Functionalized organic/inorganic composite anion exchange membranes (AEMs) were prepared with quaternized cardo-poly(etherketone) (QPEK-C) and N-(trimethoxysilylpropyl)-N,N,N-trimethylammonium chloride (TMSP-TMA(+)Cl(-)). With an optimized loading of 20wt% of TMSP-TMA(+), the sulfate ion conductivity and VO2+ permeability of the composite membrane were 8.4 +/- 0.2 mScm(-1) and 0.53 x 10(-9)cm(2)s(-1), respectively; pristine QPEK-C AEMs had a sulfate ion conductivity of 4.5 +/- 0.5 mS cm(-1) and a VO2+ permeability of 1.1 x 10(-9)cm(2)s(-1). Membranes with 10-20%wt% of TMSP-TMA(+) exhibited better mechanical properties, with an ultimate tensile strength of 26 +/- 2 MPa and an elongation at break of 32 +/- 3%. The chemical and mechanical stabilities of QPEK-C/TMSP-TMA(+) composite AEMs were investigated by immersing the membranes in a VO2+ solution at 30 degrees C and monitoring the sulfate ion conductivity and any changes in chemical structure using 1D and 2D-NMR spectroscopy. Nafion (R) 212, pristine QPEK-C AEM, and QPEK-C/20wt%TMSP-TMA(+) composite AEMs were used as separators in an all-vanadium redox flow battery (VRFB). The coulombic efficiencies (CE) (at 100 mAcm(-2)) were 99% for QPEK-C and QPEK-C/20wt%TMSP-TMA(+) and 95% for Nafion (R) 212. The battery capacity was lowered by 10% over 30 charge-discharge cycles (similar to 60 h) for QPEK-C and QPEK-C/20wt%TMSP-TMA(+)AEMs. Under similar conditions a loss of 30% of the capacity was observed for the Nafion 212 separator. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.