| Electrochemical capacitors or supercapacitors, has been considered as a candidate other than batteries for energy storage. Electrochemical capacitors have the virtue of high charge/discharge rate and long cycling life, while the batteries lacking.Combining the high energy density of batteries with the high power density and long cycling life of the electrochemical capacitors, can meet the demanding of the future energy storage. This thesis addresses the high power density and long cycling life of the electrochemical capacitors.Electrochemical capacitors can be separated into two classes by the energy storage mechanism: the electrochemical double layer capacitors store energy by the static electric attraction between the ions and the charged surface, the pseudo-capacitors rely on the fast redox reaction on the surface. For the improvement of the power density of the electrochemical double layer capacitors, two points are to be considered. The first is the improvement of the transport of the electron on the electrode, i.e. eliminating the resistance of the electrode materials. Carbon materials has been researched extensively for electrochemical double layer capacitors, new materials with higher conductivity have the opportunity to achieve higher power density. The second is the improvement of the transport and diffusion of the ions in the electrode, and then the micro- and nano-structures have to be re-designed for the fast ions transport and diffusion. MnO2 is a low cost, abundant, and environmentally friendly active materials for pseudo-capacitors. But the MnO2 suffer from the problem of low conductivity and short cycling life. Form the aspects of charge/discharge rate and cycling life, it's very hard for MnO2 reach the magnitude of the carbon based materials. Controlling the nano structured current collector and the deposition state of the pseudo-capacitive materials is a way to alter the rate ability and cycling life of the electrode.This thesis focuses on searching materials that can reach high charge/discharge rate while keeping the specific capacitance. Titanium oxynitride is a high conductive material with a conductivity of 30,000 - 35,000 S/cm. This is higher than chemically-converted graphene (210 - 1,000 S/cm) and carbon nanotubes (60 - 170 S/cm). In this thesis, the hydrothermal and anodic oxidization method has been utilized to synthesize the titanium oxide and titanate arrays with various nanostructures, and then transform them into titanium oxynitride nano-array with the corresponding structure. The titanium oxynitride nano-array has been studied as electrochemical double layer capacitor electrode and as the nanostructured current collector for the pseudo-capacitors. The main contents include:1. By the hydrothermal method, titanate with various nanostructure of nano-leaf,nano-wire, nano-belt and nano-grid array on titanium foil were synthesized without any surfactant. Under the ammonia atmosphere, these titanate nanostructured arrays were transformed into titanium oxynitride array with the same morphology to their precursor in the nitrification process. The specific capacitance of titanium oxynitride arrays are 108.3, 112.5, 95.0, and 103.1 F/g, respectively, and with high rate ability.After 8000 galvanostatic charge/discharge cycles, the capacitance retention is 96.0%,92.9%, 85.9%, and 97.0%, respectively.2. For the purpose to provide linear channel for fast ion transport, synthesized morphology controllable titania nanotubes array by anodic oxidization, and tested as the electrochemical capacitor electrodes. The specific area capacitance of the titania nanotubes array at the scan rate of 100 mV/s reaches 1.0 mF/cm2.3. After nitrification in ammonia atmosphere, the titania nanotubes array transformed to titanium oxynitride nanotubes array. The titanium oxynitride nanotube's wall is good conductor for electron transport, while linear channel benefits ion transport and diffusion. At the scan rate of 100 mV/s, the area capacitance reaches 22.3 mF/cm . TiO0.54N0.46 nanotube array electrodes were electrochemically stable in 1.0 mol/L KCl aqueous solution and in 1.0 mol/L TEA-BF4 acetonitrile electrolyte,the capacitance retention were very high after 100,000 galvanostatic charge/discharge cycles in these electrolyte.4. For the purpose to improve the power density and cycling life, the TiO0.54N0.46 has been used as a nanostructured current collector. By a modified electrochemical deposition method, a MnO2 layer of a thickness of 5 - 10 nm has been uniformly deposited on the surface of the nanotubes. The MnO2/TiO0.54N0.46 show low equivalent series resistance and high charge/discharge rate capability, the capacitance remained ca. 220 F/g even when the capacitor was operated at an ultrafast charge/discharge rate of 2000 A/g. Benefit from the electrochemical stability of the current collector and the mechanical stability of MnO2, MnO2 reach a energy density of 9.8 Wh/kg and a power density of 620 kW/kg at the current density of 100 A/g.5. The CoOx,V2O5, and BiVO4 deposited on the TiO0.54N0.46 nanotubes array show improved capacitance compared that deposited on the flat foil with the same quantity. The same quantity of CoOx deposited on the TiO0.54N0.46 nanotubes array show higher specific area capacitance and energy discharge efficiency.In this thesis,the purpose of the study about titanium oxynitride used as electrochemical double layer capacitor's active materials or used as pseudo-capacitor's current collectors were providing design ideas and theoretical instruction for the development of electrochemical capacitors with ultrahigh charge/discharge rate and long cycling life. |
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