Designing Fe-Based Oxygen Catalysts by Density Functional Theory

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Designing Fe-based Oxygen Catalysts by Density Functional Theory Calculations Chi Chen, and Francesco Ciucci Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02953 • Publication Date (Web): 13 Sep 2016 Downloaded from http://pubs.acs.org on September 17, 2016

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Chemistry of Materials

Designing Fe-based Oxygen Catalysts by Density Functional Theory Calculations

Chi Chen, a and Francesco Ciucci a, b, *

a

Department of Mechanical and Aerospace Engineering, The Hong Kong University

of Science and Technology, Hong Kong, China. Prof. Francesco Ciucci ([email protected]) b

Department of Chemical and Biomolecular Engineering, The Hong Kong University

of Science and Technology, Hong Kong, China

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ACS Paragon Plus Environment


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Abstract A cheap and effective oxygen catalyst can find a variety of applications including cathodes for solid oxide fuel cells, oxygen transport membranes, cathodes for metal air batteries and catalysts for thermal water splitting. This work studies BaFeO3 (BFO)-based perovskites as oxygen catalysts from density functional theory calculations by substituting the A and B site of the BFO perovskite structure. Na, K, Rb, Ca, Sr, Y, La, and Pb are substituted into the A site, and Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, Zr, Nb, Ag, In, and Ce are substituted into the B site. We point out that there is a tradeoff among stability, electronic conductivity, and ionic conductivity, where the material selection needs to be based on the maximization of the desired property for a given application. Specifically, the calculation results show that the A site substitution with Na and K lowers the vacancy formation energy while preserving the electronic conductivity. However the introduction of such elements may destabilize the cubic perovskite lattice structure. Alkaline-earth elements, Ca and Sr have relatively small impact on the ionic diffusion but may enhance the electronic conductivity. In addition, La substitution stabilizes the cubic perovskite phase and lowers the oxygen migration barrier but it compromises the electronic conductivity and vacancy concentrations. Regarding the B site substitution, Sc, Y, Nb, and Ce stabilize the cubic perovskite structure although they sacrifice performance. On the 2


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other hand, Ni, Cu, Zn and Ag substitution may increase the electronic conductivity and reduces the vacancy formation energy at the expense of stability.

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Introduction Owing to their high catalytic activity and relatively low cost, BaFeO3 (BFO)-based materials have found applications in both high temperature technologies, such as solid oxide fuel cells (SOFCs) and oxygen transport membranes1-4, and in low temperature electrochemical devices as oxygen reduction and evolution catalysts4, 5. Because of its simple ABO3 perovskite structure, BFO has considerable structural flexibility, allowing the substitution of various cations on the A or B site, thereby resulting in fine control of physicochemical properties. Compared to other Co-based perovskite catalysts6, 7, Fe-based materials are economically more effective due to the abundance of Fe in the Earth’s crust. On the other hand, the use of alkaline-earth elements, especially Ba, at the A site of the perovskite is beneficial to lowering the oxygen vacancy formation energy, a key factor promoting the oxygen catalytic activity5. For example, recent density functional theory (DFT) calculations have shown that in Fe-based perovskite AFeO3, in comparison to lanthanides at the A site, divalent alkaline-earth cations (Ca, Sr and Ba) substantially lower the oxygen vacancy formation energies (from 3.73 eV to 0.90-1.37 eV)8. Even though changing the A site from Ca to Sr or Ba does not substantially affect the vacancy formation energy, the practical catalytic performance of SrFeO3 is inferior to BFO, as exhibited by a lower oxygen permeation flux when using these two materials in oxygen transportation 4


