Page 321 - Adsorbents fundamentals and applications

306 SORBENTS FOR APPLICATIONS

subject of considerable controversy. For these reasons, it is only possible to
present the current understanding on this subject. An attempt will be made to
provide a fair assessment of its prospect. In addition to carbon nanotubes, metal
hydrides and other sorbents are also included.
The four general approaches for on-board vehicle applications (compressed
hydrogen gas, liquefied hydrogen, metal hydrides, and carbon sorbents) have
been discussed in detail elsewhere and will not be repeated here (Department of
Energy, 1999; Hynek et al., 1997; Schlapbach and Z¨ uttel, 2001).

10.3.1. Metal Hydrides
Extensive research on metal hydrides and their role in hydrogen storage have been
conducted over 30 years (Alefeld and Volkl, 1978; Schlapbach, 1988; Sastri et al.,
1998; Zaluska et al., 2001). Myriad metal hydrides have been studied, yet none
are practical or commercially successful. However, some promising approaches
are on the horizon and remain the subject of active research.
The most important basic properties expected in metal hydrides for hydrogen
storage are (1) high hydrogen content, (2) fast hydrogen charging and discharging
rates, and (3) moderate temperature for hydrogen desorption (discharge), prefer-
◦
ably below 100 C. The desorption rates are generally slower than the absorption
rates, and are associated with hysteresis.
In addition to these basic requirements, metal hydrides are prone to the fol-
lowing common problems: (1) deactivation or poisoning by contaminants in
hydrogen, such as CO, H 2 O, O 2 ,CO 2 ,and H 2 S; (2) deterioration upon absorp-
tion/desorption cycling — both capacity and rates decay upon cycling.
In order to discharge hydrogen at a moderately low temperature and to have
acceptable rates (i.e., diffusion rates), the H-bond energy with the metal atoms
must be low. Thus, the enthalpy of formation of the hydride should be below
∼12 kcal/mol. The metals that are known to form hydrides all have high enthalpies
of formation. This problem was circumvented by using intermetallic compounds,
from the early work of Libowitz et al. (1958) and Reilly and Wiswall (1968).
The intermetallic compounds are formed by alloying the hydride-forming met-
als with other metals (usually transition metals, Fe, Ni, Co, Cr, etc.) that do not
form hydrides. This second metal greatly lowers the enthalpy of formation while
retaining the hydride-forming capacity of the first metal.
Over the past three decades, a great number of binary and ternary alloys were
developed for hydrogen storage. They are designated AB x or A x B type, where A
denotes the hydride-forming metal and B denotes the non-hydride-forming metal.
There are four classes of alloys that have been developed: (1) AB-type based on
Ti as the hydride forming metal, (2) AB 5 -type based on rare earth metals as the
hydride forming metals, (3), A 2 B-type based on Mg as the hydride forming metal,
and (4) AB 2 -alloys using Zr as the hydride forming metal (Sastri et al., 1998).
The thermodynamic equilibria of H 2 /metal systems are described by pressure-
composition isotherms, shown as an example in Figure 10.23 for LaNi 5 (Schlap-
bach and Z¨ uttel, 2001). At a constant temperature, at low pressures, the metal