Showing papers by "Yury Gogotsi published in 2008"

Materials for electrochemical capacitors

Abstract: Electrochemical capacitors, also called supercapacitors, store energy using either ion adsorption (electrochemical double layer capacitors) or fast surface redox reactions (pseudo-capacitors). They can complement or replace batteries in electrical energy storage and harvesting applications, when high power delivery or uptake is needed. A notable improvement in performance has been achieved through recent advances in understanding charge storage mechanisms and the development of advanced nanostructured materials. The discovery that ion desolvation occurs in pores smaller than the solvated ions has led to higher capacitance for electrochemical double layer capacitors using carbon electrodes with subnanometre pores, and opened the door to designing high-energy density devices using a variety of electrolytes. Combination of pseudo-capacitive nanomaterials, including oxides, nitrides and polymers, with the latest generation of nanostructured lithium electrodes has brought the energy density of electrochemical capacitors closer to that of batteries. The use of carbon nanotubes has further advanced micro-electrochemical capacitors, enabling flexible and adaptable devices to be made. Mathematical modelling and simulation will be the key to success in designing tomorrow's high-energy and high-power devices.

Relation between the ion size and pore size for an electric double-layer capacitor.

Abstract: The research on electrochemical double layer capacitors (EDLC), also known as supercapacitors or ultracapacitors, is quickly expanding because their power delivery performance fills the gap between dielectric capacitors and traditional batteries. However, many fundamental questions, such as the relations between the pore size of carbon electrodes, ion size of the electrolyte, and the capacitance have not yet been fully answered. We show that the pore size leading to the maximum double-layer capacitance of a TiC-derived carbon electrode in a solvent-free ethyl-methylimmidazolium-bis(trifluoro-methane-sulfonyl)imide (EMI-TFSI) ionic liquid is roughly equal to the ion size (∼0.7 nm). The capacitance values of TiC−CDC produced at 500 °C are more than 160 F/g and 85 F/cm3 at 60 °C, while standard activated carbons with larger pores and a broader pore size distribution present capacitance values lower than 100 F/g and 50 F/cm3 in ionic liquids. A significant drop in capacitance has been observed in pores that w...

Desolvation of ions in subnanometer pores and its effect on capacitance and double-layer theory.

Abstract: The study of charged solid–liquid interfaces, manifested as “double layers”, represents a problem of both practical and scientific importance. Double layers are present in all electrolyte solutions and have been traditionally studied using planar noble-metal electrodes and mercury drops. However, in the ionic channels in cells or the small-diameter pores of electrochemical double-layer capacitors (EDLCs),ions are in a very confined situation, which is different from that of a planar solid/electrolyte interface. By using nanoporous carbon with pores smaller than the size of an ion and a single associated solvent molecule, we show that the implicit assumption that double layers are governed only by ion/ electrode charge separation may be short-sighted. Other factor may play a more dominant role than previously thought, for example, increasing the confinement of the ions leads to an increase in the capacitance. Including the effect of partially desolvating ions in the current double-layer theory could lead to a better understanding of the behavior of ions in confined environments.

Review : static and dynamic behavior of liquids inside carbon nanotubes

Abstract: This review deals with the static and dynamic behavior of liquids inside carbon nanotubes, a broad subject, which includes: the investigation of liquid entering inside the tubes, and the subsequent filling of them, the overall flow through tubes as well as the wetting of the nanotube walls. Although most of the numerical work has been done on small diameter nanotubes, due to computational limitations, a large wealth of experimental results have been obtained for larger diameter nanotubes, between 10 and 100 nm, or above. This review offers an overview of the major achievements in the field, with particular emphasis on the effect on liquid flow through them of the structure and chemistry of carbon nanotubes. The limit below which liquids confined in nanotubes will not obey classical fluid dynamics equations as well as the structure and state of liquids under confinement will also be reviewed. Finally, near-to commercialization and still-in-the-lab applications will be covered.

