Microscopic insight into anion conduction in covalent-organic framework membranes: A molecular simulation study

Recently there has been considerable interest in the utilization of covalent-organic frameworks (COFs) as ionexchange membranes. While rapid ion conduction is experimentally observed in COF membranes, the underlying mechanism remains elusive. Herein, we report a molecular simulation study on chloride ion (Cl- ) conduction in four COF membranes functionalized with quaternary ammonium groups (QA-2, QA-4, QA-6 and QAEO) of different side chains (ethyl, butyl, hexyl and diethyl ether). It is revealed that membrane flexibility is crucial to be incorporated in simulation for reliable predictions. The Cl- conductivities in the four membranes are predicted to decrease as COF-QA-2 > COF-QA-4 approximate to COF-QA-EO > COF-QA-6, which is in good agreement with experimental data. The pore size, rather than membrane-Cl- interaction, is unravelled to be the key factor governing the trend of conductivity. The microscopic insight provided by simulation is useful to elucidate the fundamental mechanism of ion conduction, and might facilitate the design of new COF membranes for optimal ion conduction.

Automated summary

The study highlights the importance of membrane flexibility in predicting reliable ion conductivity in COF membranes, with the pore size rather than membrane-Cl- interaction being identified as the key factor governing conductivity trends. The predicted Cl- conductivities in different COF membranes showed good agreement with experimental data, providing valuable insight into the fundamental mechanism of ion conduction. This microscopic understanding might help in designing new COF membranes for optimal ion conduction.