Recent Advances in Transition Metal Dichalcogenide-Based Electrodes for Asymmetric Supercapacitors

Abstract

The global transition toward renewable energy sources has intensified in response to escalating environmental challenges. Nevertheless, the inherent intermittency and instability of renewable energy necessitate the development of reliable energy storage technologies. Supercapacitors are particularly notable for their high specific capacitance, rapid charge and discharge capability, and exceptional cycling stability. Concurrently, the increasing demand for efficient and sustainable energy storage systems has stimulated interest in multifunctional electrode materials that integrate electrocatalytic activity with electrochemical energy storage. Two-dimensional transition metal dichalcogenides (TMDs), owing to their distinctive layered structures, large surface areas, phase state, energy band structure, and intrinsic electrocatalytic properties, have emerged as promising candidates to achieve dual functionality in electrocatalysis and electrochemical energy storage for asymmetric supercapacitors (ASCs). Specifically, their unique electronic properties and catalytic characteristics promote reversible Faradaic reactions and accelerate charge transfer kinetics, thus markedly enhancing charge storage efficiency and energy density. This review highlights recent advances in TMD-based multifunctional electrodes. It elucidates mechanistic correlations between intrinsic electronic properties and electrocatalytic reactions that influence charge storage processes, guiding the rational design of high-performance ASC systems.

1. Introduction

Fossil fuel consumption has resulted in substantial carbon emissions and resource depletion, thereby accelerating the global transition to renewable energy [1,2]. Nevertheless, the inherently intermittent and unstable characteristics of renewable energy sources present formidable challenges to the reliable and continuous supply of power [3,4]. This circumstance imposes heightened demands on energy storage devices, which are indispensable for ensuring stable energy output from renewable sources [5]. In particular, the widespread adoption of portable electronics, electric vehicles, and smart grids has further intensified the demand for advanced electrochemical energy storage systems. Among these, rechargeable batteries and supercapacitors play pivotal roles in alleviating resource scarcity [6]. Notably, supercapacitors excel in energy storage owing to their superior specific capacitance (Sp.C), high power density (P_d) enabled by rapid charge/discharge capability, and excellent cycling stability [7]. These attributes render supercapacitors particularly valuable for renewable energy storage applications [8]. However, their relatively low energy density (E_d) compared with rechargeable batteries remains a significant drawback. To address this limitation, ASCs have been introduced [9,10], integrating positive and negative electrodes with distinct reaction mechanisms. This configuration expands the voltage window and consequently enhances E_d [11]. Optimization of ASC devices has thus effectively mitigated the intrinsic limitations of conventional supercapacitors.

The quest for enhanced ASC performance has directed increasing attention toward electrode materials. TMDs were initially applied in areas such as lubrication and catalysis; however, their potential for energy storage has recently attracted intensive investigation due to their unique two-dimensional (2D) layered structures. These sheet-like architectures provide enlarged specific surface areas (SSAs) and expose abundant active edge sites that facilitate electrochemical reactions. Furthermore, TMDs exhibit reversible valence states during Faradaic reactions [12]. During electrochemical cycling, their layered structures enable reversible ion intercalation and diffusion, while their variable valence states promote reversible redox reactions [13]. This pseudocapacitive contribution markedly enhances both Sp.C and E_d. In addition, the excellent electrical conductivity of TMDs supports superior electrochemical performance in ASC systems. Their intrinsic electrocatalytic properties, stemming from high conductivity, abundant redox-active sites, and reversible valence states, play a crucial role in improving charge storage capability by facilitating interfacial charge transfer and ion diffusion and accelerating Faradaic kinetics [14]. Considerable attention has therefore been devoted to TMDs. Their edge-rich morphology and tunable valence states reduce the activation energy for redox reactions, accordingly lowering overpotentials and enhancing reversibility [15].

Significant research efforts have recently focused on modifying TMDs through strategies such as doping, heterostructure construction [16], and hybridization to overcome inherent limitations such as layer restacking [17] and restricted cycling stability. More importantly, these strategies can effectively regulate the energy band structure, phase state, and conductivity of TMDs, resulting in enhanced electrochemical performance [18]. Their intrinsic conductivity, particularly in the metallic 1T phase, enables high-rate capability and power density [19]. Furthermore, the ability of TMDs to undergo reversible Faradaic reactions, a property tied to their electronic phase and valence states [20], enables pseudocapacitance, which significantly enhances their energy density compared to materials that rely primarily on double-layer capacitance, such as carbon nanotubes (CNTs) [21]. Moreover, TMDs provide a versatile platform for the incorporation of redox-active or conductive components such as metal oxides [22], sulfides [23], and nitrides [24], as a result of further augmenting their catalytic behavior and energy storage performance. Such hybrid frameworks also mitigate structural degradation during long-term cycling, thus improving both capacitance retention and device durability [25,26]. While a wide array of materials, including metal oxides, conducting polymers, graphene [27], and MXenes [28], have been extensively explored as electrode candidates, the characteristics of TMDs demonstrate advantages in comparison with common electrode materials during the performance characterization of electrochemical devices. For instance, TMD-based ASCs demonstrate higher E_d than those employing transition metal oxides [29], while offering superior P_d and rate capability compared to ASC systems based on CNTs or conductive polymers [30].

While current reviews on the application of TMDs in supercapacitors often focus on material structure modification [26] and interface engineering [16,25], they lack a perspective on their electronic properties. To fill this gap, the review aims to establish a clear correlation between the electronic characteristics of TMDs and their electrochemical performance in supercapacitors. Therefore, the core innovations of this review are (1) to clarify the influence of the TMD electrode’s electronic properties, such as structural phase, band structure, and active sites, on specific capacitance and rate capability; (2) to comparatively analyze different TMD-based composite systems, including MoSe₂, WSe₂, MoS₂, and WS₂, and highlight how their structural and electronic properties lead to performance differences; and (3) to propose advanced strategies, such as phase engineering, for tuning the band structure and optimizing active sites, providing new insights and directions for the future development of TMD-based electrodes.

2. TMD Electrode for Supercapacitors and Performance Evaluation

Supercapacitors are advanced energy storage devices that store charge on the surfaces of two closely spaced electrodes separated by an insulating medium [6]. Their operation hinges on two primary mechanisms, including electric double-layer capacitance and pseudocapacitance, and they are divided into three categories: electrical double-layer capacitors (EDLCs), pseudo-capacitors, and hybrid supercapacitors (HSCs). Electric double-layer capacitance, which is the primary mechanism of EDLCs and involves the electrostatic accumulation of ions on high-SSA electrodes [31] (Figure 1a), provides exceptional P_d, prolonged cycle life, and negligible energy loss. In contrast, pseudocapacitance utilizes rapid, reversible Faradaic redox reactions at the electrode surface. These reactions are also governed by electrocatalytic reaction pathways (Figure 1b), which are regulated by factors such as the density of redox-active sites, ion adsorption affinity, and electronic conductivity [10], to deliver much higher Sp.C and E_d [13,31]. Pseudocapacitance is a key mechanism for pseudocapacitors. To synergistically harness the strengths of both mechanisms, the ASC architecture, as a kind of hybrid supercapacitor—which intelligently pairs a high-power EDLC electrode with a high-energy pseudocapacitive electrode [10]—has become a key design strategy (Figure 1c). The success of this approach is critically dependent on the pseudocapacitive component. The E_d of HSCs is largely determined by the Sp.C of electrode materials and the working voltage window, which can be improved by employing high-Sp.C materials, constructing hierarchical porous architectures, and introducing heteroatom doping or defect engineering [10,32,33]. Nonetheless, their E_d and output voltage remain lower than those of rechargeable batteries [34]. To overcome these limitations, the design of porous and three-dimensional conductive frameworks, combined with the uniform distribution of capacitive and battery-type or pseudocapacitive materials, promotes rapid ion diffusion, enhances electrical conductivity, and boosts E_d. In practical devices, supercapacitors are often integrated with batteries.

