Freestanding two-dimensional nanofluidic membranes modulated by zwitterionic polyelectrolyte for mono-/di-valent ions selectivity transport

Effective separation of mono-/di-valent metal ions based on membrane technology is highly desirable in the energy storage and environmental protection field, but requires membranes capable of selective recognition of ions with similar nano-scale size. In this work, we designed and synthesized the sulfonate polyethylenimine (PEI-SO3H), as a zwitterionic polyelectrolyte, and integrated it into two-dimensional (2D) graphene oxide (GO) confined channels. Stable and sub-nanometer ion-selective transport channels enable fast transport of monovalent metal ions with high mono-/di-valent ion selectivity for K+/Mg2+ (12.2), Na+/Mg2+ (11.4) and Li+/Mg2+ (10.1) under electric field. Notably, the hydration compensation-promoting diffusion (HCPD) mechanism based on the synergistic effect of amino groups as compensation sites and sulfonic acid groups as recognition sites was proposed to understand the high-efficiency ion-selective transport process. In addition, instructed by abovementioned theory, the 2D vermiculite membrane was also designed to well realize the mono-/di-valent ions separation. This study provides a new strategy to construct freestanding 2D membranes with efficient ion-selective transport channels instructed by HCPD theory, which is expected to expand the potential for using membranes in the lithium extraction.