Anode kinetics degradation in vanadium redox flow batteries-Reversible inhibition of the V2+/V3+-reaction due to V(II)-adsorption

Vanadium redox flow batteries (VRFB) typically show performance degradation due to V2+/V3+-reaction activity deterioration on their carbon anodes. The reason for the deterioration has not yet been identified. Employing different graphite rotating-disk electrodes, we demonstrate that V2+ inhibits the hydrogen evolution reaction (HER). Our findings suggest that V2+ adsorbs at cathodic potentials and deactivates the HER. Analogously, the V2+-adsorption deteriorates the V2+/V3+-activity, causing the VRFB performance degradation. With graphite felts in symmetric and full-cells, we demonstrate that the slowly degrading anode activity reflects an inhibiting V2+-coverage formation. As the rate of degradation decelerates, the coverage approaches equilibrium. We restored the V2+/V3+-activity by reversing the electrode polarity, representing V2+-desorption. At anodic potentials, the adsorbed V2+ liberates the surface, which we also confirmed in half-cells. Here, in vanadium-free sulfuric acid, the HER and V2+/V3+-activity recovered already below -0.26 V vs. NHE due to the Nernst shift. Our findings facilitate the understanding of several features of VRFB electrode behavior. Moreover, they promote operational strategies upon which the VRFB performance can be entirely restored and considerably increased.