High Performance Solid-State Asymmetric Supercapacitor using

Subscriber access provided by UNIVERSITY OF THE SUNSHINE COAST

Article

High Performance Solid-State Asymmetric Supercapacitor using Green Synthesized Graphene-WO Nanowires Nanocomposite 3

Arpan Kumar Nayak, Ashok Kumar Das, and Debabrata Pradhan ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 25, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Sustainable Chemistry & Engineering is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

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, W.B., India

Abstract Development of active materials capable of delivering high specific capacitance is one of the present challenges in supercapacitor application. 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 nanowires

nanocomposite with an optimized weight ratio has shown excellent electrochemical performance with specific capacitance of 465 F g−1 at 1 A g−1 and 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 graphene-WO3 nanowires nanocomposite as negative electrode and activated carbon as 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 graphene-WO3 nanowires 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

*

E-mail: [email protected]

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 31

Introduction Energy crisis and environment pollution not only demand 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 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 low specific capacitance and thus not suitable to achieve high energy density. On the other hand, different transition metal oxides have been widely used as active material for the fabrication of pseudocapacitors, where reversible redox reaction is being used for the charge storage. Ruthenium oxide, iron oxide, manganese oxide, cobalt oxide, tin oxide, nickel oxide, conducting polymers such as polypyrrole and polyaniline have been used for this purpose and much higher specific capacitance has been obtained compared to the EDLCs.6,7,8,9,10,11,12,13 Tungsten trioxide (WO3) has recently emerged as a potential electrode material in the development of pseudocapacitor due to its excellent capacitive behavior and earth abundance.14,15,16,17 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 Though, WO3 has proven its potential as a promising candidate for wide


Page 3 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 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 CNT-WO3 nanohybrid electrode could deliver much higher specific capacitance25 and Gao et al. demonstrated excellent electrochemical performance with carbon cloth supported WO3 nanowires arrays.26 Recently, Wang et al. demonstrated that WO3@Polypyrrole core-shell nanowires arrays with excellent supercapacitive performance.28 In addition, graphene-WO3 nanocomposite has recently emerged as a promising material for wide variety of applications with much better performance compared to either WO3 or graphene.30,31,32 Graphene-WO3 nanocomposite has also been recognized as a potential candidate for supercapacitor application 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 flowerlike WO3·H2O (140 F g−1).34 Recently, a much improved storage ability of WO3 nanoflower coated with graphene nanosheet was reported by Chu et al.17 Though, a number of reports are available on the synthesis and application of graphene-WO3 nanocomposite for wide variety of applications, there are a handful of reports on it’s application as an electrode material in the fabrication of supercapacitor requiring further study in this direction.32,33,34 Herein, we demonstrate synthesis of graphene sheet decorated with WO3 nanowires following a green solvothermal approach using ethanol as only solvent instead of most



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 31

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 solid-state asymmetric supercapacitor (ASC) device. The specific capacitance value obtained with the as-synthesized graphene-WO3 nanowires 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. Experimental section Chemicals. Tungsten (VI) chloride (WCl6) (Sigma Aldrich, USA), sulfuric acid (H2SO4), ethanol (C2H5OH), and graphene (Merck, India), 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 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-colored product was obtained. The product was collected by centrifuging followed by washing repeatedly with ethanol and Millipore water, and finally dried at 60 °C for 4 h. Synthesis of graphene-WO3 nanocomposite. The synthesis of graphene-WO3 nanocomposite was carried out following the above procedure by adding chosen quantity of graphene to a preprepared 40 mL WCl6 solution and was subjected to ultrasonication for 30 min. After


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 31

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 nanowires 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., USA). A three electrode configuration mode was used for these studies where graphite sheet electrode of thickness 0.25 mm (Nickunj Eximp Entp P. Ltd., India), Pt wire, and saturated calomel electrode (SCE) were functioned as working, counter, and reference electrode, respectively. In all the electrochemical experiments, 0.1 M H2SO4 was used as electrolyte. The active material slurry was prepared by mixing graphene-WO3 nanowires nanocomposite or WO3 nanowires with polyvinylidene fluoride (PVDF) in 2 mL 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 for overnight drying 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 polyvinyl 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.


Page 7 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fabrication of Solid-State Supercapacitor. From the preliminary electrochemical investigation, the highest specific capacitance was obtained with 10-graphene-WO3 nanowires nanocomposite (10 mg graphene added for the synthesis) modified electrode. Thus detail 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 nanowires nanocomposite as negative electrode, activated carbon (AC) as positive electrode, and H2SO4/PVA gel as solid electrolyte. First, slurry of active material was prepared by mixing either graphene-WO3 nanocomposite or AC and PVDF in 9:1 mass ratio using 2 mL NMP. The as-prepared slurries were then coated individually on the 1 × 1 cm2 area of 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 for overnight until the gel electrolyte solidified. The thickness of the solidstate 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 the equations (1−4) given below. Q+ = Q−

(1)

m+ × C s + × ∆V+ = m− × C s − × ∆V−

(2)

m+ C s − × ∆ V − = m − C s + × ∆ V+

(3)

m+ = m−

(4)

685.2 × 1 155 × 2

= 2.21

where Cs is the specific capacitance (F g−1) at 10 mV s−1 scan rate, ∆V 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 F g−1 and 155 F g−1, respectively at 10 mV s−1. The mass ratio of graphene-WO3 nanowires nanocomposite and AC was 0.41 in the ASC device. In the present work, the mass of graphene-WO3 nanowires 7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 31

nanocomposite and AC used in the fabrication of ASC device was 6.0 and 13.26 mg, respectively. Hence the total mass of the two active electrode materials was 19.26 mg. Results and discussion Structural properties. The structural property of the as-synthesized graphene, WO3, and 10graphene-WO3 nanowires nanocomposite (10 mg graphene) was studied by XRD and Raman analysis, and is shown in Figure 1. The XRD pattern of graphene [Figure 1A(a)] shows a broad diffraction feature at ~26° and a small hump at ~43.25° corresponding to the (002) and (100) plane, respectively.39 The XRD pattern of WO3 [Figure 1A(b)] shows sharp diffraction features representing the crystalline nature and are 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 No. 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 10graphene-WO3 nanowires nanocomposite shows similar monoclinic structure of WO3. The (002) plane of graphene is believed to be overlapped by (120) diffraction of WO3 nanowires in the 10graphene-WO3 nanowires nanocomposite. Moreover, XRD intensity of different WO3 crystal planes in 10-graphene-WO3 nanowires 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 10-graphene-WO3 nanowires nanocomposite, respectively. This indicates the change in growth kinetics of WO3 nanowires in presence of graphene. This was further confirmed by TEM study (discussed later). An intense (202) diffraction intensity was also observed for WO3 nanowires in 10-graphene-WO3 nanowires nanocomposite.


