Lewis Acid–Base Interaction Boosting Ion Selectivity of Poly(Ether Ether Ketone)/Polyimide Membrane Toward High–Performance Alkaline Zinc–Iron Flow Batteries

The development of advanced ion–conducting membranes is crucial for improving alkaline zinc–iron flow batteries (AZIFBs), which face a persistent tradeoff between ion selectivity and conductivity. This work presents a sulfonated poly(ether ether ketone)/polyimide (SPEEK/PI) composite membrane engineered via Lewis acid–base interaction. The specific interaction between the –SO 3 − groups of SPEEK and the –N= sites of PI creates additional cross–linking points within the polymer matrix. This cross–linked structure effectively suppresses membrane swelling and enhances mechanical stability. Consequently, the composite membrane demonstrates superior ion selectivity by significantly hindering the crossover of active species such as [Fe(CN) 6 ] 4− . When applied in AZIFBs, the SPEEK/PI membrane enables a notable improvement in electrochemical performance. The battery achieves a high coulombic efficiency (CE) of 98.3%, an energy efficiency (EE) of 89.8%, and a voltage efficiency (VE) of 91.4% during long‐term cycling at 80 mA cm −2 , outperforming devices using pristine SPEEK membranes (CE:95.4%; EE: 86.7%; VE: 90.9%). This study provides a cost‐effective and efficient membrane design strategy based on molecular interaction, offering a promising alternative for practical flow battery applications.