Sulfonated poly (ether ether ketone)-based proton exchange membranes for vanadium redox battery applications

• Synthesized and tested SPEEK membrane for vanadium bromide redox battery.
• Bromine complexing agents were used to suppress bromine vapor during charging.
• Cell performance efficiency in SPEEK membranes was slightly higher than in Nafion 117.
• The SPEEK membrane was found to be chemically stable in vanadium electrolytes.

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 G1 vanadium redox battery (VRB) before being evaluated in the G2 vanadium bromide redox flow battery (V/Br), and the performance was compared to that 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 time, the SPEEK membrane was evaluated in the 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 Nafion 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.