Transition Metal Dichalcogenides in Electrocatalytic Water Splitting

Abstract

Two-dimensional transition metal dichalcogenides (TMDs), also known as MX₂, have attracted considerable attention due to their structure analogous to graphene and unique properties. With superior electronic characteristics, tunable bandgaps, and an ultra-thin two-dimensional structure, they are positioned as significant contenders in advancing electrocatalytic technologies. This article provides a comprehensive review of the research progress of two-dimensional TMDs in the field of electrocatalytic water splitting. Based on their fundamental properties and the principles of electrocatalysis, strategies to enhance their electrocatalytic performance through layer control, doping, and interface engineering are discussed in detail. Specifically, this review delves into the basic structure, properties, reaction mechanisms, and measures to improve the catalytic performance of TMDs in electrocatalytic water splitting, including the creation of more active sites, doping, phase engineering, and the construction of heterojunctions. Research in these areas can provide a deeper understanding and guidance for the application of TMDs in the field of electrocatalytic water splitting, thereby promoting the development of related technologies and contributing to the solution of energy and environmental problems. TMDs hold great potential in electrocatalytic water splitting, and future research needs to further explore their catalytic mechanisms, develop new TMD materials, and optimize the performance of catalysts to achieve more efficient and sustainable energy conversion. Additionally, it is crucial to investigate the stability and durability of TMD catalysts during long-term reactions and to develop strategies to improve their longevity. Interdisciplinary cooperation will also bring new opportunities for TMD research, integrating the advantages of different fields to achieve the transition from basic research to practical application.

1. Introduction

The growing demand for energy and the increasing severity of environmental issues have prompted the pursuit of more sustainable and efficient energy conversion technologies [1,2,3,4,5]. Hydrogen energy, as a clean, efficient, occupies a crucial position in today’s world. The sole combustion product of hydrogen is water, without generating any pollutants or greenhouse gases, which is incomparable for addressing global climate change. Amidst the escalating atmospheric pollution from traditional energy combustion and the intensifying greenhouse effect, hydrogen energy offers us a green and sustainable path for development [6,7,8]. The competition for conventional fossil energy among countries is becoming increasingly fierce, and the instability of energy supply poses challenges for many nations. Hydrogen, with its widespread sources, can be produced through various methods such as water electrolysis, fossil fuel reforming, and biomass conversion [9]. This diversifies the energy options for countries, reduces reliance on single energy sources, and enhances the security and stability of energy supply. Particularly for nations with -relatively scarce energy resources, the development of hydrogen energy presents new opportunities. In the transportation sector, the potential of hydrogen energy is becoming increasingly evident. Hydrogen fuel cell vehicles, with their zero-emission characteristics, excellent energy conversion efficiency, and rapid hydrogen refueling speed, effectively alleviate the environmental challenges and energy consumption issues caused by traditional fuel vehicles [10]. The application of hydrogen energy is not limited to the automotive industry; it also extends to other transportation fields such as aviation and maritime transport, providing a solid foundation for the green transformation of transportation systems. In industry, hydrogen, as a clean high-temperature heat source, is beneficial for energy-intensive industries like steel and chemicals by reducing energy consumption and pollutant emissions, thus promoting the green transformation of industrial production. This is of significant importance for environmental protection [11]. Electrocatalytic water splitting stands out as a promising method to address environmental and energy concerns, capable of converting water into hydrogen and oxygen, thus providing a viable pathway for producing clean energy [12,13,14,15,16,17,18]. Compared to traditional hydrogen production techniques such as steam methane reforming and water-gas shift reaction, which are commonly used in industry, electrolytic hydrogen production does not result in carbon emissions [19,20]. It transforms surplus electrical energy into storable hydrogen, hence being regarded as an essential route for the preparation of clean, sustainable, green hydrogen [21]. Moreover, the purity of hydrogen produced via electrolysis is the highest, and the production conditions are relatively mild. However, the main challenge it faces is the requirement of a high overpotential, leading to excessive energy consumption and high costs for hydrogen production [22,23,24].

In the field of electrocatalytic water splitting, precious metal catalysts such as platinum (Pt) and iridium (Ir) have always been a hot topic of research due to their outstanding catalytic performance. These precious metals possess excellent electronic structures and surface properties, providing high catalytic activity and accelerating the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) during the water splitting process, thereby demonstrating excellent performance under laboratory conditions [25,26,27]. However, their scarcity and high cost limit their large-scale commercial applications. Particularly in industrial-scale water electrolysis, the use of these precious metals as catalysts significantly increases the cost of hydrogen production, restricting the widespread application of clean energy [28,29,30,31,32,33]. In contrast, two-dimensional transition metal dichalcogenides (TMDs), as a class of non-precious metal catalysts, are abundant and have lower production costs [34,35]. The structure of TMDs consists of two layers of chalcogen atoms sandwiching a layer of transition metal atoms, with layers interacting through weak van der Waals forces [36,37]. This layered structure allows TMDs to be easily exfoliated into ultra-thin nanosheets, greatly increasing the material’s specific surface area. An increase in specific surface area means more active sites are exposed on the surface, allowing for full contact with reactants, thereby improving the efficiency of catalytic reactions [38,39]. Therefore, when TMDs are used for electrocatalytic water splitting, their rich edge and surface active sites can effectively adsorb and activate water molecules, promoting the generation of hydrogen and oxygen. The layered structure also facilitates rapid electron transfer within the layers, and the interlayer channels are conducive to the diffusion of substances, thus enhancing the kinetic performance of the electrocatalytic reaction. This unique structural advantage gives TMDs broad application prospects in the field of electrocatalysis [40,41,42]. Researchers can precisely control their structure according to needs and catalytic reaction mechanisms, adjust the electronic structure and chemical properties of TMDs via selecting appropriate elements and controlling their ratios and combinations, and optimize catalytic activity. They can also improve catalytic performance via designing morphology and size to increase specific surface area and the degree of exposure of active sites, enabling TMDs to exhibit electrocatalytic performance comparable to that of precious metals [43,44,45,46,47].

In the quest for hydrogen and oxygen production through water electrolysis, TMDs demonstrate significant advantages over metal-based compounds containing phosphorus (P), sulfur (S), or selenium (Se). The electronic structure of TMDs can be finely tuned by adjusting the type and ratio of transition metals and chalcogen elements, providing ample room for optimizing the adsorption capacity for intermediates in the water-splitting reaction. Other metal-based compounds with P/S/Se typically lack flexibility and broadness in electronic structural adjustability compared to TMDs, which limits their potential for further enhancement in catalytic activity. The chemical stability of TMDs in air and aqueous environments is also a notable strength, as they are resistant to oxidation, maintaining long-term activity and stability during the electrocatalytic process. Moreover, TMDs retain high catalytic activity and stability under both acidic and alkaline conditions, making them adaptable to various water-splitting system configurations with a wide range of applicability. Chemical stability is a critical consideration, as catalysts must remain stable over extended operational periods to ensure the reliability and economic viability of electrolysis systems. Certain metal-based compounds containing P, S, or Se may face limitations in electrocatalytic water splitting. The catalytic activity of these materials can be significantly affected by pH levels, leading to optimal performance in specific acidic or basic environments. Some metal sulfides or selenides may exhibit greater activity under alkaline conditions but may degrade rapidly or become inactive in acidic environments. This pH dependency restricts the application of these compounds in diverse water-splitting systems, as actual water electrolysis processes may need to operate under varying pH conditions to meet specific industrial requirements or environmental contexts. In comparison, TMDs generally exhibit better chemical stability and a broader pH tolerance. TMDs maintain relatively high catalytic activity across a range of pH environments, giving them an edge in various water-splitting applications. Additionally, the layered structure of TMDs helps to preserve structural integrity under different pH conditions, reducing the risk of chemical degradation. In contrast, not all metal-based compounds containing P, S, or Se exhibit layered structures. Many of these compounds form complex crystal structures that do not allow for the same level of exfoliation or possess the same weak interlayer interactions as TMDs. Some metal phosphides or sulfides may have a more rigid, three-dimensional network structure that hinders the exposure of a large number of active sites and impedes efficient charge transfer. The limitation of charge transfer efficiency in non-layered P/S/Se-containing compounds can affect their electrocatalytic performance. While they may still participate in electrochemical reactions, the rate of electron transfer and the overall catalytic activity might not be as high as those observed with TMDs. This is because the structural rigidity can restrict the approach of reactant molecules to the active sites, and the lack of a conductive pathway between layers can slow down the electron transfer process, which is critical for the efficiency of the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in water splitting.

Black phosphorus, as a layered structured direct bandgap semiconductor, can adjust its bandgap width between 0.3 eV and 1.5 eV, providing great flexibility for the adjustment of properties, However, the instability of black phosphorus under air and water oxygen conditions limits its application; it is prone to oxidation in air and water, and its electrical properties are sensitive to the environment, which to some extent restricts its practical application [48,49,50,51]. TMDs exhibit stable structure and performance under various environmental conditions. TMDs relatively stable in air, not easily oxidized, and can maintain the stability of their structure and properties. This allows TMDs to play a more stable role in electrocatalytic reactions, ensuring the durability of the catalytic process. Moreover, the stability of TMDs also enables them to exhibit good catalytic performance in different electrolyte environments, maintaining high catalytic activity and stability under both acidic and alkaline conditions. This makes TMDs adaptable in practical applications, capable of meeting electrocatalytic needs in various scenarios [52,53,54].

