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First-Principles Study of Intrinsic Defects in Ammonia Borane Jianchuan Wang, Christoph Freysoldt, Yong Du, and Lixian Sun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07273 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 30, 2017
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The Journal of Physical Chemistry
First-principles Study of Intrinsic Defects in Ammonia Borane Jianchuan Wang*,1, Christoph Freysoldt2, Yong Du1,#, Lixian Sun3 1
State Key Laboratory of Powder Metallurgy, Central South University, 410083, Changsha, China
2
Max-Planck-Institut für Eisenforschung GmbH, D-40237 Düsseldorf, Germany
3
School of Material science and Engineering, Guilin University of Electronic Technology, 541004, Guilin, China
*Corresponding author:
[email protected] #
Corresponding author:
[email protected] Abstract Solid ammonia borane is a promising hydrogen storage material, but suffers from a slow and ill-controlled dehydrogenation process. We studied intrinsic point defects that might play a role for hydrogen release by means of first-principles calculations augmented with van der Waals (vdW) corrections. The vdW corrections proved to be crucial for structural properties, and also for energies in some cases. For vacancy and interstitial defects of single H as well as of molecular (NH3, BH3) type, we determined formation energies and local lattice structures of the defects in various charge states. Atomic H-related vacancies and interstitials exist predominantly in charged states in agreement with their chemical, i.e., protonic or hydridic character. For molecular defects, some NH3- and BH3-related neutral defects have rather low formation energies, suggesting that the dehydrogenation of undoped ammonia borane is initiated by the cleavage of the B-N bond. The relaxation pictures associated with H-related defects can explain the observation of a variety of oligomeric products in experiment. Besides, some low-energy defects are found to spontaneously form H2 molecules, and thus might catalyze the dehydrogenation reaction of doped ammonia borane.
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The Journal of Physical Chemistry
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Ⅰ Introduction The use of hydrogen as an alternative energy carrier requires efficient and safe methods for hydrogen storage. In particular for cars, widespread adoption of hydrogen fuel depends critically upon the ability to store hydrogen on-board at high volumetric and gravimetric densities, as well as on the ability to extract and insert it at sufficiently high rates. A broad range of materials have been proposed for hydrogen storage, such as conventional metal hydrides 1, 2, carbon- or boron-based sorbents1, 3, 4, metal-organic frameworks5, 6, complex hydrides1 and chemical hydrides1, 7. Among them, ammonia borane (NH3BH3, abbreviated in the following as AB), has received considerable attention as a potential hydrogen storage material due to its relatively high hydrogen gravimetric density of 19.6 wt%8. Its practical application is limited by its slow dehydrogenation kinetics below 100 °C and the concurrent release of the fuel cell poisons, such as borazine (NHBH)3 and diborane B2H69-11. Yet, these difficulties might be overcome eventually by carefully controlling and catalyzing the hydrogen release process. This calls for an in-depth understanding of possible dehydrogenation routes at the atomic level. At ambient pressure and low temperature (
