Progress in 2D MoS2-Based Advanced Materials for Hydrogen Evolution and Energy Storage Applications

Abstract

The increasing energy demand for and fast depletion of fossil fuels have driven the need to explore renewable and clean energy sources. Hydrogen production via water electrocatalysis is considered a promising green fuel technology for addressing global energy and environmental challenges while supporting sustainable development. Molybdenum disulfide (MoS₂) has emerged as a potential electrocatalyst for hydrogen evolution reactions (HERs) and super-capacitor (SC) applications due to its high electrochemical activity, low cost, and abundance. However, compared to noble metals like platinum (Pt), MoS₂ exhibit lower HER activity in water electrocatalysis. Therefore, further modification is needed to enhance its catalytic performance. To address this, MoS₂ has been effectively modified with materials such as reduced graphene oxide (rGO), carbon nanotubes (CNTs), polymers, metal oxides, and MXenes. These modifications significantly improve the electrochemical properties of MoS₂, enhancing its performance in HER and SC applications. In this review article, we have compiled recent reports on the fabrication of MoS₂-based hybrid materials for HER and SC applications. The challenges, advantages, and future perspectives of MoS₂-based materials for HERs and SCs have been discussed. It is believed that readers may benefit from the recent updates on the fabrication of MoS₂-based hybrid materials for HER and SC applications.

1. Introduction

Humanity is currently facing two major challenges: energy and environmental crises. These issues arise from rapid population growth, advances in living standards, and uncontrolled industrial development leading to a global energy crisis [1,2,3,4,5]. The increasing global energy demand is mainly fulfilled by the use of fossil fuels. However, their excessive use has led to significant negative impacts on human health and the environment, particularly in developing countries [6,7]. Furthermore, the extensive use and burning of fossil fuels release significant amounts of different harmful gasses such as CO, CO₂, Sox, and NOx into the atmosphere, contributing to environmental pollution [1]. Therefore, it is crucial to develop eco-friendly and sustainable alternatives to meet the energy demand of the fast-growing population and rising living standards, while addressing environmental pollution and ensuring the sustainable development of global economy [2,8]. At this critical juncture, among all renewable energy sources, sustainable hydrogen energy is considered as the most promising ideal and alternative energy carrier to fossil fuels due to its high energy capacity and environmentally friendly nature [9]. Moreover, hydrogen energy is also considered a green fuel because its combustion produces only water as a byproduct, which is non-polluting and does not contribute any environmental problems [10]. However, hydrogen is currently produced at the industrial level mainly through methane reforming or coal gasification. These processes release large amounts of greenhouse gases into the atmosphere, posing a significant amount of environmental challenges [9,10]. For the production of hydrogen via electrochemical water splitting, using electricity from renewable sources such as solar, wind, and tidal energy is one of the best methods that results in high-purity hydrogen [11]. This process is environmentally friendly, feasible, and cost-effective, providing a sustainable method for producing hydrogen fuel to meet global energy demands while addressing environmental problems [11]. Generally, the electrocatalysis and water splitting processes involve the decomposition of hydrogen and oxygen bonds in water, converting hydrogen bonds into hydrogen fuel [12]. This process consists of two key electrochemical half-reactions, such as hydrogen evolution reactions (HERs) at the cathode and oxygen evolution reactions (OERs) at the anode [13]. However, the splitting of water is an energy-intensive reaction with slow kinetics, which needs a large overpotential and high energy consumption, limiting its industrial application [14,15]. To make this process feasible for practical application, highly efficient electrocatalysts are essential for both HERs and OERs to enhance the activity and overall performance [15]. The theoretical overpotential for electrochemical water splitting is 1.23 V, but to effectively split water, an overpotential of around 1.8–2.0 V is needed, which is 1.5–1.6 times higher than the theoretical value [16]. Therefore, designing the effective electrocatalysts to reduce the overpotential of water splitting is crucial for enabling the efficient utilization of energy. At present, the most effective catalyst for electrocatalytic HERs is precious noble metal platinum (Pt), which can almost reach the theoretical decomposition overpotential of water splitting [17]. But the high cost and scarcity of Pt limits its industrial application [17]. Therefore, developing high-performance HER electrocatalysts with low costs and abundant reserves is crucial but challenging. To achieve this, significant efforts have been dedicated to reducing the overpotential of slow HER kinetics by employing non-Pt transitions metal electrocatalysts, including carbides, sulfides, borides, nitrides, and phosphides [18,19,20,21,22].

In recent days, transition metal dichalcogenides (TMDs), such as WS₂, WSe₂, MoS₂, MoSe₂, etc., have been extensively utilized in sensors, electronics, and the catalysis field owing to its outstanding electronic and chemical properties [23,24,25]. Among TMDs, MoS₂ has gained significant attention in the field of electrocatalysis due to its low cost, low toxicity, high durability, and unique physicochemical properties [24,25].

As a typical two-dimensional (2D) material, MoS₂, has been widely studied as a potential HER catalyst, demonstrating great potential to replace Pt both theoretically and experimentally [9]. This is primarily due to its nearly ideal theoretical hydrogen adsorption Gibbs free energy (∆G_H = 0.06 eV), which is close to zero, making it highly effective for HERs [26]. In a typical lamellar hexagonal structure, the atomic layers of MoS₂ are arranged in such a way that each monolayer is covalently bonded via Mo-S-Mo bonds, whereas adjacent layers interact through van der Walls forces [9,27]. In general, within each MoS₂, the Mo atoms are covalently bonded to six adjacent sulfur atoms, and there are no dangling bonds between layers, other than van del Walls interactions. The different stacking arrangements of Mo and S layers give rise to different crystal phases, such as 1T, 2H, and 3R. For catalysis applications, the 1T and 2H phases of MoS₂ have been extensively studied and compared [28,29]. MoS₂ naturally occurs in the 2H phase, showing a 2D lamellar structure similar to graphite, with layers connected by van der Walls forces [9,30]. The 2H phase is characterized by a triangular prismatic structure, exhibiting semiconducting properties, and is thermodynamically stable. Each monolayer consists of three atomic layers in an S-Mo-S arrangement, where two S layers are symmetrically positioned and the Mo atom resides at the center of the triangular prism formed by the S atoms. The Mo atom is coordinated with the surrounding six S atoms [9,31]. In the 2H phase of MoS₂, the symbol “2” signifies the structural periodicity, indicating that two layers of S-Mo-S monolayers are repetitively stacked as a structural unit, whereas the symbol “H” denotes the hexagonal crystal system, reflecting the coordination structure of the material. The 1T phase of MoS₂ also possesses a 2D structure but differs in its octahedral coordination. This phase is thermodynamically metastable and exhibits metallic properties, making it distinct from the semiconducting 2H phase [32]. Each monolayer is composed of three atomic layers of S-Mo-S atoms, where two S layers are asymmetrically positioned and the Mo atom is sandwiched between two layers of S atoms. The 1T phase of the MoS₂ structure can be obtained by rotating the S atomic layer by 60° around the center of the top plane, using the triangular prism structure of the 2H phase as a reference [33]. In this configuration, each Mo atom is octahedrally coordinated with six S atoms, which is denoted by the “T” in 1T, referring to trigonal. The 1T phase of MoS₂ is constructed by the repeated stacking of single atomic layers along the c-axis, with each layer serving as a fundamental unit, as indicated by the “1” in 1T [34]. Figure 1 illustrates the 2H and 1T phases of MoS₂. The distinct crystalline phases of MoS₂ exhibit varying properties due to the differences in their structural configurations. However, in the case of the 3R phase, the arrangement of the scaring method is the same as for 2H phase of S-Mo-S, but the Mo atoms occupy the center of triangular prisms [33]. The main structural distinction between the 2H and 3R phases lies in the layer arrangement along the c-axis. The 2H phase contains two layers per unit cell, whereas the 3R phase contains three layers per unit cell. Moreover, the 3R-type phase of MoS₂ readily transforms into a more thermodynamically stable 2H phase upon heating [34].

