Crystal Structures, Phase Stability, and Decomposition Reactions in the Quaternary Mg–B–N–H Hydrogen Storage System

Abstract

Using the combination of DFT-based computational approaches and experimental measurements, we have studied the crystal structure, phase stability, and decomposition products of mixed Mg(NH2)2/Mg(BH4)2 materials. We find the following: (i) DFT crystal structure prediction calculations (0 K) show the existence of a mixed Mg(NH2)2/Mg(BH4)2 phase, which is thermodynamically stable relative to its separated phases [Mg(NH2)2 and Mg(BH4)2]. (ii) The DFT calculated phonon density of states of Mg(NH2)(BH4) is in good agreement with the peak positions from experimental PAS IR measurements (at the room temperature) of a ball-milled Mg(NH2)2/Mg(BH4)2 mixture, suggesting the mixture is not merely a physical mixture of the individual compounds. (iii) The experimentally measured dehydrogenation temperature of the mixed Mg(NH2)2/Mg(BH4)2 phase is lower than that of Mg(NH2)2 or Mg(BH4)2, which further confirms that it is not a simply physical mixture of Mg(NH2)2 and Mg(BH4)2. The observed amount of H2 release is 3.4 wt % at 250° and 8.3 wt % above 280°. (iv) From a combination of DFT, the grand-canonical linear programming (GCLP) method calculations, and PAS IR measurements of dehydrogenated samples, we identify the existence of the B–H bonds and linear N–B–N units in the decomposition of Mg(NH2)2/Mg(BH4)2. (v) Experimental desorption measurements reveal that the Mg(NH2)2/Mg(BH4)2 mixed phase is irreversible, consistent with DFT calculated enthalpies in the range of −18 to +16 kJ/(mol H2), too low for near-ambient reversible storage.