Page 9 of 31

The structural property of the graphene, WO3, and 10-graphene-WO3 nanowires nanocomposite was further investigated by Raman analysis (Figure 1B). The Raman spectrum of graphene [Figure 1B(a)] shows two peaks at ~1346 cm−1 and ~1580 cm−1 corresponding to the D and G bands of graphene. The G and D band correspond to the graphitic sp2 carbon and defects in the graphitic carbon structure, respectively.40,41 The Raman spectrum of pure WO3 [Figure 1B(b)] shows three peaks at ~259 cm−1, ~695 cm−1, and ~804 cm−1 corresponding to the stretching mode of O–W–O. In the case of 10-graphene-WO3 nanowires nanocomposite [Figure 1B(c)], the same Raman vibrations were observed at ~261 cm−1, ~697 cm−1, and ~810 cm−1 along with two additional features at ~1345 cm−1 and ~1586 cm−1 contributed from graphene. The lower intensity of Raman peak of graphene in the composite was due to the high loading of WO3. Similar Raman signature for graphene-WO3 nanocomposite has also been reported elsewhere.42

B 261

Graphene - WO3 1345

(100)

(a)

Graphene

(c)

804 259

695

WO3

(b) 1346

Intensity (a.u.)

(116)

(340)

(400)

(142)

(222)

WO3

(002)

(002) (020) (120) (200) (112) (202)

(c)

(b)

1586

10-Graphene-WO3

697 810

1580

A Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Graphene

(a) 10

20

30

40

50

60

2θ (degree)

70

80

400

800 1200 1600

-1

Raman shift (cm )

Figure 1. (A) Powder XRD pattern and (B) Raman spectra of (a) pure graphene, (b) WO3 nanowires, and (c) 10-Graphene-WO3 nanowires nanocomposite synthesized by solvothermal method at 200 °C for 12 h. 9 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 31

Morphology and microstructure. The morphology of the as-synthesized pure WO3 nanostructures and 10-graphene-WO3 nanocomposite was examined by FESEM. Figure 2a and 2b show 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, FESEM image of 10-graphene-WO3 nanocomposite shows 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. 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 spot diffraction pattern suggesting its single crystalline nature. The close examination of TEM image of 10-grapheneWO3 nanowires nanocomposite (Figure 2f and 2g) 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 preventing the restacking of graphene sheets, which would result in the enhancement of active surface area leading to the better electrochemical performance. The spot selected area electron diffraction (SAED) pattern confirms the single crystalline nature of graphene-WO3 nanowires 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 nanowires 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 the XRD result with the highest diffraction intensity of (002) planes of 10-


Page 11 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


graphene-WO3 nanowires nanocomposite and ascertains the role of graphene in changing the growth orientation of WO3.

Figure 2. FESEM images of (a, b) WO3 nanowires and (c) 10-graphene-WO3 nanowires nanocomposite. (d) TEM and (e) HRTEM images of WO3 nanowires, the FFT pattern, and (f,g) TEM images of 10-graphene-WO3 nanowires nanocomposite, insets of (g) show the SAED pattern of WO3 nanowires (top) and graphene (bottom). 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 nanowires nanocomposite was measured by N2 adsorptiondesorption isotherm at 77 K as shown in Figure 3. All the samples show type IV isotherm as per Brunauer classification.30 The BET surface area (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, 10-graphene-WO3 nanowires nanocomposite (Figure 3c) displays a larger specific surface area of 186 m2 g−1 and pore volume 0.63 cm3 g−1 compared to 11 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 31

graphene and WO3 nanowires. The larger surface area of nanocomposite than 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 re-stacking of graphene sheets with each other resulting the enhancement of active surface area.46,47 In the present case, loading of WO3 nanowires onto graphene sheets thus hinders the re-stacking of graphene sheets during the synthesis of nanocomposite. Benefiting from the combined effect of graphene and WO3 nanowires, the resulting graphene-WO3 nanowires 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

nanowires

nanocomposite

is

much

higher

than

the

WO3

nanorods/graphene nanocomposite (34.8 m2 g−1) reported by An et al.30 Since the present 10graphene-WO3 nanowires nanocomposite possesses larger surface area, therefore, in principle, it can 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


−1

on

(c) 10-Graphene-WO3

tio n

200

(c)

(a) Graphene (b) WO3

Ad so rp

300

De so rp ti

3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Volume @ STP (cm g )

Page 13 of 31

100

(a)

(b) 0 0.0

0.2

0.4

0.6

0.8

1.0 −1

Relative pressure (P P0 ) Figure 3. Nitrogen adsorption/desorption isotherms of (a) graphene nanosheets, (b) WO3 nanowires, and (c) 10-graphene-WO3 nanowires nanocomposite.

