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Title: Ultrafast Bulk Diffusion of AlHxin High-Entropy Dehydrogenation Intermediates of NaAlH4

Abstract

Using first-principles molecular dynamics (FPMD) and total-energy calculations, we demonstrate low-barrier bulk diffusion of Al-bearing species in γ-NaAlH4, a recently proposed high-entropy polymorph of NaAlH4. For charged AlH4– and neutral AlH3 vacancies, the computed barriers for diffusion are <0.1 eV, and we directly observe the predicted diffusive pathways in FPMD simulations at picosecond time scales. In contrast, such diffusion in the α phase is inaccessible to FPMD, consistent with much higher barriers. The transport behavior of γ-NaAlH4, in addition to key dynamical and structural signatures, is consistent with experimental observations of high-mobility species, strongly supporting the idea that an intermediate transition from the α phase to a high-entropy polymorph facilitates the hydrogen-releasing decomposition of NaAlH4. Our results provide an answer to longstanding questions regarding the responsible agent for the experimentally observed efficient Al transport during dehydrogenation and suggest that mass transport and phase transformation kinetics are coupled. Implications for understanding the (de)hydrogenation of undoped and catalyzed NaAlH4 are discussed.

Authors:
 [1];  [2];  [3];  [1];  [1];  [3]
  1. Ames Laboratory
  2. Lawrence Livermore National Laboratory
  3. Georgia Institute of Technology
Publication Date:
Research Org.:
Ames Laboratory (AMES), Ames, IA (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1166716
Report Number(s):
IS-J 8454
Journal ID: ISSN 1932-7447
DOE Contract Number:
DE-AC02-07CH11358
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Physical Chemistry. C; Journal Volume: 118; Journal Issue: 32
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; Electrochemical, Radiational, and Thermal Energy Technology

Citation Formats

Zhang, Feng, Wood, Brandon C, Wang, Yan, Wang, Cai-Zhuang, Ho, Kai-Ming, and Chou, Mei-Yin. Ultrafast Bulk Diffusion of AlHxin High-Entropy Dehydrogenation Intermediates of NaAlH4. United States: N. p., 2014. Web. doi:10.1021/jp504550m.
Zhang, Feng, Wood, Brandon C, Wang, Yan, Wang, Cai-Zhuang, Ho, Kai-Ming, & Chou, Mei-Yin. Ultrafast Bulk Diffusion of AlHxin High-Entropy Dehydrogenation Intermediates of NaAlH4. United States. doi:10.1021/jp504550m.
Zhang, Feng, Wood, Brandon C, Wang, Yan, Wang, Cai-Zhuang, Ho, Kai-Ming, and Chou, Mei-Yin. Thu . "Ultrafast Bulk Diffusion of AlHxin High-Entropy Dehydrogenation Intermediates of NaAlH4". United States. doi:10.1021/jp504550m.
@article{osti_1166716,
title = {Ultrafast Bulk Diffusion of AlHxin High-Entropy Dehydrogenation Intermediates of NaAlH4},
author = {Zhang, Feng and Wood, Brandon C and Wang, Yan and Wang, Cai-Zhuang and Ho, Kai-Ming and Chou, Mei-Yin},
abstractNote = {Using first-principles molecular dynamics (FPMD) and total-energy calculations, we demonstrate low-barrier bulk diffusion of Al-bearing species in γ-NaAlH4, a recently proposed high-entropy polymorph of NaAlH4. For charged AlH4– and neutral AlH3 vacancies, the computed barriers for diffusion are <0.1 eV, and we directly observe the predicted diffusive pathways in FPMD simulations at picosecond time scales. In contrast, such diffusion in the α phase is inaccessible to FPMD, consistent with much higher barriers. The transport behavior of γ-NaAlH4, in addition to key dynamical and structural signatures, is consistent with experimental observations of high-mobility species, strongly supporting the idea that an intermediate transition from the α phase to a high-entropy polymorph facilitates the hydrogen-releasing decomposition of NaAlH4. Our results provide an answer to longstanding questions regarding the responsible agent for the experimentally observed efficient Al transport during dehydrogenation and suggest that mass transport and phase transformation kinetics are coupled. Implications for understanding the (de)hydrogenation of undoped and catalyzed NaAlH4 are discussed.},
doi = {10.1021/jp504550m},
journal = {Journal of Physical Chemistry. C},
number = 32,
volume = 118,
place = {United States},
year = {Thu Aug 14 00:00:00 EDT 2014},
month = {Thu Aug 14 00:00:00 EDT 2014}
}
  • Using first-principles molecular dynamics (FPMD) and total-energy calculations, we demonstrate low-barrier bulk diffusion of Al-bearing species in γ-NaAlH 4, a recently proposed high-entropy polymorph of NaAlH 4. For charged AlH 4– and neutral AlH 3 vacancies, the computed barriers for diffusion are <0.1 eV, and we directly observe the predicted diffusive pathways in FPMD simulations at picosecond time scales. In contrast, such diffusion in the α phase is inaccessible to FPMD, consistent with much higher barriers. The transport behavior of γ-NaAlH 4, in addition to key dynamical and structural signatures, is consistent with experimental observations of high-mobility species, strongly supportingmore » the idea that an intermediate transition from the α phase to a high-entropy polymorph facilitates the hydrogen-releasing decomposition of NaAlH4. Lastly, our results provide an answer to longstanding questions regarding the responsible agent for the experimentally observed efficient Al transport during dehydrogenation and suggest that mass transport and phase transformation kinetics are coupled. Implications for understanding the (de)hydrogenation of undoped and catalyzed NaAlH 4 are discussed.« less
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