Mg(BH 4) 2 is one of the few complex hydrides which have the potential to meet the requirements for hydrogen storage materials, because it contains 14.9 mass% H and has suitable thermodynamic properties. It has not been investigated for hydrogen storage applications yet. In this study, several ways to synthesize solvated and desolvated magnesium tetrahydroborate by wet-chemical and mechanochemical methods were tested and compared. A direct synthesis by a reaction of MgH 2 with aminoboranes yields magnesium tetrahydroborate quantitatively and in pure form. The method is also applicable to the synthesis of other tetrahydroborates. The products were characterised by elemental analysis, in-situ X-ray diffraction (XRD), infrared spectroscopy (FTIR), and thermal analysis methods, such as thermogravimetric analysis (TGA-DSC) and high-pressure calorimetry under hydrogen atmosphere (HP-DSC).

X-ray absorption spectroscopy is applied to investigate NaAlH 4 doped with 5 mol % TiCl 3 by ball milling. XANES analysis indicates that after the ball milling Ti species are reduced from Ti (+3) to Ti (0). Release or absorption of hydrogen does not affect the chemical state obtained after ball milling. EXAFS analysis shows that the Ti atoms are associated only with Ti as next neighbors in the ball-milled state as well as during subsequent desorption and absorption of hydrogen. Furthermore, an increase of the particle size and an ordering of the local structure is seen to evolve with the desorption and the absorption of hydrogen.

Metal borohydrides are of interest as hydrogen storage materials due to their high volumetric and gravimetric capacity. However, as with many of the complex hydrides, they are hampered by slow absorption and desorption kinetics and poor reversibility. 1À3 Among the borohydrides, Mg(BH 4 ) 2 ( Figure 1) and Ca(BH 4 ) 2 have more favorable thermodynamic stability than, for example, LiBH 4 , while maintaining attractive hydrogen storage capacities (14.9 and 11.5 mass %, respectively). 2,4 Furthermore, for these two compounds, partial reversibility has been obtained by utilizing high pressure 5À7 (60% 7 À70% 6 recovery of the borohydride) and in the case of Ca(BH 4 ) 2 at more moderate conditions by addition of catalysts (60% recovery 8 ). Kinetic properties have also been shown to improve by using composite materials like Ca(BH 4 ) 2 + MgH 2 . 9 Borohydrides are largely ionic compounds with a general formula M(BH 4 ) n , consisting of metal cations M n+ , the hydrogen atoms being covalently bound to the boron, forming tetrahedral BH 4 À . The possible hydrogen dynamics are long-range translational diffusion and localized motions such as rotations of the BH 4 À complexes along specific axes, librations of the complexes, and vibrations within the complexes. Rotational dynamics are often coupled to orderÀdisorder phase transition in coordination compounds, 10,11 and the decomposition of borohydrides could possibly involve long-range diffusion of H and/or of the whole BH 4 À complex. The first results published on the rotational reorientation of the BH 4 À unit in borohydrides date back to the 1950s. At that time, researchers were interested in understanding the nature of the interactions influencing molecular reorientations in solids. To our knowledge, the first experimental study ever published, giving the energy barriers for the reorientations of BH 4 À in sodium, potassium, and rubidium borohydrides, was performed using nuclear magnetic resonance (NMR) and date from 1955, 12 while a second was published on lithium, sodium, and potassium borohydrides 14 years after, ABSTRACT: In this work, hindered rotations of the BH 4 À tetrahedra in Mg(BH 4 ) 2 were studied by quasielastic neutron scattering, using two instruments with different energy resolution, in combination with density functional theory (DFT) calculations. Two thermally activated reorientations of the BH 4 À units, around the 2-fold (C 2 ) and 3-fold (C 3 ) axes were observed at temperatures from 120 to 440 K. The experimentally obtained activation energies (E aC 2 = 39 and 76 meV and E aC 3 = 214 meV) and mean residence times between reorientational jumps are comparable with the energy barriers obtained from DFT calculations. A linear dependency of the energy barriers for rotations around the C 2 axis parallel to the MgÀMg axis with the distance between these two axes was revealed by the DFT calculations. At the lowest temperature (120 K) only 15% of the BH 4 À units undergo rotational motion and from comparison with DFT results it is expectedly the BH 4 ...

