Spray-evaporation assembled graphene oxide membranes for selective hydrogen transport

A Facile Way to Fabricate GO-EDA/Al2O3 Tubular Nanofiltration Membranes with Enhanced Desalination Stability via Fine-Tuning the pH of the Membrane-Forming Suspensions

Pristine graphene oxide (GO)-based membranes have proven promising for molecular and ion separation owing to efficient molecular transport nanochannels, but their separation ability in an aqueous environment is limited by the natural swelling tendency of GO. To obtain a novel membrane with anti-swelling behavior and remarkable desalination capability, we used the Al₂O₃ tubular membrane with an average pore size of 20 nm as the substrate and fabricated several GO nanofiltration ceramic membranes with different interlayer structures and surface charges by fine-tuning the pH of the GO-EDA membrane-forming suspension (pH = 7, 9, 11). The resultant membranes could maintain desalination stability, whether immersed in water for 680 h or operated under a high-pressure environment. When the pH of the membrane-forming suspension was 11, the prepared GE-11 membrane showed a rejection of 91.5% (measured at 5 bar) towards 1 mM Na₂SO₄ after soaking in water for 680 h. An increase in the transmembrane pressure to 20 bar resulted in an increase in the rejection towards the 1 mM Na₂SO₄ solution to 96.3%, and an increase in the permeance to 3.7 L·m^-2·h^-1·bar^-1. The proposed strategy in varying charge repulsion is beneficial to the future development of GO-derived nanofiltration ceramic membranes.

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.

Next-generation graphene oxide additives composite membranes for emerging organic micropollutants removal: Separation, adsorption and degradation

In the past two decades, membrane technology has attracted considerable interest as a viable and promising method for water purification. Emerging organic micropollutants (EOMPs) in wastewater have trace, persistent, highly variable quantities and types, develop hazardous intermediates and are diffusible. These primary issues affect EOMPs polluted wastewater on an industrial scale differently than in a lab, challenging membranes-based EOMP removal. Graphene oxide (GO) promises state-of-the-art membrane synthesis technologies and use in EOMPs removal systems due to its superior physicochemical, mechanical, and electrical qualities and high oxygen content. This critical review highlights the recent advancements in the synthesis of next-generation GO membranes with diverse membrane substrates such as ceramic, polyethersulfone (PES), and polyvinylidene fluoride (PVDF). The EOMPs removal efficiencies of GO membranes in filtration, adsorption (incorporated with metal, nanomaterial in biodegradable polymer and biomimetic membranes), and degradation (in catalytic, photo-Fenton, photocatalytic and electrocatalytic membranes) and corresponding removal mechanisms of different EOMPs are also depicted. GO-assisted water treatment strategies were further assessed by various influencing factors, including applied water flow mode and membrane properties (e.g., permeability, hydrophily, mechanical stability, and fouling). GO additive membranes showed better permeability, hydrophilicity, high water flux, and fouling resistance than pristine membranes. Likewise, degradation combined with filtration is two times more effective than alone, while crossflow mode improves the photocatalytic degradation performance of the system. GO integration in polymer membranes enhances their stability, facilitates photocatalytic processes, and gravity-driven GO membranes enable filtration of pollutants at low pressure, making membrane filtration more inexpensive. However, simultaneous removal of multiple contaminants with contrasting characteristics and variable efficiencies in different systems demands further optimization in GO-mediated membranes. This review concludes with identifying future critical research directions to promote research for determining the GO-assisted OMPs removal membrane technology nexus and maximizing this technique for industrial application.

Novel graphene oxide/polymer composite membranes for the food industry: structures, mechanisms and recent applications

The membrane can not only be used as food packaging, but also for the separation, fractionation and recovery of food ingredients. Graphene oxide (GO) sheets are a two-dimensional (2 D) material with a unique structure that exhibit excellent mechanical properties, biocompatibility, and flexibility. The corporation of polymer matrix membrane with GO can significantly improve the permeability, selectivity, and antibacterial activity. In this review, the chemical structures of GO, GO membranes and GO/polymer composite membranes are introduced, the permeation mechanisms of molecules through the membranes are discussed and key factors affecting the permeability are presented in detail. In addition, recent applications in the food industry for filtration, bioreactions and active food packaging are analyzed, and limitations and future trends of GO membranes development are also highlighted. GO/polymer composite membranes exhibit excellent permeability, selectivity and strong barrier properties against bacterial and gas permeation. However, current food material filtration and packaging applications of GO/polymer composite membranes are still in the laboratory stage. Future work can focus on the development of large scale uniformly sized GO production, the homogeneous distribution and tight combination of GO in polymer matrixes, the sensing function of GO in packaging, and the verification method of GO toxicology.

