Strategies to improve electrocatalytic performance of MoS₂-based catalysts for hydrogen evolution reactions

Abstract

Electrocatalytic hydrogen evolution reactions (HERs) are a key process for hydrogen production for clean energy applications. HERs have unique advantages in terms of energy efficiency and product separation compared to other methods. Molybdenum disulfide (MoS₂) has attracted extensive attention as a potential HER catalyst because of its high electrocatalytic activity. However, the HER performance of MoS₂ needs to be improved to make it competitive with conventional Pt-based catalysts. Herein, we summarize three typical strategies for promoting the HER performance, i.e., defect engineering, heterostructure formation, and heteroatom doping. We also summarize the computational density functional theory (DFT) methods used to obtain insight that can guide the construction of MoS₂-based materials. Additionally, the challenges and prospects of MoS₂-based catalysts for the HER have also been discussed.

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1. Introduction

With the rapid growth of the economy, the traditional, polluting sources of energy are being consumed heavily, resulting in environmental degradation;^1–3 thus, developing sustainable and clean energy has become urgent. Among all the alternative energies, hydrogen has significant advantages due to its high energy density and environmental friendliness.⁴ Currently, mass H₂ production mainly comes from steam reforming of hydrocarbons, which is highly energy-consuming and relies on the polluting petroleum industry.^5–7 To promote the adoption of the H₂ economy, in the past several decades, new H₂ production technologies, like photocatalytic and electrocatalytic water splitting, have been developed.^8–12 Compared with photocatalytic water splitting, the electrochemical approach has more advantages in terms of energy efficiency and achieving effective product separation. Electrochemical water splitting consists of two half-reactions, which are named as the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).¹³ The OER is hard to occur, because it involves four-electron transfer.¹⁴ The reaction mechanism is as follows:

Total reaction:

In acidic solution:

Cathode: 2H⁺(aq) + 2e⁻ → H₂(g) (2)

In neutral and alkaline solution:

Cathode: 2H₂O(l) + 2e⁻ → H₂(g) + 2OH⁻(aq) (4)

Generally, the HER consists of the three following steps:

Volmer step: H⁺ + e⁻ → H* (6)

Heyrovsky step: H* + H⁺ + e⁻ → H₂ (7)

Tafel step: 2H* → H₂ + 2* (8)

In acidic electrolytes, the HER occurs when protons are adsorbed at the surface of the MoS₂ and consequently, step (6) takes place, which is then followed by step (7) or (8). The overall reaction rate relies on the Gibbs free energy of H₂ adsorption (ΔG_H*). In theory, the closer this value is to zero, the greater is the HER activity. The reactions (7) or (8) are inhibited when the binding of H to the catalyst surface is weak. Therefore, catalysts with excellent HER performance tend to exhibit good adsorption of H atoms, which is conducive to charge transfer and bond breakage, resulting in the formation of H₂.^9,15–18

Under standard conditions, initiating HER requires a dynamic overpotential, which essentially involves additional energy consumption; thus, electrocatalysts are integral to abate the overpotential. As of today, Pt-based materials are the most widely used electrocatalysts; however, Pt is scarce and costly, making it essential to develop cost-effective electrocatalysts. Currently, a wide range of materials, including transition metal alloys,^19–27 transition metal carbides,^28–36 transition metal dichalcogenides,^37–45 transition metal phosphides,^46–50 and so on, are being studied as possible alternative HER electrocatalysts. Among them, MoS₂ has attracted wide attention due to its unique two-dimensional layered structure and adjustable band gap width, which makes MoS₂ has a property of ease incorporation into the lattice (heteroatoms and vacancies) and adjustability of electron transport. However, experimental and theoretical studies reveal that the HER activity of MoS₂ remains unsatisfactory due to three main issues. First, the HER activity of MoS₂ correlates with the active edge sites rather than basal planes, but the edge sites just account for a small part of the surface area of MoS₂. Second, the semiconducting basal plane of MoS₂ has a low intrinsic electronic conductivity, which leads to poor charge transfer. Third, the aggregation and restacking of MoS₂ hinders the availability of edge sites. Herein, we discuss three important approaches that have been demonstrated to improve the intrinsic catalytic activity and HER performance of MoS₂ effectively, i.e., defect engineering, formation of the heterostructure, and heteroatom doping (Fig. 1).

Fig. 1 Structural polytypes of MoS₂:1T, 2H, and 3R. Reproduced with permission from ref. 51. Copyright 2020, Elsevier.

2. Properties of MoS₂

MoS₂ is a typical layered transition metal dichalcogenides composed of stacking S–Mo–S layers by van der Waals interactions, leading to a sandwich-like layered structure.^51,52 The basal plane of MoS₂ is generally believed to be inert toward HER, while the edge sites of MoS₂ layers possess much higher chemical reactivity than the basal plane.¹³ Based on the arrangement of the S atoms, three phases of MoS₂ can be obtained, i.e., 1T, 2H and 3R.⁵³ The 2H phase MoS₂ is thermodynamically stable, but the conductivity of 2H phase MoS₂ is poor.^54,55 Compared to 2H MoS₂, 1T MoS₂ is metallic and is 10⁷ times more conductive than 2H MoS₂.⁵⁶ Nevertheless, 1T MoS₂ is thermodynamically unstable and does not exist in nature⁵⁷ (Fig. 2). Under some specific conditions,1T MoS₂ can also exist and phase transformation is one of the most effective methods to improve the catalytic performance of MoS₂.^58,59 Zhao et al.⁶⁰ investigated the HER dynamics in the MoS₂ basal plane of the 1T and 2H phases. They calculated the absorbed H free energies on the 2H/1T structural interfaces and their work offered a new way to increase the number of MoS₂ active sites on the basal plane. DFT calculations demonstrated that MoS₂ had a similar H binding energy as that of Pt.⁶¹ In theory, MoS₂ should exhibit outstanding HER performance; however, bulk MoS₂ displays poor performance due to the following reasons. (i) The active sites of bulk MoS₂ cannot be fully exposed.⁶² (ii) The superimposition of the van der Waals force in the interior of the bulk MoS₂ leads to an incongruous effect of the active sites.⁶³ (iii) The covalently bonded atoms in the layers lead to poor HER performance.⁶⁴ (iv) The electronic conductivity of bulk MoS₂ is low.⁶⁵ Thus, many efforts have been devoted to designing a reasonable MoS₂ nanostructure and modulating the MoS₂ electronic structure.^66–68 Furthermore, many engineering methods such as phase transformation engineering, vacancy engineering, and stretching have been developed to overcome the disadvantages of bulk MoS₂.^69–73 Cao et al.⁷⁴ demonstrated that S vacancies could provide another type of active site for the HER besides the Mo and S active edge sites. They also found that the HER activity of grain boundaries was much lower than that of the S vacancy and S edge sites. Based on the structure determining properties, many efforts have been devoted to constructing specific MoS₂ nanostructures, including nanoparticles, nanoflakes, nanowires, nanoflowers and nanosheets.^75–79 These nanostructures can maximally expose active edge sites of MoS₂ and shorten the electron transfer distance.⁸⁰

