The role of oxygenic groups and sp3 carbon hybridization in activated graphite electrodes for vanadium redox flow batteries.

Graphite felt is a widely used electrode material for vanadium redox flow batteries. Electrode activation leads to the functionalization of the graphite surface with epoxy, OH, C=O, and COOH oxygenic groups and changes the carbon surface morphology and electronic structure; thus, improving the electrode's electroactivity relative to the untreated graphite. In this study, we conduct density functional theory (DFT) calculations to evaluate functionalization's role towards the positive half-cell reaction of the vanadium redox flow battery. The DFT calculations show that oxygenic groups improve the graphite felt's affinity towards the VO2+/VO2+ redox couple in the following order: C=O > COOH > OH > basal plane. Projected density of states (PDOS) calculations show that these groups increase the electrode's sp3 hybridization in the same order. We conclude that the increase in the sp3 hybridization is responsible for the improved electroactivity, while the oxygenic groups' presence is responsible for this sp3 increment. These insights can help in the better selection of activation processes and optimization of their parameters.