Recent Progress on High-Efficiency Hydrogen Evolution Electrocatalysis of Heteroatom-Doped MoS₂: A Review

Abstract

The exacerbation of the global energy crisis has brought the development of efficient and sustainable hydrogen energy to the forefront of contemporary research endeavors. Molybdenum disulfide (MoS₂), recognized for its outstanding electrocatalytic performance as a two-dimensional material, has attracted significant interest for its potential in the hydrogen evolution reaction (HER). This review delves into the heteroatom-doped modification strategy centered on MoS₂ and its effectiveness in enhancing electrocatalytic hydrogen evolution. The influence of various doping elements (including noble metals, transition metals, and non-metals) on the electronic structure and catalytic efficiency of MoS₂ is also analyzed, elucidating the mechanism by which heteroatom doping enhances the catalytic performance and stability of MoS₂. Looking ahead, the integration of multiple doping elements, utilization of advanced computational techniques, and advancement of novel synthetic methods position MoS₂ for practical applications in the field of hydrogen energy, driving the progress and improvement of sustainable energy initiatives.

1. Introduction

Energy serves as the pillar of modern society; however, the world is currently grappling with an exceptionally severe energy crisis. Fossil fuels stand as the primary energy source, yet they face challenges including rapid resource depletion [1], extraction complexities [2], low output [3], limited storage capacity [4], and non-renewability [5]. Furthermore, fossil fuel extraction results in the generation of mine water [6] and substantial greenhouse gas emissions upon usage [7], giving rise to critical environmental pollution concerns [8,9]. Consequently, the pursuit of sustainable, environmentally friendly, and highly efficient energy sources has emerged as a prominent focus in recent energy research endeavors. Hydrogen, an elemental pioneer in the universe and occupying the top position in the periodic table [10], boasts remarkable physical and chemical attributes such as colorlessness, odorlessness, and easy flammability under standard conditions [11]. Hydrogen gas exhibits a notably high energy density, with the heat released from the combustion of a unit mass of hydrogen approximately three times that of natural fossil fuels [12]. Hydrogen energy pertains to the energy liberated in reactions primarily involving hydrogen and its isotopes, which can be harnessed through diverse means like direct combustion [13], nuclear fusion [14], and fuel cells [15], among others. In contrast with conventional fossil fuels, hydrogen energy presents substantial advantages: it can be sustainably derived from water, showcasing renewability [10,12]; its extraction adheres to eco-friendly processes, enabling low to zero emissions and environmental compatibility [16,17]; and applications such as hydrogen fuel cells demonstrate elevated energy conversion rates and reduced waste heat, thereby enhancing energy utilization efficiency [18]. Researchers are actively engaged in exploring the realms of hydrogen energy development and storage. Presently, prevalent hydrogen production methods within chemical processes predominantly encompass hydrogen production from natural fossil fuels [19], industrially generated by-product hydrogen [20], renewable energy-driven hydrogen production [21], and biological hydrogen production [22]. The underlying production principle involves the conversion of primary energy directly extracted from nature into secondary energy hydrogen through a sequence of processing steps [10,23]. Nevertheless, traditional hydrogen manufacturing technologies confront obstacles like demanding process operation conditions, low hydrogen purity, modest economic gains, and constraints stemming from technological and equipment limitations [22,24,25]. The utilization of water as the exclusive reactant, coupled with electrolyzing water to yield hydrogen with electricity, culminates in a pristine and environmentally conscious process [26], boasting hydrogen purity surpassing 99.99% [27]. In comparison to traditional high-temperature and high-pressure hydrogen production methodologies, water electrolysis for hydrogen generation operates under temperate, secure, and constant conditions at standard temperature and pressure [28,29], positioning it as a focal point in hydrogen production. Nonetheless, in the pragmatic operational scenario of water electrolysis for hydrogen production, elevated overpotentials within the electrode system may lead to actual electrode potentials surpassing theoretical levels by a considerable margin [30], resulting in a notable decline in energy conversion efficiency and reaction rates [29,31].

Recognizing the persistent challenges in sustainable hydrogen production technologies [32], electrocatalytic water electrolysis has gained prominence as a promising strategy to mitigate these limitations. Within this framework, the hydrogen evolution reaction (HER) represents a crucial cathodic half-reaction where advanced electrocatalyst design plays a pivotal role in reducing the inherent overpotential of the HER process. This optimization enables substantial minimization of energy dissipation while enhancing the system’s overall energy conversion efficiency. In the electrochemical hydrogen adsorption phase of the HER (Volmer), hydrogen ions are adsorbed in acidic electrolyte conditions, while water molecules dissociate in alkaline electrolyte conditions, resulting in the creation of adsorbed hydrogen atoms (H*) [27,31,33]. Catalysts function to decrease the activation energy of chemical reactions, leading to faster reaction rates. Within HER, electrocatalysts play a crucial role not only in enhancing H₂ production rates and optimizing selectivity but also in advancing the economic feasibility of the HER process [34]. With unique surface structures and electronic properties, electrocatalysts provide a multitude of active sites and suitable adsorption energies and facilitate the efficient absorption of hydrogen ions or water molecules onto these active sites, thereby enabling effective activation [33,34]. In the electrochemical hydrogen desorption step of the HER (Heyrovsky) and the electrochemical hydrogen recombination desorption step (Tafel), highly conductive electrocatalysts establish efficient pathways for electron transfer [35]. This optimization improves electron transfer modes between active sites, adsorbed hydrogen atoms, and electrolyte ions, resulting in accelerated and smoother reaction kinetics, ultimately leading to a substantial enhancement in the rate of hydrogen evolution [30,32,33]. Commonly employed in industrial applications, electrocatalysts frequently incorporate precious metal catalysts such as Pt and Pd [36,37].