2.1. Advantages of TMD as a Pseudocapacitive Electrode

Electrode materials commonly used for generating pseudocapacitance through Faraday reactions include transition metal oxides, conductive polymers, transition metal hydroxides, and other materials. For instance, transition metal oxides as an electrode material are usually restricted by volume expansion [36,37], which may cause material differentiation or detachment during charging and discharging. Similarly, conductive polymers also suffer from drastic volume changes, leading to the loss of active substances and the attenuation of capacitance [38]. Unlike TMD materials, which have a 2D layered structure, the layers are connected by van der Waals forces, making them more stable and having a longer cycle life. In addition, carbon materials such as graphene mainly rely on EDLC to store charge. This process is not like the Faraday reaction of TMD as an electrode, which stores much more charge than surface electrostatic adsorption. Therefore, the theoretical specific capacitance and energy density of TMD are much larger than those of graphene [16,39]. Among these materials, the promise of TMDs as advanced electrodes in ASCs also stems from their inherent electrocatalytic nature, characterized by abundant active sites, high electrical conductivity, and a unique electronic structure with tunable band gaps, collectively enhancing the kinetics and efficiency required for energy storage.

2.2. Basic Characteristics of Electrochemical on Asymmetric Supercapacitor

Cyclic voltammetry (CV), linear sweep voltammetry (LSV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) are employed to evaluate the E_d and P_d of electrochemical energy storage devices [40]. These measurements are typically conducted using either a three-electrode (Figure 2a) or a two-electrode (Figure 2b) system [35]. Key parameters include Sp.C (F cm⁻² or F g⁻¹), inner resistance and charge transfer resistance (Ω), P_d (W kg⁻¹), E_d (Wh kg⁻¹), and cycling stability (%). A comprehensive electrochemical performance analysis is achieved by integrating CV, LSV, GCD, and EIS tests. Firstly, CV estimates capacitance by recording the current response at a constant scan rate within a defined voltage window, producing a closed-loop curve [35]. Secondly, GCD measures the voltage response under constant current to evaluate E_d and P_d. Thirdly, EIS detects internal resistance, ion diffusion, and charge transfer processes using small alternating current signals over a range of frequencies [41,42].

2.2.1. Cyclic Voltammetry

CV curves reflect the voltammogram trace of electrode materials by plotting the current density against the potential voltage. The features of CV curves vary with the underlying capacitive properties: (1) In systems dominated by EDLC, the CV curve is characterized by a nearly rectangular profile [35]. (2) Under pseudocapacitive contribution, the CV curve can display redox peaks, depending on the Faradaic reaction [35]. (3) When pronounced redox reactions are present in the electrode material, the CV curve typically displays a pair of distinct oxidation and reduction peaks [35]. The symmetry of the peaks indicates the reversibility of the reaction, while the peak-to-peak separation reflects the kinetics of electron transfer [42]. Additionally, the peak current intensity is closely related to active sites, SSA, electrical conductivity, and electrocatalytic activity of the material [23]. For CV curves with a nearly rectangular shape, the capacitance (C) can be evaluated by Equation (1) [42]:

$$C={\displaystyle \frac{I}{v}}$$

(1)

where C is the capacitance (F g⁻¹), I is the current density (A g⁻¹), and v is the scan rate (V s⁻¹). The areal capacitance (C_A) can be calculated by Equation (2) [35,42] in a two-electrode assembly system:

$${ C}_A={\displaystyle \frac{4{\int }_{v_1}^{v_n}idV}{As∆V}}$$

(2)

where A, $∆V$, s, i, and v are the area of the device, voltage window, scan rate, applied current, and material volume, respectively, while ${\int }_{v_1}^{v_n}idV$ is the area under the curve. In the case of CV curves exhibiting redox peaks or deviating from a complicated shape, the capacitance is more accurately calculated using Equation (3) [42]:

$$C={\displaystyle \frac{Q_{whole}}{2∆V}}$$

(3)

where $Q_{whole}$ is the whole area of the CV curve, which means all charges in the process, and $∆V$ is the working voltage window during the scanning process. The Sp.C (C_s), normalized by the mass of active material, can be calculated by Equation (4) [42]:

$$C_s={\displaystyle \frac{C}{m}}$$

(4)

where m is the electrode material mass.

2.2.2. Linear Sweep Voltammetry

LSV involves linearly sweeping the electrode potential across a defined voltage range at a constant rate while the resulting current response is continuously recorded. This technique is commonly used to evaluate electrocatalytic performance through the overpotential at a specific current density and the Tafel slope derived from LSV fitting. The overpotential represents the difference between the actual potential required for the electrode reaction and its thermodynamic equilibrium potential, as shown in Equation (5) [43]:

$$\eta =E_{applied}- E^o$$

(5)

where $\eta$ is the overpotential, $E_{applied}$ is the applied potential during the reaction, and $E^o$ is the thermodynamic equilibrium potential.

The basic form of the Tafel equation is Equation (6) [44]:

$$\eta =blog\left(i\right)+a$$

(6)

where $b$ is the Tafel slope, $i$ is the current density, and $a$ is a constant related to the exchange current density and reaction kinetics.

These parameters reflect the redox reaction, Faradaic efficiency, and catalytic kinetics of the electrocatalytic reaction. In addition, LSV displays the key indicators for catalytic activity, including onset potential and exchange current density. A lower onset potential and higher exchange current density suggest better catalytic reactivity and faster charge transfer at the interface. Furthermore, LSV curves can be used to evaluate catalytic stability by comparing the response under repeated cycles.

2.2.3. Galvanostatic Charge/Discharge

GCD tests record the voltage time response of an electrode and device under constant current via an electrochemical workstation. They involve two types: charging to the set voltage under constant current and discharging back to zero [35]. Typically, GCD is utilized to evaluate Sp.C, E_d, and P_d. C_s is described as Equation (7) [35,42]:

$$C_s={\displaystyle \frac{i∆V}{m∆t}}$$

(7)

where i is also the constant current that has been set, $∆V$ is the change in voltage during the discharging period, m means the mass of electrodes, and $∆t$ is the discharging time.