Fig. 2 (a) Schematic illustration for synthesis of MoS₂/C. Reproduced with permission from ref. 116, Copyright 2014 Elsevier. (b) Schematic illustration of 1T/2H MoS₂ phase transition. Reproduced with permission from ref. 117, Copyright 2017 Royal Society of Chemistry. (c) Schematic of the synthesis process for MoS₂/NiCoS heterostructures. Reproduced with permission from ref. 118, Copyright 2019 Royal Society of Chemistry. (d) Schematic illustration of HER process and DFT calculation results. Reproduced with permission from ref. 119, Copyright 2021 Elsevier. (e) Molybdenum oxide chain in (H₂en)Mo₃O₁₀ and the proposed formation mechanism of 1T/2H-MoS₂/RGO. Reproduced with permission from ref. 120, Copyright 2021 Elsevier.

3. Synthetic methods for MoS₂

In order to synthesize MoS₂, many strategies have been developed, including mechanical exfoliation,^81–83 solution exfoliation,^84–86 chemical vapor deposition^87–89 (CVD), hydrothermal synthesis,^90–92 electrodeposition methods,^93–95 metal–organic chemical vapor deposition (MOCVD),^96–98 and molecular beam epitaxy (MBE).^99–101 These methods can be classified as top-down methods (including mechanical exfoliation and solution exfoliation) and bottom-up methods (including hydrothermal synthesis, CVD, MOCVD and MBE). In the top-down methods, the morphology of MoS₂ can be well controlled since the structure of the final product is inherited from the original materials.¹⁰² However, top-down methods have intrinsic shortcomings; for instance, the nanoscale morphology of the final product is hard to adjust since it is difficult to change the distance between ions and atoms in top-down methods.¹⁰³ In bottom-up methods, a large and complicated system are formed by atoms, molecules and nanoparticles through self-assembly or weak interactions, and the usage of materials is more efficient.¹⁰⁴ The specific surface area and thickness of MoS₂ can be better controlled than in top-down methods. For example, in hydrothermal method, the thickness and size of MoS₂ can be modulated by simply tuning the hydrothermal time, temperature and growth parameters of solution concentration.¹⁰⁵ The main disadvantage of bottom-up methods is the difficulty of large-scale synthesis.^106,107 In the rest of this section, we try to list out the general methods of synthesizing MoS₂ for the electrochemical HER (Table 1).

Table 1 Comparison of synthetic methods of MoS₂-based catalysts

Synthetic method	Advantages	Disadvantages	Reference
Hydrothermal synthesis	Easy to operate, high purity, good dispersibility, complete crystal form, uniform particle size	Easy to agglomerate, unclear reaction mechanisms, unsuitable for water sensitive materials, poor repeatability	90–92
Chemical vapor deposition	Simple film-forming device, easy to control the product composition, good repeatability, good uniformity	May produce corrosive, toxic or explosive reaction gas, low film forming rate, easy to introduce impurities	87–89
Mechanical exfoliation	Few defects, smooth surface, high mobility	Difficult to control the thickness, random position on substrate, small output	81–83
Solution exfoliation	Low cost, high controllability, easy to realize large-scale preparation	Small product size, low yield	84–86
Electrodeposition methods	Adjustable grain size, easy to operate, low cost, high efficiency	Difficult to control growth rate, low crystallinity of product	93–95
Metal–organic chemical vapor deposition	Low reaction temperature, wide application range, suitable for mass production	May produce toxic and flammable vapor, difficult to in situ monitor the growth process	96–98
Molecular beam epitaxy	Atomic-level controlled film thickness, composition and dopants	High requirements for equipment conditions	99–101

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3.1. Hydrothermal synthesis

Hydrothermal method is to seal the reactants in the reactor, and the reaction process is completed through chemical transmission. Liquid or gaseous water is the pressure transmission medium under high temperature and high pressure, and most of the reactants can be dissolved in water, so that the reaction can be carried out in the critical state of gas phase and liquid phase solvent. Compared with other powder preparation methods, the powder prepared by hydrothermal methods has the advantages of complete grain development, small particle size, uniform distribution, light particle agglomeration, cheaper raw materials, and appropriate stoichiometry and crystal shape are easily obtained.^108–111 Typically, the ceramic powder prepared by hydrothermal methods does not need high-temperature calcination treatment, which prevents grain growth, defect formation and introduction of impurities in the calcination process, so the prepared powder has high sintering activity.^112–115

Liu et al.¹¹⁶ prepared ultrathin MoS₂ nanosheets grown on an in situ-formed polysaccharide matrix derived from glucose condensation via a hydrothermal method. 0.80 g of glucose with a varied amount of ammonium heptamolybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O) and thiourea (CS(NH₂)₂) was dissolved in 15.0 mL of distilled water, and then the solution was turned to a Teflon-lined stainless-steel autoclave, with a hydrothermal treatment at 240 °C for 48 h. After centrifugation, washing and annealing, the MoS₂/C nanospheres was successfully synthesized. Wang et al.¹¹⁷ prepared 1T/2H MoS₂ via a facile hydrothermal method with the addition of ammonium bicarbonate. During the hydrothermal process, ammonium bicarbonate was decomposed into small molecules and ions as guests, including NH₄⁺, H₂O and CO₂, which were inserted into the lamellar structures of MoS₂, inducing the formation of multiphasic 1T/2H MoS₂. Zhang et al.¹¹⁸ used the hydrothermal method to construct MoS₂/NiCoS nanosheets with ultrathin NiCo bi-metal–organic framework (NiCo-MOF) nanosheets and (NH₄)₂MoS₄ as precursors. The hydrothermal process was controlled at 200 °C for 24 h. In this reaction, (NH₄)₂MoS₄ not only acts as a precursor for the synthesis of MoS₂, but also provides a source of sulfur for in situ conversion of NiCo-MOF nanosheets to NiCoS. Li et al.¹¹⁹ prepared V-doped 1T MoS₂ nanosheets as a highly efficient HER electrocatalyst in both acidic and alkaline solutions through a hydrothermal method. Typically, 0.2 g sulfur was put into 4 beakers with different molar quantity of ammonium heptamolybdate ((NH₄)₆Mo₇O₂₄·4H₂O). Then, 4 mL of hydrazine monohydrate (N₂H₄·H₂O) was dissolved in each beaker. Then, 36 mL of DI water was added into the beakers to make the mixture 80% of autoclave volume, and different molar quantity of ammonium metavanadate (NH₄VO₃) were added into the beakers as the V source. Then the precursors were transferred to 50 mL stainless steel autoclaves. The whole set was sealed and put into the oven at 200 °C for 48 h. Xiao et al.¹²⁰ synthesized 1T/2H MoS₂@graphene via a hydrothermal method. The introduction of metallic 1T-MoS₂ endues the as-prepared materials with remarkably electrocatalytic activity and favorable kinetics, highly efficient and stable HER performance.