Recently, the two-dimensional material MoS₂ has gained significant attention in HER research due to its exceptional properties as a transition metal chalcogenide compound. In contrast to traditional electrocatalysts, which mainly rely on expensive noble metal nanomaterials with limited availability [38], MoS₂ offers notable advantages such as cost-effectiveness, high efficiency, excellent energy storage capabilities, and a large surface area [39]. The catalytic mechanism of MoS₂ centers on providing active catalytic sites for the adsorption and activation of H₂O, thereby facilitating H₂ generation [38,39]. While conventional methods of modifying MoS₂ include surface modifications [40], sulfur vacancy repairs [41], and interface engineering [42], they are hindered by challenges like inadequate product stability, lack of precise product control, and complex process workflows. As a result, there is a growing focus on developing advanced MoS₂ modification strategies, with heteroatom doping modification emerging as a particularly promising approach. By incorporating additional elements into MoS₂, changes in its electronic structure and surface properties can be achieved to enhance its catalytic performance [40,43]. This review explored the crystal phase characteristics and operational mechanisms of HER technology and MoS₂, highlighting the importance of employing element doping modification strategies to improve the effectiveness of MoS₂.

2. Electrocatalytic Hydrogen Evolution Technology (HER)

2.1. Basic Steps

The hydrogen evolution reaction (HER) occurs at the interface between the electrode and the electrolyte within an electrochemical electrolysis system. It consists of three essential steps: the adsorption step (Volmer step) [44], the electrochemical desorption step (Heyrovsky step) [45], and the composite desorption step (Tafel step) [46,47] (Figure 1A). The Volmer step initiates the HER process. Under acidic electrolyte conditions (Reaction (1)), hydrogen ions (H⁺) within the electrolyte gradually migrate to the electrode surface, where a substantial number of electrons (e⁻) are supplied by the electrode to the surrounding hydrogen ions [48]. Consequently, the physicochemical properties of the electrode material and the roughness of its surface significantly influence the reaction rate of the Volmer step. Following electron acquisition, hydrogen ions transform into adsorbed hydrogen atoms (H*) attached to the electrode surface [48,49]. In alkaline electrolyte conditions, the reaction mechanism parallels that of acidic conditions (Reaction (2)), with water molecule (H₂O) becoming the primary reactants. Post-electron acquisition, H₂O decomposes to produce H* and hydroxide ions (OH⁻). Unlike in acidic environments where H* is the primary reactant, the HER rate is notably lower under alkaline settings due to the absence of direct H* provision [49,50]. Subsequently, the process proceeds to the Heyrovsky step, where H* generated in the Volmer step combines with hydrated hydrogen ions (H₃O⁺) in the electrolyte under continuous electron supply from the electrode, resulting in hydrogen gas evolution and H₂O formation (Reaction (3)). This reaction occurs under acidic electrolyte conditions; meanwhile, under alkaline conditions, though the mechanism is akin, H₂O directly participates due to the inadequate H₃O⁺ quantity in the electrolyte (Reaction (4)). The crux of the Heyrovsky step lies in the amalgamation of H* with protons under electron conditions to yield hydrogen gas. Within the entire HER process, H* serves as a crucial intermediate in the conversion of hydrated hydrogen ions to hydrogen gas molecules. Unlike the Volmer and Heyrovsky steps, the Tafel step manifests when the H* concentration in the electrode system is excessive [51]. In this scenario, H* on the electrode surface directly combines to generate H₂ without protons and electrons (Reaction (5)), leading to a substantial overpotential in the electrode system, impacting the HER rate and hydrogen evolution rate. Although the Tafel step does not exert dominant control over the HER process, it bears profound significance in electrocatalyst selection and comprehensive HER investigations [52]. Throughout the HER process, the three core reaction steps contend and collaborate: dominance of one step weakens the reaction rates of others, while they mutually influence each other. The H* produced in the Volmer step notably influences the progression of the Heyrovsky and Tafel steps. The thorough exploration of the basic HER step mechanisms will dictate electrocatalyst selection.

$${\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to {\mathrm{H}}^{*}$$

(1)

$${\mathrm{H}}_2\mathrm{O}+\mathrm{e}\to {\mathrm{H}}^{*}+{\mathrm{OH}}^{-}$$

(2)

$${\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{H}}^{*}+{\mathrm{e}}^{-}\to {\mathrm{H}}_2+{\mathrm{H}}_2\mathrm{O}$$

(3)

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{H}}^{*}+{\mathrm{e}}^{-}\to {\mathrm{H}}_2+{\mathrm{OH}}^{-}$$

(4)

$$2{\mathrm{H}}^{*}\to {\mathrm{H}}_2$$

(5)

2.2. Kinetics of the HER Reaction

In an electrochemical system, the phenomenon where the electrode potential deviates from the equilibrium potential when current passes through is known as polarization. Polarization comprises two primary types: concentration polarization [52,53] and electrochemical polarization [54]. In the scenario of the HER, the production and release of H₂ result in a decrease in the concentration of hydrogen ions near the electrode surface [54], leading to concentration polarization. Concurrently, processes like hydrogen ion adsorption, electron transfer, and hydrogen gas generation at the electrode surface require overcoming specific activation energy [54], causing electrochemical polarization. Employing the Butler–Volmer equation (Reaction (6)) allows for a more profound comprehension of the significance and influence of each parameter on the kinetics of the HER reaction [55,56]. Polarization acts as the fundamental reason for overpotential (η), indicating the difference between the genuine electrode potential and the equilibrium electrode potential. The presence of η disturbs the balance between forward and backward chemical reactions, making it a crucial parameter for evaluating the kinetics of the HER reaction, underscoring the energy hurdle that the electrode reaction must overcome [57,58]. The primary principle in electing an electrocatalyst for the HER is to minimize the overpotential η of the system to boost the reaction rate. The exchange current density ($j_0$) measures the speeds of forward and backward reactions at the equilibrium potential. Substances with a high $j_0$ value can serve as exceptional and effective electrocatalysts in the HER, achieving heightened reaction rates at low overpotentials [58]. The transfer coefficient (α) reveals the symmetry of electron transfer in electrode reactions, demonstrating how much η influences the rates of forward and reverse reactions. The value of α impacts the controlling steps and reaction mechanisms of the HER [58]. Through the Butler–Volmer equation, a quantified depiction of the correlation between the electrode reaction rate and η in the HER is offered, providing theoretical direction for developing efficient electrocatalysts and refining reaction conditions, playing a pivotal role in the HER.