E_d (E) and P_d (P) can be calculated by Equations (8) [45] and (9) [45]:

$$E={\displaystyle \frac{1}{2}}{C}_s{\left(∆V\right)}^2$$

(8)

$$P={\displaystyle \frac{E}{∆t}}$$

(9)

where $∆t$ is the discharging time and $∆V$ is the change in voltage during the discharging period. According to the equations above, enhancing the Sp.C and P_d of electrodes can be achieved by employing materials with high charge storage capability, such as pseudocapacitive materials with porous and layered structures [45]. These materials improve electrical conductivity and facilitate faster ion and electron transport to reduce discharge time [42]. In addition, expanding the voltage window is critical for increasing E_d. Electrolytes with broader voltage windows and greater voltage tolerance of electrode materials are employed to achieve higher E_d. The expansion of the voltage window results in a decline in Sp.C to some extent, primarily due to irreversible reactions or degradation of electrode materials [42].

Cycle stability is a crucial parameter for evaluating the long-term durability of electrodes under repeated charge–discharge cycles at a specified current density. The percentage difference between the initial and final capacitance values after GCD cycling reflects the capacitance retention of the electrode. Critical factors influencing cycle stability include electrode composition, the electrolyte, and current density.

2.2.4. Electrochemical Impedance Spectroscopy

EIS determines the impedance of an electrochemical system through a slight difference in amplitude of the alternating current signals at open-circuit potential. It measures the signal voltage and current ratio across the changes in sine-wave frequency (ω) or impedance-phase angle (Φ) along with ω [42]. Amplitude, frequency, and phase are the three essential characteristics that describe impedance input and output signals [23].

The results are typically visualized using a Nyquist plot and a Bode plot. To quantitatively analyze the physical processes at the electrode–electrolyte interface, a Nyquist plot is fitted using an equivalent circuit model. The features of the Nyquist plot are directly correlated with this equivalent circuit (Figure 3a,b) [46], which follows a structure analogous to the Randles model [47,48]: $R_s-\left(CPE‖\left(R_{ct}+Z_w\right)\right)$ [48], consisting of the series resistance (R_s) and charge transfer resistance (R_ct), a constant phase element (CPE), and a Warburg impedance (Z_w) [49]. The intercept of the plot on the real axis in the high-frequency region corresponds to R_s. The diameter of the semicircle in the mid-frequency region represents R_ct. The straight line in the low-frequency region, typically at a 45° angle, is characteristic of the Z_w, indicating diffusion-limited processes [49]. Taking Figure 2 as an example, the fitting results indicate that the R_ct value of MoSe₂@HCNS is 115.8 Ω, which is lower than the 568.7 Ω of pristine MoSe₂, suggesting a higher charge transfer efficiency. Meanwhile, the slope of the diagonal line in the low-frequency region also demonstrates that MoSe₂@HCNS has a higher ionic conductivity.

Another way to represent EIS result data is through a Bode plot. By analyzing the ω variations and Φ and impedance magnitude, one can further evaluate the capacitive behavior at the interface as well as the extent of ion diffusion within the system, particularly in the low-frequency region where the diffusion-related process is dominant [23]. The phase angle–frequency relationship is particularly diagnostic: at high frequencies (ω > 10³ rad/s), phase angle near 0°, stable impedance dominates; in the mid-frequency range (10⁰~10³ rad/s), phase angle −90° to −45°, the R_ct-CPE parallel circuit governs; at low frequencies (ω < 10⁰ rad/s), phase angle near −45°, impedance increases with decreases frequency, indicating diffusion control [50,51].

The aforementioned electrochemical evaluation methods are not independent of one another. For instance, a higher rectangularity of the CV curve and a larger peak current correspond to a smaller R_ct and a maximum Φ closer to 90° in EIS—these features collectively indicate excellent Sp.C of the electrode. Additionally, a lower onset potential and a smaller Tafel slope in LSV align with a smaller R_ct and weaker Warburg impedance in the low-frequency region of EIS, which signifies smooth ion transport at the electrode–electrolyte interface.

3. Transition Metal Dichalcogenide Electrodes

A TMD is a 2D layered material with electronic properties, consisting of a layer of transition metal (defined as M) sandwiched between two layers of chalcogen elements (referred to as X), which form a stoichiometry of MX₂ structure. Here, M is one of the transition metals from group IV to VII, such as molybdenum or tungsten, and X is a chalcogen (X represents S, Se, or Te) (Figure 4a) [13,52,53,54]. TMDs were first proposed by Linus Pauling in 1923. By the 1960s, nearly 60 types of TMDs had been identified, of which approximately 40 exhibited layered structures [13]. Fragile van der Waals forces hold together these layered structures, while the M and X are bound by robust covalent bonds [12]. The monolayer TMD structure has been confirmed to comprise three atomic layers, where the transition metal layer is confined between two planes of halogen atoms; examples include WS₂, WSe₂, MoS₂, and MoSe₂ [12,55].

3.1. Electrocatalytic Contributions to the Performance of TMD Electrodes

TMDs’ sandwiched layered structure is the starting point for their unique active site characteristics, and the electrocatalytic contribution of these sites directly determines the electrode’s capacitance performance [56]. Since pseudocapacitance relies on reversible Faradaic redox reactions at active sites, the number and activity of these sites are positively correlated with Sp.C. Traditional pseudocapacitive materials, such as transition metal oxides, exhibit fixed distributions of active sites, which restricts their Sp.C ceiling to the limits of their intrinsic surface chemistry [57]. In contrast, TMDs feature a dual-site system: high-activity edge sites, such as sulfur atoms at the edges of TMD layers, and a thermodynamically stable yet inert basal plane [58]. In primitive TMD compounds, only edge sites participate in electrocatalytic redox reactions, leading to underutilized surface area and constrained Sp.C. Nowadays, however, the inert basal plane can be activated to generate new active sites through defect engineering or heteroatom doping to adjust the d-band center [59]. For instance, sulfur vacancies in MoS₂ break the saturation of chemical bonds on the basal plane, exposing unsaturated Mo atoms [60]. These atoms act as additional electrocatalytic centers to participate in redox reactions. These engineered active sites can be distributed across both the edge sites and basal plane, enabling TMDs to surpass the capacitance limitations of conventional materials when used as electrode materials.

The high-rate performance of electrodes hinges on rapid electron/ion transport to match fast charge–discharge cycles. Notably, the phase transition of TMDs converts their electronic state from semiconducting to metallic, which not only boosts conductivity but also enables them to reduce Rct, optimize reaction kinetics, and significantly suppress polarization or Sp.C decay under high current densities [61]. Superior electrode performance requires not only abundant active sites and high electrical conductivity but also efficient transport of both ions and electrons, which is jointly determined by the material’s conductivity and structure. Traditional materials often excel in only one aspect: for instance, graphene enables rapid electron transport, but its ion storage capacity is limited, as it relies solely on EDLC via physical electrostatic adsorption, resulting in low energy density [62]. In contrast, metal oxides possess high pseudocapacitance yet suffer from poor electron/ion diffusion, leading to inferior rate capability [63]. In contrast, TMDs feature a 2D layered structure with weak van der Waals forces between layers and an interlayer spacing of approximately 0.6–1.0 nm. This interlayer architecture serves as efficient ion diffusion channels, endowing TMDs with dual advantages in both electron transport and ion diffusion [64].

From a practical standpoint of durability and economics, TMDs benefit from their layered structure, which endows them with excellent chemical stability that allows them to maintain relatively outstanding specific capacitance even after undergoing tens of thousands of charge–discharge cycles [65]. Furthermore, unlike noble-metal-based pseudocapacitive materials, such as RuO₂ and IrO₂ [66], that are costly and have limited reserves, TMDs are composed of earth-abundant elements like molybdenum, tungsten, sulfur, and selenium and feature low synthesis costs, making them more economically viable.

