MXenes are attracting much attention as electrode materials due to their excellent energy storage properties and electrical conductivity, and the energy storage capacities were found to strongly depend on the surface terminal groups. Here S-functionalized Ti₃C₂ as a representative MXene material is designed. Our density functional theory (DFT) calculations are performed to investigate the geometric and electronic properties, dynamic stability, and Na storage capability of Ti₃C₂, Ti₃C₂O₂ and Ti₃C₂S₂ systems. The Ti₃C₂S₂ monolayer is proved to show metallic behavior and has a stable structure, and meanwhile it also exhibits a low diffusion barrier and high storage capacity (up to Ti₃C₂S₂Na₄ stoichiometry) for Na ion batteries (NIBs). The superior properties such as good electrical conductivity, fast charge–discharge rates, low open circuit voltage (OCV), and high theoretical Na storage capacity, make the Ti₃C₂S₂ monolayer a promising anode material for NIBs compared to the Ti₃C₂O₂ monolayer. More importantly, similar to the Ti₃C₂S₂ monolayer, other MXenes with a high charge density difference and suitable lattice constant can be formed, and thus the energy storage properties are worth further study. This finding will be useful to the design of anode materials for NIBs.

Currently, MXenes have been identified as promising electrode material candidates because of their excellent energy storage and electrical conductivity. In the present study, Na adsorptions on 2D MXenes of Zr₂C, Zr₃C₂, Zr₂CO₂, and Zr₃C₂O₂ are explored by density functional theory (DFT) calculations. We have found that Zr₂CO₂/Zr₃C₂O₂ MXenes show metallic behavior after Na adsorption and exhibit a low diffusion barrier (0.29/0.32 eV) and high storage capacity (up to Zr₂CO₂Na₄/Zr₃C₂O₂Na₄ stoichiometry) for Na-ion batteries (NIBs). Zr₂CO₂/Zr₃C₂O₂ MXenes also exhibit excellent properties such as good electrical conductivity, low diffusion barrier, decreased equilibrium open circuit voltage (OCV), and high theoretical capacity of Na storage, due to which they can be used as promising anode materials for NIBs. In addition, the physical origin of adsorption behavior of Na atoms on Zr₂CO₂/Zr₃C₂O₂ as well as on Ti₃C₄, Ti₃C₂O₂ and Ti₃C₂S₂ MXenes is studied. Our results give insights into understanding the manner in which the theoretical capacity can be efficiently tuned by charge transfer and lattice mismatch factors, which will be useful for designing other 2D anode materials for NIBs.

A monotonical relationship between the binding energies and the lattice constants of M ₃ C ₂ O ₂ was identified, which indicated that M ₃ C ₂ O ₂ MXenes with a smaller lattice constant tends to exhibit a stronger anchoring effect.

Preparing a nanoporous membrane with high density and ordered pore sizes which allows high water permeability and salt rejection rate is the key to realize highly efficient desalination. However, preparing a nanoporous membrane with high density and order pore sizes is still extremely hard due to the limitation of experimental techniques. Recently, a 3D covalent organic framework (3D-COF) material named as the 3D-OH-COF with good crystallinity and large specific surface areas has been synthesized. Based on the structural features of the 3D-OH-COF, we speculate that it may be a good candidate for the desalination application derived from its high-density sub-nanometer pore. In this work, using molecular dynamics simulations, the possibility of the 3D-OH-COF for desalination application was explored, the influence of membrane thickness on its desalination performance was also studied, and the detailed structure and dynamics of ions and water transport in the channel of the 3D-OH-COF was discussed. The results show that the rectangular channel structure and charged H atoms are responsible for the excellent salt rejection rate (100%) and high water flux (41.44 Lit cm⁻² day⁻¹ MPa⁻¹), respectively. Furthermore, the water flux is three orders of magnitude higher than that of the commercial reverse osmosis membrane and is four times higher than that of the theoretically reported monolayer nanoporous MoS₂ membrane. It is also about 28% higher than that of the recently reported 2D-CAP membrane. This work theoretically confirms that the 3D-OH-COF is a promising membrane material for desalination applications and the underlying molecular mechanisms are clarified.