(Invited) How Electrochemical Treatment History Can Change Vanadium Kinetics at Carbon Electrodes

The effect of the electrochemical treatments on the electrode kinetics of the positive and negative half-cell reactions in vanadium flow batteries was investigated by controlling treatment potentials. In general, VIV-VV electrode kinetics are enhanced at cathodic treatment potentials but are inhibited at anodic treatment potentials. In contrast, VII-VIII kinetics are enhanced after anodization but are inhibited after cathodization. However, as will be shown in this paper, the redox reaction (VIV-VV or VII-VIII) exhibiting the faster kinetics is strongly dependent on the most anodic and most cathodic treatment potential that has been applied to the electrode surface. That is, the electrochemical treatment potential window to which the electrode is exposed changes the magnitude of the activation of the electrode at all intermediate treatment potentials and the magnitude of the effect is different for the VIV-VV and VII-VIII reactions. The less negative the cathodic boundary of the treatment potential window the faster the kinetics towards both VII-VIII and VIV-VV. Whereas the less positive the anodic boundary of the treatment potential window the faster the kinetics towards VII-VIII but the slower the kinetics towards VIV-VV. Thus, the upper and lower potentials of the applied electrochemical treatment potential window can change which reaction, VIV-VV or VII-VIII, has the fastest kinetics.