Layer-by-layer self-assembly of Nafion-[CS-PWA] composite membranes with suppressed vanadium ion crossover for vanadium redox flow battery applications

A novel polymer-inorganic composite membrane, Nafion-[CS-PWA]_n (n = the number of bilayers), was prepared by a layer-by-layer self-assembly technique with polycation chitosan (CS) and negatively charged phosphotungstic acid (PWA) for vanadium redox flow battery applications. The UV-visible spectra and SEM results showed that the CS-PWA multilayer was successfully fabricated on the surface of the Nafion membrane, and the FTIR result showed that there was a strong interaction between the CS and PWA molecules. The obtained Nafion-[CS-PWA]_n showed much lower vanadium ion permeability compared with the pristine Nafion membrane. Accordingly, the VRFB with the Nafion-[CS-PWA]₃ membrane exhibited higher coulombic efficiency (CE) and energy efficiency (EE) together with a slower self-discharge rate than that of the pristine Nafion212 system.