vanadium oxide composite as binder-free electrode for asymmetrical supercapacitor

Accepted Manuscript Free-standing graphene/vanadium oxide composite as binder-free electrode for asymmetrical supercapacitor Lingjuan Deng, Yihong Gao, Zhanying Ma, Guang Fan PII: DOI: Reference:

S0021-9797(17)30705-1 http://dx.doi.org/10.1016/j.jcis.2017.06.048 YJCIS 22475

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

21 April 2017 11 June 2017 15 June 2017

Please cite this article as: L. Deng, Y. Gao, Z. Ma, G. Fan, Free-standing graphene/vanadium oxide composite as binder-free electrode for asymmetrical supercapacitor, Journal of Colloid and Interface Science (2017), doi: http:// dx.doi.org/10.1016/j.jcis.2017.06.048

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Free-standing graphene/vanadium oxide composite as binder-free electrode for asymmetrical supercapacitor Lingjuan Deng*, Yihong Gao, Zhanying Ma, Guang Fan (School of Chemistry & Chemical Engineering, Xianyang Normal University, Xianyang 712000, China)

*Correspondence should be addressed to: Lingjuan Deng School of Chemistry & Chemical Engineering, Xianyang Normal University Xianyang, Shaanxi, 712000, P. R. China Tel: ++86−29−33720704 E-mail: [email protected]

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Abstract Preparation of free-standing electrode materials with three-dimensional network architecture has emerged as an effective strategy for acquiring advanced portable and wearable power sources. Herein, graphene/vanadium oxide (GR/V2O5) free-standing monolith composite has been prepared via a simple hydrothermal process. Flexible GR sheets acted as binder to connect the belt-like V2O5 for assembling three-dimensional network architecture. The obtained GR/V2O5 composite can be reshaped into GR/V2O5 flexible film which exhibits more compact structure by ultrasonication and vacuum filtration. A high specific capacitance of 358 F g−1 for GR/V2O5 monolith compared with that of GR/V2O5 flexible film (272 F g-1) has been achieved in 0.5 mol L−1 K2SO4 solution when used as binder free electrodes in three-electrode system. An asymmetrical supercapacitor has been assembled using GR/V2O5 monolith as positive electrode and GR monolith as negative electrode, and it can be reversibly charged-discharged at a cell voltage of 1.7 V in 0.5 mol L-1 K2SO4 electrolyte. The asymmetrical capacitor can deliver an energy density of 26.22 Wh kg-1 at a power density of 425 W kg-1, much higher than that of the symmetrical supercapacitor based on GR/V2O5 monolith electrode. Moreover, the asymmetrical supercapacitor preserves 90% of its initial capacitance over 1000 cycles at a current density of 5 A g-1. Key Words： ：free-standing, GR/V2O5 composites, flexible, asymmetrical supercapacitor,

2

1.

Introduction The rapid development of portable and wearable consumer electronics such as electronic

papers, roll-up displays, flexible biosensors and implantable medical devices in human’s daily life, has raised huge demand for flexible and highly efficient energy storage devices [1-5]. Because of their high power density, fast charge/discharge rate, light-weight, easy to handle, and excellent stability, flexible supercapacitors have attracted great attention in recent years [6]. It is well known that electrode material is a crucial factor which affects capacitive properties of a supercapacitor [7], so preparing electrodes along with intriguing capacitive property and flexibility holds great promise to fulfill the need for flexible supercapacitors. Graphene (GR) is believed as a prime candidate for flexible supercapacitor electrode material due to the one-atom thick, excellent mechanical flexibility, superior electrical conductivity and so on [8]. GR could give an encouraging electrical double-layer capacitance (EDLC, which attracts charge electrostatically on electrode-electrolyte interface) of 550 F g-1 if all the theoretical specific surface area can be fully utilized [9]. Generally, pure GR commonly present a specific capacitance of about 150 F g-1 resulting from sacrificing partial specific surface area during the irreversible agglomerates [10]. Therefore, some endeavors have been made for preparing GR based flexible electrode materials including preparation of GR with three-dimensional network using natural template such as nickel foam, sponge, mesoporous molecular sieve and so on [11-14]. The other is minimizing the restacking induced property deterioration when the GR sheets are processed into bulk materials [15]. The modified GR, however, still give unsatisfied specific capacitance due to relative small theoretical capacitance. 3

