A comparative study of W, V, and Co doping in MoSe₂ for high-performance flexible all-solid-state supercapacitors

Abstract

The importance of supercapacitor electrode materials and devices undoubtedly stands out in relation to energy storage devices. Selecting a suitable electrode material is crucial for characterization and electrochemical studies. Herein, we report a one-step hydrothermal strategy for the fabrication of molybdenum diselenide (MoSe₂) and 2% tungsten-, vanadium- and cobalt-doped MoSe₂, along with characterization. The structural and morphological changes in the various nanocomposites were confirmed by powder X-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectra. Electrochemical studies reveal significant improvement in specific capacitance, with the Co@MoSe₂ electrode exhibiting the highest specific capacitance of 518 F g⁻¹ at 1 A g⁻¹ over a potential window of 0.7 V in 2 M KOH electrolyte solution. The fabricated symmetric supercapacitor device (SSD) delivered a specific capacitance of 127 F g⁻¹ at 1 A g⁻¹ over a potential window of 1.4 V. It achieved a maximum energy density of 34.54 W h kg⁻¹ with a power density of 700 W kg⁻¹. The device demonstrated excellent durability, with cycling stability of 80% even after 10 000 cycles. The results obtained are highly promising, indicating that these materials hold significant potential for commercial applications in energy storage devices.

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1. Introduction

Flexible electronics are becoming increasingly prevalent, paving the way for a more comfortable lifestyle for generations to come. There is an increasing demand for energy storage and conversion (ESC) devices with tiny volumes, compact size, superior security, outstanding technical longevity, and excellent electrochemical capabilities. High-performance ESC devices are ideal for powering flexible electronics, particularly multifunctional electronic devices that can monitor, detect, and transmit simultaneously.^1,2 Among ESC devices, supercapacitors stand out as an upgraded technology, receiving increasing attention for their high power density, fast charge–discharge rates, good efficiency, long cycle life, and flexibility. Supercapacitors can be classified into three types based on their charge-storage mechanism, namely (i) electric double-layer capacitors (EDLCs), which utilize carbon-based materials; (ii) pseudocapacitors, which are usually redox-active materials; and (iii) hybrid capacitors, which are a combination of EDLC and pseudo-materials.^3,4 Supercapacitors are now extensively used in a range of applications, particularly those requiring the storage/release of a large quantity of energy in a short period of time. They have recently been used mostly in electric vehicles due to their superior charge–discharge capacity and ability to perform efficiently over numerous cycles.^5,6 They can be used as a power backup in uninterruptible power supply (UPS) systems, in windmill tracking to smooth out the inconsistent power produced by the wind, and in robotics, to deliver short bursts of power, improving performance during variable tasks. Flexible supercapacitors have also been used in wearable electronics as batteries are too bulky and require a liquid electrolyte. All these qualities make them very appealing for a wide range of applications.^7,8 The electrochemical efficacy of supercapacitors is primarily dependent on the design and synthesis of the electrode materials, making the appropriate selection of electrode materials extremely important.⁷ Consequently, transition-metal dichalcogenides (TMDCs) are now being adopted as active electrode materials in supercapacitors owing to their distinctive characteristics, which include a substantial specific surface area, suitable dimensions, good electrical conductivity and thermal and chemical stability. Among several TMDCs, two-dimensional (2D) materials such as molybdenum diselenide (MoSe₂) have unique physical and chemical characteristics, such as high specific power, extended stability and cycle life, and enhanced safety profiles, winning out over other electrode materials for use in supercapacitors.^8,9 MoSe₂ is a significant material characterized by a substantial layer spacing of 0.65 nm, which reduces the energy barrier for the adsorption and desorption of charged ions.^4,10,11 MoSe₂ exhibits two characteristic structural phases: a metallic octahedral 1T phase and a semiconducting trigonal prismatic 2H phase. MoSe₂ naturally occurs in the 2H phase due to its thermodynamic stability; the 1T phase is metastable and can be reversibly transformed into the 2H phase. Given the robust catalytic activity attributed to the elevated conductivity of 1T-phase MoSe₂, phase transition engineering has been identified as an effective method to enhance its electrocatalytic performance.¹² With an unusual atomic structure, three layers of atoms make up MoSe₂, and they are stacked and joined by weak van der Waals¹³ contacts, with strong covalent bonding within the layers. Consequently, MoSe₂ layers are easily separated from the bulk and have been utilized in supercapacitors.^14,15 Gao et al. synthesized MoSe₂ using a facile hydrothermal method and obtained a specific capacitance of 243 F g⁻¹ at 0.5 A g⁻¹, and still kept 90.3% capacitance after 1000 cycles at 1 A g⁻¹.¹³ Regardless of the great theoretical capacity, limited electronic conductivity leads to low capacitances. To address this issue, a promising technique is to adorn MoSe₂ with highly conductive materials by doping. This offers a key method to alter its properties through the customization of the composition.¹⁶ For instance, Bhat et al. prepared W-doped MoSe₂/graphene, delivering an enhanced specific capacitance of 248 F g⁻¹ which was complimented by excellent capacitance retention capability of 102% for 20 000 cycles.¹⁶ Furthermore, Zhu et al. fabricated a supercapacitor device, which delivers a capacitance of 49.7 F g⁻¹ and a maximal energy density of 25.3 W h kg⁻¹ at a power density of 1143.6 W kg⁻¹.¹⁷ Thus motivated, this study presents a comparative analysis of a pure sample and samples with structural modifications and enhanced electrochemical properties achieved through material doping. Hence, we effectively synthesized molybdenum diselenide (MoSe₂) via a facile one-step hydrothermal method and investigated the effects of tungsten (W), vanadium (V), and cobalt (Co) doping on its electrochemical performance for supercapacitor applications. The selection of these transition metals was based on their distinct electronic configurations, which can modulate and enhance electrochemical properties. Specifically, tungsten doping is known to improve charge transfer kinetics and stability due to its high oxidation states and strong bonding with chalcogenides. Vanadium's multiple redox states can contribute to pseudocapacitance, increasing specific capacitance. Cobalt's fast redox activity and good electrical conductivity can facilitate faradaic reactions, enhancing energy storage capabilities. A comparative analysis of the structural, morphological, and electrochemical properties of pure and doped MoSe₂ samples was conducted to elucidate the impact of doping. The electrochemical results with 2 M KOH as the electrolyte solution in a three-electrode system indicated that the doped MoSe₂ electrode had enhanced storage capacity. The results demonstrated significant enhancements, with a 1- to 2-fold increase in specific capacitance compared with the pure sample. Additionally, the development of a novel supercapacitor was examined for symmetric device configurations. A significant enhancement in energy density was observed, with values increasing from 15.49 W h kg⁻¹ to 34.54 W h kg⁻¹, representing a substantial improvement and partially achieving one of the desired performance metrics for supercapacitor devices. Furthermore, the flexibility of the supercapacitor device was also tested, addressing a core obstacle for such devices and demonstrating potential for lightweight, portable, and compatible applications connected with present or next-generation energy storage and cutting-edge devices.