$$j=j_0(e^{{\displaystyle \frac{\propto nF\eta }{RT}}}-e^{{\displaystyle \frac{-\left(1-\propto \right)F\eta }{RT}}})$$

(6)

where J is the net Faradaic current density, $j_0$ is the overpotential of the system, and α denotes the transfer coefficient of the system. The corresponding values of parameters R and F are 8.314 and 96,500 or 96,485, respectively.

2.3. Thermodynamics of the HER Reaction

In an electrochemical system, the standard electrode potential governs the thermodynamic force propelling a reaction [59]. When the actual electrode potential is more negative than this standard, the reaction leans towards thermodynamic favorability, demonstrating a spontaneous trend [60]. Additionally, assessing the spontaneity of the reaction from a thermodynamic viewpoint requires consideration of the Gibbs free energy (∆G). In the case of the HER, a negative ∆G indicates a thermodynamically spontaneous advancement, whereas a positive ∆G demands external energy input to proceed with the reaction. At ∆G equals zero, the HER reaches a thermodynamic equilibrium state at a specific electrode potential, resulting in the equalization of forward and reverse reaction rates [61,62]. Hence, fine-tuning the ∆G value for the HER closer to zero or negative values is pivotal for enhancing the reaction rate in a spontaneous manner. The electronic makeup, surface arrangement, and activity centers of electrocatalysts significantly impact the ∆G of the HER. Electrochemical catalysts characterized by appropriate electronic structures, expansive specific surface areas, and plentiful active sites provide heightened reaction sites, facilitating the absorption of hydrogen ions and the subsequent generation of hydrogen gas [63,64]. Ongoing progress in researching the thermodynamic complexities of the HER reaction is rooted in experimental chemistry and computational methodologies. Employing tools like density functional theory (DFT) allows for precision in predicting ∆G for diverse electrocatalysts involved in the HER [64]. Moreover, by conducting thorough calculations on essential thermodynamic parameters such as hydrogen adsorption energy and reaction barriers [65], promising electrocatalysts with reduced ∆G values can be efficiently pinpointed and prioritized [66].

3. Molybdenum Disulfide (MoS₂)

The bilayer configuration of MoS₂ consists of single-layer sulfur atoms (S) positioned at the top and bottom, with a molybdenum atom (Mo) situated in the central layer. S in the upper and lower layers envelops the central Mo atom in a hexa-coordinated layout, forming a sandwich-like atomic structure of S-Mo-S [67]. Each Mo atom establishes robust covalent bonds with six adjacent S atoms, while each sulfur atom is surrounded by three Mo atoms, resulting in a densely packed hexagonal lattice layer. This arrangement confers significant stability on the structure, featuring an interlayer spacing of around 0.65 nm [68]. Feebler van der Waals interactions among the layers enable effortless relative slippage, enhancing the material’s flexibility. The crystal phases of MoS₂ encompass the metallic 1T phase (Figure 1B), the semiconductor 2H phase (Figure 1C), and the less common 3R phase [69].

3.1. Crystal Phase Characteristics

Extensive research in the field of the HER has focused on exploring the characteristics of both the 1T and 2H phases of MoS₂. The 1T-MoS₂ structure is defined by a tetrahedral coordination arrangement, featuring significant interlayer spacing, sparse atomic arrangement, and some level of distortion [70,71]. Its exceptional conductivity enables swift electron transfer during the HER process, thereby accelerating reaction kinetics. Typically, the 1T phase demonstrates low overpotential and outstanding catalytic activity in the HER, facilitating efficient operation under low-energy conditions. However, being a metastable phase, the 1T phase exhibits poor stability and is prone to transitioning into the 2H phase, posing challenges in the preparation and long-term preservation of 1T-MoS₂ [72]. Li et al. [73] developed a solvent-thermal approach utilizing H₂O₂ to create a flower-shaped P-1T-MoS₂ structure without amino groups. This material exhibited an overpotential of 165 mV at a current density of 10 mA cm⁻² and a Tafel slope of 44.0 mV dec⁻¹ in an acidic environment, maintaining stability through continuous cyclic voltammetry (CV) testing. Additionally, Zhang et al. [74] introduced a high-pressure microwave synthesis method (MW) for producing 1T-MW-MoS₂, which displayed an overpotential of 62 mV and a Tafel slope of 42 mV dec⁻¹ at a current density of 10 mA cm⁻² under acidic conditions.

The 2H phase of MoS₂ falls within the scope of the hexagonal crystal system and features a pseudo-hexagonal coordination structure [75]. As a semiconductor, the 2H phase of MoS₂ possesses a noticeable band gap that partly hinders electron transport efficiency. The presence of this band gap requires electrons to overcome an energy barrier of around 1.2–1.9 eV to participate in the HER, leading to reduced activity and limited catalytic performance in the HER [75]. Due to its stability as the primary phase of MoS₂, preparing the 2H phase is relatively straightforward. In typical lab settings and practical applications, the 2H phase maintains structural integrity and consistent performance without undergoing phase changes. However, the semiconductor nature of the 2H phase results in lower catalytic activity during the HER, leading to diminished hydrogen production and often necessitating higher overpotentials to initiate the reaction [76]. Consequently, the synthesis of a combined 1T/2H phase MoS₂ exhibited superior catalytic activity and structural durability, becoming a key focus in recent crystal research. Wang et al. [77] utilized Pulsed Laser Deposition (PLD) technology to fabricate uniformly distributed MoS₂ with a hybrid 1T/2H composition. This sample substantiated an overpotential of 154 mV at a current density of 10 mA cm⁻² in an acidic medium, with a Tafel slope of 38 mV dec⁻¹.