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membranes9. On the other hand, SrFeO3 and CaFeO3 are characterized by oxygen vacancy-ordered phases10, 11 that are not beneficial for oxygen catalysis. Similar to CaFeO3 and SrFeO3, pristine BFO also contains vacancy-ordered phases, such as Ba2Fe2O5 or BaFe2O4, which have low electrocatalytic activity3. However, different from CaFeO3 and SrFeO3, the vacancy-disordered cubic phase can also be found in BFO at room temperature9. The cubic phase is characterized by favorable ionic transport and oxygen catalytic activity. Therefore, attempts have been made to stabilize it. Various strategies have been used, including oxidation treatment12, strain control by growing epitaxial thin films using pulsed laser deposition13, and doping the material with other elements. A number of dopants have been used including La14-16, and Ce17 in the A site and Y18, Zr19, Nb20, Ce21, In22, Cu and Ni23 in the B site. Recent work from our research group has also investigated experimentally A and B site substitutions.7 We found that Zr doping leads to the highest oxygen reduction activity in SOFC conditions. We have also performed DFT simulations and analyzed the impact of doping 12.5% of Y, Ce and La in either the A or the B site of BFO. We found that B-site doping results in lower oxygen vacancy formation energies, while A-site doping prohibits further creation of donors, i.e., oxygen vacancies24. However, it should be noted that the B site substitution often distorts the lattice, deflecting the B-O-B chain away from 180 degrees, and reducing 5



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the electrical conductivity. Our previous calculations generally captured the experimental trends24. Therefore, additional calculations surveying a series of materials may be helpful in predicting their properties. This computational aided material selection has recently become an increasingly common research tool25. For example, previous DFT calculations have suggested a correlation between bulk electronic structure with the activity of oxygen reduction reaction (ORR) at high temperature26 and oxygen evolution reaction (OER) at low temperature27. In this article, we broadly survey the A and B site elements that may be substituted into the BFO framework, and study how the substitution may affect the electronic characteristics as well as oxygen vacancy formation and oxygen migration within these materials. These properties are key to the development of cheap, yet effective oxygen catalysts that find applications at both high and low temperature.28 Specifically, Na, K, Rb, Ca, Sr, Y, La and Pb were substituted to the A site, and Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, Zr, Nb, Ag, In, and Ce were substituted to the B site. This elemental choice was guided by the observation that alkaline, alkaline-earth, and rare-earth elements are common A site elements and transition metals are common B site elements5. Furthermore, we explored additional interesting substitutions, such as Pb at the Ba site and Y, Zr, and In at the Fe site. This was motivated by their high probability in compounds with 6


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same structures as determined by data mining the ICSD database29. It should be noted that most of the selected substitutions also have high probability. Additionally, Nb20, Ag30 and Ce21 substitutions were motivated by recent experimental findings. In summary, we wish to provide guidelines for the selection of the dopants into BFO so as to adjust its properties for specific applications.

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Computational details All DFT calculations were performed using VASP31,

32

within the projector

augmented-wave approach33 and with exchange-correlation effects described by the generalized gradient approximation taking a Perdew-Burk-Ernzerhof functional34. To account for the on-site Coulomb interactions of localized electrons of transition metals, we adopted a DFT+U approach with a rotational invariant scheme and with U parameters fitted from oxide formation enthalpy by Wang et al.35 We studied 2x2x2 cubic perovskite supercells as the structure models. Specifically, Ba was substituted with Na, K, Rb, Ca, Sr, Y, La, and Pb, and Fe was substituted with Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, Zr, Nb, Ag, In, and Ce. To limit the combinatorial complexity and total number of site configurations, we substituted half of the original cations, forming Ba0.5A0.5FeO3 or BaFe0.5B0.5O3, as shown in Figure S1. We used 4x4x4 Monkhorst-Pack grids where we set a 480 eV cut off for the energy. For all compositions, the energy tolerance was set to 10-5 eV and the calculations started by relaxing the initial structure with a 0.02 eV/Å force convergence. The oxygen anion migration barrier was calculated with the climb-image nudged elastic band method as implemented by VTST.36 Charge analysis was performed using an open-source Bader charge algorithm.37

