Degradation of Carbon Electrodes in the All-Vanadium Redox Flow Battery.

Abstract

The charge-discharge operation of the vanadium redox flow battery degrades the electrodes over time and results in a performance and efficiency decay. The impact of extended charge-discharge cycling operation on carbon electrodes is investigated using carbon paper as a model electrode. Electrode degradation along with 70 % degradation of charge-discharge capacity was observed after 100 charge-discharge cycles of a single cell vanadium redox flow battery operating at a current density of 80 mA cm-2 at room temperature (23 °C). Raman mapping of the electrodes shows a decrease in structural defects in the negative electrode, and an increase in defects in the positive electrode, indicating differences in the degradation mechanism at each electrode. Electrochemical investigation reveals an increase in the activation overpotential at both the positive and negative electrodes. However, the negative electrode showed a higher activation overpotential indicating a higher impact of electrode degradation on the negative side. X-ray photoelectron spectroscopy shows around an eightfold increase in surface oxygen functional groups after degradation in both positive and negative electrodes. The composition of oxygen functional groups was also observed to change significantly after degradation from dominantly carbonyl-based to a combination of carbonyl- and carboxyl-based groups. This study provides insight into the electrode degradation mechanism and highlights the differences in the mechanism for the positive and negative electrodes.