A review on the cooperative effect of intimate interfacial TMD/MXene (2D/2D) heterostructures for an enhanced electrocatalytic hydrogen evolution reaction

Abstract

The future energy system strongly depends on hydrogen (H₂) energy as a viable choice owing to its high energy density and environmentally benign nature. Electrocatalytic water splitting is a promising method to produce H₂, and hence, research on developing economical and efficient electrocatalysts for hydrogen generation has increased. Over the past few decades, two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as effective hydrogen evolution reaction (HER) catalysts due to their tunable electronic properties and large surface area. Recently, 2D MXene-based electrocatalysts have attracted much attention due to their distinct properties, such as high conductivity and stability. The construction of 2D/2D heterostructures using TMDs and MXenes can further boost the hydrogen evolution reaction (HER) by increasing the electrochemically active surface area, thereby inducing accelerated reaction kinetics and stability. The present review initially summarizes the important performance parameters for HER processes, followed by an extensive review of the current approaches to enhance the catalytic efficiency of TMD/MXene (2D/2D) heterostructures. In summary, this review provides insights into the synthesis of 2D/2D heterostructures, their electrochemical hydrogen evolution reaction (HER) activity, and the underlying mechanisms responsible for their enhanced catalytic performance. Finally, it highlights the remaining challenges and potential paths for developing TMD/MXene electrocatalysts.

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1. Introduction

The world's energy consumption has been rising substantially each year due to modern living standards and population growth.¹ According to the International Energy Agency (IEA) assessment, there will be a 30% increase in the world energy demand by 2040, and the annual carbon dioxide (CO₂) emissions might reach 35.7 Gt.² At present, carbon-based fuels consume approximately 90% of the world's energy supply, which are the primary source of releasing harmful greenhouse gases such as carbon dioxide (CO₂), causing global warming and various environmental-related problems.^3,4 Due to the scarcity of fossil fuel reserves and increasing CO₂ emissions, it is necessary to produce at least 10 TW of green energy by 2050, utilizing sources such as the ocean, solar energy, hydroelectricity, wind, biomass, and sea waves.⁵ Developing renewable and green energy sources from Earth's abundant components is crucial to fix the present energy and environmental problems.^6,7

Hydrogen, the lightest element, has a higher energy density (140 MJ kg⁻¹) than fossil fuel (∼50 MJ kg⁻¹) and produces non-toxic and environmentally benign water as a product when used as fuel in fuel cells. Hence, H₂ is considered one of the most efficient sustainable energy source that do not emit harmful gasses.^8,9 There are several methods for producing hydrogen, such as hydrolyzing, reforming, and breaking down fossil fuels. Every year, over 4 billion tons of hydrogen is generated by the hydrolysis of fossil fuels, which leads to CO₂ emissions. Towards the production of hydrogen, water-splitting devices and regenerative fuel cells (RFCs) have a lot of potential, contributing to the “hydrogen economy”.¹⁰ Moreover, 99.9% pure hydrogen produced by water electrolysis is used as a reactant in a variety of industrial processes.¹¹ Due to the lack of widely accessible, reasonably priced renewable energy systems, only four percent of hydrogen is currently produced by electrolysis using renewable energies.¹²

The production of “green hydrogen” via photocatalytic and electrocatalytic water splitting is believed to reach the global goal of net-zero carbon emissions by 2050 and reduce our reliance on fossil fuels. Electrocatalytic water splitting is a simple and the most efficient method for the production of H₂. Water electrolysis describes the breaking of water into hydrogen and oxygen in the presence of an electric field.^13–15 There are two major electrochemical reactions that take place during water electrolysis, which are HER and the oxygen evolution reaction (OER) (Fig. 1a). However, a suitable catalyst is needed to overcome the energy barrier required for water electrolysis due to its sluggish reaction kinetics. Hence, finding a suitable catalyst to create a cleaner, more environmentally friendly H₂ energy is pivotal.¹⁶

Fig. 1 (a) Water electrolysis. Reprinted with permission from ref. 17. Copyright {2020}, the American Chemical Society. (b) Properties of MXenes.

Noble metal electrocatalysts such as platinum and ruthenium/iridium-based materials are currently demonstrated as state-of-the-art catalysts for hydrogen production via electrochemical HER due to their high efficiency. However, their cost and scarcity limit their commercial utilization, making it necessary to find inexpensive, earth-abundant, suitable alternative electrocatalysts for HER that produce hydrogen at a minimal overpotential.¹⁸ An ideal electrocatalyst should exhibit a small overpotential for hydrogen evolution and long-term stability without a change in the potential value, and further also should not release hazardous products during electrolysis. It should also have good adherence to the substrate, free from impurities that lower the electrode sensitivity, safe from a sustainability viewpoint, and easy and inexpensive synthesis for the scale-up process.¹⁹ A significant amount of research has been dedicated to developing effective Pt-free electrocatalysts for HER, which include metals/alloys,^20–22 single atoms,^23,24 functional carbon materials,^25,26 various synergistic hybrids,^27–29 and metal phosphides,^30–35 carbides,^36–41 nitrides,^42–46 oxides,^47–50 and borides.^51–55

Two-dimensional material-based electrocatalysts including graphene and transition metal dichalcogenides (TMDs) have attracted much attention owing to their layered structure.^56–59 TMDs have the general formula MX₂, where M is a transition metal and X is tellurium (Te), selenium (Se), or sulfur (S). The structure of TMDs consists of X–M–X layers, each layer is composed of a metal atom layer sandwiched between two chalcogenide layers, and each layer is connected with an adjacent layer by weak Van der Waals interlamination forces. Another emerging group of two-dimensional material-based electrocatalysts is MXenes, which includes transition metal carbides, nitrides, and carbonitrides.^60,61 MXenes possess several advantages, including excellent flexibility, laminar structure (Fig. 1b), outstanding metallic conductivity (up to ∼15 100 S cm⁻¹), high surface hydrophilicity, and tunable surface chemistry.⁶² The electrical conductivity of MXenes can be precisely tuned by engineering their terminal groups, with –OH and –F terminations offering higher conductivity for energy conversion and storage applications.⁶³ Both TMDs and MXenes have distinct advantages. In particular, TMDs such as MoS₂, WS₂, and related sulfides have been extensively explored for H₂ production due to their natural abundance, environmental friendliness, low cost, and ease of preparation. TMDs offer significant electrical conductivity (though generally lower than that of pristine MXenes), high theoretical capacitance, and rich active sites distributed along their basal planes and edges. Their large surface area and high ionic conductivity further facilitate efficient charge transfer during HER. However, the intrinsic conductivity of many TMDs is still moderate compared to MXenes, which can be improved by defect engineering, phase transition (e.g., 2H to 1T phase), or hybridization strategies, thereby enhancing their HER activity.⁶⁴ Constructing 2D/2D heterostructures is a new gateway for enhancing the properties of individual electrocatalysts, which has several advantages, as follows: (i) improving the conductivity, (ii) intimate contact at the interface, (iii) enhancement in the number of active sites and surface area, (iv) synergistic effect in both enhancement and stability of individual components, and (v) reduction of Gibbs free energy for adsorption of atoms on the catalytic surface. Hence, combining MXenes and TMDs can be synergistically integrated to increase their active surface area and induce superior conductivity, thereby leading to an enhanced HER catalytic performance. Fig. 2a shows the trend in recent annual publications and citations of TMD/MXene-based electrocatalysts for HER, which indicates the considerable importance given to this research field.

Fig. 2 (a) Publication and citation trends related to TMD/MXene-based electrocatalysts for the HER from 2019 to 2024 (based on data retrieved from Google Scholar-accessed April 2025 with the keywords “MXene, TMDs, HER”) and (b) timeline highlighting key developments in academic research on TMD/MXene-based electrocatalysts for the HER from 2019 to recent advances. The inserted figures are reprinted with permission from ref. 65, copyright 2019, the American Chemical Society; ref. 66, copyright 2020, the American Chemical Society; ref. 67, copyright 2021, the American Chemical Society; ref. 68 copyright 2022, Elsevier; ref. 69, copyright 2023, Elsevier; ref. 70, copyright 2024, the American Chemical Society; ref. 71, copyright 2025, the American Chemical Society.

In recent years, several reviews have been published based on MXenes, TMDs and their heterostructures, focusing on energy conversion^72,73 and energy storage applications.^74–77 Heteroatom-doped MXenes as electrocatalysts for HER were reported by Abdul Hanan et al.,⁷⁸ and Jiaqi Wang et al. reviewed 2D/2D MXene-based heterostructures for electrocatalytic water splitting.⁷⁹ Zhongshui Li et al. elaborated the current research progress and challenges in the phase engineering of 2D-based nanomaterials such as MoS₂, non-MoS₂, and hybrid TMDs for HER.⁸⁰ Another interesting review by Jing Mei et al. reported a comprehensive summary on iron-based sulfides and their composites for OER and HER activity.⁸¹ Recently, Liwei Hou et al. reviewed the recent advances in MXene-based nanohybrids for electrochemical water splitting.⁸² However, to the best of our knowledge, there are no reports available presenting an in-depth discussion of the interfacial charge transfer mechanism of 2D–2D TMD/MXenes for the electrocatalytic hydrogen evolution reaction. A systematic literature and detailed description of TMD/MXene heterostructures especially, MXene heterostructures with layered TMDs such as MoS₂, WS₂, VS₂, MoSe₂, VSe₂, and WSe₂, and nonlayered TMDs such as Ni₃S₂, NiS₂, CoS₂, NiSe₂, and FeSe₂ are elaborated. This review provides insights into the current achievement of TMD/MXene heterostructures by the research community towards the synthesis, choice of TMDs and MXenes for effective charge transfer processes and other parameters to enhance the HER activity of TMD/MXene heterostructures. Initially, the present review covers a brief overview of the HER process covering kinetic parameters for evaluating catalytic performance, activity descriptors, and reaction processes. The current advancements in TMD/MXene-based electrocatalysts for the HER process are systematically analysed and the different approaches to improve their electrocatalytic HER performance (Fig. 2b) are highlighted. Also, the importance of theoretical simulations and in situ characterization of TMD/MXene heterostructures is concisely presented. A summary of the latest advancements in MXene and transition metal sulfide/selenide heterostructure electrocatalysts is provided towards the end. A brief discussion on the disadvantages, advantages, and future perspectives of TMD/MXene-based electrocatalysts for HER is presented in the last part of this review.

2. Hydrogen evolution reaction (HER) mechanism

Electrocatalytic HER involves three main steps. The first step is the Volmer reaction, in which a proton (H⁺)/water molecule and an electron interact to generate adsorbed hydrogen (H*) on the catalytic surface (eqn (1) and (4)) in the presence of an external applied current in acidic/alkaline media (Fig. 3). The next step is the generation of hydrogen (H₂) by either the Heyrovsky (eqn (2) and (5)) or Tafel step (eqn (3) and (6)). In the Heyrovsky step, adsorbed hydrogen combines with H⁺/water molecule to produce H₂, while the Tafel reaction involves the reaction of two adjacent adsorbed hydrogen (H*) to produce H₂.⁸³

Fig. 3 HER mechanism in acid (left) and alkaline (right) media. Reprinted with permission from ref. 17. Copyright {2020}, the American Chemical Society.