Over the past decades, numerous studies have been conducted to achieve high HER activity using MoS₂. However, its performance remains significantly lower than the Pt catalyst due to MoS₂’s low conductivity and inert basal planes. Early research by Tributsch and Bennett on bulk MoS₂ demonstrated that this is not an efficient HER catalyst [36]. Subsequently, in 2005, groundbreaking work by Hinnemann and co-workers revealed that the nanostructure of MoS₂ exhibits significantly improved HER activity [37]. This improvement is attributed to the hydrogen adsorption free energy (∆G_H) of hydronium at the Mo-edge, being more favorable than that on the S-edge, as supported by density function theory (DFT) [38]. The electrons localized on the Mo-H bond are readily transferred to form dihydrogen, leading to improved catalytic performance. After the report of Jaramillo and co-workers in 2007 on the effectiveness of Mo-edge (2H phase), it was found that the Mo-edge is catalytically more active [supported on Au (111) surface] than the surface [39]. Previously, MoS₂ was also grown on W-anchored graphene for HER applications [40]. In other reports, N, S-carbon dots (CDs), intercalated MoS₂/graphene [41], onion-like graphene-surrounded MoS₂ [42], and exfoliation/doping-processed MoS₂ [43] were also applied as HER catalysts. These reports demonstrate the potential of MoS₂ as an HER catalyst.

In this review, we aim to provide a comprehensive understanding of MoS₂ nanomaterials as electrocatalysts for HERs and as material for super-capacitor (SC) applications. The MoS₂ and MoS₂-based materials possess excellent electrochemical properties, which makes them suitable candidates for HER and SC applications. The SCs are the electrochemical devices which bridge the conventional capacitors and batteries. Thus, it would be of great significance to summarize the progress in MoS₂-based materials for HER and SC applications. This work summarizes the previous development in the modification strategies of MoS₂, including increasing the interlayer spacing, phase transition, introducing sulfur vacancies, doping with various elements, and constructing a heterostructure with materials such as reduced graphene oxide (rGO), carbon nanotubes (CNTs), polymers, metal oxides, and MXenes. Finally, we will discuss the challenges and future direction of the development of high-performance MoS₂ catalysts for scalable and economical hydrogen production and energy storage applications.

2. MoS₂-Based Materials in Hydrogen Evolution Reactions (HERs)

MoS₂ has emerged as a promising catalyst to replace Pt-based electrocatalysts for HERs in water electrolysis due its unique layered structure, tunable electronic properties, abundance, low cost, and excellent catalytic potential and stability. However, its intrinsic HER performance is lower than that of Pt electrodes. To address this, various modifications have been dedicated to improving its HER activity, as will be discussed. A fundamental understanding of the water electrolysis mechanism is crucial. Water electrolysis consists of two half-cell reactions, where the HER takes place at the cathode and the OER takes place at the anode, as illustrated in Figure 2. To achieve efficient water splitting, appropriate catalysts are typically applied to the cathode and anode.

Figure 3 depicts the typical polarization curves for HERs, OERs, and their respective reactions at the cathode and anode. In addition to coating catalysts on the cathode or anode, the initiation of a water splitting reaction requires overcoming a free energy (∆G) of 237.2 kJ mol⁻¹ at 25 °C and 1 atm, irrespective of whether the medium is acid, alkaline, or neutral. This energy corresponds to a theoretical cell voltage of 1.23 V.

The HER is a half-cell reaction that occurs at the cathode during electrocatalytic water splitting, producing one molecule of H₂ through a two-electron transfer process. In an acidic medium, the HER involves three elementary reactions, as described by Equations (1)–(3). The first step is the Volmer reaction, in which protons are discharged onto the electrode surface, forming the adsorbed hydrogen intermediate (H_ads). This step represents the reduction in protons on the catalyst surface. This process is also analyzed through the Tafel slopes obtained from the experimental plots of the overpotential versus logarithm of the current density (mA cm⁻²) [44,45].

Volmer reaction: H₃O⁺ + e⁻ → H₂O + H_ads(1)

The second step is the electrochemical desorption, which can take place either via the Heyrovsky reaction or the Tafel reaction, depending on the surface coverage of the adsorbed hydrogen (H_ads).

Heyrovsky reaction: H_ads + H₃O⁺ + e⁻ → H₂O + H₂(2)

(when H_ads coverage is low);

Tafel reaction: H_ads + H_ads → H₂ (3)

(when H_ads coverage is high).

The mechanism of the HER can also be identified from the experimental values from the Tafel slope. For example, a Tafel value of 120 mV dec⁻¹ indicates a Vomer step. On the other hand, Tafel slops of 40 and 30 mV dec⁻¹ correspond to the Heyrovsky and Tafel steps, respectively, whereas in the case of alkaline and neutral electrolyte solutions, the H_ads in the HER process primarily originates from H₂O molecules. The adsorbed hydrogen intermediate (H_ads) plays a key role in the HER, irrespective of the pathway it follows. As a result, the free energy of hydrogen adsorption (∆G_Hads) is widely regarded as a critical descripted for HER electrocatalysts. ∆G_Hads serves as an indicator of catalytic efficiency, reflecting the catalyst’s ability to absorb and desorb H_ads effectively. Theoretical analysis suggests that electrocatalysts achieve optimal HER performance when the value of ∆G_Hads is close to zero [46]. This relationship can be visualized in a Volcano plot, as shown in Figure 4. In this plot, noble metals such as Pt exhibit ∆G_Hads values nearly close to zero, reflecting their highest HER electrocatalytic activity [47]. Moreover, changes in the electronic structure of a catalyst can significantly influence the adsorption behavior of intermediate hydrogen (H_ads) and its binding energy, thereby improving hydrogen production [48]. For example, upon forming transition–metal–metalloid compounds, such as MoS₂, the transition metal may occupy a new electronic environment [49]. This results in enhanced catalytic activity, as MoS₂ appears higher on the Volcano plot compared to Mo alone (Figure 4) [50]. While the inherent structure of bulk MoS₂ satisfied the basic requirement for HER applications, it often fails to deliver the desired electrocatalytic performance. Consequently, various strategies have been developed to improve its electronic properties and catalytic activity.

Ultrathin materials, compared to their bulk materials, demonstrate significantly controlled charge transfers within each layer. Thus, the electronic structure of these materials can be finely tuned by reducing their thickness to an atomic scale. DFT calculations reveal ultrathin nanosheets possessing a significantly higher density of electronic states near the Fermi level compared to bulk materials [51]. This results in improved electron transfer and conductivity, enhancing their performance in the water electrolysis process. In 2005, Novoselov et al. [52] conducted groundbreaking research by using the micromechanical cleavage method to extract the individual atomic planes of MoS₂ from its bulk form. Since then, substantial advancements have been made in the synthesis of single- or multilayer MoS₂ nanosheets. This reduction in thickness can transform the indirect band gap of bulk MoS₂ into a direct band gap, significantly boosting it efficiency for electrocatalytic hydrogen production [53]. Lukowaki et al. [54] utilized a chemical exfoliation method to convert the semiconducting 2H MoS₂ nanostructure into a metallic 1T MoS₂ phase. They found that nanosheets of the metallic MoS₂ polymorph exhibited significantly higher HER activity.

