a Anhui Province Key Laboratory of Pollutant Sensitive Materials and Environmental Remediation, College of Energy Science and Engineering, Huaibei Normal University, Huaibei 235000, China
b Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid-State Physics, HFIPS, Chinese Academy of Sciences, Hefei 230031, China.
The development of efficient bifunctional materials is crucial for self-powered flexible systems. Few-layer 1T-MoS2, featuring a layered structure and abundant active sites, is an ideal candidate for supercapacitors and the hydrogen evolution reaction (HER). However, conventional synthesis methods provide limited control over active site exposure. Here, we anchor few-layer 1T-MoS2 nanosheets onto carbon cloth and subsequently apply plasma treatment. This treatment uniformly introduces active sites, facilitates the 2H to 1T phase transition, and enhances the specific surface area. As a result, the optimized MoS2-100 (plasma treatment at 100 W) electrode delivers a high areal capacitance of 646.6 mF cm-2 at 1 mA cm-2. A symmetric supercapacitor assembled with MoS2-100 achieves an energy density of 135.6 μWh cm-2 at 800 μW cm-2 and retains 92.7% of its initial capacitance after 4,000 cycles. Moreover, the electrode exhibits excellent HER activity in acidic medium, with a low overpotential of 289 mV at ~10 mA cm-2 and a Tafel slope of 90.7 mV dec-1. Linear fitting results further reveal that MoS2-100 achieves a Cdl value of 59.9 mF cm-2, approximately twice that of MoS2-0 (untreated). In summary, this work presents a simple and scalable strategy for fabricating few-layer MoS2-based self-supported electrodes with increased active site density, enabling bifunctional capabilities for flexible energy storage and electrocatalysis.
Against the backdrop of the global energy transition, countries are actively driving change due to their dependence on fossil fuels and the issue of greenhouse gas emissions [1,2]. Among these efforts, materials with dual functionality as both supercapacitor electrode materials and hydrogen evolution reaction (HER) electrocatalysts have become a research priority and key breakthrough direction in the field of clean and sustainable energy technologies [3,4]. These materials can simultaneously address the two major demands of energy storage and hydrogen production, effectively bridging the gap between renewable energy storage and clean hydrogen generation [5]. Currently, supercapacitors are regarded as an ideal solution for storing intermittent renewable energy due to their high power density, excellent reversibility, and ultra-long cycle life. However, their relatively low energy density remains a significant challenge, necessitating the support of high-performance electrode materials [6-8]. Meanwhile, hydrogen energy, as a clean and efficient energy carrier, offers great potential for the future energy transition. Water splitting is a key pathway for sustainable hydrogen production. In this context, efficient, stable, and low-cost HER electrocatalysts are central to reducing energy consumption and promoting the large-scale application of hydrogen energy [19-22]. Dual-functional materials are precisely capable of addressing the core technical bottlenecks in both above fields simultaneously.
Molybdenum disulfide (MoS2), a typical layered transition metal dichalcogenide, has garnered significant research interest in energy storage and electrocatalysis due to its unique electronic structure and tunable layer numbers [9-11]. However, traditional multilayer MoS2 exhibits poor electrochemical performance due to sluggish ion diffusion from narrow interlayer spacing, low active site accessibility, and reduced conductivity from interlayer stacking [12-14]. In recent years, few-layer MoS2 has emerged as an effective strategy to overcome these limitations[15-17]. By reducing the layer number, FL-MoS2 expands interlayer spacing to facilitate rapid ion transport, exposes more edge active sites (which serve as HER active centers), and helps stabilize the highly conductive 1T phase, thereby substantially improving reaction kinetics[18] [23,24]. Nevertheless, However, few-layer MoS2 still suffers from limited active site exposure, insufficient conductivity, and poor structural stability. [25-27].
