What decides the kinetics of V2+/V3+ and VO2+/VO2+ redox reactions - Surface functional groups or roughness?

Carbon paper (CP) electrodes are electrochemically treated in 1 M H2SO4 for various time durations to functionalize their surface with oxygenated groups. Such electrodes are subsequently used to investigate the kinetics of redox reactions relevant to the vanadium redox flow batteries (VRFBs). The results suggest that the oxidized electrodes catalyse the kinetics of V2+/V3+ reaction and impede that of the VO2+/VO2+ reaction. On cycling the electrode to lower potentials (-0.25 to -0.9 V), relevant to the negative electrode of VRFB, the V2+/V3+ reaction kinetics decelerates. Yet, the reduced electrodes show improved kinetics than that of the untreated carbon electrodes. On the other hand, the VO2+/VO2+ reaction kinetics is improved, as compared to that of the untreated carbon electrodes. The origin of such interesting and yet contradicting results are explained using XPS and Zeta-3D imaging of the oxidized and reduced carbon electrodes. The role of oxygenated functional groups and surface roughness of the electrode on the redox reaction kinetics of vanadium ions is established. (c) 2020 Elsevier B.V. All rights reserved.