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, thereby improving the electrode's electroactivity relative to the untreated graphite. In this study, density functional theory (DFT) calculations are conducted to evaluate functionalization's contribution 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, indicating that the increase in sp³ hybridization is responsible for the improved electroactivity, whereas the oxygenic groups’ presence is responsible for this sp³ increment. These insights can aid the selection of activation processes and optimization of their parameters.