Enhancement of hydrogen sorption in magnesium hydride using expanded natural graphite

Magnesium hydrogenation reaction being exothermic and limited by heat removal, the thermal conductivity of ball-milled magnesium hydride (BM MgH2) powders has to be improved. The compression of BM MgH2 associated to Expanded Natural Graphite (ENG) to form compacted disks has been investigated. Using BM MgH2 without ENG, its compression reduces the porosity and increases its volumetric hydrogen storage capacity. Incorporating ENG before compression drastically improves the thermal conductivity in the direction normal to compression axis. Moreover, the thermal conductivity increases linearly with ENG content, and can be adjusted to fulfill the loading time requirements. The thermodynamic properties and intrinsic sorption kinetics remain unchanged. However, both compression and ENG incorporation reduce the hydrogen permeability, especially in the direction parallel to the compression axis, which imposes a limit to the disk thickness. A small-size instrumented tank has been loaded with either pure BM MgH2 powder or with disks having different ENG contents. The results obtained for both cases are compared.