The HER activity of catalysts can be improved by regulating their active sites, which can be achieved by expanding the interlayer spacing of bulk MoS₂. Increasing the interlayer spacing provides more active sites, facilitates the desorption of H_ads, and improves the transfer of electrons/protons [55]. Moreover, this expansion optimized the ∆G_Hads at both the edge and the basal planes of MoS₂, facilitating intermediate H_ads adsorption and hydrogen molecule desorption, thereby boosting HER efficiency [56]. Zhang et al. [57] reported the design of 1T MoS₂ with expanded interlayer spacing using a simple solvothermal approach in N,N-dimethylformamide (DMF) derived from Mo-based organic frameworks (Mo-MOFs). The resulting 1T-MoS₂ demonstrated efficient HER activity, achieving an overpotential of 98 mV at a current of 10 mA cm⁻², along with a Tafel slope of 52 mV dec⁻¹. Hu et al. [58] synthesized a 1T/2H-MoS₂ material with an expanded interlayer spacing of 0.94 nm under reaction conditions of 200 °C in ethylene glycol (Figure 5a). This interlayer expansion was attributed to the incorporation of ammonium ions during the reaction, which creates large channels for ion transport and enhances the exposure of active sites. The resulting 1T/2H-MoS₂/NH⁴⁺-200 demonstrated outstating HER performance, achieving an overpotential of 159.9 mV at 10 mA cm⁻², with a Tafel slope of 55.5 mV dec⁻¹ in 0.5 M H₂SO₄ (Figure 5b,c). Moreover, the material demonstrated remarkable electrochemical stability, showing only a minor drop overpotential (7.2 mV) at 10 mA cm⁻² after 1000 cyclic voltammetry cycles.

Heteroatom doping is a highly effective strategy to modify the electronic structure of the original lattice, reduce the hydrogen adsorption free energy (∆G_H*), activate the basal plane of MoS₂, and enhance its catalytic activity [59,60]. Additionally, doping with atoms of varying radii and valance electrons significantly alter the electron density of Mo and S atoms, increasing the electron transfer rate and conductivity, which further boosts the HER activity of MoS₂ [61,62]. Xie and co-workers [63] proposed a novel strategy to synthesized V-doped MoS₂ using semi-metallic interlayer doping. They reported that vanadium doping alters the electronic structure of MoS₂, facilitating the activation of valance electrons, shortening the electron pathways, and enhancing electrical conductivity and carrier concentration, thereby significantly improving the catalytic performance of MoS₂. Additionally, V doping reduced the band gap of MoS₂ due to interactions between the V atoms and neighboring Mo atoms. Zhao and co-workers [64] reported a covalent doping strategy to incorporate cobalt into MoS₂, creating a bifunctional electrocatalyst for overall water splitting (both HERs and OERs). The synthesized Co-MoS₂ catalyst demonstrated superior HER performances, with an onset potential of −0.02 V (vs. reversible hydrogen electrode (RHE) in 0.1 M KOH. The improved HER activity was attributed to cobalt doping, which effectively reduced the band gap of MoS₂ from 1.70 eV to 0 eV, thereby significantly improving its conductivity.

Single-atom catalysts represent a frontier in the field catalysis, gaining significant attention for their maximum atomic efficiency and tunable electronic properties. Li and co-workers [65] applied a novel single-atom catalysis approach to developed a highly active electrocatalyst with superior HER performance across a wide pH range. Initially, 2H-MoS₂ was synthesized using a simple two-step hydrothermal approach. Subsequently, different amounts of noble Ru metal were incorporated into 2H-MoS₂, as illustrated in Figure 6.

The decoration of Ru as a single atom was confirmed through HAAD-STEM analysis, where the bright spot in yellow circles confirmed the position of Ru single atoms in the 1T/2H-MoS₂ catalyst, as shown in Figure 7a–c. To further validate the presence of Ru single atoms in MoS₂, energy dispersive spectrometer (EDS) analysis was conducted, indicating the presence of Ru, Mo, and S atoms in the Ru_0.10@2H-MoS₂ catalyst (Figure 7d–g).

The optimized Ru_0.10@2H-MoS₂ catalyst exhibited the highest HER activity, with the lowest overpotential of 137 mV at a current of 10 mAcm⁻² in 1.0 M PBS at pH 7, as shown in Figure 8a. The corresponding Tafel slope, as shown in Figure 8b, indicates a smaller slope compared to other catalysts, although a slightly higher one than that of commercial Pt/C. The electrochemical double-layer capacitance in Figure 8c demonstrates that Ru_0.10@2H-MoS₂ has better HER activity and improved charge transfer. Moreover, the charge transfer resistance of the optimized Ru_0.10@2H-MoS₂ is lower than that of other catalysts, further confirming that it has a faster HER reaction speed (Figure 8d). The incorporation of Ru single atoms in MoS₂ significantly reduces the ∆G_H of 2H-MoS₂, as confirmed by the DFT calculation.

The heterostructure coupling of two or more components is a highly efficient approach to modulate the interfacial electronic structure and improve the electrical conductivity of the base materials. This approach enhances the interfacial charge transfer, leading to improved catalytic performance [66,67]. Additionally, heterostructures create electrochemically active interfaces that enhance electrochemical properties and optimize the Gibbs free energy for hydrogen adsorption (∆G_H) driven by synergistic effects at the interface [68,69]. To improve the electrical conductivity of MoS₂, Song and co-workers [70] developed a heterostructure with single-walled carbon nanotubes (SWNT). Their study revealed that electrons migrate from the SWCT to MoS₂ and accumulate around the S atom in MoS₂ within the MoS₂/SWNT heterostructure. This process resulted in the formation of an electrochemically active interface. Consequently, the MoS₂/SWNT heterostructure exhibited excellent hydrogen atom recombination and H₂ gas release, significantly improving HER performance due to the altered electronic structure induced by the electron transfer. Li et al. [71] designed a well-defined three-dimensional MoS₂/rGO heterostructure using a facile hydrothermal approach. DFT simulations revealed that graphene possesses stronger electron affinity than n-type MoS₂, as shown in Figure 9a–d. Following the formation of the active interface, electrons are initially transferred from MoS₂ to graphene due to the higher work function of MoS₂. This transfer causes MoS₂ to acquire a positive charge and graphene to acquire a negative charge through an electrostatic interaction. Once equilibrium is achieved at the Fermi level between MoS₂ and rGO, a built-in electric field forms, directed from MoS₂ to graphene. This field facilities rapid electron transfer from the electrode to graphene and subsequently to MoS₂ during electrolysis, as shown in Figure 9e,f.

The formation of an electrochemically active interface rearranges and optimizes electron transfer pathways, enhances the conductivity of MoS₂, and improves its adsorption energy, collectively leading to a superior catalytic performance (Figure 10a,b).