The self-supported electrode structure is considered an effective strategy to address these issues [28-30]. By directly growing nanostructured arrays of electroactive materials on a conductive substrate, without the need for additional binders or conductive additives, the utilization efficiency of the active material can be significantly improved, and efficient charge and ion transport can be promoted. To further optimize electrode performance, plasma treatment has been introduced as an effective surface modification method [31,32]. Performing the treatment can enhance material conductivity, introduce structural defects, increase intercalation active sites, and promote surface conversion reactions, thereby comprehensively boosting the electrochemical activity of the electrode [33-35].
The work reports the in-situ growth of few-layer MoS2 nanostructures on carbon cloth via a hydrothermal method, followed by modification through plasma treatment. Benefiting from the rapid ion transport and abundant edge active sites provided by the few-layer structure, the efficient charge conduction of the self-supported electrode, and the synergistic enhancement effect introduced by plasma treatment, the as-prepared MoS2-100 composite material exhibits excellent performance in both energy storage and electrocatalysis. For supercapacitor applications, the MoS2-100 electrode delivers a high reversible areal capacitance of 646.6 mF cm-2 at a current density of 1 mA cm-2, representing a 52.2% increase in capacitance compared to MoS2-0. Furthermore, the electrode demonstrates good cycling stability, retaining 77.2% of its initial capacitance after 2000 cycles at a high current density of 20 mA cm-2. A symmetric flexible device assembled with MoS2-100// MoS2-100 achieves a specific capacitance of 381.5 mF cm-2 at 1 mA cm-2, along with high energy and power densities of 45.2 μWh cm-2 at a power density of 4000 μW cm-2. Moreover, the device retains 78.0% of its initial capacitance after 12000 cycles at a high current density of 30 mA cm-2, demonstrating excellent electrochemical stability. On the other hand, the MoS2-100 catalyst exhibits remarkable HER electrocatalytic activity in acidic electrolyte, with a very low overpotential of 289 mV at -10 mA cm-2 and a low Tafel slope of 90.7 mV dec-1. The reported work provides a novel and scalable strategy for constructing flexible self-supported electrodes for electrochemical energy storage and electrocatalysis.
2. Experimental section
2.1 Preparation of MoS2-0, MoS2-50, MoS2-100, and MoS2-150
Materials synthesis: 0.411 g of sodium molybdate dihydrate (Na2MoO4⋅2H2O), 0.368 g of thioacetamide (C2H5NS), and 0.259 g of sodium borohydride (NaBH4) are homogeneously mixed in 12 mL of deionized water and 6 mL of alcohol. The solution is then thoroughly stirred using a magnetic stirrer and subsequently transferred to a polytetrafluoroethylene-lined stainless-steel autoclave, into which a piece of pre-cut carbon cloth is placed. The autoclave is heated to 200℃ and held at the same temperature for 20 hours. After cooling naturally to room temperature, the carbon cloth is rinsed repeatedly with deionized water and ethanol. The sample is then dried in a constant-temperature oven at 60 °C for about 12 hours to obtain MoS2-0. The as-prepared MoS2-0 is then subjected to plasma treatment at different power levels (50 W, 100 W, and 150 W), yielding the corresponding MoS2-50, MoS2-100, and MoS2-150. The plasma treatment was performed in air using a radio-frequency plasma system. The treatment duration was 5 min, the working pressure was 50 Pa, the gas flow rate was 80 sccm, and the distance between the sample and the plasma source was 15 cm.
2.2 Electrochemical measurement and evaluations
Electrochemical performance testing of supercapacitors is conducted using a three-electrode system. The system consists of a 1×1 cm2 platinum plate as the counter electrode, an Ag/AgCl (in 1 M KCl) electrode as the reference electrode, and a working electrode composed of a self-supported molybdenum disulfide supported on carbon cloth. All electrochemical measurements are carried out in a 1 M Na2SO4 electrolyte using an Ivium Vertex. C. DC electrochemical workstation (Netherlands), from which cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) data atr collected.
