Cyclic Voltammogram Profile of Single-Walled Carbon Nanotube Electric Double-Layer Capacitor Electrode Reveals Dumbbell Shape

The electrochemical behavior of highly crystallized single-walled carbon nanotubes (SWCNTs), having a small diameter distribution, is investigated by cyclic voltammetry (CV), using triethylmethylammonium tetrafluoroborate in propylene carbonate as an electrolyte. Unlike the CV curves previously observed by other researchers and referred to as the "butterfly" shape, the CV curve observed in the present study shows large bulges on both sides of the rest potential, thereby resembling a dumbbell. By comparison of the electronic density of states (DOS) of SWCNTs and the dumbbell CV shape, it was determined that the drastic increase of current in the dumbbell shape can be explained by the van Hove singularity in the DOS of the semiconducting SWCNTs in the sample. To validate the explanation, we performed separation of metallic and semiconducting SWCNTs by the density gradient ultracentrifugation method and measured CV curves of the two separated samples. As was expected, the two SWCNT samples showed completely different CV profiles corresponding to each DOS shape. In addition, ion adsorption inside the nanotube is discussed with attention to the change in CV curves with increasing sweep rate.