Single-Crystal Covalent Organic Frameworks for Anhydrous Proton Conduction Above 200°C.

The development of fast proton-conducting materials that operate above 150°C with high chemical stability is both challenging and critically important for advancing proton-exchange membrane fuel cells (PEMFCs). In this study, we constructed two three-dimensional COFs with covalent phosphonate modification using a solvent-free, melt-phase post-synthetic modification (PSM) strategy. This approach simultaneously reduces imine to amine linkages and constructs C─P bonds, covalently anchoring phosphonate groups without disrupting crystal integrity. Single-crystal x-ray diffraction (SCXRD) analysis reveals precise geometric changes in the framework and the formation of an extended N─H···O═P hydrogen-bond network. The functionalized single-crystal COFs exhibit excellent anhydrous proton conduction along the crystallographic c-axis at exceptionally high temperatures, achieving 8.91 × 10-3 S cm-1 at 210°C for COF-300-DMP and 5.65 × 10-3 S cm-1 at 230°C for COF-300-DEP. The remarkably low activation energies (0.196‒0.229 eV) indicate a Grotthuss-type hopping mechanism. This work not only establishes a generalizable route for COF functionalization but also provides a definitive blueprint for designing advanced proton conductors for extreme environments.