The effect of organic additive in Mg/graphite composite as hydrogen storage materials

摘要

Among the metals that form hydrides, Mg and Ti are the lightest materials; however, they are covered with passive layer of oxides and hydroxides and require activation treatment under high temperature (>300 deg C) and hydrogen pressure (>3 MPa) before being used for storage and transport applications. It is well known that addition of small amounts of graphite to Ti or Mg, lead to a dramatic change in the kinetics of mechanically induced (ball milling) hydrogen sorption and significantly stimulates the Ti-hydrogen interaction. Imamura et al. in a series of publications in this journal confirmed that the magnesium/graphite (Mg/C) composite formed by mechanical milling with the organic additives not only show improved hydriding-dehydriding properties of magnesium but also additional hydrogen uptake other than that due to the magnesium component. Among the organic additives (benzene, cyclohexene, cyclohexane, and tetrahydrofurane) which were tried by Imamura et al. tetrahydrofurane was the most promising. Tetrahy-drofurane required only 1 h of ball milling and the least amount of volume used during ball milling was needed compared to the other additive. The objective of this publication is to suggest a mechanism for the effect of tetrahydrofurane. The role of THF is possibly due to the value of the mw/e (mw is the molecular weight and e is the dielectric constant) for THF being in the range in which the solvent provide a medium where complex formation between Mg molecules and the solvents is maximum. This can happen when halide impurities undergo chemical transformations to form Grignard type reagents (organomegnesium halides). Thus the values of mw/epsilon can be used as an initial guide to select the proper solvent.