Metal cation crosslinked, partially reduced graphene oxide membranes with enhanced stability for high salinity, produced water treatment by pervaporative separation.

Graphene oxide (GO)-based membranes hold significant promise for applications ranging from energy storage to protective coatings, to saline water and produced water treatment, owing to their chemical stability and unique barrier properties achieving a high selectivity for water permeation. However, unmodified GO membranes are not stable when submerged in liquid water, creating challenges with their commercial utilization in aqueous filtration and pervaporation applications. To mitigate this, we develop an approach to modify GO membranes through a combination of low temperature thermal reduction and metal cation crosslinking. We demonstrate that Zn2+-rGO and Fe3+-rGO membranes had the highest permeation flux of 8.3 ± 1.5 l m-2h-1and 7.0 ± 0.4 l m-2h-1, for saline water separation, respectively, when thermally reduced after metal cross-linking; These membranes maintained a high flux of 7.5 ± 0.7 l m-2h-1, and 5.5 ± 0.3 l m-2h-1for produced water separation, respectively. All the membranes had a salt rejection higher than 99%. Fe3+crosslinked membranes presented the highest organic solute rejections for produced water of 69%. Moreover, long term pervaporation testing was done for the Zn2+-rGO membrane for 12 h, and only a minor drop of 6% in permeation flux was observed, while Zn2+-GO had a drop of 24%. Both modifiers significantly enhanced the stability with Fe3+-rGO membranes displaying the highest mechanical abrasion resistance of 95% compared to non-reduced and non-crosslinked GO. Improved stability for all samples also led to higher selectivity to water over organic contaminants and only slightly reduced water flux across the membrane.

Femtosecond laser irradiation as a novel method for nanosheet growth and defect generation in g-C3N4.

Among the many recently developed photo-catalytic materials, graphitic carbon nitride (g-C3N4) shows great promise as a catalytic material for water splitting, hydrogen generation, and related catalytic applications. Herein, synthesized bulk g-C3N4is simply irradiated under a 35 fs pulse at mixed photon energies (800 nm and its second harmonic). g-C3N4was synthesized from melamine following a facile thermal polymerization procedure. The prepared material was introduced, in an aqueous environment, to the femtosecond laser for various lengths of time. The treated material demonstrates a significant increase in surface area, relative to the untreated samples, indicating that irradiation is a successful method for exfoliation. The subsequent characterization reveals that the mixed irradiation process drives significant defect generation and sheet growth, which is not seen under 800 nm irradiation. Extended mixed irradiation results in 4 nm thick nanosheets with lateral dimensions 4× that of the bulk material. The treated material shows improved dye absorption/removal. This novel method of defect generation and nanosheet growth shows great potential as a g-C3N4pre-treatment method for co-catalytic applications. Herein it is shown that femtosecond laser irradiation drives exfoliation beyond 100 nm particle sizes, and sheet-like morphologies under extended irradiation, which must be taken into account when using this method to improve material performance.

Aqueous, Mixed Micelles as a Means of Delivering the Hydrophobic Ionic Liquid EMIM TFSI to Graphene Oxide Surfaces.

Most ionic liquids (ILs) are not surface-active and cannot, alone, be directed to assemble at surfaces─despite their potential as nonvolatile structure-directing agents and use as advanced materials in a multitude of applications. In this work, we investigate aqueous systems of common nonionic surfactants (Triton X-100 and Tween 20), which we use to solubilize 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. The resulting solution of mixed micelle leads to spontaneous adsorption of the IL/surfactant complex onto graphene oxide (GO) surfaces, forming a compact film. Adsorption isotherms generated by fluorescence labeling of the IL and surfactant phases are used to quantify the extent of adsorption. While sensitive to the GO dispersion concentration, upwards of 3 g IL/g GO adsorb under dilute conditions. Atomic force microscopy is used to show that the adsorbed layer uniformly distributes as an ∼1 nm thick coating (per GO side) as the system reaches the first plateau of a Langmuir-type isotherm. Adsorption beyond this plateau is possible but leads to thicker (>30 nm), inhomogeneous adsorbed layers. Both micellar size in solution and adsorbed layer thickness reduce upon the addition of IL to the surfactant phase, suggesting significant interactions among the materials and nonideal mixing of the components.