HER in acid medium

H⁺ + e⁻ ⇌ H* (Volmer step) (1)

H* + H⁺ + e⁻ ⇌ H₂ (Heyrovsky step) (2)

H* + H* ⇌ H₂ (Tafel step) (3)

HER in alkaline medium

H₂O + e⁻ ⇌ H* (Volmer step) (4)

H* + H₂O + e⁻ ⇌ H₂ (Heyrovsky step) (5)

H* + H* ⇌ H₂ (Tafel step) (6)

3. Assessment of HER performance of an electrocatalyst

Several kinetic factors such as overpotential (η), Tafel slope (b), exchange current density (j₀), turnover frequency (TOF), faradaic efficiency, and long-term stability are used to assess the HER performance of an electrocatalyst.

3.1 Overpotential (η)

Overpotential (η) is one of the most important parameters considered to evaluate the HER performance of a catalyst. The HER process requires a greater potential than the standard Nernst potential (i.e., 0 V, NHE) due to its high activation energy barrier. Overpotential is defined as the difference between the Nernst potential and the experimentally observed potential of an electrocatalyst.⁸⁴ Typically, three unique overpotential values, η₁, η₁₀, and η₁₀₀, are evaluated at the current densities of 1 mA cm⁻², 10 mA cm⁻², and 100 mA cm⁻², respectively. A common definition of η₁ is the onset overpotential, where the HER begins, and η₁₀ is always utilized to compare the catalytic activity. A smaller overpotential (η₁₀) value of a catalyst suggests its higher HER activity. For practical applications, η₁₀₀ is another important parameter,⁸⁴ where a lower η₁₀₀ indicates better catalytic efficiency and durability under high-current operating conditions, factors that are essential for scalable HER applications. Linear sweep voltammetry (LSV) is used to determine the overpotential experimentally and the Nernst equation (eqn (7)) is used to calculate the overpotential, as follows:where E⁰ is the standard electrode potential of the reference electrode, R is the universal gas constant, F is the Faraday constant, the number of coulombs per mole of electrons, T is the temperature (298 K), and n is the number of moles of electrons transferred in the cell reaction or half-reaction (n = 2).⁸⁵

3.2 Tafel slope (b)

The Tafel slope is the second crucial indicator of HER activity, which indicates the rate of proton and hydrogen atom adsorption and desorption. The empirical formula (eqn (8)) can be expressed as follows:

η = a + b × log(j₀) (8)

where j₀ is the current density, a is a constant parameter, b is the Tafel slope, and η is the overpotential.⁸⁶ The Tafel slope is obtained from the linear fitting portion of the plot of log |j| vs. η. The Tafel slope is an important parameter that provides insight into the HER mechanism and used to predict the rate-determining step.^87,88 The Tafel slope values of 120 mV dec⁻¹, 40 mV dec⁻¹, and 30 mV dec⁻¹ correspond to the Volmer step, Heyrovsky step, and Tafel step as the rate-determining step,⁸⁹ respectively.

3.3 Stability

The stability of an electrocatalyst is another crucial factor for its real-time practical application. The stability of a catalyst is measured by LSV, followed by cyclic voltammetry, and chronoamperometry/chronopotentiometry analysis. A negligible change in the overpotential value after several thousands (e.g. 10 000) of cycles represents excellent stability. Similarly, chronoamperometry/chronopotentiometry is performed under a constant current/potential with respect to time (at least 10 h) with a minimal change in the current density/potential representing the excellent stability of the catalyst.^90,91

3.4 Electrochemical impedance spectroscopy (EIS)

Electrochemical impedance spectroscopy (EIS) is a method for determining the charge transfer resistance (R_ct) in an electrochemical reaction.⁹² The diameter of the Nyquist plot is used to estimate the value of R_ct. A lower charge transfer resistance (R_ct) value for HER indicates a quicker reaction rate, which lowers the overpotential and increases the activity toward HER.⁹³

3.5 Turnover frequency (TOF)

The turnover frequency is one of the suitable quantitative metrics for comparing the intrinsic catalytic activity of a catalyst.⁹⁴ The turnover frequency (TOF) for HER is defined as the number of moles of H₂ evolved per active site per unit time.

The formula for TOF (s⁻¹) is given (eqn (9)) as follows:

where A is the area of the working electrode, n is the number of moles of active materials, F is the Faraday constant, and j is the exchange current density.⁹⁵

The TOF is mostly impacted by the physical and chemical properties of the catalyst surface, which establish factors such as the binding energy of the reactive intermediates and the activation energy.⁹⁶

3.6 Electrochemical active surface area (ECSA) and double layer capacitance (C_dl)

The electrochemical active surface area is crucial for evaluating the intrinsic catalytic activity of a prepared electrode. The determination of ECSA by calculating the double-layer capacitance (C_dl) is a widely applied method, which is identified by performing cyclic voltammetry at a non-faradaic potential.⁹⁷ The ECSA value can calculated using the following formula (eqn (10)):⁹⁸

The double layer capacitance (C_dl) is calculated using the following formula (eqn (11)):

C_dl = (slope_anodic/slope_cathodic)/2 (11)

The anodic and cathodic current densities measured at the middle potential from the CV scans are linearly fitted with the CV scan rates to obtain slope_anodic and slope_cathodic, respectively, where (C_s) is the specific capacitance, which is generally considered 40 μF cm⁻².⁹⁹ The higher the C_dl, the higher the ECSA, and thereby the greater HER efficiency of the catalyst.

3.7 Exchange current density (j₀), faradaic efficiency and normalization of LSV

The exchange current density is defined as the current density measured at the equilibrium potential. The inherent nature of a catalyst is studied with the help of the exchange current density, which is taken as the point at which the linear part of the Tafel slope intersects the X-axis. The performance of a catalyst is assessed by another parameter called faradaic efficiency, which is obtained by the ratio of experimental H₂ production to the theoretical amount. The experimental amount is calculated by monitoring the evolved H₂ with the help of gas chromatography in a specified galvanostatic/potentiostatic electrolysis. The theoretical amount is predicted under the same galvanostatic/potentiostatic electrolysis conditions.

The overpotential value at 10 mA cm⁻² and the Tafel slope is directly dependent on the catalyst loading amount. To obtain a more reliable measure of specific activity, normalizing the current using the electrochemical active surface area (ECSA) and turnover frequency (TOF) is the most appropriate evaluation method, given that the overpotential value is independent of the catalyst loading. LSV normalized by the ECSA is highly helpful in determining the intrinsic HER activity.

3.8 Volcano plots

According to the Sabatier principle, the catalyst and the reactive intermediate (H⁺) should interact appropriately.¹⁰⁰ If the interaction is too strong, the products of the reaction will fail to dissociate and block the active sites for further adsorption, which prevents hydrogen evolution. Conversely, a weaker interaction causes the adsorption of very few intermediates on the catalyst surface, which slows down the reaction. Measuring the basic physical chemistry parameter Gibbs free energy for atomic hydrogen adsorption, ΔG_H*, allows the evaluation of both H* adsorption and hydrogen desorption.¹⁰¹

By plotting the measured j₀ value and the theoretical ΔG_H*, Parsons established the volcano-type trend for the HER. Later, S. Trasatti (Fig. 4) correlated the j₀ value and the theoretical ΔG_H* for various metal–hydrogen bond strengths, giving rise to a volcano-type plot to identify the efficiency of a catalyst for HER.¹⁰² It is clearly evident that ΔG_H* > 0 for weak hydrogen adsorption and ΔG_H* < 0 for too strong hydrogen adsorption. The ideal condition for an efficient HER performance by a catalyst is a ΔG_H* of zero.

Fig. 4 Volcano plot correlating the exchange current density (j₀) for the HER with the Gibbs free energy of hydrogen adsorption (ΔG_H*) on various catalysts. The peak of the plot (near ΔG_H* ∼ 0) represents optimal HER activity. Reprinted with permission from ref. 17. Copyright {2020}, the American Chemical Society.

4. MXenes

4.1 Structural properties of MXenes

MXenes are the latest member of the 2D material family derived from the selective etching of the parent MAX phase (M_n+1AX_n), which are comprised of transition metal nitrides, carbides or carbonitrides. In the general formula M_n+1AX_n, M is an early transition metal, A is a group IIIA or IVA element, and X is carbon or nitrogen. M_n+1X_n is formed by the selective etching of the A layer, with surface-functionalized –F, –O, and –OH as termination groups.¹⁰³ These surface functionalizations impart high hydrophilicity and electronic and ion transport properties. The crystal structure of MXenes is composed of M atoms arranged in a compact hexagonal lattice, with the X atoms at the interstitial sites of an octahedral geometry. MXenes can be categorized based on their stacking arrangement as follows, M₂XT_x structures have ABABAB stacking (HCP), while M₃X₂T_x and M₄X₃T_x structures have ABCABC stacking (FCC). The stability of MXenes depends on the stacking of the M layers and the position of the T_x atoms, which can be in the HCP, FCC, bridge, or top sites.^104–107 MXenes exist as multi-layered structures, where each layer is held together by Van der Waals forces, enabling the feasibility to exfoliate them into 2D nanosheets. Recent research has demonstrated the potential applications of MXenes in energy storage, energy conversion, and sensors owing^108–114 to their good electrical conductivity, tunable bandgap, and hydrophilic surfaces.^115–118 The number of known MXenes has surpassed 130, each with varying numbers of M layers, X, and surface functional atoms. The number of M layers and their arrangement influence their electronic properties and catalytic activity, whereas their surface terminal groups play a key role in their stability and functionality.⁷⁸ MXenes are broadly classified into single transition metal MXenes (Ti₂CT_x, Zr₃C₂T_x, Nb₂CT_x, Nb₄C₃T_x, V₂CT_x, Ti₃CNT_x, Mo₂CT_x, and Ti₄N₃T_x), double transition metal MXenes (i.e., Mo₄VC₄T_x, Mo₄VC₄T_x, and Mo₂ScC₂T_x) and solid–solution MXenes (i.e., (Ti_0.5, Nb_0.5)₂CT_x, (Nb_0.8, Ti_0.2)₄C₃T_x, and (Nb_0.8, Zr_0.2)₄C₃T) (Fig. 5). Recent studies have highlighted the impact of the functional groups in MXenes on their properties, for instance,¹¹⁹ the stability of MXenes with different terminal groups has been ranked as follows: Ti₃C₂ < Ti₃C₂H₂ < Ti₃C₂(OH)₂ < Ti₃C₂F₂ < Ti₃C₂O₂. The high thermodynamic stability of oxygen-functionalized Ti₃C₂ suggests significant charge transfer from its interlayer to its surface.¹²⁰ Further, the importance of structural modulation and phase engineering for enhancing the catalytic activity of MXene-based catalysts have gained much interest. For example, in solar-driven HER, heterostructures such as Mo₂C-based MXenes,¹⁵ and TiC₂T_x/Pt@CdS exhibit enhanced light absorption and charge separation.^15,121,122 A bipolar visible-light-responsive photocatalytic fuel cell (PFC) system utilizing a Z-scheme with g-C₃N₄ carbon black/BiOBr and Ti₃C₂/MoS₂ Schottky heterojunctions achieved the high-efficiency degradation of antibiotics and dyes. Remarkably, this system maintained significant activity even under dark conditions, demonstrating dual-chamber synergy and enhanced charge separation.¹²³ Cellulose nanofiber/MXene/Ni chain (CMN) aerogels exhibited an excellent solar-driven seawater evaporation performance, achieving an evaporation rate of 1.85 kg m⁻² h⁻¹ and over 99.69% pollutant rejection. The synergy among the 3D CNF structure, MXene, and Ni chains enabled efficient photothermal conversion and stability under extreme pH conditions,¹²⁴ confirming their broader role in sustainable energy and environment. In particular, to realize electrocatalytic HER activity, strategies such as phase tuning in 2D materials,⁸⁰ lattice defect engineering in carbon nanomaterials,¹¹⁶ and converting Ti₃C₂ into 1D ultrathin nanowires¹²⁵ have resulted in improved H* adsorption and charge transport. Hollow core-multi-shell architectures such as MOF-derived Co₉S₈@MXene@Bi₂O₃ electrodes offer a large surface area and high conductivity.¹¹¹ Interfacial engineering, where the synergy between MXenes and other materials significantly boosted their HER performance, evidently resulted in excellent alkaline HER activity¹²⁶ in nitrogen-doped MXenes embedded with Ru nanoparticles/single atoms. CoP–Ti₃C₂T_x nanocomposites exhibited an excellent HER performance with a low overpotential of 135 mV and Tafel slope of 48 mV dec⁻¹ in acidic media. The synergy between the metallic MXene conductivity and catalytic activity of CoP enabled efficient charge transfer and long-term stability, confirming optimized hydrogen binding.³⁵ The –OH functional groups on alk-Ti₃C₂ enhance the adsorption of H₂O and BH₄⁻, activating dual reaction pathways for HER.¹²⁷ Alternatively, MXene/TiSe₂ ¹⁰⁹ and 1T-MoS₂/MXene¹¹⁰ hybrids showed enhanced conductivity. These developments underscore the value of rational nanostructure design for catalytic optimization.

