Metallic Hydrides I: Hydrogen Storage and Other Gas- Phase Applications
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Metallic Hydrides I:
Hydrogen Storage and Other GasPhase Applications
Robert C. Bowman Jr. and Brent Fultz Abstract A brief survey is given of the various classes of metal alloys and compounds that are suitable for hydrogen-storage and energy-conversion applications. Comparisons are made of relevant properties including hydrogen absorption and desorption pressures, total and reversible hydrogen-storage capacity, reaction-rate kinetics, initial activation requirements, susceptibility to contamination, and durability during long-term thermal cycling. Selected applications are hydrogen storage as a fuel, gas separation and purification, thermal switches, and sorption cryocoolers. Keywords: hydrogen storage, metal hydrides, neutron scattering.
Introduction In the late 1960s, it was discovered that certain intermetallic compounds (e.g., Mg2Ni, LaNi5, and TiFe) would directly and reversibly react with hydrogen gas at practical temperatures (i.e., 250–650 K). These observations, coupled with the petroleum embargoes and related energy crises of the 1970s, stimulated extensive investigations on metal hydrides for energy storage and conversion.1,2 While many intermetallic hydrides can absorb substantial quantities of hydrogen (i.e., hydrogento-metal atom ratios 1), most of these alloys composed of rare-earth or transition metals have a maximum storage of about 1–2% hydrogen by weight. Alloys of Mg are an exception, having hydrogen capacities in the range of 3.3–7.7 wt%.3,4 Hundreds of intermetallic alloys have been screened for hydrogen-storage potential5,6 with respect to the following criteria: reversible hydrogen capacity, operating pressure/temperature range, reaction kinetics, degradation after repeated cycling of hydrogen, and cost. The search continues for the “Holy Grail” of hydrides that will excel at all of these requirements. Recently, this quest has been
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expanded to include light-metal complex hydrides such as catalyzed alanates6 (ternary compounds containing AlH4) and nanostructured carbon materials, as described in the article by Bogdanovic´ and Sandrock in this issue of MRS Bulletin.
Properties of Metal Hydrides The desired reaction for a generic intermetallic alloy ABz with hydrogen gas is ABz xH2 ABzH2x Q,
(1)
where Q is the heat released upon absorption of hydrogen. Typically, metal A forms stable binary hydrides, that is, A is an early transition metal, rare-earth metal, or Mg. Metal B (e.g., Ni, Co, Cr, Fe, Mn, or Al) does not form stable hydrides, although it may help dissociate the H2 molecule. Table I describes representative hydrides from the five metal hydride families (i.e., A, A2B, AB, AB2, and AB5) that have shown the most promise6 for practical hydrogen storage. The heat Q released upon absorption is usually characterized by the enthalpy parameter (Hplateau) determined from a van’t Hoff plot of the equilibrium pressures in the middle of the plateau
regions (i.e., plateau pressures) of the pressure–composition–temperature (PCT) isotherms.6 Key hydrogen-storage pr
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