Continuous Langmuir-Blodgett Deposition and Transfer by Controlled Edge-to-Edge Assembly of Floating 2D Materials.

The Langmuir-Blodgett technique is one of the most controlled methods to deposit monomolecular layers of floating or surface active materials but has lacked the ability to coat truly large-area substrates. In this work, by manipulating single-layer dispersions of graphene oxide (GO) and thermally exfoliated GO into water-immiscible spreading solvents, unlike traditional Langmuir-Blodgett deposition which requires densification achieved by compressing barriers, we demonstrate the ability to control the 2D aggregation and densification behavior of these floating materials using a barrier-free method. This is done by controlling the edge-to-edge interactions through modified subphase conditions and by utilizing the distance-dependent spreading pressure of the deposition solvent. These phenomena allow substrates to be coated by continuous deposition and substrate withdrawal-enabling roll-to-roll deposition and patterning of large-area substrates such as flexible polyethylene terephthalate. The aggregation and solvent-driven densification phenomena are examined by in situ Brewster angle video microscopy and by measuring the local spreading pressure induced by the spreading solvent acting on the floating materials using a Langmuir-Adam balance. As an example, the performance of films deposited in this way is assessed as passivation layers for Ag nanowire-based transparent conductors.

Salt-Induced Doping and Templating of Laser-Induced Graphene Supercapacitors.

The use of inexpensive and widely available CO2 lasers to selectively irradiate polymer films and form a graphene foam, termed laser-induced graphene (LIG), has incited significant research attention. The simple and rapid nature of the approach and the high conductivity and porosity of LIG have motivated its widespread application in electrochemical energy storage devices such as batteries and supercapacitors. However, nearly all high-performance LIG-based supercapacitors reported to date are prepared from costly, petroleum-based polyimide (Kapton, PI). Herein, we demonstrate that incorporating microparticles of inexpensive, non-toxic, and widely abundant sodium salts such as NaCl and Na2SO4 into poly(furfuryl alcohol) (PFA) resins enables the formation of high-performance LIG. The embedded particles aid in carbonization and act as a template for pore formation. While increasing both the carbon yield and surface area of the electrodes, the salt also dopes the LIG formed with S or Cl. The combination of these effects results in a two- to four-order-of-magnitude increase in device areal capacitance, from 8 µF/cm2 for PFA/no salt at 5 mV/s to up to 80 mF/cm2 for some PFA/20% Na2SO4 samples at 0.05 mA/cm2, significantly higher than that of PI-based devices and most other LIG precursors.

Spontaneous clustering of exfoliated two-dimensional materials at the air-water interface.

HYPOTHESIS: Coating approaches which trap nanoparticles at an interface have become popular for depositing single-layer films from nanoparticle dispersions. Past efforts concluded that concentration and aspect ratio dominate the impact on aggregation state of nanospheres and nanorods at an interface. Although few works have explored the clustering behaviour of atomically thin, two-dimensional materials, we hypothesize that nanosheet concentration is the dominant factor leading to a particular cluster structure and that this local structure impacts the quality of densified Langmuir films. EXPERIMENTS: We systematically studied cluster structures and Langmuir film morphologies of three different nanosheets, namely chemically exfoliated molybdenum disulfide, graphene oxide and reduced graphene oxide. FINDINGS: We observe cluster structure transitions from island-like domains to more linear networks in all materials as dispersion concentration is reduced. Despite differences in material properties and morphologies, we obtained the same overall correlation between sheet number density (A/V) in the spreading dispersion and cluster fractal structure (df) is observed, with reduced graphene oxide sheets showing a slight delay upon transitioning into a lower-density cluster. Regardless of assembly method, we found that cluster structure impacts the attainable density of transferred Langmuir films. A two-stage clustering mechanism is supported by by considering the spreading profile of solvents and an analysis of interparticle forces at the air-water interface.

Encapsulating a Responsive Hydrogel Core for Void Space Modulation in High-Stability Graphene-Wrapped Silicon Anodes.

