Abstract
In this dissertation study, PAN-based fibers provided from ITRI were under various thermal treatments including oxidative stabilization, carbonization, and activation were performed on Polyacrylonitrile-based carbon fibers (PAN-CFs) to enhance their supercapacitive performances. In the oxidative stabilization and carbonization process, thermal treatment duration was controlled to observe the change in the electrochemical characteristics of PAN-CFs. In the activation process, both thermal treatment duration and pressure of carrier gas were varied to investigate their effects on the morphology and electrochemical performances of PAN-CFs. Activation process is found to favor the formation and dispersion of pores on the surface of the activated PAN-CFs, which increases the specific surface areas and subsequently improves the specific capacitances from 0.25 mF/cm2 for the pristine PAN-CFs to 34.7 mF/cm2 for the activated PAN-CFs, operated at 100 mV/s scan rate. In addition, we used cyclic-voltammetric method to deposit RuO2 on the surface of the PAN-based fibers with oxidation and carbonization treatments to further improve the supercapacitive performance of them. Finally, we try several deposition parameters such as concentrations of aqueous solution and deposition cycles to obtain the optimum deposition recipe for the supercapacitors. For the optimal electrodeposition condition, substrates were placed in an aqueous solution containing 0.01 M RuCl3, 0.1 M KCl and 0.01 M HCl. The depositions were carried out in a potential window of -0.2 to 1V for 50 scan cycles at 50 mV/s. The specific capacity is 218 mF/cm2 at a lower scan rate of 10 mV/s, and decayed to 145 mF/cm2 at a higher scan rate of 1000 mV/s. The capacity could be maintained 67％ at high scan rate. Due to the high cost of RuO2, MnO2 with low cost and good performance was considered to replace RuO2. The result shows that the amount of needle-like MnO2 coated on the surface of CFs was increasing with increasing deposition cycles and decreasing deposition rate. Meanwhile, the specific capacity of composite electrodes was increasing significantly and the capacity maintenance was decreasing resulting from bad electrical conductivity of MnO2. The hybrid electrode with highest specific capacitance was under the condition of deposition rate of 10 mV/s and 150 deposition cycles. The specific capacity is 405 mF/cm2 at a lower scan rate of 10 mV/s, and sharply dropped to 2 mF/cm2 at a higher scan rate of 1000 mV/s. In the other hand, the hybrid electrode with best capacity maintenance was under the condition of deposition rate of 150 mV/s and 30 deposition cycles. The specific capacity is 42 mF/cm2 at a lower scan rate of 10 mV/s, and decreased to 13 mF/cm2 at a higher scan rate of 1000 mV/s. The capacity could be maintained 31％ at high scan rate.