The flexible asymmetric supercapacitor (ASC) device is assembled as follows: First, 1 g of polyvinyl alcohol powder is dissolved in 10 mL of deionized water, and the mixture is stirred continuously while heating at 90°C until the solution becames clear. Subsequently, 3-5 mL of an aqueous 1 M Na2SO4 solution is added dropwise under continuous stirring. Using MoS2-100 as both the positive and negative electrode, a flexible symmetric supercapacitor device is constructed by sandwiching a separator between the electrodes and filling the assembly with the prepared gel electrolyte.
Electrocatalytic tests are also performed in a three-electrode configuration, with a 1×1 cm2 platinum plate as the counter electrode, a Hg/HgO (in 1 M KCl) electrode as the reference electrode, and the tested electrode as the working electrode. The electrolyte used is 0.5 M H2SO4. Measurements, including linear sweep voltammetry (LSV), EIS, and cyclic voltammetry (CV), are carried out using an Ivium Vertex. C. DC electrochemical workstation (Netherlands). The HER polarization curves were recorded at a scan rate of 5 mV s-1 using an exposed electrode area of 1 cm2, and the electrolyte was purged with N2 for 30 min before measurement. The measured potentials were converted to the reversible hydrogen electrode scale according to ERHE = EHg/HgO + 0.095 + 0.059pH. The HER polarization curves were reported without iR compensation to avoid overestimating the intrinsic activity, and this point is explicitly stated in both the Experimental section and the Figure 6 caption. Because MoS2-0, MoS2-50, MoS2-100 and MoS2-150 were tested under identical conditions, the relative activity trend remains reliable.
3. Results and discussion
As illustrated in Scheme 1, the synthesis of MoS2-100 involves two key steps. First, a uniform nanoflower-like MoS2-0 is grown in situ on the carbon cloth fibers via a hydrothermal method, exhibiting well-ordered lattice fringes and a homogeneous distribution of two-dimensional nanosheets. Subsequently, MoS2-0 is subjected to surface modification using 100 W plasma treatment, resulting in the formation of MoS2-100, which displays a dispersed nanosheet morphology and significantly reduced lattice regularity. During the plasma treatment, the continuous bombardment of high-energy particles not only disrupts the periodic arrangement of the original lattice and induces abundant structural defects but also promotes the phase transformation from the semiconducting 2H phase to the metallic 1T phase [36]. Acting through such a dual modulation mechanism, the system effectively increases the density of electrochemically active sites and significantly enhances surface conductivity and charge transfer efficiency. Consequently, the plasma-optimized MoS2-100 demonstrates superior comprehensive performance in both supercapacitor energy storage and electrocatalytic hydrogen evolution reactions.
The morphologies of the MoS2-0 and MoS2-0 are observed by scanning electron microscopy (SEM). From Figure 1a, after hydrothermal treatment, the carbon cloth fiber surface is uniformly and continuously coated with MoS2 nanosheets. As shown in Figure 1b and Figure 1c, significant morphological changes in MoS2 are observed after 100 W plasma treatment. As shown in Figure S1, the untreated MoS2 exhibits a nanoflower-like structure composed of randomly oriented and extended nanosheets. With increasing plasma power, the random orientation of the nanosheets gradually decreases. The observed morphological evolution is primarily attributed to the bombardment of the material surface by high-energy particles in the plasma. Within the nanoflower structure, the protruding, loosely bound, and edge-curved regions possess higher surface energy and weaker binding forces, making them preferentially subjected to selective sputtering and etching under plasma exposure. The energy-dispersive X-ray spectroscopy (EDS) elemental mapping in Figure 1e shows that Mo, S, C, and O elements are all distributed continuously and uniformly in the two-dimensional space of the material. The C signal mainly originates from the carbon cloth substrate, while the presence of O can be attributed to mild oxidation occurring during the material preparation and plasma treatment processes. These results indicate that even after plasma bombardment, no significant regional enrichment or segregation of elements is observed, demonstrating that while plasma treatment regulates the morphology and phase structure of the material, it does not disrupt the overall homogeneity of its chemical composition. Such uniform elemental distribution contributes to maintaining the structural stability of the material during electrochemical processes, laying an important foundation for enhancing its cycling stability and catalytic activity [37].
