Backbone structure design of poly (aryl Isatin) membranes for enhanced performance of aqueous organic redox flow...

Aqueous organic redox flow batteries (AORFBs) have garnered widespread attention for the great potential in large-scale energy storage technology. However, as a crucial component of AORFBs, ion exchange membranes (IEMs) with high ionic conductivity and low electrolyte redox-species crossover still remains challenging. Herein, an effective polymer backbone design strategy by regulating polymer chain segments with different hydrophobic properties and rigidities is reported. Such strategy facilitates the formation of interconnected and well-ordered ion transport channels across the membrane, further resulting in a comprehensive enhancement in membrane applications. The prepared side-chain-type poly (m-terphenyl-co-diphenylethane isatin) membranes (sPTD-X) possessed remarkable water uptake, ionic conductivity and tensile strength. Compared to Nafion 212, the optimal sPTD-30 exhibited higher K⁺ conductivity and lower redox-active molecules permeation. Furthermore, the single-cell of AORFB assembled with sPTD-30 demonstrated an energy efficiency (EE) of 77.4 % (100 mA cm^-2), surpassing that of Nafion 212. Also, the sPTD-30 membrane-based cell showed outstanding cycling stability with a minimal capacity decay rate of 0.0174 % per cycle over 350 h and no significant change in the chemical structure was observed after the test. These results offer an innovative approach to designing the structure of IEMs, further improving the battery performances of AORFBs.