Synthesis and characterization of highly durable hydrocarbon-based composite membrane for zinc-bromine redox flow battery

Low-cost, durable, and high-performance membranes are urgent requirements for zinc bromine redox flow battery (ZBFB) applications. Sulfonated poly (ether ether ketone), SPEEK is a low-cost, ion-exchange membrane with excellent ionic conductivity, but its backbone is susceptible to the harsh bromine environment. Herein, the successful incorporation of a multifunctional nanofiller, perovskite-structured cerium titanate nanoparticles dispersed on smooth carbon nanofibers (p-CT/CNF) with varying carbon content into the polymer matrix protects the SPEEK backbone, reduces the crossover of active species and improves the selectivity of the composite membrane. At high current density of 100 mA cm⁻², the SP-p-CT₃₅₀ membrane in ZBFB shows an energy efficiency of 73.34% compared to pristine SPEEK and SP-p-CT₃₂₀ with 70.32 and 69.71%, respectively. Interestingly, the excellent dispersion of the nanofiller notably reduces the self-discharge rate of SP-p-CT₃₅₀, retaining the OCV at 1.70 V for 117 h which is 1.86 and 1.75 times greater than pristine SPEEK and SP-p-CT₃₂₀. Furthermore, with the effect of loading to determine the durability of the SP-p-CT₃₅₀ membrane, SP-p-CT₃₅₀ with a nanofiller loading of 0.5 wt% demonstrates stable battery performance across 500 cycles at a current density of 40 mA cm⁻² with an outstanding energy efficiency of 81.13%.