CoWO4 nanocomposites with enhanced electrochemical performances for supercapacitors

Accepted Manuscript Title: Facile synthesis of reduced graphene oxide/CoWO4 nanocomposites with enhanced electrochemical performances for supercapacitors Author: Xiaowei Xu Jianfeng Shen Na Li Mingxin Ye PII: DOI: Reference:

S0013-4686(14)02160-4 http://dx.doi.org/doi:10.1016/j.electacta.2014.10.139 EA 23655

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

13-8-2014 22-10-2014 27-10-2014

Please cite this article as: Xiaowei Xu, Jianfeng Shen, Na Li, Mingxin Ye, Facile synthesis of reduced graphene oxide/CoWO4 nanocomposites with enhanced electrochemical performances for supercapacitors, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2014.10.139 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Facile synthesis of reduced graphene oxide/CoWO4 nanocomposites with enhanced electrochemical performances for supercapacitors Xiaowei Xu, Jianfeng Shen, Na Li and Mingxin Ye* Center of Special Materials and Technology, Fudan University, Shanghai 200433, Highlights ► China Highlights ► • RGO/CoWO4 composites were successfully prepared through a facile hydrothermal method. ► • RGO/CoWO4 composites show much higher specific capacitances than pure CoWO4. ► • Enhanced electrical conductivity leads to superior electrochemical performance.

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Abstract

A facile one-pot hydrothermal method was provided for synthesis of the reduced

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graphene oxide-cobalt tungstate (RGO/CoWO4) nanocomposites with the enhanced

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electrochemical performances for supercapacitors for the first time. The resulting

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nanocomposites are comprised of CoWO4 nanospheres that are well-anchored on

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graphene sheets by in situ reducing. The prepared RGO/CoWO4 nanocomposites have been thoroughly characterized by Fourier-transform infrared spectroscopy, X-ray

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diffraction, Raman spectroscopy, Thermogravimetric analysis, Scanning electron

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microscopy, Transmission electron microscopy, X-ray photoelectron spectroscopy

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and N2 adsorption-desorption. Importantly, the prepared nanocomposites exhibit superior electrochemical performance to CoWO4 as electrodes for supercapacitors. As

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a result, RGO/CoWO4 nanocomposites with 91.6 wt% CoWO4 content achieved a specific capacitance about 159.9 F g-1 calculated from the CV curves at 5 mV s-1,

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which was higher than that of CoWO4 (60.6 F g-1). The good electrochemical

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performance can be attributed to the increased electrical conductivity and the creation of new active sites due to the synergetic effect of RGO and CoWO4 nanospheres. The cyclic stability tests demonstrated capacitance retention of about 94.7% after 1000 cycles, suggesting the potential application of RGO/CoWO4 nanocomposites in energy-storage devices.

Key

words:

Graphene;

Cobalt

tungstate;

Electrochemical

performances;

Nanocomposite.

* Corresponding author. Tel.: +86 021 55664095; fax: +86 021 55664094 E-mail address: [email protected]

1. Introduction Nowadays, the imminent shortage of fossil fuels and growing environmental concerns have triggered tremendous research efforts for energy storage and

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conversion from sustainable and renewable clean energy sources [1-3]. As one of the most important electrochemical energy storage systems, supercapacitors (SCs), also

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called electrochemical capacitors (ECs), have attracted worldwide attention in the

SC

field of electronic devices, electric vehicles and hybrid electric vehicles due to their high power density, long lifespan, and fast charge/discharge process [4-6]. In general,

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according to the charge conversion/storage mechanisms, SCs can be divided into two

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main types: (i) electrical double-layer capacitors (EDLCs) and (ii) Faradaic

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pseudocapacitors [7]. In EDLCs, the capacitance is generated from the electrostatic

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charges which are adsorbed at the interface between the electrode materials and the electrolyte solution [8]. Carbon-based materials, regarded as typical EDLCs, have

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been extensively studied [9, 10]. Because there is no electrochemical reaction

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between the electrode of carbon and electrolyte, the capacitance of EDLCs depends

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on the available surface area, pore structure, connectivity and electrical conductivity [11]. In contrast, pseudocapacitors generate capacitance from the fast and reversible

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redox reaction between electrolyte and electrode materials on the electrode surface [12]. During the reversal processes of charging and discharging, the faradaic current

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not only increases the specific capacitance but also extends the operation voltage of the supercapacitor [13]. Thus, transition metal (Ru, Mn, Ni, Fe and Co) oxides and hydroxides with pseudocapacitive behavior are of extensive interest for the application as electrode materials in pseudocapacotors [14-23]. Although in recent years, a lot of progress has been made with the development of both types of supercapacitors, the main challenges are still the low energy density and poor overall

performance, which have to be overcome in order to widen the applications of supercapacitors. In order to achieve the enhanced energy density, an effective approach is to increase the capacitance and /or broad the operation voltage, since the energy density can be calculated by the equation of E = 0.5CV2, where E is the energy density, C is the specific capacitance, and V is the cell voltage, respectively [24, 25]. Therefore, the development of novel electrode materials with excellent physical and electrochemical performances is the key factor in resulting in high energy density for supercapacitors.

