| Summary: | 碩士 === 國立臺灣科技大學 === 化學工程系 === 98 === ABSTRACT
In the present study, we develop the ultracapacitor cells of miniature size, which are prepared with photolithography, chemical vapor deposition of carbon nanotubes (CNT), and electrochemical deposition of pseudocapacitive NiCo2O4. The performance of the ultracapacitors is analyzed by cyclic voltammetry (CV), impedance, and charge-discharge experiments. The most important factor of capacitor performance is identified as whether the electrode configuration is symmetric or asymmetric. The second important factor is the resistance of bottom electrode, manipulated with the addition of gold in the Al-Fe seed layer for CNT growth.
Before the investigation of patterned cell of interdigital electrodes, we synthesize CNT forest with blanket growth and electrodeposit the oxide of nickel and cobalt oxide, which is later identified as microcrystalline NiCo2O4. The electrolyte of 0.1 M KOH and 0.09 M Na2SO4 is chosen for its high conductivity and pH value. The alkaline electrolyte is necessary for pseudocapacitive NiCo2O4, which slowly degrades in acidic solution.
The structure of CNT is damaged with the addition of gold in the Al-Fe seed layer, since the sputtered gold interferes with the guided growth of CNT. Without gold, the CNT demonstrates vertical aligned growth with an impressive height of 20-30 ?慆, along with a high population density > 109 cm-2. With the addition of gold, the improvement on electrode resistance is accompanied with a small CNT height 1-2 ?慆, randomly orientation, and a low population density. Thus, the electrode capacitance of symmetric CNT + NiCo2O4 cell on the Al-Fe layer, measured by CV, is 199 mFcm-2, while that of symmetric CNT + NiCo2O4 cell on the Al-Fe-Au layer 109 mFcm-2 at 10 mVs-1. But the capacitance of the electrode without Au decreases more rapidly with increasing sweep rate, compared with the electrode with Au. The more rapid decreasing trend is attributed to the larger electrode resistance, for the electrode without Au, the resistance is measured 4167 ? by impedance spectroscopy, higher than that of the electrode with Au 333 ??
Ragone plots of power versus energy for the symmetric and asymmetric cells show the typical hooked-shape curve. Built on either Al-Fe or Al-Fe-Au bottom electrodes, the asymmetric cell exhibits the highest energy and power capability. On the Al-Fe bottom electrode, the asymmetric electrode of CNT + NiCo2O4 cell displays an energy capacity of 9170 ?寙 and a power level of 119 ?巰. On the Al-Fe-Au bottom electrode, the asymmetric electrode of CNT + NiCo2O4 cell exhibits an energy capacity of 756 ?寙, and a power level of 94 ?巰.
Keywords：Ultracapacitor; Cobalt-nickel oxides; Carbon nanotubes; Interdigitated electrode; Symmetric electrode; Asymmetric electrode.
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