A novel composite vanadium-blocking proton exchange membrane for vanadium redox flow battery (VRB) was designed and constructed by immobilizing phosphotungstic acid (PWA) on Nano Kevlar Fibers (NKFs) via a solution-casting method. The proton/vanadium selectivity of the composite membrane is greatly improved by incorporating the complex formed by NKFs and PWA into the Nafion matrix. Simple tuning of the complex doping quantity results in different composite membranes with superior vanadium barrier properties (namely, a minimum vanadium permeability of 2.46 x 10(-7) cm(2) min(-1)). This coupled with proton conductivities reaching 0.061 S cm(-1) at room temperature indicates a proton/vanadium selectivity of 2.48 x 10(5) S min cm(-3), which is 6.3 times higher than that of recast Nafion (0.34 x 10(5) S min cm(-3)). When tested in a VRB, the performance of the single cell assembled with the composite membrane greatly outperforms that with recast Nafion at current densities ranging from 40 to 100 mA cm(-2). (C) 2019 Elsevier Inc. All rights reserved.