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Abstract A carbon nanotubes (CNTs) reinforced Nafion membrane (CNT/N) with reduced interfacial resistance was prepared and served as a promising ion exchange membrane for vanadium redox flow batteries (VRFBs). The reinforcement of CNT effectively enhanced the tensile properties of Nafion membranes. The electrochemical properties of CNT/N membrane were analyzed by electrochemical impedance spectroscopy (EIS) and fitted with an analytical model to investigate the ionic and interfacial resistances. The EIS measurement reveals that the exposed CNT on the composite membrane surface significantly reduced the interfacial resistance of the membrane. The VRFB single cell performance of the CNT/N shows higher voltage efficiency (93% vs 89%) and energy efficiency (86% vs 83%) than recast Nafion at a current density of 40 mA cm⁻². The cycling stability measurement showed that the discharge capacity retention of the VRFB cell equipped with CNT/N was greatly enhanced. The results suggest that the incorporation of CNT in ion exchange membrane is an effective approach for achieving lower membrane bulky and interfacial resistance, and thus, improving capacity retention, voltage, and energy efficiency.