MoS₂‐Based Electrocatalysts Across Major Water‐Related Energy Reactions: Advances, Challenges, and Future Prospects

Abstract

Water electrochemical reactions have garnered increasing attention due to their central role in sustainable energy conversion. Amid the intensifying global energy crisis, the development of renewable pathways for green hydrogen production has become imperative. Numerous materials have been investigated as electrocatalysts for water splitting and fuel cell technologies, with molybdenum disulphide (MoS ₂) emerging as a particularly promising candidate owing to its versatility in major aqueous electrochemical processes. This review provides an integrated perspective on recent advances in MoS ₂ as a multifunctional electrocatalyst for hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction and hydrogen oxidation reaction, the key half‐reactions that govern water splitting and fuel cell systems. The kinetics of these reactions, governed by the Sabatier principle, can be optimised through careful control of proton‐coupled electron transfer pathways. MoS ₂ has attracted significant attention not only as a non‐noble catalyst but also as a robust support for constructing highly active catalytic systems. Its layered framework, tuneable electronic structure and inherent defect sites provide a versatile platform that can be engineered into efficient catalytic systems, enhancing reaction kinetics, intermediate binding and overall activity. This review examines and compares the activity of MoS ₂ ‐based catalysts and summarises recent progress and challenges in their development as electrocatalysts for water‐related energy applications.

MoS₂ as a multifunctional electrocatalyst for sustainable energy conversion.

1. Introduction

In the context of diminishing fossil fuel reserves and an escalating global energy crisis, there is an urgent need to transition from conventional fossil‐based energy sources to sustainable hydrogen technologies. Rapid population growth and increasing environmental pollution further emphasise the importance of adopting renewable energy pathways [1, 2, 3]. Green hydrogen produced from renewable resources such as solar and wind energy represents a clean and sustainable fuel option characterised by zero carbon emissions, environmental compatibility, and high energy density. Within this framework, electrochemical water splitting remains one of the most viable routes for large‐scale hydrogen production [4, 5, 6, 7, 8, 9, 10]. The development of efficient electrocatalysts is central to advancing water electrolysis, fuel cells, and related energy conversion technologies. Key water‐related electrochemical reactions include the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR). State‐of‐the‐art catalysts for HER, HOR and ORR are predominantly based on Pt, while Ir‐ and Ru‐based oxides are widely employed for OER [11, 12, 13]. Although these noble‐metal catalysts have demonstrated high activity and commercial deployment, their widespread use is constrained by high cost, scarcity, and durability issues under practical operating conditions [14, 15]. Consequently, the design of low‐cost, earth‐abundant electrocatalysts capable of delivering high activity, selectivity, and long‐term stability across multiple electrochemical reactions has become a major research focus. In this regard, bifunctional and multifunctional catalysts that can efficiently drive both hydrogen‐ and oxygen‐related reactions are particularly attractive for integrated energy conversion and storage systems, including water electrolysers, fuel cells, and rechargeable metal–air batteries [16, 17, 18, 19, 20, 21].

The scientific community has therefore explored earth‐abundant materials to replace costly noble‐metal catalysts. A wide range of promising electrocatalysts has been developed, particularly those based on metal‐free catalysts and non‐noble transition‐metal‐based catalysts of oxides, chalcogenides, nitrides, carbides, phosphides, and borides [22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40]. As part of narrowing this broad research area, this review focuses on the widely studied transition metal disulphides (TMDs), i, e., MoS₂ [41, 42]_. MoS₂ is an individual S‐Mo‐sandwich layer structure weakly interacted by van der Waals forces. MoS₂ is a naturally semiconducting material with a thermodynamically favourable 2H phase and an inactive basal plane. The bulk MoS_2, being semiconducting, has less HER activity, and its (0001) basal planes are electrocatalytically inert due to the poor electrical conductivity, which hinders charge–transfer kinetics [43, 44]. Moreover, a direct experimental comparison between the basal and edge planes of MoS₂ revealed that the edge plane exhibits much higher electrochemical catalytic activity and faster heterogeneous electron transfer. This results in improved conductivity and enhanced chemical kinetics, ultimately leading to superior catalytic performance [45, 46]. The discovery of MoS₂‐based HER catalysts opens an exciting field for research activity. MoS₂ exhibits several polymorphs, including the semiconducting 1H‐ MoS₂, the semi‐metallic 1T′‐ MoS₂, and the metallic 1T‐ MoS₂, which differ in the interlayer stacking arrangement of S atoms relative to Mo atoms within the unit cell, as illustrated in Figure 1a. Furthermore, multilayered 1H‐phase MoS₂ can adopt multiple polytypes, as shown in Figure 1b, such as bilayer 2H, bilayer 3R, and trilayer 3R, with interlayer stacking sequences following AA′, AB, and ABC configurations, respectively. These polytypes introduce subtle differences in interlayer coupling and band structure, which modulate the charge transport and active site exposure, thereby impacting their suitability for catalysis [47, 48]. Among them, the common 1T phase has a distorted octahedral structure and exhibits metallic behaviour, making it highly conductive and catalytically active, particularly for HER. However, it is thermodynamically metastable and can transition to the 2H phase over time or with thermal treatment. In contrast, the 2H phase has a trigonal prismatic structure and is a stable semiconductor, making it ideal for electronic and optoelectronic applications. Due to its poor electrical conductivity and fewer active edge sites, 2H‐MoS₂ is less efficient in electrocatalytic applications compared to the 1T phase. However, phase engineering techniques can convert the more stable phase of 2H‐MoS₂ into the more catalytically active 1T phase, enhancing its performance in electrocatalysis and energy storage applications [49, 50]. There is significant interest in the investigations and developments of MoS₂ nanostructures with a high percentage of active edge sites and increased conductivity through doping with different metal ions as electrocatalysts for achieving better HER performance. MoS₂ has emerged as a versatile and promising electrocatalyst across several key energy conversion reactions. For ORR and OER, metallic 1T‐phase MoS₂ nanosheets demonstrate notable activity, outperforming their semiconducting 2H counterparts in both experimental and theoretical studies [51]. In addition to metal doping, the enrichment of MoS₂ with the more active 1T phase can enhance its performance in water‐splitting reactions, owing to the higher electrical conductivity of the 1T phase compared to the 2H phase. For instance, the 1T phase of MoS₂ improves the electron transfer and reduces the HER free energy, leading to improved HER catalytic activity [45, 52]. Although direct studies on MoS₂'s role in the HOR are still limited, findings collectively indicate MoS₂'s potential as a multifunctional catalyst in renewable energy technologies.

FIGURE 1.

Atomic configurations of different MoS₂ polymorphs are shown, including (a) monolayer phases: 1H, 1T, and 1T′, and (b) bilayer phases: 2H, 3R, as well as a trilayer with 3R stacking. Side views are presented in the lower panels, while perspective views are displayed in the right panels. Reproduced with permission from Ref [47]. Copyright 2018, Wiley‐VCH.

The novelty of this review lies in its comprehensive analysis of MoS₂ and MoS₂‐based composites across four energy‐related electrochemical water‐related reactions, namely HER, OER, ORR, and HOR. Unlike previous reviews, this review emphasises recent advances, engineering strategies, and fundamental mechanistic insights while highlighting the intrinsic connectivity among these reactions. HER and OER constitute the reversible half‐reactions of water splitting, whereas HOR and ORR form the complementary half‐reactions in fuel cells. The core principle linking their activity is the Sabatier principle, which dictates that maximum catalytic activity is achieved when the catalyst binds the crucial reaction intermediates neither too strongly nor too weakly [45, 53]. For HER and HOR, this critical intermediate is adsorbed hydrogen (H*), where highly efficient kinetics occur when the Gibbs free energy of adsorption (ΔG H*) is near zero. Similarly, OER and ORR activity is strongly coupled to the binding energy of adsorbed oxygenated intermediates such as *OH, *O, *OOH [54, 55]. These reactions are mechanistically interconnected through their shared reliance on efficient proton‐coupled electron transfer (PCET), often involving water or hydroxide (OH−) species. From a fundamental perspective, PCET processes at MoS₂ electrodes can be described within extended Butler–Volmer and Marcus–Hush formulations, in which the rate constant depends primarily on the applied potential and the coupled free‐energy surfaces associated with proton and electron transfer at S‐rich active sites [56]. In these models, the degree of coupling between proton and electron motion determines whether charge transfer proceeds via concerted PCET or stepwise pathways, with nonadiabatic effects becoming increasingly relevant when proton tunnelling contributes to the overall kinetics [57, 58]. Consequently, the Volmer step is more appropriately described as a PCET process, with deviations from classical Butler–Volmer behaviour, such as small apparent transfer coefficients, indicative of concerted proton–electron transfer [56]. The catalyst surface must effectively handle water activation/dissociation; for instance, in alkaline HER, the initial Volmer reaction involves the discharge of H₂ O to form MH_ads and OH−. Conversely, HOR requires the H₂ oxidation via H∗ to form water, often involving a reaction between H∗ and hydroxyl (OH∗) intermediates. Enhancing hydrogen reactions (HER/HOR) can be achieved by maximising hydrogen spillover, which facilitates surface transport of protons (H∗) across the catalyst surface [59]. In parallel, improved oxygen reactions (OER/ORR) require effective deprotonation steps (e.g., *OH → *O), which are critical for overcoming kinetically sluggish pathways [60]. MoS₂ can be rationally designed to support all four reactions by tailoring its electronic and structural properties to accommodate both H∗ and O∗ intermediates. For HER and HOR, the semiconducting 2H‐MoS₂ is largely limited to catalytic activity primarily at its edge sites whereas phase engineering to metallic 1T phases activates basal planes and enhances its catalytic activity. Density functional theory (DFT) calculations further indicate that transition metal single‐atom doping can generate dual‐active sites capable of stabilising both H∗ and OH∗ adsorption [61]. For oxygen‐related reactions, pristine MoS₂ generally exhibits limited activity; however, multifunctionality can be achieved by tuning the electronic structure of Mo and S atoms, which weakens OH∗ and H∗ adsorption, thereby enhancing catalytic rates for OER and ORR [62, 63]. Studying these reactions simultaneously is crucial because the required binding energy profiles are fundamentally linked, allowing researchers to predict activity across coupled reversible reactions.

This review focuses on MoS₂ and MoS₂‐based composites in electrochemical water‐related reactions (as represented in Scheme 1), highlighting their underlying mechanisms and relevance as multifunctional catalysts for energy applications. The general mechanisms of HER, OER, ORR, and HOR are discussed together with the intrinsic properties of pristine MoS₂, the electrocatalytic activity of MoS₂‐based systems, and the contributions of engineered MoS₂ systems. The review further addresses catalyst synthesis strategies and their influence on electrocatalytic performance, with particular emphasis on recent MoS₂‐based electrocatalysts.

SCHEME 1.

Schematic representation of MoS₂‐based electrocatalysts for water‐related energy reactions.

