Composition of the Electrode Determines Which Half-Cell's Rate Constant is Higher in a Vanadium Flow Battery

Vanadium flow batteries are a promising system for stationary energy storage. One of their shortcomings is a low power density caused by slow kinetics of the redox reactions. To alleviate this drawback, many studies tried to catalyze the redox reactions. However, up to now, there is no consensus in the literature on which of the two half-cell reactions, the V²⁺/V³⁺ or the VO²⁺/VO₂⁺reaction, features the slower electron transfer. The present study is the first showing that reaction rates for the half-cells are of the same order of magnitude with their respective rate constants depending on the composition of the electrode material. The surface functional groups hydroxyl, carbonyl, and carboxyl on carbon increase the wetted surface area, catalyze the V²⁺/V³⁺ redox reaction, but impede the VO²⁺/VO₂⁺ redox reaction. This complex situation was unraveled by using a newly developed procedure based on electrochemical impedance spectroscopy. Reaction mechanisms based on these results are discussed.