Switching on the proton transport pathway of a lanthanide metal–organic framework by one-pot loading of tetraethylene glycol for high proton conduction

Abstract

A one-pot hydrothermal approach has been developed to introduce tetraethylene glycol (TEG) molecules into a two-dimensional (2D) layered lanthanide metal–organic framework ([Sm(H₅C₂P₂O₇)(H₂O)₂]·Guest, denoted SmHEDP-Guest). Through the straightforward loading of TEG, the proton conductivity of SmHEDP-TEG (1.21 × 10⁻³ S cm⁻¹) is increased by 3 orders of magnitude compared with its analogue SmHEDP-H₂O (1.22 × 10⁻⁶ S cm⁻¹) under 100% relative humidity at room temperature. More excitingly, SmHEDP-TEG exhibits very high proton conductivity of 9.17 × 10⁻² S cm⁻¹, even higher than commercial Nafion, when the temperature is increased to 333 K, which is significantly higher than SmHEDP-H₂O (3.38 × 10⁻⁵ S cm⁻¹). The single crystal XRD reveals that the adjacent water molecules located in the channels of SmHEDP-H₂O are isolated without hydrogen bonding interactions owing to their long distances. However, interestingly, the guest TEG molecules of SmHEDP-TEG behave as hydrogen bonded connected bridges, which switch on the proton transport pathway to promote proton hopping. This discovery may provide a facile strategy to design and synthesize more promising candidates for novel proton conductors.