期刊
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
卷 105, 期 10, 页码 3673-3677出版社
NATL ACAD SCIENCES
DOI: 10.1073/pnas.0709224105
关键词
density-functional theory; hydrogen storage; kinetics; metal hydride; molecular dynamics
Clarification of the mechanisms of hydrogen release and uptake in transition-metal-doped sodium alanate, NaAlH4, a prototypical high-density complex hydride, has fundamental importance for the development of improved hydrogen-storage materials. In this and most other modern hydrogen-storage materials, H-2 release and uptake are accompanied by long-range diffusion of metal species. Using first-principles density-functional theory calculations, we have determined that the activation energy for Al mass transport via AlH3 vacancies is Q = 85 kJ/mol.H-2, which is in excellent agreement with experimentally measured activation energies in Ti-catalyzed NaAlH4. The activation energy for an alternate decomposition mechanism via NaH vacancies is found to be significantly higher: Q = 112 kJ/mol.H-2. Our results suggest that bulk diffusion of At species is the rate-limiting step in the dehydrogenation of Ti-doped samples of NaAlH4 and that the much higher activation energies measured for uncatalyzed samples are controlled by other processes, such as breaking up of AlH4- complexes, formation/dissociation of H-2 molecules, and/or nucleation of the product phases.
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