To further elucidate the effect of plasma treatment on the microstructure of MoS2, transmission electron microscopy (TEM) observations are conducted on MoS2-0 and MoS2-100. As shown in Figure S2 and Figure 1f, after 100 W plasma treatment, MoS2-100 retains its typical few-layer structure, with the number of layers generally below 10. The finding indicates that although plasma treatment significantly alters the macroscopic morphology of the material, it does not fundamentally disrupt the intrinsic layered stacking structure of MoS2. High-resolution TEM observations (in Figure 1g and Figure 1h) reveal that the untreated MoS2-0 exhibits clear and continuous lattice fringes, demonstrating a long-range ordered atomic arrangement. In contrast, the plasma-treated MoS2-100 (in Figure 1i and Figure 1j) shows a distinct reduction in lattice ordering, presenting a structural feature characterized by long-range disorder and short-range order. These findings suggest that while plasma treatment largely preserves the local lattice structure, it introduces a high density of defects such as grain boundaries, dislocations, and vacancies, thereby disrupting the long-range periodic order of the crystal [38]. Such alterations in microstructure contribute to exposing more edge-active sites, enhancing surface reactivity, and optimizing charge carrier transport pathways, thus providing a key structural foundation for improving the electrochemical and catalytic performance.
Figure 2a presents the X-ray diffraction (XRD) patterns of MoS2-0, MoS2-50, MoS2-100, and MoS2-150. Diffraction peaks are observed at 10.1°, 32.5°, and 57.3°, corresponding to the (002), (100), and (110) crystal planes of MoS2, respectively, confirming the characteristic layered crystal structure of the material. The peak around 25.5° originates from the carbon cloth substrate, indicating successful loading of the samples onto the conductive support. By comparing the XRD patterns after plasma treatment at different powers, it is observed that the positions and intensities of the main diffraction peaks show no significant changes, suggesting that plasma treatment does not cause substantial distortion of the MoS2 lattice parameters. As demonstrated, within the power range employed in the current study, plasma treatment primarily modulates the surface morphology and micro defect structure, without fundamentally disrupting its bulk layered crystal structure. In the Raman spectra, the peaks located at 377.3 cm-1 and 405.4 cm-1 are assigned to the ${\mathrm{E}}_{2\mathrm{g}}^{1}$ and A1g vibrational modes of the 2H phase MoS2, respectively, while the characteristic peaks observe at 153.6 cm-1, 197.3 cm-1 and 220.8 cm-1, 284.0 cm-1, and 341.3 cm-1 correspond to the J1, J2, E1g, J3, and other vibrational modes of the 1T phase MoS2. The intensity of the 1T‑related peaks reflects the relative content of the metallic phase in the materials. Notably, the 1T‑phase features in MoS2-100 are significantly stronger than those in MoS2-0, indicating that plasma treatment effectively promotes the phase transition from 2H to 1T.
To quantitatively analyze the relative content of the 1T and 2H phases in the materials, X-ray photoelectron spectroscopy (XPS) is performed. Figure 2c displays the Mo 3d XPS spectra of all samples. The region typically shows two prominent peaks corresponding to the binding energies of the Mo 3d5/2 and Mo 3d3/2 orbitals in MoS2. According to literature, the Mo 3d binding energies of 1T-MoS2 are shifted toward lower values by approximately 0.9 eV compared to those of the 2H phase [39]. As shown in Figure 2c, the Mo 3d5/2 and Mo 3d3/2 peaks of the 1T and 2H phases in MoS2-0 are located at 228.7 eV, 229.5 eV and 231.9 eV, 233.0 eV, respectively. Based on XPS peak fitting analysis, the 1T phase content in MoS2-100 reaches as high as 60%, significantly exceeding that of MoS2-0 (48%), MoS2-50 (55%), and MoS2-150 (42%). These results indicate that appropriate plasma treatment (e.g., 100 W) effectively promotes the formation of the 1T phase in MoS2. In contrast, the notable decrease in 1T phase content observed for MoS2-150 may be attributed to excessive plasma bombardment, which could induce structural collapse, thereby driving partial reversion of the 1T phase back to the more thermodynamically stable 2H phase. There is a significant correlation between the XPS signal intensity of the Mo6+ species in the spectrum and the degree of oxidation layer on the sample. The more severe the oxidation, the stronger the Mo6+ peak.
