Atomically selective oxidation of double transition-metal TiVCTX MXene for highly selective Mg2+/Ca2+ separation

• SO-TiVC showed high Ca 2+ , Mg 2+ removal capacity (1.38 mmol/g and 1.44 mmol/g, respectively). • SO-TiVC showed high ion selectivity for Ca 2+ and Mg 2+ (S Ca/Na = 13.67 and S Mg/Na = 11.12, respectively). • The slight oxidation strategy improved the ion transport kinetics of TiVC and increased its active sites. • Increased layer spacing is the key to the enhanced selectivity of SO-TiVC for divalent ions . • Selective oxidation is realized due to the different oxidation tendency of Ti and V. Realizing efficient and selective separation of monovalent and divalent cations in water is the key to the development of next-generation water treatment and resource recovery technologies. Herein, we report a simple slight oxidation strategy that results in selective oxidation of Ti atoms and retention of V atoms on the surface of the double transition-metal TiVCT x (abbreviation TiVC) MXene. The TiO 2 nanoparticles partially derivatized on the surface due to slight oxidation extended the layer spacing of MXene and improved its ion diffusion kinetics, while providing abundant active sites. The retention of V atoms ensures the integrity and good electronic conductivity of the MXene 2D lamellae. Benefiting from the above synergistic effect, SO-TiVC (slightly oxidized TiVCT x MXene) possesses good electrochemical performance as well as hybridized CDI performance, showing high ion selectivity for divalent cations Ca 2+ and Mg 2+ (S Ca/Na = 13.67, S Mg/Na = 11.12), which is superior to that of the reported MXene-based electrodes. Density-functional theory calculations show that the expansion of the MXene layer spacing and the change in the surface end-groups combine to promote the higher divalent cation selectivity of SO-TiVC. This study provides a theoretical basis for the development of efficient divalent cation-selective electrodes utilizing the intrinsic properties of the bis-transition metal MXene for environmental separation applications.