Tuning the Morphology of Transition Metal Disulfides: Advances in Electrocatalysts for Hydrogen Evolution Reaction

Abstract

The hydrogen evolution reaction (HER) in the renewable energy system has gained a lot of attention from researchers as hydrogen is assumed to be a clean and renewable carrier. Transition metals and their compounds have been used as promising alternatives to precious noble metals for the HER, offering low cost, more availability, and high activity. In this work, we discussed the mechanisms of the HER and how morphology influenced the catalytic performance of transition metal disulfide (TMD), focusing on structures that range from zero-dimensional (0D) to three-dimensional (3D) TMD materials. Notably, two-dimensional (2D) TMDs, like nanosheets, exhibit the lowest overpotential and a very small Tafel slope, which can be ascribed to their inherent layered structure and large surface area. According to recent research reports, the efficacy and efficiency of the HER process are influenced by surface chemistry, electrochemical characteristics, and the existence of active sites.

1. Introduction

Over the past centuries, immense amounts of fossil fuels have been used for energy storage and conversion technologies. However, burning fossil fuels emits large quantities of CO₂ into the atmosphere, which results in increasing climate change and environmental deterioration [1,2]. To overcome these problems, we need to generate clean and renewable energy from renewable energy sources, such as wind, water, solar, and geothermal power [3,4].

Hydrogen is considered a high-density, clean, and renewable energy source; it is produced from fossil fuels, biomass, and water splitting [5]. Among these, water splitting is a promising way to generate sustainable pure hydrogen, which can serve as an important alternative to conventional fossil fuels in specific applications. There are two significant reactions involved in water splitting, including the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) at the anode and cathode, respectively [6,7]. During the water splitting process, an energy output of 237.1 kJ mol⁻¹ is required, with a theoretical potential of 1.23 V. But the real potential required to achieve this exact current density varies with theoretical potential from 1.23 to 2.5 V due to the sluggish kinetics of the HER and OER. Precious noble metals like platinum are used to overcome this obstacle and enhance the reaction kinetics. However, the noble metals lead to agglomeration, dissolution, and low durability during water splitting [8].

Researchers have gained a lot of interest in transition metals and their compounds, such as oxides, hydroxides, phosphides, carbides, nitrides, and sulfides, as catalysts for water splitting applications [9,10,11,12,13,14]. Due to their cost-effectiveness, generous availability, and good catalytic nature, these materials are beginning to replace noble metals. Their novelty and potential enhance energy storage and conversion technologies [15,16]. Of these transition metal compounds, TMDs, such as VS₂, CoS₂, NiS₂, MoS₂, WS₂, TaS₂, etc., have frequently been utilized as catalysts in recent studies because of their huge surface area, flexible electronic structure, and intrinsic catalytic capacity [17,18,19,20,21]. In addition to water splitting, these TMDs are used in various applications, such as field effect transistors, lubrication, electrocatalysis, and photocatalysis [22,23,24,25].

For the improvement of the catalytic efficiency of TMDs, researchers are extensively introducing new strategies, such as the doping of metal and the incorporation of carbonaceous materials. The doping of metals can alter the electronic structure, provide active sites, and increase the conductivity of TMDs [26,27]. Similarly, carbonaceous materials, including graphene, carbon nanotubes (CNTs), and activated carbon, are combined with TMDs to increase surface area and conductivity and to provide more active sites [28,29,30,31,32]. Even though these strategies modify the morphology of TMDs, the morphology plays a vital role in the enhancement of catalytic HER activity by exposing more edge sites. Each dimension, from 0D to 3D, offers additional advantages in hydrogen evolution. In short, 0D nanoparticles provide a high density of active sites; the one-dimensional (1D) nanowire facilitates charge transport; 2D nanosheets have more exposed edge sites for catalytic reactions; and 3D structures offer a high surface area, high active site density, and favorable scalability for application [33,34,35,36].

Here, we discussed the development of various morphologies of TMDs towards the electrochemical HER. First and foremost, we explained the principle of HER mechanisms and the details of the intermediate steps and pathways of the HER in different pH solutions. Also, we summarized the reported TMD composites with their morphology, synthesis method, and electrochemical performance. The specific dimensions of TMDs were then included, and their morphologies were evaluated for their effect on the catalytic efficiency of the HER. We also mention the advancements made in tuning these morphologies for higher performance, providing insight into research opportunities in HER applications.

2. HER Mechanism

The HER is one of the basic electrochemical reactions in water splitting, where the proton reduces to produce hydrogen gas at the cathode. Two different mechanisms are mainly involved in hydrogen evolution, namely the Volmer–Heyrovsky and Volmer–Tafel mechanisms [37]. These mechanisms are influenced by several factors, such as electrode materials, electrolyte pH, electrode surface, the concentration of the reactants, and overpotentials [8,38]. Based on the electrolyte pH conditions (acidic and alkaline), the HER involves different intermediate steps and pathways:

In acidic conditions,

H₃O⁺ + e⁻ → H* + H₂O         (via Volmer reaction)

H* + H₃O⁺ + e⁻ → H₂ + H₂O       (via Heyrovsky reaction)

Or

H* + H* → H₂               (via Tafel reaction)

In alkaline conditions,

H₂O + e⁻ → H* + OH⁻         (via Volmer reaction)

where H₃O⁺ is a hydronium ion and H* is a hydrogen intermediate [39]. In alkaline conditions, the Heyrovsky reaction and the Tafel reaction follow the same path as in acidic conditions. H₃O⁺ from the acidic electrolyte solution reduces in the Volmer reaction and the hydrogen atom absorbs on the surface of the electrode. In contrast, water molecules from an alkaline electrolyte solution absorb electrons and simultaneously adhere the hydrogen on the surface of the electrode and release OH⁻ molecules in the solution. In an alkaline solution, hydrogen evolution is slower because it requires water dissociation to occur before H* creation [40]. Hence, overpotential is required for each of those reactions to overcome the inherent barrier at the cathode [7].

