High Performance Solid-State Asymmetric Supercapacitor using

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Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10128-10138

High Performance Solid-State Asymmetric Supercapacitor using Green Synthesized Graphene−WO3 Nanowires Nanocomposite Arpan Kumar Nayak, Ashok Kumar Das, and Debabrata Pradhan* Materials Science Centre, Indian Institute of Technology, Kharagpur 721 302, West Bengal, India

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S Supporting Information *

ABSTRACT: Development of active materials capable of delivering high specific capacitance is one of the present challenges in supercapacitor applications. Herein, we report a facile and green solvothermal approach to synthesize graphene supported tungsten oxide (WO3) nanowires as an active electrode material. As an active electrode material, the graphene−WO3 nanowire nanocomposite with an optimized weight ratio has shown excellent electrochemical performance with a specific capacitance of 465 F g−1 at 1 A g−1 and a good cycling stability of 97.7% specific capacitance retention after 2000 cycles in 0.1 M H2SO4 electrolyte. Furthermore, a solid-state asymmetric supercapacitor (ASC) was fabricated by pairing a graphene−WO3 nanowire nanocomposite as a negative electrode and activated carbon as a positive electrode. The device has delivered an energy density of 26.7 W h kg−1 at 6 kW kg−1 power density, and it could retain 25 W h kg−1 at 6 kW kg−1 power density after 4000 cycles. The high energy density and excellent capacity retention obtained using a graphene−WO3 nanowire nanocomposite demonstrate that it could be a promising material for the practical application in energy storage devices. KEYWORDS: Graphene, Tungsten oxide, Green synthesis, Nanowires, Asymmetric supercapacitor

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INTRODUCTION

Tungsten trioxide (WO3) has recently emerged as a potential electrode material in the development of pseudocapacitors due to its excellent capacitive behavior and earth abundance.14−17 WO3 is also recognized as a candidate for dye sensitized solar cells,18 photocatalysis,19 gas sensing,20,21 electrochromism,22 lithium ion batteries,23 and electrocatalysis.24 However, while WO3 has proven its potential as a promising candidate for a wide variety of applications, its low electrical conductivity is a major shortcoming. To be an ideal candidate for supercapacitor application, the active material should have high conductivity and be capable of delivering high electrochemical performance. Therefore, several attempts have been made to enhance the performance of WO3. Among them, integration of WO3 with highly conducting materials such as carbon nanotubes (CNTs),25 carbon cloths,26 carbon fibers,27 conducting polymers,28 and graphene29 is recognized as one of the effective ways. Sun et al. demonstrated that the CNT-WO3 nanohybrid electrode could deliver much higher specific capacitance,25 and Gao et al. demonstrated excellent electrochemical performance with carbon cloth supported WO3 nanowire arrays.26 Recently, Wang et al. demonstrated WO3@polypyrrole core−shell nanowire arrays with excellent supercapacitive performance.28 In addition, the graphene-WO3 nanocomposite has recently emerged as a promising material for a wide variety of applications with much better performance

The energy crisis and environment pollution demand not only the production of energy through cleaner means but also development of energy storage devices for its efficient utilization.1 Among various energy storage devices, supercapacitors have recently emerged as promising candidates due to their high power density, long life, ultrafast charging− discharging rate, and excellent reversibility.2 As per charge storage mechanism, supercapacitors are classified in two types, i.e., (i) electrical double layer capacitors (EDLCs) and (ii) pseudocapacitors.3 For EDLCs, various carbon-based materials have been widely used in which charge storage occurs electrostatically by reversible adsorption of ions at the electrode and electrolyte interface and are known to deliver low specific capacitance.3,4 In recent years, graphene, a two-dimensional sheet of sp2 hybridized carbon with excellent physical and chemical properties has been used as an active electrode material for EDLCs.5 As an active material, it delivers a low specific capacitance and thus is not suitable to achieve high energy density. On the other hand, different transition metal oxides have been widely used as active materials for the fabrication of pseudocapacitors, where a reversible redox reaction is being used for charge storage. Ruthenium oxide, iron oxide, manganese oxide, cobalt oxide, tin oxide, nickel oxide, and conducting polymers such as polypyrrole and polyaniline have been used for this purpose, and much higher specific capacitance has been obtained compared to those of the EDLCs.6−13 © 2017 American Chemical Society

Received: June 29, 2017 Revised: September 7, 2017 Published: September 25, 2017 10128

DOI: 10.1021/acssuschemeng.7b02135 ACS Sustainable Chem. Eng. 2017, 5, 10128−10138