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Results and discussion Lattice parameter with changing A and B site In our earlier work, we used similar computational methods to study the effects of dopants on BFO, and our results compared well with experiments.24 Here we present our computational results and compare the insights and trends to existing experiments, and, when no experiments are available, we use computations to provide guidelines. We first analyzed how the lattice parameter changes by substituting different elements. The lattice parameter is correlated, along with the cation size, to the size of the bottleneck size for oxygen diffusion as shown by Chen et al.38 Interestingly, in spite of their larger ionic radii (rK+ = 1.64 Å and rRb+ = 1.72 Å) in comparison to Ba2+ (rBa2+ = 1.61 Å), see Figure S2a, (1.61 Å), both K and Rb reduce the lattice parameter. As expected, within the same group of possible substitutions, increasing the ionic radius results in larger lattices, and within the same row the alkaline metal substitution gives a larger lattice than the alkaline-earth metal substitution.

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(a)

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(b)

Figure 1. Lattice parameter change by substituting A site (a) and B site (b) of BFO. The dashed line indicates the lattice parameter of BFO. In contrast, transition metal substitution at the B site is more complicated and the trend is elusive, as shown in Figure 1b. This is because transition metals usually have multiple valence and spin states, see in Figure S2b. Generally, the substitution of Fe with first-row transition metals results in similar lattice parameters, as shown for B = Sc to Zn in Figure 1b. Y, Zr, Ag, In and Ce substitution greatly increases the lattice parameter. In summary, A site substitution reduces the lattice parameter and B site substitution generally increases it. In fact, Ba has a large cationic radius (1.61 Å), while Fe, in the nominal Fe4+ state in BFO, is relatively small (0.585 Å). The unexpected smaller lattice parameters for K and Rb substitution may be due to the charge redistribution brought by their lower valence compared to the host Ba.

For a model cubic perovskite structure, the lattice parameter of a unit cell is 10


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approximately twice the B-O bond length. It is generally accepted that the B-O-B framework forms an electronically conducting path in the material, therefore the B-O length is expected to affect the electronic conductivity when the electronic conducting mechanism is based on the hopping of polarons. This mechanism assumes that the electronic charge is localized on ions and the electronic conduction occurs when the charge hops from one ion potential well to another39. Unlike other semiconductors, metal oxide semiconductors tend to form small polarons40. This small polaron model has been applied to explain the electronic conduction in Fe-based perovskite, for example in

SrFeO3-δ and BFO41. Regarding the A-site substitution with

alkaline-earth elements, isovalent Ca and Sr substitutions are expected to increase the conductivity compared to pristine BFO due to the shorter Fe-O lengths. Experimental results showed that SrFeO3-δ has a much higher conductivity compared to oxygen deficient BFO and with an electronic mobility 3 orders of magnitude higher at 100 o 41

C . For B site substitutions, the charge concentration differs due to the differences in

the number of outer electrons. However, we expect that first-row transition metal substitutions from Sc to Mn would reduce the conductivity due to the expanded lattices and the fewer outer electrons compared to Fe.

Formation energies As explained earlier, the cubic phase shows oxygen vacancy disordering that is 11



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beneficial to high temperature ORR. Therefore, the stability of the cubic phase is of great importance for the selection of dopants. Conventional analysis uses the Goldschmidt tolerance factor42 to predict the stability of the cubic perovskite, i.e., t=

rA + rO ( r is the ionic radius and the subscripts indicate the A, B, or O ions), 2 ( rB + rO )

where a t close to 1 favors the cubic perovskite structure42. However, the chemical environment can complicate the determination of the ionic radius. For example, the B site transition metals have different radii at different charge states and spin states and the latter information may not be available beforehand. To quantify the stability of the cubic lattice, we studied the formation energy of the final cubic perovskite structure starting from the binary oxides by the following formula 1 1 1  2x - y  BaO+ Fe 2O 3 + A x O y +   O 2 ⇔ Ba 0.5A 0.5 FeO 3 2 2 2x  4x 

(1)

1 1  5x − 2 y  BaO+ Fe 2O 3 + B x O y +   O 2 ⇔ BaFe 0.5 B0.5O 3 4 2x  8x 

(2)

