Hydrogen vacancies facilitate hydrogen transport kinetics in sodium hydride nanocrystallites

Hydrogen vacancies facilitate hydrogen transport kinetics in sodium hydride nanocrystallites

Singh, S.
Eijt, S.W.H.

Applied Sciences

Radiation, Radionuclides and Reactors

2008-12-30

We report ab initio calculations based on density-functional theory, of the vacancy-mediated hydrogen migration energy in bulk NaH and near the NaH(001) surface. The estimated rate of the vacancy mediated hydrogen transport, obtained within a hopping diffusion model, is consistent with the reaction rates of H-D exchange in nano-NaH at the relatively low temperatures observed in recent neutron studies on TiCl3-doped NaAlH4. We further obtained the formation energy for hydrogen vacancies and interstitials in NaH in all relevant charged states. These formation energies are too high to lead to the abundant hydrogen concentrations seen experimentally. Ab initio calculations on the NaCl//NaH interface are presented to provide an insight into the mechanism which may lead to high hydrogen concentrations. We show that the formation of an fcc-Na interlayer during the growth of NaH on top of NaCl is plausible, providing a source of vacancies and leading to fast hydrogen transport. The low interface energies for NaCl//NaH are consistent with an easy growth of NaH crystallites on NaCl nucleation centers, which may, therefore, act as grain refiners.

ab initio calculations
charge exchange
density functional theory
diffusion
hydrogen
interstitials
nanostructured materials
reaction rate constants
sodium compounds
surface energy
vacancies (crystal)

http://resolver.tudelft.nl/uuid:3632cb10-4454-49ab-91c4-6df5dfcfd5b4

American Physical Society

0163-1829

Physical Review B, 78 (22), 2008

Institutional Repository

journal article

(c) Singh, S. ; Eijt, S.W.H. ; American Physical Society