Porous Graphene Membranes Research Articles

Mechanism and Prediction of Gas Permeation through Sub-Nanometer Graphene Pores: Comparison of Theory and Simulation.

Due to its atomic thickness, porous graphene with sub-nanometer pore sizes constitutes a promising candidate for gas separation membranes that exhibit ultrahigh permeances. While graphene pores can greatly facilitate gas mixture separation, there is currently no validated analytical framework with which one can predict gas permeation through a given graphene pore. In this work, we simulate the permeation of adsorptive gases, such as CO2 and CH4, through sub-nanometer graphene pores using molecular dynamics simulations. We show that gas permeation can typically be decoupled into two steps: (1) adsorption of gas molecules to the pore mouth and (2) translocation of gas molecules from the pore mouth on one side of the graphene membrane to the pore mouth on the other side. We find that the translocation rate coefficient can be expressed using an Arrhenius-type equation, where the energy barrier and the pre-exponential factor can be theoretically predicted using the transition state theory for classical barrier crossing events. We propose a relation between the pre-exponential factor and the entropy penalty of a gas molecule crossing the pore. Furthermore, on the basis of the theory, we propose an efficient algorithm to calculate CO2 and CH4 permeances per pore for sub-nanometer graphene pores of any shape. For the CO2/CH4 mixture, the graphene nanopores exhibit a trade-off between the CO2 permeance and the CO2/CH4 separation factor. This upper bound on a Robeson plot of selectivity versus permeance for a given pore density is predicted and described by the theory. Pores with CO2/CH4 separation factors higher than 102 have CO2 permeances per pore lower than 10-22 mol s-1 Pa-1, and pores with separation factors of ∼10 have CO2 permeances per pore between 10-22 and 10-21 mol s-1 Pa-1. Finally, we show that a pore density of 1014 m-2 is required for a porous graphene membrane to exceed the permeance-selectivity upper bound of polymeric materials. Moreover, we show that a higher pore density can potentially further boost the permeation performance of a porous graphene membrane above all existing membranes. Our findings provide insights into the potential and the limitations of porous graphene membranes for gas separation and provide an efficient methodology for screening nanopore configurations and sizes for the efficient separation of desired gas mixtures.

Propylene/propane separation by porous graphene membrane: Molecular dynamic simulation and first-principle calculation

Molecular dynamic (MD) simulation and first-principle density functional theory (DFT) calculation are applied to study the C3H6/C3H8 separation performance of porous graphene membrane. It was found that porous graphene membrane with appropriate pore size and shape can be employed to separate propylene and propane. Among the ten kinds of pores investigated, the graphene membrane with NH modified pores exhibit extremely superior selectivity for propylene while maintaining high permeance. According to the study on adsorption mechanism, porous graphene (PG) membranes show preferential adsorption towards propylene mainly attributed to van der Waals interaction, which gives C3H6 molecules more chances to approach the pores. On the other hand, interaction energy and electron density by DFT demonstrate that the attractive potential energy well and the less pronounced overlap of PG membrane towards propylene molecules do facilitate their penetration through the pores. On the contrary, the case is totally different with propane. We anticipate that our work will accelerate the application of porous PG membrane in the C3H6/C3H8 separation.

Computational Design of Porous Graphenes for Alkane Isomer Separation

Using first-principles calculations, we systematically evaluated a series of single-layer porous graphene membranes with different sized pores passivated by hydrogen atoms for separating short alkane isomers (C = 5–7). We found that graphene membranes with appropriate pore size (e.g., the pore19 model whose pore size is 8.0 × 5.8 A) could efficiently separate dibranched isomers from their monobranched and linear counterparts. When alkane molecules diffused through a membrane, the porous graphene might exhibit significant distortion. At the same time, the passing molecule would be forced to change its own geometry as well. More importantly, we found that the geometric deformations of both the penetrating molecule and the membrane concertedly lowered the diffusion barrier by similar magnitudes. Therefore, when designing two-dimensional (2D) separation materials, it is necessary to consider the geometric flexibility of both the separation material and the molecules to be separated. Our results theoretically ...

