An Ultrathin Ionic COF Self‐Supporting Proton Exchange Membrane Through Stacking of Few‐Layer Nanosheets Grown From High‐Curvature Micro‐Interface for Fuel Cell

Ultrathin self‐supporting ionic covalent organic framework (iCOF) proton exchange membranes (PEM) are crucial for low‐resistance sustainable energy applications but remain challenging to fabricate. The key to fabricating ultrathin self‐supporting iCOF membranes lies in the compact stacking of iCOF nanosheets, which remains inaccessible through micro‐interfacial polymerization systems. Herein, we report for the first time an ultrathin self‐supporting iCOF PEM constructed by growing few‐layer iCOF nanosheets at high‐curvature micro‐interfaces, alongside a proposed growth mechanism. The curved iCOF exhibits reduced interlayer attraction due to a twisted configuration, and enhanced solvation‐driven exfoliation arising from greater exposure of sulfonic acid groups to water. It avoids the commonly reported nanosphere‐like morphologies in previously reported micro‐interfacial assembly systems by regulating the oil‐in‐water microemulsion interface using weakly surface‐active octanoic acid, which effectively balances between the confined oil phase growth and the unconfined aqueous phase exfoliation. As‐prepared iCOF few‐layer nanosheets (1.3–1.5 nm) expose abundant reactive terminal groups for interlayer crosslinking under the vacuum filtration pressure gradient, leading to ultrathin self‐supporting iCOF PEM (3.2–15.1 µm) with compact layer stacking, good mechanical robustness, and excellent scalability (20 cm in diameter). A fuel cell peak power density of 2017.7 mW cm −2 at 80°C is achieved, surpassing recently reported iCOF‐based membranes.