Fig. 5 Schematic representation of structural diversity of MXenes showing single and multi-transition metal configurations (solid solution and ordered) with surface terminations (T_x). Reprinted with permission from ref. 128. Copyright {2024}, Elsevier.

4.2 Synthesis of MXenes

Choosing an appropriate precursor for M_n+1AX_n is crucial, as it directly influences the structural characteristics of the final MXene product. The type and stoichiometry of the MAX phase determine the elemental composition, layer thickness, and etching behavior. Additionally, the selection impacts the conductivity, stability, and surface functionalization of the resulting MXene. The synthesis of 2D MXene materials is broadly classified into three methods,¹²⁹ as shown in Fig. 6. They include the etching method, a top down approach, chemical vapour deposition method, a bottom up approach, and high-temperature solid state reaction, a direct synthesis method.

Fig. 6 Schematic representation of the different methods for the synthesis of MXenes.

4.2.1 HF etching method. The formation of MXenes requires the selective removal of the A element from the M_n+1AX_n phase by etching with HF.

M_n+1AX_n + 3HF = AF₃ + 3/2H₂ + M_n+1X_n (12)

M_n+1X_n + 2H₂O = M_n+1X_n(OH)₂ + H₂ (13)

M_n+1X_n + 2HF = M_n+1X_n(F)₂ + H₂ (14)

here, eqn (12) shows the removal of the ‘A’ atoms, and eqn (13) and (14) represent the surface functionalization with hydroxyl and fluorine, respectively.^123,130 After etching, the MXene obtained is often multilayered, which can limit its effectiveness in catalytic applications. The multilayer MXene is stepped down to a few or single layer through the typical exfoliation process. These single or few-layer MXenes have better interaction with other components in catalytic systems, thereby improving their overall activity.

4.2.2 Salt etching. Instead of HF etching, a mixture of LiF and HCl is used to etch the MAX phase. The use of the LiF–HCl system facilitates the pre-insertion of lithium ions (Li⁺), which helps expand the interlayer spacing and weakens the interlayer bonding. The LiF–HCl system is not only milder and safer than concentrated HF but also yields MXenes with improved structural integrity and dispersibility.^{35,121,124,131} Notably, this method has also been successfully extended to the synthesis of carbonitride-based MXenes. For example, Ti₃CN(OH)_x has been synthesized via the in situ etching of Ti₃AlCN using an LiF–HCl mixture. Furthermore, other mild etching agents such as NH₄HF₂ have also shown potential for the synthesis of MXenes under safer operating conditions.¹³²

4.2.3 Alkali, molten salt and Lewis acid etching methods. Etching can also be conducted in a basic medium, where hydroxyl ions replace the Al atoms within the MAX phase. The efficiency and quality of this method are highly dependent on both the concentration of the alkali and the temperature conditions. Moreover, this method requires high temperatures.¹³³ In certain MAX phases, especially those forming MXene nitrides, HF-based etching is ineffective due to the high bonding energy between their constituent atoms. In these cases, molten salt etching offers a better solution. This method involves etching of the MAX phase using fluoride salts, viz. LiF + NaF + KF, HCl + NaF, KF, LiF and NH₄F, under an inert argon atmosphere.^134,135 Furthermore, Lewis acid salts (ZnCl₂) are also used to etch the MAX phases via the ion exchange etching method. However, this needs highly controlled and rigorous synthesis conditions.¹³⁶

4.2.4 Electrochemical etching method. Electrochemical etching represents a promising, eco-friendly alternative to conventional chemical methods. This technique involves anodic etching, where the Al layer of the MAX phase is selectively dissolved in an electrolyte under an applied voltage using HCl and ammonium chloride combined with tetramethylammonium hydroxide (NH₄Cl + TMAOH). Electrochemical approaches offer advantages such as ambient reaction conditions, reduced chemical hazards, and the potential for large-area, few-layer MXene synthesis with precise control over the reaction time and product morphology. Electrochemical etching is considered a scalable and cost-effective route for MXene production.^137,138

4.2.5 Chemical vapor deposition (CVD) method. CVD is a bottom-up synthesis strategy that allows the direct growth of MXene nanosheets on suitable substrates. The resulting materials typically exhibit large lateral dimensions, high crystallinity, and minimal lattice defects. This method is especially valuable for studying the intrinsic physical and chemical properties of MXenes, which are often influenced by their structural quality. However, despite these advantages, the synthesis of monolayer MXenes using the CVD method remains challenging.^139,140

4.2.6 Direct synthesis of MXene. Wang et al.¹⁴⁰ reported the direct synthesis of DS-Ti₂CCl₂ through a high-temperature solid-state reaction involving Ti, graphite, and TiCl₄. The precursors in a 3 : 1.8 molar ratio of Ti to graphite with 1.1 molar equivalent TiCl₄ were sealed in a quartz ampoule and heated to 950 °C for 2 h. This method enables the direct formation of halogen-terminated MXenes without requiring an etching process. Also, it is scalable to multigram quantities and provides a fluoride-free approach for the synthesis of MXenes, offering a clean alternative for large-scale production. In recent years, a novel fluoride-free approach has been developed for synthesizing Mo₂C MXene, which involves hydrothermal pre-treatment of Mo₂Ga₂C in NH₃·H₂O to break its layered grains, followed by selective etching using cetyltrimethylammonium bromide (CTAB). CTAB is used directly as an etchant in MXene synthesis, offering a milder and environment-friendly route.¹¹⁷ A summary of key developments in the MXenes synthesis is given in Table 1.

Table 1 Key developments and highlights in the synthesis of MXenes

S. no.	Key development	Year	Synthesis conditions	Highlights	Reference
1	Hydrofluoride (HF) etching	2011	RT to 55 °C	Discovery of MXene	61
2	Intercalation & delamination	2013	80 °C	Separation of MXene layers using DMSO	141
3	Bifluoride salts etching (NH4F2) & (HCl + LiF)	2014	35–55 °C	Milder conditions, non-uniform O-based surface group	142 and 143
4	New organic intercalation agent	2015	—	Delamination using isopropylamine	144
5	Fluorine-containing molten salt etching (LiF + NaF + KF)	2016	550 °C	Few layered nanosheets by selectively eliminating the A element, first nitride-based MXenes	134
6	Electrochemical etching (HCl)	2017	RT	Milder conditions	135 and 138
	Fluoride salts etching (HCl + NaF, KF, LiF, NH4F)		30–60 °C
7	Electrochemical etching (NH4Cl + TMAOH), hydrothermal (NaBF4 + HCl)	2018	180 °C	HF-free synthesis	133, 137 and 145
	Hydrothermal alkali etching (NaOH)		270 °C
8	Lewis acid (ZnCl2)	2019	550 °C	Element replacement	136
9	Water-free etching	2021	—	Water-free method improves MXene stability	146 and 147
10	CVD	2023	950 °C	Direct synthesis method	140

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5. 2D transition metal dichalcogenides (TMDs)

5.1 Structure of 2D transition metal dichalcogenides

Transition metal dichalcogenides (TMDs) are typically denoted as MX₂ (M = transition metal from groups IVB–VIB, such as Mo, W; X = chalcogen elements: S, Se, Te). Structurally, each TMD monolayer adopts a three-atom “X–M–X” sandwich model. These layers are stacked with an interlayer spacing of ∼6–7 Å and are weakly bonded via Van der Waals forces, allowing easy exfoliation into two-dimensional nanosheets.¹⁴⁸ TMDs feature catalytically active basal planes and edges, along with excellent mechanical flexibility and strong chemical resilience, making them well-suited for use in flexible electronics and demanding chemical environments.^149–151 When integrated with MXenes or other conductive substrates,^152,153 they exhibit synergistic interactions, which enhance the charge mobility, facilitate interfacial electron transfer, and boost the overall catalytic efficiency, thereby expanding their application scope as versatile electrocatalyst composites.^154,155 The rich chemical diversity of TMDs (different M and X atoms) offers tunability in catalytic properties, which is crucial for optimizing their hydrogen evolution performance. TMDs can crystallize into several polymorphic phases, most notably 2H (hexagonal, trigonal prismatic coordination), 1T (octahedral coordination), and distorted 1T′ (monoclinic or distorted octahedral) structures. The 2H phase is generally semiconducting, limiting the intrinsic electrical conductivity, while the 1T and 1T′ phases are metallic, significantly improving the electron transport, which is beneficial for the HER kinetics.¹⁵⁶ Additionally, Ni- and Co-based chalcogenides exhibit good electrical conductivity, multiple redox states, and strong catalytic activity. Their ability to easily switch oxidation states (e.g., Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺) makes them effective for HER and OER, respectively. When combined with other materials, they show enhanced performance due to the improved charge transfer and active site exposure.

5.2 Synthesis of TMDs

Synthesis methods play a vital role in tuning the catalytic performance of two-dimensional (2D) TMDs. Generally, these methods can be categorized into top-down exfoliation techniques and bottom-up synthesis techniques (Fig. 7).

Fig. 7 Methods for the synthesis of 2D transition metal dichalcogenides (TMDs).

