The prerequisite for widespread use of hydrogen as an energy carrier is the development of new materials that can safely store it at high gravimetric and volumetric densities. Metal borohydrides M(BH₄)_n (n is the valence of metal M), in particular, have high hydrogen density, and are therefore regarded as one such potential hydrogen storage material. For fuel cell vehicles, the goal for on-board storage systems is to achieve reversible store at high density but moderate temperature and hydrogen pressure. To this end, a large amount of effort has been devoted to improvements in their thermodynamic and kinetic aspects. This review provides an overview of recent research activity on various M(BH₄)_n, with a focus on the fundamental dehydrogenation and rehydrogenation properties and on providing guidance for material design in terms of tailoring thermodynamics and promoting kinetics for hydrogen storage.

Conditions have been found whereby it is possible to reversibly store >11 wt% hydrogen through the direct hydrogenation of MgB₂ to Mg(BH₄)₂.

Thermal decomposition of magnesium borohydride, Mg(BH₄)₂, in the solid state was studied by a combination of PCT, TGA/MS and NMR spectroscopy. Dehydrogenation of Mg(BH₄)₂ at 200 °C en vacuo results in the highly selective formation of magnesium triborane, Mg(B₃H₈)₂. This process is reversible at 250 °C under 120 atm H₂. Dehydrogenation at higher temperature, >300 °C under a constant argon flow of 1 atm, produces a complex mixture of polyborane species. A borohydride condensation mechanism involving metal hydride formation is proposed.

The activity of the PCP pincer complex IrH₂{C₆H₃-2,6-(OPBu^t ₂)₂}, (3) as a catalyst for the dehydrogenation of the potential liquid organic hydrogen carriers: perhydro-dibenzofuran (4), perhydro-indole (5), N-methyl perhydro-indole (6), 4,4′-bipiperridine (7), 4-amino-methylpiperridine (8), and aminomethylcyclohexane (9) was investigated. Only low levels of dehydrogenation were achieved with 4 at 150 and 200 °C except in the presence a hydrogen acceptor, 3,3-dimethyl-1-butene (tbe). At temperatures above 150 °C, 3 showed high activity with 5 but only for the release of H₂ across the C–N bond. Dehydrogenation of 6 at 150 °C was found to release up to 2.9 wt% H₂, giving N-methyl tetrahydroindole in 92% yield, but only 7% N-methyl indole. However, efficient dehydrogenation of the aliphatic hydrogens occurred at 200 °C giving mixtures that approached a ∼1 : 3 equilibrium between N-methyl indole and N-methyl tetrahydroindole at longer reaction times. The pincer catalyst was observed to be effective for the dehydrogenation of 7 and 8 at 200 °C but the products polymerize. The catalytic dehydrogenation of 9 was very efficient at 200 °C but gave primarily the undesired products dibenzylamine (68%), benzylidenebenzylamine (22%), ammonia and only minor amount of benzonitrile (10%). At 160 °C, 85% of the substrate was consumed after 24 h of heating but only a minor amount of cyclohexanecarbonitrile (2%) was detected with the major products being biscyclohexylmethylamine (45%) and cyclohexylmethyl-cyclohexylimine (38%). The nitrile yield is remarkably improved to 97% when the dehydrogenation was carried out at 160 °C in the presence of 10 mol% NaOBu^t but heating the reaction mixtures containing the base to 200 °C resulted in the decomposition of the pincer catalyst.