3.2. Chemical vapor deposition

Chemical vapor deposition (CVD) is a method of synthesizing coatings and nanomaterials deposited on a substrate surface by chemical gas or vapor reaction. It is the most widely used technology for depositing a variety of materials in the semiconductor industry, including a wide range of insulating materials, most metal materials and metal alloy materials.^121–123 Theoretically, the CVD process is very simple: two or more gaseous raw materials are introduced into a reaction chamber, which then react with each other to form a new material that is deposited on the wafer surface. However, experimentally, the reaction that takes place in the reaction chamber is very complex, and there are many factors that must be considered. The deposition parameters, which can vary over a wide range, include the pressure in the reaction chamber, the temperature of the wafer, the flow rate of the gas, the distance of the gas through the wafer, the chemical composition of the gas, the ratio of one gas to another, the role of the intermediate product of the reaction, and whether other reactions are needed.^124–128

Pumera et al.¹²⁹ synthesized MoS₂ films via powderless gas deposition. The CVD fabrication method they used is suitable for industry because this powderless and one-step process eliminates the deviations of MoS₂ growth which are likely to arise from the usage of a powder precursor. Yu et al.¹³⁰ hybridized MoS₂ micro-flowers on VS₂ by a one-pot CVD method and their catalyst exhibited excellent HER performance. Atomic layer deposition (ALD) is a method in which substances can be deposited on the surface of the substrate, layer by layer, in the form of a monoatomic film.¹³¹ ALD has similarities with ordinary chemical deposition, except that the chemical reaction of the new layer of atomic film is directly dependent on the previous layer. In this way, only one layer of atoms is deposited per reaction. Due to its highly controllable deposition parameters (thickness, composition, and structure), atomic layer deposition technology, excellent deposition uniformity and consistency, it has wide application potential in the field of micro-nanoelectronics and nanomaterials. Tan et al.¹³² successfully synthesized layered MoS₂ via the ALD method. Typically, MoS₂ films were prepared by alternating exposure to molybdenum chloride (MoCl₅) and hydrogen disulfide (H₂S) vapors. Due to the self-limiting reactions of the vapors, the number of MoS₂ film layers can be precisely controlled by the number of deposition cycles.

3.3. Exfoliation method

At present, the most effective preparation methods of single- or few-layer MoS₂ are exfoliation methods, which can be divided into mechanical exfoliation, lithium-ion intercalation exfoliation and liquid phase ultrasonic exfoliation. In the mechanical exfoliation method, MoS₂ powder is directly stripped to form a thin layer of MoS₂ nanoflakes with a special gummed tape. This is the most original and traditional preparation method. The thickness of MoS₂ that was first peeled off in 1965 ranged from a few layers to dozens of layers.¹³³ After the improvement of this stripping process, monolayer MoS₂ was successfully obtained.¹³⁴ The most obvious advantage of the mechanical exfoliation method is that the process is simple, can be obtained manually, and the stripping efficiency is high. Since this is direct exfoliation, the monolayer MoS₂ can maintain a good single crystal structure and offer high carrier mobility. However, this method has obvious disadvantages, such as low yield and poor repeatability. The lithium-ion intercalation-based exfoliation method can be used, for instance the Li_xMoS₂ (x ≥ 1) intercalation compound is obtained by the reaction of the lithium-ion intercalation agent (such as n-butyl lithium) and MoS₂ powder (intercalation process). Then the intercalation compound reacts violently in water and other protonic solvents, and the stripping process occurs. Thus, multilayer and monolayer MoS₂ can be obtained by ultrasonic extraction. In 1986, Morrison et al.¹³⁵ prepared monolayer MoS₂ by this method. The efficiency of the Li ion intercalation stripping was found to be very high, with a yield close to 100%. The quality of the prepared nanosheets was very good, making it suitable for mass-production, and the method could be used to peel many other inorganic layered compounds. However, this preparation method is complex, the intercalation agent is sensitive to the environment, leading to a high production cost, and long preparation time. Moreover, it is difficult to control the degree of intercalation, which makes the post-treatment difficult. For this reason, Zeng et al.¹³⁶ used an electrochemical lithium battery device to control the lithium ion insertion and stripping process, so as to prepare a variety of monolayer materials such as MoS₂. This electrochemical lithium-ion intercalation exfoliation method can effectively avoid many of the abovementioned problems and obtain high yield monolayer MoS₂. The basic principle of the liquid-phase ultrasonic exfoliation method is to put MoS₂ and other layered materials into a suitable solvent, use the ultrasonic wave to vibrate and peel off the lower layer of materials from the block, and then centrifuge to obtain thin MoS₂ nanosheets. In 2011, Coleman et al.¹³⁷ prepared monolayer and multilayer nanosheets such as MoS₂, BN and WS₂. The solvent selected by the liquid phase ultrasonic exfoliation method is generally a polar solvent with good dispersity and solubility for layered materials and similar surface energy. Coleman et al.¹³⁸ prepared thin MoS₂ material by adding a sodium cholate surfactant and stripping it in an aqueous phase. However, Zhou et al.¹³⁹ used ethanol/water mixed low-boiling solvents to successfully peel off nanosheets such as MoS₂, making the peeling method cheaper and environmentally friendly, and peeling MoS₂ under water phase conditions has become a new choice for people. The liquid-phase ultrasonic peeling method is simple and fast, and can be suitable for large-scale production, but generally the peeling degree is not high, the concentration of the obtained nanosheet solution is small, and the dependence on the ultrasonic conditions is high, and the ultrasonic power is too large or too small, which is not conducive to the formation of nanosheets.