5.2.1 Top-down approaches. Mechanical exfoliation, liquid phase exfoliation, and intercalation are the most common top-down approaches for the synthesis of TMDs. Using mechanical cleavage methods, such as Scotch tape exfoliation, high-quality monolayer or few-layer TMD crystals can be produced.¹⁵⁷ These method yield ultraclean, defect-free flakes ideal for electronic applications, but are limited by their low scalability and poor control over the flake size and thickness. Direct exfoliation of bulk TMDs in solvents such as N-methyl-pyrrolidone (NMP) and dimethylformamide (DMF) through sonication or shear forces allows the scalable production of nanosheets. This method is simple, cost-effective, and solution-processable, although it suffers from low monolayer yield, small lateral size, and solvent toxicity concerns.^158,159 In the intercalation-based liquid exfoliation strategy, foreign ions or molecules are first intercalated between the TMD layers to expand the interlayer spacing, followed by gentle exfoliation. Lithium-ion (Li⁺) intercalation is common but can induce undesirable 2H to 1T phase transitions and basal-plane defects. Larger cations (e.g., tetraalkylammonium) can preserve the structure but lower the intercalation rates. This method improves the exfoliation yield and enables large-area monolayer production with low energetic cost.¹⁵⁶

5.2.2 Bottom-up approaches. Bottom-up methods provide catalytic active site-rich TMDs with distinct shapes, sizes and morphologies. Chemical vapor deposition growth allows the wafer-scale synthesis of TMD monolayers or few-layer films with high crystallinity and controlled thickness. Vaporized precursors and a sulfur source react and nucleate onto heated substrates under a controlled atmosphere. However, although it yields high-purity materials, it requires high temperatures and vacuum conditions.^160,161 Wet chemical methods such as hydrothermal, solvothermal and hot-injection processes offer simpler, scalable alternatives for producing 2D TMDs. These techniques allow the formation of defect-rich, catalytically active nanosheets, but with relatively lower crystallinity compared to CVD products. Particularly, hydrothermal synthesis facilitates doping and defect engineering by adjusting the reaction parameters.^162,163 Conversely, some advanced techniques such as molecular beam epitaxy (MBE) and atomic layer deposition (ALD) provide more precise control over the material quality and structure, though they may require high temperatures or specific growth conditions.

6. Analysis of TMD/MXene electrocatalysts for HER

6.1 Characterization methods

Several characterization techniques are employed to gain a comprehensive understanding of the structural, morphological, and surface chemical properties of TMD/MXene-based heterostructures. For instance, Chen et al. analyzed the X-ray powder diffraction (XRD) pattern of MoS₂–Ti₃C₂T_x, which showed a distinct (002) reflection at ∼7°, indicating that Ti₃C₂T_x is not damaged during the solvothermal reaction. Meanwhile, the presence of MoS₂ was difficult to detect by XRD due to its ultrafine size and low loading (Fig. 8a). In the case of MoS₂–F structures, the characteristic peaks corresponding to the (002), (100), and (110) planes of MoS₂ were identified (Fig. 8b) .¹⁶⁴ In another report the in situ SEM images reveal that Ti₃C₂ exhibited an accordion-like, compact morphology with expanded interlayer spacing after HF etching. Upon hydrothermal decoration, CuS nanoparticles densely populated the MXene surface, and MoS₂ nanosheets uniformly coated the CuS/MXene structures, greatly enhancing the available surface area and exposing abundant active sites of HER.¹⁶⁵ Similarly, in WS₂/Ti₃C₂T_x hybrids, pristine WS₂ maintained a nanosheet-like morphology, while Ti₃C₂T_x preserved an open multilayer structure. The hybrid samples showed intimate interfacial contact between WS₂ and Ti₃C₂T_x, promoting efficient electron transfer across the interface. This further verified the morphological features in the WS₂/Ti₃C₂T_x composites with closely stacked nanosheets of WS₂ and Ti₃C₂ supporting strong interfacial coupling. The elemental mapping is used to confirm the uniform distribution of elements throughout the hybrid structures.¹⁶⁶ In MoS₂/CuS/MXene electrocatalyst, high-resolution TEM (HRTEM) images revealed clear lattice fringes corresponding to the (111) plane of MXene (0.24 nm), the (002) plane of MoS₂ (0.305 nm), and the (102) plane of CuS (0.64 nm), respectively, evidencing the successful integration of these three components.¹⁶⁵ Additionally, in VSe₂/MXene heterostructures, HRTEM images displayed expanded (002) fringes of MXene (∼1.1 nm) and the (011) plane of VSe₂ (0.26 nm), which was further supported by the selected-area electron diffraction (SAED) patterns, indicating the coexistence of VSe₂, Ti₃C₂, and minor anatase TiO₂ phases.¹⁶⁷

Fig. 8 (a) XRD patterns of MoS₂-F, MoS₂-Ti₃C₂T_x-P, MoS₂ QDs/Ti₃C₂T_x, Ti₃C₂T_x-180 °C and Ti₃C₂T_x. (b) Enlarged XRD pattern of MoS₂–F. Reproduced with permission from ref. 164. Copyright 2023, Elsevier, Inc.

X-ray photoelectron spectroscopy studies provide insights into the surface chemistry and bonding environments. The successful incorporation of nitrogen in the N-doped MoS₂/Ti₃C₂T_x heterostructure was confirmed by the deconvoluted N 1s peaks corresponding to pyridinic N, pyrrolic N, and quaternary N, along with Mo–N bonds. The presence of nitrogen at the defect sites was shown to create additional catalytic active sites and enhance the conductivity. The Mo 3d spectra demonstrated multiple oxidation states (Mo⁴⁺, Mo⁶⁺, Mo³⁺, and Mo⁰), further confirming the formation of the heterostructure and surface oxidation. Additionally, the S 2p, and Ti 2p spectra revealed the chemical environment of sulfur and titanium, respectively, validating the chemical integrity of the heterostructure (Fig. 9). Ultraviolet photoelectron spectroscopy (UPS) measurements are employed to study the electronic structure modulation. The valence band maximum (E_V) of N-doped MoS₂/Ti₃C₂T_x shifted to a higher energy compared to that of pure N-doped MoS₂, indicating an upward shift in its d-band center relative to the Fermi level.⁶⁷

Fig. 9 High-resolution XPS spectra of (a) Mo 3p and N 1s of N-doped MoS₂/Ti₃C₂T_x heterostructures on an Ni foam, (b) Mo 3d of MoS₂/Ti₃C₂T_x heterostructures on Ni foam, and (c) Mo 3d of N-doped MoS₂ and N-doped MoS₂/Ti₃C₂T_x heterostructures on Ni foam. (d) C 1s, (e) S 2p, and (f) Ti 2p of N-doped MoS₂/Ti₃C₂T_x heterostructures on Ni foam. Reprinted with permission from ref. 67. Copyright (2021), the American Chemical Society.

6.2 Theoretical calculations

Density functional theory (DFT) calculations are used to further elucidate the superior HER performance of MXene/TMD-based catalysts. The Gibbs free energy of hydrogen adsorption (ΔG_H*) is widely adopted as the key descriptor for HER activity, with an ideal catalyst exhibiting ΔG_H* values close to zero. In the case of Ti₃C₂O₂/Ni₃S₂ heterostructures, DFT results revealed a ΔG_H* of −0.328 eV, which is notably close to that of Pt (−0.09 eV) (Fig. 10b), confirming their enhanced HER kinetics.⁶⁵ Similarly, the VS₂@V₂C hybrid demonstrated improved hydrogen adsorption behavior (ΔG_H* = −0.07 eV) due to strong interfacial electron transfer, as indicated by charge density difference analysis.¹⁶⁸ In the case of WS₂–Ti₃C₂F₂ hybrids (Fig. 10a), DFT calculations demonstrated that the heterostructure stabilized the active sites and achieved a ΔG_H* value of 0.13 eV.¹⁶⁹ Furthermore, computational models of WS₂@MXene/GO composites revealed that Ti–S covalent bonding and metallic conductivity from the Ti d-states facilitated charge redistribution, reducing ΔG_H* to around −0.46 eV, supporting their superior HER performance.⁶⁹ Additionally, DFT studies on Ru/1T-MoS₂ interfaces uncovered that electron transfer from Ru to MoS₂ (1.3 e⁻) significantly decreased. The hydrogen adsorption energy is around −0.37 eV, further boosting the HER kinetics.¹⁷⁰ These studies validate the critical role of electronic modulation, interfacial engineering, and synergistic charge transfer effects in achieving high-efficiency HER catalysis in TMD/MXene hybrid structures.

Fig. 10 Calculated free energy diagram for the HER, (a) WS₂/Ti₃C₂T_x. Reproduced from ref. 169. ACS Appl. Mater. Interfaces, 2024, licensed under CC BY 4.0. (b) Ti₃C₂T_x/Ni₃S₂. Reprinted with permission from ref. 65. Copyright (2019), the American Chemical Society.

7. Transition metal disulfide/MXene-based electrocatalysts

Theoretical studies and experimental results clearly evidence that the properties of MXenes such as large surface area, high conductivity, unique basal plane activity, restacking, formation of oxides and hydrophilicity make them predominant candidates for electrocatalytic water splitting. However, their very strong surface hydrogen adsorption hinder their HER performance, and hence research is focused on improving their HER performance and stability. Constructing a hybrid structure with a suitable cocatalyst/protective material is the most efficient strategy to enhance the HER performance and stability. Similarly, TMDs have been demonstrated as excellent HER catalysts owing to their high surface area and exposed active sites; however, their poor durability hinders their practical application. Considering this, recently MXenes have been coupled with 2D TMDs to enhance their HER performance and stability through the interface synergetic effect. Herein, we highlight some of the TMD/MXene hybrids and their HER performances in both acidic and alkaline media.

Panyong Kuang et al. reported a novel NiS₂/V₂CT_x composite for efficient electrocatalytic HER. V₂CT_x was obtained from the selective etching of V₂AlC powder, followed by the hydrothermal growth of the Ni(OH)₂/V₂CT_x composite. Later, the Ni(OH)₂/V₂CT_x composite was sulfurized using the chemical vapor deposition method. NiS₂/V₂CT_x composites with different proportions of V₂CT_x in wt% (2.6%, 13%, 26%, 52%) were prepared and evaluated for their HER performance. Combined experimental and theoretical studies on the electrochemical properties and structural properties of V₂CT_x suggest that V₂CT_x is the ideal candidate with the best HER activity due to its lower hydrogen adsorption-free energy. NiS₂/V₂CT_x (wt% = 13%) exhibited superior HER activity with an overpotential of 179 mV at 10 mA cm⁻² and Tafel slope value of 85 mV dec⁻¹ in 1 M KOH electrolyte. The construction of the NiS₂ and V₂CT_x composite enhanced the electron transfer, and a larger electrochemical active surface area and more active sites were evidenced from the Nyquist plots and turnover frequency analysis, respectively. Here, the NiS₂ nanoparticles covering V₂CT_x prevented the restacking of V-MXene, exposed additional active sites, and facilitated further electronic transfer, resulting in the enhanced HER performance. This work demonstrates the efficiency of V-MXene in electrocatalytic HER activity.¹⁷¹ Jiapeng Liu and coworkers prepared a hierarchical “nanoroll”-like MoS₂/Ti₃C₂T_x hybrid via liquid nitrogen-freezing and subsequent annealing. The HER performance of MoS₂/Ti₃C₂T_x was evaluated using a typical three-electrode system in 0.5 M H₂SO₄. MoS₂/Ti₃C₂T_x exhibited superior activity with a small onset overpotential of 30 mV, which is slightly higher than that of Pt/C electrocatalysts. Further, this catalyst showed an overpotential of 152 mV and Tafel value of 70 mV dec⁻¹ compared with MoS₂ and Ti₃C₂T_x, which need a higher overpotential. This enhancement in the HER activity is due to the smaller charge transfer resistance of MoS₂/Ti₃C₂T_x than pure MoS₂, as confirmed by electrochemical impedance spectroscopy (EIS) measurements. Further, the composite exhibited excellent stability, as evidenced by the negligible change in the overpotential values even after 3000 cycles. Hence, this composite construction played a crucial role in tuning the catalytic activity and improvement in durability.¹⁷² Hierarchical MoS₂/Ti₃C₂T_x (1%, 5%, 10%, and 20%) was fabricated by using a hydrothermal strategy, in which nanosheets of MoS₂ were grown vertically on planar Ti₃C₂T_x nanosheets to form a heterostructure.¹⁷³ In the TEM image (Fig. 11), the presence of densely packed MoS₂ nanosheets on Ti₃C₂T_x suggests that the obtained heterostructures have facile electron transfer, which is beneficial for improving the HER activity. Furthermore, the ripples and crumpled edges indicate the presence of ultrathin and insufficient crystallization-induced defects on the prepared hierarchical MoS₂/Ti₃C₂T_x heterostructure.

