Single-walled carbon nanotubes as electrodes in supercapacitors

Abstract

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 300-550°C. The as-grown and modified samples have been characterized by Raman spectroscopy, N2 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 550°C. Heat-treatment at 350°C leads to the highest specific surface area (644 m2 g–1) and the highest micropore volume (0.288 cm3 g–1). TEM images of the as-grown and 350°C-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 350°C-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 350°C-oxidized SWCNTs and when KOH is used as electrolyte. Typical values of the equivalent series resistance (ESR) are in the range 1-5 , 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.