A mechanistic study of VO2 reduction on graphene-based electrodes: insights from computational quantum chemistry.

Abstract

The identity of active sites for redox reactions within vanadium redox flow batteries (VRFB) are still controversial despite decades of research into the matter. Here, we use density functional theory to examine the promise of select surface functional groups as active sites and provide mechanistic insights into the reaction pathway for the positive electrode reaction. The adsorption of electroactive species to phenol and carbene-like edge carbon sites were compared using model aromatic clusters. Though phenol groups were not favorable sites for the chemisorption of VO2+ in either a V-down or O-down approach, 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 were a series of proton adsorptions and reaction products desorption. The rate determining step for a reaction pathway using an edge site was the VO2+ desorption step with a Gibbs energy of activation of +84 kcal/mol.