Cardo-polyetherketone anion exchange membranes with suitable chemical/mechanical stability and performance for all-vanadium redox flow battery

Cardo-polyethereketone (PEK-C) based anion exchange membrane (AEM) was prepared by quaternization using trimethylammonium. The structures of intermediates and AEMs were confirmed by NMR spectroscopy. The ion exchange capacity of the PEK-C AEMs was 1.4±0.1 mmol g^-1, which resulted in membranes with an ionic conductivity (sulfate form, 30 °C) of 5.6 ± 0.5 mS cm^-1 and a vanadium (IV) permeability (at room temperature) of 8.2±0.2 × 10-9 cm² s^-1. Stability tests performed by immersion of PEK-C based AEMs in 1.5M VO ₂⁺ oxidizing solution during two months resulted in no change in the chemical structure of the polymer as demonstrated by ¹H NMR, ¹³C NMR and ¹H-¹³C heteronuclear multiple quantum correlation (HMQC). PEK-C based AEMs were also stable during vanadium redox flow battery (VRFB) operation (30 mAcm^-2) for 50 charge/discharge cycles and showed coulombic and energy efficiency of 98% and 80%, respectively.