Graphene-based Membranes Research Articles

Graphene-based membranes for CO2 separation

Increasing concentration of carbon dioxide (CO2) in the atmosphere is responsible for global warming in the world. Up to date, many technologies have been investigated to reduce the concentration of CO2 in the environment but no technology has got significant successes yet. Recently graphene based membrane has attracted increasing attention for their potential applications in gas separation. In this review, we focus on separation of CO2 from gas mixtures through graphene-based membranes. More specifically, the separation of gas mixtures such as CO2/H2, CO2/CH4, CO2/N2 and CO2/O2 which have similar compositions in flue gas treatment, hydrogen purification and natural gas purification will be discussed. The advantages and disadvantages of these membranes will also be part of discussion and finally, the review ends with several recommendations and suggestions on the future prospect for CO2 separation.

Water desalination through fluorine-functionalized nanoporous graphene oxide membranes

Abstract This study evaluates the application of fluorinated graphene-based water desalination membranes using molecular dynamics simulations. The studied membranes are fluorinated nanoporous graphene oxide (F–NPGO) and nanoporous graphene (F–NPG) sheets. The applied pressure, as a driving force, was used in the range of 5–150 MPa in order to transfer the water molecules through the reverse osmosis (RO) membranes. The obtained results show that the water permeability of F–NPGO membranes (G1 and G2) is much greater than that of F–NPG membranes (G3 and G4) due to the existence of hydrophilic groups on their surface. The maximum value of water permeability was obtained for the G2 system (i.e., 3345 L/(m2 hr bar)) at 150 MPa applied pressure. By fluorine functionalization of the pore edge, the passage of ions through the membrane, especially Cl− ions was prevented. This is the main issue of salt rejection performance. The minimum of obtained salt rejection percentage for F–NPGO membranes with the large pore (G2) was 94.31%, and small pore (G1) did not allow for crossing all of the ions. Many analyses such as water density map, water density profile, RDF, and PMF were computed for precisely understand and prove the performance of membrane systems.

Fabrication of Three-Dimensional Graphene-Based Polyhedrons via Origami-Like Self-Folding.

The assembly of two-dimensional (2D) graphene into three-dimensional (3D) polyhedral structures while preserving the graphene's excellent inherent properties has been of great interest for the development of novel device applications. Here, fabrication of 3D, microscale, hollow polyhedrons (cubes) consisting of a few layers of 2D graphene or graphene oxide sheets via an origami-like self-folding process is described. This method involves the use of polymer frames and hinges, and aluminum oxide/chromium protection layers that reduce tensile, spatial, and surface tension stresses on the graphene-based membranes when the 2D nets are transformed into 3D cubes. The process offers control of the size and shape of the structures as well as parallel production. In addition, this approach allows the creation of surface modifications by metal patterning on each face of the 3D cubes. Raman spectroscopy studies show the method allows the preservation of the intrinsic properties of the graphene-based membranes, demonstrating the robustness of our method.

Photocatalytic self-cleaning carbon nitride nanotube intercalated reduced graphene oxide membranes for enhanced water purification

Crumpled laminar nanostructures with nanoscale water transport channels constructed by two-dimensional (2D) reduced graphene oxide nanosheets (rGO NS) have been considered as a promising membrane separation layer for water purification. However, efficient tuning of rGO laminar structures with enhanced membrane performance remains challenging. In this work, we report a novel 1D graphitic carbon nitride nanotube (g-C3N4 NT) intercalated rGO nanofiltration (NF) membrane with enhanced photo-induced self-cleaning performance. The g-C3N4 NT photocatalyst could not only enlarge the rGO interlayer spacing for improved water permeability, but also endow the composite membrane with visible-light photocatalytic activity for efficient removal of organic dyes from polluted water. Our g-C3N4 NT/rGO membrane exhibits superior water permeability (4.87 L·m−2·h−1·bar−1) and synergetic Rhodamine B (RhB) removal rate (>98%) for long-term operation, compared with pristine rGO membrane and 2D g-C3N4 NS intercalated rGO membrane prepared by the same method. The RhB removal mechanism on the as-prepared composite membranes under integrated photocatalytic filtration process is presented. Our findings offer new insights into designing and tuning laminar structures of photocatalytic graphene-based NF membranes for enhanced water purification.

