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;were synthesized from:（1） Ullmann coupling extension of tetra-benzylmethyl-containing bisphenol A;（2） condensation polymerization with activated dihalide in the presence of K;CO;;（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;exhibited a high SO;conductivity of 11.4 mS cm;and a low VO;permeability of 0.06×10;m;s;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;, 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.