNx@GDY NS) to improve the catalytic performance. This method resulted in an impressive current density of 10 mA cm⁻² at low overpotentials of only 70 and 260 mV for the HER and OER, respectively, surpassing the performance of commercial Pt/C and RuO₂ products. Moreover, when utilized as a bifunctional catalyst in a two-electrode electrolytic cell, it exhibited an unusually low cell voltage of 1.48 volts at a current density of 10 mA cm⁻². This was significantly lower than the Pt/CeRuO₂ pair and the catalyst maintained stability for more than 20 hours. Wu et al.¹⁵⁵ integrated active OER (Ni₃N) and HER (NiMoN) catalysts as heterostructures for successful overall water splitting, as shown in Fig. 9c. They used a sufficient ratio to encapsulate Ni₃N and NiMoN on CC and modulated the crystalline degree and crystalline size by modifying the preparation parameters. The improved Ni₃N–NiMoN electrocatalysts demonstrated good stability and low overpotentials of 277 and 31 mV at a current density of 10 mA cm⁻² for both the OER and HER in the alkaline electrolyte. Lu et al.¹⁵⁶ produced cobalt–molybdenum nitride nanosheet arrays (CoMoN_x NSAs/NF) on NF (Fig. 9d) using a simple hydrothermal and nitridation technique.

Fig. 9 (a) Atomic structure of Cu₃N. (b) Water splitting by Cu₃N||Cu₃N on NF.¹⁵³ (c) Schematic of the formation of the Ni₃N–NiMoN heterostructure.¹⁵⁵ (d) SEM image of CoMoN_x-500 NSAs/NF. (e) Polarization curves of overall water splitting by CoMoN_x-500 NSAs/NF. (f) The potential comparison of CoMoN_x-500 NSAs/NF with electrodes from the literature.¹⁵⁶ (g) Schematic diagram of the fabrication of Co₆W₆C@NC/CC. (h) SEM image of Co₆W₆C@NC/CC. (i) Polarization curve of Co₆W₆C@NC/CC.¹⁵⁷ All the corresponding images have been reproduced with permission from Elsevier, ACS, and Wiley © Copyright.

The resultant CoMoN_x NSAs/NF catalyst has more active sites, a larger surface area, and also good electrical conductivity, accelerating the reaction rate. Also, the synergistic action of Co₂N and Mo₂N cooperatively enhances charge transfer, boosting the electrocatalytic activity even further. Due to these advantages, the optimum CoMoN_x-500 NSAs/NF catalyst demonstrates good HER and OER activities with overpotential values of 91 and 231 mV at a current density of 10 mA cm⁻². Additionally, the CoMoN_x-500 NSAs/NF combination has a surprising property. It requires a considerably low cell voltage of 1.55 V at a current density of 10 mA cm⁻² (Fig. 9e), outperforming other TMN catalysts reported in the literature (Fig. 9f). Chang et al.¹⁵⁸ investigated nanomaterials with highly reactive exposed facets, which have sparked considerable attention due to their large catalytic performance improvement. By performing an in situ N/O exchange, the (100) facet of NiMoN nanowires was preferentially exposed, and the shape was adjusted using a NiMoO₄ precursor. Under both alkaline and acidic conditions, the facet-tuned NiMoN nanowires demonstrated remarkable electrocatalytic activity for the HER, identical to the noble metal platinum.