Surface composition.The surface chemical composition and valence state of WO3 nanowires and 10-graphene-WO3 nanowires nanocomposite were further studied by XPS. The survey spectra (Figure S2a, SI) show presence of no other element except W, O, and C. Part a, b, and c of Figure 4 show the W 4f, O 1s, and C 1s region XPS spectra of 10-graphene-WO3 nanowires nanocomposite, respectively. The symmetric features at binding energy of 37.6 eV 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. 13 ACS Paragon Plus Environment

42 40 38 36 34

534

532

530

528

C− O

35.5 eV

531.8 eV

W 4f5/2

C− C

(c) C 1s

C= C

(b) O 1s

Page 14 of 31

O − C= C

W 4f7/2

530.3 eV

(a) W 4f

37.6 eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Counts per sec


296 292 288 284 280

Binding energy (eV) Binding energy (eV) Binding energy (eV)

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

Synthesis mechanism. The formation mechanism of WO3 nanowires by the solvothermal process can be explained as follows. The addition of ethanol to WCl6 could form tungsten chloride alkoxide and HCl (equation 5).49 The subsequent solvothermal treatment of tungsten alkoxide at 200 °C for 12 h produces WO3 nanowires (equation 6) as obtained in the present work. WCl6 + xC2H5OH → WCl6-x(OC2H5)x + xHCl

(5)

WCl6-x(OC2H5)x + 3O2 → WO3 + H2O + CO2 + CO + 3HCl + Cl2 (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, Ham et al. reported formation of WO3 nanoplates from sodium tungstate dihydrate and ammonium nitrate hydrothermally at 200 °C for 24 h and upon adding 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. 14 ACS Paragon Plus Environment

Page 15 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Electrochemical supercapacitor properties. The electrochemical energy storage performance of graphene-WO3 nanowires nanocomposite was investigated by both the three and 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 nanowires nanocomposite (synthesized by adding different quantities of graphene i.e. 2 mg, 5 mg, 10 mg, 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 graphene-modified electrode was quasi-rectangular in shape. On the other hand, the CV obtained with WO3 nanowires and graphene-WO3 nanowires nanocomposite-based electrodes were quasi-rectangular in shape consisting of distinct redox peaks, demonstrating their pseudo-capacitance behavior.34 This confirms the successful formation of graphene-WO3 nanowires 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 (Equation 7).26 WO3 + xH+ + xe − ↔ 4HxWO3

(7)

In addition, the CV profiles shows a wider voltammetry area for 10-graphene-WO3 nanowires nanocomposite compared to 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 equation (8) and shown in Figure 5b.

Cs =

∫ Idv m ×ν × ∆V

(8)

where 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 ∆V is the potential window (V). From the specific capacitance value,



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 31

it is concluded that 10-graphene-WO3 nanowires nanocomposite has the highest electrochemical storage activity than graphene, WO3 nanowires, and their composites with either lower or higher graphene quantity. The higher specific capacitance of the composite is believed to be originated from the synergistic effect of conducting nature of graphene and 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/de-intercalation reactions.32,34 The effect of scan rate (1 to 100 mV s−1) on the electrochemical performance of WO3 nanowires and graphene-WO3 nanowires nanocomposites was investigated in the potential range of −0.4 to 0.6 V [Figures S3(a−e), SI]. The CV profiles retained their shape irrespective of the scan rates indicating excellent capacitive behaviour of the graphene-WO3 nanowires nanocomposites. At the 1, 5, 10, 20, 40, 60, 80, and 100 mV s−1 scan rate, the specific capacitance of 10-graphene-WO3 nanowires nanocomposite was estimated to be 980 F g−1, 800 F g−1, 685.2 F g−1, 544.7 F g−1, 397.1 F g−1, 309.6 F g−1, 253.3 F g−1, and 204.8 F g−1, respectively. The specific capacitance was found to be decreased 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 nanowires nanocomposite-based electrodes [Figures S4(a−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 behaviour of the latter. In particular, WO3 nanowires and graphene-WO3 nanowires nanocomposites show 16 ACS Paragon Plus Environment

Page 17 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


steeper charge/discharge in the positive potential range (0 to 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 grapheneWO3 nanowires (2 mg, 5 mg, 10 mg, and 15 mg), respectively, at 1 A g−1, from the GCD plots. Similar charge/discharge profiles have also been observed with other WO3-based materials in H2SO4 electrolyte.33,56 The specific capacitance value was also calculated from the charge/discharge profiles using equation (9).33 Specific capacitance, Cs (F g−1) =

I × ∆t m × ∆V

(9)

where Cs, I, m, and ∆V has earlier mentioned meaning and ∆t is the total discharge time. To prove the superiority of graphene-WO3 nanowires nanocomposite over graphene and WO3 nanowires, the specific capacitance vs. current density plot is shown in Figure 5d as estimated from the GCD plots. With 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 10-graphene-WO3 nanowires nanocomposite was calculated to be 465 F g−1, 418 F g−1, 390 F g−1, 372 F g−1, 355 F g−1, 336 F g−1, 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 the reported for WO3based materials.33,34,58,59 The higher specific capacitance obtained here is due to the larger surface area of the active materials that has profound effect on the electrochemical performance.60 Furthermore, the rate capability of 10-graphene-WO3 nanowires nanocomposite (67.74%) was found to be much higher than WO3 nanowires (32.98%) as measured from GCD data (Figure 5d). This underlines the important role of graphene in the nanocomposite for energy storage application. 17 ACS Paragon Plus Environment

Scan rate - 10 mV s 0.1 M H2SO4

(v)

(vi)

(iv)

4.0

(iii)

(ii) (i)

0.0

(i) Graphene (ii) WO3 (iii) 2-Graphene-WO3 (iv) 5-Graphene-WO3 (v) 10-Graphene-WO3 (vi) 15-Graphene-WO3

-4.0 -8.0 -0.4

-0.2

0.0

0.2

0.4

0.6

−1

−1

(a)

−1

Current density (A g )

8.0

1000 800

(b)

(iii)

(i) Graphene (ii) WO3 (iii) 2-Graphene-WO3 (iv) 5-Graphene-WO3 (v) 10-Graphene-WO3 (vi) 15-Graphene-WO3

0.4 0.2 0.0

−1

Current density 1 A g 0.1 M H2SO4

(i)

-0.2 -0.4

(vi) (v)

(ii) (iii) (iv)

0

200

400

600

800

1000

2-Graphene-WO3

(iv)

5-Graphene-WO3

(v)

10-Graphene-WO3

(vi)

15-Graphene-WO3

(ii)

400 200 (i)

0 0

−1

(c)

Graphene WO3

(iii)

(iv)

600

(i) (ii)

Page 18 of 31

25

50

75

100

−1

Scan rate (mV s ) Specific capacitance (F g )

0.6

(v)

(vi)

Potential (V vs SCE) Potential (V vs SCE)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Specific capacitance (F g )


(d)

(i) (ii)

600

Graphene WO3

(iii)

400

2-Graphene-WO3

(iv)

5-Graphene-WO3

(v)

10-Graphene-WO3

(vi)

15-Graphene-WO3

200

0 1

2

3

4

5

6

−1

7


Time (s)

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)

2-graphene-WO3

nanowires

nanocomposite,

(iv)

5-graphene-WO3

nanowires

nanocomposite, (v) 10-graphene-WO3 nanowires nanocomposite, and (vi) 15-graphene-WO3 nanowires nanocomposite.