In

particular,

the

effective

combination

of

EDLCs

and

to the high energy density and better overall performance [26].

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pseudocapacitors can generate synergistic effect, which can be realized to contribute

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As is well known, graphene with its one-atom thick layer two-dimensional

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nanostructure, is newly found and has been widely investigated for its unique features, such as good chemical stability, excellent electric conductivity, and high surface area

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[27]. These encouraging features make such new material the most ideal electrode

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material for EDLCs among carbon-based materials, such as active carbon [28], carbon

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nanotubes [29], carbon film [30], and mesoporous carbon [31]. However, graphene

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usually suffers from restacking and agglomeration which result in great loss of effective surface area and exhibit poorer electrochemical performance than as

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expected. Therefore, it is important to improve the two-dimensional nanostructure and

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surface modification of graphene in order to enlarge the specific surface area and

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electrochemical performance. To overcome this issue, metal oxides have been added into graphene to prepare graphene/metal oxide nanocomposites [32-34]. These metal

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oxides uniformly distributed on graphene sheets can prevent the restacking and agglomeration of graphene. Recently, much work has been done on hybrid binary

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metal oxides such as NiMoO4 [35], CoMoO4 [36], ZnWO4 [37], and NiWO4 [38] because of their feasible oxidation states and high electrical conductivity. At present, it is still a research hot spot to design and synthesis novel binary transition metal oxides with enhanced capacitive behavior. As reported in some literatures, CoWO4, one of the most important compounds, has excellent catalytic and electrochemical characteristics [39, 40]. In addition, it also

offers many advantages such as low cost, abundant resources and environmental friendliness. Thus, CoWO4 is predicted to be a very promising electrode material for SCs. However, to the best of our knowledge, compared with other binary metal oxides, CoWO4 as candidate electrode material for pseudocapacitor has never been studied, especially the electrochemical properties of CoWO4 combined with graphene. So reduced

graphene

oxide-CoWO4

(RGO/CoWO4)

nanocomposites

could

be

synthesized to explore their potential as a supercapacitor. In the present work, we report for the first time a simple one-pot hydrothermal to

synthesize

RGO/CoWO4

nanocomposites

and

investigate

their

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method

electrochemical performance as electrode materials for SCs. This method avoids the

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use of any seeds, hazardous catalysts, harmful surfactants or templates, and thus can

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be considered as a promising method for the large scale and low-cost production with high-quality crystals. The RGO/CoWO4 nanocomposites showed higher specific

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capacitance and better cycling stability and rate capability than CoWO4. It is believed

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that the RGO/CoWO4 nanocomposites could serve as a promising candidate for

2. Experimental

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2.1 Materials

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environmental friendliness.

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supercapacitor materials because of the high capacity and low-cost as well as

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Pristine graphite was purchased from Qingdao BCSM. CO., Ltd. Cobalt chloride (CoCl2·6H2O) and Sodium tungstate (Na2WO4·2H2O) were supplied by Sinopharm

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Chemical Reagent Co., Ltd. All other reagents were at least of analytical reagent

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grade and used without further purification. 2.2 Preparation of RGO/CoWO4 nanocomposites

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The RGO/CoWO4 nanocomposites were prepared via one-pot hydrothermal

method with different ratios of graphene oxide (GO) and CoWO4. The different ratios were controlled by changing the amount of GO and keeping the constant amount of CoWO4. The product was named as RGO/CoWO4-x, where x is the amount of GO. For example, the composite RGO/CoWO4-80 was typically synthesized as follows: graphite oxide was obtained by the modified Hummers method as described

elsewhere [41]. 80 mg graphite oxide was exfoliated to GO in 40 mL deionized water by ultrasonication for 1 h. 2 mmol (10 mL) solution of CoCl2 was added into the GO dispersion under magnetic stirring. After stirring for about 1 h, 2 mmol (10 mL) solution of Na2WO4 was added directly into the above solution and stirred for another 1 h at room temperature. Subsequently, the suspension was transferred into a Teflon-lined stainless autoclave. The autoclave was sealed and maintained at 180 ºC for 12 h, then allowed to cool to room temperature. When the reduction reaction was finished, the as-prepared product was isolated by centrifugation, washed with

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deionized water and ethanol for several times, and dried at 80 ºC for 12 h. For comparison, CoWO4 nanospheres were prepared using the same hydrothermal method

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without the adding of GO.

