Fluorinated sulfonated poly(fluorenyl ether ketone) (F-SPFEK) membranes and composites with 3-aminopropyltriethoxylsilane (F-SPFEK-APTES) were prepared for evaluation in vanadium redox flow battery (VRB) devices. Improved oxidative stability as well as increased water uptake and proton conductivity were found after direct fluorination of a sulfonated poly(fluorenyl ether ketone) (SPFEK) sample with fluorine gas. The incorporation of a polysilsesquioxane network within the fluorinated membrane effectively suppressed vanadium permeation without a large decrease in proton conductivity, leading to a membrane with a more optimized balance of proton conductivity and VO2+ permeability for good VRB performance. Both F-SPFEK and F-SPFEK-APTES membranes showed promising characteristics for VRB application, but the coulombic efficiency of the VRB with the F-SPFEK-APTES composite membrane was 21.5% higher than the VRB with a SPFEK membrane. (c) 2012 Elsevier B.V. All rights reserved.