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

Abstract

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...