High-Performance Flexible Energy Storage Devices Based on Graphene Decorated with Flower-Shaped MoS2 Heterostructures

MoS2, owing to its advantages of having a sheet-like structure, high electrical conductivity, and benign environmental nature, has emerged as a candidate of choice for electrodes of next-generation supercapacitors. Its widespread use is offset, however, by its low energy density and poor durability. In this study, to overcome these limitations, flower-shaped MoS2/graphene heterostructures have been deployed as electrode materials on flexible substrates. Three-electrode measurements yielded an exceptional capacitance of 853 F g−1 at 1.0 A g−1, while device measurements on an asymmetric supercapacitor yielded 208 F g−1 at 0.5 A g−1 and long-term cyclic durability. Nearly 86.5% of the electrochemical capacitance was retained after 10,000 cycles at 0.5 A g−1. Moreover, a remarkable energy density of 65 Wh kg−1 at a power density of 0.33 kW kg−1 was obtained. Our MoS2/Gr heterostructure composites have great potential for the development of advanced energy storage devices.


Introduction
Due to their exceptional chemical and physical properties, transition metal dichalcogenides (TMDCs) have been widely employed as active materials in supercapacitors (SCs) [1][2][3]. Of the numerous TMDCs that have been synthesized, MoS 2 is a remarkable electrode material for use in SCs as it has a unique structure, high electrical conductivity, and is eco-friendly [4,5]. However, the electrochemical performance of MoS 2 is significantly limited by its low energy density and poor long-term cycling stability. To address these issues, extensive efforts have been directed toward improving the electrochemical performance of MoS 2 via a range of different strategies, including modification of MoS 2 's structure [6,7], doping with non-metal ions [8,9], hybridization with metal oxides [10,11], and integrating it with highly conductive materials [12][13][14]. Despite significant advances in the synthesis of MoS 2 , the overall electrochemical behavior of MoS 2 -based SCs is still far from satisfactory.
Recently, graphene (Gr), an archetypal two-dimensional (2D) material, has attracted a considerable amount of attention due to its superior properties, such as its high stability and large specific surface area, as well as high thermal and electrical conductivities [15][16][17]. Due to these excellent properties, Gr has been explored in many research fields, including nanogenerators, SCs, field-effect transistors, photodetectors, etc. [18][19][20][21]. In particular, utilizing Gr as a co-active material in combination with MoS 2 to construct MoS 2 /Gr composites has proven to be an efficient strategy in significantly improving the electrochemical

Fabrication of MoS 2 /Gr Heterostructures
The Gr oxide was fabricated via the modified Hummers approach [27]. The MoS 2 /Gr heterostructures samples were synthesized via a facile hydrothermal method (Scheme 1). First, 50 mg of Gr powder was dispersed in 15 mL of distilled water through ultrasonication. Second, 121 mg of Na 2 MoO 4 ·H 2 O and 191 mg of CS(NH 2 ) 2 were added to the above suspension, and ultrasonication was performed for 40 min. Third, 50 mg of NH 2 OH·HCl and 0.5 mL of ice acetate acid were added to the above suspension solution, while continuous ultrasonication was performed for 20 min. The mixed solution was then transferred to a Teflon autoclave and kept at 210 • C for 24 h. It should be noted that, after the hydrothermal reaction process, the Gr oxide was reduced to Gr under NH 2 OH·HCl as a reducing agent. Subsequently, the samples were collected via centrifugation and rinsed three times using distilled water and ethanol, separately. The product was then dried at 70 • C for 10 h in a vacuum oven to obtain MoS 2 /Gr heterostructures. The product characterizations are displayed in the Supporting Information. Micromachines 2023, 14, x FOR PEER REVIEW 3 of 12 Scheme 1. Preparation procedure of the MoS2/Gr heterostructure.

