Robert A. W. Dryfe

Oxygen Reduction Reaction in a Droplet on Graphite: Direct Evidence that the Edge Is More Active than the Basal Plane†

Carbon-based metal-free electrocatalysts for the oxygen reduction reaction (ORR) in alkaline medium have been extensively investigated with the aim of replacing the commercially available, but precious platinum-based catalysts. For the proper design of carbon-based metal-free electrocatalysts for the ORR, it would be interesting to identify the active sites of the electrocatalyst. The ORR was now studied with an air-saturated electrolyte solution droplet (diameter ca. 15 μm), which was deposited at a specified position either on the edge or on the basal plane of highly oriented pyrolytic graphite. Electrochemical measurements suggest that the edge carbon atoms are more active than the basal-plane ones for the ORR. This provides a direct way to identify the active sites of carbon materials for the ORR. Ball-milled graphite and carbon nanotubes with more exposed edges were also prepared and showed significantly enhanced ORR activity. DFT calculations elucidated the mechanism by which the charged edge carbon atoms result in the higher ORR activity.

Characterization of MoS2–Graphene Composites for High-Performance Coin Cell Supercapacitors

Two-dimensional materials, such as graphene and molybdenum disulfide (MoS2), can greatly increase the performance of electrochemical energy storage devices because of the combination of high surface area and electrical conductivity. Here, we have investigated the performance of solution exfoliated MoS2 thin flexible membranes as supercapacitor electrodes in a symmetrical coin cell arrangement using an aqueous electrolyte (Na2SO4). By adding highly conductive graphene to form nanocomposite membranes, it was possible to increase the specific capacitance by reducing the resistivity of the electrode and altering the morphology of the membrane. With continued charge/discharge cycles the performance of the membranes was found to increase significantly (up to 800%), because of partial re-exfoliation of the layered material with continued ion intercalation, as well as increasing the specific capacitance through intercalation pseudocapacitance. These results demonstrate a simple and scalable application of layered 2D materials toward electrochemical energy storage.

Visible-Light-Mediated Generation of Nitrogen-Centered Radicals: Metal-Free Hydroimination and Iminohydroxylation Cyclization Reactions.

The formation and use of iminyl radicals in novel and divergent hydroimination and iminohydroxylation cyclization reactions has been accomplished through the design of a new class of reactive O-aryl oximes. Owing to their low reduction potentials, the inexpensive organic dye eosin Y could be used as the photocatalyst of the organocatalytic hydroimination reaction. Furthermore, reaction conditions for a unique iminohydroxylation were identified; visible-light-mediated electron transfer from novel electron donor–acceptor complexes of the oximes and Et3N was proposed as a key step of this process.

Electrochemical Behavior of Monolayer and Bilayer Graphene

Results of a study on the electrochemical properties of exfoliated single and multilayer graphene flakes are presented. Graphene flakes were deposited on silicon/silicon oxide wafers to enable fast and accurate characterization by optical microscopy and Raman spectroscopy. Conductive silver paint and silver wires were used to fabricate contacts; epoxy resin was employed as a masking coating in order to expose a stable, well-defined area of graphene. Both multilayer and monolayer graphene microelectrodes showed quasi-reversible behavior during voltammetric measurements in potassium ferricyanide. However, the standard heterogeneous charge transfer rate constant, k°, was estimated to be higher for monolayer graphene flakes.

Chromium redox couples for application to redox flow batteries

Abstract Both anodic and cathodic chromium–EDTA (ethylenediaminetetra-acetate) complex redox processes have been studied using cyclic voltammetry. Their potential use in a redox battery has been evaluated by comparing the charge and discharge performance of a simple redox battery employing several redox couples including the conventional Fe–Cr redox couples. The cyclic voltammetry experiments suggested that oxidation of Cr(III)–EDTA formed Cr(V)–EDTA rather than a hexavalent chromium species. It was found that the kinetics of the Cr(III)–EDTA/Cr(II)–EDTA redox reaction are fast at a graphite rod electrode, whereas the Cr(V)–EDTA/Cr(III)–EDTA redox reaction is relatively slow. In spite of the slow kinetics, the battery employing solely these chromium–EDTA based redox couples provided higher energy output and longer life than the conventional Fe–Cr redox system.