3.4. Electrodeposition method

Electrodeposition method is an attractive way to synthesize MoS₂ thin film in large scale because it uses cheap equipment, enables the deposition in large area and easy control of growth parameters through applied potential, current, pH and temperature of the bath.¹⁴⁰ Many researchers have studied the mechanism of an electrochemical deposition of MoS₂ from aqueous and little from non-aqueous solutions. A variety of electrodeposition techniques have been reported for the preparation of MoS₂ films using anodic oxidation or cathodic reduction.

Ping et al.¹⁴¹ developed an edge-guided electrodeposition approach for the construction of high quality edge contacted metal-2D MoS₂ and further demonstrated the potential of this approach in the fabrication of high performance electrocatalyst. Hwang et al.¹⁴² developed a novel MoS₂ electrode with a nanocarbon (NC) coating as a catalytic cathode for hydrogen production in a microbial electrolysis cells (MEC), while treating simulated urine. The MEC performance of HER using the MoS₂-NC cathode was characterized and evaluated under different applied voltages and various dilution factors for adjusting conductivity. Lyndi et al.¹⁴³ prepared MoS₂ films via an electrodeposition method. Electrodeposition of molybdenum disulfide films was carried out with a 10 mM (NH₄)₂MoS₄ (ammonium tetrathiomolybdate) with 0.1 M LiClO₄ adjusted to pH 10 with NH₄OH using chronoamperometric deposition. Jiang et al.¹⁴⁴ prepared the MoS₂–Co₃S₄/NF heterostructure layer by Co-MOF vulcanization and cyclic voltammetry (CV) method, which showed better electrochemical performance than the single-layer Co₃S₄/NF. The as-prepared MoS₂–Co₃S₄ heterostructure layer was decorated by ZnCo-LDH, the hydrogen evolution performance was further improved.

Though the above synthetic methods have been demonstrated to be an effective way to synthesize bulk MoS₂, the HER performance of bulk MoS₂ is still not satisfactory. In the following part, we summarize three main strategies to promote the HER performance of MoS₂ (Fig. 3).

Fig. 3 (a) Schematic diagram of preparing MoS₂/VS₂ hybrid by a one-pot CVD process. The SEM images of MoS₂/VS₂ hybrid observed at different magnifications. (b) Low magnification; (c) middle magnification; and (d) high magnification. The morphological evolutions of MoS₂/VS₂ hybrid intercepted at different temperatures; (e) formation of pristine interfaced flake-like VS₂ crystals at 300 °C, with an inset shows the high-magnification of VS₂ crystals; (f) initial nucleation of flower-like MoS₂ on the existing VS₂ crystals at 730 °C; and (g) further growth of flower-like MoS₂ on underlying flake-like VS₂ crystals at 750 °C. (h) The EDX mapping of MoS₂/VS₂ hybrid, in which the elemental distributions of vanadium, molybdenum, and sulfur are illustrated in red, green, and yellow, respectively. Reproduced with permission from ref. 130, Copyright 2021 Elsevier. (i) Schematic illustration for a one-step powderless deposition growth process of 2H-MoS₂. Reproduced with permission from ref. 129, Copyright 2021 Elsevier.

4. Main strategies to promote the HER performance of MoS₂

4.1. Defect engineering

Many physical and chemical properties of materials are closely related to the existence of defects.^145–147 Generally, defects can be classified as vacancies, grain boundaries and disorders.^148–150 Furthermore, vacancies can consist of, for instance, Mo vacancies, S vacancies, O vacancies, N vacancies and C vacancies.^151–155 Thermodynamically, it's unavoidable to introduce vacancies in the process of synthesizing MoS₂.¹⁵⁶ Many studies have examined the impact of vacancies on the HER performance of MoS₂.^157–167 Theoretically, the rational design of defects on the basal plane of MoS₂ can enhance the HER performance of MoS₂ because the basal plane of MoS₂ is inert while the edge sites are significantly more active.

Li et al.¹⁶⁸ designed vertically aligned MoS₂ with a selectively cleaved Mo–S bond. Their work showed a new way to synthesize MoS₂ with a vacancy and their synthesis route is shown in Fig. 4(a). Fig. 4(b) shows that the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images confirm the existence of micro-cubic c-MoS₂–C morphology. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC HAADF-STEM) images show the existence of the vacancy. This unique defect is beneficial to a thermoneutral H-adsorption energy, which leads to accelerate HER kinetics. The energy-dispersive X-ray spectroscopy (EDS) elemental mapping results further confirm the formation of MoS₂ nanosheets with Mo and S defects. The electrocatalytic performance of the defect-rich MoS₂ is shown in Fig. 5. Further DFT calculations reveal that the newly generated interior edge sites break the periodic electronic structure of the MoS₂ basal plane, leading to an optimized surface charge configuration for H adsorption and desorption.

Fig. 4 (a) Schematic representation of the formation of c-MoS₂−C; (b) SEM images of HZIF-Zn/Mo and c-MoS₂−C. TEM image, HRTEM images, FFT-generated image of c-MoS₂−C. Atomic-resolution HAADF-STEM images of c-MoS₂−C with multiple Mo_xS_y vacancies. EDS elemental mapping for c-MoS₂−C. Reproduced with permission from ref. 168, Copyright 2021 American Chemical Society.

Fig. 5 The HER performance of c-MoS₂−C and the reference samples. Reproduced with permission from ref. 168, Copyright 2021 American Chemical Society.

Li et al.⁷⁴ conducted a study of the catalytic activities of all potential reaction sites, such as edge sites, S vacancies and grain boundaries. For the first time, they demonstrated that S vacancies provide another active site for HER in addition to the well-known active edge sites (Fig. 6(a)). In order to identify the influence of S vacancy states and concentration on catalysis, Wang et al.¹⁶⁹ synthesized MoS₂ with homo-vacancies, with an H₂O₂ chemical etching strategy (Fig. 6(b)). They performed DFT calculations of ΔG_H* to explore the effect of S vacancy concentration and distribution on the HER performance. The results showed that 12.50% of the single S-vacancies yielded the optimal HER performance. They synthesized a vertically aligned MoS₂ nanosheet guided by these theoretical DFT calculations. The X-ray diffraction (XRD) results in Fig. 7(a) demonstrate that the sample phase was firm and did not change after H₂O₂ etching. To determine the S-vacancy concentration as a function of the etching process, three X-ray photoelectron spectroscopy (XPS) spectra of P-MoS₂ and MoS_2−x are shown in Fig. 7(a). The S : Mo ratio in each spectrum was calculated and it was converted to the corresponding S-vacancy concentration. The electrochemical results are shown in Fig. 7(b) indicate that MoS₂ with 12.11% S-vacancy exhibited the best HER performance and the ratio of S-vacancy was very close to the theoretically predicted optimal value of 12.50%. Furthermore, the superiority of single S-vacancies over agglomerate S-vacancies was found to originate from more effective engineering of the surface electronic structure. The synergistic modulation of catalytic performance from both vacancy concentration and distribution broadens the vacancy design field and can be further extended to other types of TMDs and for other catalytic reactions.

