Insight from First-Principles Calculations - ACS Publications

Subscriber access provided by University of Missouri-Columbia

Article

Structural and Electronic Features of Nb-Doped SrCoO: Insight From First-Principles Calculations 3

Xueling Lei, Bo Xu, Chuying Ouyang, and Kevin Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08852 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 26, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Structural and Electronic Features of Nb-Doped SrCoO3: Insight from First-principles Calculations

Xueling Lei a,1, Bo Xu a, Chuying Ouyang a, Kevin Huang b a

Department of Physics, Laboratory of Computational Materials Physics, Jiangxi

Normal University, Nanchang, Jiangxi 330022, China b

Department of Mechanical Engineering, University of South Carolina, Columbia,

South Carolina 29201, USA

Abstract: The present work reports a systematic investigation into the structural and electronic features of tetragonal and cubic structured undoped and Nb-doped SrCoO3 using first principles calculations. The results suggest that the tetragonal structure is more stable than the cubic structure as a result of the Jahn-Teller distortion effect. In the Nb-Doped SrCoO3 system, the structural analysis reveals a gradual tetragonal-tocubic transition with Nb-doping. Nb-ions show no occupational preference over Co-sites in the SrCoO3 host structure. Co-ions become more easily reduced by Nb-doping due to the filling of excess electrons from Nb-ions into Co-3d orbitals, thus decreasing the magnetic moment of Co-ions. Furthermore, the rich electronic states at the Fermi level in Nb-doped SrCoO3 enable a high electronic conductivity, making the material suitable for a solid oxide fuel cell cathode material.

Keywords: Structure; Electronic Properties; First-principles calculations; Cathode; Solid oxide fuel cells;

1

Corresponding author: Email: [email protected] 1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 19

1. Introduction Recently, SrCoO3−δ (SCO) perovskite has been increasingly studied as a promising oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) bifunctional material for intermediate temperature reversible solid oxide fuel cells (IT-RSOFCs), largely due to its unique reversible oxygen storage and release capability 1-4. However, the pure SCO with oxygen-deficient cubic structure is unstable at a typical IT-RSOFC operating condition (e.g. ambient pressure and 500-700 ͦ C); it readily decomposes into a more stable but less conductive and active brownmillerite (BM) structure SrCoO2.5 5. To avoid the formation of BM phase, a common strategy is to dope higher oxidation state cations (known as donor doping or electron doping), which has been proven effective in stopping the BM formation. Donor doping of higher oxidation-state cations into SCO, including on the Co site with Nb, Ta, Fe, Sc and Sb

6-24

or on the Sr site with Y and La

25-27

, can stabilize the

wanted cubic structure. Of these donor dopants, Nb has been shown the most effective structural stabilizer and performance retainer as an IT-SOFC cathode. For example, Wang et al. 14 reported that SrCo0.9Nb0.1O3-δ (SCN10) exhibits a stable cubic structure between 500 and 700 ͦ C and a reversible oxygen redox reaction at 350 ͦ C. At T < ~500 ͦ C, a tetragonal phase containing ordered oxygen-vacancy superlattice appears. While it appears that extensive experimental studies on structural, thermal and electrochemical properties of Nb-doped SrCoO3−δ have been carried out in the literatures 6-16, an in-depth atomic-level understanding on the structural and electronic properties of Nb-doped SCO system is still lacking. However, atomic scale simulation is an important theoretical technique in the research of materials science. With the information collected from simulations, we can better understand the fundamental physical and chemical characteristics of the materials and therefore find ways to modify the materials and design new materials from the atomic level 28. In the present work, we report structural and electronic properties of tetragonal and cubic structures in both pure SCO and Nb-doped SCO investigated by first-principles calculations, aiming to understand the effect of Nb-doping on the structural and electronic properties of SCO-based perovskites. The results reveal the origin of the 2


