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 center dot 6H(2)O center dot 2DMF (compound 1), which is constructed through the self-assembly of zwitterionic-based ligands H(2)CcbpBr (H(2)CcbpBr = 4-carboxy-1-(4-carboxybenzyl)pyridinium bromide) and (NH4)(2)Ce(NO3)(6). During the investigation of humidity dependent proton conduction behavior, we.observed a rate case of rapid water-induced single-crystal-to-single-crystal phase transformation from compound 1 to a neutral chain [Ce(Ccbp)(3)(H2O)(3)]center dot 8H(2)O (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 X 10(-4) S cm(-1) at 368 K and 95% relative humidity.