Purity of MgH₂ Improved by the Process of Pre-milling Assisted Hydriding of Mg Powder under a Hydrogen Pressure of 0.5 MPa

Abstract: Magnesium hydride (MgH₂) is a very promising hydrogen storage material due to its high hydrogen storage capacity (7.6 wt %) and low cost. It has been paid increasing attention as it may be employed in supplying hydrogen on-board. However, the production of MgH₂ in high purity is still a challenging issue because the process requires high pressure and prolonged time. In this paper, high purity of MgH₂ was achieved by the process of pre-milling assisted hydriding of Mg powder under a hydrogen pressure as low as 0.5 MPa. The effects of hydrogen pressure as well as hydriding and mechanical milling parameters, such as ball-to-powder ratio and milling time were investigated systematically. The relationship between the morphologies and the purities of products were discussed in detail. Our results revealed that MgH₂ purity of 94.19 wt % can be achieved by hydriding with heating up to 853 K followed by cooling down to 593 K and keeping this temperature for 5 h after the milling pretreatment of Mg powder, at 400 rpm milling speed, 20 : 1 ball-to-powder ratio and 40 min milling time. SEM observation of the morphology of products indicated the achievement of the purity of MgH₂ under a hydrogen pressure as low as 0.5 MPa was related to the structural defects and activation of Mg powders caused by the mechanical milling.