Ph. Mauron

Hydrogen storage in carbon nanostructures

Abstract Carbon nanotubes have been known for more than 10 years. It is a challenge to fill their unique tubular structure with metals and gases. Especially, the absorption of hydrogen in single wall nanotubes has attracted many research groups worldwide. The values published for the quantity of hydrogen absorbed in nanostructured carbon materials varies between 0.4 and 67 mass%. With the assumption that the hydrogen condenses in the cavity of the nanotube or forms an adsorbed monolayer of hydrogen at the surface of the tube, the potential of nanotubes as a host material for hydrogen storage can be estimated. The hydrogen storage density due to condensed hydrogen in the cavity of the tube depends linearly on the tube diameter and starts at 1.5 mass% for a 0.671 nm single wall carbon nanotube. The surface adsorption of a monolayer of hydrogen leads to a maximum storage capacity of 3.3 mass%. We have investigated a large number of nanostructured carbon samples, i.e. high surface area graphite, single wall and multiwall nanotubes, by means of volumetric gas adsorption, galvanostatic charge/discharge experiments and temperature programmed desorption spectroscopy. The reversible hydrogen capacity of the carbon samples measured in an electrochemical half-cell at room temperature correlates with the specific surface area (BET) of the sample and is 1.5 mass% /1000 m 2 / g .

Investigation of electrochemical double-layer (ECDL) capacitors electrodes based on carbon nanotubes and activated carbon materials

The carbon nanotubes (CNT) show promising electrochemical characteristics particularly for electrochemical energy storage. The electrochemical double-layer (ECDL) capacitor is a new type of capacitor with features intermediate between those of a battery and a conventional capacitor. ECDL capacitors have been made using various types of CNT and activated carbon (a-C) as electrode material. The specific capacitance per surface area of the electrodes depends on the thickness and the specific surface area of the active material. The CNT electrodes show a specific capacitance from 0.8 and 280 mF cm −2 and 8t o 16 Fc m −3 , respectively. Increasing the mass density also helps to increase the capacitance. Commercially available activated carbon (a-C) electrodes were also tested in order to study their specific capacitance as a function of their physical properties. The various a-C electrodes have specific capacitance per surface area ranging from 0.4 to 3.1 F cm −2 and an average specific capacitance per volume of 40 F cm −3 due to their larger mass density.

Hydrogen sorption by carbon nanotubes and other carbon nanostructures

Abstract We have analyzed the hydrogen storage capability of a set of carbon samples including a variety of carbon nanotubes, in the gas phase and in the electrolyte as well. The nanotube samples synthesized in our laboratory by pyrolysis of acetylene are of the multi-wall type. The hydrogen sorption properties of our synthesized nanotubes were compared with the properties of commercially available nanotubes and high surface area graphite as well. The nanotube samples and the high surface area graphite as well absorb hydrogen up to 5.5 mass% at cryogenic temperatures (77 K). However, at room temperatures this value drops to ≈0.6 mass%. The electrochemical experiments on the carbon samples showed a maximum discharge capacity of 2.0 mass% at room temperature (298 K). The hydrogen tends to covalently bind to carbon when the absorption takes place at elevated temperatures (>573 K). Therefore, hydrocarbons desorbed from the sample were analyzed by means of temperature programmed desorption measurements. We conclude that the adsorption of hydrogen on nanotubes is a surface phenomenon and is similar to the adsorption of hydrogen on high surface area graphite.

Physisorption of hydrogen in single-walled carbon nanotubes

Abstract The interaction of hydrogen with single-walled carbon nanotubes (SWNTs) was analysed. A SWNT sample was exposed to D 2 or H 2 at a pressure of 2 MPa for 1 h at 298 or 873 K. The desorption spectra were measured by thermal desorption spectroscopy (TDS). A main reversible desorption site was observed throughout the range 77 to 320 K. The activation energy of this peak at about 90 K was calculated assuming first-order desorption. This corresponds to physisorption on the surface of the SWNTs (19.2±1.2 kJ/mol). A desorption peak was also found for multi-walled carbon nanotubes (MWNTs), and also for graphite samples. The hydrogen desorption spectrum showed other small shoulders, but only for the SWNT sample. They are assumed to originate from hydrogen physisorbed at sites on the internal surface of the tubes and on various other forms of carbon in the sample. The nanosized metallic particles (Co:Ni) used for nanotube growth did not play any role in the physisorption of molecular hydrogen on the SWNT sample. Therefore, it is concluded that the desorption of hydrogen from nanotubes is related to the specific surface area of the sample.

