Several lightweight metal hydrides such as MgH2 and NaAlH4 have high gravimetric hydrogen contents. However, they suffer from relatively slow kinetics and high hydrogen-release temperatures. One strategy for improving the hydrogen-release properties of these materials is to reduce the particle size and add small amounts of highly dispersed catalysts.

Testing the idea

Researchers at the Debye Institute for Nanomaterials Science at Utrecht University have pioneered a melt infiltration method to prepare carbon-supported light metal hydride nanomaterials such as nanosized MgH2. In a paper published recently in Nanotechnology, the group describes the preparation method and reveals the subsequent hydrogen sorption properties of its carbon supported Mg(Ni)Hx nanocrystallites. The materials were prepared by magnesium melt infiltration of nanoporous carbon-containing nickel nanoparticles, followed by cooling and hydrogenation.

Bulk MgH2 contains 7.7wt% of hydrogen, although the gas is released typically only at 400–450 °C. From the bulk phase diagram full phase segregation into MgH2 and Mg2NiH4 would be expected, but the supported nanomaterials showed no hydrogen release in this temperature range.

Favourable result

Even for high MgH2 loadings (50wt%) and a relatively low nickel content (corresponding to Mg0.95Ni0.05 atomic composition) hydrogen-release temperatures were effectively lowered by 100–200 °C compared with bulk MgH2. The favorable hydrogen desorption properties could be attributed mainly to the excellent kinetics brought about by the efficient mixing of magnesium, nickel and carbon on the nanoscale.

The results illustrate the potential of this preparation technique for designing nanoscale mixtures of metals and provide access to studies on the thermodynamic and kinetic hydrogen sorption properties of nanocrystalline mixed light-metal alloy (hydrides).