Due to its formidably high theoretical capacity (3590 mAh/g at room temperature), silicon (Si) is expected to replace graphite as the dominant anode for higher energy density lithium (Li)-ion batteries. However, stability issues stemming from silicon's significant volume expansion (∼300%) upon lithiation have slowed down commercialization. Herein, we report the design of a scalable process to engineer core-shell structures capable of buffering this volume expansion, which utilize a core made up of a poly(ethylene oxide)-carboxymethyl cellulose hydrogel and silicon protected by a crumpled graphene shell. The volume expansion of the hydrogel upon exposure to water creates a void space between the Si-Si and Si-rGO interfaces within the core when the gel dries. Unlike sacrificial spacers, the dehydrated hydrogel remains in the core and acts as an elastic Li-ion conductor, which improves the stability and high rate performance. The optimized composite electrodes retain ∼81.7% of their initial capacity (1055 mAh/(grGO+gel+Si)) after 320 cycles when an active material loading of 1 mg/cm2 is used. At more practical mass loadings (2.5 mg/cm2), the electrodes achieve 2.04 mAh/cm2 and retain 79% of this capacity after 200 cycles against a lithium half-cell. Full cells assembled using a lithium ion phosphate cathode lose only 6.7% of their initial capacity over 100 cycles, demonstrating the potential of this nanocomposite anode for use in next-generation Li-ion batteries.

Imparting conformational memory for material adhesion.

Adhesion between similar and dissimilar materials is essential to many biological systems and synthetic materials, devices, and machines. Since the inception of adhesion science more than five decades ago, adhesion to a surface has long been recognized as beyond two-dimensional. Similarly, molecular conformation - the three-dimensional arrangement of atoms in a molecule - is ubiquitous in biology and fundamental to the binding of biomolecules. However, the connection between these concepts, which could link molecular conformation in biology to micro- and macroscopic adhesion in materials science, remains elusive. Herein, we examine this connection by manipulating the molecular conformation of a mussel-inspired universal coating, which imparts a memory for recognizing different hydrogels. This approach leads to significantly (several fold) increased interfacial adhesion between the coating and hydrogels across a broad range of length scales, from molecular to macroscopic. Furthermore, we demonstrate that imparting memory is a general and facile noncovalent approach for enhancing interfacial adhesion that, with suitable energy dissipation, can be used for the bonding of materials.

Probing Sulfur Deposition onto Carbon Nanomaterials from Aqueous, Elemental Sulfur Sols for Lithium-Sulfur Batteries.

Sulfur cathodes for lithium-sulfur batteries often rely on integrating sulfur with high surface area carbonaceous materials. Nanoscale mixing is typically achieved by a lengthy, high-temperature melt imbibition approach that employs carbon nanomaterials in an aggregated solid form. In this work, we present a simple strategy to coat carbon nanomaterials with sulfur in a cost-effective, room-temperature process using inexpensive elemental sulfur. Our results are based on hydrophobic sulfur sols, which have rarely been examined for use in the preparation of sulfur cathodes. We study the deposition mechanism on different carbon materials and find that sulfur dissolves from the sol into the aqueous phase and coats the surface of reduced graphene oxide (rGO) by heterogeneous nucleation and growth, but that this mechanism is not favored for carbon materials such as Ketjen black (KB) and graphene oxide (GO), for which undesirable homogeneous nucleation of micron-sized, insulating sulfur crystals is observed. High loading (3-4 mgsulfur/cm2) rGO-based cathodes prepared using this approach achieve discharge capacities of 1300 mAh/gsulfur (∼4.8 mAh/cm2) at 0.1C and achieve capacities 7-fold higher than cells prepared via traditional melt imbibition approaches at higher rates of 0.8C and 1C. Cells prepared without the need for added binder or conductive additive achieve projected full cell energy densities of 468 Wh/kg at 0.1C when taking into account all inactive components and assuming no lithium metal degradation, indicating that the deposition of sulfur from hydrophobic sols onto carbon nanomaterials can serve as a simple, aqueous-based, one-step process to prepare high sulfur loading cathodes with high projected energy densities.

Controlling Void Space in Crumpled Graphene-Encapsulated Silicon Anodes using Sacrificial Polystyrene Nanoparticles.