In general, TMDs follow Volmer–Heyrovsky mechanisms because hydrogen desorption is easier in the Heyrovsky reaction compared to the Tafel reaction. Nevertheless, some differences are exhibited in the mechanisms due to the dependency of the transition metal and sulfide vacancies, their active sites, electronic structure, and hydrogen binding energy [41,42]. Researchers intend to add dopants or other materials, boost H₂ adsorption rates, and decrease desorption rates on the TMD surface to give the best pathway for the HER.

3. Crystal Structure and Conductivity of TMDs

Electrochemical water splitting has gained a lot of attention with regard to producing clean hydrogen energy. However, concerns were raised about the HER regarding its electronic and catalytic properties, such as low conductivity, low electrochemical stability, and slower reaction kinetics due to the simultaneous participation of four electrons. To overcome these concerns, TMDs are extensively used for the HER because of their interesting mechanism and exceptional properties. However, TMDs have a significant crystal structure, which enhances the conductivity and catalytic activity in the HER.

The crystal structure plays a vital role in determining the conductivity and the catalytic behavior of TMDs for the HER. For example, MoS₂ is a quasi-two-dimensional layered TMD, which consists of several layers of S−Mo−S coupled with weak van der Waals interactions [43]. In MoS₂, molybdenum (Mo) atoms are sandwiched between two sulfur (S) atoms with strong covalent bonds between Mo and S atoms, as illustrated in Figure 1, where X represents the Mo atom and M represents the S atoms [42,44,45]. It has two polymorphs, namely 2H-MoS₂ (hexagonal) and 1T-MoS₂ (octahedral). The 2H phase of MoS₂ is a semiconductor material with a band gap of about 1.2–1.8 eV, while the 1T phase of MoS₂ is metallic in nature [46,47]. In the 1T phase of MoS₂, the (002) plane enhances the conductivity of the material, which is characterized by efficient charge transfer behavior. Additionally, the 2D structure of the materials has a high density of catalytic active sites that minimize ion diffusion distance and increase surface area for electrocatalytic processes [48,49]. In addition, more active edge sites were revealed by adding defects to 2H MoS₂ nanosheets, which significantly offered structural and electrical advantages for HER activity. Notably, the conversion of the 2H to the 1T phase influences the performance of the HER [1,36,50].

CoS₂ is favorable for the advancement of the HER based on both computational and experimental studies [8,51]. CoS₂ is a metallic compound with a pyrite structure, where Co is face-centered cubic and S atoms are S-S dimers located at the edge of the unit cell. This material is relatively stable due to the overlapping Co d-orbitals with the S p-orbitals. Moreover, the CoS₂ nanoparticles improve the rate of the HER because they facilitate minimal interference in electron transport during the catalytic processes. Consequently, the conductivity is extended by adding defects, impurities, and exact crystal orientation of the TMDs because these factors increase the electronic and catalytic properties of the HER [52,53,54].

The top-down and bottom-up methods are the two basic approaches to synthesizing TMD nanomaterials. Various synthesis methods, such as colloidal synthesis, chemical vapor deposition, pyrolysis, thermal decomposition, solid-state synthesis, liquid phase synthesis, coprecipitation, electrospinning, and hydrothermal and solvothermal methods, are used to synthesize TMDs in the 0D–3D range [55,56,57,58,59,60,61,62,63,64,65]. Among them, the hydrothermal method is frequently utilized to produce all the dimensional TMDs with fine, elaborate, and often specific architectures due to the parameters of the reaction, the interaction between precursors, and the solvent effects. Several factors, such as precursor concentration, microwave power, the exfoliation agent, pH, temperature, pressure, and duration, affect the morphology of materials throughout each step of the synthesis process that converts the material from 0D to 3D.

Table 1. Summary of different TMDs and their electrochemical measurements for HER with different morphologies.

Sr No.	Materials	Morphology of TMDs	Synthesis Method	Overpotential η [mV] at Current Density 10 mA cm−2	Tafel Slope [mV dec−1]	Electrolyte	Ref.
1.	1T’ phase WS2	Nanoparticles	Colloidal synthesis	200	50.4	0.5 M H2SO4	[64]
2.	MoS2	Nanoparticles	Pyrolysis and sulfurization methods	200	62	0.5 M H2SO4	[66]
3.	CNFS@rGO	Nanoparticles	Hydrothermal	138	49	0.5 M H2SO4	[67]
4.	MoS2/rGO	Nanoparticles	Hydrothermal	70	44	0.5 M H2SO4	[68]
	MoS2/MWCNT			190	73
5.	Graphdiyne-CoS2	Nanowires	-	97	56	1 M KOH	[52]
6.	NbS2-PPy	Nanowires	Chemical vapor deposition	219	56	-	[65]
7.	Ni-doped CoS2 (Ni0.33Co0.67S2 NN/SS)	Nanoneedle	Hydrothermal	350	76	1 M KOH	[69]
8.	N−CoS2	Nanowires	Hydrothermal	152	58	0.5 M H2SO4	[70]
9.	MoS2	Nanosheets	-	200	65	-	[71]
10.	W2C@WS2	Nanoflowers	Hydrothermal	170	55.4	0.5 M H2SO4	[72]
11.	Cu-doped NiS2	Nanosheets	-	139	77	1 M KOH	[73]
12.	MoS2.ZnO	Nanoflakes	Microwave-assisted method	239	62	0.5 M H2SO4	[74]
13.	Pt-decorated VS2	Nanosheets	Hydrothermal	77	54.27	0.5 M H2SO4	[75]
14.	Mo-Co-Pt	Nanosheets nanocubes	Hydrothermal	20	34	0.5 M H2SO4	[76]
15.	MoS2/MoN	Nanospheres	Hydrothermal	117	87	0.5 M H2SO4	[77]
				132	98	1 M KOH
16.	WS2TN/CC	Triangular nanoplates	Hydrothermal	196	87	0.5 M H2SO4	[78]
				150	200	1.0 M PBS
				193	170	1 M KOH
17.	MoS2-rGO@Mo	Nanosheets	Hummers method Hydrothermal	123	62	1 M KOH	[79]
18.	VF-MoS2-rGO-CNTs	Hybrid	Hydrothermal	400	39	0.5 M H2SO4	[80]
19.	CoS2/MWCNTs	Nanospheres	Hydrothermal	257	83	0.5 M H2SO4	[81]
20.	MoS2/GF	Flower-like structure	Hydrothermal	82	48	0.5 M H2SO4	[82]

presents the various TMD materials, morphologies, synthesis methods, overpotentials, Tafel slopes, and electrolytes.