It has been demonstrated that faradaic supercapacitors which employ pseudocapacitive (electrochemical active) material as electrodes (where fast and reversible faradaic reactions and involve the passage of charge across the double layer will take place) can provide much higher specific capacitance and energy density than those of EDLC which usually use carbon materials as electrodes [7]. Many transition metal oxides, such as RuO2, MnO2, NiO, SnO2, V2O5 and so on, have been used as electrode materials for supercapacitors [16-19]. Among the pseudocapacitive transition metal oxides containing metal atoms capable of various valence states, V2O5 is one of the most promising electrode materials for supercapacitors ascribe to its low cost, abundant resources and ease of synthesis [19, 20]. As a kind of extrinsic pesudocapacitive material, V2O5 only exhibits pesudocapacitance when nanostructured [21]. V2O5-based electrode materials, however, suffer from poor cycle stability and low rate capability due to the poor electrical conductivity (10−2~10 −3 S cm−1) and the large specific volume changes upon cycling [22]. Therefore, fabricating V2O5/carbon composites , has been developed to improve the structure integrity and electrical conductivity of V2O5-based electrode materials [23]. Up to now, some V2O5/carbon (such as GR, carbon black, and carbon nanotubes) composites have been prepared for supercapacitors. However, most of the V2O5/carbon electrode materials are in the form of powder, and thus the conductive and binder additives must be added when preparation of electrodes. Ultimately, the specific capacitance of the V2O5/carbon electrode would be reduced due to the increasing of electrode mass [24]. Flexible V2O5/GR composite, therefore, is expected to improve the conductivity and stability of V2O5, and therefore (thus) making it possible to achieve high energy density and 4

remarkable cycling stability. Although several V2O5/carbon electrode materials in the form of flexible

film

such

as

V2O5-carbon

nanofibers

[23],

V2O5-carbon

cloth

[25],

V2O5/Polyindole-carbon cloth [19] and so on, have been prepared for electrode materials of supercapacitors, their flexibility strongly depends on the base substance, and the preparation process is complicated, In the present work, GR/V2O5 composite monolith with GR amounts of 22% is prepared by one-step hydrothermal technology. The flexible GR sheets decorated with ultralong V2O5 nanobelts were self-assembled into monolith with interpenetrated three-dimensional network architecture during the hydrothermal process. The obtained GR/V2O5 composite can be further reshaped to flexible film with a compact structure. We further show both GR/V2O5 composite monolith and GR/V2O5 flexible film can function as binder-free supercapacitor electrodes with ultrahigh capacitive energy storage performances. Moreover, the GR/V2O5 composite monolith has been used as positive electrode for assembling asymmetrical supercapacitor, while GR monolith acted as negative electrode. The assembled asymmetrical supercapacitor can work in a voltage of 1.7 V, and give much better capacitive properties than that symmetrical supercapacitor based on GR/V2O5 composite monolith electrodes. 2. Experimental 2.1 Material preparation Graphite oxide (GO) was fabricated from crude flake graphite using a modified Hummers method [26]. The as-prepared GO was ultrasonic treated by a KQ-5200kDE digital ultrasonic cleaning device (200 W, 100 % amplitude) for 3 hours to obtain GO homogeneous dispersion (1.5 mg mL-1). 5

The GR/V2O5 composite monolith was prepared as follows: 0.1 g NH4 VO3 powder was added into 35 mL GO dispersion (1.5 mg mL-1), followed by violently stirred for 10 min, and then 4 mL acetic acid was added to the above suspension. After being vigorously stirred for another 10 min, the brown dispersion was transferred into an autoclave and heated at 180 °C for 12 h. Finally, the resulted monolith was removed, dialysis with distilled water to neutral. Compared with GR/V2O5 composite monolith, the GR and V2O5 monolith were also prepared through the same process using only GO or NH4VO3 as precursors. It is interesting that only the GR monolith compared with ooze-like V2O5 is obtained. The GR/V2O5 composite flexible film was prepared as follows: the slice with a thickness of ~5 mm was first cut from the purified cylindrical GR/V2O5 composite monolith. Subsequently, stirred for 10 min, and finally obtained by vacuum filtration of the GR/V2O5 composite dispersion through an anodic membrane filter (50 mm in diameter, 0.45 µm pore size) followed by drying at room temperature. The typical thickness of the GR/V2O5 composite flexible film was ~10 µm. In comparison with GR/V2O5 composite flexible film, GR and V2O5 flexible film were also prepared using the same methods. Unfortunately, a V2O5 flexible film in comparison with a very fragile GR film was only obtained. 2.2. Characterization X-ray diffraction (XRD) measurements were carried out by an Ultima IV diffractometer. A Quanta 600 FEG field emission scanning electron microscope (FSEM) and transmission electron microscope (TEM) (JEM2010-HR) were used to observe the morphology of the obtained materials. The surface electronic states about the obtained materials were investigated by X-ray photoelectron spectroscopy (XPS; AXIS ULTRA Kratos Analytical 6

