Influence of Pretreatment on Kinetics at Carbon Electrodes and Consequences for Flow Battery Performance

Using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), it was shown for four different types of carbon that electrode treatments at negative potentials enhance the kinetics of V-IV-V-V and inhibit the kinetics of V-II-V-III while electrode treatments at positive potentials inhibit the kinetics of V-IV-V-V and enhance the kinetics of V-II-V-III. These observations may explain conflicting reports in the literature. The potentials required for activation and deactivation of electrodes were examined in detail. The results suggest that interchanging the positive and negative electrodes in a vanadium flow battery (VFB) would reduce the overpotential at the negative electrode and so improve the performance. This is supported by flow-cell experiments. Thus, periodic catholyte-anolyte interchange, or equivalent alternatives such as battery overdischarge, show promise of improving the voltage efficiency of VFBs.