The roles of native defects and transition metal additives in the dehydrogenation mechanism of Mg(AlH4)2

Abstract

The roles of native defects and transition-metal additives (Ti, Sc and Ni) in the dehydrogenation of Mg(AlH4)2 hydride were investigated by First-principles calculations based on density functional theory. The elementary native defects including Hi−, VH+, VH−, (H2)i0, VMg2−, Mgi2+, and VAlH4+ in Mg(AlH4)2 were identified. Based on the formation and migration of dominant defects, we proposed a dehydrogenation mechanism of Mg(AlH4)2. The formation of the dominant defect VAlH4+ is the rate-limiting step in the dehydrogenation process. The highly mobile Hi− diffuses into the lattice and binds with Mg2+ to produce MgH2 phase. In the transition metal doped hydrides, interstitial defects Tii and Sci cause the Fermi-level shift to the left and lead to the decrease of the formation energy and activation energy of VAlH4+, which is beneficial to the dehydrogenation of Mg(AlH4)2. This study presents an in-depth understanding on the roles of native defects and transition metal additives in the dehydrogenation process of Mg(AlH4)2 hydride.