A novel imidazolium-based amphoteric membrane for high-performance vanadium redox flow battery

Abstract

A novel amphoteric membrane was prepared using imidazolium-functionalized polysulfone (ImPSf) as the base polymer and sulfonated poly (ether ether ketone) (SPEEK) as the acid polymer for vanadium flow battery applications. The introduction of the ImPSf effectively resists the migration of vanadium ions owing to the Donnan repelling effect of the imidazolium cations on the vanadium ions and the low swelling ratio of the amphoteric membranes due to the ionic cross-linked interactions between the imidazolium and sulfonic groups. The amphoteric membrane with an ImPSf mass ratio of 17% shows a vanadium permeability of 1.5 × 10−8cm2s−1 that is much lower than that of Nafion212 (21 × 10−8cm2s−1). Having a high IEC of 2.04mmolg−1, it also exhibits a low area resistance of 0.48Ωcm−2 that is comparable to that of Nafion212 membrane (0.41Ωcm−2). As a result, the VFB cell with the amphoteric membrane exhibits high performances even at high current densities, i.e. at a current density of 200mAcm−2, and the amphoteric membrane achieves a coulombic efficiency of 97.5% and an energy efficiency of 77.3%, both of which are higher than those of Nafion212 (92.4% and 73.4%). These results indicate that the ImPSf/SPEEK amphoteric membrane is promising for VFB applications.