Silicon anodes have a theoretical capacity of 3590 mAh g-1 (for Li15 Si4 , at room temperature), which is tenfold higher than the graphite anodes used in current Li-ion batteries. This, and silicon's natural abundance, makes it one of the most promising materials for next-generation batteries. Encapsulating silicon nanoparticles (Si NPs) in a crumpled graphene shell by spray drying or spray pyrolysis are promising and scalable methods to produce core-shell structures, which buffer the extreme volume change (>300 vol %) caused by (de)lithiaton of silicon. However, capillary forces cause the graphene-based materials to tightly wrap around Si NP clusters, and there is little control over the void space required to further improve cycle life. Herein, a simple strategy is developed to engineer void-space within the core by incorporating varying amounts of similarly sized polystyrene (PS) nanospheres in the spray drier feed mixture. The PS completely decomposes during thermal reduction of the graphene oxide shell and results in Si cores of varying porosity. The best performance is achieved at a 1 : 1 ratio (PS/Si), leading to high capacities of 1638, 1468, and 1179 mAh g-1 Si+rGO at 0.1, 1, and 4 A g-1 , respectively. Moreover, at 1 A g-1 , the capacity retention is 80.6 % after 200 cycles. At a practical active material loading of 2.4 mg cm-2 , the electrodes achieve an areal capacity of 2.26 mAh cm-2 at 1 A g-1 .

Graphene Oxide Membranes for High Salinity, Produced Water Separation by Pervaporation.

Oil and gas industries produce a huge amount of wastewater known as produced water which contains diverse contaminants including salts, dissolved organics, dispersed oils, and solids making separation and purification challenging. The chemical and thermal stability of graphene oxide (GO) membranes make them promising for use in membrane pervaporation, which may provide a more economical route to purifying this water for disposal or re-use compared to other membrane-based separation techniques. In this study, we investigate the performance and stability of GO membranes cast onto polyethersulfone (PES) supports in the separation of simulated produced water containing high salinity brackish water (30 g/L NaCl) contaminated with phenol, cresol, naphthenic acid, and an oil-in-water emulsion. The GO/PES membranes achieve water flux as high as 47.8 L m-2 h-1 for NaCl solutions for membranes operated at 60 °C, while being able to reject 99.9% of the salt and upwards of 56% of the soluble organic components. The flux for membranes tested in pure water, salt, and simulated produced water was found to decrease over 72 h of testing but only to 50-60% of the initial flux in the worst-case scenario. This drop was concurrent with an increase in contact angle and C/O ratio indicating that the GO may become partially reduced during the separation process. Additionally, a closer look at the membrane crosslinker (Zn2+) was investigated and found to hydrolyze over time to Zn(OH)2 with much of it being washed away during the long-term pervaporation.

Enhanced Cycle Stability of Crumpled Graphene-Encapsulated Silicon Anodes via Polydopamine Sealing.

Despite silicon being a promising candidate for next-generation lithium-ion battery anodes, self-pulverization and the formation of an unstable solid electrolyte interface, caused by the large volume expansion during lithiation/delithiation, have slowed its commercialization. In this work, we expand on a controllable approach to wrap silicon nanoparticles in a crumpled graphene shell by sealing this shell with a polydopamine-based coating. This provides improved structural stability to buffer the volume change of Si, as demonstrated by a remarkable cycle life, with anodes exhibiting a capacity of 1038 mA h/g after 200 cycles at 1 A/g. The resulting composite displays a high capacity of 1672 mA h/g at 0.1 A/g and can still retain 58% when the current density increases to 4 A/g. A systematic investigation of the impact of spray-drying parameters on the crumpled graphene morphology and its impact on battery performance is also provided.

Intrinsic Capacitance of Molybdenum Disulfide.

The metallic, 1T polymorph of molybdenum disulfide (MoS2) is promising for next-generation supercapacitors due to its high theoretical surface area and density which lead to high volumetric capacitance. Despite this, there are few fundamental works examining the double-layer charging mechanisms at the MoS2/electrolyte interface. This study examines the potential-dependent and frequency-dependent area-specific double-layer capacitance (Ca) of the 1T and 2H polymorphs of MoS2 in aqueous and organic electrolytes. Furthermore, we investigate restacking effects and possible intercalation-like mechanisms in multilayer films. To minimize the uncertainties associated with porous electrodes, we carry out measurements using effectively nonporous monolayers of MoS2 and contrast their behavior with reduced graphene oxide deposited layer-by-layer on atomically flat graphite single crystals using a modified, barrier-free Langmuir-Blodgett method. The metallic 1T polymorph of MoS2 (Ca,1T = 14.9 µF/cm2) is shown to have over 10-fold the capacitance of the semiconducting 2H polymorph (Ca,2H = 1.35 µF/cm2) near the open circuit potential and under negative polarization in aqueous electrolyte. However, under positive polarization the capacitance is significantly reduced and behaves similarly to the 2H polymorph. The capacitance of 1T MoS2 scales with layer number, even at high frequency, suggesting easy and rapid ion penetration between the restacked sheets. This model system allows us to determine capacitance limits for MoS2 and suggest strategies to increase the energy density of devices made from this promising material.

