Showing papers on "Hydrogen storage published in 1999"

Hydrogen Storage in Single-Walled Carbon Nanotubes at Room Temperature

Abstract: Masses of single-walled carbon nanotubes (SWNTs) with a large mean diameter of about 1.85 nanometers, synthesized by a semicontinuous hydrogen arc discharge method, were employed for hydrogen adsorption experiments in their as-prepared and pretreated states. A hydrogen storage capacity of 4.2 weight percent, or a hydrogen to carbon atom ratio of 0.52, was achieved reproducibly at room temperature under a modestly high pressure (about 10 megapascal) for a SWNT sample of about 500 milligram weight that was soaked in hydrochloric acid and then heat-treated in vacuum. Moreover, 78.3 percent of the adsorbed hydrogen (3.3 weight percent) could be released under ambient pressure at room temperature, while the release of the residual stored hydrogen (0.9 weight percent) required some heating of the sample. Because the SWNTs can be easily produced and show reproducible and modestly high hydrogen uptake at room temperature, they show promise as an effective hydrogen storage material.

Hydrogen adsorption and cohesive energy of single-walled carbon nanotubes

Abstract: Hydrogen adsorption on crystalline ropes of carbon single-walled nanotubes (SWNT) was found to exceed 8 wt.%, which is the highest capacity of any carbon material. Hydrogen is first adsorbed on the outer surfaces of the crystalline ropes. At pressures higher than about 40 bar at 80 K, however, a phase transition occurs where there is a separation of the individual SWNTs, and hydrogen is physisorbed on their exposed surfaces. The pressure of this phase transition provides a tube-tube cohesive energy for much of the material of 5 meV/C atom. This small cohesive energy is affected strongly by the quality of crystalline order in the ropes.

Costs of Storing and Transporting Hydrogen

Abstract: An analysis was performed to estimate the costs associated with storing and transporting hydrogen. These costs can be added to a hydrogen production cost to determine the total delivered cost of hydrogen. Storage methods analyzed included compressed gas, liquid hydrogen, metal hydride, and underground storage. Major capital and operating costs were considered over a range of production rates and storage times.

Hydrogen Adsorption in Carbon Materials

Abstract: Recent reports of very high, reversible adsorption of molecular hydrogen in pure nanotubes, alkali-doped graphite, and pure and alkali-doped graphite nanofibers (GNFs) have aroused tremendous interest in the research community, stimulating much experimental work and many theoretical calculations worldwide. The U.S. Department of Energy (DOE) Hydrogen Plan has seta standard for this discussion by providing a commercially significant benchmark for the amount of reversible hydrogen adsorption. This benchmark requires a system-weight efficiency (the ratio of stored H2 weight to system weight) of 6.5-wt% hydrogen and a volumetric density of 63 kg H2/m. If the encouraging experimental reports (summarized in Table I) are reproducible, it may be possible to reach the goals of the DOE Hydrogen Plan. On the other hand, the community still awaits confirmation of these experimental results by workers in other laboratories. Of additional concern is the fact that theoretical calculations have been unable to identify adsorption mechanisms compatible with the requirements of the DOE Hydrogen Plan.An economical, safe, hydrogen-storage medium is a critically needed component of a hydrogen-fueled transportation system. Hydrogen storage in a carbon-based material offers further advantages associated with its low mass density. Furthermore, fuel cell technology involving the conversion of hydrogen into protons, or hydrogen and oxygen into electric current, is being vigorously researched for both transportation and small power-plant applications.

Further Studies of the Interaction of Hydrogen with Graphite Nanofibers

Abstract: Catalytically grown graphite nanofibers (GNF) are molecularly engineered structures that are produced by the interaction of carbon-containing gases with small metal particles at temperatures around 600 °C. The fibrous solids consist of minuscule graphene sheets stacked at various angles with respect to the fiber axis. This arrangement generates a material possessing unique chemical properties because unlike conventional graphite crystals, only edges are exposed. Such a conformation produces a material composed entirely of nanopores that can accommodate small-sized adsorbate molecules, such as hydrogen, in the most efficient manner. In addition, the nonrigid pore walls can expand to accommodate the gas in a multilayer conformation. GNF exhibit extraordinary behavior toward the sorption and retention of hydrogen at high pressures and abnormally high temperatures. In this paper we discuss some of the critical factors involved in the adsorption of molecular hydrogen and the influence that this process exerts ...

Optimization of Carbon Nanotube Arrays for Hydrogen Adsorption

Abstract: The amount of hydrogen adsorbed in arrays of single-walled carbon nanotubes has been studied as a function of the geometry of the array. The tube lattice spacing has been varied to optimize hydrogen uptake. Two different lattice geometries have been examined, namely, the triangular lattice and the square lattice. None of the geometries studied are capable of achieving adequate hydrogen storage capacity for use in vehicular fuel cells at room temperature. The strength of the solid−fluid interaction potential has been increased in order to identify a combination of potential and geometry that will meet the DOE targets for hydrogen storage for fuel cell vehicles. The DOE target values cannot be reached even by tripling the fluid-wall potential at ambient temperature. However, it is possible to achieve the DOE targets at a temperature of 77 K, but only if the strength of the interaction potential is increased by about a factor of 2 and the lattice spacing of the tubes is optimized. On the basis of these obser...