Materials' Characterizations
The structures and crystalline phases of the as-prepared powder were examined using X-ray diffraction (XRD, 40 kV × 40 mA, Rigaku Ultima III, Tokio, Japan) with Cu-Kα radiation (λ = 1.5418 Å). The measured 2θ range was from 10° to 70° with a scanning speed of 5°/min. The XRD result was corrected to an apparatus line width of 0.08. The chemical composition of the samples was analyzed via X-ray photoelectron spectroscopy (XPS, Thermo Scientific, UK). The XPS was carried out with a standard Al-Kα source (1486.6 eV) in an ultra-high vacuum (~8 × 10 −9 Pa). The binding energies were corrected for the charging effects by considering the adventitious C 1s line at 284.6 eV. The morphology of the product was examined by scanning electron microscopy (SEM, JEOL JSM-7400E, Japan) with a working voltage and emission current of 10 kV and 10 μA, respectively. Energy-dispersive X-ray spectroscopy (EDS) was performed under the working voltage of 15 kV for analyzing the elemental composition of the product. The microstructure was tested by using transmission electron microscopy (TEM, FEI Tecnai G2 F20, 200 kV, MA, USA). The pore size distributions and specific surface areas of the as-prepared products were evaluated via the Brunauer-Emmett-Teller (BET, GEMINI VII 2390, USA) method at 77 K. The pore size distribution was studied via the Barrett-Joyner-Halenda approach using desorption isotherms.

Electrochemical Performance Measurements
The CHI660E electrochemical workstation was employed to assess the electrochemical performance of the as-prepared samples. Polytetrafluoroethylene, acetylene black, and Gr/MoS2 heterostructures (8 mg) at a weight ratio of 5:15:80 were deposited on a piece of Ni foam (1 cm × 1 cm), and the Ni foam was pressed down by applying a hydraulic pressure of 8.0 MPa (working electrode). Pt and Hg/HgO were employed as the counter and reference electrodes, respectively. In addition, a 3.0-M KOH solution was used as the electrolyte. Galvanostatic charge-discharge (GCD), cyclic voltammetry (CV), and long-term cycle stability tests were performed using a three-electrode measurement technique. Electrochemical impedance spectroscopy (EIS) was performed using a PARSTAT2273 electrochemical workstation.

Fabrication of the Asymmetric Supercapacitive Device
The electrochemical properties of the MoS2/Gr heterostructures were examined using a two-electrode system. The working electrode was fabricated via an approach identical to the three-electrode technique. In addition, 12 mL of distilled water, 1.65 g of polyvinyl alcohol (PVA), and 2.15 g of KOH were mixed at 80 °C for 40 min to produce a gel electrolyte. The activated materials (MoS2/Gr heterostructures (8 mg) and activated carbon (8 mg)) were then wrapped with a gel electrolyte. After 5-7 min, a layer of solid electrolyte (PVA-KOH) was formed. Finally, the two-electrode devices were fabricated by pressing the two electrodes together using a sheet out roller.

Materials' Characterizations
The structures and crystalline phases of the as-prepared powder were examined using X-ray diffraction (XRD, 40 kV × 40 mA, Rigaku Ultima III, Tokio, Japan) with Cu-K α radiation (λ = 1.5418 Å). The measured 2θ range was from 10 • to 70 • with a scanning speed of 5 • /min. The XRD result was corrected to an apparatus line width of 0.08. The chemical composition of the samples was analyzed via X-ray photoelectron spectroscopy (XPS, Thermo Scientific, Nottingham, UK). The XPS was carried out with a standard Al-Kα source (1486.6 eV) in an ultra-high vacuum (~8 × 10 −9 Pa). The binding energies were corrected for the charging effects by considering the adventitious C 1s line at 284.6 eV. The morphology of the product was examined by scanning electron microscopy (SEM, JEOL JSM-7400E, Tokyo, Japan) with a working voltage and emission current of 10 kV and 10 µA, respectively. Energy-dispersive X-ray spectroscopy (EDS) was performed under the working voltage of 15 kV for analyzing the elemental composition of the product. The microstructure was tested by using transmission electron microscopy (TEM, FEI Tecnai G2 F20, 200 kV, Boston, MA, USA). The pore size distributions and specific surface areas of the as-prepared products were evaluated via the Brunauer-Emmett-Teller (BET, GEMINI VII 2390, Norcross, GA, USA) method at 77 K. The pore size distribution was studied via the Barrett-Joyner-Halenda approach using desorption isotherms.