Synthesis of oriented nanotube films by chemical vapor deposition

Abstract Oriented nanotube films (20–35 μm thick) were synthesised on flat silicon substrates by chemical vapor deposition (CVD) of a gas mixture of acetylene and nitrogen. For the CVD we used metal oxide clusters formed by spin coating an iron(III) nitrate ethanol solution onto a silicon substrate and subsequent heating. The cluster density and its effects on the nanotube density were investigated as a function of the iron(III) nitrate concentration and the synthesis temperature. A high nanotube density was achieved with a high density of iron oxide clusters as nucleation centres for the growth of nanotubes. The cluster density was controlled by the iron(III) concentration of the ethanolic coating solution and by the synthesis temperature. The perpendicular orientation of the nanotubes with respect to the substrate surface is attributed to a high density of nanotubes.

Metal nanoparticles for the production of carbon nanotube composite materials by decomposition of different carbon sources

Carbon nanotube composite materials were produced by catalytic decomposition of gaseous carbon sources (such as carbon monoxide or hydrocarbons) on nanometer-size metal clusters of iron, cobalt and nickel embedded in matrices of inert metal oxide particles. The resulting multiwalled carbon nanotubes are several micrometers long with tube diameters ranging from 5 to 20 nm. A fluidised bed reactor was developed for a large-scale synthesis of the carbon nanotube/metal oxide composite (CMC) material. Hydrogen storage capacities of these materials were tested by volumetric and electrochemical methods.

Solid-state synthesis of LiBD4 observed by in situ neutron diffraction

The synthesis of Li[(11)BD(4)] from LiB and D(2) (p = 180 bar) is investigated by in situ neutron diffraction. The onset of the Li[(11)BD(4)] formation is observed far below the temperatures reported so far for the reaction from the pure elements, indicative of a lower activation barrier. We attribute the improved formation behavior to the breaking of the rigid boron lattice and intermixing of the elements on an atomic level when forming the binary compound LiB. The reaction starts with the decomposition of the initial LiB compound and the formation of LiD. At 623 K LiBD(4) starts to form. However, under the given experimental conditions (maximal temperature = 773 K) a complete reaction was not achieved; there is still residual LiD present.

Hydrogen Interaction with Carbon Nanostructures

Note: Times Cited: 06th International Symposium on Chemical and Electrochemical Reactivity of Amorphous and Nanocrystalline MaterialsFeb 07-09, 2001Mt tremblant, canadaHydro Quebec; Minist Rech, Sci & Technol; McGill Univ, Ctr Phys Mat; HQ CapiTech Reference EPFL-CHAPTER-206095View record in Web of Science URL: ://WOS:000172513700011 Record created on 2015-03-03, modified on 2017-12-03

High-pressure and high-temperature x-ray diffraction cell for combined pressure, composition, and temperature measurements of hydrides

We present the design and construction of a high-pressure (200 bars) and high-temperature (600 °C) x-ray diffraction (XRD) cell for the in situ investigation of the hydrogen sorption of hydrides. In combination with a pressure, composition, and temperature system, simultaneous XRD and volumetric measurements become accessible. The cell consists of an x-ray semi-transparent hemispherical beryllium (Be) dome covering a heatable sample stage, which simultaneously allows sample temperatures of up to 600 °C in an applied hydrogen atmosphere of up to 200 bars. The system volume is as low as possible to maximize the precision of the volumetric measurements. Due to the high thermal conductivity of hydrogen, and in order to preserve the mechanical stability of the beryllium, the cell is water cooled. Its operability was studied on the example of the hydrogen absorption of Mg(2)Ni. The advantages and limitations of the proposed design are discussed.

Towards room temperature, direct, solvent free synthesis of tetraborohydrides

Due to their high hydrogen content, tetraborohydrides are discussed as potential synthetic energy carriers. On the example of lithium borohydride LiBH4, we discuss current approaches of direct, solvent free synthesis based on gas solid reactions of the elements or binary hydrides and/or borides with gaseous H2 or B2H6. The direct synthesis from the elements requires high temperature and high pressure (700?C, 150bar D2). Using LiB or AlB2 as boron source reduces the required temperature by more than 300 K. Reactive milling of LiD with B2H6 leads to the formation of LiBD4 already at room temperature. The reactive milling technique can also be applied to synthesize other borohydrides from their respective metal hydrides.