In situ spectroelectrochemical Raman studies of vanadyl-ion oxidation mechanisms on carbon paper electrodes for vanadium flow batteries

The elucidation of redox reaction mechanisms utilized in vanadium flow battery opens broad opportunities for targeted improvement of performance in specific power and energy efficiency. However, there is no unified agreement with regard to the catholyte reaction mechanism as common electrochemical methods such as cyclic voltammetry show general current-voltage response without distinguishing the various vanadium reaction intermediates involved in the redox reaction process. The literature describes several mechanisms that vary from electrode carbon composition, surface functionality, and electrolyte composition. Still, all proposed reaction pathways were derived from indirect models, and thus the introduction of an additional method for monitoring the speciation of vanadium moieties and for spatially resolving redox processes at the electrode/electrolyte interface is required for reliable validation of the reaction mechanism. Herein, we introduce a new spectroelectrochemical technique, which combines Raman spectroscopy with cyclic voltammetry as a powerful tool for deducing complex redox mechanisms at carbon paper electrodes. In particular, the vanadyl oxidation reaction mechanism was investigated on heated carbon paper with the help of in situ Raman Spectroelectrochemistry. Significant differences in the reactivity of carbon paper was found for electrodes activated at different temperatures and show that the fibers are more susceptible to oxidation, while the binder burns, which leads to a decrease in electrochemical active surface area. The monitoring of vanadyl oxidation showed two different reaction mechanisms on the fiber surface and the alternative one for the binder phase. Thus, this new spectroelectrochemical technique is an effective tool to track species transformation during redox reaction not only for catholyte in Vanadium Flow batteries but also for other systems.