Liu et al. [72] synthesized a MoO₂/MoS₂/C hollow nanoreactor with a monodispersed sandwich structure and a unique triple-layer “conductor/catalyst/protector” configuration. The design effectively enhances the HER performance across all pH values. The metallic MoO₂ substrate, known for its ultrahigh intrinsic conductivity, facilitates rapid charge transfer, whereas sulfur vacancies were introduced to activate the highly exposed (002) facets of MoS₂, enhancing catalytic activity. The optimized MoO₂/MoS₂/C hollow nanoreactor achieved overpotentials of 77, 91, and 97 mV at a current of 10 mA in acidic, alkaline, and neutral media, respectively, with corresponding Tafel slopes at 41, 49, and 53 mV dec⁻¹, demonstrating its excellent catalytic efficiency. Fu et al. [73] developed an ultrathin MoS₂/g-C₃N₄ heterostructure using an in situ interfacial engineering approach. The synthesized MoS₂/g-C₃N₄ heterostructure demonstrated outstanding HER performance, with overpotential and Tafel slope values comparable to that of commercial Pt catalysts. The excellent HER activity was attributed to van der Waal interactions and the strong interfacial coupling via Mo-N bonds, which significantly enhanced the hydrogen adsorption and reduction kinetics, thereby boosting overall HER efficiency. Chen et al. [74] dementated the design of a 1T-MoS₂/Ni(OH)₂ heterostructure via an interface engineering strategy to achieve excellent HER activity. The optimized heterostructure exhibited superior HER performance in an alkaline electrolyte (1 M KOH), achieving a low overpotential of 57 mV at a current of 10 mA and a corresponding Tafel slope of 30 mV dec⁻¹, highlighting its excellent catalytic efficiency. Huang and colleagues [75] demonstrated a method for creating a high performance HER electrocatalyst by combining MoS₂ with Ni₂O₃H (MoS₂/Ni₂O₃H). Their strategy involved modifying the interface electronic structure to significantly improve the HER activity. The resulting heterostructure demonstrated excellent alanine HER activity, with a low overpotential of 84 mV at 10 mA cm⁻² and a small charge transfer resistance of 1.5 Ω. Additionally, the catalyst exhibited outstanding long-term stability. DFT calculations revealed that the Ni 3d of Ni2O3H were significantly activated, serving as an electron depletion center to facilitate rapid electron transfer during HERs. Wu et al. [76] synthesized the MoS₂/Co₉S₈ heterostructure nanosheets on carbon cloth (MoS₂/Co₉S₈/CC) using a sulfurization method. This approach effectively enhanced the HER activity by leveraging the synergistic effects of the heterostructure and the conductive carbon cloth substrate. The synthesized MoS₂/Co₉S₈/CC catalyst was thoroughly analyzed to confirm its distribution and heterostructure formation. As shown in Figure 11a,b, the XRD pattern and Raman spectra confirm the successful synthesis of a MoS₂/Co₉S₈/CC heterostructure catalyst. Figure 11c,d show the scanning electron microscopic images of MoS₂/Co₉S₈/CC catalysts, indicating that the heterostructure nanosheets were uniformly grown on carbon cloth. This growth formed a network-like structure, which is conductive to the improved electron transport and increased exposure of active sites. Transmission electron microscopy (TEM) revealed that MoS₂ nanosheets, consisting of 4–8 layers, were distributed on the surface of Co₉S₈, as shown in Figure 11e,f. The formation of a heterostructure between MoS₂ and Co₉S₈ was evident, with atomic-level connections that facilitate enhanced electron transfer due to the presence of additional boundary active sites in MoS₂. The lattice spacing was measured as 0.61 nm for the (002) planes of MoS₂ and 0.35 nm for the (220) planes of Co₉S₈ (Figure 11 g). The EDS elemental mapping results in the scanning transmission electron microscopy (STEM) mode further confirmed the uniform distribution of Co, Mo, and S elements (Figure 11g–k). The catalytic performance of developed MoS₂/Co₉S₈/CC catalysts was tested in a three-electrode electrochemical system using a 1 M KOH electrolyte. As shown in Figure 12a, the linear sweep voltammeter (LSV) curve of MoS₂/Co₉S₈/CC, Co₉S₈/CC, and MoS₂CC indicted an overpotential of 73 mV at a current density of 10 mA cm⁻², which is significantly lower than those of Co₉S₈/CC (199 mV), MoS₂/CC (157 mV), and CC (373 mV). The corresponding Tafel slope, shown in Figure 12b, revealed that MoS₂/Co₉S₈/CC has a much smaller Tafel slope of 78 mV dec⁻¹ compared to other catalysts, indicating faster HER kinetics. These results demonstrated that the MoS₂/Co₉S₈/CC heterostructure significantly enhances HER activity.

Table 1 shows the previous reports on MoS₂-based materials for HER applications [77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98].

3. MoS₂-Based Materials for Energy Storage

3.1. MoS₂ as Electrode Material

MoS₂ possesses excellent electrochemical characteristics, which make it a promising electrode modifier for SCs. In this connection, Gupta et al. [99] reported a specific capacitance (C_sp) of 205.41 F/g at a current density of 0.25 A/g. An electrode modifier (MoS₂) was synthesized using a hydrothermal method and was explored for the construction of working electrodes for SCs. This work indicated that MoS₂ is a suitable candidate for electrochemical energy storage devices. Joseph et al. [100] prepared 1 T-MoS₂/carbon microspheres (CMSs) using the hydrothermal method and applied them for the construction of an SC electrode. The constructed electrode exhibited a C_sp of 692 F/g at 1 A/g under a three-electrode assembly system. This work also achieved a high power density of 775 W/kg and a high energy density of 46 Wh/kg. Authors observed that the prepared electrode had a decent stability of up to 20,000 cycles. Varasteanu et al. [101] systematically investigated the electrochemical performance of the electrochemically co-deposited MoS₂ nanoflakes and poly-naphthalenemethylamine. The fabricated polymer/MoS₂ composite facilitates the charge transport and intercalation of the ions, and an improved C_sp of 130 mF/cm³ was obtained at a scan rate of 5 mV/s. Lee et al. [102] reported a benign hydrothermal method for the preparation of novel oxygen (O)-incorporated MoS₂ with a 1T/2H hybrid phase on a graphite foil (GF) substrate. The preparation of the 1T/2H-O-MoS₂@GF can be seen in Figure 13. It was observed that the 1T/2H hybrid phase prepared by employing Mo-blue clusters simultaneously demonstrated excellent stability in the 2H phase and high electrical conductivity in the 1T phase. The 1T/2H-O-MoS₂@GF exhibited a Csp of 280 F/g at 1 A/g with a stability of 10,000 at 10 A/g.

In another article [103], 2H-MoS₂ was fabricated using the hydrothermal method route and its electrochemical performance was studied as electrode modifier for the development of electrochemical devices. Authors obtained C_sp of 115.42 F/g at 10 mV/s and stability of 5000 cyclic voltammetry (CV) cycles. Kour et al. [104] also proposed the synthesis of mixed phase of 1T/2H MoS₂ as shown in Figure 14. It was believed that presence of synergistic effects in the prepared mixed phase of 1T/2H MoS₂ may exhibit better electrochemical properties. Authors used two different techniques, i.e., conventional hydrothermal (HT) and microwave assisted hydrothermal (MHT) methods for the preparation of electrode materials (1T/2H MoS₂). Authors used CV, electrochemical impedance spectroscopy (EIS) and gravimetric charge–discharge (GCD) techniques to investigate the super capacitive performance of the prepared MS-HT and MS-MW samples. The MS-MW based electrode delivered C_sp of 421 F/g whereas MS-HT delivered C_sp of 742 F/g at 5 mV/s. The MS-HT based electrode also demonstrated excellent stability of 1200 cycles.

Teli et al. [105] electro-deposited MoS₂ on nickel foam (NF) and applied it as working electrode for SCs. A C_sp of 416.9 mF/cm² was obtained at 1 mAcm⁻² in 1 M Na₂SO₄ electrolyte. According to Naz et al. [106], covalently functionalized 1T-MoS₂ (Fc-1T-MoS₂) nanoheets (NSs) exhibits decent electrochemical properties and C_sp of 501 F/g was achieved at 1 A/g with acceptable stability of 2000 cycles. Xu et al. [107] employed the hydrothermal method to prepare the multi-layered 1T-MoS₂ with high purity. The authors adopted the combination of sodium borohydride and ethanol to control the purity of the multi-layered 1T-MoS₂. The multi-layered 1T-MoS₂ shows a C_sp of 250.3 F/g at 1 A/g and an excellent stability of 5000 cycles.

