BiVO4 Hybrid Nanostructures for High-Energy

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Fern-like rGO/BiVO4 hybrid nanostructures for high-energy symmetric supercapacitor Santosh S Patil, Deepak Prakash Dubal, Virendrakumar Gorakhnath Deonikar, Mohaseen S Tamboli, Jalindar D. Ambekar, Pedro Gomez-Romero, Sanjay S Kolekar, Bharat Bhanudas Kale, and Deepak Rajaram Patil ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08165 • Publication Date (Web): 01 Nov 2016 Downloaded from http://pubs.acs.org on November 1, 2016

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ACS Applied Materials & Interfaces

Fern-Like rGO/BiVO4 Hybrid Nanostructures for High-Energy Symmetric Supercapacitor Santosh S. Patil,‡,a,c Deepak P. Dubal,‡,b Virendrakumar G. Deonikar, a Mohaseen S. Tamboli, a Jalindar D. Ambekar, a Pedro Gomez-Romero, b,* Sanjay S. Kolekar, c Bharat B. Kale, a,* Deepak R. Patil a,*

a

Centre for Materials for Electronics Technology, Ministry of Electronics and Information Technology

(MeitY), Govt. of India. Pune-411008 b

Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC-BIST, Campus UAB, Bellaterra,

08193 Barcelona, Spain c

Analytical Chemistry and Material Science Laboratory, Department of Chemistry, Shivaji University,

Kolhapur- 416004 India ‡These authors contributed equally *Corresponding Author Emails [email protected], [email protected], [email protected]

1 Environment ACS Paragon Plus


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Abstract Herein, we demonstrate the synthesis of rGO/BiVO4 hybrid nanostructures by facile hydrothermal method. Morphological studies reveal that rGO sheets are embedded in the special dendritic fern-like structures of BiVO4. The rGO/BiVO4 hybrid architecture shows the way to a rational design of supercapacitor, since these structures enable easy access of electrolyte ions by reducing internal resistance. Considering the unique morphological features of rGO/BiVO4 hybrid nanostructures, their supercapacitive properties were investigated. The rGO/BiVO4 electrode exhibits a specific capacitance of 151 F/g under the current density of 0.15 mA/cm2. Furthermore, we have constructed rGO/BiVO4 symmetric cell which exhibits outstanding volumetric energy density of 1.6 mWh/cm3 (33.7 Wh/kg) and ensures rapid energy delivery with power density of 391 mW/cm3 (8.0 kW/Kg). The superior properties of symmetric supercapacitor can be attributed to the special dendritic fern-like BiVO4 morphology and intriguing physicochemical properties of rGO. Keywords: rGO/BiVO4, Fern/dendritic structures, Hydrothermal method, High energy density, Symmetric supercapacitor, Ragone plot


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1. Introduction Nowadays, the use of fossil fuels for energy has become a serious societal concern due to environment pollution, global warming, and rapid resource depletion.1,2 Therefore, finding sustainable and cost-effective energy storage devices is urgently needed to overcome the increasing energy crisis.3 Recently, supercapacitors have gained an intense research attention from the scientific community owing to their elevated power density and extensive cycle life.4 Supercapacitors are generally divided as Electric double layer capacitors (EDLC) wherein electrical energy is stored through charge accumulation at electrode-electrolyte interface and Pseudocapacitors where Faradaic redox reactions are involved. Metal oxides (binary or tertiary),5 metal sulphides,6 carbonaceous materials (graphene, reduced graphene oxide),7 and their composites8 have been exemplified as high-performance supercapacitor electrodes. However, the low energy density (˂10 Wh/kg) of these supercapacitors is still remained a major problem to use them as primary power sources in place of batteries.9 Therefore, increasing energy density of supercapacitor is of prime importance in order to use them for large scale practical applications. Two most commonly used strategies to get high energy density supercapacitors are to design high-voltage supercapacitors with wide potential windows and to build hybrid supercapacitors with one faradaic electrode and other capacitive electrode.9 Currently, research in this field is mostly focused on asymmetric capacitors fabricated with carbonaceous negative electrode and metal oxides/conducting polymers based positive electrode. Asymmetric supercapacitors present high energy densities with good cycling stabilities. However, the electrochemical supercapacitive performances of carbon based negative electrodes are insufficient to act as suitable counter electrodes.10,11 Moreover, because of the different charge storing mechanisms, positive and negative electrodes has to face the complex step of charge balancing.10,11 In this context, symmetric supercapacitor is an effective approach to replace an asymmetric supercapactors, however; the electrodes and electrolytes were need to be carefully chosen to obtain wide voltage window and high specific capacitance with affordable rate capability. 10