Ltd.). A Beckman coulter-type nitrogen adsorption–desorption apparatus (ASAP 2020 HD88) was used to investigate the pore property degassing at 120 °C for 3 h below 10-3 mmHg. The GR content in the composite was determined by weighting the residual sediment after the GR/V2O5 composite was dissolved in a 6 mol L−1 HCl solution. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements of different electrodes were carried out on a CHI660E electrochemical workstation. Galvanostatic charge/discharge and cycle stabilities measurements were measured using a battery testing system (LAND, ModelCT2001A). 2.3. Electrochemical measurement Electrodes used for electrochemical test were prepared as follows: The GR/V2O5 composite flexible film pieces with size of ~1 cm2 and a mass about 2 mg were first cut from the pristine obtained flexible film, and then were sandwiched between two stainless steel net layers (500 mesh, one piece: 1.5cm ×1.5 cm, and the other piece: 1.5 cm ×10 cm) current collector under a pressure of ~ 10 MPa for 1 min. Slices of GR/V2O5 composite monolith with a thickness of ~1 mm with a mass of 3 mg was first cut from the purified GR/V2O5 composite monolith, and then were sandwiched between two stainless steel net layers (500 mesh, one piece: 1.5cm× 1.5 cm, and the other piece: 1.5 cm ×10 cm) under a pressure of ~ 5 MPa for 30 seconds. Electrodes used for asymmetrical supercapacitor were prepared as follows: Slices of GR/V2O5 composite monolith with a thickness of ~1 mm with a mass of 3 mg was first cut from the purified GR/V2O5 composite monolith, then the slices was pressed onto stainless steel net (2 cm2) under a pressure of ~ 5 MPa for 1 minute and dried at 120 °C for 1 h. The 7

assembled GR/V2O5//GR and GR/V2O5//GR/V2O5 supercapacitors separated by a glass paper fiber saturated by 0.5 mol L−1 K2SO4 electrolyte without removal of oxygen from the solution were performed in a two-electrode cell. The electrode and glass fiber layers were then pressed between two nickel current collectors by the mean of stainless steel clamps. The electrochemical test of the individual electrode was performed in a three-electrode cell, in which platinum foil, saturated calomel electrodes (SCE) and 0.5 mol L-1 K2SO4 aqueous solution was used as counter reference electrodes, referenced electrode and electrolyte, respectively. The specific capacitance C (F g–1) of the electrode and the assembled supercapacitors, energy density E (Wh kg–1) and power density P (W kg–1) of the supercapacitors were determined by means of galvanostatic charge-discharge cycles as follows:

C=

It ( ∆V ) m

E=

C ( ∆V ) 2 2 × 3600

(2)

P=

E t

(3)

(1)

Where ∆V = (Vmax-Vmin), Vmax is the potential at the end of charge and Vmin at the end of discharge, m is the active mass of the electrode or two composite electrodes in the assembled supercapacitors (kg), I is the applied current (A) and t is the time of the discharge stage (s).

τ0 =

1 f0

(4)

τ 0 indicating time relaxation constant is calculated from EIS date, where f0 is a frequency corresponding to at a phase angle of -45°。

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3. Results and discussion

The microstructure of the GR/V2O5 composite was analyzed by XRD, Raman spectroscopy, and XPS (Fig. 1). The characteristic peaks of XRD pattern are well indexed for the V2O5 crystal structure, matching with the JCPDF no. 40-1296. As expected, the characteristic (001), (003), and (004) peaks are located at 7.9°, 24.4°, and 32.7° [27], respectively (Fig. 1 a). In addition, the characteristic XRD pattern of GR in GR/V2O5 composite disappears, suggesting that V2O5 prevent the restacking of GR nanosheets and GR naonosheets are well dispersed in the final product. There are seven well-indexed peaks for both GR/V2O5 composite and V2O5 in Raman spectroscopy (Fig. 1b), which clearly demonstrates that V2O5, rather than other compound, was obtained as final products. The simplified hydrothermal reactions are proposed as follow. 2VO3− +2H + → V2 O5 +H 2 O