Page 3 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


enhanced structural stability and electrical conductivity in SCO by Nb-doping. 2. Computational Details and Models All calculations in the present work were performed using the projector augmented-wave (PAW) method 29-30 with plane-wave basis set as implemented in the Vienna Ab initio Simulation Package (VASP)

31-32

. The generalized gradient

approximation (GGA) with the Perdew−Burke−Ernzeh (PBE)

33

exchange-correlation

functional was considered in the calculations. The O 2s22p4, Co 3d84s1, Sr 4s24p65s2 and Nb 4s24p65s14d4 were treated as valence electrons, and the cut-off energy of 500 eV was used for the plane wave basis. The LDA+U approach was applied in all calculations for the corrections of on-site Coulombic interactions of Co 3d orbitals. In this work, the effective Hubbard value Ueff was set to 3.3 eV 34-35. The original structures of tetragonal and cubic phase were taken from the X-ray diffraction results 15.To model the 10% Nb-doped SCO (SrNb0.1Co0.9O3-δ or SCN10 in short), we first constructed a supercell of 3×3×1 unite cells with 90 atoms (O: 50; Sr: 18; Co: 18) based on the optimized tetragonal structure since it is more stable than the cubic structure. Then, the SrNb0.11Co0.89O3 structure was created by replacing two Nb atoms with two Co atoms, which is the nearest doping ratio to the SCN10 synthesized in experiment. The Monkhorst–Pack scheme 36 of 2×2×3 k-point grid was used for the integration over the Brillouin zone. The total ground state energy and the final force on each atom was converged within 10-4 eV and 0.01 eV/Å, respectively. In the calculation procedure, we considered only the ferromagnetism (FM) state since the FM state was found to be the ground state from our test calculations. The total energy of the FM state calculated is about 0.015 eV lower than that of the antiferromagnetic (AFM) state, which is also in line with the previous reports 37. 3. Results and Discussions 3.1 Structures and electronic properties of tetragonal and cubic phase SrCoO3 First, the crystal structures of tetragonal (S.G.: P4/mmm) and cubic (S.G.: Pm-3m) phase SrCoO3 were carefully optimized; the results are shown in Fig. 1. For easy comparison, the 1×1×2 supercell structure for the cubic phase SrCoO3 is shown in Fig. 1 (b), where Co1 and Co2 are equivalent in the cubic structure (Co1cubic = Co2cubic). 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

For the tetragonal phase, the lattice parameters are a=b=3.894 Å and c=7.747 Å, which is in good agreement with experimental findings (a=b=3.874 Å and c=7.787 Å) 15

. A further examination of the local structure of CoO6 octahedron in the tetragonal

structure indicates that the flattened and elongated CoO6 octahedron alternate along the c-axis. The Co1O6 octahedrons are elongated with two longer Co1-O bonds of 2.034 Å along the c-axis and four normal Co1-O bonds of 1.939 Å in the x-y plane. In contrast, the Co2O6 octahedrons are flattened along the c-axis direction, displaying two very short Co2-O bonds of 1.829 Å and four equivalent Co2-O bonds of 1.939 Å in the x-y plane. Compared with the tetragonal structure, the cubic structure has the lattice constant of 3.866 Å, which agrees well with the experiment observed 3.900 Å 15

. All the Co-O bond lengths are equal to 1.931 Å in the cubic structure. To understand the structural features described above, the Bader charge and