Fig. 11 TEM images (a and b), HRTEM image (c) and line profile analysis (d) of the MoS₂/Ti₃C₂ heterostructures. Reproduced with permission from ref. 173. Copyright 2023, Elsevier Inc.

These heterostructures exhibited excellent activity in HER compared with bare MoS₂ and Ti₃C₂T_x. MoS₂/Ti₃C₂T_x (5 wt%) showed superior HER activity with an overpotential of 280 mV at a current density of 10 mA cm⁻² and Tafel slope of 68 mV dec⁻¹ in acidic electrolyte. It is also noteworthy that the catalytic current density induced by these heterostructures is 6.2 times higher than that of the bare MoS₂, and they also exhibited improved stability in the long-term durability test. The enhanced HER activity of the heterostructure is well supported by the EIS analysis, where it shows a lower charge transfer resistance (14.74 ohms) than MoS₂ (187.3 ohms), and also an increase in the ECSA, which provides more active sites. The as-formed external porous networks and opened channels of the vertically grown MoS₂ on Ti₃C₂T_x are highly favorable for the adsorption and diffusion of the reactants. These results showcased the enhancement in HER activity with a suitable wt% loading of MXene with transition metal dichalcogenides and improvement in the stability of the TMD/MXene heterostructure.

Similar to the above-mentioned results, several research groups demonstrated the use of TMD/MXene heterostructures for efficient HER electrocatalysis. For instance, Junmei Liang et al. reported the synthesis of a Co-doped MoS₂ combined with Mo₂CT_x electrocatalyst and its HER activity in alkaline media (1 M KOH). The Co-MoS₂/Mo₂CT_x nanohybrids achieved an overpotential value of 112 mV at 10 mA cm⁻² and a Tafel slope value of 82 mV dec⁻¹ with excellent stability over 18 h in 1 M KOH. Their improved performance is attributed to the enhanced number of water dissociation sites and efficient electron conductivity. The as-formed Co-MoS₂/Mo₂CT_x heterostructure possesses excellent electron-transport property, functional group-terminated structure of MXene, increased active sites, and synergistically improved low charge transfer resistance, providing a strategy for the development of novel heterostructure electrocatalysts.¹⁷⁴ Another interesting work demonstrated the synthesis of an Mo₂CT_x/2H-MoS₂ nanohybrid through the oxidation of Mo₂CT_x MXene, followed by two-step in situ sulfidation. This study eliminates the introduction of additional Mo precursors, thereby avoiding the formation of MoO₃ and the in situ sulfidation of Mo₂CT_x MXenes provides 22-times reduced charge transfer resistance than a physical mixture of d-Mo₂CT_x + 2H-MoS₂. DFT studies reveal the cleavage of the topmost S atoms, which produces vacancy sites, and subsequently reduces the overpotential value. The structure of Mo₂CT_x MXenes is preserved during the in situ sulfidation, which is evidenced from the XRD and Raman analyses. The HRTEM images of the Mo₂CT_x MXenes show a flake-like structure and their EFTEM mapping suggests the existence of O atom due to the oxidation process. However, after the sulfidation process, the S atom density increased in the place of the previously dominant O species. The nanohybrids exhibit superior HER activity, with an overpotential 119 and 182 mV to reach the current density 10 and 100 mA cm⁻² in 0.5 M H₂SO₄, respectively. Crucially, the intimate interface formation between Mo₂CT_x and 2H-MoS₂ enhances the adhesion and prevents further oxidation, ensuring the stability of the catalyst at high current densities exceeding 450 mA cm⁻² with excellent durability. This enhanced electrocatalytic performance of the nanohybrid paves the way for the synthesis of other classes of TMD/MXene nanocomposites through in situ sulfidation.¹⁷⁵

A modified (MD) Ti₃C₂-supported MoS_x (MD-Ti₃C₂/MoS_x) 2D/2D structure nanohybrid electrocatalyst was prepared by a facile one-step gamma radiation strategy.⁶⁶ The MD-Ti₃C₂/MoS_x hybrid electrocatalysts showed the best performance, with a Tafel slope of 41 mV dec⁻¹ and overpotential (η) of 196 mV at 50 mA cm⁻² in 0.5 M H₂SO₄, which is 137-times higher than that of MoS_x. This improvement is attributed to the enhanced electron transfer, strong interfacial coupling between MXene and MoS_x, and increase in the number of electrochemically active sites. More importantly, the facile one-step gamma radiation strategy is an efficient way for producing uniform interfacial composites with rich active sites.

Jie Ren et al. reported a hybrid material composed of organ-like Mo₂C MXene with MoS₂ nanoflowers. At first, Mo₂CT_x was obtained from Mo₂Ga₂C and coupled with MoS₂ through the typical hydrothermal process. The SEM image obtained from the side view of the composite material shows MoS₂ nanoflowers at the interlayers of the organ-like Mo₂CT_x matrix. Further, the organ morphology prevents the agglomeration of the MoS₂ nanoflowers, which are distributed homogenously on the Mo₂CT_x matrix. The MoS₂@Mo₂CT_x nanohybrids demonstrate remarkably improved electrocatalytic performance than Mo₂Ga₂C, Mo₂CT_x, and MoS₂ with an overpotential of 176 mV and 197 mV and Tafel slope of 207 mV dec⁻¹ and 113 mV dec⁻¹ at 10 mA cm⁻² in alkaline medium (1 M KOH) and acidic medium (0.5 M H₂SO₄), respectively. This is attributed to the strong interface coupling observed in MoS₂@Mo₂CT_x, resulting in better conductivity and reduced charge transfer resistance. In addition, the organ-like Mo₂CT_x and MoS₂ nanoflower morphology provide abundant active sites for catalytic activity. DFT calculations suggested the enhanced intrinsic conductivity of MoS₂@Mo₂CT_x due to the considerable reduction in the hydrogen adsorption energy.¹⁷⁶

Another interesting work demonstrated few-layer, thick, high percentage (85%) 1T′-phase MoS₂ nanosheets on cation-modified Ti₃C₂T_x, MoS₂–Ti₃C₂T_x, with an excellent electrocatalytic performance. MoS₂–Ti₃C₂T_x exhibited a very low overpotential of 98 mV at 10 mA cm⁻² and a Tafel slope of 45 mV dec⁻¹ in 0.5 M H₂SO₄. The HER efficiency of the MoS₂–Ti₃C₂T_x composite originated from the accessible rich active sites of the few-layer, thick edge-terminated MoS₂ nanosheets and facile electron transfer in the 1T phase. Further, MoS₂ stabilizes Ti₃C₂T_x against spontaneous oxidation, ensuring long-term durability.¹⁷⁷ Similarly, Long Chen et al. prepared 1T phase-rich quasi 0D MoS₂ quantum dots on 2D Ti₃C₂T_x nanosheets using a one-step solvothermal process and a freeze-dried mixture of (NH₄)₂MoS₄ precursor for MoS₂ and 10 mL of Ti₃C₂T_x suspension.¹⁶⁴ The MoS₂ quantum dots/Ti₃C₂T_x are a few layers thick and rich in 1T MoS₂, which provide high conductivity and abundant available active sites. However, the mass loading percentage of MoS₂ is restricted to 10%, as beyond it, the deterioration of the catalytic activity of the composite was observed. MoS₂ QDs/Ti₃C₂T_x-10% exhibited the best HER activity with an overpotential value of 220 mV at 10 mA cm⁻² and a Tafel slope of 72 mV dec⁻¹. The higher conductivity and enriched active sites are evidenced by the EIS and TOF analysis, respectively, with a five-fold enhanced exchange current density compared to pristine MoS₂, revealing the importance of the optimum level of composite construction for enhanced catalytic performance.

Some other MoS₂/MXenes are also well documented, for instance, MoS₂ was coupled with Ti₃C₂ and carbon nanofibers (CNFs) to prepare a “plane-line” skeleton structure MoS₂/Ti₃C₂@CNF hybrid. This hybrid material showed an excellent HER catalytic performance with 142 mV@10 mA cm⁻² and Tafel slope of 113 mV dec⁻¹. The “plane-line” skeleton structure prevents flake stacking and enhances the electron transfer. CNFs act as a synergistic substrate to expand the layers of Ti₃C₂T_x, which facilitates the loading content of MoS₂.⁶⁸ Huiting Hu et al. prepared an accordion-like MoS₂/CuS/Ti₃C₂T_x, which exhibited an overpotential of 115 mV@10 mA cm⁻² and Tafel slope of 58.4 mV dec⁻¹ in 0.5 M H₂SO₄ (Fig. 12). This is attributed to the reduced charge transfer resistance and optimized active site exposure. MoS₂ exhibits poor electrical conductivity, whereas CuS conducts and transfers electrons more effectively. The synergistic interaction between these materials, combined with the conductive support provided by MXene, enhances the HER activity of the catalyst, while ensuring excellent stability.¹⁶⁵ These works offer a new pathway for the preparation of efficient MoS₂-based HER catalysts.

Fig. 12 (a) LSV curves of MoS₂/CuS/MXene, MoS₂/MXene, CuS/MXene, and MXene normalized by ECSA. (b) Schematic of the possible electrocatalysis of the MoS₂/CuS/MXene electrocatalyst for the HER. Reprinted with permission.¹⁶⁵ Copyright {2023}, the American Chemical Society.

