Dresden 2011 – scientific programme

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MM: Fachverband Metall- und Materialphysik

MM 3: Computational Materials Modelling I

MM 3.8: Talk

Monday, March 14, 2011, 12:45–13:00, IFW B

Point-defect-mediated dehydrogenation of alane — •Lars Ismer, Anderson Janotti, and Chris G Van de Walle — University of California at Santa Barbara, CA 93106, United States

For the engineering of better hydrogen storage materials a systematic understanding of their hydrogen sorption kinetics is crucial. We present a systematic analysis of the dehydrogenation kinetics of alane (AlH3), one of the prime candidate materials for hydrogen storage. Using hybrid-density functional calculations we determine the concentrations and mobilities of point defects and their complexes. Kinetic Monte Carlo simulations are used to describe the full dehydrogenation reaction. We show that under dehydrogenation conditions charged hydrogen vacancy defects form in the crystal, which have a strong tendency towards clustering. The vacancy clusters denote local nuclei of Al phase, and the growth of these nuclei eventually drives the AlH3/Al transformation. However, the low concentration of vacancy defects limits the transport of hydrogen across the bulk, and hence acts as the rate-limiting part of the process. The dehydrogenation is therefore essentially inactive at room temperature, explaining why AlH3 is metastable for years, even though it is thermodynamically unstable. Our derived activation energy and dehydrogenation curves are in excellent agreement with the experimental data, providing evidence for the relevance of bulk point-defect kinetics. Based on our results we argue that manipulating vacancy defect concentrations, e.g., by the usage of irradiation, will allow control over hydrogen sorption kinetics, opening up new engineering strategies.