Research Article

ACS Sustainable Chemistry & Engineering

Scheme 1. Schematic Representation on the Synthesis of Graphene-WO3 Nanowire Nanocomposite Using Solvothermal Technique

compared to that of either WO3 or graphene.30−32 GrapheneWO3 nanocomposite has also been recognized as a potential candidate for supercapacitor applications that delivered higher specific capacitance (143.6 F g−1) than pure WO3 (32.4 F g−1) at 0.1 A g−1.33 Ma et al. reported flower-like WO3·H2O decorated over reduced graphene oxide sheets that delivered a specific capacitance of 244 F g−1 at 1 A g−1 which was higher compared to flower-like WO3·H2O (140 F g−1).34 Recently, a much improved storage ability of a WO3 nanoflower coated with graphene nanosheet was reported by Chu et al.17 While a number of reports are available on the synthesis and application of graphene-WO3 nanocomposite for a wide variety of applications, there are a handful of reports on its application as an electrode material in the fabrication of a supercapacitor requiring further study in this direction.32−34 Herein, we demonstrate a synthesis of a graphene sheet decorated with WO3 nanowires following a green solvothermal approach using ethanol as the only solvent instead of the most commonly used corrosive and hazardous chemicals such as HCl,24,35 H2SO4,36 and HNO3.37,38 The as-synthesized active material was then employed as an active material to study supercapacitor behavior and for the fabrication of a solid-state asymmetric supercapacitor (ASC) device. The specific capacitance value obtained with the as-synthesized grapheneWO3 nanowire nanocomposite is much higher compared to the reported results in which WO3-based materials were used.33,34 This is attributed to much finer nanowires and larger effective surface area of the synthesized active composite materials. In addition, the fabricated solid-state ASC device exhibits excellent electrochemical performance with an energy density of 26.7 W h kg−1 at 6 kW kg−1 power density and 93.6% capacitance retention after 4000 cycles.

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containing graphene and WCl6 precursor was transferred into a 50 mL Teflon-lined stainless steel autoclave and was solvothermally treated at 200 °C for 12 h (Scheme 1). Then, the autoclave was cooled to room temperature naturally, and a gray product was collected by centrifuging followed by repeated washing with Millipore water and ethanol. Finally, the gray product was dried at 60 °C for 4 h. In order to study the effect of graphene quantity in the nanocomposites, a varying quantity of graphene (2, 5, 10, and 15 mg) was added keeping the initial precursor concentration (0.025 M of WCl6) for WO3 fixed. The as-prepared nanocomposites were labeled as 2-, 5-, 10-, 15graphene-WO3 nanowire nanocomposites. Characterization. The as-synthesized materials were characterized by X-ray diffraction (XRD) using a PANalytical High Resolution XRD (PW 3040/60) with Cu Kα radiation (λ ≈ 1.54 Å) operated at 40 kV and 30 mA. The effective surface area was measured by the Brunauer− Emmett−Teller (BET) method using nitrogen adsorption−desorption isotherms at 77 K with autosorb iQ2 volumetric physisorption analyzer (Quantachrome Instruments). Morphology and microstructure were examined by field emission scanning electron microscopic (FESEM, JEOL JSM-7610F) and transmission electron microscope (TEM, FEI TECNAI G2), respectively. The Raman spectra were obtained using a fiber-coupled micro-Raman spectrometer (Model TRIAX550, JY) and a CCD detector with 488 nm of 5 mW air-cooled Ar+ laser as the excitation light source. The surface elemental composition and chemical states of selected samples were measured by X-ray photoelectron spectroscopy (XPS) (Kratos Analytical, UK, SHIMADZU group) with a monochromatic (Al Kα) 600 W X-ray source (1486.6 eV). Electrochemical Measurement. The electrochemical performance of the as-synthesized graphene-WO3 nanowire nanocomposite and WO3 nanowires was investigated by cyclic voltammetry (CV), galvanostatic charge−discharge (GCD), and electrochemical impedance spectroscopy (EIS) techniques using a CHI 760D electrochemical workstation (CH Instruments, Inc.). A three-electrode configuration mode was used for these studies where the graphite sheet electrode of thickness 0.25 mm (Nickunj Eximp Entp P. Ltd., India), Pt wire, and saturated calomel electrode (SCE) functioned as working, counter, and reference electrodes, respectively. In all the electrochemical experiments, 0.1 M H2SO4 was used as electrolyte. The active material slurry was prepared by mixing a graphene-WO3 nanowire nanocomposite or WO3 nanowires with polyvinylidene fluoride (PVDF) in 2 mL of N-methyl-2-pyrrolidone (NMP) through ultrasonication for 30 min. The graphene-WO3 nanocomposite or WO3 slurry was then painted on the graphite sheet and allowed to dry overnight at 60 °C. This active material coated graphite sheet was used as working electrode for the electrochemical investigation. Synthesis of PVA/H2SO4 Gel. The PVA/H2SO4 gel electrolyte was prepared as follows: 2 mL of H2SO4 (98% v/v) was added into 20 mL of distilled water, and then 2 g of poly(vinyl alcohol) (PVA) powder was added. The whole mixture was heated to 80 °C while stirring until the solution became clear and finally cooled down to room temperature. Fabrication of Solid-State Supercapacitor. From the preliminary electrochemical investigation, the highest specific capacitance was obtained with the 10-graphene-WO3 nanowire nanocomposite (10 mg graphene added for the synthesis) modified electrode. Thus, detail

