Activation characteristics of graphite modified hydrogen absorbing materials

Abstract

The activation characteristics for hydrogen absorption of pure Mg, Mg 2Ni, Ti, V and FeTi is compared with that of the materials obtained by adding 10 wt.% of graphite and milling the mixture during 30 min. In all cases, there is a dramatic improvement of the activation characteristics of the material containing graphite. For example, with Mg/G (at 300°C and 10 bar of H 2), the hydrogen contents [H]=4, 5 and 6 wt.% are reached after 0.73, 1.97 and 10.5 h after the first initial exposure to hydrogen, respectively. About 92% of the maximum hydrogen capacity of the material is reached after 20 h. In the same conditions, pure Mg does not absorb. A similar behavior is also observed in the case of Ti, V, Mg 2Ni and FeTi. As far as we can tell from the analysis of the X-ray diffraction pattern, the introduction of graphite does not lead to the formation of any new phase, although milling with graphite is responsible for a sizeable reduction of the crystallite size, even if the milling operation is performed for only 30 min. This reduction of the crystallite size is not responsible for the improved activation characteristics, as materials with similar crystallite size prepared without graphite do not show the same behavior. Instead, it is proposed that graphite inhibits the formation of a new oxide layer of the surface of the materials once the native oxide layer is broken during the milling process.