2. Electrochemical Testing

When evaluating electrocatalysts for energy‐related reactions, multiple parameters must be considered to assess activity, efficiency, and practical relevance. Thermodynamically, water splitting requires a minimum cell voltage of 1.23 V to produce hydrogen and oxygen. In practical systems, however, additional energy is required to overcome kinetic barriers associated with charge transfer and reaction intermediates. This additional energy is expressed as the overpotential (η), which represents the excess potential required to drive the reaction at a given rate. Consequently, overpotential is a key metric of catalytic efficiency, with lower values indicating superior performance. Most studies, therefore, benchmark electrocatalysts by comparing the overpotential required to reach a current density of 10 mA cm⁻². Kinetic information is commonly obtained from the Tafel slope, which provides insight into the reaction mechanism for HER and OER. The Tafel slope is derived from linear sweep voltammetry (LSV) data by fitting the linear region of the plot of overpotential versus the logarithm of current density according to the Tafel equation (1),

$$\eta =b\log j+a$$ (1)

where ‘η’ is the overpotential, ‘j’ is the current density, ‘b’ represents the Tafel slope and ‘a’ is the Tafel constant [64]. High j ₀ and low b values are indicative of an efficient catalyst. Another parameter, Exchange current density (j ₀), is associated with the fundamental reaction activation rate derived from Equation (2),

$$j_0=nF{K}_{\mathrm{h}}(1‐\Theta )$$ (2)

where n is the number of electrons transferred, F is the Faraday constant, K _h is the heterogeneous rate constant, and Θ represents the hydrogen coverage fraction of active sites determined by extrapolating the log (j) versus overpotential (Tafel plot) to zero overpotential.

A comprehensive assessment of electrocatalysts requires consideration of both intrinsic and extrinsic properties. Intrinsic properties are inherent to the active sites and define their fundamental catalytic capability, including Faradaic efficiency, exchange current density (j ₀), and turnover frequency or turnover number. In contrast, extrinsic properties determine practical performance under operating conditions and include the electrochemically active surface area (ECSA), catalyst loading, porosity, and support conductivity. Even catalysts with high intrinsic activity may exhibit limited overall performance if these extrinsic factors are not optimised, highlighting the need for balanced evaluation [65]. In addition to activity, stability is a critical parameter for commercial viability and is commonly assessed using established electrochemical protocols such as cyclic voltammetry (CV), chronopotentiometry, and chronoamperometry. During CV‐based durability testing, the catalyst is subjected to repeated potential cycles, and changes in onset potential or overpotential are monitored to evaluate degradation. Chronopotentiometry measures the potential variation at a constant current density over extended operation, whereas chronoamperometry tracks the current response at a fixed potential. Catalysts exhibiting minimal shifts in these parameters are generally considered to possess acceptable long‐term electrochemical stability [12, 66]. In general electrocatalysis, durability is defined by the ability of a catalyst to maintain consistent reaction rates over repeated cycles without significant loss of efficiency or structural integrity [67]. Performance degradation is often considered acceptable when the overpotential or onset potential increases by no more than ~30 mV at 10 mA cm⁻² and when activity loss remains below 5% following potential cycling. Sustained operation at constant current densities (typically 10–100 mA cm⁻²) during extended chronoamperometric measurements, often exceeding 12 h, is also widely used as a benchmark for long‐term stability [68, 69, 70]. In addition, reusability and reproducibility are essential considerations, as effective catalysts should deliver consistent performance across multiple cycles and batches to support scalable and sustainable energy production [71]. To standardise such evaluations, the US Department of Energy (DOE) has developed protocols for assessing the stability and efficiency of fuel cells and energy devices [72]. The DOE accelerated stress test protocols provide a standardised framework for evaluating the durability of proton exchange membrane (PEM) fuel cell components under simulated automotive conditions. These tests target specific degradation mechanisms: electrocatalyst stability is assessed through voltage cycling, catalyst support durability via high‐potential holds, and membrane integrity through chemical and mechanical stress tests. Diagnostic measurements, including polarisation curves and hydrogen crossover analysis, are performed using standardised single‐cell protocols to ensure consistency [72].

3. General Properties of MoS₂

TMDs like MoS₂, WS₂, and MoSe₂ represent a large family of layered materials with the general formula MX₂ [73], where M is a transition metal element from group IV (Ti, Zr or Hf), group V (V, Nb or Ta) or group VI (Mo, W), and X is a chalcogen atom (S, Se or Te). These materials consist of covalently bonded X–M–X layers stacked through weak van der Waals interactions, which enable exfoliation into atomically thin sheets and allow versatility in their structure as illustrated by various its polymorphs in Figure 1 [47]. Among TMDs, MoS₂ has been the most extensively investigated owing to its favourable combination of electronic, chemical, and structural properties. It has attracted considerable attention owing to its significant roles in batteries [74], energy conversion applications [75], and supercapacitors [76]. Following graphene, MoS₂ is regarded as one of the most important 2D materials, receiving substantial interest due to its distinctive physicochemical properties [77]. MoS₂ exists in both thermodynamically stable and metastable polymorphs, with properties that depend strongly on crystal structure and thickness. Bulk MoS₂ exhibits an indirect bandgap, whereas monolayer MoS₂ displays a direct bandgap of approximately 1.8–1.9 eV, making it attractive for post‐silicon electronics and optoelectronic devices. Thin MoS₂ films deposited on substrates often show improved charge transport compared to bulk crystals and have been explored for nanoelectronic applications compatible with existing semiconductor technologies [78, 79].

Beyond its electronic characteristics, MoS₂ possesses intrinsic properties such as strong anisotropy, chemical inertness, and excellent lubricity, which influence its behaviour in diverse environments. Consequently, MoS₂ and related TMDs have been widely studied as solid lubricants [80], transistors [81], solid‐state secondary lithium‐ion battery cathodes [82] and industrial catalysts. To support these applications, several synthesis strategies have been developed for TMDs, including chemical vapour deposition (CVD) [83, 84, 85, 86], electrospinning [87, 88], and hydrothermal methods [89]. MoS₂ exhibits highly tuneable properties that can be further engineered through exfoliation, compositing, and hybridisation. Its layered structure allows facile exfoliation into few‐layer or monolayer nanosheets, leading to pronounced changes in electronic, optical, and electrochemical behaviour. For example, exfoliated MoS₂ nanocomposites enhance lithium‐ion storage by increasing accessible surface area and the density of active sites. Incorporation of secondary components such as graphene or iron oxide further improves electrical conductivity and structural stability, as demonstrated in MoS₂/graphene composites synthesised using L‐cysteine and MoS₂/Fe₃O₄ hybrids [90, 91, 92, 93, 94]. MoS₂ functions as an atomically thin semiconductor with a tuneable bandgap, exhibiting an indirect gap in the bulk and a direct bandgap in monolayer nanosheets, as evidenced by strong photoluminescence. This intrinsic semiconducting nature addresses key limitations of graphene for electronic and optoelectronic applications. Beyond electronics, MoS₂ serves as an effective host for ion insertion and transport, finding widespread use as an anode material in lithium‐ion batteries, particularly in exfoliated and composite architectures. Furthermore, its dangling‐bond‐free surface and van der Waals–bonded layers enable seamless integration with diverse materials, facilitating the construction of atomically sharp heterostructures with tailored electronic and optoelectronic functionalities [95].

4. Preparation Strategies of MoS₂

The synthesis route plays a decisive role in determining the phase composition, morphology, defect density, and interfacial characteristics of MoS₂, all of which directly influence its electrocatalytic performance. Different preparation strategies offer distinct advantages in terms of structural control, scalability, crystallinity, and process complexity. Consequently, a wide range of synthetic approaches has been explored to tailor MoS₂ for water‐related electrochemical reactions. This section summarises the most commonly employed preparation strategies for MoS₂‐based catalysts, with emphasis on hydrothermal/solvothermal synthesis, CVD, and microwave‐assisted methods.

4.1. Hydrothermal Technique

The hydrothermal method is a versatile and widely adopted synthesis technique that involves chemical reactions in aqueous media under elevated temperatures and pressures, typically within a sealed autoclave. Under these conditions, water acts as an effective solvent, enabling the dissolution, transport, and recrystallisation of precursors that are otherwise poorly soluble under ambient conditions. This environment allows precise control over nucleation and crystal growth, making the method well‐suited for the synthesis of nanostructured materials with tailored morphologies, crystal phases, and particle sizes [96, 97]. One of the key advantages of the hydrothermal route for MoS₂ synthesis is its ability to produce crystalline materials without the need for extremely high external heating, as illustrated in Figure 2a. The elevated vapour pressure within the autoclave facilitates low‐temperature crystallisation and phase formation [98]. As a result, the hydrothermal method has been successfully applied to the synthesis of a broad range of functional materials, including metal oxides, sulphides, phosphates, TMDs and hybrid nanocomposites. By adjusting parameters such as temperature, pressure, reaction time, pH, and precursor concentration, the surface area, porosity, and morphology of the resulting materials can be finely tuned, directly influencing their catalytic, electronic, and electrochemical properties [101].

FIGURE 2.

Schematic illustration of the common synthesis routes of MoS₂. Representation of Hydrothermal synthesis of MoS₂ (a). Reproduced with permission from Ref [98]. Copyright 2022, American Chemical Society. Stepwise synthesis of MoS₂ from MoO₃ by CVD (b). Reproduced with permission from Ref [99]. Copyright 2018, American Chemical Society. Representation of microwave‐assisted synthesis of MoS₂ (c). Reproduced with permission from Ref [100]. Copyright 2015, Springer Nature.

In the context of electrocatalysis, hydrothermal synthesis is particularly advantageous for fabricating MoS₂ nanostructures with high surface area and well‐exposed active sites, including nanosheets, nanorods, and hierarchical architectures. These structural features enhance mass transport, charge transfer, and overall catalytic performance. Moreover, the method is relatively simple, cost‐effective, and scalable, making it attractive for the preparation of advanced catalysts for energy conversion and storage applications such as water splitting, fuel cells, and batteries [102, 103, 104, 105].

4.2. CVD

CVD is a widely used materials synthesis technique, primarily employed for the deposition of high‐quality thin films on various substrates. In addition to thin‐film growth, CVD is also utilised for producing high‐purity bulk materials and powders, including composite materials, through controlled infiltration processes under defined temperature and pressure conditions [106, 107]. The method relies on the thermal decomposition or reaction of gaseous precursors at elevated temperatures, leading to continuous film growth on the substrate surface with precisely controllable thickness, morphology, and composition. The CVD process is typically conducted under a controlled inert or reducing gas atmosphere, such as Ar, H₂, N₂, or CH₄, which serves multiple functions, including carrier transport of precursor vapours, regulation of reaction kinetics, and suppression of unwanted oxidation. The specific gas composition and flow rates are tailored according to the targeted material phase, thickness, and crystallinity. For instance, Pondick et al. employed a CVD‐based stepwise sulfurisation strategy in which MoO₃ was first exposed to S vapour at elevated temperature, leading to the formation of intermediate MoOS₂ crystals. Subsequent sulfurisation under controlled conditions resulted in complete conversion to MoS₂, enabling improved control over phase evolution and film quality, as illustrated in Figure 2b [99]. CVD is often combined with other synthesis approaches, including hydrothermal, sol–gel, and sonication methods, to further tailor material properties. In such hybrid strategies, materials initially prepared via hydrothermal synthesis can be deposited onto substrates using CVD, with subsequent optimisation of reaction parameters to enhance crystallinity, phase composition, or interfacial properties [19]. These combined approaches are particularly useful for producing MoS₂ films and composites with controlled thickness and improved electrochemical performance.

