Liquid-Liquid Phase Separation of Aqueous Ionic Liquids in Covalent Organic Frameworks for Thermal Switchable Proton Conductivity

Covalent organic frameworks (COFs) have regular channels that can accommodate guest molecules to provide highly conductive solid electrolytes. However, designing smart, conductive COFs remains a great challenge. Herein, we report the first example of PEG-functionalized ionic liquids (ILs) anchored on the COF walls by strong hydrogen bonding to fabricate thermally responsive COFs (ILm @COF). We found that similar to the traditional IL/water mixture, the ILs undergo lower critical solution temperature (LCST)-type phase behavior within COF nanopores under high moisture levels. However, the phase separation temperature of aqueous IL decreases in COF channels due to the strong interaction between the IL and COF. Thus, the proton conductivity of ILm @COF can be reversibly switched by phase miscibility and separation in COF nanopores, and there is no obvious decrease even after 20 switching cycles. Our work provides important clues for understanding liquid-liquid phase separation in a confined nanospace and opens a new pathway to switchable proton conductivity.

Automated summary

In this study, a new type of thermally responsive material was fabricated by anchoring polyethylene glycol functionalized ionic liquids (ILs) on the walls of covalent organic frameworks (COFs) through strong hydrogen bonding. The ILs exhibited lower critical solution temperature (LCST)-type phase behavior within the COF nanopores under high moisture levels, similar to traditional IL/water mixtures. The strong interaction between the IL and COF led to a decrease in the phase separation temperature of the aqueous IL within the COF channels. The proton conductivity of ILm @COF could be reversibly switched by phase miscibility and separation in the COF nanopores, with no obvious decrease observed even after 20 switching cycles.