EXPERIMENTAL SECTION

Chemicals. Tungsten(VI) chloride (WCl6) (Sigma-Aldrich), sulfuric acid (H2SO4), ethanol (C2H5OH), and graphene (Merck) were analytical grade and used without further purification. Synthesis of WO3 Nanowires. WO3 nanowires were synthesized by the solvothermal method. In a typical synthesis, 0.4 g (0.025 M) of WCl6 was added to 40 mL of ethanol and was stirred for 5 min at room temperature. After stirring, the solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and subjected to solvothermal treatment at 200 °C for 12 h. After the completion of the reaction, the autoclave was cooled to room temperature naturally, and a light blue product was obtained. The product was collected by centrifuging followed by washing repeatedly with ethanol and Millipore water, and it was finally dried at 60 °C for 4 h. Synthesis of Graphene-WO3 Nanocomposite. The synthesis of a graphene-WO3 nanocomposite was carried out following the above procedure by adding a chosen quantity of graphene to a pre-prepared 40 mL WCl6 solution; this reaction mixture was subjected to ultrasonication for 30 min. After ultrasonication, the dispersion 10129

DOI: 10.1021/acssuschemeng.7b02135 ACS Sustainable Chem. Eng. 2017, 5, 10128−10138

Research Article

ACS Sustainable Chemistry & Engineering study was performed on 10-graphene-WO3 using different characterization techniques. The solid-state asymmetric supercapacitor (ASC) was also fabricated by using 10-graphene-WO3 nanowire nanocomposite as negative electrode, activated carbon (AC) as positive electrode, and H2SO4/PVA gel as solid electrolyte. First, a slurry of active material was prepared by mixing either graphene-WO3 nanocomposite or AC and PVDF in a 9:1 mass ratio using 2 mL of NMP. The as-prepared slurries were then coated individually on the 1 × 1 cm2 area of the graphite sheet and dried at 80 °C for 1 h. Finally, each electrode was sandwiched together by PVA/H2SO4 gel, and the device was kept at room temperature overnight until the gel electrolyte solidified. The thickness of the solid-state ASC was around 0.8 mm. The mass ratio of negative to positive electrode was calculated using the charge balance theory (Q+ = Q−) according to eqs 1−4 given below.

Q+ = Q−

(1)

m+ × Cs + × ΔV+ = m− × Cs − × ΔV −

(2)

m+ C × ΔV − = s− m− Cs + × ΔV+

(3)

m+ 685.2 × 1 = = 2.21 m− 155 × 2

(4)

(Figure 1Ab) shows sharp diffraction features representing the crystalline nature and is readily indexed to the monoclinic phase of WO3 with lattice parameters a = 7.31 Å, b = 7.53 Å, and c = 7.69 Å, which are in good agreement with the standard JCPDS data (JCPDS File 01-083-0951, a = 7.3013 Å, b = 7.5389 Å, and c = 7.6893 Å). In this XRD pattern, no other phases and/or impurities corresponding to either WO3·H2O or WO3·2H2O were observed indicating the phase pure WO3 nanowires. The XRD pattern of 10-graphene-WO3 nanowire nanocomposite shows a monoclinic structure similar to that of WO3. The (002) plane of graphene is believed to be overlapped by the (120) diffraction of WO3 nanowires in the 10-grapheneWO3 nanowire nanocomposite. Moreover, XRD intensity of different WO3 crystal planes in 10-graphene-WO3 nanowire nanocomposite was found to be different than that of pure WO3 nanowires. In particular, (200) and (002) diffraction intensities were the highest for WO3 nanowires and the 10graphene-WO3 nanowire nanocomposite, respectively. This indicates the change in growth kinetics of WO3 nanowires in the presence of graphene. This was further confirmed by TEM study (discussed later). An intense (202) diffraction intensity was also observed for WO3 nanowires in the 10-graphene-WO3 nanowire nanocomposite. The structural property of the graphene, WO3, and 10graphene-WO3 nanowire nanocomposite was further investigated by Raman analysis (Figure 1B). The Raman spectrum of graphene (Figure 1Ba) shows two peaks at ∼1346 and ∼1580 cm−1 corresponding to the D and G bands of graphene, respectively. The G and D bands correspond to the graphitic sp2 carbon and defects in the graphitic carbon structure, respectively.40,41 The Raman spectrum of pure WO3 (Figure 1Bb) shows three peaks at ∼259, ∼695, and ∼804 cm−1 corresponding to the stretching mode of O−W−O. In the case of the 10-graphene-WO3 nanowire nanocomposite (Figure 1Bc), the same Raman vibrations were observed at ∼261, ∼697, and ∼810 cm−1 along with two additional features at ∼1345 and ∼1586 cm−1 contributed from graphene. The lower intensity of the Raman peak of graphene in the composite was due to the high loading of WO3. A similar Raman signature for graphene-WO3 nanocomposite has also been reported elsewhere.42 Morphology and Microstructure. The morphology of the as-synthesized pure WO3 nanostructures and 10-grapheneWO3 nanocomposite was examined by FESEM. Figure 2a,b shows the FESEM images of pure WO3 with nanowire-like morphology. These nanowires were found to be highly uniform and several micrometers in length with ∼20 nm diameter. On the other hand, the FESEM image of the 10-graphene-WO3 nanocomposite shows a random distribution of WO3 nanowires on the graphene sheets (Figure 2c). To support the FESEM results, TEM analysis of pure WO3 and 10-graphene-WO3 nanocomposite was carried out. The TEM image of pure WO3 also shows nanowire-like morphology (Figure 2d) which is very similar to that observed by FESEM. Figure 2e shows a high resolution TEM (HR-TEM) image with fringe spacing of 0.36 nm corresponding to the (200) planes of WO3 monoclinic structure.43 The fast Fourier transform (FFT) pattern (inset of Figure 2e) of a single WO3 nanowire shows a spot diffraction pattern suggesting its single crystalline nature. The close examination of TEM image of 10-graphene-WO3 nanowire nanocomposite (Figure 2f,g) reveals that WO3 nanowires were uniformly decorated on the thin graphene sheet. Anchoring of WO3 nanowires onto the graphene surface could act as a spacer

