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Encyclopedia of Physical Science and Technology EN009A-426 July 6, 2001 20:44

Metal Hydrides 443

FIGURE 1 Pressure–composition isotherms (left) and van’t Hoff plot (right) for the reaction of a metal or intermetallic
compound with hydrogen. p H2 , hydrogen pressure; c H , hydrogen concentration in the solid phase; T, absolute
temperature; α, solid solution phase of hydrogen in the metal or intermetallic compound; β, metal hydride phase;
p eq , equilibrium pressure of metal hydride formation; T c , critical temperature; H, enthalpy of hydride formation.
[From Schlapbach, L., Meli, F., and Z¨uttel, A. (1995). Intermetallic hydrides and their applications. In “Intermetallic
Compounds: Principles and Practice” (J. H. Westbrook and R. L. Fleischer, eds.), Vol. 2, pp. 475–488. Reproduced
with permission from John Wiley & Sons, New York.]

interdisciplinary work involving chemists, crystallogra- entropy S can be derived from a van’t Hoff plot ln p eq
phers, engineers, materials scientists, metallurgists, and vs 1/T using the equation ln p eq = H/RT − S/R (R,
physicists. gasconstant;Fig.1,right).Thermodynamicaldataofsome
metal hydrides are summarized in Table II. For stability
considerations it is sufficient to discuss H values, since
S is fairly constant for most systems and corresponds to
II. PREPARATION AND
CHARACTERIZATION the entropy of hydrogen gas (130 J/K mol) that is lost on
absorption by the metal or intermetallic compound. Thus,
A. Hydrogenation of Metals and Intermetallic
Compounds: Thermodynamics
TABLE II Thermodynamic and Hydrogen Stor-
Most binary metal hydrides are synthesized by a solid–gas age Properties for Selected Metal Hydrides a
reaction between the metal and hydrogen. As shown in the ∆H Weight fraction H density
idealized pressure–composition isotherms (Fig. 1), at low Hydride (kJ/mol H 2 ) of H (%) (g/L)
hydrogen pressure p H2 a solid solution of hydrogen in the
metal forms in which H occupies interstitial places in the LiH −180 12.7 98
metal host lattice (α-phase). When the equilibrium pres- NaH −112 4.2 58
sure p eq is reached a metal hydride (β-phase, sometimes MgH 2 −74 7.7 109
−188 4.8 93
called α ) forms and the pressure remains constant until

CaH 2
the α-phase is entirely converted into the β-phase at the AlH 3 −8 10.1 149
end of the plateau region. Temperature and pressure of the TiH 2 −136 4.0 152
plateau region define the conditions for the preparation of MnH 0.5 −16 0.9 62
metal hydrides and of reversible hydrogen storage in these FeH 0.5 +20 0.9 59
materials. The hydrogen concentration (c H ) at the end of PdH 0.7 −41 0.7 72
the plateau and the length of the plateau region determine LaH 2 −208 1.4 73
the maximum hydrogen uptake and the capacity useful for UH 3 −127 1.3 137
reversible storage, respectively. In the single-phase region Mg 2 FeH 6 −98 5.5 150
β following the plateau, the pressure rises again drastically Mg 2 CoH 5 −86 4.5 125
with only small compositional changes. This region may Mg 2 NiH 4 −64 3.6 97
be followed by further plateau regions at higher pressures. FeTiH 2 −30 1.9 96
Above a critical temperature T c the two-phase region van- LaNi 5 H 6 −31 1.4 92
ishes and the hydrogen concentration in the metal hydride ZrCr 2 H 3.8 −96 2.0 111
can vary continuously. The enthalpy of metal hydride for- a For comparison, the density in liquid and gaseous
mation H which is in general negative and the reaction hydrogen in 70.9 and 0.084 g/L, respectively.