4.3. Microwave‐Assisted Synthesis

Microwave‐assisted synthesis is an energy‐efficient and rapid materials processing technique that has gained increasing attention for the preparation of two‐dimensional materials such as MoS₂. Unlike conventional heating methods, microwave irradiation enables direct and volumetric heating of the reaction medium through dielectric loss mechanisms, resulting in rapid temperature rise and uniform heat distribution [108]. The reaction kinetics are primarily governed by the applied microwave power, operating frequency, and irradiation duration, allowing precise control over nucleation and growth processes [109]. In a typical microwave‐assisted synthesis of MoS₂ materials, precursor compounds are mixed in stoichiometric ratios and sealed in an inert container like pyrex under vacuum conditions, as shown in Figure 2c [100]. The sealed system is then exposed to controlled microwave irradiation for a defined time and temperature, promoting rapid nucleation and crystallisation of layered MoS₂ structures. Commonly employed Mo precursors include ammonium molybdate tetrahydrate and molybdenum pentachloride, combined with S sources such as thioacetamide, thiourea, or L‐cysteine [19, 110]. The principal advantage of microwave‐assisted synthesis lies in its ability to rapidly produce MoS₂ with controlled morphology and phase composition using significantly reduced reaction times [111]. While further optimisation is required to consistently match or exceed the performance of materials prepared by hydrothermal methods, the scalability and time efficiency of microwave‐assisted synthesis make it an attractive complementary approach for the development of MoS₂‐based electrocatalysts.

5. General Electrochemical Water Reaction Mechanisms

Water electrolysis is considered a perfect way to produce clean hydrogen gas, which is carbon‐free without global warming‐causing gases. The water‐splitting reaction comprises the OER and the HER as the anodic and cathodic processes, respectively, both requiring a minimum thermodynamic potential of 1.23 V to drive the decomposition of water into H₂ and O₂ under standard conditions. So far, the activity Ir/Ru Oxides offered for OER and Pt‐based catalysts for HER are considered state‐of‐the‐art. However, the scarcity of the precursors and high cost of these catalysts significantly hinder their large‐scale adoption³.

Total Reaction is

$$2{\mathrm{H}}_2\mathrm{O}\hspace{0.5em}\to \hspace{0.5em}2{\mathrm{H}}_2+{\mathrm{O}}_2\hspace{0.17em}(\mathrm{\Delta G}°=237.2\mathrm{kJ}/\mathrm{mol})$$ (3)

5.1. HER

HER is employed to produce hydrogen gas through the straightforward process of water splitting driven by electrolysis, with the aid of an electrocatalyst to enhance the cathodic reaction [55, 112]. HER involves H atom adsorption at the electrode surface. The adsorption energy depends on the nature of the electrode materials used, as does HER kinetics. All metallic elements and the H atom tend to pass into the solution as a positive ion. This property of the metal is known as the solution pressure of the metal and is constant at a given temperature. Cathodic HER and anodic OER are crucial half‐cell reactions in electrochemical water splitting [113]. The decomposition of pure water into H₂ and O₂ at standard temperature and pressure (25°C, 1 atm) is not favourable in thermodynamic terms, as described by the equations:

$$\mathrm{Anode}:2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to {\mathrm{O}}_2\left(\mathrm{g}\right)+4{\mathrm{H}}^{+}\left(\mathrm{aq}\right)+4{\mathrm{e}}^{-}\hspace{0.17em}({{\mathrm{E}}^{\mathrm{o}}}_{\mathrm{ox}}=-1.23\mathrm{V})$$ (4)

$$\mathrm{Cathode}:2{\mathrm{H}}^{+}\left(\mathrm{aq}\right)+2{\mathrm{e}}^{-}\to {\mathrm{H}}^2\left(\mathrm{g}\right)\hspace{0.17em}({{\mathrm{E}}^{\mathrm{o}}}_{\mathrm{red}}=0.00\mathrm{V})$$ (5)

The most popular and widely accepted mechanisms of HER are the Volmer–Heyrovsky and Volmer–Tafel mechanisms, as shown in Figure 3. Generally, the following equations represent the HER process in different electrolytes.

FIGURE 3.

HER mechanisms on the surface of electrocatalysts in acidic, neutral, and alkaline solutions.

$${\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{e}}^{-}\to {\mathrm{H}}^{\ast }\left(\text{Volmer step}\right)$$ (6)

$${\mathrm{H}}^{\ast }+{\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{e}}^{-}\to {\mathrm{H}}_2\left(\text{Heyrovsky step}\right)$$ (7)

$${\mathrm{H}}^{\ast }+{\mathrm{H}}^{\ast }\to {\mathrm{H}}_2\left(\text{Tafel step}\right)$$ (8)

In neutral and alkaline solutions

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}\to {\mathrm{H}}^{\ast }(\text{Volmer step})$$ (9)

$${\mathrm{H}}^{\ast }+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}\to {\mathrm{H}}_2\left(\text{Heyrovsky step}\right)$$ (10)

$${\mathrm{H}}^{\ast }+{\mathrm{H}}^{\ast }\to {\mathrm{H}}_2(\text{Tafel step})$$ (11)

The reaction starts with an adsorption process of a hydrated proton or water molecule on the surface of the electrocatalyst via an electrochemical reduction process to generate hydrogen intermediates of adsorbed hydrogen atom H*, denoted as the Volmer step. Subsequently, the hydrogen gas is produced either by combining the adsorbed hydrogen with the water molecule, the Heyrovsky mechanism, or through the chemical combination of two adsorbed H* intermediates, which leads to forming H₂. The electrochemical HER is a two‐step process regardless of Volmer–Heyrovsky and Volmer–Tafel mechanisms [114, 115]. The Tafel slope values for the Volmer, Heyrovsky, and Tafel reaction mechanisms are around 118.2, 39.4, and 29.6 mV dec⁻¹, respectively, in HER, which is influenced by the rate‐determining step dependent [66]. For example, a slope near 118.2 mV dec⁻¹ indicates that slow hydrogen adsorption kinetics are the Volmer step. At the same time, values close to 39.4  and 29.6 mV dec⁻¹ suggest the Heyrovsky and Tafel steps, respectively [64].

5.2. OER

OER occurs at the anode in electrochemical systems and, in an alkaline medium, involves the adsorption of hydroxide ions onto the catalyst surface. This is followed by electron transfer steps that lead to the coupling of intermediates, and ultimately, the release of an oxygen molecule as it desorbs from the catalyst. However, the reaction typically proceeds with slow kinetics, making it challenging. The generally accepted OER mechanism in alkaline conditions proceeds through a series of steps that are commonly outlined as follows:

$$\mathrm{A}+{\mathrm{OH}}^{-}\to \mathrm{A}-{\mathrm{OH}}_{\mathrm{ads}}+{\mathrm{e}}^{-}$$ (12)

$$\mathrm{A}-{\mathrm{OH}}_{\mathrm{ads}}+{\mathrm{OH}}^{-}\to \mathrm{A}-{\mathrm{O}}_{\mathrm{ads}}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}$$ (13)

$$\mathrm{A}-{\mathrm{O}}_{\mathrm{ads}}+\mathrm{A}-{\mathrm{O}}_{\mathrm{ads}}\to 2\mathrm{A}+{\mathrm{O}}_2\left(\mathrm{g}\right)$$ (14)

$$\mathrm{A}-{\mathrm{O}}_{\mathrm{ads}}+{\mathrm{OH}}^{-}\to \mathrm{A}-{\mathrm{OOH}}_{\mathrm{ads}}+{\mathrm{e}}^{-}$$ (15)

$$\mathrm{A}-{\mathrm{OOH}}_{\mathrm{ads}}+{\mathrm{OH}}^{-}\to \mathrm{A}+{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}$$ (16)

where ’A’ is the active site of the catalyst and ’ads’ represents the adsorbed species on the catalyst's surface used in the process. Considering the OER in an alkaline medium, the process is carried out in two different ways. The first one follows the combination of two adsorbed oxygen molecules to form oxygen gas, which follows the (12) → (13) → (14) reaction pathway. The other OER pathway includes the formation of intermediates like hydroxides, just as in the equations mentioned above (12) → (13) → (15) → (16) [11, 54].

5.3. ORR

ORR is an essential electrochemical process in fuel cells and metal‐air batteries, converting O₂ into water or OH^‐ ions, during the galvanic discharge reaction. This complex reaction can proceed through two main pathways: the four‐electron and two‐electron pathways. In the four‐electron pathway, O₂ is directly reduced to water (in acidic media) or OH^‐ (in alkaline media) through a four‐electron transfer, which is desirable in fuel cells due to its efficiency and high energy yield, producing fewer intermediates. The reactions for this pathway are:

In acidic electrolyte:

$${\mathrm{O}}_2+4{\mathrm{H}}^{+}+4{\mathrm{e}}^{-}\to 2{\mathrm{H}}_2\mathrm{O}$$ (17)

In alkaline electrolyte:

$${\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}\to 4{\mathrm{OH}}^{-}$$ (18)

In contrast, the two‐electron pathway involves O₂ reduction to H₂O₂ or other intermediates before a complete reduction to water, involving two electron transfers in each step. This pathway is less efficient due to intermediate products like H₂O₂, which may disproportionate into reactive hydroxyl radicals that damage the electrode, membranes and structural materials, including the catalyst performance. The reactions for partial reduction in this pathway are:

In acidic electrolyte:

$${\mathrm{O}}_2+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2$$ (19)

In alkaline electrolyte:

$${\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}\to \mathrm{H}{{\mathrm{O}}_2}^{-}+{\mathrm{OH}}^{-}$$ (20)

The ORR typically proceeds through several elementary steps: (1) O₂ adsorption, where O₂ binds to the catalyst surface to facilitate its reduction; (2) O₂ dissociation or intermediate reduction, where O₂ dissociates or forms hydroperoxide (OOH*) intermediates, which are further reduced; and (3) Electron and proton transfer to the adsorbed oxygen or intermediates, forming OH or O species that yield water or hydroxide ions. The pathway choice largely depends on the catalyst material and reaction conditions, with catalysts like platinum favouring the four‐electron pathway due to their high affinity for O₂ binding and stabilising the oxide intermediates, while other materials may favour the two‐electron pathway, producing H₂O₂ [116].