Electron Transfer Kinetics on Mono- and Multilayer Graphene

Understanding of the electrochemical properties of graphene, especially the electron transfer kinetics of a redox reaction between the graphene surface and a molecule, in comparison to graphite or other carbon-based materials, is essential for its potential in energy conversion and storage to be realized. Here we use voltammetric determination of the electron transfer rate for three redox mediators, ferricyanide, hexaammineruthenium, and hexachloroiridate (Fe(CN)(6)(3-), Ru(NH3)(6)(3+), and IrCl(6)(2-), respectively), to measure the reactivity of graphene samples prepared by mechanical exfoliation of natural graphite. Electron transfer rates are measured for varied number of graphene layers (1 to ca. 1000 layers) using microscopic droplets. The basal planes of mono- and multilayer graphene, supported on an insulating Si/SiO(2) substrate, exhibit significant electron transfer activity and changes in kinetics are observed for all three mediators. No significant trend in kinetics with flake thickness is discernible for each mediator; however, a large variation in kinetics is observed across the basal plane of the same flakes, indicating that local surface conditions affect the electrochemical performance. This is confirmed by in situ graphite exfoliation, which reveals significant deterioration of initially, near-reversible kinetics for Ru(NH3)(6)(3+) when comparing the atmosphere-aged and freshly exfoliated graphite surfaces.

Graphene oxide-assisted deposition of carbon nanotubes on carbon cloth as advanced binder-free electrodes for flexible supercapacitors

We successfully developed a graphene oxide-assisted electrophoretic deposition (EPD) method to prepare the porous hybrid graphene–carbon nanotube (G–CNT) layer on the carbon fiber surface of carbon cloth (CC). The as-fabricated flexible supercapacitor based on the G–CNT/CC electrodes shows significantly enhanced supercapacitor performance.

Alkali reduction of graphene oxide in molten halide salts: Production of corrugated graphene derivatives for high-performance supercapacitors

Herein we present a green and facile approach to the successful reduction of graphene oxide (GO) materials using molten halide flux at 370 °C. GO materials have been synthesized using a modified Hummers method and subsequently reduced for periods of up to 8 h. Reduced GO (rGO) flakes have been characterized using X-ray-diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR), all indicating a significantly reduced amount of oxygen-containing functionalities on the rGO materials. Furthermore, impressive electrical conductivities and electrochemical capacitances have been measured for the rGO flakes, which, along with the morphology determined from scanning electron microscopy, highlight the role of surface corrugation in these rGO materials.

Sieving hydrogen isotopes through two dimensional crystals

Separating H+ from D+ In many respects, hydrogen and deuterium show similar properties because they share the same number of protons and electrons and only differ by one neutron. However, when you strip away the electron, a proton ends up having less than half the radius of a deuterion. Lozada-Hidalgo et al. used two-dimensional membranes of graphene or hexagonal boron nitride to separate these two charged isotopes, with a separation factor of about 10. Science, this issue p. 68 Isotopes of hydrogen are separated using membranes based on two-dimensional crystals of hexagonal boron nitride and graphene. One-atom-thick crystals are impermeable to atoms and molecules, but hydrogen ions (thermal protons) penetrate through them. We show that monolayers of graphene and boron nitride can be used to separate hydrogen ion isotopes. Using electrical measurements and mass spectrometry, we found that deuterons permeate through these crystals much slower than protons, resulting in a separation factor of ≈10 at room temperature. The isotope effect is attributed to a difference of ≈60 milli–electron volts between zero-point energies of incident protons and deuterons, which translates into the equivalent difference in the activation barriers posed by two-dimensional crystals. In addition to providing insight into the proton transport mechanism, the demonstrated approach offers a competitive and scalable way for hydrogen isotope enrichment.

Structural and electrochemical characterisation of Pt and Pd nanoparticles electrodeposited at the liquid/liquid interface

We present the first reported characterisation by x-ray diffraction and high resolution transmission electron microscopy of metals electrodeposited at the bare and templated liquid/liquid interfaces. Additional structural information is also obtained using ion transfer voltammetry as an in situ characterisation tool. In particular, the metallic deposits are shown to consist of aggregates of discrete nanoparticles, predominantly between 3 and 5 nm in diameter. Deposition of platinum at the liquid/liquid interface is reported for the first time, which enables a preliminary comparison to be made between the growth mechanism of this metal and the growth of palladium, previously reported at this interface.