Aramid nanofiber-reinforced three-dimensional graphene hydrogels for supercapacitor electrodes.

HYPOTHESIS: Self-assembled graphene hydrogels are notable in the field of electrochemical energy storage for their unique combination of excellent specific surface area, high porosity, and electrically conductive continuous network. However, graphene hydrogels suffer from poor mechanical integrity compared to layered architectures like graphene buckypapers, limiting their applications in practical devices. We propose the use of high strength, Kevlar®-derived polymeric nanofillers, aramid nanofibers (ANFs) as structural fillers to enhance graphene hydrogel's shear modulus in the context of multifunctional (mechanical and electrochemical) architectures. EXPERIMENTS: Graphene hydrogels are fabricated using sol-gel self-assembly of graphene oxide (GO) nanosheets in presence of ammonium hydroxide. Colloidal dispersion of ANFs and GO are integrated using a novel combination of solvent exchange and dialysis approach to fabricate GO-ANF hydrogels with 0-15 wt.% of ANFs loading (dry weight basis). Shear modulus and electrochemical properties of resulting hydrogel composites are evaluated using rheology and symmetric supercapacitor cell. FINDINGS: The addition of 2 wt.% ANFs resulted in an 80% improvement in shear modulus compared to neat graphene hydrogel. Addition of ANFs resulted in gradual reduction of specific capacitance, with the specific capacitance of 190 F/g for neat graphene hydrogel, reducing to 128 F/g for an ANF loading of 15 wt.% (dry weight basis). This work shows the broader concept that adding high-strength nanofibers to a nanomaterial gel can add reinforcement provided that the gelation process itself is not disrupted.

Ultra-compact high-sensitivity plasmonic sensor based on Fano resonance with symmetry breaking ring cavity.

Miniaturizing optical devices with desired functionality is a key prerequisite for nanoscale photonic circuits. Based on Fano resonance, an on-chip high-sensitivity sensor, composed of two waveguides coupling with a symmetry breaking ring resonator, is theoretically and numerically investigated. The established theoretical model agrees well with the finite-difference time-domain simulations, which reveals the physics of Fano resonance. Differing with the coupled cavities, the Fano resonance originates from the interference between symmetry-mode and asymmetry-mode in a single symmetry-broken cavity. The spectral responses and sensing performances of the plasmonic structure rely on the degree of asymmetry of cavity. In particular, the plasmonic sensor can detect the refractive index changes as small as 10-5, and the figure of merit (FOM) of symmetry-breaking cavity structure is 17 times larger than that of symmetrical cavity system. Additionally, the sensitivity to temperature of ethanol analyte achieves 0.701 nm/○C. Compared with the coupled cavities, the on-chip high-sensitivity sensor using a single cavity is more compact, which paves the way toward highly integrated photonic devices.

Calorimetry of explosive thermal decomposition of graphite oxide.

Graphite oxide (GO) has shown immense potential in energy storage and composite filler applications, and large-scale production of GO is of increasing commercial and academic interest. However, prior studies show that GO has the potential to undergo explosive decomposition. In this study, Advanced Reactive System Screening Tool was used to track the temperature and pressure of the explosive decomposition of GO. The data showed that the explosive decomposition temperature of GO strongly depends on sample size. The temperature and pressure generation are on the order of 1000s of °C per minute and 1000s of psig per minute respectively for less than a gram of material. Therefore, the rapid decomposition of bulk GO can lead to catastrophic consequences. The paper further compared the thermal stability of GO from different sources and found that the GO surface area has significant effects on GO stability. Finally, the Frank-Kamenetskii model was used to predict the critical mass necessary for GO to undergo explosive decomposition, the model predicted the mass within a factor of experimental data. The results of this study are beneficial in assessing and predicting the hazards of bulk GO during storage and handling.

High-yield scalable graphene nanosheet production from compressed graphite using electrochemical exfoliation.