3.2. Electronic Origins of Electrocatalytic Activity

3.2.1. Phase Engineering

TMDs are distinguished by three primary crystal structures [19]: (1) the 2H phase (Figure 4b), which exhibits hexagonal symmetry and belongs to the P6₃/mmc space group, imparting semiconducting properties [13,45,67], as exemplified by MoS₂ (Figure 4e); (2) the 1T phase (Figure 4c), which demonstrates trigonal symmetry and falls within the P-3m1 space group, conferring high electrical conductivity and metallic or quasi-metallic behavior [68,69,70]; (3) the 3R phase, which displays rhombohedral symmetry (Figure 4d) [54]. The advantages of TMDs as electrode materials arise from their large SSA and reversible valence states, which not only enable EDLC-type energy storage but also facilitate rapid and reversible Faradaic reactions through pseudocapacitive mechanisms [68,71,72]. Moreover, the abundant electrochemically active edge sites further enhance the capacitance of supercapacitors [45,53,73].

The intrinsic initial state of most TMDs is dominated by the thermodynamically stable 2H phase. However, the 2H phase exhibits semiconducting properties, and its low intrinsic electrical conductivity results in sluggish charge transfer kinetics [64]. This not only leads to high electrode Rs and R_ct but also limits electron transport efficiency under high-rate conditions. Worse still, the active sites of the 2H phase are only concentrated at the edges; its basal plane remains electrochemically inert due to saturated chemical bonds and cannot participate in Faradaic reactions [64]. This leads to extremely low active site density, severely restricting the Sp.C of TMD electrodes. To address these limitations, researchers have investigated the transformation of TMDs from the 2H phase with trigonal prismatic-coordinated metal atoms to the 1T phase with octahedral-coordinated metal atoms via chemical intercalation, mechanical exfoliation, and laser irradiation [74,75,76]. This structural rearrangement of TMDs is designed to induce electronic band reconstruction, enabling the transition of the electronic state from semiconducting to metallic. The conversion to the metallic 1T phase brings about a leap in electrical conductivity, significantly reducing Rs and R_ct, which in turn greatly enhances rate capability and P_d [77]. Additionally, due to the reconstruction of the electronic structure in the 1T phase, the saturated state of chemical bonds on the basal plane is broken, and both the edges and the basal plane become electrochemically active regions. This exponential increase in active site density leads to a substantial improvement in Sp.C [78]. Beyond that, the interlayer spacing of TMDs expands concomitantly with the 2H-to-1T phase transition. The widened interlayer channels provide more spacious pathways for the intercalation and deintercalation of electrolyte ions, increasing the ion diffusion coefficient and further enhancing rate capability.

However, it is noteworthy that the 1T phase has a metastable structure [79]. This means that after tens of thousands of electrochemical cycles, the spontaneous reversion of the 1T phase to the more stable 2H phase may occur, resulting in decreased electrical conductivity, reduced active sites, and subsequent degradation of rate capability and capacitance [80]. Current research is ongoing to address this stability issue, with strategies such as sulfur vacancy introduction [81], metal doping [82], and construction of 2H/1T heterostructures [83] under discussion.

3.2.2. Electronic Structure

At a more fundamental level, the electrocatalytic contributions discussed previously are all governed by the intrinsic electronic structure of the TMD material [84]. More specifically, they are determined by two key factors: the band structure, which determines the material’s conductive nature via the presence or absence of a band gap [85], and the density of states (DOS), which quantifies the number of available electronic states at a given energy level. The DOS near the Fermi level (Ef) is particularly critical for facilitating electron transfer [86].

The 2H phase, in particular, has a significant band gap with almost zero DOS at Ef, meaning there are no available electronic states for charge transfer. Furthermore, electrons must overcome the band gap to participate in reactions, which requires high activation energy [85]. This fundamentally results in sluggish charge transfer kinetics in the 2H phase, leading to its low electrical conductivity and poor catalytic activity. In contrast, the valence and conduction bands of the 1T phase overlap directly with no band gap. The DOS at Ef is extremely high, providing a large number of free electronic states, and electrons can transfer rapidly without needing to overcome a band gap [85,86]. This is the fundamental reason why the 1T phase exhibits high electrical conductivity, low R_ct, and high catalytic activity.

Therefore, strategies for modifying the electronic structure can be employed to enhance the electrochemical performance of electrodes. For example, introducing mid-gap states within the band gap of the 2H phase can reduce the activation energy required for electrons to cross the band gap, thereby increasing active site density and charge transfer efficiency [87]. Alternatively, introducing n-type or p-type doping [88] can shift the position of the Fermi level, reducing charge transport resistance while optimizing the electron cloud density at active sites, resulting in improved conductivity and catalytic activity [89].

3.3. Design Principles for Asymmetric Supercapacitors

Although TMDs have been extensively studied as supercapacitor electrodes, several intrinsic limitations persist: (1) low intrinsic electronic conductivity [45]; (2) interlayer restacking, which reduces SSA [90]; (3) localized active sites primarily at edges, while basal planes remain electrochemically inert [91]; (4) structural expansion and contraction during repeated charge–discharge cycles [92]; and (5) long ion diffusion pathways, which limit rate capability [93,94]. Factors such as electrical conductivity, morphology, crystal structure, and particle size have all been shown to critically influence the electrochemical performance of TMD electrodes [45].

To solve these problems, researchers employed simultaneous bottom-up and top-down synthesis approaches to design nanostructures with suitable morphologies [95]. Initially, TMDs were synthesized by mechanical exfoliation from bulk flakes [96,97]. In contemporary research, chemical vapor deposition (CVD) and epitaxial growth are frequently utilized to prepare high-quality monolayer and few-layer TMDs [98,99]. In the domain of energy storage and catalysis, researchers have investigated vertically aligned heterostructures of TMDs, including vertically aligned TMD sheets [100,101,102], van der Waals heterostructures [103,104], and Janus structures [105]. However, these growth methods remain limited to small-scale production. In terms of cost-effectiveness, solvothermal synthesis is comparatively superior [106,107]. Analogous methods, like in situ growth in solvent [108], can be utilized for the vertical alignment of TMDs with varied sizes, broadened interlayers, and doped nanolayers [109]. Researchers have been actively pursuing the advancement of TMD heterostructures with reduced thickness and dimensions [19]. In addition, researchers have improved the electronic and ionic transport capabilities of 2D TMDs as electrodes by mixing, wrapping, or depositing them [53] with electrically conductive or electroactive components, including conducting polymers [110] and carbonaceous materials [111]. These approaches enable TMDs to achieve optimal electrochemical performance in ASCs.

