Dehydrogenation kinetics of pure and nickel-doped magnesium hydride investigated by in situ time-resolved powder X-ray diffraction

Abstract

The dehydrogenation kinetics of pure and nickel (Ni)-doped (2 w/w%) magnesium hydride ( MgH 2 ) have been investigated by in situ time-resolved powder X-ray diffraction (PXD). Deactivated samples, i.e. air exposed, are investigated in order to focus on the effect of magnesium oxide (MgO) surface layers, which might be unavoidable for magnesium (Mg)-based storage media for mobile applications. A curved position-sensitive detector covering 120 ∘ in 2 θ and a rotating anode X-ray source provide a time resolution of 45 s and up to 90 powder patterns collected during an experiment under isothermal conditions. A quartz capillary cell allowed the in situ study of gas/solid reactions. Three phases were identified: Mg, MgH 2 and MgO and their phase fractions were extracted by Rietveld refinement or integration of selected reflections from each phase. Dehydrogenation curves were constructed and analysed by the Johnson–Mehl–Avrami formalism in order to derive rate constants at different temperatures. The apparent activation energies for dehydrogenation of pure and Ni-doped magnesium hydride were E A ≈ 300 and 250 kJ / mol , respectively. Differential scanning calorimetry gave, E A = 270 kJ / mol for dehydrogenation of the Ni-doped sample. The relatively high activation energies are due to MgO surface layers, retarding the diffusion of hydrogen ( H 2 ) out of MgH 2 / Mg . The observed difference in E A of ca. 50 kJ/mol is likely due to the catalytic effect of Ni on the recombination of H atoms to H 2 molecules verified by theoretical considerations.