Graphene Oxide Membranes Research Articles

Rearrangement of GO nanosheets with inner and outer forces under high-speed spin for supercapacitor

The self-standing graphene membranes are considered as ideal electrode materials for supercapacitors. However, maintaining highly regularized and uniform graphene membranes with satisfied electrochemical performance is still a challenge. Herein, with the chelation of metal cation and the radial shear force introduced by high-speed spinning, the uniform interlayer channels and shrunken cracks between adjacent nanosheets can be achieved in the metal-intercalated graphene oxide (GO) membranes, thus realizing regularization both in normal and radial direction. With the promotion in electron transfer and electrolyte penetration, the iron cross-linked GO membrane with spin coating for 40 cycles exhibits a high specific capacitance (427 F g−1 at 1 A g−1) and rate capability (42.6% capacitance retention from 1 to 40 A g−1), as well as excellent cyclic capability (90.5% capacitance retention after 20,000 cycles). Particularly, a 21% increasement in capacitance can be achieved after high-speed spinning treatment. Moreover, the spin regularization strategy can be extended to GO membranes cross-linked by various multi-valence metal cations, the electrochemical performance of metal-cation cross-linked GO membrane electrodes after high-speed spinning treatment can also be improved obviously. Therefore, this paper provides a novel method to fabricate highly ordered GO membranes with promising electrochemical performance, which presents an immense potential application in membrane materials applied in energy storage, separation and catalysis.

A single functionalized graphene nanocomposite in cross flow module for removal of multiple toxic anionic contaminants from drinking water.

Polyether sulfone (PES)-based thin-film nanofiltration (TFN) membranes embedded with ferric hydroxide (FeIII(OH)x) functionalized graphene oxide (GO) nanoparticles were fabricated through interfacial polymerization for a generalized application in removal of a plethora of anionic and toxic water contaminants. Following the most relevant characterization, the newly synthesized membranes were fitted in a novel flat sheet cross-flow module, for experimental investigation on purification of live contaminated groundwater collected from different affected areas. The separation performances of the membranes in the flat sheet cross-flow module demonstrated that GOF membranes had higher selectivity for monovalent and divalent salt rejections than pristine GO membranes. Furthermore, both membranes were tested for simultaneously removing widely occurring hazardous ions of heavy metals and metalloids in groundwater, such as arsenic, selenium, chromium, and fluoride. Compared to the pristine GO and the reported membranes in the literature, the GOF membrane exhibited remarkable performance in terms of rejection efficiency (Cr (VI): 97.2%, Se (IV): 96.6%, As(V): 96.3%, F- 88.4%) and sustained flux of 184 LMH (Lm-2h-1) at an optimum transmembrane pressure of 16bar. The investigated membrane module equipped with the GOF membrane proved to be a low-cost system with higher anionic rejection and sustained high flux at a comprehensive pH range, as evident over long hours of study vis-à-vis reported systems.

Confined and mediated intercalation of nanoparticles in graphene oxide membrane to fine-tune desalination performance

Multilayered graphene oxide (GO) membranes with interlayered sub-nanometer channels have attracted considerable interest for selective water and ion transport. However, the tortuous transport pathways and water-swelling characteristics of GO laminates hinder the desalination performance of such membranes. Modifying the interlayer through nanoparticle intercalation is one of the most prevalently employed methods of enlarging the nanochannels for rapid transport. However, this generally results in significantly reduced separation precision. In this study, we develop a confined interlayer with mediated nanoparticle intercalation to precisely modulate GO membranes. Enhanced water transport and high ion retardation were simultaneously achieved. We investigated the roles of confinement and mediation in the intercalation modification of GO laminates and highlighted their indispensability in realizing an efficient and effective desalination process. This strategy may also be applicable to the fine-structure control of other two-dimensional laminates.

Emulsion Assembly of Graphene Oxide/Polymer Composite Membranes.

