Selectivity enhancement of quaternized poly(arylene ether ketone) membranes by ion segregation for vanadium redox flow batteries

Quaternary ammonium densely functionalized octa-benzylmethyl-containing poly(arylene ether ketone)s (QA-OMPAEKs) with ion exchange capacities (IECs) ranging from 1.23 to 2.21 mmol g−1 were synthesized from: (1) Ullmann coupling extension of tetra-benzylmethyl-containing bisphenol A; (2) condensation polymerization with activated dihalide in the presence of K2CO3; (3) selective bromination using N-bromosuccinimide; and (4) quantitative quaternization using trimethylamine. Both smallangle X-ray scattering (SAXS) and transmission electron microscope (TEM) characterizations revealed distinct nano-phase separation in QA-OMPAEKs as a result of the dense quaternization. The QA-OMPAEK-20 with an IEC of 1.98 mmol g−1 exhibited a high SO42− conductivity of 11.4 mS cm−1 and a low VO2+ permeability of 0.06×10−12 m2 s−1 at room temperature, leading to a dramatically higher ion selectivity than Nafion N212. Consequently, the vanadium redox flow battery (VRFB) assembled with QA-OMPAEK-20 achieved a Coulombic efficiency of 96.9% and an energy efficiency of 84.8% at a current density of 50 mA cm−2, which were much higher than those of the batteries assembled with Nafion N212 and a home-made control membrane without distinct nano-phase separation. Therefore, ion segregation is demonstrated to be a strategical route for the design of high performance anion exchange membranes (AEMs) for VRFBs.