Charged layered double hydroxides with sub-nanometer channel for efficient monovalent cation sieving

Abstract

The design of monovalent cation selective membranes for precise separation requires a comprehensive understanding of the geometry and chemical environment of the transport channels. Here, a charged cation transfer channel with sub-1-nanometer is constructed by layer-by-layer self-assembly of layered double hydroxides. To effectively improve the separation performance, the thickness of the membrane and the separation performance test temperature are explored. The enhanced size confinement effect and wall Coulomb interaction lead to better migration of K⁺/Na⁺ than of Li⁺, which allows the selectivity of K⁺/Li⁺ or Na⁺/Li⁺ as 5.7 and 4.0 for 3 μm thickness at 293 K, respectively. Molecular dynamics simulation and density functional theory calculation further illustrate the root cause of different transfer rates and high selectivity due to the different Coulomb effects between various cations and channel walls. These results provide insight into the ion transfer behavior and separation mechanism in a charged confinement regime.