Tuning the Crowding\nEffect of Water and Imidazole\nin Covalent Organic Frameworks for Proton Conduction

The proton conduction of imidazole under confined conditions\nhas\nattracted widespread attention from researchers. Under anhydrous conditions,\nthe proton transfer behavior is primarily governed by the molecular\ndynamics of imidazole. However, within a water-mediated system, the\ncrowding effect of water and imidazole in a confined space may outweigh\nthe intrinsic properties of imidazole itself. In this study, we have\nmeticulously adjusted the structural fragments within the covalent\norganic frameworks (COFs), fine-tuning the saturation level of imidazole\nloading and adjusting the crowding degree of imidazole and water molecules.\nAs a result, the two COF composites exhibit distinctly different proton\nconduction mechanisms from 32 to 100% relative humidity (RH), of which\none possesses proton conduction progressively shifting from the Grotthuss\nmechanism to the vehicle mechanism, while the other has proton conduction\nundergoing a transition from the vehicle mechanism at 32% RH through\nthe Grotthuss mechanism at 75% RH and finally back to the vehicle\nmechanism at 100% RH. These results highlight the critical role of\nthe crowding effect of water and imidazole within confined spaces\nin proton conduction.