Here, the following abbreviations apply: Cs is the specific capacitance (F g−1) at 10 mV s−1 scan rate, ΔV is the potential window (V), and m is the mass of the electrode (g). From the CV profile, the specific capacitance of graphene-WO3 nanocomposite and AC was calculated to be 685.2 and 155 F g−1, respectively, at 10 mV s−1. The mass ratio of graphene-WO3 nanowire nanocomposite and AC was 0.41 in the ASC device. In the present work, the masses of graphene-WO3 nanowire nanocomposite and AC used in the fabrication of ASC device were 6.0 and 13.26 mg, respectively. Hence the total mass of the two active electrode materials was 19.26 mg.

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RESULTS AND DISCUSSION Structural Properties. The structural property of the assynthesized graphene, WO3, and 10-graphene-WO3 nanowire nanocomposite (10 mg graphene) was studied by XRD and Raman analysis, and is shown in Figure 1. The XRD pattern of graphene (Figure 1Aa) shows a broad diffraction feature at ∼26° and a small hump at ∼43.25° corresponding to the (002) and (100) planes, respectively.39 The XRD pattern of WO3

Figure 1. (A) Powder XRD patterns and (B) Raman spectra of (a) pure graphene, (b) WO3 nanowires, and (c) 10-graphene-WO3 nanowire nanocomposite synthesized by solvothermal method at 200 °C for 12 h. 10130

DOI: 10.1021/acssuschemeng.7b02135 ACS Sustainable Chem. Eng. 2017, 5, 10128−10138

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Figure 2. FESEM images of (a, b) WO3 nanowires and (c) 10-graphene-WO3 nanowire nanocomposite. (d) TEM and (e) HRTEM images of WO3 nanowires, the FFT pattern, and (f, g) TEM images of 10-graphene-WO3 nanowire nanocomposite. Insets of part g show the SAED pattern of WO3 nanowires (top) and graphene (bottom).

surface area of the nanocomposite compared to that of the individual component can be attributed to the incorporation of WO3 nanowires between graphene sheets preventing their stacking, thus creating more gaps. The anchoring of metal oxide nanostructures onto the graphene sheet can prevent the restacking of graphene sheets with each other resulting in the enhancement of active surface area.46,47 In the present case, loading of WO3 nanowires onto graphene sheets thus hinders the restacking of graphene sheets during the synthesis of the nanocomposite. Benefiting from the combined effect of graphene and WO3 nanowires, the resulting graphene-WO3 nanowire nanocomposite delivered a higher specific surface area compared to both graphene and WO3 nanowires. It is to be noted that the specific surface area of the 10-graphene-WO3 nanowire nanocomposite is much higher than the WO3 nanorod/graphene nanocomposite (34.8 m2 g−1) reported by An et al.30 Since the present 10-graphene-WO3 nanowire nanocomposite possesses a larger surface area, it can, therefore, in principle, offer more active sites for the electrochemical reaction. At the same time, due to its highest pore volume compared to either graphene or WO3 nanowires, easy access of electrolyte ions to the electrode surface can occur, giving rise to higher supercapacitive performance.48 Surface Composition. The surface chemical composition and valence state of WO3 nanowires and 10-graphene-WO3 nanowire nanocomposite were further studied by XPS. The survey spectra (Figure S2a, SI) show the presence of no other element except W, O, and C. Parts a−c of Figure 4 show the W 4f, O 1s, and C 1s region XPS spectra of the 10-graphene-WO3 nanowire nanocomposite, respectively. The symmetric features at binding energies of 37.6 and 35.5 eV were assigned to W 4f5/2 and W 4f7/2 of W6+ oxidation state in WO3, respectively.20 The O 1s spectrum (Figure 4b) was deconvoluted to two features at 530.3 eV for lattice oxygen bonding (WO) and 531.8 eV for surface oxygen/hydroxide. The C 1s XPS spectrum was deconvoluted to peaks at 284.3 eV (CC), 285.6 eV (CO), 288.5 eV (CC), and 290.8 eV (OC C).30 This suggests the bonding between carbon atoms of graphene with oxygen atoms of WO3 that facilitates the charge transportation and conductivity of the nanocomposite. Similar W 4f (Figure S2b, SI) and O 1s (Figure S2c, SI) region spectra were obtained for WO3 nanowires. Synthesis Mechanism. The formation mechanism of WO3 nanowires by the solvothermal process can be explained as