5.4. HOR

HOR includes two half‐reactions in the basic electrolyte. The HOR is carried out in alkaline media through either the Tafel–Volmer or the Heyrovsky–Volmer mechanism. The mechanism is shown below, where M denotes surface sites with a high affinity for H atoms, and H_ad denotes adsorbed hydrogen on the M site.

$${\mathrm{H}}_2+2\mathrm{M}\to 2\mathrm{M}‐{\mathrm{H}}_{\mathrm{ad}}\left(\mathrm{Tafel}\right)$$ (21)

$${\mathrm{H}}_2+{\mathrm{OH}}^{‐}+\mathrm{M}\to \mathrm{M}‐{\mathrm{H}}_{\mathrm{ad}}+{\mathrm{H}}_2\mathrm{O}+\mathrm{e}‐\left(\mathrm{Heyrovsky}\right)$$ (22)

$${\mathrm{H}}_{\mathrm{ad}}+\mathrm{OH}‐\to {\mathrm{H}}_2\mathrm{O}+\mathrm{e}‐+\mathrm{M}\left(\mathrm{Volmer}\right)$$ (23)

One way to find the rate‐determining step is the Tafel slope. A slope of ~30 mV dec⁻¹ indicates that hydrogen recombination in the Tafel step is the rate‐determining step, a Tafel slope of ~120 mV dec⁻¹ indicates either the Volmer or Heyrovsky as the rate‐determining step.

6. The Role of MoS² in the HER

HER is a key half‐reaction in water splitting and underpins sustainable hydrogen production. MoS₂ has emerged as a promising alternative to Pt‐based catalysts owing to its elemental abundance, stability, and competitive activity [117, 118, 119, 120, 121]. The following subsections discuss the most widely adopted engineering strategies, which have been employed to systematically regulate active‐site exposure, electronic conductivity, and hydrogen adsorption behaviour in MoS₂‐based catalysts.

6.1. Active Sites and Phase Engineering

Early experimental work by Jaramillo et al. established that HER activity in MoS₂ originates predominantly from S edge sites rather than the basal planes, with catalytic performance scaling directly with edge‐site density [45]. Phase engineering plays a decisive role in activating MoS₂ for HER. While the thermodynamically stable 2H phase is semiconducting and largely inactive at the basal plane, the metallic 1T phase exhibits substantially enhanced conductivity and catalytic activity. Zhang et al. demonstrated a strong positive correlation between HER performance and 1T‐phase concentration in polycrystalline 1T/2H MoS₂, reporting an onset overpotential of 146.6 mV for samples enriched in the 1T phase [122]. DFT calculations further confirm that single‐phase 1T‐MoS nanosheets possess more exposed active sites and superior HER kinetics compared to their 2H counterparts [123]. Mixed‐phase interfaces also contribute significantly to activity enhancement. Zhang et al. identified zigzag (ZZ) 1T/2H interfacial configurations with lower energy barriers and faster HER kinetics than either pure phase alone, with improvement in HER activity through interfacial doping at Mo or S sites [124]. Complementary theoretical studies by Nørskov et al. confirmed that even in 2H‐MoS₂, HER activity is dominated by metallic edge states rather than basal‐plane sites [125]. The effect of organic S sources on MoS₂ phase formation was investigated using thioacetamide, L‐cysteine, and thiourea. Thioacetamide and L‐cysteine produce metallic MoS₂, whereas thiourea yields semiconducting MoS₂. Consequently, MoS₂ synthesised using thioacetamide exhibits enhanced HER activity, achieving an overpotential of 210 mV at a current density of 10 mA cm⁻² with a Tafel slope of 44 mV dec⁻¹. This enhanced performance is attributed to the lower decomposition temperature of thioacetamide, which facilitates rapid S release, stabilises the metallic phase, and suppresses excessive particle growth, as evidenced by the comparative transmission electron microscopy (TEM) images shown in Figure 4a–c [126]. Wu et al. compared crystalline 2H, 1T, and amorphous MoS₂ using X‐ray absorption spectroscopy and electrochemical analysis, demonstrating that amorphous MoS₂ outperforms both crystalline polymorphs due to its high defect density and abundance of unsaturated edge sites [128]. Operando studies further reveal that both amorphous and 1T‐MoS₂ share short Mo–Mo bond motifs that remain stable during HER, providing a common structural origin for their enhanced activity [129]. Huang et al. visualised HER active sites on MoS₂ using electrochemical tip‐enhanced Raman spectroscopy (EC‐TERS). The potential‐dependent EC‐TERS measurements reveal distinct spectral evolution at the basal plane and edge sites during HER. A reconstruction region extending ~40 nm from the edge into the adjacent basal plane was observed, accompanied by variations in lattice structure and electron density. This progressive edge‐induced reconstruction generates active sites, where lattice deformation lowers activation energy barriers and enhances HER kinetics, providing direct insight into active‐site dynamics [130]. Liu et al. further demonstrate the modulation of MoS₂ by hydrothermally growing 1T‐MoS₂ as nanosheets onto carbon cloth, achieving an overpotential of 151 mV at 10 mA cm⁻², outperforming 2H‐MoS_2. The scanning electron microscopy (SEM) and TEM images of vertically grown 1T‐MoS₂, shown in Figure 4d,e respectively, depict the vertically and uniformly grown MoS₂ nanosheets. Similarly, 2H‐MoS₂ grown onto carbon cloth resembles the morphology as shown in its SEM and TEM images in Figure 4f,g, respectively. Figure 4h,i indicates that the HER activity of 1T‐MoS₂ nanosheets outperforms its 2H‐MoS₂ counterpart along with their bulk counterparts, the enhanced activity of 1T‐MoS₂/CC is attributed to its basal plane that serves as an active site and improves its conductivity [127].

FIGURE 4.

TEM images of MoS₂ synthesised using (a) thioacetamide, (b) thiourea, and (c) L‐cysteine, respectively. Reproduced with permission from [126]. Copyright 2023 American Chemical Society. SEM images of 1T‐MoS₂ (d) and 2H‐MoS₂ (f) nanosheets grown on CC. TEM images of 1T‐MoS₂ (e) and 2‐H MoS₂ (g). Comparison of LSV (h) and Tafel plots (i) of MoS₂ grown on CC and bulk MoS₂. Reproduced with permission from [127]. Copyright 2019, Elsevier.

6.2. Interfacial and Composite Engineering

Interfacial engineering provides an effective route to enhance charge transfer and HER kinetics. Jian et al. reported that the incorporation of aluminium onto hydrothermally synthesised MoS₂ via atomic layer deposition, as illustrated in Figure 5a, resulted in the formation of MoS₂ nanosheets (Figure 5b,c). The presence of Al on MoS₂ reduced the bandgap of the electrocatalysts, thereby enhancing their HER activity, delivering an overpotential of 198 mV at 10 mA cm⁻² with a Tafel slope of 134 mV dec⁻¹ [120]. The composite engineering of Ni₃S₂ with MoS₂ composite resulted in an overpotential of 110 mV to reach 10 mA cm⁻², which indicates that the Ni–S bond with MoS₂ can enhance HER with impressive activity [132]. Incorporating metal sulphides, metal oxides and transition metals to MoS₂ has been found to be an effective strategy of improving the electron transfer kinetics in MoS₂‐based and improving the HER activity [133, 134]. Likewise, the composite of MoS₂‐NiS₂ displayed higher HER activity than over individual metal sulphide counterparts with an overpotential of 62 mV at 10 mA cm⁻² [135]. The growth of MoS₂ supported on conductive substrates further benefits from improved electron transport [136]. Beyond conventional carbon supports, biomass‐derived architectures have also been explored to realise pH‐universal HER catalysts. Du et al. reports a wood‐derived electrode hosting ternary MoS₂–MoP–Mo₂C nanoparticles was fabricated via a two‐step hydrothermal treatment followed by carbonisation, the associated SEM images of the stepwise development of the catalysts are displayed in Figure 5d‐k. The interconnected wood channels facilitate uniform nanoparticle loading, while the carbonised wood (CW) framework provides high electrical conductivity together with resistance to acidic and alkaline corrosion. The synergistic interaction among MoS₂, MoP, and Mo₂C generates abundant unsaturated edge coordination sites, promoting efficient adsorption and desorption of hydrogen intermediates. As a result, the MoS₂–MoP–Mo₂C@CW electrode exhibits low HER overpotentials of 46 mV (alkaline), 84 mV (neutral), and 65 mV (acidic) at 10 mA cm⁻², along with durability over 200 h of continuous operation [131].

FIGURE 5.

(a) Schematic representation of Al‐MoS₂ by atomic layer deposition. (b and c) SEM and TEM images of Al‐MoS₂. Reproduced with permission from [120]. Copyright 2020 American Chemical Society. (d) Photograph and (e) SEM images of natural wood (NW). (f) Photograph and (g,h) SEM images of MoS₂@NW. (i) Photograph and (j,k) SEM images of MoS₂–MoP–Mo₂C@CW. Reproduced with permission from [131]. Copyright 2025 American Chemical Society.

6.3. Doping and Defect Engineering

Defect engineering and hetero‐elemental doping offer powerful strategies for activating MoS₂. Introducing S vacancies, either intrinsically or via dopant‐assisted stabilisation, lowers hydrogen adsorption free energy and enhances HER kinetics [137, 138]. Joyner et al. engineered rGO‐supported MoS₂ with tailored morphologies featuring extensively exposed edge sites and a high density of basal‐plane defects, which act as active centres for HER, resulting in an onset potential of 197 mV [139]. Similarly, Ye et al. introduced controlled defects into monolayer MoS₂ through oxygen plasma exposure and subsequent hydrogen treatment, generating a high density of catalytically active edge sites [140]. Wang et al. theoretically predicted and experimentally realised an optimal S‐vacancy configuration through defect engineering, demonstrating that controlled vacancy concentration and spatial distribution significantly enhance HER performance. DFT calculations confirmed that S vacancies in MoS₂ lower the hydrogen adsorption free energy and promote catalytic activity [123]. Experimentally, S vacancies can be introduced through chemical etching using H₂O₂ or mild NaClO treatment, while Mo vacancies can be generated via Ar irradiation [141, 142].

Doping represents an effective strategy for modulating the electronic structure, defect chemistry, and phase composition of MoS₂, thereby enhancing charge–transfer kinetics and increasing the density of catalytically active sites for HER. Wu et al. demonstrated that Zn doping in MoS₂ promotes the formation of S vacancies, leading to enhanced HER activity. Theoretical calculations showed that Zn incorporation lowers the formation energy of S vacancies and reduces the free energy of adjacent vacancy sites, thereby facilitating charge transfer. Consequently, the Zn@MoS₂ catalyst delivers an HER overpotential of 194 mV at 10 mA cm⁻² with a Tafel slope of 78 mV dec⁻¹ [143]. A similar effect was observed for V(II) and V(IV) co‐doped MoS₂, where V doping induced S vacancies, structural defects, and an increased fraction of the 1T phase, collectively to enhance HER activity. The resulting V–MoS₂ catalyst exhibits pH‐universal activity, achieving overpotentials of 111 mV in acidic, 218 mV in alkaline, and 266 mV in saline media at a current density of 10 mA cm⁻² [144].

Overall, MoS₂ is a highly adaptable HER catalyst whose activity is governed by phase composition, edge exposure, defect density, and interfacial electronic structure. Strategies including phase engineering, defect creation, heterostructure formation, and conductive support integration have enabled MoS₂‐based systems to approach or rival noble‐metal catalysts. These advances position MoS₂ as a viable platform for scalable hydrogen production and broader electrochemical energy applications. Recent comparisons of MoS₂‐based HER catalysts are summarised in Table 1.

