Crystal structures, phase stability, and decomposition reactions in the quaternary Mg-B-N-H hydrogen storage system

Yongsheng Zhang*, David Farrell, Jun Yang, Andrea Sudik, C. Wolverton

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

7 Scopus citations

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(NH 2)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(NH 2)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 H 2 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.

Original languageEnglish (US)
Pages (from-to)11193-11202
Number of pages10
JournalJournal of Physical Chemistry C
Volume118
Issue number21
DOIs
StatePublished - May 29 2014

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Energy(all)
  • Physical and Theoretical Chemistry
  • Surfaces, Coatings and Films

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