Improvement in hydrogen sorption kinetics of MgH2 with Nb hydride catalyst

Nanocrystalline MgH2 with fine and evenly dispersed Nb hydride was prepared by ball milling a mixture of MgH2 and 1 mol.% NbF5. This NbH-catalyzed MgH2 desorbed 6.3 wt.% H-2 in 15 min and absorbed more than 90% of its initial hydrogen capacity within 5 min at 573 K. Moreover, this fast sorption kinetics was maintained after 10 cycles. Based on X-ray diffraction and transmission electron microscopy/energy-dispersive spectroscopy analyses, it is suggested that NbF5 melts during high-energy ball milling and this promotes the formation of extremely fine, film-like Nb hydride preferentially along the grain boundaries of nanocrystalline MgH2 by a liquid/solid reaction. This unique nanostructured Nb hydride is believed to suppress the grain growth of MgH2 quite effectively and thus maintain its initial catalytic effect throughout repeated hydrogenation-dehydrogenation cycles. (C) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.