Recent advances in polymeric nanostructured ion selective membranes for biomedical applications

Nano structured ion-selective membranes (ISMs) are very attractive materials for a wide range of sensing and ion separation applications. The present review focuses on the design principles of various ISMs; nanostructured and ionophore/ion acceptor doped ISMs, and their use in biomedical engineering. Applications of ISMs in the biomedical field have been well-known for more than half a century in potentiometric analysis of biological fluids and pharmaceutical products. However, the emergence of nanotechnology and sophisticated sensing methods assisted in miniaturising ion-selective electrodes to needle-like sensors that can be designed in the form of implantable or wearable devices (smartwatch, tattoo, sweatband, fabric patch) for health monitoring. This article provides a critical review of recent advances in miniaturization, sensing and construction of new devices over last decade (2011-2021). The designing of tunable ISM with biomimetic artificial ion channels offered intensive opportunities and innovative clinical analysis applications, including precise biosensing, controlled drug delivery and early disease diagnosis. This paper will also address the future perspective on potential applications and challenges in the widespread use of ISM for clinical use. Finally, this review details some recommendations and future directions to improve the accuracy and robustness of ISMs for biomedical applications.

Tight Covalent Organic Framework Membranes for Efficient Anion Transport via Molecular Precursor Engineering

Fabricating covalent organic frameworks (COFs) membranes with tight structure, which can fully utilize well-defined framework structure and thus achieve superior conduction performance, remains a grand challenge. Herein, through molecular precursor engineering of COFs, we reported the fabrication of tight COFs membrane with the ever-reported highest hydroxide ion conductivity over 200 mS cm^-1 at 80 °C, 100 % RH. Six quaternary ammonium-functionalized COFs were synthesized by assembling functional hydrazides and different aldehyde precursors. In an organic-aqueous reaction system, the impact of the aldehyde precursors with different size, electrophilicity and hydrophilicity on the reaction-diffusion process for fabricating COFs membranes was elucidated. Particularly, more hydrophilic aldehydes were prone to push the reaction zone from the interface region to the aqueous phase of the reaction system, the tight membranes were thus fabricated via phase-transfer polymerization process, conferring around 4-8 times the anion conductivity over the loose membranes via interfacial polymerization process.

Graphene-based Membranes for H2 Separation: Recent Progress and Future Perspective

Hydrogen (H2) is an industrial gas that has showcased its importance in several well-known processes such as ammonia, methanol and steel productions, as well as in petrochemical industries. Besides, there is a growing interest in H2 production and purification owing to the global efforts to minimize the emission of greenhouse gases. Nevertheless, H2 which is produced synthetically is expected to contain other impurities and unreacted substituents (e.g., carbon dioxide, CO2; nitrogen, N2 and methane, CH4), such that subsequent purification steps are typically required for practical applications. In this context, membrane-based separation has attracted a vast amount of interest due to its desirable advantages over conventional separation processes, such as the ease of operation, low energy consumption and small plant footprint. Efforts have also been made for the development of high-performance membranes that can overcome the limitations of conventional polymer membranes. In particular, the studies on graphene-based membranes have been actively conducted most recently, showcasing outstanding H2-separation performances. This review focuses on the recent progress and potential challenges in graphene-based membranes for H2 purification.

Mass Transfer Through Graphene-Based Membranes

The problems related to the transport of gases through nanoporous graphene (NG) and graphene oxide (GO) membranes are considered. The influence of surface processes on the transport of gas molecules through the aforementioned membranes is studied theoretically. The obtained regularities allow finding the dependence of the flux of the gas molecules passing through the membrane on the kinetic parameters which describe the interaction of the gas molecules with the graphene sheets. This allows to take into account the influence of external fields (e.g., resonance radiation), affecting the aforementioned kinetic parameters, on the transport of gas molecules through the membranes. The proposed approach makes it possible to explain some experimental results related to mass transfer in the GO membranes. The possibility of the management of mass transfer through the NG and GO membranes using resonance radiation is discussed.

Effects of cationic concentration on controlling the interlayer spacings for highly effective ion rejection via graphene oxide membranes

The interlayer spacings of graphene oxide membranes can be finely controlled using different concentrations of cations.

A facile gold nanoparticles embeded hydrogel for non-enzymatic sensing of glucose

The assembly of nanoparticle into electrodes with precise structure and uniform core sizes is important for electrocatalysis. In this study, we reported on a simple strategy for in-situ preparation of gold nanoparticles embedded D-sorbitol hydrogel (D-gel@AuNPs). D-sorbitol hydrogel with acyl hydrazide (D-gel) was synthesized and characterized. AuNP's stable electronic structure, high surface coverage and good conductivity was achieved enabled D-gel@AuNPs exhibits the enhanced electrocatalytic performance. The electrochemical results reveal that the catalytic progress is highly promoted by the D-gel@AuNPs with a detection limit of 0.067 mM and detection range of 0.1-30 mM. The high enzymatic activity and stability provide the high possibility for the development of high value glucose sensors. This mechanistically novel strategy expands the scope of assembly of NPs method for the development of enhanced other electrochemical properties such as amperometric sensing and photcatalysis applications, as well as electrocatalysis.

