In this study, a series of the commercial Nafion membranes (equivalent weight of 1100 g mol(-1)) with thickness of 50 mu m (Nafion 112), 88 mu m (Nafion 1135), 125 mu m (Nafion 115), and 175 mu m (Nafion 117) are selected to investigate the thickness impact of Nafion on the performance of vanadium redox flow battery (VRFB). VRFBs are evaluated at a broad current density range from 40 to 320 mA cm(-2). The self discharge process, cell efficiencies, electrolyte utilization, and long-term cycling stability of VRFBs based on various Nafion membranes are compared comprehensively. Owing to the good balance between vanadium ions permeability and membrane resistance, Nafion 115 membrane presents the highest energy efficiency and electrolytes utilization at current density from 120 to 240 mA cm(-2). A simple and powerful online monitoring system is developed to monitor the electrolyte imbalance process during the cycle-life test of VRFB. The capacity fading mechanism of VRFBs with different Nafion membranes is also discussed. (C) 2016 Elsevier B.V. All rights reserved.