Sulfonated PEEK-based proton exchange membranes for semi-organic AQDS/bromine redox flow batteries

This work presents the first example in which sulfonated poly(ether ether ketone)s (SPEEKs) with different degrees of sulfonation (DS) in the range 50–84 mol% were used as is or blended with PVDF (up to 30 wt%) to prepare proton exchange membranes (PEMs), alternative to Nafion, for a novel energy storage system technology, namely semi-organic AQDS/bromine RFBs. By varying the DS of SPEEKx as well as the composition of the blends, it was possible to modulate the membrane properties in terms of thermal, mechanical, water uptake, ion exchange capacity and in-situ electrochemical performance. The results show that the SPEEK60 membrane was the top performer among the fluorine-free membranes, while the SPEEK60PVDF-9010 membrane outperformed all other investigated membranes, including the Nafion 115 benchmark. Indeed, they showed higher values of coulombic efficiency and energy efficiency and lower crossover of the redox-active species with respect to Nafion 115.