2. Experimental section

2.1. Materials

Sodium molybdate dihydrate (Na₂MoO₄·2H₂O), hydrazine hydrate (N₂H₄·H₂O), sodium tungstate dihydrate (Na₂WO₄·2H₂O), selenium powder (Se), ammonium metavanadate (NH₄VO₃), and cobalt chloride hexahydrate (CoCl₂·6H₂O) were purchased from Sigma Aldrich, and were 99% pure.

2.2. Synthesis of MoSe₂ and W-, V-, and Co-doped MoSe₂

Using the technique of hydrothermal heating, a MoSe₂ sample with a 1 : 2 weight ratio is obtained. Specifically, 160 mg of Se in 4 mL of N₂H₄·H₂O is allowed to age for 24 h, and this is denoted as solution A. Concurrently, 250 mg of Na₂MoO₄·2H₂O is dissolved in 55 mL of DIW, which is labelled as solution B. Solution A is then slowly added dropwise into solution B under continuous stirring; a red-colour solution appears. The resulting mixture is transferred to a 100-mL Teflon-lined stainless-steel autoclave and heated at 180 °C for 24 h, followed by cooling to room temperature. The sample is subsequently washed several times with DIW and ethanol with centrifugation. Finally, the sample is allowed to dry at 60 °C overnight. The entire process is illustrated in Scheme 1. To synthesize W- (Mo_0.98Se₂W_0.02), V- (Mo_0.98Se₂V_0.02), and Co-doped (Mo_0.98Se₂Co_0.02) MoSe₂, the precursors Na₂WO₄·2H₂O, NH₄VO₃, and CoCl₂·6H₂O, respectively, are added, replacing equal molar percentages of Na₂MoO₄·2H₂O, using the same technique as before (Scheme 1), and these are designated W@MoSe₂, V@MoSe₂, and Co@MoSe₂, accordingly.

Scheme 1 (a) An illustration of the synthesis procedure for the preparation of powder samples via a hydrothermal method, (b) an illustration of the 3-electrode setup and (c) the construction of the symmetric supercapacitor device.

