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

T.R. Jensen, A. Andreasen, Tejs Vegge, Jens Wenzel Andreasen, Kenny Ståhl, Allan Schrøder Pedersen, Martin Meedom Nielsen, A.M. Molenbroek, F. Besenbacher

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Abstract

The dehydrogenation kinetics of pure and nickel (Ni)-doped (2w/w%) magnesium hydride (MgH2) 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 degrees in 20 and a rotating anode X-ray source provide a time resolution of 45 s and up to 90 powder pattems 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, MgH2 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-Mehi-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 approximate to 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 MgH2/Mg. The observed difference in E-A of ca. 50 kJ/moI is likely due to the catalytic effect of Ni on the recombination of H atoms to H-2 molecules verified by theoretical considerations.
Original languageEnglish
JournalInternational Journal of Hydrogen Energy
Volume31
Issue number14
Pages (from-to)2052-2062
ISSN0360-3199
DOIs
Publication statusPublished - Nov 2006

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Keywords

  • in situ powder X-ray diffraction
  • hydrogen storage materials
  • gas-solid reactions
  • nickel-doped magnesium hydride
  • differential scanning calorimetry
  • dehydrogenation kinetic
  • magnesium hydride

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