(201ab) Preparation of Carbon-MnO2 Nanocomposites By Chemical Redox Deposition for Application to Asymmetric Electrochemical Capacitor

Asymmetric electrochemical capacitors are characterized by Faradaic reactions (pseudocapacitance) and charge-storage (electrical double layer capacitance, EDLC). They have drawn intense research interests due to their high energy density, large power density, fast charging-discharging rate, excellent reversibility, and long cycle-life. According to the charge storage mechanisms, EDLC is realized by the electrostatic storage via ion adsorption at the electrode/electrolyte interface of the porous carbon materials such as carbon nanotubes, carbon nanofibers, activated carbon and so on. Such EDLC systems have been known to exhibit relatively low energy density and capacitance. To achieve high power and high energy density of the energy storage system, several metal oxides with fast and reversible redox reactions such as RuO₂, MnO₂, NiO, and Co₃O₄ have been used as the electrode materials. Among those metal oxides, MnO₂ has attracted much attention because of its low cost, eco-friendly, rich redox activity, and high theoretical specific capacitance (1232 F/g). In this study, carbon materials such as carbon nanotube (CNT), carbon sphere (CS), and hollow carbon sphere (HCS) were utilized for preparation of carbon-MnO₂ composites by a chemical redox deposition method and their capacitive behaviors were compared by an electrochemical measurement. In addition, in order to improve their electrochemical properties, fGNS-CNT-MnO₂ composites were prepared by wrapping the CNT-MnO2 composite by functionalized graphene. fGNS-CNT-MnO₂ electrode exhibited the best pseudocapacitance of 202 F g^−1, a cycling stability of 95%, and power and energy density of AC//fGNS-CNT-MnO₂ asymmetric capacitor are 720 W kg^−1 and 46 Wh kg^−1, respectively, at 8 mA cm^−2.