Graphene oxide membranes with narrow inter-sheet galleries for enhanced hydrogen separation

This paper reports synthesis of graphene oxide (GO) membranes with narrow interlayer free spacing on scalable polyester substrates using GO sheets prepared by Brodie's method. The GO membranes show interlayer free spacing of ∼3.2 Å with significantly improved hydrogen perm-selectivity than the GO membranes with the large inter-sheet spacing reported in literature.

Spray deposition and characterization of carbon nanoflower and gold-doped carbon nanoflower thin films

Herein, we present an aerosol filtration method for the fabrication of carbon nanoflower (CNF) thin films. The method was based on generation, evaporation and filtration of solvent encapsulated CNF droplets. The particles were collected on polytetrafluoroethylene membranes and roll-transferred at room temperature onto flexible polyethylene terephthalate substrates. Suspensions for spraying were made in low vapor pressure mixtures of EtOH/Hex (50/50 v/v%). Doping of starter suspensions was made by the addition of organometallic 1-dodecanethiol-coated gold nanoparticles (AuNPs). The produced films displayed substrate surface coverage of up to 83.3% ± 13.9% and a film thickness of up to 2.4 μm. The deposition of doped suspensions resulted in uniform distribution of AuNPs in the volume of the CNF film, which enables film application for flexible photovoltaics.

Graphene-based Membranes

Owing to their unique one-atom-thick structure, graphene and its derivatives (e.g., graphene oxide) have become emerging nano-building blocks for developing separation membranes. Extraordinary molecular separation properties for purifying water and gases have been demonstrated by graphene-based membranes, which has attracted a huge surge of interest during the last few years. Graphene and its derivatives can be processed into separation membranes with three types: porous graphene membranes, graphene laminate membranes and graphene-based hybrid membranes. This chapter will present the latest ground-breaking advances in both theoretical and experimental studies related to these graphene-based membranes, including their design, fabrication, characterization, as well as application for pressure filtration, pervaporation and gas separation.

Permselective H2/CO2 Separation and Desalination of Hybrid GO/rGO Membranes with Controlled Pre-cross-linking

Covalent bonding is widely adopted in graphene oxide (GO) membrane to improve structural integrity and restrict swelling, while it comes with a price of enlarged d-spacing and sacrifices membrane selectivity. This work offers a facile strategy to break the trade-off between membrane stability and selectivity. Specifically, graphene oxide (GO)/reduced graphene oxide (rGO) hybrid membranes were fabricated by a controlled pre-cross-linking method. With this method, restricted swelling by cross-linking and reduced d-spacing by GO reduction can be achieved simultaneously by controlling reaction time. Membranes were prepared on porous alumina support by vacuum filtration method. Two different d-spacing values (∼12.0 and ∼7.5 Å) were found in the hybrid membrane, representing the layer structures of expanded GO interspacing with inserted cross-linker and reduced layer spacing after GO reduction. The presence of such mixed layer structures enables restricted swelling, excellent mechanical strength, and unique separation property. The hybrid membrane shows excellent permselective H₂/CO₂ separation with a separation factor of 22.93 ± 1.57 and H₂ permeance of 2.46 ± 0.01× 10^-8 mol m^-2 s^-1 Pa^-1. In desalination test with 3.5 wt % sea salt solution, the hybrid membrane shows high ion (Na⁺, K⁺, Mg²⁺, Cl^-, and SO₄^2-) rejection rate of above 99%, as well as excellent durability.

A remarkable two-dimensional membrane for multifunctional gas separation: halogenated metal-free fused-ring polyphthalocyanine

We theoretically explore the structural and mechanical properties of metal-free fused-ring polyphthalocyanine (H2PPc) and halogenated H2PPc (F-H2PPc and Cl-H2PPc) membranes, and the energy profiles for gaseous H2, CO, CH4, CO2 and N2 molecules adsorbing on and passing through these monolayers. Importantly, we reconsider in depth the values of the parameters in the definition of permeance, and corroborate the validity of the model from first-principles theory with the results of H2 diffusion from classic molecular dynamics simulations. With well-defined nanosized pores, halogenated H2PPc monolayers turn out to be multifunctional gas separation membranes, i.e. F-H2PPc for H2/CO, H2/CH4, CO2/N2, CO2/CH4, CO/CH4 and N2/CH4 separation as well as Cl-H2PPc for H2/CO, H2/CH4 and H2/CO2 separation, which should be of great potential in energy and environmental fields.

