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

ABSTRACT

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.