(3)

Two peaks locating at 1349 and 1596 cm−1 in Raman spectra provide the evidence that GR remains in the GR/V2O5 composite. The C1s peak in XPS spectra of GR/V2O5 composite (Fig.1c) further reniforces the existence of GR, and the amount of GR in the composite is 22 % according to the weighing analytic result. The XPS spectra (Fig. 1d) also give the information that V2O5 crystals were formed in the GR/V2O5 composite, where the 2p3/2 and 2p1/2 bands of V5+ are located at the binding energy values of 517.2 and 524.7 eV, respectively (Fig. 1d). The V2p3/2 band which could be deconvoluted into two peaks, is broad and asymmetric, indicating the existence of different vanadium species. The peak position at a binding energy of 517.4 eV is attributed to +5 oxidation state, while the peak at

9

515.9 eV is due to +4 oxidation state [28], and the +4/+5 vanadium ratio is 0.4 on the basis of the peak area (insert in Fig. 1d). As shown in Fig. 2a, GR/V2O5 monolith exhibits a nearly dumbbell-like shape with diameter of 1.9 cm and length of 2.9 cm (Fig. 2a) compared with the cylindrical-like of the GR monolith (Fig. 2e). GR/V2O5 monolith reveals a porous network which is mainly constructed by a large quantity of ultralong V2O5 nanobelts with diameters of about 50-200 nm and length up to the millimeter scale (Fig. 2b). Transparent GR sheets acted as binder to connect the belt-like V2O5 for forming the three-dimensional network architecture (Fig. 2c), and that is why V2O5 monolith is absent in the compared experiment. Crumpled plate-like morphology GR (Fig. 2g) closely associated with each other can be seen in Fig. 2f, which contrasted sharply with that of GR/V2O5 monolith and suggesting an inevitable agglomeration. TEM image (Fig. 2d) further confirms the twining and intersect overlap between GR and V2O5 nanobelts in GR/V2O5 composite. This is because that the V2O5 nanobelts are decorated with both sides of the GR plate, and thus an interpenetrating structure for GR/V2O5 composite is obtained due to the existence of π−π interaction between the GR sheets. The restacking of GR nanosheets can be partly hindered ascribe to the interpenetrated V2O5 nanobelts during preparation process. Due to absence of V2O5 nanobelts, the GR nanosheets become more compacter than that of GR/V2O5 composite and ultimately form a regular cylindrical monolith. The porous nature of the GR/V2O5 monolith, GR monolith and V2O5 was investigated by nitrogen adsorption and desorption measurements (Fig. 3a). The typical IV isotherm with H3 hysteresis loop at high relative pressure, indicating the existence of plentiful mesopores in 10

GR/V2O5 monolith. The BET method reveals a specific surface area of 172.9 m2 g-1 for GR/V2O5 monolith, which is much higher than that of GR monolith (77.5 m2 g-1) and V2O5 (35.1 m2 g-1). The pores constructed by GR nanosheets and V2O5 nanobelts are mainly mesopores with a pore size distribution of 0.9-20 nm (calculated from desorption data using the Barrette-Joynere-Halenda model) and an average pore size of about 3.8 nm (Fig. 3b). The BET surface area and pore-size distribution combined with the FSEM image (Fig. 2b, 2c) strongly confirm the fact that the GR/V2O5 monolith has an interconnected porous structure. V2O5 nanowires inserted between the GR nonosheets effectively reduce the stacking of the GR and well develop interconnected pores in the GR/V2O5 monolith. It is well known that the pore size at the range of 0.8-5 nm is the effective one required to increase either the pseudocapacitance or EDLC [29]. The hierarchical structure of GR/V2O5 monolith with high specific surface area and appropriate pore size is favorable for improving both the main pseudocapacitance of V2O5 and the EDLC capacitance of GR since the hydrated ions in the electrolyte are easily accessible to the exterior and interior pore surfaces during the charge-discharge process. Fig. 4 gives the digital photo of GR, V2O5 and GR/V2O5 composite film. The black GR film is broken (Fig. 4a) and fragile (Fig. 4b) due to the serious agglomeration of GR nanosheets. The khaki V2O5 film, however, is smooth and flexible ascribe to the stacking and interpenetration of ultralong V2O5 nanobelts. It is happened just as expected that the dark green GR/V2O5 composite film is also smooth and flexible resulting from the V2O5 nanobelts interpenetration between the GR nanosheets, like the longitude and latitude lines in spinning cloth. 11

