Investigation Of Gas Separation And Adsorption Performance Of Graphene Membranes

Graphene,the new carbon based nanomaterials,has received numerous scientific interest due to its exceptional mechanical,thermal,optical and electrical properties since its discovery in 2004.Now it has been successfully applied in solar cells,dielectric materials,membrane materials,energy storage materials and related areas.It is very difficult to characterize their properties due to the nanosize of graphene,while theoretical simulation method has much more advantages on studying the nanosized materials.Therfore,the main purpose of this article is to study the gas separation and adsorption performance of graphene by using theoretical simulations.We will explore the influence factors for gas separation of porous graphene and gas adsorption of metal-doped graphene,describe the separation and adsorption process and also give the separation and adsorption mechanism.Firstly,the hydrogen purification process of bottom-up porous graphene（PG-ES,PG-ES1,PG-ES2）is simulated and the influence of pore size of porous graphene on hydrogen selectivity and permeability is investigated.It is found that PG-ES and PG-ES1 membrane both exhibit high selectivity for H₂ over other gases,but the permeability of PG-ES membrane is much lower that the PG-ES1 membrane because of the smaller pore size.The PG-ES2 membrane with bigger pores demonstrates low selectivity for H₂ over other gases.DFT calculation is employed to calculate the pore size of three different porous graphene,the energy barrier for gas molecules passing through the membranes and electron density around the pores.After comparing the DFT results with the kinetic diameters of gas molecules,we found that both bigger molecular kinetic diameter and smaller pore size contribute to the higher energy barrier,due to the electron overlap between gas and membrane pores.PG-ES1 membrane can effectively separate H₂ from other gases and can satisfy the selectivity and permeability at the same time,which may be useful for designing new concept membranes for other gases.Then,using DFT calculations,the adsorption process of CO₂ and other gases on Ca-doped graphene（Ca-graphene）is simulated and the effects of the intensity and direction of an external electric field is investigated.The results show that the adsorption energy of gas molecules（ΔEgas）（including CO₂,N₂ and CH₄ molecules）on pristine graphene is largely enhanced by Ca-decoration,especially for CO₂ adsorption.Interestingly,ΔEgas become adjustable in the presence of an external electric field.CO₂ molecules are more preferentially adsorbed on Ca-graphene than N₂ and CH₄ ones whether under a positive or negative electric field.Bearing in mind that ΔEN₂ decreases under a positive electric field while ΔECH₄ decreases under a negative electric field,we can predict that a positive elelctric filed is favorable for CO₂/N₂ separation while a negative electric field is appropriate for CO₂/CH₄ separation.Our results provide a general picture on the interaction between gas molecules and Ca-graphene with or without an external electric field.We predict that carbon dioxide capture and sequestration process can be modulated via proper metal decoration and controlling electric field intensity and direction.Our simulation results have broadened our understanding on the interaction between graphene and gas molecules at the micro level.It can also provide some theoretical guidance for designing graphene membrane and its application in gas separation and adsorption.Theoretical simulation methods can reduce the research period and the cost.Our study also makes it possible to use this kind of graphene in gas separation and adsorption areas more quickly.