magnetic moment of Co ions of tetragonal and cubic structures were calculated, respectively; the results are listed in Table 1. It needs to be pointed out that Bader charge is an approximation to the total electronic charge of an ion, and the charge distribution can be used to analyze the interaction strength of interacting ions. From Table 1, we can see that the Bader charges of Co1, Co2 of tetragonal structure and Co1, Co2 of cubic structure are 7.458 e, 7.339 e, 7.393 e, and 7.393 e, respectively, implying that there exists charge transfer from the corresponding cobalt ions to the oxygen ions. This is reasonable because Co is in the oxidation state of Co2+, and about 2 electrons are transferred to O ions for each Co ion. Further analysis revealed that the less charge transfer (1.542 e) of Co1 of tetragonal structure shows the weaker electrostatic interaction between Co1 and O, leading to an elongated bond length of 2.034 Å. On the contrary, the more charge transfer (1.661 e) of Co2 of tetragonal structure suggests a stronger chemical bonding between Co2 and O, resulting in a shortened bond of 1.829 Å. As for the cubic structure, the same charge transfer (1.607 e) for Co1 and Co2 contributes to the equal Co-O bond length. Further, the structural features of tetragonal and cubic phases are also reflected in the projected density of states (PDOSs) of 3d orbital of Co1tetra, Co2tetra, and Co1cubic ions. As shown in Fig. 2, the electronic states of Co1tetra at the vicinity of Fermi level 4


Page 4 of 19

Page 5 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


are mainly from dyz and dxz states, while considerable amounts of dxy states are unoccupied vacant orbitals of spin down at about 1.5 eV above the Fermi level. In contrast, the major contributions of electron states of Co2tetra close to the Fermi level are dxy states, while dyz and dxz states show unoccupied vacant orbitals of spin down at about 1.0 eV above the Fermi level. As a result, Co1tetra and Co2tetra forms elongated and flattened octahedrons with oxygen along z-axis (or c-axis in the lattice coordinate), respectively. For the 3d states of Co1cubic, the PDOS of dz2 and d(x2-y2) are completely degenerate, and so are dxy, dyz and dxz orbitals. More importantly, the PDOSs near the Fermi level are mainly contributed by the dxy, dyz and dxz states. Next, the stability of tetragonal and cubic structure was examined. The total energy of the cubic structure calculated is ~ 0.332 eV higher than that of the tetragonal structure, meaning that the tetragonal structure is more stable. This result can be reasonably understood by the Jahn-Teller (JT) effect. From Fig. 1, it is evident that the JT distortion exists in the tetragonal structure, e.g. the elongated Co1O6 octahedron. In general, the JT effect can induce variation of the local atomic structure, and weaken or eliminate the energy degeneracy of orbitals, resulting in a more stable structure. Figure 2 shows that the PDOSs of 3d orbitals of Co1cubic are degenerate, e.g., the energy level of dxy, dyz, and dxz are equal, and so are dz2 and d(x2-y2) states. On the other hand, the PDOSs of 3d orbitals of Co1tetra and Co2tetra are non-degenerate, implying that the JT distortion occurs in the tetragonal structure, resulting in the elimination of degeneracy in d-orbitals. Therefore, a more stable structure of tetragonal SrCoO3 is produced. 3.2 Structures and electronic properties of tetragonal SrNb0.5Co0.5O3 As aforementioned, the tetragonal structure is more stable than the cubic structure. Therefore, the tetragonal structure is considered as the host structure to understand the Nb doping effect. For the simplest case, a 50% Nb doped SCO (SrNb0.5Co0.5O3) was constructed by replacing one Nb ion with one Co ion in the unit cell of the tetragonal phase. Although this composition may not be experimentally achievable, it still can be used to understand the effect of Nb doping from the viewpoint of theoretical calculation. Figure 3 shows the optimized structures together with the bond length of 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Co-O and Nb-O of Nb doping on Co1 site and Co2 site, respectively. Interestingly, after a full relaxation, the geometrical structure of Nb-doping on Co1 site is exactly the same as that of Nb-doping on Co2 site. In addition, the total energy of Nb-doping on Co1 site is equal to that of Nb-doping on Co2 site. This result implies that Nb ions are randomly distributed at Co-sites in the SrNb0.5Co0.5O3 structure, regardless of Co1 site or Co2 site. This result is in good agreement with the observation from experiment 38, where the authors claimed that the Nb-dopant has no preferred Co-site to occupy. Further observation of the structural features shows that all the bond lengths in the SrNb0.5Co0.5O3 are approximately equal to 2.0 Å (listed in Table 2, to be shown later), indicating that a transition from tetragonal phase to cubic phase taking place. This finding indicates that Nb-doping can facilitate the transition from tetragonal structure to cubic structure. On the other hand, Nb-doping is accompanied by a lattice expansion (a=b=3.981 Å and c=7.967 Å) due to a larger covalent radius of Nb than that of Co (Nb: 137 pm; Co:126 pm). To understand the effect of Nb-doping on the structure, the Bader charge and magnetic moment of Co and Nb ions for Nb-doping at Co1 and Co2 site were also calculated, respectively, the results of which are listed in Table 2. It is clear from the calculations that the Bader charges of Co and Nb ions for Nb-doping at Co1 site are equal to those of Co and Nb ions for Nb-doping at Co2 site, which again confirms that Nb-doping is random in the tetragonal SrCoO3 structure. In addition, an important finding from Table 2 is that the magnetic moment of Co ions is smaller than that of Co ions in pure tetragonal structure (as listed in Table 1), and the magnetic moment of Nb is almost zero. This can be understood from the PDOSs of Co and Nb ions in the SrNb0.5Co0.5O3 structure. Figure 4 (a) shows that the electronic states near the Fermi level are mainly from the Co ions, the contribution from Nb ions is almost zero. In the SrNb0.5Co0.5O3 structure, the Nb ion is in Nb5+ and the magnetic moment of Nb5+ is zero. On the other hand, the Co4+ in the pure tetragonal structure of SrCoO3 is reduced to the Co3+ in the Nb-doped SrNb0.5Co0.5O3 structure due to the introduction of Nb5+, resulting in an extra electron filling into the 3d orbitals of Co ion. As shown in Fig.4 (b), the empty 3d orbitals at about 1.0 eV above the Fermi level of Co2 in pure 6


