Showing papers on "Graphite published in 2001"

Hydrogen Sensors and Switches from Electrodeposited Palladium Mesowire Arrays

Abstract: Hydrogen sensors and hydrogen-activated switches were fabricated from arrays of mesoscopic palladium wires. These palladium "mesowire" arrays were prepared by electrodeposition onto graphite surfaces and were transferred onto a cyanoacrylate film. Exposure to hydrogen gas caused a rapid (less than 75 milliseconds) reversible decrease in the resistance of the array that correlated with the hydrogen concentration over a range from 2 to 10%. The sensor response appears to involve the closing of nanoscopic gaps or "break junctions" in wires caused by the dilation of palladium grains undergoing hydrogen absorption. Wire arrays in which all wires possessed nanoscopic gaps reverted to open circuits in the absence of hydrogen gas.

Graphite Nanofibers as an Electrode for Fuel Cell Applications

Abstract: The potential of graphite nanofiber supported platinum catalysts as an electrode for fuel cell applications was investigated using the electrochemical oxidation of methanol at 40 °C as a probe reac...

Studies on the Mechanism by Which the Formation of Nanocomposites Enhances Thermal Stability

Abstract: Polystyrene-clay and polystyrene-graphite nanocomposites have been prepared and used to explore the process by which the presence of clay or graphite in a nanocomposite enhances the thermal stability of polymers. This study has been designed to determine if the presence of paramagnetic iron in the matrix can result in radical trapping and thus enhance thermal stability. Nanocomposites were prepared by bulk polymerization using both iron-containing and iron-depleted clays and graphites, and they were characterized by X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, and cone calorimetry. The presence of structural iron, rather than that present as an impurity, significantly increases the onset temperature of thermal degradation in polymer-clay nanocomposites. Intercalated nanocomposites show an iron effect, but this is less important for exfoliated systems. Polymer-graphite nanocomposites show no difference between iron-free and iron-containing nanocomposites, presumably because the iron is not nanodispersed in the graphite.

Chemical Composition and Morphology of the Elevated Temperature SEI on Graphite

Abstract: The relationship between the elevated temperature performance of Li/graphite half-cells and the composition and morphology of the solid electrolyte interphase (SEI) formed on the graphite surface has been investigated for two electrolyte systems: I M LiPF 6 in ethylene carbonate/dimethyl carbonate EC/DMC (2:1) and 1 M LiBF 4 in EC/DMC (2:1). Precycled cells were stored at different temperatures up to 80°C, and the graphite electrodes were analyzed chemically (by X-ray photoelectron spectroscopy) and electrochemically under continued cycling. Loss of charge (for both salts) and of intercalation capacity (for LiBF 4 ) occurred after elevated temperature storage. The charge loss could be coupled to disappearance of the R-OCO 2 Li phase from the surface, with subsequent exposure of the graphite surface. The amount of LiF increased with increased storage temperature, but the LiF morphology differed between the two electrolyte systems. A model for the morphological changes of the SEI layer on storage at elevated temperature is proposed.

Growth of Pd, Pt, Ag and Au nanoparticles on carbon nanotubes

Abstract: By thermal decomposition of metal salts, palladium, platinum, silver and gold nanoparticles, with average size of 7 nm, 8 nm, 17 nm and 8 nm, respectively, were grown on carbon nanotubes. TEM observations showed that most of the nanoparticles synthesized stuck on the outer surface of the carbon nanotubes. The size of nanoparticles can be adjusted by changing the metal to carbon ratio. Small nanoparticles are prone to agglomerate under intense bombardment from the electron beam. By comparison, nanoparticles were scarcely found on graphite or active carbon using the same synthesis process. HRTEM and XRD investigations revealed the well-crystallized structure and a lattice contraction of these nanoparticles.