The electronic structure of TMDs can be adjusted in various ways. Through creating more active sites, doping, heterojunctions, and phase engineering, their band structure and electron density of states can be altered, thereby optimizing their catalytic performance [55,56,57,58,59,60,61]. In the reactions of electrocatalytic water splitting, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) are two key steps. TMDs exhibit good catalytic performance in both reactions. For HER, the surface active sites of TMDs can effectively adsorb and activate hydrogen ions, promoting the generation of hydrogen. At the same time, their appropriate hydrogen adsorption Gibbs free energy (ΔG) allows for the generation and desorption of hydrogen to occur at a lower overpotential, improving the efficiency of energy conversion [62,63,64,65]. In OER, TMDs can provide abundant active sites to promote the adsorption and decomposition of water molecules, and the adjustment of their electronic structure can reduce the reaction barrier and increase the reaction rate [65,66,67]. TMDs, due to their cost-effectiveness, unique layered structure, good stability, and adjustable electronic and surface chemical properties, have significant application value in the field of electrocatalytic water splitting. In-depth research on the electrocatalytic performance of TMDs and the development of effective strategies to enhance their catalytic performance are of great importance for achieving efficient and sustainable energy conversion. This article will explore in detail the basic structure, properties, reaction mechanisms, and measures to improve the catalytic performance of TMDs in electrocatalytic water splitting, including creating more active sites, doping, phase engineering, and constructing heterojunctions. Through research in these areas, we hope to provide a deeper understanding and guidance for the application of TMDs in the field of electrocatalytic water splitting, promote the development of related technologies, and contribute to solving energy and environmental issues.

Future research can further explore the relationship between the structure and performance of TMDs, and optimize their catalytic performance via precisely controlling the structure and composition of TMDs. At the same time, combining theoretical calculations with experimental research to deeply understand the mechanism of action of TMDs in electrocatalytic reactions will provide stronger theoretical support for the design of catalysts. In addition, developing efficient and scalable preparation methods for TMD catalysts is also an important direction for future research, which will help promote the popularization and application of TMDs in practical applications. It is believed that with the continuous deepening of research, TMDs will play an increasingly important role in the field of sustainable energy and provide strong support for the production and application of clean energy.

2. Basic Structure and Properties of TMDs

TMDs characterized via the molecular formula MX₂, are a class of two-dimensional materials that have garnered considerable interest for their diverse applications in catalysis, energy storage, and conversion. Here, M represents transition metals from groups IVB to VIIB, such as molybdenum (Mo), tungsten (W), and titanium (Ti), while X represents chalcogen elements such as sulfur (S), selenium (Se), or tellurium (Te) [68]. The structure of a single layer of TMD consists of a transition metal layer flanked via two chalcogen layers, forming a sandwich-like configuration. The layers are held together via covalent bonds within the plane and via van der Waals forces between the planes, leading to a layered structure that allows for adjustable physical and chemical properties [69].

TMDs exhibit multiple crystal phases, exemplified via molybdenum disulfide (MoS₂), which can exist in octahedral (1T phase), trigonal prismatic (2H phase, or 1H phase as a single layer), and rhombohedral (3R phase) structures as shown in Figure 1. The 1T phase features trigonal symmetry with an ABC stacking sequence, resulting from a lateral shift of one chalcogen layer in the 1H phase. The 1T’-MoS₂, a triclinic variant, is distinguished via its reduced symmetry due to additional electrons. The 2H phase, with an ABA stacking sequence, is stabilized via the triangular prismatic coordination of each transition metal atom via three chalcogen atoms, leading to an ABAB stacking order in the bulk material. The 3R phase, a metastable polytype, can be derived from modifications to the 2H phase [70,71,72,73,74], as shown in Figure 1a–c.

1T-MoS₂, due to its metallic nature, exhibits excellent electronic conductivity characteristics. The overlap of the valence and conduction bands results in the absence of a bandgap, allowing electrons to move freely. This electron mobility is a key factor contributing to its high rate of electron transfer and excellent electrocatalytic performance, particularly in reactions such as electrocatalytic water splitting and hydrogen evolution. Notably, the best 1T-MoS₂ catalysts have demonstrated an overpotential of merely 98 mV at a current density of 10 mA cm⁻², along with a Tafel slope of 52 mV dec⁻¹, indicating a rapid and efficient electrocatalytic process [75]. However, the metallic nature of 1T-MoS₂ also introduces the challenge of lower chemical stability, which may compromise its stability and reliability for long-term applications.

On the other hand, 2H-MoS₂ has semiconductor properties and a direct bandgap, where electron transport is limited by the bandgap [76]. Yet, this limitation also brings better chemical stability [77]. The hexagonal crystal structure and strong interatomic bonding of 2H-MoS₂ provide structural integrity under harsh conditions such as acidic and alkaline environments and high temperatures, which is conducive to long-term use [78]. Moreover, the preparation process of 2H-MoS₂ is relatively mature, allowing for large-scale production, making it an important molybdenum disulfide material in a variety of application fields [79].

The hexagonal crystal structure and the hexagonal coordination of Mo atoms provide strong interatomic bonding, while the tetragonal metallic structure of 1T-MoS₂ and the octahedral coordination of Mo atoms lead to a more unstable electronic state. From a thermodynamic perspective, the 2H phase is the most stable state at room temperature and pressure, while the 1T phase is thermodynamically unstable and prone to phase transformation into the 2H phase [79].

The catalytic performance of 1T-MoS₂ is superior to that of 2H-MoS₂ because the former has a higher density of active sites and better electron transfer compared to the latter. However, compared to the thermodynamically favorable 2H-MoS₂, 1T-MoS₂ exhibits poorer stability, which will limit its practical applications. When considering the application of these two materials, it is necessary to choose the appropriate material according to specific application requirements and environmental conditions. If the application requires a high rate of electron transport and electrocatalytic performance, 1T-MoS₂ may be a better choice; whereas, if the application requires long-term stability and chemical stability, 2H-MoS₂ may be more suitable. At the same time, the preparation process of materials is also a key factor in realizing their application potential. Therefore, optimizing the preparation process of 1T-MoS₂ to achieve large-scale production is also an important direction for future development.

The 3R phase has the semiconducting properties of a tunable band gap. This band gap, while allowing electron mobility, also provides the necessary energy barrier for charge separation, which is crucial for water splitting reactions. The unique electronic structure and morphology of 3R-MoS₂ can strike a balance between the high electron transfer rate of 1T-MoS₂ and the chemical stability of 2H-MoS₂, compared to 1T and 2H phases. The stability of the 3R phase, while challenging due to its thermodynamic instability, can be enhanced by various synthesis strategies and post-synthesis treatments such as doping or surface modification, which may also enhance its catalytic activity for water splitting. In terms of catalytic performance, due to its different electron configurations, 3R-MoS₂ may not exhibit the same high-density reactivity check point as the 1T phase.

In addition to MoS₂, WS₂ also exhibits a rich diversity of crystal phases, including the 1T, 2H, and 3R polymorphs. The 1T phase of WS₂ is characterized by an octahedral coordination of tungsten atoms by sulfur atoms, with a stacking sequence of ABC. In contrast, the 2H phase features a trigonal prismatic coordination, with an ABA stacking sequence. The 3R phase, which is metastable and can be accessed through modifications of the 2H phase, offers additional structural nuances [80,81]. Molybdenum diselenide (MoSe₂) shares a similar crystallographic landscape with its counterpart MoS₂, existing in 1T, 2H, and 3R phases. The 1T phase, with its octahedral coordination and ABC stacking, is structurally analogous to the 2H phase, which presents a trigonal prismatic coordination and ABA stacking. The 3R phase, derived from the 2H phase, represents a metastable state with unique properties [82]. Titanium disulfide (TiS₂) predominantly adopts the 1T phase, where titanium atoms are octahedrally coordinated by sulfur atoms, and the layers stack in an ABC sequence. The weak van der Waals forces between these layers facilitate the exfoliation process, enabling the production of two-dimensional nanosheets [83,84]. Beyond these examples, a plethora of TMDs, such as niobium disulfide (NbS₂), tantalum disulfide (TaS₂), and vanadium disulfide (VS₂), exhibit analogous crystal structures and phase transitions. The distinct physical and chemical properties associated with each phase render these materials highly versatile for applications spanning catalysis, energy storage, and conversion technologies. The ability to manipulate the crystal structure of TMDs through controlled synthesis and post-treatment methods is a cornerstone of materials science. This tunability not only broadens the scope of their applications but also opens avenues for the discovery of new phenomena and functionalities. As research in TMDs continues to evolve, the exploration of phase transitions and the engineering of novel structures promise to further expand the horizons of material science and nanotechnology.

The layered structure of TMDs facilitates the modulation of surface area, porosity, and conductivity, which are pivotal for efficient charge transfer and mass transport in water electrolysis. The surface of TMDs is replete with active sites that enhance the adsorption and activation of water molecules, potentially accelerating reaction kinetics and reducing the overpotential for water splitting. The electronic configurations resulting from the interaction between transition metal and chalcogen atoms are conducive to the catalytic selectivity and specificity required for the hydrogen and oxygen evolution reactions [85,86,87,88,89].

3. Fundamentals of TMDs in Electrocatalytic Water Splitting

The quest for sustainable energy conversion and storage has propelled the scientific community to explore innovative materials and processes that can efficiently harness and utilize renewable resources. Among the various strategies, electrocatalytic water splitting stands as one of the most promising approaches, offering a clean and direct route to produce hydrogen [90,91,92,93,94,95].

3.1. The Electrochemical Mechanisms of HER and OER

Water splitting, a process that holds promise for clean energy production, is fundamentally composed of two half-reactions: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). These reactions, when synergistically catalyzed, allow for the efficient conversion of water molecules into hydrogen and oxygen gases, thereby enabling a clean and renewable energy cycle. The HER involves the reduction of protons to form hydrogen gas, a process that typically occurs at the cathode in an electrolytic cell. Conversely, the OER, which takes place at the anode, involves the oxidation of water molecules, requiring the removal of electrons and the incorporation of oxygen into the product.