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Insert Scheme 1 2.3 Characterization of sample

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Fourier transform infrared spectra (FTIR) were recorded on a Nicolet IS10

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spectrometer. Solid samples were imbedded in KBr disks. The spectrum was

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generated, collected 16 times, and corrected for the background noise. Powder X-ray

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diffraction (XRD) analyses were performed on D/max-γB diffractometer using Cu Kα radiation. Raman spectra were recorded on a Dilor LABRAM-1B multi-channel

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confocal microspectrometer with 514 nm laser excitation. Thermo-gravimetric

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analysis (TGA) was conducted in air atmosphere with a heating rate of 10 ºC/min

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using a Netzsch TG 209F1. Before the tests, all the samples were carefully grinded to powders to ensure sufficient diffusion of heat. X-ray photoelectron spectra (XPS)

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were recorded on XR5VG spectrometer. Peak deconvolution was performed using Gaussian components after a Shirley background subtraction. Scanning electron

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microscopy (SEM) was performed with a Philips XL30 FEG FE-SEM instrument at an accelerating voltage of 25 kV. Sample was sputter-coated with gold to improve the contrast. Transmission electron microscopy (TEM) was performed with a JEOL JEM-2100F. The specific surface area and pore size distribution were obtained from the nitrogen adsorption-desorption isotherm (Micromeritics, Tristar3000 porosimeter). 2.4 Electrochemical measurements

The working electrodes were prepared by thoroughly mixing the as-prepared composites, acetylene black, and poly(vinylidenefluoride) (PVDF) with the mass ratio 80 : 10 : 10, and were dispersed in N-methylpyrrolidone (NMP). The mixture was stirred adequately to form a homogeneous slurry, and then was coated and pressed onto nickel foam and dried under vacuum at 80 °C for 24 h. The loading mass of each working electrode was about 3 - 4 mg, and each working electrode had a geometric surface area of about 1 cm2. In a three-electrode system, 2 M KOH was used as the electrolyte, platinum foil and Ag/AgCl (KCl-saturated) electrodes were used as the and

reference

electrodes,

respectively.

Cyclic

voltammetry (CV),

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counter

electrochemical impedance spectroscopy (EIS) measurements, and galvanostatic

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charge-discharge testing were performed by an electrochemical workstation (Autolab

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PGSTAT128N). The scan rates of CV were in the range from 5 mV s-1 to 100 mV s-1 at the potential rang of -0.25 to 0.45 V. EIS was recorded under the following

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conditions: AC voltage amplitude of 5 mV, frequency range of 1×105 to 0.1 Hz, and

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open circuit potential.

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3. Results and discussion

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The synthesis of RGO/CoWO4 nanocomposites is summarized in Scheme 1. Considering the outstanding properties of graphene and binary metal oxides, we

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speculate that the combination of these two materials via a one-pot synthesis method

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opposing to simple physical mixing or other complicated methods will lead to a

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strong synergetic effect with contributions from both of their unique properties. During the preparation of RGO/CoWO4 nanocomposites, Co2+ cations were firstly

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adsorbed on the negatively-charged graphene oxide sheets via simple electrostatic interaction, followed by adding WO42- anions, then the reduction of GO and the

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crystallization of CoWO4 crystals happened in a one-pot process by the hydrothermal method, as shown in the following Eqs. (1)-(3): After dissolution of the salts at room temperature: (1)

(2) After hydrothermal processing:

(3) Insert Fig. 1

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The FTIR spectroscopy was used to examine the chemical bonds and chemical composition of the as-prepared samples. The change of the oxygen-containing

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functional groups from GO to RGO can be reflected in FTIR spectroscopy, as shown

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in Fig. 1. The characteristic IR features of GO indicate the presence of oxygen-containing functional groups on its surface (Fig. 1a).The characteristic bands

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at 1075 cm-1, 1225 cm-1, 1400 cm-1, 1625 cm-1 and 1725 cm-1 correspond to the epoxy

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C-O-C stretching vibrations, the alkoxy C-O stretching peak, the O-H deformation of

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the C-O group, the C-C stretching mode, and the carboxyl C=O and C-O stretching

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vibrations, respectively. The broad and intense peak at 3200 cm-1 is according to the -OH vibration stretching. As to RGO (Fig. 1b), after GO was hydrothermally reduced,

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the intensities of all absorption peaks corresponding to oxygen-containing functional

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groups in RGO decreased significantly, especially the peak at 3200 cm-1 according to

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the -OH vibration. This result indicates that the majority of the oxygen-containing functional groups in GO have been successfully removed by the hydrothermal process,

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which is consistent with the XRD result. Furthermore, a new absorption peak located at 1612 cm-1 corresponding to the aromatic skeletal of C=C stretching vibration is

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observed, which further instructs the reduction of GO [21]. Fig. 1c shows the typical CoWO4 absorption features. Intense peaks appeared in the low frequency region of 400-1000cm-1 which belongs to the characteristic deformation modes of Co-O, W-O and W-O-W bridges. It can be found that two main bands (at 822 cm-1 and 656 cm-1), which are associated to the O-W-O vibration mode and the W-O bond stretching, respectively [42]. As to RGO/CoWO4 samples (Fig. 1d-h), the IR spectra show small

differences. Some peaks of oxygen-containing functional groups such as epoxy still remains after reduction, which is beneficial to improve the dispersion of the composite and the stability of the CoWO4 anchored onto the surface of RGO sheets. Besides, there still exhibit the typical absorption features of CoWO4. This clearly confirms that CoWO4 particles are strongly attached to the surface of RGO. Insert Fig. 2 To further investigate the formation of CoWO4, the typical power XRD patterns of GO, CoWO4 and RGO/CoWO4 nanocomposites are given in Fig. 2. The XRD

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pattern of GO reveals that the most intensive peak of GO appears at 2θ = 10.5° corresponds to the reflections of the (002) plane (interlayer d-spacing = 0.8 nm).

