Storage alloys hydride elements

Although the above hydrogen storage alloys can absorb and desorb hydrogen reversibly under a lower hydrogen pressure at lower temperature than those of stable ionic and covalent hydrides, the gravimetric density of hydrogen is small because the alloys are composed of heavy elements. 

Nowadays, such hydride electrodes are used widely to make alkaline storage batteries which in their design are similar to Ni-Cd batteries but exhibit a considerably higher capacity than these. These two types of storage battery are interchangeable, since the potential of the hydride electrode is similar to that of the cadmium electrode. The metal alloys used to prepare the hydride electrodes are multicomponent alloys, usually with a high content of rare-earth elements. These cadmium-free batteries are regarded as environmentally preferable. 

Two factors currently limit reversible metal hydride storage applications cost and weight. The relatively high cost of the elemental constituents for the hydriding alloys (Ni, 3. 5/lb ... 

Table 3.7 lists some important metals and alloys that have been studied for hydrogen storage applications. To date, no material (neither metals nor nonmetals) has been discovered that satisfies all of the above five constraints. Although hydrides exist for most elements of the Periodic Table, only the light elements (e.g., Li, Mg, Al) are able to meet criterion 1 above. 

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