P. Sudan

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.

Thermodynamic aspects of the interaction of hydrogen with Pd clusters

Abstract Clusters are agglomerates of a few to some hundred atoms. A large fraction of the atoms find themselves on a surface site. The geometrical structure of a cluster, in contrast to the structure of crystalline bulk material, cannot be described with a repeated, space filling unit cell. Clusters appear often in regular geometrical polyhedrons, e.g. cuboctahedron or icosahedron. The interaction between hydrogen and palladium clusters of different sizes was investigated. The sorption properties of the clusters are compared with the well-known properties of Pd bulk material. The Pd clusters reversibly absorb and desorb hydrogen. The miscibility gap (plateau in the pressure concentration isotherm) narrows with decreasing cluster size and therefore, an increased hydrogen solubility was found for Pd clusters as compared to bulk Pd. The hysteresis between the absorption and the desorption pressure decreases with decreasing cluster size.

Hydrogen adsorption on a single-walled carbon nanotube material: a comparative study of three different adsorption techniques

Hydrogen adsorption measurements on mixed carbon material containing single-walled carbon nanotubes (SWNTs) obtained by the electric-arc method were carried out by three different techniques: a volumetric system, a gravimetric system and an electrochemical method. The found H2 gas adsorption capacity (volumetric and gravimetric) is very low, around 0.01 wt% at room temperature and pressure, increasing to 0.1 wt% at 20 bar. Electrochemical measurements show a slightly higher capacity (0.1–0.3 wt%) than volumetric and gravimetric data. The results obtained by the three different techniques are compatible within each other and they are also in good agreement with other previously reported data from different researchers.

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

Carbon nanostructures: Growth, electron emission, interactions with hydrogen

We introduce the concept of the field enhancement distribution function f(β) for a useful characterization of the field emission properties of thin film emitter and show how this distribution function can be measured by scanning anode field emission microscopy. Using f(β) measured on a thin film of randomly oriented multiwalled carbon nanotubes we show that even these kinds of low cost emitters can show a field emission performance comparable to micro-tip arrays. However the large spread in field enhancement values between the individual emitter prevent this performance to be fully exploited. This because the field-range in which such thin film emitters can be operated is limited due to emitter disruption and triggering of vacuum arcs. Further we briefly discuss the possibilities of using carbon nanostructures for hydrogen storage applications.