F. Buchter

In situ synchrotron diffraction studies of phase transitions and thermal decomposition of Mg(BH4)(2) and Ca(BH4)(2)

Mg(BH4)2 and Ca(BH4)2 with 14.9 and 11.6 mass% hydrogen, respectively, are among the most promising materials for mobile hydrogen storage, but until now very little has been known about their hydrogen desorption properties. In this work the materials have been studied by time-resolved in situ synchrotron powder X-ray diffraction, thermal desorption spectroscopy and energy dispersive X-ray spectroscopy, and details of the phase transitions and decomposition routes are reported.

Structure of Ca(BD4)2 β-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.

The effect of Al on the hydrogen sorption mechanism of LiBH4

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.

Low‐Temperature Synthesis of LiBH4 by Gas–Solid Reaction

The solvent-free synthesis of LiBH(4) from LiH in a borane atmosphere at 120 degrees C and ambient pressures is demonstrated. The source of borane is a milled LiBH(4)/ZnCl(2) mixture, in which Zn(BH(4))(2) is generated by a metathesis reaction. The yield of the reaction of about 74 % LiBH(4) shows that a bulk reaction is taking place upon borane absorption by LiH. This indicates that the formation of B-H bonds is the limiting step for the formation of LiBH(4) from the elements. Therefore, the use of diborane as a starting reactant allows one to circumvent the reaction barrier for the B-B bond dissociation and explains the rather moderate synthesis conditions.

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.

Solid-state synthesis of LiBD4 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.

Hydrogen Dynamics in Lightweight Tetrahydroborates

Abstract The high hydrogen content in complex hydrides such as M[AlH4]x and M[BH4]x (M = Li, Na, K, Mg, Ca) stimulated many research activities to utilize them as hydrogen storage materials. An understanding of the dynamical properties on the molecular level is important to understand and to improve the sorption kinetics. Hydrogen dynamics in complex hydrides comprise long range translational diffusion as well as localized motions like vibrations, librations or rotations. All the different motions are characterized by their specific length- and timescales. Within this review we give an introduction to the physical properties of lightweight complex hydrides and illustrate the huge variety of dynamical phenomena on selected examples.