Polyether Ether Ketone-Based Anion Exchange Membranes with Bis-imidazolium Cations for All-Vanadium Redox Flow Batteries

In this study, a series of polyether ether ketone (PEEK)-based un-cross-linked and cross-linked anion exchange membranes (AEMs) with bis-imidazolium cations are prepared to fabricate all-vanadium redox flow batteries (VRFBs). Compared with a Nafion 212 membrane, these AEMs exhibit significantly lower vanadium-ion permeabilities because of the Donnan exclusion effect of bis-imidazolium cations. Moreover, owing to their denser structure, the vanadium permeability of the cross-linked PEEK-based AEMs (CImPEEK) is less than that of the un-cross-linked PEEK-based AEMs (ImPEEK) with similar ion exchange capacity (IEC) values. The ImPEEK and CImPEEK membranes exhibited the vanadium-ion permeabilities of 4.7 × 10–10 and 5.5 × 10–11 cm2 s–1 at room temperature, respectively, which are considerably lower than the permeability of the Nafion 212 membrane (8.9 × 10–9 cm2 s–1) under the same conditions. Furthermore, the PEEK-based AEMs are more chemically stable in an electrolyte solution than the Nafion 212 membrane. The VRFB containing a CImPEEK membrane demonstrate a Coulombic efficiency of 99.5% at a current density of 120 mA cm–2, whereas the corresponding value obtained for the VRFB with a Nafion 212 membrane under the same conditions was 95.8%. Finally, the capacity retention of the former battery is superior to those of the VRFBs containing Nafion 212 and ImPEEK membranes. The results of this work indicate that CImPEEK is a promising membrane material for VRFB applications.