Tracing proton conduction pathways in polycrystalline MOF-based core–shell systems

Abstract

Metal–organic frameworks (MOFs) have emerged as proton conductors, however, understanding conduction mechanisms and pathways in MOFs is limited by the common polycrystalline form of these materials, resulting in the presence of grain boundaries. Herein, we report model core–shell systems based on the UiO-68 platform with incorporated tetrazine function for studying proton conduction in polycrystalline MOFs. The solvent-assisted linker exchange (SALE) method, applied to two model MOFs of the UiO-68 family, enables core shell swapping, control over the degree of linker exchange, and stability of the system. The subsequent use of the inverse electron-demand Diels–Alder (iEDDA) reaction with an acid dienophile precisely in the core or shell of microcrystals, followed by encapsulation of imidazole (HIm), resulted in the formation of a diverse group of polycrystalline proton-conducting systems differing in the distribution of pendant carboxylic groups and concentration of imidazole charge carriers. Comprehensive impedance studies of these covalently modified model systems revealed minor differences between their proton conductivity values, and considerable differences between activation energies, and pseudocapacitances. These unprecedented observations demonstrate the significant role of external surface conduction in polycrystalline MOFs.