Fig. 6 (a) Schematic illustration for HER activity of MoS₂ with an S vacancy. Reproduced with permission from ref. 74, Copyright 2016 American Chemical Society. (b) Schematic of the chemical etching process to introduce single S-vacancies.

Fig. 7 (a) XRD patterns with magnified regions and Raman spectra with magnified regions of P-MoS₂ and MoS_2−x. High-resolution Mo 3d XPS spectrum, high-resolution S 2p XPS spectrum, and S L-edge XANES spectra of P-MoS₂ and MoS₂-60s. (b) Electrochemical HER performance of P-MoS₂ and MoS_2−x. Reproduced with permission from ref. 169, Copyright 2020 American Chemical Society.

Forming pits on MoS₂ monolayers has been found to be desirable for promoting HER performance. Park et al.¹⁷⁰ investigated the effect of adventitious C on the pit formation in the MoS₂ layers. They used an aperture type in situ ETEM to study the reaction between MoS₂ monolayers and O₂. The HER activity of MoS₂ was enhanced in the presence of pits, which can be ascribed to the increase of active edge sites (Fig. 8(b)). They also conducted DFT calculations and found that the presence of C nanoparticles on monolayer MoS₂ was favorable for progressive oxidation (Fig. 8(a)).

Fig. 8 (a) DFT calculation of adsorption energies of three sequential O atoms, for three different model structures. (b) Comparisons of HER activity between the pristine monolayer MoS₂ and monolayer MoS₂ with pits. Reproduced from ref. 170, Copyright 2020 John Wiley and Sons.

Although the existence of defects can significantly improve the HER performance of MoS₂, the key is the ability to detect the existence of defects. Thus, we need to study the relationship between defect types and electronic modes. Up till now, the existence of defects was observed indirectly because the concentration of defects is typically very low. One of the detecting methods is Raman spectroscopy, which can effectively detect edge defects and show distinct electronic structure. The vital task of defect engineering is to enhance the electrochemical activity of the MoS₂ basal plane.

4.2. Formation of heterostructure

The construction of heterostructured composites is an effective way to enhance the activity of MoS₂.^171–181 Wei et al.¹⁸² synthesized core–shell structured CoS₂@MoS₂ by a hydrothermal method. As shown in Fig. 9(a), the CoS₂@MoS₂ nanoparticles had a uniform size distribution, ranging from 20–30 nm. The HER performance of CoS₂@MoS₂ in Fig. 9(b) shows that the overpotential of HER was only 276 mV and the Tafel slope was 60 mV dec⁻¹. Mei et al.¹⁸³ constructed a PCN@MoS₂@C sandwich-like heterostructure by a facile hydrothermal method. The PCN@MoS₂@C exhibited excellent HER performance within a wide pH range (Fig. 9(c)).

Fig. 9 (a) SEM images of bulk MoS₂ and CoS₂@MoS₂ nanoparticles. (b) HER performance of CoS₂@MoS₂. Reproduced with permission from ref. 182, Copyright 2020 Elsevier B.V. (c) The HER performance of PCN, PCN@MoS₂, PCN@MoS₂@C and Pt/C. Reproduced with permission from ref. 183, Copyright 2020 the Royal Society of Chemistry.

Constructing heterostructures with multiple electrocatalytically active components simultaneously to improve HER performance is regarded as an effective strategy. He et al.¹⁸⁴ synthesized CoS₂/SnO₂@MoS₂ (denoted as CSM) nanotubes by a fast template-engaged hydrothermal treatment. The SEM and TEM images of the obtained CSM are shown in Fig. 10(a) and (b). These images revealed a porous structure, which enlarged the surface area of the material and boosted the mass transport process, leading to excellent HER performance (Fig. 10(c)). Huang et al.¹⁸⁵ designed an exfoliated few-layer FePS₃ decorated with vertical MoS₂ nanosheets through a facile hydrothermal reaction. The synergistic effect between the FePS₃ and MoS₂ led to excellent HER performance. This effect can regulate the electronic structure and improve the electron conductivity. As shown in Fig. 10(d). The overpotentials of FeS₃@MoS₂ were 175 mV and 168 mV in alkaline and acidic media, respectively. Furthermore, the heterostructure could prevent agglomeration during electrocatalytic cycling, which led to high stability.

Fig. 10 (a) FESEM images of CSM1 nanocubes, CSM2 nanocubes and CSM3 nanocubes (b) TEM image, HRTEM image, the corresponding SAED pattern and TEM-EDX elemental mapping images of CSM1 nanocubes, respectively. (c) HER performance of CSMy (y = 0.5, 1, 2, 3), MoS₂ and Pt/C. Reproduced with permission from ref. 184, Copyright 2020 the Partner Organisations. (d) HER performance of FePS₃, MoS₂ MoS₂@FePS₃ and 20 wt% Pt/C. Reproduced with permission from ref. 185, Copyright 2020 Elsevier B.V.

Tahira et al.¹⁸⁶ reported a stable electrocatalyst composed of TiO₂ nanorods decorated with MoS₂ nanosheets by a two-step hydrothermal method. This electrocatalyst showed an enhanced HER performance compared with bulk MoS₂ (Fig. 11(a)). Cao et al.¹⁸⁷ prepared cobalt molybdenum nitride with a nanoscale morphology using a two-step solid-state reaction. This compound did not belong to any layered structural family prepared for HER; however, it exhibited HER activity comparable to MoS₂ (Fig. 11(b)).

Fig. 11 (a) HER performance of TiO₂/MoS₂, TiO₂, MoS₂, Pt/C. Reproduced with permission from ref. 188, Copyright 2019 American Chemical Society. (b) HER performance of Co_0.6Mo_1.4N_2. Reproduced with permission from ref. 187, Copyright 2013 American Chemical Society.

