Fabriacation And Characterization Of Anodic Metal Oxide Nannotube Arrays And Their Applications In Energy Storage And Conversion Devices

Electrochemical energy storage and conversion devices, such as Li-ion batteries, supercapacitors and dye-sensitized solar cells, are of great interest for their potential applications in renewable-energy-related fields. The electrode materials is the critical element to determinate the performance of these devices. Highly ordered metal oxide nanotube arrays have special advantages in electrochemical energy storage and conversion devices. How to realize the high controllability of the morphology of these nanomaterials is the prerequisite for practical applications. Due to its simplicity, reliability, low cost and facile large-scale fabrication, anodization is one of the most effectively methods to prepare nanostructure arrays.In this dissertation, great efforts have been made to accomplish the morphology control of the anodic metal oxide nanotube arrays, to discuss the formation mechanism, and to realize the use of these special nanomaterials as electrodes for high-performance electrochemical energy storage and conversion devices. The effects of anodization process parameters and annealing on the morphology, constituent and structure of anodic TiO2and Fe2O3were systematically investigated. Meanwhile, the formation mechanism and dimension evolution of TiO2and Fe2O3nanotube arrays (TiO2-NT,Fe2O3-NT) were also investigated. Moreover, the electrochemical characteristics of PANI-TiO2/Ti composite electrodes have been studied systematically. In addition, the TiO2nanotube photonic crystal (TiO2PC), fabricated by anodization under periodic current pulses, was proposed to as photoanode for dye-sensitized solar cell (DSSC), while the mechanism of photonic crystal on the enhancement in the power conversion efficiency of DSSCs was also investigated herein. Furthurmore, a novel design and facile synthesis of a unique and low-cost C@Fe3O4/Fe3D composite electrode for Li-ion battery has been demonstrated, the electrochemical characteristics of the composite electrodes have also been studied systematically. At last, the electrochemical characteristics of α-Fe2O3/Fe electrode for supercapacitor have also been studied. The major research results obtained are as follows:(1) The formation mechanism and dimension evolution of TiO2-NT were investigated. In both aqueous and organic electrolytes, there are three major stages during the anodization process, the formation of dense oxide barrier layer, the transformation to a porous structure, and the steady growth of TiO2nanotubular structure. The onset of nanotube formation was found to be strongly dependent on rate of field-assisted oxide growth, field-assisted oxide dissolution and chemical dissolution. The preparation process of TiO2-NT has been established. The topologies of the anodized TiO2were strongly determinated by the electrolyte systems, fluoride concentration, applied potential, oxidation time and so on. The nanotube architecture will disrupt and aggregat to form porous nanorod structure when the temperature during the anneal process is too high.(2) A novel composite electrode made of polyaniline nanowire-titania nanotube array was synthesized via a simple and inexpensive electrochemical route by electropolymerizing aniline onto an anodized titania nanotube array. The specific capacitance was as high as732F g-1at1A g-1. An excellent long cycle life of the electrode was observed with a retention of86%of the initial specific capacitance after2000cycles. The special microstructure of the nanocomposite electrode is the key to the excellent electrochemical performance:the disordered PANI nanowire array in small diameter and short length provides high capacitance and high rate performance, while the highly-ordered TiO2nanotube array not only relieves the mechanical stress of PANI during the doping/de-doping process, but also offers good contact between the PANI nanowires and the substrate.(3) The TiO2PC was obtained under current-pulse anodization, and used to fabricate two types of photoanodes for DSSC. DSSCs equipped with the photoanode Ⅰ showed a significantly enhanced power conversion efficiency, an increase of over50%compared with the cells without a PC layer. While the power conversion efficiency of DSSCs equipped with the photoanode Ⅱ showed was6.96%, and39.5%superior to its TiO2NP counterpart. This improvement is attributed to the enhanced light harvesting of the DSSCs in the spectral range corresponding to the photonic bandgap of the PC, due to the reflection of lights and localization of photons.(4) Also the formation mechanism and dimension evolution of Fe2O3-NT were investigated. There are three major stages during the anodization process of iron, the formation of dense oxide barrier layer, the transformation to a porous structure, and the steady growth of Fe2O3nanotubular structure. The onset of nanotube formation was strongly dependent on rate of field-assisted oxide growth, field-assisted oxide dissolution and chemical dissolution. The preparation process of Fe2O3-NT has been established. The topologies of the anodized Fe2O3were determinated by fluoride concentration, applied potential, oxidation time and so on. Compared with the preparation of TiO2, more strict process control was required. Iron oxide nanotube arrays have been only achieved over a potential range of30V-50V, using an electrolyte comprised of0.25wt%-0.5wt%NH4F and1%-3%DI water between40℃to60℃. The nanotube architecture was found to be stable up to500℃, then disrupted and aggregated to form porous nanorod structure at the temperature of700℃.(5) A novel design and facile synthesis of a unique and low-cost C@Fe3O4/Fe3D composite electrode for LIB has been demonstrated. In this new design strategy, each part of the composite provides its desired functionality, with the Fe foil being a low cost and stable current collector, the Fe3O4as an active material offering high capacity, the carbon coating layer forming an electron conducting network for high rate capability and stable SEI films for enhanced cycle stability. A balance between high capacity (1020μAh·cm-2at20μA·cm-2) and high rate capability (176μAh·cm-2at1000μA·cm-2) were achieved in this rationally designed electrode.(6) The α-Fe2O3/Fe electrode has been prepared by annealing amorphous Fe2O3-NT in air directly. The electrochemical behaviors of α-Fe2O3/Fe have been investigated as electrode in supercapacitor. The specific capacitance obtained in this study was as high as138F·g-1at a current density of1.3A·g-1, which remained at91F·g-1when the current density was increased to12.8A·g-1. The capacitance retention of the electrode was as high as88.9%after500cycles, indicating that the α-Fe2O3NTAs electrode has good electrochemical stability.