Understanding Solid Electrolyte Interface Film Formation on Graphite Electrodes

Abstract: Using ac impedance, we studied the formation process of solid electrolyte interface (SEI) film on graphite electrode during initial cycles. Results show that the SEI formation takes place through two major stages. The first stage takes place at voltages above 0.25 V (before lithiation of graphite), during which a loose and highly resistive film is formed. The second stage occurs at a narrow voltage range of 0.25-0.04 V, which proceeds simultaneously with lithiation of graphite electrode. In the second stage, a stable, compact, and highly conductive SEI film is produced. © 2001 The Electrochemical Society. All rights reserved.

Lithium intercalation into opened single-wall carbon nanotubes: storage capacity and electronic properties.

Abstract: The effects of structure and morphology on lithium storage in single-wall carbon nanotube (SWNT) bundles were studied by electrochemistry and nuclear magnetic resonance techniques. SWNTs were chemically etched to variable lengths and were intercalated with Li. The reversible Li storage capacity increased from LiC(6) in close-end SWNTs to LiC(3) after etching, which is twice the value observed in intercalated graphite. All the nanotubes became metallic upon intercalation of Li, with the density of states at the Fermi level increasing with increasing Li concentration. The enhanced capacity is attributed to Li diffusion into the interior of the SWNTs through the opened ends and sidewall defects.

Formation of Multilayers on Glassy Carbon Electrodes via the Reduction of Diazonium Salts

Abstract: The functionalization of carbon electrodes with aryl films can be achieved via the electrochemical reduction of the corresponding diazonium salt. We have previously shown that this deposition procedure will produce multilayer films on ordered graphite under certain conditions. We examine here the formation of multilayer films on glassy carbon (GC) electrodes by potential step electrolysis of diazonium salts for longer periods of time. The deposition of diethylaniline (DEA) is tracked with the use of infrared reflectance absorption spectroscopy and scanning force microscopy. DEA is continually deposited over a 30 min potential step and the film thickness approaches 20 nm. Phenylacetic acid and nitrophenyl films that are 15−25 nm thick can also be deposited in this way. We also find that, despite the presence of a relatively thick DEA film on a GC electrode, electron transfer to benchmark redox systems is not completely blocked. We attribute the compromised blocking to a high density of defects in the film ...

Hydrogen storage in sonicated carbon materials

Abstract: The hydrogen storage in purified single-wall carbon nanotubes (SWNTs), graphite and diamond powder was investigated at room temperature and ambient pressure. The samples were sonicated in 5 M HNO3 for various periods of time using an ultrasonic probe of the alloy Ti-6Al-4V. The goal of this treatment was to open the carbon nanotubes. The maximum value of overall hydrogen storage was found to be 1.5 wt %, as determined by thermal desorption spectroscopy. The storage capacity increases with sonication time. The sonication treatment introduces particles of the Ti alloy into the samples, as shown by X-ray diffraction, transmission electron microscopy, and chemical analysis. All of the hydrogen uptake can be explained by the assumption that the hydrogen is only stored in the Ti-alloy particles. The presence of Ti-alloy particles does not allow the determination of whether a small amount of hydrogen possibly is stored in the SWNTs themselves, and the fraction of nanotubes opened by the sonication treatment is unknown.

Preparation of polymer/graphite conducting nanocomposite by intercalation polymerization

Abstract: An in situ polymerization was conducted in the presence of expanded graphite obtained by rapid heating of the graphite intercalation compound (GIC) to form a polymer/expanded graphite conducting composite. Study showed that the graphite was dispersed in the form of nanosheets in the polymer matrix. The transition from an electrical insulator to an electrical semiconductor for the composite occurred when the expanded graphite content was 1.8 wt %, which was much lower than that of conventional conducting polymer composite. The composite exhibited high electrical conductivity of 10−2 S/cm when the graphite content was 3.0 wt %. This great improvement of conductivity could be attributed to the high aspect ratio (width-to-thickness) of the graphite nanosheets. Study suggested that extensive rolling of the blend greatly affected the conductivity of the composite. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2506–2513, 2001

