Ion-retention properties of graphene oxide/zinc oxide nanocomposite membranes at various pH and temperature conditions

Laminar graphene oxide (GO) is a promising candidate material for next-generation highly water-permeable membranes. Despite extensive research, there is little information known concerning GO's ion-sieving properties at high acidic/basic pH and temperatures. In this study, the ion-blockage properties of the pristine GO and GO/zinc oxide (ZnO) nanocomposite membranes were tested using a non-pressure-driven filtration setup over a wide range of pH and temperatures. The ZnO nanoparticles within the composite membranes were synthesized via the room-temperature oxidation of zinc acetate and zinc acrylate precursors and were uniformly distributed across the composite membrane. It is observed that partially replacing the zinc acetate precursor with zinc acrylate improves the blockage performance of the composite membranes under extreme basic conditions by 42%. Moreover, photocatalytically-reduced composite membranes blocked copper sulfate ions 28% more than as-prepared composite membranes. Further, it was discovered that the composition of the membrane plays a vital role in its ion blockage performance at higher temperatures.

Polyoxometalate ionic liquid between graphene oxide surfaces as a new membrane in the desalination process: a molecular dynamics study

In this study, the performance of the positioning of polyoxometalate ionic liquid ([Keggin][emim]₃ IL) between graphene oxide (GO) plates with different concentrations (nIL-GO (n = 1-4)) were examined in the desalination process at different external pressures using molecular dynamics (MD) simulations. The use of Keggin anions with charged GO layers was also investigated in the desalination process. The potential of the mean force, average number of hydrogen bonds, self-diffusion coefficient, and angle distribution function were calculated and discussed. The results showed that although the presence of polyoxometalate ILs between the GO plates decreases water flux, they efficiently increase salt rejection. The positioning of one IL increases salt rejection to two times at lower pressure and increases it up to four times at higher pressure. Moreover, the positioning of four ILs results in almost complete salt rejection at all pressures. The use of only Keggin anions between the charged GO plates (n[Keggin]-GO⁺³ⁿ) presents more water flux and a smaller salt rejection rate than the nIL-GO systems. However, the n[Keggin]-GO⁺³ⁿ systems show a nearly complete salt rejection at high concentrations of Keggin anions. These systems also have a smaller risk of the contamination of the desalinated water by the probable escape of cations from the nanostructure to the desalinated water at very high pressures.

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.

High-Ionic-Conductivity Sodium-Based Ionic Gel Polymer Electrolyte for High-Performance and Ultrastable Microsupercapacitors

Due to the lower cost and greater natural abundance of the sodium element on the earth than those of the lithium element, sodium-based ionic gel polymer electrolytes (IGPEs) are becoming a more cost-effective and popular material choice for portable and stationary energy solutions. The sodium-based IGPEs, however, appeared relatively inferior to their lithium-based counterparts for use in high-performance microsupercapacitors in terms of ionic conductivity and electrochemical stability. To tackle these issues, poly(ethylene glycol) diacrylate (PEGDA) with fast polymerization to build a polymer matrix and sodium perchlorate (NaClO₄) with high chemical stability and high thermal stability are employed to generate free ions for an ionic conducting phase with the support of tetramethylene glycol ether (G4) and 1-ethyl-3-methylimidazolium bis(triflouromethylsulfonyl)imide (EMIM-TFSI). It was found that the ionic conductivity (σ_dc) of this sodium-based IGPE reaches up to 0.54 mS/cm at room temperature. To manifest a high-conductivity sodium-based IGPE (SIGPE), a microsupercapacitor (MSC) with an area of 5 mm² is designed and fabricated on an interdigital reduced graphene oxide electrode. This MSC demonstrates prominent performance with a high power density of ∼2500 W/kg and a maximum energy density of ∼0.7 Wh/kg. Furthermore, after 20,000 cycles at an operating potential window from 0.0 to 1.0 V, it retains approximately 98.9% capacitance. An MSC array in 3 series × 3 parallels (3S × 3P) was successfully designed as a power source for a basic circuit with an LED. Therefore, we believe that our sodium-based IGPE microsupercapacitor holds its promising role as a solid-state energy source for high-performance and high-stability energy solutions.

