Investigation of dehydrogenation mechanism of MgH2-Nb nanocomposites

Nanocomposites of composition MgH2-V and MgH2-Nb have been shown to have very fast hydrogen sorption kinetics. This could be explained by the presence of vanadium, which eases hydrogen penetration into the material, and by the particular microstructure of this nanocomposite. In this paper, we report a systematic structural study of the nanocomposite MgH2-Nb5at.%. To see the effect of hydrogenation/dehydrogenation process on the crystal structure, X-ray diffraction under hydrogen pressure was carried out at different temperatures. Crystallite size was evaluated by X-ray powder diffraction peak broadening. In order to see the evolution of crystal phases during the dehydrogenation process, real time X-ray investigation of dehydrogenation in MgH2-Nb nanocomposite was carried out using synchrotron radiation. It was found that a niobium hydride metastable phase, closely related to the low temperature epsilon-NbH phase, acts as a gateway for hydrogen. Activation energies of the hydrogenation and dehydrogenation processes are also estimated. (C) 2002 Elsevier Science B.V. All rights reserved.