An Ab Initio Study of the Effect of Strain on the Permeability of N₂ and CO₂ in N-Graphdiyne: Implication for Gas-Selective Membranes

Utilizing nanoporous membranes for efficient CO2/N2 separation presents a promising strategy for addressing climate change and related environmental issues. In this work, we employed first-principles calculations and molecular dynamics simulations to investigate the separation efficiency between CO2 and N2 of the rhombic N-Graphdiyne (r-N-GDY) monolayer under uniaxial tensile strain. Our simulation results demonstrate that, at room temperature, strain applied in the zigzag direction of the r-N-GDY membrane enables efficient CO2 separation from N2. At strains between 3 and 3.5%, the membrane exhibits ultrahigh selectivity ranging from 1.4 × 103 to 2.9 × 104 for CO2/N2, alongside superior CO2 permeance of approximately 1.3 × 10–4 ∼ 1.3 × 10–3 mol m–2 s–1 Pa–1, likely due to the enhanced confinement of the nanopore to N2 molecules under strain. This is supported by quantitative free energy barrier calculations, which indicate that the outstanding separation performance originates from higher energy barriers for N2 (i.e., 47.3 kJ/mol at 3.5%) compared to lower energy barriers for CO2 (i.e., 19.3 kJ/mol at 3.5%) under equivalent strain levels. Additionally, density of states analysis reveals significantly enhanced high-frequency rotational modes for both N2 and CO2 under strain along the short axis of the nanopore, indicating that the confinement is primarily imposed by the negatively charged nitrogen atoms defining this axis. In conclusion, this study proposes the r-N-GDY monolayer, as a strain-tunable, high-performance material for the efficient separation of CO2 from N2. The findings highlight the potential of using strain engineering to enhance membrane separation technologies, offering a significant advancement toward sustainable and effective gas separation solutions.