Atomic-scale understanding of the Na and Cl trapping on the Mo_1.33C(OH)₂-MXene

Drinking water scarcity in arid and semi-arid regions is a reality that may turn into a global healthcare problem in the next few years. The scientific community is always looking for new materials to achieve effective sea and brackish water desalination to reduce water scarcity. Commonly, theoretical, and experimental methods make a synergy to better understand and explain the chemical and physical processes in water desalination electrodes. In this way, experimental evidence pointed Mo_1.33CT_x MXene as an efficient ion intercalation material, in which both Na⁺ and Cl^- are removed. However, the atomic scale understanding of the physicochemical processes due to the Na and Cl interaction with the MXene is still unknown. We report the Na⁰ and Cl⁰ interaction with an OH functionalized Mo_1.33C monolayer through a comprehensive first-principles density functional theory assessment. Results demonstrate that Na atoms attach to Oxygen, whereas Cl atoms bond through hydrogen bonds to the functional groups in the MXene, these bonds have two energy contributions: electrostatic and charge transfer, which increases its adsorption energy. Electrostatic potential isosurfaces, Bader charge analysis, and non-covalent interactions index help clarifying the way Na⁰ and Cl⁰ attach to the MXene layer. Oxygen atoms have an affinity for the electropositive Na⁰ atoms, which after interaction oxidizes to Na⁺, whereas hydrogen atoms-of the hydroxyl groups-interact with the electronegative Cl⁰ atoms, which upon adsorption reduce to Cl^-. Our findings explain why OH-functionalized Mo_1.33C can efficiently remove both Na and Cl atoms based on their affinities with the functional groups present in the MXene layer.