Thermally Robust 1D Cu(I) Phosphonate Coordination Polymer Exhibiting Enhanced Proton Conductivity via Humidity‐Driven Pathways

The development of thermally stable solid‐state proton conductors (SSPCs) is crucial for advancing energy‐conversion devices such as proton‐exchange membrane fuel cells (PEMFCs). In this work, we report the solvothermal synthesis and characterization of a novel, 1D Cu(I) coordination polymer, {Cu(ADP) 0.5 (BPY)} n ( BAM‐5 ), based on anthracenediylphosphonate (H 2 ADP) and 4,4′‐bipyridine (BPY). Single‐crystal X‐ray diffraction revealed that BAM‐5 crystallizes in the triclinic space group and shows a 1D ladder structure connected by the H 2 ADP and organic BPY linkers, which is assembled into a 2D layer via O−H···O hydrogen bonding interactions between uncoordinated oxygen and the O−H of the phosphonate group. Thermogravimetric and dynamic water sorption analysis demonstrated exceptional thermal robustness of BAM‐5 until 230°C and notable water affinity. Proton conductivity measurements found increasing proton conductive properties with increasing temperature and relative humidity. The latter is correlated with the material's water uptake since the structure itself does not contain any permanent lattice water molecules. A maximum proton conductivity of 6.6 × 10 −6 S cm −1 was found at 80°C and 98% RH. To the best of our knowledge, no dense, nonporous 1D coordination polymer without lattice or coordinated solvent molecules has shown comparable proton conductivity. The high activation energy suggests a combination of both, a Grotthuss‐type proton hopping through the hydrogen‐bonded framework, and a vehicular process, in which protons are carried along with absorbed water molecules.