V 2 O 5 /Carbon Nanotube/Polypyrrole Based Freestanding Negative Electrodes for High-Performance Supercapacitors

: In this study, the vanadium pentoxide (V 2 O 5 ), functionalized carbon nanotubes ( f -CNT), and polypyrrole (PPy) based composites ﬁlms have been prepared through a facile synthesis method and their electrochemical performance were evaluated as freestanding negative electrodes of supercapacitor. A hydrous V 2 O 5 gel prepared by treating V 2 O 5 powder with H 2 O 2 was mixed with f -CNT to obtain V 2 O 5 / f -CNT composite ﬁlm. V 2 O 5 / f -CNT composite was then coated with PPy through vapor phase polymerization method. The PPy deposited on the V 2 O 5 / f -CNT prevented the dissolution of V 2 O 5 and thus resulted in an improved the capacitance and cycle life stability for V 2 O 5 / f -CNT/PPy composite electrode. V 2 O 5 / f -CNT/PPy freestanding negative electrode exhibited a high areal capacitance value (1266 mF cm − 2 at a current density of 1 mA cm − 2 ) and good cycling stability (83.0% capacitance retention after 10,000 charge-discharge cycles). The superior performance of the V 2 O 5 / f -CNT/PPy composite electrode can be attributed to the synergy between f -CNT with high conductivity and V 2 O 5 and PPy with high-energy densities. Thus, V 2 O 5 / f -CNT/PPy composite based electrode can effectively mitigate the drawbacks of the low speciﬁc capacitance of CNTs and the poor cycling life of V 2 O 5 .


Introduction
Electrochemical supercapacitors (SCs) have recently attracted a lot of attention due to their high power density, fast charging/discharging rates, high cycling stability, low fabrication costs, and environmental friendliness, making them a good candidate for use as the energy storage system for hand-held portable devices, wearable electro-optical devices, and sensors [1,2]. According to the energy storage mechanism, SCs can be divided into electrical double-layer capacitors (EDLCs) and pseudocapacitors [3][4][5][6]. The carbon materials based EDLCs store energy by the formation of the electrical double layer via electrostatic attraction at the electrode/electrolyte interface, which exhibits fast charging/discharging kinetics, wide potential window, high power density, and high cycling stability, but EDLC is limited by their low specific capacitance [3,7]. The conducting polymer, transition metal compounds, and redox-active materials-based pseudocapacitors store energy by the fast and reversible redox reaction at the electrode/electrolyte interface [8]. Although the pseudocapacitors have a high capacitance behavior, they have limited cycle stability and rate capability [9][10][11][12][13]. In order to improve the energy storage properties, the hybrid SCs fabricated from the electrode consisting of carbon materials, conducting polymer, transition metal compound, and redox-active material have been developed [14][15][16][17].
Transition-metal oxides (TMOs), such as ruthenium oxide (RuO 2 ), manganese dioxide (MnO 2 ), vanadium pentoxide (V 2 O 5 ), iron oxide (Fe 2 O 3 ), and Co 3 O 4 , have been explored for use in pseudocapacitors [18][19][20][21]. TMOs based SCs exhibited better electrochemical stability than the conducting polymer and higher specific capacitance than conventional carbon materials [22]. Among the TMOs, RuO 2 is one of the promising performers among the electrode materials, but high cost, scarcity, and toxicity hinder its extensive usage [23]. In contrast, V 2 O 5 is inexpensive, which has been studied as cathode materials for SCs, due to its high theoretical capacitance (2020 F g −1 ), large voltage window (up to −2.8 V), low cost, and abundance [24,25]. The balanced oxidation states and layered structures with a large surface area of V 2 O 5 allow a much higher charge storage capacity than other cheap TMOs [26][27][28]. Since V 2 O 5 exhibits a low electrical conductivity and cycling instability, V 2 O 5 /active carbon fabric (ACF), V 2 O 5 /CNT, and V 2 O 5 /graphene oxide (GO) nanocomposites have been developed to improve electrode performance for SCs [29][30][31][32][33][34][35][36][37][38]. The 3D array of layered V 2 O 5 grown on activated carbon fabric (ACF) for SC has been reported by Panigrahi et al. [29]. The 3D network with the porous structure on the ACF can significantly maximize the active material's performance, making it a potential contender in the modern flexible electronics arena. Moreover, the supercapactance of the functionalized CNTs coated with a thin layer of V 2 O 5 has been reported by Sathiya et al. [39]. The V 2 O 5 /CNT composite exhibits both high power density and high energy density, stressing the benefits of using carbon substrates to design high-performance supercapacitor electrodes. In addition, Pande et al. report a simple chemical bath deposition method for the preparation of the V 2 O 5 /CNT composite with nanoscale architecture [40]. The V 2 O 5 /CNT with unique nanostructured morphology exhibits the larger electrochemical potential window, which favors greater electrochemical features for SC. High-performance supercapacitors based on intertwined CNT/V 2 O 5 nanowire nanocomposites have been reported by Chen et al. [41]. The intertwined CNT/V 2 O 5 nanocomposite electrode exhibits excellent rate capability, high capacity, and cycling stability. On the other hand, free-standing three-dimensional V 2 O 5 nanobelts coated with conducting poly(3,4-ethylenedioxythiophene) (PEDOT) on vertically aligned CNTs (VA-CNTs)/graphene foam (GF) for flexible energy storage devices has been reported by Jiang et al. [42]. The well-aligned structure of VA-CNTs, and light-weight, highly conductive GF leads to highly efficient ionic and electronic transport channels, which are of scientific and practical significance for energy storage/conversion applications.
