Characterization of equivalent series resistance of electric double-layer capacitor electrodes using transient analysis

In this study, transient equations based on chronoamperometry (CA), chronopotentiometry (CP), electrochemical impedance spectroscopy (EIS) and imaginary capacitance analysis (ICA) are proposed using two equivalent circuit models for the purpose of accurate estimation of the equivalent series resistance (ESR) in electric double-layer capacitor (EDLC) electrodes. After examining transient equations based on a simple resistance–capacitance series connection, alternative equations with a more complicated form are proposed using the transmission line model. From these equations, it is theoretically predicted that one-third of the electrolyte resistance within the pores contributes to the total ESR, irrespective of the electrochemical analysis method employed. As EDLC electrode materials, mesoporous carbons with different pore structure (size, surface area) are prepared by the direct template method. After fabrication of EDLC electrodes using these materials, transient experiments using CA, CP, EIS, and ICA are conducted, and a consistent ESR is obtained. From ESR comparison, it is observed that the increase in ESR is mostly attributable to the electrolytic resistance in the pores and is highly correlated with the pore structure of the carbon electrodes. Additionally, it is found that a mesoporous carbon electrode with a 2-h reaction time exhibits an improved rate performance comparable with that of ordered mesoporous carbon electrodes prepared by the templating of ordered mesoporous silica.