The DFT energy of triplet O2 was corrected for the overbinding problem38 and the most thermodynamically stable structures of the corresponding oxides were obtained from the Materials Project via the software pymatgen43. Furthermore, the formation energy is defined as the energy on the left hand side reactants of the equation (1) and (2), minus the energy of the right hand side products. Therefore, a positive formation energy indicates a stable cubic perovskite, while a negative energy suggests that the perovskite decomposes into binary oxides. 12


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(a)

(b)

Figure 2. The perovskite formation energy from binary oxides for A site (a) and B site (b) substitutions. The formation energies for both A and B site substitutions are shown in Figure 2. Among the selected common A site dopants, Rb and Mg reduced the stability of cubic perovskites, and Y substitution at the A site was also unfavorable compared to pristine BFO. It is important to note that a previously published experimental work on Rb-substituted BFO did not report a cubic phase3. For Mg and Y, the 12-coordinated ionic radii are not available in the Shannon table44, supporting our results that 12-coordinated Mg and Y may not be stable. The B site substitution calculations show that three elements, Sc, Y and In, have noticeably higher formation energies. One would expect that these three cations favor the cubic phase once substituted to the BFO lattice. Previous studies have confirmed that Y18 and In22 stabilize the cubic phase, while Sc has not been studied. It should be noted that Sc stabilizes the cubic phase BiFeO3, a perovskite similar to BFO45. On the other hand, V, Cr, and Mn 13



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destabilize the cubic phase, as shown by the negative formation energies. Co substitution reduces the formation energy substantially compared to the pristine BFO. Experimentally, cubic BaVO346, BaCrO347 can only be obtained under extreme conditions for example by high pressure-induced phase transition. In addition, BaMnO348 and BaCoO349 are hexagonal with ordered vacancies. Therefore, in the following part of this article, the A site substitution with Rb, Mg, and Y, and B site substitution with V, Cr, Mn and Co will not be discussed due to the substantially reduced cubic lattice stability after their introduction into the BFO lattice.

Density of states analysis To further study the electronic properties, density of states (DOS) calculations were performed on relaxed structures. Partial DOS (PDOS) are shown in Figure 2 and 3 and the correspondingly full DOS plots are shown in the supplementary information, Figure S3 and S4.

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Figure 2. The PDOS of Fe d-band, O p-band, Ba s-band and A dopant s-band. Overall, the valence band near the Fermi level can be attributed primarily to the O p-band while the conduction band has a major contribution from the Fe d-band. This highlights the importance of Fe-O interaction on forming the electronic conductivity. The seemingly dominating oxygen character around the Fermi level also suggests that oxygen anions may play a major role in the electron exchange with the reaction intermediates. In fact, a recent experimental work by Mueller and coworkers studying Fe-based perovskites with operando XPS found that during electrocatalysis, the O band has reversible systematic change. These results suggest that the anionic redox process is responsible for the oxygen activity50. In addition, the O p-band shows 15



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unoccupied states above the Fermi level, indicating that ligand holes may be formed in the oxygen sublattice. In particular, Na and K have a likely positive impact on the holes’ concentration, while La hampers their presence. Isovalent substitution with Ca, Sr and Pb has little impact on the DOS. This could be justified by noting that the A-site substitution mainly shifts the Fermi level and does not alter the hybridization of the transition metal and oxygen bond. Specifically, acceptor substitution with alkaline elements lowers the Fermi level, while the donor substitution raises it.