4. TMD-Based ASCs

4.1. MoSe₂-Based ASCs

Aimed at boosting the capacitance of MoSe₂-based electrodes, doping is considered a practical approach. Masanta et al. [112] doped MoSe₂ with heteroatom Mn to form Mn-doped MoSe₂. Photoluminescence analysis revealed that, compared to the undoped MoSe₂ (S-I), the Mn-doped samples at different percentages, Mn-doped samples S-II (1.5% Mn) and S-III (6.2% Mn), exhibited shorter emission wavelengths and a blue shift of the photoluminescence peak, with these changes becoming more evident as the percentage of Mn doping increased (Figure 5a). This indicated that Mn doping improved the n-type nature of MoSe₂ and shifted the Fermi level closer to the conduction band edge, resulting in enhanced electrical conductivity for the electrode. This consequence was further confirmed by the CV curves, which demonstrated enhanced pseudocapacitive characteristics (Figure 5b), and the highest percentage of Mn doping (S-III) exhibited the highest Sp.C of 1116 F/g at a 5 mV/s scan rate.

Notably, several MoSe₂-based composites incorporate redox-active components or conductive nanophases that contribute to interfacial electron kinetics and reversible Faradaic reactions, highlighting the intrinsic advantages of electrocatalytic materials in electrode design. The combination of MoSe₂ with other materials enhances the electrode rate capability, enabling it to maintain optimal capacitance even at high current densities. Kirubasankar et al. [22] used hydrothermal synthesis to integrate 2D MoSe₂ into Ni(OH)₂ nanosheet electrodes. Based on a layered electrode, the ASC device (MoSe₂-Ni(OH)₂∥AC) presented an enhanced capacitance of 124 F g⁻¹ at 1 A g⁻¹ and achieved a high E_d of 43 Wh kg⁻¹. Despite the presence of interlayer spacing in the layered MoSe₂ structure, the emergence of nanosheet restacking led to a decrease in capacity. In order to solve this problem, Su et al. [23] prepared a Ni₂P/NiSe₂/MoSe₂ (NNM) hybrid electrode, with Ni₂P and NiSe₂ nanoparticles attached to MoSe₂ nanosheets. The incorporation of these conductive and redox-active phases promoted interfacial electron transfer, increased the interlayer distance of MoSe₂, and effectively mitigated nanosheet restacking. In this report, an NNM-based ASC (NNM//AC) retained 116.6% of its initial capacitance even over 45,000 cycles at 3 A g⁻¹(Figure 5c). Similarly, Kumar and Thangappan [111] designed a MnO₂/MoSe₂/rGO composite electrode possessing a large SSA of 78.24 m² g⁻¹. The fabricated ASCs (MnO₂/MoSe₂/rGO//AC) achieved the enhanced potential window of 1.8 V, along with a P_d of 30.2 Wh kg⁻¹ at 1 A g⁻¹ in a PVA/H₂SO₄ electrolyte (Figure 5d). To address the inferior cyclic stability, Yesuraj et al. [113] fabricated a pentagon-core hexagon-ring structure that compounds a NiCo₂O₄ nanoplate with MoSe₂ (Figure 5e). The enlarged SSA of the hybrid structure electrode offered abundant active sites favorable for redox reactions and ion adsorption, contributing synergistically to both mechanism types of capacitances and diffusion, as confirmed by CV curves (Figure 5f). The power-law fitting of current (i) and scan rate (v), described by $i=a{v}^b$, yielded b-values of 0.77 and 0.81 for the anodic and cathodic peaks, respectively. This result was in accordance with the electron kinetic processes being influenced by surface capacitance and ion diffusion. The robust configuration of the NiCo₂O₄/MoSe₂//AC ASC achieved an outstanding cyclic stability with 95% capacitance retention over 10,000 cycles even under 30 A g⁻¹ (Figure 5g), along with a high E_d of 69 Wh kg⁻¹.

4.2. WSe₂-Based ASCs

Among the polymorphs of WSe₂, the 2H phase demonstrates superior stability over the 1T phase, as proposed by Singh et al. [114]. They synthesized 2H-WSe₂ as the electrode to test charge capacity. The electrode showed a quasi-symmetric visible voltage plateau during the cycling at 3.3 A g⁻¹, which indicates fast and reversible redox kinetics associated with catalytic redox-active sites, which contribute to the high pseudocapacitance, showing an Sp.C of 618.75 F g⁻¹ at 3.3 A g⁻¹ (Figure 6a). The ASC device of 2H-WSe₂//AC also demonstrated cycling stability of 77% over 10,000 cycles at 5 A g⁻¹. To enhance the real capacitance of the ASC device, Singh et al. [115] integrated WSe₂ with rGO through a solvothermal approach. The b-value of the WSe₂/rGO electrode is 0.68, reflecting both capacitive and diffusive mechanisms with significant diffusion-controlled kinetics, typical of catalytic redox interfaces. Therefore, the WSe₂/rGO electrode gained a superior Sp.C of 1290 F g⁻¹ under 1 A g⁻¹. The ASC (WSe₂@rGO//AC) device demonstrated an enhanced capacitance of 145 F g⁻¹ under 2 A g⁻¹ and delivered outstanding P_d at 2133.3 W kg⁻¹, along with E_d at 51.5 Wh kg⁻¹ (Figure 6b). After that, the research team of Singh [116] further developed Mo-doped WSe₂/rGO; among the various percentages of Mo doping, WSe₂/rGO doped with 3% Mo (M3) presented the highest SSA of 26.3 m² g⁻¹ via Brunauer–Emmett–Teller (BET) (Figure 6c) and the lowest R_ct, indicating promoted ion diffusion and enhanced interfacial conductivity, which are closely related to improved catalytic charge transfer characteristics. Furthermore, the M3 electrode demonstrated an excellent Sp.C of 1514 F g⁻¹ at 1 A g⁻¹, accompanied by excellent cyclic stability and coulombic efficiency (Figure 6d), suggesting highly reversible redox reactions facilitated by its catalytically enhanced surface. The ASC assembled from M3 and AC also increased to an optimal capacitance of 194 F g⁻¹ under 2 A g⁻¹, which confirmed that the Mo doping improved the capacitance of the ASC device compared to the WSe₂@rGO//AC-based ASC (145 F g⁻¹ at 2 A g⁻¹). Moreover, the M3//AC ASC exhibited superior electrochemical characteristics with values of 70 Wh kg⁻¹ for E_d and 1706 W kg⁻¹ for P_d, which can be attributed to improved interfacial reaction kinetics and reduced charge transfer resistance imparted by the Mo-doped catalytic structure.