In this study, a hydrous V 2 O 5 gel prepared by treating V 2 O 5 powder with H 2 O 2 was mixed with f -CNT to obtain V 2 O 5 /f -CNT composite. In order to improve the conductivity and electrochemical properties of the V 2 O 5 /CNTs composites, the conducting polymer PPy was deposited on the surface of V 2 O 5 /f -CNT electrode by the vapor phase polymerization of pyrrole. The deposition of PPy by vapor phase polymerization favors for the uniform coating of PPy polymer chains on the surface of V 2 O 5 /f -CNT composite. The presence of conducting polymers, such as PPy, polyaniline, and PEDOT have been proven to improve the electrical conductivity of V 2 O 5 [43][44][45]. Bi et al. have reported that the oxygen vacancies could be induced on the surface of V 2 O 5 during the oxidative polymerization of the conducting polymer [46,47]. The induced oxygen vacancies promote the charge transfer kinetics of the resulting oxygen vacancies-V 2 O 5 /conducting polymer nanocables by forming a local electric field. The accompanying V4 + and V3 + may also catalyze the redox reactions and improve the supercapacitor performance [48]. Here, we have investigated the effect of the pyrrole deposition on the morphology, surface area, and electrochemical properties of the resulting V 2 O 5 /f -CNT/PPy composite. The chemical structure of the composites was confirmed by using Fourier transform infrared (FTIR) spectroscopy. X-ray photoelectron spectroscopy (XPS) was carried out to analyze the chemical components of the composites [49]. The morphology of the composites was studied by using the scanning electron microscopy (SEM) and X-ray diffractometer (XRD). The Brunauer-Emmett-Teller (BET) method was used to determine the specific surface areas and pore size distributions of the V 2 O 5 /f -CNT/PPy. The electrochemical properties of the V 2 O 5 /f -CNT/PPy composites based electrodes were evaluated through cyclic voltammetry (CV), the galvanostatic charge/discharge (GCD) method, and electrochemical impedance spectroscopy (EIS) in a three-electrode system using aqueous 1 M Na 2 SO4 as the electrolyte. High capacitance value (1266 mF cm −2 at a current density of 1 mA cm −2 ) and good cycling stability (with the retention of 83.0% of its initial capacitance after 10,000 charge-discharge cycles) were observed for the V 2 O 5 /f -CNT/PPy based electrode.

Preparation of the V 2 O 5 /f-CNT/PPy Composite Film
The schematic illustration of the V 2 O 5 /f -CNT/PPy composite film fabrication process is shown in Figure 1a.  (2) and (3)). Upon aging, no free hydrogen peroxide can be detected in the solution, and an aqueous solution of dioxovanadium cation [VO 2 ] + (Equation (4)) is obtained. The increase in the viscosity of the solution indicates the onset of hydrous vanadium oxide gel formation.    species such as HVO 4 2− and H 2 VO 4 − , produced during the charge-discharge processes are readily soluble in aqueous electrolytes leading to poor cycling stability [25]. Therefore, the V 2 O 5 /f -CNT composite film was further coated with PPy by vapor phase polymerization to improve the conductivity and electrochemical stability of the composite film. The fabrication process of the V 2 O 5 /f -CNT/PPy composite film (Figure 1d) without any binder and additives is easy, low-cost, and scalable.    Figure 2 shows the FTIR spectra of f-CNT, V2O5 gel, V2O5/f-CNT, and V2O5/f-CNT/PPy. f-CNT spectra exhibited prominent characteristic peaks at 1697 and 1528 cm −1 , assigned to the stretching vibrations of C=O and COO-, respectively. The FTIR spectrum of V2O5 gel exhibited absorption bands at 1005 cm −1 corresponding to the stretching vibrations of V=O. The bands which appeared at 765 and 516 cm −1 are attributed to the asymmetric and symmetric stretching vibrations of V-O-V, respectively [52]. The peaks at 3438 and 1635 cm -1 correspond to the bending and stretching vibrations of H-O-H and O-H bonds due to the absorbed or bound water content in the material [46]. Major characteristic peaks of V2O5 gel and f-CNT are present in the FTIR spectrum of V2O5/f-CNT composite, indicating the successful integration of V2O5 and f-CNT in the composite. However, after the deposition of PPy, the intensity of absorption bands corresponding to V2O5 gel were reduced, which was corresponded to the coverage of PPy. In addition to the absorption peaks of V2O5 and f-CNT, the weak absorption peaks of PPy were observed ranged from 1250 to 1600 cm −1 .  