Stable fluorinated sulfonated poly(arylene ether) membranes for vanadium redox flow batteries

Partially fluorinated sulfonated poly(arylene ether) (SFPAE) copolymers were investigated as chemically stable proton exchange membranes for application in vanadium redox flow batteries (VRFB). The membranes' proton conductivity and vanadium ion permeability were quantified and correlated to other membrane properties such as water uptake and tensile modulus to provide insight into the tradeoffs in the design of new membranes for flow battery applications. The SFPAE-1.8 sample with optimized proton conductivity to vanadium permeability selectivity was selected for evaluation in a VRFB device and compared to the performance of a cell with a NAFION (R) N212 membrane. The VRFB cell with a SFPAE-1.8 membrane had higher coulombic efficiency, voltage efficiency, and energy efficiency compared to a VRFB with a N212 membrane under all tested current densities. The capacity fade of a VRFB with the SFPAE-1.8 membrane was 1.1 mA h per cycle, which was about 7 times lower than the fade experienced for a VRFB with a N212 membrane. The performance characteristics of the device could be correlated directly to the membrane properties and this work demonstrates our progress towards high-performance, low-cost, long-lifetime ion exchange membranes for electrochemical energy storage devices.