2.3. Electrochemical measurements and fabrication of an all-solid-state flexible supercapacitor

To eliminate any potential oxide layer from the surface, a piece of nickel foam (NF) (1 cm × 1.5 cm) was immersed in 3 M HCl solution for 30 min. After multiple ultrasonic cleaning cycles with DI water, acetone, and ethanol, it was dried at room temperature. The active electrode material was prepared by mixing the active material with PVDF, as a binder, and activated charcoal at a mass ratio of 80 : 10 : 10 and grinding the mixture with a mortar and pestle. N-Methyl pyrrolidone (NMP) was then added to form a slurry. Following that, the resulting slurry was coated onto clean NF (1 cm × 1 cm) and set aside to dry at 70 °C for 6 h. About 3 mg of active material was coated on each piece of NF. A standard three-electrode electrochemical cell is employed with the prepared electrode, platinum wire, and Ag/AgCl as the working electrode, counter electrode, and reference electrode, respectively, in 2 M KOH solution, as depicted in Scheme 1(b). In a two-electrode system, the SSD was fabricated by sandwiching two identical prepared electrodes, separated by a Whatman filter paper with a thickness of 5 mm, with PVA–KOH gel as the electrolyte, as illustrated in Scheme 1(c). The gel electrolyte was prepared by dissolving 2 g of PVA and 2 g of KOH in 20 mL of DI water separately and stirring for 30 min. These two solutions were mixed together and maintained at 80 °C under vigorous stirring for 2 h. The positive and negative electrodes are constructed with prepared MoSe₂, W@MoSe₂, V@MoSe₂, and Co@MoSe₂. Both electrodes, with equal active material loading, had uniform contact with the electrolyte, ensuring effective utilization of the active material and balanced charge transfer during the tests. The electrochemical performance data are evaluated utilizing a Biologic SP-300 electrochemical workstation for cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS). The specific capacitance is calculated using eqn (1):¹⁸where, I is the current, dt is the discharge time, m is the active mass, dV is the potential range, and C indicates the specific capacitance.

The energy density (E, W h kg⁻¹) and power density (P, W kg⁻¹) of the assembled SSD are calculated as follows:

where V is the voltage window, E represents the energy density, and P is the power density.

3. Results and discussion

3.1. Structural and morphological studies

Verification of the condition and phase of the synthesized material has been accomplished using PXRD. Fig. 1(a) shows peaks at 2θ angles of 13.38°, 33.49°, 41.48°, 54.98°, and 65.54°, which correspond to the (002), (102), (006), (110), and (200) crystal planes of MoSe₂, respectively, and are matched with JCPDS no. 29-0914. The reaction equation, based on the above discussion, is given below:

Mo + 2Se + N₂H₄·H₂O → MoSe₂ + N₂ + 2H₂O (4)

Fig. 1 (a) Powder X-ray diffraction (PXRD) patterns of a pure MoSe₂ sample, W@MoSe₂, Co@MoSe₂ and V@MoSe₂. (b) Rietveld refinement performed on the XRD data of a pure MoSe₂ sample. (c) Raman spectra of MoSe₂, W@MoSe₂, Co@MoSe₂, and V@MoSe₂. (d) BET results and the corresponding BJH plots (inset).

The PXRD patterns of the W@MoSe₂, Co@MoSe₂, and V@MoSe₂ nanostructures show comparable peaks to those of pure MoSe₂ with enhanced peak intensities, despite the presence of a weak Se ring (*) and a small shift in the 2θ positions to the left.¹⁶ Additionally, MoSe₂ peaks are visible at angles of 27° and 56° for the (004) and (008) planes in the PXRD patterns of the doped nanostructures. These findings show that transition metals have been successfully doped into the host lattice and that crystallinity has increased. Fig. 1(b) shows refinement conducted on the pure sample, and the low chi-square value of 1.08 indicates the goodness of fit. The development of nanostructured multilayer MoSe₂ is seen in the PXRD patterns, which display a hexagonal phase of MoSe₂ with a space group of P6₃/mmc.¹⁹ According to the Debye–Scherrer equation given below, the average crystallite size of MoSe₂ was determined to be 2 nm:

D = kλ/β cos θ (5)

