Improved CO₂/CH₄ Separation Performance in Negatively Charged Nanoporous Graphene Membranes

We propose a negatively charged nanoporous graphene (NPG) membrane which presents an improved separation performance of CO₂/CH₄ gas mixtures. The CO₂ permeance and the selectivity of CO₂ molecules over CH₄ molecules both increase after adding negative partial charges on each carbon atom. The CO₂ permeance can increase from 6.81 × 10⁵ GPU for the neutral NPG membrane up to 5.65 × 10⁶ GPU for a partial charge of -0.125 e, and the selectivity increases correspondingly from 10.4 to 42.8. The improved separation performance can be attributed to the relatively enhanced adsorption of CO₂ molecules and weakened adsorption of CH₄ molecules on the graphene surface at high partial charges. The enhanced adsorption of CO₂ molecules can enhance their permeation abilities by improving the surface contributions, especially on the molecular deliverability from surface to pore area for permeation. The weakened adsorption of CH₄ molecules can weaken the permeation abilities of themselves due to the limited surface contributions and simultaneously improve the CO₂ permeance by abating their blocking effects for the permeation of CO₂ molecules. Meanwhile, the enhanced adsorption layer on the graphene surface can further enhance the net CO₂ permeance by preventing the permeated molecules from moving back to the feed side.