Electrochemical ion adsorption behavior of single-wall carbon nanotube capacitor electrodes

The ion adsorption/desorption behavior of single-wall carbon nanotube capacitor electrodes revealed by galvanostatic charge/discharge tests, cyclic voltammetry (CV), X-ray diffraction (XRD), Raman spectroscopy, and electrochemical quartz-crystal microbalance (EQCM) measurements are reviewed. It was found that capacitance of SWCNT capacitor electrodes depends not only on the surface area, but also on the electrical density of states (DOS) of the SWCNTs. It was also found that the inner tube space of SWCNTs is used for ion-storage. Smaller-diameter SWCNTs showed a higher gravimetric capacitance at low current densities. On the other hand, large diameter SWCNTs showed a better rate performance. The ion adsorption/desorption dynamics of SWCNT electrodes were examined by the EQCM method. The results obtained indicate that in addition to traditional ion adsorption-type charge compensation, ion desorption-type charge compensation dynamics occur inside the SWCNTs. The dependence of the rate performance of SWCNT electrodes on the tube-diameter can be explained by this mechanism. By combining the above knowledge, we propose a new type of electrochemical energy storage device which uses the redox reaction of iodine ions in the hollow space of SWCNTs. (author)