3.2. Doped Mo_S2 as Electrode Material

Doping with cobalt (Co) or manganese (Mn), etc., may improve the electrochemical properties of the MoS₂. Thus, Rohith et al. [108] employed Co as a dopant and MoS₂ as a host to fabricate the Co-doped MoS₂ as an electrode material for electrochemical energy storage applications. Authors achieved a C_sp of 86 F/g at 1 A/g for a two-electrode assembly system with excellent stability of 10,000 GCD cycles. Fayed et al. [109] proposed that nitrogen (N) and Co-doped MoS₂ may exhibit better electrochemical features and adopted a hydrothermal synthetic procedure for the preparation of the N and Co-doped MoS₂. The N and Co-doped MoS₂ shows a C_sp of 1400 F/g at 1 A/g. Asha and co-workers [110] have prepared MoS₂ and iron (Fe)-, nickel (Ni)-, and Co-doped MoS₂ on carbon cloth (CC) using the hydrothermal method. The synthesized materials were used for SC applications. The GCD curves of the MoS₂/CC and Ni-MoS₂/CC electrodes are shown in Figure 15a,b, respectively. It can be seen that the Ni-MoS₂/CC electrode exhibits better charge storage features compared to the MoS₂/CC electrode.

The GCD curves of the Fe-MoS₂-, MoS₂-, Co-MoS₂-, and Ni-MoS₂-based electrodes are displayed in Figure 15c. It was observed that the Ni-MoS₂-based electrode shows improved charge storage properties compared to the other electrodes at a current density of 1 A/g (Figure 15d). A decent C_sp of 305.9 F/g was obtained for the Ni-MoS₂/CC-based electrode, which is higher than MoS₂/CC (170.2 F/g). Authors also reported acceptable stability for 5000 cycles. In another report [111], manganese (Mn)-doped MoS₂ was prepared by the hydrothermal method for the preparation of working electrodes for SCs. The obtained Mn-MoS₂ shows a C_sp of 430 F/g with a stability of 5000 cycles. Charapale et al. [112] also reported the fabrication of Fe-doped MoS₂ and achieved a C_sp of 545 F/g at 1 A/g with a cyclic stability of 2000 cycles. Khichi et al. [113] proposed the fabrication of novel MoS_{2(1 − x)}Se_2x (x = 0, 0.25, 0.5, 0.75, and 1) electrode materials, as demonstrated in Figure 16. This optimized MoS₁Se₁ electrode material shows excellent electrochemical properties and an improved C_sp of 266.51 F/g at 0.5 A/g. Authors also observed that the proposed electrode is stable up to 6000 cycles.

3.3. MoS₂-Based Hybrid Materials as Electrode Material

According to Zhang et al. [114], the MnO₂/MoS₂-based electrode showed a C_sp of 224 mF cm⁻² with a cyclic stability of 3000 cycles. In another work [115], a MoS₂-RuO₂ composite was prepared using a facile hydrothermal-assisted chemical reduction approach followed by calcination. The prepared hybrid material showed a C_sp of 972 F/g at 1 A/g with a decent stability of 10,000 cycles. A tin oxide-modified MoS₂ (SnO₂/MoS₂)-based electrode showed a C_sp of 567 F/g at 2 mV/s [116]. In another report [117], a SnO₂-MoS₂-based electrode exhibited a decent C_sp of 415 F/g at 1 A/g. This may be ascribed to the synergistic interactions between the SnO₂ and MoS₂. MoS₂ was also modified with SnO₂ quantum dots (SQD) to further investigate the effects of SQD for energy storage applications [118]. The proposed electrode material shows a C_sp value of 505 F/g at 1 A/g and a cyclic stability of 5000 cycles at 10 A/g. This interesting performance may be attributed to the structural integrity, high conductivity, and better electron electrochemical reactivity of the prepared electrode. Iqbal et al. [119] reported the hydrothermal preparation of a MoS₂@TiO₂ composite and explored it as an electrode material for SCs. The investigations showed that the MoS₂@TiO₂ composite has a decent Csp of 210 F/g at 10 mV/s with an acceptable cyclic stability of 2000 cycles. Ma et al. [120] grew a zinc sulfide (ZnS)/MoS₂ hybrid composite material on Mo foil via the hydrothermal method. The prepared electrode showed a Csp of 956.3 F/g at 10 mA/cm² and a long-term stability of 2000 cycles. Liu et al. [121] reported the novel electrode material of nickel sulfide (NiS₂)/MoS₂ on NF using an in situ hydrothermal method. This electrode demonstrated a Csp of 4.46 F/cm³ at 5 mA/cm² and an acceptable cyclic stability of 3000 cycles. An iron sulfide (FeS₂)/MoS₂-based electrode revealed the presence of high conductivity and synergistic interactions [122]. A FeS₂/MoS₂-based electrode showed a C_sp of 495 mF/cm², which is higher than that of pristine MoS₂ (132 mF/cm²) and FeS₂ (315 mF/cm²) at 1 mA/cm². Authors also stated that the proposed electrode material has a decent cyclic stability of 5000 cycles. In 2020, a nickel molybdenum oxide (NiMoO₄)@MoS₂ core–shell structure was prepared using a two-step hydrothermal method [123]. The presence of synergism in the prepared hybrid electrode material lead to the generation of a high C_sp of 2246.7 F/g with 5000 cycles. Wu et al. [124] reported the fabrication of three-dimensional (3D) flower bud-like phosphorus (P) and tungsten (W) co-doped nickel cobalt sulfide (NiCo₂S₄)/MoS₂ composites (P, W-NCS@MS) via the hydrothermal method (Figure 17). The synthesized P, W-NCS@MS was explored as an electrode modifier, which demonstrated a Csp of 1250 C/g at 1 A/g with a stability of 10,000 cycles.

3.4. MoS₂/Carbon-Based Composites as Electrode Material

In another article [125], single-layered MoS₂/graphene NSs were fabricated on activated carbon nanofibers (MoS₂@G/AC) using an electrospinning process. The prepared electrode material exhibited a high specific surface area of 850.2 m²/g with a diameter of around 100 nm. Authors found that 0.5 MoS₂@G/AC had a high C_sp of 334 F/g at 0.5 A/g and, contrastingly, of 246.3 F/g at 10 A/g (electrolyte 3 M KOH). The 0.5 MoS₂@G/AC also retained excellent stability of 5000 cycles with a retention of 83.8%. Authors believed that the improved electrochemical activity of the electrode may be ascribed to the high surface area, promising pore structure, and synergistic interactions between the three components. A C_sp of 13.6 mF/cm² and cyclic stability of 1000 cycles were reported for the MoS₂/electrochemically exfoliated graphene (EEG)-based electrode [126]. The MoS₂/rGO composite also showed a C_sp of 460 F/g at 1 A/g with a decent cyclic stability of 5000 cycles [127]. In another article [128], a flake-like MoS₂/r-GO composite-based electrode exhibited a C_sp of 441 F/g at 1 A/g and a cyclic stability of 1000 cycles at 80 mV/s.

It is well known that mixed-phase MoS₂ has impressive electrochemical properties, which originate from the generation of electrochemical active sites and higher structural stability compared to the other materials with a crystalline nature. In this connection, Bokhari et al. [129] reported the one-pot synthesis of N-doped, rGO-modified, mixed-phase (MP) MoS₂ nanoflowers (NGM). The preparation of the electrode material can be seen in Figure 18. The NGM composite-based electrode exhibited a Csp of 539.5 F/g and a power density of 25.4 kW/kg. It was stated that the robust ion transport and low charge transfer resistance at the electrode–electrolyte interface may boost the electrochemical charge storage properties of the NGM-based electrode.