where the crystallite size D is measured in nanometers, the constant k is equal to 0.94, the material's wavelength λ is equal to 0.15 nm, the line expansion, β (FWHM), is the full width at half maximum in radians, and the Bragg angle is indicated by θ. PXRD analysis indicates an enhancement in the electrochemical performance by revealing a balanced crystalline peak pattern, resulting in structural stability and better conductivity with improved ionic transport pathways. The synthesis of MoSe₂ was further verified using Raman spectroscopy, as shown in Fig. 1(c). Two separate MoSe₂ peaks can be found in the spectrum at about 237.85 and 284.86 cm⁻¹. These peaks correspond to the out-of-plane A_1g and in-plane E¹_2g vibrational modes. Furthermore, the Raman spectra of W@MoSe₂, Co@MoSe₂, and V@MoSe₂ nanostructures show a minor shift to the right, demonstrating successful integration via MoSe₂ doping. This leads to improved electrochemical adaptability and more active sites for the charge-storage mechanism. The Brunauer–Emmett–Teller (BET) technique was employed to determine the specific surface areas and average pore volumes of the synthesized materials, as illustrated in Fig. 1(d). The analysis revealed that the Co@MoSe₂ sample exhibited the highest surface area of 113.03 m² g⁻¹ with a corresponding pore volume of 0.21 cm³ g⁻¹. In comparison, the V@MoSe₂ and W@MoSe₂ samples showed moderate surface areas of 51.6 m² g⁻¹ and 50.2 m² g⁻¹, respectively, with pore volumes of 0.18 cm³ g⁻¹ and 0.11 cm³ g⁻¹, respectively. The pristine MoSe₂ sample displayed the lowest surface area and pore volume, measured at 13.9 m² g⁻¹ and 0.03 cm³ g⁻¹, respectively. The BET analysis confirms that the incorporation of Co, V, and W atoms into the MoSe₂ matrix via hydrothermal synthesis occurs through substitutional doping at Mo sites. This atomic substitution induces a redistribution of electronic density, increases the density of electrochemically active sites, and strengthens the interactions among chemical bonds within the host lattice. These structural and electronic modifications collectively enhance the electrochemical performance of the electrode material. Moreover, the mesoporous frameworks of the doped samples facilitate improved electrolyte ion diffusion and maximize the electrode–electrolyte interfacial contact area, thereby contributing to superior charge-storage characteristics.²⁰ Fig. 2 shows magnified FESEM images of MoSe₂ and doped materials, indicating that they are laminated with varying sizes heaped together. Some nanorods with average diameters of 55 nm are also detected, along with nanoflowers assembled from ultra-thin nanosheets that tend to congregate and overlap with each other, as illustrated in the HRTEM image (Fig. 3) of MoSe₂.^10,16,19 This indicates time-dependent nanostructure growth with a subsequent increase in surface coverage by 3D nanoflower-like structures.²¹ As seen in Fig. 2(a), pristine MoSe₂ exhibits a flower-like morphology composed of agglomerated nanoparticles and occasional nanorods (∼42–58 nm). These features facilitate the formation of interconnected porous networks, offering enhanced ion accessibility and diffusion channels. In the case of V@MoSe₂ (Fig. 2(b)), the morphology evolves into larger spherical aggregates with particle sizes ranging from approximately 45 to 75 nm. This increase in particle size and surface roughness suggests enhanced nucleation and growth mechanisms during synthesis, potentially improving interactions between the electrode surface and the electrolyte, though it may slightly reduce overall porosity. For W@MoSe₂ (Fig. 2(c)), the sample exhibits a denser structure, with prominent nanorods (∼47–60 nm) intertwined within the matrix. The presence of these rod-like features indicates a more ordered structural arrangement, which could facilitate improved electron transport and contribute to greater mechanical stability. The Co@MoSe₂ sample (Fig. 2(d)) reveals a loosely packed configuration characterized by a mixture of thin nanorods and a sheet-like texture, with feature sizes in the range of 58–76 nm. This porous and layered morphology is likely to enhance the effective surface area and expose more electroactive sites, thereby promoting better ion diffusion and charge-storage capabilities. These findings suggest that while the nanoflower structure of MoSe₂ is generally retained across all doped samples, each dopant undergoes unique morphological transformations that directly influence the material's surface area, porosity, and electrochemical properties. In addition, as shown in Fig. 2(b)–(d), FESEM examinations of the doped samples demonstrate that minimal doping of the pure sample does not alter the morphology but results in increased size and reduction in agglomeration. This confirms the effective growth of rod- and sheet-like nanostructures on MoSe₂, which present a larger surface to the electrolyte, create more active sites and allow ions to rapidly move into and out of the material.²² This results in an increased surface area and more active sites, with electrochemical performance showing better capacitance and rate performance. HRTEM measurements, as shown in Fig. 3(a)–(f), present nanosheets of the as-synthesized MoSe₂, which closely match the observations from FESEM study. The interplanar spacing of MoSe₂ in Fig. 3(c) is 0.27 nm, from the (102) plane, indicating the polycrystalline nature of the material. Fig. 3(d) and (e) depicts the individual proportions of homogeneous Mo and Se elements over the designated area, with the distributions indicated in red and green, respectively. The rings in Fig. 3(f), indexed to the (002), (102), (110) and (200) planes, correspond to MoSe₂ and are in good agreement with the XRD results.^10,21,22 XPS analysis was performed to investigate the chemical states and confirm the successful doping of the pristine sample. The deconvoluted Mo 3d spectrum is displayed in Fig. 4(a), where two prominent peaks are visible at binding energies of around 228.14 and 231.27 eV, which correspond to 3d_5/2 and 3d_3/2 Mo⁴⁺ states, respectively.²³ Minor shoulders at higher binding energies may indicate slight oxidation (Mo⁶⁺), possibly due to surface exposure. The oxidation state of the selenium (Se²⁻) is indicated by the two deconvoluted peaks in the Se 3d plot in Fig. 4(b) at binding energies of 53.49 and 54.38 eV, showing Se 3d_5/2 and Se 3d_3/2, respectively.²⁴ Bulk sp³-bonded C–C carbon appears at 285.05 eV in the C 1s profile in Fig. 4(c). Similarly, Fig. 4(d) shows the W 4f spectrum, displaying doublet peaks at ∼35.6 eV (W 4f_7/2) and ∼37.7 eV (W 4f_5/2), corresponding to W⁶⁺ species, and indicating that W likely substitutes Mo in the lattice while preserving the structural integrity of MoSe₂. The W⁶⁺ state is known to influence electrical conductivity and increase the active surface area.¹⁹ The formation of V–Se bonds is responsible for the V 2p_1/2 and 2p_3/2 peaks corresponding to V⁴⁺/V⁵⁺ oxidation states, which are located at 520.03 and 512.45 eV, respectively. The coexistence of multiple valence states suggests the potential for battery behavior due to reversible surface redox reactions. This confirms that MoSe₂ was successfully doped with V atoms (Fig. 4(e)). The presence of Co²⁺/Co³⁺, likely substituted into Mo sites or present in interstitial positions and contributing to enhanced electrochemical activity via redox reactions, is indicated by the peaks at 777.64 and 792.41 eV, which are attributed to the electron-binding energies of Co 2p_3/2 and Co 2p_1/2, respectively, as seen in Fig. 4(f).²² These dopants likely create additional electroactive sites and modulate the electronic structure, thereby enhancing charge transport and boosting the overall electrochemical performance.