Surface Film Formation on a Graphite Negative Electrode in Lithium-Ion Batteries: Atomic Force Microscopy Study on the Effects of Film-Forming Additives in Propylene Carbonate Solutions

Abstract: In situ electrochemical atomic force microscopy (AFM) observation of the basal plane of highly oriented pyrolytic graphite (HOPG) was performed during cyclic voltammetry in 1 M LiClO4/propylene carbonate (PC) containing 3 wt % vinylene carbonate (VC), fluoroethylene carbonate (FEC), and ethylene sulfite (ES) in order to clarify the roles of these additives in the formation of a protective surface film on a graphite negative electrode in lithium-ion batteries. Particle-like precipitates appeared on the HOPG surface at the potentials 1.35, 1.15, and 1.05 V versus Li+/Li in PC + VC, PC + FEC, and PC + ES, respectively, and covered the whole surface at lower potentials. No evidence for cointercalation of solvent molecules was observed in the presence of each additive. It was concluded that the layer of the precipitates functions as a protective surface film, which suppresses cointercalation of PC molecules as well as direct solvent decomposition on the surface of the graphite negative electrode.

Theoretical Study of Oxygen Adsorption on Graphite and the (8,0) Single-walled Carbon Nanotube

Abstract: Spin-polarized density functional calculations are used to study the adsorption of O atoms and O2 molecules on graphite and on a (8,0) single-walled carbon nanotube An O atom is found to bind to g

In situ multiphase fluid experiments in hydrothermal carbon nanotubes

Abstract: Hydrothermal multiwall closed carbon nanotubes are shown to contain an encapsulated multiphase aqueous fluid, thus offering an attractive test platform for unique in situ nanofluidic experiments in the vacuum of a transmission electron microscope. The excellent wettability of the graphitic inner tube walls by the aqueous liquid and the mobility of this liquid in the nanotube channels are observed. Complex interface dynamic behavior is induced by means of electron irradiation. Strong atomic-scale interactions between the entrapped liquid phase and the wetted terminated graphite layers are revealed by means of high-resolution electron microscopy. The documented phenomena in this study demonstrate the potential of implementing such tubes in future nanofluidic devices.

Carbon Nanofilaments in Heterogeneous Catalysis: An Industrial Application for New Carbon Materials?

Abstract: Special carbon! Carbon nanofilaments differ from graphite and soot catalysts in their high stability during the oxidative dehydrogenation of ethylbenzene to styrene. The high yields of styrene achieved suggest that a first industrial application of carbon nanofilaments in catalysis is possible.

Hydrogenation of carbon nanotubes and graphite in liquid ammonia

Abstract: We have prepared hydrogenated single-wall and multiwall carbon nanotubes, as well as graphite, via a dissolved metal reduction method in liquid ammonia. The hydrogenated derivatives are thermally stable up to 400 °C. Above 400 °C, a characteristic decomposition takes place accompanied with the simultaneous formations of hydrogen and a small amount of methane. Transmission electron micrographs show corrugation and disorder of the nanotube walls and the graphite layers due to hydrogenation. The average hydrogen contents determined from the yield of evolved hydrogen correspond to the compositions of C11H for both types of nanotubes and C5H for graphite. Hydrogenation occurred even on the inner tubes of multiwall nanotubes as shown by the chemical composition and the overall corrugation. The thermal stability and structural results suggest the formation of C−H bonds in nanotubes and graphite.

Hydrogen desorption property of mechanically prepared nanostructured graphite

Abstract: Two desorption peaks of hydrogen molecule (mass number=2), starting at about 600 and 950 K, respectively, are observed in thermal desorption mass spectroscopy of nanostructured graphite mechanically milled for 80 h under hydrogen atmosphere. It follows from a combined analysis of thermal desorption mass spectroscopy and thermogravimetry, that ∼6 mass % of hydrogen (corresponding to 80% of the total amount of hydrogen) is desorbed at the first desorption peak as a mixture of pure hydrogen and hydrocarbons. Below the temperature of the second desorption peak, at which recrystallization related desorption occurs, nanostructured graphite is expected to retain its specific defective structures mainly with carbon dangling bonds as suitable trapping sites for hydrogen storage. The formation process of the nanostructures during milling under hydrogen atmosphere is also discussed on the basis of the profile of Raman spectroscopy.