WS₂ is another important and widely used TMD catalyst owing to its unique electronic properties, exceptional catalytic activity, and excellent stability. Tekalgne et al. reported enhanced HER activity in a WS₂/Ti₃C₂ MXene hybrid nanosheet prepared via a single-step hydrothermal method. The close packing of Ti₃C₂ and WS₂ nanosheets led to the formation of an intimate hybrid structure, which provides better charge transfer between MXene and WS₂. The 10% Ti₃C₂-loaded composite exhibited the best catalytic performance with an overpotential of 150 mV and Tafel slope of 62 mV dec⁻¹ in 0.5 M H₂SO₄. This enhancement in HER activity is attributed to the large surface area provided by the nanosheets, and the rapid charge transfer between MXene and WS₂ through the intimate hybrid structure. Further, the composite also showed outstanding stability during the long-term durability test.¹⁶⁶ Recently, a porous WS₂-embedded Ti₃C₂T_x/GO nanocomposite with excellent HER performance in both acidic and alkaline electrolytes was reported. The layered architecture of GO facilitates the insertion and decoration of WS₂ and MXene during the synthesis, which increases the surface area of the resulting composite (Fig. 13a). Further, the porous-structured grains of the composite promoted rapid transfer and diffusion of ions during electrocatalysis. The WS₂@Ti₃C₂T_x/GO composites exhibited low overpotential values of 42 and 45 mV@10 mA cm⁻² (Fig. 13b and c) and small Tafel slope values of 43 and 58 mV dec⁻¹ in acidic and alkaline medium (Fig. 13d and e), respectively. The exemplary HER activity of the WS₂@Ti₃C₂T_x/GO nanocomposites is attributed to the prevention of oxidation of MXenes, making them defect rich, providing abundant accessible active sites, and lowering the Gibbs free energy for hydrogen adsorption through the interfacial synergistic effects between WS₂ and Ti₃C₂T_x/GO.⁶⁹ More recently, a 2D–2D interface was created by WS₂ nanopetals on Ti₃C₂T_x MXene via the solvothermal method. The 5% WS₂–Ti₃C₂T_x nanocomposite demonstrated remarkable HER activity with a low overpotential value of 66 mV@10 mA cm⁻² and low Tafel slope value of 46.7 mV dec⁻¹ in 1 M KOH.¹⁶⁹ WS₂ nanopetals were grown at the interlayer space of MXene, forming a wrinkled nanosheet composite interface. The 2D–2D interaction between WS₂ and MXene enhanced the electrochemical active area, thereby providing more active sites and reduced overpotential barriers. Further, efficient charge transfer was observed in the composite due to the intimate interfacial contact of WS₂ and MXene. Guozheng Li et al. reported the synthesis of a triple-interface Ru@1T-MoS₂/MXene composite for electrocatalytic HER activity. The formed triple-interface composite exhibited a superior catalytic performance to its individual counterparts such as Ru/MoS₂, Ru/MXene, MoS₂/MXene, and Ru nanoparticles. Ru@1T-MoS₂-MXene exhibited a superior performance with overpotentials of 44, 42, and 106 mV@10 mA cm⁻² and Tafel slope values of 47 mV dec⁻¹, 38 mV dec⁻¹ and 64 mV dec⁻¹ in acidic, alkaline, and neutral media, respectively. The synergic effect between Ru@MoS₂, MXene, and MoS₂/MXene in the formed triple interface composite, the enhanced conductivity at the interface, excellent oxidation resistivity of MXene from the covered MoS₂ sheets and low Gibbs free energy of hydrogen adsorption resulted in the superior activity of the catalyst. These results suggest that the optimization of the activity of HER catalysts can be done through multi-interface integration (Ru/MoS₂, Ru/MXene, and MoS₂/MXene) on Ti₃C₂T_x, achieving high stability and activity in acidic conditions, which is also validated by comprehensive electrochemical and theoretical analyses.¹⁷⁰

Fig. 13 (a) Synthesis of WS₂@MXene/GO hybrid composites. (b) Polarization (LSV) curves of MXenes, GO, WS₂, WS₂@MXene and WS₂@MXene/GO at a sweep rate of 10 mV s⁻¹ in acidic and (c) alkaline media and (d) Tafel plots of MXenes, GO, WS₂, WS₂@MXene, and WS₂@MXene/GO in acidic and (e) alkaline media. Reproduced with permission from ref. 69. Copyright 2023, Elsevier Inc.

Another important but less-studied TMD is vanadium disulfide (VS₂), which suffers from a reduction in specific surface area during electrocatalysis due to bulk structure formation, decreased carrier mobility, and lower cross-sectional current density caused by size effects. Hence, it is of great interest to improve the HER activity of VS₂ through heterostructure construction by coupling it with MXenes. In this regard, Zhenguo Wang et al. constructed a novel 2D VS₂@V₂C through the uniform growth of T-VS₂ on V₂C MXene using a one-step hydrothermal process and evaluated its HER activity. The composite material showed an overpotential value and Tafel slope of 164 mV and 47.6 mV dec⁻¹ in an alkaline medium (1 M KOH) and 138 mV and 37.9 mV dec⁻¹ in an acidic medium (0.5 M H₂SO₄) at 20 mV cm⁻², respectively. The vertical growth of VS₂ on the surface of V₂C avoided the stacking of the V₂C matrix, thereby providing a high specific surface area in the composite material, which is beneficial for HER activity. The composite interface provided rapid charge transfer and excellent electrical conductivity, along with the enhanced transfer of adsorbed H₂ from V₂C to the VS₂ surface, resulting in the enhanced HER activity of VS₂@V₂C than pristine VS₂.¹⁶⁸ A recent study by Loh et al. reported a 2D/2D heterostructure electrocatalyst composed of phase-engineered 1T/2H MoS₂ and Ti₃C₂T_x MXene, which was synthesized via a DMF-assisted hydrothermal method, followed by ultrasonication and stirring. The optimized hybrid 1T/2H MoS₂ (25D)/Ti₃C₂T_x-1 (MTC-1) prepared using 25% of DMF and 1 wt% Ti₃C₂T_x exhibited high HER activity with an overpotential value and Tafel slope of 280 mV and 83.8 mV dec⁻¹ and 300 mV and 117.2 mV dec⁻¹ in acidic and alkaline media, respectively. Further, excellent long-term stability was achieved for the composite. The enhanced performance was attributed to the synergistic interaction between the conductive Ti₃C₂T_x layers and the active basal and edge sites of 1T/2H MoS₂, which improved the charge transport and increased the surface area. This prevented restacking, demonstrating its promise as a pH-universal electrocatalyst.¹⁷⁸ Some other TMD/MXene composites and their HER performances are summarized in Table 2.

Table 2 Electrocatalytic HER activity of TMD/MXene composites

S. no.	Catalyst	Synthesis method	Electrolyte	Over potential (10 mA cm^−2) (mV)	Tafel slope (mV dec^−1)	R ct (Ω)	C dl (mF cm^−2)	Advantages/limitations	Ref.
MoS 2 /MXene composites
1	MoS2/Ti3C2Tx	HF etching, ultrasonication	0.5 M H2SO4	152	70	—	—	Low onset potential, excellent stability over 3000 cycles	172
2	MoS2/Ti3C2Tx	HF etching, hydrothermal method	0.5 M H2SO4	280	68	14.74	61.3	High Cdl	173
3	Co–MoS2/Mo2CTx	HF etching, ultrasonication	1 M KOH	112	82	50	53.7	Enhanced water dissociation	174
4	2H-MoS2/Mo2CTx	HF etching, two-step in situ sulfidation	0.5 M H2SO4	119	60	28	—	Industrial current tolerance (−450 mA cm^−2)	175
5	MD-Ti3C2/MoSx	HCl + LiF etching, gamma radiation	0.5 M H2SO4	165, 196 (50 mA cm^−2)	41	—	29	Low Tafel slope	66
6	MoS2@Mo2CTx	HF etching, hydrothermal method	1 M KOH	176	207	26	106	Abundant active sites, high Cdl	176
7	1T-MoS2-Ti3C2 MXene	HF etching, hydrothermal method	0.5 M H2SO4	98	45	—	26.0	High % of active 1T′ MoS2 phase, strong structural stability	177
8	N-Doped MoS2/Ti3C2Tx	HF etching, hydrothermal method	1 M KOH	80	100	1.6	80	High capacitance and low Rct/catalytic effectiveness depend on N-doping	67
9	MoS2 QDs/Ti3C2Tx (quantum dots)	HCl + LiF etching, ultrasonication, hydrothermal	0.5 M H2SO4	220	72	—	80.74	High Cdlvia QDs, ∼76% 1T phase improves conductivity; over time, quantum dots can cluster, reducing surface activity	164
10	MoS2/Ti3C2@CNFs (carbon nanofiber)	HCl + LiF etching, CVD	0.5 M H2SO4	142	113	—	36.4	Enhanced active sites: the CNF skeleton helps expose MoS2 edge site	68
11	MoS2/CuS/Ti3C2Tx	HF etching, hydrothermal method	0.5 M H2SO4	115	58.4	41.4	14.8	Edge-oriented MoS2 nanosheets create more active sites	165
12	ZnS-ZnO-MoS2/Ti3C2Tx MXene	NH4F + HCl etching, hydrothermal method	0.5 M H2SO4	327.6	79.5	—	7.13	Sharp edges and petal-like morphology on MoS2 NSs provide enhanced exposure to active sites	70
13	Ru@1T-MoS2-Ti3C2Tx	HF etching, two-step hydrothermal method	0.5 M H2SO4	44	47	1.8	–	Enhanced HER activity, improved stability (160 h, 5000 cycles in acid); cost of Ru a concern	170
			1 M KOH	42	38	2	26
			1 M PBS	106	64	4.3	—
WS 2 /MXene composites
14	WS2@Ti3C2Tx/GO graphene oxide	HF etching, hydrothermal method, sonication	0.5 M H2SO4	42	43	0.45	9.1	Dual-function electrode (supercapacitor, HER), excellent capacitance	69
			1 M KOH	45	58	1.9	34
15	WS2/Ti3C2	HF etching, hydrothermal method	0.5 M H2SO4	150	62	—	1.2	Enhanced hybrid interface	166
16	2D–2D WS2/Ti3C2Tx	HCl in PTFE + NH4F, ultrasonication, hydrothermal	1 M KOH	66	46.7	2.56	6.19	Intercalation of WS2 nanopetals into the Ti3C2Tx MXene layers increases the number of active sites	169
17	WS2 QDs/Ti3C2Tx	HCl + LiF etching, solvothermal method	0.5 M H2SO4	247	90	∼7.1	7.2	Synergistic 0D/2D interface, QD active sites	71
VS 2 /MXene composites
18	VS2@V2C	HF etching, hydrothermal method	0.5 M H2SO4	138	37.9	—	—	Improved charge transfer	168
			1 M KOH	164	47.6
Ni- and Co-based sulfide/MXene composites
19	NiS2/V2CTx	HF etching, hydrothermal method	1 M KOH	179	85	—	—	V-sites boost activity	171
20	Ti3C2Tx/Ni3S2/NF	HF etching, hydrothermal method	1 M KOH	72	45	17.67	43.4	Fast charge transfer	65
21	NiS/Mo2CTx	NH4F + HCl etching, solvothermal method	0.5 M H2SO4	157	77	49.5	12.4	Intercalated NiS clusters and 2D Mo2CTx layers expose a larger ECSA	179
22	Ti3C2Tx@NiS	HF etching, hydrothermal method	1 M KOH	173	157	—	56	Larger active surface area	180
	Ti3C2Tx@Co4S3			142	126	—	68
23	CoS2/Ti3C2Tx	(LiF + HCl) etching, hydrothermal method + sulfurization	0.1 M KOH	175	97	—	—	Low voltage splitting (1.63V), trifunctional activity(OER,ORR,HER/Requires multistep synthesis which can be time-consuming and not scalable	181
24	CoS2@Ti3C2Tx	HCl + LiF etching, wet chemical/coprecipitation method	1 M KOH	286 mV at 20 mA cm^−2	78	115	19.8	ECSA of 495 cm^2, CoS2 provides active catalytic sites	182
25	Cu2MoS4/Ti3C2Tx	HF etching, hydrothermal method	0.5 M H2SO4	317	96	208	—	Solvothermal synthesis is straightforward and cost-effective	183