3.1.1. Mechanism of HER

In both acidic and alkaline systems, the electrocatalytic hydrogen evolution reaction (HER) is initiated through a two-step mechanism, commencing with the Volmer step, also known as the discharge reaction step [96]. This inaugural phase of HER facilitates the reduction of a proton, resulting in the formation of an adsorbed hydrogen atom. Particularly in acidic environments, the chalcogen atoms on the surface of transition metal dichalcogenides (TMDs), such as sulfur, selenium, and tellurium, exhibit a higher electronegativity that enables strong interactions with protons. These interactions enhance the absorption of protons to form adsorbed hydrogen atoms, thereby accelerating the rate of the hydrogen evolution reaction [97,98]. For instance, sulfur atoms in molybdenum disulfide (MoS₂) can form hydrogen bonds with protons, facilitating their adsorption. Moreover, the high electron cloud density of chalcogen atoms allows them to accept electrons from an external circuit, participating in the reduction of protons [99,100]. Furthermore, the layered structure of TMDs provides a rapid pathway for charge transfer during the electrocatalytic process. Comprising alternating layers of transition metal and chalcogen atoms, TMDs are held together by weak van der Waals forces, which enable efficient electron transfer between layers. The specific electronic configuration of the transition metal atoms also contributes to the charge transfer process; the d-orbital electrons of the metal interact with the p-orbital electrons of the chalcogen atoms to form covalent or metallic bonds, thereby promoting electron transfer within the catalyst [101,102]. The Volmer reaction can be represented by the following equation:

$$H^{+}+e^{-}\text{→}{H}_{\mathit{ads}}^{*}$$

(1)

In this reaction, a proton ($H^{+}$) and an electron ($e^{-}$) will combine on the surface of the catalyst to form an adsorbed hydrogen atom ($H_{\mathit{ads}}^{*}$), as shown in Figure 2a. This step is crucial for initiating the HER, as it sets the stage for the subsequent evolution of hydrogen.

In alkaline system, the Volmer step may be slightly different, involving the adsorption of water molecules ($H_{2}O$) instead of proton adsorption, as in Figure 2b. The surface characteristics of transition metal dichalcogenides (TMDs), particularly their affinity for water molecule adsorption, play a crucial role in facilitating the Volmer step. Taking molybdenum disulfide (MoS₂) as an example, its surface, composed of molybdenum and sulfur atoms, features a prismatic structure where molybdenum atoms are centered within hexagonal arrangements of sulfur atoms, characteristic of the typical 2H phase of MoS₂. This structural feature imparts a certain roughness and reactive site density to the surface, which is conducive to the adsorption of water molecules [104,105]. Moreover, the differing electronegativities between the transition metal and chalcogen atoms in TMDs lead to electronic shifts, endowing the surface with a charge that can engage in electrostatic interactions with the hydrogen and oxygen atoms of water molecules, thereby promoting adsorption. In the case of MoS₂, the higher electronegativity of sulfur atoms relative to molybdenum results in an electron shift towards the sulfur, leaving the molybdenum atoms with a partial positive charge and the sulfur atoms with a partial negative charge, which in turn affects the adsorption of water molecules [106]. Regarding the electronic band structure, the band structure of TMDs consists of valence and conduction bands, primarily formed by the p-orbitals of chalcogen elements and the d-orbitals of transition metals for the valence band, and the d-orbitals of transition metals for the conduction band. Upon contact with water molecules, the electron cloud of the water molecules can interact with the band structure of TMDs, altering the electron state density and consequently affecting the adsorption capacity for water molecules, which further promotes the progression of the Volmer step [107]. The reaction can be described as follows:

$$H_{2}O+e^{-}\text{→}{H}_{\mathit{ads}}^{*}+O{H}^{-}$$

(2)

Subsequent to the Volmer step, the adsorbed hydrogen atoms engage in a reaction with an additional proton and electron, culminating in the formation of hydrogen. This reaction is encapsulated via the Heyrovsky reaction:

$$H_{\mathit{ads}}^{*}+H^{+}+e^{-}\text{→}{H}_2$$

(3)

The Heyrovsky reaction describes the union of hydrogen atoms adsorbed on the electrode with protons and electrons derived from the electrolyte, resulting in the production of hydrogen. Within a alkaline system, the Heyrovsky step may involve the participation of water molecules, rather than the direct involvement of hydroxide ions:

$$H_{\mathit{ads}}^{*}+H_{2}O+e^{-}\text{→}{H}_2+{\mathit{OH}}^{-}$$

(4)

Under acidic conditions in the Heyrovsky reaction, the chalcogen atoms on the surface of transition metal dichalcogenides (TMDs) act as active sites for protons and electrons. These atoms, characterized by their elevated electronegativity, are capable of attracting protons and accepting electrons. In the case of molybdenum disulfide (MoS₂), sulfur atoms can engage in hydrogen bonding with protons, which enhances the stability of adsorption and accelerates the hydrogen evolution reaction rate. Once protons receive electrons at these active sites, they transform into adsorbed hydrogen atoms. The surface architecture and electronic characteristics of TMDs can regulate the adsorption energy of hydrogen atoms to a favorable state, and their conductive properties expedite the transfer of electrons across the catalyst surface, facilitating the combination of adsorbed hydrogen atoms with additional protons and electrons to form hydrogen molecules. In alkaline media, the surface of TMDs is capable of adsorbing water molecules and promoting their dissociation. Defects and edges on the surface offer active sites where the electronic structure can modulate the adsorption and dissociation energies of water molecules. The adsorbed hydrogen atoms then react with water molecules and electrons to produce hydrogen molecules and hydroxide ions. The surface features of TMDs influence the binding energy of hydrogen atoms and the reaction pathway. Their layered structure and two-dimensional attributes provide a conduit for the transport of hydrogen atoms, hastening the desorption and formation of hydrogen gas [108,109,110,111].

In addition to the Heyrovsky reaction, there is another parallel step: the Tafel step, which describes the process where two adsorbed hydrogen atoms recombine to form hydrogen:

$$2H_{\mathit{ads}}^{*}\text{→}{H}_2$$

(5)

The Tafel step is a surface-sensitive reaction involving the direct combination of two adsorbed hydrogen atoms to form hydrogen gas, a process that does not require the participation of additional protons or electrons. ($H^{+}$ represents a hydrogen ion, $e^{-}$ represents an electron, $H_{\mathit{ads}}^{*}$ represents a hydrogen atom adsorbed on the catalyst surface, ($H_{2}O$) represents a water molecule, and ${\mathit{OH}}^{-}$ represents a hydroxide ion).

The distinction between HER in acidic and alkaline systems primarily stems from the availability of hydrogen ions ($H^{+}$) and water molecules ($H_{2}O$) in the reaction medium, as well as their roles in the reaction.

In the acidic system, protons ($H^{+}$) directly participate in the reaction, as illustrated in Equation (1), where protons combine with electrons on the surface of the catalyst to form adsorbed hydrogen atoms. This is because the concentration of hydrogen ions is higher in the acidic medium, making it easier for protons to combine with electrons to undergo the reaction [112,113,114,115].

In the alkaline system, due to the presence of hydroxide ions (${\mathit{OH}}^{-}$), water molecules ($H_{2}O$) are involved in the initial Volmer step, as shown in Equation (2) [116,117,118,119]. Water molecules adsorb on the catalyst surface and combine with electrons to form adsorbed hydrogen atoms and hydroxide ions. This is because the concentration of hydroxide ions is higher in the alkaline medium, and water molecules need to interact with electrons first to provide a basis for the subsequent hydrogen generation reaction [120].

3.1.2. Mechanism of OER

The oxygen evolution reaction (OER) occurring at the anode is a complex, multi-step process involving the transfer of four electrons, with the Adsorption Evolution Mechanism (AEM) serving as the pivotal theoretical framework for describing the progressive transformation of oxygen precursors in the OER. This mechanism delineates the sequential adsorption of oxygen precursors onto the catalyst surface, their deprotonation, the coupling of intermediates, and ultimately, the generation of molecular oxygen followed by desorption, as illustrated in Figure 2c,d. The overall reaction is succinctly encapsulated by the following equation:

$$2H_{2}O\left(l\right)\text{→}{O}_2\left(g\right)+4H^{+}+4e^{-}$$

(6)

The initiation step of the oxygen evolution reaction (OER) involves the active sites on the surface of transition metal dichalcogenide (TMD) catalysts that facilitate the effective adsorption of water molecules ($H_{2}O$). These active sites, often associated with sulfur atoms or other coordinating atoms on the TMDs surface, form strong interactions with water molecules, thereby promoting proton dissociation and electron transfer. On the TMD catalyst, water molecules are initially adsorbed onto these active sites and subsequently lose a proton to form adsorbed hydroxyl intermediates (${HO}^{*}$), as shown in Figure 2c, accompanied by the transfer of electrons (where ∗ represents the active site of the catalyst, g refers to the gas phase, l represents the liquid phase, and ${\mathit{HO}}^{*}$, $O^{*}$, and ${\mathit{HO}O}^{*}$ represent the species adsorbed on the active site):

$$H_{2}O\left(l\right)\text{→}{\mathit{HO}}^{*}+H^{+}+e^{-}$$

(7)

The hydroxyl intermediate (${\mathit{HO}}^{*}$) further loses a proton, resulting in the formation of an adsorbed oxygen intermediate ($O^{*})$. In this process, the surface active sites of TMDs provide a conducive environment for the reaction, enabling the hydroxyl intermediate to stably exist and further react. The active sites, such as sulfur atoms or other coordinating atoms, interact with the hydroxyl intermediate, facilitating the loss of a proton and subsequently forming an oxygen intermediate. This step can be represented by the following equation:

$${\mathit{HO}}^{*}\text{→}{O}^{*}+H^{+}+e^{-}$$

(8)

Next, the adsorbed oxygen intermediate reacts with another water molecule to form an adsorbed peroxide intermediate ($\mathit{HO}{O}^{*}$):

$$O^{*}+H_{2}O\left(l\right)\text{→}\mathit{HO}{O}^{*}+H^{+}+e^{-}$$

(9)

In this process, the active sites of the transition metal dichalcogenide (TMD) catalyst offer a favorable environment that enables the oxygen intermediates to interact effectively with water molecules, leading to the formation of adsorbed peroxide intermediates ($\mathit{HO}{O}^{*}$). This step involves not only the adsorption and activation of water molecules but also the transfer of electrons, which are typically supplied by the electrolyte and rapidly conducted within the conductive framework of the TMD catalyst. The conductivity and metallic character of TMDs play a decisive role in the efficiency of this step. Good electronic conductors facilitate the swift transfer of electrons from the electrolyte to the catalyst surface, thereby reducing the activation energy barrier. Moreover, the surface active sites of TMDs can optimize the adsorption and stabilization of intermediates, making the binding of oxygen intermediates with water molecules more effective. Additionally, the layered structure of TMDs provides the necessary spatial environment for subsequent coupling reactions to form peroxide intermediates. Therefore, the role of TMDs in this step is multifaceted: they not only provide the electrons and active sites required for the reaction but also optimize the adsorption and transformation processes of intermediates through their electronic structure and surface properties. This synergistic effect is key to the high catalytic activity exhibited by TMD catalysts in the oxygen evolution reaction (OER).

