Composite anion exchange membranes based on quaternized cardo-poly(etherketone) and quaternized inorganic fillers for vanadium redox flow battery applications

• Quaternized cardo-poly(etherketone) AEMs were evaluated as separators for VRFB.
• The composite AEMs remained mechanically stable in VO2+ solution for 2 weeks.
• NMR spectroscopy confirmed the degradation of TMSP-TMA+ filler in VO2+ solution.
• Composite AEMs reduce vanadium species cross-mixing.

Functionalized organic/inorganic composite anion exchange membranes (AEMs) were prepared with quaternized cardo-poly(etherketone) (QPEK-C) and N-(trimethoxysilylpropyl)-N,N,N-trimethylammonium chloride (TMSP-TMA+Cl−). With an optimized loading of 20wt% of TMSP-TMA+, the sulfate ion conductivity and VO2+ permeability of the composite membrane were 8.4 ± 0.2 mScm−1 and 0.53 × 10−9cm2s−1, respectively; pristine QPEK-C AEMs had a sulfate ion conductivity of 4.5 ± 0.5 mS cm−1 and a VO2+ permeability of 1.1 × 10−9cm2s−1. Membranes with 10–20%wt% of TMSP-TMA+ exhibited better mechanical properties, with an ultimate tensile strength of 26 ± 2 MPa and an elongation at break of 32 ± 3%. The chemical and mechanical stabilities of QPEK-C/TMSP-TMA+ composite AEMs were investigated by immersing the membranes in a VO2+ solution at 30 °C and monitoring the sulfate ion conductivity and any changes in chemical structure using 1D and 2D-NMR spectroscopy. Nafion® 212, pristine QPEK-C AEM, and QPEK-C/20wt%TMSP-TMA+ composite AEMs were used as separators in an all-vanadium redox flow battery (VRFB). The coulombic efficiencies (CE) (at 100 mAcm−2) were 99% for QPEK-C and QPEK-C/20wt%TMSP-TMA+ and 95% for Nafion® 212. The battery capacity was lowered by 10% over 30 charge–discharge cycles (∼60 h) for QPEK-C and QPEK-C/20wt%TMSP-TMA+AEMs. Under similar conditions a loss of 30% of the capacity was observed for the Nafion® 212 separator.