Hydrogen Generation

As opposed to reversible hydrogen storage (characterized by hydrogenation and dehydrogenation cycles), hydrogen generation means “one-shot” operation, where hydrogen is produced from a source hydride material. It involves production of hydrogen through a chemical reaction of hydrides with, for example, water or alcohol. In this case, hydrogen is not only coming from the decomposed hydride, but also from water molecules. For each hydrogen molecule formed, one hydrogen atom is taken from the hydride and one from water.  This significantly improves the overall hydrogen output..

General reaction of hydrolysis of hydrides

MH + H2O           MOH + H     where M is a metal

These systems provide significantly larger hydrogen capacities, combined with more convenient temperature of operation. In practice, at least 5 wt.% is provided, with decomposition temperature well below 100oC. These conditions become available for certain hydride systems, as for example in magnesium hydride or sodium borohydride.

Hydrolysis of hydrides is in practice often too rapid and vigorous, as in the case of such alkali metal hydrides as LiH, LiAlH4 or NaH. Combination of these hydrides with water is extremely unstable and they decompose almost explosively. On the other hand, another group of hydrides (such as magnesium hydride, MgH2, sodium borohydride, NaBH4) exhibit very high stability in water. In those cases,  hydrolysis does not proceed without heating water to hot, pressurized steam or applying special catalysts.

Hydrogen generation capacities of various hydrides in the hydrolysis reactions. Water weight as an external component is not taken into account.

     



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