Significant Proton Conductivity Enhancement through Rapid Water-Induced Structural Transformation from a Cationic Framework to a Water-Rich Neutral Chain

Searching for new host materials tailored for the high proton conductivity is highly desirable for the new generation of fuel cell system. We report here an anion-exchangeable cationic metal organic framework with the formula of [Ce­(Ccbp)2]­Br0.25Cl0.75·​6H2O·​2DMF (compound 1), which is constructed through the self-assembly of zwitterionic-based ligands H2Ccbp­Br (H2Ccbp­Br = 4-carboxy-1-(4-carboxy­benzyl)­pyridinium bromide) and (NH4)2­Ce­(NO3)6. During the investigation of humidity-dependent proton conduction behavior, we observed a rare case of rapid water-induced single-crystal-to-single-crystal phase transformation from compound 1 to a neutral chain [Ce­(Ccbp)3­(H2O)3]·​8H2O (compound 2). This structural transformation originates from the coordination of water to Ce­(III) metal centers, distortion of ligands, and the soft nature of the cationic framework 1, as probed and confirmed by a variety of investigations including color change, water vapor adsorption measurement, powder X-ray diffraction, single-crystal X-ray diffraction, humidity-dependent proton-conducting measurements, IR and UV–vis spectroscopies, and thermogravimetric analysis. As a consequence, this process introduces significant amounts of both coordinated and lattice water molecules into the structure, further giving rise to a decent water-assisted proton conductivity of 1.104 × 10–4 S cm–1 at 368 K and 95% relative humidity.