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There are very few transition metal oxide (TMOs) materials such as RuO2,12 Co(OH)2,13 MnO2,14 VO215 and NiO16 have been reported for symmetric supercapacitor application which exhibited high voltage window and excellent supercapacitor performance. Among them, vanadium (IV) oxide (VO2) based symmetric supercapacitors showed better electrochemical performance.15 Most recently, BiVO4 (belonging to the vanadate family) has been reported as one of the novel anode material for supercapacitor application.10,17 Owing to excellent physicochemical properties and stability, BiVO4 has also been used previously for gas sensing, ferroelasticity and photocatalysis applications.17 However, previous studies have proven that BiVO4 possesses relatively poor electronic conductivity which hampers capacitance retention and rate capability due to the low charge transfer rate during fast charge/discharge cycles.10,17,18 Therefore, electronic conductivity of BiVO4 needs to be improved. In this regards, Khan et al. have recently reported the hybrid composite of BiVO4 with SWCNT as supercapacitor electrode (three electrode system).17 They have indeed found the improved specific capacitance for SWCNT/BiVO4 than that of parent BiVO4 electrode. However, the study provides only preliminary data on the use of SWCNT/BiVO4 as supercapacitor electrode (three electrode system) with study focused only on the electrochemical performances for single electrode not for the device. Similarly, our recent study on Ag:BiVO4 hybrid architectures shows very high energy density for symmetric supercapacitor device owing to the improved electronic conductivity of the hybrid architecture.10 However, it is important to note that the cost of both the SWCNT and Ag is quite expensive therefore; it is not viable to use them for cost-effective high performance supercapacitor applications. The cost of the raw material must be taken into account for large scale practical applications. In this context, one of the most promising approaches to improve the electronic conductivity of BiVO4 is to prepare hybrid composites by integrating BiVO4 with cost effective carbonaceous materials.4 These carbon based materials possesses good electrical conductivity and can be act as a buffer layer to overcome the volume change of BiVO4.19 Reduced graphene oxide (rGO) is one of the best carbonaceous material since it shows good electrochemical properties and possesses intriguing surface properties which helps to compromise the volume change and avoids the agglomeration of particles during


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charge/discharge process.20 In addition, rGO also ensures easy access to electrolyte ions and induces pseudocapacitive effect.21 Most importantly, rGO is cost-effective, stable and mass produced in the

laboratory. Thus, the synergic combination of rGO with BiVO4 would have several advantages due to distinctive features of rGO and could represent a rational design of hybrid material for high performance supercapacitor device. In this respect, innovative rGO/BiVO4 hybrid architectures were prepared by a simple hydrothermal method. These rGO/BiVO4 hybrid nanostructures did undergo different physical-chemical characterizations such as structural, compositional and morphological etc. Later, the electrochemical performance of rGO/BiVO4 hybrid material was tested by fabricating symmetrical supercapacitors with KOH electrolyte. Certainly, the rGO/BiVO4 based symmetric cell exhibits an extensive voltage window (1.6 V) in aqueous electrolyte. The rGO/BiVO4 based symmetric cell exhibits notable volumetric energy density of 1.6 mWh/cm3 owing to the improved electrical conductivity and broad voltage window of rGO/BiVO4 symmetric cell

2. Experimental 2.1 Preparation of reduced graphene oxide (rGO) Commercial graphite (Sigma-Aldrich) was used as source to prepare reduced graphene oxide. First, graphene oxide (GO) was acquired by modified Hummers method22 through exfoliation of a chemically oxidized flake graphite powder. The GO powder is synthesized by adding 1 g of graphite into concentrated H2SO4 (23 mL) in a round-bottom flask and stirred on magnetic stirrer under an ice bath. Then, 3 g of KMnO4 were gradually added under intensive stirring under cooled condition and maintained at 20 oC for 10 minutes. Further, the solution was stirred for 4 h at a constant temperature of 35 °C using magnetic stirrer to achieve a dark brown color paste. After this, 46 mL of distilled water was poured into the mixture and the temperatures is raised to 98 oC and kept for 15 minutes. Subsequently, 140 mL of distilled water was added along with 10 ml H2O2 (30%; Fisher Scientific) solution. The final product was subjected to centrifugation and thoroughly washed 3-4 times with 5% HCl (38%; SDFCL). The obtained