Electrochemical exfoliation is a promising bulk method for producing graphene from graphite; in this method, an applied voltage drives ionic species to intercalate into graphite where they form gaseous species that expand and exfoliate individual graphene sheets. However, a number of obstacles have prevented this approach from becoming a feasible production route; the disintegration of the graphite electrode as the method progresses is the chief difficulty. Here we show that if graphite powders are contained and compressed within a permeable and expandable containment system, the graphite powders can be continuously intercalated, expanded, and exfoliated to produce graphene. Our data indicate both high yield (65%) and extraordinarily large lateral size (>30 µm) in the as-produced graphene. We also show that this process is scalable and that graphene yield efficiency depends solely on reactor geometry, graphite compression, and electrolyte transport.

Tailored Network Formation in Graphene Oxide Gels.

Graphene oxide (GO)-based gels are attractive because of their ability to retain individual nanosheet properties in a three-dimensional (3D) bulk material. The final morphology and properties of these 3D gel networks depend strongly on the type and density of cross-links, and these gels can be dried and annealed to form aerogels with both high conductivity (560 S/m) and high surface area (1700 m2/g). The results show that both ammonia content and the parent nanosheet morphology (crumpled vs flat) have a strong influence on the cross-linked structure and composition; notably, nitrogen is found in the gels, suggesting that ammonia actively participates in the reaction rather than as a mere catalyst. The GO nanosheet morphology may be altered using spray-drying to obtain crumpled GO (cGO) nanosheets and form cGO gels; this allows for an additional handle in the creation of GO-based gels with tunable density, electrical conductivity, and surface area.

Overexpression of PtABCC1 contributes to mercury tolerance and accumulation in Arabidopsis and poplar.

Mercury (Hg) is a highly biotoxic heavy metal that contaminates the environment. Phytoremediation is a green technology for environmental remediation and is used to clean up Hg contaminated soil in recent years. In this study, we isolated an ATP-binding cassette (ABC) transporter gene PtABCC1 from Populus trichocarpa and overexpressed it in Arabidopsis and poplar. The transgenic plants conferred higher Hg tolerance than wild type (WT) plants, and overexpression of PtABCC1 could lead to 26-72% or 7-160% increase of Hg accumulation in Arabidopsis or poplar plants, respectively. These results demonstrated that PtABCC1 plays a crucial role in enhancing tolerance and accumulation to Hg in plants, which provides a promising way for phytoremediation of Hg contamination.

Oxygen- and Nitrogen-Enriched 3D Porous Carbon for Supercapacitors of High Volumetric Capacity.

Efficient utilization and broader commercialization of alternative energies (e.g., solar, wind, and geothermal) hinges on the performance and cost of energy storage and conversion systems. For now and in the foreseeable future, the combination of rechargeable batteries and electrochemical capacitors remains the most promising option for many energy storage applications. Porous carbonaceous materials have been widely used as an electrode for batteries and supercapacitors. To date, however, the highest specific capacitance of an electrochemical double layer capacitor is only ∼200 F/g, although a wide variety of synthetic approaches have been explored in creating optimized porous structures. Here, we report our findings in the synthesis of porous carbon through a simple, one-step process: direct carbonization of kelp in an NH3 atmosphere at 700 °C. The resulting oxygen- and nitrogen-enriched carbon has a three-dimensional structure with specific surface area greater than 1000 m(2)/g. When evaluated as an electrode for electrochemical double layer capacitors, the porous carbon structure demonstrated excellent volumetric capacitance (>360 F/cm(3)) with excellent cycling stability. This simple approach to low-cost carbonaceous materials with unique architecture and functionality could be a promising alternative to fabrication of porous carbon structures for many practical applications, including batteries and fuel cells.

High Surface Area Tunnels in Hexagonal WO3.

High surface area in h-WO3 has been verified from the intracrystalline tunnels. This bottom-up approach differs from conventional templating-type methods. The 3.67 Å diameter tunnels are characterized by low-pressure CO2 adsorption isotherms with nonlocal density functional theory fitting, transmission electron microscopy, and thermal gravimetric analysis. These open and rigid tunnels absorb H(+) and Li(+), but not Na(+) in aqueous electrolytes without inducing a phase transformation, accessing both internal and external active sites. Moreover, these tunnel structures demonstrate high specific pseudocapacitance and good stability in an H2SO4 aqueous electrolyte. Thus, the high surface area created from 3.67 Å diameter tunnels in h-WO3 shows potential applications in electrochemical energy storage, selective ion transfer, and selective gas adsorption.