Ligand-Level Heteroatom Tuning in Robust Aluminum–Organic Frameworks for High Proton Conductivity

Metal-organic frameworks (MOFs) for proton exchange membrane fuel cells suffer from a lack of clear design principles linking atomic structure to transport efficiency. Herein, we report a simple, scalable ligand-level heteroatom strategy to achieve high proton conductivity without postsynthetic modification. Two isostructural Al-MOFs, Al-IPA (benzene) and Al-PDC (pyridine), were synthesized and compared. At 80 °C and 100% RH, Al-PDC exhibits a conductivity of 2.28 × 10-1 S cm-1, 5.16-fold higher than Al-IPA (4.42 × 10-2 S cm-1) and rivaling the best-performing MOFs. Mixed-ligand analogues (Al-xIPA-(100 - x)PDC) confirm a monotonic increase with pyridine content, directly correlating transport with nitrogen density. Theoretical simulations reveal that framework-embedded pyridyl nitrogen acts as a weak-binding/fast-exchange proton relay, lowering the barrier for Grotthuss-type hopping and enhancing pore hydrophilicity to promote continuous water hydrogen-bond networks. In contrast, benzene rings lack such relay capability and rely on bulk-like water transport. This work establishes that proton conductivity in confined frameworks is governed not by intrinsic basicity, but by the interplay of hydration structure and acid-base site topology─a design principle enabling ligand-level tuning of high-performance MOF solid electrolytes.