Destabilizing the dehydriding thermodynamics of MgH2 by reversible intermetallics formation in Mg−Ag−Zn ternary alloys

In this study, Mg−Ag−Zn ternary alloys are employed as carriers for hydrogen storage, which show improved thermodynamic and kinetic properties compared with pure Mg. Mg90Ag7.5Zn2.5 alloy exhibits a reversible phase transformation during hydriding-dehydriding process with a maximum hydrogen storage capacity of approximately 4.2 wt%. Two-step reactions occur in the dehydriding process of Mg90Ag7.5Zn2.5. In the first step, a portion of MgH2 reacts with MgAg and MgZn1.8Ag0.2 phases to transform to Mg54(Ag, Zn)17 solid solution and release hydrogen. After that, the remaining MgH2 decomposes to Mg. Because of the modified reaction pathway in the first step dehydrogenation, the dehydriding equilibrium pressure at 300 °C increases to 0.28 MPa. Besides, the apparent activation energy (Ea) for dehydrogenation of the Mg90Ag7.5Zn2.5 alloy is lowered to 118.7 kJ mol−1. By fully utilizing the first step reaction, another alloy Mg78Ag16.5Zn5.5 is designed, which presents intact destabilized dehydriding thermodynamics and a lower dehydrogenation Ea of 116.5 kJ mol−1. Our work demonstrates that alloying Mg with elements that can form compounds with low formation enthalpies, and the hydrides of the elements have low stabilities, are the main guidelines for the designing of the Mg-based hydrogen storage alloys with destabilized dehydriding thermodynamics by reversible intermetallics formation.