The electrochemical property of graphene, WO3 nanowires, and graphene-WO3 nanowires nanocomposite was 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 18 ACS Paragon Plus Environment

Page 19 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


surface.61,62 The Rct value of graphene, WO3 nanowires, and 10-graphene-WO3 nanowires nanocomposite were calculated to be 36.37 Ω, 11.94 Ω, and 4.15 Ω, respectively. Although, graphene has high electrical conductivity, a 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 nanowires 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 nanowires 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 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 nanocomposite giving rise to faster electron transfer between the active material and charge collector making it a suitable candidate for supercapacitors. Study on the long-term cyclic stability of electrode material is another important criterion for the practical application of supercapacitors. Therefore, the cyclic stability of WO3 nanowires and 10-graphene-WO3 nanowires nanocomposite modified electrodes were investigated using GCD technique at 6 A g−1 for 2000 cycles. Figure 6b shows the specific capacitance of WO3 nanowires and graphene-WO3 nanowires nanocomposite as a function of cycle number. After 2000 GCD cycles, the specific capacitance of WO3 nanowires was decreased from 75 F g−1 to



65.95 F g−1 and graphene-WO3 nanowires nanocomposite decreased from 335.33 F g−1 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 2.3% decrease in the specific capacitance was observed on graphene-WO3 nanowires nanocomposite electrode after 2000 GCD cycles, demonstrating its excellent long-term cyclic stability. Furthermore, the Nyquist plot was obtained after the 2000 cyclic 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

−1

(a)

80

(iii)

''

(i) (ii)

60

40 (i) Graphene (ii) WO3

20

(iii) 10-Graphene-WO3

0

0


4.15 Ω to 6.47 after 2000 cycles of stability test.

Z (ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 31

20

40

'

60

Z (ohm)

80

−1

(b)

350

Current density - 6 A g

300 250

WO3

200

10-Graphene-WO3

150 100 50 0

500

1000

1500

2000

Cycle number

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

To further access the practical application of graphene-WO3 nanowires nanocomposite in the development supercapacitor, a solid-state asymmetric supercapacitor (ASC) device was fabricated using 10-graphene-WO3 nanowires 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 nanowires nanocomposite


Page 21 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and AC were recorded using 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 10graphene-WO3 nanowires nanocomposite. Combining the individual potential range of AC and graphene-WO3 nanowires 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 two-electrode solid-state ASC device. The CVs were then recorded with two electrode 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 behaviour 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 CV profile of the solid-state 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 quasi-rectangular shape of 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. Similar type of GCD profile has been observed in which WO3 mesoscopic microspheres was 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,

0.0 -4.0 (i) 10-Graphene-WO3

-8.0

(ii) Activated Carbon

-0.8 -0.4

0.0

0.4

Scan rate 50 mV s

4.0

0.8

0.4 0.0

(i)(ii)(iii)(iv)(v)

(vi)

0

600

300

(vii)

900

Time (s)

1200

−1

(ii) (i)

(i) 0.8 V (ii) 1.0 V (iii) 1.2 V (iv) 1.4 V (v) 1.6 V (vi) 1.8 V (vii) 2.0 V

-2.0 -4.0

0.0

0.5

1.0

1.5

2.0

(e)

−1

(i) 1 A g −1 (ii) 2 A g −1 (iii) 3 A g −1 (iv) 4 A g −1 (v) 5 A g −1 (vi) 6 A g

1.0 0.5 0.0

(vi)(v)(iv)(iii)

0

(ii)

300

0.0 -4.0

−1

Scan rate (mV S ) (i) 1, (ii) 5, (iii) 10, (iv) 20, (v) 40, (vi) 60, (vii) 80, (viii) 100

-8.0 0.0

0.5

1.0

1.5

2.0

2.5

Potential (V vs SCE)

(i)

600

(v) (iv) (iii) (ii) (i)

4.0

2.0

10-Graphene-WO3

1.5

(vii) (vi)

0.1 M H2SO4

Potential (V vs SCE) Potential (V vs SCE)

−1

0.8


1.2

(v)

0.0

1.2

10-Graphene-WO3

(i) 0.8 V (ii) 1.0 V (iii) 1.2 V (iv) 1.4 V (v) 1.6 V (vi) 1.8 V (vii) 2.0 V

1.6

(iv)

-6.0

0.1 M H2SO4

(d)

(vi)

−1

(iii)

0.1 M H2SO4

2.0

Potential (V vs SCE) 2.0

10-Graphene-WO3

(viii)

(c) 8.0 10-Graphene-WO 3

900

−1

(ii)

6.0

(vii)


(i)

(b)

0.1 M H2SO4

4.0

8.0

−1

Scan rate - 10 mV s 0.1 M H2SO4


−1

(a)


8.0

−1


which is much higher than the ASC device based on WO3 nanostructures reported recently.69

Potential (V vs SCE)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 31

Time (s)

(f)

10-Graphene-WO3

160 120 80 40 0

1

2

3

4

5

6

−1


Figure 7. Cyclic voltammograms of (a) 10-graphene-WO3 nanowires nanocomposite and activated carbon recorded using three electrode system in 0.1 M H2SO4 at 10 mV s−1, (b) 10graphene-WO3 nanowires nanocomposite in different potential windows using two-electrode solid-state ASC device, (c) 10-graphene-WO3 nanowires nanocomposite at different scan rates. GCD curves of 10-graphene-WO3 nanowires 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.