From Fig. 5a and 5b, we can see V2O5 film is composed of nanobelts which can be further assembled into large-area flexible paper with a compact and neat structure (Fig. 5c) during the vacuum filtration process. For GR/V2O5 film (Fig. 5d), we can hardly see GR nanosheet except many belt-like V2O5 due to the highly dispersion of GR. From cross-sectional FSEM image of GR/V2O5 flim (Fig. 5f), the neat structure becomes loose and wavy due to several GR sheets (see the white arrows in the Fig. 5f) sandwiched between the V2O5 belts. The thicknesses of GR/V2O5 films were estimated to be about 10 µm according to the SEM images. Furthermore, it is worthy to be note that GR/V2O5 flexible film can be tailored with any desired dimension by change the suction funnel during the vacuum filtration process. The capacitive property of GR/V2O5, GR and V2O5 film electrodes are investigated by cyclic voltammetry (CV) and galvanostatic charge/discharge measurements in 0.5 mol L-1 K2SO4 aqueous solution. An almost rectangular CV curve is obtained for GR electrode, suggesting that the dominant charging contribution adheres to EDLC mechanism between -0.2 and 0.8 V vs. SCE (Fig. 6a). The V2O5 film electrode shows eggplant-like CV curve indicating

the

unsatisfied

capacitive

performance

resulting

from

the

awful

electroconductivity. The GR/V2O5 film electrode shows three defined peaks of anodic oxidation (0.08, 0.23 and 0.56 V) and reduction (0.04, 0.12 and 0.37 V), indicating the pseudocapacitive behaviour characteristic of the crystalline V2O5 to the overall capacitance. Such three-redox pairs may resulting from K+ insertion and de-insertion reactions at different energy states have been reported for V2O5 nanowire/CNT composite electrode in Na2SO4 aqueous electrolyte system [30]. According to the view of Patrice Simon, such behavior should be regard as battery performance [31]. The redox interaction occurs not only at the 12

interface but also in bulk of the electrode materials. Due to the introduction of GR, the electric conductivity of GR/V2O5 composite is well improved and thus a bigger current density than the pure V2O5 electrode is obtained in the same potential. Moreover, as shown in Fig. 6 a, the current of the GR/V2O5 composite electrode responses to the switching potential rapidly, indicating the composite electrode exhibits a lower equivalent series resistance (ESR) than that of the V2O5 electrode. A small ESR is vital to achieve a high rate capability and power density for supercapacitors. To further quantify their specific capacitance, galvanostatic charge-discharge tests were measured in the same potential window. The V2O5 electrode shows nearly linear curve with a slightly increased curvature, indicating that EDLC is still the primary contribution to the overall capacitance (67 F g-1 at a current density of 0.25 A g-1). Although V2O5 can provide a pseudocapacitance as high as 530 F g−1, poor conductivity obstructs its use in supercapacitors [32]. The charge-discharge plot of the GR/V2O5 film electrode indicates the overall capacitance (272 F g-1 at a current density of 0.25 A g-1) is composed by EDLC and pseudocapacitance. The variation in the specific capacitance as a function of the current densities indicates that the specific capacitance of GR/V2O5 electrode is higher than those of GR and V2O5 electrodes at the same current density (Fig. 6c). GR/V2O5 electrode shows high specific capacitance values from 272 to 142 F g−1 as the current densities increase from 0.25 to 10 A g−1, which are higher than those of V2O5 electrode (from 67 to 2 F g−1) and GR electrode (from 152 to 60 F g−1). These results clearly indicate intimately weaving V2O5 with the highly conductive materials (such as CNT and GR) into composites led to dramatically increase in capacitances. 13