TMCs have gathered a lot of attention for the OWS process because of their excellent corrosion resistance, great mechanical toughness, high electrical conductivity, exceptional stability, and low cost.¹⁵⁹ TMCs and TMNs are compounds in which carbon and nitrogen atoms are integrated inside the parent metal's interstitial sites. Face-centered cubic (fcc), hexagonally closed packed (hcp), and even simple hexagonal (hex) crystal structures are often found in these materials. TMCs and TMNs are especially intriguing for various applications in electrocatalytic water splitting due to their unique electrical structure and intrinsic characteristics.¹⁶⁰ Numerous research studies have been published in recent years, and EWS has advanced quickly. Despite certain catalysts showing promising characteristics, they cannot meet actual application demands because of the difficult preparation process and their unsatisfactory stability. Modifying the catalyst's structure and shape makes it possible to expose additional active sites, which helps improve the electrocatalytic reaction rate. Due to their distinct electronic structure and characteristics, TMCs, particularly TMNs, offer interesting applications in electrocatalytic water splitting.¹⁶¹ Several methods, including carbothermal reaction, chemical vapour deposition, metallothermic reduction, self-propagating high-temperature synthesis, and mechanical alloying, have been used to produce TMCs and TMNs.¹⁶² The most popular techniques for developing active TMCs and TMNs for electrocatalysis typically include carbothermal reaction and chemical vapour deposition. Novel methods also received significant attention due to the development of nanotechnology to produce nanostructures. Giordano et al.¹⁶³ used a soft urea glass process to manufacture metal carbides and nitrides. Compared to other synthesis methods, this method requires a lower reaction temperature, a faster reaction process, a more homogenous dispersion, and a larger surface area. Li et al.¹⁶⁴ introduced nitrogen-doped carbon nanofibers (NCNFs) that were effectively manufactured for use as a bifunctional electrocatalyst using a cost-effective and simple electrospinning approach, followed by a carefully regulated carbonization procedure. These NCNFs were subsequently combined with Ni and Mo₂C NPs, resulting in Ni/Mo₂C-NCNFs. The optimized Ni/Mo₂C (1 : 2)-NCNFs showed the best electrocatalytic performance for total water splitting in a 1.0 M KOH electrolyte among the several compositions examined. The overpotentials recorded for the HER and the OER at a current density of 10 mA cm⁻² were 143 mV and 288 mV, respectively. Kou et al.¹⁶⁵ reported a remarkable bifunctional catalytic performance for both the HER and the OER by efficiently anchoring Co single atoms onto the surface of a Mo₂C nanosheet (referred to as Co SAs/Mo₂C). Notably, compared to Co single atoms supported by N-doped carbon, the Co SAs/Mo₂C catalyst demonstrated a greater TOF per active site. This discovery implies that the single Co sites connected to the Mo₂C nanosheets have higher intrinsic catalytic activity, emphasizing the Co SAs/Mo₂C system's strong catalytic capabilities. Based on theoretical calculations, the author hypothesized that the Co-Mo₃ coordination is accountable for the noticeably improved intrinsic catalytic activity and that metal carbides have a greater application potential than the extensively researched carbon-based substrates. In another work, Rabi et al.¹⁶⁶ demonstrated the electrocatalytic ability of a hybrid heterostructure of molybdenum carbide (Mo₂C) and ZIF-67 produced by high-temperature annealing. This bifunctional electrocatalyst demonstrated exceptional water-splitting performance with high activity for both HER and OER processes. The hybrid material MCZ-2, composed of ZIF-67 and Mo₂C in a ratio of 1 : 2, exhibited enhanced catalytic activity for both the HER and OER. At a current density of 10 mA cm⁻², MCZ-2 demonstrated remarkably low overpotentials of 169 mV and 208 mV for the HER, while for the OER, the overpotentials were 340 mV and 370 mV at current densities of 10 mA cm⁻² and 50 mA cm⁻², respectively. Furthermore, MCZ-2 displayed excellent stability over 24 hours at a current density of 100 A cm⁻². The improved HER and OER performances of the MCZ-2 hybrid can be attributed to the synergistic interaction between ZIF-67 and Mo₂C and the modification of active sites due to the presence of N and Co in the hybrid structure. Chen et al.¹⁵⁷ successfully synthesized a highly efficient bifunctional electrocatalyst, Co₆W₆C@NC, which consists of vertically aligned porous cobalt tungsten carbide nanosheets embedded in an N-doped carbon matrix, using a MOF-derived method (as depicted in Fig. 9g and h). The interaction of Co and W atoms in the Co₆W₆C@NC system changes the electronic state of the carbide synergistically, resulting in optimum hydrogen-binding energy. Consequently, Co₆W₆C@NC performs better in the HER, with a low overpotential of 59 mV at a current density of 10 mA cm⁻². Furthermore, during the OER, Co₆W₆C@NC is converted in situ into tungsten-activated cobalt oxide/hydroxide, which acts as an active species for the OER with a 286 mV overpotential at a current density of 10 mA cm⁻². The Co₆W₆C@NC electrocatalyst has a low cell voltage of 1.585 V at 10 mA cm⁻² and displays high alkaline stability (Fig. 9i). A solid-state co-reduction approach was used to manufacture the Ni/Mo₂C@NC electrocatalyst, involving the synthesis of hybrid nanoparticles composed of multiple-interfacial nickel (Ni) and molybdenum carbide (Mo₂C). Following this, the nanoparticles were embedded in N-doped carbon nanosheets (Fig. 10a).¹⁶⁷ Because of its high conductivity, numerous interface active sites, and the synergistic interaction of Ni and Mo₂C nanoparticles (Fig. 10b), the electrocatalyst exhibits extraordinary activity in the HER. In an alkaline solution, it has an extremely low overpotential of 91 mV at a current density of 10 mA cm⁻² and a Tafel slope of 74 mV dec⁻¹. Furthermore, the catalyst is very stable, making it an ideal option for long-term use in electrolysis cells. It acts as a bifunctional electrode with durability and dependability, needing an applied voltage of 1.64 V at a current density of 10 mA cm⁻² (Fig. 10c–e).

Fig. 10 (a) Schematic illustration of the synthesis process of the Ni/Mo₂C@NC hybrid. (b) HRTEM image of Ni/Mo₂C@NC. (c) Overall water splitting by Ni/Mo₂C@NC. (d) Illustration of the reaction mechanism of the water splitting reaction on Ni/Mo₂C@NC. (e) Stability test of Ni/Mo₂C@NC.¹⁶⁷ (f) and (g) SEM images of Fe_xNi_1−xB/SSG. Polarization curves of Fe_0.2Ni_0.8B/SSG for the (h) OER and (i) HER.¹⁶⁸ All the corresponding images have been reproduced with permission from Elsevier © Copyright.

TMBs are of great interest due to their diverse compositions, environmentally benign nature, and excellent bifunctional catalytic performances facilitated by the unique electronic effect between the transition metal (M) atom and B atom.¹⁶⁹ The main research elements of TMBs as new electrocatalysts in recent years include Fe, Co, Ni and Mo. Chunduri et al.¹⁷⁰ developed a new hybrid Co–P–B electrocatalyst by employing a straightforward chemical reduction technique to combine the phosphide and boride for the water-splitting reactions. Under optimized conditions, the catalyst exhibited overpotentials of 145 mV and 290 mV, respectively, to achieve a current density of 10 mA cm⁻² in the HER and the OER. Hu et al.¹⁷¹ used 3D bark-like N-doped carbon (BNC) as a nanoreactor for water splitting to create metal borate Co–Bi and Ru-doped Co–Bi (Ru–Co–Bi) nanomeshes with a lot of nanosized pores. A low voltage of 1.53 V produced a current density of 10 mA cm⁻² when Co–Bi/BNC and Ru–Co–Bi/BNC were used as the anode and cathode materials, respectively. These materials outperformed the noble RuO₂/Pt/C counterpart (1.61 V). Li et al.¹⁷² used a straightforward one-pot NaBH₄ reduction process to construct a series of Co-based multi-metal boride nanochains by adding other metal elements (Ni and Fe) at room temperature for the overall water splitting. NiCoFeB nanochains with low overpotentials of 345 and 284 mV at a current density of 10 mA cm⁻² and Tafel slopes of 98 and 46 mV dec⁻¹, respectively, outperformed other Co-based metal borides under investigation in terms of electrocatalytic performances for the HER and OER. In a single step, Lao et al.¹⁶⁸ effectively created nickel boride (NiB) and iron-doped nickel boride (Fe_xNi_1−xB) by growing them on 3D self-supporting graphene (SSG) electrodes (Fig. 10f and g). The Fe_0.2Ni_0.8B/SSG electrode only needed a relatively small overpotential of 263 mV, which is the best OER activity achieved to date for a metal boride. The NiB/SSG electrode showed strong HER activity with average OER performance (Fig. 10h and i). The Fe_0.2Ni_0.8B/SSG and NiB/SSG water electrolysers produced a current density of 10 mA cm⁻² at a voltage of just 1.62 V. Furthermore, both the NiB/SSG and Fe_0.2Ni_0.8B/SSG catalysts demonstrated outstanding stability over a 14-hour testing period with no deactivation being noted. Doping electrocatalysts based on TMBs with heteroatoms is a sensible technique for regulating the local disorder and improving catalytic activity, just as for electrocatalysts based on other types of materials. In addition, TMB-based electrocatalysts might be grown on carbon materials via a strong interaction, which would increase both the electrical structure and the electrocatalytic performance of the material.