Graphene oxide/polymer composite water filtration membranes were developed via coalescence of graphene oxide (GO) stabilized Pickering emulsions around a porosity-generating polymer. Triptycene poly(ether ether sulfone)-CH2NH2:HCl polymer interacts with the GO at the water-oil interface, resulting in stable Pickering emulsions. When they are deposited and dried on polytetrafluoroethylene substrate, the emulsions fuse to form a continuous GO/polymer composite membrane. X-ray diffraction and scanning electron microscopy demonstrate that the intersheet spacing and thickness of the membranes increased with increasing polymer concentration, confirming the polymer as the spacer between the GO sheets. The water filtration capability of the composite membranes was tested by removing Rose Bengal from water, mimicking separations of weak black liquor waste. The composite membrane achieved 65% rejection and 2500 g m-2 h-1 bar-1. With high polymer and GO loading, composite membranes give superior rejection and permeance performance when compared with a GO membrane. This methodology for fabrication membranes via GO/polymer Pickering emulsions produces membranes with a homogeneous morphology and robust chemical separation strength.

Two-Dimensional Interlayer Space Induced Horizontal Transformation of Metal-Organic Framework Nanosheets for Highly Permeable Nanofiltration Membranes.

Laminar membranes comprising graphene oxide (GO) and metal-organic framework (MOF) nanosheets benefit from the regular in-plane pores of MOF nanosheets and thus can support rapid water transport. However, the restacking and agglomeration of MOF nanosheets during typical vacuum filtration disturb the stacking of GO sheets, thus deteriorating the membrane selectivity. Therefore, to fabricate highly permeable MOF nanosheets/reduced GO (rGO) membranes, a two-step method is applied. First, using a facile solvothermal method, ZnO nanoparticles are introduced into the rGO laminate to stabilize and enlarge the interlayer spacing. Subsequently, the ZnO/rGO membrane is immersed in a solution of tetrakis(4-carboxyphenyl)porphyrin (H2 TCPP) to realize in situ transformation of ZnO into Zn-TCPP in the confined interlayer space of rGO. By optimizing the transformation time and mass loading of ZnO, the obtained Zn-TCPP/rGO laminar membrane exhibits preferential orientation of Zn-TCPP, which reduces the pathway tortuosity for small molecules. As a result, the composite membrane achieves a high water permeance of 19.0Lm-2 h-1 bar-1 and high anionic dye rejection (>99% for methyl blue).

Green Methods for the Fabrication of Graphene Oxide Membranes: From Graphite to Membranes.

Graphene oxide (GO) has shown great potential as a membrane material due to its unique properties, including high mechanical strength, excellent thermal stability, versatility, tunability, and outperforming molecular sieving capabilities. GO membranes can be used in a wide range of applications, such as water treatment, gas separation, and biological applications. However, the large-scale production of GO membranes currently relies on energy-intensive chemical methods that use hazardous chemicals, leading to safety and environmental concerns. Therefore, more sustainable and greener approaches to GO membrane production are needed. In this review, several strategies proposed so far are analyzed, including a discussion on the use of eco-friendly solvents, green reducing agents, and alternative fabrication techniques, both for the preparation of the GO powders and their assembly in membrane form. The characteristics of these approaches aiming to reduce the environmental impact of GO membrane production while maintaining the performance, functionality, and scalability of the membrane are evaluated. In this context, the purpose of this work is to shed light on green and sustainable routes for GO membranes' production. Indeed, the development of green approaches for GO membrane production is crucial to ensure its sustainability and promote its widespread use in various industrial application fields.

Improving safety and efficiency in graphene oxide production technology

Chemical conversion of natural graphite into graphene based nanomaterials is achievable and scalable by designed Hummers’ method. Conventionally, high amount of manganyl compound (Mn2O7 or MnO3+) and explosive risk of thermal runaway in the reaction are still the key concerns of efficiency and safety in the production technology. In this manuscript, reaction mechanism and innovative strategy are applied in the cascade design of graphite oxide synthesis. By optimizing the amounts of reactive chemicals (KMnO4, H2SO4 and water), utilizing exothermic energy (for self-heating reaction and exfoliation) and combining reaction divisions (for control of thermal runaway), the principal issues of safety and efficiency are improved for simple and practical preparation of graphene oxide nanomaterials. The cascade synthesis occurs in just 8 h without external supply of thermal energy, producing solid yield of more than 130%wt. Obtained graphene oxide nanosheets have good colloidal property, uniform decoration of functional groups, appropriate C/O atomic ratio of 1.62 and optoelectronic band gap energy of 3.33 eV. Characterization of graphene oxide membranes showed elemental oxygen mapping, C/O atomic ratio of 2.12, ultimate tensile strength of 59.17 MPa and strong elastic modulus of 10.123 GPa. Hydrogel of partially reduced graphene oxide was prepared and analyzed, revealing the C/O atomic ratio of 2.63 and band gap energy of 2.88 eV. The technological improvements open a safe and efficient route of graphene oxide production for scientific and industrial purposes.