Electrochemical Performance Measurements
The CHI660E electrochemical workstation was employed to assess the electrochemical performance of the as-prepared samples. Polytetrafluoroethylene, acetylene black, and Gr/MoS 2 heterostructures (8 mg) at a weight ratio of 5:15:80 were deposited on a piece of Ni foam (1 cm × 1 cm), and the Ni foam was pressed down by applying a hydraulic pressure of 8.0 MPa (working electrode). Pt and Hg/HgO were employed as the counter and reference electrodes, respectively. In addition, a 3.0-M KOH solution was used as the electrolyte. Galvanostatic charge-discharge (GCD), cyclic voltammetry (CV), and longterm cycle stability tests were performed using a three-electrode measurement technique. Electrochemical impedance spectroscopy (EIS) was performed using a PARSTAT2273 electrochemical workstation.

Fabrication of the Asymmetric Supercapacitive Device
The electrochemical properties of the MoS 2 /Gr heterostructures were examined using a two-electrode system. The working electrode was fabricated via an approach identical to the three-electrode technique. In addition, 12 mL of distilled water, 1.65 g of polyvinyl alcohol (PVA), and 2.15 g of KOH were mixed at 80 • C for 40 min to produce a gel electrolyte. The activated materials (MoS 2 /Gr heterostructures (8 mg) and activated carbon (8 mg)) were then wrapped with a gel electrolyte. After 5-7 min, a layer of solid electrolyte (PVA-KOH) was formed. Finally, the two-electrode devices were fabricated by pressing the two electrodes together using a sheet out roller.

Crystal phase, Structure, and Microstructure Properties
The crystallinity and elemental composition of the surface of the fabricated samples were assessed using XRD and XPS, respectively. Figure 1a shows the XRD pattern of the pristine MoS 2 and MoS 2 /Gr heterostructure. In this XRD pattern, the peaks at 26.58 and 54.56 • can be attributed to the (002) and (004) planes of Gr, respectively (JCPDS No. 75-1621) [18,28], while those at 14.  [29]. In comparison, after the combination of MoS 2 with Gr, the intensity of MoS 2 in MoS 2 /Gr was reduced, confirming that the MoS 2 /Gr heterostructure was successfully synthesized. The full survey XPS spectrum is displayed in Figure S1 in the Supporting Information, which reveals that the Mo, S, and C elements are present in the as-prepared sample. The high-resolution XPS spectra of the MoS 2 /Gr heterostructure are shown in Figure 1b [30]. The S 2p XPS spectrum ( Figure 1c) exhibits two peaks at 161.9 eV (S 2p 1/2 ) and 160.7 eV (S 2p 3/2 ) corresponding to the S 2− in MoS 2 [31]. In the C 1s XPS spectrum, the two main peaks present at 284.7 and 287.7 eV can be attributed to the sp 2 and sp 3 hybridized carbon in Gr, respectively [32].