4.5. Transition metal chalcogenide-based electrocatalysts

Transition metal chalcogenides are compounds composed of transition metals linked to chalcogen atoms including sulfur (S), selenium (Se), tellurium (Te), and polonium (Po). Such materials display a variety of features such as semiconducting, magnetic, and catalytic behaviours. These materials have been used in a range of applications including electronics, catalysis, and energy storage.¹⁷³ Moreover, TM chalcogenides include a wide range of compounds with different stoichiometries that go beyond having just two chalcogen atoms in every transition metal.¹⁷⁴ This category may consist of ternary or more complex compounds. They often demonstrate a variety of characteristics and uses that extend beyond those associated with the dichalcogenides. TMDs are compounds consisting of a transition metal atom bound to precisely two chalcogen atoms. These materials consist of layered structures held together by robust covalent bonds within the layers and less strong van der Waals forces between the layers.¹⁷⁵ Specifically, TMDs are of great interest because of their distinctive electrical and optical characteristics, particularly when in a 2D state. Examples include MoS₂, WS₂, and their analogues, as mentioned in the next section.

Furthermore, TMSs have gained significant attention and have been widely explored as a highly promising group of bifunctional catalysts for efficient and durable overall water splitting, primarily due to their superior performance and stability. TMSs have a higher intrinsic electroconductivity than metal oxides.¹⁷⁶ Therefore, various synthesis techniques, such as chemical vapour deposition, solid phase synthesis, template approach, solvothermal method, and others, have been employed to design sulphide heterojunctions. The solvothermal approach is the most popular synthetic technique due to its simplicity in adjusting heterojunction particle size, shape, and crystal plane exposure. Sulphide structures change depending on the coordination mode of the metal atom.⁴⁰ There are currently two basic methods for boosting the performance of transition metal sulphide catalysts: speeding up electron transport to increase conductivity and maximizing the number of active sites to enhance the electrocatalytic activity. For example, in a straightforward solvothermal procedure, Li et al.¹⁷⁷ synthesized N-doped carbon layer-coated NiCo₂S₄ hollow nanotubes (NCT–NiCo₂S₄) using polyacrylonitrile (PAN) as the template. Three-dimensional hollow nanotubes with uniform N-doping layers (Fig. 11a–j) were designed to optimize catalytically active sites, promote mass transfer, avoid corrosion from the electrolyte, and improve activity and stability (Fig. 11k and l). The findings show that the efficient NCT–NiCo₂S₄ catalyst demonstrated low overpotentials of 295 and 330 mV to deliver a current density of 10 mA cm⁻² for the HER and OER, respectively, and exhibited outstanding stability. The low Tafel slope also reflected the rapid HER and OER electrocatalytic reactions. Ma et al.¹⁷⁸ developed a straightforward, multi-step method for preparing a porous CuCo₂S₄/NiCo₂S₄ core–shell material as a bifunctional electrocatalyst for OWS (Fig. 11m). The porous NiCo₂S₄-sheets and CuCo₂S₄ rods on NF can also maintain their structural integrity and offer more active sites and a larger specific surface area (Fig. 11n and o). The CuCo₂S₄/NiCo₂S₄ core–shell electrode showed reduced overpotentials of 271 and 206 mV for the OER and HER at a current density of 10 mA cm⁻², respectively, as shown in Fig. 11p and q. Kong et al.¹⁷⁹ developed novel 3D hierarchical NiMo₃S₄ nanoflowers (NiMo₃S₄/CTs) with many defects and reactive sites produced directly on carbon textiles. By doping Ni²⁺ in the Mo–S crystal lattice to create defect-rich NiMo₃S₄ nanoflakes, it was possible to increase their electrocatalytic activity for the HER and OER under alkaline conditions. Small overpotentials of 149.5 and 126.2 mV for the HER and OER at 10 mA cm⁻² are exhibited by the self-supported NiMo₃S₄/CT electrode, respectively. Wang et al.¹⁸⁰ used a unique two-step hydrothermal technique involving hydrazine monohydrate and thiourea treatment to create a new form of heterostructure Co₃S₄/Fe₃S₄ (referred to as CoFe-HM-T). This heterostructure was created using CoFe LDHs and exhibited novel characteristics. The CoFe-HM-T electrode demonstrated outstanding catalytic activity with overpotentials of 320 and 200 mV for the OER and HER at a current density of 100 mA cm⁻², respectively. CoFe-HM-T also had the lowest Tafel slopes, which were calculated to be 38.1 and 144.9 mV dec⁻¹ for the OER and HER, respectively. In another work, Liu et al.¹⁸¹ successfully synthesized the Fe-doped Ni_0.85Se (Fe_xNi_0.85−xSe, x = 0.02, 0.04, 0.05, 0.06, 0.08, 0.1, 0.15, 0.2, 0.3 and 0.4) catalysts by a one-step hydrothermal method and investigated them as highly active bifunctional catalysts for the OER and HER in electrocatalytic water splitting. The findings suggest that sufficient doping can enhance catalytic performance by creating a synergistic interaction between various metal ions. Adding CNTs to create the Fe_0.08Ni_0.77Se/CNT composite significantly improves the electrocatalytic activity due to the high electronic conductivity of the carbon nanotube network. In comparison to other composites for the OER and HER, the resulting Fe_0.08Ni_0.77Se/CNT₃ composite demonstrates reduced overpotentials of 204 and 108 mV at a current density of 10 mA cm⁻² and low Tafel slopes of 58 and 59 mV dec⁻¹, respectively. Similar to TMSs, TMSes have drawn much interest in the last ten years. Selenium (in the fourth period) and sulphur (in the third period) have the same valence electrons. However, in contrast to S, Se exhibits distinct chemical properties due to various periods: Se differs from S in several ways, including its slightly bigger atomic radius, higher metallicity, improved conductivity, and lower ionization energy.¹⁸² These factors proved that Se showed fascinating activities for the HER and OER. Wang et al.¹⁸³ constructed stacked flower-like NiTe–NiSe nanosheets using a simple hydrothermal technique, bringing the usage of transition metal tellurides in clean and renewable energy closer (Fig. 12a and b). The optimized electrode material has an active surface area 20 times larger than single NiTe/NFF or NiSe/NFF, which results in low overpotentials of 164 and 76 mV for the OER and HER, delivering a current density of 10 mA cm⁻². These results are also in good correlation with Gibbs free energy DFT calculations (Fig. 12c and d). Zhou et al.¹⁸⁴ used a two-step hydrothermal approach to create a Co, Fe co-doped Ni₃Se₄ nano-flake array (Ni_0.62Co_0.35Fe_0.03)₃Se₄ on conductive CF. Co, Fe co-doping can significantly increase the hydrogen evolution activity of Ni₃Se₄ in acidic and alkaline environments. This study revealed the overpotentials of 87 and 53.9 mV for the HER and OER, respectively, and the computed Tafel slopes of (Ni_0.62Co_0.35Fe_0.03)₃Se₄/CC were 122.6 and 262 mV dec⁻¹ at a current density of 10 mA cm⁻². Wang et al.¹⁸⁵ described a simple method for creating a selenide/sulfide hetero-structured NiCo₂Se₄/NiCoS₄ catalyst for the HER and OER (Fig. 12e–h). The NiCo₂Se₄/NiCoS₄ heterostructure demonstrated outstanding OER and HER performances, as evidenced by low overpotentials of 248 mV and 180 mV at 10 mA cm⁻² for the OER and HER, respectively, and superior long-term stability to the benchmark IrO₂ and Pt/C catalysts for the OER and HER as shown in Fig. 12i and j. Pan et al.¹⁸⁶ reported a simple and efficient polymerization–pyrolysis–selenization (PPS) approach for the on-site manufacture of nitrogen-doped carbon nanosnakes (NCNSs) containing iron-doped cobalt selenide NPs as depicted in Fig. 12k. These NPs are made of trimetallic networks of zinc/iron/cobalt polyphthalocyanine coupled polymers (Fig. 12l–r). The FeCoSe@NCNS catalyst showed good electrocatalytic activity for the HER with minimal overpotentials (142 and 99 mV in 0.5 M H₂SO₄ and 1.0 M KOH) due to the synergistic effect regulated by Fe atoms and CoSe NPs and the confinement effect of the in situ generated porous conductive carbon nanosnakes for the OER (320 mV in 1.0 M KOH) at a current density of 10 mA cm⁻².

