A highly stable membrane for vanadium flow batteries (VFBs) enabled by the selective degradation of ionic side chains

Vanadium flow batteries (VFBs) are attracting increasing attention due to their promising prospect for large-scale energy storage. However, the high cost and low selectivity of current membranes limit their further application. In this work, a highly conductive (1122 mS cm−1) and stable microporous membrane was obtained via oxidizing a dense anion exchange membrane by an in situ or off situ process. Poly[(fluorene alkylene)-co-(biphenyl alkylene)] (PFBA-QA) with pendant quaternary ammonium alkyl groups was designed and prepared. The difference in stability between the main and side chains of the ether-free polymer allows the transformation of ion channels from aggregated ion clusters in the dense membrane to micropores by sacrificing the side chain in a strong oxidative environment. As a result, a VFB assembled with the resulting membrane demonstrates a CE of 97% and an EE of 88% at a current density of 80 mA cm−2, which can stably run for more than 290 cycles. This paper provides a new option for the membrane design in VFBs.