Figure 2d presents the S 2p XPS spectra of all samples. The S 2p region similarly exhibits a doublet structure, corresponding to the binding energies of the S 2p3/2 and S 2p1/2 orbitals in MoS2. For MoS2-0, the S 2p3/2 peaks of the 1T and 2H phases are positioned at 161.6 eV and 162.9 eV, respectively, while the corresponding S 2p1/2 peaks are observed around 162.8 eV and 164.0 eV [40]. The phase ratios (1T/2H) derived from the peak fitting of the S 2p spectra are consistent with those obtained from the Mo 3d fitting (in Figure 2e), further confirming the reliability of the XPS phase quantification. These findings demonstrate that plasma treatment not only modifies the electronic structure of the material but also significantly regulates its phase composition, thereby influencing its electrochemical and catalytic performance.
Given the significant influence of plasma treatment on the structure of MoS2, nitrogen adsorption-desorption (BET) measurements are conducted to further investigate the changes in specific surface area and pore structure. As shown in Figure S3a, the adsorption isotherms of MoS2-0, MoS2-50, MoS2-100, and all exhibit typical type-IV isotherms with H3-type hysteresis loops, indicating the presence of mesoporous structures in all samples. Notably, plasma treatment within a certain power range effectively increases the specific surface area, with MoS2-100 displaying the highest value of 17.5 m2 g-1(MoS2-0: 12.1 m2 g-1; MoS2-50: 15.9 m2 g-1; MoS2-150: 9.8 m2 g-1), in Figure f. Pore size distribution analysis (in Figure S3b) further reveals that after plasma treatment, the proportion of mesopores decreases (in Figure 2g), while the proportion of macropores increases accordingly. The observed phenomenon may be attributed to the restructuring of nanosheets and the collapse or interconnection of partial mesoporous structures induced by plasma bombardment. Such structural changes are conducive to enhancing the transport efficiency of electrolyte ions, thereby potentially improving the electrochemical performance of the material [41]. The increased macropore fraction in MoS2-100 can facilitate electrolyte penetration into the self-supported nanosheet network, provide low-resistance ion-buffering channels, shorten the effective ion-diffusion pathway, and reduce concentration polarization at high current density, thereby improving the utilization of electrochemically active surfaces during supercapacitor operation.
The electrochemical performance of MoS2 as a supercapacitor electrode is evaluated using a three-electrode configuration in 1 M Na2SO4 electrolyte. The material maintains stable operation within a wide potential window of -0.6 V to 0.2 V (vs. Ag/AgCl), indicating its potential for achieving high energy density. As shown in Figure 3a, the cyclic voltammetry (CV) curves of MoS2-0, MoS2-50, MoS2-100, and MoS2-150 all exhibit nearly rectangular shapes at a scan rate of 20 mV s-1, suggesting a mixed charge-storage response involving both electric-double-layer-like surface contribution and pseudocapacitive/diffusion-controlled processes. The excellent rectangularity and symmetry of the CV curves confirm fast charge-storage kinetics and high electrochemical reversibility. Among them, MoS2-100 displays the largest enclosed CV area, implying a higher specific capacitance. To quantitatively compare the areal capacitance of different samples, GCD tests are performed (in Figure 3b). The areal specific capacitance is calculated using the formula (1) yielding values of 424 mF cm-2, 581 mF cm-2, 646.6 mF cm-2, and 515 mF cm-2 for MoS2-0, MoS2-50, MoS2-100, and MoS2-150, respectively.