Crystal phase, Structure, and Microstructure Properties
The crystallinity and elemental composition of the surface of the fabricated samples were assessed using XRD and XPS, respectively. Figure 1a shows the XRD pattern of the pristine MoS2 and MoS2/Gr heterostructure. In this XRD pattern, the peaks at 26.58 and 54.56° can be attributed to the (002) and (004) planes of Gr, respectively (JCPDS No. 75-1621) [18,28], while those at 14.36°, 33.75°, 39.92°, 43.66°, 49.56°, and 58.96° correspond to the (002), (100), (103), (104), (105), and (008) crystallographic planes of MoS2, respectively (JCPDS No. 77-1716) [29]. In comparison, after the combination of MoS2 with Gr, the intensity of MoS2 in MoS2/Gr was reduced, confirming that the MoS2/Gr heterostructure was successfully synthesized. The full survey XPS spectrum is displayed in Figure S1 in the Supporting Information, which reveals that the Mo, S, and C elements are present in the as-prepared sample. The high-resolution XPS spectra of the MoS2/Gr heterostructure are shown in Figure 1b-d. The Mo 3d XPS spectrum ( Figure 1b) shows two prominent peaks at binding energies of 227.4 eV (Mo 3d5/2) and 230.6 eV (Mo 3d3/2), which can be assigned to the Mo 4+ in MoS2 [30]. The S 2p XPS spectrum ( Figure 1c) exhibits two peaks at 161.9 eV (S 2p1/2) and 160.7 eV (S 2p3/2) corresponding to the S 2− in MoS2 [31]. In the C 1s XPS spectrum, the two main peaks present at 284.7 and 287.7 eV can be attributed to the sp 2 and sp 3 hybridized carbon in Gr, respectively [32]. Furthermore, SEM and TEM were employed to assess the surface morphology and microstructure of the as-prepared samples. Figure S2 (Supporting Information) presents the SEM image of pristine MoS2, suggesting that flower-shaped structures were formed. The low-resolution SEM images in Figure 2a,b indicate that flower-shaped MoS2 was successfully grown on the Gr layer. It is worth noting that the growth of flower-like MoS2 on the Gr layer improves the surface area of the MoS2/Gr heterostructure, providing it with abundant surface active sites, enhancing the structural stability of the electrode and further significantly improving the electrochemical performance of the material [33][34][35]. The high-resolution SEM image in Figure 2c shows that the flower-like MoS2 is made up of a large number of MoS2 nanoflakes, of 20 nm in thickness. Such thin MoS2 nanoflakes not only facilitate ion transfer between the electrolyte and electrodes, but also boost electrical conductivity. The TEM image in Figure 2d reveals that the Gr layer is decorated with flower-shaped MoS2, which is in good agreement with the SEM images shown in Figure Furthermore, SEM and TEM were employed to assess the surface morphology and microstructure of the as-prepared samples. Figure S2 (Supporting Information) presents the SEM image of pristine MoS 2 , suggesting that flower-shaped structures were formed. The low-resolution SEM images in Figure 2a,b indicate that flower-shaped MoS 2 was successfully grown on the Gr layer. It is worth noting that the growth of flower-like MoS 2 on the Gr layer improves the surface area of the MoS 2 /Gr heterostructure, providing it with abundant surface active sites, enhancing the structural stability of the electrode and further significantly improving the electrochemical performance of the material [33][34][35]. The high-resolution SEM image in Figure 2c shows that the flower-like MoS 2 is made up of a large number of MoS 2 nanoflakes, of 20 nm in thickness. Such thin MoS 2 nanoflakes not only facilitate ion transfer between the electrolyte and electrodes, but also boost electrical conductivity. The TEM image in Figure 2d reveals that the Gr layer is decorated with flowershaped MoS 2 , which is in good agreement with the SEM images shown in Figure 2a,b. The high-resolution TEM (HRTEM) and corresponding selected area electron diffraction (SAED) image is shown in Figure 2e and the inset of Figure 2e. As shown, there is a lattice spacing of the material of around 0.62 nm that can be assigned to the (002) crystallographic planes of the hexagonal lattice of MoS 2 . The lattices of MoS 2 and Gr can be seen to be well-interfaced, indicating that the MoS 2 /Gr heterostructure was successfully fabricated. It should be noted that such a heterostructure significantly promotes the structural stability of the electrode, further leading to an improvement in its electrochemical cycling stability. Figures 2f and S3 (Supporting Information) show the energy dispersive X-ray (EDX) mapping and spectrum of the MoS 2 /Gr heterostructure, in which the uniform distribution of the Mo, C, and S elements corroborates the formation of the material.   Figure 3a shows the CV curves of the MoS2/Gr electrode recorded at different scan rates in the range of 5-100 mV s −1 , from which it can be seen that the current density gradually increases with an increase in the scan rate. The CV curve remained the same shape, even at a high scan rate of 100 mV s −1 , indicating the remarkable capacitive behavior of the MoS2/Gr heterostructure [22,36]. The GCD curves (Figure 3b) of the MoS2/Gr electrode were recorded at various current densities in the range of 1-8 A g −1 . Figure 3b shows the electrochemical capacitance of the MoS2/Gr electrode at various current densities (the equation used to calculate the electrochemical capacitance is provided in the Supporting Information). The electrochemical capacitance of the MoS2/Gr electrode was found to reach 853 F g −1 at 1 A g −1 , as shown in Figure 3c. From the electrochemical measurements, it can be seen that as the current density was increased to 8 A g −1 and the electrochemical capacitance reached 225 F g −1 . This phenomenon is due to the MoS2/Gr electrode having high electrical conductivity and a large surface area, as well as the synergistic effect of the Gr layer and flower-like MoS2 resulting in the fast diffusion and transfer of ions in the electrolyte. Moreover, the electrochemical performance of the MoS2-based electrode is shown in Figure S4 (Supporting Information); it shows an electrochemical capacitance of 227 F g −1 at 1 A g −1 . In comparison, the MoS2/Gr electrode presents a higher electrochemical capacitance than that of MoS2, indicating that Gr can greatly enhance the electrochemical capacitance of MoS2. As shown in Figure S4a (Supporting Information) and Figure 3a, the CV curves of MoS2 and MoS2/Gr present a typical electrical double layer property. In general, for electrical double layer SCs, the electrochemical current is caused by the adsorption/desorption of the ions on the surface of the electrode. Therefore, for the electrode with a large surface area, more ions will be accumulated, and the reaction kinetic can be further  Figure 3a shows the CV curves of the MoS 2 /Gr electrode recorded at different scan rates in the range of 5-100 mV s −1 , from which it can be seen that the current density gradually increases with an increase in the scan rate. The CV curve remained the same shape, even at a high scan rate of 100 mV s −1 , indicating the remarkable capacitive behavior of the MoS 2 /Gr heterostructure [22,36]. The GCD curves (Figure 3b) of the MoS 2 /Gr electrode were recorded at various current densities in the range of 1-8 A g −1 . Figure 3b shows the electrochemical capacitance of the MoS 2 /Gr electrode at various current densities (the equation used to calculate the electrochemical capacitance is provided in the Supporting Information). The electrochemical capacitance of the MoS 2 /Gr electrode was found to reach 853 F g −1 at 1 A g −1 , as shown in Figure 3c. From the electrochemical measurements, it can be seen that as the current density was increased to 8 A g −1 and the electrochemical capacitance reached 225 F g −1 . This phenomenon is due to the MoS 2 /Gr electrode having high electrical conductivity and a large surface area, as well as the synergistic effect of the Gr layer and flower-like MoS 2 resulting in the fast diffusion and transfer of ions in the electrolyte. Moreover, the electrochemical performance of the MoS 2 -based electrode is shown in Figure S4 (Supporting Information); it shows an electrochemical capacitance of 227 F g −1 at 1 A g −1 . In comparison, the MoS 2 /Gr electrode presents a higher electrochemical capacitance than that of MoS 2 , indicating that Gr can greatly enhance the electrochemical capacitance of MoS 2 . As shown in Figure S4a (Supporting Information) and Figure 3a, the CV curves of MoS 2 and MoS 2 /Gr present a typical electrical double layer property. In general, for electrical double layer SCs, the electrochemical current is caused by the adsorption/desorption of the ions on the surface of the electrode. Therefore, for the electrode with a large surface area, more ions will be accumulated, and the reaction kinetic can be further optimized, thereby resulting in high electrochemical performance. To evalu-ate the surface area of the MoS 2 /Gr heterostructure, nitrogen (N 2 ) adsorption-desorption isothermal analysis was performed at 77 K. The BET analysis ( Figure S5 in the Supporting Information) indicates that the MoS 2 /Gr heterostructure has a large specific surface area of 95.2 m 2 /g, which significantly enhances its electrochemical performance [37,38]. Furthermore, compared with studies reported in the literature [39][40][41][42][43][44][45][46][47][48] (please see Table 1), the electrochemical capacitance of the MoS 2 /Gr heterostructure in this work is much higher, indicating that it is an excellent candidate for use in high-performance SCs. To investigate the electrical conductivity of the MoS 2 /Gr electrode, EIS measurements were performed before and after cycling stability tests were carried out over a frequency range from 0.01 Hz to 100 kHz with an alternating current voltage amplitude of 5 mV. The EIS curves and the corresponding equivalent circuit of the MoS 2 /Gr electrode before and after the 10,000 cycles test are shown in Figure 3e and the inset of Figure 3e. Compared with that of before cycling, the MoS 2 /Gr electrode exhibited a slight change in the solution resistance (R s ) and charge-transfer resistance (R ct ) after 10,000 cycles, indicating its excellent electrical conductivity [49]. Figure 3f shows an SEM image of the MoS 2 /Gr heterostructure after long-term cycling testing. A comparison of the SEM images shown in Figure 2b,c reveals that there are no significant changes in the structure of the MoS 2 /Gr heterostructure observed compared to before the cycling performance testing, suggesting that the MoS 2 /Gr heterostructure exhibits excellent structural stability.

