Simultaneous, monovalent ion selectivity with polyelectrolyte multilayers and intercalation electrodes in capacitive deionization

Selective ion separation via capacitive deionization (CDI) is of relevance because of its environmental and industrial applications in water purification and softening, heavy metal removal, and resource recovery. Conventionally, carbon electrodes and ion-selective membranes have been used for selective removal of anions and cations. In this study, we engineered a CDI cell with two identical NiHCF electrodes, separated by an anion–exchange membrane coated with a polyelectrolyte multilayer (PEM), for simultaneous and selective separation of monovalent over divalent anions and cations. The modified membrane rejects divalent over monovalent anions while the NiHCF electrodes selectively adsorb monovalent over divalent ions. A separation factor (β) of 7 < β < 14 was obtained for Cl⁻ over SO₄²⁻, while an average β of ≈ 17 was obtained for Na⁺ over Mg²⁺, reflecting the preference of the modified membrane and the electrodes, respectively. Moreover, this preference was preserved at low concentrations of monovalent ions as well.