Page 6 of 19

Page 7 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


tetragonal structure (blue line) are occupied by electrons from Nb after Nb doping (red line), resulting in the rich electronic states around the Fermi level. Similar situation was also found for the Co1 case. As a result, the magnetic moment of Co ions in Nb-doped SrNb0.5Co0.5O3 structure is decreased. This result is also consistent with the Bader analysis on Co ions. Because of electrons transfer from Nb ions to Co ions, the Bader charge on Co ions of SrNb0.5Co0.5O3 structure (7.515 e and 7.518 e, as listed in Table 2) is larger than that of pure tetragonal structure (7.458 e and 7.339 e, as listed in Table 1). 3.3 Structures and electronic properties of tetragonal SrNb0.1Co0.9O3 We have also examined the structural and electronic properties of SCN10 crystalized in the tetragonal structure. From the analysis of the symmetry of SCN10 crystal structure (see Computational Details and Models), three non-equivalent doping cases are considered. Figure 5 (a)-(c) represent the nearest, the second nearest, and the third nearest cases for the two Nb ions, respectively. The calculated total energy shows that the case (c) is the most stable with the lowest energy, indicating that Nb ions prefer to distribute randomly in the tetragonal SrCoO3 host structure. To further understand Nb-doping effect on the geometry of SrCoO3, local structures of SCN10 and pure tetragonal SrCoO3 were examined and compared. Figure 6 (a) shows that along c-axis direction, the bond length of Co-O is very close to that of Nb-O in Nb-doped one. At the a-b plane, both Co-O and Nb-O bonds display the characteristics of two longer bonds and two shorter bonds. Also, the Nb-O bond length is shorter in the direction of a longer Co-O bond, and vice versa. These structural features indicate that the Nb-doping induces a gradual transition from tetragonal structure to cubic structure. To further understand Nb-doping effect on the electronic properties of the tetragonal SCN10, the d-orbital magnetic moment and the total magnetic moment of each Co ion and Nb ion were examined, the results of which are listed in Table 3. It is clear that the magnetic moment of Co8 and Co15 is approximately 1μB smaller than those of other Co ions, and the magnetic moment of Nb1 and Nb2 is almost zero. This result is reasonable since in the structure of SCn10, the oxidation states of two 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