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(a)

(b)

Figure 3. The PDOS of Fe d-band, O p-band, Ba s-band and B dopant d-band for first-row transition metal substitution (a) and other B site substitutions (b). The B site substitution is much more complex and no general trends can be derived. For Sc and Ti substitutions, the d electrons do not contribute to the valence band near EF and the main features of O p-band remain intact. In contrast, first-row transition metals after Fe, i.e., Ni, Cu, and Zn, visibly contribute to the valence band, as shown in Figure 3a. This could be explained by noting that the electronegativity increases from Sc to Zn and this in turn lowers the d-band and promotes metal and oxygen hybridization. In the valence section of the DOS, the Fe d-band has a lower energy than the O p-band, indicating a negative ligand to metal charge transfer energy ∆ p−d , in agreement with experiments12. This supports experimental evidence that it is difficult to obtain fully oxidized BFO and oxygen vacancies are likely to form. The 17



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computational study by Ribeiro et al.51 found that after removing 0.5 oxygen per unit formula, Ba2+Fe3+(O2.5)5- shows a band gap at the Fermi level with negligible hole content. Taking this oxygen-deficient material as the reference, more oxygen ions can be seen as source of holes in the O p-band due to the negative ∆ p−d . Following the same rationale, BFO could be viewed as hole-doped semiconductors, where the A and B substitutions further tune the hole concentration, in spite of the DOS showing metallic behavior. We note that during electrical conduction, only electrons close to the Fermi level can participate. Therefore, studying the available states at the Fermi level may provide additional insights.

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(a)

(b)

Figure 4. The spin up, spin down and total DOS at the Fermi level for A site substitution (a) and B site substitution (b). In the Ba0.5A0.5FeO3 series, with the exception of La, all DOSs around the Fermi level are similar as the DOSs of the pristine BFO (Ba in Figure 4a), as shown in Figure 4a. La seems to lower the DOS, especially for the spin down states which approach 0. This is because the donated electrons from La fill the holes in the Fe-O band. Consistent with the DOS analysis outlined above, K and Pb do not change the DOS substantially. For B site substitution with first-row transition metals, the elements before Fe generally reduce the number of states at Fermi level and if the elements are first-row transition metal after Fe, more states are observed at the Fermi level. This trend is consistent with the number of valence electrons. Therefore, to achieve higher electronic conductivity, Ni, Cu and Zn substitution may be beneficial. Y, Zr, Nb and Ce substitutions give lower DOS at the Fermi level, suggesting relatively poor electronic conductivity. Experiments found that Y substitution in BFO decreases the 19



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electronic conductivity18, and similar results were also observed for Nb15 and Zr52.

Previous research by Lee et al. suggested that a higher O p-band center, closer to the Fermi level, is correlated with better high temperature ORR activity26 and low temperature OER activity27, 53, 54. A higher O p-band center of the material, however, enables a facile oxygen exchange with the environment. In the alkaline solution environment, the oxygen loss of the catalysts may cause cation leaching and further dissolution or amorphization of the material. The oxygen loss from the material and the subsequent removal of the products may cause the material to decompose. This physical phenomenon is different from previously described thermodynamic stability, which is assessed by analyzing the formation energies of the perovskites from binary oxides. The stability of the catalyst in an alkaline solution environment was shown to be inversely correlated to the O p-band center18. On the other hand, as explained earlier, a lower O p-band center is linked to the materials’ resistance to oxygen release, and therefore may be indicative of the materials’ stability against the formation of vacancy ordered phases. We calculated the O p-band center of the valence band as shown in Figure 5. Relative to A site elements, La shows a strong ability to stabilize the material, followed by Ca and Sr. The effect of Pb is not noticeable. In terms of the B site substitution, Cu, Zn and Ag likely improve the activity while Sc, Ti, Y, Zr, Nb, 20


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In, and Ce stabilize the material. (a)

(b)

Figure 5. O p-band center for BFO with A site (a) and B site (b) substitutions.

