Sulfonated polyetheretherketone/polyvinylidene fluoride blend polymeric cation exchange membrane as separator in zinc-iodine flow batteries

Abstract

Zinc-iodine flow batteries (ZIFBs) are very appealing owing to their abundance of active materials, low cost, high safety and environmental friendliness. The cation exchange membrane (CEM) separator contributes a significant part to the cost and determines battery performance. Sulfonated poly (ether ether ketone) (SPEEK) is a promising candidate as a CEM separator. The ionic conductivity of SPEEK CEM can be tuned by adjusting a degree of sulfonation (DS), but the mechanical property of CEM could be deteriorated. Blending poly (vinylidene fluoride) (PVDF) with SPEEK can improve the mechanical strength of SPEEK membrane. Therefore, this study examined the effect of PVDF contents (5, 10, 15 and 20 wt.%) on the physicochemical properties of SPEEK/PVDF membranes for using as CEMs in ZIFBs. ZIFBs were fabricated as static H cell and flowing stack cell configuration. Moderate DS of SPEEK (53%) was used in this study. The physicochemical properties, which are surface morphology, cross section, water uptake, electrolyte uptake, swelling ratio, ionic conductivity and iodine species crossover preventability, of the synthesized membranes and battery performances were investigated and compared with commercial Nafion115 membrane. It was found that the SPEEK, blend membrane with 5 wt.% PVDF (denoted as SP 5) and Nafion115 membrane were dense, uniform and homogeneous. The SPEEK and SP 5 membranes are further investigated for their good physicochemical properties and good battery performances. ZIFBs using SPEEK/PVDF membranes were able to be operated over 700 cycles indicating long cycle life performance. ZIFB with SP 5 membrane showed higher energy efficiency compared with Nafion115 membrane (77.52% vs. 69.71%). In addition, ZIFB with SP 5 membrane also showed significantly better stability than Nafion115 membrane corresponding to the mechanical property of membranes. Furthermore, the cost of SP 5 membrane is lower than Nafion115 membrane by around 2.5 times. Due to its low cost and other good features, the SPEEK based membranes prove to have the quality for use as CEM separator in ZIFBs.