Hydrogenation Properties of Mg_83.3Cu_7.2Y_9.5 with Long Period Stacking Ordered Structure and Formation of Polymorphic γ-MgH₂

Abstract

Nanosizing is known to affect the hydrogenation properties of magnesium. For this reason, the long period stacking ordered (LPSO) structures, made of the stacking of nanolayers of magnesium and nanolayers of Mg-A-B (with A = rare earth and B = transition metal), were herein considered. A Mg83.3Cu7.2Y9.5 LPSO compound with 18R structure was successfully synthesized. Its hydrogenation properties were investigated at temperatures between 150 and 400 °C. The X-ray diffraction (XRD) analysis indicates that the LPSO structure decomposes into magnesium hydride, yttrium hydride, and an intermetallic compound (Mg2Cu or MgCu2). The pressure composition (PC) isotherm for Mg83.3Cu7.2Y9.5 at 400 °C combined with XRD analysis allows one to understand the three-step hydrogenation pathway, detailed in this paper. At this hydrogenation temperature, the fully hydrogenated compound contains magnesium hydride exclusively crystallized in the most stable tetragonal structure (100% of α-MgH2 was formed). When the pristine LPSO was hydrogenated at lower temperature, the amount of α-MgH2 decreased, while its polymorphic structure, γ-MgH2, appeared. Finally, hydrogenation of Mg83.3Cu7.2Y9.5 at 150 °C led to the formation of γ-MgH2 with a high phase fraction (82% of γ-MgH2/MgH2). These results suggest that the crystallographic structure of the magnesium hydride can be controlled by the hydrogenation temperature of LPSO compounds.

Fabrication of Multiple-Phase Magnesium-Based Hydrides with Enhanced Hydrogen Storage Properties by Activating NiS@C and Mg Powder

“…In the past few years, researchers had tried many approaches to accelerate the thermodynamics and kinetics of MgH 2 , such as nanocrystallization, , alloying, − adding catalysts, − and constructing complex hydrides systems. , Among them, multiphase Mg-based hydrides possessed more thermal instability than the single MgH 2 phase. , However, the reduction of the hydrogen storage capacity for multiphase Mg-based hydrides is the major deficiency that needs to be addressed. Facing this problem, a simple and effective method is to establish MgH 2 -rich complex hydrides, such as MgH 2 –Mg 2 NiH 4 , which exhibits significant thermodynamic instability without an obvious loss in capacity .…”