Hydration shell energy barrier differences of sub-nanometer carbon pores enable ion sieving and selective ion removal.

[Display omitted] Ion electrosorption in sub-nanometer carbon micropores enables ion separation. • Studies ion selectivity in nanoconfinement applying online chemical monitoring. • Ion selectivity based on pore size effect, charge effect, and desolvation energy effect. • Observed ion sieving effect and the influence on ion selectivity. Subnanometer pores of carbon discriminate against ions based on their size. Capitalizing on such nuanced differences enables ion separation via charge/discharge cycling during ion electrosorption. Different ion uptake capacities in aqueous media with multiple, competing ions are also of high importance to understand capacitive deionization of surface water or industrial process water. In our experiments, we observed divalent cations sieving in pores smaller than 0.6 nm. By applying this phenomenon, a desalination cell with online concentration monitoring was used to study the ion-selectivity. We concluded that in pores below 0.6 nm, divalent Mg2+ and Ca2+ are entirely blocked, and the K+ over Na+ selectivity corresponds with their size ratio. Larger micropores show a preference for divalent cations with higher charge numbers. In both materials, a dynamic monovalent cation and divalent cation replacement dependent on the potential variation is observed. [ABSTRACT FROM AUTHOR]