Electrode Kinetics and Electrolyte Stability in Vanadium Flow Batteries

Two aspects of vanadium flow batteries are reviewed: electrochemical kinetics on carbon electrodes and positive electrolyte stability. There is poor agreement between reported values of kinetic parameters; however, most authors report that kinetic rates are faster for V-IV/V-V than for V-II/V-III. Cycling the electrode potential increases the rates of both reactions initially due to roughening but when no further roughening is observed, the V-II/V-III and V-IV/V-V reactions are affected oppositely by the pretreatment potential. Anodic pretreatment activates the electrode for the V-II/V-III reaction, and deactivates it for V-IV/V-V. Three states of the carbon surface are suggested: reduced and oxidized states R and O, respectively, both with low electrocatalytic activity, and an intermediate state M with higher activity. The role of surface functional groups and the mechanisms of electron transfer for the V-II/V-III and V-IV/V-V reactions are still not well understood. The induction time for precipitation of V2O5 from positive electrolytes decreases with temperature, showing an Arrhenius-type dependence with an activation energy of 1.79 eV in agreement with DFT calculations based on a VO(OH)(3) intermediate. It also decreases exponentially with increasing V-V concentration and increases exponentially with increasing sulphate concentration. Both arsenate and phosphate are effective additives for improving thermal stability.