Point-defect kinetics in alpha - and gamma -MgH sub(2)

• Published in: International journal of hydrogen energy 2016-04, Vol.41 (13), p.5688-5692
• Main Authors: Sander, Jonas M, Ismer, Lars, Van de Walle, Chris G
• Format: Article
• Language: English
• Subjects: Barriers; Crystal structure; Desorption; Hydrogen storage; Hydrogen-based energy; Mathematical analysis; Transport; Vacancies
• ISSN: 0360-3199
• DOI: 10.1016/j.ijhydene.2016.01.156

The kinetics of hydrogen desorption from storage materials in principle depend on the crystalline phase of the material. In MgH sub(2), desorption rates may be higher in the crystalline gamma phase compared to the equilibrium bulk alpha phase. It has been suggested [R. A. Varin, T. Czujko, Z. Wronski, Nanotechnol., 17 (15) (2006) 3856-3865] that this effect is responsible for enhanced desorption from ball-milled MgH sub(2), since smaller particles contain a higher proportion of the metastable gamma phase. We investigate hydrogen transport kinetics in these phases of MgH sub(2) by using first-principles calculations based on density functional theory. Imposing charge neutrality, we find that the formation energy of hydrogen vacancies in gamma -MgH sub(2) is smaller by 0.032 eV compared to alpha -MgH sub(2). Our calculations of migration barriers show that the only relevant point defect for mass transport in both crystal structures is the positively charged hydrogen vacancy, and that its lowest migration barrier in gamma -MgH sub(2) is 0.02 eV lower than in alpha -MgH sub(2). We conclude that hydrogen vacancies exist in higher concentrations and are also more mobile in the gamma phase than in the alpha phase, thus explaining the faster dehydrogenation kinetics of gamma -MgH sub(2).