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Title: Ultrafast bulk diffusion of AlHx in high-entropy dehydrogenation intermediates of NaAlH4 [Highly mobile AlHx species and the dehydogenation kinetics of NaAlH4]

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. 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 NaAlH4 are discussed.
Authors:
 [1] ;  [2] ;  [3] ;  [4] ;  [4] ;  [5]
  1. Georgia Inst. of Technology, Atlanta, GA (United States); Ames Lab., Ames, IA (United States)
  2. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
  3. Georgia Inst. of Technology, Atlanta, GA (United States)
  4. Ames Lab., Ames, IA (United States)
  5. Georgia Inst. of Technology, Atlanta, GA (United States); Academia Sinica, Taipei (Taiwan)
Publication Date:
OSTI Identifier:
1325879
Report Number(s):
LLNL-JRNL--644033
Journal ID: ISSN 1932-7447
Grant/Contract Number:
AC52-07NA27344
Type:
Accepted Manuscript
Journal Name:
Journal of Physical Chemistry. C
Additional Journal Information:
Journal Volume: 118; Journal Issue: 32; Journal ID: ISSN 1932-7447
Publisher:
American Chemical Society
Research Org:
Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; 08 HYDROGEN; 37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; 75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; 25 ENERGY STORAGE