Nyquist plots of GR/V2O5, GR and V2O5 electrodes in the frequency range of 100 kHz-0.1 Hz at open circuit potential with an ac perturbation of 5 mV are similar to one another, which are composed of an arc at the higher frequency region and a straight line at the lower frequency region (Fig. 6d). It can be seen that the Nyquist plot of GR/V2O5 electrode locates in the left of V2O5 electrode and near close to that of GR electrode, suggesting a synergistic effect between V2O5 nanobelts and GR nanosheets. After fitting EIS spectra using the equivalent circuit diagram (inset of Fig. 6d), Rct are 3.67, 55.55 and 97.58 Ω for GR, GR/V2O5 and V2O5 electrodes, respectively. The Rct of GR/V2O5 hybrid electrode is larger than that of GR electrode while smaller than that of V2O5 electrode, suggesting that the electron conductivity of GR/V2O5 electrode can be improved by adding GR nanosheets into V2O5 nanobelts, and the excellent conductivity of GR enables the electron to transfer fast from GR nanosheets to V2O5 belts. The relationship of the total impedance to the frequency for GR/V2O5, GR and V2O5 electrodes is shown in Fig. 6e. At low frequency, electrolyte ions can migrate from bottom of the porthole to all the electrode materials pores, and therefore the alternating current signal must penetrate through different depth to form big resistance. The electrolyte ions, however, can only migrate near the porthole of the pores to give a small resistance value at high frequency. The penetration degree is inversely proportional to the frequency [33]. As exhibits in Fig. 6e, V2O5 electrode shows tremendous resistance below 1 Hz, suggesting an appalling electrical conductivity. Compared with that of V2O5 electrode, GR electrode gives the smallest resistance resulting from the excellent electrical conductivity. The obtained GR/V2O5

14

electrode locating between V2O5 and GR electrodes is illustrative of improvement of electrical conductivity. After 1000 consecutive galvanostatic charge-discharge test at a current density of 5.0 A g−1, the specific capacitances of GR, GR/V2O5 and V2O5 electrodes are 153, 218, and 40 F g−1, corresponding to 101 %, 80 % and 60 % of capacitance retention for these electrodes, respectively. The improved cycling performance improvement of GR/V2O5 electrode is probably ascribed to the introduction of GR nanosheets in the electrode, which makes the electrode/electrolyte contact better and partly prevents vanadium dissolution into the aqueous electrolyte. Although the flexible GR/V2O5 flexible film electrode shows high specific capacitance than that of V2O5 electrode, its capacitive property is still discouraging. The GR/V2O5 monolith demonstrates unique hierarchical porosity compared with that of the flexible film counterpart, and the electrochemical performances of GR/V2O5 monolith as binder-free electrode is studied next. The GR/V2O5 monolith electrode shows more distinguishable redox peaks and higher corresponding current density than that of film-like electrode (Fig. 7a), suggesting that the GR/V2O5 monolith has a higher ion accessible specific surface area and faster ion diffusion rate than that of flexible film counterpart. The CV curves indicate a complex supercapacitance, namely pseudocapacitance and EDLC for both GR/V2O5 monolith and film. The GR/V2O5 monolith electrode exhibits a specific capacitance of 358 F g-1 at a current density of 0.25 A g-1, 30% higher than that of flexible film counterpart (272 F g-1) (Fig. 7b). When the current density was increased up to 10 A g-1, the GR/V2O5 monolith could retain as high as ∼63% of its initial value (224 F/g), while the GR/V2O5 film electrode gives a ∼52% 15

capacitance retention (142 F/g) (Fig. 7c). The ion diffusion dynamics within the GR/V2O5 monolith was further probed by electrochemical impedance spectroscopy (EIS) (Fig. 7d). The Nyquist plots obtained over a frequency range from 100 kHz to 0.01 Hz showed a nearly vertical line compared with that of GR/V2O5 film in the low-frequency regime, indicating a nearly ideal capacitive property for GR/V2O5 monolith. GR/V2O5 monolith electrode shows smaller Rct value (5.97 Ω) than that of GR/V2O5 film electrode (55.55 Ω), confirming a lower charge transfer resistance and more rapid ion diffusion within the GR/V2O5 monolith due to its hierarchical porosity. Fig. 6e shows the dependence of impedance phase angle on the frequency of GR/V2O5 monolith and GR/V2O5 film electrodes in three-electrode system. The relaxation time constant τ0 of supercapacitor, which is defined as the 1/f0 at a phase angle of -45°, represents the minimum time needed to discharge all the energy from the device with an efficiency of more than 50% [33]. The characteristic frequencies f0 at the phase angle of -45° was measured to be 0.21 and 8.92 Hz in aqueous K2SO4, corresponding to a time constants τ0 of 4.75 and 0.11 s for GR/V2O5 film and GR/V2O5 monolith electrodes, respectively. The smaller relaxation time of GR/V2O5 monolith also indicates the bigger accessibility of the electrolyte ions to the outer surface and the improved electrical conductivity compared with GR/V2O5 film. From the relationship of the total impedance to the frequency for GR/V2O5 monolith and GR/V2O5 film electrodes (Fig. 7f), we can see GR/V2O5 monolith electrode shows bigger total impedance than that of GR/V2O5 film at low frequency which is relate to the rich channels of the three dimensional network architecture.