To study the crystallographic structure of the prepared materials, XRD analysis was conducted, and the data is shown in Figure 3. The pure V 2 O 5 powder exhibited a highly crystalline orthorhombic phase [53]. On the other hand, the lack of sharp diffraction peaks in the XRD patterns of V 2 O 5 .nH 2 O gel indicates its poor crystallinity. The characteristics diffraction peaks of V 2 O 5 .nH 2 O gel appeared at 6.2 • , 12.7 • , 19.3 • , and 25.8 • . The intense peak at 6.2 • corresponding to the (001) plane indicates the characteristic layered structure of V 2 O 5 ·nH 2 O [50]. f -CNT exhibited a major diffraction peak at 25.5 • which corresponds to the (002) lattice plane [54]. V 2 O 5 /f -CNT composite exhibited the characteristic peaks of both V 2 O 5 and f -CNT, indicating their successful integration. The intensity of the diffraction peaks of V 2 O 5 at 6.2 • has been decreased after the deposition of PPy.

Characterization of the V2O5/f-CNT/PPy Composite Films
in the XRD patterns of V2O5.nH2O gel indicates its poor crystallinity. The characteristics diffraction peaks of V2O5.nH2O gel appeared at 6.2°, 12.7°, 19.3°, and 25.8°. The intense peak at 6.2° corresponding to the (001) plane indicates the characteristic layered structure of V2O5·nH2O [50]. f-CNT exhibited a major diffraction peak at 25.5° which corresponds to the (002) lattice plane [54]. V2O5/f-CNT composite exhibited the characteristic peaks of both V2O5 and f-CNT, indicating their successful integration. The intensity of the diffraction peaks of V2O5 at 6.2° has been decreased after the deposition of PPy.  Figure 4. V2O5 powder exhibited high thermal stability with a char yield of 98.4%, indicating its highly crystalline nature. The volatilization of adsorbed and interlayer water molecules causes a progressive weight loss in V2O5 gel at temperatures below 200 °C, showing its hygroscopic character. At temperatures above 200 °C, the removal of inter-layer water results in the conversion of the disordered layer phase to crystalline orthorhombic phase, resulting in high thermal stability for V2O5 gel at higher temperatures [55]. The improved thermal stability of V2O5/f-CNT composite compared with pure V2O5 gel is due to the presence of f-CNT. The weight loss in the V2O5/f-CNT composite at higher temperatures is mainly due to the removal of carboxyl groups from the surfaces of f-CNT. The continuous weight loss pattern observed for the V2O5/f-CNT/PPy composite can be ascribed to the degradation and decomposition of PPy in the composite.  crystalline orthorhombic phase [53]. On the other hand, the lack of sharp diffraction peaks in the XRD patterns of V2O5.nH2O gel indicates its poor crystallinity. The characteristics diffraction peaks of V2O5.nH2O gel appeared at 6.2°, 12.7°, 19.3°, and 25.8°. The intense peak at 6.2° corresponding to the (001) plane indicates the characteristic layered structure of V2O5·nH2O [50]. f-CNT exhibited a major diffraction peak at 25.5° which corresponds to the (002) lattice plane [54]. V2O5/f-CNT composite exhibited the characteristic peaks of both V2O5 and f-CNT, indicating their successful integration. The intensity of the diffraction peaks of V2O5 at 6.2° has been decreased after the deposition of PPy.  Figure 4. V2O5 powder exhibited high thermal stability with a char yield of 98.4%, indicating its highly crystalline nature. The volatilization of adsorbed and interlayer water molecules causes a progressive weight loss in V2O5 gel at temperatures below 200 °C, showing its hygroscopic character. At temperatures above 200 °C, the removal of inter-layer water results in the conversion of the disordered layer phase to crystalline orthorhombic phase, resulting in high thermal stability for V2O5 gel at higher temperatures [55]. The improved thermal stability of V2O5/f-CNT composite compared with pure V2O5 gel is due to the presence of f-CNT. The weight loss in the V2O5/f-CNT composite at higher temperatures is mainly due to the removal of carboxyl groups from the surfaces of f-CNT. The continuous weight loss pattern observed for the V2O5/f-CNT/PPy composite can be ascribed to the degradation and decomposition of PPy in the composite.  