Hydrogen Storage Alloys with PuNi3 ‐ Type Structure as Metal Hydride Electrodes

Abstract: A powder sintering method was used to synthesize the intermetallic compounds , , , and . The microstructure and primary phases were observed by scanning electron microscopy and X‐ray diffraction. The pressure‐composition isotherms showed that all alloys could reversibly absorb and desorb up to hydrogen at and a hydrogen pressure of . The sintered samples were employed as the active materials of metal hydride electrodes. The hydride stability and electrochemical performance, combined with low cost raw materials, make these compounds attractive for metal hydride electrodes. ©2000 The Electrochemical Society

Recent developments in the applications of nanocrystalline materials to hydrogen technologies

Abstract: The paper discusses the application of nanocrystalline alloys as hydrogen storage materials and as electrocatalysts for solid polymer electrolyte fuel cells. After reviewing some of the requirements of metal hydrides for hydrogen fueled vehicles, the paper presents new results on the structure and hydrogen sorption properties of high storage capacity ball-milled magnesium hydride. The great advantages of milling the hydride instead of the pure metals for producing novel nanostructures with high surface area and for improving hydrogen sorption kinetics are presented. In the second part of the paper, the same technique has been extended to the milling of carbides and chlorides and coupled to a lixiviation process to produce new electrocatalysts for polymer electrolyte fuel cells. This new technology offers the possibility of producing nanoparticles with metastable structures whose specific surface area is much larger than that of any nanocrystalline powders made by conventional ball milling techniques. Pt-based nanoparticles were fabricated and tested as anode in fuel cells under pure and CO-contaminated hydrogen feedstreams.

Fuel cell system with hydrogen purification subsystem

Abstract: The invention relates to a fuel cell system with a hydrogen purification subsystem The hydrogen purification subsystem can concentrate hydrogen from the fuel exhaust for recirculation or storage The hydrogen purification subsystem can also concentrate hydrogen from a fuel supply for input into a fuel cell or for storage The hydrogen purification subsystem can also concentrate hydrogen for quantitative comparison with a second stream containing hydrogen The hydrogen purification subsystem can also charge a hydrogen storage device for system use such as meeting transient fuel cell load increases

An Overview of Hydrogen Interaction with Amorphous Alloys

Abstract: Theories, experimental results and applications associated with hydrogen behavior in amorphous metals and alloys are reviewed An emphasis is made on the potential use of these advanced materials for hydrogen storage technology Therefore, several properties that are especially relevant for such applications are assessed These include structural models for hydrogen occupancy, sorption characteristics, solubility, diffusion behavior and thermal stabilities Hydrogen effects on the mechanical properties and fracture modes of glassy metals are also presented, and possible mechanisms of hydrogen embrittlement are discussed Similarities and differences between hydrogen behavior in amorphous and crystalline metals and alloys are discussed in detail

Molecular modeling of adsorptive energy storage: Hydrogen storage in single-walled carbon nanotubes

Abstract: In this paper, density functional theory is used to estimate hydrogen adsorption in a novel carbonaceous material, single-walled carbon nanotubes. An idealized adsorbent structure for the nanotubes is assumed. We have mapped out the regime of operating pressures and temperatures where an adsorption-based storage system is expected to deliver more hydrogen than a similar system of compressed gas. This regime is also a function of pore size. We have calculated the overall hydrogen volumetric and gravimetric density within the framework of a typical high-pressure gas storage system. Within the regime of operating conditions where adsorptive storage seems attractive, the storage properties of hydrogen in a carbon nanotube system appear to fall far short of the targets of 62 kg of H2/m3 and 6.5 wt % H2 set by the Department of Energy. The computed gravimetric storage densities also fall short of those reported in the literature (Nature 1997, 386, 377). We discuss several possible mechanisms by which higher gra...

Hydrogen Storage in Magnesium-Based Alloys

Abstract: Magnesium can reversibly store about 7.7 wt% hydrogen, equivalent to more than twice the density of liquid hydrogen. This high storage capacity, coupled with a low price, suggests that magnesium and magnesium alloys could be advantageous for use in battery electrodes and gaseous-hydrogen storage systems. The use of a hydrogen-storage medium based on magnesium, combined with a fuel cell to convert the hydrogen into electrical energy, is an attractive proposition for a clean transportation system. However, the advent of such a system will require further research into magnesium-based alloys that form less stable hydrides and proton-conducting membranes that can raise the operating temperature of the current fuel cells.Following the U.S. oil crisis of 1974, research into alternative energy-storage and distribution systems was vigorously pursued. The controlled oxidation of hydrogen to form water was proposed as a clean energy system, creating a need for light and safe hydrogen-storage media. Extensive research was done on inter-metallic alloys, which can store hydrogen at densities of about 1500 cm3-H2 gas/ cm3-hydride, higher than the storage density achieved in liquid hydrogen (784 cm3/cm3 at –273°C) or in pressure tanks (˜200 cm3/cm3 at 200 atm). The interest in metal hydrides accelerated following the development of portable electronic devices (video cameras, cellular phones, laptop computers, tools, etc.), which created a consumer market for compact, rechargeable batteries. Initially, nickel-cadmium batteries fulfilled this need, but their relatively low energy density and the toxicity of cadmium helped to drive the development of higher-energy-density, less toxic, rechargeable batteries.