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 the equation (10) and (11), respectively. 3,71 E = ½ Cs ∆V2

(10) 22 ACS Paragon Plus Environment

Page 23 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


P = E / ∆t

(11)

The present two-electrode solid-state ASC device with 10-graphene-WO3 nanowires 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 ASC device fabricated with 10-graphene-WO3 nanowires nanocomposite as a function of cycle number at 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 several earlier 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 10-graphene-WO3 nanowires nanocomposite also showed excellent performance compared to 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 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 nanowires 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 solid-state 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).


(a)

This work Ref 72 Ref 73 Ref 74 Ref 75 Ref 69 After 4000 cycles

80 60 40 20 0

0

1

2

3

4

5

6

−1

Power density (kW kg )

7

120 100 80 60

(b)

Page 24 of 31

10-Graphene-WO3 Potential (V vs SCE)

−1

−1

100

Energy density (Wh kg )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



−1

th

2.0

1-10 cycles at 6 A g th −1 3990 - 4000 cycles at 6 A g

1.0 0.0 0

100

200 Time (s)

300 −1


40 20 0 0

1000

2000

3000

4000

Cycle number

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 two-electrode solid-state ASC cell with 10-graphene-WO3 as negative electrode and activated carbon as positive electrode.

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.

Conclusions In summary, here we demonstrate the facile and green solvothermal synthesis of graphene-WO3 nanowires nanocomposite and its application as an active material in the fabrication of all solid-state ASC device. Morphological characterization shows the random 24 ACS Paragon Plus Environment

Page 25 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


distribution of WO3 nanowires on the graphene sheets, which prevents stacking of graphene sheets and thus exhibit large surface area. The large surface area of graphene-WO3 nanowires nanocomposite facilitates to deliver 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 nanowires 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 nanowires nanocomposite as a promising active electrode material for ASC devices and that could serve in the development of high performance energy storage devices.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. HR-TEM image of WO3 nanowires in graphene-WO3 nanowires nanocomposite, XPS spectra, cyclic voltammograms at different scan rates, GCD and Nyquist plots.

AUTHOR INFORMATION Corresponding Author * E-Mail: [email protected] Notes The authors declare no competing financial interest.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 31

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

References 1

Wang, G.; Zhang, L.; Zhang, J. A Review of Electrode Materials for Electrochemical Supercapacitors. Chem. Soc. Rev.2012, 41, 797−828.

2

Peng, X.; Peng, L.; Wu C.; Xie, Y. Two Dimensional Nanomaterials for Flexible Supercapacitors. Chem. Soc. Rev. 2012, 43, 3303−3323.

3

Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7, 845−854.

4

He, Y.; Chen, W.; Gao, C.; Zhou, J.; Li, X.; Xie, E. An Overview of Carbon Materials for Flexible Electrochemical Capacitors. Nanoscale 2013, 5, 8799−8820.

5

Stoller, M.D.; Park, S.; Zhu, Y.; An, J.; Ruoff, R.S. Graphene-based Ultracapacitors. Nano Lett. 2008, 8, 3498−3502.

6

Hu, C.C.; Chang, K.H.; Lin, M.C.; Wu, Y.T. Design and Tailoring of the Nanotubular Arrayed Architecture of Hydrous RuO2 for Next Generation Supercapacitors, Nano Lett. 2006, 6, 2690−2695.

7

Wang, L.; Ji, H.; Wang, S.; Kong, L.; Jiang, X.; Yang, G. Preparation of Fe3O4 with High Specific Surface Area and Improved Capacitance as a Supercapacitor. Nanoscale, 2013, 5, 3793−3799.

8

Yang, P.; Li, Y.; Lin, Z.; Ding, Y.; Yue, S.; Wong, C.P.; Cai, X.; Tan, S.; Mai, W. Worm-like Amorphous MnO2 Nanowires Grown on Textiles for High-performance Flexible Supercapacitors. J. Mater. Chem. A 2014, 2, 595−599.

9

Wang, Y.; Lei, Y.; Li, J.; Gu, L.; Yuan, H.; Xiao, D. Synthesis of 3D-nanonet Hollow Structured Co3O4 for High Capacity Supercapacitor. ACS Appl. Mater. Interfaces 2014, 6, 6739−6747.

10

Liu, Y.; Jiao, Y.; Zhang, Z. Qu, F.; Umar, A.; Wu, X. Hierarchical SnO2 Nanostructures Made of Intermingled Ultrathin Nanosheets for Environmental Remediation, Smart Gas Sensor, and Supercapacitor Applications. ACS Appl. Mater. Interfaces 2014, 6, 2174−2184.

11

Cao, F.; Pan, G.X.; Xia, X.H. Tang, P.S.; Chen, H.F. Synthesis of Hierarchical Porous NiO Nanotube Arrays for Supercapacitor Application. J. Power Sources 2014, 264, 161−167.

12

Wang, K.; Meng, Q. Zhang, Y.; Wei, Z.; Miao, M. High-performance Two-ply Yarn Supercapacitors Based on Carbon Nanotubes and Polyaniline Nanowires Arrays. Adv. Mater. 2013, 25, 1494−1498.

13

Mi, H.; Zhang, X.; Ye, X.; Yang, S. Preparation and Enhanced Capacitance of Core–shell Polypyrrole/polyaniline Composite Electrode for Supercapacitors. J. Power Sources 2008, 176, 403−409.


Page 27 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


14

Xu, J.; Ding, T.; Wang, J.; Zhang, J.; Wang, S.; Chen, C.; Fang, Y.; Wu, Z.; Huo, K.; Dai, J. Tungsten Oxide Nanofibers Self-assembled Mesoscopic Microspheres as High-performance Electrodes for Supercapacitor. Electrochim. Acta 2015, 174, 728−734.