3.1. Zero-Dimensional (0D) TMDs

Zero-dimensional materials, particularly nanoparticles and nanospheres, showed significant improvement in HER performance compared to bulk materials [60,83]. They have large active sites, a huge surface area/volume ratio, and quantum confinement, which may alter the electronic structure of the catalyst to make it more catalytically favorable. Due to the lower particle sizes, the materials help to improve electron transfer during hydrogen evolution. Furthermore, a large number of edge sites are available in nanoparticles to improve proton adsorption and desorption in the HER [84].

MoS₂ and WS₂ nanoparticles are widely used to improve the catalytic activity of the HER due to the existence of unique phases (1T and 2H phases). However, the 1T phase is superior to the 2H phase by virtue of its excellent electrical conductivity, availability of more active sites, favorable hydrogen binding energy, larger surface area, and favorable charge transfer characteristics. For instance, Liu et al. described the fact that 1T/1T’ phase-dominated WS₂ has a higher HER activity than the 2H phase. They used the colloidal synthesis method to synthesize controllable 1T’ phase-dominated WS₂ nanoparticles. The synthesized WS₂ nanoparticles exhibited superior catalytic activity with an overpotential of 200 mV at the 10 mA cm⁻² current density, a small Tafel slope of 50.4 mV dec⁻¹, and excellent stability [64]. Wang et al. prepared MoS₂ nanoparticles on the 3D CFP substrate to improve the HER activity. MoS₂ converted its phase from 2H to 1T through lithiation and layer expansion by electrochemical intercalation. This conversion provided more active sites and enhanced the activity of the catalyst by reducing the overpotential to 200 mV (200 mA cm⁻²) and providing long-term stability [66].

MXene serves as a unique 2D layered structure of transition metal substrate, which has weak van der Waals interaction with excellent electric conductivity. Wang et al. created MoS₂ nanodots anchored on MXene as an electrocatalyst for the HER. Polydopamine (PDA)-coated Ti₃C₂T_x MXene flakes and chelated Mo ions equally dispersed MoS₂ nanodots on the MXene surface. During the annealing process, the PDA encapsulated on the MXene surface may assist prevent surface oxidation while maintaining good electrical conductivity (Figure 2). MoS₂/MXene demonstrated outstanding catalytic HER performance due to the high exposure of active sites. The composite required a low overpotential of 94 mV at a current density of 10 mA cm⁻² and a Tafel slope of 59 mV dec⁻¹ in 1.0 M KOH [85]. Chen et al. prepared a high percentage of a 1T phase of MoS₂ quantum dots on a Ti₃C₂T_x MXene substrate for the HER. The MoS₂ QDs/Ti₃C₂T_x nanohybrid composite facilitated the charge transport process during hydrogen evolution because it provided more active sites and superior electronic conductivity. Thus, the composite showed an overpotential of 220 mV at 10 mA cm⁻² and a Tafel slope of 72 mV dec⁻¹ [86].

Researchers are widely using CNTs to create composites with TMDs, increasing the mechanical strength, modifying the electronic structure, and enhancing the conductivity of TMDs for hydrogen evolution. For instance, Lin et al. synthesized a novel CoS₂ nanoparticle embedded in an N-doped CNT hollow polyhedron (CoS₂-NCNHP) for both hydrogen evolution and Al-S batteries. The CNT acted as a substrate to improve the conductivity and stability of CoS₂, while the hollow polyhedron structure offered a larger active surface. CoS₂-NCNHP showed superior HER electrocatalysis in both acid and alkaline electrolytes due to the strong synergic effect between the active CoS₂ nanoparticles and the N-doped CNT hollow polyhedron. It required an overpotential of 158 and 192 mV at 10 mA cm ⁻² and a Tafel slope of 84 and 95 mV dec⁻¹ in 0.5 M H₂SO₄ and 1 M KOH, respectively [87]. Ji et al. prepared NiS₂ nanoparticles anchored on the surface of nitrogen-doped CNT (Co-CNTs) encapsulated with cobalt nanoparticles for both supercapacitor and water splitting applications. Co-CNTs provided a larger surface area and better conductivity, which facilitated the charge transport kinetics of composite structures. The composite exhibited overpotentials of 235 and 309 mV in the HER and OER to drive a current density of 10 mA cm⁻² [88].

Reduced graphene oxide (r-GO) itself shows good catalytic activity towards the HER. So, researchers make r-GO-TMD composites to increase the two-fold efficiency of the catalyst. The r-GO acts as a good substrate that can accommodate the dispersion of TMDs, reducing their agglomeration, and can offer the largest exposed surface area. It also minimizes the overpotential due to the optimization of the transfer of charges, which is required in the process. For example, Cui et al. fabricated Co-Ni-Fe disulfide nanoparticles on the surface of rGO as an electrocatalyst for the HER. It showed superior catalytic performance with a low overpotential of 138 mV at 10 mA cm⁻² and a small Tafel slope of 49 mV dec⁻¹, due to the synergic effect of r-GO and doping with Ni and Fe metals. These doping metals offered numerous active sites, and adding r-GO improved the anti-aggregation and electron transport properties (Figure 3). Additionally, the material had a large electrochemically active surface area due to the modulation of the crystal lattice and interaction of rGO [67]. Salarizadeh et al. prepared MoS₂ nanoparticles with coatings on different carbonaceous materials, such as rGO, hollow carbon nanotubes (HCNs), single-walled carbon nanotubes (SWCNTs), and multi-walled carbon nanotubes (MWNTs). In addition, they studied and compared the electrochemical performance of these composites for the HER. Among these composites, MoS₂/rGO had a porous nature and possessed many more active sites, good electronic conductivity, and larger catalytic activity than other carbon material-derived MoS₂ composites due to the uniformly distributed MoS₂ nanoparticles on rGO. It resulted in an outstanding overpotential of 70 mV for the required current density of 10 mA cm⁻² and a smaller Tafel slope of 44 mV dec⁻¹ [68]. Sun et al. fabricated MoS₂ nanodots supported by rGO for both the HER and sodium-ion battery applications. The size-controlled MoS₂ nanodots were synthesized with a narrow size distribution ranging from 2.2 to 5.3 nm using a heating-up method by controlling reaction time. These monodispersed MoS₂ nanodots were supported on graphene via an ultrasonication technique and subsequent thermal treatment. The composite displayed a synergic effect due to the high surface area of MoS₂ NDs and the electrical conductivity of rGO, resulting in a good catalytic performance with an overpotential of 222 mV at 10 mA⁻² and a Tafel slope of 59.8 mV dec⁻¹ [89].