Graphene-Based Membranes for Water Desalination: A Literature Review and Content Analysis

Graphene-based membranes have unique nanochannels and can offer advantageous properties for the water desalination process. Although tremendous efforts have been devoted to heightening membrane performance and broadening their application, there is still lack of a systematic literature review on the development and future directions of graphene-based membranes for desalination. In this mini-review, literature published between 2011 and 2022 were analyzed by using the bibliometric method. We found that the major contributors to these publications and the highest citations were from China and the USA. Nearly 80% of author keywords in this analysis were used less than twice, showing the broad interest and great dispersion in this field. The recent advances, remaining gaps, and strategies for future research, were discussed. The development of new multifunctional nanocomposite materials, heat-driven/solar-driven seawater desalination, and large-scale industrial applications, will be important research directions in the future. This literature analysis summarized the recent development of the graphene-based membranes for desalination application, and will be useful for researchers in gaining new insights into this field.

Nitrogenated Graphene Oxide-Decorated Metal Sulfides for Better Antifouling and Dye Removal

Nitrogenated graphene oxide-decorated copper sulfide nanocomposites (Cu _x S-NrGO, where x = 1 and 2) are designed to be incorporated in polysulfone (PSF) membranes for effective fouling resistance of PSF membranes and their dye removal capacity. The developed membranes possess more hydrophilicity and an enhancement in pure water flux (PWF). Also, the highest bovine serum albumin (BSA) rejection of 89% was observed when compared to membranes with pristine PSF (5 L/m² h PWF and 88% BSA rejection) and CuS-incorporated PSF membranes (14 L/m² h PWF and 83% BSA rejection) because of N doping and enhanced permeability. It is also found that the Cu _x S-NrGO-incorporated PSF membranes exhibited a significantly higher fouling resistance, a larger permeate flux recovery ratio (FRR) of nearly 82%, and a congo red dye rejection of 93%. Cu _x S-NrGO nanoparticles thus demonstrate the potential efficacy of enhancing the hydrophilicity, leading to a better flux, dye removal capacity, and antifouling capacity with a very high FRR value of 82% because of a strong interaction between the N-active sites of the NrGO, Cu _x S, and polysulfone matrix, and negligible leaching of nanoparticles is observed.

Influence of substrate on ultrafast water transport property of multilayer graphene coatings

The ultrafast water transport in graphene nanoplatelets (GNPs) coating is attributed to the low friction passages formed by pristine graphene and the hydrophilic functional groups which provide a strong interaction force to the water molecules. Here, we examine the influence of the supporting substrate on the ultrafast water transport property of multilayer graphene coatings experimentally and by computational modelling. Thermally cured GNPs manifesting ultrafast water permeation are coated on different substrate materials, namely aluminium, copper, iron and glass. The physical and chemical structures of the GNPs coatings which are affected by the substrate materials are characterized using various spectroscopy techniques. Experimentally, the water permeation and absorption tests evidence the significant influence of the substrate on the rapid water permeation property of GNPs-coating. The water transport rates of the GNPs coatings correspond to the wettability and the free surface energy of their substrates where the most hydrophilic substrate induces the highest water transport rate. In addition, we conduct molecular dynamics (MD) simulations to investigate the transport rate of water molecules through multilayer GNPs adjacent to different substrate materials. The MD simulations results agree well with the experimental results inferring the strong influence of the substrate materials on the fast water transport of GNPs. Therefore, selection of substrate has to be taken into consideration when the GNPs-coating is placed into applications.