3.2. Catalytic Mechanism

The sites that exhibit catalytic activity of MoS₂ include grain boundary sites [78], defect sites [79], and edge sites [80]. Among them, the edge sites exhibit the highest catalytic activity due to their unsaturated coordination environment. However, the “sandwich-like structure” of MoS₂ results in a decreased number of exposed surface sites and relatively low reactivity in the bulk phase [80]. Enhancing the catalytic activity by modifying the structure to increase active sites has become a research focus for improving the HER performance of MoS₂ from the catalytic mechanism perspective. Research showed that single-layer MoS₂ with the right band gap and dangling bonds at the edge layers can form a two-dimensional nanostructure, and the exfoliation of MoS₂ into fewer or single layers can furnish supplementary catalytic active sites for the HER. Tsai et al. [81] successfully synthesized single-layer MoS₂ nanosheets using a liquid-phase exfoliation technique. After continuous potential assessments under 100 mA or 500 mA current in acidic conditions for 24 h, the electrochemical performance remained stable. Furthermore, exfoliation can induce a phase transition. Shi et al. [82] effectively exfoliated 2H-MoS₂ into a bilayer 1T phase using ether and ester electrolytes. In acidic conditions, the overpotential of this material decreased to 199 mV at a current density of 10 mA cm⁻², showing a Tafel slope of 54 mV dec⁻¹, significantly lower than that of the original 2H-MoS₂.

4. Heteroatom Doping Modification Strategy of MoS₂

4.1. Metal Element Doping

4.1.1. Noble Metal Element Doping

Noble metals like platinum (Pt) and palladium (Pd) are frequently used as commercial catalysts to enhance the performance of the HER. However, the usage of Pt faces challenges due to high costs, scarcity, and inefficiencies in energy conversion. A promising alternative to Pt-based catalysts is the modification of MoS₂ with the addition of Pt nanoparticles. Razavi et al. [83] employed a hydrothermal technique using chloroplatinic acid (H₂PtCl₆) as the platinum source, successfully creating Pt/MoS₂ composites with various Pt loading levels. Transmission electron microscopy (TEM) analysis indicated that the average diameter of the Pt/MoS₂-90 sample is around 1 μm, with Pt nanoparticles evenly distributed on the MoS₂ nanosheets (Figure 2A). In acidic environments, the Pt/MoS₂-90 catalyst exhibited an overpotential of −0.01 V at a current density of −10 mA cm⁻² (Figure 2B), alongside a Tafel slope of 41 mV dec⁻¹ (Figure 2C), demonstrating remarkable catalytic activity similar to commercial Pt/C. After 2000 CV at a scan rate of 50 mV s⁻¹, the loss in the final cycle compared to the initial scan was minimal, indicating significant stability for Pt/MoS₂-90 (Figure 2D). When maintained at a constant potential of −0.25 V for 20 h, Pt/MoS₂-90 showed outstanding durability and greater stability than commercial Pt/C.

Pd-based catalysts have received considerable interest as viable alternatives to Pt-based catalysts; however, their stability poses a major challenge for extensive use in the HER. The similarity in atomic radii of Pd and Mo allows for the theoretical possibility of doping Pd into MoS₂. Research utilizing DFT indicated that Pd tends to occupy vacancy sites in Mo, especially at grain boundaries, defects, and edges in MoS₂. Zhao et al. [84] created a solvothermal approach to combine two-dimensional MoS₂ with a vertical graphene network (VGN), leading to the development of the composite material VGN@MoS₂, into which various concentrations of Pd were introduced (Figure 3A). In acidic conditions, the VGN@Pd_0.2-MoS₂ displayed an overpotential of 106 mV at a current density of 10 mA cm⁻² (Figure 3B) and a Tafel slope of 60 mV dec⁻¹ (Figure 3C), outperforming many commercial Pd-based catalysts in terms of catalytic activity. The findings suggest that sulfur vacancies generated by Pd doping optimize hydrogen adsorption energy, subsequently enhancing the catalytic performance of MoS₂. VGN@Pd₀.₂-MoS₂ showed remarkable stability for up to 1100 h and 180 h at current densities of 10 mA cm⁻² and 80 mA cm⁻², respectively (Figure 3D), indicating better stability than the catalysts of undoped Pd. The presence of Pd species such as Pd–S and Pd–S–Mo enhanced the catalytical corrosion resistance in acidic environments, while the introduction of sulfur vacancies improved the interaction between MoS₂ and VGN, thus increasing stability. Moreover, Sultana et al. [85] synthesized Pd-MoS₂ nanorods with 1 wt% and 2 wt% Pd content, alongside pure MoS₂ nanorods using a colloidal solvothermal technique. SEM and TEM images showed small Pd nanoparticles on the 1 wt% Pd-MoS₂ nanorods, whereas the 2 wt% Pd-MoS₂ nanorods displayed a uniform distribution of Pd particles on their surfaces. Under acidic conditions, the 2 wt% Pd-MoS₂ nanorods exhibited an overpotential of 119 mV at a current density of 100 mA cm⁻² and a Tafel slope of 48 mV dec⁻¹, exceeding the performance of MoS₂ nanorods in similar tests. This implies that Pd doping substitutes nearby Mo sites and introduces conductive electronic states around the Fermi level, thereby improving the metallic characteristics and catalytic activity of MoS₂. Among the synthesized samples, the 2 wt% Pd-MoS₂ nanorods had the highest double-layer capacitance (Cdl) value and the largest electrochemical surface area (ECSA), reflecting a high dispersion of the amorphous nanocrystalline structure. The doping of Pd promoted a more effective transition from the 2H phase to the 1T phase, resulting in an increase in Pd-S-Mo sites and active site density. After 3000 cycles, the polarization curve of the 2 wt% Pd-MoS₂ nanorods did not change significantly and exhibited stable performance following continuous operation at −0.4 V for 10 h.