Ultrahigh permeability of graphene-based membranes by adjusting D-spacing with poly (ethylene imine) for the separation of dye wastewater

Abstract Graphene oxide (GO) as a unique 2D material attracts much attention. It can be employed to prepare graphene-based membranes by vacuum filtration on microporous substrates. However, the permeability of graphene-based membranes is mainly limited by its interlamellar spacing. In this work, the interlamellar spacing of graphene-based membranes is expanded by grafting poly (ethylene imine) (PEI) on GO sheets. The novel graphene-based membranes present ultrahigh permeability without sacrificing the rejection for the filtration of dye wastewater. The original GO membrane (GOM) only has a pure water permeation flux (PWP) value of 27.0 L·m−2·h−1 bar−1. However, the novel graphene-based membranes prepared with the modified GO by PEI-600 and PEI-10000 (recorded as PGOM-600 and PGOM-10000) markedly increase to 208.1 and 450.2 L·m−2·h−1·bar−1 respectively. In the separation of dye wastewater, GOM, PGOM-600 and PGOM-10000 exhibit much higher rejections for direct red and methyl blue. However, the modified graphene-based membranes (PGOM-600 and PGOM-10000) present more than 7–20 times higher permeation flux than that of original GOM without sacrificing dye rejection. Thus, this work provides graphene-based membranes with ultrahigh permeation flux for dye removal from wastewater.

Synthesis of graphene oxide/polyacrylamide composite membranes for organic dyes/water separation in water purification

To obtain nanofiltration membranes with high-performance in desalination and water purification, membranes of graphene oxide (GO), reduced graphene oxide (rGO) and GO/polyacrylamide (PAM) are prepared by a vacuum filtration method. This method is conducted in aqueous solution without any organic solvents. The graphene-based membranes (GBMs) are characterized by UV–visible spectroscopy, Fourier-transform infrared spectroscopy, transmission electron microscopy, atomic force microscopy, scanning electron microscopy, thermogravimetric analysis and X-ray photoelectron spectroscopy. The hydrophilicity of GBMs is also evaluated by contact angle measurement. The interlayer spacing of GO membrane (0.85 nm), GO/PAM membrane (0.68 nm) and rGO membrane (0.36 nm) are measured by X-ray diffraction. The performance of the GBMs is evaluated on a dead-end filtration device. The water flux and retention of rhodamine B of the membranes are 399.04 L m−2 h−1 bar−1 and 85.03% (GO), 188.89 L m−2 h−1 bar−1 and 95.43% (GO/PAM), 85.85 L m−2 h−1 bar−1 and 97.06% (rGO), respectively. The GO/PAM membrane has the best comprehensive separation performance because of its proper interlayer spacing. GO/PAM membranes provide potential advantages in the design of high-performance membranes for molecular separation and water purification.

Tunable Ion Sieving of Graphene Membranes through the Control of Nitrogen-Bonding Configuration.

Graphene-oxide (GO) membranes with notable ionic-sieving properties have attracted significant attention for many applications. However, the swelling and unstable nanostructure of GO laminates in water results in enlarged interlayer spacing and a low permeation cut-off, limiting their applicability for water purification and desalination. Herein, we propose novel nitrogen-doped graphene (NG) membranes for use in tunable ion sieving that are made via facile fabrication by a time-dependent N-doping technique. Doping reaction time associated variation in atomic content and bonding configurations strongly contributed to the nanostructure of NG laminates by yielding narrower interlayer spacing and a more-polarized surface than GO. These nanostructural features subsequently allowed ion transport through the combined mechanisms of size exclusion and electrostatic interaction. The stacked NG membranes provided size-dependent permeability for hydrated ions and improved ion selectivity by 1-3 orders of magnitude in comparison to that of a GO membrane. For ions small enough to move through the interlayer spacing, the ion permeation is determined by electrostatic properties of NG membranes with the type of N configuration, especially polarized pyridinic N. Due to these properties, the NG membrane functioned as an unconventionally selective graphene-based membrane with better ion sieving for water purification.

Interfacial Force-Assisted In-Situ Fabrication of Graphene Oxide Membrane for Desalination.