Efficient Permeance Recovery of Organically Fouled Graphene Oxide Membranes

The emergence of facile approaches for the large-scale production of graphene oxide (GO) membranes necessitates a clearer understanding of their potential to foul and, more importantly, strategies for efficient recovery of membrane performance following fouling. Here, we systematically investigated the feasibility of water, ethanol, and hypochlorite as cleaning agents to remove organic foulants over a GO membrane. Among them, 100 ppm hypochlorite solution showed a remarkable ability to remove bovine serum albumin (BSA) and could recover the membrane flux up to 98% after five cycles of BSA filtration and cleaning. The potential of hypochlorite was also demonstrated for permeance recovery during molecular filtration of tannic acid and methyl blue. Scanning electron microscopy, attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), and X-ray diffraction (XRD) analyses were used to study the oxidative effects of hypochlorite on the GO membrane, and it was determined that exposure to higher concentrations of hypochlorite (>1000 ppm) degrades the structure of GO membrane and deteriorates the membrane performance after three cycles of cleaning. The studies demonstrate that the use of a modest concentration of hypochlorite is effective in restoring permeance of this class of high flux nanofiltration membranes.

Water transport confined in graphene oxide channels through the rarefied effect

This work combines classic hydrodynamics with kinetic theory to provide a molecular insight into water transport between graphene oxide sheets.

Charge-Gated Ion Transport through Polyelectrolyte Intercalated Amine Reduced Graphene Oxide Membranes

Charge-gated channels are nature's solutions for transport of water molecules and ions through aquaporins in biological membranes while excluding undesired substances. The same mechanism has good potentials to be adopted in pressure or electrically driven membrane separation processes. Herein, we report highly charged nanochannels created in polyelectrolyte (PE) intercalated amine reduced graphene oxide membrane (PE@ArGO membrane). The PE@ArGO membrane, with a rejection layer of ∼160 nm in thickness, features a laminate structure and a smooth top surface of a low roughness (typically ∼17.2 nm). Further, a modified PE@ArGO membrane (mPE@ArGO membrane) was developed in situ using free chlorine scavenging post-treatment method, which was designed to alter the charge while keeping alteration to the layered structure minimal. The surface charge of the PE@ArGO and mPE@ArGO membrane was +4.37 and -4.28 mC/m² respectively. In pressure driven processes, the pure water permeability for PE@ArGO and mPE@ArGO was 2.9 and 10.8 L m^-2 h^-1 bar^-1, respectively. Salt rejection is highly dependent on the charge density of the membrane surface, the valence of the co-ions and the size of ions in hydrated form. For example, in the positively charged PE@ArGO membranes, the rejection of the salts follows the order of: R(MgCl₂), 93.0% > R(NaCl), 88.2% ≈ R(MgSO₄), 88.1% > R(Na₂SO₄), 65.1%; while in the negatively charged mPE@ArGO membranes, the rejection of the salts follows the order of: R(Na₂SO₄), 90.3% > R(NaCl), 85.4% > R(MgSO₄), 68.3% > R(MgCl₂), 42.9%. To the best knowledge of the authors, this is the first study to report graphene oxide based membranes (GOBMs) with high density positive/negative charge gated ion transport behavior. What's more, the high rejection rate along with high water permeability of the PE@ArGO and mPE@ArGO membranes has not been achieved by other types of GOBMs.

Molecular Insight into Water Desalination across Multilayer Graphene Oxide Membranes

Transport of ionic solutions through graphene oxide (GO) membranes is a complicated issue because the complex and tortuous structure inside makes it very hard to clarify. Using molecular dynamics (MD) simulations, we investigated the mechanism of water transport and ion movement across multilayer GO. The significant flow rate is considerably influenced by the structural parameters of GO membranes. Because of the size effect on a shrunken real flow area, there is disagreement between the classical continuum model and nanoscaled flow. To eliminate the variance, we obtained modified geometrical parameters from density analysis and used them in the developed hydrodynamic model to give a precise depiction of water flow. Four kinds of solutions (i.e., NaCl, KCl, MgCl₂, and CaCl₂) and different configurational GO sheets were considered to clarify the influence on salt permeation. It is found that the abilities of permeation to ions are not totally up to the hydration radius. Even though the ionic hydration shell is greater than the opening space, the ions can also pass through the split because of the special double-deck hydration structure. In the structure of GO, a smaller layer separation with greater offsetting gaps could substantially enhance the membrane's ability to reject salt. This work establishes molecular insight into the effects of configurational structures and salt species on desalination performance, providing useful guidelines for the design of multilayer GO membranes.