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.