Highly stable membranes based on sulfonated fluorinated poly(ether ether ketone)s with bifunctional groups for vanadium flow battery application

A facile strategy for fabricating a sulfonated poly (ether ether ketone) ion exchange membrane with high chemical stability, excellent ion selectivity and battery performance is presented. The essence of this strategy lies in the introduction of benzotrifluoride bifunctional groups that function to protect the ether bond (through the conjugation effect of the benzene ring) and restrict swelling behaviour of the membrane (resulting from the –CF3 hydrophobic groups). With the introduction of benzotrifluoride bifunctional groups, the SFPEEK membranes exhibited excellent ion selectivity and cell performance under vanadium flow battery (VFB) operation conditions, exhibiting a columbic efficiency of 98.32% and an energy efficiency of 87.74% at a current density of 80 mA cm−2, which is much higher than commercial Nafion 115 (CE = 94.83%, EE = 83.25%). A VFB single cell assembled with a SFPEEK membrane shows a stable performance after continuously running more than 1050 cycles, which is by far the longest cycle life reported on sulfonated aromatic cation exchange membranes. These results demonstrated that the sulfonated poly (ether ether ketone) ion exchange membranes with benzotrifluoride bifunctional groups are promising candidates for VFB systems.