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Because of the introduction of oxygen-containing functional groups, the interlayer

SC

spacing is much wider than that of pristine graphite [43]. As for CoWO4 and RGO/CoWO4 nanocomposites prepared with different experimental conditions, all the

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diffraction patterns are similar, as shown in Fig. 2. The sharp diffraction peak at 30.5°

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corresponds to the reflections of the (-111) plane. The other diffraction peaks at 15.5°,

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18.9°, 23.6°, 24.5°, 36.2°, 38.4°, 41.2°, 44.0°, 45.8°, 48.4°, 50.5°, 51.8°, 53.9°, 61.4°,

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and 64.5° can be assigned to the reflections of the (010), (001), (-110), (011), (200), (002), (-201), (-211), (-112), (-220), (022), (031), (-202), (-311), and (-113), planes,

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respectively. The observed diffraction peaks are in good agreement with the standard

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patterns for monoclinic structure phase of CoWO4 (according to the JCPDC card no.

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15-0867). Moreover, there were no peaks due to impurities or other residuals, revealing the high purity of the prepared CoWO4 nanostructures. However, the

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diffraction peaks of RGO did not appear. It is not surprising that the characteristic diffraction peaks of graphene were shielded by the strong diffraction peaks of

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CoWO4.

Insert Fig. 3 Raman spectroscopy, regarded as a powerful, nondestructive technique, has been

extensively used for examining the ordered and disordered crystal structures of the samples. The Raman spectra of GO, RGO, CoWO4 and RGO/CoWO4 composites are shown in Fig. 3. From the Raman spectra, we clearly observed that all of the

graphene-containing samples have three main features. The D band peak, which is attributed to local defects and disorder especially at the edges of graphene and graphite platelets, usually centered at 1350 cm-1, the G band peak, which is related to the first-order scattering of the E2g phonons of the graphitic structure of carbon atoms, is usually observed at 1575 cm-1, and the 2D band peak appeared at 2700 cm-1, which is also a Raman signature of graphitic sp2-bonded carbon atoms and is a second order vibration caused by the scattering of phonons at the zone boundary [44]. Generally, the ratio of the relative intensities of D and G band (ID/IG) in the Raman spectra is

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usually considered as a signal to evaluate the disorder degree in a carbon material. As shown in Fig. 3, the spectrum of GO shows broadened D-band and G-band. When GO

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was chemically converted to RGO, the ratio of ID/IG clearly increased from 1.43 for

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GO to 1.74 for RGO. This change suggests that the reduction caused an increase in the number of defects by the removal of oxygen-containing functional groups, which

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resulted in the increase of D band. The Raman spectrum of CoWO4 exhibits an

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intense peak at 879 cm-1 along with some medium intensity peaks appearing at 767

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cm-1, 689 cm-1, 528 cm-1，406 cm-1，333 cm-1，273 cm-1 and 198 cm-1, which can be

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attributed to the characteristic peaks of CoWO4 [45]. In the spectra of RGO/CoWO4 nanocomposites, the characteristic peaks of CoWO4 and RGO still exist, indicating

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the successful synthesis of their composites. In addition, the ratios of ID/IG for

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RGO/CoWO4 composites are 1.74, 1.72, 1.71, 1.73, and 1.73, respectively.

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Furthermore, the broad 2D band in RGO and RGO/CoWO4 nanocomposites indicates a few layers of graphene. Interestingly, in the RGO/CoWO4 nanocomposites, all the

CC

peaks of CoWO4 appear slightly blue-shifted, indicating that graphene act not as a basal plane for the crystalline growth of CoWO4, but instead there is some sort of

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chemical/electrostatic interaction between the CoWO4 and graphene. This interaction is usually considered as the synergistic, which plays an important role in the electrochemical behavior. Insert Fig. 4 The chemical composition and the weight percentage of CoWO4 in the RGO/CoWO4 composites were also determined by TGA. Fig. 4 shows the TGA

curves of the RGO and RGO/CoWO4 composites. As shown in Fig. 4a, there is a weight loss of 3.58% below 100 °C, which is owing to the removal of surface absorbed water, and a weight loss of 94.5% from the onset temperature of 571 °C to the end temperature of 609 °C owing to the burning of RGO in air (which is consistent with the previous result [21]). As shown in Fig. 4b, CoWO4 shows a relatively good thermal stability, thus there is not a weight loss for CoWO4. It can be seen that after CoWO4 nanoparticles were bonded onto RGO, the weight loss greatly decreased. From the TGA curve of RGO/CoWO4-40 (Fig. 4c), an obviously weight

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loss of 5.64% from the temperature of 429 °C - 580 °C is owing to the burning of the graphene in the composites. We can find that the onset oxidation temperature of RGO

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in the RGO/CoWO4-40 composites is 429 °C, much lower than that of the RGO (571

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°C), which indicates that the antioxidation of the RGO in the RGO/CoWO4-40 composites becomes worse after hydrothermal treatment. The reason maybe that the

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presence of cobalt tungstate nanoparticles damage the surface structure of graphene,

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and the junctions of cobalt tungstate nanoparticles and graphene are oxidized firstly

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during the heating process. From the TGA curves of RGO/CoWO4 composites,

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considering the weight loss of RGO, the weight percentage of CoWO4 nanoparticles of RGO/CoWO4-40 to RGO/CoWO4-100 nanocomposites are about 93.36%, 91.86%,

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Insert Fig. 5

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91.79%, 91.62 % and 90.27%, respectively.

