Metal hydride materials for solid hydrogen storage: a review

The poor performance of hydrogen storage materials continues to hinder development of fuel cell-powered automobiles. Nanoscale carbons, in particular (activated carbon, exfoliated graphite, fullerenes, nanotubes, nanofibers, and nanohorns), have not fulfilled their initial promise. Here we show that carbon materials can be rationally designed for H2 storage. Carbide-derived carbons (CDC), a largely unknown class of porous carbons, are produced by high-temperature chlorination of carbides. Metals and metalloids are removed as chlorides, leaving behind a collapsed noncrystalline carbon with up to 80% open pore volume. The detailed nature of the porosity-average size and size distribution, shape, and total specific surface area (SSA)-can be tuned with high sensitivity by selection of precursor carbide (composition, lattice type) and chlorination temperature. The optimum temperature is bounded from below by thermodynamics and kinetics of chlorination reactions and from above by graphitization, which decreases SSA and introduces H2-sorbing surfaces with binding energies too low to be useful. Intuitively, pores of different size and shape should not contribute equally to hydrogen storage. By correlating pore properties with 77 K H2 isotherms from a wide variety of CDCs, we experimentally confirm that gravimetric hydrogen storage capacity normalized to total pore volume is optimized in materials with primarily micropores ( approximately 1 nm) rather than mesopores. Thus, in agreement with theoretical predictions, a narrow size distribution of small pores is desirable for storing hydrogen, while large pores merely degrade the volumetric storage capacity.

Tailoring of nanoscale porosity in carbide-derived carbons for hydrogen storage.

A breakthrough in hydrogen storage technology was achieved by preparing nanocrystalline hydrides using high-energy ball milling and the use of suitable catalysts/additives. These new materials show fast or in case of Mg-based hydrides very fast absorption and desorption kinetics within minutes, thus qualifying lightweight Mg- or Al-based hydrides for storage applications. This article summarizes our current understanding of the kinetics of Mg-based light metal hydrides, describes an approach for a cost-effective processing technology and highlights some promising new developments in lightweight metal hydride research.

Tailoring hydrogen storage materials towards application

Rechargeable metal hydrides are being used for a wide variety of applications: mobile and stationary hydrogen storage, hydrogen purification, hydrogen recovery and separation, thermal compression, chemical heat pumps and refrigeration. Successful selection of metal hydrides for these and other proposed applications requires the knowledge of many engineering properties. In many cases these data are scattered widely in the literature or are simply not available.

In this paper we bring together engineering properties for three classes of hydride-forming alloys (AB, AB5 and A2B) manufactured and marketed by Ergenics under the HY-STOR trade name. The properties discussed are the plateau pressure-temperature relationship, hysteresis, the plateau slope, the hydrogen capacity and the heat capacity.

Engineering properties of metal hydrides

A group of twenty-nine scientists and engineers convened a 'Workshop on Shielding Strategies for Human Space Exploration' at the Lyndon B. Johnson Space Center in Houston, Texas. The provision of shielding for a Mars mission or a Lunar base from the hazards of space radiations is a critical technology since astronaut radiation safety depends on it and shielding safety factors to control risk uncertainty appear to be great. The purpose of the workshop was to define requirements for the development and evaluation of high performance shield materials and designs and to develop ideas regarding approaches to radiation shielding. The workshop was organized to review the recent experience on shielding strategies gained in studies of the 'Space Exploration Initiative (SEI),' to review the current knowledge base for making shield assessment, to examine a basis for new shielding strategies, and to recommend a strategy for developing the required technologies for a return to the moon or for Mars exploration. The uniqueness of the current workshop arises from the expected long duration of the missions without the protective cover of the geomagnetic field in which the usually small and even neglected effects of the galactic cosmic rays (GCR) can no longer be ignored. It is the peculiarity of these radiations for which the inter-action physics and biological action are yet to be fully understood.

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Shielding Strategies for Human Space Exploration

The storage of hydrogen as a metal hydride is discussed. Properties of some representative hydrogen-storage mediums (MgH/sub 2/, Mg/sub 2/NiH/sub 4/, VH/sub 2/, FeTiH/sub 1/ /sub 95/, LaNi/sub 5/H/sub 7/, liquid hydrogen, and gaseous hydrogen) are compared. Magnesium hydride, the more promising of the two binary hydrides, is thought to be only a borderline possibility as a hydrogen-storage medium at present. Of the three ternary hydrides listed the leading contender is iron-titanium hydride. It's main advantage over lanthanum-pentanickel hydride is one of cost. All the metal hydrides listed have a higher hydrogen storage capacity than an equal volume of liquid or gaseous hydrogen has. Energy densities (watt-hours per kilogram) of various automotive power sources either already in existence or proposed are tabulated. Metal hydrides lag far behind gasoline in terms of energy density, but they are competitive with electric batteries in this respect.

Gary D. Sandrock

Papers

Hydrogen storage in metal hydrides