dopants Nb1 and Nb2 are 5+ with zero magnetic moment. To ensure the electrical neutrality of the entire system, two Co4+ ions must be reduced into Co3+ ions. Then, the two extra electrons fill into the 3d states of Co4+ ions, respectively, leading to the smaller magnetic moment of Co8 and Co15. The magnetic moments can also be explained by the PDOSs of Co8, Co15 and other Co ions. Here the PDOS of Co15 as an example is shown in Fig.7 (b) (blue line). For comparison, the PDOS of Co1 was selected and is shown in Fig.7 (b) (red line). The corresponding geometrical structure together with the number of ions is shown in Fig. 7 (a). Clearly, a unique feature in Fig.7 (b) is the appearance of a notable spin down peak for Co15 at the Fermi level, whereas a distinct spin down peak for Co1 appears at ~ 1eV above the Fermi level. This result again demonstrates that the 3d orbital of Co15 is filled by extra electrons from Nb. Similar situation was also found in the Co8 case. 4. Conclusions In summary, the structural and electronic properties of tetragonal and cubic phases in undoped and Nb-doped SrCoO3 are systematically investigated using first principles calculations. The structural features of tetragonal and cubic phase SrCoO3 are understood by Bader analysis and PDOS. The tetragonal structure in SrCoO3 is more stable than its cubic counterpart, which can be well explained by the Jahn-Teller effect. In Nb-doped SrCoO3 systems, the structural analysis shows a gradual tetragonal-to-cubic transition by Nb-doping. Nb ions prefer to randomly distribute over Co-sites in the SrCoO3 host structure. The examination of Bader charge, PDOS, and the magnetic moment of Co- and Nb-ions in Nb-doped SrCoO3 systems indicates that the reduction of Co-ions becomes easier due to the Nb-doping with excess electrons filling into 3d orbitals of Co-ions and decreasing the magnetic moments of Co-ions. Last, the rich electronic states at the Fermi level suggests a higher electronic conductivity of Nb-doped SrCoO3, making it suited for cathode materials in IT-SOFCs. Acknowledgments The authors thank the National Science Foundation of China (Grant No. 11404149, 8


Page 8 of 19

Page 9 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


11764019, 11234013) for major financial support of the current work. Partial support to B. Xu from the Science and Technology Department of Jiangxi Province (Grant No. 20152ACB21014) and to C.Y. Ouyang from 863 Key Program (Grant No. 2015AA034201) is also appreciated. The partial computations were performed on TianHe-1 (A) at the National Supercomputer Center in Tianjin. We would also like to thank National Science Foundation for supporting this work (NSF-DMR-1464112). References 1. Jeen, H.; Choi, W. S.; Biegalski, M. D.; Folkman, C. M.; Tung, I. C.; Fong, D. D.; Freeland, J. W.; Shin, D.; Ohta, H.; Chisholm, M. F. and Lee, H. N. Reversible redox reactions in an epitaxially stabilized SrCoOx oxygen sponge. Nat Mater, 2013, 12, 1057-1063. 2. Jeen, H.; Choi, W. S.; Freeland, J. W.; Ohta, H.; Jung C. U. and Lee, H. N. Topotactic Phase Transformation of the Brownmillerite SrCoO2.5 to the Perovskite SrCoO3−δ. Adv Mater, 2013, 25, 3651-3656. 3. Jeen, H.; Bi, Z. H.; Choi, W. S.; Chisholm, M. F.; Bridges, C. A.; Paranthaman, M. P. and Lee, H. N. Orienting Oxygen Vacancies for Fast Catalytic Reaction. Adv Mater, 2013, 25, 6459-6463. 4. Aguadero, A.; Perez-Coll, D. J.; Alonso, A.; Skinner S. J. and Kilner, J. A New Family of Mo-Doped SrCoO3−δ Perovskites for Application in Reversible Solid State Electrochemical Cells. Chem Mater, 2012, 24, 2655-2663. 5. Mitra, C.; Meyer, T.; Lee, H. N. and Reboredo, F. A. Oxygen diffusion pathways in brownmillerite SrCoO2.5: Influence of structure and chemical potential. J Chem Phys, 2014, 141. 6. Cascos, V.; Martinez-Coronado R. and Alonso, J. A. New Nb-doped SrCo1-xNbxO3−δ perovskites performing as cathodes in solid-oxide fuel cells. Int J Hydrogen Energ, 2014, 39, 14349-14354. 7. Wang, F.; Zhou, Q. J.; He, T. M.; Li, G. D. and Ding, H. Novel SrCo1−yNbyO3−δ cathodes for intermediate-temperature solid oxide fuel cells. J Power Sources, 2010, 195, 3772-3778. 8. Zhou, W.; Shao, Z.; Ran, R.; Jin W. and Xu, N. A novel efficient oxide electrode for electrocatalytic oxygen reduction at 400–600 ºC. Chem. Commun. 2008, 5791–5793.