XPS measurement was used to analyze the surface composition and chemical structure of the V 2 O 5 /f -CNT/PPy composite. As shown in Figure 5a, the XPS survey spectrum exhibited four distinct peaks, indicating the presence of C, V, O, and N elements. The deconvoluted profile of V 2p 3/2 core peak spectrum (Figure 5b) is composed of two components located at 517.2 eV and 515.8 eV, respectively, as shown in the fitting data. These two binding energy values indicate the existence of +5 and +4 oxidation states of vanadium in the V 2 O 5 /f -CNT/PPy composite with V 5+ accounts for the majority of the spectra. Moreover, V2p 3/2 and V2p 1/2 have a binding energy difference of about 7.2 eV, confirming the predominance of +5 valence state of vanadium in V 2 O 5 /f -CNT/PPy composite [29,56]. The deconvoluted XPS spectrum for the O1s orbital exhibited three  [29,32]. A single peak appeared at 400 eV in the XPS spectrum for the N1s orbital (Figure 5d) was attributed to pyrrolic nitrogen [57]. exhibited four distinct peaks, indicating the presence of C, V, O, and N elements. The deconvoluted profile of V 2p3/2 core peak spectrum (Figure 5b) is composed of two components located at 517.2 eV and 515.8 eV, respectively, as shown in the fitting data. These two binding energy values indicate the existence of +5 and +4 oxidation states of vanadium in the V2O5/f-CNT/PPy composite with V 5+ accounts for the majority of the spectra. Moreover, V2p3/2 and V2p1/2 have a binding energy difference of about 7.2 eV, confirming the predominance of +5 valence state of vanadium in V2O5/f-CNT/PPy composite [29,56]. The deconvoluted XPS spectrum for the O1s orbital exhibited three bands at 532.2, 531.2, and 530.2 eV, corresponding to the O-C=O, V-OH, and V-O bonds, respectively ( Figure  5c) [29,32]. A single peak appeared at 400 eV in the XPS spectrum for the N1s orbital (Figure 5d) was attributed to pyrrolic nitrogen [57].  Figure 6 shows SEM images of f-CNT, V2O5 gel, V2O5/f-CNT, and V2O5/f-CNT/PPy. In Figure 6a, the pure f-CNT film showed a densely packed network structure of f-CNT that was tightly interconnected. Sheet-like structures of V2O5 were visible in the SEM image of V2O5 gel (Figure 6b). Despite their high electronic conductivity and surface area, f-CNTs have a very low specific capacitance due to the electrical double-layer charge-storage process. As a result, pseudocapacitive materials with high theoretical capacitance are often combined with f-CNTs to improve the composites' total capacitance. Therefore, using a simple solution-based mixing technique, V2O5, a highly pseudocapacitive metal oxide, was integrated into the f-CNT network. V2O5 embedded in the f-CNT matrix can be seen in the SEM image of V2O5/f-CNT (Figure 6c). Figure 6d reveals the spherical PPy nanoparticles deposited on the V2O5/f-CNT/PPy composite after vapor phase polymeriza-  Figure 6 shows SEM images of f-CNT, V 2 O 5 gel, V 2 O 5 /f-CNT, and V2O 5 /f-CNT/PPy. In Figure 6a, the pure f-CNT film showed a densely packed network structure of f-CNT that was tightly interconnected. Sheet-like structures of V 2 O 5 were visible in the SEM image of V 2 O 5 gel (Figure 6b). Despite their high electronic conductivity and surface area, f-CNTs have a very low specific capacitance due to the electrical double-layer charge-storage process. As a result, pseudocapacitive materials with high theoretical capacitance are often combined with f -CNTs to improve the composites' total capacitance. Therefore, using a simple solution-based mixing technique, V 2 O 5 , a highly pseudocapacitive metal oxide, was integrated into the f -CNT network. tion. In Figure 7, the elemental mapping images of the V2O5/f-CNT/PPy reveal the homogeneous distribution of four elements C, V, O, and N in the composite. The C element comes from f-CNT, the V and O elements come from V2O5, and the N element comes from PPy.  Nitrogen adsorption/desorption test was used to analyze the porous structures of f-CNT, V2O5/f-CNT, and V2O5/f-CNT/PPy, as seen in Figure 8. Type IV isotherms with distinct hysteresis loops at moderate pressures revealed that all composite materials had porous structures dominated by mesopores [58]. The BET specific surface areas for f-CNT, V2O5/f-CNT, and V2O5/f-CNT/PPy were 270.66, 188.43, and 163.33 m 2 g −1 , respectively. As compared to the f-CNT, lower specific surface areas was observed for the V2O5/f-CNT due to the coverage of V2O5 gel. Moreover, the surface area of the V2O5/f-CNT/PPy composite was slightly lower than the V2O5/f-CNT composite, which may be due to the PPy content partly blocking the pores in the composite. The average pore diameters estimated from the pore size distribution curves using the BJH technique for f-CNT, V2O5/f-CNT, and V2O5/f-CNT/PPy were 21.4, 19.7, and 12.2 nm, respectively. The distinctive hierarchical mesoporous architecture of V2O5/f-CNT/PPy composite with high specific surface areas allows for efficient use of the active materials by providing fast electrolyte diffusion pathways. tion. In Figure 7, the elemental mapping images of the V2O5/f-CNT/PPy reveal the homogeneous distribution of four elements C, V, O, and N in the composite. The C element comes from f-CNT, the V and O elements come from V2O5, and the N element comes from PPy.  