Selective gas diffusion in two-dimensional MXene lamellar membranes: insights from molecular dynamics simulations

Membrane gas separation has become increasingly important for modern industry, and the emerging two-dimensional (2D) lamellar membranes provide unprecedented possibilities to overcome the well-known permeability-selectivity trade-off of traditional membranes. However, the 2D materials currently available for lamellar membrane fabrication are very limited, and relevant experimental or simulation studies are scarce. Consequently, the understanding of gas diffusion in 2D nanochannels, though critical for developing more efficient lamellar membranes, is still quite poor. Very recently, we fabricated a 2D MXene lamellar membrane with exceptional gas separation performance (L. Ding, Y. Wei, L. Li, T. Zhang, H. Wang, J. Xue, L. X. Ding, S. Wang, J. Caro and Y. Gogotsi, Nat. Commun., 2018, 9, 155), and the gas transportation mechanism was studied thoroughly with Molecular Dynamics (MD) simulations in this work. The diffusion of different gases, such as H-2, He, CH4, CO2 and N-2, was simulated in 2D MXene nanogalleries with structural factors (e.g., interlayer distance and intercalating water) adjusted systematically. These gases were found to diffuse in the nanogalleries mainly via two mechanisms, activated diffusion and Knudsen diffusion. The main features of both diffusion mechanisms were discussed through studying the simulation trajectories carefully. The simulations also revealed that the MXene membrane structure significantly influenced the gas diffusion, such as the mechanism, diffusivity, and selectivity, and could thus be tuned to boost the gas separation performance as our recent experiment did. This simulation work provides a detailed microscopic understanding of gas diffusion in 2D nanochannels, and useful guidance for developing new 2D lamellar gas separation membranes with high performance.