MoS₂ doped with noble metals showed significant benefits in enhancing catalytic activity and stability in the HER. However, the widespread use of noble metals is constrained by their high cost and limited availability. Additionally, a notable drawback is the low energy conversion efficiency associated with these metals. On the other hand, transition metals provide a promising solution to enhance the catalytic performance of MoS₂ due to their similar atomic radius to molybdenum and advantageous doping characteristics, all while reducing costs. Therefore, opting for transition metal doping as an alternative method not only improves catalytic efficiency but also strengthens economic feasibility and sustainability.

4.1.2. Transition Metal Element Doping

Introducing transition metal elements into MoS₂ can effectively modify its electronic structure, leading to lattice distortion and subsequent changes in electronic properties. This altered MoS₂ demonstrates a notable enhancement in catalytic performance, especially in terms of boosted reaction rates during the HER. Among the different transition metals, nickel (Ni) stands out for its ability to catalyze the breakdown of H₂O and assist in the release of intermediates via weak hydrogen bonding interactions, consequently refining the key processes in the HER.

Dharman et al. [86] utilized sodium molybdate dihydrate (Na₂MoO₄·2H₂O), thiourea (CH₄N₂S), and nickel nitrate hexahydrate (Ni(H₂O)₆(NO₃)₂) as start materials to synthesize MoS₂ nanosheets with different Ni concentrations by a solvothermal method (Figure 4A). SEM images exhibited a nanoflower structure in the prepared samples (Figure 4B). The samples, named NM1 (3%), NM2 (6%), and NM3 (9%), demonstrated varied catalytic performances, with NM3 showing the best performance, characterized by the lowest overpotential of 127 mV at a current density of 10 mA/cm² in an acidic environment (Figure 4C) and a Tafel slope of 86 mV dec⁻¹ (Figure 4D). Particularly, NM3 displayed stability for more than 15 h, surpassing both pure MoS₂ and other Ni-doped samples, suggesting the involvement of the Volmer–Heyrovsky reaction pathway and significant improvements in the kinetics of the HER. Additionally, NM3 exhibited 3.4 Ω Cdl and a 97.14 mF cm⁻² electrochemical surface area (ECSA) (Figure 4E), indicating an increase in potential electrocatalytic active sites due to Ni doping. Moreover, NM3 showed minimal charge transfer resistance (Rct) (Figure 4F), implying enhanced conductivity from Ni doping, thereby enhancing the efficiency of the charge transfer process in NM3. Ghanashyam et al. [87] developed a simple hydrothermal method for synthesizing Ni-doped MoS₂ at different concentrations. Under acidic conditions, the MoS₂ sample with 11% Ni showed an overpotential of −160 mV at a current density of −10 m A cm⁻², with a Tafel slope of 79 mV dec⁻¹. After 3000 cycles of CV, minimal degradation in the performance of the HER was observed. These results indicate that the introduction of Ni promotes the formation of the 1T phase, leading to increased surface area, improved charge transfer kinetics, and enhanced capacitance of MoS₂, thereby enhancing its HER catalytic activity. Subsequent studies explored the doping of MoS₂ with various transition metals under the same conditions, enabling a comparative assessment of their catalytic performances. Venkatesh et al. [88] employed a hydrothermal method to synthesize MoS₂ nanosheets doped with Fe, Co, and Ni using Na₂MoO₄, thiourea (NH₂CSNH₂), oxalic acid (C₂H₂O₄), Ni(NO₃)₂·6H₂O, cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O), and iron nitrate nonahydrate (Fe(NO₃)₃·9H₂O) as precursors (Figure 5A). After Fe doping, Fe atoms tended to aggregate on the surface of the MoS₂ nanosheets, disrupting the layered structure and reducing the available active edge sites, resulting in an inferior HER performance compared to pure MoS₂. Among the doped variants, Co-MoS₂ exhibited higher catalytic activity than both Fe-MoS₂ and pure MoS₂, while Ni-MoS₂ showed the most significant catalytic performance enhancement. The introduction of Ni increased the number of catalytically active sites on MoS₂, thereby improving its catalytic efficiency. In acidic conditions, the Tafel slope for Ni-MoS₂ was measured at 66.27 mV dec⁻¹ (Figure 5B) at a current density of 10 mA/cm². Furthermore, Rct decreased to a minimum of 37 Ω (Figure 5C), which was only one-sixth that of pure MoS₂, indicating substantial improvements in charge transfer dynamics due to Ni doping. Following 1000 cycles of linear sweep voltammetry (LSV) (Figure 5D) and a 10,000 s CA test (Figure 5E), the current density remained stable, underscoring the outstanding stability and reliability of Ni-MoS₂ in HER applications.