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XPS measurements were performed to further investigate the chemical bonding state and composition of the as-synthesized nanocomposites, and the spectra of

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RGO/CoWO4-80 are shown in Fig. 5. As expected, a wide range survey spectrum of RGO/CoWO4-80 (Fig. 5a) displayed four peaks situated at 284.6 eV, 531.2 eV, 783.2

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eV and 36.0 eV, corresponding to C 1s, O 1s, Co 2p and W 4f levels, respectively. According to the semi-quantitative analysis of XPS, the C/O atomic ratio of GO was about 2.3 [46]. In the as-prepared RGO/CoWO4-80 composite, the C/O atomic ratio was found to be ~ 1.9, including the content of oxygen from CoWO4. This observation further indicates that most of the oxygen-containing functional groups from GO were reduced during the hydrothermal process, whereas a few of the

residual functional groups provide a stable dispersion of the nanoparticles in the RGO/CoWO4 composites [47]. Fig. 5b shows the C 1s core level spectrum of the as-prepared RGO/CoWO4-80 composite, which can be curve-fitted into three peaks with binding energies of 284.2 eV, 286.8 eV and 289.2 eV, corresponding to the sp2 hybridized carbon in graphene, C-OH and O=C-OH functional groups, respectively [48]. The O 1s core level spectrum (Fig. 5c) contains two peaks at binding energies of 531.0 eV and 532.9 eV, representing the O 1s level in CoWO4. The peak at 532.5 eV is likely attributed to the surface O-H group [49] and the water molecule attached to

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the CoWO4. From the high resolution Co 2p spectrum shown in Fig. 6d, the Co 2p3/2 peak at 783.2 eV implies the presence of Co ions in the form divalent state [50].

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Moreover, the W 4f7/2 peak appearing at 35.2 eV (Fig. 6e) and combining with the Co

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2p3/2 peak at 783.2 eV together suggests the formation of CoWO4 binary metal oxide [50]. In addition, element analysis by XPS reveals that the atomic ratio of Co to W is

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close to 1:1, confirming the successful synthesis of CoWO4.

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Insert Fig. 6

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The surface morphologies and microstructure characters of GO, RGO, CoWO4

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and RGO/CoWO4-80 are imaged by SEM, as shown in Fig. 6. From Fig. 6a, it can be seen that GO shows flat and layered structure, with some wrinkles on the surface and

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edge. Compared with the SEM image of GO, RGO is crumpled to a curly and wavy

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shape, and some of the RGO sheets are stacked with each other (Fig. 6b). It can be

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clearly seen from Fig. 6c that the CoWO4 is composed of nanosphere-like architecture, and each sphere is randomly assembled with other individuals. After graphene is

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incorporated in CoWO4 matrix, RGO/CoWO4 composites display an interesting and distinctive morphology. Fig. 6d and e show SEM images of the RGO/CoWO4-80

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composites at various magnifications, revealing a random distribution of the CoWO4 nanosphere on the graphene surface. Interestingly, there are no obvious differences in the morphology of CoWO4, indicating that the introduction of graphene oxide sheets is effective. Besides, the energy dispersive spectroscopy (EDS) analysis (Fig. 6f) also demonstrates the presence of C, O, Co, and W elements, which is in good accordance with the XPS results.

Insert Fig. 7 To further study the detailed microstructure of RGO, CoWO4 and RGO/CoWO4-80, the TEM images are shown in Fig. 7. A typical TEM image of RGO (Fig. 7a) illustrates that RGO is winked transparent flakes. The TEM image of CoWO4 (Fig. 7b) suggests its nanosphere like morphology, and the corresponding SAED pattern (inset of Fig. 7b) shows a number of random and continuous bright

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spots, revealing the formation of nanocrystals. After the hydrothermal reaction, a

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large number of nanospheres were formed on the RGO sheets surface, as is evidenced

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by the low magnification TEM image of RGO/CoWO4-80 shown in Fig. 7c. In addition, the low contrast between graphene sheet and carbon of carbon-copper grids

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reveals that an ultrathin thickness of the graphene sheets is obtained by loading

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CoWO4 nanospheres. Fig 7d further reveals that these nanospheres have sizes in the

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range of 20-60 nm. The high-resolution TEM (HRTEM) images taken from one

D

nanosphere (inset of Fig 7d) reveal clear lattice fringe with interplanar spacing of 0.46