Knudsen effusion through polymer-coated three-layer porous graphene membranes

Graphene membranes have the potential to exceed the permeance and selectivity limits of conventional gas separation membranes. Realizing this potential in practical systems relies on overcoming numerous scalability challenges, such as isolating or sealing permeable defects in macroscopic areas of graphene that can compromise performance and developing methods to create high densities of selective pores over large areas. This study focuses on a centimeter-scale membrane design, where leakage is reduced by substrate selection, permeable polymer film coating, and stacking of three independent layers of graphene, while (selective) pores are created by high density ion bombardment. The three-layer graphene provides high resistance to gas flow, which decreases with ion bombardment and results in selectivity consistent with Knudsen effusion. The results suggest that the permeable pores created in three layer graphene were larger than those required for molecular sieving and that designs based on single layer graphene may lend themselves more easily to molecular sieving of gases.

Efficient ethanol/water separation via functionalized nanoporous graphene membranes: insights from molecular dynamics study

Systematic molecular dynamics simulations are conducted to study the separation of ethanol/water mixture through single-layer graphene with designed nanoscale pores. The effects of pore size, chemical functionalization, and applied pressure were investigated. It was found that the diameter of pore plays a key role for efficient separation of ethanol from water. With appropriate diameter, water molecules can pass through but flow of ethanol is essentially blocked. Compared to hydrophobic, hydrophilic functionalization is found to be more efficient for ethanol/water separation as energy barrier for water molecule is less than ethanol in case of hydrophilic porous graphene membrane. Overall, our results indicate that the flux through hydrophilic functionalized (P2_OH) graphene membrane is nearly four times higher than conventional reverse osmosis membranes with a good selectivity for ethanol/water separation. This simulation study provides molecular-level understanding of ethanol/water separation through functionalized nonporous graphene and reveals the key governing factors that are essential for designing novel graphene membranes for bioethanol purification.

Graphene-based membranes: status and prospects.

Recently, graphene-based membranes have been extensively studied, represented by two distinct research directions: (i) creating pores in graphene basal plane and (ii) engineering nanochannels in graphene layers. Most simulation results predict that porous graphene membranes can be much more selective and permeable than current existing membranes, also evidenced by some experimental results for gas separation and desalination. In addition, graphene oxide has been widely investigated in layered membranes with two-dimensional nanochannels, showing very intriguing separation properties. This review will cover state-of-the-art of graphene-based membranes, and also provide a material guideline on future research directions suitable for practical membrane applications.

Ion current rectification effect of porous graphene membrane

AbstractGraphene, as an important ideal two‐dimensional material, is driving more and more attention because of its unique mechanical strength and chemical stability. In this work, the rectification effect of nanopores in graphene/PET structure at different KCl concentration is investigated. Porous graphene membrane is obtained by using heavy ion irradiation technology. During investigating the ion current rectification effect, PET membrane with conical pores acts as porous graphene's substrate. The rectification coefficient of nanopores in graphene/PET is deduced from the current‐voltage curves at KCl solution with different concentrations. The porous graphene/PET shows a relative higher rectification ratio and is more resistive to pH value while comparing the nanopores in PET. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Programmed synthesis of freestanding graphene nanomembrane arrays.

Freestanding graphene membranes are unique materials. The combination of atomically thin dimensions, remarkable mechanical robustness, and chemical stability make porous and non-porous graphene membranes attractive for water purification and various sensing applications. Nanopores in graphene and other 2D materials have been identified as promising devices for next-generation DNA sequencing based on readout of either transverse DNA base-gated current or through-pore ion current. While several ground breaking studies of graphene-based nanopores for DNA analysis have been reported, all methods to date require a physical transfer of the graphene from its source of production onto an aperture support. The transfer process is slow and often leads to tears in the graphene that render many devices useless for nanopore measurements. In this work, we report a novel scalable approach for site-directed fabrication of pinhole-free graphene nanomembranes. Our approach yields high quality few-layer graphene nanomembranes produced in less than a day using a few steps that do not involve transfer. We highlight the functionality of these graphene devices by measuring DNA translocation through electron-beam fabricated nanopores in such membranes.

