Kinetics of double-layer charging/discharging of the activated carbon fiber cloth electrode

Abstract

The effects of pore length distribution (PLD) and solution resistance $R_sol$ on the kinetics of double-layer charging/discharging of the porous activated carbon fiber cloth electrode were investigated in a 30 wt % $H_2SO_4$ solution using scanning electron microscopy (SEM), nitrogen gas adsorption, ac-impedance spectroscopy, current transient technique, and cyclic voltammetry. SEM micrograph revealed that the pores comprising the carbon electrode were cylindrical in shape. The Nyquist plot of the impedance spectrum experimentally measured on the carbon electrode hardly followed the ideal impedance behaviour of a cylindrical pore over the whole frequency range. From the Nyquist plots of the impedance spectra theoretical calculated based upon the transmission line model in consideration of pore size distribution (PSD) and PLD, it was suggested that such non-ideal impedance behaviour is mainly due to PLD, rather than due to PSD. The cathodic current transients and cyclic voltammograms were theoretically calculated based upon the transmission line model in consideration of PLD as functions of standard deviation σ of PLD and $R_sol$. From the results of the derivatives of the calculated logarithmic cathodic current transients, it was found that σ and $R_sol$ significantly affect the ion penetration into the pores during double-layer charging/discharging of the carbon electrode. The rate capability γ defined as the quotient of the reduced charge for the carbon electrode divided by the reduced charge for the ideal double-layer capacitor decreased with increasing σ and $R_sol$. This suggests that the ion penetration into the pores during double-layer charging/discharging of the carbon electrode is more retarded as σ and $R_sol$ increase.