High energy efficiency and stability of vanadium redox flow battery using pore-filled anion exchange membranes with ultra-low V⁴⁺ permeation

Anion exchange membranes for vanadium redox flow battery are prepared by filling pore of polytetrafluoroethylene (PTFE) substrates with poly(arylene ether ketone) with imidazole (PAPI). For facile impregnation of PAPI molecules into pore of hydrophobic PTFE, catechol and polyethyleneimine (PEI) are co-deposited to hydrophilize the surface of PTFE. Compared with the pristine PAPI membranes, dimensional and mechanical properties of the pore-filled membranes improved due to the reinforcing effect of PTFE substrate. While this process reduces water uptake and swelling ratio by 10 % and 8 %, respectively, it enhances the tensile strength by 5 folds. Strikingly, the ion conductivity even slightly increases along with the huge reduction of vanadium ion permeability. Pore filed membranes reveal vanadium ion permeability of 1.4 × 10⁻⁷ cm² min⁻¹–2.08 × 10⁻⁷ cm² min⁻¹ at room temperature, which is 10 times and 3 times lower than those of the commercial Nafion117 and FAP450 membranes, respectively. For the cycling test of VRFB cell with 200 cycles, it shows the coulombic and energy efficiency over 96.5 % and 85 %, respectively, because of the low vanadium permeability. In association with this, the capacity retention behavior is more stable than the commercial membranes.