Revealing Degradation Mechanisms by Spectroelectrochemical Investigations on the Intercalation and Deintercalation Behavior of Graphitic Electrodes During the V(IV)/V(V) Conversion

The spectroelectrochemical behavior of graphitic electrode materials, with a particular focus on intercalation and deintercalation phenomena, was studied under conditions close to the positive half-cell of a vanadium redox flow battery (VRFB). In situ Raman spectroscopic investigations reveal the development of the G, Gi, and 2D bands of a graphite foil due to the intercalation of SO42–-based ions at increasingly positive potentials. Besides that, the D band also increases significantly, but since the latter changes are (at least partially) reversible, they are not attributed to permanent modifications of the graphitic host structure but rather to distortions in the sp2-hybridized sixfold ring structure during intercalation and deintercalation. These processes directly influence the electrochemical activity and stability under reaction-relevant conditions, and “long-term” measurements clearly show a negative impact on the electrical conductivity due to gradual exfoliation processes. In general, this study demonstrates the importance of combining fundamental electrochemical research with time-resolved Raman spectroscopic in situ measurements. Key proposals for further model-based research approaches are provided with the intention of directly promoting the development of long-term stable electrode materials and, consequently, the efficiency of a VRFB.