Fig. 5: Selective transport of monovalent cations through COF membranes.

a Diffusion coefficient of the monovalent cation with a different diameter across TpPa–PO₃H₂, TpPa–SO₃H, and TpPa–CO₂H membranes. Chloride with a concentration of 0.1 M was used as a counterion in all cases. The diffusion coefficients for TpPa–PO₃H₂ and TpPa–SO₃H membranes had a sharp cutoff between the diameter of 6.6 Å and 7.2 Å. b Schematic illustration of the proposed confined cascade separation within COF membranes for selective ion transport. The cascade acid domains conferred selectivity for monovalent cations and the acid-free-domains rendered high permeability of monovalent cations. c Transport performances of COF membranes under different hydration energy of confined stage. Overall, increasing the hydration energy could increase the ideal K⁺/Li⁺ selectivity of COF membranes. d Transport performances of COF membranes under different group density of confined stage. Increasing the group density brought about the increase of ideal K⁺/Li⁺ selectivity and decrease of permeation rates. e Transport performances of COF membranes under different group distance of confined stage. The smaller group distance (~6 Å) and the larger group distance (~25 Å) had no obvious sieving performances for ion transport. In contrast, ~14 Å of group distance was found appropriate for selective ion transport. f Transport performances of COF membranes under different stage number. Increasing stage number led to the elevated ideal K⁺/Li⁺ selectivity. All the error bars in this figure represent the standard deviation of the experiments. Source data are provided as a Source Data file.