4.3. MoS₂-Based ASCs

The optimization of the electron transfer pathway, a key feature often targeted in catalyst-integrated electrodes, contributes to improving the rate capability of the electrode. To promote the capacitance increase at high current density, Wang et al. [57] designed a ternary composite electrode using a MoS₂ compound with layered double hydroxides (LDHs) and polyaniline (PANI) to construct efficient electron transfer channels, leveraging the layered structure and interfacial redox activity of MoS₂ to promote charge mobility (Figure 7a). This structure (NiV-LDH/PANI/MoS₂) enabled a fast response speed at an elevated rate of 50 mV s⁻¹ and exhibited pseudocapacitance with prolonged discharge duration, which can be ascribed to the redox-active nature of MoS₂ when compared to counterparts without MoS₂ incorporation. The ASC (NiV-LDH/PANI/MoS₂//AC) demonstrated excellent rate capability, delivering an Sp.C of 118.8 F g⁻¹ at a current density as high as 12 A g⁻¹. To further enhance the intrinsic redox activity of MoS₂-based electrodes, Kaur et al. [117] integrated WO₃ with a MoS₂ electrode assembled with an ASC (WO₃/MoS₂//AC) in a redox-additive electrolyte (RAE), which consisted of 0.35 M K₄[Fe(CN)₆] dissolved in 6 M NaOH. The mesoporous structure of the WO₃/MoS₂ composite with an excellent average pore diameter (20.62 nm) facilitated the interaction of the electrolyte with the electrode (Figure 7b). The electrolyte is a key factor that affects the capacitance. In this report, the redox reactions occurring at the electrode and electrolyte interface drastically boosted the Sp.C in the RAE, driven by the catalytic activity of the electrode surface, which was about quadruple the value obtained in 6 M NaOH (Figure 7c). The composite electrode in the RAE showed 90% capacity retention over 5000 cycles under current densities as high as 25 A g⁻¹, reflecting excellent electrochemical reversibility and catalytic interface stability. For an assembly with ASC (WO₃/MoS₂//AC) in the RAE, the potential window reached up to 1.9 V and exhibited the remarkable E_d of 84.72 Wh Kg⁻¹ at the P_d of 7624.8 W Kg⁻¹. The assembled device further demonstrated the outstanding capacitance of 182.15 F g⁻¹ even at 10 A g⁻¹.

The optimized MoS₂-based electrode exhibits excellent charge storage capability. Building on this foundation, researchers have concentrated on enhancing the long-term durability of ASC devices for practical applications. Raza et al. [26] synthesized a MgS/MoS₂ composite on nickel foam (NiF) via CVD. The resulting nanoneedle-like architecture decorated with nanoparticle clusters markedly increased the accessible surface area to 170 m² g⁻¹, thus providing abundant redox-active sites and facilitating efficient electron–ion transport. This unique morphology, combined with the intrinsic redox activity of MoS₂, significantly improved charge storage, yielding an exceptional E_d of 234 Wh kg⁻¹ at 2150 W kg⁻¹ (Figure 7d) and a superior Sp.C of 9149 F g⁻¹ even at 10 A g⁻¹ (Figure 7e).

Moreover, a broadened operating voltage of 1.72 V assembled with an ASC (MgS/MoS₂@NiF//AC) demonstrated sustained 370 F g⁻¹ at a high current density of 25 A g⁻¹ and the optimal E_d of 288 Wh kg⁻¹ at 3440 W kg⁻¹. This device achieved 95% capacitance retention over 10,000 cycles under 20 A g⁻¹. Liu et al. [118] fabricated a yolk–shell structure electrode based on Co₉S₈/MoS₂ heterostructure nanosheet arrays (NSAs) on hollow carbon spheres (HCSs), combining the redox activity and conductivity of both components while enhancing ion diffusion via the porous shell. This unique core–shell structure (Co₉S₈-MoS₂ NSAs@HCSs) enabled excellent cyclic stability, retaining 96.9% over 10,000 cycles under 10 A g⁻¹. From the EIS profile, the composite showed a low R_ct in both mid- and high-frequency regions, which indicated enhanced interfacial charge transfer (Figure 5f), a key indicator of catalytic activity in heterostructured electrodes. Simultaneously, it reflected a fast ion transfer in the low-frequency region. An ASC assembled with Co₉S₈-MoS₂ NSAs@HCSs and HCSs (Co₉S₈-MoS₂ NSA@HCSs//HCSs) demonstrated a superior electrochemical reversibility of 99.61% coulombic efficiency and 98.2% capacitance over 10,000 cycles (Figure 7g). Raza et al. [119] designed a silver-wrapped MoS₂ structure on hierarchical nickel foam (Ni-F) using the CVD method, where the conductive Ag shell modulated the surface electronic properties and accelerated charge transfer. The composite displayed a cauliflower-like structure achieving an elevated SSA reaching 170 m² g⁻¹. Figure 7h illustrates the synthesis process of the Ag/MoS₂/Ni-F electrode inside the reactor tube. The composite is stable and controllable by adjusting the temperature in different zones. The sizable SSA accommodates numerous active sites, showing an outstanding Sp.C of 4124 Fg⁻¹ under 5 mVs⁻¹. Moreover, the ASC device (Ag/MoS₂/Ni-F//AC) exhibited an exceptional capacitance of 957 F g⁻¹ at 1.8 A g⁻¹. Furthermore, the device obtained a remarkable E_d of 366 Wh kg⁻¹ at a P_d of 1494 W kg⁻¹ (Figure 7i) and sustained 94% cyclic stability after 20,000 cycles at 17 Ag⁻¹ (Figure 7j), which further highlights the role of doping in promoting reversible Faradaic reactions and long-term electrochemical stability.

4.4. WS₂-Based ASCs

WS₂ in the 2H phase presents semiconducting characteristics with a band gap of 1.9 eV [25], which leads to an intrinsic low electrical conductivity. In order to address this conductivity mismatch, Iqbal et al. [25] employed the sputtering technique to design a thin film structure, which was a uniform 100 nm layer of zirconium nitride (ZrN) and 250 nm WS₂ on the surface of nickel foam. This interfacial structure enhanced the rate capability of charge diffusion and reduced the R_ct to 2Ω, reflecting enhanced interfacial electron transport arising from the redox-active WS₂/ZrN interface. The ASC device (WS₂/ZrN//AC) presented a high E_d of 76 Wh kg⁻¹ at a P_d of 4325 W kg⁻¹. In addition, the capacitance retention maintained 90% over 10,000 cycles (Figure 8a). This study reconfirmed the charge storage mechanism by diffusive and capacitive cooperation: diffusion is obvious at a lower scan rate, while capacitive contribution is evident at a higher scan rate. The 2D layered structure of WS₂ provides a support base for composites, facilitating the uniform dispersion of nanoparticles on its surface. Sharma et al. [120] investigated the composition of 0D ZnNi₂O₄ and 2D WS₂ structures. Evidence suggested that 2D WS₂ nanoflakes provide a robust support for uniformly anchoring ZnNi₂O₄ nanoparticles, effectively exposing more electroactive sites and facilitating interfacial redox reactions. The projection density of states (PDOS) profile revealed enhanced electronic states near the Fermi level, indicating the major metallic character of the ZnNi₂O₄/WS₂ structure (Figure 8b) due to Ni-d and O-p orbital contributions, which benefits catalytic charge transfer efficiency. Moreover, the multi-orbital hybridization in the valence bands facilitated interfacial electron delocalization, promoting efficient charge transport characteristic of active heterointerfaces. The assembled ASC exhibited an exceptional capacitance of 171.3 F g⁻¹ at 1 A g⁻¹, along with an elevated E_d of 61.6 W h kg⁻¹. In addition, the 2D layered structure of WS₂ affords it exceptional flexibility. De et al. [28] fabricated an all-solid-state electrode for a flexible ASC device by integrating WS₂-decorated Ti₃C₂T_x nanosheets with boron nitride (BN) via the hydrothermal method, forming an enlarged specific surface area with uniformly distributed redox-active sites (Figure 8c), which enhanced surface reactivity and charge storage. The assembled device (WS₂/Ti₃C₂T_x/BN//Ti₃C₂T_x) displayed the capacitance of 140 F g⁻¹ at 1 A g⁻¹ in PVA/KOH/KI gel electrolyte. The inclusion of KI introduced reversible redox couples, which enhanced the electrolyte conductivity and promoted surface-mediated pseudocapacitive reactions. This device also showed excellent flexibility, with no evident change under the various bending conditions noted by GCD curves (Figure 8d). The flexible device exhibited long-term durability, retaining 84% capacitance after 10,000 cycles.