A stable TiO2-graphene nanocomposite anode with high rate capability for lithium-ion batteries.

A rapid microwave hydrothermal process is adopted for the synthesis of titanium dioxide and reduced graphene oxide nanocomposites as high-performance anode materials for Li-ion batteries. With the assistance of hydrazine hydrate as a reducing agent, graphene oxide was reduced while TiO2 nanoparticles were grown in situ on the nanosheets to obtain the nanocomposite material. The morphology of the nanocomposite obtained consisted of TiO2 particles with a size of ∼100 nm, uniformly distributed on the reduced graphene oxide nanosheets. The as-prepared TiO2-graphene nanocomposite was able to deliver a capacity of 250 mA h g-1 ± 5% at 0.2C for more than 200 cycles with remarkably stable cycle life during the Li+ insertion/extraction process. In terms of high rate capability performance, the nanocomposite delivered discharge capacity of ca. 100 mA h g-1 with >99% coulombic efficiency at C-rates of up to 20C. The enhanced electrochemical performance of the material in terms of high rate capability and cycling stability indicates that the as-developed TiO2-rGO nanocomposites are promising electrode materials for future Li-ion batteries.

Ultrasensitive Electrochemical Methane Sensors Based on Solid Polymer Electrolyte-Infused Laser-Induced Graphene.

Methane is a potent greenhouse gas, with large emissions occurring across gas distribution networks and mining/extraction infrastructure. The development of inexpensive, low-power electrochemical sensors could provide a cost-effective means to carry out distributed sensing to identify leaks for rapid mitigation. In this work, we demonstrate a simple and cost-effective strategy to rapidly prototype ultrasensitive electrochemical gas sensors. A room-temperature methane sensor is evaluated which demonstrates the highest reported sensitivity (0.55 µA/ppm/cm2) with a rapid response time (40 s) enabling sub-ppm detection. Porous, laser-induced graphene (LIG) electrodes are patterned directly into commercial polymer films and imbibed with a palladium nanoparticle dispersion to distribute the electrocatalyst within the high surface area support. A pseudo-solid-state ionic liquid/polyvinylidene fluoride electrolyte was painted onto the flexible cell yielding a porous electrolyte, within the porous LIG electrode, simultaneously facilitating rapid gas transport and enabling the room temperature electro-oxidation pathway for methane. The performance of the amperometric sensor is evaluated as a function of methane concentration, relative humidity, and tested against interfering gases.

Decorating Graphene Oxide with Ionic Liquid Nanodroplets: An Approach Leading to Energy-Dense, High-Voltage Supercapacitors.

A major stumbling block in the development of high energy density graphene-based supercapacitors has been maintaining high ion-accessible surface area combined with high electrode density. Herein, we develop an ionic liquid (IL)-surfactant microemulsion system that is found to facilitate the spontaneous adsorption of IL-filled micelles onto graphene oxide (GO). This adsorption distributes the IL over all available surface area and provides an aqueous formulation that can be slurry cast onto current collectors, leaving behind a dense nanocomposite film of GO/IL/surfactant. By removing the surfactant and reducing the GO through a low-temperature (360 °C) heat treatment, the IL plays a dual role of spacer and electrolyte. We study the effect of IL content and operating temperature on the performance, demonstrating a record high gravimetric capacitance (302 F/g at 1 A/g) for 80 wt % IL composites. At 60 wt % IL, combined high capacitance and bulk density (0.76 g/cm3), yields one of the highest volumetric capacitances (218 F/cm3, at 1 A/g) ever reported for a high-voltage IL-based supercapacitor. While achieving promising rate performance and cycle-life, the approach also eliminates the long and costly electrolyte imbibition step of cell assembly as the electrolyte is cast directly with the electrode material.