Electrochemical Performance of the MoS 2 /Gr Heterostructure
optimized, thereby resulting in high electrochemical performance. To evaluate the sur area of the MoS2/Gr heterostructure, nitrogen (N2) adsorption-desorption isothermal a ysis was performed at 77 K. The BET analysis ( Figure S5 in the Supporting Informat indicates that the MoS2/Gr heterostructure has a large specific surface area of 95.2 m which significantly enhances its electrochemical performance [37,38]. Furthermore, c pared with studies reported in the literature [39][40][41][42][43][44][45][46][47][48] (please see Table 1), the electroch ical capacitance of the MoS2/Gr heterostructure in this work is much higher, indica that it is an excellent candidate for use in high-performance SCs. To investigate the e trical conductivity of the MoS2/Gr electrode, EIS measurements were performed be and after cycling stability tests were carried out over a frequency range from 0.01 H 100 kHz with an alternating current voltage amplitude of 5 mV. The EIS curves and corresponding equivalent circuit of the MoS2/Gr electrode before and after the 10,000 cles test are shown in Figure 3e and the inset of Figure 3e. Compared with that of be cycling, the MoS2/Gr electrode exhibited a slight change in the solution resistance (Rs) charge-transfer resistance (Rct) after 10,000 cycles, indicating its excellent electrical c ductivity [49]. Figure 3f shows an SEM image of the MoS2/Gr heterostructure after lo term cycling testing. A comparison of the SEM images shown in Figure 2b,c reveals there are no significant changes in the structure of the MoS2/Gr heterostructure obser compared to before the cycling performance testing, suggesting that the MoS2/Gr het structure exhibits excellent structural stability.    To assess the electrochemical capacitance performance of the MoS 2 /Gr heterostructure electrode for use in practical applications, a flexible ASC device was fabricated using the MoS 2 /Gr heterostructure and activated carbon as positive and negative electrodes, respectively. Figure S6

