Low cost sulfonated poly (ether ether ketone) membranes have been successfully prepared and optimized at various sulfonation conditions by casting method for vanadium redox battery applications. The optimized SPEEK membrane was initially tested in the Cl vanadium redox battery (VRB) before being evaluated in the G2 vanadium bromide redox flow battery (V/Br), and the performance was compared to that L of Nafion 117. From the G1 VRB performance tests, the energy efficiency of the membrane was found to be 77%, slightly higher than Nafion 117 which gave 73% at current density 40 mA cm(-2). For the first Lime, the SPEEK membrane was evaluated in he G2 V/Br at two different ratios of bromine complexing agents and the performance was assessed from the measured cell efficiencies. At 4 mA cm(-2), the optimum energy efficiency was 76% using SPEEK in the presence of 0.19 M MEM and 0.56 M MEP, when compared to 75% obtained with Nation 117. Similar to Nafion 117, SPEEK also exhibits excellent chemical stability in the highly oxidizing electrolytes. The SPEEK membrane thus appears to be a promising candidate for both G1 and G2 VRB applications. (C) 2013 Elsevier B.V. All rights reserved.