Breaking the passivation--the road to a solvent free borohydride synthesis.

We describe a new method for the solvent-free synthesis of borohydrides at room temperature and demonstrate its feasibility by the synthesis of three of the most discussed borohydrides at present: LiBH(4), Mg(BH(4))(2) and Ca(BH(4))(2). This new gas-solid mechanochemical synthesis method is based on the reaction of metal hydrides with diborane to form the corresponding borohydrides. The synthesis will facilitate the preparation of a wide range of different borohydrides, including mixed borohydride systems, with tuneable sorption properties. We propose that diborane is an intermediate compound for the hydrogen sorption in borohydrides and may be the key for a reversible hydrogen ab- and desorption reaction under moderate conditions.

Density functional theory based screening of ternary alkali-transition metal borohydrides: a computational material design project.

We present a computational screening study of ternary metal borohydrides for reversible hydrogen storage based on density functional theory. We investigate the stability and decomposition of alloys containing 1 alkali metal atom, Li, Na, or K (M(1)); and 1 alkali, alkaline earth or 3d/4d transition metal atom (M(2)) plus two to five (BH(4))(-) groups, i.e., M(1)M(2)(BH(4))(2-5), using a number of model structures with trigonal, tetrahedral, octahedral, and free coordination of the metal borohydride complexes. Of the over 700 investigated structures, about 20 were predicted to form potentially stable alloys with promising decomposition energies. The M(1)(Al/Mn/Fe)(BH(4))(4), (Li/Na)Zn(BH(4))(3), and (Na/K)(Ni/Co)(BH(4))(3) alloys are found to be the most promising, followed by selected M(1)(Nb/Rh)(BH(4))(4) alloys.

The effect of Al on the hydrogen sorption mechanism of LiBH(4).

We demonstrate the synthesis of LiBH(4) from LiH and AlB(2) without the use of additional additives or catalysts at 450 degrees C under hydrogen pressure of 13 bar to the following equation: 2LiH + AlB(2) + 3H(2)<--> 2LiBH(4) + Al. By applying AlB(2) the kinetics of the formation of LiBH(4) is strongly enhanced compared to the formation from elemental boron. The formation of LiBH(4) during absorption requires the dissociation of AlB(2), i.e. a coupled reaction. The observed low absorption-pressure of 13 bar, measured during hydrogen cycling, is explained by a low stability of AlB(2), in good agreement with theoretical values. Thus starting from AlB(2) instead of B has a rather low impact on the thermodynamics, and the effect of AlB(2) on the formation of LiBH(4) is of kinetic nature facilitating the absorption by overcoming the chemical inertness of B. For desorption, the decomposition of LiBH(4) is not indispensably coupled to the immediate formation of AlB(2). LiBH(4) may decompose first into LiH and elemental B and during a slower second step AlB(2) is formed. In this case, no destabilization will be observed for desorption. However, due to similar stabilities of LiBH(4) and LiBH(4)/Al a definite answer on the desorption mechanism cannot be given and neither a coupled nor decoupled desorption can be excluded. At low hydrogen pressures the reaction of LiH and Al gives LiAl under release of hydrogen. The formation of LiAl increases the total hydrogen storage capacity, since it also contributes to the LiBH(4) formation in the absorption process.

Solid-state synthesis of LiBD(4) observed by in situ neutron diffraction.

The synthesis of Li[(11)BD(4)] from LiB and D(2) (p = 180 bar) is investigated by in situ neutron diffraction. The onset of the Li[(11)BD(4)] formation is observed far below the temperatures reported so far for the reaction from the pure elements, indicative of a lower activation barrier. We attribute the improved formation behavior to the breaking of the rigid boron lattice and intermixing of the elements on an atomic level when forming the binary compound LiB. The reaction starts with the decomposition of the initial LiB compound and the formation of LiD. At 623 K LiBD(4) starts to form. However, under the given experimental conditions (maximal temperature = 773 K) a complete reaction was not achieved; there is still residual LiD present.

Structure of Ca(BD4)2 beta-phase from combined neutron and synchrotron X-ray powder diffraction data and density functional calculations.

We have investigated the crystal structure of Ca(BD4)2 by combined synchrotron radiation X-ray powder diffraction, neutron powder diffraction, and ab initio calculations. Ca(BD4)2 shows a variety of structures depending on the synthesis and temperature of the samples. An unknown tetragonal crystal of Ca(BD4)2, the beta phase has been solved from diffraction data measured at 480 K on a sample synthesized by solid-gas mechanochemical reaction by using MgB2 as starting material. Above 400 K, this sample has the particularity to be almost completely into the beta phase of Ca(BD4)2. Seven tetragonal structure candidates gave similar fit of the experimental data. However, combined experimental and ab initio calculations have shown that the best description of the structure is with the space group P4(2)/m based on appropriate size/geometry of the (BD4)tetrahedra, the lowest calculated formation energy, and real positive vibrational energy, indicating a stable structure. At room temperature, this sample consists mainly of the previously reported alpha phase with space group Fddd. In the diffraction data, we have identified weak peaks of a hitherto unsolved structure of an orthorombic gamma phase of Ca(BD4)2. To properly fit the diffraction data used to solve and refine the structure of the beta phase, a preliminary structural model of the gamma phase was used. A second set of diffraction data on a sample synthesized by wet chemical method, where the gamma phase is present in significant amount, allowed us to index this phase and determine the preliminary model with space group Pbca. Ab initio calculations provide formation energies of the alpha phase and beta phase of the same order of magnitude (delta H < or = 0.15 eV). This indicates the possibility of coexistence of these phases at the same thermodynamical conditions.