Ion adsorption on the inner surface of single-walled carbon nanotubes used as electrodes for electric double-layer capacitors

In the present study, ion adsorption on the outer and inner surfaces of single-walled carbon nanotubes (SWCNTs) in different aqueous and organic electrolytes was analysed. It was found that the fundamental properties of tube size and electronic structure, particularly the transition between van Hove singularities (the band gap), reflected by the shape of the cyclic voltammogram and increase in the number of charge carriers upon doping, apparently provided additional energy for ion adsorption inside open-end SWCNTs. In addition, when cyclic voltammograms recorded at different potential scan rates were observed, the outer surface of the tubes demonstrated the behaviour of a flat electrode with less dependence on the potential scan rate when compared to the inner surface, which acts as a porous electrode showing an ohmic drop and a distorted voltammogram at high scan rates. Mathematical analysis showed that opening the inner channel of the tubes increases electrode resistance, and that the magnitude of variation in the resistance depends on the type of electrolyte.