Effect of carbon on hydrogen desorption and absorption of mechanically milled MgH2

The use of MgH2, instead of pure Mg, in the mechanical synthesis of Mg-based hydrogen storage materials offers added benefit to powder size refinement and reduced oxygen contamination. Alloying additions can further improve the sorption kinetics at a relatively low temperature. This paper examines the effect of graphitic carbon on the desorption and absorption of MgH2. Graphite powder of different concentrations were mechanically milled with MgH2 particles. The milled powder was characterised by XRD, SEM and simultaneous TG and DSC techniques. The results show that graphite poses little influence on the desorption properties of MgH2. However, it does benefit the absorption process, leading to rapid hydrogen uptake in the re-hydrogenated sample. After dehydrogenation, 5 wt.% of hydrogen was re-absorbed within 30 min at 250 °C for the (MgH2+10 G) mixture prior-milled for 8 h, while only 0.8 wt.% for the pure MgH2 milled for 8 h, the effect may be attributed to the interaction between crystalline graphite with H2 disassociation close to the MgH2 or Mg surface. Moreover, graphite can also inhibit the formation of a new oxide layer on the surface of Mg particles.