TABLE 1.

HER reactivity and kinetic parameters of recently reported catalysts.

CATALYST	Cathodic overpotential @ 10 mA cm−2, mV	Tafel slope,  mV/dec	Ref.
Pd@MoS2/Mo2TiC2Tx	92	60	[4]
MoS2/NiS	244	97	[37]
MoS2/Graphene	143	71	[61]
Co, Pd‐MoS2	49.3	43	[117]
MoS2 Quantum dots	241	163	[118]
Al‐MoS2	248	82	[120]
1T‐MoS2 (thioacetamide)	210	44	[126]
Cu2S/MoS2/Cu foam	91	41	[145]
Ni3S2/MoS2	330	83	[132]
MoS2/NiS2	62	50	[135]
0D/3D MoS2‐NiS2/N‐Graphene	172	70	[134]
Ni/MoS2	162	37	[136]
MoS2‐MoP‐Mo2C@CW	46	57	[131]
V‐MoS2	111	71	[144]
CoSx@Cu2MoS4	118.1	53.5	[146]
(Ni2P, MoS2)/MWCNT	101	47	[147]
Ni‐doped MoS2	168	86	[148]
ZnS@C@MoS2	118	55	[149]
Cu2MoS4	96	61	[150]
MoS2/WSe2	116	76	[151]
MoS2/CuS	150	63	[152]
MoSx@NiO	406	43	[153]
CuS@ MoS2	135	50	[154]
Mn‐MoS2/CNT	150	39	[155]
P‐MoS2 in N‐carbon nanofibers	98	66	[156]
SrTiO3@MoS2	165	81	[157]
Zn0.25Co0.75/NC@MoS2	130	60	[158]
Ni0.85Se/MoS2	118	49	[159]
1T‐MoS2	175	41	[160]
MoS2@CoFe@N‐carbon	64	45	[161]
rGO/MoS2	122	132	[162]
Ni–MoS2@N‐carbon	122	98	[163]
PtSA@MoS2	25	43	[164]

7. The Role of MoS² in OER

Unlike its well‐established activity toward the HER, pristine MoS₂ is generally considered an inefficient standalone catalyst for the OER. This limitation originates from unfavourable adsorption energetics of key oxygenated intermediates and necessitates targeted engineering strategies to activate MoS₂ toward OER. Recent studies demonstrate that rational modulation of active sites, phase composition, defects, and interfacial chemistry can substantially improve its OER performance.

7.1. Active Sites and Phase Engineering

DFT calculations on MoS₂ monolayers reveal that its poor intrinsic OER activity arises from weak binding of adsorbed *OH and *OOH intermediates, which are critical to OER kinetics. Even after oxygen or phosphorus doping, monolayer MoS₂ exhibits unfavourable OER activity, and both Mo‐ and S‐edge sites display relatively high overpotentials compared to benchmark OER catalysts [165]. A study from Karmodak et al. however, presents an activation pathway for the MoS₂ edges themselves, which were previously largely dismissed as inactive for OER. It addresses the limitation of earlier studies by incorporating ab initio molecular dynamics simulations and hybrid explicit/implicit descriptions of the aqueous electrochemical environment. Their study revealed that ZZ MoS₂ edges can be activated for OER through surface oxidation, where S atoms rapidly oxidise under oxidative potentials to form a stable oxygenated ZZ‐Mo edge with 0% S coverage, exhibiting markedly lower overpotentials than both the basal plane and S‐covered ZZ edges [166]. This study supported the work of Mohanty et al., they developed MoS₂ quantum dots with an overpotential of 370 mV at 10 mA cm⁻², outperforming pristine MoS₂. DFT calculations further confirmed that, although the basal plane of MoS₂ is relatively inactive for the OER, S vacancies generate active sites at the edges, while the vertices of the quantum dots modulate S coverage and thereby regulate the energy barriers of the intermediates, as reflected in the free energy profile shown in Figure 6a [167]. Metal–organic frameworks (MOFs) have gained considerable attention due to their distinctive structural characteristics and tuneable compositions [169]. Dharman reported a strategy of employing Ti MOFs to induce the phase transformation of 2H‐MoS₂ into its more active 1T phase by the hydrothermal method, achieving a conversion ratio of 78.3%. The resulting MoS₂/Ti MOF heterostructure exhibits enhanced OER activity, with an overpotential of 290 mV at 10 mA cm⁻² as compared to its precursors of MoS₂ and Ti‐MoF. Beyond promoting phase transition, the MOF facilitates charge transfer from Ti to Mo, which stabilises the 1T‐MoS₂ phase by lowering its transition energy and enhances OH* intermediate adsorption, a critical step in OER kinetics [170].

FIGURE 6.

(a) Free energy diagrams for OER at vertex, edge, and surface sites of Mo‐edge MoS₂ quantum dots with half (top) and full (bottom) S coverage. Calculations performed at U = 0.6 V, pH 14. Reproduced with permission from Ref [167]. Copyright 2018, American Chemical Society. Correlation between bond orders and overpotentials for OER. Theoretical (b,c) and experimental (d,e) overpotentials are plotted against averaged M—S bond orders and M—S/M—O bond order differences, with experimental OER activity defined by onset overpotential at 5.0 mA/cm². Reproduced with permission from Ref [168]. Copyright 2019, Elsevier.

7.2. Interfacial and Composite Engineering

Over the years, there have been several strategies that have been developed for developing MoS₂‐based catalysts for OER. A straightforward yet simple strategy is the direct growth of MoS₂ onto nickel foam by the hydrothermal method and subsequent calcination at a higher temperature, with the synthesised catalysts exhibiting an overpotential of ≈310 mV @20 mA cm⁻² and a Tafel slope of 105 mV/dec [171]. The combination of MoS₂ with other metal sulphides can also enable the heterostructure as an effective OER catalysts, Li et al. reports the synthesis of a CoS₂/MoS₂ nanosheet supported on carbon cloth via a solvothermal method. The CoS₂/MoS₂ heterostructure electrocatalyst exhibits an overpotential of 243 mV at 10 mA cm⁻² with a Tafel slope of 109 mV/dec. The formation of OOH* is the rate‐determining step and is attributed to the heterojunction arising between MoS₂ and CoS₂ [172]. Huang reports that the introduction of CeO₂ nanoparticles in the CoS/MoS₂ interface results in activating the Co/Mo sites as well as introducing oxygen defects and vacancies, resulting in an OER overpotential of 247 mV at 10 mA cm⁻². More importantly, the incorporation of CeO₂ nanoparticles effectively protects the CoS/MoS₂ couple from corrosion via the Ce³⁺/Ce⁴⁺ redox couple [173]. Composite engineering represents another effective strategy to enhance OER performance. The synergistic interaction between MoS₂‐based materials and transition metal composites modulates the adsorption energies of oxygen‐containing intermediates and lowers the ΔG value of the rate‐determining step in the OER. Notably, combining Ni with MoS₂ can enhance the activity of MoS₂ toward OER by reducing the charge transfer resistance and enhancing the charge transfer kinetics [174, 175]. Bao et al. reports a composite of MoS₂ and NiCo₂O₄ supported on Ti mesh, the interface between the two components complements the OER activity with an overpotential of 313 mV at 10 mA cm⁻² [176]. Chakraborty reports that the OER activity of NiFe layered double hydroxides (LDH) was improved when grown with 1T MoS₂. The NiFe LDH/MoS₂ composite displays an OER activity of 190 mV at 10 mA cm⁻² with a Tafel slope of 31 mV/dec [177]. Jiang et al. reports Ni–Co–MoS₂ composite with varying micronanostructures prepared by magnetic field‐assisted jet electrodeposition. The optimised Ni–Co–MoS₂ composite, present as coral‐like micronanostructures with rough surface, displays an OER activity of 227 mV at 10 mA cm⁻² [178]. MoS₂ also effectively enhances the interfacial interaction and directs the redistribution of the electrons at the heterojunction interfaces, which enables the optimisation of the local Co spin state to a higher valence state. The elements Mo and Co interact to form open latent vacancies and Co–Mo bonds. These changes have been observed in aiding the synergistic effect of the elements in the local interfacial interaction with the MoS₂ phase and effectively improving the electrochemical stability [63, 179, 180].

The incorporation of carbon‐based materials with MoS₂‐based catalysts has also seen success in improving their OER activity. This effect was demonstrated by Zhao et al. where Ni‐MoS₂/rGO exhibited a low overpotential of 349 mV at a current density of 10 mA cm⁻² for OER [181]. Zhao et al. also demonstrates this by growing MoS₂@CoS₂ nanospheres on GO, the resulting heterostructure exhibits better OER activity with an overpotential 260 mV at 10 mA cm⁻² by means of its larger ECSA [182]. MOFs have also observed to serve as effective platforms for constructing MoS₂‐based heterostructures directly onto electrode substrates, Muthurasu et al. demonstrates this by development of Co₃O₄/MoS₂ hybrid derived from calcined Co MOF deposited onto NF. The developed catalysts exhibited an enhanced OER activity of 230 mV at 20 mA cm⁻², attributed to the surface redox activity of Co(II)/Co(III) and Co(III)/Co(IV) [183].

