Freestanding Phosphonium Covalent Organic Frameworks with Efficient Hydroxide Conduction for Zinc-Air Batteries

Owing to their well-defined crystalline pore structures and ordered functional ionic groups along the skeleton, ionic covalent organic frameworks (iCOFs) exhibit excellent performance and have significant potential for use in energy storage and conversion devices. Herein, we for the first time developed cationic phosphonium COFs with high hydroxide conduction even with low ion exchange capacity (IEC). Specifically, we synthesized COFs containing quaternary phosphonium groups as excellent ion transport moieties. Then, we fabricated freestanding phosphonium membranes through a vapor-assisted method, which exhibited high hydroxide conductivity of 126 mS cm^-1 at 80 °C from a minimal IEC of 1.17 mmol g^-1. The resulting film was successfully applied to zinc-air batteries, demonstrating energy density of 96.1 mW cm^-2, specific capacity of 95.0 mAh cm^-2, and stable operation over 2,300 min. Overall, in addition to investigating a novel cationic functional group, we demonstrated a freestanding film formation method of COF-based materials. The findings can provide a solid foundation for advancing the field of iCOFs to ion transport and promoting electrochemical applications.