| Summary: | 碩士 === 國立臺灣大學 === 生物產業機電工程學研究所 === 106 === Recently, energy storage devices continue to develop with the expanding of renewable energy sources. Among energy storage systems, supercapacitors are receiving increasing attention becaue of their high power density, high columbic efficiency, greater stability and long lifespan. The main challenge facing supercapaciors for further applications to portable electronics, electric vehicles, and smart grids is their low energy density.
In this study, we raise the supercapacitor energy desnsity by incrasing the specific capacitance and widening the electrochemical operation window. In addition, we replace the aqueous electrolyte with a gel polymer to improve safety and widen applications of super capacitors. Serving as electrode materials in a supercapacitor, graphene possesses high electrical conductivity but the capacity it supplies could be low due to its re-staking phenomenon, while nickel hydroxide has excellent specific capacitance capability but low electrical conductivity. We aim to synthesize the Ni(OH)2/rGO composite electrode via the hydrothermal process to raise overall specific capacitance.
First, material characteristics of the composite electrode were investigated by XRD and FT-IR. The electrochemical performance of the composite electrode in the alkaline aqueous electrolyte were examined. A composite electrode reacted for three hours at 180 °C via the hydrothermal method reaches the highest specific capacitance value of 5404 F g-1.
To fabricate an all-solid-state symmetric supercapacitor, the optimized composite electrodes were combined with lithium-ion gel polymer electrolyte, which is to widen the operation window. Cyclic voltammetry was used to identify redox reactions occur within the supercapactior, and we found that to avoid the EC/PC decomposition the operation potential is limited to 3 V to preserve the reversibility of the system.
The highest operation potential is thus set as 3 V for long-term galvanostatic charge and discharge. After activation, the specific capacitance of the all all-solid-state supercapacitor based on Ni(OH)2/rGO composite electrode is as high as 97 F g-1, which is attributed to the redox couple of nickel hydroxide and the oxidation of organic electrolyte. The supercapacitor exhibits a battery-like high energy density of 121 Wh kg-1and a high power density of 231W kg-1; however, its columbic efficicency is only of 54%.
To raise columbic efficiency, the highest operation potential of the supercapacitor is controlled at 2.5 V. In turn, the specic capacitance of the system is 6.8 F g-1. After 100 cycles, the colmbic efficiency remains over 95% and achieves a better energy density of 12 Wh kg-1and high power density of 6.8 kW kg-1. The capacitance retention of the system is 67% after 5000 cycles.
Finally, through electrochemical impedance analysis we found that the symmetric composite electrodes in lithium-ion gel polymer electrolyte behaves more similar to an ideal capacitor than in the 1 M KOH system. It also means the all-solid-state symmetric supercapacitor have better reversibility and stability than its 1 M KOH counter part.
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