Computational Quantum Chemistry Insights into the Mechanism of VO₂⁺ Reduction on Graphene-Based Electrodes

The identity of active sites for redox reactions within vanadium redox flow batteries (VRFBs) isstill controversial despite decades of research into the matter. Here, we use density functional theory to examine the premise of selected surface functional groups as active sites and provide mechanistic insights into the reaction pathway for the positive electrode reaction. The adsorption of electroactive species on phenol and carbene-like edge carbon sites was compared using model aromatic clusters. Phenol groups were not favorable sites for the chemisorption of VO₂⁺ in either V-down or O-down approach In contrast, carbene-like edge carbon sites readily adsorbed VO₂⁺ via an oxygen-down approach, mimicking gas-phase CO₂ adsorption mechanisms. Subsequent steps to complete the reaction pathway are a series of proton adsorptions and reaction products desorption. The rate-determining step for a reaction pathway using an edge site is VO²⁺ desorption step with a Gibbs energy of activation of +84 kcal mol⁻¹.