Finally, the peroxide intermediate undergoes deprotonation to release molecular oxygen and regenerate the active sites:

$${\mathit{HOO}}^{*}\text{→}{O}_2\left(g\right)+H_{2}O+e^{-}$$

(10)

In alkaline media, the mechanism of the oxygen evolution reaction (OER) is slightly different, as shown in Figure 2d. In the first step, hydroxide ions ($O{H}^{-}$) will be adsorbed onto the active sites of the catalyst to form adsorbed intermediates (${\mathit{HO}}^{*}$):

$$O{H}^{-}+*\to {\mathit{HO}}^{*}+e^{-}$$

(11)

The adsorbed intermediate (${\mathit{HO}}^{*}$) then reacts with another hydroxide ion to form an adsorbed peroxide intermediate ($O^{*}$):

$${\mathit{HO}}^{*}+O{H}^{-}\to O^{*}+H_{2}O+e^{-}$$

(12)

($O^{*}$) reacts with another ($O{H}^{-}$) to form a (${\mathit{HOO}}^{*}$) intermediate:

$$O^{*}+O{H}^{-}\to {\mathit{HOO}}^{*}+e^{-}$$

(13)

Finally, the adsorbed peroxide intermediate decomposes to produce oxygen gas and regenerate the active sites:

$${\mathit{HOO}}^{*}\to O_2\left(g\right)+H_{2}O+e^{-}$$

(14)

Compared to the acidic conditions, the presence of hydroxide ions in the alkaline medium alters the reaction pathway and kinetics of the OER [120,121,122]. The adsorption and reaction of hydroxide ions on the catalyst surface play a crucial role in the overall process [123,124,125].

It is important to note that the exact mechanism of the OER can vary depending on the specific catalyst and reaction conditions. Further studies are often required to elucidate the detailed reaction steps and intermediate species in both acidic and alkaline environments.

In addition to the traditional AEM, the lattice oxygen-mediated mechanism (LOM) has garnered significant attention in recent years [126,127]. Distinct from the conventional AEM, the LOM emphasizes the direct involvement of lattice oxygen atoms in the electrocatalytic process. Under the influence of this unique mechanism, oxygen atoms within the lattice of TMDs are activated under the effect of an electric field, transforming into high-valent reactive oxygen species. These reactive oxygen species possess robust reactivity, enabling them to react with water molecules or hydroxide ions dissociated therefrom, leading to the formation of peroxide or superoxide intermediates. Subsequently, these activated lattice oxygen atoms commence migration within the lattice, a process intimately connected with the material’s electronic structure and lattice characteristics. The migration capability of lattice oxygen atoms largely depends on the conductivity of TMDs. When TMDs exhibit good electrical conductivity, electrons can be rapidly transported within the material, facilitating the migration of lattice oxygen atoms. Concurrently, the stability of the lattice plays a pivotal role in the migration process. A stable lattice structure can provide suitable channels for the migration of lattice oxygen atoms, enabling them to more smoothly locate appropriate active sites for further reactions. Moreover, the coordination environment of oxygen atoms in the lattice is also crucial. Different coordination environments affect the electronic structure and chemical reactivity of oxygen atoms, thereby influencing their migration capabilities. Once the lattice oxygen atoms successfully migrate to active sites, they combine with another lattice oxygen atom or oxygen species adsorbed on the catalyst surface to form an oxygen-oxygen bond. This step is critical in the overall OER as it directly leads to the generation of oxygen molecules. Ultimately, the formed oxygen molecules are released from the catalyst surface into the solution, completing the OER process.

3.2. Primary Parameters of Electrochemical Water Splitting

3.2.1. Gibbs Free Energy

The Hydrogen Evolution Reaction (HER) is pivotal in the process of hydrogen atom adsorption and desorption, with the Gibbs free energy of adsorption (ΔG) being a key indicator of the strength of hydrogen atom adsorption on the catalyst surface. Consequently, the value of ΔG is closely linked to the HER activity of the catalyst [128,129,130]. Ideally, a catalyst exhibits optimal HER activity when ΔG is close to 0.0 eV, indicating a thermodynamically neutral state where hydrogen atoms can freely bind to and detach from the catalyst surface, facilitating efficient catalysis and the coupling of protons and electrons to release molecular hydrogen [131,132,133,134]. However, achieving a perfect balance of thermodynamic neutrality is challenging, as it requires the adsorption and desorption free energy changes to be perfectly balanced, which is typically difficult to attain in practice [135,136,137].

When ΔG is greater than zero, the adsorption requires additional energy, which typically reduces the activity of the Hydrogen Evolution Reaction (HER); this situation often indicates that the adsorption energy of the catalyst surface for hydrogen atoms is insufficient to effectively promote HER, possibly because the interaction between the catalyst and hydrogen atoms is not strong enough [138,139,140,141]. Conversely, when ΔG is less than 0, the adsorption process is exothermic, suggesting that hydrogen atoms are more inclined to bind to the catalyst surface [142,143]. However, if the adsorption is too strong, it may hinder the desorption of hydrogen atoms from the catalyst surface, affecting the activity and reversibility of the HER. Therefore, a moderate ΔG value helps maintain a balance between adsorption and desorption, enabling efficient catalysis.

Thus, identifying catalysts with an ideal adsorption Gibbs free energy ΔG is crucial for enhancing the efficiency of the HER. According to Sabatier’s principle, catalytic activity results from a balance between adsorption strength and reaction kinetics. The Sabatier plot, commonly known as the volcano plot, visually demonstrates the relationship between the adsorption Gibbs free energy ΔG and catalyst activity [144,145,146,147,148]. Catalysts located near the top of the volcano plot have a ΔG value close to 0.0 eV, indicating an optimal balance of adsorption and desorption, and thus exhibit high catalytic activity in the HER process. In contrast, catalysts at the base or sides of the volcano plot, with ΔG values deviating from this ideal range, typically show lower activity. Platinum group metals, due to their superior electronic structure and surface properties, are usually positioned at the top of the volcano plot, demonstrating high HER catalytic performance, as shown in Figure 3d. However, the scarcity and high cost of these noble metals limit their widespread application. As an alternative, TMDs are considered promising substitutes for platinum group metals due to their lower ΔG values and moderate intermediate adsorption energy [149,150,151].

In the study of the Oxygen Evolution Reaction (OER), the Gibbs free energy changes (ΔG) of intermediates on the catalyst surface, such as hydroxyl radicals (${\mathit{HO}}^{*}$), oxygen atoms ($O$), and hydroxide ions ($\mathit{OOH}$), also affect their adsorption strength and desorption on the catalyst surface, a moderate ΔG value can promote the effective adsorption and transformation of intermediates, thereby reducing the reaction energy barrier and enhancing the OER rate. This phenomenon, depicted in the volcano plot, shows a similar pattern to the catalytic efficiency of HER, reflecting the analogous regularity in the catalytic activity between the two reactions [155,156,157,158,159].

3.2.2. Overpotential

Overpotential refers to the difference between the actual potential applied and the theoretical potential required in an electrochemical reaction to achieve the desired reaction rate [160]. This difference reflects the additional part of the actual potential exceeding the theoretical potential, which is usually related to kinetic barriers in the reaction process. These barriers may include charge transfer resistance, the availability of active sites, and mass transport limitations of the reactants, as shown in Figure 3a.

For the Hydrogen Evolution Reaction (HER), the overpotential is calculated based on the following formula:

$${\eta }_{\mathrm{HER}}=E_{\mathrm{RHE}}-E_0$$

(15)

where ($E_{\mathrm{RHE}}$) represents the potential relative to the Reversible Hydrogen Electrode (RHE), and ($E_0)$ is the standard potential for the hydrogen electrode reaction, conventionally set at 0 V [161,162,163]. This indicates that the overpotential for HER is the additional potential required to generate hydrogen gas under specific conditions beyond the standard potential.

Similarly, the overpotential for the Oxygen Evolution Reaction (OER) is calculated based on a similar principle but involves a different standard potential difference:

$${\eta }_{\mathrm{OER}}=E_{\mathrm{RHE}}-1.23\mathrm{V}$$

(16)

where 1.23 V represents the standard reversible thermodynamic potential for oxygen evolution. It is important to note that these formulas provide a practical method for assessing overpotential without impedance (IR) compensation [164,165].

In experimental operations, to achieve the required current density, researchers often need to apply a voltage higher than the theoretical potential to overcome the kinetic barriers in electrochemical reactions. When assessing the performance of HER and OER electrocatalysts in different pH media, a current density of 10 mA cm⁻² is commonly used as a standard benchmark in experiments.

3.2.3. The Tafel Slope

During the process of electrolytic water splitting, the Tafel slope (Tafel Slope) plays a pivotal role as a commonly utilized parameter. The Tafel slope reflects the kinetic characteristics of the electrocatalytic reaction and is described via the Tafel equation [166,167,168,169,170,171]:

$$\eta =a+{\displaystyle \frac{b}{log\left(j\right)}}$$

(17)

Here, ($\eta$) represents the overpotential, ($j$) denotes the current density, and ($a$) and ($b$) are parameters of the Tafel equation. This equation reveals the logarithmic relationship between overpotential and current density, which can be employed to ascertain the rate-determining step (RDS) of the electrocatalytic reaction [172,173]. However, due to the complexity of the Oxygen Evolution Reaction (OER), determining its precise RDS and Tafel slope is relatively challenging and necessitates the comprehensive consideration of various factors, such as the structure, composition, surface properties of the catalyst, and the nature of the electrolyte [174,175,176,177]. Despite this, the Tafel slope remains an important metric for assessing the performance of OER catalysts; a smaller Tafel slope typically indicates that the catalyst possesses faster reaction kinetics, enabling higher current densities at lower overpotentials, which is crucial for improving the efficiency of electrolytic water splitting, as presented in Figure 3b. Similarly, when comparing the catalytic activity of different catalysts on HER, a catalyst with a smaller Tafel slope is generally considered to have superior catalytic performance [178,179,180,181,182,183,184,185,186,187,188,189].