Although cobalt (Co) doping did not significantly improve the catalytic performance of MoS₂, recent studies have started to investigate the use of carbon-based materials as carriers for Co-based composite MoS₂ catalysts. Fu et al. [89] employed super-light porous carbon (NPC) as the carrier for electrocatalysts and chose cobalt acetate (Co(CH₃COO)₂·4H₂O) as the cobalt source. Through a hydrothermal method, they successfully doped Co into MoS₂ nanosheets, producing Co/MoS₂@NPC composite materials with varying Co content (Figure 6A). TEM and high-resolution TEM (HRTEM) images verified the successful incorporation of Co, with Co/MoS₂ effectively supported on NPC, resulting in a flower-like nanostructure formation (Figure 6B). Notably, the catalytic performance of the 0.2Co/MoS₂@NPC sample was optimal. In acidic conditions, it displayed an overpotential of 139 mV at a current density of 10 mA cm⁻² (Figure 6C), a Tafel slope of 69 mV dec⁻¹ (Figure 6D), an ECSA of 98.8 mF cm⁻² (Figure 6E), and a Rct of 0.034 Ω (Figure 6F). Even after 1000 cycles of CV, the overpotential remained relatively stable, indicating good cyclic and long-term stability. However, under alkaline conditions, the overpotential increased to 163 mV at a current density of 10 mA cm⁻², leading to decreased stability compared to that observed in acidic conditions. Conversely, Gyawali et al. [90] prepared graphene oxide (GO) using the Brodie method and subsequently produced nitrogen-doped reduced graphene oxide (N-rGO) via the thermal reduction of a melamine mixture, which acted as a carrier for electrocatalysts. They successfully synthesized Co_x/1T-Mo_1−xS₂ by utilizing NH₂CSNH₂, Na₂MoO₄, ethylthioamide (C₂H₅NS), and cobalt nitrate hexahydrate (Co(NO)₃·6H₂O) as precursors, followed by deposition onto N-rGO. SEM analysis revealed that the synthesized MoS₂ exhibited a floral microsphere morphology, with Co doping causing morphological changes in 1T-MoS₂, resulting in the unique composite structure of flower-like microspheres and N-rGO observed in Co_0.18/1T-Mo_0.82S₂@N-rGO. This material showed an overpotential of 142 mV at a current density of −10 mA cm⁻² in an alkaline environment, with a Tafel slope of 48 mV dec⁻¹. Notably, it displayed minimal potential degradation after continuous operation at a current density of 100 mA cm⁻² for 24 h, highlighting its exceptional stability and catalytic activity in the HER. Moreover, the issue of limited enhancement in the catalytic performance of MoS₂ through sole cobalt doping can be effectively addressed by incorporating other transition metal elements into MoS₂. Li et al. [91] devised a one-step hydrothermal approach to produce Ni and Co-doped MoS₂ nanosheets. HRTEM analysis revealed that, upon the introduction of Ni and Co elements, the samples exhibited a floral structure with an interlayer spacing of approximately 0.61 nm corresponding to the (002) crystal plane of MoS₂, along with numerous defects on the transverse and basal surfaces (Figure 7A). In an alkaline environment, Ni,Co-MoS₂ displayed an overpotential of 160 mV at a current density of 10 mA cm⁻² (Figure 7B) and a Tafel slope of 78 mV dec⁻¹ (Figure 7D), considerably lower than MoS₂, Ni-MoS₂, and Co-MoS₂. These findings suggest that Ni doping enhances the water dissociation step, while Co doping aids in the adsorption and desorption of H₂ on the S edge, notably boosting the HER rate in an alkaline setting through the incorporation of dual cations. Furthermore, the dual cation doping slightly increases the active surface area of MoS₂ (43.66 MF cm⁻²) (Figure 7C), underscoring that the heightened HER activity of double cation-doped MoS₂ is primarily attributed to the enhancement of intrinsic activity rather than a rise in surface area.

Apart from Ni and Co, various other transition metal elements are extensively employed for the doping modifications of MoS₂. Sahoo et al. [92] devised a chemical vapor deposition (CVD) technique to produce thin films of vanadium-doped MoS₂ (VMS). By adjusting the ratios of the molybdenum precursor MoO₃ and the vanadium precursor V₂O₅, they successfully generated samples with vanadium concentrations of 1% (VMS-L) and 9% (VMS-H). The introduction of vanadium notably decreased the onset potential, and the optimized structure where vanadium atoms replaced the doped monolayer MoS₂ (VMS-L) exhibited enhanced HER activity. Remarkably, VMS-L showcased superior HER performance compared to VMS-H, achieving an overpotential as low as 130 mV at a current density of 10 mA/cm² in acidic conditions, while maintaining stability for over 12 h at a current density of 5.72 mA cm⁻². These results indicated that the cooperative effect between vanadium and molybdenum on the basal plane optimized the ΔG in the HER process, thereby improving conductivity without influencing the phase of MoS₂.

