Stable isomeric layered indium coordination polymers for high proton conduction

Abstract

Coordination polymers have received immense attention for proton conduction owing to their highly tunable and designable structures. Herein, based on the flexible ligand 5-(4-pyridine)-methoxyisophthalic acid (H₂L), two novel highly chemically stable indium coordination polymers (In-CPs), InL(H₂O)Cl·H₂O (In–L–Cl) and InL(H₂O)(NO₃) (In–L–NO₃), were synthesized through solvothermal methods. In–L–Cl and In–L–NO₃ are isostructural, in which every individual In(III) atom connects to L²⁻ to form a single layer, and coordinated Cl⁻ and NO₃⁻ are inserted into the windows of the interlayers. Both of them possess high water and chemical stability. More importantly, the abundant Cl⁻ and NO₃⁻ exposed in the interlayer can act as hydrogen bond acceptors to facilitate the formation of a dense hydrogen bonding network, thus improving proton transfer efficiently. Electrochemical impedance spectroscopy shows that In–L–Cl and In–L–NO₃ display optimized proton conductivities of 1.88 × 10⁻³ and 1.00 × 10⁻³ S cm⁻¹ at 100% relative humidity (RH) and 80 °C, respectively. The difference in proton conductivity of In–L–Cl and In–L–NO₃ likely stems from the variations in the hydrogen bonding networks formed by lattice water and the different electronegativities of the coordinated anionic functional sites. This study provides an alternative way to prepare highly proton-conductive CPs.