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The summary of recent studies given in Table 2 highlights that etching techniques using HF or HCl/LiF, followed by hydrothermal or solvothermal synthesis are commonly employed to fabricate these hybrids. These methods have proven effective in constructing vertically aligned or intimate interfacial heterostructures that enhance the active site exposure and charge transport. The observed enhanced performance in the WS₂@Ti₃C₂T_x/GO composite underscores the role of multi-phase hybridization and conductive carbon support in promoting the electrochemical kinetics. Decorating TMD/MXene hybrids with noble metals is also another strategy for realizing an enhanced performance, as observed in Ru@1T-MoS₂–Ti₃C₂T_x with ultra-low overpotentials of 44 mV in 0.5 M H₂SO₄ and 42 mV in 1 M KOH, reflecting the synergy between the Ru and MoS₂ active sites. Similarly, the synergistic catalytic activity and efficient charge transfer across the MXene–sulfide interface observed in Ti₃C₂T_x/Ni₃S₂/NF resulted in a remarkably low overpotential in an alkaline medium (1 M KOH). The most sought after noble-metal-free systems, such as 1T-MoS₂/Ti₃C₂T_x and MoS₂/CuS/Ti₃C₂T_x, also showed competitive performances, suggesting that well-engineered heterostructures can emulate or even surpass the activity of conventional Pt-based catalysts. Nevertheless, some hybrids such as ZnS–ZnO–MoS₂/Ti₃C₂T_x and Cu₂MoS₄/MoS₂/Ti₃C₂T_x exhibit relatively higher overpotentials (above 300 mV), likely due to their suboptimal phase distribution or interfacial mismatches. These limitations highlight the importance of precise compositional control and synthesis reproducibility. Recent research also demonstrates the utility of quantum dot incorporation, heteroatom doping, and carbon nanofiber integration in enhancing the electrocatalytic behavior. For example, in the MoS₂ QD/Ti₃C₂T_x system, the quantum dots contribute to a higher catalytic site density and modified electronic structures.^116,184

8. Transition metal diselenide/MXene electrocatalysts

Similar to sulfides, selenides also have a great potential in electrocatalytic HER activity, as shown by Arulkumar et al. who synthesized an MoSe₂/Ti₃C₂T_x hybrid material via the hydrothermal method, which demonstrated outstanding HER activity and electrochemical performance due to the enhanced charge transfer and strong interfacial interactions between the MoSe₂ nanosheets and MXene. Compared to the unmodified MXene and MoSe₂, the hybrid exhibited a lower overpotential of 245.1 mV at 10 mA cm⁻² and a Tafel slope value of 37.7 mV dec⁻¹, indicating a Volmer–Heyrovsky mechanism. Electrochemical impedance spectroscopy (EIS) confirmed a low charge transfer resistance (R_ct of 68.1 Ω), suggesting improved reaction kinetics and high stability owing to the strong interfacial contact between MoSe₂ and MXene.¹⁸⁵ The 3D nanoflower structure of MoSe₂/Ti₃C₂ (2 : 1, 4 : 1, 20 : 1, and 50 : 1) hybrids fabricated by Huang et al. significantly boosted the HER activity, exposing numerous electrochemical active sites and facilitating change transport. The HER activity was found to be highly dependent on the MoSe₂/Ti₃C₂ ratio, with the catalytic performance improving up to the ratio of 10 : 1 (Mo₁₀/Ti), and then declining at higher ratios. It showed an onset potential of 61 mV, which is significantly lower than that of the MoSe₂ nanoflowers (∼200 mV) and Ti₃C₂ (∼380 mV) and a 6-fold increase in current density at 300 mV compared to MoSe₂ alone. Also, 2000 electrochemical cycles carried out in an acidic medium showed negligible changes, confirming the excellent long-term stability of the hybrid. The hybrid nanoflower exhibited a significantly reduced Tafel slope of 91 mV dec⁻¹, indicating improved charge transfer kinetics. Cyclic voltammetry (CV) was used to determine the C_dl, an indicator of the electrochemical active surface area (ECSA). The increase in C_dl from 33 μF cm⁻² (MoSe₂) to 44 μF cm⁻² (Mo₁₀/Ti) suggested the availability of more active sites for HER. EIS measurements demonstrated that the Mo₁₀/Ti hybrid had a significantly lower charge transfer resistance (16.6 Ω) with remarkable cycling stability.¹⁸⁶ In another work, Shao et al. reported the synthesis of 1T/2H-MoSe₂/Ti₃C₂ ¹⁸⁷ composites to optimize the phase tuning and charge transfer, synthesizing MoSe₂/Ti₃C₂ composites containing different Ti₃C₂ amounts. The optimized M/100 catalyst demonstrated an overpotential of 150 mV@10 mA cm⁻² and a Tafel slope of 90 mV dec⁻¹, significantly outperforming MoSe₂, and Ti₃C₂ facilitated H₂O dissociation in the Volmer step. The double-layer capacitance (C_dl) value of 66.2 mF cm⁻² revealed the presence of more active sites. This composite also displayed a higher turnover frequency (TOF), reaching 0.015 s⁻¹ at 100 mV and 0.127 s⁻¹ at 200 mV, outperforming the individual MoSe₂ and Ti₃C₂. EIS revealed that M/T₁₀₀ had a lower charge transfer resistance (R_ct) of 6.53 Ω compared to MoSe₂, demonstrating its improved conductivity and electron transfer. Stability tests showed negligible degradation even after 2000 cycles, and stable activity for over 30 h in alkaline medium. This shows that Ti₃C₂ is an effective conductive support that enhances the HER performance.

Similarly, Hussain et al. synthesized an Ti₃C₂/MoSe₂ hybrid structure via ultrasonication and a selenization method.¹⁸⁸ Its evaluation using iR-compensated LSV in 1 M KOH and 0.5 M H₂SO₄ solutions revealed overpotentials of 149 mV at 10 mA cm⁻² and 136 mV and Tafel slopes of 83 mV dec⁻¹ and 88 mV dec⁻¹, respectively, outperforming the its individual counterparts, which is due to its enhanced conductivity and active site exposure. Also, the hybrid exhibited a significantly reduced charge transfer resistance (2.98 Ω in acid, 2.08 Ω in base), signifying faster electron transport. Electrochemical stability tests carried out over 24 h showed minimal current loss and a high capacitive double layer value of 485 mF cm⁻² in acidic and 454 mF cm⁻² in alkaline media, confirming the presence of a large electrochemically active surface area. Overall, the hierarchical MXene/MoSe₂ enhances the active area and porosity, achieving excellent HER activity and long-term stability in both acidic and alkaline electrolytes. Chaudhary et al. fabricated VSe₂@G/Ti₃C₂T_x using hydrothermal and freeze-drying methods, forming a 3D sponge-like structure that prevents VSe₂ aggregation and enhances the electrocatalytic performance. The combination of VSe₂ with the Ti₃C₂T_x/rGO matrix offers a high surface area, conductivity, and robust meso/macro-pore channels for efficient mass transport and gas diffusion. It also has potential for efficient bifunctional water splitting. The polarization curves revealed that VSe₂@G/Ti₃C₂T_x exhibited the lowest onset potential (0.11 V) and an overpotential of 153 mV at 10 mA cm⁻², outperforming VSe₂ (261 mV) and VSe₂@G (202 mV). The Tafel slope analysis indicated a Volmer–Heyrovsky mechanism, with VSe₂@G/Ti₃C₂T_x showing the fastest kinetics (84 mV dec⁻¹). Turnover frequency (TOF) measurements further confirmed its superior catalytic activity, achieving 0.15 s⁻¹ at 0.3 V. The Nyquist plot demonstrated the lowest charge transfer resistance of 9.1 Ω, indicating efficient electron transport. Stability tests showed minimal performance degradation over 1000 cycles, while chronoamperometry studies confirmed its stable HER activity for 24 h.¹⁶⁷ An FeSe₂/Ti₃C₂T_x electrocatalyst with a 0D/2D heterostructure was synthesized via in situ, seed-induced growth with varying (20, 40, 60, 80 wt%) FeSe₂ loadings. Among them, the 40% FeSe₂-loaded Ti₃C₂T_x exhibited the lowest onset potential of 89 mV at a current density of 10 mA cm⁻², requiring an overpotential of only 199 mV and a Tafel slope of 78 mV dec⁻¹, outperforming the others. Its enhanced catalytic efficiency was attributed to its balanced FeSe₂/ratio, given that insufficient FeSe₂ resulted in fewer active sites, whereas excessive FeSe₂ led to aggregation, and thereby reduced activity. The uniform dispersion of FeSe₂ nanoparticles on ultrathin Ti₃C₂T_x resulted in strong electronic coupling, enhanced reactive sites and high conductivity. The double layer capacitance (C_dl) for this composition was measured to be 11.5 mF cm⁻², indicating a higher density of accessible catalytic active sites. Long-term electrochemical tests conducted on FeSe₂(40%)/Ti₃C₂T_x revealed an HER current over 18 h, demonstrating its excellent durability with minimal degradation even after 3000 LSV cycles, maintaining its onset potential and cathodic current. Electrochemical impedance spectroscopy studies revealed an R_ct value of 19.2 Ω for the FeSe₂(40%)/Ti₃C₂T_x electrode, which was slightly higher than that of Ti₃C₂T_x but substantially lower than that of FeSe₂ (178.9 Ω). This demonstrated that the Ti₃C₂T_x nanosheets significantly enhanced the electron transport, contributing to the overall HER efficiency.¹⁸⁹

Recently, Du et al. reported¹⁹⁰ a 3D bowl-shaped Ti_3−xC₂T_y (B-TCT) nanocavity, functionalized with boron and coupled with MoSe₂ nanoflakes, achieving an exceptional HER performance across acidic, alkaline, and neutral conditions. Its high efficiency is attributed to its heteroatom functionalization, self-adapting Ti vacancies, and spatial configuration, which synergistically enhanced its electronic structure and active sites. The B-TCT@MoSe₂ nanohybrid exhibited the best HER activity with a low overpotential of 49.6 at 10 mA cm⁻². The Tafel slope analysis indicated that B-TCT@MoSe₂ followed the Volmer–Heyrovsky mechanism in acidic medium with a value of 64.3 mV dec⁻¹. Stability tests confirmed its exceptional durability under all pH conditions, with minimal current loss after 6000 cycles. The ΔG_H* value of B-TCT@MoSe₂ is close to zero, which is significantly lower than that of pristine MXene and B-TCT, suggesting that the strong interfacial coupling facilitates efficient hydrogen adsorption and desorption.

9. Transition metal ditelluride/MXene electrocatalysts

In recent years, transition metal tellurides have also attained great interest in electrocatalytic HER application. Smal et al. fabricated 2D NiTe₂/Ti₃C₂T_x and CoTe₂/Ti₃C₂T_xvia the hydrothermal approach with overpotential values of 200 and 241 mV and Tafel slope values of 95 mV dec⁻¹ and 109 mV dec⁻¹ for CoTe₂/Ti₃C₂T_x and NiTe₂/Ti₃C₂T_x, respectively, indicating the Volmer–Heyrovsky pathway for HER. The lowest charge transfer resistance (R_ct) was observed for NiTe₂/MXene (4.02 Ω), while CoTe₂/MXene exhibited a commendable R_ct of 16.2 Ω, indicating efficient electron transport. Stability tests through chronoamperometry confirmed the robustness of CoTe₂/MXene over a continuous 9 h operation with minimal degradation. Additionally, the observed 24 mV reduction in the overpotential of the post-stability test suggested activation of its catalytic sites over prolonged use.¹⁹¹ Shinde et al. developed a 2D/2D MoTe₂/Ti₃C₂T_x hybrid electrocatalyst through a simple hydrothermal method, where semimetallic 1T′ MoTe₂ petal clusters were integrated onto conductive Ti₃C₂T_x MXene sheets. The hybrid exhibited a remarkable HER performance with a low overpotential of 293 mV and a Tafel slope of 65 mV dec⁻¹, suggesting a Volmer–Heyrovsky reaction pathway. Its enhanced activity is mainly attributed to the synergistic effects between MoTe₂ and Ti₃C₂T_x. The high electrical conductivity of Ti₃C₂T_x promoted faster electron transport, while MoTe₂ contributed abundant active sites due to its petal-like morphology. The strong interfacial contact between the two components reduced the charge transfer resistance to 14.7 Ω, improving the reaction kinetics. Additionally, this hybrid showed excellent durability over 12 h with minimal degradation. BET analysis confirmed a larger surface area, facilitating better electrolyte access. DFT calculations further revealed charge transfer from Ti₃C₂T_x to MoTe₂, enhancing the catalytic efficiency. These combined features make MoTe₂/Ti₃C₂T_x a promising non-precious HER catalyst.¹⁹²

Table 3 summarizes the various transition metal selenide/MXene composite electrocatalysts, which clearly evidenced that the construction of a heterostructure boosts the electrocatalytic HER performance of TMDs and MXenes. Notably, B-TCT/MoSe₂ shows outstanding catalytic activity with an extremely low overpotential of 49.9 mV in 0.5 M H₂SO₄, highlighting the impact of its unique bowl-shaped morphology. Similarly, MoSe₂/Ti₃C₂T_x in 0.5 M H₂SO₄ achieves an impressively low overpotential of 61 mV, showcasing how Se-based 2D materials synergize well with MXene sheets for improved charge transport and active site exposure. Across all the samples, the catalysts that underwent combined hydrothermal treatments and post-thermal annealing generally demonstrate enhanced conductivity and interface integrity. Overall, the performance of these heterostructures reinforces the idea that TMD/MXene combinations offer promising pathways toward designing low-cost, efficient electrocatalysts for HER, through precise control of their structure and surface modifications.