7.3. Doping and Defect Engineering

It is crucial to examine the structural tuning strategies systematically applied to MoS₂, along with an updated understanding of their catalytic mechanisms for OER enhancement [184]. A study from Hai et al. supplemented that the OER activity of MoS₂ nanosheets can additionally be tuned by transition‐metal doping. Using high‐throughput DFT calculations, they showed that activity is strongly governed by the M—S and M—O bond with the oxygen intermediates. Notably, a smaller difference between the theoretical and experimental M–S (Figure 6b,d) as well as M–O (Figure 6c,e) bond orders correlate with enhanced OER performance, both theoretically and experimentally. This relationship, illustrated in Figure 6b–e, demonstrates that the bond order difference serves as an effective activity descriptor for transition metal–doped MoS₂ nanosheets [168]. Additionally, Maiti and Srivastava report Ru‐doped CuO/MoS₂ nanostructures resulted in improving the OER activity with an overpotential of 201 mV at 10 mA cm⁻² [185]. The vital role of oxygen in modulating the OER activity of MoS₂ was demonstrated by Mu et al. by the oxidation of M–O species and the introduction of oxygen vacancies to significantly enhance the catalytic performance. This was achieved using a bimetallic AlCo₃–MoS₂ system, wherein surface reorganisation engineering triggered self‐reconstruction of the pre‐catalyst into crystalline CoAl–OOH intermediates, while simultaneously redistributing surface electronic states to optimise the adsorption and conversion of OER intermediates. These changes optimised the adsorption of OER intermediates such as OH* and OOH* on metal oxyhydroxides while weakening the strong Mo—S bonds. The resulting AlCo₃–MoS₂ outperformed its counterparts, MoS₂, Al–MoS₂ and Co–MoS₂, with an overpotential of 323 mV at 30 mA cm⁻² for OER [186]. Wang et al. demonstrated through in situ FT‐IR and Raman spectroscopy that MoS₂ undergoes oxidative transformation to MoOOH during the OER. In their study, Ce‐doped dual‐phase (1T/2H) MoS₂ was converted to MoOOH when subjected to anodic potentials around 1.6 V. Rather than causing deactivation, this oxidation generated surface sites capable of promoting oxygen‐radical adsorption, thereby accelerating the reaction kinetics and improving overall OER performance [187]. Doping of MoS₂ nanosheets has also proven to be an effective strategy for enhancing its OER activity, Tang et al. demonstrate this by doping MoS₂ nanosheets with Fe, resulting in the formation of smaller‐sized MoS₂ nanosheets that are present as highly rippled sheets, enabling maximum exposure of the active edges. The Fe‐MoS₂ nanosheets exhibit an overpotential of 290 mV @ 50 mA  cm⁻² with a Tafel slope of 72 mV/dec. The DFT studies further revealed that the substitution of Fe at the Mo site provided the most stable configuration with enhanced OER activity [188]. Nguyen et al. establishes that heteroatom doping of MoS₂ nanosheets with Co and Nb influences its electronic structure for optimum OER activity. Furthermore, the interaction of the heterostructure between the doped MoS₂ and TiO₂ presents effective pathways for electron transfer as well as increases its surface area of interaction with the reaction intermediates. The Co, Nb‐MoS₂/TiO₂ catalysts exhibit an OER overpotential of 260 mV at 10 mA/cm²; the developed catalysts further exhibit HER and ORR activity [189]. Gong et al. reports the development of a novel yolk–shell O–MoS₂ nanoreactor decorated with Pt nanoparticles (O–MoS₂@Pt), where S vacancies induced by oxygen doping enabled the uniform anchoring of Pt nanoparticles (ca. 10.9 nm) onto the outer shell. The unique yolk–shell architecture (563 ± 14.8 nm) enhanced the activity of the basal plane, prevented agglomeration, and facilitated gas release, thereby optimising the electron structure for OER catalysis. The O–MoS₂@Pt nanoreactor exhibited an overpotential of 244 mV @ 10 mA cm⁻² with a low Tafel slope of 53 mV dec⁻¹, significantly surpassing IrO₂ (335 mV, 77 mV dec⁻¹) and most MoS₂‐based catalysts [190].

Consequently, while MoS₂ was initially dismissed as an ineffective OER catalyst due to its poor binding with key oxygen intermediates, recent advances in engineering strategies discussed in this section highlight its promise, with optimised systems achieving impressive overpotentials for OER. Table 2 presents a comprehensive comparison of overpotential (η ₁₀) and Tafel slopes for different MoS₂‐based materials as OER catalysts.

TABLE 2.

OER reactivity and kinetic parameter of recently reported MoS₂‐based catalysts.

Catalyst	Anodic overpotential η = 10 mA cm −2 ,  mV	Tafel slope,  mV/dec	Reference
MoS2 Quantum Dots	370	39	[167]
MoS2/Ti MOF	290	85	[170]
MoS2 on NF	η 20: ≈310	105	[171]
CoS2/MoS2	243	109	[172]
CeO2@CoS/MoS2	247	63	[173]
(Ni, Fe)S2@MoS2	270	43.	[175]
NiCo2O4/MoS2	313	66.8	[176]
NiFe LDH/MoS2	190	31	[177]
Ni–Co–MoS2	227	59	[178]
MoS2@CoS2/GO	260	54	[182]
AlCo3‐MoS2	η 30: 323	46.7	[186]
O–MoS2@Pt	244	53	[190]
Co3O4/MoS2	η 20: 230	98	[183]
MoS2 nanoislands	300	84	[191]
CdS/MoS2/CrS2	248	37	[192]
CoS2−C@MoS2	391	46	[193]
Ru‐CuO/MoS2	201	229	[185]
Ru‐CoMOF@MoS2	240	87.9	[194]
NiO/MoS2/BiVO4	300	–	[195]
Fe‐MoS2	η 50: 290	72	[188]
Co, Nb‐MoS2/TiO2	260.1	65	[189]

8. MoS² as a Bifunctional Electrode for Overall Water Splitting (OWS)

The preceding discussion outlined various approaches to enhance MoS₂‐based catalysts for the separation of hydrogen and OERs. In practical electrochemical water splitting applications, there is significant value in developing materials capable of catalysing both half‐reactions efficiently, which can streamline device architecture and reduce manufacturing expenses. Creating dual‐purpose electrocatalytic activity in MoS₂ systems involves addressing inherent design conflicts, since the ideal material properties for HER and OER are often contradictory. Developing bifunctional MoS₂ catalysts thus demands sophisticated engineering approaches that can reconcile these conflicting material requirements through targeted structural and compositional modifications. This section examines how MoS₂‐based materials can be engineered to exhibit dual catalytic functionality, enabling them to serve as effective electrodes for OWS.

The MoS₂/Ni₃S₂ system has emerged as a leading platform for water splitting applications through strategic interfacial engineering. Zhang et al. developed MoS₂/Ni₃S₂ heterostructures using MoS₂ nanosheets and Ni₃S₂ nanoparticles as bifunctional catalysts, assembled an alkaline electrolyser achieved OWS with a cell voltage of 1.56 V at 10 mA cm⁻². DFT chemisorption models (Figure 7a) show that creating MoS₂/Ni₃S₂ and MoS₂/NiO heterostructures substantially enhances intermediate binding compared with the pristine surfaces. The H‐intermediate exhibits its strongest adsorption on Ni‐MoS₂, while OH‐ adsorption is most favourable on Mo‐Ni₃S₂. At the Mo–NiO interface, hydroxide affinity increases even further, giving the lowest ΔG _OH. Accordingly, the proposed mechanisms in Figure 7b show that MoS₂/Ni₃S₂ promotes HER, while MoS₂/NiO favours OER. The interfaces combine the strong H‐chemisorption of MoS₂ with the enhanced OH‐chemisorption of Ni₃S₂ and NiO, thereby lowering the Gibbs free energies of key intermediates, thereby accelerating the OWS process [196]. Yang et al. introduced a different approach to modulate the MoS₂/Ni₃S₂ system by creating hetero nanorods through hierarchical integration of Ni₃S₂ nanorods onto MoS₂ nanosheets, resulting in enhanced active site exposure and charge transport with driving OWS with a cell voltage of 1.50 V at 10 mA cm⁻² [198]. Wang et al. introduced a MoS₂/Ni₃S₂ system of engineering Mo–S–Ni interfaces that promote intermediate chemisorption and enable light‐enhanced performance through photogenerated charge transfer, achieving OWS at 1.53 V at 10 mA cm⁻² under solar irradiation [199]. Similar strategies for promoting OWS in MoS₂ systems have been successfully extended to MoS₂/NiS₂ systems. where Lin et al. demonstrated this by defect engineering of MoS₂/NiS₂ to create disordered structures that facilitate electron transfer, delivering OWS at 1.59 V at 10 mA cm⁻² [135]. Additionally, MoS₂‐NiS₂ nanoparticles on 3D nitrogen‐doped foam utilised interconnected hollow architecture (MoS₂‐NiS₂/NGF) for improved performance of OWS at 1.64 V at 10 mA cm⁻² [134].

FIGURE 7.

Theoretical models showing chemisorption of H and OH species on individual components (MoS₂, Ni₃S₂, NiO) and heterostructured surfaces (MoS₂/ Ni₃S₂, MoS₂/NiO) (a); mechanistic pathways for water dissociation and intermediate formation on MoS₂/ Ni₃S₂ heterostructures (b). Reproduced with permission from Ref [196]. Copyright 2016 Wiley‐VCH. Laboratory alkaline electrolyser prototype employing CoSAs‐MoS₂/TiN nanorod electrodes along with setup for collection of H₂ and O₂ produced during electrolysis (c). Reproduced with permission from Ref [197]. Copyright 2021 Wiley‐VCH.

Building upon the success of MoS₂/Ni₃S₂ systems, researchers have developed increasingly sophisticated heterostructures by coupling MoS₂ with other transition metal sulphides. The strategy involves pairing MoS₂ with transition metal sulphides of Ni, Fe, or Co, with the key design principle across these systems involving interface engineering and defect modulation to promote OWS. For instance, Liu et al. reported (Ni, Fe)S₂@MoS₂ heterostructures, where interfacial electron transfer from (Ni, Fe)S₂ to MoS₂ lowered the chemisorption energies of H* and OH^‐ intermediates, resulting in an OWS cell voltage of 1.56 V @ 10 mA cm⁻² [175]. The interlayer space between MoS₂ was effectively utilised by Jiang et al. to confine NiFe dual atoms within MoS₂ nano scrolls, where the confinement effect optimised adsorption of the reaction intermediates and protected the active sites in acid, yielding an exceptional OWS of 1.49 V@ 10 mA cm⁻² in 0.5 M H₂SO₄ [200].

Comparable progress has been made with Co‐based MoS₂ heterostructures. Hou et al. developed vertically aligned oxygenated‐CoS₂–MoS₂ hetero nanosheets (O‐CoMoS) grown on carbon cloth with advantageous disordered architectures and faster charge transport pathways. This resulted in O‐CoMoS exhibiting more favourable HER and OER activity and OWS with a cell potential of 1.6 V @ 10 mA cm⁻² as compared to O‐FeMoS and O‐NiMoS [201]. Defect engineering of the Co _x S _x /MoS₂ system also proved effective for promoting OWS He et al. reports S‐vacancy engineered Co₃S₄/MoS₂ nanosheets that required 1.67 V @ 10 mA cm⁻² for OWS [202]. The Se doping of CoS₂@MoS₂ heterostructure grown on multiwalled carbon nanotubes (MWCNT) (Se‐CoS₂@MoS₂/CNTs) displayed an OWS cell voltage of 1.56 V @ 10 mA cm⁻². The performance arises from synergistic CoS₂–MoS₂ interfaces, abundant active sites, and Se‐induced electronic modulation that optimises intermediate adsorption and reaction kinetics [203]. Whilst doping improved the OWS activity of MoS₂ systems, cationic Co doping in FeS/MoS₂ stabilised the Fe active sites and redistributed the interfacial field; the resulting Co–FeS/MoS₂ derived from a MOF precursor achieved an alkaline OWS cell voltage of 1.45 V at 10 mA cm⁻² [204]. Yang further reports a hierarchical heterostructure consisting of MoS₂ and Co₉S₈ nanosheets on Ni₃S₂ nanorods, which exhibited remarkable all‐pH OWS activity, requiring only 1.54, 1.45, and 1.80 V at 10 mA cm⁻² in alkaline, acidic, and neutral media, respectively. Its superior activity originates from abundant active sites and interfacial charge transfer from Co₉S₈ to MoS₂, which optimises intermediate adsorption and lowers overpotentials [205].

