Poly(fluorenyl ether ketone)s with densely distributed multi-cationic side chains for anion exchange membranes

The anion conductivity is one of the key properties of anion exchange membranes (AEMs), which is affected by the content and distribution of ionic groups in the ionomers. Herein, a series of poly(fluorenyl ether ketone)s with multiple phenylmethyl groups were synthesized. After bromination, the phenylmethyl groups were attached with linear tri- and di-quaternary ammonium side chains. As a result, the quaternary ammonium groups were tethered closely at the aliphatic side chains which emanate from the specific segments of the aromatic backbone. The ion exchange capacity, water affinity, Br− conductivity, morphology, thermal property, mechanical property, oxidative stability, and VO2+ permeability of the obtained AEMs were studied. It has been found that both types of AEMs have well-developed hydrophilic-hydrophobic phase separation. When comparing at similar ion exchange capacity (IEC), the AEMs bearing tri-quaternary ammonium side chains have higher Br− conductivity than the AEMs bearing di-quaternary ammonium side chains, presumably because of the better phase separation in the former. All of the AEMs have significantly reduced VO2+ permeability than Nafion 212, owing to the positive effect of Donnan repulsion. Along with the other characterizations, the obtained AEMs were proven to be promising candidates for potential vanadium redox flow battery applications.