Mechanism of Dioxovanadium Ion Reduction on Oxygen-Enriched Carbon Surface

The reduction of dioxovanadium(5+) ion (VO2+) was fundamentally investigated using glassy carbon rotating disk electrodes (GC RDEs) as a model of the carbon electrodes of vanadium redox flow batteries (VRFBs). The enrichment of oxygen-containing functional groups on the GC surface was performed by the electrochemical oxidation and checked by X-ray photoelectron spectroscopy (XPS). The XPS results also suggested the coordination of the functional groups to the oxo/dioxovanadium(4+/5+) ions (VO2+/VO2+) after immersion into the VO2+/VO2+-dissolved solution. The enrichment enhanced the VO2+ reduction and changed the Tafel slope from −0.161 V decade−1 to −0.087 V decade−1, indicating the reduction mechanism alteration. The Tafel slope was explained by a multi-step reaction mechanism, in which the reduction of the quinone-like functional group generated the hydroxylic functional group that served as the reaction site and coordinated to the oxo/dioxovanadium(4+/5+) species.