Carbon Nanotube Capacitor Using Redox Reaction of Iodide Ions in Electrolyte
Authors: Yoshida Yukihiro , Taniguchi Yoshimitsu , Ishii Yosuke , Kawasaki Shinji 
Published: February 2020

Owing to their high specific surface area, single-walled carbon nanotubes (SWCNTs) have been expected as electrode materials of electric double layer capacitors (EDLCs). Many researchers have investigated and reported EDLC electrode properties of SWCNTs. However, unfortunately, the reported EDLC capacitance values were not very high comparing with the capacitance of conventional activated carbon electrodes.
On the other hand, recently new types of electrochemical capacitors using redox reactions of electrolytes have been reported. In this paper, we use the abbreviation EREC (Electrolyte Redox Electrochemical Capacitor) to express such capacitor. EREC is very attractive because it can store high energy in spite of the simple construction. In EREC, redox reaction of electrolyte occurs on one electrode while the other electrode works as a capacitor electrode (i.e. physi-sorptions of ions occur). Therefore, the redox electrode of EREC shows almost constant potential independent of the amount of charge, while the potential of the capacitor electrode is proportional to the charge. If we plot the potential difference between two electrodes as a function of accumulated charge, the plots show a straight line having half the slope value of the conventional symmetrical EDLC. It means the apparent capacity of EREC is twice as high as that of EDLC. Furthermore, only a small amount of active materials would be required for redox electrode of EREC compared to the capacitor electrode, because the redox electrode can use the entire active materials while the capacitor electrode can use only the surface of the materials. Taking it into account, it is expected that the energy density of EREC would be 2-4 times greater than that of EDLC. However, incidental shuttle reaction or charge migration of active redox species in an electrolyte between electrodes would cause the deterioration of the capacitor property of EREC. Probably due to the above reason, cycle performance has not been well discussed for the previous EREC cell using redox reaction of iodine molecules. To avoid such undesired migrations, the electrodes should have a trapping system for redox species. In the present study, we attempted to use hollow cores of SWCNTs as redox reaction sites. As well discussed in the previous reports, owing to their unique nanostructure, SWCNTs have fascinating physical and chemical properties for various kinds of application field such as energy storage, energy conversion, electronics, sensor, filter, gas storage, and medicine. Interestingly, various kinds of organic and inorganic molecules can be introduced into the hollow cores of SWCNTs, and the obtained encapsulation systems show unique electrochemical properties. We reported that it is very easy and effectively to prepare iodine molecules encapsulated in SWCNTs by an electrochemical method [1]. Using this encapsulation reaction, new type of EREC can be developed. We will talk about EREC electrode properties of SWCNTs for redox reaction of iodide ions [2].
References
[1] H. Song, Y. Ishii, A. Al-zubaidi, T. Sakai, S. Kawasaki, Temperature-dependent water solubility of iodine-doped single-walled carbon nanotubes prepared using an electrochemical method, Phys. Chem. Chem. Phys. 15, 5767-5770 (2013).
[2] Y. Taniguchi, Y. Ishii, A. Al-zubaidi, S. Kawasaki, New type of pseudo-capacitor using redox reaction of electrolyte in single-walled carbon nanotubes, J. Nanosci. Nanotechnol. in press.