Determination and enhancement of the capacitance contributions in carbon nanotube based electrode systems for CNT based electrochemical capacitors/supercapacitors

摘要

Carbon nanotubes (CNTs) have been proposed for electrodes in electrochemical capacitors (ECCs)/supercapacitors primarily due to their large surface area and abundance of reaction sites[l] with the possibility of large charge storage capacity and capacitance (C). Consequently, in an electrolyte where electrochemical reactions can occur over a wide voltage range (V), large energy densities (W) per unit mass (m) can be achieved, through W = CV~2/2m. While possessing superior power densities due to the capability of fast charge/discharge, presently CNT based ECCs have lower energy densities (1-10 Wh/kg) compared to batteries (with 10-100 Wh/kg), making them less competitive compared to the latter technology. In this paper, we aim to probe the total CNT capacitance (C_T) and suggest ways in which it can be improved. Consequently, we focus on the accurate characterization and analysis of the electrostatic/double layer[3] (C_(dl)) and faradaic/pseudo-capacitive (C_p) components of the C_T. The C_(dl) arises primarily due to charge separation across the electrode/electrolyte interface while C_p requires adsorption of electroactive species coupled with charge transfer. We then suggest a method aimed at increasing C_(dl) and C_p, based on the controlled introduction of defects into the CNTs through argon irradiation. Characterization of the CNTs through Raman spectroscopy and Cyclic Voltammetry (CV) was used.