| Summary: | 碩士 === 國立臺灣科技大學 === 化學工程系 === 99 === We have investigated preparation and properties of the miniaturized ultracapacitors loaded with carbon nanotubes (CNT) , α-Co(OH)2, and RuO2˙xH2O on a flexible substrate. These ultracapacitors are featured with 20 μm–spaced comb-like electrodes, which are patterned using the standard technologies of photolithography, chemical vapor deposition, and electrodeposition. Owing to the mini size and the synthesis techniques, they can be easily integrated into portable electronics.
Correlation between the structure and energy storage property is studied with two ultracapacitors, a symmetric CNT_CNT capacitor and an asymmetric capacitor of CNT_α-Co(OH)2/CNT or CNT_RuO2•xH2O/CNT. The electrode capacitance of each electrode is measured with cyclic voltammetry (CV). Energy and power performance of symmetric and asymmetric capacitor are evaluated using galvanostatic charge-discharge experiment. If appropriate, the voltage and the capacitance of individual electrode of the working cell are analyzed during the cell discharge process.
On synthesis, a major achievement in this study is to demonstrate that the interdigital pattern of CNT electrode can be transferred from a rigid substrate to a flexible (polymeric) substrate using polyester tape. The transfer step allows us to pre-sputter a layer of gold on the top surface of CNT pattern electrode, after inversion and transfer, the gold-sputtered surface turns into a conductive bottom of CNT pattern. The conductive CNT bottom assures every nanotube is electrically connected to the current collector, reduces the electrode resistance significantly, and make possible a high-power cell.
Consequently, the power performance of these ultracapacitors is far better than that of the previous ultracapacitors synthesized in our research group. At a current density of 30 Ag-1 and a potential window 1.8 V, the CNT_CNT cell discharges at a power level 20.6 kWkg-1 with energy density 2.2 Wh kg-1. At a current density of 35 Ag-1 and a potential window of 1.8 V, the CNT_α-Co(OH)2/CNT cell discharges at a power level 14.5 kWkg-1 with energy density 5.9 Whkg-1. At current density of 32 Ag-1 and a potential window of 1.8 V, the CNT_RuO2•xH2O/CNT cell discharges at a power level 16.2 kWkg-1 with energy density 16.5 Whkg-1. Although ruthenium costs more than cobalt in material price, its asymmetric cell excels in both energy and power density. Furthermore, the CNT_RuO2•xH2/CNT cell can be operated at a potential window of 2.0 V, at current density of 40 Ag-1, it discharges at a power level 22.9 kWkg-1 with energy density 24.0 Whkg-1.
We attribute the higher energy density of CNT_RuO2•xH2O /CNT cell to the higher capacitance of its oxide electrode. The CV results indicate the capacitance of RuO2•xH2O/CNT is much higher than that of α-Co(OH)2 /CNT.
|
|---|
