Understanding the Cation and Anion Trapping in the Water Desalination Processes Mediated by 2D Mo_1.33C (i-MXene)

Fresh-water scarcity in arid and semi-arid regions is a global problem that is increasing due to population growth, climate change, and environmental degradation. 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 to Mo1.33C MXene as an efficient ion intercalation material, in which both Na cations and Cl anions are removed. However, the atomic-scale understanding of the Physico-chemical processes due to the cation and anion interaction with the MXene is still unknown. We report the Na cation and Cl anion interaction with OH functionalized Mo1.33C monolayer through a comprehensive first-principles density functional theory assessment. Results demonstrate that Na atoms attach to Oxygen, whereas Cl atoms bond through multiple van der Waals interactions with the MXene, which increases its adsorption energy. Electrostatic potential isosurface calculations evidence reactive sites. Oxygen atoms have an affinity for the electropositive Na atoms, whereas hydrogen atoms -of the hydroxyl groups- interact with the electronegative Cl atoms. Bader change analysis and projected densities of states help clarify the way cations and anions attach to the MXene layer. Our findings explain why OH-functionalized Mo1.33C can efficiently remove both anions and cations based on their affinities with the functional groups present in the MXene.<br/>We thank DGAPA-UNAM projects IA100822, IN110820, and IN105722 for partial financial support. Calculations were performed in the DGCTIC-UNAM Supercomputing Center, projects LANCAD-UNAM-DGTIC-051, LANCAD-UNAM-DGTIC-150 and LANCADUNAM-DGTIC-368. J.G.S. acknowledges THUBAT-KAAL IPICYT supercomputing center for computational resources. The authors thankfully acknowledge the computer resources, technical expertise, and support provided by the Laboratorio Nacional de Supercomputo del Sureste de Mexico, LNS-CONACYT member of the network of national laboratories. DMMP thanks CONACyT for the postdoctoral fellowship. We thank Aldo Rodriguez-Guerrero for his technical support.