Lewis Acid‐Driven Weak Electrostatic Interaction of Polybenzimidazole‐Based Membrane for Alkaline Zinc‐Iron Redox Flow Batteries

Alkaline zinc-iron flow batteries (AZIFBs) are one of the promising aqueous redox chemistries for large-scale energy storage due to their intrinsic safety and low cost. However, the energy efficiency (EE) and power density of batteries with low-cost polybenzimidazole (PBI) membranes are still limited due to the relatively poor ionic conductivity of PBI in an alkaline medium. Here, this study proposes a novel chemical approach for regulating the chemical environment of the PBI membrane. Specifically, AlF₃, as a representative Lewis acid, is employed to reduce the electron cloud density of the benzimidazole (BI) ring, weaken the electrostatic interactions between the BI ring and charge-balancing ions, and thereby accelerate the diffusion of charge-balancing ions. Characterizations on the AlF₃-PBI membranes reveal that the enhanced ion transport can be ascribed to decreased electron cloud density of BI rings and reduced ion diffusion resistance. Charge density difference analysis further illustrates the electron-withdrawing feature of AlF₃ as a Lewis acid. AZIFBs using AlF₃-PBI membrane demonstrate excellent rate capability from 40 to 240 mA cm^-2 with EE of more than 80%, and an outstanding power density of nearly 1000 mW cm^-2. The battery also performs decent cycle stability of 500 cycles at a high current density of 140 mA cm^-2 with EE over 80%, resulting from the robust stability of the AlF₃-PBI membrane and the reliability of the strategy. This study highlights the potential of Lewis acid-based chemical regulation to optimize PBI-based membranes, offering a new perspective for enhancing the performance of AZIFBs.