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Energy storage materials based on an electrochemical double layer and pseudocapacitance for use in supercapacitors must satisfy the requirements of high electrical conductivity and specific surface area, and short diffusion paths for protons in order to reduce resistance. Electrode materials with well-designed three- dimensional (3D) architectures not only meet these requirements but also improve the energy/power density of the supercapacitors. Significant efforts have been made to establish methods for the preparation of transition metal-oxide nanostructures. Among transition metal oxides, tungsten oxide has been widely studied for applications such as photocatalysts, gas sensing, and electrochromic devices.
In chapters 3 and 4, the growth mechanism of 3D hollow urchin-like W18O49 by a hydrothermal method and the control of its phase and morphology by nanostructured molybdenum-tungsten oxide are described, respectively. In chapter 5, the applicability of W18O49 and WO3 nanowires grown on carbon fibers for flexible supercapacitor applications is discussed.
3D urchin-like W18O49 nanostructures were prepared by a template-free hydrothermal synthetic route. The detailed morphology and crystallinity were dependent on the synthesis temperature and synthesis time. The mechanism underlying the formation of the urchin-like nanostructures essentially followed the Ostwald ripening process.
Substoichiometric mixed molybdenum-tungsten oxide nanostructures (MMTONs) with diverse phases and morphologies are prepared using a solvothermal method to investigate the impact of these structural features on the electrochemical performance of the MMTON as supercapacitor electrodes. With an increase in the Mo content of the mixed Mo and W source materials, mixed phases of W18O49 and MoO2 appear, and the morphology changes from crystalline nanourchins to amorphous nanospheres. Crystalline molybdenum oxides are formed only in the presence of a tungsten precursor that serves as the framework for the formation of the crystalline molybdenum phase. The MMTONs synthesized with a 50:50 (W:Mo) molar ratio exhibit significant peaks originating from H+ insertion into tungsten and molybdenum oxide, and redox reaction of Mo (IV)/Mo(VI) for MoO2 in the cyclic voltammograms. The cyclic voltammograms indicate that the diverse phases of the MMTONs synergistically contribute to the charge storage process.
For supercapacitor applications, W18O49 nanowires have been extensively grown on graphitic carbon felt using a facile solvothermal method. The diameter and length of the nanowires are about 7 and 300 nm, respectively. The nanowires consist of monoclinic W18O49 grown along the [010] direction, as shown by TEM and XRD analyses. The W18O49 nanowires assembled on the carbon felt exhibit a high capacity of 588.33 F/g at a current density of 1 A/g, as well as excellent cycling performance and low internal resistance during electrochemical testing. This outstanding performance may originate from the 3D porous nanostructure of these W18O49 nanowires, which leads to a reduction in the resistance and fast reaction kinetics due to the high specific surface area and electrolyte accessibility. Furthermore, sufficient oxygen deficiencies in the substoichiometric tungsten oxide contribute to electrochemical activity, as confirmed by comparison of the cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) data of the W18O49 nanowires with those of WO3 nanowires.
The results of this thesis could enhance our understanding of the growth mechanism of the hollow structures and tailoring of nanostructured tungsten oxides. In addition, nanostructured tungsten oxides tuned by adjusting the phase and morphology are promising materials for the working electrodes of supercapacitors.
Thesis Advisor: Prof. Dohyung Kim