Giving physical insight into the Butler-Volmer model of electrode kinetics: Application of asymmetric Marcus-Hush theory to the study of the electroreductions of 2-methyl-2-nitropropane, cyclooctatetraene and europium(III) on mercury microelectrodes

The asymmetric Marcus-Hush (MH) model for electrode kinetics is applied to the kinetic study of the electroreduction of 2-methyl-2-nitropropane in acetonitrile, cyclooctatetraene in dimethylsulfoxide and europium(III) in aqueous solution, using mercury microhemispheres as working electrodes. This kinetic model includes the possibility of the oxidative and reductive processes having different reorganization energies due to differences between the force constants of the electroactive species. For each redox couple, the response obtained in cyclic and square wave voltammetries can be fitted satisfactorily with the four-parameter asymmetric MH model. From the fitting of the voltammograms the values of the kinetic parameters are extracted and analyzed in terms of physical properties of the electroactive species. A comparison of the asymmetric model against the simpler, phenomenological Butler-Volmer (BV) approach is discussed, as well as a possible physical interpretation for the BV transfer coefficient.