N-doped porous graphene for carbon dioxide separation: a molecular dynamics study

Using molecular dynamics simulations, it is shown that N-doped porous graphene membrane can efficiently separate CO2 from the CO2/CH4 mixture. The effects of pore rim modifications, N-doping sites, initial gas pressure, and feed gas compositions on CO2 separation performance of N-doped porous graphene membrane are also examined. It is found that gas permeability increases with increasing initial gas pressure or the feed gas percentages. Pore rim modifications with nitrogen atoms (pyridinic N) can significantly improve the selectivity of CO2 over CH4 owing to the enhanced electrostatic interactions compared to the unmodified one, and the all-N-modified pore-16 shows the highest CO2 selectivity over CH4 (~29). Doping N atoms on the graphene sheets (quaternary N) has little effect on the CO2 selectivity. Our study demonstrates that the N-doped porous graphene is an excellent candidate for CO2 separation, which may be beneficial for the realization of a low carbon society.

Ultimate permeation across atomically thin porous graphene.

A two-dimensional (2D) porous layer can make an ideal membrane for separation of chemical mixtures because its infinitesimal thickness promises ultimate permeation. Graphene--with great mechanical strength, chemical stability, and inherent impermeability--offers a unique 2D system with which to realize this membrane and study the mass transport, if perforated precisely. We report highly efficient mass transfer across physically perforated double-layer graphene, having up to a few million pores with narrowly distributed diameters between less than 10 nanometers and 1 micrometer. The measured transport rates are in agreement with predictions of 2D transport theories. Attributed to its atomic thicknesses, these porous graphene membranes show permeances of gas, liquid, and water vapor far in excess of those shown by finite-thickness membranes, highlighting the ultimate permeation these 2D membranes can provide.

Fluorine-Modified Porous Graphene as Membrane for CO2/N2 Separation: Molecular Dynamic and First-Principles Simulations

It is demonstrated that the fluorine-modified porous graphene membrane has excellent selectivity for CO2/N2 separation by using molecular dynamic (MD) simulations. We also investigated in detail the mechanism of the fluorine-modified porous graphene membrane for CO2/N2 separation by using first-principles simulations. We find that the diffusion barriers for CO2 and N2 to pass through the pore-22 (with 22 carbon atoms drilled out) graphene membrane are relatively small, which indicates that the pore-22 has a low selectivity for CO2/N2 separation. After fluorine modification, the diffusion barrier for CO2 to pass through decreases to 0.029 eV, while the diffusion barrier for N2 greatly increases to 0.116 eV. Therefore, N2 gets more difficult, while CO2 gets easier to penetrate through the fluorine-modified pore-22. The fluorine-modified pore-22 porous graphene shows a great enhancement of selectivity for CO2/N2 separation, which is consistent with the MD results. Our studies show that first-principles simul...

Tunable Hydrogen Separation in Porous Graphene Membrane: First-Principle and Molecular Dynamic Simulation

First-principle density functional theory (DFT) calculation and molecular dynamic (MD) simulation are employed to investigate the hydrogen purification performance of two-dimensional porous graphene material (PG-ESX). First, the pore size of PG-ES1 (3.2775 Å) is expected to show high selectivity of H2 by DFT calculation. Then MD simulations demonstrate the hydrogen purification process of the PG-ESX membrane. The results indicate that the selectivity of H2 over several other gas molecules that often accompany H2 in industrial steam methane reforming or dehydrogenation of alkanes (such as N2, CO, and CH4) is sensitive to the pore size of the membrane. PG-ES and PG-ES1 membranes both exhibit high selectivity for H2 over other gases, but the permeability of the PG-ES membrane is much lower than 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. Energy barrier and electron density have been used to explain the difference of selectivity and permeability of PG-ESX membranes by DFT calculations. The energy barrier for gas molecules passing through the membrane generally increase with the decreasing of pore sizes or increasing of molecule kinetic diameter, due to the different electron overlap between gas and a membrane. The PG-ES1 membrane is far superior to other carbon membranes and has great potential applications in hydrogen purification, energy clean combustion, and making new concept membrane for gas separation.

Separation of the hydrogen sulfide and methane mixture by the porous graphene membrane: Effect of the charges

The effect of the charges around the pore on the separation of H2S/CH4 mixture using a porous graphene membrane is investigated by molecular dynamics simulations. Due to the electrostatic interactions between H2S and membrane, the potential energy of H2S around the charged pore reduces. As a consequence, more H2S molecules accumulate around the pore and its selectivity is improved as well, while the permeation ratio of H2S is lower. The present study is valuable for designing the novel efficient membranes for the mixture separation.

