The effect of electrochemical ageing of glassy carbon electrodes on V^II-V^III and V^IV-V^V kinetics

Electrode activity towards the negative and positive half-cell reactions of a vanadium flow battery were investigated. Cyclic voltammetry and electrochemical impedance spectroscopy were employed to monitor electrode activity of glassy carbon electrodes towards V^II-V^III and V^IV-V^V redox reactions during electrochemical ageing through repeated anodic and cathodic treatments. Electrode activity is found to increase with number of treatment steps, showing little difference initially between anodised and cathodised electrodes. However, after several treatments, a potential-differentiated behaviour emerges, with distinct enhanced and inhibited states. For V^II-V^III, anodised electrodes showed enhanced activity, while cathodised electrodes were inhibited. Conversely, for V^IV-V^V, cathodised electrodes had enhanced activity. In almost all cases, the activity is greater than that of an untreated electrode. Eventually, electrode activities stabilise in a steady-state region where activity depends on the final treatment potential rather than the number of steps. In this region, activity can be toggled reproducibly between enhanced and inhibited states. Therefore, it can be concluded that functional groups, rather than surface roughening or defect formation, are responsible for this toggling capability. Furthermore, for V^IV-V^V, steady-state activity levels and the number of treatment steps required to reach this region are found to be dependent on the upper and lower treatment potentials.