Highly selective sulfonated poly(ether ether ketone)/polyvinylpyrrolidone hybrid membranes for vanadium redox flow batteries

A novel amphoteric membrane was designed by blending polyvinylpyrrolidone (PVP) with sulfonated poly(ether ether ketone) (SPEEK) to fabricate a vanadium redox flow battery. Acid-base pairs were formed by sulfonic acid and heterocyclic nitrogen, and the dense network structures were interwoven by the hydrogen bonds between the acid-base pairs. Vanadium ions were blocked by these network structures such that their penetration was weakened. Compared with Nafion115 and SPEEK membranes, the ion selectivity of SPEEK/PVP (S/P) hybrid membranes of various proportions was improved, of which the S/P-30% membrane performed best (106.1*10(4) S min cm(-3)). At the same time, these hybrid membranes showed far superior performances than the single-component membranes in a single-cell system. Most notably, the coulombic efficiency and energy efficiency achieved by the battery with a S/P-30% hybrid membrane were 97.5% and 84.5% at 60 mA cm(-2), respectively, with notable stability after 50 cycles. The corresponding open-circuit voltage of the battery was maintained above 0.8 V for more than 73 h. This acid-base hybrid membrane has reached a high level of overall performance in binary non-fluorine proton exchange membranes in recent years.