Nitrogen adsorption/desorption test was used to analyze the porous structures of f-CNT, V2O5/f-CNT, and V2O5/f-CNT/PPy, as seen in Figure 8. Type IV isotherms with distinct hysteresis loops at moderate pressures revealed that all composite materials had porous structures dominated by mesopores [58]. The BET specific surface areas for f-CNT, V2O5/f-CNT, and V2O5/f-CNT/PPy were 270.66, 188.43, and 163.33 m 2 g −1 , respectively. As compared to the f-CNT, lower specific surface areas was observed for the V2O5/f-CNT due to the coverage of V2O5 gel. Moreover, the surface area of the V2O5/f-CNT/PPy composite was slightly lower than the V2O5/f-CNT composite, which may be due to the PPy content partly blocking the pores in the composite. The average pore diameters estimated from the pore size distribution curves using the BJH technique for f-CNT, V2O5/f-CNT, and V2O5/f-CNT/PPy were 21.4, 19.7, and 12.2 nm, respectively. The distinctive hierarchical mesoporous architecture of V2O5/f-CNT/PPy composite with high specific surface areas allows for efficient use of the active materials by providing fast electrolyte diffusion pathways. Nitrogen adsorption/desorption test was used to analyze the porous structures of f -CNT, V 2 O 5 /f -CNT, and V 2 O 5 /f -CNT/PPy, as seen in Figure 8. Type IV isotherms with distinct hysteresis loops at moderate pressures revealed that all composite materials had porous structures dominated by mesopores [58]. The BET specific surface areas for f -CNT, V 2 O 5 /f -CNT, and V 2 O 5 /f -CNT/PPy were 270.66, 188.43, and 163.33 m 2 g −1 , respectively. As compared to the f -CNT, lower specific surface areas was observed for the V 2 O 5 /f -CNT due to the coverage of V 2 O 5 gel. Moreover, the surface area of the V 2 O 5 /f -CNT/PPy composite was slightly lower than the V 2 O 5 /f -CNT composite, which may be due to the PPy content partly blocking the pores in the composite. The average pore diameters estimated from the pore size distribution curves using the BJH technique for f -CNT, V 2 O 5 /f -CNT, and V 2 O 5 /f -CNT/PPy were 21.4, 19.7, and 12.2 nm, respectively. The distinctive hierarchical mesoporous architecture of V 2 O 5 /f -CNT/PPy composite with high specific surface areas allows for efficient use of the active materials by providing fast electrolyte diffusion pathways.

Electrochemical Properties of the Freestanding Negative Electrodes
The CV plots measured at different scan rates of the f-CNT, V2O5/f-CNT, and V2O5/f-CNT/PPy composites in a three-electrode set-up are shown in Figure 9a-c. The CV curves of the f-CNT electrode were almost rectangular, indicating an ideal electrochemical double layer (EDL) capacitance. The quasi rectangular-shaped curves with high peak current response for V2O5/f-CNT and V2O5/f-CNT/PPy electrodes indicate that the capacitance was contributed by the pseudocapacitance of V2O5 and PPy in addition to the double-layer capacitance of f-CNTs. At all measured scan rates, the V2O5/f-CNT/PPy electrode had a much greater integrated area within the CV curve than the f-CNT and V2O5/f-CNT electrodes, demonstrating its superior performance. Figure 9d-f provides the GCD curves recorded at different current densities. The typical EDL capacitance of the f-CNT electrode was confirmed by GCD curves with symmetric triangular shapes at all current densities. The triangular-shaped GCD curves of V2O5/f-CNT, and V2O5/f-CNT/PPy composites with slight distortion and longer discharge times than the f-CNT electrode confirmed the pseudocapacitance contribution of V2O5 and PPy towards the total capacitance. In comparison to f-CNT electrode (490 mF cm -2 ) and V2O5/f-CNT electrode (1061 mF cm -2 ), V2O5/f-CNT/PPy electrode had the highest areal capacitance (1266 mF cm -2 ). Moreover, a high volumetric capacitance of 126.6 F cm -3 was also achieved by the V2O5/f-CNT/PPy electrode. In addition to the partial contribution of pseudocapacitance from PPy, the electrochemical Na + insertion process occurring at V2O5/f-CNT and V2O5/f-CNT/PPy electrodes during the charge-discharge process can be expressed as V O + xNa + xe ↔ Na V O [59,60]. The superior capacitance of the V2O5/f-CNT/PPy electrode can be ascribed to the improved conductivity as well as its highly porous architecture with large active surface area, which allowed a better use of the inner active redox sites in the electrode material. The electrochemical performance of the V2O5/f-CNT/PPy electrode was superior to those of many previously reported freestanding electrodes (see Table S1).

