Borohydrides as hydrogen storage materials

7 Elements M formed M(BH4)n, their gravimetric hydrogen densities in the unit of mass% (middle) and the Pauling electronegativity P of M (bottom). Asterisks indicate compounds stabilized at room temperature by co-ordination with ligands. Brackets indicate compounds which have been reported to be unstable at room temperature but may be Isolated at lower temperature.  

The first report of pure alkali metal borohydride appeared in 1940 by Schlesinger and Brown ° who synthesized LiBH4 by the reaction of ethyllithium with B2H6. The direct reaction of the corresponding metal/metal hydrides with diborane in etheral solvents under suitable conditions produces high yields of the borohydrides  

Industrially, NaBH4 is an important starting material for the production of other borohydrides. Thus far, over 100 methods of preparation for NaBH4 have been described. Industrial production of NaBH4 is produced by the reaction of extremely fine sodium hydride with trimethyl borate in high boiling hydrocarbon oil at 523-553 

Another process developed by Bayer uses finely ground borosilicate glass, sodium and hydrogen  

NaBH4 is extracted from the borohydride-silicate mixture with liquid ammonia under pressure. 

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Borohydrides as hydrogen storage materials 439 Trivalent and tetravalent borohydrides... 

G. Severa, Development of novel borohydrides as hydrogen storage materials, PhD Thesis, 2010. 

Sodium borohydride contains 10.8 wt.% of hydrogen, but it shows unfavorable thermodynamics for use as reversible hydrogen storage material. Over 100 synthesis methods for the preparation of NaBH4 have been described, but only two have reached practical significance. In the Schlesinger process (Eq. (5.7)), trimethyl borate is boiled together with NaH in hydrocarbon oil at 250 °C [19] ... 

The complex hydrides composed of iVT+ and (BH4) are "Borohydride." The borohydrides are quite attractive for hydrogen storage materials because of their extremely high hydrogen capacities due to high hydrogen composition in the anion as shown in Table 5.6,... 

Thus, it was demonstrated that calcium borohydride can be prepared from its proposed decomposition products, suggesting that it may have potential for use as a reversible hydrogen storage material... 

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