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GO powder was dried and used for further reaction. The GO powder was annealed at 800 oC under N2 atmosphere to achieve the reduced graphene oxide (rGO) nanosheets. 2.2 Synthesis of rGO/BiVO4 hybrid nanostructures The synthesis of rGO/BiVO4 hybrid nanostructures was carried out by facile hydrothermal method. In a detailed synthesis, a mixed solvent was prepared by using 65 ml distilled H2O and 5 ml HNO3 solution. To the as prepared solvent, the rGO powder (0.14 mg/ml) was dispersed and ultrasonicated for 2h in order to exfoliate rGO. Then, solvent is divided into two portions of 35 ml. Then, 5 mmol of both Bismuth (III) nitrate (Bi(NO3)3.5H2O, 98.5%; SDFCL) and Ammonium metavanadate (NH4VO3, 99%; Fisher Scientific) were separately dissolved in each 35 ml of solvent. Subsequently, NH4VO3 solutions was slowly added to the solution of Bi (NO3)3.5H2O under vigorous magnetic stirring for 1 h. Further Ammonia (NH3, 25%; Qualigen Chemicals Limited) was added in order to make the pH of the solution neutral. Then the resulting suspension was poured into a Teflon reactor embedded in the hydrothermal cell and kept into an oven at constant temperature (180 oC for 24 hours). The precipitate was collected, washed thoroughly using distilled water followed by ethanol. The obtained product was finally dried at 60 oC for 3 h and used for further characterization. For comparison pristine BiVO4 powder was also prepared under similar reaction conditions. 2.3 Materials characterization: Rigaku-Ultima III X-ray diffractometer with CuKα radiation (λ = 1.5418 Å) was used to determine the phase formation of the as prepared materials. The XPS spectra were acquired by using Xray photoelectron spectroscopy (XPS, SPECS Germany, PHOIBOS 150). The surface morphological features of as-prepared samples were studied by using FE-SEM, Hitachi, S-4800 II, Japan. The elemental detection was done by using energy-dispersive X-ray spectroscopy (EDS) analyzer. Microstructure analysis was carried out using FETEM with a JEOL JSM 2200 FS microscope operating at 200 kV. Thermal analysis (TGA) was done using a TG analyzer in the range of room temperature to 800 oC with increment of 10 K/min in air atmosphere.


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2.4 Electrochemical measurements Doctor Blade technique is used to prepare the working electrodes. For this, active material: PVDF: acetylene black (85:10:5 by weight) were mixed to make a thick slurry. Then, appropriate amount of N-Methyl-2-pyrrolidone was introduced in the slurry thoroughly mixed using agate mortar. The supercapacitor electrodes were then prepared by coating the slurry on flexible carbon cloth. The resultant films were then annealed at 180 oC for two hours in order to remove the binder. The estimated mass loading of the electrode material was around 0.5-0.9 mg/cm2. The electrochemical properties of the electrodes were measured by using standard three cell system which contains working electrodes of BiVO4 and rGO/BiVO4 samples, platinum counter electrode and Ag/AgCl reference electrode in 6 M KOH electrolyte. Symmetric cell was constructed in a 3-way Teflon Swagelok cell using two identical electrodes of rGO/BiVO4 with polypropylene separator sandwiched between them and few drops of 6 M KOH electrolyte. Two channels from potentiostat were connected together in such a way that one channel records voltage between two electrodes (positive and negative) and other channel records potential contributed from positive and negative electrodes with respect to reference electrode. Cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) studies were done by means of a Biologic VMP3 potentiostat.

3. Results and discussion: Figure 1a depicts the X-Ray diffraction (XRD) patterns of BiVO4 and rGO/BiVO4 nanostructures. XRD patterns confirm the single phase formation of BiVO4 with monoclinic crystal structure. The lattice constants were calculated to be a = 5.185 Å, b = 11.713 Å and c = 5.102 Å which match well with literature data (JCPDS # 014-0688). In case of rGO/BiVO4 nanostructures no any diffraction peaks correspond to rGO was observed which might be due to the weak diffraction intensities of rGO compared to those of BiVO4 or due to the low content of graphene oxide employed in the reaction.