Electrochemical Properties of the Freestanding Negative Electrodes
The CV plots measured at different scan rates of the f -CNT, V 2 O 5 /f -CNT, and V 2 O 5 /f -CNT/PPy composites in a three-electrode set-up are shown in Figure 9a-c. The CV curves of the f -CNT electrode were almost rectangular, indicating an ideal electrochemical double layer (EDL) capacitance. The quasi rectangular-shaped curves with high peak current response for V 2 O 5 /f -CNT and V 2 O 5 /f -CNT/PPy electrodes indicate that the capacitance was contributed by the pseudocapacitance of V 2 O 5 and PPy in addition to the double-layer capacitance of f -CNTs. At all measured scan rates, the V 2 O 5 /f -CNT/PPy electrode had a much greater integrated area within the CV curve than the f -CNT and V 2 O 5 /f -CNT electrodes, demonstrating its superior performance. Figure 9d-f provides the GCD curves recorded at different current densities. The typical EDL capacitance of the f -CNT electrode was confirmed by GCD curves with symmetric triangular shapes at all current densities. The triangular-shaped GCD curves of V 2 O 5 /f -CNT, and V 2 O 5 /f -CNT/PPy composites with slight distortion and longer discharge times than the f -CNT electrode confirmed the pseudocapacitance contribution of V 2 O 5 and PPy towards the total capacitance. In comparison to f -CNT electrode (490 mF cm -2 ) and V 2 O 5 /f -CNT electrode (1061 mF cm -2 ), V 2 O 5 /f -CNT/PPy electrode had the highest areal capacitance (1266 mF cm -2 ). Moreover, a high volumetric capacitance of 126.6 F cm -3 was also achieved by the V 2 O 5 /f -CNT/PPy electrode. In addition to the partial contribution of pseudocapacitance from PPy, the electrochemical Na + insertion process occurring at V 2 O 5 /f -CNT and V 2 O 5 /f -CNT/PPy electrodes during the charge-discharge process can be expressed as V 2 O 5 + xNa + + xe − ↔ Na x V 2 O 5 [59,60]. The superior capacitance of the V 2 O 5 /f -CNT/PPy electrode can be ascribed to the improved conductivity as well as its highly porous architecture with large active surface area, which allowed a better use of the inner active redox sites in the electrode material. The electrochemical performance of the V 2 O 5 /f -CNT/PPy electrode was superior to those of many previously reported freestanding electrodes (see Table S1). EIS was used to assess the resistance associated with charge storage at the electrode/electrolyte interface and to better understand the charge transfer mechanism in the electrode materials. The Nyquist plots of f-CNT, V2O5/f-CNT, and V2O5/f-CNT/PPy electrodes comprising a straight line in the low-frequency region and a semi-circle in the high-frequency region are shown in Figure 10a. By intersecting the semi-circle with the X-axis, the equivalent series resistance (Rs), which is the inner resistance of the electrode and electrolyte, is obtained, while the diameter of the semi-circle provides the charge transfer resistance (Rct) at the electrode/electrolyte interface. The V2O5/f-CNT/PPy composite electrode exhibited good charge transfer properties due to the self-standing electrode structure that eliminates the need for an insulating binder for electrode fabrication. Moreover, the highly porous structures of the electrodes enable easy infiltration of electrolyte resulting in less ion diffusion resistance. The areal capacitances of the composite electrodes at various current densities are summarized in Figure 10b. The capacitances of all freestanding electrodes decreased as the current intensity rose. This reduction in the areal capacitance is due to the reduced electrolyte ions accessibility to the inner active sites EIS was used to assess the resistance associated with charge storage at the electrode/electrolyte interface and to better understand the charge transfer mechanism in the electrode materials. The Nyquist plots of f -CNT, V 2 O 5 /f -CNT, and V 2 O 5 /f -CNT/PPy electrodes comprising a straight line in the low-frequency region and a semi-circle in the high-frequency region are shown in Figure 10a. By intersecting the semi-circle with the X-axis, the equivalent series resistance (R s ), which is the inner resistance of the electrode and electrolyte, is obtained, while the diameter of the semi-circle provides the charge transfer resistance (R ct ) at the electrode/electrolyte interface. The V 2 O 5 /f -CNT/PPy composite electrode exhibited good charge transfer properties due to the self-standing electrode structure that eliminates the need for an insulating binder for electrode fabrication. Moreover, the highly porous structures of the electrodes enable easy infiltration of electrolyte resulting in less ion diffusion resistance. The areal capacitances of the composite electrodes at various current densities are summarized in Figure 10b. The capacitances of all freestanding electrodes decreased as the current intensity rose. This reduction in the areal capacitance is due to the reduced electrolyte ions accessibility to the inner active sites of the electrode materials, resulting in inadequate use of the V 2 O 5 and PPy at higher current densities.