9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 19

9. Usiskin, R. E.; Davenport, T. C.; Wang, R. Y.; Guan, W. and Haile Bulk, S. M. Properties of the Oxygen Reduction Catalyst SrCo0.9Nb0.1O3−δ. Chem. Mater. 2016, 28, 2599−2608. 10. Chen, D.; Chen, C.; Gao, Y.; Zhang, Z.; Shao, Z.; Ciucci, F.; Evaluation of pulsed laser deposited SrNb0.1Co0.9O3−δ thin films as promising cathodes for intermediate-temperature solid oxide fuel cells. J Power Sources 2015, 295, 117-124. 11. Zhu, Y.; Lin, Y.; Shen, X.; Sunarso, J.; Zhou, W.; Jiang, S.; Su, D.; Chen, F. and Shao, Z. Influence of crystal structure on the electrochemical performance of A-site-deficient Sr1-sNb0.1Co0.9O3−δ perovskite cathodes. RSC Adv. 2014, 4, 40865–40872. 12. Zhu, Y.; Chen, Z. G.; Zhou, W.; Jiang, S.; Zou, Jin and Shao, Z. An A-Site-Deficient Perovskite offers High Activity and Stability for Low-Temperature Solid-Oxide Fuel Cells. ChemSusChem 2013, 6, 2249-2254. 13. Zhu, Y.; Sunarso, J.; Zhou, W.; Jiang S. and Shao, Z. High-performance SrNb0.1Co0.9-xFexO3−δ perovskite cathodes for low-temperature solid oxide fuel cells. J. Mater. Chem. A, 2014, 2, 15454-15462. 14. Wang, J.; Jiang, L.; Xiong, X.; Zhang, C.; Jin, X.; Lei, L. and Huang, K. A Broad Stability Investigation

of

Nb-Doped

SrCoO2.5+δ

as

a

Reversible

Oxygen

Electrode

for

Intermediate-Temperature Solid Oxide Fuel Cells. J Electrochem Soc. 2016, 163 (8), F891-F898. 15. Wang, J.; Jin, X.; Huang, K. A New Defect Chemistry Model for Nb-doped SrCoO2.5+δ: The Role of Oxygen Interstitials and Delocalized-to-Localized Electron Holes. J. Solid State Chem. 2017, 246, 97–106. 16. Zhou, W.; Jin, W. Q.; Zhu, Z. H. and Shao, Z. P. Structural, electrical and electrochemical characterizations of SrNb0.1Co0.9O3−δ as a cathode of solid oxide fuel cells operating below 600 ºC. Int J Hydrogen Energ, 2010, 35, 1356-1366. 17. Li, M.; Zhao, M.; Li, F.; Zhou, W.; Peterson, V. K.; Xu, X.; Shao, Z.; Gentle, I. & Zhu, Z. A niobium and tantalum Co-doped perovskite cathode for solid oxide fuel cells operating below 500 ºC. Nature Communications 8, 2017, Article number: 13990. 18. Wang, J.; Yang, T.; Lei, L. and Huang, K. Ta-Doped SrCoO3−δ as a promising bifunctional oxygen electrode for reversible solid oxide fuel cells: a focused study on stability. J. Mater. Chem. A, 2017, 5, 8989-9002. 10