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Figure 1b-d represents the XPS spectra of rGO/BiVO4 hybrid nanostructures which confirm the C, O, Bi, and V elements in a sample. The appearance Carbon peaks confirm the existence of rGO in rGO/BiVO4 hybrid nanostructures. A major peak at 284.6 eV in C 1s XPS spectrum (Figure 1b) is characteristic peak of graphitic sp2 carbon atoms. Similarly, another two peaks at 286.6 eV and 288.9 eV clearly indicate existence of epoxy/ C-O and carbonyl (C=O) residual oxygenate groups on the surface of rGO.23,24 The spin orbit component of Bi 4f is deconvoluted into two peaks with binding energies of Bi 4f7/2 (159.0 eV) and Bi 4f7/2 (165 eV) (Figure 1c). Similarly, the split peaks for V2p were observed at binding energy 524.2 eV (V 2p1/2 ) and 516.7 eV (V 2p3/2 ).25 The XPS spectrum of O1s showed asymmetric behavior which indicates presence of various oxygen states with peak at 529.9 eV is attributed to lattice oxygen of BiVO4 (Figure 1d)26,

27

and another one at 531.6 eV is ascribed to the

surface hydroxyl groups.27


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Figure 1 (a) Comparative XRD patterns of BiVO4 and rGO/BiVO4. XPS spectra of (b) C 1s (c) Bi 4f and (d) O 1s and V 2p, respectively. Figure 2a-f shows FESEM images of as-prepared BiVO4 and rGO/BiVO4 hybrid nanostructures. A special dendritic fern-like morphology was observed for BiVO4 (see, Figure 2a) which has gained a renowned research interest due to the significant connectivity among crystals.10,28,29 Importantly, dendrite consists of a number of branches densely packed together with affordable high porosity which favors the facile access of the electrolyte ions.28 Figures 2a and b clearly evidence the typical dendrite fern-like morphology for BiVO4 which is formed by number of branches with length size in 200-500 nm. The average length of intact single fern structure is found to be around 4–5 µm which is supported by unique backbone.



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Figure 2 FESEM micrographs of BiVO4 (a, b) and rGO/BiVO4 hybrid nanostructures (c-f) with corresponding EDS mapping images During hydrothermal reaction, due to rotation of particles via Brownian motion or short-range interaction between the particles, branches (rod like structures) are formed which act as building blocks for fern development. The growth of these branches takes place via Ostwald ripening and always tries to adjust the structures in order to achieve a minimal total surface free energy.30,31 It should be noted that an isotropic growth of branches along [001] direction is responsible for formation of special BiVO4 dendrite structures.31 A close look at FESEM images (Figure 2c-f) clearly revealed that the rGO thin sheets were embedded into the BiVO4 dendrite structures. The corresponding EDS mapping further assures the uniform growth of BiVO4 on the rGO nanosheets. In particular, rGO improves the electrical performance of the hybrid material as well as it also facilitates fast transportation of electrons during electrochemical


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reaction. It was previously assumed that the rGO nanosheets can effectively shorten the ion diffusion length so that both ions can easily transfer during the charge-discharge process.6,20 Owing to unique dendrite morphology of BiVO4 and appropriate rGO (high surface area material) combination, rGO/BiVO4 hybrid nanostructures could bear better electrochemical performance for supercapacitor. Further, to determine the size, crystal structure and existence of rGO and BiVO4 phase contact, FETEM images of both rGO and rGO/BiVO4 hybrid architectures were acquired. Figure 3a depicts the FETEM image of rGO which showed formation of thin sheet like structures. FETEM images of rGO/BiVO4 hybrid nanostructures (Figure 3b and c) indicate the presence of both the phases with intimate contact between typical BiVO4 fern/dendritic structure and rGO thin sheets. As seen from Figure 3, the rGO thin sheets were embedded into the surface of BiVO4 structures and their sizes are consistent with FESEM results. Furthermore, Figure 3d shows the high resolution TEM image of rGO/BiVO4 hybrid nanostructures which confirms the interplanar spacing of 0.308 nm corresponding to (121) lattice plane of BiVO4. The additional FESEM and FETEM images of rGO and rGO/BiVO4 hybrid structures are provided in supporting information S1 and S2 (Figure S1 and Figure S2).



