Selective electrosorption of Ca²⁺ by MXene cathodes coupled with NiAl-LMO anodes through ion intercalation

Capacitive deionization (CDI), or electrosorption, is a desalination technology that exhibits significant potential; however, its major technical requirement of selective ion separation poses a challenge for its further practical application. Herein, a titanium carbide (MXene)-layered electrosorption electrode with high selectivity for Ca²⁺ was fabricated. The prepared MXene electrode had many surface hydroxyl functional groups that serve as adsorption sites for Ca²⁺. Ca²⁺ was successfully inserted into the interlayers of the MXene cathode and formed a strong interaction with [Ti-O] bonds during the capacitive deionization process. When a Ni-Al layered metal oxide anion intercalation electrode was employed as the counter electrode, Ca²⁺ adsorption by the MXene electrode was significantly enhanced due to the valence compensation balance effect. The maximum Ca²⁺ electrosorption capacity of the MXene electrode reached 1011.82 mg per gram effective MXene material, which is 6.3 times higher than that of Na⁺ based on the Langmuir adsorption isotherm model. The MXene electrode exhibited prominent selectivity for Ca²⁺ ions in the presence of Na⁺ and Mg²⁺. The Ca²⁺/Mg²⁺ selectivity factor for electrosorption reached 2.63, and Ca²⁺/Na⁺ selectivity factor could achieve 9.84, respectively. After five electrosorption/desorption cycles, the Ca²⁺ removal rate only decreased from 46.96% to 45.34%, suggesting that the MXene electrode has excellent stability. Our study demonstrated a novel CDI electrode and technical approach for softening water.