16

On the basis of the above experimental results, it can be seen that the GR/V2O5 composite monolith electrode exhibit excellent capacitive property than that of GR/V2O5 composite film electrode. In order to further evaluate the capacitive of the obtained GR/V2O5 monolith composite, an asymmetrical supercapacitor is assembled employing GR/V2O5 monolith as positive electrode and GR monolith as negative electrode in 0.5 mol L−1 K2SO4 electrolyte. In the asymmetric supercapaciotr, the loading mass ratio of the negative to the positive electrode is 1.5. Take advantage of the higher specific capacitance of GR/V2O5 monolith and excellent stability of GR, favorable capacitive property of assembled asymmetrical supercapacitor could be achieved. The oxygen evolution potential for GR/V2O5 monolith electrode is 0.8 V vs. SCE and the hydrogen evolution potential for GR monolith electrode is -0.9 V, so it is expected that the operating cell voltage could be extended to about 1.7 V in K2SO4 aqueous solution (Fig. s1). Just as expected, a typical CV profile of the asymmetrical supercapacitor (GR/V2O5//GR) with a voltage of 1.7 V compared with the symmetrical supercapacitor (GR/V2O5//GR/V2O5) with a voltage of 1.0 V at a scan rate of 10 mV s−1 is shown in Fig. 8a, and which make it possible to offer high energy densities. Resulting from faradic pseudocapacitive nature of V2O5 nanobelts and EDLC nature of GR in the composites, a combination capacitance traced by unconspicuous redox peaks is observed. The galvanostatic charge-discharge curves of the asymmetrical supercapacitor at different current densities show that the potentials of charge-discharge lines are nearly proportional to the charge or dischgarge time (Fig. 8b). Power density (P) and energy density (E) are two vital factors to characterize the capacitive performance of supercapacitors. The Ragone plot of the assembled asymmetrical 17

supercapacitor at different current densities is shown in Fig. 8c. Although the energy densities reduce slowly with increasing the power densities, the energy density of the GR/V2O5//GR cell at the same power density is much higher than that of GR/V2O5//GR/V2O5, which means that the energy and power densities of GR/V2O5//GR cell is superior to that of GR/V2O5//GR/V2O5 cell. Based on the total mass of the two electrodes, the maximum energy density for GR/V2O5//GR cell is 26.22 Wh kg−1 at a power density of 425 W kg−1, which is superior to those of the asymmetrical supercapacitors reported by references, such as V2O5//SWCNT (18 Wh kg-1 at 315 W kg-1) [35], RG(1.0)/VO2//GR (22.8 Wh kg-1 at 425 W kg-1) [36], V2O5//AC (<20 Wh kg-1 at 400 W kg-1 )[37]. AC//V2O5·0.6H2O (29 Wh kg-1 at 70 W kg-1) [38]. Moreover, the asymmetrical supercapacitor still shows a moderate energy density of 7.0 Wh kg−1 even at a power density of 8.5 kW kg−1. Long cycle life is a requirement for supercapacitors. Consecutive galvanostatic charge-discharge on the supercapacitors of GR/V2O5//GR and GR/V2O5//GR/V2O5 at a current density of 5 A g−1 are performed. It can be seen that the asymmetrical GR/V2O5//GR supercapacitor shows a high capacitance retention of 90 % after 1000 cycles, while the GR/V2O5//GR/V2O5 symmetrical supercapacitor only exhibits 50 % retention after 1000 cycles (Fig. 8d). Such cycle stability confirms the electrochemical reversibility of GR/V2O5//GR asymmetrical supercapacitor, and also is superior to that of some asymmetrical supercapacitors based on vanadium oxides, such as V2O5/CNT//MnO2/C supercapacitor (90 % retention after 100 cycles) [39], V2O5 nanowire-graphene//carbon supercapacitor (70 % retention after 70 cycles) [40] and V2O5/AC supercapacitor (< 70 % retention after 1000 cycles) [41]. 18

