Magnesium-Rich Compounds and LPSO Phases for Hydrogen Storage: A Review

This review provides an overview of magnesium-rich compounds and Long-Period Stacking Ordered (LPSO) phases for their hydrogen storage properties. Thanks to their high volumetric density, safety, and exceptional purity, metal hydrides are promising for hydrogen storage. Magnesium is a great candidate as it can form MgH2, which has a weight capacity of 7.6 wt.%. However, due to its high stability (at 283 °C, equilibrium pressure is 1 bar (i.e., atmospheric pressure)) and slow hydrogen sorption kinetics, Mg is alloyed with TMs (transition metals) and/or REs (rare earths) to overcome these problems. Some alloys that are synthesized with both TMs and REs (ternary system) form LPSO phases, which irreversibly decompose under hydrogenation. The LPSO phases discussed in this review are mostly the 14H- and 18R-type phases, although, rarely, other types of LPSO phases can still be observed as well. These discussed phases may lead to good hydrogen sorption properties depending on the REs and TMs used. This review focuses on the recent literature addressing Mg-rich binary Mg-TM and Mg-RE alloys and ternary (TMx-REy-Mgz) systems and their hydrogen storage properties with an emphasis on LPSO phases.