Since cycling stability is an important parameter for electrode materials, V2O5/f-CNT and V2O5/f-CNT/PPy electrodes were charged and discharged 10,000 times at a current density of 10 mA cm −2 to determine cycling performance. Figure 10c shows that the areal capacitance of both electrodes decreases with the increasing charge-discharge cycles. However, after 10,000 charge/discharge cycles, the V2O5/f-CNT/PPy electrode retained 83% of its original capacitance, whereas V2O5/f-CNT electrode could retain only 45% of its initial capacitance. The poor cycling performance of the V2O5/f-CNT electrode is due to the dissolution of V2O5, as indicated by the color change of the electrolyte solution during the charge-discharge process. The PPy protection layer prevents the dissolution of V2O5, resulting in improved stability for V2O5/f-CNT/PPy electrode.

Materials
CNTs were obtained from C-nano Technology (Beijing, China). Nitric Acid (HNO3), urea [CO(NH2)2], and vanadium oxide (V2O5) powder were purchased from J.T.Baker Chemicals (Phillipsburg, NJ, USA). Sodium hydroxide (NaOH), iron(III) chloride hexahydrate (FeCl3.6H2O), and sodium sulfate (Na2SO4) were procured from Honeywell Fluka (Charlotte, NC, USA). Thiourea [SC(NH2)2], hydrogen peroxide (H2O2, 30% w/w), and sulfuric acid (H2SO4) were obtained from Showa Chemical Industry (Minato-ku, Tokyo, Japan). Acetic acid (CH3COOH), pyrrole (C4H4NH), and sodium chlorite (NaClO2) were purchased from Sigma-Aldrich (St. Louis, MO, USA). High-purity deionized water was used in all experimental processes. Since cycling stability is an important parameter for electrode materials, V 2 O 5 /f -CNT and V 2 O 5 /f -CNT/PPy electrodes were charged and discharged 10,000 times at a current density of 10 mA cm −2 to determine cycling performance. Figure 10c shows that the areal capacitance of both electrodes decreases with the increasing charge-discharge cycles. However, after 10,000 charge/discharge cycles, the V 2 O 5 /f -CNT/PPy electrode retained 83% of its original capacitance, whereas V 2 O 5 /f -CNT electrode could retain only 45% of its initial capacitance. The poor cycling performance of the V 2 O 5 /f -CNT electrode is due to the dissolution of V 2 O 5, as indicated by the color change of the electrolyte solution during the charge-discharge process. The PPy protection layer prevents the dissolution of V 2 O 5 , resulting in improved stability for V 2 O 5 /f -CNT/PPy electrode.

Preparation of Vanadium Pentoxide Gel
First, 0.2 g of V 2 O 5 powder was added into 20 mL of deionized water and stirred at 60 • C at high speed. Then, 2 mL of H 2 O 2 was slowly dropped into the V 2 O 5 suspension and continued stirring for 20 min until a red solution was obtained. After cooling the reaction mixture to room temperature, 20 mL of ethanol was added and allowed to age for 7 days to obtain a homogeneous green viscous gel.

