The importance of wetting in carbon paper electrodes for vanadium redox reactions

As demand for renewable energy storage increases, flow batteries such as those based on the resilient vanadium redox reactants and inexpensive carbon electrode materials have become more widespread. Thus far, however, there are many conflicting results in the literature regarding the reaction kinetics of the V2+/V3+ and VO2+/VO2+ redox couples and various methods to enhance these reactions. The present work demonstrates how the misinterpretation of cyclic voltammetry for porous carbon materials may account for many of these inconsistencies. Several oxidation treatments investigated here are observed to have a significant effect on the cyclic voltammogram of the V2+/V3+ redox couple reactions. Using electrochemical impedance spectroscopy and a recently developed analytical flow cell technique, these changes are shown to be due almost entirely to the effective wetting of the porous carbon paper rather than any electroactivation effect of surface functional groups on the intrinsic kinetics of the V2+/V3+ reaction. A similar methodology is recommended for any future assessment of electrode treatments.