Graphene-like Membranes: From Impermeable to Selective Sieves

ABSTRACTRecently, it was proposed that graphene membranes could act as impermeable atomic structures to standard gases. For some other applications, a higher level of porosity is needed, and the so-called Porous Graphene (PG) and Biphenylene Carbon (BPC) membranes are good candidates to effectively work as selective sieves. In this work we have used classical molecular dynamics simulations to study the dynamics of membrane permeation of He and Ar atoms and possible selectivity effects. For the graphene membranes we did not observe any leakage through the membrane and/or membrane/substrate interface until a critical pressure limit, then a sudden membrane detachment occurs. PG and BPC membranes are not impermeable as graphene ones, but there are significant energy barriers to diffusion depending on the atom type. Our results show that this kind of porous membranes can be effectively used as selective sieves for pure and mixtures of gases.

An overview of the engineered graphene nanostructures and nanocomposites

This critical review focuses on the property analysis of graphene and graphene nanocomposites (GNCs) and their demonstrated superior performances in energy storage and conversion, electrochemical- and bio-sensing, environmental remediation and flame retardant application and atomic thickness membrane separation. The performances of graphene and GNCs are strongly dependent on their chemical component, synthetic method, nanoscale morphology, and assembling structure of the hybrids. The current progress in supercapacitor energy storage density, solar cell power conversion efficiency, thermoelectric energy conversion efficiency, electrochemical sensing capability, biosensor sensitivity, heavy metal adsorption capacity and efficiency, photocatalytic degradation rate of organic dye, flame retardant polymer nanocomposites, graphene and porous graphene membranes is discussed with detailed examples through extensive analysis of the literature.

Influence of chemical functionalization on the CO2/N2 separation performance of porous graphene membranes

The separation of CO₂ from a mixture of CO₂ and N₂ using a porous graphene membrane was investigated using molecular dynamics (MD) simulations. The effects of chemical functionalization of the graphene sheet and pore rim on the gas separation performance of porous graphene membranes were examined. It was found that chemical functionalization of the graphene sheet can increase the absorption ability of CO₂, while chemical functionalization of the pore rim can significantly improve the selectivity of CO₂ over N₂. The results show that the porous graphene membrane with all-N modified pore-16 exhibits a higher CO₂ selectivity over N₂ (∼11) due to the enhanced electrostatic interactions compared to the unmodified graphene membrane. This demonstrates the potential use of functionalized porous graphene as single-atom-thick membrane for CO₂ and N₂ separation. We provide an effective way to improve the gas separation performance of porous graphene membranes, which may be useful for designing new concept membranes for other gases.

Separation of Hydrogen and Nitrogen Gases with Porous Graphene Membrane

We designed a series of porous graphene as the separation membrane of H2/N2. The selectivity and permeability could be controlled by drilling various nanopores with different shapes and sizes. The mechanisms of hydrogen and nitrogen to permeate through the porous graphene are different. The small nanopore (pore-11) can only allow the hydrogen molecules to permeate due to the size restriction. In the systems of bigger nanopores (e.g., pore-13, pore-14, etc.), where the pore size is big enough to allow nitrogen molecules to permeate without any restriction, we observed more permeation events of nitrogen than that of hydrogen molecules. The reason is that the van der Waals interactions with the graphene membrane make the nitrogen molecules accumulate on the surface of graphene. When the pore size further increases, the flow of hydrogen molecules exhibits the linear dependence on the pore area, while there is no obvious correlation between the flow of nitrogen molecules and the pore area.

Water Transport through Ultrathin Graphene

Graphene can be considered as an ideal membrane since its thickness is only one carbon diameter. In this study, using molecular dynamics simulations, we investigate water transport through a porous graphene membrane and compare the results with water transport through thin (less than 10 nm in thickness/length) carbon nanotube (CNT) membranes. For smaller diameter pores, where a single-file water structure is obtained, CNT membranes provide higher water flux compared to graphene membranes. For larger diameter pores, where the water structure is not single-file, graphene membranes provide higher water flux compared to CNT membranes. Furthermore, in thin CNT membranes, the water flux did not vary significantly with the thickness of the membrane. We explain the results through a detailed analysis considering pressure distribution, velocity profiles, and potential of mean force. This work opens up opportunities for graphene-based membranes in molecular sieving, water filtration, fuel cells, and so forth.