preventing the restacking of graphene sheets, which would result in the enhancement of active surface area leading to better electrochemical performance. The spot selected area electron diffraction (SAED) pattern confirms the single crystalline nature of graphene-WO3 nanowire nanocomposite (top inset in Figure 2g). The SAED pattern of graphene is shown in the bottom inset of Figure 2g. The HR-TEM image of a WO3 nanowire in 10-graphene-WO3 nanowire nanocomposite shows a lattice spacing of 0.38 nm [Figure S1, Supporting Information (SI)], which corresponds to (002) crystal planes of monoclinic WO3.44 This correlates to the XRD result with the highest diffraction intensity of (002) planes of 10-graphene-WO3 nanowire nanocomposite and ascertains the role of graphene in changing the growth orientation of WO3. Surface Area. Specific surface area plays an important role on the electrochemical performance of the active material.45 The specific surface area of the as-synthesized graphene, WO3 nanowires, and 10-graphene-WO3 nanowire nanocomposite was measured by a N2 adsorption−desorption isotherm at 77 K as shown in Figure 3. All the samples show a type IV isotherm

Figure 3. Nitrogen adsorption/desorption isotherms of (a) graphene nanosheets, (b) WO3 nanowires, and (c) 10-graphene-WO3 nanowire nanocomposite.

as per Brunauer classification.30 The BET surface areas (and pore volume) of graphene (Figure 3a) and WO3 nanowires (Figure 3b) were measured to be 60 m2 g−1 (0.3 cm3 g−1) and 132.16 m2 g−1 (0.12 cm3 g−1), respectively. Moreover, 10graphene-WO3 nanowire nanocomposite (Figure 3c) displays a larger specific surface area of 186 m2 g−1 and pore volume 0.63 cm3 g−1 compared to graphene and WO3 nanowires. The larger 10131

DOI: 10.1021/acssuschemeng.7b02135 ACS Sustainable Chem. Eng. 2017, 5, 10128−10138

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Figure 4. Region (a) W 4f, (b) O 1s, and (c) C 1s XPS spectra of 10-graphene-WO3 nanowire nanocomposite.

Figure 5. (a) Cyclic voltammograms at 10 mV s−1, (b) specific capacitance vs scan rate, (c) galvanostatic charge/discharge plots at current density of 1.0 A g−1, and (F) specific capacitance as a function of current densities of different active materials (i) graphene, (ii) WO3 nanowires, (iii) 2graphene-WO3 nanowire nanocomposite, (iv) 5-graphene-WO3 nanowire nanocomposite, (v) 10-graphene-WO3 nanowire nanocomposite, and (vi) 15-graphene-WO3 nanowire nanocomposite.

Ham et al. reported the formation of WO3 nanoplates from sodium tungstate dihydrate and ammonium nitrate hydrothermally at 200 °C for 24 h, and upon addition of polyethylene glycol, WO3 nanowires were formed.50 We believe that an ethanolic medium in the present work facilitates the growth of WO3 nanowires with {100} orientation. Electrochemical Supercapacitor Properties. The electrochemical energy storage performance of graphene-WO3 nanowire nanocomposite was investigated by both the threeand two-electrode solid-state configurations in 0.1 M H2SO4 electrolyte using cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance study (EIS). Figure 5a shows the CV profile of graphene, WO3 nanowires, and graphene-WO3 nanowire nanocomposite (synthesized by adding different quantities of graphene i.e., 2, 5,

follows. The addition of ethanol to WCl6 could form tungsten chloride alkoxide and HCl (eq 5).49 The subsequent solvothermal treatment of tungsten alkoxide at 200 °C for 12 h produces WO3 nanowires (eq 6) as obtained in the present work. WCl 6 + xC2H5OH → WCl 6 − x(OC2H5)x + x HCl

(5)

WCl 6 − x (OC2H5)x + 3O2 → WO3 + H 2O + CO2 + CO + 3HCl + Cl 2

(for x = 1)

(6)

It should be noted that the morphology and orientation of WO3 nanostructures in the solvothermal growth process depend on several factors including the solvent medium (and pH), reaction temperature, and duration.44,50,51 In particular, 10132

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Figure 6. (a) Nyquist plots in the frequency range of 1 Hz to 1 MHz, and (b) cycling performance of WO3 nanowires and 10-graphene-WO3 nanocomposite at a current density of 6 A g−1.

