Fabrication of SPAEK-cerium zirconium oxide nanotube composite membrane with outstanding performance and durability for vanadium redox flow batteries

Herein, an alternative sulfonated poly(arylene ether ketone) (SPAEK)-cerium zirconium oxide nanotube (Ce₂Zr₂O₇NT) composite (SPAEK/Ce₂Zr₂O₇) membrane has been proposed as a potential polymer electrolyte membrane for all vanadium redox flow batteries (VRFBs) performance. The as-prepared composite membrane showed higher ion selectivity and reduced vanadium ion crossover than the pristine SPAEK and NRE-212 (Nafion-212) membranes in VRFB operation. The vanadium ion permeability rate of SPAEK/Ce₂Zr₂O₇ (2%) composite membrane was found to be 27-fold and 12-fold lower than that of commercial NRE-212 and pristine SPAEK membranes, respectively. VRFBs operated with SPAEK/Ce₂Zr₂O₇ (441 h) were shown to have a lower self-discharge rate than pristine SPAEK (172 h) and NRE-212 (42 h) membranes, respectively. Finally, an electrochemical test of VRFB exhibited high retention capacity after 100 cycles for SPAEK/Ce₂Zr₂O₇ membrane as compared to NRE-212 and pristine SPAEK, while the coulombic efficiency (99.42%) at 40 mA cm^-2. The structure and morphology of Ce₂Zr₂O₇NT and the composite membrane were investigated by means of XRD, SEM, and TEM, and a consistent chemical structure of SPAEK/Ce₂Zr₂O₇ (2%) composite membrane before and after the chemical stability test was confirmed using ¹H NMR. The composite with 2% filler exhibits improved thermal, mechanical, oxidative, and chemical stability with superior ion selectivity. Also, X-ray photoelectron spectroscopy (XPS) analysis of SPAEK/Ce₂Zr₂O₇ (2%) composite membrane reveals the existence of a vanadium peak after 100 VRFB cycles.