Chemical Storage Based on Metal Hydrides and Hydrocarbons - Hydrogen Generation, Storage, and Utilization - page 93

FIGURE 6.2 The typical PCT curves for the hydrogenation and dehydrogenation of a metal hydride
under a fixed temperature T . (See color insert.)

where Q is the heat of hydride formation. In general, the hydrogenation is
exothermic, while the dehydrogenation is endothermic. Practically, the
hydrogenation process happens at high hydrogen pressure, while dehydro-
genation occurs at low pressure. Figure 6.2 shows a typical pressure-
composition-temperature (P-C-T) curve during a hydrogenation and
dehydrogenation cycle. With the increasing hydrogen pressure, a metal starts
to adsorb hydrogen to form metal-hydrogen solid solution ( α -phase). When
the pressure reaches “A” location shown in Figure 6.2, the metal starts to
form hydride ( β -phase). At this stage, the hydrogen pressure ( P A ) almost
remains as a constant while the hydrogen content increases significantly. The
hydrogenation process will be complete at “B” location. This A-B adsorption
plateau characterizes the effective hydrogen storage capacity at a fixed tem-
perature. In general, the adsorption plateau pressure will increase with tem-
perature, and follow the van 't Hoff relation [2],

∆ H

RT

∆ S

R

ln P = − ,

(6.5)

where P is the hydrogen pressure, Δ H and Δ S are the enthalpy and entropy
of hydride formation or decomposition, R is the universal gas constant, and
T is the temperature. The heat of formation can be obtained by plotting the
plateau pressure ln P versus 1/ T (van 't Hoff plot), as shown in Figure 6.3