Fig. 2 FESEM images of (a) MoSe₂, (b) V@MoSe₂, (c) W@MoSe₂, and (d) Co@MoSe₂, and (e–h) EDX spectra confirming the elemental compositions of the samples.

Fig. 3 (a–c) HRTEM images of a pure sample, (d and e) element mapping of Mo and Se, and (f) the SAED pattern of MoSe₂.

Fig. 4 XPS high-resolution core-level deconvoluted spectra: (a) Mo 3d, (b) Se 3d, (c) C 1s, (d) W 4f, (e) V 2p and (f) Co 2p.

3.2. Electrochemical analysis

Three-electrode electrochemical tests were carried out to explain the electrochemical storage properties of the fabricated electrodes, as described above in the experimental section. The specific capacity of the electrodes is obtained from the CV profiles using the relationship:²⁵where C_p is the specific capacity (mA h g⁻¹) of the electrode, is the area under the CV curve, v is the scan rate (mV s⁻¹), and m is the active mass (g). Fig. 5(a) shows the CV curves at 10 mV s⁻¹ over a potential window of −0.2 to 0.5 V for MoSe₂, W@MoSe₂, V@MoSe₂, and Co@MoSe₂, which have specific capacities of 77.05, 103.10, 107.58 and 113.09 mA h g⁻¹, respectively. The redox couplings of MoSe₂ electrodes are investigated, which demonstrate battery properties due to the electrochemical storage mechanism. The Co@MoSe₂ electrode has a higher area under the curve, whereas W@MoSe₂ and V@MoSe₂ also exhibit more catalytic edge sites with rapid conductance behavior, resulting in better electrodes than the bare MoSe₂ electrode. The observed large redox profiles could be derived from the involvement of distinct Mo atom valence states.²⁶ Fig. 5(b)–(e) displays the CV curves for all the prepared electrodes as the scan rate changes from 10 to 100 mV s⁻¹. These images show the intensification of the scan rate and the expansion of the area associated with the CV curve. The results support both the enhanced rate performance and current capabilities of the electrodes. Both the negative and positive versions of the CV curve exhibit symmetry. The fact that the peak current changes in direct proportion to the increase in scan rate suggests that the energy storage mechanism at the electrode/electrolyte interface is highly reversible. The capacitive effect of an electrode material can be analyzed using the CV profiles measured at different scan rates according to the following power law:

i = av^b (7)

where v is the scan rate (mV s⁻¹) and a and b are adjustable parameters. The b value can be calculated from the slope of log i vs. log v, as shown in Fig. 5(f). There are two boundary conditions: (i) b = 0.5 indicates a diffusion-controlled process due to ion intercalation/de-intercalation; and (ii) b = 1 represents a surface capacitive nature. The value for the Co@MoSe₂ electrode is found to be 0.54 from the log(i) vs. log(v) plot, indicating that the charge-storage mechanism is predominantly diffusion-controlled, which is characteristic of battery-type behavior. However, slight deviations in the CV profile at higher scan rates suggest a minor pseudocapacitive contribution. These surface-related processes, while not dominant, can facilitate faster ion/electron transport, thereby enhancing the rate capability and supporting fast-charging performance. This synergistic behavior is advantageous for high-power applications where both energy density and charge/discharge speed are critical. It is shown that the capacitive route influences charge storage on the Co@MoSe₂ electrode, while diffusion processes dominate the storage process. The current value at a specific scan rate (v) and potential (V) can be divided between surface-controlled (k₁v) and diffusion-driven (k₂v^1/2) reactions as follows:.^24,25

I(V) = k₁v + k₂V^1/2 (8)

Fig. 5 Three-electrode mode: (a) comparative CV plots at a constant scan rate of 10 mV s⁻¹ for all samples, (b–e) CV plots for MoSe₂, W@MoSe₂, V@MoSe₂, and Co@MoSe₂ at varied scan rates of 10–100 mV s⁻¹, (f) a b-value estimation of the CoMo@Se₂ electrode from a plot of log(peak current) against log(scan rate), (g) the diffusion and capacitive current contributions at scan rates from 10–100 mV s⁻¹, and (h) a CV curve of the Co@MoSe₂ material depicting the capacitive/diffusive current contributions at 100 mV s⁻¹.