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respectively.

and (011) planes of CoWO4,

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nm and 0.36 nm corresponding to the (001)

Insert Fig. 8

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Further information on the specific surface area and pore structure of the as-prepared

CoWO4

and

RGO/CoWO4-80

were

obtained

from

N2

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adsorption-desorption isotherm measurements, as shown in Fig. 8. The data shows that the nitrogen adsorption isotherm is a typical IV-type curve (Fig. 8a). Additionally, the loop nature of the nitrogen adsorption isotherm suggests a uniform mesoporous feature. The Brunauer-Emmett-Teller (BET) surface area of the CoWO4 was calculated to be 29 m2 g-1 and the pore volume was 0.13 cm3 g-1. Due to the extremely high surface area of RGO (Fig. S1, Supplementary data), the added graphene

increases the surface area as well as the porosity of the composite, and an increased specific surface area of 32 m2 g-1 with a pore volume of 0.17 cm3 g-1 was obtained for RGO/CoWO4-80. In addition, according to the corresponding Barrett-Joyner-Halenda (BJH) analysis, the pores have a mainly mesoporous structure (Fig. 8b). The pore sizes are about 27 nm and 20 nm for CoWO4 and RGO/CoWO4-80, respectively. Insert Fig. 9 On the basis of the characterization results, the RGO/CoWO4 nanocomposites with nanopores and mesopores structures may have potential applications on

RGO/CoWO4

nanocomposites

have

been

evaluated

by

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supercapacitors. The electrochemical capacitive properties of the obtained CV,

galvanostatic

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charge-discharge and EIS measurements in 2 M KOH aqueous electrolyte. Fig. 9a

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displays the typical CV curves of RGO, CoWO4 and RGO/CoWO4 electrodes at a scan rate of 5 mV s-1 within the operation voltage of -0.25 - 0.45 V. All the CV curves

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of CoWO4 and RGO/CoWO4 nanocomposites obviously present a pair of redox peaks,

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indicating the pseudocapacitive nature, while that of RGO shows rectangular shapes,

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representing the double-layer capacitive behavior. The observed redox peaks are due

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to the charge-transfer kinetics of Co2+/Co3+ in the metal tungstate. Previous work by Niu et al. also demonstrated a similar redox reaction in NiWO4 with the aid of

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Pourbaix diagrams [38]. Obviously, RGO/CoWO4 composites show higher

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electrochemical activity than CoWO4. Besides, the CV curves of RGO/CoWO4-80

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shows the highest integrated area of all the electrodes, suggesting that it has the highest energy storage capacity. The specific capacitance can be calculated from CV

(4)

A

CC

curves according to the following equation [51]:

where I is the current density, V is the potential, v is the potential scan rate, m is the mass of the electroactive materials in the electrodes. The corresponding specific capacitances calculated from the CV curves at 5 mV s-1 of CoWO4 and RGO/CoWO4 nanocomposites electrodes are 60.6, 87.9, 121.9, 125.1, 159.9, and 133.6 F g-1, respectively, while that of RGO is 46.1 F g-1 (Fig. 9b). The good electrochemical

performance of RGO/CoWO4-80 can be attributed to the appropriate experimental conditions leading to the less aggregation of graphene sheets and good dispersion of CoWO4 nanospheres on the surface of graphene, thus resulting in a higher active surface area for charge storage as reported [52]. From the SEM and TEM images of RGO/CoWO4 nanocomposites (Fig. S2, Supplementary data), it can be clearly seen that the RGO/CoWO4-x (x = 40, 50, 60, 100) nanocomposites show some agglomerations on the graphene sheets, which may decrease the specific capacitance. However, in the RGO/CoWO4-80 nanocomposties, CoWO4 nanoparticles are

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uniformly distributed on the surface of graphene sheets. Fig. 9c shows the CV curves of RGO/CoWO4-80 at different scan rates of 5, 10, 25, 50, 75, 100 mV s-1. It can be

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seen that with the increase of the sweep rate, a slight deviation in the shape of the CV

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curves was observed at high scan rates (100 mV s-1), which mainly resulted from the low interaction between the electrolyte ions and the electrode. Besides, with the

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increase of the scan rate from 5 to 100 mV s-1 (Fig. 9c), the anodic and the cathodic

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peaks shift to positive and to negative potentials, respectively. It is worth noting that

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the total peak current density of RGO/CoWO4-80 increases obviously with increasing

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potential scan rate, demonstrating the good rate property and excellent capacitance behavior. Fig. 9d shows the specific capacitances of RGO, CoWO4 and RGO/CoWO4

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nanocomposites electrodes under different scan rates. Importantly, the specific

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capacitances of the RGO/CoWO4 nanocomposites electrodes are much larger than

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that of RGO and CoWO4 electrodes at the same scan rate. This improvement can be attributed to (i) the increase in specific surface area of the resultant composites, (ii)

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the introduction of conductive graphene, which facilitate the charge transfer in the RGO/CoWO4 nanocmposites, and ensure high electrochemical utilization of the

A

CoWO4, (iii) the contribution of double-layer capacitance from RGO and pseudocapacitance from CoWO4 nanospheres and (iv) the close bonding and synergistic coupling effects afforded by the direct nucleation and growth of CoWO4 nanospheres

onto

the

conductive

RGO

sheets

[53-55].