Theoretically, heterostructure engineering of MoS₂ with graphene is an effective and facile method and the enhanced catalytic performance can be attributed to two factors. First, the induced graphene substrate prevents the agglomeration of MoS₂ nanosheets; thus, creating sufficient edge sites for HER. Second, the excellent electronic conductivity of graphene encourages rapid electron transfer, which is beneficial for the absorption of the H atom and reduces the Gibbs free energy. Furthermore, the heterojunction of the MoS₂@graphene hybrid provides new chemical properties and may be beneficial for an enhanced HER performance. Wang et al.¹⁸⁹ constructed a MoS₂@rGO hybrid catalyst by growing MoS₂ arrays on the rGO surface via a one-step hydrothermal method. The MoS₂@rGO presented a three-dimensional flower-like morphology, with plentiful active edge sites exposed, which contributed to good HER performance. Yan et al.¹⁹⁰ reported a bottom-up method for the large-scale synthesis of 3D hybrid architectures constructed from graphene, MoS₂ and graphitic carbon nitride nanosheets by self-assembly. Due to the distinct architectural features such as 3D interconnected porous networks and low charge-transfer resistance, the MoS₂-CN/G exhibited superior HER performance with low overpotential, small Tafel slope and long-term durability. Wu et al.¹⁹¹ designed MoS₂/MoN heterostructures with tuned components and the catalyst showed excellent HER performance in acid and alkaline electrocatalyst. DFT calculations suggested that the MoS₂/MoN interface could optimize the H atom adsorption kinetic energy, accelerating the HER process. Liu et al.¹⁹² reported a hierarchical ZnS@C@MoS₂ core–shell nanostructure in a bottom-up strategy combined with a selective etching method. The co-existence of ZnS and porous carbon shell not only improved the electrical conductivity but also separated and loaded the MoS₂ nanosheets, which effectively exposed active edge sites. This catalyst showed an excellent HER performance with an overpotential of 118 mV and a Tafel slope of 55.4 mV dec⁻¹. Li et al.¹⁹³ reported a 1T-MoS₂/CoS₂ heterostructure via a self-sacrificing template method. In this work, they compared the cohesive energy of in-plane heterostructure and out-plane heterostructure by DFT calculation and revealed that the cohesive energy of in-plane heterostructure (−5.1 eV) is much lower than that of out-plane (−4.5 eV), which demonstrated that the in-plane heterostructure is thermodynamically more stable.

4.3. Heteroatom-doping method

4.3.1. Metal-doping method. Metal doping is an effective method to promote the HER activity of MoS₂. Transition metals (Fe, Co, Ni, etc.) can significantly enhance the conductivity of MoS₂, and modulate the electronic structure of MoS₂. Co doping can enhance the conductivity of MoS₂, which can decrease the H atom adsorption energy of MoS₂ for HER, and also add new active catalytic sites for HER.

Xiong et al.¹⁹⁴ reported Co-doped MoS₂ nanosheets via a facile one-step hydrothermal method. As shown in Fig. 12(a), the obtained Co-MoS₂ exhibited superior HER activity with an overpotential of 60 mV/90 mV at a current density of 10mA cm⁻² for HER in 0.5 M H₂SO₄ and 1.0 M KOH, respectively. They conducted DFT calculations (Fig. 12(b)) and revealed that the ΔG_H* value of pristine MoS₂ was about ∼2.06 eV while the ΔG_H* value decreased to −0.20 eV after Co doping, demonstrating that Co doping is an effective method for enhancing HER activity of MoS₂. They also studied the OER activity origin of Co-doped MoS₂ by recording the ex situ Co and Mo K-edge EXAFS spectra of Co-MoS₂.

Fig. 12 (a) HER performance of Co-doped MoS₂, MoS₂ and Pt/C. (b) The H₂ adsorption free energy (ΔG_H*) of MoS₂ and Co-doped MoS₂ and the cell voltage of different electrode couples in a two-electrode system in both acidic and alkaline media. Reproduced from ref. 194, Copyright 2018 The Royal Society of Chemistry. (c) HER performance of Fe-MoS₂, MoS₂ and Pt/C. (d) XPS spectra of Fe-MoS₂. Reproduced from ref. 197, Copyright 2019 American Chemical Society. (e) HER performance of Ni-MoS₂, MoS₂ and Pt/C. Reproduced from ref. 198, Copyright 2019 The Authors.

Iron is a typical nonprecious and earth-abundant element, and Fe-containing materials are more effective in adsorbing hydroxyl, which leads to its unique conductivity to water splitting.¹³ Previous studies^195,196 have verified these observations through XPS characterizations. Xue et al.¹⁹⁷ synthesized a series of Fe-doped MoS₂ nanomaterials via a facile one-pot solvothermal method. The Fe-MoS₂ displayed high cycling stability and high HER activity with a small overpotential of 173 mV at a current density of 10 mA cm⁻² (Fig. 12(c)). As shown in Fig. 12(d), the XPS full spectrum confirmed the existence of Fe, S, Mo and O.

Ni doping has a significant impact on surface topography and valence state of Mo in MoS₂, which can accelerate the HER reaction kinetics. Wang et al.¹⁹⁸ prepared Ni-doped MoS₂ via a hydrothermal method. The layer number of MoS₂ can be estimated by the difference in frequency between two characteristic Raman peaks. The results indicate that Ni doping decreases the vibration frequency of MoS₂ and increases the number of layers of MoS₂. Fig. 12(e) shows that the Ni-MoS₂ displayed enhanced HER performance with an overpotential of 164 mV to reach a current density of 10 mA cm⁻² in acidic media and a small charge transfer resistance (R_ct) of 89 Ω.

