Introduction of economically viable hydrogen cars is hindered by the need to store large amounts of hydrogen. Metal borohydrides [LiBH(4), Mg(BH(4))(2), Ca(BH(4))(2)] are attractive candidates for onboard storage because they contain high densities of hydrogen by weight and by volume. Using a set of recently developed theoretical first-principles methods, we predict currently unknown crystal structures and hydrogen storage reactions in the Li-Mg-Ca-B-H system. Hydrogen release from LiBH(4) and Mg(BH(4))(2) is predicted to proceed via intermediate Li(2)B(12)H(12) and MgB(12)H(12) phases, while for Ca borohydride two competing reaction pathways (into CaB(6) and CaH(2), and into CaB(12)H(12) and CaH(2)) are found to have nearly equal free energies. We predict two new hydrogen storage reactions that are some of the most attractive among the presently known ones. They combine high gravimetric densities (8.4 and 7.7 wt % H(2)) with low enthalpies [approximately 25 kJ/(mol H(2))] and are thermodynamically reversible at low pressures due to low vibrational entropies of the product phases containing the [B(12)H(12)](2-) anion.

The effects of nanoconfinement on the structural phase transition, H 2 release and uptake, and the emission of toxic diborane (B 2 H 6 ) on desorption of LiBH 4 have been comprehensively investigated in the presence of various porous hard carbon templates at a variety of pore sizes. Calorimetry signatures of both the structural phase transition and melting of nanoconfined LiBH 4 shifted to a lower temperature with respect to the bulk, finally vanishing below a pore size around 4 nm. The desorption of LiBH 4 confined in these nanoporous carbons shows a systematic and monotonic decrease in the desorption temperature and concomitantly, mass spectroscopic analysis indicated a gradual reduction of the partial pressure of B 2 H 6 with decreasing pore size, suggesting that formation of stable closoborane salts may be avoided by interrupting the reaction pathway. This represents a major breakthrough in the reversibility of boron-based hydrogen storage systems, where capacity is lost in the formation of stable B-H species on cycling. Different carbon preparation techniques suggest that the confinement size, and not solely surface interactions, may be used to tune the properties of complex hydrides for kinetic and reaction pathway improvements for hydrogen storage applications.

We demonstrate a new solid-state synthesis route to prepare calcium borohydride, Ca(BH4)2, by reacting a ball-milled mixture of CaB(6) and CaH(2) in a molar ratio of 1:2 at 700 bar of H2 pressure and 400-440 degrees C. Moreover, doping with catalysts was found to be crucial to enhance reaction kinetics. Thermogravimetric analysis and differential scanning calorimetry revealed a reversible low-temperature to high-temperature endothermic phase transition at 140 degrees C and another endothermic phase transition at 350-390 degrees C associated with hydrogen release upon formation of CaB(6) and CaH(2), as was evident from X-ray diffraction analysis. Thus, since Ca(BH(4))(2) here is shown to be prepared from its anticipated decomposition products, the conclusion is that it has potential to be utilized as a reversible hydrogen storage material. The theoretical reversible capacity was 9.6 wt % hydrogen.