Superprotonic Conductivity in Metal–Organic Frameworks by Charged-Layer-Mediated Proton Conduction

Through a stepwise functionalization strategy, LiCl@UiO-66-F2(SO3H)2 manifests a superprotonic conductivity of 2.86 S cm–1 (at 90% RH and 90 °C), a record breaker so far. Supported by joint experimental–theoretical studies, the ultrahigh conductivity originates from conduction by protons rather than ions and is rationalized to result from the enhanced acidity of −SO3H by introduction of electronegative −F followed by the formation of double charged layers composed of Li+ and −SO3– layers after encapsulation of LiCl. Importantly, the effect of charged layers has been verified, and charged-layer-mediated proton conduction is unprecedentedly proposed to fill in the missing pieces in existing proton conduction mechanisms, giving insight into the rational design of superprotonic conducting framework materials potentially applied as proton exchange membranes in fuel cells.