The Influence of Electron Deficiencies on the Activity and Stability of Graphene-Based Model Electrodes for the Vanadium(IV/V) Redox Reaction

Abstract

Carbon-based materials are widely used as electrode materials for the all-vanadium redox flow battery (VRFB) because of their superior stability and electrochemical activity over other material classes. As a result of the required high potentials, different degradation effects at the positive side (VIVO2+/VVO2+) of the battery, such as electrochemical oxidation, ion intercalation, and surface blocking by adsorbed vanadium species, have to be considered but are addressed only in a few studies. To understand the degradation mechanisms, a single graphene sheet is probed by cyclic voltammetry (CV) in a battery-like electrolyte using scanning electrochemical cell microscopy (SECCM). Ex situ Raman microscopy shows increased defect formation of the respective spots after passing higher upper vertex potentials (Euv) of the CV. To gain deeper insights into this process, additional in situ Raman probing during SECCM experiments was conducted at discrete potential steps and showed graphene G band splitting to G1 and G2 (1587 and 1620 cm–1) indicating a contribution of electron-deficient carbon (Cx+) to the active sites of the VO2+/VO2+ conversion reaction. Consequently, the graphene defects of the CV experiments can be the result of increased Cx+ formation in the graphene structure, which damages the 6-fold ring structure and finally forms oxygen-containing functional groups.