Fundamental Understanding of Ultrathin, Highly Stable Self-Assembled Liquid Crystalline Graphene Oxide Membranes Leading to Precise Molecular Sieving through Non-equilibrium Molecular Dynamics

Self-assembled graphene oxide lyotropic liquid crystal (GO LLC) structures are mostly formed in aqueous medium; however, most GO derivatives are water insoluble, so processing GO LLCs in water poses a practical limitation. The use of polar aprotic solvent (like dimethyl sulfoxide) for the formation of GO LLC structures would be interesting, because it would allow incorporating additives, like photoinitiators or cross-linkers, or blending with polymers that are insoluble in water, which hence would expand its scope. The well-balanced electrostatic interaction between DMSO and GO can promote and stabilize the GO nanosheets' alignment even at lower concentrations. With this in mind, herein we report mechanically robust, chlorine-tolerant, self-assembled nanostructured GO membranes for precise molecular sieving. Small-angle X-ray scattering and polarized optical microscopy confirmed the alignment of the modified GO nanosheets in polar aprotic solvent, and the LLC structure was effectively preserved even after cross-linking under UV light. We found that the modified GO membranes exhibited considerably improved salt rejection for monovalent ions (99%) and water flux (120 LMH) as compared to the shear-aligned GO membrane, which is well supported by forward osmosis simulation studies. Additionally, our simulation studies indicated that water molecules traveled a longer path while permeating through the GO membrane compared to the GO LLC membrane. Consequently, salt ions permeate slowly across the GO LLC membrane, yielding higher salt rejection than the GO membrane. This begins to suggest strong electrostatic repulsion with the salt ions, causing higher salt rejection in the GO LLC membrane. We foresee that the ordered cross-linked GO sheets contributed to excellent mechanical stability under a high-pressure, cross-flow, chlorine environment. Overall, these membranes are easily scalable, exhibit good mechanical stability, and represent a breakthrough for the potential use of polymerized GO LLC membranes in practical water remediation applications.

Regulating interlayer spacing of aminated graphene oxide membranes for efficient flow reactions

Regulating interlayer spacing of aminated graphene oxide membranes for efficient flow reactions

Engineering fast water-selective pathways in graphene oxide membranes by porous vermiculite for efficient alcohol dehydration

Graphene oxide (GO) lamellar membranes with well-aligned architecture hold promising potential for molecular separations. However, the tortuous transport pathways and weak interlamellar interactions between stacked GO nanosheets cause low flux and poor stability, which are major drawbacks for their practical applications. Herein, we report a strategy to engineer fast and robust water-selective pathways within GO-based lamellar membranes by porous vermiculite (PVMT), a naturally layered magnesium aluminosilicate. PVMT nanosheets conferred abundant in-plane pores, which decreased the tortuosity and shortened the mass transport distance inside lamellar membranes. Meanwhile, PVMT nanosheets enhanced the hydrophilicity and fixed the interlayer distance, contributing to robust water-selective transport. The physicochemical properties of membranes, such as thickness and hydrophilicity, were manipulated by the loading amount of PVMT nanosheets. The resulting lamellar membranes exhibited excellent n-butanol dehydration performance with a permeation flux of 9554 g m−2 h−1 and a separation factor of 2678, which increased by up to 91% and 328% compared to pristine GO/PTFE membranes. Moreover, membranes remain stable for 192 h operation. Our study may stimulate further research on precise construction of mass-transfer pathways within lamellar membranes.

