Acta Phys. -Chim. Sin. ›› 2020, Vol. 36 ›› Issue (11): 1911044.doi: 10.3866/PKU.WHXB201911044

• Article • Previous Articles     Next Articles

Electrostatic Effect-based Selective Permeation Characteristics of Graphene Nanopores

Chengzhen Sun(), Runfeng Zhou, Bofeng Bai   

  • Received:2019-11-25 Accepted:2019-12-16 Published:2019-12-20
  • Contact: Chengzhen Sun E-mail:sun-cz@xjtu.edu.cn
  • Supported by:
    the National Natural Science Foundation of China(51876169);the National Natural Science Foundation of China(51425603);the General Open Project of Key Laboratory of Thermal Power Technology, China(TPL2017BB009)

Abstract:

Two-dimensional graphene nanopores have proved to be a very effective molecular sieve with ultra-high molecular permeance due to the atomic thickness of graphene sheets. The mechanism of graphene nanopores for molecular sieving is generally the size-sieving effect of different molecules. However, high-selective molecular separation is difficult to realize based only on the size-sieving effect. Therefore, graphene nanopore-based membranes usually present high permeance but a moderate selectivity, such that the separation performance cannot far exceed those of traditional separation membranes. In this study, the effects of charges on graphene surfaces on the selective permeation of CO2/N2 mixtures through a graphene nanopore is studied using molecular dynamics simulations; its purpose to realize electrostatic effect-based selective molecular permeation through graphene nanopores and find a promising method to improve the selectivity of molecular separation. The simulation results show that graphene nanopores with negative charges have higher CO2 permeance and lower N2 permeance and, thus, present a high selectivity for the separation of the CO2/N2 mixtures. The graphene nanopore with positive charges, however, does not improve the selectivity. The electrostatic effect-based selectivity of graphene nanopores is related to the different molecular adsorption abilities on the graphene surface with charges. For negative charges, the adsorption ability of CO2 molecules increases and the number of permeated molecules via surface mechanism increases and the experience time during the permeation process also increases; ultimately the CO2 permeance increases with increasing the charge density. For the molecules permeated through the surface mechanism, they are firstly adsorbed onto the graphene surface and then diffuse to the pore region for the ultimate permeation; thus, their experience time is longer than that of the molecules permeated through a direct mechanism. Therefore, a longer experience time means a more significant contribution of the surface flux to the total flux. At high surface charge densities, the contribution of surface flux is dominated and thus the experience time is longer. For CO2 molecules, the permeation rates increase with increasing the surface charge density. Namely, a higher experience time corresponds to a higher permeation rate for CO2 molecules. A decrease of N2 permeance with increasing the charge density is correlated to the increasing CO2 permeance via the inhibition effects of non-permeating components on the permeation of permeating components. For positive charges, the adsorption abilities of CO2 and N2 molecules have no obvious variation with the charge density and their permeance is constant; therefore, the graphene nanopore still has no electrostatic effect-based selectivity.

Key words: Graphene nanopore, Selective permeation, Electrostatic effect, Gas molecule, Molecular dynamics, Molecular sieve

  • O647