Sulfur and nitrogen containing groups have been bonded to carbon in graphite felt electrodes using a hydrothermal treatment with ammonium persulfate (APS) for use in an aqueous all-vanadium redox flow battery (VRB). Our results reveal that the hydrothermal treatment with APS is a very effective means to attach both sulfur and nitrogen groups to the surface of graphite felt, and as a result, the inclusion of these groups significantly improved the voltaic efficiencies (∼95%), discharge capacities (>400 mAh), and capacity retention (∼83%) of these batteries at current density of 50 mA/cm2. The direct addition of reactive sulfur and nitrogen groups also substantially extended the range of current densities available to the battery (in excess of 200 mA/cm2) without sacrificing capacity and capacity retention. Linear sweep voltammetry indicated that APS treated graphite felts showed an electrochemical enhancement factor over double that of untreated pristine graphite felts. These results suggest that the direct addition of sulfur and nitrogen containing groups on the graphite felt as a simple and effective method for improving the performance of all-vanadium redox flow batteries.