Insights into High Li+/Mg2+ Separation Performance Using a PEI-Grafted Graphene Oxide Membrane

Lithium extraction from brine or seawater using membrane technology has attracted extensive attention in recent years. Graphene oxide (GO), as one of the two-dimensional materials, has been proven as a competitive candidate for membranes. However, the GO membranes still suffer challenges for ion sieving due to the swelling in the aqueous solution. In this work, a GO–PEI membrane with positively charged channels was constructed by polyelectrolyte polyethyleneimine (PEI) molecular chain-grafted GO nanosheets. The GO–PEI membrane showed a high selectivity of 22.2 for Li+/Mg2+, together with a competitive Li+ permeation rate of 0.09 mol m–2 h–1 in a binary permeation test. In addition, the membrane showed excellent stability during the separation process. The enhanced Li+/Mg2+ separation performance of the GO–PEI membrane could be mainly attributed to the synergistic effect of size sieving and electrostatic repulsion. We further systematically studied the influence of various variables, such as PEI molecular weight, PEI content, membrane thickness, and ion concentration, on separation performance. This work deepens the understanding of membrane ion selectivity from multiple perspectives and puts forward a strategy to design a two-dimensional membrane structure from microscopic materials.

Host-Guest Recognition Boosts Biomimetic Mono/Multivalent Cation Separation.

Biomimetic ion permselective membranes with ultrahigh ion permeability and selectivity represent a research frontier in ion separation, yet the successful fabrication of such membranes remains a formidable challenge. Here, we demonstrate a 4-sulfocalix[4]arene (4-SCA)-modified graphene oxide (GO) membrane that shows extraordinary performance in separating mono-from multivalent cations, as well as having reversible pH-responsiveness. The resulting 4-SCA-modified GO (SCA-GO) membrane preferentially transports potassium ions (K+) over radionuclide cations (Co2+, UO22+, La3+, Eu3+, and Th4+). The ion selectivities are an order of magnitude higher than that of the unmodified GO membrane. Theoretical calculations and experimental investigations demonstrate that the much-improved ion selectivity arises from the specific recognition between 4-SCA and radionuclide cations. The transport of multivalent radionuclides is impeded by a binding-obstructing mechanism from the host-guest interactions. Interestingly, the host-guest interactions are responsive to the protonation/deprotonation transformation of the 4-SCA. Therefore, the SCA-GO membrane mimics pH-regulated ion selective behavior found in biological ion channels. Our strategy of designing a biomimetic permselective GO membrane may allow efficient nuclear wastewater treatment and, more importantly, deepen our understanding of biomimetic ion transport mechanisms.

A two-dimensional nanochannel facilitates ionic conductivity of a deep eutectic solvent for an efficient supercapacitor

As an environmentally benign promising emerging class of versatile solvents, deep eutectic solvents (DESs) are inexpensive, non-corrosive, compatible with electrodes and eco-friendly, and attractive for cost-efficient energy storage systems. However, the limited ionic conductivity and liquid state constrict the wide application of bulk DESs in energy storage. Herein, we present a nanoconfined system by confining a common DES, choline chloride-ethylene glycol, into the two-dimensional nanochannel of graphene oxide (GO) membranes. Under infusing DES into GO nanochannels, the ionic conductivity of DESs has increased 64 times as that of the bulk one. The molar ratio and temperature influence of DES components on the ionic conductivity of nanoconfined GO-DES membranes have been investigated. As a proof of conception, the resulting GO-DES membranes were utilized as quasi-solid liquid electrolytes for supercapacitors. The GO-DES-based supercapacitor demonstrates a 50.88% energy density improvement than bulk one. The nanoconfined system of GO-DES10 exhibits high specific capacitance of 120 F g−1 at 1 A g−1 and an energy density of 10.23 Wh kg−1 at power density of 1 kW kg−1. These are the best performance among similar ionic liquid supercapacitors. This enhanced ionic conductivity in nanoconfined DESs will offer a new way for DESs in the electrochemical area.

LBL assembled graphene oxide-based nanofiltration membranes with tunable surface charges and high selectivity for charged organic dye molecules

In this work, 2D ultrathin positively-charged α-nickel hydroxide (α-Ni(OH)2) nanosheets composite with negatively-charged graphene oxide (GO) to prepare composite membrane by LBL assembly technology. The strong electrostatic interaction between α-Ni(OH)2 and GO improved the structural stability of the interlayer channels of GO/α-Ni(OH)2 composite membranes. Moreover, the surface charge and thickness of the membrane can be easily regulated by assembly layer numbers. Compared with the unmodified GO membrane, the GO/α-Ni(OH)2 membrane with nanometer thickness inhibited the membrane swelling and achieved controlled sieving of charged molecules.