In addition to Co, Fe, Ni doping,^198–201 some other transition metals can also promote the HER activity of MoS₂. Huang et al.²⁰² prepared a self-supporting electrode composed of Cu mesh-supported graphene and MoS₂ by electroplating zinc, followed by a hydrothermal reaction. The self-supporting catalytic electrode has the advantages of avoiding complex pre-treatment of the active substance and the usage of binders. As shown in Fig. 13(b), the MoS₂–rGO–Cu exhibited excellent HER performance with a low Tafel slope of 54 mV dec⁻¹ and a small overpotential of −300 mV at a current density of 400 mA cm⁻². The excellent HER performance can be ascribed to the good electron transport capability of the Cu mesh, and specifically, the self-supporting electrode can be fabricated into any desired shape, which is well maintained while the HER takes place. Luo et al.²⁰³ designed Pd-doped MoS₂ via a spontaneous interfacial redox technique. The Pd doping converted 2H phase MoS₂ into a stabilized 1T structure. Furthermore, Pd atoms occurred at the Mo site, which simultaneously introduced an S vacancy. As shown in Fig. 13(a), the Pd-doped MoS₂ exhibited good cycling stability and excellent HER activity with an overpotential of 78 mV at a current density of 10 mA cm⁻². The DFT calculations demonstrated that the S atom next to the Pd sites had a low H atom adsorption energy of −0.02 eV. Sun et al.²⁰⁴ designed a novel intralayer V-doping method to synthesize semi-metallic V-doped MoS₂ nanosheets. The introduction of V atoms regulated the intrinsic electrical properties of MoS₂, leading to an enhanced in-plane conductivity and shortened electron transfer paths. The V-doped MoS₂ showed excellent catalytic activity with an overpotential of 130 mV and a small Tafel slope (Fig. 13(c)). Zhang et al.²⁰⁵ reported a single Ru atom-doped MoS₂ electrocatalyst for HER. As shown in Fig. 14(b) and (c), the DFT calculations revealed that the synergistic effects of the single Ru atom doping, S vacancies and phase transition of MoS₂ modulated the electronic structure of MoS₂ and significantly reduced the energy barrier of the Volmer step, leading to enhanced HER performance of Ru-MoS₂ (Fig. 14(a)). Multi-doping methods are also a promising way to enhance the HER activity of MoS₂. Yu et al.²⁰⁶ reported a typical MOF-engaged synthetic method to prepare Ni and Co incorporated MoS₂ nano-boxes. The ultrathin nanosheets obtained exhibited enhanced electrochemical activity for HER (Fig. 13(d)). In recent years, materials with single-atom structure have been recognized as promising HER catalysts due to the maximized atom efficiency and tunable electronic properties. Ni et al.²⁰⁷ reported that Ru-based single-atom catalysts are promising HER catalysts due to its excellent HER performance, which is comparable to commercial Pt/C catalysts. However, Ru-based single atom catalysts have problems such as large amount of Ru and unsatisfactory stability. Ge et al.²⁰⁸ synthesized dual-metallic Ru and Ni atoms decorated MoS₂, which resolve the above problems well. The introduction of Ni has two key roles. On the one hand, the introduction of highly electronegative heteroatom Ni could regulate the electronic structure of Ru and optimize the H* adsorption during the water splitting process, thus vastly enhancing the HER property. On the other hand, the incorporation of highly electronegative heteroatom Ni in the substrate exposes abundant active sites and improves electron transfer of the catalyst, and further anchors the single atom Ru and reduces the consumption of precious metal Ru atoms.

Fig. 13 (a) HER performance of Pd-MoS₂/CP and its comparison. Reproduced with permission from ref. 203, Copyright 2020 Elsevier Ltd. (b) HER performance of MoS₂–rGO–Cu and its comparison. Reproduced with permission from ref. 202, Copyright 2020 Elsevier. (c) HER performance of V-MoS₂ and its comparison. Reproduced with permission from ref. 204, Copyright 2014 The Royal Society of Chemistry. (d) HER performance of Ni–Co–MoS₂ and its comparison. Reproduced with permission from ref. 206, Copyright 2016 WILEY.

Fig. 14 (a) HER performance of Ru–MoS₂ and its comparison. (b) Free energy diagrams of the different MoS₂ model catalysts under alkaline solution. (c) Contour maps of the electron localization function (ELF) in 2H/1T-Ru-MoS₂-Sv. Reproduced with permission from ref. 205, Copyright 2019 WILEY.

4.3.2. Nonmetal-doping method. Nonmetal doping can enhance the electrocatalytic activity of MoS₂. Wang et al.²⁰⁹ synthesized amorphous P-doped MoS₂ nanoparticles hydrothermally. The P-doping promoted the amorphization of MoS₂; thus, affecting the HER performance of MoS₂. Deng et al.²¹⁰ prepared O-doped MoS₂ via an electrochemical anodic activation method. The HER activity of O-doped MoS₂ significantly improved at an overpotential of 50 mV, at a current density of 1 mA cm⁻² (Fig. 15(a)). As shown in Fig. 15(b), the XPS images confirmed the strong electron interactions after O doping, which indicated the existence of Mo–O bonds and demonstrated that the O doping was successful. Liu et al.²¹¹ reported O and P dual-doped MoS₂ nanosheets, which have a porous structure and conductive network via a one-pot hydrothermal approach. As shown in Fig. 15(c), the O, P multi-doped MoS₂ showed superior electrocatalytic performances compared with O-doped MoS₂ nanosheets. Wang et al.²¹² prepared MoS₂ decorated Ni–Fe–N-doped carbon nanotubes via a hydrothermal method. According to the SEM results shown in Fig. 15(d), the catalysts were hierarchically structured. Due to the synergistic effect of the NiFe alloy and N-doped carbon nanotubes, the catalysts exhibited excellent HER performance with a cell voltage of 1.6 V at a current density of 10 mA cm⁻². Tian et al.²¹³ synthesized N, P dual-doped MoS₂ on hollow carbon spheres by a template and hydrothermal process. The synergistic effect of N, P dual-doped carbon and MoS₂ led to the enhanced electrocatalytic performance of MoS₂ with an overpotential of 147 mV, at a current density of 10 mA cm⁻² and Tafel slope of 72 mV dec⁻¹. As shown in Fig. 15(e), the XPS spectra confirmed the co-existence of N and P atoms.

Fig. 15 (a) HER performance of O-doped MoS₂. (b) Mo 3d XPS spectra of O, P-MoS₂ (R = 10%) and O-MoS₂. Reproduced with permission from ref. 210, Copyright Elsevier Ltd. (c) HER performance of O, P dual-doped MoS₂ and its comparison. Reproduced with permission from ref. 211, Copyright 2019 WILEY. (d) SEM images and elemental mapping of NiFe–NCNT–MoS₂. Reproduced with permission from ref. 212, Copyright 2019 The Royal Society of Chemistry. (e) XPS survey spectra and high-resolution scans of B, C 1s, C, N 1s, D, P 2p, E, S 2p, and F, Mo 3d electrons of the P-MoS₂@NCs-2. Reproduced with permission from ref. 213, Copyright 2021 John Wiley& Sons Ltd.

