Page 270 - Handbook of Battery Materials

240 9 Metal Hydride Electrodes

on the storage of hydrogen for use as a gaseous fuel, although the possibility
of electrochemical applications was also recognized. In about 1980 that focus
dramatically shifted toward electrochemical applications where a metal hydride
(MH x ) electrode is used to replace the cadmium electrode in Cd/Ni batteries. The
driving force for such replacement was the environmental problems associated
with cadmium. An additional benefit was the higher energy density of the MH x
electrode, and in this chapter we discuss the electrochemical, thermodynamic, and
structural properties of MHs that pertain to their use in Ni–MH x batteries.
A hydrogen–metal system may be defined as consisting of an amorphous or
crystalline metal phase containing dissolved hydrogen in interfacial contact with
molecular, atomic, or ionized hydrogen. In many cases, depending on temperature
and pressure, an MH phase will form of which there are three general categories:
ionic, covalent, and metallic. Intermetallic hydrides are, of course, a sub-group of
the latter class, where hydrogen occupies interstitial sites in the metal lattice and the
hydride phase is crystalline. There are a large number of intermetallic compounds,
many of which will form a hydride via the direct and reversible reaction with
hydrogen. Consequently, even though most may not be of interest for practical
applications, the sheer number of intermetallic hydride systems constitutes a great
advantage over binary systems with respect to the formulation of attractive energy
storage materials.

9.2.1
Thermodynamics

Flanagan and Oates have extensively reviewed the thermodynamics of intermetallic
hydrides [1]; also recommended are the classic work of Libowitz [2] and the
comprehensive text of Muller, Blackledge, and Libowitz [3] which treats the
properties of binary hydrides. The properties of a metal–hydrogen system can be
conveniently summarized by a pressure-temperature-composition (PTC) diagram
of which an idealized version is shown in Figures 9.1 and 9.2. The former is

T 4
H 2 P T 3
Figure 9.1 Ideal pressure-composition-
T 2
isotherms showing the hydrogen solid
α α + β β solution phase, α, and the hydride phase,
β. The plateau marks the region of the co-
T 1
existence of the α and β phases. As the
temperature is increased the plateau narrows
and eventually disappears at some consolute
H CONTENT temperature, T c .