The vanadium redox flow battery (VRFB) is considered a promising candidate for large-scale energy storage because of their advantages such as long lifetime, flexible design, and safety against explosion. However, the high overpotential induced by high polarization and inferior reversibility of its electrode remain challenges to its application. Therefore, several groups have studied methods of electrode surface modification aimed at increasing the number of active sites. There are two widely used approaches to modify the electrode surface, namely, (i) introduction of oxygen-containing functional groups, such as C–OH or C=O, as active sites through thermal treatment, acid treatment, or electrochemical oxidation, and (ii) introduction of an electrocatalyst such as a noble metal, metal oxide, heteroatom, or carbon-based material. Recently, some studies reported effect of dual functionalization onto electrode surface, thus manifesting synergistic effect and improving energy efficiency of VRFB. Representative examples include dual functionalization of nitrogen and metal oxide (WO3 and Mn3O4) or nitrogen and another heteroatom (oxygen and boron).In this study, we report a simple and easy method for nitrogen-sulfur dual functionalization on the graphite felt surface to enhance the energy efficiency of VRFB via self-polymerization and graft method. In addition, surface properties were investigated using various analytical techniques, and effect of dual functionalization was examined through comparison with the nitrogen functionalized graphite felt