Estimation of charge transfer kinetic parameters from irreversible cyclic voltammograms at carbon fiber electrodes

Abstract

As an example of cyclic voltammograms associated with heterogeneous electrode kinetics at microcylinder electrodes, reduction waves of Fe 3+ were measured at carbon fiber electrodes. They showed the following three distinctive forms depending on the potential sweep rate: (a) peak-shaped waves as are often observed at a large planar electrode; (b) peak potentials too vague to be determined; (c) a loss of the anodic peak due to convection at very slow potential sweep rates. A graphical method of determining the peak potentials of waves (b) is presented. Peak potentials and half-peak potentials do not depend on the potential sweep rate. This fact is in contrast with the significant dependence on the sweep rate at a large planar electrode. The charge transfer rate constant for Fe 3+/Fe 2+ was evaluated from the peak potential and the half-peak potential. The value obtained from the peak potential was half of that from the half-peak potential because of heterogeneity of the carbon fiber surface. The rate constant from the peak potential provides a quantitative measure of surface activation of carbon fiber electrodes. Application of this evaluation was directed to carbon fabric electrodes employed in redox flow batteries.