Graphene-based membranes have shown great potential application prospects in many fields, especially for water purification. Except for the current relatively low salt rejection rate, another main factor restricting application of such membranes is the lack of applicable preparation processes. In this work, a facile and cost-effective method was developed that can be used to in situ fabricate a graphene oxide (GO)-based membrane inside a filtration apparatus. Novel partial reduction and cross-linking was employed to adjust the surface properties and interlayer distance of GO membranes at the subnanometer range. A simple compacting process was applied to promote the integrity and compactness of the GO-based membranes by making full use of the interfacial tensions of gas/liquid/solid, which enables the in-situ fabrication. The as-prepared PrGO membranes show good water permeability (17.2-86.5 L m-2 h-1 MPa-1), reasonable desalination rates (27.7-62.6% for NaCl and 68.4-86.1% for Na2SO4), and good rejection rates of 92.3-96.8% for methyl orange. The method is appropriate for large-scale preparation and is theoretically not restricted by the shape or texture of the basement membrane, which represents another step forward in the fabrication of GO-based membranes toward wide-ranging applications.

Fluorine-functionalized nanoporous graphene as an effective membrane for water desalination

Most water in the world is as saline water in seas and oceans. Desalination technology is a promising method to solve the global water crisis. Recently, many attentions have been paid to the graphene-based membranes in water desalination due to their low production cost and high efficiency. In this paper, molecular dynamics simulations are employed to investigate the effect of functionalized graphene nanosheet (GNS) membranes on the performance of salt separation from seawater in terms of water permeability and salt rejection. For this purpose, the hydrogenated (–H) and fluorinated (–F) pores were created on the GNS membrane. Then, the functionalized graphene membrane was placed in the middle of the simulation box in an aqueous ionic solution containing Na+ and Cl− ions. The applied pressure (in the range of 10–100 MPa) was used as the driving force for transport of water molecules across the reverse osmosis (RO) graphene-based membrane in order to obtain the water permeability and salt rejection. Also, radial distribution functions (RDFs) of ion–water and water–water as well as the water density map around the membrane were obtained. The results indicated that the hydrophilic chemical functions such as fluorine (–F) can improve the water permeability at low pressures.

Graphene-Based Composite Membrane for Water Desalination

Graphene-Based Composite Membrane for Water Desalination

Low-voltage electrostatic modulation of ion diffusion through layered graphene-based nanoporous membranes.

Ion transport in nanoconfinement differs from that in bulk and has been extensively researched across scientific and engineering disciplines1-4. For many energy and water applications of nanoporous materials, concentration-driven ion diffusion is simultaneously subjected to a local electric field arising from surface charge or an externally applied potential. Due to the uniquely crowded intermolecular forces under severe nanoconfinement (<2 nm), the transport behaviours of ions can be influenced by the interfacial electrical double layer (EDL) induced by a surface potential, with complex implications, engendering unusual ion dynamics5-7. However, it remains an experimental challenge to investigate how such a surface potential and its coupling with nanoconfinement manipulate ion diffusion. Here, we exploit the tunable nanoconfinement in layered graphene-based nanoporous membranes to show that sub-2 nm confined ion diffusion can be strongly modulated by the surface potential-induced EDL. Depending on the potential sign, the combination and concentration of ion pairs, diffusion rates can be reversibly modulated and anomalously enhanced by 4~7 times within 0.5 volts, across a salt concentration gradient up to seawater salinity. Modelling suggests that this anomalously enhanced diffusion is related to the strong ion-ion correlations under severe nanoconfinement, and cannot be explained by conventional theoretical predictions.

Graphene-Based Membrane Technology: Reaching Out to the Oil and Gas Industry

This paper presents a critical review and the state of the art of graphene porous membranes, a brand-new technology and backdrop to discuss its potential application for efficient water desalination in low salinity water injection (LSWI). LSWI technology consists in injecting designed, adequately modified, filtered water to maximize oil production. To this end, desalination technologies already available can be further optimized, for example, via graphene membranes, to achieve greater efficiency in water-oil displacement. Theoretical and experimental applications of graphene porous membranes in water desalination have shown promising results over the last 5-6 years. Needless to say, improvements are still needed before graphene porous membranes become readily available. However, the present work simply sets out to demonstrate, at least in principle, the practical potential graphene membranes would have in hydrocarbon recovery processes.

