Single-walled carbon nanotubes as electrodes in supercapacitors

As-grown and modified single-walled carbon nanotubes (SWCNTs) have been used as electrochemical active materials of electrode for electrochemical capacitors (supercapacitors). A series of modified SWCNTs has been obtained by oxidation of the as-grown SWCNTs in air at increasing temperatures in the range 300-550degreesC. The as-grown and modified samples have been characterized by Raman spectroscopy, N-2 adsorption, and transmission electron microscopy (TEM). Raman spectra of the modified samples show that heating at increasing temperature produces a decrease in the intensity of radial and tangential breathing modes of SWCNTs and an alteration of the structure of SWCNTs up to complete destruction at 550degreesC. Heat-treatment at 350degreesC leads to the highest specific surface area (644 m(2) g(-1)) and the highest micropore volume (0.288 cm(3) g(-1)). TEM images of the as-grown and 350degreesC-oxidized samples show a multicomponent structure with bundles of SWCNTs that are thinner in the oxidized sample compared to the as-grown one. The as-grown and 350degreesC-oxidized SWCNTs were mixed with polyvinylidenefluoride and compacted to get electrodes. 6 M KOH and 2 M H2SO4 aqueous solutions were utilized as electrolytes. The specific capacitances (30-140 F g(-1)) are discussed on the basis of the electrode material and the electrolyte chosen. The specific capacitance is higher for the 350degreesC-oxidized SWCNTs and when KOH is used as electrolyte. Typical values of the equivalent series resistance (ESR) are in the range 1-5 Omega, except for the oxidized sample when KOH is used as electrolyte; in this case, ESR is higher. The thermal treatment improves the energy density but does not affect the power density, this last being higher when H2SO4 is used. (C) 2004 The Electrochemical Society.