Furthermore, the Tafel slope plays a significant role in studying the behavior of electrolytic water reactions under various conditions. Via precisely measuring the changes in the Tafel slope under different electrolyte concentrations, temperatures, and pH values, researchers can gain in-depth insights into the evolution of the reaction mechanism and the stability of the catalyst.

The impact of varying electrolyte concentrations is notable; changes in electrolyte concentration affect ion transport and the structure of the electric double layer at the electrode surface, thereby influencing the kinetics of the electrocatalytic reaction [190,191,192,193,194]. When the electrolyte concentration increases, the ionic strength rises, which may alter the charge distribution at the electrode surface and subsequently affect the adsorption and desorption behavior of reactants and intermediates on the electrode surface. This could lead to changes in the Tafel slope, reflecting alterations in the rate-determining step [195,196,197,198,199,200,201,202].

The influence of temperature on the Tafel slope is also significant [203,204,205]. An increase in temperature typically leads to an increase in reaction rate via providing additional energy to overcome the activation barrier. This could result in a decreased Tafel slope, indicating that the reaction proceeds more readily at higher temperatures. However, excessively high temperatures may negatively impact the stability of the catalyst, such as causing structural changes or deactivation of active sites. Therefore, studying the variation of the Tafel slope at different temperatures can inform us about the thermal stability of the catalyst and the sensitivity of the reaction to temperature, which is important for optimizing the operating temperature of electrolytic water splitting systems [206,207,208,209,210,211,212,213,214].

Changes in pH value also significantly affect the Tafel slope [215]. In electrolytic water reactions, pH influences the concentration of hydrogen ions and hydroxide ions, thereby altering the reaction pathway and kinetics. Under acidic conditions, HER may primarily proceed through the Volmer-Heyrovsky mechanism, while under alkaline conditions, water dissociation may become the rate-determining step [216,217,218,219,220,221,222,223,224,225,226,227,228,229]. These variations are reflected in changes in the Tafel slope. Via examining the Tafel slope at different pH values, we can deepen our understanding of the pH-dependence of the reaction mechanism, facilitating the development of catalysts that operate efficiently across a range of pH conditions.

The distinctive architecture and electronic characteristics of transition metal dichalcogenides (TMDs) facilitate the manipulation of reaction dynamics, which in turn can modulate the Tafel slope. TMDs exhibit a layered configuration that endows them with an expansive surface area replete with numerous catalytically active sites. These sites are instrumental in bolstering the adsorption and activation of reactant molecules, thereby accelerating the reaction kinetics and diminishing the Tafel slope. Furthermore, the electronic configuration of TMDs can be tailored through the incorporation of specific metallic dopants, such as nickel (Ni) or cobalt (Co) in the context of MoS₂. Such doping introduces novel electronic states that can reshape the local electronic milieu surrounding the active sites, thereby amplifying the catalyst-reactant interaction. Consequently, this enhancement in interaction translates to an improved adsorption and desorption process of hydrogen atoms, culminating in an accelerated reaction pace and a reduced Tafel slope.

3.2.4. Other Useful Parameters

In the field of electrochemical water splitting, in addition to parameters such as Gibbs free energy, overpotential, and Tafel slope, a series of other parameters are also crucial for an in-depth understanding and performance optimization of the reaction. For example, Electrochemical impedance spectroscopy (EIS) is a common technique for measuring this parameter [230,231,232,233]. The double-layer capacitance ($C_{\mathit{dl}}$) reflects the structure and properties of the electrode surface’s double layer and is closely related to the catalyst’s surface area and the ionic concentration of the electrolyte; catalysts with a higher double-layer capacitance, due to their larger surface area and ion adsorption capacity, are often able to increase the reaction rate [234,235,236], as depicted in Figure 3c.

Stability is a critical factor to be considered when evaluating whether a catalyst can maintain its activity and structural integrity over an extended period. Prolonged electrochemical assessments are crucial for evaluating the durability of TMDs. This process entails exposing the catalysts to successive cycles within electrochemical reactions, including the electrocatalytic decomposition of water. Throughout these cycles, it is imperative to meticulously track pivotal metrics such as catalytic efficiency, onset potential, and Tafel kinetics. Specifically, in the realm of water electrolysis, the endurance of TMD catalysts can be gauged by monitoring the rates of hydrogen and oxygen evolution across an extensive series of cycles. A sustained level of performance, devoid of any significant decline in catalytic vigor, is indicative of superior stability. Physical characterization tools are also paramount in deciphering the stability of transition metal dichalcogenides (TMDs). Techniques like X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) offer profound insights into the structural and compositional evolution of catalysts following extended utilization. XRD is adept at identifying any modifications in the crystalline framework of TMDs, whereas TEM unveils details regarding the catalyst’s morphology, including any potential aggregation or deterioration of the nanoparticulates. XPS, on the other hand, is instrumental in scrutinizing shifts in the surface elemental composition and the oxidation states, factors that can profoundly influence the catalyst’s efficacy. Moreover, the stability of TMDs can be influenced by a variety of factors, including the operating conditions (e.g., temperature, pH, and current density) and the presence of impurities or contaminants. Extreme operating conditions, such as high temperatures or extreme pH values, can induce structural changes or degradation of the catalyst. Therefore, it is imperative to study the stability of TMDs under different operating conditions to ensure their reliability in practical applications. To enhance the stability of TMDs, various strategies can be employed. Doping TMDs with other elements can also improve their resistance to degradation by altering the electronic structure and enhancing the stability of the material. Additionally, forming composite materials with other stable phases can provide additional support and stability to the TMD catalyst [237,238].

TMDs can contribute to increasing the double-layer capacitance. Their layered structure provides a large surface area for ion adsorption, thereby enhancing the capacitance. This, in turn, improves the charge transfer efficiency at the interface, leading to a more efficient electrochemical reaction. The layered structure of MoS₂ allows for the easy intercalation of ions between the layers, increasing the ion storage capacity and thus the double-layer capacitance.

The electrochemical active surface area (ECSA) measures the actual surface area of the catalyst that participates in the electrochemical reaction.

It is a crucial parameter that quantifies the extent of the catalyst’s involvement in electrochemical reactions, and it is particularly significant for evaluating the performance of TMDs. Their two-dimensional layered structure can be engineered into various nanostructures like nanoparticles, nanowires, and nanosheets, which inherently possess a larger specific surface area. This increased surface area, coupled with a high density of surface defects, leads to a higher ECSA, thereby offering a greater number of active sites for electrochemical reactions. TMDs, with two-dimensional (2D) layered structure, present a unique platform for enhancing ECSA. The role of TMDs in improving ECSA is multifaceted. Firstly, the 2D structure of TMDs allows for the creation of nanostructures with a high surface-area-to-volume ratio. When these TMDs are crafted into nanoparticles, nanowires, or nanosheets, the specific surface area is significantly amplified. This architectural manipulation translates to a larger ECSA, providing an expanded platform for electrochemical reactions to occur, thereby increasing the reaction kinetics and lowering the overpotential required for reactions like the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Furthermore, the high density of surface defects in TMD nanostructures, including edges, vacancies, and dopants, is instrumental in enhancing ECSA. These defects act as active sites for the adsorption and transformation of reactant molecules, thereby increasing the overall catalytic activity. They also serve to facilitate electron transfer by offering pathways that lower the energy barrier for electron transport, which is particularly beneficial for multi-electron transfer reactions. In addition to structural modifications, the electronic properties of TMDs can be tailored through doping with heteroatoms or by creating heterostructures, which can modulate the electronic band structure of TMDs. This electronic tuning makes TMDs more conducive to electron transfer and reactant adsorption, inducing new active sites or enhancing the reactivity of existing sites, and thus further boosting ECSA. Lastly, ensuring the stability of TMDs is crucial for maintaining a high ECSA over time. Strategies such as surface coating or the formation of hybrid structures with more stable materials can protect TMDs from corrosion or degradation, ensuring that the ECSA remains high even under prolonged electrochemical operation. This focus on stability is essential for the longevity and reliability of TMD-based catalysts in practical applications.

The ECSA can be measured via cyclic voltammetry (CV) and calculated using the following formula:

$$\mathrm{ECSA}={\displaystyle \frac{C_{\mathit{dl}}}{C_s}}$$

(18)

where ($C_s$) is the capacitance value per unit area. It should be noted that a single parameter usually cannot reflect the catalytic performance systematically and objectively. Taking overpotential ($\eta$) as an example, its value can be influenced via factors such as the catalyst loading and the electrochemical active surface area of the specific electrode. If one focuses solely on the value of overpotential without considering other related parameters, the interpretation of catalytic performance may not be precise. Therefore, to accurately understand the catalytic mechanism and performance of the hydrogen evolution reaction, a comprehensive application of various descriptors should be employed [239,240,241,242,243,244,245,246].

4. Strategies for Enhancing the Electrocatalytic Performance of Transition Metal Dichalcogenides in Water Splitting

The efficiency and selectivity of TMDs in the catalytic process are often limited via their inherent characteristics [247,248,249]. TMDs face issues such as a relative scarcity of active sites and limitations in their electronic structure. To overcome these limitations, researchers have adopted various strategies to enhance the electrocatalytic performance of TMDs. These include creating additional active sites to increase the reaction surface area, modulating the electronic structure strengthening active sites through metal and non-metal doping, utilizing phase engineering to fabricate more stable metallic phase TMDs, and constructing heterostructures to generate synergistic effects [103,250,251,252,253]. These strategies not only significantly enhance the catalytic activity and stability of TMDs but also help to reduce the energy input required during the catalytic process, thereby achieving more efficient electrocatalytic water splitting.