Stability and Flexibility Properties
It is well known that electrochemical durability is a crucial parameter of SCs. Hence, the electrochemical cycling stability of the MoS2/Gr electrode was also assessed at 1 A g −1 over 10,000 cycles, with the results shown in Figure 3d. The CV curves of before and after the 10,000 cycles test are shown in the inset of Figure 3d. It was found that 89.6% of the initial electrochemical capacitance of the MoS2/Gr electrode was retained after long-term cycling, indicating that the MoS2/Gr heterostructure exhibits excellent electrochemical cycling stability. This superior electrochemical cycling stability can be attributed to the growth of flower-shaped MoS2 on the Gr, which significantly enhances the structural stability of the electrode.
Furthermore, the ASC device was tested at various angles (0°, 30°, 60°, 90°, and 180°) to examine its flexibility. Figure 4e shows the GCD curves of the flexible ASC device twisted at different bending angles at 2 A g −1 . The corresponding CV curves are presented in Figure S7 in the Supporting Information. Note that the shapes of the GCD and CV curves are almost identical after multiple bending tests, indicating the excellent flexibility of the device. Figure 4f presents the long-term cycling stability of the flexible ASC device at 0.5 A g −1 , in which it can be seen that 86.5% of the electrochemical capacitance was re- (f) Cycling performance of the flexible asymmetric device at a current density of 1 A g −1 and EIS data of the flexible ASC device before and after 10,000 cycles (inset).