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Figure 3 TEM micrographs of (a) rGO, (b, c, d) rGO/BiVO4 hybrid nanostructure

The mass ratio of rGO and BiVO4 in the rGO/BiVO4 was determined by TGA (Figure S3). From TGA, the residual quantity of rGO, BiVO4 and rGO/BiVO4 was obtained and the mass ratio is calculated using formula given in the supporting information S3. The ratio of BiVO4 to rGO in rGO/BiVO4 is found to be 22.5:1. For detail calculations please see the supporting information S3. Furthermore, the electrochemical performance of BiVO4 and rGO/BiVO4 hybrids were evaluated in two electrode Swagelok cells in 6 M KOH electrolyte. Figure 4a,b depicts CV curves of BiVO4 and rGO/BiVO4 electrodes at various scan rates exhibiting a broad voltage window from -1.0 to 0.6 V. Both the BiVO4 and rGO/BiVO4 electrodes show pseudo capacitive behavior which is clearly evident from the


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non-ideal rectangular shapes. Two pairs of redox current peaks were observed for rGO/BiVO4 electrode corresponding to the redox pairs Bi(II)/Bi(III) and V(IV)/V(V), which were distinct from EDLCs, indicating the presence of a faradaic reaction.32 It should be noted that such feature was not observed for BiVO4 electrode, where only one redox peak is observed. This might be due to strong chemical interaction between the BiVO4 dendrites and the residual oxygen on the rGO or Vander Waals interactions.31 Furthermore, with increasing scan rate, area under the curves is also increased. Appearance of redox peaks yet at increased scan rates indicating the superior rate capability.10 The galvonostatic charge/discharge (GCD) curves of BiVO4 (Figure 4c) and rGO/BiVO4 (Figure 4d) electrodes were recorded at different current densities which showed that the rGO/BiVO4 hybrid electrode has a longer discharge time, relatively higher specific capacitance owing to the improved electrical conductivity of rGO/BiVO4 electrode contributed from rGO (supporting information Figure S4), thereby facilitate the easy transfer of electrons during the charge/discharge process.



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Figure 4 CV curves of (a) BiVO4 and (b) rGO/BiVO4 electrodes, GCD curves of (c) BiVO4 and (d) rGO/BiVO4 electrodes. Figure 5a depicts the CV curves of BiVO4 and rGO/BiVO4 electrodes with a scan rate of 40 mV/s. It is noteworthy that although there is no significant change in operational voltage window for electrodes, the current density of rGO/BiVO4 hybrid electrode was increased drastically.

Figure 5 (a) CV curves of BiVO4 and rGO/BiVO4 electrodes at fixed scan rate (40 mV/s), (b) Plot of Areal capacitance as a function of scan rates (c) Chronopotentiometry curves of BiVO4 and rGO/BiVO4 electrodes at constant current density (15 mA/cm2) (d) Plot of specific capacitance with that of scan rates.

Similarly, Figure 5c indicates the comparison of the discharge curves at 15 mA/cm2 for BiVO4 and rGO/BiVO4 electrodes. The hybrid rGO/BiVO4 based electrode showed substantially longer (almost double) discharge time as compared to BiVO4 electrode, indicating a significantly larger specific