Insight into hydrogen bonds and characterization of interlayer spacing of hydrated graphene oxide

The number of hydrogen bonds and detailed information on the interlayer spacing of graphene oxide (GO) confined water molecules were calculated through experiments and molecular dynamics simulations. Experiments play a crucial role in the modeling strategy and verification of the simulation results. The binding of GO and water molecules is essentially controlled by hydrogen bond networks involving functional groups and water molecules confined in the GO layers. With the increase in the water content, the clusters of water molecules are more evident. The water molecules bounding to GO layers are transformed to a free state, making the removal of water molecules from the system difficult at low water contents. The diffuse behaviors of the water molecules are more evident at high water contents. With an increase in the water content, the functional groups are surrounded by fewer water molecules, and the distance between the functional groups and water molecules increases. As a result, the water molecules adsorbed into the GO interlamination will enlarge the interlayer spacing. The interlayer spacing is also affected by the number of GO layers. These results were confirmed by the calculations of number of hydrogen bonds, water state, mean square displacement, radial distribution function, and interlayer spacing of hydrated GO. Graphical Abstract This work research the interaction between GO functional groups and confined water molecules. The state of water molecules and interlayer spacing of graphene oxide were proved to be related to the number of hydrogen bonds.

Robust Covalently Cross-linked Polybenzimidazole/Graphene Oxide Membranes for High-Flux Organic Solvent Nanofiltration

Robust, readily scalable, high-flux graphene oxide (GO) mixed matrix composite membranes were developed for organic solvent nanofiltration. Hydroxylated polybenzimidazole was synthesized by N-benzylation of polybenzimidazole with 4-(chloromethyl)benzyl alcohol, which was confirmed by FTIR and NMR spectroscopy. Flat-sheet composite membranes comprising of polybenzimidazoles and 1 or 2 wt % GO were fabricated via conventional blade coating and phase inversion. Subsequently, GO was covalently anchored to the hydroxyl groups of the polymer using a diisocyanate cross-linking agent. The even distribution of GO in the membranes was mapped by visible-light microscopy. Hydroxylation and incorporation of GO in the polymer matrix increased the permeance up to 45.2 ± 1.6 L m^-2 h^-1 bar^-1 in acetone, nearly 5 times higher than the unmodified benchmark membrane. The enhancement in permeance from the addition of GO did not compromise the solute rejection. The composite membranes were found to be tight in seven organic solvents, having molecular weight cut-offs (MWCO) as low as 140 g mol^-1. Permeance increased with increasing solvent polarity, while rejection of a 420 g mol^-1 pharmaceutical remained over 93%. The covalent anchoring resulted in robust composite membranes that maintained constant performance over 14 days in a continuous cross-flow configuration.

Incorporating Graphene Oxide into Alginate Polymer with a Cationic Intermediate To Strengthen Membrane Dehydration Performance

Two-dimensional graphene oxide (GO) in hybrid membranes provides fast water transfer across its surface due to the abundant oxygenated functional groups to afford water sorption and the hydrophobic basal plane to create fast transporting pathways. To establish more compatible and efficient interactions for GO and sodium alginate (SA) polymer chains, cations sourced from lignin are employed to decorate GO (labeled as cation-functionalized GO (CG)) nanosheets via cation-π and π-π interactions, providing more interactive sites to confer synergetic benefits with polymer matrix. Cations from CG are also functional to partially interlock SA chains and intensify water diffusion. And with the aid of two-dimensional pathways of CG, fast selective water permeation can be realized through hybrid membranes with CG fillers. In dehydrating aqueous ethanol solution, the hybrid membrane exhibits considerable performance compared with bare SA polymer membrane (long-term stable permeation flux larger than 2500 g m^-2 h^-1 and water content larger than 99.7 wt %, with feed water content of 10 wt % under 70 °C). The effects of CG content in SA membrane were investigated, and the transport mechanism was correspondingly studied through varying operation conditions and membrane materials. In addition, such a membrane possesses long-term stability and almost unchanged high dehydration capability.

Separation and purification using GO and r-GO membranes

Many materials with varied characteristics have been used for water purification and separation applications. Recently discovered graphene oxide (GO), a two-dimensional derivative of graphene has been considered as a promising membrane material for water purification due to its excellent hydrophilicity, high water permeability, and excellent ionic/molecular separation properties. This review is focussed on the possible versatile applicability of GO membranes. It is also known that selective reduction of GO results in membranes with a pore size of ∼0.35 nm, ideally suited for desalination applications. This article presents the applicability of graphene-based membranes for multiple separation applications. This is indeed the first review article outlining a comparison of GO and r-GO membranes and discussing the suitability for applications based on the porosity of the membranes.

This review article outlines a comparison of GO and r-GO membranes for separation and purification applications.