Wang et al. [130] obtained a C_sp of 392 F/g at 5 mV/s using a hydrothermally prepared MoS₂/graphene composite modified electrode. Authors also obtained a C_sp of 76 F/g at 5 mV/s for a 2D printing-based fabrication process using MoS₂/graphene ink. Pilathottathil et al. [131] prepared AC/MoS₂, graphene/MoS₂, and MWCNTs/MoS₂ using benign approaches. The fabrication of SC devices are demonstrated in Figure 19. Authors observed that AC/MoS₂ has superior electrochemical properties and exhibited a decent C_sp of 216 F/g with an energy density of 6.2 Wh/kg. In contrast, graphene/MoS₂ showed a C_sp of 202 F/g, whereas MWCNTs/MoS₂ exhibited a C_sp of 161 F/g.

Kour et al. [132] used mixed-phase MoS₂ anchored carbon nanofibers (CNFs) by employing the hydrothermal route. Authors prepared a series of MS/CNF_-x composites (where x = 1, 1.5, 2, and 3) to optimize the electrochemical performance of the proposed electrode materials. It was believed that the introduction of CNFs may offer a conductive path to enhance the diffusion of ions, provide better structural support, and reduce the restacking of the MoS₂ layer during GCD cycles. Authors found that the MS/CNF-2 sample showed higher electrochemical behavior and an improved C_sp of 626.08 F/g at 1 A/g. In contrast, pristine MS showed a C_sp of 159.35 F/g, which is almost four times lower compared to the MS/CNF-2 sample. Tiwari et al. [133] stated that the hydrophilicity of the SC electrodes with minimal resistive losses are the most crucial parameters for the development of energy storage devices. In this connection, authors demonstrated the incorporation of MnO₂ and MoS₂ film-based heterostructures on CNTs. The MnO₂/CNT electrode showed a real capacitance and volumetric capacitance of 0.34 F/cm² and 6.5 F/cm³, respectively. In a report by Chen et al. [134], the CNT@MoS₂ core–shell structure was obtained using the hydrothermal method. Authors found that MoS₂ has a hexagonal 2H-MoS₂ structure, which was confirmed by X-ray diffraction (XRD) and Raman spectroscopic measurements. The CNT@MoS₂ was used as an electrode modifier, and a C_sp of 87 F/g was obtained at 10 mV/s. Kumar et al. [135] prepared MoS₂-rGO (varying percentage of rGO; 5, 10, 15, and 20%) composites and explored them for energy storage applications. Authors obtained a C_sp of 1023 F/g at 1 A/g and a stability of 1000 cycles under optimized conditions. Farshadnia et al. [136] prepared a spongy nanostructured GO with a high surface area and decent porosity. The CoNi₂S₄ and MoS₂ were deposited on porous GO to improve the capacity and performance of the working electrode. The prepared CoNi₂S₄@MoS₂@rGO exhibited a C_sp of 3268 F/g at 1 A/g with 3000 stability cycles in a 3 M KOH electrolyte. Wei et al. [137] coated rGO onto NF and Ni₃S₂/MoS₂ composites under simple strategies at different times (18 h, 24 h, 30 h, 36 h, and 42 h). The time was optimized to investigate the effect of reaction time on the surface morphology of the electrode materials. It was observed that at 18 h and 24 h, Ni₃S₂/MoS₂ was formed with a 2D NS-like shape with a 3D network. The nanorods (NRs) obtained with increasing reaction time and hierarchical core–shell configuration were formed at 36 h at 150 °C. A C_sp of 6451 mF/cm² was obtained at 40 mA/cm² with a stability of 5000 cycles under optimized conditions for rGO/Ni₃S₂/MoS₂-based studies. Serrapede et al. [138] used 3D graphene aerogel-decorated MoS₂ as an electrode modifier and achieved a decent C_sp of 210 F/g. In another report, Yan et al. [139] explored polymers as conductive supports to enhance the electrochemical performance of the MoS₂-based SCs. Authors prepared polypyrrole (PPy)-coated MoS₂ on CC and explored them for SC applications. The fabricated PPy/MoS₂/CC electrode showed a C_sp of 1150.4 mF/cm² at 5 mA/cm². Rajapriya et al. [140] used the hydrothermal method for the preparation of MoS₂/CNFs. Authors found that MoS₂/CNFs have a high surface area of 48 m²/g, and the electrochemical investigation yielded a Csp of 903.9 F/g at 1 A/g with a cyclic stability of 5000 cycles. It was stated by the authors that nanoflowers of MoS₂ provided a void space between the NSs for the intercalation of electrolyte ions, which enhanced the performance of the MoS₂/CNFs. Arun et al. [141] proposed the hydrothermal preparation of Ni₃S₂/MoS₂/rGO hybrids on NF for SCs. The fabricated electrode with the Ni₃S₂/MoS₂/rGO hybrid on NF demonstrated a C_sp of 2580 F/g at 1 A/g and a stability of 10,000 cycles with 100% capacitance retention. Zhang et al. [142] reported the synthesis of a MoS₂-PPy@rGO composite using molybdenum oxide as the Mo source. The PPy was adopted as an intercalation agent to enlarge the interlayer space in MoS₂. The fabricated SCs exhibited an energy density of 134.4 µWh/cm² with a power density of 800 µW/cm². Rani et al. [143] adopted the hydrothermal approach for the fabrication of MoS₂/CNTs. The fabricated MoS₂/CNTs composite showed a C_sp of 436 F/g at 1 A/g. Liu et al. [144] reported the formation of N/O-co-doped carbon nanocages (NOCNs) modified with MoS₂ using the hydrothermal method. Authors found that ultrathin MoS₂ NSs were vertically grown on the NOCN surface, which can provide a fast pathway for electron transportation and prevent sheet aggregation. Authors observed that MoS₂@NOCN-3 demonstrated a C_sp of 240.8 F/g at 0.5 A/g. Aftab et al. [145] prepared a MoO₂/MoS₂@GO ternary hybrid composite by employing a wet chemical-aided method. The prepared MoO₂/MoS₂@GO ternary hybrid composite displayed decent electrochemical performance, and authors reported an acceptable C_sp of 1530 F/g at 2 A/g. Khandare et al. [146] reported a LiMoS₂/rGO-based electrode for energy storage applications, which demonstrated a power density of 686.3 W/kg and an energy density of 10.6 Wh/kg.

3.5. MoS₂ and Polymer Composite as Electrode Material

Li et al. [147] also used the in situ oxidative polymerization method for the preparation of a MoS₂/PPy composite. Authors obtained a Csp of 677.8 F/g at 1 A/g using an MP-2 electrode. Wang et al. [148] proposed the formation of a silicon dioxide (SiO₂) peristaltic supporting carbon (SPSC)/polyaniline (PANI)/MoS₂ composite for energy storage devices. Authors found that the proposed electrode displayed a C_sp of 637.5 F/g at 0.5 A/g with an energy density of 22.6 Wh/kg and a power density of 550 W/kg. A CuS/PANI@MoS₂ (CSPM) composite was prepared by Dai et al. [149]. This prepared CSPM composite showed a Csp value of 759.2 F/g at 1 A/g. Authors also reported a decent stability of 5000 cycles. Jasna et al. [150] have designed and fabricated a PANI/CNT/e-MoS₂ composite for SCs, which exhibited a C_sp of 532 F/g at 1 A/g. The proposed work also showed a stability of 4000 cycles with a high energy density of 11.8 Wh/kg. Suresh et al. [151] reported a C_sp of 177 F/g at a 0.5 current density using a cerium dioxide (CeO₂)/PANI/MoS₂ tri-composite as the electrode. Jhanjhariya et al. [152] fabricated a MoS₂/MWCNTs (MC) composite using the hydrothermal method, and PPy was incorporated using the in situ oxidation polymerization of a pyrrole monomer. The fabricated electrode (MCP1.5) exhibited a C_sp of 3352.08 F/g at 2 mV/s.