Intrinsic Catalytic Activity of Graphene Defects for the Co(II/III)(bpy)3 Dye-Sensitized Solar Cell Redox Mediator.

We demonstrate that functionalized graphene, rich with lattice defects but lean with oxygen sites, catalyzes the reduction of Co(III)(bpy)3 as well as platinum does, exhibiting a rate of heterogeneous electron transfer, k0, of ∼6 × 10(-3) cm/s. We show this rate to be an order of magnitude higher than on oxygen-site-rich graphene oxide, and over 2 orders of magnitude higher than on the basal plane of graphite (as a surrogate for pristine graphene). Furthermore, dye-sensitized solar cells using defect-rich graphene monolayers perform similarly to those using platinum nanoparticles as the catalyst.

Anomalous Capacitance Maximum of the Glassy Carbon-Ionic Liquid Interface through Dilution with Organic Solvents.

We use electrochemical impedance spectroscopy to measure the effect of diluting a hydrophobic room temperature ionic liquid with miscible organic solvents on the differential capacitance of the glassy carbon-electrolyte interface. We show that the minimum differential capacitance increases with dilution and reaches a maximum value at ionic liquid contents near 5-10 mol% (i.e., ∼1 M). We provide evidence that mixtures with 1,2-dichloroethane, a low-dielectric constant solvent, yield the largest gains in capacitance near the open circuit potential when compared against two traditional solvents, acetonitrile and propylene carbonate. To provide a fundamental basis for these observations, we use a coarse-grained model to relate structural variations at the double layer to the occurrence of the maximum. Our results reveal the potential for the enhancement of double-layer capacitance through dilution.

Electrochemical sensing of nitric oxide with functionalized graphene electrodes.

The intrinsic electrocatalytic properties of functionalized graphene sheets (FGSs) in nitric oxide (NO) sensing are determined by cyclic voltammetry with FGS monolayer electrodes. The degrees of reduction and defectiveness of the FGSs are varied by employing different heat treatments during their fabrication. FGSs with intermediate degrees of reduction and high Raman ID to IG peak ratios exhibit an NO oxidation peak potential of 794 mV (vs 1 M Ag/AgCl), closely matching values obtained with a platinized Pt control (791 mV) as well as recent results from the literature on porous or biofunctionalized electrodes. We show that the peak potential obtained with FGS electrodes can be further reduced to 764 mV by incorporation of electrode porosity using a drop-casting approach, indicating a stronger apparent electrocatalytic effect on porous FGS electrodes as compared to platinized Pt. Taking into consideration effects of electrode morphology, we thereby demonstrate that FGSs are intrinsically as catalytic toward NO oxidation as platinum. The lowered peak potential of porous FGS electrodes is accompanied by a significant increase in peak current, which we attribute either to pore depletion effects or an amplification effect due to subsequent electrooxidation reactions. Our results suggest that the development of sensor electrodes with higher sensitivity and lower detection limits should be feasible with FGSs.

Ternary self-assembly of ordered metal oxide-graphene nanocomposites for electrochemical energy storage.

Surfactant or polymer directed self-assembly has been widely investigated to prepare nanostructured metal oxides, semiconductors, and polymers, but this approach is mostly limited to two-phase materials, organic/inorganic hybrids, and nanoparticle or polymer-based nanocomposites. Self-assembled nanostructures from more complex, multiscale, and multiphase building blocks have been investigated with limited success. Here, we demonstrate a ternary self-assembly approach using graphene as fundamental building blocks to construct ordered metal oxide-graphene nanocomposites. A new class of layered nanocomposites is formed containing stable, ordered alternating layers of nanocrystalline metal oxides with graphene or graphene stacks. Alternatively, the graphene or graphene stacks can be incorporated into liquid-crystal-templated nanoporous structures to form high surface area, conductive networks. The self-assembly method can also be used to fabricate free-standing, flexible metal oxide-graphene nanocomposite films and electrodes. We have investigated the Li-ion insertion properties of the self-assembled electrodes for energy storage and show that the SnO2-graphene nanocomposite films can achieve near theoretical specific energy density without significant charge/discharge degradation.