Intermetallics for hydrogen storage

Hydrogen reacts with many elements and alloys, providing a range of metal hydrides to choose for hydrogen storage [4,5]. Metal hydride formation involves dissociative chemisorption of H2 on to a metal surface and then H atom diffusion starts into the crystal lattice. The formation of metal hydrides is typically exothermic and hydrogen desorption from the hydrides can be achieved endothermally under appropriate thermodynamic conditions. Over 

A special class of metal hydrides of importance are Laves phase (GdEe2) 

A comprehensive review up to about 1960s is given in the classic Metal Hydrides book by Mueller, Blackledge, and Libowitz [15]. There are many other excellent reviews on classical intermetallic hydrides such as Schlapbach 

In many cases a sloping plateau appears, possibly due to different equilibrium pressure lattice expansions, relaxation of residual forces to relieve the stress in the metal matrix are attributed to this distinctive feature of metal hydride systems as shown in Fig. 12.1(a), the slope of the plateau has been defined as dlnpd/dhi(H/M). 

7 (a) An isotherm showing a sloped plateau for hydrogen absorption and desorption isotherms. Hysteresis between absorption and desorption isotherms is also shown, (b) The effect of temperature on the isotherm plateau pressure and phase transitions regions from a— a+ 3— 3 are shown, (c) The van t Hoff plot derived from the isotherms obtained at various temperatures whose slope yields the enthalpy of hydriding. 

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Zhao J-C, Andrus M, Cui J, Gao Y, Lemmon J, Raber T, Rijssenbeek J, Rubinsztajn G, Soloveichik G, Lightweight Intermetallics for Hydrogen Storage, US DOE, Washington DC, 2006, http //www.hydrogen.energy.gov/pdfs/progress06/ iv a 4c zhao. pdf. 

A relatively simple set of rules have been found to hold for all intermetallic hydrides useful for hydrogen storage [11]. They may be stated as follows ... 

Disproportionation of intermetallics is indicated by the changes in the shape of the isotherms and associated loss in the hydrogen capacity [20]. This phenomenon may be attributed to short-range diffusion of the atoms, the thermodynamic stability of one of the elemental species from an elemental hydride, and dissociation of the non-hydride former second atom as a metal. Note that it is not necessary for all the binary intermetallic hydride to completely dissociate into elemental hydrides. Many variants of AB5 alloys have been used for hydrogen storage over the years and these hydrides were tested for intrinsic and extrinsic degradation, mainly LaNis and FeTi-type hydrides these hydrides exhibit a propensity for disproportionation. A hydride that would hydride/dehydride really well during the first few cycles may not... 

Five groups of materials based on Ti may be distinguished [1.51-53] commercially pure (i. e., commercially available) Ti (cp-Ti), low-alloy Ti materials, Ti-base alloys, intermetallic Ti-Al materials, and highly alloyed functional materials TiNi shape memory alloys, Nb-Ti superconducting materials (Sect. 4.2.1), and Ti-Fe-Mn materials for hydrogen storage. 

The intermetallic compound LaNi is well-known as a suitable material for hydrogen storage. Pseudobinary compounds with the same CaCu -type structure can be easily formed by partial substitution of nickel. Depending upon the rate and the nature of substitution, the hydriding properties (stability, maximum H-content, interstitial sites occupied by hydrogen. ..) are modified (1). 

In the case of metallic systems, there were early indications that metallic glasses and disordered alloys may be more corrosion resistant, less susceptible to embrittlement by hydrogen and have a higher hydrogen mobility than ordered metals or intermetallics. All of these properties are desirable for hydrogen storage. Subsequent research has shown that thermodynamic instability is a severe problem in many amorphous metal hydrides. The present ASI has provided an appropriate forum to focus on these issues. 

The nickel metal hydride battery functions on the principle that some intermetallic compounds can react directly and reversibly with hydrogen to form a hydride phase. Nickel is the most common metal for hydrogen storage. The half-cell reactions are ... 

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