3.2. One-Dimensional (1D) TMDs

One-dimensional morphology, like nanowires, nanotubes, or nanorods, has inherent structural advantages that enhance the performance of the HER. It has a high aspect ratio, which gives a large surface area and a more exposed active site for hydrogen evolution. The elongated straight structure of these materials facilitates easy electron transportation from end to end, thus enhancing catalytic activity. Also, it maintains high catalytic activity for a long time due to the flexible material endurance strain and anti-aggregation properties [90,91,92,93,94].

Xie et al. developed graphdiyne-CoS₂ nanowires on carbon cloth as an electrocatalyst for a highly efficient HER. CoS₂ nanowires supported on graphdiyne increase the number of active sites, resulting in a low overpotential of 97 mV to obtain a current density of 10 mA cm⁻² and a small Tafel slope of 56 mV dec⁻¹. This excellent electrochemical performance is mainly due to the synergic effect of the SP¹ hybridized carbon and unsaturated planar sulfurs, which promote electron transfer kinetics (Figure 4) [52]. Gong et al. explained that niobium disulfide (NbS₂) nanowires increased the stability and conductivity of the HER through the addition of polypyrrole. The thickness of polypyrrole was controlled by the process of radical polymerization of pyrrole under different reaction durations of 2 min (NbS₂-PPy-0), 3 min (NbS₂-PPy-1), and 5 min (NbS₂-PPy-2). The NbS₂-PPy-1 sample showed a higher surface area (65.8 μF cm⁻²) and good electrochemical performance, with an overpotential of 219 mV at 10 mA cm⁻² and a Tafel slope of 56 mV dec⁻¹ compared to NbS₂-PPy-0 and NbS₂-PPy-2 [65]. Nguyen et al. investigated different compositions of materials, such as WO₃ nanorods (WO₃ NR), heterostructures of WS₂/WO₃ nanobricks (WS₂/WO₃ NB), and WS₂/WO₃ nanorods (WS₂/WO₃ NR), using batch reactors for the HER. Among them, the WS₂/WO₃ NR showed a significant improvement in catalytic activity due to the uniform formation of a heterostructure between the WS₂ and WO₃ elements in the material. The WS₂/WO₃ NR composite delivered an overpotential of 224 mV at 10 mA cm⁻² with a Tafel slope of 82.7 mV dec⁻¹ and decreased the resistance of 397.7 Ω compared to those of WO₃ NR (1816 Ω) and WS₂/WO₃ NB (3597 Ω) [95]. Abdullah et al. synthesized a NiS₂@SnS₂ nanocomposite using the hydrothermal method for the HER. The composite exhibited nanoparticle (SnS₂) and nanorod (NiS₂) structures that showed several benefits, such as greater active surface area, increased edge-terminated structures, and better conductivity. Also, it displayed good catalytic performance with a low overpotential of 155 at 10 mA cm⁻² and a very small Tafel slope of 34 mV dec⁻¹ [96].

Hu et al. developed CoS₂ nanostructures with different morphologies and stoichiometries using the cobalt nitrate precursor under low sulfur (LS) and high sulfur (HS) concentrations. Moreover, they explained the role of the sulfur concentration in the synthesis of CoS₂ and structural analysis. CoS₂ (LS) showed straight and smooth nanowires that were transformed into crooked nanowires with a roughened surface, while CoS₂ (HS) displayed rougher nanowires that were covered by a large number of dispersed nanoparticles. CoS₂ (LS) facilitated the charge and mass transfer processes due to its smaller size with a larger surface area, whereas CoS₂ (HS) exhibited better electrocatalytic activity because of its low overpotential of 163 mV at 10 mA cm⁻² and small Tafel slope of 70 mV dec⁻¹[54]. Xue et al. designed tubular carbon matrix-supported NiCoP-NiS₂ nanowires as a heterogenous electrocatalyst for water splitting applications. The carbon microtube substrates, derived from Metaplexis japonica fluff biomass, enhanced the exposure of more active areas and promoted charge transfer during the reaction. The inner and outer surfaces of the hollow tubular biomass were completely covered by dense and inconsistently orientated nanowires, which provided structural stability and delivered good electrocatalytic performance for both the HER and OER. The composite required overpotentials of 134 and 308 mV at a current density of 10 mA cm⁻² for the HER and OER, respectively [97]. Liu et al. developed porous CoS₂ nanowires enriched with sulfur vacancies on carbon cloth (Vs-CoS₂/CC) via an argon plasma-assisted process for the HER. The introduction of sulfur vacancies into CoS₂ played a beneficial role in controlling the electron density surrounding the S sites, effectively activating the inert S sites for the HER and increasing their intrinsic activity. By adjusting the material’s charge density, the Gibbs free energy of hydrogen adsorption (ΔG_H*) can also be maximized. Vs-CoS₂/CC showed porous nanowires with aggregated nanoparticles, resulting in an enhanced surface area and exposing more active sites. This structural feature exhibited a lower overpotential (170 mV) at a current density of 10 mA cm⁻² compared to pure CoS₂/CC (236 mV) [98].