Preparation of V 2 O 5 /f-CNTs Composite Film
To prepare the f -CNTs, CNTs were treated with 3 M nitric acid under reflux for 14 h at 90 • C, followed by 1 h ultrasonication. The dried f -CNTs (50 mg) were then ultrasonically dispersed in DI water. Then, 45 mg of f -CNTs were dispersed well in 45 mL of ethanol by ultrasonication for 1 h. Next, 5 mL (25 mg of V 2 O 5 ) of V 2 O 5 gel was then dropped slowly into the f -CNT dispersion at room temperature under magnetic stirring. After 1 h reaction, the reaction mixture was filtered by vacuum suction to form V 2 O 5 /f -CNTs composite film. The composite film was washed thoroughly with deionized water several times and dried in a vacuum oven at 60 • C for 12 h. The mass ratio of V 2 O 5 /f -CNTs was 36/64 (w/w).

Preparation of V 2 O 5 /f-CNT/PPy Composite Film
PPy was coated on V 2 O 5 /f -CNT composite film through a simple vapor phase polymerization method at room temperature. The V 2 O 5 /f -CNT composite film was first immersed in a 1 M aqueous FeCl 3 ·6H 2 O solution for 10 min. After drying the composite film at room temperature, it was placed in a vacuum flask. Then, 2 mL of purified pyrrole monomer was placed in a Petri plate and allowed to evaporate inside the vacuum flask. The interaction of the pyrrole vapors with FeCl 3 ·6H 2 O initiates the polymerization process to form a PPy layer on the composite film. The polymerization time was adjusted to 10 min to obtain the V 2 O 5 /f -CNT/PPy composite film. The as-prepared composite film was washed thoroughly with DI water and then dried in a vacuum oven at 60 • C for 12 h.

Characterizations
FTIR spectra of the samples were recorded using a PerkinElmer RX1 IR spectrophotometer (PerkinElmer, Taipei, Taiwan). Thermogravimetric analysis (TGA, PerkinElmer Pyris1, PerkinElmer, Taipei, Taiwan) was performed under an N 2 atmosphere at a heating rate of 10 • C min -1 . SEM (Jeol-JSM-7401F, Jeol, Tokyo, Japan) was used to observe the morphologies and microstructures of the prepared samples. The crystallographic structures of the prepared materials were analyzed using XRD (Bruker AXS, Bruker Taiwan Co. Ltd., Hsinchu, Taiwan, Cu Kα radiation). XPS (ESCALAB 250Xi, Thermo Fisher, Waltham, MA, USA) was used to examine the surface chemical components of the samples. Nitrogen adsorption/desorption isotherms of the composite samples were recorded using a surface area and porosity analyzer (ASAP 2010, Micromeritics, Merit Sci. Corp., Taipei, Taiwan). From the linear regions of the isotherms, the Brunauer-Emmett-Teller (BET) technique was applied to calculate the specific surface areas. The Barret-Joyner-Halenda (BJH) technique was used to determine pore size distributions from the desorption branches of the isotherms.

Electrochemical Measurements
CV, GCD, and EIS measurements were performed using a three-electrode system on an electrochemical work station (CH Instruments, Austin, TX, USA) to evaluate the electrochemical properties of the prepared composite materials. The reference electrode, counter electrode, and electrolyte were saturated calomel electrodes (SCE), platinum foil, and 1 M Na 2 SO 4 , respectively. As the working electrode, a piece of composite film (1.0 by 1.0 cm) was utilized directly. CV and GCD analyses were performed in the voltage range from 0 to −1 V at various scan rates and applied current densities, respectively. The areal capacitances (C A , F cm -2 ) of freestanding negative electrodes were calculated from the GCD curves using Equation (6) [7]: where I is the discharge current (A), ∆t is the discharge time (s), A s is the area of the freestanding electrode (cm 2 ), and ∆V is the potential window (V) excluding the IR drop.
The volumetric capacitance (C v , F cm -3 ) was calculated using the Equation (7): where d is the thickness (cm) of the single electrode.

Conclusions
A simple, straightforward, and efficient approach was developed for the production of V 2 O 5 /f -CNT/PPy composite electrode. Firstly, a hydrous V 2 O 5 gel was prepared by treating V 2 O 5 powder with H 2 O 2 . The as-prepared V 2 O 5 gel was then dispersed uniformly in the f -CNT matrix to obtain V 2 O 5 /f -CNT composite electrode. V 2 O 5 /CNT was then coated with a PPy layer by the vapor-phase polymerization method. The PPy coating on the V 2 O 5 /CNT prevented the dissolution of V 2 O 5 and thus improved the capacitance and cycle life stability for V 2 O 5 /CNT/PPy composite electrode. The V 2 O 5 /f -CNT/PPy negative electrode exhibited outstanding electrochemical properties due to the synergistic effects of highly conductive f -CNTs and pseudocapacitive V 2 O 5 . The V 2 O 5 /f -CNT/PPy electrode exhibited a high areal capacitance of 1266 mF cm −2 and excellent cycling stability with 83% capacitance retention after 10,000 charge-discharge cycles.