Nanoporous multilayer graphene oxide membrane for forward osmosis metal ion recovery from spent Li-ion batteries

The recovery of Li and rare metals from the spent electrodes of Li-ion batteries (LIBs) is becoming increasingly important. Herein, nanoporous multilayer graphene oxide (NMG) membranes were fabricated via the confined thermal annealing method to achieve Li-ion selectivity. A graphene oxide (GO) membrane was prepared by coating the membrane with an aqueous layer of GO liquid crystal via the scalable bar coating method, followed by hot-pressing. The influence of the GO interlayer spacing and nanopore presence on the ion separation performance was investigated. The NMG membrane shows different ion permeance phenomena depending on ion concentration. The permeation rate of divalent ions (Co, Ni, Mn) through the NMG membrane was faster than that of Li ions in solution with low ionic strength, whereas the membrane was Li-ion selective in mixed ionic solutions or at high ionic strength. This result indicates that electrostatic interaction by oxygen groups is important at low ionic strength, but size exclusion by interlayer spacing and adsorption by nanopore is critical at high ionic strength. The ion permeation mechanisms were further examined based on molecular calculations, which revealed that the binding energies between the divalent ions and nanopores or basal plane of graphene were high, resulting in slow ion permeation. When the NMG membrane was combined in a forward osmosis system, Li-ion can be separated continuously from the mixture solution simulated from the spent Li-battery electrode. This approach shows the high potential of nanoporous multilayer graphene membrane for Li extraction in particular with dense pore structure and around 7 Å of d-spacing.

Hydrogen isotopic water separation in membrane distillation through BN, MoS2 and their heterostructure membranes

The development and application of nuclear energy involves a challenging process of separating H2O/HTO to treat tritium-containing radioactive wastewater (HTO). Recently graphene oxide (GO)-based membrane distillation process has shown good performance for the separation of H2O/D2O used as a laboratory model research process for H2O/HTO separation, but further development of novel membrane materials or structures for membrane distillation process to give higher permeation flux or separation factor is still desirable. Here, boron nitride (BN) and molybdenum disulfide (MoS2) were investigated as membrane materials in the type of PTFE supported membrane, and PTFE/BN membrane showed higher permeation flux than PTFE/MoS2 and PTFE/GO membrane, and higher separation factor than PTFE/GO membrane that was higher than PTFE/MoS2 membrane. Furthermore, binary PTFE/BN/GO and PTFE/MoS2/GO heterostructure composite membranes were explored. The permeation flux of PTFE/BN/GO membrane was higher than that of PTFE/MoS2/GO and PTFE/GO/GO membrane. The separation factor of PTFE/BN/GO membrane was higher than that of PTFE/GO/GO membrane, which was higher than that of PTFE/MoS2/GO membrane. These results suggest that other 2D material- or heterostructure membranes can give performance comparable with or even better than GO membrane for the separation of H2O/D2O. This can guide for further exploration of novel materials or membrane structures for membrane distillation process with higher performance to provide a solution for the treatment of tritium-containing radioactive wastewater in the nuclear energy sector.

Effective cross-linking strategy for graphene oxide membrane with high structural stability and enhanced separation performance

In order to solve the poor structural stability of graphene oxide (GO) membrane, a facile and effective cross-linking technology was employed to create a high-performance GO membrane. Herein, DL-Tyrosine/amidinothiourea and (3-Aminopropyl) triethoxysilane were used to crosslink GO nanosheets and porous alumina substrate, respectively. The group evolution of GO with different cross-linking agents was detected via Fourier transform infrared spectroscopy. Ultrasonic treatment and soaking experiment were conducted to explore the structural stability of the different membranes. The GO membrane cross-linked with amidinothiourea exhibits exceptional structural stability. Meanwhile, the membrane has superior separation performance, with the pure water flux reaching approximately 109.6 l·m−2·h−1·bar−1. During the treatment of 0.1 g l–1 NaCl solution, its permeation flux and rejection for NaCl are about 86.8 l·m−2·h−1·bar−1 and 50.8%, respectively. The long-term filtration experiment also demonstrates that the membrane exhibits great operational stability. All these indicate the cross-linking graphene oxide membrane has promising potential applications in water treatment.