Table 3 Electrocatalytic HER activity of transition metal diselenide/MXene composites

S. no.	Catalyst	Synthesis method	Electrolyte	Overpotential (mV)	Tafel slope (mV dec^−1)	R ct (Ω)	C dl (mF cm^−2)	Advantages/limitations	Ref.
MoSe 2 /MXene composites
1	Ti3C2Tx/MoSe2	HF etching, hydrothermal method	1 M KOH	200	37.7	4.3	205.8	Excellent double layer conductivity/HF etching	185
2	MoSe2/Ti3C2Tx	HF etching, hydrothermal method	0.5 M H2SO4	61	91	16.6	44	Low overpotential, abundant active sites/HF etching	186
3	1T/2H-MoSe2/Ti3C2Tx	HCl + LiF etching hydrothermal method	1 M KOH	150	90	6.53	66.2	Polymorphic catalytic site diversity	187
4	Ti3C2Tx/MoSe2	HF etching, thermal treatment or annealing	0.5 M H2SO4	136	88	2.98	485	Excellent stability, abundant active site/HF etching	188
			1 M KOH	149	83	2.08	454
5	Bowl-shaped Ti3−xC2Ty (B-TCT)/MoSe2	Etching, hydrothermal	0.5 M H2SO4	49.9	64.3	—	16.5	Activation of basal planes through doping/Synthesis involves a multistep template-assisted method	190
			1 M KOH	52.7	77.4	—	6.3
			1 M PBS	67.8	82.3	—	5.1
VSe 2 /MXene composites
6	VSe2@G/Ti3C2Tx	HF etching, hydrothermal, freeze-dry	1 M KOH	153	84	9.1	—	3D porous structure/HF etching	167
Ni, Fe-Based selenides/MXene composites
7	Ti3C2Tx/NiSe2/rGO	HF etching, solvothermal	1 M KOH	97	89	7	19.8	Charge transfer enhancement by 2D rGO/HF etching	193
8	Ti3C2Tx/NiSe2	HCl + LiF etching one-pot hydrothermal method	0.5 M H2SO4	200	37.7	33.9	—	Strong interfacial bonding enhances conductivity and active site exposure	194
9	NiCoSe2/2D Ti3C2Tx	HF etching, hydrothermal method	0.5 M KOH	65	86	110.35	17.83	2D heterostructure enables fast charge transfer/HF etching	195
10	Ti3C2Tx/NiMoSe2	HF etching, hydrothermal method	0.5 M H2SO4	328	100	74	0.18	1D/2D hybrid enables more active edge sites/HF etching	196
11	FeSe2 (40%)/Ti3C2Tx	LiF/HCl etching, ultrasonication, hydrothermal method	0.5 M H2SO4	89	78	19.2	11.5	0D/2D interface ensures uniform FeSe2 dispersion and fast charge transport	189

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In short, MoS₂-based MXene 2D-2D composites offer an excellent electrocatalytic HER performance compared to other layered dichalcogenides. The catalytic activity of MoS₂/MXene is tuned via the following strategies: (i) stabilising the 1T metallic phase, offering better conductivity and catalytic edge activity than the more inert basal planes of 2H-MoS₂. (ii) Quantum dot (QD)-based hybrids, which have more exposed edge sites, and the further agglomeration of their particle provides long-term stability. (iii) Heteroatom doping and trace amount of noble metal doping improve the performance of MoS₂/MXene composites owing to the defect-induced electronic structural modulation, decreasing the hydrogen adsorption free energy (ΔG_H*), and increased conductivity. Similarly, among the non-MoS₂-based layered dichalcogenides, WS₂ typically exhibits stronger catalytic activity, especially in acidic media. For instance, WS₂/Ti₃C₂T_x, a 2D–2D interface, provided a large contact area and abundant interfacial active sites, enhanced electron mobility and hydrogen adsorption. Further the formation of conductive metallic 1T′ phases also improved the performance of WS₂-based HER electrocatalysts. In the case of non-layered systems, the Ni-based dichalcogenide/MXene composites show excellent HER performances due to their fast charge transfer, high conductivity of MXene and high catalytic activity of Ni-chalcogenides, which favor the water splitting kinetics in alkaline media. The non-layered/MXene structures often benefit from superior electrical pathways and structural rigidity, making them stable under high-current operations, and further these composites show excellent kinetics and stability in alkaline media. Apart from sulfide-based TMDs, selenide systems, for example MoSe₂/Ti₃C₂T_x and NiCoSe₂/2D Ti₃C₂T_x, also showed reasonable HER performances, driven by their high edge site density and efficient charge transport at the 2D/2D interfacial conductivity benefitting from their 2D heterostructure. It is noteworthy to mention the excellent performance of the B-TCT/MoSe₂ and NiCoSe₂/Ti₃C₂T_x composites, but still sulfide-based TMDs dominate in terms of HER and enhanced interfacial tuning opportunities. Furthermore, telluride-based dichalcogenides also show some improved performances with MXene composites by improving their low conductivity and intrinsic activity. However, compared to sulfide and selenide TMD, tellurides are less explored and need to be the focus in the future. In general, layered TMD/MXene composites outperform their non-layered counterparts and show comparable performances with noble metal catalysts in HER owing to their structural resemblance, compatibility with tunable electronic properties, large active surface area, and facile electron transfer through their 2D–2D interface. MXenes and TMDs are mostly prepared using the conventional HF etching and hydro/solvothermal method, respectively, and most TMDs/MXenes are prepared using the hydrothermal method, leading to unique morphologies having more exposes active sites. The cooperative effect between TMDs and MXenes is frequently cited as a key reason for their enhanced HER activity due to their structural compatibility to form a 2D–2D interface and the high conductivity of TMDs and MXenes. Some TMD/MXene composites are demonstrated as excellent HER catalysts due to this synergic effect. However, in-depth mechanistic studies on this synergy remain scarce, which hinder the construction of TMD/MXene heterostructures with remarkable HER performances to compete with noble metal catalysts. Detailed insights into their interfacial charge transfer, electronic coupling, and hydrogen adsorption dynamics must be elucidated through advanced in situ characterization techniques such as XPS, EPR, XAS, Raman and Mössbauer spectroscopy. Further, strong theoretical studies can validate the construction and function of TMD/MXene 2D–2D heterostructures to unravel the underlying mechanisms governing this cooperative enhancement (Fig. 14).

Fig. 14 Cooperative effect of TMD/MXene heterostructures.

10. Conclusion

In summary, the integration of TMDs with MXenes provides an efficient pathway for developing highly efficient electrocatalysts for HER. The excellent properties of MXenes, i.e., high electrical conductivity, large surface area, and stability, coupled with the versatile catalytic characteristics of TMDs enable significant improvements in their HER performance. The effective catalytic activity of TMD/MXene hybrids has been significantly improved, which is comparable with that of conventional noble metal catalysts, by recent developments in synthesis techniques and structural changes. Both TMDs and MXenes compensate for their individual drawbacks, while making use of their advantages, resulting in the overall high efficiency of the heterostructure/composite. However, despite this success, there are still several issues to be resolved, including cost-effectiveness, scalability, and long-term stability. Future studies need to improve the composite architectures and investigate novel combinations of MXenes and TMDs.

11. Future outlook

The enhanced inherent characteristics of MXenes over other two-dimensional electrocatalysts make their commercialization as HER electrocatalysts highly promising. However, further enhancement of their intrinsic HER activity is essential to completely realize their catalytic potential. Similarly, it is time to explore the structural engineering of TMDs and the hidden active sites in the intercalated interface of the heterostructure. The synthesis of novel MXenes, particularly nitride based, can be possible by employing solutions that avoid hazardous etchants such as hydrofluoric acid. Current synthesis procedures frequently result in oxidation and performance deterioration. Research on bottom-up synthesis techniques is crucial as they have the potential to provide MXenes with distinct electrical characteristics. Surface termination species such as halogens, S, B, P, and Se play an important role in the HER kinetics and need more research to fully comprehend their effects and make the most use of them (Fig. 15). Research towards these termination groups other than the widely utilized –O, –F, and –OH may lead to the discovery of more stable and effective HER electrocatalysts.

Fig. 15 Future outlook on TMDs/MXenes in the HER.

Defect engineering and hybrid structures can also be used to increase the electrocatalytic HER activity of MXenes. Combining MXenes with other active materials by doping and compositing techniques, such as TMDs, layered double hydroxides (LDH), and noble metals, has shown considerable promise. Defect engineering is another way to enhance their electrocatalytic capabilities. To comprehend the mechanisms behind their performance, in-depth kinetic investigations at their interface are required. Easy scale-up synthesis methods are needed for the large-scale preparation of TMD/MXene composites. Long-term stability evaluations are necessary to guarantee the commercialization and real-world use of MXene-based electrocatalysts towards hydrogen production for net zero emissions.

12. Challenges of MXenes and TMDs in electrochemical water splitting (EWS) for hydrogen production

MXenes and TMDs have shown significant promise in electrochemical water splitting (EWS) for hydrogen production. However, several key challenges limit their practical deployment. One of the major obstacles is the complexity and limited scalability of current synthesis techniques, which are often time-intensive and lack standardized procedures. Further, the use of hazardous chemicals and etchants in the preparation of these materials raises environmental and safety concerns, which must be addressed to ensure sustainable and eco-friendly hydrogen production. Chemical instability, particularly the tendency of these materials to oxidize or degrade in aggressive electrolytic environments, further hampers their long-term operational viability. Additionally, tailoring surface terminations to enhance catalytic activity remains a technical hurdle, as precise control over surface functionalities is difficult to achieve. The high cost of raw materials and precursors, especially those with high purity, also restricts their large-scale utilization. Moreover, the structural defects introduced during synthesis can adversely impact the electronic conductivity and the availability of active catalytic sites. Finally, the fabrication of electrodes is a significant bottleneck in the real-time water splitting application due to the weak catalyst substrate interactions, poor charge transfer efficiency, and limited mechanical stability under the operational conditions (Fig. 16).

Fig. 16 Challenges of TMDs/MXenes in electrochemical water splitting.

Conflicts of interest

There are no conflicts to declare.

Data availability

No primary research results, software or code has been included and no new data were generated or analysed as part of this review.

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