Non-fluorinated sulfonated poly(indole ketone) (SPIK) membranes for aqueous organic redox flow batteries (AORFBs)

Redox flow batteries (RFBs) are recognized as a safe and cost-effective technology for large-scale electrical energy storage (EES). The ion exchange membrane (IEM) is a crucial component of RFBs, facilitating the rapid conduction of charge carrier ions while minimizing the crossover of redox active species. In this study, a high-performance sulfonated poly(indole ketone) (SPIK) membrane was designed and synthesized for the application in aqueous organic redox flow batteries (AORFBs) . The main chain of the SPIK consists entirely of rigid indole rings and carbonyl groups without any ether bonds. Such structure provides strong resistance to free radical oxidation. Also, the incorporation of nitrogen-containing heterocyclic rings reduced the swelling rate (SR) of membrane and enhanced its mechanical strength (SR<10% at 25 °C, mechanical strength >58 MPa in the dry state). Additionally, the introduction of sulfonic acid groups improved the ion conductivity of the membrane (>120 mS cm -1 at 25 °C). Among all SPIK membranes, SPIK 50/50 demonstrated exceptional performance in AORFBs, exhibiting an area-specific resistance as low as 0.93 Ω cm 2 and achieving a power density of 66.06 mW cm -2 . Furthermore, at a current density of 20 mA cm -2 , the voltage efficiency (VE) reached 92.48%, highlighting its potential for high-efficiency energy storage applications. In terms of the membrane stability, SPIK 50/50 maintained an energy efficiency (EE) over 80% without obvious decay in Coulombic efficiency (CE) over 600 cycles at a current density of 60 mA cm -2 . Also, such membrane was evaluated in iron-chromium redox flow battery and vanadium flow battery to further prove its applicability in RFBs.