Influence of cycling on the thermodynamic and structure properties of nanocrystalline magnesium based hydride

We have investigated the effect of prolonged cycling on the hydriding/dehydriding properties and on the structure of nanocrystalline MgH2-V composite produced by high-energy ball milling. The hydrogen charge and discharge kinetics of the nanocomposite hydride were tested at 300 degrees C using up to 2000 cycles. Pressure composition isotherms at 300 degrees C were also carried out. The nanocomposite exhibits good reversibility in its hydrogenation/dehydrognation curves after 2000 cycles. The results show some improvements in hydrogen capacity during cycling; this enhanced H-solubility is believed to be the result of structural relaxation. The sample resistance to hydrogen decrepitation was also evaluated via additional experiments involving SEM, BET specific surface area and X-ray crystal structure characterisations. These observations indicate that the nanostructured Mg-based composite does not decrepitate much upon cycling. However, a slight deterioration in the discharge rate of the nanocrystalline magnesium hydride is observed, apparently related to the crystal growth during cycling. (C) 2000 Elsevier Science S.A. All rights reserved.