Effect of carbon-containing compounds on the hydriding behavior of nanocrystalline Mg2Ni

Nanocrystalline Mg2Ni was prepared by high-energy ball milling during 40 h and then further milled during 30 min with 3, 5 and 10 wt.% of carbonaceous compounds. Three carbonaceous compounds were used, namely fullerene (C-60), graphite and a highly conducting carbon support (Vulcan XC72). The structural and hydriding characteristics of the resulting materials were compared with those of the primitive material. The specific surface area of nanocrystalline Mg2Ni increases from 1.06 m(2) g(-1) to 3.45 m(2) g(-1) or 6.00 m(2) g(-1) when it is milled with 5 or 10 wt.% C-60. The dissolution of C-60 in toluene yields a further increase of the specific surface area. The specific surface area of Mg2Ni milled with 5 wt.% graphite is 3.61 m(2) g(-1), but almost no change in the specific surface area of Mg2Ni is observed when it is milled with Vulcan. No change in the crystallographic structure of Mg2Ni could be evidenced by X-ray diffraction as a result of the milling step with either carbonaceous compound. When normalized for the total amount of hydride-forming material, Mg2Ni/C-60, Mg2Ni/graphite and Mg2Ni/Vulcan have the same hydrogen absorption capacity as that of nanocrystalline Mg2Ni. On the contrary an irreversible loss of hydrogen absorption capacity is observed following the leaching of C-60 from the material. This most probably results from a slight oxidation of magnesium during the leaching process. At 275 degrees C, there is no noticeable change in the kinetics of absorption between the various compounds, but the time needed to complete desorption of Mg2Ni under vacuum is drastically reduced by a factor of 2-3 for Mg2Ni/C-60, Mg2Ni/graphite and Mg2Ni/Vulcan. (C) 2000 Elsevier Science S.A. All rights reserved.