15

Yang, P.; Sun, P.; Du, L.; Liang, Z.; Xie, W.; Cai, X.; Huang, L.; Tan, S.; Mai, W. Quantitative Analysis of Charge Storage Process of Tungsten Oxide that Combines Pseudocapacitive and Electrochromic Properties. J. Phys. Chem. C 2015, 119, 16483−16489.

16

Qiu, M.; Sun, P.; Shen, L.; Wang, K.; Song, S.; Yu, X.; Tan, S.; Zhao, C.; Mai, W. WO3 Nanoflowers with Excellent Pseudo-capacitive Performance and the Capacitance Contribution Analysis. J. Mater. Chem. A 2016, 4, 7266−7273.

17

Chu, J.; Lu, D.; Wang, X.; Wang, X.; Xiong, S. WO3 Nanoflower Coated with Graphene Nanosheet: Synergetic Energy Storage Composite Electrode for Supercapacitor Application. J. Alloys Compd. 2017, 702, 568−572.

18

Prabhu, N.; Agilan, S.; Muthukumarasamy, N.; Senthil, T.S. Enhanced Photovoltaic Performance of WO3 Nanoparticles Added Dye Sensitized Solar Cells. J. Mater. Sci.: Mater. Electron. 2014, 25, 5288−5295.

19

Liu, L.; Guo, S.Q.; Zhang, X.; Sun, M.; Zhao, Y. Designable Fabrication of Hierarchical WO3·H2O Hollow Microspheres for Enhanced Visible Light Photocatalysis. RSC Adv. 2015, 5, 16376−16385.

20

Nayak, A.K.; Ghosh, R.; Santra, S.; Guha, P.K.; Pradhan, D. Hierarchical Nanostructured WO3–SnO2 for Selective Sensing of Volatile Organic Compounds. Nanoscale 2015, 7, 12460−12473.

21

Li, J.; Liu, X.; Cui, J.; Sun, J. Hydrothermal Synthesis of Self-assembled Hierarchical Tungsten Oxides Hollow Spheres and Their Gas Sensing Properties. ACS Appl. Mater. Interfaces 2015, 7, 10108−10114.

22

Xiao, W.; Liu, W.; Mao, X.; Zhu, H.; Wang, D. Na2SO4-assisted Synthesis of Hexagonal-phase WO3 Nanosheet Assemblies with Applicable Electrochromic and Adsorption Properties. J. Mater. Chem. A 2013, 1, 1261–1269.

23

Li, W.J.; Fu, Z.W. Nanostructured WO3 Thin Film as a New Anode Material for Lithium-ion Batteries. Appl. Surf. Sci. 2010, 256, 2447−2452.

24

Ham, D.J.; Phuruangrat, A.; Thongtem, S.; Lee, J.S. Hydrothermal Synthesis of Monoclinic WO3 Nanoplates and Nanorods used as an Electrocatalyst for Hydrogen Evolution Reactions from Water. Chem. Eng. J. 2010, 165, 365−369.

25

Sun, P.; Deng, Z.; Yang, P.; Yu, X.; Chen, Y.; Liang, Z.; Meng, H.; Xie, W.; Tan, S.; Mai, W. Freestanding CNT–WO3 Hybrid Electrodes for Flexible Asymmetric Supercapacitors. J. Mater. Chem. A 2015, 3, 12076−12080.

26

Gao, L.; Wang, X.; Xie, Z.; Song, W.; Wang, L.; Wu, X.; Qu, F.; Chen, D.; Shen, G. High-performance Energy-storage Devices Based on WO3 Nanowires Arrays/Carbon Cloth Integrated Electrodes. J. Mater. Chem. A 2013, 1, 7167−7173.

27

Taha, A.A.; Li, F. Porous WO3–carbon Nanofibers: High-performance and Recyclable Visible Light Photocatalysis. Catal. Sci. Technol. 2014, 4, 3601−3605.

28

Wang, F.; Zhan, X.; Cheng, Z.; Wang, Z.; Wang, Q.; Xu, K.; Safdar, M.; He, J. Tungsten Oxide@polypyrrole Core–shell Nanowires Arrays as Novel Negative Electrodes for Asymmetric Supercapacitors. Small 2015, 11, 749–755.

29

Zhou, M.; Yan, J.; Cui, P. Synthesis and Enhanced Photocatalytic Performance of WO3 Nanorods@ graphene Nanocomposites. Mater. Lett. 2012, 89, 258−261. 27 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

30

An, X.; Jimmy, C.Y.; Wang, Y.; Hu, Y.; Yu, X.; Zhang, G. WO3 Nanorods/graphene Nanocomposites for High-efficiency Visible-light-driven Photocatalysis and NO2 Gas Sensing. J. Mater. Chem. 2012, 22, 8525−8531.

31

Khan, M.E.; Khan, M.M.; Cho, M.H. Fabrication of WO3 Nanorods on Graphene Nanosheets for Improved Visible Light-induced Photocapacitive and Photocatalytic Performance. RSC Adv. 2016, 6, 20824−20833.

32

Kim, D.M.; Kim, S.J.; Lee, Y.W.; Kwak, D.H.; Park, H.C.; Kim, M.C.; Hwang, B.M.; Lee, S.; Choi, J.H.; Hong, S.; Park, K.W. Two-dimensional Nanocomposites Based on Tungsten Oxide Nanoplates and Graphene Nanosheets for High-performance Lithium Ion Batteries. Electrochim. Acta, 2015, 163, 132−139.

33

Cai, Y.; Wang, Y.; Deng, S.; Chen, G.; Li, Q.; Han, B.; Han, R.; Wang, Y. Graphene NanosheetsTungsten Oxides Composite for Supercapacitor Electrode. Ceram. Int. 2014, 40, 4109−4116.

34

Ma, L.; Zhou, X.; Xu, L.; Xu, X.; Zhang, L.; Ye, C.; Luo, J.; Chen, W. Hydrothermal Preparation and Supercapacitive Performance of Flower-like WO3·H2O/Reduced Graphene Oxide Composite. Colloids Surf., A 2015, 481, 609−615.

