Ion Transport through Permselective Nanochannels of 2D-Covalent Organic Frameworks: Insights from Nonequilibrium Atomistic MD Simulations

Covalent organic frameworks (COFs), with their long-range crystallinity, rigidity, and highly tunable molecular porosity, are evolving as promising next-generation ion-permselective membranes for ion separation, reverse electrodialysis, and osmotic energy conversion. Herein, we employ steady-state nonequilibrium molecular dynamics (SS-NEMD) simulations to accurately and realistically describe the concentration-driven ion permeation process through functionalized and charged COF membranes and gain a fundamental understanding of the factors governing the ion transport in COFs. To this end, we model the diffusion of K+ and Cl– ions through the COF membranes that are suspended in aqueous electrolyte and are decorated with either propanesulfonic acid or sulfonic acid-terminated oligo(n)-ethylene glycol chains. We systematically vary the percentage of charged groups present within the nanoporous domains and demonstrate that the ion permeation, and thus the ion selectivity in these systems, depends on the concentration gradient of ions across the membrane, as modulated by the presence and degree/percentage of charged groups within the COF membranes. We further demonstrate that COF membranes decorated with charged oligo(n)-ethylene glycol chains show enhanced ion selectivity ratios, even when partially charged, reaching efficiencies of around 99% when compared to COFs with alkylated chains that show efficiencies of around 80%.