Chemical Storage Based on Metal Hydrides and Hydrocarbons - Hydrogen Generation, Storage, and Utilization - page 90

6

Chemical Storage Based on Metal
Hydrides and Hydrocarbons

6.1 BASICS ON HYDROGEN STORAGE OF METAL HYDRIDES

Many metals, intermetallic compounds, and alloys can react with hydrogen
to form solid metal hydrides. Most metal hydrides have hydrogen very
strongly bound to the metal. Some of the metal hydrides have even higher
volumetric hydrogen storage capability than that of liquid hydrogen, which
makes the metal hydride very attractive for on-board hydrogen storage appli-
cations. For example, Mg 2 FeH 6 and Al(BH 4 ) 3 have the highest volumetric
hydrogen density known today, 150 kgm −3 .
Figure 6.1 shows a comparison of both the volumetric and gravimetric
storage capacities of different materials summarized by Züttel recently [1].
In general, most metal hydrides have very high volumetric density compared
with that of liquid hydrogen, but the gravimetric density is usually low.
Also, metal hydrides have the advantage for low pressure hydrogen adsorp-
tion and storage, thus making them more attractive from safety point of
view. Since metal hydrides are in solid state, they do not require a compli-
cated container to store, which makes them even more attractive. However,
the formation of metal hydride and the dehydrogenation process are both
chemical processes, which involve the formation or breaking of metal-
hydrogen bonds, and the hydrogen atoms often occupy interstitial sites of
metal. As a result, relatively high temperatures, around 120-300°C, are