Effect of Carbon Particle Size on Electrochemical Performance of EDLC

Abstract: The effect of particle size on the electrochemical performance of electrical double-layer capacitors (EDLCs) has been studied using carbon derived from silicon carbide powders with 20 nm to 20 μm grains at temperatures from 800 to 1200°C. For the same synthesis temperature, similar pore texture and microstructure of carbide-derived carbons produced from different powders have been observed. Nanoparticles exhibited a slight porosity modification with a larger pore volume (1.8 cc/g) and surface area (1300 m 2 /g) as compared to micrometer particles (0.4 cc/g and 1100 m 2 /g). Capacitances as high as 135 F/g associated with a small resistance and time constant have been reached for nano- and sub-micrometer particles synthesized at low temperatures and tested in a tetraethylammonium tetrafluoroborate in acetonitrile solution. This result suggests that small particles facilitate the migration of the ions inside the porous electrodes, allowing them to reach the whole pore volume due to a shorter transport distance within the particle.

SERS intensity optimization by controlling the size and shape of faceted gold nanoparticles

Abstract: In this work, we experimentally investigated the surface-enhanced Raman spectroscopy (SERS) activity of faceted gold nanoparticles, which have been theoretically predicted to yield giant enhancements. Glycine was used to determine the SERS activity as a function of pH and ionic strength and to estimate the corresponding enhancement factor (EF). By optimizing the synthesis conditions of the flat prismatic nanoparticles, it was possible to control their size and shape. We demonstrate that the maximum SERS intensity increases with the edge length of the triangle, reaching a maximum EF of ∼1013 for 1.9 µm triangles (the largest tested). The corresponding glycine detection limit was as low as 10−12M, close to the single-molecule threshold. Copyright © 2007 John Wiley & Sons, Ltd.

Temperature dependence of silicon hardness: Experimental evidence of phase transformations

Abstract: The hardness of silicon is known to be nearly independent of temperature below a certain transition point, and to decrease steeply thereafter Based on high-temperature Berkovich nanoindentation at 25–500 °C and Raman microanalysis of Vickers indentations produced in singlecrystal silicon at 25–750 °C, we present evidence of a transformation into a high-pressure metallic Si phase during indentation at temperatures up to about 350 °C We show that this transformation pressure determines silicon hardness below the transition temperature We also report the temperature stability ranges of different metastable phases of silicon, including a new Si-XIII phase

Magnetostatic interactions between carbon nanotubes filled with magnetic nanoparticles

Abstract: The magnetostatic interactions between carbon nanotubes filled with magnetic particles have been experimentally and theoretically studied. By making nanotubes uniformly magnetized, one eliminates the attraction caused by periodicity of nanoparticles in magnetic chains. The discreteness of individual nanoparticles in the nanoneedles is not observed and these nanoneedles interact by their magnetic poles. Since the attraction/repulsion events are predictable, the suspensions of magnetic nanotubes are attractive candidates for active elements in changeable diffraction gratings, filters, and polarizers.

Carbon nanopipettes characterize calcium release pathways in breast cancer cells.

Abstract: Carbon-based nanoprobes are attractive for minimally invasive cell interrogation but their application in cell physiology has thus far been limited. We have developed carbon nanopipettes (CNPs) with nanoscopic tips and used them to inject calcium-mobilizing messengers into cells without compromising cell viability. We identify pathways sensitive to cyclic adenosine diphosphate ribose (cADPr) and nicotinic acid adenine dinucleotide phosphate (NAADP) in breast carcinoma cells. Our findings demonstrate the superior utility of CNPs for intracellular delivery of impermeant molecules and, more generally, for cell physiology studies. The CNPs do not appear to cause any lasting damage to cells. Their advantages over commonly used glass pipettes include smaller size, breakage and clogging resistance, and potential for multifunctionality such as in concurrent injection and electrical measurements.

Laser-induced light emission from carbon nanoparticles

Abstract: Strong absorption of light in a broad wavelength range and poor thermal conductance between particles of carbon nanomaterials, such as nanotubes, onions, nanodiamond, and carbon black, lead to strong thermal emission (blackbody radiation) upon laser excitation, even at a very low (milliwatts) power. The lasers commonly used during Raman spectroscopy characterization of carbon can cause sample heating to very high temperatures. While conventional thermometry is difficult in the case of nanomaterials, Raman spectral features, such as the G band of graphitic carbon and thermal emission spectra were used to estimate the temperature during light emission that led to extensive graphitization and evaporation of carbon nanomaterials, indicating local temperatures exceeding 3500 °C.