35

Kalanur, S. S.; Hwang, Y. J.; Chae, S. Y.; Joo, O. S. Facile Growth of Aligned WO3 Nanorods on FTO Substrate for Enhanced Photoanodic Water Oxidation Activity. J. Mater. Chem. A 2013, 1, 3479−3488.

36

Zhu, J.; Wang, S.; Xie, S.; Li, H. Hexagonal Single Crystal Growth of WO3 Nanorods Along a [110] Axis with Enhanced Adsorption Capacity. Chem. Commun. 2011, 47, 4403–4405.

37

Li, N.; Teng, H.; Zhang, L.; Zhou, J. Liu, M. Synthesis of Mo-doped WO3 Nanosheets with Enhanced Visible-light-driven Photocatalytic Properties. RSC Adv. 2015, 5, 95394−95400.

38

Xie, Y. P.; Liu, G.; Yin, L.; Cheng, H.-M.Crystal Facet-dependent Photocatalytic Oxidation and Reduction Reactivity of Monoclinic WO3 for Solar Energy Conversion. J. Mater. Chem. 2012, 22, 6746−6751.

39

Das, A. K.; Srivastav, M.; Layek, R. K.; Uddin, M. E.; Jung, D.; Kim, N. H.; Lee, J. H. IodideMediated Room Temperature Reduction of Graphene Oxide: A Rapid Chemical Route for The Synthesis of a Bifunctional Electrocatalyst. J. Mater. Chem. A 2014, 2, 1332−1340.

40

Wen, C.; Li, X.; Sun, D.Y.; Guan, J.Q.; Liu, X.X.; Lin, Y.R.; Tang, S.Y.; Zhou, G.; Lin, J.D.; Jin, Z.H. Raman Spectrum of Nano-graphite Synthesized by Explosive Detonation. Spectrosc. Spect. Anal. 2005, 25, 54−57.

41

Graf, D.; Molitor, F.; Ensslin, K.; Stampfer, C.; Jungen, A.; Hierold, C.; Wirtz, L. Spatially Resolved Raman Spectroscopy of Single-and Few-layer Graphene. Nano Lett. 2007, 7, 238−242.

42

Fu, C.; Foo, C.; Lee, P. S. One-Step Facile Electrochemical Preparation of WO3/Graphene Nanocomposites with Improved Electrochromic Properties. Electrochim. Acta 2014, 117, 139−144.

43

Van Hieu, N.; Van Quang, V.; Hoa, N.D.; Kim, D. Preparing Large-scale WO3 Nanowires-like Structure for High Sensitivity NH3 Gas Sensor Through a Simple Route. Curr. Appl. Phys. 2011, 11, 657−661.

44

Zhang, N.; Chen, C.; Mei, Z.; Liu, X.; Qu, X.; Li, Y.; Li, S.; Qi, W.; Zhang, Y.; Ye, J.; Roy, V. A. L.; Ma, R. Monoclinic Tungsten Oxide with {100} Facet Orientation and Tuned Electronic Band Structure for Enhanced Photocatalytic Oxidations. ACS Appl. Mater. Interfaces 2016, 8, 10367−10374.


Page 29 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


45

Zhang, L.; Zhang, F.; Yang, X.; Long, G.; Wu, Y.; Zhang, T.; Leng, K.; Huang, Y.; Ma, Y.; Yu, A.; Chen, Y. Porous 3D Graphene-Based Bulk Materials with Exceptional High Surface Area and Excellent Conductivity for Supercapacitors. Sci. Rep. 2013, 3, Article No. 1408.

46

Williams, G.; Seger, B.; Kamat, P. V. TiO2-Graphene Nanocomposites. UV-Assisted Photocatalytic Reduction of Graphene Oxide. ACS Nano 2008, 2, 1487–1491.

47

Hao, L.; Song, H.; Zhang, L.; Wan, X.; Tang, Y.; Lv, Y. SiO2/graphene Composite for Highly Selective Adsorption of Pb(II) Ion. J. Colloid Interface Sci. 2012, 369, 381–387.

48

Yan, J.; Liu, J.; Fan, Z.; Wei, T.; Zhang, L. High-performance Supercapacitor Electrodes Based on Highly Corrugated Graphene Sheets. Carbon 2012, 50, 2179−2188.

49

Klejnot, O. J. Chloride Alkoxides of Pentavalent Tungsten. Inrog. Chem. 1965, 4, 1668−1670.

50

Ham, D. J.; Phuruangrata, A.; Thongtem, S.; Lee, J. S. Hydrothermal Synthesis of Monoclinic WO3 Nanoplates and Nanorods used as an Electrocatalyst for Hydrogen Evolution Reactions from Water. Chem. Eng. J. 2010, 165, 365–369.

51

Zhang, H.; Yang, J.; Li, D.; Guo, W.; Qin, Q.; Zhu, L.; Zheng, W. Template-free Facile Preparation of Monoclinic WO3 Nanoplates and Their High Photocatalytic Activities. Appl. Surf. Sci. 2014, 305, 274– 280.

52

Qi, T.; Jiang, J.; Chen, H.; Wan, H.; Miao, L.; Zhang, L. Synergistic Effect of Fe3O4/Reduced Graphene Oxide Nanocomposites for Supercapacitors with Good Cycling Life. Electrochim. Acta 2013, 114, 674−680.

53

Darmawi, S.; Burkhardt, S.; Leichtweiss, T.; Weber, D. A.; Wenzel, S.; Janek, J.; Elm, M.T.; Klar, P.J. Correlation of Electrochromic Properties and Oxidation States in Nanocrystalline Tungsten Trioxide. Phys.Chem.Chem.Phys. 2015, 17, 15903-15911.

54

Jo, C.; Hwang, I.; Lee, J.; Lee, C. W.; Yoon, S. Investigation of Pseudocapacitive Charge-Storage Behavior in Highly Conductive Ordered Mesoporous Tungsten Oxide Electrodes. J. Phys. Chem. C 2011, 115, 11880-11886.

55

Li, G.R.; Feng, Z.P.; Zhong, J.H., Wang, Z.L.; Tong, Y.X. Electrochemical Synthesis of Polyaniline Nanobelts with Predominant Electrochemical Performances. Macromolecules 2010, 43, 2178−2183.