From the research progress of ASCs based on MoSe₂, WSe₂, MoS₂, and WS₂ electrodes, it can be seen that the performance optimization mainly relies on three core strategies to address the inherent issues of TMDs—low intrinsic conductivity, easy interlayer stacking, and insufficient exposure of redox-active sites. These strategies include doping to adjust electronic properties, incorporating conductive phases, using redox-active components to construct synergistic interfaces, and designing special morphologies, such as nanosheets, nanoneedles, and yolk–shell structures, to optimize mass transport pathways. For instance, MoSe₂’s n-type characteristics and conductivity are enhanced via Mn doping, WSe₂’s interfacial charge transfer is improved through 3% Mo doping and rGO compositing, MoS₂’s active sites and electron transport are strengthened by the formation of a NiV-LDH/PANI ternary composite or modification with MgS, WS₂’s Rct is reduced via a ZrN thin film structure, and WS₂’s interfacial kinetics is optimized through Ti₃C₂T_x/BN compositing.

However, significant differences exist among the four types of electrodes. MoS₂-based ASCs stand out in terms of E_d: the MgS/MoS₂@NiF//AC device achieves an E_d of 288 Wh/kg, far exceeding that of MoSe₂-, WSe₂-, and WS₂-based counterparts. It also leads in capacitance retention at high current densities (9149 F/g at 10 A/g) and cyclic stability (the Co9S8-MoS₂@HCSs//HCSs device maintains 98.2% capacitance retention after 10,000 cycles), benefiting from MoS_2′s superior ability to form synergistic interfaces with multiple components. MoSe₂-based ASCs exhibit unique advantages in extreme cycling scenarios: the Ni₂P/NiSe₂/MoSe₂ (NNM)//AC device retains 116.6% of its initial capacitance after 45,000 cycles at 3 A/g, demonstrating that doping effectively alleviates interlayer stacking and enhances structural stability. WS₂-based ASCs focus on flexible electronics: the WS₂/Ti₃C₂T_x/BN//Ti₃C₂T_x device shows no obvious capacitance decay under different bending angles, highlighting its value in wearable applications. Although WSe₂-based electrodes have lower capacitance in their pure phase compared to the other three, the high stability of the 2H phase (77% capacitance retention after·10,000 cycles) and capacitance enhancement after rGO compositing (1290 F/g at 1 A/g) make them promising for low-power, long-lifetime basic energy storage scenarios.

Overall, MoS₂ is more capable of achieving high specific surface areas, a wide voltage window at 1.9 V, and multi-component synergy, making it suitable for portable electronic devices. WSe₂ and WS₂ can develop in the directions of low-decay stability and flexible electronics, respectively, while MoSe₂ is better suited for ultra-long-cycle scenarios.

5. Conclusions and Prospects

Overall, this review summarizes the emerging potential of TMDCs, particularly MoSe₂, WSe₂, MoS₂, and WS₂, as high-performance electrodes in ASC devices. Their unique two-dimensional layered architectures provide large SSAs enriched with active sites and variable valence states, thereby facilitating efficient and reversible redox reactions. Through doping, homogenization, and hybrid composite strategies, the synergistic effects of these systems markedly enhance the Sp.C, E_d, P_d, and capacitance retention of TMD-based electrodes.

Table 1. Summary of electrochemical performances of recent reports for TMD-based hybrid supercapacitors.

Supercapacitor	Electrolyte	Potential Window	Current Density	Specific Capacitance of the Device	Energy Density	Power Density	Retention	Ref.
		V	A g−1	F g−1	Wh kg−1	W kg−1	% (cycles)
MoSe2-Ni(OH)2//AC	6 M KOH	1.6	1	124	43	817	85 (5000)	[22]
Ni2P/NiSe2/MoSe2//AC	4 M KOH	1.6	0.5	66.1	23.5	400	116.6 (45,000)	[23]
NiSe2@MoSe2//AC	1 M KOH	1.5	1	305	79	738	87.35 (5000)	[121]
MoSe2@NiSe/NF//AC	0.4 M Fe(CN)63− /Fe(CN)64−	1.6	1	150.9 C g−1	54	806	92.8 (50,000)	[122]
Mn-doped MoSe2//AC	3 M KOH	1.2	1	112	22.25	1400	90 (5000)	[112]
MoSe2/MWCNT//MnO2	1 M LiCl	1.6	1.5	112	35.6	964	80 (2000)	[123]
NiCo2O4/MoSe2//AC	PVA/KOH	1.6	1	121.25	68.9	1280	95 (10,000)	[113]
MnO2/MoSe2/rGO//AC	PVA/H2SO4	1.8	1	85.1	30.2	807	80 (10,000)	[111]
MoSe2/rGO//MoSe2/rGO	PVA/KOH	1	0.3	35.1	4.88	150	83.1 (10,000)	[124]
MoSe2-GFs-25//MoSe2-GFs-25	PVA/KOH	0.6	1	243	48.7	600	78 (13,000)	[125]
MoSe2/e-Ti3C2Tx//MoSe2/e-Ti3C2Tx	1 M KOH	1	1	93 C g−1	12.92	1001.02	80 (5000)	[126]
MoSe2/MWCNT//MoSe2/MWCNT	1 M LiCl	1	3	129	17.9	1500	-	[123]
MoSe2@WSe2//MoSe2@WSe2	PVA/H2SO4	1	1	-	14.44	397	91.94 (10,000)	[127]
WSe2//AC	PVA/KOH	1.5	2	81.6	25.5	1111	77 (10,000)	[114]
WSe2@rGO//AC	PVA/KOH	1.5	2	145	51.5	2133.3	82 (3000)	[115]
WSe2-Mo-3@rGO//AC	PVA/KOH	1.6	2	194	70	1706	87 (3000)	[117]
WSe2@graphite//WSe2@graphite	1 M HCl	1.5	4 mA cm−2	88 mF cm−2	27.5 μWh cm−2	3000 μW cm−2	75.36 (5000)	[128]
MoS2//MoS2	0.5 M TEABF4	3	0.75	14.75	18.43	1125	91.2 (5000)	[129]
NiCo-LDH@MoS2CuS//AC	PVA/KOH	1.6	1	46.66 mAh g−1	152.6	539.9	90.05 (7000)	[130]
MoS2/Ni3S4@NiCr-LDH41//AC	6 M KOH	1.6	1	71.37	25.37	800	85.01 (10,000)	[131]
NiV-LDH/PANI/MoS2//AC	3 M KOH	1.2	6	182.5	36.51	3600	78.84 (8000)	[110]
MoS2/MWCNTs/polypyrrole//AC	1 M H2SO4	1.2	5 mV s−1	633.33	93.33	240.17	-	[132]
N-3DG//3D-IEMoS2@G	1 M NaClO4 in EC/DMC	1–4.3	-	-	140	630	99 (10,000)	[133]
1T-MoS2/Graphene-0.8//AC	1 M LiPF6	1–4	5	93	235.4	249.6	89.9 (2000)	[134]
MoS2/Fe2O3/Graphene//AC	3 M KOH	1.5	1	150.1	46.8	750	77 (10,000)	[7]
WO3/MoS2//AC	RAE	1.9	10	182.15	84.72	7624.8	-	[117]
ZnS/MoS2/NF//AC	2 M KOH	1.72	1	494	203	860	97 (5000)	[135]
MgS/MoS2@NF//AC	2 M KOH	1.72	4	701	288	3440	95 (10,000)	[26]
Co9S8-MoS2 NSA@HCSs//HCSs	6 M KOH	1.6	1	198	45.6	770.4	98.2 (10,000)	[118]
Ag/MoS2/NF//AC	2 M KOH	1.66	1.8	957	366	1494	94 (20,000)	[119]
WS2/ZrN//AC	1 M KOH	1.6	-	200	76	4325	90 (10,000)	[25]
WS2-MWCNT//WS2-MWCNT	PVA/H2SO4	1.2	1	275	46.15	500	89.14 (10,000)	[136]
ZnNi2O4/WS2//AC (ASC)	KOH	1.6	1	171.3	61.6	1236.5	68.4 (3000)	[120]
ZnNi2O4/WS2//AC (QSSASC)	PVA/KOH	1.6	1	56.8	20.4	921.2	97.2 (3000)	[120]
CuSe/WS2//CuSe/WS2 (SSC)	1 M (NH4)2SO4	1.5	1	100	31.3	750	73.2 (5000)	[137]
CuSe/WS2//CuSe/WS2 (QSSC)	PVA/(NH4)2SO4	1.6	1	135.6	48.2	800	80 (5000)	[137]
WS2/Ti3C2Tx/BN//Ti3C2Tx	PVA/KOH/KI gel	1	1	140	19.4	997.7	84 (10,000)	[28]
Mx-WS2-Hal@Ni-Adsorbed// Mx-WS2-Hal@Ni-Adsorbed	1 M Na2SO4	1.7	1.75	251.86	59.47	583	90 (10,000)	[138]
PANI/Graphene/WS2// PANI/Graphene/WS2	1 M H2SO4	1	1	71.7	9.96	250.04	71.6 (10,000)	[139]
WS2/Z8-800//Z8-800	1 M H2SO4	1.4	1	88	25	801	78 (3000)	[140]

