Edge Site Catalyzed Vanadyl Oxidation Elucidated byOperandoRaman Spectroscopy

Abstract

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 vibra-tional 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 graphiterelated feature shows direct electronic interactions between vanadium ions and carbon atoms during charge transfer.

Defective Carbon for Next‐Generation Stationary Energy Storage Systems: Sodium‐Ion and Vanadium Flow Batteries

“…It is applied for diverse phenomena, such as the oxygen evolution reaction in metal oxides, [129] the lithiation [130] and sodiation [131] in carbonbased electrodes, carbon dioxide reduction on copper, [132] and the vanadyl oxidation on carbon paper [123] and graphite. [124] Reddy et al [131] and Euchner et al [133] used operando Raman spectroscopy to investigate the intercalation mechanism and the storage process in HC. Similar to graphite, HC also exhibits a G-band associated with a first-order longitudinal optical phonon mode (E 2g ), which resembles the in-plane stretching of C=C bonds, and a D-band caused by disorder such as edges, stacking faults or atomic defects, which corresponds to an A 1g breathing mode of a C ring.…”

“…Raman spectroscopy proves particularly well‐suited for this purpose due to its utilisation of visible light, which makes experiments possible under ambient conditions Hence, it has garnered significant favour for numerous electrochemical systems, such as batteries and electrocatalysis. It is applied for diverse phenomena, such as the oxygen evolution reaction in metal oxides, [129] the lithiation [130] and sodiation [131] in carbon‐based electrodes, carbon dioxide reduction on copper, [132] and the vanadyl oxidation on carbon paper [123] and graphite [124] …”

“…An important factor in understanding how defects work is not only controlled fabrication, but also the application of advanced characterisation techniques. While techniques such as NMR or operando Raman spectroscopy have been used to analyse HC electrodes, there are few studies reporting such mechanistic experimental investigations for the felt electrodes used in VFBs [123,124] . Therefore, we will shortly discuss some general ideas and topics for understanding defects in HC and encourage the interested reader to consider this as a topic for future review articles.…”