The values of k₁ and k₂ are obtained through CV analysis at varying scan rates (10–100 mV s⁻¹). The contributions of capacitive and diffusion-controlled processes to the overall electrochemical response were quantified, revealing a small shift towards capacitive dominance at higher scan rates, as shown in Fig. 5(g). As anticipated, because of the shorter time for ion diffusion at higher sweep speeds, the capacitive contribution gradually rises with scan rate. Even though diffusion processes predominate, the growing surface-controlled contribution at high scan rates points to the existence of pseudocapacitive behavior. Fig. 5(h) presents the capacitive contribution at a representative scan rate, where the surface-controlled current is highlighted. The claim that capacitive effects are active and grow more noticeable at high rates is supported by this, even though diffusion controls the overall charge-storage process. Taken together, our findings show that Co@MoSe₂ has a synergistic charge-storage mechanism. Fig. 6(a) shows the generated GCD curves over a potential range of −0.2 to 0.5 V for MoSe₂, W@MoSe₂, V@MoSe₂, and Co@MoSe₂ electrodes, which have specific capacitances of 331.71, 372.34, 412.71, and 518.4 F g⁻¹ at 1 A g⁻¹, respectively. All electrodes display nonlinear charge/discharge curves due to their battery-like electrochemical storage properties. The extended charging/discharging duration for the Co@MoSe₂ electrode is consistent with its high capacitance value and matches the corresponding CV results.²⁷ The GCD curves further highlight that doping significantly enhances the charge-storage capability of MoSe₂. Specifically, V@MoSe₂ increases the capacitance by improving charge carrier density and reducing internal resistance, as reflected in its lower R_ct value (1 Ω). W@MoSe₂, though not as effective as V or Co, also contributes to improved charge transport, likely due to a modified electronic structure and increased conductivity. Co@MoSe₂ demonstrates the most remarkable improvement, suggesting the presence of additional redox-active centers (Co²⁺/Co³⁺) that may provide pseudocapacitive contributions beyond the electric double-layer mechanism. Fig. 6(b) displays the GCD curves of Co@MoSe₂ at different current densities. The calculated specific capacitances were 518.4, 389.14, 315, and 264.83 F g⁻¹ at 1, 2, 3, and 4 A g⁻¹, respectively, with plots of current density versus specific capacitance presented in Fig. 6(c)–(d). The gradual decrease in capacitance with increasing current density is expected and indicates good rate capability. Nyquist plots for all four electrodes are shown in Fig. 6(e), collectively affirming that doping significantly influences the electrochemical characteristics of MoSe₂. The dopants modify the electronic environment and surface reactivity of MoSe₂, and the bulk resistance (R_b) values for MoSe₂, W@MoSe₂, V@MoSe₂, and Co@MoSe₂ are determined to be 0.9, 1.1, 0.9, and 0.8 Ω, respectively, while the charge transfer resistance (R_ct) values are 1.4, 1.3, 1.0, and 0.6 Ω, respectively. The low R_b and R_ct values across all samples confirm facile ionic movement and electron transport. Among them, Co@MoSe₂ demonstrates the lowest R_ct, indicating it has the most efficient interfacial charge transfer kinetics. Fig. 6(f) depicts the experimental and fitted EIS data for Co@MoSe₂, using Zsim software. The inset shows the equivalent circuit [R_b(QR_ctW)C], where Q represents a constant phase element accounting for surface irregularities and non-ideal capacitive behavior. The low impedance response and good fitting support the superior electrochemical behavior of the Co-doped electrode. These results point to reduced resistance and enhanced charge storage. Among all the samples, Co@MoSe₂ appears to be the most beneficial due to its ability to introduce additional active sites and improve electron mobility, which is directly reflected in its high specific capacitance and low impedance parameters. Fig. 7(a) displays CV scans from assembled SSDs using pure MoSe₂, W@MoSe₂, V@MoSe₂, and Co@MoSe₂ electrodes at 10 mV s⁻¹. As observed in the three-electrode system, Co@MoSe₂ still holds an advantage in terms of a SSD, exhibiting large-area CV curves and a high capacity compared with the other devices. The observed Co@MoSe₂ (Fig. 7(b)) CV current increases with increasing sweep rate, confirming the high-rate capability of the designed symmetric device. Furthermore, the observed CV curves maintain their shape throughout cycling, demonstrating the device's excellent storage kinetics and reversible performance. Fig. 7(c) presents the GCD curves of the fabricated SSDs within a voltage window of 0 to 1.4 V, showing specific capacitances of 56.92, 70.99, 81.60 and 127 F g⁻¹ at a current density of 1 A g⁻¹ for MoSe₂, W@MoSe₂, V@MoSe₂ and Co@MoSe₂, respectively. Among these, the Co@MoSe₂ SSD produces the optimal storage capacitances of 127, 79.37, 74.91, and 68.57 F g⁻¹ at 1, 2, 3 and 4 A g⁻¹, respectively, as observed in the GCD plots in Fig. 7(d). The specific capacitances of all prepared electrodes are higher than those of the devices due to the ideal and isolated conditions of the electrodes. The nonlinear profiles clearly indicate the prolonged charge/discharge span for the devices, and the electrochemical performance of Co@MoSe₂ is detailed in Table 1. The energy densities are calculated to be 15.49, 19.32, 22.21, and 34.54 W h kg⁻¹ for MoSe₂, W@MoSe₂, V@MoSe₂ and Co@MoSe₂, respectively, with Co@MoSe₂ having a power density of 700 W kg⁻¹. Fig. 7(e) shows Nyquist plots for all the fabricated devices, where the R_ct values are calculated to be 2.5, 2, 1.8 and 1.6 Ω for MoSe₂, W@MoSe₂, V@MoSe₂ and Co@MoSe₂, respectively. Additionally, the common PVA–KOH gel electrolyte used in all devices was evaluated using eqn (9):