$C =\frac{ I\Delta t}{S\Delta V}$
The results indicate that the specific capacitance of MoS2 does not follow a simple linear relationship with the plasma treatment power; instead, an appropriate power (100 W) can effectively enhance the capacitive performance. As shown in Figure 3c, MoS2-100 consistently delivers the highest specific capacitance across current densities ranging from 1 mA cm-2 to 20 mA cm-2. Furthermore, EIS reveals that MoS2-100 possesses lower contact resistance, suggesting improved electrical conductivity (in Figure 3d). In Equation (1), C is the areal capacitance (mF cm-2), I is the discharge current (mA), Δt is the discharge time (s), S is the geometric area of the working electrode (cm2), and ΔV is the potential window after excluding the iR drop (V). The values in the three-electrode test are calculated on the basis of the single working electrode.
To elucidate the charge-storage mechanism, CV curves are measured at different scan rates ranging from 5 to 100 mV s-1. It evaluated the capacitor-like properties using the equation
$i = a{v}^{b}$
where i denotes the current, v is the scan rate, and a and b are constants. When the value of b approaches 1, the electrode exhibits capacitive characteristics, while when the value of b approaches 0.5, the electrode exhibits battery-like characteristics. As shown in Figure 3e, kinetic analysis reveals that the slope (b-value) derived from the linear fitting of log(i) versus log(v) is approximately 0.75, indicating a charge storage mechanism involving both surface-controlled capacitive contribution and diffusion-controlled ion transport, which is characteristic of pseudocapacitive behavior. Moreover, the b-values across different applied potentials show little variation among all samples, suggesting that plasma treatment does not significantly alter the fundamental energy storage mechanism of MoS2. In Figure 3f, the capacitive contribution is further quantified by separating the current response using the equation (3).
$i\left(v\right)= {k}_{1}v + {k}_{2}{v}^{1/2}$
The results show that the capacitive contributions of MoS2-100 at 30 mV s-1, 50 mV s-1 and 100 mV s-1 are 33.6%, 38.4% and 50.4%, respectively. Quantitative analysis reveals that diffusion-controlled processes dominate charge storage. Cycling stability is a critical parameter for supercapacitor. After 2000 cycles at 20 mA cm-2 (in Figure 3g), the capacitance retention rates for MoS2-0, MoS2-50, MoS2-100, and MoS2-150 are 84.2%, 79.1%, 77.2%, and 86.4%, respectively. This result reflects an activity-stability trade-off caused by plasma-induced defect engineering. A moderate plasma power (e.g., 100 W) introduces a large number of defects into MoS2, which improves capacitance but may also reduce structural integrity during repeated ion insertion/extraction, leading to lower capacitance retention than the more structurally stable MoS2-150 sample. In contrast, the recovery of cycling stability for MoS2-150 results from the partial structural collapse and rearrangement induced by excessively high-power plasma bombardment, driving the material toward a more stable structural state. This result is highly consistent with the XPS and BET analysis results presented earlier.
To evaluate the potential of MoS2-100 for wearable applications, a flexible symmetric supercapacitor (denoted as MoS2-100//MoS2-100) is assembled using the material as both positive and negative electrodes (in Figure 5a), and its electrochemical performance is systematically tested. The device operates stably within a voltage window of -0.6 V to 1.0 V (in Figure 5b). Cyclic voltammetry tests at different scan rates reveal that the MoS2-100//MoS2-100 symmetric device maintains well-shaped, symmetrical rectangular CV curves, indicating typical electric double-layer capacitive behavior. Galvanostatic charge-discharge measurements further show that the device delivers an areal specific capacitance of 381.5 mF cm-2 at a current density of 1 mA cm-2. The variation of specific capacitance with current density is presented in Figure S4. At a power density of 800 μW cm-2, a maximum energy density of 135.6 μWh cm-2 is achieved; even when the power density is increased to 16000 μW cm-2, the energy density remains at 12.8 μWh cm-2. Table 1 and Figure 5d compare the key performance metrics of the MoS2-100//MoS2-100 supercapacitor developed in the present work with those of other symmetric devices reported recently. The data demonstrate that the device exhibits significant advantages in both energy density and power density. The excellent comprehensive performance results from the effective optimization of the conductive network and ion transport pathways in MoS2 through plasma treatment. For the assembled two-electrode device, the areal capacitance is calculated from the full-device discharge curve using the same geometric area basis. The areal energy density and areal power density are calculated as the equation (4) and (5).
