Electrochemical studies of single-wall carbon nanotubes in aqueous solutions

Studies on the voltammetric responses, capacitance, and charge storage capability of single-wall carbon nanotube sheets or papers are described. Broad redox responses have been observed probably due to the presence of oxygen-containing functional groups attached to the surface of the nanotubes or to the impurities produced during nanotube purification in nitric acid. Annealing the material at 900°C eliminates these responses. The voltammetry and capacitance of the nanotube paper varies little with either anion or cation molar mass, ion charge, or ion hydrophobicity. Acids as a group tend to produce higher capacitance but similarity between the electrochemical responses in various acids is also observed. The small variation of the capacitance within a wide range of scan rates suggests that in the time scale investigated the double layer charging process is controlled by the RC time constant and not by electrolyte ion diffusion into the pores of the nanotube paper.