Selective Monovalent Cation Transport in Two-Dimensional Polymer Membranes via Charge Control of Nanochannels

Monovalent cations like Li⁺, Na⁺, and K⁺ are now regarded as important components for advanced electrochemical energy storage technologies for achieving high energy density and power density. However, due to their identical charge number and similar hydration diameter, the low separation efficiency of monovalent cations during their recovery and use presents a major bottleneck for the rapidly sustainable energy development. Herein, we propose a charge-engineering strategy for efficiently tuning the cation-channel interaction via varying the membrane charges toward rapid and selective transmembrane transport of monovalent Li⁺ and K⁺ cations. In this respect, we develop charged polyketoenamine-based two-dimensional polymer membranes (2DPMs) at liquid-liquid interface, which feature high crystallinity with pH-responsive quasi-1D porous channels functionalized by acyl oxime groups. Notably, the positively charged 2D polyketoenamine simultaneously exhibits ultrahigh K⁺ permeation rate (361.7 mmol·m^-2·h^-1) and high K⁺/Li⁺ selectivity (6.6), as well as a record-high K⁺ diffusion coefficient (2.9 × 10^-10 m²·s^-1), which are superior to the state-of-the-art graphene- and polymer-based membranes (< 1.7 × 10^-10 m²·s^-1) as well as neutral and negatively charged 2D polyketoenamine membranes.