Metalo Hydrogen-Bonded Organic Frameworks Self-Assembled by Charge-Assisted Synthons for Ultrahigh Proton Conduction

Abstract

Hydrogen-bonded organic frameworks (HOFs) have been well recognized for constructing solid-state proton-conducting materials due to their inherent well-defined H-bonded networks; however, the design and synthesis of stable HOFs showing ultrahigh superprotonic conductivity is still challenging. Herein, we reported a facile and effective synthetic strategy to build high proton-conducting metalo hydrogen-bonded organic–inorganic frameworks (MHOFs) through charge-assisted inorganic and organic synthons. The supramolecular self-assembly of simple metal salts and tetra-carboxylic acids gives rise to two hydrogen-bonded frameworks, [M(H2O)6][H2tcba] (M = Zn2+, Ni2+; H4tcba = 1,2,4,5-benzenetetracarboxylic acid). X-ray crystallography and gas adsorption confirmed the nonporous nature of the HOIFs. Variable-temperature single-crystal and powder X-ray diffraction experiments reveal the thermal and water stability of the supramolecular structures sustained by exclusively charge-assisted hydrogen bonds. The high density of carboxylic acid groups and coordinated water molecules provide ample protons and hydrophilic environments for efficient proton transport; therefore, variable-temperature and -humidity electrochemical impedance spectroscopy revealed that MHOFs are good supramolecular proton conductors with conductivities up to 1.1 × 10–2 and 2.1 × 10–2 S·cm–1 for 1 and 2, respectively, at 80 °C under 97% RH. Our study demonstrates great potential of charge-assisted inorganic and organic synthons in the design and construction of proton-conducting HOIFs.