4. Conclusions

GR/V2O5 free-standing composite monolith was prepared using one-step hydrothermal technology. The unique three-dimensional network architecture of the GR/V2O5 free-standing composite monolith, incorporating high capacitive performance of ultralong V2O5 nanobelts and excellent conductivity of GR, endows it to be a suitable electrode for flexible and lightweight supercapacitors. The obtained GR/V2O5 composite, moreover, can be reshaped into GR/V2O5 flexible film by ultrasonication and vacuum filtration. The GR/V2O5 composite can be used as binder-free electrode for supercapacitor, and manifest many advantageous features including outstanding specific capacitance (358 F g-1 at a current density of 0.25 A g-1), enhanced cycle stability (20% decay in specific capacitance after 1000 cycles at a current density of 5 A g-1), and excellent mechanical flexibility. An asymmetrical supercapacitor is assembled by using GR/V2O5 monolith as positive electrode and GR monolith as negative electrode, and it can be reversibly charged-discharged at a cell voltage of 1.7 V in 0.5 mol L−1 K2SO4 electrolyte. The asymmetrical supercapacitor can deliver an energy density of 26.22 Wh kg−1 at power density of 425 W kg−1, much higher than those of symmetrical supercapacitors based on the GR/V2O5 monolith electrodes. Moreover, the asymmetric supercapacitor preserves 90% of its initial capacitance over 1000 cycles at a current density of 5 A g−1. The excellent capacitive performance makes the GR/V2O5 monolith comoposite a promising electrode material for flexible supercapacitors. V. Acknowledgment

The project was supported by the Scientific Research Funds from Shaanxi Province Ministry of Education (15JK1783) and the Special Research Funding of Xianyang Normal 19

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Figure caption Fig. 1 XRD pattern (a), Raman spectra (b), and XPS spectrs (c, d) of V2O5, GR/V2O5 composite and GR, respectively. Fig. 2 Digital photos (a, e), FSEM images (b, c, f, g) and TEM images (d, h) of GR/V2O5 (a) and GR monolith. Fig. 3 (a) N2 adsorption/desorption isotherms of GR/V2O5 monolith, GR monolith, and V2O5, respectively. (b) Pore size distribution of GR/V2O5 monolith. Fig. 4 Digital photos of GR (a, b), V2O5 (c, d) and GR/V2O5 film (e, f). Fig. 5 FSEM images of V2O5 (a, b) and GR/V2O5 film (e, f). Cross-sectional FSEM images of V2O5 (c) and GR/V2O5 film (f). Fig. 6 Electrochemical capacitive properties of GR/V2O5, GR and V2O5 film electrodes measured using a three-electrode system: (a) CV curves at scan rate of 5 mV s-1. (b) Galvanostatic discharge profiles at a current density of 0.25 A g-1. (c) Specific capacitance at different current densities. (d) Nyquis plots measured at frequent range of 100 kHz to 0.1 Hz. Inset: A simplified equivalent circuit (RS: ohmic resistance of solution and electrodes, Rct: charge transfer resistance, C: double layer capacitance, W: Warburg impedance). (e) Plot of total impedance versus frequency. (f) Variation of specific capacitance with cycle numbers. Fig. 7 Electrochemical capacitive properties of GR/V2O5 composite monolith and GR monolith electrode using a three-electrode system: (a) CV curves at scan rate of 5 mV s-1. (b) Galvanostatic discharge profiles at a current density of 0.25 A g-1. (c) Specific capacitance at different current densities. (d) Nyquis plots measured at frequent range of 100 kHz to 0.1 Hz. (e) Impedance phase angle versus frequency. (f) Plot of total impedance versus frequency. 26

Fig. 8 Capacitive performance of the asymmetrical supercapacitor with GR/V2O5 composite monolith as positive electrode and GR monolith as negative electrode: (a) CV curve at a scan rate of 20 mV s-1 with a cell voltage of 1.7 V, (b) galvanostatic charge-discharge curves at different current densities, (c) Ragone plot of the assembled supercapacitor of GR/V2O5//GR with a cell potential of 1.7 V and GR/V2O5//GR/V2O5 a cell potential of 1.0 V, and (d) the cycle stability of GR/V2O5//GR and GR/V2O5//GR/V2O5, respectively. Fig. s1 CV curves of GR/V2O5 and GR monolith electrodes in a three electrode cell using 0.5 mol L-1 K2SO4 electrolyte at a scan rate of 5 mV s-1.

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Graphical abstract

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