Edge Site Catalyzed Vanadyl Oxidation Elucidated by Operando Raman Spectroscopy

The kinetic processes responsible for the efficient oxidation of dissolved vanadyl oxide species in the positive half-cell of a vanadium flow battery are far from being understood. Despite recent evidence that the reaction is most strongly favored at hydrogen-terminated graphite edge sites, a mechanism involving oxygen-containing surface groups has still been frequently reproduced to date. In this work, operando Raman spectroscopy follows the reaction at the interface between graphite-based model electrodes and vanadium-containing sulfuric acid as the electrolyte. The potential-dependent growth of different vibrational modes is related to the electrocatalytic activity of the sample and allows to track the oxidation of the electrolyte species. Moreover, the results express vanadium reaction intermediates of dimeric origin only on the edge-exposed surface of graphite, which exhibits significantly higher electrochemical activity. No interaction with surface oxygen postulated before could be observed for the active electrodes at potentials relevant to the reaction. Instead, a new growing graphite-related feature shows direct electronic interactions between vanadium ions and carbon atoms during charge transfer.