4.1. Creating More Active Sites

The significant advantage of TMDs over bulk materials lies in their layered crystal structure, which allows TMDs to form nanosheets as thin as the atomic level [254,255,256,257,258]. Liquid phase exfoliation is a common technique that can produce monolayer or few-layer TMD nanosheets. The TMD nanosheets prepared via liquid-phase exfoliation technology possess a larger specific surface area, providing more active sites on their surfaces and edges, which effectively promotes the progress of catalytic reactions. These nanosheets are not only easy to prepare but also fully expose the active sites required for catalytic reactions [259,260,261,262,263]. I-Wen Peter Chen et al. used an innovative method in their research, using a specific amino acid—histidine (His)—as an exfoliating agent, to exfoliate layered materials layer via layer in water, successfully preparing monolayer MoSe₂ nanosheets (MoSe₂/PtNPs). This exfoliation method not only produces nanosheets with a high surface-to-volume ratio modified with Pt nanoparticles but also shows excellent performance in the hydrogen evolution reaction (HER). Specifically, MoSe₂/PtNP nanosheets exhibit an ultra-low overpotential (198 mV @ 100 mA cm⁻²), a small Tafel slope (24 mV dec⁻¹), as shown in Figure 4d,e, and long-term electrochemical stability (over 25 h) [264].

In addition to liquid-phase exfoliation (LPE), solid-phase exfoliation (SPE) is also an efficient exfoliation technique that uses physical methods to overcome van der Waals forces, achieving layer-by-layer exfoliation of materials. This method is particularly suitable for layered materials that are unstable or difficult to handle in liquid media [267]. Compared with LPE, SPE avoids the use of toxic organic solvents, showing higher environmental friendliness and operational safety.

Zhang et al. adopted an innovative solid-phase exfoliation strategy, successfully preparing 18 types of MX₂-type two-dimensional TMD nanosheets, including TiTe₂, MoTe₂, WTe₂, VTe₂, NbTe₂, TaTe₂, TiSe₂, MoSe₂, WSe₂, VSe₂, NbSe₂, TaSe₂, TiS₂, MoS₂, WS₂, VS₂, NbS₂, and TaS₂. They first synthesized crack-induced bulk crystals (C-TMD) through molten salt cleavage synthesis technology, as illustrated in Figure 4b,c. These crystals, due to the introduction of molten salts during the synthesis process, formed cracks, thereby weakening the van der Waals forces between layers. Subsequently, mechanical forces such as ball milling were used to exfoliate these bulk crystals into thin and uniform nanosheets [266].

A significant advantage of this method is its high efficiency and scalability. Via optimizing the conditions of ball milling, such as the speed and time, the size and thickness of the nanosheets can be precisely controlled, thereby adjusting the physical and chemical properties of the materials. Moreover, since SPE does not rely on chemical reagents, the resulting nanosheets have better phase purity and crystalline quality.

In addition to diluting the layers of TMDs, Luo et al. treated the basal plane of MoS₂ with an oxidizing agent, introducing oxygen atoms (O), which replaced some sulfur atoms (S), forming active sites on the basal plane of MoS₂. Then, via controlling the current and electrolyte conditions, cobalt atoms (Co) were precisely introduced to the basal plane of MoS₂. These Co atoms occupy positions above the Mo atoms, together with the previously introduced O atoms and the original S atoms, constructing an “O-Co-S₂” three-dimensional protruding coordination configuration, as illustrated in Figure 4a. This structure not only increases the diversity of active sites but also optimizes the catalytic reaction dynamics through the synergistic action of atoms; the design of the “O-Co-S₂” coordination configuration significantly increases the number of active sites on the basal plane of MoS₂ and optimizes the catalytic reaction dynamics through the synergistic action of atoms. Cobalt atoms, as the active center, effectively promote the dissociation of water molecules, generating more protons ($H$) and hydroxide ions ($O{H}^{-}$). These protons are then adsorbed on the active sites of S atoms and converted into hydrogen gas, thereby significantly improving the hydrogen evolution reaction (HER) activity of MoS₂ nanosheets in alkaline media. The Co-O@MoS₂ catalyst exhibits an ultra-low overpotential of only 81 mV to reach a current density of 100 mA cm⁻², and a Tafel slope as low as 42 mV dec⁻¹. Specifically, the addition of Co atoms promotes the dissociation of water molecules, generating more protons ($H$) and hydroxide ions ($O{H}^{-}$), while S atoms provide an ideal active site for the adsorption and conversion of these protons into hydrogen gas. This innovative design makes the Co-O@MoS₂ catalyst exhibit excellent HER performance in alkaline media, including an ultra-low overpotential of only 81 mV to reach a current density of 100 mA cm⁻², and outstanding stability at high current density of 600 mA cm⁻² for up to 300 h [265].

The catalytic performance of TMD nanosheets largely depends on the quantity and accessibility of their active sites. Methods such as liquid-phase exfoliation, solid-phase exfoliation, and atomic-level regulation of active sites can effectively increase the number of active sites and improve their accessibility, thereby significantly enhancing the catalytic performance of TMD nanosheets. Future research can further explore the optimization and synergistic effects of these methods to develop more efficient and selective catalysts, providing strong support for the development of fields such as clean energy conversion.

In the domain of transition metal dichalcogenides (TMDs), future research directions are poised to significantly expand, aiming to substantially increase the number of active sites to optimize their catalytic performance. Continuously refining liquid-phase exfoliation techniques to enhance the efficiency of active site generation in materials is an effective strategy. Investigating various combinations of exfoliants and solvents, such as exploring novel amino acids or biomolecules as exfoliants, while precisely controlling exfoliation conditions to regulate the size and thickness of nanosheets, can increase the specific surface area, thereby creating more surface and edge active sites. Regarding the potential of solid-phase exfoliation techniques, different mechanical forces, such as ultrasonic-assisted ball milling or high-pressure homogenization, can be attempted. Advanced material characterization techniques can be employed to study structural changes and the formation mechanisms of active sites, such as using in situ transmission electron microscopy (TEM) to observe the ball milling process. Furthermore, expanding methods for atomic-level regulation of active sites is crucial. Beyond the existing “O-Co-S₂” structure, exploring the introduction of elements like nitrogen and phosphorus to form new three-dimensional protruding coordination structures with the metal and chalcogen atoms of TMDs is a promising avenue. Theoretical calculations and simulation methods can be utilized to predict the stability and catalytic performance of different elemental combinations and coordination structures. Additionally, self-assembly strategies represent a direction worth pursuing. Leveraging the self-assembly properties of TMD nanosheets to construct nanocomposites with specific structures and active sites, and introducing other functional nanomaterials for synergistic self-assembly, can enhance the quantity of active sites and catalytic performance. This approach not only enriches the structural diversity of TMDs but also paves the way for the development of advanced catalysts with tailored properties for various applications, including energy conversion, environmental catalysis, and more.

4.2. Doping

Optimizing the electronic structure and conductivity of TMDs through elemental doping strategies has become an effective means to improve electrocatalytic performance; via precisely regulating the chemical composition and physical properties of the catalyst, elemental doping strategies significantly improve the adsorption capacity and reaction kinetics of the catalyst to the reaction intermediate.

Metal doping has been proven to be an extremely efficient modification method [268,269,270,271,272,273]. Via introducing metal atoms with different electronic properties into the lattice of TMDs, metal doping can not only effectively regulate the electronic structure of the host material but also significantly increase the number of active sites, thereby greatly enhancing the catalytic performance of TMDs [274,275]. This modification method has brought a series of positive changes to TMDs, such as significantly improving the efficiency of charge transfer within the material, reducing the activation energy of the reaction, enhancing the material’s adsorption capacity for reactants, and optimizing the binding energy of reaction intermediates. These changes enable TMDs to play a more efficient catalytic role in water splitting reactions, providing strong support for the realization of efficient water splitting reactions.

Among many studies, Lee et al. detailed the transformation process from the 2H phase to the 1T and 1T’ phases through the synthesis of Re-doped WS₂ monolayer quantum dots (MQDs), and demonstrated the superior performance of 1T’ WS₂ MQDs in the hydrogen evolution reaction (HER). Experimental results show that 1T’ WS₂ MQDs have a lower Tafel slope of 44 mV dec⁻¹, while the Tafel slopes of other catalysts in comparison are usually above 70 mV dec⁻¹, as illustrated in Figure 5a. At the same time, their overpotential at a current density of 10 mA cm⁻² is also relatively low.

In addition to singlemetal doping, multi-metal doping is also an effective strategy. Via introducing two or more metals, a synergistic effect can be achieved, further optimizing the electrocatalytic performance of TMD materials.

The achievements of Guowei Wang et al. are particularly prominent. They proposed a low-temperature hydrothermal synthesis method (Figure 5b), successfully prepared stable 1T-MoS₂ (the planar HRTEM image of 1T MoS₂ is shown in Figure 5e), and significantly improved the material’s HER and OER performance in alkaline media through doping with transition metals such as Ni, Co, and Fe (the HRTEM image of Ni-1T-MoS₂ is shown in Figure 5f). For example, Ni-1T-MoS₂ exhibits an overpotential of only 112 mV and a Tafel slope of 48.2 mV dec⁻¹ at a current density of 10 mA cm⁻², while the undoped 1T-MoS₂ has an overpotential of 173 mV and a Tafel slope of 109 mV dec⁻¹ under the same conditions. Multi-metal doping changes the electronic structure of 1T-MoS₂, increases the number of active sites, and reduces the hydrogen adsorption free energy (G), thereby improving HER performance. At the same time, doping also promotes the dissociation and oxidation of water, enhancing OER performance [277].

