Superionic Conduction over a Wide Temperature Range in a Metal–Organic Framework Impregnated with Ionic Liquids

Abstract

Most molecules in confined spaces show markedly different behaviors from those in the bulk. Large pores are composed of two regions: an interface region in which liquids interact with the pore surface, and a core region in which liquids behave as bulk. The realization of a highly mobile ionic liquid (IL) in a mesoporous metal-organic framework (MOF) is now reported. The hybrid shows a high room-temperature conductivity (4.4×10^-3 S cm^-1 ) and low activation energy (0.20 eV); both not only are among the best values reported for IL-incorporated MOFs but also are classified as a superionic conductor. The conductivity reaches over 10^-2 S cm^-1 above 343 K and follows the Vogel-Fulcher-Tammann equation up to ca. 400 K. In particular, the hybrid is advantageous at low temperatures (<263 K), where the ionic conduction is superior to that of bulk IL, making it useful as solid-state electrolytes for electrochemical devices operating over a wide temperature range.