presents a comparative analysis of the previously discussed TMD-based composites.

The structure exerts a significant influence on performance. For instance, the MoSe₂-Ni(OH)₂ [22,25] 2D layered heterojunction structure detailed in Table 1 is formed by the combination of 5–8 nm MoSe₂ nanosheets and 3–5 nm Ni(OH)₂ nanosheets via van der Waals forces and electronic interactions. This structure maximizes the exposure of active sites while shortening the ion diffusion paths in the electrolyte. The two components form a heterostructure, and electronic coupling between Mo⁴⁺/Mo⁶⁺ and Ni²⁺/Ni³⁺ exists at the heterojunction interface, which reduces the interfacial charge transfer resistance. As a result, a capacitance retention of 82% is maintained even at 10 A g⁻¹, far exceeding the 55% retention of pure Ni(OH)₂. Furthermore, the Ni₂P/NiSe₂/MoSe₂ hybrid [23] is synthesized via a stepwise phosphidation–selenization method, forming a ternary multi-heterojunction. Ni₂P nanoparticles are loaded on the surface of the NiSe₂-MoSe₂ heterojunction (composed of ultrathin nanosheets), presenting three-layer heterointerfaces. The heterojunction’s BET specific surface area reaches 156 m² g⁻¹; this ultra-high specific surface area accommodates a large number of active sites, which facilitates the multivalent redox reactions of Ni²⁺/Ni³⁺/Se²⁻ to provide pseudocapacitance. The multiple heterojunction interfaces form continuous charge transfer paths, with an interfacial barrier of only 2.5 Ω. This enables a specific capacitance of 2100 F g⁻¹ at 1 A g⁻¹, which is much higher than the 1175 F g⁻¹ of the aforementioned MoSe₂-Ni(OH)₂ binary composite system.

As demonstrated by the examples above, MoSe₂, WSe₂, MoS₂, and WS₂ exhibit significant differences in their electronic properties and stability due to their different central metal atoms and chalcogen elements. Under identical structural and phase conditions, MoSe₂- and WSe₂-based electrodes typically exhibit superior rate capability and lower internal resistance compared to MoS₂ and WS₂, consequently delivering higher P_d at high current densities. This is attributed to the stronger covalent nature of the metal–selenium bonds, which results in a narrower band gap and better electrical conductivity. Furthermore, molybdenum-based composites show higher Sp.C values than their tungsten-based counterparts. This is due to a greater contribution from pseudocapacitance, as molybdenum-based materials possess more abundant redox-active sites for Faradaic reactions. In addition to these findings, it is evident from the aforementioned studies that for a given material, the 1T phase demonstrates an enhancement in both specific capacitance and power density compared to the 2H phase. Lastly, the composites of all four materials show relatively excellent cycling stability.

In general, ASC devices exhibit broader potential windows and higher E_d values compared with symmetrical supercapacitors, primarily due to differences in electrolyte composition, electrode structure, and material configuration. First, the electrochemical performance of ASC systems is strongly influenced by electrolyte–electrode interactions, as an optimized electrolyte can effectively expand both the potential window and Ed. Moreover, hierarchically porous structures and defect engineering have been demonstrated to accelerate charge transport, while optimized architectures further improve electrode stability.

Furthermore, the integration of redox-active TMDs with conductive frameworks or metallic phases has proven highly effective in reducing R_ct and facilitating interfacial electron transport. Such catalytic interfaces contribute not only to enhanced pseudocapacitance but also to improved energy efficiency and rate capability. Recent studies further emphasize the importance of electronic structure regulation in TMD-based hybrids. By modulating the density of states near the Fermi level through orbital hybridization or heterojunction formation, researchers have achieved improved electronic delocalization and accelerated Faradaic kinetics. These insights underscore the growing significance of electronic structure engineering as a complementary strategy to morphological optimization, both of which are essential for enhancing interfacial charge transfer and overall electrochemical performance.

In addition, the inherent mechanical flexibility of layered TMDs facilitates their application in solid-state and flexible ASC devices. The development of hybrid electrodes combining WS₂, MXenes, and BN has demonstrated the feasibility of achieving high capacitance under conditions of mechanical deformation, thereby extending the applicability of TMD-based ASCs to wearable energy storage technologies.

Future directions for TMD-based electrodes include the following:

• Improving catalytic activity by increasing active site density, ensuring that catalytic activity occurs across the entire basal plane of 2H-phase TMD nanosheets rather than being limited to edge sites.
• Preventing interlayer restacking of TMD nanosheets to improve electrode stability and electrical conductivity.
• Synthesizing well-designed heterostructures, such as vertically and laterally structured hetero lattices, to achieve specific applications.
• Exploring redox-additive electrolytes and interface-coupled reaction systems to extend the potential window and enable synergistic redox reactions at the electrode–electrolyte interface.
• Investigating the application of TMDs in asymmetric supercapacitors under real-world conditions, such as temperature variation and long-term mechanical stress, especially in miniaturized and flexible energy storage platforms.