Fabrication of Graphene-based Membranes for Solvent-resistant Nanofiltration

Solvent-resistant nanofiltration (SRNF) is a promising technology which developed rapidly in recent years, which exhibits broader application prospects in separation of industries organic solvent system. Among various materials used for SRNF-membranes, graphene-based nanomaterials have aroused great interests due to the unique molecular sieve properties and excellent tolerance to organic solvents and harsh chemical environments. This paper emphatically introduces the progresses of membrane preparation with graphene-based nanomaterials for SRNF and their application in separation processes. The performances including permeability and stability of different kinds of graphene-based SRNF membranes in the process of filtration are also evaluated and discussed.

Effect of Oxidized-Group-Supported Lamellar Distance on Stability of Graphene-Based Membranes in Aqueous Solutions

Graphene oxide (GO) membranes with diverse lamellar distances can achieve sieving of molecules of different sizes in aqueous solutions. Without the extra assistance of cross-linking, the increase of lamellar distance supported by self-contained oxidized groups could diminish the internal π–π attraction force of GO membranes, resulting in the instability of GO membranes. However, the critical value of oxidized-group-supported lamellar distance of unstable GO membranes in aqueous solutions, which is crucial in applications for precise sieving of molecules, still remains unknown. In this study, the effect of oxidized-group-supported lamellar distance on the stability of graphene-based membranes in aqueous solutions is systematically investigated. By controlling the content of oxidized groups via changing reducing condition, different lamellar distances of graphene-based membranes are obtained. With changing the lamellar distance in aqueous solutions under different pH conditions, the graphene-based membranes...

Enhanced electricity generation and effective water filtration using graphene-based membrane air-cathodes in microbial fuel cells

Air-cathodes in microbial fuel cells that can also filter wastewater provide the dual benefits of electricity production and reduction of the effluent chemical oxygen demand. Air-cathodes prepared using a novel activated carbon/graphene membrane (2, 5 or 10% graphene by weight) prepared by phase inversion have good conductivities (5.6 ± 0.5 to 7.3 ± 0.6 mS cm−1) compared to control (3.0 ± 0.4 mS cm−1, activated carbon, no graphene). The cathode with 5 wt% graphene produces the highest maximum power density of 1460 ± 10 mW m−2, which is 58% higher than that the control (928 ± 8 mW m−2). The increased power is due to an 88% reduction in charge transfer resistance of 6.0 ± 0.3 Ω (cathode with 5 wt% graphene) compared to the control. Following a cycle of treatment and current generation, 60 ± 1% of the chemical oxygen demand is removed from the remaining chemical oxygen demand, producing an effluent chemical oxygen demand concentration of 20 ± 1 mg L−1. Biomass (4.99 ± 0.02 mg-protein cm−2) is decreased by 33% compared to the control. These results demonstrate that cathodes made with graphene can produce electricity and a high quality effluent with low cathode biofouling.

RGO/protonated g-C3N4 hybrid membranes fabricated by photocatalytic reduction for the enhanced water desalination

Expected to be the next generation separation membranes, graphene-based functional membranes are springing up in the domains of water desalination and purification. Considering the relatively large interlayer spacing and the swelling effect of the pure GO membranes in solution, feasible strategies must be adopted to overcome these obstacles. Based on the electrostatic attraction between GO and protonated g-C3N4, uniform and compact GO/g-C3N4 hybrid membranes are firstly prepared by a vacuum filtration method and then rGO/g-C3N4 hybrid membranes are fabricated under mild UV irradiation. During the in situ photocatalytic reduction of GO, oxygen functionalities are gradually removed by virtue of g-C3N4 without any chemicals, resulting in the decreased interlayer spacing, the restrained swelling effect of the hybrid membranes and the weakened electrostatic screening of ion hydration shells, and then the outstanding water desalination is obtained. As-prepared rGO/g-C3N4 hybrid membranes irradiated under UV for 18 h show high rejections towards several ion salts under maintaining the appropriate water flux, especially for sodium salts (67.5% for NaCl, 89.2% for Na2SO4). Based on characterization results, the reason of the difference of the rejections towards various salts and the desalination mechanisms are discussed. This work offers valuable clues for preparing all‑carbon based membranes.

Quantum tunneling of thermal protons through pristine graphene.

