The reduction reaction kinetics of vanadium(V) in acidic solutions on a platinum electrode with unusual difference compared to carbon electrodes

With the consideration of understanding the interplays between the electrolyte and electrode for the VO₂⁺/VO²⁺ redox reactions at various electrodes and developing high-activity electrode materials for an all-vanadium redox flow battery, the reduction reaction kinetics of vanadium(V) ions on a platinum (Pt) electrode in comparison with that on carbon electrodes is investigated by steady-state potentiodynamic polarization and impedance spectroscopy measurements in sulfuric acid solutions with various pH and vanadium concentrations. No abnormal increase is observed neither in the cathodic Tafel slope nor the charge transfer resistance (R_ct) at a transition potential (E_K) on the Pt electrode. However, an abrupt change in the Tafel slope and R_ct of vanadium(V) ions is observed on carbon electrodes. The above results indicate that the reduction reaction kinetics of vanadium(V) ions in acidic solutions on the Pt electrode show some unusual difference with that on carbon electrodes, and this is the first study to report such a new phenomenon. Additionally, possible mechanism pathways for the reduction reaction of vanadium(V) ions in acidic solutions are proposed for the above-mentioned carbon and Pt electrodes in this work.