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.

We investigated the electrochemical lithium-ion storage properties of 9,10-anthraquinone (AQ) and 9,10-phenanthrenequinone (PhQ) molecules encapsulated in the inner hollow core of single-walled carbon nanotubes (SWCNTs). The structural properties of the obtained encapsulated systems were characterized by electron microscopy, synchrotron powder X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy. We found that almost all quinone molecules encapsulated in the SWCNTs can store Li-ions reversibly. Interestingly, the undesired capacity fading, which comes from the dissolution of quinone molecules into the electrolyte, was suppressed by the encapsulation. It was also found that the overpotential of AQ was decreased by the encapsulation, probably due to the high-electric conductivity of SWCNTs.

A change in temperature can modify the ion density near the electrode surface, and create a potential difference across a supercapacitor.

We demonstrate that iodine-doping into single-walled carbon nanotubes (SWCNTs) can be effectively done using an electrochemical method. It is shown by in situ Raman measurements that the iodine-doping level can be easily and finely controlled because de-doping is also possible by changing the polarity. In situ synchrotron XRD measurements reveal that iodine molecules are mainly inserted into the hollow core of SWCNTs. The dispersion state of the iodine-doped SWCNTs in water as a function of temperature is also investigated. It is shown that the iodine-doped SWCNTs can be homogeneously dispersed in water at low temperature (ca. <15 °C).