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

Abstract

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.