Recent Progress in our Understanding of the Degradation of Carbon-based Electrodes in Vanadium Redox Flow Batteries - Current Status and Next Steps -

This mini-review summarises and discusses recent findings form the literature on the degradation of carbon-based electrodes for vanadium redox flow batteries (VRFBs). It becomes evident that the focus of current investigations is on carbon paper, carbon felt and graphite felt electrodes, which is understandable from a practical point of view. However, the structural complexity of these materials often prohibits doubtless attribution of observed performance reduction (or increase) to changes in the electrode materials. Among the discussed major causes for degradation are formation or change of surface functional groups, changes in the carbon sp2/sp3 ratio, intercalation of ions as well as formation of inhibiting adsorbates. In order to gain deeper insight into the changes of carbon electrodes in VRFBs under relevant operation conditions, the authors suggest reducing complexity of the investigated materials and applying in situ-studies under well-defined and controllable conditions on model electrodes. These studies then should be extended towards more practical systems and may finally help to reduce degradation phenomena including enhanced overvoltages and thus could improve cycling and energy efficiency as well as long-term stability of vanadium redox flow batteries. The recent progress in our understanding of electrode degradation in VRFBs is reviewed and contextualised. Future work aiming at a comprehensive and fundamental understanding of electrode degradation based on model systems is suggested. image