The doping of metal is a good strategy to enhance the overall catalytic performance of the material and alter the electronic structure. For example, He et al. synthesized a pyrite-type bimetallic Ni-doped CoS₂ nanoneedle on stainless steel to improve the bifunctional electrocatalytic performance, i.e., the mechanical flexibility and structural integrity. Figure 5 illustrates the SEM images of the synthesized Ni-Co precursors. Doping Ni into CoS₂ resulted in a decrease in the geometric size of the nanoneedle, which improved both the surface roughness and the specific surface area. This material delivered superior catalytic activity for both the OER (overpotential of 286 mV at 50 mA cm⁻² and Tafel slope of 55 mV dec⁻¹) and the HER (overpotential of 350 mV at 30 mA cm⁻² and Tafel slope of 76 mV dec⁻¹) [69]. Chen et al. revealed the catalytic activity for the HER using nitrogen anion-modified CoS₂ (N−CoS₂) nanowires as an electrocatalyst. The incorporation of nitrogen in the CoS₂ system altered the morphology and improved the catalytic properties, including the active sites, electronic structure, and reaction dynamics of the material. The porous nature of the N−CoS₂ nanowire displayed an overpotential of 152 mV to drive the current density of 50 mA cm⁻² and a Tafel slope of 58 mV dec⁻¹ [70]. Nan et al. obtained an efficient P-doped CoS₂ hybrid (P-doped CoS₂-h) nanoelectrocatalyst for the HER. After sulfurization, CoS₂-h displayed a hybrid structure consisting of octahedra and nanotubes, which increased the number of active sites. The synergic effect between the hybrid structure and P-doping enhanced the catalytic performance in both acidic and alkaline media. The P-doped CoS₂-h showed an overpotential of 129.4 mV in acidic media and 170 mV in alkaline media at a current density of 10 mA cm⁻². The catalyst also exhibited a long stability of 24 h in both acidic and alkaline media [99].

3.3. Two-Dimensional (2D) TMDs

Researchers focus heavily on 2D materials for the HER because of their remarkable features. Two-dimensional materials, especially nanosheets, have an inherent layered structural feature, which can promote the electrocatalytic activity of the HER. Additionally, their large surface area, small bandgap, and rich edge site make the materials excellent HER catalysts [100,101,102,103].

Wang et al. activated the basal plane of multilayered MoS₂ via Li-ion intercalation, which resulted in atomic defects and an increased HER. Desulfurization on the MoS₂ basal plane would produce nanopores, increasing the number of active sites. Due to the phase transition from 2H to 1T, the MoS₂ nanosheets exhibited a low overpotential of 200 mV to drive a current density of 10 mA cm⁻² and a small Tafel slope of 65 mV dec⁻¹ [71]. Nguyen et al. created W₂C@WS₂ nanoflowers using a hydrothermal method with a size of about 200–400 nm for the HER. W₂C@WS₂ alloys have improved electronic and catalytic properties due to the combination of tungsten carbide and tungsten disulfide [72]. Dinh et al. demonstrated the electrocatalytic performance of Cu-doped NiS₂ for both the HER and OER. NiS₂ improved the electrical conductivity, surface area, active sites, and electron transport by introducing Cu into NiS₂ (Figure 6). The material showed excellent catalytic activity for both the HER (overpotential of 139 mV at 10 mA cm⁻²) and OER (overpotential of 232 mV at 10 mA cm⁻²) [73]. S. K. Sumesh developed ZnO-functionalized MoS₂ heterostructures for the HER. Due to the incorporation of ZnO, MoS₂ increased the surface area and active sites, which showed higher electrochemical performance with an overpotential of 239 mV at 10 mA cm⁻², a Tafel slope of 62 mV dec⁻¹, and excellent cycling stability [74]. Do et al. reported the comparative study of different morphologies of WS₂ for the HER. WS₂ nanoflowers were prepared by using ammonium metatungstate hydrate and thioacetamide precursors, while WS₂ nanoflakes were fabricated from ammonium tetrathiotungstate through the thermolysis technique. The WS₂ nanoflakes showed a larger surface area compared to the WS₂ nanoflakes, which implies higher catalytic activity and faster electrochemical reaction kinetics. This resulted in an overpotential of 325 mV at a current density of 10 mA cm⁻² and a small Tafel slope of 92.3 mV dec⁻¹ [104].

Homayounfard et al. synthesized a flower-like MoS₂ nanosheet on the heteroatom-doped activated carbon derived from a rose petal biomass for the HER. The F108 surfactant was used as a soft template to control MoS₂ morphology with a higher active edge. Also, this surfactant decreased the crystallite and particle size of MoS₂ flowers. This resulted in a decrease in the overpotential of 136 mV in 10 mA cm⁻² with a Tafel slope of 72 mV dec⁻¹ [105]. Li et al. introduced polyvinylpyrrolidone (PVP) and r-GO into MoS₂ dispersion for liquid phase exfoliation to prepare a 3D MoS₂/PVP/RGO microstructure on Cu film by inkjet printing. This fabrication exposed high-density edge sites, partial 1T-phase MoS₂, reduced charge transfer impedance, and enhanced electron injection. Thus, this novel architecture improved HER performance with a lower overpotential of 51 mV at 10 mA cm⁻² and a smaller Tafel slope of 32 mV dec⁻¹ [106]. Wang et al. reported on the tungsten (W) nanoparticle incorporated on the 2H-WS₂ film by pulsed laser deposition for the HER. Furthermore, they investigated the effect of laser deposition, which significantly reduced the size of WS₂ and the formation of films composed of small nanoparticles and fragments. The catalyst exposed several edge sites with the fragmentation of WS₂ because the incorporation of W nanoparticles on the W-WS₂ film provided abundant electrons on the S-edge. Thus, W-WS₂ film delivered an overpotential of 210 mV at 10 mA cm⁻² in 0.5 M H₂SO₄ compared to the commercial WS₂ powder (526 mV) or pure WS₂ film (341 mV) [107]. Li et al. investigated the phase engineering of W-doped MoS₂ nanosheets as an efficient electrocatalyst for the HER. The high proportion of 1T-phase MoS₂ was formed under a low magnetic field due to the synergistic effect of a micro-strain induced by W doping and the introduction of magnetic free energy. Thus, the material displayed an overpotential of 195 mV at a current density of 10 mA cm⁻², a Tafel slope of 66 mV dec⁻¹ and long-term stability over 50 h [108].

