Doctor of Philosophy
Institute for Superconducting and Electronic Materials
Salari, Maryam, Functionalization of nanostructured titania (TiO2) as electrode materials for supercapacitors, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials, University of Wollongong, 2013. http://ro.uow.edu.au/theses/3824
The many applications of high energy storage devices have forged an increasing interest in research areas related to the electrochemical capacitors. This thesis presents a method to fabricate self-organized titanium dioxide (titania; TiO2) nanotube arrays and offers a facile technique to modify the crystal structure of titania nanotubes in order to overcome their high resistivity and harvest high charge storage accumulation for energy storage applications. Since the as-synthesized nanotubes are amorphous, more attention is given to the crystallization process. The titania nanotubes were grown by anodic oxidation of titanium foil in a fluorine-containing electrolyte and subsequently exposed to an inert atmosphere under various heat-treatment regimes. Integrating these nanostructures into a binder-free working electrode resulted in improved capacitance of up to 2.6 mF cm-2, which far exceeds the values so far reported for different forms of titania in the literature. The specific capacitance results were mainly extracted from both cyclic voltammetry and galvanostatic charge-discharge curves. The increase in the capacitance of these highly ordered titania nanotubes confirms the pseudocapacitive contribution due to the modification of the crystal structure. Physical characterization of the obtained nanostructures was carried out by employing field emission scanning electron microscopy, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy techniques. The oxygen depletion phenomenon, the interstitial presence of titanium in the anatase to rutile phase transformation, and also the role of phase transformation in the electrochemical charge-discharge behaviour of nanocrystalline titania nanotubes were investigated and are discussed in detail for the first time. The ease of synthesis and exceptional electrochemical properties make these nanotube arrays a promising alternative candidate for use in energy storage devices. A two-electrode configuration was used to study these nanostructured materials, and their electrochemical properties were measured in different electrolyte environments. It was found that the capacitance could be further increased in an acidic electrolyte up to 17.5 mF cm-2, which is comparable with the values reported for commercial and conventional electric double layer capacitors. In addition, the results show that capacitance as high as 6.4 mF cm-2 can be achieved by employing biocompatible electrolytes. These results are important, as they offer a new type of material for biological energy storage applications
Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.