Both the experimental and first-principles modeling results revealed the dependence of defect energetics on oxygen non-stoichiometry and magnetic coupling of Fe in the Fe-based perovskite oxides. A ...generalized defect thermodynamic model of the proton-conducting (La,Ba)Fe
1-x
M
x
O
3-δ
perovskite oxide is developed to allow inclusion of nonlinear δ dependent terms in three key defect reaction energies, namely, the oxygen vacancy formation, hydration, and charge disproportionation reactions. A transition from a large polaron description at lower δ values to a small polaron expression at higher δ is also considered in our analysis. Based on the functional forms of defect energetics on δ as guided by first principles modeling and literature data, the Brouwer diagrams of BaFe
0.9
Y
0.1
O
3-δ
are assessed to provide information on electronic and ionic defect concentration (including the proton species) as a function of O
2
and H
2
O pressure at different temperatures for solid-oxide cell applications.
In this work, the diffusion barriers of A-site cation and dopants in bulk LaMnO3±δ (LMO) are determined based on density functional theory (DFT) simulations. These properties are highly relevant for ...solid oxide fuel cell (SOFC) cathode applications. Due to the weak repulsive interactions (0.2~0.3 eV) between the nearest neighbor A-site and B-site cation vacancies, the A-site and B-site cation vacancy clusters can readily form in LMO. The DFT modeling unveils a facile A-site cation diffusion pathway via a cation vacancy cluster mechanism with a migration barrier reduction of about 1.3 eV versus the direct A-site vacancy migration. By combining these results with the bulk LMO defect modeling, the temperature and oxygen partial pressure dependences of the A-site cation tracer diffusion coefficients are assessed based on the concentration of transport carriers and the migration barriers associated with the A-site cation transport in bulk LMO. The trends in the migration barriers vs. various types of metal cations relevant to SOFC applications i.e. La3+, Sr2+, Zr4+, Y3+, are also investigated, and the results obtained suggest that both the ionic charge and the ionic radius correlate with the calculated cation migration barriers
Density functional theory (DFT) based modeling was performed to investigate the defect energetics of the proton conducting oxides BaFe
1-x
M
x
O
3-δ
(with M=Zr, Y, and Zn at the B-site). ...High-throughput defect modeling was carried out to obtain the defect energetics ensembles as a function of B-site x (0 – 0.25) and oxygen δ (0 – 0.5) compositions for assessment of oxygen vacancy formation (E
vac
) and hydration reaction (E
hyd
) energies in complex defect-dopant configurations. The modeling effort aims to elucidate the complex coupling among oxygen vacancies, the protons and metal dopants in various concentrations and their combined effect upon defect energetics in BaFe
1-x
M
x
O
3-δ
perovskite. This information complements the current experimental measurements with essential data related to the energetics of various defect configurations that can play a role in solid-oxide-cell (SOC) applications.
Preliminary results among more than 3,000 configurations investigated are summarized in Figure 1. All calculations were performed with the Perdew-Burke-Ernzerhof (PBE) functional in charge neutral 2 × 2 × 2 perovskite supercells. An effective Hubbard U
eff
correction of 4 eV was applied to the Fe 3d shell, with all the Fe atoms being in a high-spin state and having a ferromagnetic ordering. Stability analysis of each of the BaFe
1-x
M
x
O
3-δ
systems investigated at specified x and δ composition values reveals that the oxygen vacancy (V
O
) is energetically unfavorable when placed adjacent to the M dopant. Overall, the Fe-V
O
-Fe configurations are identified to be more stable than the Fe-V
O
-M and M-V
O
-M configurations, while one with a M-V
O
-M cluster arrangement is the least stable. Such a short-range ordering leads to the blocking of the oxygen vacancy migration toward the oxygen sites around the M dopant, whereas the proton hopping between the oxygens around the M dopants is energetically not hindered. Based on the stability of various defect-dopant configurations of the BaFe
0.75
M
0.25
O
2.875
systems with or without hydration, it is revealed that the stability range of analogous configurations with varying dopant types can be an indicator of the defect-dopant ordering strength and can be correlated with the ionic radii of the dopants in the BaFe
1-x
M
x
O
3-δ
systems.
Taking the energy of the most stable BaFe
1-x
M
x
O
3-δ
structures at various x and δ values analyzed as representative for the respective compositions, the corresponding derivations of the E
vac
and E
hyd
energies as a function of δ (and x) are summarized in Figures 1(a) and 1(b), respectively. Overall, the results indicate a stronger oxygen non-stoichiometry (δ) dependence in the E
vac
values of the BaFeO
3-δ
and BaFe
1-x
M
x
O
3-δ
systems (M=Zr, Y, and Zn at x=0.125 and x=0.25), whereas the E
hyd
data exhibit a much weaker δ dependence. This contrasting behavior in the δ dependences between E
vac
and E
hyd
values may be attributed to the charge transfer characteristics of the oxygen vacancy formation reaction, which is associated with the Fe 3d and the O 2p energy levels of the materials at various δ values while the charge transfer involved in the hydration reaction is relatively minimal. The effect of dopant concentration (x) further shows an opposite trend in the E
vac
values vs. x with the dopant types, with an increase of E
vac
upon increasing the Zr doping concentration and a decrease of E
vac
upon increasing the Y and Zn doping content. Due to the strong oxygen non-stoichiometry dependence in the E
vac
values, the results indicate an increased proton affinity of the BaFe
1-x
M
x
O
3-δ
oxides. Further assessments of the E
vac
and E
hyd
data on δ values and their impact upon the defect equilibria of the BaFe
1-x
M
x
O
3-δ
systems under the SOC operating conditions will be discussed.
Disclaimer
This project was funded by the U.S. Department of Energy, National Energy Technology Laboratory, in part, through a site support contract. Neither the United States Government nor any agency thereof, nor any of their employees, nor the support contractor, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
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