Building on these developments, further progress has been realised through hybrid and structurally engineered MoS₂ systems tailored with common metal‐based compounds for OWS. A CoO/N–MoS₂ hybrid supported on carbon fibre paper achieved an impressively low cell voltage of 1.5 V at 53 mA cm⁻² in alkaline media. The OWS activity is attributed to the intimate coupling between CoO nanoparticles and N‐doped MoS₂ nanosheets, which not only provides a high density of catalytically active sites but also creates favourable electronic interactions that accelerate charge transfer across the interface [206]. Notably, single‐atom Co‐decorated MoS₂ nanosheets grown on TiN nanorods (CoSAs–MoS₂/TiN) enabled highly efficient pH‐universal OWS, requiring 1.70, 1.65, and 1.66 V to drive a current density of 10 mA cm⁻² in acid 0.5 M H₂SO₄, 1 M KOH, and 1 M PBS, respectively. The interfacial electronic modulation at the CoSAs–MoS₂ junction, the mesoporous architecture that maximised accessible active sites, and the conductive TiN framework that accelerated charge transport, collectively ensuring robust bifunctional performance across acidic, alkaline, and neutral conditions. Figure 7c represents a typical lab‐scale alkaline electrolyser in an H‐cell set‐up with a volumetric gas collection setup and the formation of H₂ and O₂ bubbles on the cathode and anode, respectively [197]. The incorporation of Ir onto MoS₂ nanosheets induced a 2H‐to‐1T phase transition, forming an Ir/MoS₂ heterostructure that achieved OWS with a cell voltage of 1.57 V at 10 mA cm⁻², surpassing IrO₂//Pt/C. This activity resulted from the conductive 1T lattice and strong interfacial coupling between Ir and MoS₂, demonstrating how phase control and electronic interactions can synergistically accelerate OWS [207]. Majority of the reports mentioned so far primarily focused on strategies for MoS₂ catalyst development for OWS, innovative electrode fabrication route can also provide an additional avenue for efficient OWS. For instance, screen printing of NiO–MoS₂ gels onto carbon cloth has been demonstrated as an effective approach to produce robust and high‐performance electrodes for OWS [208].

Table 3 presents a comprehensive comparison of different bifunctional MoS_2‐based electrocatalysts, including their cell voltages for OWS. Advances in MoS₂‐based electrocatalysts through various engineering strategies have enabled efficient and durable OWS across a wide pH range. Collectively, these strategies demonstrate the significant potential for developing cost‐effective, high‐performance MoS₂‐based bifunctional electrocatalysts for practical water electrolysis applications.

TABLE 3.

Comparison of bifunctional MoS₂‐based catalysts for OWS.

Catalyst	OVERPOTENTIAL @ 10 mA cm −2 ,  mV		Cell voltage V @ 10 mA cm −2	Ref
	HER	OER
MoS2‐NiS2/NGF	172	370	1.64	[134]
MoS2/NiS2 nanosheets	62	278	1.59	[135]
(Ni, Fe)S2@MoS2	130	270	1.56	[175]
MoS2/Ni3S2 heterostructures	110	218	1.56	[196]
MoS2/Ni3S2 heteronanorods	98	249	1.50	[198]
MoS2/Ni3S2	78	260	1.53	[199]
NiFe MoS2	67	201	1.49	[200]
Oxygenated‐CoS2–MoS2	97	272	1.6	[201]
Sv‐Co3S4/MoS2	156	209	1.67	[202]
Se‐CoS2@MoS2/CNTs	85	240	1.56	[203]
Co–FeS/MoS2	63	η 100 : 230	1.45	[204]
MoS2/Co9S8/Ni3S2	103	228	1.45	[205]
CoO/N–MoS2	78	η 50 : 458	1.5 @ 53 mA/cm2	[206]
CoSAs–MoS2/TiN	131.9	340.6	1.65	[197]
Ir/MoS2	44	330	1.57	[207]
Co‐MoS2	297	312	1.86	[209]
La0.6Sr0.4CoO3/MoS2	248	192	148	[210]

9. The Role of MoS² in ORR

In sustainable energy conversion and storage devices, the ORR represents a critical electrochemical process that significantly impacts device performance. The ORR involves a sluggish four‐electron (4e^‐) transfer mechanism, which creates a substantial kinetic barrier and limits the overall efficiency of energy devices such as fuel cells and metal‐air batteries [211]. Pristine MoS₂ exhibits inherently poor catalytic activity toward ORR, similar to OER, due to its weak binding to the ORR intermediates. Furthermore, the semiconducting nature of its commonly occurring 2H phase, limited active edge sites, and poor electrical conductivity collectively hinder its electrocatalytic performance [165]. Consequently, various modification strategies have been developed to overcome these limitations and enhance ORR activity.

Ling et al. reports the 1T metallic phase of MoS₂ contributes more effectively toward ORR due to its superior electronic conductivity and modified surface properties. Their first‐principles studies revealed that 1T‐MoS₂ can effectively stabilise single transition metal atoms through its electrophilic properties, creating highly active single‐atom catalytic sites that substantially improve ORR performance [212]. Experimental investigations have demonstrated that MoS₂ can be modulated to enhance its ORR activity. The Co and Nb dual‐doped MoS₂ nanosheets shelled micro‐TiO₂ hollow spheres with a TiO₂ shell also exhibited ORR with an impressive onset potential of 0.96 V while maintaining a direct 4e^‐ transfer pathway. The enhanced performance originates from the synergistic effects between the core–shell architecture and the dual metal doping, which optimise both electronic structure and mass transport properties [189]. Transition metal incorporation with MoS₂ offers a promising approach to address the material's intrinsically poor electrical conductivity, a critical limitation that significantly hinders its electrocatalytic ORR activity. Solomon et al. report MoS₂ decorated with Ag NPs, the Ag‐MoS₂ catalyst operates through a mixed 4 and 2 + 2 serial route reduction mechanism, where O₂ molecules are initially reduced to H₂O₂ intermediates within the diffusion‐limited region. Notably, the Ag‐MoS₂ catalyst exhibits around 2.5 times faster conversion rates from HO₂ ^‐ to OH^‐ compared to pristine MoS₂, with Ag serving as an effective co‐catalyst that accelerates peroxide reduction kinetics and enhances overall chemical‐to‐electrical energy conversion efficiency [213].

The incorporation of carbon derivatives with MoS₂ offers significant advantages in terms of synthesis convenience, active site dispersion and electron transfer enhancement, collectively improving ORR activity. According to Huang et al. MoS₂ integrated with N, S co‐doped porous carbon exhibited a half‐wave potential of 0.82 V through a direct 4e^‐ transfer mechanism, with nitrogen incorporation into the MoS₂ crystal lattice identified as the key factor responsible for the improved ORR catalytic performance [214]. Additionally, Zhang et al. demonstrated that the MoS₂/Vulcan XC‐72R composite as a catalyst for ORR achieves onset potentials of 0.78 V in 0.1 M KOH and 0.92 V in 0.1 M HClO₄ electrolytes [215]. In addition, the straightforward dispersion of MoS₂ on Ketjenblack carbon was found to favour ORR performance with an onset potential of 0.83 V, coupled with high durability and excellent methanol tolerance [216].

Advanced composite architectures have demonstrated remarkable improvements in both catalyst stability and performance. Sun et al. reported S‐doped rGO aerogels that facilitate 1T/2H hybrid phase MoS₂ formation, creating effective catalyst supports that significantly enhance Pt nanoparticle durability. The developed Pt/MoS₂@S‐rGO catalyst, with low Pt nanoparticles mass loading, exhibited excellent ORR activity with a half‐wave potential of 0.90 V versus RHE in 0.1 M HClO₄ [217]. Similarly, Ramakrishnan et al. reported Pt nanoparticles stabilised by MoS₂/N‐doped rGO architectures (Pt@MoS₂/NrGO) that demonstrate bifunctional catalytic performance. The catalyst exhibited alcohol oxidation activity at the anode while displaying ORR performance at the cathode with a half‐wave potential of 0.895 V versus RHE, highlighting the versatility of MoS₂‐carbon composite supports in fuel cell applications [218]. A vacancy‐engineering strategy was employed to successively anchor atomically dispersed Pt and Pd at S vacancies in hierarchical porous 2H–MoS₂, forming stable Pt–S–Mo and Pd–S–Mo coordination sites as represented in Figure 8a. This controlled dual‐metal single‐atom configuration optimises the local electronic structure while preserving the individual catalytic characteristics of Pt and Pd. As a result, Pd, Pt–MoS₂ exhibits superior ORR activity, long‐term stability, and resistance to common poisons under neutral conditions, outperforming commercial Pt/C. The catalyst further demonstrates excellent performance in the microbial fuel cell setup shown in Figure 8b, delivering a power density of 1398.6 ± 43.5 mW m⁻² and a coulombic efficiency of 22.5 ± 0.5% [219].

FIGURE 8.

(a) Schematic illustration of the synthetic procedure for engineering Pt–S–Mo and Pd–S–Mo sites in hierarchical porous MoS₂. (b) Schematic representation of a microbial fuel cell used for ORR performance evaluation. Reproduced with permission from [219]. Copyright 2024 Elsevier. (c) HOR polarisation curves of FeMo₂S₄–T samples (where T denotes different hydrothermal temperatures) recorded in H₂‐saturated 1 M KOH at a rotation speed of 1600 rpm. (d) Corresponding Tafel plots. (e) Linear current–potential region near the equilibrium potential. (f) Comparison of the exchange current densities of FeMo₂S₄–T samples. Reproduced with permission from [220]. Copyright 2022 American Chemical Society.

While the engineering of the MoS₂ catalyst is important, the choice of electrode substrate also plays an important role in its ORR activity. Samuel et al. demonstrate the ORR performance of 2D‐MoS₂ in 0.1 M H₂SO₄ across various substrates, including boron doped diamond, edge plane pyrolytic graphite (EPPG), glassy carbon (GC) and screen‐printed electrodes (SPE) with optimised 2D‐MoS₂ coverages. Significant ORR improvements were achieved through careful selection of underlying carbon substrates, with onset potentials reduced to ca. +0.10 V for EPPG, GC and SPE at optimal 2D‐MoS₂ loading (1524 ng/cm²) [221]. A comparison of MoS₂ based catalysts reported for ORR has been summarised in Table 4.

TABLE 4.

Comparative table for ORR performance of recently reported catalysts.

Catalyst	Onset potential,  V	Half‐wave potential, V	Tafel slope,  mV/dec	Ref
NiFe2O4/MoS2	0.82	—	—	[50]
MoS2@ NSC	0.93	0.82	87	[214]
MoS2@S‐rGO	0.9	—	80	[217]
Pt@MoS2/N‐rGO	—	0.89	55	[218]
Pd, Pt–MoS2	0.24 (VAg/AgCl)	− 0.01 (VAg/AgCl)	72.4	[219]
Mo–N/C@MoS2	0.9	0.81	—	[222]
MoS2@Fe‐N‐C	—	0.84	84	[223]
Ni/MoS2‐rGO	0.87	0.74	—	[224]
1T/2H‐MoS2	0.85	—	57	[225]

While MoS₂‐based materials show promise for ORR applications, it is important to note that they typically require the presence of co‐catalysts or substantial modification through doping and composite formation to achieve competitive ORR catalytic activity. These modified MoS₂ systems function primarily as catalyst supports that enhance the dispersion and utilisation of active sites rather than as standalone catalysts. Nevertheless, their potential applications in metal‐air batteries and other energy storage devices warrant continued investigation, particularly in developing cost‐effective alternatives to traditional Pt‐basedcatalysts.