Page 11 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


19. Li, M. R.; Zhou W. and Zhu, Z. H. Comparative Studies of SrCo1−xTaxO3−δ (x=0.05–0.4) Oxides as Cathodes for Low-Temperature Solid-Oxide Fuel Cells. Chemelectrochem, 2015, 2, 1331-1338. 20. Qu, B. P.; Long, W.; Jin, F. J.; Wang S. Z. and He, T. M. SrCo0.7Fe0.2Ta0.1O3−δ perovskite as a cathode material for intermediate-temperature solid oxide fuel cells. Int J Hydrogen Energ, 2014, 39, 12074-12082. 21. Chen, D.; Chen, C.; Zhang, Z.; Baiyee, Z. M.; Ciucci, F. and Shao, Z. Compositional Engineering of Perovskite Oxides for Highly Efficient Oxygen Reduction Reactions. Appl. Mater. Interfaces 2015, 7, 8562−8571. 22. Aguadero, A.; Alonso, J. A.; Perez-Coll, D.; de la Calle, C.; Fernandez-Diaz, M. T. and Goodenough, J. B. SrCo0.95Sb0.05O3−δ as Cathode Material for High Power Density Solid Oxide Fuel Cells. Chem Mater, 2010, 22, 789-798. 23. Aguadero, A.; de la Calle, C.; Alonso, J. A.; Escudero, M. J.; Fernandez-Diaz M. T. and Daza, L. Structural and Electrical Characterization of the Novel SrCo0.9Sb0.1O3–δ Perovskite: Evaluation as a Solid Oxide Fuel Cell Cathode Material. Chem Mater, 2007, 19, 6437-6444. 24. Aguadero, A.; Pérez-Coll, D.; de la Calle, C.; Alonso, J. A.; Escudero, M. J.; Daza, L. SrCo1−xSbxO3−δ perovskite oxides as cathode materials in solid oxide fuel cells. J Power Sources 2009, 192, 132-137. 25. Ahvenniemi, E.; Matvejeff, M. and Karppinen, M. Atomic layer deposition of quaternary oxide (La, Sr)CoO3−δ thin films. Dalton T, 2015, 44, 8001-8006. 26. Yoo, S.; Kim, J.; Song, S. Y.; Lee, D. W.; Shin, J.; Ok, K. M. and Kim, G. Structural, electrical and electrochemical characteristics of La0.1Sr0.9Co1-xNbxO3−δ as a cathode material for intermediate temperature solid oxide fuel cells. Rsc Adv, 2014, 4, 18710-18717. 27. Jiang, L.; Wang, J.; Xiong, X.; Jin, X.; Pei Q. and Huang, K. Thermal and Electrical Stability of Sr0.9Y0.1CoO2.5+δ as a Promising Cathode for Intermediate-Temperature Solid Oxide Fuel Cells. J Electrochem. Soc., 2016, 163, F330-F335. 28. Shi, S.; Gao, J.; Liu Y.; Zhao, Y.; Wu Q.; Ju, W.; Ouyang, C.; Xiao, R. Multi-scale computation methods: Their applications in lithium-ion battery research and development. Chin. Phys. B 2016, 25, 018212. 29. Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B 1994, 50, 17953. 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