where E is the areal energy density (μWh cm-2), P is the areal power density (μW cm-2), and Δt is the discharge time (s).
Additionally, the flexible freestanding structure eliminates the need for inactive binders, thus enabling higher energy storage capacity while maintaining high power output. To investigate the mechanical stability of the device under conditions relevant to wearable applications, the influence of different bending angles (0°, 90°, 180°) on its electrochemical performance is systematically studied via cyclic voltammetry at a scan rate of 30 mV s-1. As shown in Figures 5e and 5f, both the shape and integrated area of the CV curves remain essentially unchanged under different bending states, indicating that the flexible device maintains stable electrochemical response and structural integrity during mechanical deformation. Such a characteristic suggests strong adhesion between the electrode and the substrate. It also indicates that no significant delamination or structural damage of the active material occurs during repeated bending, thereby ensuring reliable electrochemical performance of the device in practical flexible applications.
Furthermore, a symmetric flexible supercapacitor assembled based on MoS2-100 (MoS2-100//MoS2-100) undergoes 4000 charge-discharge cycles at a high current density of 30mAcm-2 (in Figure5g). The results show that the device retains 92.7% of its initial capacitance after cycling (pink curve), while the Coulombic efficiency remains close to 100% throughout the test (blue curve). It exhibits outstanding cycling stability, coupled with highly reversible electrochemical behavior. This excellent cycling performance is mainly attributed to the optimization of the MoS2 microstructure by plasma treatment. The optimized microstructure not only improves the conductivity and ion transport efficiency of the material but also enhances its structural stability during repeated charge and discharge processes. Consequently, it effectively suppresses the detachment of active materials and irreversible structural degradation.
As a typical transition metal dichalcogenide, MoS2 shows great potential for HER. Its catalytic activity is primarily determined by active sites located at the material edges. Plasma treatment effectively increases the density of these active sites, imparting a significant catalytic advantage. Notably, the HER pathways differ depending on the electrolyte: in acidic media, the MoS2-catalyzed HER typically proceeds via either the Volmer‑Heyrovsky or Volmer‑Tafel route[48]. The detailed steps can be described as follows:
In the present step, hydronium ions migrate from the electrolyte to the active sites on the catalyst surface (* represents a vacant and available active site on the surface of the catalyst), accept electrons to form adsorbed hydrogen atoms (H), and release water molecules.
The adsorbed hydrogen atom combines with another hydronium ion and an electron to generate hydrogen gas, which desorbs from the catalyst surface, simultaneously restoring the active site.
Under conditions of relatively high active site density or favorable reaction kinetics, two adjacent adsorbed hydrogen atoms can directly combine to form a hydrogen molecule, thereby freeing two active sites.
