Sulfonated poly(indole ketone)/polyaniline (SPIK/PANI) composite membranes for aqueous organic redox flow batteries (AORFBs)

Redox flow batteries (RFBs) represent a promising solution for large-scale energy storage. These systems offer inherent operational safety, decoupled power and energy scaling and extended cycle lifetimes. As a pivotal component in RFBs, the ion exchange membrane (IEM) ensures efficient and selective ion conduction while effectively blocking the crossover of redox-active species, which is vital for the efficiency and longevity of RFBs. This study presents a composite membrane based on sulfonated poly(indole ketone) and polyaniline (SPIK/PANI) for aqueous organic redox flow batteries (AORFBs). The incorporation of PANI nanoparticles facilitates the formation of robust acid-base pairs between their -NH groups and the -SO 3 H groups within the SPIK matrix. These molecular-scale interfacial interactions act as dynamic physical cross-links, enhancing interchain cohesion and leading to a compact membrane architecture that effectively suppresses excessive swelling under high hydration conditions. Concurrently, this interfacial engineering optimizes the nanostructure of hydrophilic domains, establishing continuous ion transport pathways with reduced energy barriers. Moreover, the hollow tubular architecture of PANI creates well-defined hydrated nanochannels that enable efficient long-range ionic conduction, yielding a high proton conductivity of 129.76 mS cm -1 at 25 °C. Among all prepared compositions, the SPIK/PANI-0.6% membrane demonstrated superior performance, enabling an AORFB to achieve a peak power density of 78.01 mW cm -2 . Furthermore, the membrane exhibited remarkable stability. At a current density of 60 mA cm -2 , the battery assembled with SPIK/PANI-0.6% maintained an energy efficiency (EE) above 81%, while its coulombic efficiency (CE) showed no observable decay over 2500 cycles.