Surface Film Formation on Graphite Negative Electrode in Lithium-Ion Batteries: AFM Study in an Ethylene Carbonate-Based Solution

Abstract: In situ atomic force microscopic (AFM) observation of the basal plane of highly oriented pyrolytic graphite was performed during cyclic voltammetry at a slow scan rate of 0.5 mV in 1 mol dissolved in a mixture of ethylene carbonate and diethyl carbonate. In the potential range 1.0-0.8 V, atomically flat areas of 1 or 2 nm height (hill-like structures) and large swellings of 15-20 nm height (blisters) appeared on the surface. These two features were formed by the intercalation of solvated lithium ions and their decomposition beneath the surface, respectively, and may have a role in suppressing further solvent cointercalation. At potentials more negative than 0.65 V, particle-like precipitates appeared on the basal plane surface. After the first cycle, the thickness of the precipitate layer was 40 nm, and increased to 70 nm after the second cycle. The precipitates were considered to be mainly organic compounds that are formed by the decomposition of solvent molecules, and they have an important role in suppressing further solvent decomposition on the basal plane. © 2001 The Electrochemical Society. All rights reserved.

Preparation of polystyrene–graphite conducting nanocomposites via intercalation polymerization

Abstract: In situ polymerization of styrene was conducted in the presence of expanded graphite obtained by rapid heating of a graphite intercalation compound (GIC), to form a polystyrene–expanded graphite conducting composite. The composite showed excellent electrically conducting properties even though the graphite content was much lower than in normal composites. The transition of the composite from an electrical insulator to an electrical semiconductor occurred when the graphite content was 1.8 wt%, which is much lower than that of conventional conducting polymer composites. TEM, SEM and other studies suggest that the graphite was dispersed in the form of nanosheets in a polymer matrix with a thickness of 10–30 nm, without modification of the space between carbon layers and the structure of the graphite crystallites. The composite exhibited high electrical conductivity of 10−2 S cm−1 when the graphite content was 2.8–3.0 wt%. This great improvement of conductivity could be attributed to the high aspect ratio (width-to-thickness) of the graphite nanosheets. The rolling process strongly affected the conductivity and the mechanical properties of the composite. © 2001 Society of Chemical Industry

Intercalation of copper underneath a monolayer of graphite on Ni(111)

Abstract: Angle-resolved photoemission and scanning tunneling microscopy have been applied to study the intercalation of copper underneath a monolayer of graphite (MG) on Ni(111). The room-temperature deposition of copper on MG/Ni(111) in the coverage range 4--12 \AA{} leads to the growth of Cu islands on the MG. Annealing of the ``as-deposited'' system at a temperature of 400 \ifmmode^\circ\else\textdegree\fi{}C results in the intercalation of all Cu atoms underneath the MG. The intercalation of Cu is followed by a shift of the graphite-derived valence bands toward energies that are characteristic of pristine graphite. This observation is understood in terms of a weakening of chemical bonding between the MG and the substrate in the MG/Cu/Ni(111) system.

Effects of the Diffusion Layer Characteristics on the Performance of Polymer Electrolyte Fuel Cell Electrodes

Abstract: Several carbon blacks and graphite were investigated as candidates for diffusion layer preparation in polymer electrolyte fuel cell electrodes (PEFC). Single cell electrochemical characterizations under different working cell conditions were carried out on the electrodes by varying the kind of carbon in the diffusion layer. An improvement in cell performance was found by using Shawinigan Acetylene Black (SAB) as carbon, resulting in a measured power density of about 360 mW cm−2 in H2/air operation at 70°C and 1/1 bar. Pore size distribution and scanning electron microscopy analyses were carried out to help the understanding of the different behaviour of the investigated carbon diffusion layers.