Field controlled nematic-to-isotropic phase transition in liquid crystal–carbon nanotube composites

Abstract: A nematic-to-isotropic transition has been observed in suspensions of carbon nanotubes (CNTs) and a cyanobiphenyl-based liquid crystal (LC) confined within an indium tin oxide glass sandwich cell. Upon the application of electric field, CNTs rotate out of plane short-circuiting the electrodes and producing a current flow through the CNTs. The resulting Joule heating leads to a local increase in temperature of the LC-CNT medium. Hence, starting from a metastable nematic phase, a complete transition to the isotropic phase is observed. On removal of the electric field, the transition is reversed with the LC-CNT medium returning to the nematic phase.

Nanodiamond compositions and methods of making and using thereof

Abstract: Provided are functionalized nanodiamonds. Also provided are methods for fabricating such functionalized nanodiamonds. Also provided are composites including nanodiamonds and polymers. Also provided are methods for fabricating such composites including nanodiamonds and polymers. Also provided are electrospun fibers including nanodiamonds and polymers. Also provided are methods for fabricating such electrospun fibers including nanodiamonds and polymers.

Polyamide nanofibers and methods thereof

Abstract: Provided are methods for dissolving polyamides under ambient conditions and for forming polyamide nanofibers by electrospinning. Also provided are methods for incorporating nanoparticles, including nanotubes, into such nanofibers.

Electrocatalysts for fuel cells

Abstract: Disclosed are metallized carbonaceous materials, processes for forming such materials, and electrodes and fuel cells comprising the disclosed materials.

Bactericidal activity of chlorine‐loaded carbide‐derived carbon against Escherichia coli and Bacillus anthracis

Abstract: The authors investigated the bactericidal activity of high-chlorine-content nanoporous carbide-derived carbon (CDC) against the Gram-positive, spore-forming bacterium Bacillus anthracis and the common Gram-negative enteric bacterium Escherichia coli. Chlorine-loaded nanoporous CDC produced by thermochemical etching of metals and metalloids by chlorination of carbides can retain up to 40 wt % of chlorine. Etching temperature and the structure and composition of carbides allow tuning the porosity of CDC. The CDC chlorine content depends on the synthesis temperature, pore size, and metal carbide used during preparation. It was observed that chlorine-loaded CDC killed up to 100% of exposed E. coli and B. anthracis spores and vegetative cells in a dose and time-dependent manner. CDC containing higher concentrations of chlorine killed bacteria to a greater extent and faster than did CDC containing lesser concentrations of chlorine. The results suggest that chlorine-loaded CDC can be used in several commercial, defense, and industrial activities and processes to kill bacteria.

Smoothing of nanoscale roughness based on the Kelvin effect

Abstract: A novel method of smoothing surfaces with nanoscale roughness is described, based on the Kelvin effect. The problem of vapor redistribution in cylindrical channels and over rough planar walls with nanoscale texture is posed and solved analytically. Vapor deposition (condensation) on the walls initially produces a deposit emulating the surface landscape. After a saturated state at the deposit surface is reached, the Kelvin effect should result in higher vapor pressure/ concentration near the convex sections of the wall and in lower vapor pressure/ concentration near the concave sections. As a result, local vapor fluxes should arise directed from the locally convex to the locally concave regions. Accordingly, the deposited layer at the wall should vaporize (or sublimate) at the convex sections due to depletion and vapor should condense at the concave sections, thus causing smoothing of physical surface unevenness. This mechanism of smoothing of nanoscale roughness has not been considered in detail or used before, even though the basic physics of the Kelvin effect is well known. In the present work, the smoothing kinetics is predicted and the characteristic timescales are calculated in the general case of axisymmetric and non-axisymmetric perturbations of the cylindrical channel walls, as well as for planar surfaces. In addition, experimental data are presented to show that the theoretically motivated approach is also practically realizable.