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capacitance. The areal and specific capacitances of BiVO4 and rGO/BiVO4 electrodes were determined from the CV data (supporting information S5) and depicted in Figure 5b and d, respectively. BiVO4 and rGO/BiVO4 electrodes show maximum areal (specific) capacitances of 0.75 F/cm2 (116.3 F/g) and 1.33 F/cm2 (196 F/g) respectively, at 5 mV/s. The rGO/BiVO4 electrode shows enhanced electrochemical performance than that of BiVO4. The enhanced electrochemical performance for rGO/BiVO4 electrode indicates that rGO extensively influence the capacitive behavior of BiVO4 in the hybrid nanostructure. There are several factors endow the superior electrochemical performance of rGO/BiVO4 hybrid nanostructures. First, the controlled assembly of dendritic BiVO4 structures which enables easy access of electrolyte ions by reducing internal resistance and also presence of electronically conducting rGO nanosheets. Second, the intimate interface interaction of rGO/BiVO4 hybrid provides electron superhighways which could led to facile and efficient charge transport, thereby largely increases the electronic conductivity (see Figure S4).33 We have also studied the effect of different composition of rGO in the rGO/BiVO4 on the supercapacitor performance and we found that 1 wt. % rGO loaded sample has shown better supercapacitor performance than the samples with 2 wt. % and 3 wt. % of rGO loading (Figure S5). Therefore, we have chosen present hybrid sample (1 wt. % of rGO) for further study. Since the rGO/BiVO4 hybrid electrode shows extended voltage window between -1.0 to 0.6 V and can show superiority for electrochemical charge storage, we constructed a rGO/BiVO4//rGO/BiVO4 symmetric cell. Here, rGO/BiVO4 electrode is used as both cathode and anode for supercapacitor study. The electrochemical performance of symmetric cell was evaluated by CV, GCD, cyclic stability and impedance measurements. Figure 6a shows cyclic polarization curves of rGO/BiVO4//rGO/BiVO4 symmetric cell. Interestingly, the symmetric cell exhibits wide voltage window of 1.6 V. In addition, as evident from the CV curves, a fabricated symmetric cell shows pure capacitive behavior at higher scan rate of 100 mV/s. Moreover, the acquired GCD curves for rGO/BiVO4 symmetric cell (Figure 6b) showed longer discharge time, indicating elevated rate capability of cell.



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Figure 6 (a) CV curves of rGO/BiVO4//rGO/BiVO4 symmetric cell, (b) GCD curves of rGO/BiVO4//rGO/BiVO4 symmetric cell (c) Plot of specific capacitance and volumetric capacitance vs. current densities of rGO/BiVO4//rGO/BiVO4 symmetric cell (d) Gravimetric energy/power densities of rGO/BiVO4// rGO/BiVO4 symmetric cell.

The specific and volumetric capacitances of symmetric cell were calculated (supporting information S6 & Table S1) and plotted in Figure 6c. The maximum volumetric capacitance of 4.63 F/cm3 (94.83 F/g, at total mass of 4.9 mg/cm2) is observed at 1.40 A/g. There is gradual reduction in capacitance up to 2.10 F/cm3 (43 F/g) was observed with increasing current density due to diffusion limitation aroused by ionic movement of electrolyte.10 The volumetric/gravimetric energy density and power density of rGO/BiVO4 symmetric cell was determined from CD curves and shown as a Ragone plot in Figure 6d and 7a. The cell exhibits maximum volumetric/gravimetric energy density of 1.63


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mWh/cm3 (33.7 Wh/kg) at lower power density of 55 mW/cm3 (1.14 kW/kg). Even at 391 mW/cm3 (8.00 kW/kg), rGO/BiVO4 symmetric cell exhibits energy density of 0.75 mWh/cm3 (15.33 Wh/kg). Although, the rGO/BiVO4//rGO/BiVO4 symmetric supercapacitor shows slightly lower energy density of 1.6 mWh/cm3 (33.7 Wh/kg) than previously reported Ag:BiVO4 symmetric cell with energy density of 2.63 mWh/cm3 (38.4 Wh/kg), it promises cost-effective and efficient approach to mass produce the high performance supercapacitor devices. Moreover, the values of energy density are considerably superior than other rGO based and metal oxide based symmetric and asymmetric supercapacitors such as HTiO2@C//H-TiO2@MnO2 (0.30 mWh/cm3, 5M LiCl),34,35 Co9S8//Co3O4@RuO2 (1.21 mWh/cm3, 3M KOH),34 VOx//VN (0.61 mWh/cm3, 5M LiCl),37 MnO2//MnO2 (0.023 mWh/cm3, polyvinylpyrrolidone (PVP)-6 M LiClO4 gel electrolyte)14 laser-scribed graphene (LSG)//LSG (0.09 mWh/cm3, 1 M H3PO4)38, RGO//RGO (0.0031 mWh/cm3 2M H2SO4),39 RGO–RuO2//RGO–RuO2 (0.012 mWh/cm3 2M H2SO4),39 RGO–PANi//RGO–PANi (0.013 mWh/cm3, 2M H2SO4)39 (Please also see Table S2 for more details). The high energy and power densities of the rGO/BiVO4//rGO/BiVO4 symmetric cell is a result of high voltage window (1.6 V) and improved electrical conductivity due to rGO incorporation into BiVO4 dendritic nanostructures. Figure 7b shows the Cycling stability of rGO/BiVO4 symmetric cell investigated by charge/discharge technique. rGO/BiVO4 symmetric cell was subjected to 2000 GCD cycles at 4 A/g. The rGO/BiVO4 symmetric cell shows 80.3 % capacitance retention up to 2000 cycles which is much better than BiVO4 (42% after 200 cycles)17 and VO2//VO2 based supercapacitor (78.7 % after 4500 cycles).15 It is important to note that, previously Shivakumara et al.40 have demonstrated 86% capacitance retention for rGO//rGO symmetric cell after 3000 cycles however, other electrochemical properties of the same are poor compared to our rGO/BiVO4//rGO/BiVO4 symmetric cell.



