The Role of Charge Redistribution in the Self-discharge of Electrochemical Capacitor Electrodes

Abstract

This work examines the role of charge redistribution in the self-discharge of electrochemical capacitor electrodes. Electrochemical capacitors are charge storage devices which have high power capability and a long cycle life, but have a low energy density compared to other devices, coupled with a high rate of self-discharge which further diminishes the available energy. The mechanisms of self-discharge in electrochemical capacitors are poorly understood, and it is important to gain a better understanding of the electrode processes which lead to self-discharge, in order to minimize self-discharge and enhance electrochemical capacitor performance. To learn more about charge redistribution and its role in the self-discharge of electrochemical capacitors, multiple self-discharge experiments were performed on carbons with various surface areas/pore structures and in various electrolytes. Charge redistribution was also examined in a model pore (a transmission line circuit based on de Levie?s model of a porous electrode) and results from this model were compared to the self-discharge of a high surface-area carbon. Results demonstrate that charge redistribution is a major component of the self-discharge in high surface-area carbons. Results also indicate that charge redistribution requires a much longer time than previously thought (tens of hours rather than minutes) which further highlights the importance of charge redistribution during self-discharge. Therefore when performing mechanistic studies of self-discharge in electrochemical capacitors, it is important that effects of charge redistribution are not neglected. The self-discharge profiles of various pore shapes were also examined using the model pore, and results emphasize the superiority of cone and cylindrically shaped pores, and the disadvantages of restrictive pore mouths and bottlenecks for high power applications.