3.6. MoS₂ and MXene Composites as Electrode Material

In previous years, MXenes such as Ti₃C₂T_x have attracted researchers because of their excellent electrochemical and conducting properties. Pan et al. [153] reported the synthesis of a Ti₃C₂T_x/MoS₂ composite, which showed a Csp of 439 F/g at 5 mV/s with a stability of 10,000 cycles. Qiao et al. [154] designed and fabricated a flexible MXene/1T-MoS₂ composite paper electrode for SC applications, which exhibited specific area and volumetric capacitances of 561.2 mF/cm² and 1268.17 F/cm³ at 0.8 mA/cm², respectively. Li et al. [155] reported that electrostatic spray deposition (ESD) is the most efficient method for the preparation of high-quality materials with improved electrochemical properties. Thus, authors used the ESD method for the preparation of a 1T MoS₂/Ti₃C₂T_x composite, which showed C_sp of 1198 mF/cm² at 3 mA/cm² and cyclic stability for up to 7000 cycles. Ali et al. [156] designed and reported the synthesis of MoS₂@Ti₃C₂T_x for SC applications. The fabricated electrode showed a Csp of 1022.7 F/g at 1 A/g with a cycle life of 5000 cycles. In another report by Bera et al. [157], a MoS₂ QDs/Ti₃C₂T_x-based electrode showed a decent Csp of 985 F/g and an energy density of 165.53 Wh/kg with a power density of 1100 W/kg. Hayat et al. [158] fabricated MoS₂@MXene//MXene flexible asymmetric SCs, which demonstrated a high energy density of 1.21 Wh/kg and a power density of 54.45 W/kg. The above reports show that MoS₂-based materials are promising candidates for SC applications. The electrochemical performance of the MoS₂-based electrode for SC applications is summarized in Table 2 [99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,153,157,159,160].

4. Conclusions and Perspectives

Herein, we have reviewed MoS₂-based hybrid materials and concluded that they are promising for HER and SC applications. In the case of SCs, we have discussed the formation of various composites of MoS₂ with polymers, MXenes, metal oxides, CNTs, and rGO for the development of high-performance energy storage devices. It was observed that MoS₂-based materials with MXene show decent performance and stability. The observations suggest that MoS₂-based materials have the potential for the construction of long-term stable SCs with decent performance. Various strategies have been employed to enhance the catalytic activity of MoS₂ for hydrogen production. One of the most effective approaches is modifying its electronic structure, which directly influences its physical properties and overall performance. On one hand, this modification increases the charge density, accelerates electron transport, and enhances conductivity. On the other hand, tuning the electronic structure optimizes hydrogen adsorption energy, a key factor in improving intrinsic catalytic activity. Moreover, the catalytic activity of hydrogen evolution reactions (HERs) can be further enhanced through several strategies. These include improving the intrinsic activity of catalytic sites by optimizing the electronic structure at the edges or activating the inert basal plane to enhance hydrogen adsorption, increasing the number of active sites by introducing defects such as cracks or holes in the MoS₂ layers, and boosting electrical conductivity by creating new electronic states near the Fermi level, thereby narrowing the bandgap. While significant progress has been made in designing and developing MoS₂-based electrocatalysts, there is still ample room for further advancements in the field. Current laboratory methods for synthesizing MoS₂ nanosheets typically yield only small quantities, limiting their feasibility for industrial applications. Therefore, developing scalable and efficient fabrication techniques is essential for enabling the mass production of MoS₂-based electrocatalysts. Literature reports indicate that the current method for evaluating the HER activity of MoS₂-based powder materials involves coating them onto glassy carbon electrodes (GCEs). This process requires binders like Nafion, which can hinder catalytic performance. A more effective approach to overcome this limitation is the direct growth of MoS₂ nanosheets on conductive substrates such as metal foils, fluorine-doped tin oxide (FTO), tin-doped indium oxide (ITO), carbon paper, or carbon cloth, ensuring better conductivity and stability. We believe that further advancements in this research field require enhancing the HER performance of MoS₂ by optimizing its electronic structure. This approach can effectively improve its physical properties, increase charge density, enhance conductivity, and facilitate electron transport.

Author Contributions

Conceptualization, K.A. and W.R.; writing—original draft preparation, K.A. and W.R.; writing—review and editing, T.H.O. and F.G.A.; supervision, T.H.O. and F.G.A. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Tables

Figure 1. The typical ball and stick model of the 2H and 1T phases of MoS2, where pale blue represents Mo atoms and the yellow color denotes S atoms. Reproduced with permission from Ref. [35], copyright 2024, MDPI.

Figure 2. Schematic illustration of electrocatalytic water splitting for the H2 and O2 production.

Figure 3. Schematic illustration of typical polarization curves for HERs and OERs and corresponding half-reactions. Reproduced with permission from Ref. [30], copyright, 2017, WILEY-VCH.

Figure 4. HER Volcano plot of the relationship of the exchange current density of certain metals and MoS2 and their corresponding ∆GH*. Reproduced with permission from Ref. [46], copyright 2019, Elsevier.

Figure 5. (a) Schematic presentation for the synthesis of 1T/2H-MoS2 electrocatalysts with ammonium ions at different temperatures, (b) polarization, and (c) corresponding Tafel slopes of 1T/2H-MoS2 electrocatalysts with ammonium ions at different temperatures at a 5 mV/S scan rate in 0.5 M H2SO4. Reproduced with permission from Ref. [58], copyright 2023, Elsevier.

Figure 6. Schematic illustration for the synthesis of Ru single-atom-doped MoS2 catalysts for HER applications. Reproduced with permission from Ref. [65], copyright 2021, Elsevier.

Figure 7. HAADF-STEM image of (a) optimized Ru0.10@2H-MoS2, (b,c) high resolution images of 1T and 2H phases decorated with Ru single atoms (marked by yellow circles), (d–g) EDS elemental mapping image of optimized Ru0.10@2H-MoS2 catalysts indicating the presence of Ru, Mo, and S atoms. Reproduced with permission from Ref. [65], copyright 2021, Elsevier.

Figure 8. HER performance of Ru@2H-MoS2 catalysts with varying loadings of Ru in 1.0 M PBS solution at pH 7, (a) polarization curves (b) corresponding Tafel slopes, (c) the electrochemical double-layer capacitance (Cdl), and (d) electrochemical impedance spectroscopy (EIS). Reproduced with permission from Ref. [65], copyright 2021, Elsevier.

Figure 9. Charge density difference between (a) pristine MoS2 and (b) MoS2/graphene heterostructure. Blue color indicates depletion space and yellow color indicates accumulation space. (c,d) Plot of (110) surfaces of the charge density difference contour for pristine MoS2 and MoS2/graphene, (e) x-y plane averaged charge density, with charge density on the y-axis and distance along z-direction on the x-axis. (f) Average electrostatic potential of pristine MoS2 and MoS2/graphene. Reproduced with permission from Ref. [71], copyright 2015, Nature.

