Tunable monovalent cation separation in polymeric carbon nitride membranes via multivalent ions†

Synthetic membrane technology plays an increasingly dominant role in modern industry, boasting remarkable efficiency and low carbon attributes. The ever-growing demand for molecular-level separation necessitates precise structures at the angstrom range with a concomitant low transport resistance, but it still remains a great challenge. Here, we demonstrate an enhanced separation performance towards monovalent cations of two-dimensional (2D) conjugated polymeric carbon nitride (PCN) membranes with angstrom pores, achieved through the strategic incorporation of multivalent ions. Based on the additional ions, the energy barrier of transmembrane transport for individual alkali metal ions could be effectively manipulated. Remarkably, the presence of LaCl₃ substantially improves monovalent cation selectivity ratios, improving from 7 to 22 for K⁺/Li⁺ in mixtures. More importantly, under an initial concentration gradient, the transport rate of K⁺ was further enhanced over 1 kmol m⁻² h⁻¹, primarily attributed to the low ion transfer barrier.