Thermodynamic and kinetic properties of potassium alanate (KAlH 4 ) are investigated. Its pressure-compositionisotherm measurement exhibits two plateaus for hydrogen absorption/desorption in KAlH 4 , with gravimetric hydrogen densities of 1.2 ( 0.1 and 2.6 ( 0.2 mass% and reaction enthalpies of 81 and 70 kJ • mol -1 H 2 , respectively. However, the nonisothermal decomposition of KAlH 4 occurs through three endothermic events at temperatures of 294, 311, and 347 °C with the release of hydrogen. Whereas the high temperature event is clearly attributed to K 3 AlH 6 decomposition, the low temperature events occur by two reactions, denoting the existence of an intermediate phase during KAlH 4 decomposition. FTIR measurements suggest that this intermediate phase is a K y AlH x compound (y g 1, x g 4) with a high coordination about the aluminum. TiCl 3 -doped KAlH 4 also exhibits three decomposition events, but with significant reduction of desorption temperatures (∼50 °C) as well as activation energies that is attributed to particle size reduction and creation of charged vacancies.

Structural properties and reversible deuterium uptake of MgD 2 −TiD 2 nanocomposites have been studied by joint X-ray and neutron diffraction analyses to shed light on the extremely fast hydrogenation kinetics of these materials. (1 − x)MgD 2 −xTiD 2 nanocomposites with compositions ranging between x = 0 and 0.5 have been prepared by reactive ball milling of Mg and Ti powders under deuterium pressure. They consist of mixtures of MgD 2 (β-and γ-polymorphs) and ε-TiD 2 phases homogenously distributed at the nanoscale with crystallite sizes below 15 nm. Minor phase miscibility is detected with Mg solubility in the TiD 2 phase up to 8 at.% and Ti solubility in the β-MgD 2 up to 7 at.% Ti. At moderate temperatures and pressures (T < 600 K, P D2 < 1 MPa) reversible deuterium loading in MgD 2 −TiD 2 nanocomposites only occurs through the β-MgD 2 to Mg transformation. Mg/MgD 2 thermodynamics is not modified as γ-MgD 2 and Ti solubility in β-MgD 2 are metastable and do not operate during reversible deuterium loading. However, the TiD 2 phase allows for outstanding D-sorption kinetics in the Mg/MgD 2 system. This paper demonstrates that TiD 2 inclusions limit the grain growth of Mg and MgD 2 phases allowing for short D-diffusion paths. Furthermore, we provide evidence that the TiD 2 phase also favors H-mobility through the existence of coherent coupling between TiD 2 and Mg/MgD 2 phases and the presence of sub-stoichiometric MgD 2-η and TiD 2-η phases.

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Tribochemical decomposition of magnesium hydride (MgH2) induced by deformation at room temperature was studied on a micrometric scale, in situ and in real time. During deformation, a near-full depletion of hydrogen in the micrometric affected zone is observed through an instantaneous (t < 1 s) and huge release of hydrogen (3-50 nmol/s). H release is related to a nonthermal decomposition process. After deformation, the remaining hydride is thermally decomposed at room temperature, exhibiting a much slower rate than during deformation. Confocal-microRaman spectroscopy of the mechanically affected zone was used to characterize the decomposition products. Decomposition was enhanced through the formation of the distorted structure of MgH2 with reduced crystal size by mechanical deformation.