Sulfonated carbon nanotubes (CNTs) was prepared by direct hydrothermal treatment in chlorosulfonic acid and employed as electrocatalysts for V3+/V2+ redox reaction for vanadium redox flow battery. The sulfonic groups introduced on the surface of CNTs not only significantly promote the accessibility of vanadium electrolyte, but also provide more active sites for V3+/V2+ redox reaction. Therefore, a series of sulfonated CNTs exhibit higher electrochemical activity and reversibility toward V3+/V2+ redox reaction compared with pristine CNTs. In particular, sulfonated CNTs treated for 10 hrs (CNTs-10) demonstrate the best electrocatalytic performance. The cell using CNTs-10 as negative catalysts achieves excellent charge-discharge performance with voltage efficiency and energy efficiency of 68.34% and 65.56% at a current density of 120 mA cm(-2), respectively, which are higher than those (60.74% and 60.50%) for the pristine cell. The excellent electrocatalytic performance of sulfonated CNTs toward V3+/V2+ redox reaction is ascribed to the promoted electrochemical kinetic process of V3+/V2+ redox reaction and the accelerated mass transfer of vanadium ions. (c) 2018 The Electrochemical Society.