30 Kresse, G.; Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 1999, 59, 1758. 31. Kresse, G.; Hafner, J. Ab initio molecular dynamics for liquid metals. Phys. Rev. B 1993, 47, 558. 32. Kresse, G,; Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 1996, 54, 11169. 33. J. P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865–3868. 34. Wang, L.; Maxisch, T.; Ceder, G. Oxidation Energies of Transition Metal Oxides within the GGA+U Framework. Phys. Rev. B 2006, 73, 195107. 35 Lee, Y.-L.; Kleis, J.; Rossmeisl, J.; Morgan, D. Ab Initio Energetics of LaBO3(001) (B=Mn, Fe, Co, and Ni) for Solid Oxide Fuel Cell Cathodes. Phys. Rev. B 2009, 80, 224101. 36. Monkhorst, H. J.; Pack, J. D. Special points for brillouin-zone integrations. Phys. Rev. B 1976, 13, 5188–5192. 37. Ali, Z.; Ahmad, I. Band Profile Comparison of the Cubic Perovskites CaCoO3 and SrCoO3. J. Electron. Mater. 2013, 42, 438−444. 38. Yang, T.; Wang, J.; Chen, Y.; An, K.; Ma, D.; Vogt, T.; Huang, K.). A Combined Variable Temperature Neutron Diffraction and Thermogravimetric Analysis Study on a Promising Oxygen Electrode SrCo0.9Nb0.1O3-δ for Reversible Solid Oxide Fuel Cells. ACS Appl. Mater. Interfaces. in press, DOI: 10.1021/acsami.7b08697.

12


Page 12 of 19

Page 13 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The captions for all figures Fig. 1 The optimized structures of tetragonal (a) and cubic (b) phase of SrCoO3. Fig. 2 The PDOS of 3d orbitals of Co1tetra, Co2tetra and Co1cubic ions in the tetragonal and cubic structures of SrCoO3. For the Co1cubic ion, the PDOS of dz2 and d(x2-y2) are completely degenerate, and so are the PDOS of dxy, dyz and dxz . The Fermi level is set to 0 eV. Fig. 3 The optimized structures of Nb-doped tetragonal structure; (a) Nb doping at Co1 site; (b) Nb doping at Co2 site. Fig. 4 (a) PDOS of Co and Nb ions in the SrNb0.5Co0.5O3 structure, respectively; (b) PDOS of Co ion in the pure and Nb-doped tetragonal structure, respectively. The Fermi level is set to 0 eV. Fig. 5 The optimized structures for tetragonal 10% Nb-doped SCO , Sr ions are invisible; (a) the nearest for two doped Nb ions; (b) the second nearest for two doped Nb ions; (c) the third nearest for two doped Nb ions. Fig. 6 The local structures of (a) Nb-doped tetragonal structure SrNb0.1Co0.9O3 and (b) pure tetragonal structure. The bond length is in Å. Fig. 7 (a) the optimized structure of SrNb0.1Co0.9O3 with number labelled for Co8 and Co15 ions, Sr ions are invisible; (b) the PDOS of Co1 and Co15. The Fermi level is set to 0 eV.

13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 1 The Bader charge (Q) on Co ions, bond length of Co-O along the c-axis direction (dCo-O), and the magnetic moment (M) of Co ions for tetragonal and cubic structures of SrCoO3. Co1tetra and Co2tetra present the Co ions of tetragonal structure. Co1cubic and Co2cubic represent the Co ions of cubic structure.

Table 2 The Bader charge (Q) on Co and Nb ions, bond length of Co-O and Nb-O along c axis (dCo-O), and magnetic moment(M) of Co and Nb ions for Nb doping at Co1 and Co2 site of tetragonal structure, respectively.

14


Page 14 of 19

Page 15 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3 The d orbital magnetic moment (dM) and the total magnetic moment (TM) of Co and Nb ions in the tetragonal SrNb0.1Co0.9O3; the corresponding structure is shown in Fig.7 (a).

Fig. 1

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 2

Fig. 3

16


Page 16 of 19

Page 17 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 4

Fig. 5

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 6

Fig. 7

18


Page 18 of 19

Page 19 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC

19


Insight from First-Principles Calculations - ACS Publications

Recommend Documents