Fig. 11 (a) and (b) TEM images, (c) SEM images, and (d) EDS mapping images of NCT-NiCo₂S₄. XPS profiles of NCT-NiCo₂S₄ for (e) survey, (f) Ni 2p, (g) Co 2p, (h) S 2p, (i) C 1s, (j) N 1s. (k) Overall water splitting by NCT-NiCo₂S₄. (l) Stability test at 10 mA cm⁻² for NCT-NiCo₂S₄.¹⁷⁷ (m) Schematic diagram illustrating the synthesis process of CuCo₂S₄/NiCo₂S₄ electrodes directly grown on NF. SEM images of (n) CuCo₂S₄ and (o) CuCo₂S₄/NiCo₂S₄. Polarization curves of CuCo₂S₄/NiCo₂S₄ for the (p) OER and (q) HER.¹⁷⁸ All the corresponding images have been reproduced with permission from Elsevier © Copyright.

Fig. 12 (a) and (b) SEM images of NiTe–NiSe nanosheets. DFT calculation of free energy for the (c) HER and (d) OER.¹⁸³ (e) The illustration of the synthetic method for NiCo₂Se₄/NiCoS₄. SEM images of (f) NiCo₂Se₄, (g) NiCoS₄, and (h) NiCo₂Se₄/NiCoS₄. The polarization curves for the (i) HER and OER and (j) overall water splitting by NiCo₂Se₄/NiCoS₄.¹⁸⁵ (k) The schematic illustration of synthesis of NCNS encapsulated Fe-doped CoSe NPs (FeCoSe@NCNSs). XANES of (l) Co K-edge and (n) Fe K-edge. The fitted average oxidation states of (m) Co and (o) Fe from XANES spectra. k3-weighted FT-EXAFS spectra of (p) Co and (q) Fe at R spaces. (r) Wavelet transform contour plots of Co and Fe samples and reference samples.¹⁸⁶ All the corresponding images have been reproduced with permission from Elsevier © Copyright.

Gong et al.¹⁸⁷ created a CoSe/Co(OH)₂ carbonaceous material (CoSe/Co(OH)₂-CM (AE)) by acid etching and in situ selenization. CoSe/Co(OH)₂-CM (AE) demonstrates outstanding electrocatalytic activity in the 1.0 M KOH electrolyte, with low overpotentials of 299 mV for the OER and 207 mV for the HER at a current density of 10 mA cm⁻².

