Tunable surface chemistry of carbon electrodes and the role of surface functionalities towards vanadium redox reactions

The surface chemistry of carbon electrodes plays a vital role towards the kinetics of vanadium redox reactions of vanadium redox flow batteries (VRFB). In this study, O₂, CO₂, N₂, and NH₃ plasmas have been employed to modify the surface chemistry of graphite felt electrodes, and the effect of various surface functional groups on negative and positive electrode reactions of VRFB has been explored. Various analytical techniques, electrochemical characterizations and asymmetric charge–discharge experiments reveal that the surface functional groups have a specific role towards vanadium redox reactions on each side of a VRFB cell. The C=O and O-C=O groups show catalytic effects while the C-O group has an inhibiting effect on the kinetics of negative and positive electrode reactions. Pyrrolic-N shows catalytic effects on the positive electrode reactions whereas oxidized-N has catalytic effects on the negative electrode reactions. Amine groups introduced by NH₃ plasma worsen the sluggish kinetics of the negative electrode reactions by boosting hydrogen evolution reaction. Based on the specific behavior, an optimal arrangement of each plasma-treated felt has also been proposed by either utilizing it as negative electrode, positive electrode or both in a VRFB cell, leading to an overall improved performance.