Stability and Flexibility Properties
It is well known that electrochemical durability is a crucial parameter of SCs. Hence, the electrochemical cycling stability of the MoS 2 /Gr electrode was also assessed at 1 A g −1 over 10,000 cycles, with the results shown in Figure 3d. The CV curves of before and after the 10,000 cycles test are shown in the inset of Figure 3d. It was found that 89.6% of the initial electrochemical capacitance of the MoS 2 /Gr electrode was retained after long-term cycling, indicating that the MoS 2 /Gr heterostructure exhibits excellent electrochemical cycling stability. This superior electrochemical cycling stability can be attributed to the growth of flower-shaped MoS 2 on the Gr, which significantly enhances the structural stability of the electrode.
Furthermore, the ASC device was tested at various angles (0 • , 30 • , 60 • , 90 • , and 180 • ) to examine its flexibility. Figure 4e shows the GCD curves of the flexible ASC device twisted at different bending angles at 2 A g −1 . The corresponding CV curves are presented in Figure S7 in the Supporting Information. Note that the shapes of the GCD and CV curves are almost identical after multiple bending tests, indicating the excellent flexibility of the device. Figure 4f presents the long-term cycling stability of the flexible ASC device at 0.5 A g −1 , in which it can be seen that 86.5% of the electrochemical capacitance was retained after 10,000 cycles, indicating that the ASC device exhibits outstanding cycling durability. The EIS results obtained before and after cycling durability testing are shown in the inset of Figure 4f. The charge-transfer resistance (R ct ) of the ASC device exhibits no appreciable changes after cycling durability testing, confirming that it exhibits good electrical conductivity.

Ragone Plot Properties
The energy and power densities ( Figure 5) of the ASC device were estimated from Figure 4b. A high specific energy (65 Wh kg −1 ) was achieved at a specific power of 0.33 kW kg −1 , and the specific energy reached 28.7 W h kg −1 at a specific power of 11.5 kW kg −1 . These values are much higher than those of the results in the literature ( Figure 5) [6,25,47,[50][51][52][53][54][55][56][57][58], suggesting that the MoS 2 /Gr heterostructure composite is an outstanding candidate for use in eco-friendly, sustainable, and high-performance SCs.
The energy and power densities ( Figure 5) of the ASC device were estimated from Figure 4b. A high specific energy (65 Wh kg −1 ) was achieved at a specific power of 0.33 kW kg −1 , and the specific energy reached 28.7 W h kg −1 at a specific power of 11.5 kW kg −1 . These values are much higher than those of the results in the literature ( Figure 5) [6,25,47,[50][51][52][53][54][55][56][57][58], suggesting that the MoS2/Gr heterostructure composite is an outstanding candidate for use in eco-friendly, sustainable, and high-performance SCs.

Conclusions
A MoS2/Gr heterostructure was successfully fabricated via a cost effective and simple strategy. The SEM and TEM images indicate the successful growth of flower-like MoS2 on the Gr layer, which increases the surface area, enhances the structural stability, and boosts the electrical conductivity of the electrode. In three-electrode measurements performed on the MoS2/Gr heterostructure, the electrochemical capacitance reached 853 F g −1 at 1.0 A g −1 . Furthermore, the flexible ASC device composed of the MoS2/Gr heterostructure and activated carbon exhibited a high electrochemical specific capacitance of 208 F g −1 at 0.5 A g −1 , in addition to a high specific energy of 65 Wh kg −1 at a specific power of 0.33 kW kg −1 . More importantly, after 10,000 cycles, 86.5% of the electrochemical capacitance was retained, indicating the long-term cycling durability of the device. These excellent electrochemical properties indicate that the MoS2/Gr heterostructure composite is a promising candidate for use in flexible, sustainable, and advanced SCs.

Conclusions
A MoS 2 /Gr heterostructure was successfully fabricated via a cost effective and simple strategy. The SEM and TEM images indicate the successful growth of flower-like MoS 2 on the Gr layer, which increases the surface area, enhances the structural stability, and boosts the electrical conductivity of the electrode. In three-electrode measurements performed on the MoS 2 /Gr heterostructure, the electrochemical capacitance reached 853 F g −1 at 1.0 A g −1 . Furthermore, the flexible ASC device composed of the MoS 2 /Gr heterostructure and activated carbon exhibited a high electrochemical specific capacitance of 208 F g −1 at 0.5 A g −1 , in addition to a high specific energy of 65 Wh kg −1 at a specific power of 0.33 kW kg −1 . More importantly, after 10,000 cycles, 86.5% of the electrochemical capacitance was retained, indicating the long-term cycling durability of the device. These excellent electrochemical properties indicate that the MoS 2 /Gr heterostructure composite is a promising candidate for use in flexible, sustainable, and advanced SCs.