Sulfonated poly(ether sulfone)/short-carboxylic multi-walled carbon nanotube composite membrane applied for vanadium redox flow battery

Sulfonated poly(ether sulfone) (SPES) is a potential polymer for replacing Nafion membranes in vanadium redox flow batteries (VRB); however, at a high degree of sulfonation, SPES membrane displays high swelling, poor mechanical stability, and high vanadium crossover. In this study, to improve membrane performance, composite membranes of SPES and short-carboxylic multi-walled carbon nanotube (denoted as SPES/SCCT membrane) were prepared and employed as the separator for VRB to evaluate the vanadium ions permeability and cell performance. The composite membrane showed the lowest vanadium ion permeability and highest selectivity compared with the Nafion221 and SPES membranes. The VRB with SPES/SCCT membrane also showed higher coulombic efficiency of about 91.7% and energy efficiency of about 84.1% but lower capacity loss and self-discharge than that of VRB with Nafion221 membrane during the cycling test, which further indicated the promising prospects of SPES/SCCT composite membrane application in VRB systems.