Superprotonic Conductivity in Metal–Organic Framework via Solvent-Free Coordinative Urea Insertion

Abstract

Highly stable superprotonic conductivity (>10^-2 S cm^-1) has been achieved through the unprecedented <i>solvent-free-</i>coordinative urea insertion in MOF-74 [M₂(dobdc), M = Ni²⁺, Mg²⁺; dobdc = 2,5-dioxido-1,4-benzenedicarboxylate] without an acidic moiety. The urea is bound to open metal sites and alters the void volume and surface functionality, which triggers a significant change in proton conductivity and diffusion mechanism. Solid-state ²H NMR revealed that the high conductivity was attributed to the strengthening of the hydrogen bonds between guest H₂O induced by hydrogen bonds in the interface between H₂O and the polarized coordinated urea.