High-Capacitance Supercapacitor Using a Nanocomposite Electrode of Single-Walled Carbon Nanotube and Polypyrrole

Recent applications of carbon nanotubes ~CNTs! as an electrode material for supercapacitors have ignited significant worldwide investigation on their microscopic and macroscopic porous structures and electrochemical behavior.1-11 The CNT lectrodes exhibit a unique pore structure and high usage efficiency of specific surface areas.3,6 The CNT electrodes for the electric double layer type of supercapacitor ~EDLC! have excellent absorption characteristics due to the accessible mesopores formed by the entangled individual CNTs.3,7 However, there is a limitation to increasing the effective surface area of the CNTs, although some processes for breaking the bundle structure of the CNTs may enhance the utilization efficiency of their surface area. In previous papers, we have reported that supercapacitors using single-walled carbon nanotube ~SWNT! electrodes have a maximum specific capacitance of 180 F/g in a solution of 7.5 M KOH.3,8 However, the specific capacitance of supercapacitors using SWNT electrodes is still smaller than that of supercapacitors using activated carbons and activated carbon fibers, which are commonly used as electrode materials for supercapacitors. Therefore, it is necessary to improve the capacitance of SWNTs for practical applications. Nanocomposites of CNTs and conducting polymers have been intensively studied these last few years to improve the conductivity, electronic transport, and electromagnetic properties of the conducting polymers for applications in nanoelectronic elements and electro-optical devices.12-19 Recently, Frackowiak et al.20 have reported that multiwalled CNT electrodes modified by either an oxidative treatment or polypyrrole ~Ppy! show a maximum specific capacitance of 170 F/g in 1 M H2SO4 used as an electrolyte solution. However, in this case the pseudocapacitance by Ppy is insufficiently utilized because of the thick and rigid structure and less entanglement of the multiwalled CNTs. We report here the nanocomposite of SWNTs and Ppy for supercapacitors with high capacitance. SWNT-Ppy nanocomposite electrodes are fabricated to improve the specific capacitance of the supercapacitor.