Kinetics of V ⁵⁺ /V ⁴⁺ Redox Reaction—Butler-Volmer and Marcus Models

Abstract

Kinetics of the V 5+ /V 4+ redox reaction on Vulcan XC-72 modified glassy carbon disk electrode is investigated in a three-electrode configuration. Cyclic voltammograms of V 5+ /V 4+ redox couple suggest that the overpotential range for the kinetic analysis is limited to ±300 mV, after excluding V 4+ /V 3+ redox reaction at the negative overpotential and the oxygen evolution reaction at the positive overpotential. The linear sweep-voltammograms (LSVs) are corrected for potential drop due to solution resistance ( iR s ), mass-transfer resistance, and most importantly, for the back reaction current. These corrections are imperative to estimate the Tafel slope in the limited range of overpotential for V 5+ /V 4+ redox reaction. The charge-transfer coefficient ( α ) estimated from the Tafel slope deviates significantly from the expected value of 0.5 for the single electron-transfer reaction. Moreover, the instantaneous slope of the Tafel plot suggests that the α is overpotential dependent. Therefore, Marcus theory of electrochemical kinetics is applied to estimate the α . The reorganization energy ( λ ) calculated from the Arrhenius plots is in the range of values reported in the literature for the other redox couples.