4.5.1. Transition metal dichalcogenide-based electrocatalysts. A significant class of inorganic materials known as TMDCs exhibit various catalytic, magnetic, electrical, and optical properties. Researchers have widely studied the amazing characteristics of this family of materials. As the name TMDCs suggests they consist of TMs (Mo and W) combined with chalcogenides (S and Se). A TMD like MoS₂ has the chemical formula MX₂, where M and X are the transition metal and the chalcogen, respectively. Due to the weak van der Waals connections between neighbouring layers, bulk TMDs quickly exfoliate into 2D flakes.¹⁸⁸ The chalcogen atoms are arranged in two hexagonal planes of the monolayer TMD's sandwich-like X–M–X structure, which is divided by a plane of metal atoms. Between the two chalcogen atom layers is the layer of transition metal atoms. Strong covalent bonds hold together the atoms of one layer, whereas weak van der Waals bonds attract two different layers, allowing these separate sheets to be segregated. The TMDCs are stable and exist as trigonal (1T), hexagonal (2H), and rhombohedral (3R) structures with various stacking configurations.¹⁸⁹ A variety of techniques are employed to synthesize TMDCs. These techniques can be broadly categorized into top-down and bottom-up approaches. The type of material, application, and simplicity of the synthesis process are the key determinants of method selection. The layered 2D TMDCs display several desirable characteristics, ranging from high surface area and electrical conductivity to a heterogeneously fast transfer of electrons (depending on composition and layer thickness), which can be found in semimetals, semiconductors, true metals, and superconductors.⁷¹ Due to their high binding energies, robust photoluminescence, and variable bandgaps, TMDCs are excellent materials for several applications due to their distinct characteristics, such as optoelectronic devices, photodetectors, LEDs, phototransistors, solar cells, and photocleavable monomers.¹⁹⁰ For instance, Zhu et al.¹⁹¹ described the creation of CoS₂C@MoS₂ core–shell nanofibers by a one-step hydrothermal reaction and a sulfurization procedure using electrospun Co-carbon nanofibers as templates. The resulting CoS₂C@MoS₂ core–shell nanofibers exhibit outstanding electrocatalytic activity and stability towards the HER and OER, taking advantage of the distinctive structure and the synergistic impact of the components. Sun et al.¹⁹² created a special WS₂/WSe₂ flower-like heterostructure by using heterojunction engineering (Fig. 13a). Cobalt diselenide laminated with molybdenum diselenide (MOF-CoSe₂@MoSe₂), a novel core–shell structure, was synthesized by Patil et al.¹⁹³ Its effectiveness as a bifunctional electrocatalyst for the HER and OER in alkaline media was evaluated. The CC/MOF-CoSe₂@MoSe₂ core–shell structure showed low overpotentials (η₁₀) of 109.87 and 183.81 mV for the HER and OER, respectively. It was made directly on a flexible carbon cloth substrate. For the first time, Lv et al.¹⁹⁴ reported the use of hybrid CoSe₂/WSe₂/WO₃ nanowires (NWs) on CC as a very effective HER catalyst, as depicted in Fig. 13b. The developed CoSe₂/WSe₂/WO₃ NWs/CC catalyst showed superior catalytic characteristics against the HER with a low onset potential and Tafel slope. Unlike WSe₂/WO₃ NWs/CC, CoSe₂/WSe₂/WO₃ NWs/CC only needs a 115 mV overpotential to reach a current density of 10 mA cm⁻². The compositional and structural benefits of the WS₂/WSe₂ heterojunction are combined in this multiscale morphological control design into a hierarchical architecture, which may alter the electronic structure, dramatically facilitating the exposure of more electrochemical active spots. As expected, the hetero-structured WS₂/WSe₂ catalyst exhibits exceptional HER performance with a small Tafel slope of 74.08 mV dec⁻¹ and a low overpotential of 121 mV at 10 mA cm⁻². Zou et al.¹⁹⁵ have created evenly distributed triangular WSe₂ utilizing electrospun carbon nanofiber mats as the substrate and a CVD reaction that functioned as the selenation process. WSe₂ was created under high-temperature conditions with added Ar flow. As a result, the obtained sample exhibited low overpotential for the HER. Recently, Gao et al.¹⁹⁶ fabricated MoSe₂ with various shapes (nanoplates, nanosheets, and microparticles) at various temperatures (880 °C, 940 °C, and 980 °C, respectively). Because of this, the MoSe₂ nanosheets demonstrated exceptional electrochemical characteristics like minimal overpotential and long-term stability for the HER. Huang et al.¹⁹⁷ successfully constructed the NiS/MoS₂ complex grown in situ on carbon paper (NiS/MoS₂/CP) through a single hydrothermal step. It displays relatively low overpotentials of 119 mV (at a current density of 10 mA cm⁻²) and 314 mV (at a current density of 100 mA cm⁻²) for the HER and the OER, respectively. The mixed metal dichalcogenide of WS₂–MoS₂ was electroless plated onto a Cu substrate for HER investigation.¹⁹⁸ Due to the surface-active sites linked to the combination of mixed dichalcogenides, the electrocatalytic activity of the WS₂-MoS₂/NiMoP coating increased, and a synergistic effect developed between the various elements in the coating system. Niyitang et al.¹⁹⁹ used a simple hydrothermal process to prepare MoS₂, which is then modified utilizing ambient solution plasma for bifunctional hydrogen and oxygen evolution processes (HER and OER). The synthesized MoS₂ displays low overpotentials with small Tafel slopes of 33 and 62 mV dec at 10 mA cm⁻², respectively. Gao et al.²⁰⁰ fabricated cobalt-doped NiS@MoS₂ core–shell nanorods (Co–NiS@MoS₂) as a bifunctional catalyst in a straightforward one-step hydrothermal process. The Co–NiS@MoS₂ catalyst showed low overpotentials at 50 mA cm⁻² for both the HER (139.9 mV) and OER (170.6 mV) in 1.0 M KOH solution. Li et al.²⁰¹ created a Cu film that supported an MoS₂-based electrocatalyst with a partial 1T phase and 3D architecture using inkjet printing (Fig. 13c–e). Results show that (i) nanosized few-layer MoS₂ that has been spatially designed by inkjet printing provides adequate active site exposure, (ii) the 1T-MoS₂ and RGO conductive network lowers the charge-transfer impedance, and (iii) the Cu support improves the catalyst-electrode charge injection. The corresponding electrocatalyst exhibits low overpotentials (51 mV at 10 mA cm⁻² and 126 mV at 100 mA cm⁻²) and a very low Tafel slope (32 mV dec⁻¹) for the HER, including high stability (over 5000 cycles) and TOF (Fig. 13f and g). Rai et al.²⁰² presented a quick and easy wet-chemical method to create many exposed edges on few-layered MoSe₂ and WSe₂ nanoflowers. In contrast to WSe₂, temperature-controlled reactions show that MoSe₂ can be synthesized more quickly. They created a hierarchical heterostructure of MoSe₂@WSe₂ using a one-step synthesis approach by taking advantage of the quicker kinetics of the synthesis of MoSe₂. Among the produced nanostructures, the hierarchical nanostructure has a higher electrocatalytic activity.²⁰³ The hierarchical nanostructure needs overpotentials of 231 mV (HER) and 300 mV (OER) to produce a current density of 10 mA cm⁻². Xiong et al.²⁰⁴ developed a method of cobalt covalent doping that can induce HER and OER bifunctionality in MoS₂ for effective overall water splitting. The findings show that covalent cobalt doping into MoS₂ can significantly increase HER activity while also causing outstanding OER activity. The catalyst can easily reach HER and OER onset potentials of 0.02 and 1.45 V (vs. RHE) in 1.0 m KOH when cobalt doping density is optimized. A brief comparative study has been carried out specifically in alkaline media to explore the materials with high electrochemical efficiency, as tabulated in Table 2.