where σ is the ionic conductivity (S cm⁻¹), L is the thickness of the gel (cm), R_b is the bulk resistance (Ω), and A is the cross-sectional area of contact (in cm²). The ionic conductivity was calculated to be 0.0188 S cm⁻¹, confirming good ionic transport in the gel matrix. The experimental and fitted data for Co@MoSe₂ are displayed in Fig. 7(f), based on Zsim software, with the inset showing the equivalent circuit. The energy density and power density were assessed using the Ragone plot in Fig. 8(a). Certainly, our SSD acquires a considerably higher energy density, outperforming similar reported transition-metal chalcogenides in other studies, as statistically summarized in Table 2. CV profiles are obtained in various potential windows spanning from 0–0.6 V to 0–1.4 V (Fig. 8(b)) in which the potential steps confirm the faradaic behaviour of the device. At a fixed current density of 4 A g⁻¹ (Fig. 8(c)), 80% cyclic stability was demonstrated even after 10 000 GCD cycles, and 95% coulombic efficiency is also achieved. The cycling stability of a SSD is crucial for commercially viable applications. To assess the material's flexibility, it underwent several bending cycles (∼15–5 mm) and manual twisting (∼180°) under various types of mechanical deformation to assess its flexibility and structural integrity; there were no signs of fracturing, as observed in Fig. 8(d)–(g), and the CV curves of the SSD measured at a scan rate of 10 mV s⁻¹ within a potential window of 0–1.4 V, as presented in Fig. 8(h), remarkably remain virtually unchanged, maintaining exceptional flexibility and stability even under such conditions, highlighting the superior performance for energy storage applications and reaffirming the robustness for practical applications. Fig. 8(i) shows a photograph demonstrating the successful illumination of a red LED powered by two SSDs connected in series. This proves the practical energy output of the device, showing its ability to power low-voltage electronics and emphasizing the potential of such flexible energy storage systems for use in portable technologies.

Fig. 6 (a) GCD plots for all samples at 1 A g⁻¹, (b) GCD plots of Co@MoSe₂ at varied current densities (A g⁻¹), (c and d) current density vs. specific capacitance plots, (e) Nyquist plots corresponding to all prepared samples, and (f) experimental and fitted data for Co@MoSe₂ (the inset displays the equivalent fitted circuit).

Fig. 7 Device mode: (a) comparative CV plots at a constant scan rate of 10 mV s⁻¹ for all samples, (b) CV plots for Co@MoSe₂ at varied scan rates (10–100 mV s⁻¹), (c) GCD plots for all samples at a current density of 1 A g⁻¹, (d) GCD plots of Co@MoSe₂ at current densities of 1, 2, 3, and 4 A g⁻¹, (e) Nyquist plots corresponding to all prepared samples, and (f) Nyquist plots for Co@MoSe₂ using both the experimental and fitted data (inset displays the equivalent fitted circuit).

Table 1 The electrochemical performance parameters of the Co@MoSe₂-based SSD

Current density (A g^−1)	Specific capacitance (F g^−1)	Energy density (W h kg^−1)	Power density (W kg^−1)
1	127	34.54	700
2	79.37	21.6	1413
3	74.91	20.39	2099
4	68.57	18.66	2799

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Fig. 8 (a) Ragone plot of the device compared with the reported literature, (b) CV curves observed over different potential windows, (c) cycling stability and coulombic efficiency of the Co@MoSe₂ SSD at a current density of 4 A g⁻¹ (the inset shows the GCD curves from the last ten cycles), (d–g) pictorial illustrations of the bending and twisting of the SSD and (h) related CV curves, and (i) photographic evidence of a red LED being lit by SSDs in series.

