Role of solvent dynamics on kinetics of electro-oxidation of ferrocene at platinum electrode

Abstract

The one-electron oxidation of ferrocene to ferricenium ion was used as a model system to study the role of solvent dynamics on the kinetics of heterogeneous electrochemical electron-transfer reactions. Similar types of polar solvents (methanol to heptan-1-ol) with systematically changing dielectric properties were used. Diffusion coefficients were determined using the rotating-disc electrode method. Standard rate constants for the electrooxidation of ferrocene were obtained from cyclic voltammograms. The observed diffusion coefficients were found to follow the Stokes–Einstein theory. An inverse correlation was found between the experimental and theoretical values of the standard rate constants when the theoretical results were obtainded by considering the electron-transmission coefficient, k_e1= 1, and the solvent dependence was taken into account only in the exponential factor. However, the experimental rate constants in different solvents agreed well with the rate constants obtained from the theoretical expression only when the latter was modified to take into account the solvent dependence [in terms of the dielectric and the dynamic (relaxation properties)] in the electron-transmission coefficient term at the interface as well as in the frequency factor.