Ma et al. prepared M-Sv-MoS₂ via introducing sulfur vacancies (Sv) and 3D transition metal dopants (such as Ni, Co, Mn, etc.) on the basal plane of MoS₂, as illustrated in Figure 5g. This material shows a significant increase in activity in both HER and OER. Ni-Sv-MoS₂ in HER has an overpotential of 101 mV and a Tafel slope of 66 mV dec⁻¹ at a current density of 10 mA cm⁻², while the overpotential of the original MoS₂ is 183 mV, and the Tafel slope is 109 mV dec⁻¹, as shown in Figure 5c. In OER, Co-Sv-MoS₂ has an overpotential of 190 mV at a current density of 10 mA cm⁻², which is lower than many other MoS₂₋based catalysts, as presented in Figure 5d. The introduction of sulfur vacancies (Sv) leads to changes in the local electronic structure of MoS₂, thereby exposing Mo atoms that were originally covered by sulfur atoms. These Mo atoms, as new active sites, contribute to enhancing the activity of the catalyst.

In the realm of transition metal dichalcogenides (TMDs), the exploration of metal doping strategies is paramount for tailoring their electronic structure and catalytic properties. Future research must delve into the mechanisms by which metal doping influences the electronic configuration and catalytic performance of TMDs. By scrutinizing the effects of varying doping concentrations and positions of different metals, we can establish a robust theoretical foundation for the design of high-performance TMD electrocatalysts. Metal doping can significantly alter the electronic structure of TMDs, thereby modulating their adsorption capacity for reaction intermediates and reaction kinetics. It is undeniable that different doping ratios and positions induce varying degrees of electronic structural changes in TMDs, which subsequently affect their catalytic activity, selectivity, and stability. Certain metal dopants may introduce new electronic states or alter the distribution of existing states, optimizing the interaction between TMDs and reaction intermediates and significantly enhancing catalytic performance. The development of new multi-metal doping systems, aimed at identifying metal combinations with superior synergistic effects, and the study of their fabrication methods, are of utmost importance for precise control over the structure and properties of TMDs. Multi-metal doping can generate synergistic effects, potentially due to electronic interactions between different metals, which alter the surface electronic density of states and charge distribution of TMDs, thereby optimizing the catalytic active sites and reaction pathways. By judiciously selecting metal combinations and controlling the doping process, precise regulation of the electronic structure and catalytic properties of TMDs can be achieved, enhancing their efficiency and stability in electrocatalytic reactions. Further exploration of the synergistic mechanisms between defects (such as sulfur vacancies) and transition metal doping is essential for a deeper understanding of the principles behind the enhancement of TMD catalytic performance. The introduction of defects can alter the local electronic structure of TMDs, exposing new active sites and enhancing their adsorption and activation capabilities for reaction intermediates. Transition metal doping, on the other hand, can regulate the electronic structure and chemical reactivity of TMDs, facilitating charge transfer and reaction kinetics. The synergistic effect of these two factors may create active sites and electronic structures more conducive to catalytic reactions, thereby significantly improving the catalytic performance of TMDs. For example, sulfur vacancies can interact with transition metal dopants to form unique electronic structures that enhance the conductivity and catalytic activity of TMDs. Combining theoretical calculations with experimental research to systematically optimize TMD doping is key to enhancing their electrocatalytic performance. Predicting the performance of doped TMDs, such as electronic structure, band structure, and adsorption energy, through theoretical calculations, followed by experimental validation and feedback optimization based on experimental results, allows for precise control of TMD doping. Theoretical calculations can be based on methods such as density functional theory to simulate the electronic structure and catalytic reaction processes of doped TMDs, providing crucial guidance for experiments. Experimental research can then verify the results of theoretical calculations and optimize the performance of TMDs by adjusting doping conditions and fabrication methods.

4.3. Phase Engineering

Phase engineering plays a crucial role in the electrocatalytic water splitting applications of TMDs. The chemical formula of TMDs is typically written as MX₂, where M represents transition metal elements such as molybdenum (Mo), tungsten (W), niobium (Nb), etc., and X represents chalcogen elements like sulfur (S), selenium (Se), or tellurium (Te) [279]. These materials exhibit various polytypes, including 1H, 2H, 3R, 1T, 1T’, etc., among which the 2H phase is a semiconductor, while the 1T and 1T’ phases are metallic, possessing higher electron conductivity and active site density, thus showing superior catalytic activity in the hydrogen evolution reaction (HER). The 1T and 1T’ polytypes of TMDs are superior to the 2H phase in HER due to their metallic properties, which endow them with higher active site density, metallic conductivity, and better electrode kinetics [280,281,282].

Selvaraj Venkateshwaran et al. have achieved the phase transformation of 2H-MoS₂ to 1T-MoS₂ through a solvothermal treatment process, significantly enhancing the material’s electrocatalytic water reduction (HER) performance in acidic environments. In the experiment, 2H-MoS₂ nanoflowers were dispersed in solvents such as water, ethanol, N,N-dimethylformamide (DMF), and benzene, and then subjected to thermal treatment at 160 °C for 6 h (Figure 6b). After treatment with DMF, the X-ray diffraction (XRD) pattern of the sample showed a shift of the (002) peak from 14.0° to 8.8° (2θ), indicating a significant increase in interlayer spacing. Raman spectroscopy analysis revealed the weakening of the characteristic peaks of the 2H phase and the emergence of characteristic peaks of the 1T phase, while X-ray photoelectron spectroscopy (XPS) analysis showed a decrease in the binding energy of the Mo 3D orbital, further confirming the occurrence of phase transformation Figure 6c. Moreover, the 1T-MoS₂ treated with DMF exhibited excellent electrochemical performance, requiring only an overpotential of 262 mV to reach a current density of 10 mA cm⁻², with a Tafel slope of 53 mV/dec. The electrochemical active surface area (ECSA) was 0.18 mF cm⁻², and the exchange current density (j0) and turnover frequency (TOF) were 0.7 × 10⁻⁶ A cm⁻² and 0.0015 × 10⁻⁶ S⁻¹, respectively. Through cyclic stability tests and long-term stability tests (PSTAT analysis), the 1T-MoS₂ catalyst demonstrated excellent stability over continuous 30-h operation. Mun Kyoung Kim et al. have successfully improved the electrocatalytic performance of molybdenum disulfide (MoS₂) through an electrochemical cation intercalation (ECI) process. This process induces phase transformation and defect formation in MoS₂, causing the 2H-MoS₂ catalyst to partially transition from the 2H phase to the 1T phase, forming a mixed phase (p-2H/1T-MoS₂), which further transitions into vacancy-rich 1T-MoS₂ (vr-1T-MoS₂), ultimately leading to the degradation of the 1T-MoS₂ phase and the formation of sd- (I)MoS₂ and sd- (II)MoS₂ (Figure 6c). During this process, the prepared vr-1T MoS₂ catalyst exhibited excellent performance under different pH conditions, with specific data as follows: in a 0.5 M H₂SO₄ electrolyte, an overpotential of 144 mV was required at a current density of 10 mA cm⁻², with a Tafel slope of 81 mV dec⁻¹; in a 1.0M KOH electrolyte, an overpotential of 184mV was required at a current density of 10 mA cm⁻², with a Tafel slope of 92 mV dec⁻¹; in a 1.0 M PBS electrolyte, an overpotential of 212 mV was required at a current density of 10 mA cm⁻², with a Tafel slope of 133 mV dec⁻¹, shown in Figure 6d.

However, the stability issue of the 1T phase limits its widespread application. To address this issue, researchers have adopted various strategies, such as element doping, heterostructure construction, and electron injection, to improve the stability and catalytic performance of the 1T phase. Binjie Li et al. systematically studied the coordination of Co atoms on the 1T-MoS₂ matrix and their impact on the phase transformation of MoS₂ through a superlattice heterostructure of single-atom cobalt-anchored 1T-MoS₂/carbon (Co-1T-MoS₂/C). Their research results show that Co atoms form strong Co-S covalent bonds with S atoms in 1T-MoS₂, and this bonding, supported via charge density waves (CDWs), induces a coordination reconstruction of MoS₂, allowing it to exist in the 1T phase and effectively enhancing the stability of the 1T phase of MoS₂ [285].

Through phase engineering, especially the preparation and stability optimization of the 1T and 1T’ phases, the application of TMDs in electrocatalytic water splitting has been significantly enhanced. Future research needs to further explore efficient phase conversion and stabilization methods, while optimizing synthesis strategies to achieve the commercial application of TMDs in electrocatalytic water splitting.

Addressing the stability issues of the 1T phase is one of the primary challenges currently faced. To effectively enhance the stability and catalytic performance of the 1T phase, in addition to delving into the specific effects of doping different elements on the phase and properties of transition metal dichalcogenides (TMDs), a strategy of multi-element synergistic doping can be introduced. By precisely controlling the ratio and interaction of various doping elements, a more refined control over the phase of TMDs can be achieved, further optimizing their catalytic performance. Advanced material characterization techniques, such as in situ transmission electron microscopy (TEM) and synchrotron radiation techniques, can be utilized to monitor the structural changes of TMDs in real-time during the doping process, thereby gaining a deeper understanding of the interaction mechanisms between doping elements and TMDs. Furthermore, combining theoretical calculations with machine learning algorithms can predict the optimal doping element combinations and ratios, providing more accurate guidance for experimental research. In terms of heterostructure construction, attempts can be made to develop more complex heterostructures, such as multilayer heterostructures or core-shell structures. These structures can provide more interfaces and active sites, further promoting charge transfer and reaction kinetics. At the same time, advanced fabrication techniques, such as atomic layer deposition (ALD) and molecular beam epitaxy (MBE), can be used to precisely control the composition and interface structure of heterostructures, aiming to maximize the enhancement of TMDs’ catalytic performance. Regarding electron injection technology, more efficient electron injection methods can be explored, such as using electric or magnetic fields to enhance electron injection efficiency. Additionally, studying the dynamic impact of electron injection on the electronic structure and catalytic active sites of TMDs, and achieving real-time optimization of TMDs’ catalytic performance by monitoring and regulating the electron injection process, is crucial. In the realm of theoretical calculations and experimental research, in addition to traditional methods of theoretical computation and experimental validation, artificial intelligence and big data analysis can be integrated to deeply mine and analyze a vast array of experimental data to uncover potential patterns and optimization directions. Concurrently, establishing more accurate theoretical models that consider additional factors, such as surface defects and interfacial stress, is essential for more precisely predicting the electronic structure and catalytic performance of TMDs. In terms of large-scale experimental research, strengthening collaboration with the industrial sector to jointly develop efficient synthesis routes and scalable production technologies is vital. Utilizing advanced reactor designs and automated control technologies can facilitate the large-scale, high-quality production of TMD catalysts. Moreover, focusing on environmental protection and sustainable development by exploring green, low-cost synthesis methods is imperative.

