Sulfur-based functional group doped mesoporous carbon (MC) catalysts are prepared to investigate their effects on V(II)/V(III) and V(IV)/V(V) redox reactions as well as on the performance of vanadium redox flow battery (VRFB). According to the electrochemical analysis, although all of the catalysts applied have a similar influence on the V(IV)/V(V) reaction, for the V(II)/V(III) reaction, the sulfenic acid-doped MC shows better catalytic activity with a faster electron transfer rate and narrower peak potential separation. Nyquist plots also support the superiority of the sulfenic acid-doped MC catalyst with a smaller charge transfer resistance. When the catalysts are loaded onto a negative electrode for measuring the performance of VRFB, the VRFB using sulfenic acid-doped MC catalyst shows 6 similar to 10% higher voltage efficiency and 5 similar to 12% higher energy efficiency than other VRFBs. The excellent catalytic activity of the catalyst for the V(II)/V(III) reaction and superior performance of VRFB are due to an increase in the-SOxH bond that acts as an active site for the reaction. (C) 2018 The Electrochemical Society.