To enhance performance of vanadium redox flow battery (VRFB), selecting appropriate polymer electrolyte membrane (PEM), which is a main element of VRFB, is important. For the purpose, we adopt sulfonated poly (ether ether ketone) (sPEEK) as the PEM and evaluate its potential application in the VRFB system. As main originality of this work, we suggest sPEEK membrane thickness effect on VRFB performance and performance of the sPEEK membranes is compared with that of Nafion membrane. For chemical characterization of sPEEK membrane, FT-IR and TGA are used, while SEM and AFM measurements are implemented for morphological analysis. UV-vis spectrophotometer is used to measure vanadium crossover flux, while membrane resistance is measured by electrochemical impedance spectroscopy. For VRFB performance tests, four different thick sPEEKs and Nafion 212 are prepared. According to the VRFB performance tests, 50 mu m thick sPEEK (sPEEK-50) exhibits low overpotential, high energy efficiency, high maximum power density and small capacity loss rate during charge-discharge cycles. Such an excellent performance of VRFB including sPEEK-50 is attributed to enhancements (i) in voltage efficiency (VE) regarding membrane thickness effect and (ii) in both VE and charge efficiency regarding membrane type effect. (C) 2015 The Electrochemical Society. All rights reserved.