Capacitor Properties of Pure Single-walled Carbon Nanotube Electrodes

Introduction Electric double layer capacitors (EDLCs) have excellent properties in power density and durability compared with secondary batteries. Recently, development of advanced capacitors with high energy density is active because the storage devices with high power density have huge market in the fields of hybrid vehicles, mobile instruments and so on. The singlewalled carbon nanotubes (SWNTs) have high surface area theoretically up to 2600 m/g inside and outside of the tubes. The material is considered to be ideal for capacitor electrodes and hence many authors have reported the EDLC properties (Niu 1997 and Shiraishi 2002). However, the SWNTs used in the previous attempts contain considerable amounts of impurities and defects and have low surface area. Hata et al. (2004) have reported a highly efficient synthesis method of impurity-free SWNTs. The “Super growth” method named gives vertically aligned SWNT forests with millimeter-scale height (SG-SWNTs). The SG-SWNTs can be easily separated from the catalyst, providing nanotube material with carbon purity above 99.98%. The excellent capacitor performance of SG-SWNTs has already been reported. The capacitance per unit surface area was 1.5 times higher than that of commercially available activated carbons for EDLC at the potential of 3V in two electrode cell system (Tanaike 2006). Densely packed SG-SWNTs prepared by controlling the fabrication process showed excellent energy density and power density as capacitor electrodes (Futaba 2006a). In the present study, electrochemical behaviors of the SG-SWNTs were studied and the characteristic mechanism of energy storage as capacitor electrodes was revealed.