Peak potential shift of fast cyclic voltammograms owing to capacitance of redox reactions

Abstract

Cyclic voltammograms of a ferrocenyl derivative in aqueous solution showed diffusion-controlled peak, of which currents deviated downward from the proportionality of the square-root of potential scan rates, v, and of which potentials were shifted with an increase in v. The dependence of the peak on v can be attributed conventionally to sluggish charge transfer rates and the solution resistance. This redox species has been proved to exhibit extremely fast transfer rates by microelectrode voltammetry. The solution resistance was suppressed by using small currents less than 3 μA at a thin cylindrical electrode even at scan rates of 8 V s−1. The dependence was inconsistent with the theory for the heterogeneous kinetics. It can be explained in terms of the negative capacitance associated with the redox reaction. Electrochemical reactions occur in the direction of relaxing the applied electric field, as for the double layer charging currents. However, the electrochemically generated charge enhances the electric field, so that the negatively charging current should flow. The negative capacitance was close to ca. −64 μF cm−2 for 1 mM redox concentration. The values varied linearly with the concentration.