Tuning the Crowding Effect of Water and Imidazole in Covalent Organic Frameworks for Proton Conduction

Abstract

The proton conduction of imidazole under confined conditions has attracted widespread attention from researchers. Under anhydrous conditions, the proton transfer behavior is primarily governed by the molecular dynamics of imidazole. However, within a water-mediated system, the crowding effect of water and imidazole in a confined space may outweigh the intrinsic properties of imidazole itself. In this study, we have meticulously adjusted the structural fragments within the covalent organic frameworks (COFs), fine-tuning the saturation level of imidazole loading and adjusting the crowding degree of imidazole and water molecules. As a result, the two COF composites exhibit distinctly different proton conduction mechanisms from 32 to 100% relative humidity (RH), of which one possesses proton conduction progressively shifting from the Grotthuss mechanism to the vehicle mechanism, while the other has proton conduction undergoing a transition from the vehicle mechanism at 32% RH through the Grotthuss mechanism at 75% RH and finally back to the vehicle mechanism at 100% RH. These results highlight the critical role of the crowding effect of water and imidazole within confined spaces in proton conduction.