56

Yoon, S.; Kang, E.; Kim, J.K.; Lee, C.W.; Lee, J. Development of High-performance Supercapacitor Electrodes using Novel Ordered Mesoporous Tungsten Oxide Materials with High Electrical Conductivity. Chem. Commun. 2011, 47, 1021−1023.

57

Wu, R.; Wang, D.P.; Kumar, V.; Zhou, K.; Law, A.W.; Lee, P.S.; Lou, J.; Chen, Z. MOFs-derived Copper Sulfides Embedded within Porous Carbon Octahedra for Electrochemical Capacitor Applications. Chem. Commun. 2015, 51, 3109−3112.

58

Zhu, M.; Meng, W.; Huang, Y.; Huang, Y.; Zhi, C. Proton-insertion-enhanced Pseudocapacitance Based on the Assembly Structure of Tungsten Oxide. ACS Appl. Mater. Interfaces 2014, 6, 18901−18910.

59

Jo, C.; Hwang, J.; Song, H.; Dao, A.H.; Kim, Y.T.; Lee, S.H.; Hong, S.W.; Yoon, S.; Lee, J. Blockcopolymer-assisted One-pot Synthesis of Ordered Mesoporous WO3–x/Carbon Nanocomposites as High-rate-performance Electrodes for Pseudocapacitors. Adv. Funct. Mater. 2013, 23, 3747– 3754.

60

Xiao, X.; Liu, X.; Zhao, H.; Chen, D.; Liu, F.; Xiang, J.; Hu, Z.; Li, Y. Facile Shape Control of Co3O4 and the Effect of the Crystal Plane on Electrochemical Performance. Adv. Mater. 2012, 24, 5762−5766.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 31

61

Paek, S.M.; Yoo, E.; Honma, I. Enhanced Cyclic Performance and Lithium Storage Capacity of SnO2/Graphene Nanoporous Electrodes with Three-dimensionally Delaminated Flexible Structure. Nano Lett. 2008, 9, 72−75.

62

Lei, Z.; Shi, F.; Lu, L. Incorporation of MnO2-coated Carbon Nanotubes Between Graphene Sheets as Supercapacitor Electrode. ACS Appl. Mater. Interfaces 2012, 4, 1058−1064.

63

Taherian, F.; Marcon, V.; van der Vegt, N. F. A. What Is the Contact Angle of Water on Graphene? Langmuir 2013, 29, 1457–1465.

64

Miyauchi, M.; Shibuya, M.; Zhao, Z.-G.; Liu, Z. Surface Wetting Behavior of a WO3 Electrode under Light-Irradiated or Potential-Controlled Conditions. J. Phys. Chem. C 2009, 113, 10642–10646.

65

Zaid, N. A. M.; Idris, N. H. Enhanced Capacitance of Hybrid Layered Graphene/Nickel Nanocomposite for Supercapacitors. Sci. Rep. 2016, 6, 32082.

66

Shahnavaz, Z.; Woi, P. M.; Alias, Y. Electrochemical Sensing of Glucose by Reduced Graphene Oxide-Zinc Ferrospinels. App. Surf. Sci. 2016, 379, 156−162.

67

Wang, G.; Xu, H.; Lu, L.; Zhao, H. One-step Synthesis of Mesoporous MnO2/carbon Sphere Composites for Asymmetric Electrochemical Capacitors. J. Mater. Chem. A 2015, 3, 1127–1132.

68

Wu, Y.; Gao, G.; Wu, G. Self-assembled Three-dimensional Hierarchical Porous V2O5/Graphene Hybrid Aerogels for Supercapacitors with High Energy Density and Long Cycle Life. J. Mater. Chem. A 2015, 3, 1828–1832.

69

Sun, K.; Peng, H.; Mu, J.; Ma, G.; Zhao, G.; Lei, Z. High Energy Density Asymmetric Supercapacitors Based on Polyaniline Nanotubes and Tungsten Trioxide Rods. Ionics 2015, 21, 2309−2317.

70

El-Kady, M.F.; Kaner, R.B. Scalable Fabrication of High-power Graphene Micro-supercapacitors for Flexible and On-chip Energy Storage. Nat. Commun. 2013, 4, 1475.

71

Fan, Z.; Yan, J.; Wei, T.; Zhi, L.; Ning, G.; Li, T.; Wei, F. Asymmetric Supercapacitors Based on Graphene/MnO2 and Activated Carbon Nanofiber Electrodes with High Power and Energy Density. Adv. Funct. Mater. 2011, 21, 2366−2375.

72

Lei, Z.; Zhang, J.; Zhao, X.S. Ultrathin MnO2 Nanofibers Grown on Graphitic Carbon Spheres as Highperformance Asymmetric Supercapacitor Electrodes. J. Mater. Chem. 2012, 22, 153−160.

73

Tang, Y.; Liu, Y.; Yu, S.; Mu, S.; Xiao, S.; Zhao, Y.; Gao, F. Morphology Controlled Synthesis of Monodisperse Cobalt Hydroxide for Supercapacitor with High Performance and Long Cycle Life. J. Power Sources 2014, 256, 160−169.

74

Huang, M.; Zhang, Y.; Li, F.; Zhang, L.; Wen, Z.; Liu, Q. Facile Synthesis of Hierarchical Co3 O4@ MnO2 Core–shell Arrays on Ni Foam for Asymmetric Supercapacitors. J. Power Sources 2014, 252, 98−106.

75

Peng, H.; Ma, G.; Sun, K.; Mu, J.; Luo, M.; Lei, Z. High-performance Aqueous Asymmetric Supercapacitor Based on Carbon Nanofibers Network and Tungsten Trioxide Nanorod Bundles Electrodes. Electrochim. Acta 2014, 147, 54−61.


Page 31 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Content Use Only

A greener process to synthesize WO3 nanowires using ethanol only and its application in solid-state asymmetric supercapacitor device is demonstrated.


High Performance Solid-State Asymmetric Supercapacitor using

Recommend Documents