MXene membrane with directionally functionalized channel entrances for enhanced ion selectivity and permeability.

Abstract

Ion-separation membranes that combine high selectivity with high permeability are important for various practical applications. Molecular functionalization of sub-nanochannel membranes is widely used to improve ion separation selectivity; however, it often reduces ion permeation. Herein, inspired by the structural features of biological ion channels, we introduce a channel-entrance-directional functionalization strategy that integrates selective recognition with rapid ion transport. By directionally anchoring α-cyclodextrin molecules at the entrances of sub-nanochannel in two-dimensional MXene (Ti₃C₂T_x) lamellar membranes, we construct biomimetic ion channels in which entrance-localized recognition sites guide ion separation while unobstructed pathways enable rapid monovalent cation transport. The resulting membranes exhibit mono-/divalent cation selectivity exceeding 10³ while preserving high permeability for monovalent ion, thereby surpassing the selectivity-permeability trade-off limit of reported ion-separation membranes. Combined experiments and simulations reveal that alterations in ion transport pathways caused by differences in ion hydration structures within sub-nanochannels are the primary mechanism for ion separation. This mechanism contrasts with conventional interpretation that primarily attributes ion separation to differences in ion-molecule binding energies. This work establishes channel-entrance engineering as a useful strategy for the design of ion-separation membranes for mono-/divalent cation separation in brine upgrading and resource-recovery.