Coordination polymers or metal organic frameworks (CPs/MOFs) composed of metal ions and organic ligands have been much attracted as a new class of crystalline microporous solid with gas storage function.[1,2] Recently, dynamics of organic ligands or guest molecules in pores provides functions such as ferroelectricity, spin-crossover, luminescence, and ion conduction. For the characterization of insides pores, solid-state NMR is a powerful method that provides local structure with disordering and molecular dynamics.
In this work, we focused on proton conductive coordination polymers, which have potential as electrolyte in fuel cell. We synthesized two proton conductive coordination polymers and revealed proton transfer mechanisms through single crystal X-ray diffraction and solid-state NMR.
We synthesized Ca2+ based CP, which has straight 1D channels with a narrow pore diameter (c.a. 3.0×3.0 Å2). The 1D channels incorporating H+ and Li+ ion provides high ion conductivity (10 mS cm-1) at 25 ℃ with water support. To elucidate the ion conduction behavior, we measure pulse field gradient (PFG) NMR. PFG NMR revealed fast 1H and 7Li diffusion in the pores and elucidated that fast H+ conductivity was achieved by the support of Li+ ion movements in the channel.
State-of-the-art solid electrolytes in fuel cell require high thermal (above 100 ℃) and water stability. We synthesized proton conducting coordination polymer supported by cationic azole guest. The cationic guest provides thermal and water stabilization of the framework, and directs anhydrous proton hopping pathway. The proton conductivity of the compound is 0.1 mS cm-1 at 190 ℃ without water support. Single crystal X-ray and 2H solid-state NMR study visualized the effective proton transporting pathway and the mechanism.