graphene-WO3 nanowire nanocomposites. At the 1, 5, 10, 20, 40, 60, 80, and 100 mV s−1 scan rate, the specific capacitance of 10-graphene-WO3 nanowire nanocomposite was estimated to be 980, 800, 685.2, 544.7, 397.1, 309.6, 253.3, and 204.8 F g−1, respectively. The specific capacitance was found to decrease as the scan rate increased (Figure 5b) for all the samples, which is ascribed to the diffusion effect.55 Further, the GCD profile of WO3 nanowires and graphene-WO3 nanowire nanocompositebased electrodes [Figure S4a−e, SI] was recorded as a function of current density. Figure 5c shows the GCD plots of different electrodes at 1 A g−1 current density. The charge/discharge profiles of graphene were found to be more symmetric than others indicating pseudocapacitive behavior of the latter. In particular, WO3 nanowires and graphene-WO3 nanowire nanocomposites show steeper charge/discharge in the positive potential range (0−0.6 V). It is to be noted that the IR drop remains small as shown in the magnified plots (Figure S5, SI). The IR drops (potential drops) were measured to be 0.12, 0.12, 0.09, 0.09, and 0.06 V for WO3 nanowires, and graphene-WO3 nanowire nanocomposites (2, 5, 10, and 15 mg graphene), respectively, at 1 A g−1, from the GCD plots. Similar charge/ discharge profiles have also been observed with other WO3based materials in H2SO4 electrolyte.33,56 The specific capacitance value was also calculated from the charge/discharge profiles using eq 9.33

10, and 15 mg) modified electrodes in the potential range from −0.4 to 0.6 V at 10 mV s−1. The CV obtained with the graphene-modified electrode was quasirectangular in shape. On the other hand, the CV obtained with WO3 nanowires and graphene-WO3 nanowire nanocomposite-based electrodes were quasirectangular in shape consisting of distinct redox peaks, demonstrating their pseudocapacitance behavior.34 This confirms the successful formation of the graphene-WO3 nanowire nanocomposite. It is well-known that, in acidic medium, WO3 shows a pair of well-defined redox peaks corresponding to the conversion of different valence states of WO3 (eq 7).26 WO3 + x H+ + x e− ↔ 4HxWO3

(7)

In addition, the CV profiles show a wider voltammetry area for the 10-graphene-WO3 nanowire nanocomposite compared to those of all other electrodes at same scan rate, indicating the optimum quantity of graphene (10 mg) needed to obtain the highest specific capacitance.52 The specific capacitance was calculated using eq 8 and shown in Figure 5b. Cs =

∫ I dv m × ν × ΔV

(8)

Here, the following abbreviations apply: Cs is specific capacitance (F g−1), I is current (A), m is the mass of active material (g), ν is the scan rate (mV s−1), and ΔV is the potential window (V). From the specific capacitance value, it is concluded that the 10-graphene-WO3 nanowire nanocomposite has the highest electrochemical storage activity compared to those of graphene, WO3 nanowires, and their composites with either lower or higher graphene quantity. The higher specific capacitance of the composite is believed to have originated from the synergistic effect of the conducting nature of graphene and the pseudocapacitive behavior of WO3 nanowires.34 Moreover, in H2SO4 electrolyte, WO3 shows two peaks corresponding to the reduction of W6+ to W5+ followed by further reduction of W5+ to W4+.53 This change of oxidation state also enhances the electrical conductivity as well as faster proton transport within the electrode interface, which increases the charge storage ability of WO3.54 In addition, integration of WO3 nanowires onto graphene hinders the restacking of graphene sheets that provides more active sites for the intercalation/deintercalation reactions.32,34 The effect of scan rate (1 to 100 mV s−1) on the electrochemical performance of WO3 nanowires and graphene-WO3 nanowire nanocomposites was investigated in the potential range −0.4 to 0.6 V [Figure S3a−e, SI]. The CV profiles retained their shape irrespective of the scan rates indicating excellent capacitive behavior of the

specific capacitance, Cs(F g −1) =

I × Δt m × ΔV

(9)

Here, Cs, I, m, and ΔV have their previously mentioned meanings, and Δt is the total discharge time. To prove the superiority of the graphene-WO3 nanowire nanocomposite over graphene and WO3 nanowires, the specific capacitance versus current density plot is shown in Figure 5d as estimated from the GCD plots. With an increase in applied current density, the discharge time was found to be decreased. At 1, 2, 3, 4, 5, 6, and 7 A g−1, the specific capacitance value of the 10-graphene-WO3 nanowire nanocomposite was calculated to be 465, 418, 390, 372, 355, 336, and 315 F g−1, respectively. The low specific capacitance obtained at high current density was due to insufficient migration of ions into the core of the electrode material.57 The specific capacitance value obtained in this case is quite higher than that reported for WO3-based materials.33,34,58,59 The higher specific capacitance obtained here is due to the larger surface area of the active materials that has a profound effect on the electrochemical performance.60 Furthermore, the rate capability of the 10-graphene-WO3 nanowire nanocomposite (67.74%) was found to be much 10133

DOI: 10.1021/acssuschemeng.7b02135 ACS Sustainable Chem. Eng. 2017, 5, 10128−10138

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Figure 7. Cyclic voltammograms of (a) 10-graphene-WO3 nanowire nanocomposite and activated carbon recorded using a three-electrode system in 0.1 M H2SO4 at 10 mV s−1, (b) 10-graphene-WO3 nanowire nanocomposite in different potential windows using two-electrode solid-state ASC device, (c) 10-graphene-WO3 nanowire nanocomposite at different scan rates. GCD curves of 10-graphene-WO3 nanowire nanocomposite (d) in different potential windows at a current density of 1.0 A g−1 and (e) at different current densities with a fixed potential window. (f) Specific capacitance as a function of current densities.