10. The Role of MoS² in HOR

The HOR is a critical anodic process in fuel cells that converts hydrogen to electrical energy. While platinum exhibits exceptional HOR efficiency, its scarcity and high cost necessitate developing alternative catalysts with comparable performance. This challenge is particularly acute in alkaline fuel cells, where sluggish HOR kinetics severely limit device performance, making non‐precious metal catalyst development essential.

The HOR includes two half‐reactions in a basic electrolyte, and the mechanism of HOR was added in the general mechanism part explained above. Mo‐based electrocatalysts offer distinct advantages for HORs, including high chemical stability and excellent corrosion resistance [226]. Despite these favourable properties, MoS₂‐based catalysts have received limited attention for HOR applications [13]. Recent advances have demonstrated the potential of MoS₂‐based materials through strategic metal incorporation and structural engineering. Theoretical investigations using DFT have provided crucial insights into the enhancement mechanisms of MoS₂‐based HOR catalysts. Studies of 3d transition metal atoms anchored at the edge of MoS₂ nanosheets (M_SA‐MoS₂, where M_SA = Fe, Co, and Ni) reveal that tetra‐coordinated M_SA and penta‐coordinated Mo atoms at the MoS₂ edge form synergistic dual‐active sites for H* and OH* adsorption. The interaction between M_SA and MoS₂ efficiently optimises the adsorption free energy of H* (ΔG_H*) and OH* (ΔG_OH*) by tuning the d‐band centre of the active sites. Among the investigated catalysts, Ni_SA‐MoS₂ exhibits the highest theoretical HOR activity due to optimal ΔG _H* and ΔG _OH* values, with strong formation energy and high oxidation potential confirming its thermodynamic and electrochemical stability [227]. In HOR, Mo‐based electrocatalysts have the advantages of high chemical stability and corrosion resistance [226]. Tian et al. demonstrated that bio‐inspired FeMo₂S₄ microspheres achieved superior HOR performance with high mass activity (1.85 mA/mg), excellent CO tolerance, and current density of 2.05 mA/cm² at 50 mV overpotential, significantly outperforming both FeS₂ (0.53 mA/cm²) and pristine MoS₂ (0.87 mA/cm²) under identical conditions. Figure 8c‐f compares the polarisation curves, Tafel plots and j _0, of FeMo₂S₄ and its counterparts prepared at different temperatures, establishing that targeted compositional modification can dramatically enhance MoS₂‐based catalyst performance [220]. DFT provides a more detailed explanation, suggesting that 3d metal atoms anchored on the edge sites of MoS₂ during the rate‐determining steps create dual active sites. Transition metal atoms like M_sa (Co, Fe, and Ni) and meta‐Mo atoms on M_SA‐MoS₂ can avoid the competitive H* and OH* adsorption, along with synergistically optimising the adsorption strength of H* and OH*, thus enhancing the HOR performance. Several studies have also reported that doping a single 3d metal atom into the edge of MoS₂ is an effective strategy in designing efficient catalysts for HOR in alkaline conditions [13, 220, 227, 228].

11. Challenges and Future Outlook

MoS₂‐based catalysts offer several intrinsic advantages for electrochemical energy conversion, including earth abundance, chemical stability and a layered structure that enables extensive tunability through phase engineering, defect modulation, and heterostructure formation. These attributes have enabled MoS₂ to exhibit activity across multiple water‐related electrochemical reactions. However, despite this versatility, there are several fundamental challenges that hinder the advancement of MoS₂ as electrocatalysts.

A primary limitation is the difficulty of producing phase‐pure 1T‐MoS2. Most reported synthetic strategies yield mixed 1T/2H polymorphs, despite frequent claims of exclusive 1T formation. This intrinsic phase ambiguity, together with the metastable nature of the 1T polymorph and the susceptibility of both phases to transformation under aqueous electrochemical conditions, raises concerns regarding reproducibility and long‐term stability [229, 230]. As discussed in Section 6, mixed phase 1T/2H MoS₂ can achieve HER activity comparable to either pure 2H or 1T, with performance strongly dependent on the phase ratio. Therefore, it is recommended that reports carefully quantify the phase composition when evaluating catalytic activity [122, 124]. Under OER conditions, MoS₂ can undergo oxidation and surface reconstruction, complicating mechanistic interpretation and raising questions about the true nature of the active phase (see Section 7) [187]. Beyond structural transformation, catalyst degradation can also proceed through chemical dissolution pathways that are not captured by short‐term electrochemical measurements alone. The dissolution of Mo and S species can be strongly allotrope‐dependent and may occur not only under HER‐relevant potentials but also under open‐circuit conditions. These observations indicate that, in addition to monitoring structural and phase transformations, quantifying the dissolution behaviour of MoS₂ during operation is essential for a comprehensive assessment of catalyst stability [231].

Addressing these challenges demands a multifaceted approach centred on rational materials design and rigorous characterisation. Strategies such as interface engineering, composite formation, defect modulation, heteroatom doping, and deliberate nanostructuring have proven effective in improving electrical conductivity, enhancing durability, and increasing the density of catalytically accessible sites [232, 233]. When appropriately implemented, these approaches not only mitigate intrinsic limitations but also impart multifunctionality, enabling MoS₂‐based catalysts to operate across both oxidative and reductive electrochemical reactions.

Future research should place greater emphasis on standardised reporting practices, including comprehensive pre‐ and post‐catalytic characterisation, systematic stability assessments under relevant operating conditions, and transparent documentation of synthetic reproducibility across multiple batches. The broader adoption of operando techniques, including in situ Raman spectroscopy and X‐ray absorption fine structure analysis, will be critical for elucidating surface reconstruction processes and guiding the design of more robust catalyst architectures. The inclusion of electrochemical cell‐coupled techniques for the analysis of the dissolution of the MoS₂‐based catalysts is crucial for designing catalysts that are durable and will be subsequently suitable for scaling up reactors. The incorporation of electrochemical cell–coupled techniques for monitoring catalyst dissolution in real time is essential for quantitatively evaluating the chemical stability of MoS₂‐based catalysts under operating conditions. Such measurements enable direct correlation between electrochemical performance and material loss, providing insight into degradation pathways beyond structural transformation. This information is critical for the rational design of durable catalysts and assessing their suitability for scale‐up in practical electrochemical reactor systems. In addition, integration of high‐throughput computation and machine‐learning‐assisted screening may accelerate the identification of optimal dopants, interfaces, and supports prior to experimental validation.

As the field matures, emphasis must shift from laboratory‐scale demonstrations to engineering‐relevant evaluations that address mass transport limitations, mechanical durability under gas‐evolving conditions, and economic feasibility at industrial current densities. Through the combined advancement of fundamental understanding and practical engineering considerations, MoS₂‐based catalysts can progress from promising laboratory materials toward viable components of sustainable energy conversion technologies.

12. Conclusion

Overall, the electrocatalytic performance of MoS₂ across OER, HER, ORR, and HOR is governed by interconnected structure–activity relationships that depend on phase composition, electronic conductivity, and dynamic surface evolution under operating conditions. While HER and HOR are largely associated with hydrogen adsorption at S edge sites and defect‐rich regions, ORR and OER involve oxygenated intermediates whose formation is influenced by surface reconstruction and partial oxidation of MoS₂. These reactions therefore, share common physicochemical descriptors rather than occurring independently. From a catalyst design perspective, achieving multifunctional activity requires a balanced optimisation of phase composition, defect density, and interfacial charge transfer. Strategies such as controlled 1T/2H phase modulation, heteroatom incorporation, defect engineering, and interface formation with conductive or redox‐active components provide effective routes to regulate intermediate binding energies while maintaining sufficient electrical conductivity and stability. Importantly, the dynamic nature of MoS₂ under electrochemical operation must be considered, as surface reconstruction can alter the identity of the active sites during catalysis. These insights suggest that MoS₂‐based materials should be designed as adaptive catalytic platforms rather than static phases. Integrating rational materials engineering with operando characterisation and stability evaluation will be essential for advancing MoS₂ toward multifunctional electrocatalysts capable of operating across both oxidative and reductive electrochemical reactions.

Funding

This work was supported by UGC‐DAE (Grant UGC‐DAE‐CSR‐KC/CRS/19/RC08/0485); SERB: CRG (Grant CRG/2021/001506); UGC (Grant SPARC Grant Nos. P930, P1400, P1429, P1460); DST Nanomission (Grant SR/NM/NS‐1420‐2014(C), SR/NM/NS‐54/2009); DST: Fund for Improvement of S&T Infrastructure (Grant FIST Grant No. SR/FST/P SI‐143/2009); Board of Research in Nuclear Sciences (Grant BRNS Grant No. 39/29/2015‐BRNS/39009); DST Promotion of University Research and Scientific Excellence (Grant PURSE Grant No. SR/S9/Z‐23/2010/22(C, G)); CEFIPRA (Grant Project 6408‐1); Rashtriya Uchchatar Shiksha Abhiyan; Mahatma Gandhi University; Council for Higher Education of Israel, Knowledge Centre in Energy and Renewable Fuels; Quantum materials for applications in sustainable technologies (QM4ST) (Grant CZ.02.01.01/00/22_008/0004572).

Conflicts of Interest

The authors declare no conflicts of interest.

Biographies

Merin Mary Sebastian is currently pursuing her Ph.D. at the School of Pure and Applied Physics, Mahatma Gandhi University, Kerala, India. She served as a visiting student under the Sandwich Programme (2021–22) at Ariel University, Israel. Since 2020, she has been affiliated with the School of Pure and Applied Physics under the supervision of Prof. (Dr.) Nandakumar Kalarikkal. Her research interests include two‐dimensional (2D) materials, water splitting, and fuel cell technologies.

Zhoveta Yhobu is a postdoctoral fellow at Ariel University, Israel. He received his Ph.D. from the Centre for Nano and Material Science, JAIN University, Bangalore, where his research focused on designing molecular systems and developing functional materials, including metal disulphides and oxides for water splitting reactions and electrochemical sensing. His current work centres on developing sustainable electrochemical systems for advanced energy conversion applications.

Alex Schechter is a full professor at Ariel University, Israel, and Head of the Renewable Energy Centre at the New Technology Centre, University of West Bohemia, Czech Republic. His research spans electrocatalysis for water splitting reactions, hydrogen evolution, and advanced catalytic materials, alongside alternative fuels such as ammonia and innovative energy storage systems. Prof. Schechter has published extensively on electrochemical energy conversion and sustainable technologies, bridging fundamental science with practical applications to advance global renewable energy solutions and decarbonisation strategies.

Nandakumar Kalarikkal is a materials physicist with 30 years of experience in teaching and multidisciplinary research. He is currently serving as Director of the International Centre for Ultrafast Studies and Professor of Eminence at Mahatma Gandhi University, India. A fulbright fellow and former senior professor at MGU, he has published over 350 papers, edited more than 45 books, supervised 25 Ph.D. scholars, and received major international and national grants along with prestigious recognitions while holding significant administrative roles.