Platinum is one of the most studied catalysts for the HER because of the high intrinsic activity and the effectiveness of the adsorption and desorption of hydrogen atoms. Decorating TMDs with Pt provides an excellent performance of the Pt and TMD materials while also lowering the cost of the catalyst. For example, Zhu et al. developed a Pt-decorated VS₂ nanosheet electrocatalyst for the HER. The Pt catalysts were prepared using a simple optothermal reaction with different structures, including single atoms, clusters, and nanoparticles, which were well controlled and anchored on VS₂ nanosheets. This material delivered a low overpotential of 77 mV to obtain a current density of 10 mA cm⁻² and a small Tafel slope of 54.27 mV dec⁻¹ [75]. Zainal et al. prepared a ternary molybdenum disulfide-cobalt oxide-platinum (Mo-Co-Pt) composite as an efficient electrocatalyst for the HER. As shown in the TEM images of Figure 7, Pt was decorated on the surface of Co₃O₄ nanocubes, and a large amount of these Co₃O₄-Pt nanocubes were grown on the surface of MoS₂, resulting in an increased surface area and numerous active sites. This composite exhibited outstanding electrochemical performance, with the lowest overpotential of 20 mV at 10 mA cm⁻², the smallest Tafel slope of 34 mV dec⁻¹, and long-term durability over 13 h [76]. Wang et al. reported on Pt nanoparticles anchored onto the VS₂ nanosheets via the colloidal method under acidic conditions for the HER. The VS₂ nanosheets functioned as stable substrates, providing a larger surface area and high electrical conductivity. This boosted the interfacial electron transfer and electrochemical contact with Pt nanoparticles. Thus, the Pt/VS₂ composite delivered the lowest overpotential of 26 mV at a current density of 10 mA cm⁻² compared to pure VS₂ and long-term stability up to 110 h [109]. ERH.

3.4. Three-Dimensional (3D) TMDs

From recent studies, 3D nanomaterials open new possibilities for designing and attaining the high performance of the catalytic activity of the HER. These materials enhance the catalytic properties by using different strategies, like doping, defect engineering, and the creation of heterojunctions. These strategies optimize the number of available active sites, facilitate electron transfer, and improve mass transport to make 3D nanostructured materials an effective catalyst for HER application [110,111,112,113].

Wu et al. designed a hierarchical MoS₂/MoN heterostructure as an efficient electrocatalyst for the HER. As shown in Figure 8, the spherical structure of MoS₂ nanosheets with a large number of surface folds exposed more catalytic sites and enabled interfacial mass transfer. The MoS₂/MoN heterostructure exhibited excellent electrocatalytic activity due to the synergic effect of its hierarchical architecture and the interfaces between MoS₂ and MoN. The material showed overpotentials of 117 and 132 mV to drive a current density of 10 mA cm⁻² in 0.5 M H₂SO₄ and 1 M KOH, respectively [77]. Liu et al. constructed novel WS₂ triangular nanoplates on carbon cloth for the HER. For sulfurization and to improve material properties, the sample was calcinated at different temperatures, including 800 °C (WS₂TN/CC-800), 900 °C (WS₂TN/CC-900), and 1000 °C (WS₂TN/CC-1000). Among them, WS₂TN/CC-900 exhibited higher surface roughness, more active sites, better kinetics, and good stability. It required overpotentials of 196, 150, and 193 mV at a current density of 10 mA cm⁻² in acidic, alkaline, and neutral media, respectively [78]. Huang et al. synthesized phosphorus-doped Co₃S₄/NiS₂ heterostructures embedded in N-doped carbon nanoboxes via a pyrolysis–sulfidation–phosphorization strategy as an electrocatalyst for water splitting. The core–shell structure of the material provided a large inner surface area, which increased the number of active sites and facilitated charge transfer. Due to the synergistic effect between NiS₂ and Co₃S₄, the material showed excellent performance for both the HER (overpotential of 150 mV at 10 mA cm⁻² and Tafel slope of 115.91 mV dec⁻¹) and OER (overpotential of 257 mV at 10 mA cm⁻² and Tafel slope of 75.8 mV dec⁻¹) [114].

Fang et al. designed Fe-doped CoS₂ nanocages with nitrogen-doped carbon wrapping (CN/Fe-CoS₂) as an electrocatalyst for overall water splitting. Fe doping in COS₂ enhanced the charge transfer, making a reduced band gap, expanded the electrochemical surface area, and regulated the adsorption of intermediates during the reaction. These facilitated the exposure of a more active site and improved the intrinsic catalytic activity of CoS₂. In these composites, CoS₂ aggregation was prevented by the protective layer of the nanocage architecture and nitrogen-doped carbon wrapping. These results showed superior electrochemical activity and stability towards the HER and OER. The material displayed overpotentials of 186 and 304 mV to drive the current density of 10 mA cm⁻² in 1.0 M KOH media for the HER and OER, respectively [115]. Xia et al. developed WS₂-decorated Co-N-co-doped carbon hollow nanocages (WS₂-3/Co-N-CN) using a one-pot liquid phase mixing and calcination process for the HER. At the higher calcination temperature (750 °C), Co particles precipitated on the surface of the nanocage, improving electron transport efficiency. The microstructure and pore structure of the hybrid materials were influenced by the calcination temperature; thus, WS₂-3/Co-N-CN-750 showed better pore volume and average pore size compared to WS₂-3/Co-N-CN-650 and WS₂-3/Co-N-CN-850. In addition, electrochemical testing was conducted in different electrolytes, such as 0.5 M H₂SO₄, 0.05 M H₂SO₄, 1 M KOH, 0.1 M KOH, and 1 M PBS. Among these, WS₂-3/Co-N-CN-750 exhibited good electrochemical performance in 0.05 M H₂SO₄, as proved by an overpotential of 204 mV at a current density of 10 mA cm⁻² and a small Tafel slope of 79.1 mV dec⁻¹ [116].