Fig. 13 (a) Schematic illustration of the synthesis of the flower-like WS₂/WSe₂ heterostructure.¹⁹² (b) Scheme illustration of the synthesis of CoSe₂/WSe₂/WO₃ NWs/CC by a hydrothermal method and selenation treatment.¹⁹⁴ Atomic structures of (c) Cu-MoS₂ and (d) Cu-RGO-MoS₂. (e) Scheme illustration of the synthesis of MoS₂/PVP/RGO. (f) Overpotential and Tafel slope of MoS₂/PVP/RGO compared with literature values. (g) Illustration of the HER process of MoS₂ nanostructures surrounded by RGO on Cu film.²⁰¹ The corresponding images have been reproduced with permission from Elsevier © Copyright.

Table 2 TM-based electrocatalysts for the HER and OER in an alkaline medium

Catalyst	Reaction type	Electrolyte	Overpotential (mV)	Tafel slope (mV dec^−1)	Current density (mA cm^−2)	Ref.
Noble metal-based electrocatalysts
Au33Pt67	HER	1.0 M KOH	171	81	10	205
PtCo	HER	1.0 M KOH	76.2	76	20	206
O-Pt3Co-NWs	HER	1.0 M KOH	56.1	30	—	207
Pt24Cu76 NFs	HER	0.5 M KOH	18	52	10	208
CoFe–Pt1%	HER	1.0 M KOH	18	29	10	209
Pd3Ru	HER	1.0 M KOH	42	—	10	210
IrFe@NC	HER	1.0 M KOH	850	30	1000	211
RuNi	HER	1.0 M KOH	15	28	10	212
PtRu NCs/BP	HER	1.0 M KOH	22	19	10	213
Pt–NiFe LDH/CC	HER	1.0 M KOH	28	39	10	214
Pt/Ni-MOF	HER	1.0 M KOH	25	42.1	10	215
Ru@MWCNT	HER	1.0 M KOH	17	27	10	216
PdO@Co2FeO4	HER	1.0 M KOH	269	49	10	116
PdO@Co2FeO4	OER	1.0 M KOH	259	59	20	116
PdO@CoSe2	OER	1.0 M KOH	260	57	20	109
Pd NPs/Co3O4	OER	1.0 M KOH	250	58	20	116
RuCo@NC	OER	1.0 M KOH	25	—	10	217
Ru-0/CeO2	OER	1.0 M KOH	420	—	10	218
RuNi	OER	1.0 M KOH	327	—	10	219
Ru–Ni SNs	OER	1.0 M KOH	1.45 Onset	—	—	220
Ir–O–Co	OER	1.0 M KOH	178	—	10	221
Ir-NR/C	OER	1.0 M KOH	296	—	10	222
Transition metal oxide-based electrocatalysts
RuO2–Fe2O3/HrGO NSs	HER	1.0 M KOH	239	97	10	223
Ni3ZnC0.7/NCNT-700	HER	1.0 M KOH	203	91	10	224
Ru/RuO2@N-rGO	HER	1.0 M KOH	11	44.2	10	225
Pt2@Ni-rGO	HER	1.0 M KOH	∼200	327.9	—	226
Pd30Ni70/C	HER	1.0 M KOH	393	291	—	227
Pt–Ni (1 : 1)/rGO	HER	1.0 M KOH	147	72	—	228
LaCoO3@rGO	OER	1.0 M KOH	280	104	10	229
MnO2@Co3O4	OER	1.0 M KOH	310	72	20	230
Co3O4–CoS2	OER	1.0 M KOH	280	74	20	231
Cu0.19Mo0.19/Co0.62O	OER	1.0 M KOH	250	61	10	232
NiCo2O4/PGR	OER	1.0 M KOH	234	110	100	233
Co3O4/PGR	OER	1.0 M KOH	242	132	100	233
NiO/PGR	OER	1.0 M KOH	272	115	100	233
CuCo2O4/NrGO	OER	1.0 M KOH	360	64	10	234
Co@CoO-PNC/CC	OER	1.0 M KOH	289	83	10	235
Co3O4/NrGO	OER	0.5 M KOH	270	190	—	236
Transition metal phosphide-based electrocatalysts
Au/CoP@NC-3	HER	1.0 M KOH	140.9	—	10	237
Co2P	HER	1.0 M KOH	280	—	10	238
CoP/NiCoP NTs	HER	1.0 M KOH	39	—	—	239
O-CoP	HER	1.0 M KOH	98, 310	—	10	240
CoP–InNC@CNT	HER	1.0 M KOH	270	—	—	193
Fe–CoP/CoO	OER	1.0 M KOH	219	—	10	241
Mo2C–MoP/NPC	OER	1.0 M KOH	320	44.7	10	242
Ce–Co (MoP)/MoP@C	OER	1.0 M KOH	287	74.4	10	243
Co–MoP@NCNNS-600	OER	1.0 M KOH	270	60.3	10	244
MoP@Ni3P/NF	OER	1.0 M KOH	331	50	10	245
Transition metal nitride, carbide and boride-based electrocatalysts
Mo2C/N–C	HER	1.0 M KOH	189	58	—	246
W2N/W	HER	1.0 M KOH	148.5	47.4	10	247
Co–NC@Mo2C	HER	1.0 M KOH	99	65	10	248
Ni3Mo3C@NPC NWs/CC	HER	1.0 M KOH	215	74.8	100	249
MoNix–MoCx@NC	HER	1.0 M KOH	168	71	10	250
Co–TiN@NG-x/CC	HER	1.0 M KOH	208	170	10	251
Ni3ZnC0.7/NCNT	HER	1/0.1 M KOH	203/380	91/45	—	224
Ni/MoN@NCNT/CC	HER	1.0 M KOH	207	93	10	252
Co4N@NC	HER	1.0 M KOH	62	37	10	253
Co–NC@Mo2C	OER	1.0 M KOH	347	61	10	248
W2N/WC	OER	1.0 M KOH	320	94.5	10	247
2D-NC Fe2Ni2N/rGO NHSs	OER	0.1 M KOH	290	49.1	10	254
Co6W6C@NC/CC	OER	1.0 M KOH	220	53.96	10	255
Ni/Ni0.2Mo0.8N@N–C	OER	1.0 M KOH	260	46	10	256
Co5.47N@N-rGO	OER	1.0 M KOH	143	80	—	257
Ni/MoN@NCNT/CC	OER	1.0 M KOH	252	91.8	10	252
Co4N@NC	OER	1.0 M KOH	257	58	10	253
Transition metal chalcogenide/dichalcogenide-based electrocatalysts
MoS2/NiSe2/rGO	HER	1.0 M KOH	127	73	10	258
MnSxSe1−x@N,F-CQDs	HER	1.0 M KOH	129	78.19	10	259
CSCB	HER	1.0 M KOH	150	250	35	260
Fe7Se8/CNT/C	HER	1.0 M KOH	138	114	10	261
MoSe2/NG-4	HER	1.0 M KOH	120	69	10	262
FeSe2@CoSe2/rGO-2	OER	1.0 M KOH	260	36.3	10	263
FeNi2Se4–NrGO	OER	1.0 M KOH	170	62.1	10	264
Co0.85Se/rGO	OER	1.0 M KOH	274	62.4	100	265
MoS2/NiSe2/rGO	OER	1.0 M KOH	277	107	20	258
Ni0.85Se/rGO	OER	1.0 M KOH	280	40.2	100	266
MoS2–NiS2/NGF	HER	1.0 M KOH	172	70	10	267
Ni1.5Co0.5@N–C NT/NF	HER	1.0 M KOH	114	117	10	268
NPZFNS@C/NF	HER	1.0 M KOH	129	66.8	10	269
1TNi0.2Mo0.8S1.8P0.2NS/CC	HER	1.0 M KOH	55	61.5	10	270
NiFeOx(OH)y@MoS2/rGO	HER	1.0 M KOH	170	80	10	271
CoNi2S4–CNFs	HER	1.0 M KOH	228	42.1	20	272
FeNiS–MWCNTs	OER	1.0 M KOH	260	50	10	273
Fe–Co9S8@SNC	OER	1.0 M KOH	273	55.8	10	274
CoSx@Cu2MoS4–	OER	1.0 M KOH	351.4	41.1	10	275
NiS2–MoS2/rGO/N	OER	1.0 M KOH	210	58	10	276
MoS2–NiS2/NGF	OER	1.0 M KOH	370	13.7	10	267
Ni1.5Co0.5@N–C NT/NF	OER	1.0 M KOH	243	103	10	268