Table 2 An electrochemical performance comparison with literature examples

Sample	Method	Morphology	Electrolyte	Specific capacitance (F g^−1)	Energy density (W h kg^−1)	Power density (W kg^−1)	Cycling stability/cycles	Reference
M@AC/MWCNT	Hydrothermal	Nanoflower	6 M KOH	133 (1 A g^−1)	14.9	496	86.6% for 10 000 cycles	19
MoSe2/FeOOH	Hydrothermal	Nanorod	6 M KOH	132 (1 A g^−1)	18.3	1174	100% for 3000 cycles	24
2H-MoSe2-sheets	Hydrothermal	Sheets	0.5 M TEABF4/AN	16.2 (0.75 A g^−1)	20.31	750	87% for 10 000 cycles	27
2D-2D-MoSe2/N G	Solvothermal	Nanosheets	6 M KOH	75 (1 A g^−1)	26.6	800	88% for 3000 cycles	28
NiSe@rGO//rGO	Hydrothermal	Sphere	2 M KOH	115 (1 A g^−1)	34.3	937	93% for 6500 cycles	29
MoSe2@Ti3C2Tx	Hydrothermal	Nanoflower	3 M KOH	223.12 (1 A g^−1)	7.74	292.60	98.69 for 10 000 cycles	30
MoSex	Electrode deposition	Nanosheets	3 M KOH	49.7 (1 A g^−1)	25.3	1143.6	50.1% for 10 000 cycles	17
L-MoS2	Hydrothermal	Nanoflowers	2 M KOH	356.7 (1 A g^−1)	6.5	413	79.6% for 3000 cycles	31
Co@MoSe2	Hydrothermal			127 (1 A g^−1)	34.54	700		Present work

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4. Conclusions

In a nutshell, we successfully synthesized and fabricated MoSe₂, W@MoSe₂, V@MoSe₂ and Co@MoSe₂. Structural analysis using PXRD, Raman, and XPS techniques strongly confirmed the MoSe₂ nanostructures. The nanoflower-cum-nanorod structures with dopants were precisely captured by FESEM micrographs and have shown improved surface areas and charge-storage mechanisms. HRTEM investigations explored the expanded layer spacing compared with the pure MoSe₂ sample for samples doped with transition metals. The highest electrochemical performance among the prepared samples was achieved by Co@MoSe₂, while the other dopants also demonstrated improved performance compared to the pure sample, which is due to the strong synergistic interaction between MoSe₂ and the dopants for energy storage. A high specific capacitance of 518 F g⁻¹ shows the enhanced electrochemical properties of Co@MoSe₂, which are attributed to the large-area CV curves and improved conductive pathways. Building on the promising results from the Co@MoSe₂ electrode, a SSD was fabricated, yielding a specific capacitance of 127 F g⁻¹, an energy density of 34.54 W h kg⁻¹, and a power density of 700 W kg⁻¹, displaying extremely superior capacitance retention of 80% of the initial capacity even after 10 000 cycles. The nanostructures of the prepared samples exhibited robust battery-like kinetics during the charge-storage process in a basic medium. Flexibility studies proved that there was no distortion after multiple bending and twisting cycles, which makes the materials suitable for upcoming lightweight and cutting-edge devices. The findings of this study hold significant promise for advancing the development of practical energy storage devices. This study indicates that the materials support efficient and sustained energy storage, mimicking the behavior of batteries in terms of charge transfer and retention. The demonstrated performance of the SSD, marked by high specific capacitance, excellent rate capability, and mechanical stability, highlights its suitability for applications in flexible and wearable electronics, where lightweight, reliable, and bendable energy sources are essential. Moreover, the rapid charge–discharge characteristics and long cycling life make these devices ideal for high-power applications, such as short-duration energy buffering and power backup systems. From a manufacturing perspective, the symmetric configuration simplifies fabrication and reduces costs, thereby facilitating scalable device integration. Such characteristics make these nanostructures particularly suitable for high-performance energy storage applications, where stable and reliable charge–discharge cycles are essential. Therefore, we conclude that our work provides a novel approach for creating affordable, efficient ESC devices and other power-source systems for the next generation. Looking forward, further research should focus on optimizing the electrode–electrolyte interface to enhance ion transport, exploring solid-state and gel-based electrolytes to improve safety and flexibility, and investigating the mechanical and electrochemical durability of these devices under real-world conditions.

Data availability

The data supporting this article has been included as part of the article.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by an Anusandhan National Research Foundation (ANRF) grant funded by Govt. of India (SERB project no: EEQ/2022/000360). This research (or a portion thereof) was performed using facilities at CeNSE, Indian Institute of Science, Bengaluru, funded by Ministry of Education (MoE), Ministry of Electronics and Information Technology (MeitY) and Nanomission, DST, Govt. of India.

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