Kinetics of double-layer charging/discharging of activated carbon electrodes - Role of surface acidic functional groups

This work involves the role of surface acidic functional groups (SAFG) in the kinetics of double-layer charging/discharging of activated carbon powder electrode specimens in a 30 wt % H2SO4 solution using nitrogen gas adsorption, the Boehm method, ac impedance spectroscopy, a potentiostatic current transient technique, and cyclic voltammetry. For this study, two kinds of as-activated and as-reactivated carbon powder specimens with nonuniformly distributed pore size were prepared, which were characterized by roughly the same pore size distribution, but by appreciably different concentrations of the SAFG. The resistive and capacitive elements were roughly estimated using complex nonlinear least-squares fitting of the ac impedance spectra to a six-RC-element ladder network. The cathodic current transients and cyclic voltammograms (CVs) were simulated from the circuit analysis based upon the ladder network at a potential step and a potential scan, respectively, during double-layer charging/discharging using six sets of calculated time constants. Both the simulated current transients and the CVs agreed well in shape and value with those experimentally measured. From the results, we conclude that the SAFG increase the time constant and decrease the rate capability, thus causing considerable retardation of ion penetration into the pores during double-layer charging/discharging of the carbon electrodes. (C) 2001 The Electrochemical Society.