In order, to probe the effect of the infiltration time with chitosan solution, a series of sulfonated polyimide/chitosan (SPI/CS) composite membranes with different infiltration time including 6 h, 12 h, 24 h and 36 h were prepared and applied in vanadium redox flow battery (VRB) system. SEM images demonstrate the presence of thin CS layer on the surface of SPI membrane, and the thickness of the CS layer increases with the infiltration time. The proton conductivity and the vanadium ion barrier property of SPI/CS composite membranes increase with the increment of the infiltration time. The SPI/CS-24 and SPI/CS-36 membranes show similar proton selectivity which is over eight times of that of Nafion (R) 117 membrane. VRB single cell using SPI/CS membrane reveals higher coulombic efficiency (CE) and energy efficiency (EE) than that using Nafion (R) 117 membrane. In particular, the CE and EE for SPI/CS-24 membrane achieve 97.8% and 88.6% respectively. All experimental results indicate that the SPI/CS composite membranes are promising proton conducting membranes for VRB, among which SPI/CS-24 membrane exhibits the best combination property. (C) 2012 Elsevier B.V. All rights reserved.