Charge redistribution after substitution (a)

(b)

Figure 6. Bader charge magnitude of substitution (A or B), Ba, Fe and O after A-site substitution (a) and B-site substitution (b). The electronic band structure is closely related to the way charge distributes among the various atoms that make up the structure. Figure 6a shows the Bader charge of A, Ba, Fe, O in Ba0.5A0.5FeO3. The dash lines are the BFO reference values. As expected, 21



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Na and K have a nominal +1 charge, Ca, Sr and Pb maintain a +2 charge, and La shows a +3 charge, as reported in Figure 6a. Alkaline elements Na and K accept electrons from the band upon substitution. Such electrons are principally taken from O, as shown by the decrease of the nominal charge magnitude around O. Interestingly, O releases more electrons than required to compensate for the presence of Na and K; extra electrons go directly to either Ba or Fe, lowering their valence. It should be noted that Na is different from K. For the Na substitution, the charge on Ba does not vary, while Fe is reduced. On the other hand, K substitution reduces Ba but maintains Fe at a high valence state. Divalent substitution with Ca, Sr, or Pb, however, does not change the charge around Ba, but moves electrons from O to Fe, reducing Fe. For the trivalent element substitution, La donates electrons to O while maintaining a higher Fe valence. For oxygen catalysis, a higher valence transition metal is preferred due to its relatively low d-band that forms stronger covalent orbitals with oxygen. Therefore, K and La substitution could maintain the oxygen catalysis properties. The charge on O is also critical since the charge state is correlated to the Coulomb interaction of O with the lattice. A lower charge magnitude for O is potentially beneficial to the formation of oxygen vacancies.

For B site substitution, the charge of the B site element correlates with the 22


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electropositivity, as shown in Figure 6b, i.e., the valence states of the substituted B elements decrease from Sc to Zn. This is accompanied by the oxidation of O. This is perhaps due to the lowering of d-band of transition metals from Sc to Zn54, facilitating the electron transfer from O to the metal. In particular, Ni, Cu, Zn, and Ag substitutions result in a lower O charge magnitude in comparison to its counterpart in pristine BFO, suggesting that oxygen vacancy formation might be easier. In addition, the substitutes themselves can also act as active catalytic centers. Therefore, the transition metals Ni, Cu and Zn substitutions may have interesting applications. It should be noted that BaNiO3-based materials have recently been reported as good oxygen catalysts after losing lattice oxygen55. In addition, Kida and coworkers substituted Ni and Cu into BFO, and found improved oxygen diffusion in the material23. The more favorable oxygen diffusion is correlated to the ease of oxygen removal, consistent with our computational findings. Ti, Y, Zr and Nb slightly increase the charge on O but maintain higher Fe states compared to BFO. We would expect that they have lower performance, but higher lattice stability. Regarding Ti substitution, BaTiO3 displays a high oxygen catalytic ability towards OER and ORR56. Y18, Zr19 and Nb20 substitutions have been studied in BFO or its derivatives as well. As expected, their electrocatalytic activity decreases as the amount of Y, Zr or Nb in the material increases. 23



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Oxygen vacancy formation energies The formation of oxygen vacancies is needed for oxygen migration. Additionally, surface oxygen vacancies provide sites for the oxygen incorporation during electrocatalysis57, 58. The adsorption of oxygen on the surface of a material is also greatly influenced by vacancies. For example, cation segregation at the surface of perovskites is shown to hinder oxygen transport59, and the presence of vacancies at the surface can facilitate oxygen incorporation and transport towards the bulk, thereby improving the electrocatalytic activity60. Therefore, we calculated oxygen vacancy formation energies for the materials under investigation.

Figure 7. The oxygen vacancy formation energies of substituted BFO, plotted against the O p-band center. The calculation method for oxygen vacancy formation energy has been shown to compare well with published work38 and a recent work on BFO gives similar results8. 24


