Abstract
Tungsten oxide possesses capacitive-like behavior which has been synthesized within a short reaction time by a novel microwave-assisted hydrothermal (MAH) process and a microwave-assisted solvothermal (MAS) process, respectively. Crystalline WO3-WO3 0.5H2O mixtures exhibit good capacitive characteristics at 200 mV s-1 and CS 290 F g-1 at 25 mV s-□1 in 0.5 M H2SO4 between -0.6 and 0.2 V. Oxide rods can be obtained via the MAH process even when the synthesis time is only 0.75 h while WO3□0.5H2O sheets with poor capacitive performances are obtained by a conventional hydrothermal synthesis process at the same temperature for 24 h. The aspect ratio of tungsten oxide rods is found to increase with prolonging the MAH time while all oxides consist of WO3 and WO3□0.5H2O. The oxide mixtures prepared by the MAH method with annealing in air at temperatures □ 400oC show promising performances for electrochemical capacitors (ECs). The MAH synthesis is carried out in the presence of capping agent, which plays an important role in the growth of tungsten oxide structure. In this work, capping agents such as NaCl, CH3COONa, NaNO3, LiCl, KCl and CsCl are employed. It is found that the morphology, crystallinity, oxide composition, and electrochemical performance of tungsten oxide are significantly dependent on the kind of the capping agents. From the comparison of cyclic voltammograms (CVs) among all tungsten oxides, the samples prepared with CH3COONa and KCl as capping agents exhibit higher symmetry and charge storage capacity, however, thier cycle lifes are quite poor. In order to improve the cycle life, the highly ordered self-assembly W18O49 nanobundle consisting of individual nanowires obtained by the MAS method with various solvents has been pursued. The products demonstrate that the solvents have influence on nanostructured alignment. It is found that the nanowires within W18O49 bundle synthesized with n-propanol are oriented parallel to each other with a distance of 1 nm; meanwhile, this W18O49 bundle shows excellent capacitor behavior. Furthermore, the bundle disperses in formamide/ethanol mixture resulting in well-separated nanowires and worse charge storage capacity. Due to the narrow working potential of tungsten oxide, the asymmetric capacitor with a W18O49 anode and a polyaniline (PANI) cathode is applied, and the specific energy and specific power of this device respectively equals 12.0 Wh kg−1 and 2.9 kW kg−1, which is promising in the application of ECs. The products have been characterized by X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), wide angle X-ray scattering (WAXS), surface area and pore size analyzer (BET), and cyclic voltammetry (CV).