X-ray absorption and X-ray photoelectron spectroscopy are used to investigate NaAlH4 doped with 5 mol % of Ti on the basis of either TiCl3 or Ti13.6THF by ball milling. X-ray photoelectron spectroscopy (XPS) analysis of TiCl3 or Ti colloid doped samples indicates that Ti species do not remain on the sample surface but are driven into the material with increasing milling time. The surface concentration of Ti continues to decrease during subsequent cycles under hydrogen. After several cycles, it reaches a constant value of 0.5 at. % independently of the nature of the precursor. Moreover, metallic aluminum is already present at the surface after 2 min of ball milling in the case of TiCl3 doped Na-alanate, whereas it is totally absent in the case of Ti colloid doped samples at any milling time. Upon cycling, the atomic concentration of metallic Al at the surface evolves with the reaction under hydrogen, in contrast to the Ti concentration. Analysis of the binding energies of samples doped with TiCl3 or Ti colloid, after eight desorption/absorption cycles, reveals that the Na, O, and Ti environment remains the same, while the Al environment undergoes changes. According to the extended X-ray absorption fine structure (EXAFS) analysis of TiCl3 doped Na-alanate, the local structure around Ti during the first cycle is close to that of metallic Ti but in a more distorted state. In the case of the Ti colloid doped sample, a stripping of the oxygen shell occurs. After eight cycles, a similar intermetallic phase between Ti and Al is present in the hydrogenated state of TiCl3 or Ti colloid doped samples. The local structure around Ti atoms after eight cycles consists of Al and Ti backscatterers with a Ti-Al distance of 2.79 angstroms and a Ti-Ti distance of 3.88 angstroms. This local structure is not exactly the TiAl3 phase because it differs significantly from the alloy phase in its fine structure and lacks long-range order. Volumetric measurements performed on these samples indicate that the formation of this local structure is responsible for the reduction of the reversible hydrogen capacity with the increasing number of cycles. Moreover, the formation of the alloy-like phase is correlated with a decrease of the desorption/absorption reaction rate.

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Hydrogen dynamics in crystalline calcium borohydride can be initiated by long-range diffusion or localized motion such as rotations, librations, and vibrations. Herein, the rotational and translational diffusion were studied by quasielastic neutron scattering (QENS) by using two instruments with different time scales in combination with density functional theory (DFT) calculations. Two thermally activated reorientational motions were observed, around the 2-fold (C 2 ) and 3-fold (C 3 ) axes of the BH 4 -units, at temperature from 95 to 280K. The experimental energy barriers (Ea C2 ) 0.14 eV and Ea C3 ) 0.10 eV) and mean residence times are comparable with those obtained from DFT calculations. Long-range diffusion events, with an energy barrier of E aD ) 0.12 eV and an effective jump length of ∼2.5 Å were observed at 224 and 260 K. Three vacancymediated diffusion events, H jumps between two neighboring BH 4 -, and diffusion of BH 4 -and BH 3 groups were calculated and finally discarded because of their very high formation energies and diffusion barriers. Three interstitial diffusion processes (H, H 2 , and H 2 O) were also calculated. The H interstitial was found to be highly unstable, whereas the H 2 interstitial has a low energy of formation (0.40 eV) and diffusion barrier (0.09 eV) with a jump length (2.1 Å) that corresponds well with the experimental values. H 2 O interstitial has an energy of formation of -0.05 eV, and two different diffusion pathways were found. The first gives a H jump distance of 2.45 Å with a diffusion barrier of 0.68 eV, the second one, more favorable, exhibits a H jump distance of 1.08 Å with a barrier of 0.40 eV. The correlation between the QENS and DFT calculations indicates that, most probably, it is the diffusion of interstitial H 2 that was observed. The origin of the interstitial H 2 might come from the synthesis of the compound or a side reaction with trapped synthesis residue leading to the partial oxidation of the compound and hydrogen release.

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