Therefore,

their

nanocomposites exhibit an unexpected, considerably higher electrochemical performance for supercapacitors than CoWO4 and RGO alone. However, the

electrochemical property of RGO/CoWO4-100 is much less than RGO/CoWO4-80 electrode. Considering that the electrochemical performance of RGO/CoWO4 materials largely depends on its surface microstructure, we believed that both the components change and the surface area reduction contributed to the significant loss in the specific capacitance of RGO/CoWO4-100. With increasing the amount of GO to a certain extent, less CoWO4 nanospheres cannot effectively prevent the agglomeration of graphene sheets, which may decrease the specific capacitance. Thus RGO/CoWO4-80 is more suitable to be applied as electrode material for

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supercapacitors. Insert Fig. 10

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In order to further investigate the performances of the electrode materials,

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galvanostatic charge-discharge tests were performed in 2 M KOH with different constant current densities. The charge-discharge behaviors of RGO, CoWO4 and

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RGO/CoWO4 electrodes were evaluated by galvanostatic charge-discharge tests at 0.5

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A g-1 (Fig. 10a). It is found that the galvanostatic charge-discharge curves of CoWO4

A

and RGO/CoWO4 electrodes are distinct from that of RGO, in which the shape is

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normally close to an ideal linearity, indicating a pseudocapacitive nature. As can be obviously seen in the galvanostatic charge/discharge curves, RGO/CoWO4-80

D

electrode possesses the maximum charging and discharging time, resulting in the

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highest specific capacitance. This is coincidence with the CV results. Fig. 10b shows

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the galvanostatic charge-discharge curves of RGO/CoWO4-80 electrode at different current densities ranging from 0.2 to 2.0 A g-1. The specific capacitances were

(5)

A

CC

calculated using the following equation [ 32]:

where Cs is the special discharge capacitance in F g-1, i is the current density in A g-1, ∆t is the discharging time in s, and ∆V represents the potential drop during the discharge process. With increasing current density, the specific capacitance gradually decreased, this is a consequence of the fact that the partial surface of the electrode is inaccessible at a high current density. It is worth mentioning that the shapes of the

charge-discharge curves of the RGO/CoWO4-80 deviate from the ideal voltage-time curves. The factors contributing to the non-ideal behavior can be illustrated as (i) the pseudocapacitance from binary metal oxides and (ii) the redistribution of charge within the pores of the activated electrodes during charging and discharging. Insert Fig. 11 EIS was used to further investigate the electrochemical conductivity behaviors of the electrode materials. Fig. 11 shows the typical Nyquist plots of RGO, CoWO4 and RGO/CoWO4 nanocomposites. The Nyquist plots represent the frequency response of

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the electrode/electrolyte system by examining the imaginary component (-Z‫ )״‬of the impedance compared to the real component (Z‫[ )׳‬56]. All the Nyquist plots are similar,

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being composed of a semicircle at the high-frequency region, a linear section at the

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low-frequency region and a transition zone between two regions. As shown in the inset of Fig. 11, the impedance curve can be explained by an equivalent circuit that

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includes solution resistance, double layer capacitance, charge transfer resistance, and

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Warburg impedance. The intercept with the real axis at very high frequency

A

represents a combinational resistance (Rs) of ionic resistance of electrolyte, intrinsic

M

resistance of substrate, and contact resistance at the active material/current collector surface. The impedance semicircle in the high-frequency region is attributed to the

D

interfacial charge-transfer resistance (Rct) occurring at the electrode/electrolyte

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interface and double layer capacitance (Cdl). The diameter of this semicircle

EP

corresponds to the interfacial charge transfer resistance, which is mainly associated with the faradic reactions. The almost straight line of the slope of 45̊ at the

CC

low-frequency region is referred to as the Warburg resistance (Zw), which is related to the ion diffusion or transport from electrolyte to the electrode surface. A near vertical

A

line at low-frequency demonstrates that the electrodes based on RGO/CoWO4 nanocomposites can be used as ideal capacitors. As shown in Fig. 11, RGO/CoWO4 nanocomposites show a smaller semicircle than CoWO4, which corresponds to the lower Rct, owing to the good conductivity and the large surface area of RGO. RGO also provides easy access and more space for electrolyte diffusion, which corresponds to the low Zw. The low Rct and Zw reveal the excellent electrochemical capacitive

properties of RGO/CoWO4 nanocomposites. Insert Fig. 12 Long-term cyclic stability is another important parameter required for practical applications of a supercapacitor. Fig. 12 presents the cyclic stabilities of CoWO4 and RGO/CoWO4-80 electrodes with cyclic number over 1000 times at 1 A g-1. It is clearly observed that the specific capacitances of CoWO4 and RGO/CoWO4-80 slightly decreased over 1000 cycles, indicating the excellent cycle performance. Approximately, 94.7% of the specific capacitance was retained after 1000 cycles for

