Colloidal silicalite-Nalion composite membranes have been synthesized with a two-layer structure consisting of a top layer of colloidal silicalite in Nation matrix and a base layer of pure Nation. The composite membrane with an overall thickness of 120-130 mu m exhibited higher proton-to-vanadyl ion transport selectivity and lower electrical resistance than the 183 mu m-thick commercial Nafion-117 (R) membrane. The improved proton selectivity in the composite membrane is due primarily to the ability of the sub-nanometer-sized zeolitic pores to block the hydrated metal ions while permitting the small hydrated protons to enter and diffuse through. The low electrical resistance of the composite membrane is attributed to the reduced thickness of the Nafion base film and the thinness of the layer containing silicalite nanoparticles ( <30 mu m). When being used as ion exchange membrane in the vanadium redox flow battery, the composite membrane with a silicalite content of 5 wt% has achieved an energy efficiency of 77% at 60 mA/cm(2) which is significantly higher than the 65% efficiency obtained by the Nafion-117 membrane. The composite membrane also has shown excellent stability after one month of cyclic operation of the battery. (C) 2015 Elsevier B.V. All rights reserved.