Page 322 - Adsorbents fundamentals and applications

HYDROGEN STORAGE 307

b-phase
T c

100
a-phase
100°C
P eq (bar) 10 a + b-phase

25°C
1
0°C

0.1
0.0 0.2 0.4 0.6 0.8 1.0
C H (H/M)
Figure 10.23. Pressure-concentration-temperature equilibrium curves for H 2 /LaNi 5 ,forming
◦
LaNi 5 H 6 at 25 C (Schlapbach and Zuttel, 2001, with permission).

Table 10.5. Intermetallic compounds and their hydrogen storage properties
Type Metal Hydride wt % P eq ,T

Elemental Pd PdH 0.6 0.56 0.02 bar, 298 K
AB FeTi FeTiH 2 1.89 5 bar, 303 K
AB 5 LaNi 5 LaNi 5 H 6 1.37 2 bar, 298 K
A 2 B Mg 2 Ni Mg 2 NiH 4 3.59 1 bar, 555 K
AB 2 ZrV 2 ZrV 2 H 5.5 3.01 10 −8 bar, 323 K
Body-centered cubic TiV 2 TiV 2 H 4 2.6 10 bar, 313 K
AB NaAl NaAlH 4 8.0 90 bar, 403 K
Data taken from Schlapbach and Zuttel, 2001, except NaAl, from Zaluska et al., 2001.

dissolves some hydrogen-forming solid solution (or α phase). As the hydrogen
pressure is increased, nucleation and growth of the hydride phase (or β phase)
takes place, and coexists with the α phase. At still higher pressures, little increase
in hydrogen concentration can occur. The width of the plateau on the phase dia-
gram indicates the amount of hydrogen storage. From the isotherms, a van’t
Hoff plot would yield the enthalpy of formation, or −7.2 kcal/mol H 2 for LaNi 5 .
The plateau pressure at room temperature is near 2 atm and hence is conve-
nient for application. The best known binary alloys are listed in Table 10.5. The
mechanism of H 2 absorption starts from dissociation into H atoms, followed by
diffusion into the metal lattice. In desorption, the process is revered, and 2 H
atoms combine to form H 2 .
It became clear by 1990 that metal hydrides would not be practical for hydro-
gen storage without new breakthroughs (Zaluska et al., 2001). The developments