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Title: Redox Defect Thermochemistry of FeAl2O4 Hercynite in Water Splitting from First-Principles Methods

Journal Article · · Chemistry of Materials
ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [2]
  1. Univ. of Colorado, Boulder, CO (United States)
  2. National Renewable Energy Lab. (NREL), Golden, CO (United States)
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Solar thermochemical hydrogen (STCH) production is a promising route to produce fuels from sunlight via high-temperature water splitting. However, efficient and technologically viable implementations of this process only allow a narrow window of thermodynamic boundary conditions that can be used to cycle the system, thus limiting the design space for suitable metal oxide redox mediators. An oxygen defect redox mechanism can contribute a favorable reduction entropy to expand this window, and computational evaluation of materials with high oxygen defect entropies could play a pivotal role in guiding the discovery and design of suitable oxides. This study employs first-principles calculations to investigate the redox mediating defect mechanism of the STCH candidate material, hercynite (FeAl2O4). We compare the results of total energy calculations from density functional theory (DFT) with beyond-DFT approaches, including hybrid functionals and the random phase approximation, which are among the most advanced methods currently feasible for supercell defect calculations. Using the predicted formation energies, we perform thermodynamic modeling of FeAl2O4 reduction and oxidation via free energy minimization that incorporates ideal gas, configurational, and vibrational entropy contributions evaluated within the quasi-harmonic approximation. Special attention is devoted to understanding interactions among co-existing defects, such as the association of pairs and complexes of O vacancies and cation antisite defects, and the effect of mutually compensating defect charges. Our results corroborate the notion that the details of defect interactions can be decisive for the viability of hydrogen production within the desirable STCH process window.

Research Organization:
National Renewable Energy Lab. (NREL), Golden, CO (United States)
Sponsoring Organization:
USDOE Office of Energy Efficiency and Renewable Energy (EERE), Hydrogen and Fuel Cell Technologies Office (EE-3F)
Grant/Contract Number:
AC36-08GO28308
OSTI ID:
1840916
Report Number(s):
NREL/JA-5K00-79573; MainId:35794; UUID:558f9692-e373-4638-a4f1-72692e288afb; MainAdminID:63622
Journal Information:
Chemistry of Materials, Vol. 34, Issue 2; ISSN 0897-4756
Publisher:
American Chemical Society (ACS)Copyright Statement
Country of Publication:
United States
Language:
English

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37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
first principles calculations
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transition metal oxides