4.4. Construction of Heterojunctions

The catalytic activity of TMDs is often limited via their interlayer stacking and the availability of active sites [286,287]. To overcome these limitations and significantly enhance catalytic performance, constructing heterostructures has become an effective strategy. Via integrating TMDs with other materials, the construction of heterostructures can maximize the comprehensive advantages of each component, produce synergistic effects, and thus improve catalytic efficiency [288,289,290,291,292].

Cheng-Yu Tsai et al. [293]. have successfully synthesized a unique flower-like WS₂/WSe₂ heterostructure through a one-step solvothermal reaction at 230 °C for 24 h by mixing tungsten hexacarbonyl (W(CO)₆) with sulfur (S) and selenium (Se) powders in para-xylene. Initially, smooth nanospheres formed within the first 5 h, which then evolved into nanosheets between 5 and 12 h, culminating in the formation of nanoflower-like WS₂/WSe₂ heterostructures after 24 h. XRD analysis confirmed the characteristic diffraction peaks of both WS₂ and WSe₂, indicating the successful integration of the two disulfides. SEM observations revealed ultrathin nanosheets, approximately 5 nm thick and 200 nm wide, assembled into 3D nanoflowers with diameters ranging from 700 to 900 nm, providing a rich platform for catalytic activity. HRTEM images further delineated the lattice fringes of 0.647 nm and 0.689 nm, corresponding to the (002) planes of WS₂ and WSe₂, respectively, validating the precise construction of the heterostructure. This WS₂/WSe₂ heterostructure exhibited exceptional performance in electrocatalytic hydrogen evolution, with a low overpotential of 121 mV at 10 mA/cm² and a small Tafel slope of 74.08 mV/dec. Its stability was also remarkable, showing negligible degradation after 1000 cyclic voltammetry scans and maintaining 95% of the current density after 30 h of continuous operation. Lei Sun et al. [294] proposed a method involving the exfoliation of MoS₂ using sodium-functionalized chitosan (Na-Chitosan), followed by the electropolymerization of polyaniline (PANI) and electrochemical activation to construct Na-Chitosan/MoS₂/PANI/NF electrodes with P-N heterojunction interfaces. By exfoliating pristine MoS₂ in Na-Chitosan solutions with varying concentrations, they optimized the solid content of MoS₂ nanosheets, achieving a maximum concentration of 1.85 mg/mL at an Na-Chitosan concentration of 5 mg/mL. The 1/5 Na-Chitosan/MoS₂ ratio yielded the most stable dispersion with an average hydrodynamic size of 135.08 ± 0.64 nm and a zeta potential of −46.56 ± 1.99 mV. The integration of MoS₂ nanosheets into the PANI matrix through electropolymerization and the formation of P-N heterojunctions resulted in electrodes with enhanced electrocatalytic performance. Electrochemical tests in 0.5 M sulfuric acid demonstrated that the Na-Chitosan/MoS₂/PANI/NF electrodes exhibited a resistance of 1.7 ohms, an overpotential of −42.7 mV at −10 mA/cm², and a Tafel slope of 26.3 mV/dec, showing excellent stability over 24 h of operation at both 100 and 500 mA. Jiang-Yan Xue et al. [290] developed a hydrothermal selenization strategy to fabricate two novel MoSe₂/NiSe heterojunction nanocomposites: the triphase MoSe₂/NiSe-1 and the tetraphase MoSe₂/NiSe-2. The MoSe₂/NiSe-1, composed of 1T-MoSe₂, 2H-MoSe₂, and hexagonal NiSe (H-NiSe), and MoSe₂/NiSe-2, which includes an additional rhombohedral phase of NiSe (R-NiSe), were synthesized by selenizing NiMoO₄ nanorods under controlled conditions. Structural and compositional analyses confirmed the formation of these heterostructures, with MoSe₂/NiSe-1 showing a low overpotential of 30 mV at 10 mA/cm² and excellent stability, even after 40 h of operation at 200 mA/cm². DFT calculations elucidated the enhanced HER activity of MoSe₂/NiSe-1, attributing it to the uniform electron distribution at the interfaces, which facilitates electron transfer and improves conductivity, aligning with the experimental observations of superior electrocatalytic performance.

In the future, research on transition metal dichalcogenide (TMD) heterostructures is expected to achieve atomic-level precision control, thereby customizing their electronic structure and catalytic properties to meet the needs of specific reactions. Concurrently, in-depth studies on their stability and catalytic performance under extreme conditions, such as high pressure, high temperature, and intense radiation, will expand their application scope. Additionally, developing wearable or flexible catalytic devices based on TMD heterostructures to accommodate the development needs of future portable and mobile energy devices is also an important direction. Exploring the integration of TMD heterostructures with biological systems, such as functionalizing them with biomolecules or cells for biocatalysis or biomimetic catalysis, will bring new breakthroughs to the field of catalysis. Constructing multifunctional integrated platforms based on TMD heterostructures to simultaneously perform multiple catalytic reactions or energy conversion processes will enhance energy utilization efficiency. Advanced scanning probe techniques or in situ characterization methods can be employed to achieve precise measurement and control of the atomic structure and electronic properties of TMD heterostructures. High-pressure synthesis techniques or high-temperature thermal treatment methods can be used to prepare TMD heterostructures with special structures and properties suitable for applications under extreme conditions. Developing fabrication technologies for TMD heterostructures on flexible substrates, such as printing, spraying, or roll-to-roll processing, can enable large-scale production of wearable or flexible catalytic devices. Utilizing biotechnological engineering to immobilize biomolecules (such as enzymes, antibodies, etc.) or cells on the surface of TMD heterostructures can construct bio-inorganic hybrid catalysts. Employing quantum computing and simulation techniques can predict the electronic structure, band structure, and catalytic reaction pathways of TMD heterostructures, guiding experimental design and material optimization. These strategies will not only advance the fundamental understanding of TMD heterostructures but also pave the way for their practical applications in various fields, including energy conversion, environmental remediation, and biomedical engineering.

5. Summary and Outlook

This article provides a comprehensive and in-depth exploration of the application of TMDs in the field of electrocatalytic water splitting. With their unique molecular structure MX₂ (where M represents transition metal elements and X represents elements such as sulfur, selenium, or tellurium) and a variety of crystal phases (1H, 2H, 3R, 1T, and 1T’), TMDs have shown tremendous potential in electrocatalysis. Their layered structure not only facilitates the regulation of surface area, porosity, and conductivity but also enriches the surface with active sites that can effectively enhance the adsorption and activation of water molecules, thereby accelerating the reaction kinetics and reducing overpotential.

In terms of the core reaction mechanisms of electrocatalytic water splitting, the detailed pathways and key parameters of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have been deeply studied. To further enhance the catalytic performance of TMDs, researchers have employed various innovative strategies, including creating more active sites, doping, phase engineering, and constructing heterojunctions. These methods effectively modulate the electronic structure of TMDs, increase the number of active sites, improve conductivity, and produce synergistic effects, thereby significantly enhancing their catalytic activity and stability.

Despite the remarkable progress made in the field of TMD electrocatalytic water splitting, there are still many challenges to be overcome in future research. First and foremost, it is crucial to delve deeper into the catalytic mechanisms. Our understanding of the catalytic mechanisms of TMDs is still limited and requires further exploration using advanced characterization techniques and theoretical calculations, such as in situ characterization and density functional theory calculations, to reveal their intrinsic nature. Via conducting in-depth studies on the properties of active sites, changes in electronic structure, and the adsorption and transformation processes of reaction intermediates, we can provide more precise theoretical guidance for the design and optimization of catalysts, thus promoting the development of more efficient TMD catalysts.

The development of new TMD materials holds broad prospects, but the types of TMD materials studied to date are relatively limited. It is still necessary to actively explore more new materials in the future, such as TMDs with special structures or functions, to meet the diverse needs of different electrocatalytic reactions. In addition, continuing to optimize the performance of catalysts is an ongoing goal. Although the performance of current TMD catalysts has been significantly improved, further optimization can be achieved via precisely controlling their structure, composition, and morphology, as well as constructing heterostructures. It is also important to continue exploring new synthesis methods and post-treatment techniques. Strengthening research on practical applications is also a key direction. Most of the current research on TMD electrocatalytic water splitting is at the laboratory stage, and there is a need to enhance research on their performance and stability in large-scale electrolytic water devices, as well as their compatibility with other components. Developing effective catalyst loading and integration technologies will help transition from the laboratory stage to practical applications. At the same time, in-depth research on the durability of catalysts is also indispensable. It is necessary to explore the stability and durability of TMD catalysts during long-term reactions, reveal their deactivation mechanisms, and develop strategies to improve durability. Existing methods include surface modification, doping, etc., which can enhance the stability of the catalyst and extend its service life. Expanding the application fields of TMDs is also of great significance. In addition to electrocatalytic water splitting, TMDs also have potential application value in the fields of carbon dioxide reduction, nitrogen reduction, photocatalysis, electrocatalytic organic synthesis, etc., providing more effective solutions for solving clean energy and environmental protection issues. Finally, strengthening interdisciplinary cooperation will bring new opportunities for TMD research, involving multiple disciplines such as materials science, chemistry, and physics. Therefore, it is necessary to integrate the advantages of different disciplines, carry out close cooperation, and jointly tackle key issues to achieve the transition from basic research to practical application.

TMDs have broad application prospects in the field of electrocatalytic water splitting. Through continuous in-depth research and continuous innovation, we are confident in developing more efficient and stable TMD catalysts, making an important contribution to achieving sustainable energy conversion and solving environmental issues. It is believed that in future research, TMDs will play a more critical role in the field of energy, promoting continuous breakthroughs and progress in related technologies.