PT

RGO/CoWO4-80, which was higher than that of CoWO4 (88.1%). A decrease in specific capacitance can be attributed to dissolution, aggregation, and the volume

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change occurred in the electrode materials [57]. In order to investigate this，we

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examined the TEM images (Fig. S3, supplementary data) of CoWO4 and RGO/CoWO4-80 electrodes materials before and after long-term stability tests. The

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TEM images of CoWO4 (Fig. S3b) and RGO/CoWO4-80 (Fig. S3d) electrodes after

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1000 cycles revealed some aggregation compared with their no used counterparts,

A

which can decrease the specific capacitance of the electrodes. In addition, after adding

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graphene in CoWO4 matrix, the TEM image of RGO/CoWO4-80 electrode (Fig. S3d) after 1000 cycles showed less aggregation than that of CoWO4 electrode. This was

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attributable to the excellent interconnection of CoWO4 at the surface and interior of

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the electrodes resulted from the uniform dispersion of CoWO4 nanospheres on the

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graphene sheets provided enough spaces to buffer the volume change of CoWO4 nanospheres during the reversible redox reaction.

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4. Conclusions

In summary, a series of RGO/CoWO4 nanocomposites were successfully

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synthesized via a facile one-pot method, and the structure, morphology, and bonding nature were investigated. The RGO sheets provide a favorable surface for depositing the CoWO4 nanospheres and preventing their agglomeration. In addition, graphene further enhances the conductivity of the composites and therefor provides fast charge transport. The resulting nanocomposites serve as the electrodes in supercapacitor exhibit higher specific capacitance, lower resistance and better rate capability, which

result from the benefits of the pseudocapacitive nature of the binary metal oxide and conductive EDLCs nature of the RGO, implying a good application potential for supercapacitors as well as other power source systems.

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Fig. 1

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A D

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A

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Scheme 1

Fig. 3

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EP

CC

A D

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RI

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A

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Fig. 2

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EP

CC

A D

PT

RI

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A

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Fig. 4

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EP

CC

A D

PT

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A

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Fig. 5

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EP

CC

A D

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A

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Fig. 6

Fig. 8

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EP

CC

A D

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Fig. 7

Fig. 9

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EP

CC

A D

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SC

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A

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CC

A D

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Fig. 10

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Fig. 11

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A

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Fig. 12

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List of Captions for scheme and figures:

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Scheme 1. Illustration for the formation process of the RGO/CoWO4 nanocomposites. Fig. 1. FTIR spectra of GO (a), RGO (b), CoWO4 (c) and RGO/CoWO4 nanocomposites (d-h). Fig. 2. XRD patterns of GO (a), CoWO4 (b), and RGO/CoWO4 nanocomposites (c-g). Fig. 3. Raman spectra of GO (a), RGO (b), CoWO4 (c), and RGO/CoWO4 nanocomposites (d-i). Fig. 4. TGA curves of RGO (a), CoWO4 (b), and RGO/CoWO4 nanocomposites (c-g). Fig. 5. XPS survey spectrum (a), C 1s core level spectrum (b), O 1s core level spectrum (c), Co 2p

core level spectrum (d) and W 4f core level spectrum (e) of RGO/CoWO4-80. Fig. 6. SEM images of GO (a), RGO (b), CoWO4 (c), RGO/CoWO4-80 at low magnification (d), RGO/CoWO4-80 at high magnification (e), and EDX spectrum of RGO/CoWO4-80(f). Fig. 7. TEM images of RGO (a), CoWO4 (inset shows the corresponding SAED pattern) (b), TEM images of RGO/CoWO4-80 (insets show the HRTEM images of one CoWO4 nanoparticle) (c) and (d). Fig. 8. N2 adsorption-desorption isotherm (a) and BJH adsorption pore size distribution (b) of CoWO4 and RGO/CoWO4-80.

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Fig. 9. CV curves at 5 mV s-1 (a), the relationship between the specific capacitance and the scan rate (d) of RGO, CoWO4 and RGO/CoWO4 nanocomposites, the specific capacitance calculated

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from CV curves at 5 mV s-1 of CoWO4 and RGO/CoWO4 nanocomposites (b), and CV curves at

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different scan rates (5, 10, 25, 50, 75, 100 mV s-1) of RGO/CoWO4-80 (c).

Fig. 10. Galvanostatic charge/discharge curves at 0.5 A g-1 of RGO, CoWO4 and RGO/CoWO4

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current densities of 0.2, 0.5, 1.5, and 2 A g-1

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nanocomposites (a), and galvanostatic charge/discharge curves of RGO/CoWO4-80 (b) at the

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circuit of RGO/CoWO4-80 electrode).

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Fig. 11. EIS plots of RGO, CoWO4 and RGO/CoWO4 nanocomposites (inset shows the equivalent

Fig. 12. Cycling performances of CoWO4 and RGO/CoWO4-80 electrodes at 1 A g-1 for 1000

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cycles.