Sulfonated graphene oxide-based pervaporation membranes inspired by a tortuous brick and mortar structure for enhanced resilience against silica scaling and organic fouling

A novel tortuous brick-and-mortar structure utilizing intercalation of polyvinyl alcohol (PVA) on sulfonated graphene oxide (SGO) membranes was specifically tailored for brine treatment by pervaporation to ensure excessive resistance to silica scaling and organic fouling, as well as ultrafast water transport without compromising salt rejection. The synthesized SGO membrane showed a smoother surface morphology, improved zeta potential, and a higher hydration capacity than the graphene oxide (GO) membrane. Further intercalation of PVA through glutaraldehyde (GA) crosslinking, confirmed by Fourier transform infrared spectroscopy and X-ray diffraction analysis, conferred increased cohesiveness, and the SGO-PVA-GA membrane was therefore able to withstand ultrasonication tests without any erosion of the coating layer. According to a pervaporative desalination test, the SGO-PVA-GA membrane exhibited 62 kg m−2 h−1 of permeate flux, with an extraordinary salt rejection of 99.99% for a 10 wt% NaCl feed solution at 65 °C. The 72 h organic fouling, silica scaling, and combined fouling and scaling tests proved that the SGO-PVA-GA membrane sustains a stable flux with less scaling and fouling than the GO-PVA-GA membrane, attributable to dense surface negative charges and great hydration capacities caused by sulfonic acid. Thus, the SGO-PVA-GA membrane offers superlative advantages for long-term brine treatment by pervaporation, related to its ability to withstand silica scaling and organic fouling.

Tunable reduced graphene oxide nanofiltration membrane for efficient retention and separation of dye/salt based on molecular interactions

Effective retention and separation of mixed dyes are critical to treating textile wastewater. Graphene oxide (GO) is an ideal material for treating polluted water attributed to its unique structure and abundant oxygen-containing groups. However, GO still has some limitations, such as unstable performance, poor dye retention and no selective separation performance. Here, we report a weakly reduced graphene oxide (rGO) membrane that can modulate its water permeability and dye retention by controlling the reduction temperature. The water flux (84.35 L m−2 h−1 bar−1) of rGO membranes decreased with increasing reduction temperature but the methylene blue (MB) dye rejection rate (99.09 %) increased with increasing reduction temperature. More interestingly, the rGO membrane has completely different dye rejection rates for MB and methyl orange (MO), allowing the separation of MB and MO mixtures, effectively replacing traditional column chromatography. In addition, we investigated the reasons for the different retention rates and selective separation of dyes by rGO membranes through reduction temperature, contact angle, interlayer distance, electrostatic interactions and molecular dynamics simulations. This work uses a non-expensive and low-energy method to prepare nanofiltration membranes that can be easily extended to construct other stable and efficient filtration membranes for the separation of different dyes and salts.

Systematic covalent crosslinking of graphene oxide membranes using 1,3,5 triazine 2,4,6 triamine for enhanced structural intactness and improved nanofiltration performance

From its physicochemical characteristics, graphene oxide (GO) is a promising versatile next generation membrane material. Its unique characteristics like ultrafast permeation and hydrophilicity makes it a favourable separation membrane nanomaterial in water purification. However, a fundamental problem in the use of GO in nanofiltration is decreased performance overtime due to the pore size widening phenomenon. This paper explored the use of an amine group containing compound, 1,3,5 triazine, 2,4,6 triamine (melamine) to covalently interlink the GO nanosheets to counteract this swelling phenomenon. Prior to membrane fabrication, covalent interactions between GO and the crosslinker, melamine were successfully confirmed through thermogravimetric analysis (TGA), x-ray photoelectron spectroscopy (XPS), X-ray Diffraction (XRD) and Fourier Transform Infra-Red (FTIR) spectroscopy characterisations. Following these characterisations, crosslinked membranes were successfully fabricated and enhanced nanofiltration performance was confirmed. Resultantly, the surface morphology of the membranes was recorded via Scanning Electron Microscopy (SEM) characterisations while a lab-scale nanofiltration device was constructed for flux and rejection analysis. Evidently, performance improvement with covalent crosslinking was imminent as an up to a 100% rejection of methylene blue was achieved for the crosslinked membranes. Structural integrity of GO membranes has indeed been improved through crosslinking.