region is vertically more linear for composite compared to graphene and WO3, indicating that nanocomposite has lower diffusion resistance. Thus, integration of graphene with WO3 ensured the decrease in intrinsic resistance of the nanocomposite giving rise to faster electron transfer between the active material and charge collector making it a suitable candidate for supercapacitors. Study of the long-term cyclic stability of electrode material is another important criterion for the practical application of supercapacitors. Therefore, the cyclic stabilities of WO3 nanowires and 10-graphene-WO3 nanowire nanocomposite modified electrodes were investigated using the GCD technique at 6 A g−1 for 2000 cycles. Figure 6b shows the specific capacitance of WO3 nanowires and graphene-WO3 nanowire nanocomposite as a function of cycle number. After 2000 GCD cycles, the specific capacitance of WO3 nanowires was decreased from 75 to 65.95 F g−1, and graphene-WO3 nanowire nanocomposite decreased from 335.33 to 333.7 F g−1. The decrease in the specific capacitance could be ascribed to the partial dissolution and material pulverization due to repeated intercalation/deintercalation of ions during the charging and discharging cycles.67,68 Only a 2.3% decrease in the specific capacitance was observed on a graphene-WO3 nanowire nanocomposite electrode after 2000 GCD cycles, demonstrating its excellent long-term cyclic stability. Furthermore, the Nyquist plot was obtained after the 2000 cycle test (Figure S6, SI), and the Rct value was measured by fitting with an equivalent circuit model as shown in the inset of Figure S6. The Rct value was found to be slightly increased from 4.15 to 6.47 Ω after 2000 cycles of the stability test.

higher than that for WO3 nanowires (32.98%) as measured from GCD data (Figure 5d). This underlines the important role of graphene in the nanocomposite for energy storage applications. The electrochemical properties of graphene, WO3 nanowires, and graphene-WO3 nanowire nanocomposite were further investigated using EIS study (Figure 6a). The impedance Nyquist plots of these electrodes consist of a semicircle in the high frequency region and a straight line in the low frequency region. The semicircle is ascribed to the charge-transfer resistance (Rct) whereas the straight line is ascribed to the diffusion of ions from the electrolyte to the electrode surface.61,62 The Rct value of graphene, WO3 nanowires, and 10-graphene-WO3 nanowire nanocomposite were calculated to be 36.37, 11.94, and 4.15 Ω, respectively. However, graphene has high electrical conductivity, and the higher Rct value (bigger semicircle in EIS plot) of graphene is due to its hydrophobic nature.63 On the other hand, the wetting behavior of WO3 is known to be much higher with a smaller contact angle.64 Thus, the hydrophilic nature of WO3 plays a significant role in reducing the Rct value between electrolyte and electrode despite having lower electrical conductivity to WO3. Nevertheless, the lowest Rct of 10-graphene-WO3 nanowire nanocomposite is due to the synergistic effect of graphene (high electrical conductivity) and WO3 (higher wetting behavior). Similar charge transport behavior has been reported earlier.65,66 The low Rct value obtained with the 10-graphene-WO3 nanowire nanocomposite-based electrode indicates that the rate of electron transfer is faster than WO3 nanowires and graphene. In addition, the straight line obtained in the low frequency 10134

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ACS Sustainable Chemistry & Engineering

Figure 8. (a) Ragone plot, i.e., energy density vs power density. (b) Cyclic stability study by measuring specific capacitance as a function of cycle numbers at 6.0 A g−1 current density using a two-electrode solid-state ASC cell with 10-graphene-WO3 as a negative electrode and activated carbon as positive electrode.

higher than that of the ASC device based on WO 3 nanostructures reported recently.69 The energy and power density are two important parameters that decides the applicability of the supercapacitor devices.70 Therefore, the energy and power density were calculated using eqs 10 and 11, respectively.3,71

To further access the practical application of a grapheneWO3 nanowire nanocomposite in the development of a supercapacitor, a solid-state asymmetric supercapacitor (ASC) device was fabricated using a 10-graphene-WO3 nanowire nanocomposite as negative electrode, AC as positive electrode, and solidified PVA/H2SO4 gel as an electrolyte. Prior to the evaluation of ASC performance, individual voltammograms of 10-graphene-WO3 nanowire nanocomposite and AC were recorded using a three-electrode system in 0.1 M H2SO4 at 10 mV s−1 (Figure 7a). The AC has a potential window from −0.8 to 1.2 V, whereas it was −0.4 to 0.6 V for the 10graphene-WO3 nanowire nanocomposite. Combining the individual potential ranges of AC and a graphene-WO3 nanowire nanocomposite, the total cell voltage was found to be 2.0 V in 0.1 M H2SO4 indicating that the operating cell voltage could be extended up to 2.0 V for a two-electrode solidstate ASC device. The CVs were then recorded with a twoelectrode solid-state ASC device using different potential windows at a fixed scan rate of 10 mV s−1 (Figure 7b). The quasirectangular CV profiles were observed irrespective of the potential windows indicating the capacitive behavior of the ASC device. The close examination of the CV profile obtained at the highest potential window does not show any signature of the decomposition of electrolyte resulting to the hydrogen or oxygen evolution indicating that the ASC device can bear a high working voltage up to 2.0 V. Thus, the CV profile of the solidstate ASC device was further recorded as a function of scan rates keeping the potential window fixed at 0 to 2.0 V (Figure 7c). The quasirectangular shape of the CV profiles was retained irrespective of the scan rate, demonstrating the high rate capability and good reversibility of the ASC device. GCD measurements were further carried out in different potential ranges at a fixed current density of 1.0 A g−1 (Figure 7d). As can be seen in Figure 7d, though the potential window of ASC device can be extended up to 2.0 V, quite symmetrical GCD profiles were obtained up to 1.6 V. Additionally, GCD profiles at different current densities (1.0−6.0 A g−1) keeping the potential window range fixed from 0.0 to 2.0 V (Figure 7e) were recorded. A similar type of GCD profile has been observed in which WO3 mesoscopic microspheres were used as active material for the ASC device.14 The specific capacitances calculated at different current densities are presented in Figure 7f. In the present case, a maximum capacitance value of 171 F g−1 was achieved at 1.0 A g−1 current density, which is much