4.2. Non-Metal Element Doping

Non-metal doping, as opposed to metal doping, typically does not introduce new metal sites. Instead, it enhances the adsorption capacity and reaction rate of reactants by altering the band gap and conductivity of the material. The addition of non-metal elements like nitrogen (N) into MoS₂ enables the adjustment of the electronic structure, thus creating more active sites to improve the catalytic activity of MoS₂. Wang et al. [93] devised a simple hydrothermal method to produce phosphorus-doped MoS₂ nanocomposites (P-MoS₂). SEM images revealed that P-MoS₂ exhibited a spherical shape with a particle size of approximately 50 nm, displaying a uniform distribution of elements without the typical layered structure seen in crystalline MoS₂. In an acidic environment, at a current density of 10 mA cm⁻², the overpotential was measured at −219 mV with a Tafel slope of 39 mV dec⁻¹, significantly lower than pure MoS₂ and approaching that of commercial Pt/C catalysts. After 1000 cycles of CV, P-MoS₂ showed minimal loss in current density, indicating excellent cycling stability. Additionally, Le et al. [94] synthesized 1T/2H mixed-phase MoS₂ heterostructure nanosheets by utilizing thiosemicarbazide (CH₄N₂S) and ammonium molybdate ((NH₄)₂MoO₄) precursors in a hydrothermal process on carbon cloth (CC). They introduced N into the samples using different treatment times under N₂ plasma atmosphere to obtain N-MoS₂ nanosheets. SEM images demonstrated that N₂ plasma treatment induced defects in MoS₂, thereby providing additional catalytic active sites (Figure 8A). In an acidic medium, N-MoS₂-10 displayed an overpotential of 131 mV at a current density of 10 mA cm⁻² (Figure 8B) and a Tafel slope of 93 mV dec⁻¹ (Figure 8C), showcasing superior catalytic performance compared to other samples. The results suggested that N doping results in the formation of Mo-N bonds in the sample, optimizing the surface electronic structure, enhancing conductivity, and increasing the number of active sites. The Rct of N-MoS₂-10 was measured at 1.53 Ω, with only a slight loss in current density after 1000 cycles of CV (Figure 8D), indicating that N doping altered the ratio of the 1T and 2H phases. This appropriate phase structure is beneficial for maintaining the physicochemical properties of the sample, further enhancing the stability of N-MoS₂-10 in the HER. Feng et al. [95] introduced a one-step sintering method for the fabrication of N-MoS₂ and N,P-MoS₂ samples (Figure 9A). In an acidic setting, the N, P-MoS₂ sample exhibited overpotentials of 179 mV at a current density of 10 mA cm⁻² and 376 mV at 100 mA cm⁻², both lower than the corresponding values for N-MoS₂ under the same conditions (225 mV at 10 mA cm⁻² and 442 mV at 100 mA cm⁻²) (Figure 9B). Moreover, the Tafel slope of N, P-MoS₂, registered at 143 mV dec⁻¹, was marginally lower than that of N-MoS₂ at 148 mV dec⁻¹ (Figure 9C), indicating that P doping increased the interlayer spacing from 0.41–0.49 nm in N-MoS₂ to 0.51–0.65 nm in N, P-MoS₂ (Figure 9B). This expanded interlayer distance facilitated the movement of reactants and products during the HER process, thereby amplifying the efficiency of the catalytic reaction. The introduction of P doping reduced the Rct from 62 Ω in N-MoS₂ to 41 Ω in N, P-MoS₂ (Figure 9D), showcasing the outstanding semimetal attributes of N, P-MoS₂, which further boosted the conductivity and enabled swift electron transfer within N, P-MoS₂. Additionally, the electrochemical surface area of N,P-MoS₂, quantified at 65.3 mF cm⁻², exceeded that of N-MoS₂ at 42.2 mF cm⁻² (Figure 9E), signaling a notable rise in S vacancies within the sample post P doping. This surplus of active sites enhanced the effectiveness of the HER process, fostering the development of more reactive centers conducive to hydrogen adsorption and desorption mechanisms.

The incorporation of non-metallic elements into the MoS₂ catalyst displayed substantial promise in the HER. By gaining a comprehensive understanding of its electronic structure and reaction mechanisms, there is potential for future improvements in catalytic activity and stability by further optimizing dopant elements and controlling the structure.

4.3. Metal and Non-Metal Element Doping

Metal–non-metal co-doped MoS₂ enhanced catalytic activity by providing a greater number of reactive sites, improving surface adsorption capacity, and enhancing interaction with water. This research area has gained significant attention recently. Initially, researchers explored the potential of metal–non-metal co-doping in MoS₂ for the HER from a theoretical perspective. Guo et al. [96] employed DFT to assess the performance of metal–non-metal co-doped MoS₂ as an efficient HER electrocatalyst. By achieving a negative ΔG_h for co-doped MoS₂ (Figure 10A), other non-metal X elements were introduced to form Co-X co-doped MoS₂ (CoX@MoS₂). The calculated ΔG_h values for Co-C, Co-N, and Co-Se co-doped systems were 0.12, 0.23, and 0.16 eV (Figure 10B), respectively, surpassing commercial Pt electrocatalysts and aligning closely to the peak of the volcano curve (Figure 10C), indicating notable HER efficiency. The findings demonstrated a direct relationship among ΔG_h, S site P-band center (ε_p) (Figure 10D), and electron transfer (Q) (Figure 10E). Various Co-X co-doping configurations enhanced the HER reaction rate by adjusting the charge of the S site, while the charge transfer behavior of H* was influenced by the electronegativity of element X, affecting ε_p and H adsorption energy (Figure 10F). Theoretical insights from DFT calculations prompted Liu et al. [97] to develop a novel micelle-confined microemulsion technique, utilizing Na₂MoO₄·2H₂O, CH₄N₂S, and Co(NO₃)₂ as precursors and a glucose solution as a carbon source to successfully produce Co@MoS₂/C composites. FESEM and TEM images depict uniformly dispersed microspheres composed of ultra-thin nanosheets irregularly adhered to form a three-dimensional nanostructure. The spherical size remained unchanged post co-doping, exhibiting a yolk-shell structure where the shell, comprising nanosheets, retains its original form after doping. In an acidic condition, the sample Co@MoS₂/C displayed the lowest overpotential (70 mV) at an applied current density of 10 mA cm⁻²; the system achieved optimal kinetics characterized by a Tafel slope as low as 50 mV dec⁻¹, minimal charge transfer resistance, and a double-layer capacitance value of 22.6 mF cm⁻². With carbon embedding reducing the interlayer spacing to 0.920 nm, resulting in an inverted volcano curve of overpotential with increasing interlayer spacing. The results suggest that the collaborative electronic structure modulation by co-doping and C embedding expedites charge transfer, exposing more catalytically active sites. Ghanashyam et al. [98] devised a hydrothermal approach to synthesize Ni/Fe-MoS₂/CC composites on carbon cloth using Ni(NO₃)₂·6H₂O and Fe₂O₃ as Ni and Fe sources (Figure 11A), respectively. SEM imagery revealed a thicker and smoother surface with robust particle adhesion, showcasing a three-dimensional floral morphology featuring both 2H and 1T phases with varying interlayer spacing (Figure 11B). The outcomes imply that thin and interconnected nanosheets offer an increased surface area and a larger number of active sites for the HER. In an acidic media, the overpotential of the sample at −10 mA cm⁻² registered at −116 mV (Figure 11C), with a Tafel slope of 43 mV dec⁻¹ (Figure 11D), suggesting that Fe and Co doping promoted the emergence of the 1T phase and generated surface defects, significantly enhancing the catalytic competence of the 1T phase fundamental plane and the conductive metal phase to escalate the HER reaction rate. Post 5000 CV cycles, the sample exhibited minimal performance deterioration, maintaining structural stability and delivering satisfactory performance in neutral HER conditions (Figure 11E). The outcomes implied that the incorporation of Ni and Fe into the intermediate layer of MoS₂ yielded a confined structure, notably reducing the ∆G of the sample. Ma et al. [99] employed a hydrothermal sulfurization method to deposit MoS₂ nanosheets on CoFe@NC substrates, successfully fabricating MoS₂/CoFe@NC composite samples (Figure 12A). In alkaline environments, the overpotential of a sample at a current intensity of 10 mA/cm² equated to 172 mV (Figure 12B), with a Tafel slope of 122.4 mV dec⁻¹ (Figure 12C), indicating that the CoFe@NC substrate enhanced the conductivity of MoS₂. The chemical coupling and collaborative effect between MoS₂ and CoFe@NC not only provided additional active sites but also boosted the electron transfer rate following the Volmer–Heyrovsky mechanism. Despite timed potential measurements conducted with a current flux of 20 mA cm⁻², negligible alterations were observed in the structure and performance, affirming that carbon doping bolstered the crystal stability, thereby enhancing the durability of the metal phase (Figure 12D).