Plasma treatment is considered an effective strategy for introducing active sites. To confirm this and to investigate the influence of different treatment powers, a systematic HER performance evaluation is conducted on MoS2 subjected to plasma treatment at various power levels. Among all tested samples, MoS2-100 demonstrates the most outstanding HER catalytic activity. As shown in Figure 6a, MoS2-100 requires an overpotential of only 289 mV (η10 = 289 mV) to achieve a current density of 10 mA cm-2. Further results in Figure 6b show that MoS2-100 reaches current densities of -10 mA cm-2 and -20 mA cm-2 at low overpotentials of 289 mV and 320 mV, respectively. In comparison, the other samples exhibit higher overpotentials (MoS2-0: 304 mV/353 mV; MoS2-50: 304 mV/342 mV; MoS2-150: 309 mV/343 mV). A lower overpotential means that less external energy is required to drive the HER, and the energy conversion efficiency of the catalyst is higher. The significantly reduced overpotential of MoS2-100 indicates that plasma treatment effectively increases the density of active sites and accelerates the reaction kinetics, thereby enabling the material to exhibit superior electrocatalytic hydrogen evolution performance. The improved HER activity is attributed to the synergistic effect of the metallic 1T phase and plasma-induced edge/defect active sites: the 1T phase enhances electrical conductivity and electron transfer, while plasma etching exposes more accessible reaction centers for proton adsorption and hydrogen evolution.
The HER kinetics are further evaluated through Tafel slope analysis. As shown in Figure 6c, MoS2-100 exhibits a significantly lower Tafel slope than the other comparative samples, indicating faster reaction kinetics. Additionally, to assess the electrochemical active surface area (ECSA) of the catalysts, the double-layer capacitance (Cdl) is measured in the non-Faradaic region based on the positive correlation between ECSA and Cdl. As presented in Figure 6d and Figure 6e, after linear fitting, MoS2-100 shows a high Cdl value of 59.9 mF cm-2, which is notably greater than that of the other samples (MoS2-0: 27.4 mF cm-2; MoS2-50: 48.7 mF cm-2; MoS2-150: 18.4 mF cm-2). The finding also suggests that plasma treatment exposes more active sites, thereby enhancing its electrocatalytic performance.
In addition to catalytic activity, electrochemical stability is also a critical metric for assessing the practical application potential of catalysts. As shown in Figure 6f, after 25h of continuous testing at a constant current density of -10 mA cm-2, MoS2-100 exhibits only a slight potential decay of about 23 mV, demonstrating good electrochemical stability. However, compared with MoS2-0 (14 mV), MoS2-50 (9 mV), and MoS2-150 (13 mV), the potential decay of MoS2-100 is relatively higher, indicating that its stability is not the most favorable among the compared samples. This arises from the double-edged sword effect of plasma treatment: low-to-moderate power introduces defects that compromise structural stability, whereas excessively high power leads to structural collapse and densification, which in turn enhances long-term cycling stability, a finding consistent with the cycling test results shown in Figure 3g.
4. Conclusion
In this work, a plasma treatment strategy is successfully employed to introduce abundant active sites into few-layer MoS2 self-supporting materials. This treatment effectively modulates the material structure, significantly enhancing both its energy storage and electrocatalytic performance. As a result, the as-prepared self-supporting electrode exhibits excellent bifunctional electrochemical activity. Among the tested samples, MoS2-100 (treated at 100 W) delivers the best performance, achieving a high specific areal capacitance of 646.6 mF cm-2 at 1 mA cm-2. A symmetric flexible supercapacitor assembled with MoS2-100 electrodes demonstrates a remarkable areal capacitance of 381.5 mF cm-2 at 1 mA cm-2, along with high energy densities of 135.6 μWh cm-2 and 45.2 μWh cm-2 at power densities of 800 μW cm-2 and 4000 μW cm-2, respectively. The device also exhibits excellent long-term cycling stability, retaining 92.7% of its initial capacitance after 4000 cycles at 30 mA cm-2. Notably, the MoS2-based electrode shows high electrocatalytic activity in acidic medium, with a low overpotential of 289 mV at 10 mA cm-2. Linear fitting results reveal that MoS2-100 achieves a Cdl value of 59.9 mF cm-2, approximately twice that of MoS2-0, further confirming that plasma treatment enhances electrocatalytic performance through the combined effects of improved conductivity and increased accessible active sites. This work provides new insights into the development of high-performance MoS2-based electrode materials through defect engineering and interface optimization, and offers a valuable reference for advancing flexible energy storage and electrocatalysis technologies.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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