Engineering of atomically thin membranes for hydrogen isotope separation is an actual challenge which has a broad range of applications. Recent experiments [M. Lozada-Hidalgo et al., Science 351, 68 (2016)] unambiguously demonstrate an order-of-magnitude difference in permeabilities of graphene-based membranes to protons and deuterons at ambient conditions, making such materials promising for novel separation technologies. Here we demonstrate that the permeability mechanism in such systems changes from quantum tunneling for protons to quasi-classical transport for heavier isotopes. Quantum nuclear effects exhibit large temperature and mass dependence, modifying the Arrhenius activation energy and Arrhenius prefactor for protons by more than 0.5 eV and by seven orders of magnitude correspondingly. Our findings not only shed light on the separation process for hydrogen isotope ions passing through pristine graphene but also offer new insights for controlling ion transport mechanisms in nanostructured separation membranes by manipulating the shape of the barrier and transport process conditions.

Experimental study on graphene-based nanocomposite membrane for hydrogen purification: Effect of temperature and pressure

In the last five recent years, molecular-sieve Graphene Oxide (GO) membranes have shown a great potential to realize high-flux and high-selectivity mixture separation, at low energy cost. Hence, in this study, GO nanocomposite membranes were fabricated on modified γ-alumina tubes for hydrogen separation. As a high-performance process, H2-permselective GO membrane derived from graphite source was developed by using a modified hummer method. In fact, the GO nanocomposite membranes, referred to here as carbon membranes, were prepared via vacuum dip coating of the modified γ-alumina tubes using single-layer GO solution. Special attention was devoted to obtain high H2/CO2 selectivity, high H2 permeance, and good stability in separation process. Hence, the effects of the operating conditions (temperature, and pressure on quality and performance of the GO membranes) were investigated. At the best condition, the synthesized GO membrane exhibited good H2/CO2 selectivity (>57) and high H2 permeance (in the order of 7.9 × 10−7 mol Pa-1 m-2 s-1). Moreover, it was found that the GO membrane selectivities (H2/CO2 and H2/N2) were decreased by temperature increasing, while selectivity trends were improved by increasing pressure gradient. It should be noted that all permeation tests were reported after time durability of around 48 h.

Development of advanced nanocomposite membranes using graphene nanoribbons and nanosheets for water treatment

Water-intensive industries have to comply with stringent environmental regulations and evolving regulatory frameworks requiring the development of new technologies for water recycling. Development of polymeric membranes may provide an effective solution to improve water recycling, but require finely-tuned pore size and surface chemistry for ionic and molecular sieving to be efficient. Additionally, fouling is a major challenge that limits the practical application of the membranes in water recycling in these industries. In this work, four different graphene oxide (GO) derivatives were incorporated into a polyethersulfone (PES) matrix via a non-solvent induced phase separation (NIPS) method. The GO derivatives used have different shapes (nanosheets vs nanoribbons) and different oxidation states (C/O = 1.05–8.01) with the potential to enhance water flux and suppress fouling of the membranes through controlled pore size, hydrophilicity, and surface charge. The permeation properties of the PES/GO membranes were evaluated using a water sample from the Athabasca oil sands of Alberta. The results for contact angle and streaming potential measurements indicate the formation of more hydrophilic and negatively charged PES/GO nanocomposite membranes. All graphene-based nanocomposite membranes demonstrated better water flux and rejection of organic matter compared to the unmodified PES membrane. The fouling measurement results revealed that fouling was impeded due to enhanced membrane surface properties. Longitudinally unzipped graphene oxide nanoribbons (GONR-L) at an optimum loading of 0.1 wt% (wt%) provided the maximum water flux (70 LMH at 60 psi), organic matter rejection (59%) and antifouling properties (30% improvement compared to pristine PES membrane). Flux recovery ratio experiments indicated a remarkable enhancement in the fouling resistance property of PES/GO nanocomposite membranes.

(Invited) Graphene-Based Membranes for Nanofiltration

Nanoporous graphene-based films have recently emerged as promising materials for separations. Graphene-oxide is a 2D material with high mechanical strength, chemical resilience, favorable surface properties, and the potential for scalable low-cost manufacturing. In this talk, I will highlight our computational and experimental work on the properties, synthesis, and performance of graphene-oxide based membranes.