Sub-nanometer resolution for anion conduction in a covalent-organic framework membrane: A hierarchical approach

Ion conduction in covalent-organic framework (COF) membranes is vital for energy conversion and storage. Conventional phenomenological methods based on the Arrhenius equation offer micrometer-scale cognition of ion conduction, whereas they ignore atomic details of ion-pore interactions and sophisticated conduction mechanisms, leaving gaps in high-resolution and bottom-up understanding of ion conduction in a nanoconfined space. In this study, we develop a hierarchical approach by holistically synergizing electronic structure calculations, first-principles molecular dynamics simulations, and thermodynamic integration methods to investigate the conduction of chloride (Cl−) and hydroxide (OH−) ions in a COF membrane. It is revealed that Cl− ion with symmetric charge distribution undergoes weak solvation and tight ion-pore binding, which results in a tortuous conduction pathway, a high energy barrier, and slow diffusion based on the vehicular mechanism. In remarkable contrast, OH− ion with heterogeneous charge distribution features strong solvation and weak ion-pore binding, and it jumps frequently via a smooth pathway and a low energy barrier. Moreover, OH− ion conduction follows a mixed vehicular and Grotthuss mechanism, causing highly mutable ion identity and number, as well as superior dynamics due to proton transfer. This hierarchical approach provides sub-nanometer resolution insights into ion conduction, guiding intelligent membrane design and performance regulation to control ion conduction for emerging applications.