In conclusion, creating three-dimensional nanostructures by doping with both metals and non-metals enhances electrical conductivity and stability, consequently improving the efficiency of the HER. The combined impact of metals and non-metals renders doped MoS₂ a highly effective electrocatalytic material.

5. Summary and Prospects

Electrocatalysis of Heteroatom-Doped MoS₂ have been extensively studied (

Table 1. The state-of-the-art MoS₂-based HER catalysts.

Dopant	Catalyst	Catalyst Performance	Condition	Ref
Pt	Pt/MoS2-90 nanocomposite	Achieved η = −10 mV (@-10 mA cm−2) with Tafel slope 41 mV dec−1; retained 98% activity after 2000 CV cycles	0.5 M H2SO4	[83]
Pd	2 wt% Pd-MoS2 nanorods	Delivered η = 119 mV at 100 mA cm−2; no LSV shift after 3000 cycles	0.5 M H2SO4	[85]
Ni	NM3 (9% Ni-doped MoS2)	Exhibited η = 127 mV (@10 mA cm−2) with ECSA 97.14 mF cm−2; stability > 15 h via Volmer–Heyrovsky pathway	H2-saturated H2SO4	[86]
Co	0.2Co/MoS2@NPC composite	Demonstrated η = 139 mV (@10 mA cm−2) and ultra-low Rct (0.034 Ω); 93% stability after 1000 CV cycles	Acidic condition	[89]
Ni-Co	Bimetallic Ni, Co-MoS2	Synergistic η = 160 mV (@10 mA cm−2); enhanced water dissociation (Ni) and H* adsorption (Co)	Alkaline solution	[91]
P	P-MoS2 nanospheres	Near-Pt/C performance: η = −219 mV (@10 mA cm−2); 99% current retention after 1000 CV cycles	Acidic environment	[93]
N,P	N,P-MoS2 heterostructure	Expanded interlayer (0.65 nm); η = 179 mV (@10 mA cm−2); enhanced ECSA via S-vacancy engineering	Acidic medium	[95]
Co-C	Co@MoS2/C microspheres	Superior activity: η = 70 mV (@10 mA cm−2); synergistic charge transfer via Co-C dual-anchoring	0.5 M H2SO4	[96]
Ni-Fe	Ni/Fe-MoS2/CC	Achieved η = −116 mV (@-10 mA cm−2); 1T phase-dominated (81%); stable in neutral media	pH 7 PBS	[98]

). By utilizing heteroatom doping strategies, MoS₂ demonstrates promising potential as an efficient electrocatalyst. This not only introduces innovative concepts for the sustainable utilization of hydrogen energy but also establishes a strong foundation for the development and deployment of future energy sources. Further research aims to optimize the microstructure and composition of materials, enhancing the efficiency and cost-effectiveness of MoS₂ and its doped derivatives as electrocatalysts. This optimization will facilitate their practical application in hydrogen energy and drive the advancement of green energy.

Future research on heteroatom-doped MoS₂ for the HER may focus on three key areas: (1) Exploring the synergistic effects of multi-element doping for increased catalytic efficiency. Doping systems will exhibit reduced hydrogen adsorption energy and enhanced charge transfer efficiency, attributed to ligand effects and lattice strain modulation. (2) Employing advanced computational methods to understand catalytic reaction mechanisms and predict reactivity, such as integration of machine learning (ML) with ab initio molecular dynamics (AIMD), could unveil solvent effects on active site stability, addressing discrepancies between theoretical predictions and experimental overpotentials. (3) Developing new synthesis techniques to create MoS₂ materials with ideal morphology and structure, such as microwave-assisted hydrothermal methods or chemical vapor deposition (CVD) with sulfur-rich precursors, are critical to fabricating MoS₂ with tailored edge-to-basal plane ratios and defect densities. Additionally, ensuring long-term stability and corrosion resistance in practical applications will be crucial for advancing the commercialization of hydrogen energy.