Researchers make composites with carbonaceous materials, including graphene, CNTs, and activated carbon, for the enhancement of catalytic efficiency because these carbonaceous materials possess a high surface area, high electrical conductivity, and stability for the HER. Therefore, introducing the carbonaceous material highlighted that the material resulted in two-fold increased utility for the HER process. He et al. synthesized 3D MoS₂ nanosheets coated with rGO on a Mo mesh to improve the Volmer reaction of the HER. Amorphous MoS₂ nanosheets exposed more active sites, and the interaction with r-GO enhanced the surface area by the space confinement effect for the stacked layers of graphene. MoS₂-rGO@Mo nanohybrids exhibited superior catalytic performance for the HER in 1.0 M H₂SO₄ with a low overpotential of 123 mV to achieve the current density of 10 mA cm⁻², a small Tafel slope of 62 mV dec⁻¹, and excellent stability for more than 15 h [79]. Huang et al. developed a layer-by-layer 3D MoS₂-rGO-CNT structure for the HER. CNTs facilitated the non-restacking of the 2D MoS₂-rGO nanosheets and the bridge between the fragmented MoS₂-rGO nanosheets for electron transport. Thus, the MoS₂-rGO-CNT composite showed an overpotential of 400 mV at the high current density of 1200 mA cm⁻² with a Tafel slope of 39 mV dec⁻¹ [80]. Liu et al. designed CoS₂ nanospheres anchored on MWCNTs using a hydrothermal route for the HER. CoS₂ were deposited on the surface of MWCNTs to fix the CoS₂ particles and to avoid their excessive growth, which would lead to the increased surface area and the number of electrochemical active sites. This composite enhanced the HER activity in 0.5 M H₂SO₄ with an overpotential of 257 mV at 10 mA cm⁻² and a Tafel slope of 83 mV dec⁻¹ [81]. Haung et al. explained the synergistic effect of proton intercalation and electron transfer via electro-activated molybdenum disulfide/graphite felt (MoS₂/GF) for the HER. This synergic effect is mainly due to an active functional group in the GF, the crystalline structure of MoS₂ nanoflowers, electron and proton transfer between MoS₂ and GF, intrinsic conductivity, and high catalytic activity with good stability (Figure 9). The MoS₂/GF material showed the smallest overpotential of 82 mV at 10 mA cm⁻² and the smallest Tafel slope of 48 mV dec⁻¹ [82]. Peng et al. prepared highly stable 3D vertically orientated NbS₂ nanosheets on the CNT film substrate by the atmospheric pressure chemical vapor deposition (APCVD) route for the HER. In addition, they examined the thickness and nanosheet size, which were controlled by fine-tuning the growth parameters, including temperature, type, and ratio of carrier gases and growth time. In a unique 3D hierarchical structure, CNTs provided high structural stability and conductivity, while 2H NbS₂ nanosheets exposed more active sites. The composite exhibited remarkably low overpotentials of ≈55 mV at 10 mA cm⁻², a small Tafel slope of 92.1 mV dec⁻¹, and superior stability after 200 h [117].

Table 2. Recent reports on TMDs and their composites for HER.

Sr No.	Materials	Morphology of TMDs	Synthesis Method	Overpotential η [mV] at Current Density 10 mA cm−2	Tafel Slope [mV dec −1]	Electrolyte	Ref.
1.	Ru–VS2/CC	Nanoparticles	Hydrothermal	89	63	0.5 M H2SO4	[118]
2.	Pt1%−CoMoS2/C	Nanoclusters	Incipient wetness impregnation method	118	68	0.5 M H2SO4	[119]
3.	MSOR1 (Ru co-doped MoS2)	Flower-like nanosphere	Hydrothermal	197	63.1	0.5 M H2SO4	[120]
				43	75.8	1.0 M KOH
4.	MoS2-CoFeLDH	Nanoneedle (cactus-like spherical)	Hydrothermal, Electrodeposition	36	235	1.0 M KOH	[121]
5.	SL-Ni-Ru-VS2	Nanosheets	Colloidal	20	34.14	0.5 M H2SO4	[122]
6.	NiFe LDH/NiS2/VS2	Nanosheets	Hydrothermal	76	79	1.0 M KOH	[123]
7.	NiCo2S4/ReS2	Nanosheets	Hydrothermal	126	67.8	0.5 M H2SO4	[124]
				85	78.3	1.0 M KOH
8.	Pt/ReS2	Nanoflowers	Microwave-assisted hydrothermal method, Wet-impregnation method	20	31.5	Ar-saturated 0.1 M HClO4	[125]
9.	Co-NiS2/CeO2/NF	Nanosheets	Hydrothermal	84	115.3	1.0 M KOH	[126]
10.	ReS2/NiS	Nanosheets	Hydrothermal	78	76	1.0 M KOH	[127]
11.	CoS2/WS2	Nanosheets	Hydrothermal	79	52	0.5 M H2SO4	[128]
12.	ReS2/CoS	Nanosheets	Hydrothermal	187	61	1.0 M KOH	[129]
13.	Ru MIs-doped MoS2	Nanosheets	Hydrothermal	17	63	1.0 M KOH	[130]
14.	RuSA-NiS2-FeS2	Nanosheets	Hydrothermal	57	59.4	1.0 M KOH	[131]
15.	FeS2/FeOOH-ZnO@NF	Nanoplates	Hydrothermal	74	105.79	1.0 M KOH	[132]
16.	2H–VS2	Nanoflower	Hydrothermal	181	52	0.5 M H2SO4	[133]
17.	Pt@1T-MoS2−Ni	Petal-like nanosheets	Hydrothermal	76	46	1.0 M KOH	[134]
18.	CoS2@NHCs-800	Microspheres	Hydrothermal	98	85	0.5 M H2SO4	[135]
				118	157	1.0 M KOH
19.	MnS2/MnO2-CC	Heterostructure	Electrodeposition	66	26.72	0.5 M H2SO4	[136]
20.	FeS2@CoS2	Heterostructure	Hydrothermal	136	58	1.0 M KOH	[137]