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Bifunctional electrocatalysts
Catalyst	Electrolyte	Overpotential HER	Overpotential OER	Current density (mA cm^−2)	Ref.
CoFe-LDH/rGO/NF	1.0 M KOH	110 mV	250 mV	10	277
CoNiN@NiFe LDH	1.0 M KOH	150 mV	227 mV	10	278
NiTe@CoFe-LDH	1.0 M KOH	103 mV	218 mV	10	279
Mo2C@g-C3N4@NiMn-LDH	1.0 M KOH	116 mV	290 mV	10	280
Ti3C2–CoS2	—	276 mV	376 mV	10	281
Mo–CoP	1.0 M KOH	112 mV	330 mV	100	282
Ni-MOF	1.0 M KOH	330 mV	366 mV	10	283
rGO/Ni3Se2/NF	1.0 M KOH	251 mV	292 mV	10	284
P-30-doped Fe/NF	1.0 M KOH	158 mV	284 mV	10	285
Co-CoO@3DHPG	—	260 mV	1.59 V	10	286
2D MoS2	1.0 M KOH	248 mV	300 mV	10	287
NiCo nanospheres	—	105 mV	330 mV	10	288

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Additionally, MOFs are crystalline materials that can be 1D, 2D, or 3D. They are produced by coordinating metal ions or clusters with rigid organic molecules.²⁸⁹ They possess high crystallinity, mesoporous or microporous structures, and strong metal–ligand interactions. The coordination bond between the metal and the organic linker is stronger than the hydrogen bond, resulting in increased stability of the produced MOFs.²⁹⁰ MOFs often possess high specific surface areas because of their well-organized crystal structures, enabling many active sites necessary for catalytic reactions and enhancing the turnover frequencies (TOFs).²⁹¹ Li et al.²⁹² selected a neutral N-donor heterocyclic ligand having unsaturated coordination sites to capture MnO₄⁻ and CrO₄²⁻via the anion exchange technique in pristine MOFs. The Cr@NiCP and Mn@NiCP MOFs, following anion exchange, showed superior HER performance compared to the NiCP MOFs. Unlike classical metal substitution reactions, the adsorption of metal anions through organic ligands may provide more metal active sites, leading to improved synergistic effects and performance in the HER process. Moreover, Ravi Nivethe et al.²⁹³ used a customised hydrothermal technique to produce Fe-based MIL-100 MOFs that are extremely porous, having a BET surface area of 2551 m² g⁻¹. Pristine MOFs feature several scattered metal sites; however, they are not suitable as effective HER active sites because of their saturated coordination.²⁹⁴

5. Challenges faced by transition metals

TM-based electrocatalysts have some key drawbacks, which need to be addressed effectively to make them competitive catalysts compared to noble metal-based electrocatalysts. The intrinsic activity of any material is the most notable and significant factor. For example, Pt and Ir/Ru are excellent catalysts for the HER and OER and highly selective in converting water into hydrogen at the cathodic and oxygen at the anodic applied potential. Durability in practical operation is another crucial aspect that should be considered for TMs to catalyze a reaction for a long time at high electrochemical reaction rates without decreasing their electrocatalytic performance. It should be noted that for electrolysis to be deemed optimal, electrocatalytic oxidation (or corrosion) must be suppressed. A catalyst is more stable when it operates at a continuously applied current density (ideally at current densities >500 mA cm⁻²) without performance degradation or even under conditions where the current density is reversed during electrolysis.

