Molecularly Modulated SPEEK Membranes With Engineered Dual-Ion Transport for High-Performance Alkaline Zinc-Iron Flow Batteries.

Alkaline zinc‑iron flow batteries (AZIFBs) are attractive for large‑scale energy storage due to their inherent safety and low cost, where the ion‑exchange membrane (IEM) plays a decisive role in energy efficiency and cycle life. Although commercially available IEMs with microphase‑separated structures are well‑developed, they suffer from an intrinsic trade‑off between conductivity and selectivity. Microporous polymer membranes offer a promising alternative, yet their practical adoption is hampered by complicated synthesis and harsh processing. Herein, we report a molecularly engineered membrane by incorporating cyclohexyl units into poly(ether‑ether‑ketone) with precisely tuned sulfonation degree, which creates a tailored free‑volume structure and uniform ionic channels. The membrane operates via a size-selective dual-ion conduction mechanism, enabling fast shuttling of charge‑balancing ions while efficiently suppressing crossover of bulky Fe(CN)6 3 -. Molecular dynamics simulations confirm that uniformly distributed sulfonate groups and a fully percolated water network promote efficient ion transport, while enhanced polymer chain packing ensures high selectivity. As a result, the AZIFB with m‑SPEEK‑HMBC exhibits outstanding overall performance: energy efficiency >76.7% across 40-400 mA cm- 2, a peak power density of ∼600 mW cm- 2, and stable capacity retention for >1000 cycles at 200 mA cm- 2. This work provides a viable molecular‑design strategy toward next‑generation IEMs for cost‑effective AZIFBs.