provides recent reports on several TMDs and their composites for the HER. It depicts the overpotential, the Tafel slope, and the various electrolytes employed in the electrochemical reactions. The TMD morphologies varied, and several synthesis methods were discussed. The majority of the papers used a hydrothermal method to synthesize TMD. The investigations commonly utilized 0.5 M H₂SO₄ and 1.0 M KOH electrolytes for electrochemical reactions. The table reveals that the superior overpotential reported in the Ru MIs-doped MoS₂ nanosheet material was 17 mV at a current density of 10 mA cm⁻². The MnS₂/MnO₂-CC material has remarkable kinetics and provides an extremely small Tafel slope of 26.72 mV dec⁻¹ in 0.5 M H₂SO₄. Pt/ReS₂ was synthesized as nanoflowers using microwave-assisted hydrothermal and wet-impregnation techniques, which demonstrate outstanding catalytic performance with a low overpotential of 20 mV at 10 mA cm⁻² and a very small Tafel slope of 31.5 mV dec⁻¹ in Ar-saturated 0.1 M HClO₄ electrolyte. Some reports describe electrochemical testing in both acidic (0.5 M H₂SO₄) and alkaline (1.0 M KOH) conditions. MSOR1 (Ru, O-co-doped MoS₂) and NiCo₂S₄/ReS₂ materials perform well in alkaline solutions, whereas CoS₂@NHCs-800 material shows good performance in acidic solutions. In the investigations, 2D materials, such as nanosheets, demonstrate notable electrochemical performance.

4. Current Challenges and Future Perspectives

Developing TMDs as efficient electrocatalysts for the HER presents challenges. These problems include the synthesis of materials with controlled morphologies; this is critical for enhancing catalytic activity because they have a large surface area where the HER occurs. Noble metals, such as Pt and Ru, decorated the TMDs and showed excellent electrochemical performance towards the HER because of their high intrinsic activity. However, these materials present cost and scaling challenges. Scaling up the catalyst is a major challenge, and most synthesis methods are restricted to the laboratory level due to the scaling up of materials with similarity and reproducibility, and these syntheses modify the electrochemical properties and cost of the materials. Furthermore, intrinsic HER activity in TMDs is constrained since most active sites are associated with the edges, whereas basal planes are normally inactive for the HER. This constraint reduces the overall effectiveness of TMDs and needs a high overpotential for the HER in comparison to noble metals like platinum. While the integration of defects and doping materials in TMDs improves the HER’s electrochemical performance, an improper quantity of these components might degrade the catalyst’s structure and properties. Stability is also important for the HER, but many catalysts deteriorate quickly and are therefore unsuitable for commercial use.

Future advancements will necessitate enhancing active materials and electron transport through the use of hybrid structures, such as core–shell structures, which improve the performance of the HER. They also improve the conductivity, stability, and availability of active sites. The development of ternary and quaternary TMDs is a good alternative for noble metal-based electrocatalysts because they have excellent redox reversibility and high electronic conductivity. Compositing TMDs with carbonaceous materials shows promise for the HER since it improves electrical conductivity and texture. It reveals more active sites because of the tight interaction between the metal ions and carbon base; the resulting metal nanoparticles are smaller and more widely distributed than the bulk metal. The active metal phase and carbon substrates have a synergistic coupling effect that makes charge transfer easier and boosts catalytic activity. Moreover, their scalability and suitability for industrial applications further enhance their potential in the HER. Heteroatom doping and phase engineering are effective strategies for improving electrochemical properties, such as electrical conductivity, charge transfer kinetics, and stability. The electrocatalytic HER activity can be increased through phase engineering since the 1T phase often shows higher HER activity than the 2H phase. We will have to use proper element doping and alloying to change the electrical structure in order to achieve good electrochemical characteristics for HER applications. Moreover, heteroatom doping facilitates and optimizes the chemical-adsorption energies of reactive intermediates during hydrogen evolution. Co-doping, which combines heteroatoms with other elements, is also a viable method for improving material catalysis activity. Another strategy is to develop effective catalysts with specific sites for the adsorption of hydroxide and hydrogen in the HER process. These strategies developed efficient catalysts; future developments will grow high-performing catalytic materials and will provide an intrinsic understanding of the alkaline HER. When employing TMDs in industrial HER applications, we must use clean and sustainable synthetic approaches. However, the cost optimization of TMDs will be exclusively determined by the development of solutions, and their implementation into commercial devices will be vital to the widespread adoption of TMDs in the energy industry. Finally, the development of new TMD materials is needed; this would most likely be guided by increasingly advanced computational approaches. To predict the impacts of doping, defect formation, and nanostructuring on HER activity, we must trust in the previously mentioned approaches, which will aid in understanding and expediting their practical implementation in renewable energy systems.

5. Conclusions

In conclusion, the advancement of sustainable energy is crucial in energy conversion and storage technologies. Developing research works have shown that TMDs have better conductivity, lower band gap energy, and lower overpotential. The crystal structure of TMDs plays an important role in enhancing their catalytic activities with respect to band structure, electron mobility, and electrical conductivity. A discussion of the electrochemical properties of various TMD materials according to their morphology for the HER is provided here. Thus, nanosheet TMDs have been recognized as good 2D nanomaterials for the HER because of their large surface area, higher number of active sites, and exceptional quantum confinement effect. In addition, their higher density of reactive edge sites and their tendency to intercalate between layers raise their activity. The properties of doped or composite materials that change the electrical and structural properties of TMDs are also described in this review. Doping the metals improves the binding energies of intermediates with TMDs and adjusts the electrical structure to obtain quick electron transfer kinetics. In order to reveal more active sites and accelerate electron transmission, the composite material exhibits a synergistic impact between its components as well as the activation of additional interfaces and defects. Finally, several future developments that can contribute to the improvement of the efficiency and life of TMD-based HER catalysts are pointed out.