LetterWO3 nanoflower coated with graphene nanosheet: Synergetic energy storage composite electrode for supercapacitor application
Introduction
In face of the rapid increase of global energy demand, developing alternative energy have become very urgent. Supercapacitor has drawn considerable attention due to its long service life, great power density, high energy density and green environmental protection [1], [2]. The charge storage mechanism of the supercapacitor involves both electrical double layer charge (EDLC) and pesudoprocess charge storage. The total amount of charge stored in pesodocapacitor is higher than that of the EDLC materials, however, the latter exhibits better electrochemical stability than the pseudocapacitors [3]. In recent years, transition metal oxides/hydroxides, such as, MnO2, NiO, SnO2, WO3, Co(OH)2 and Ni(OH)2 have been studied extensively as supercapacitor electrode materials because of their redox activity and effective pesudocapacitance [4]. Amongst these pesudocapacitive material tungsten oxide (WO3) is of most interest due to its excellent chemical stability, various morphology and low-cost [5]. However, the low electrical conductivity and poor capacitive performance have greatly limited the electrochemical performances for supercapacitor. If the conductivity of the WO3 can be increased, the low charge transfer resistance can result in high specific capacitance. In this respect, several efforts have been devoted to overcoming these drawbacks and improving the performance, including optimizing morphology and structure, incorporating WO3 with polymer or other highly conductive materials and establishing a hybrid type material containing both pseudocapacitive and EDLC materials due to their high capacitance and outstanding long cycle life [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Amongst the conductive materials graphene is the most promising candidate due to the unique properties of high surface area, more stability and higher conductivity [16]. WO3 can acts as psedocapacitor, graphene acts as EDLC source and the graphene in graphene-WO3 have become more attractive because they enhance the effective surface area, provide short diffusion path length to ions and improve the charge accessibility on the electrode surface. In recent years, some hybrid architecture have demonstrated enhanced specific capacitance and better cycling stability. For example, Huang et al. [9] reported that a great improvement in specific capacitance is achieved with the Gr-WO3 hybrid. Wang et al. [17] has reported the preparation of the GNS-W and the composite showed a better electrochemical performance. Ma et al. [18] combined a graphene with WO3 under hydrothermal condition and the composites exhibit high specific capacitance, good cycling stability and excellent rate capability. However, these composites suffer from the problems of limited interfacial contact area between graphene and WO3. It is clear that the intimate interfacial contact between graphene and WO3 is much more desirable to harness the structure feature and excellent electron conductivity of graphene, by which the supercapacitive performance of WO3 could be improved more effectively. Therefore, it is of great interest to design a binary system supercapacitors uniformly coated on the surface of transition metal oxides. Such an architecture design with a graphene coating would provide substantial electroactive surface area, fast electron transport and the synergistic effect between two constituents, effectively improving the electrochemical performance.
Herein, we designed and fabricated the composite of WO3 with RGO nanosheets by a simple electrostatic self-assembly strategy. The fabrication procedure mainly consists of two steps: Firstly, WO3 nanoflowers were prepared by hydrothermal synthesis and modified with cationic surfactant 3-aminopropyl triethoxysilane (APTES). Then WO3 was coated with the GO sheets via electrostatic attraction, followed by reduction of GO using hydrothermal method. The results indicate that the WO3 nanoflowers are uniformly coated with RGO nanosheets. The electrochemical tests demonstrated that the WO3-RGO composites deliver a very high specific capacity and excellent cycle stability compared with pure WO3.
Section snippets
Synthesis of WO3 and WO3-RGO
All the reagents used in the experiment were of analytical grade, and used without further purification. Graphene oxide (GO) was synthesized from natural graphite by the modified Hummer's method as previous reaction process [21]. The WO3 was synthesized by a facile hydrothermal method. 11 mmol Na2WO4·2H2O was dissolved in 20 ml of deionized water under continuous stirring. Then, 6 M HCl solution was added slowly into the prepared solution to keep the pH at 1.5, followed by the addition of 22
Results and discussion
The XRD patterns of the as prepared WO3 and WO3-RGO are shown in Fig. 1a. All these peaks confirmed a mixture with most monoclinic WO3 (JCPDS 84–0886) and a little cubic WO3·H2O (JCPDS 41–0905) and hexagonal WO3 (JCPDS 33–1387) [22]. In the XRD pattern of the as prepared WO3-RGO no obvious diffraction peaks for GO can be observed and all the peaks of WO3 re appears without any significant change conforming the reduction of the GO to RGO under hydrothermal condition. Nevertheless, the presence
Conclusions
We have successfully synthesized WO3-RGO composite via a facile method by attaching RGO to the surface of WO3, which is afforded by electrostatic attraction between WO3 and RGO. It was found that WO3-RGO exhibit significantly enhanced electrochemical performance than the bare WO3 toward the specific capacitance, which can be ascribed to the synergistic effects of conductive RGO and the pseudocapacitive behavior of WO3 imparted long term cycling stability. This novel method can be used to
Acknowledgments
This study was financially supported by the National Natural Science Foundation of China (NSFC, No. 51503169, 515313734, 51402231 and 21406176). Natural Science Foundation of Shaanxi Province (2014JQ2072).
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