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Figure 7 (a) volumetric energy/power densities of rGO/BiVO4 symmetric cell (b) Cycle performance of rGO/BiVO4 symmetric cell, (c) Nyquist plots of rGO/BiVO4 symmetric cell after 2000 cycles, (d) Plot of phase angle with frequency rGO/BiVO4 symmetric cell.

Impedance spectra were recorded to investigate the bulk solution resistance (Re), Warburg resistance (Zw) and charge transfer resistance (Rct) of as synthesized electrode materials. Figure 7c shows the Nyquist plot and inset shows equivalent circuit for the Nyquist plot of the rGO/BiVO4 symmetric cell. Low ESR value (0.5 ῼ) of rGO/BiVO4 symmetric cell demonstrates a small internal resistance through a good ion response in high frequency ranges.10 The frequency dependence of phase angle of the symmetric cell is plotted in Figure 7d. The phase angle of 40o with wide-ranging capacitive region signifies a increased

capacitive

behavior

with

facile

charge

transfer

electrode/electrolyte interface.41


caused

by

the

complimentary

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From the above discussion, it becomes obvious that our rGO/BiVO4 hybrid system for symmetric supercapacitor shows better capacitive properties which could be assigned to synergetic effect of rGO and BiVO4 nanostructures. The present study demonstrates assembling of symmetric cells which have advantage of eliminating the use of complex chemistries involved and difficulty in balancing of charge/mass of both electrodes in asymmetric cells.10 Therefore, the rGO/BiVO4 symmetric cell promises the development of the high-voltage asymmetric counterpart with same power to meet high energy/power density for supercapacitors.

4. Conclusion In summary, a facile and cost-effective hydrothermal method is used for the synthesis of rGO/BiVO4 hybrid fern/dendrite structures. A symmetric cell based on rGO/BiVO4 hybrid electrode has been successfully fabricated representing excellent supercapacitive properties. Most specifically, the rGO/BiVO4 based symmetric cell exhibiting wider potential window of 1.6 V leading to significantly higher volumetric (gravimetric) energy density of 1.60 mWh/cm3 (33.7 Wh/kg) with a high volumetric (gravimetric) power density of 391 mW/cm3 (8.0 kW/Kg). Moreover, our symmetric cell promises an excellent volumetric capacitance of 4.63 F/cm accompanied by improved cycling stability (80% retention after 2000 cycles) and has a great potential in supercapacitor applications. ASSOCIATED CONTENT Supporting Information FESEM analysis of RGO, FETEM analysis of rGO and rGO/BiVO4 hybrid structures, TGA analysis, electrochemical evaluation and calculation methods, CV and CD curves for BiVO4 and rGO-BiVO4 at different percentage of rGO, Nyquist plots for rGO, BiVO4 and rGO-BiVO4, Table of electrochemical properties of rGO/BiVO4 symmetric supercapacitor, Table for comparison of rGO/BiVO4 supercapacitor to reported supercapacitors (PDF).



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Acknowledgement: BBK and DRP would like to thank Ministry of Electronics and Information Technology (MeitY, New Delhi) and INSPIRE faculty program (DST, New Delhi) for their financial support. DPD and PGR acknowledge partial support from the Spanish Miisterio MINECO (Grant MAT2015-68394-R, MINECO/FEDER) and Secretary for Universities and Research of the Ministry of Economy and Knowledge of the Government of Catalonia and the Co-fund programme of the Marie Curie Actions of the 7th R&D Framework Programme of the European Union. DPD and PGR also acknowledge AGAUR (Generalitat de Catalunya) for Project NESTOR (Nanomaterials for Energy STORage) 2014_SGR_1505


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