Figure 10. (a) Polarization curves obtained for pristine MoS2 and MoS2/graphene heterostructure at 140, 160, 180, and 200 °C (denoted MG1–4), and (b) corresponding Tafel plots. Reproduced with permission from Ref. [71], copyright 2015, Nature.

Figure 11. (a). XRD pattern and (b) Raman spectra of Co9S8, Co9S8/CC, MoS2/CC, and Co9S8/MoS2/CC; (c,d) SEM, (e,f) TEM, and (g) HRTEM images of Co9S8/MoS2/CC at different magnifications; and (h) corresponding EDS mapping of (i–k) Co, Mo, and S. Reproduced with permission from Ref. [76], copyright 2021, Elsevier.

Figure 12. (a) HER polarization curves and (b) corresponding Tafel plots of synthesized pristine and composite catalysts (Co9S8/CC, MoS2/CC, and Co9S8/MoS2/CC). Reproduced with permission from Ref. [76], copyright 2021, Elsevier.

Figure 13. Pictorial illustration for the hydrothermal preparation of 2H-MoS2 and 1T/2H-O-MoS2 on GF. Reproduced with permission [102].

Figure 14. Schematic view for the synthesis of MS-HT and MS-MW samples. Reproduced with permission [104].

Figure 15. (a) GCD graphs of MoS2/CC (a) and Ni-MoS2/CC (b) at different current densities. GCD curves (c) and Csp versus current density variations (d) of Fe-MoS2-, MoS2-, Co-MoS2-, and Ni-MoS2-based electrodes. Reproduced with permission [110].

Figure 16. Schematic representation of the preparation of MoS2(1 − x)Se2x (x = 0, 0.25, 0.5, 0.75, and 1). Reproduced with permission [113].

Figure 17. Schematic graph shows the synthetic protocols. Reproduced with permission [124].

Figure 18. Schematic view of the preparation of the electrode material. Reproduced with permission [129].

Figure 19. Schematic graph shows the synthesis and fabrication of SCs. Reproduced with permission [131].

Table 1

Electrochemical performance of MoS₂-based electrodes for HER applications.

Catalysts	Synthesis Method	Electrolyte	Stability	Overpotential (mV)	Tafel Slope (mV/dec)	Ref.
MoS2−MoO3−x/Ni3S2	Electrochemical/electrodeposition	1.0 M KOH	17 h	76	53.2	[77]
3D-Co-MoS2/G	Hydrothermal	0.5 M H2SO4	5000Cycles	143	71	[78]
MoS2−CoS−Zn	Solvothermal	0.5 M H2SO4	60 h	72.6	37.6	[79]
Ni2P/MoS2/N:RGO	Heating method	0.5 M H2SO4	3000 Cycles	39.5	39.52	[80]
MoS2/CoNi2S4	Hydrothermal	1.0 M KOH	48 h	78	67	[81]
NiFeOx(OH)y@MoS2/rGO	Sol–gel	1.0 M KOH	12 h	170	80	[82]
Co9S8−MoS2/NF	Hydrothermal	1.0 M KOH	60 h	167	81.7	[83]
MoS2/NiCo/CC	Hydrothermal	0.5 M H2SO4	17 h	153	51.1	[84]
Mo6Te6/MoS2(1 − x)Te2x	CVD	0.5 M H2SO4	1000 Cycles	320	55.7	[85]
MoS2/MoP	Hydrothermal	1.0 M KOH	30 h	92	59.8	[86]
MoS2/CoNi@G	Hydrothermal	0.5 M H2SO4	10,000Cycles	150	66	[87]
MoS2-MoP/NC	Hydrothermal	1.0 M KOH	10,000Cycles	35	30	[88]
Co9S8@MoS2/N-carbon	Calcination in argon	1.0 M KOH	12 h	126	74.1	[89]
CoS2@MoS2/CC	Hydrothermal	1.0 M KOH	5000Cycles	31	39.4	[90]
NCT@CoP@MoS2	Pyrolysis method	1.0 M KOH	5000Cycles	195	74	[91]
MoS2-Ti3C2 MXene	Hydrothermal	0.5 M H2SO4	1000 Cycles	98	45	[92]
MoS2–Ti3C2Tx-P	Solvothermal	0.5 M H2SO4	-	298	120	[93]
MoS2 QDs/Ti3C2Tx	Solvothermal	0.5 M H2SO4	500 Cycles	220	72	[93]
Co-MoS2/V2C	Hydrothermal	1 M KOH	10,000 Cycles	70.1	98.6	[94]
MoS2/Ti3C2@CNFs	CVD	0.5 M H2SO4	3000 Cycles	142	113	[95]
MoS2/CoS2-2@Mo2TiC2Tx	Hydrothermal	0.5 M H2SO4	1000 Cycles	80	108.3	[96]
MoS2 QDs/MXene	Hydrothermal	0.5 M H2SO4	1000 Cycles	307	68.5	[97]
MoS2/graphdiyne nanotube/MXene	Hydrothermal	0.5 M H2SO4	3000 Cycles	109	55	[98]

Table 2

Electrochemical performance of MoS₂-based electrode for SC applications.

Electrode Modifier	Synthesis Method	Electrolyte	Stability Cycles	Csp (F/g)	Current Density (A/g)	Refs.
MoS2	Hydrothermal	3 M KOH	1000	205.41	0.25	[99]
1 T-MoS2/CMS	Hydrothermal	1 M KOH	20,000	692	1	[100]
Polymer/MoS2	Electro-deposition	PVA-H2SO4	-	130 mF/cm3	5 mV/s	[101]
1T/2H-O-MoS2@GF	Hydrothermal	1 M Na2SO4	10,000	280	1	[102]
2H-MoS2	Hydrothermal	0.5 M H2SO4	5000	115.42	10 mV/s	[103]
MS-HT	Hydrothermal	6 M KOH	1200	742	5 mV/s	[104]
MS-MW	Hydrothermal	6 M KOH	1200	421	5 mV/s	[104]
MoS2	Electro-deposition	1 M Na2SO4	-	416.9 mF/cm2	1 mAcm−2	[105]
Fc-1T-MoS2	Electrochemical exfoliation	1 M H2SO4	2000	501	1	[106]
Few-layered 1T-MoS2	Hydrothermal	Na2SO4	5000	250.3	1	[107]
Co-MoS2	Hydrothermal	1 M KOH	10,000	86	1	[108]
N, Co doped MoS2	Hydrothermal	6 M KOH	3000	1400	1	[109]
Ni-MoS2/CC	Hydrothermal	0.5 M H2SO4	5000	305.9	1	[110]
Mn-MoS2	Hydrothermal	0.5 M Na2SO4	5000	430	1	[111]
Fe-MoS2	Hydrothermal	3 M KOH	2000	545	1	[112]
MoS1Se1	Hydrothermal	6 M KOH	6000	266.51	0.5	[113]
MnO2/MoS2	Ultrasonic/shearing	1 M Na2SO4	3000	224 mF cm−2	-	[114]
MoS2-RuO2	Hydrothermal/Reduction	1 M KOH	10,000	972	1	[115]
SnO2-MoS2	Hydrothermal	3 M KOH	5000	415	1	[116]
Ti3C2Tx/MoS2	Ultrasonic and heat treatment	3 M H2SO4	10,000	439	5 mV/s	[153]
S-rich MoS2 QDs/MXene	Hydrothermal	1 M H2SO4	10,000	985	1	[157]
MoS2–Ti3C2Tx	Hydrothermal	1 M H2SO4	10,000	303.8	1	[159]
Laser scribed graphene (LSG)/boron (B)-1 T MoS2/MXene	Hydrothermal	PVA/K2SO4	10,000	72.31 mF/cm2	0.075 mA/cm2	[160]

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