In water electrolysis devices, noble metal-based catalysts in the form of NPs are frequently placed on conductive supports to avoid stability issues like peeling, deactivation, dissolution, sintering, or poisoning. The ideal catalyst support exhibits exceptional thermal and chemical stability, mechanical strength, and low surface energy.⁹² These properties can significantly increase the electrochemically active surface area and catalyst efficiency. TMOs face a number of challenges, including intrinsic poor conductivity, slow reaction kinetics, a lack of adsorption sites for H intermediates, and an inappropriate H-binding energy. One of the biggest challenges faced by TMPs is surface oxidation, which happens when the materials are kept in ambient settings. While the OER performance of TMPs may not be impacted by partial oxidation, the oxide formed in situ may reduce conductivity and obstruct HER active areas.²⁹⁵ The slow OER process observed for TMNs, TMCs and TMBs is a significant barrier reducing their use in overall water splitting.^160,296 The lack of sufficient surface-active sites is the primary drawback of TM chalcogenides. Numerous approaches have been tried to enrich reactive sites. Initially, considering architecture, a hierarchical porous linked structure could be able to balance mass diffusion pathways, exposed active surfaces, and charge transfer. The proposed HER and OER reaction mechanisms described in the previous literature are still unclear. Thus, to gain more information regarding the dynamic variations in the HER/OER process further systematically, it is important to develop consistent multi-scale fundamental and theoretical measurement models to define analytically the dynamic reaction involvement of the whole catalyst surface, which could lead to the fabrication of high-efficiency electrocatalysts.

6. Conclusions and future perspectives

Development of low-cost, high-efficiency electrocatalysts for energy transformation and renewable energy storage through the electrochemical process is a key research area. To sum up, we have given an overview of the most recent developments in TM-based electrocatalysts for the HER and OER. TM-based electrocatalysts have emerged as promising candidates for catalysing water splitting owing to their unique physiochemical properties and modulated compositions and structures. It has been demonstrated that strategies like surface engineering, combining with other TMs, and even heteroatom dopant incorporation are effective ways to improve the electrocatalytic performance of TM catalysts and provide more electrochemically active surface area to make them industrially effective. More prominent and stable TM-based catalysts will be developed due to a thorough understanding of the reaction mechanisms and the actual reaction active centres, accelerating the development of water electrolysis as an industrial process. Finally, combining theoretical calculations with experimental results can help us find and create more effective and low cost catalysts, given the rapidly developing field of computational chemistry. Theoretical calculations could guide us in choosing and researching appropriate electrocatalytic materials in addition to assisting us in understanding the mechanism. In the realm of electrocatalysis, enhancing the performance and applicability of TM-based electrocatalysts involves a multifaceted approach. Catalyst design and engineering methods, including nano-structuring, alloying, and surface modification, have emerged as pivotal strategies to enhance activity and stability, particularly in alkaline media where avoiding sluggish behaviour is imperative. Exploring novel TM composition configurations and introducing non-metal elements are other strategies for enhancing electrocatalytic performance. Achieving an atomic scale understanding of catalytic processes becomes paramount, guiding the rational design of electrocatalysts with comprehensive improvements. Diversifying materials and architectures, such as metal oxides, nitrides, phosphides, and organic compounds, alongside innovative catalyst topologies like hierarchical structures and core–shell configurations, presents novel opportunities for heightened catalytic performance. Effectively addressing the stability and conductivity of electrocatalysts involves the development of support materials, a crucial aspect in optimizing their functionality. The scalability and cost-effective synthesis of these electrocatalysts are critical for their commercial viability and broader application, especially in alkaline environments. Furthermore, integrating transition metal-based electrocatalysts with energy storage systems, such as water electrolyzers and fuel cells, holds promise for creating sustainable and efficient energy conversion technologies. Amidst these advancements, a critical perspective involves considering the environmental impact and sustainability of electrocatalysts, encompassing raw material extraction, synthesis, and end-of-life disposal. Scheme 2 presents a schematic illustration that depicts future perspectives on TM-based electrocatalysts for the HER and OER.

Scheme 2 Schematic illustration reflecting future perspectives on TM-based electrocatalysts toward the HER and OER.

Author contributions

Muhammad Nazim Lakhan and Abdul Hanan: conceptualization, recent progress analysis, writing – reviewing draft, resources; Altaf Hussain: writing – reviewing and editing of text related to factors affecting the hydrogen evolution reaction and the oxygen evolution reaction in alkaline media; Irfan Ali Soomro: writing – reviewing and editing sections dealing with challenges and future perspectives; Yuan Wang: writing – reviewing and editing, supervision, project administration; Mukhtiar Ahmed: writing – reviewing and editing the section dealing with transition metal oxides, formal analysis; Umair Aftab: writing – reviewing and editing, project administration, supervision; Hongyu Sun: writing – reviewing and editing, project administration; and Hamidreza Arandiyan: writing – reviewing and editing, project administration, supervision.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the support from the DCCEEW International Clean Innovation Researcher Networks Grant (ICIRN000011). Y. Wang acknowledges the International Hydrogen Research Fellowship Program financially supported by CSIRO, DCCEEW, the Australian Hydrogen Research Network (AHRN), ARC DECRA (DE230100327), and ARC Linkage Project (LP220200583).

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Footnote

† These authors contributed equally to this work.

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