E=

1 CsΔV 2 2

P = E /Δt

(10) (11)

The present two-electrode solid-state ASC device with a 10graphene-WO3 nanowire nanocomposite could deliver an energy density of 26.7 W h kg−1 at 6 kW kg−1 power density and was capable of retaining energy density up to 25 W h kg−1 at 6 kW kg−1 power density after 4000 cycles suggesting high cyclic stability as shown in the Ragone plot (Figure 8a). Figure 8b shows the GCD plots of the ASC device fabricated with a 10-graphene-WO3 nanowire nanocomposite as a function of cycle number at a current density of 6.0 A g−1. The inset of Figure 8b shows the initial and final GCD profiles. The energy density obtained in the present case is higher than those of several previously reported ASC devices based on graphitic carbon spheres-MnO2 nanofibers (22.1 W h kg−1 at 7 kW kg−1),72 cobalt hydroxide (13.6 W h kg−1 at 153 W kg−1),73 and Co3O4@MnO2 core−shell arrays (17.7 W h kg−1 at 158 W kg−1).74 Except the aforementioned active materials, the ASC device based on a 10-graphene-WO3 nanowire nanocomposite also showed excellent performance compared to that of the other WO3-based ASC. For example, Peng et al. developed a carbon nanofiber and WO3 nanorod bundle-based ASC which delivered an energy density of 35.3 W h kg−1 at a power density of 0.31 kW kg−1.75 Sun et al. demonstrated the use of a polyaniline nanotube and WO3 nanorods in the ASC application and achieved an energy density of 41.9 W h kg−1 at 0.26 kW kg−1.69 The superior electrochemical performance in the present work demonstrates that graphene-WO3 nanowire nanocomposite could be a promising active material for the development of high performance energy storage devices. A schematic representation of the flexible two-electrode all-solidstate ASC device is shown in Figure 9a which was further tested to study the viability for practical application by lighting eight red light-emitting diode indicators (Figure 9b). 10135

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CONCLUSIONS In summary, here we demonstrate the facile and green solvothermal synthesis of a graphene-WO3 nanowire nanocomposite and its application as an active material in the fabrication of an all-solid-state ASC device. Morphological characterization shows the random distribution of WO3 nanowires on the graphene sheets, which prevents stacking of graphene sheets and thus exhibits a large surface area. The large surface area of a graphene-WO3 nanowire nanocomposite facilitates delivery of higher specific capacitance making it an efficient active electrode material for the fabrication of supercapacitor devices. The ASC device was fabricated by pairing the graphene-WO3 nanowire nanocomposite with AC, and the device delivered a high specific capacitance, good rate capability, and excellent cyclic stability. Using this device, an energy density of 26.7 W h kg−1 was achieved at a power density of at 6 kW kg−1, and it could retain 93.6% of its initial specific capacitance after 4000 continuous charge/discharge cycles. The present investigation demonstrates that the graphene-WO3 nanowire nanocomposite is a promising active electrode material for ASC devices that could serve in the development of high performance energy storage devices. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02135. HR-TEM image of WO3 nanowires in graphene-WO3 nanowire nanocomposite, XPS spectra, cyclic voltammograms at different scan rates, GCD, and Nyquist plots (PDF)

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REFERENCES

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Figure 9. (a) Schematic of a flexible solid-state asymmetric supercapacitor device. (b) Demonstration of real ASC device to light a few red LEDs after charging.

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Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Debabrata Pradhan: 0000-0003-3968-9610 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Science and Engineering Research Board (SERB), Department of Science and Technology, New Delhi, India, through Grant SB/S1/IC-15/2013. We acknowledge CENTRAL SURFACE ANALYTICAL FACILITY, IIT Bombay, for the XPS measurements. 10136

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DOI: 10.1021/acssuschemeng.7b02135 ACS Sustainable Chem. Eng. 2017, 5, 10128−10138

Research Article

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DOI: 10.1021/acssuschemeng.7b02135 ACS Sustainable Chem. Eng. 2017, 5, 10128−10138