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456 Metal Hydrides

entities, which leads to the formation of distortion vari- ing hydrogen as an ecologically clean, cheap fuel that
ants of the K 2 PtCl 6 type as shown in Fig. 6. On the would replace fossil fuels with their limited availability
other hand, the dynamical behavior of hydrogen with and ecological problems. In a hydrogen economy electric-
its high mobility gives rise to order–disorder transi- ity would be produced preferably by renewable energies,
tions. Mg 2 CoH 5 transforms from the tetragonal room- such as solar, wind, and water. Electricity cannot be stored
temperature structure with ordered hydrogen distribution and transported very efficiently. These shortcomings are
at 488 K into a disordered cubic high-temperature mod- overcome by using hydrogen as an energy carrier that can
ification in the K 2 PtCl 6 type (Fig. 6) in which five hy- be produced using electricity, for instance by electrolysis
drogen atoms are disordered over six crystallographically of water. For the storage and transportation of hydrogen
equivalent positions resulting in an occupation proba- the metal hydrides come into play as more volume- and
4−
bility of 5/6 ([CoH 6 5/6 ] ). A similar order–disorder weight-efficient alternatives to tanks for gas or liquid hy-
∗
transition occurs at 483–513 K for Mg 2 NiH 4 from the drogen. Hydrogen can be used in most of today’s end-user
monoclinic to a cubic modification with four hydro- systems as a fuel without major modifications, e.g., in au-
gen atoms on six sites ([NiH 6 2/3 ] ). Further transitions tomobiles, heating systems, or ovens. In constrast to fossil
4−
∗
with ordered low-temperature structures derived from fuels, hydrogen is nonpolluting, as it burns to H 2 O with
the cubic K 2 PtCl 6 type occur for A 2−x Eu x IrH 5 (A = Ca, only trace amounts of NO x and a high efficiency (close to
Sr; 0 ≤ x ≤ 2) and A 2 PtH 4 (A = Na–Cs, square planar 100% in catalytic converters).
2−
[PtH 4 ] in two different orientations depending on A). Instead of producing heat by combustion, the energy of
For K 2 PtH 4 a rigid motion-type disorder was evidenced hydrogen can also be retransformed to electricity in bat-
by NMR spectroscopy. For other disordered hydrogen- teries. Commercially very successful is the nickel–metal
deficient cubic K 2 PtCl 6 -type structures such as Mg 2 IrH 5 hydride rechargeable battery (Ni-MH), which can be con-
and Sr 2 RhH 5 , no transition to an ordered phase was ob- sidered as a successor of the nickel–cadmium battery. It
has several advantages over the latter, e.g., the prevention
served. Further order–disorder transitions occur in A 3 PtH 5
(A = K–Cs, [PtH 4 ] /[PtH 6 2/3 ] 2− for ordered and disor- of ecologically problematic Cd, a higher storage capac-
2−
∗
deredphase,respectively,K 3 PdH 3 ([PdH 2 ] /[PdH 6/3 ] ) ity, a greater energy density (80 Wh/kg, 250 Wh/L), faster
2−
2−
and in A 3 ReH 10 (A = K, Rb, [ReH 9 ] /[ReH 24 9/24 ] ). charge, and a high cycle rate (500) with a comparable
2−
2−
∗
In contrast to these examples some hydrogen-deficient operating voltage to that of Ni–Cd. A general cell reac-
compounds do not transform to an ordered modification, tion involves the reversible intercalation (on charging, C)
suchasthemetal-richCaPdH 2 ,SrPdH 2.7 (cubicperovskite and reintercalation (on discharging, D) of hydrogen in a
type),MgRhH 1−x ,MgIrH x (derivedfromcubicperovskite storage material M in an aqueous potassium hydroxide
type), and Mg 4 IrH 5 (unique type). Such compounds have solution as electrolyte:
a metallic appearance and are border cases between com-
D
plex hydrides and interstitial hydrides. MH + NiOOH M + Ni(OH) 2

C
Ni-MH rechargeable batteries are available commer-
IV. APPLICATIONS
cially containing hydrogen storage materials such as
M = LaNi 5 (MH = LaNi 5 H 6 ) and M = ZrCr 2 (MH =
A. Hydrogen Storage
ZrCr 2 H 3.8 ) and their substitutional variants, e.g.,
The most important application of metal hydrides is hy- LaNi 3.55 Mn 0.4 Al 0.3 Co 0.75 and ZrMn 0.5 Cr 0.2 V 0.1 Ni 1.2 . The
drogen storage. As pointed out in the preceding chaptes substituents on Ni and Cr sites improve the reversibility
many metals and intermetallic compounds can take in con- of the cell reaction, the cycle stability, and the corrosion
siderable amounts of hydrogen reversibly. The hydrogen behavior. Ni-MH batteries have been widely used since
density of metal hydrides often exceeds that in liquid hy- the 1990s in portable devices such as cellular phones or
drogen (Table II). An ideal material for hydrogen storage laptops, in battery-driven cars, and in aeronautics.
applications would have broad plateau regions (high re- For use in vehicles with internal combustion engines or
5
versible hydrogen storage capacity), p eq close to 10 Pa fuel cells, a high volume and weight efficiency is required.
at room temperature, fast absorption–desorption kinetics, Shortcomings of the commercially available systems are
no deviation from the idealized thermodynamic behavior their weight, bulkiness, and high cost. New developments
(Fig.1),agoodcyclelife(manyhundredsofcycles),ahigh in the area include hydrogen storage in quasicrystals and
weight and volume efficiency, a high resistance to surface- carbon nanotubes and the catalytic decomposition of light-
poisoning gases such as oxygen and water, and low pro- weight hydrides such as LiAlH 4 . Several producers are
duction costs, and it would not contain toxic materials. running test cars and trucks with liquid hydrogen, metal
The need for safe and convenient hydrogen storage sys- hydride tanks, or hydrogen fuel cells. DaimlerChrysler
tems is driven by the idea of a hydrogen economy us- recently announced the first commercially available