Metal-doping and nonmental-doping can be synergistically used to improve the HER performance of MoS₂. Meng et al.²¹⁴ reported a 3D mesoporous hybrid structure of Co-doped MoS₂ and graphene. The Co-doping enhanced the intrinsic electrocatalytic activity of the in-plane S sites. The graphene network was found to be electronically conductive and robust, which improved the conductivity of electrons and enhanced the stability of the hybrid structure. The catalysts showed good cycling stability and an excellent HER performance, with an overpotential of 143 mV at a current density of 10 mA cm⁻². Wu et al.²¹⁵ designed Co, Se dual-doped electrocatalysts via a solvothermal and post-low-temperature selenylation process. Due to the abundance of interfaces, the electrocatalysts exhibited excellent HER performance, with a low overpotential of 58 mV at a current density of 10 mA cm⁻² and a small Tafel slope of 84 mV dec⁻¹. Sun et al.²¹⁶ reported plasma-induced N, Pt-doping and phase modulation of MoS₂ nanosheets. The synergetic doping of N and Pt heteroatoms regulates the electronic and coordinated structures of MoS₂ and further facilitates water adsorption and dissociation during the HER. Notably, this plasma bombardment method is highly feasible for the large-area fabrication of MoS₂-derived HER active electrodes.

4.4. Theoretical study of the HER

The studies of electronic structures of MoS₂ offer theoretical guidance for designing efficient catalysts. For MoS₂, a typical 2D material, the theoretical calculation of the basal plane positions, Mo edge and S edge is quite significant. Furthermore, it is also beneficial to investigate the adsorption energy of intermediates. Jacob et al.²¹⁷ proposed an H-binding mechanism for S atoms in the basal plane of MoS₂. This model was tested and verified by DFT calculations; herein, the energy barrier can be tuned by introducing metal dopants and defects. Their study revealed the electronic structure mechanism which tends to influence the basal plane S atoms activation (Fig. 16(a) and (b)). Xian et al.²¹⁸ synthesized the monodispersed single-crystal Pt nanoparticles decorated on 2H phase MoS₂ nanosheets via a wet chemical method. They construct the atomic model of Pt truncated octahedron loaded on MoS₂, and choose four Pt sites (Fig. 16(c)). The charge-density difference is showed in Fig. 16(d), indicating the variations of the charge distributions for Pt and S at the interface of Pt-MoS₂ atomic model from those for pure Pt and S. The results reveal that the Pt–S bonds are formed. Then, they calculate the free energy of adsorbed atomic hydrogen (ΔG_H*), including MoS₂ (001), Pt (100), Pt (100) _interface, Pt (1–11) and Pt (1–11) _interface (Fig. 16(e)). The results indicate that the Pt (100) _interface owns the smallest ΔG_H* as well as the best HER activity. Further, they found that with the studied Pt atoms changing to the interface, the D-band center move away from the Fermi level (Fig. 14(f)), indicating the decrease of ΔG_H*, which confirms the superiority for Pt (100) _interface in HER performance. In published DFT studies, the electronic structures of MoS₂-based materials are usually computed by VASP and the electron–ion interactions are described by the projector-augmented wave method. All calculations are typically implemented with PBE exchange–correlation functional on periodically repeated slabs. The details of the molecular model-building, HER mechanism and free-energy calculations are provided in the relevant ref. 219–222.

Fig. 16 (a) Left: ball model of a Mo/WS₂ particle exposing both S-edge and Mo/W-edge. Right: differential free energies of H atom adsorption. (b) The charge of the irreversible oxidation peak as a function of the amount of Mo used during the synthesis of MoS₂ Reproduced with permission from ref. 217, Copyright 2008 The Royal Society of Chemistry. (c) The side views of the atomic model of Pt truncated octahedron loaded on MoS₂. (d) Charge-density difference for Pt-MoS₂. The decrease of electron density is shown in yellow, and increase is shown in blue. (e) The adsorption Gibbs free energy of hydrogen on the above Pt positions and the surface for MoS₂. (f) The partial density of states (PDOS) of Pt atoms on the above positions. Reproduced with permission from ref. 218, Copyright 2022 Elsevier.

5. Conclusions and outlook

With the depletion of traditional fossil energy, the development of green energy such as hydrogen energy has become urgent. Electrocatalytic water splitting to produce hydrogen offers a means to develop new energy sources. Up to now, significant progress has been made in achieving effecting hydrogen evolution reactions (HERs) using promising electrocatalysts such as transition metal sulfides (TMDs), transition metal carbides (TMCs), transition metal phosphides (TMPs) and transition metal nitrides (TMNs). As a typical electrocatalyst, MoS₂ has been widely studied and much progress has been made. In this review, we summarize the synthetic methods of MoS₂ and the effective strategies adopted to optimize the HER performance of MoS₂. The introduction of defects significantly enhanced the HER activity of the MoS₂-based materials, which was also verified theoretically. The formation of heterostructures is another effective way to enhance the HER activity of MoS_2, which can modulate the electronic structure. Furthermore, doping metal atoms (Fe, Co, Ni, Cu, Mn, V, Ru etc.) or nonmetal atoms (C, N, F, P, S, Se etc.) into MoS₂ can also improve its electrocatalytic HER performance.

However, there are still some challenges for the commercial application of MoS₂-based materials. Firstly, even though ultrathin MoS₂-based materials can be synthesized by CVD and hydrothermal methods, the large-scale production of MoS₂ is still a challenge. Secondly, the standards for evaluating electrocatalytic stability have not yet been developed. Thirdly, while DFT calculations can provide a detailed theoretical insight into the reaction mechanism of a single active center, research involving the existence of multiple active centers is still lacking. Fourthly, the catalytic influence of H₂O on the MoS₂ surface and interlayers is still not completely understood because of the difficulty to develop bifunctional electrocatalysts for all electrolytes. Lastly, vacancies, doping atoms, and heterojunctions usually have a positive effect on the electrocatalytic process. However, the reaction mechanism needs to be further explored.

While there is still a long way to go for the development of MoS₂-based electrocatalytic materials, the research interest in this area is increasing. Moreover, with the rapid development of computational tools, it is possible to make advances in many experimental areas, including MoS₂-based materials. From this point of view, we can foresee that MoS₂-based materials will be very promising for hydrogen production applications.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This research was supported by National Science and Technology Major Project (Grant No. 2020YFB1506001) and Department of Science and Technology of Sichuan Province (Grant No. 2021YFG0231).

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