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Consistent with the charge analysis, the alkaline elements facilitate the formation of oxygen vacancies, as shown in Figure 7. However, while the Na, K, Ag, Cu, Ni and Zn substitutions into cubic BFO improve the activity, they also decrease the stability, since the vacancy formation energies are close to 0. Previous analysis by Grimaud et al. pointed out that the perovskites with O p-band centers above -1.75 eV are likely to have amorphous structure at their surface in OER condition in alkaline media27. This amorphization may result in a trade-off between improved performance and decreased stability27. For the isovalent substitutions, Ca (overlapped with Ti in the plot) and Sr show similar results as the pristine BFO, and Pb substitution slightly decreases the vacancy formation energy, but maintains a virtually identical O p-band center to that of pristine BFO (marked as Fe). Different from other substitutions, La and Nb raises the vacancy formation energy substantially. The high vacancy formation energies brought by La and Nb are particularly interesting for thermal water splitting applications61. The feasibility of forming vacancies is, to some extent, inversely related to the lattice stability since the lattice frame is formed by BO6 octahedral. In SOFCs, however, the trade-off between vacancy formation energy and lattice stability has to be carefully assessed, since these fuel cells operate at high temperatures with fast material degradation speed and long-term operation is key to this technology. Therefore, for this application the stability is as important as the activity of the 25



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catalyst. From the stability considerations, La, Ce, Sc, Zr and Y seem to be suitable choices. In addition, there seems to exist a linear correlation between the oxygen vacancy formation energy and the O p-band center, which is consistent with the intuition from a rigid band understanding, as described in a previous work26.

Oxygen migration energies (a)

(b)

Figure 8. Oxygen migration energy for BFO with A site (a) and B site (b) substitutions. We also calculated oxygen migration barriers for A and B site substituted BFO, as shown in Figure 8a and Figure 8b, respectively. Within the A site elements, Na, Ca, La, Sr and Pb lower the migration barrier, while K increases it. This trend could be explained using the bottleneck radius used by Chen et al.,38 where as shown in Figure S5, a larger bottleneck radius enables a more facile oxygen diffusion. Interestingly, Ca and La substitutions induce an out-of-plane transport path for the migrating oxygen, in comparison to other cases where O diffuses within a plane, as shown in Figure S6. 26


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On the other hand, B site substitutions with Ti, Ni, Cu and Zr are likely to reduce the migration barriers.

Conclusions A (Na, K, Rb, Mg, Ca, Sr, Y, La and Pb) or B (Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, Zr, Nb, Ag, In, and Ce) substituted BFO is studied by DFT calculations to evaluate their properties for applications as oxygen catalysts. Table S1 and S2 summarize the trends in an intuitive form. For A site substitution, the cubic lattice stability is generally reduced, where Mg, Rb, and Y are less stable in the substituted perovskites compared to the corresponding binary oxides. Na, K, Ca, Sr and Pb reduce oxygen vacancies formation energies, where the acceptors Na and K substantially promote the vacancy formation. With the exception of K, all other studied A site elements seem also to reduce the oxygen migration barriers. In particular, La substitution shows greater lattice stability, as exhibited by a low O p-band center and high vacancy formation energy. In addition, La substitution lowers the oxygen migration barrier. For B site substitution, Mn, Co, Cr and V are not likely to form a stable cubic lattice after substitution into BFO, while Sc, Y and In show strong tendency to stabilize the cubic perovskite structure. Oxygen vacancy formation energies are reduced by Ni, Cu, Zn and Ag, while they are increased by Y, Zr, Zb and Ce. In addition, Ni, Cu, Zn and Ag may increase the electronic conductivity. Lastly, the oxygen migration barriers are 27



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reduced by Ti, Ni and Zr, but increased by Sc, Y, Nb, Ag and In. In short, we have shown that our DFT calculations are consistent with experimental results and we attempt to use the calculations to predict the impact of substitution on the performance and stability of BFO. We point out the tradeoff between lattice stability and catalytic activity. The choice of material can be based on striking a balance between these two properties, and the selection of material should depend on the given application.

Supporting information Structural illustrations, DOS with different dopants, bottleneck radii of structures, migration paths and a tabular summary of trends. This material is freely available at http://pubs.acs.org/.

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Acknowledgements The authors gratefully acknowledge the Research Grants Council of Hong Kong for support through the projects DAG12EG06 and ECS 639713.

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Designing Fe-Based Oxygen Catalysts by Density Functional Theory

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