Space charge region induced self-evolution dehydrogenation of polymorphic magnesium-based hydrides

In the field of metal hydride hydrogen storage, Mg₂NiH₄ is favoured for its high hydrogen storage capacity, fast hydrogenation/dehydrogenation kinetics, and unique electronic structure. The diversity of polymorphic Mg₂NiH₄ and complexity in phase transformation pose challenges on study of controllable preparation, structural analysis and hydrogen storage properties. Besides, one of the major factors plaguing metal hydride applications is air stability. In this work, an in-depth analysis of the Mg₂NiH₄ isomers was conducted by examining their thermodynamic, electrical, and microstructural differences. Differs from other metal hydrides, it is astonishing that the onset dehydrogenation temperature of Mg₂NiH₄ decreases after air exposure. This phenomenon, which was called self-evolution dehydrogenation of Mg₂NiH₄, occurred in both the high and low temperature phases of Mg₂NiH₄. The main components of the surface passivation layer resulting from air exposure of polymorphic Mg₂NiH₄ were clarified in this work. Self-evolution dehydrogenation can be attributed to the electronic structure variation of Ni/Ni(OH)₂ and Ni/NiO in the passivation layer. Our findings provide some insights on phase composition determination of multi-metal hydrides. Furthermore, the findings also benefit for design of high-active and oxidation-resistant metal hydrides.