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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Transition-Metal Distribution in Brownmillerite Ca2FeCoO5 Kei Nakayama,† Ryo Ishikawa,†,‡ Akihide Kuwabara,§ Shunsuke Kobayashi,§ Teruki Motohashi,∥ Naoya Shibata,†,§ and Yuichi Ikuhara*,†,§ †
Institute of Engineering Innovation, University of Tokyo, Bunkyo, Tokyo 113-8656, Japan Japan Science and Technology Agency, PRESTO, Kawaguchi, Saitama 332-0012, Japan § Nanostructures Research Laboratory, Japan Fine Ceramics Center, Atsuta, Nagoya 456-8587, Japan ∥ Department of Materials and Life Chemistry, Kanagawa University, Kanagawa, Yokohama 221-8686, Japan Downloaded via UNIV OF SOUTHERN INDIANA on July 17, 2019 at 09:04:50 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: Ca2Fe2−xCoxO5 (0 ≤ x ≤ 1) with higher Co content, which crystallizes in a brownmillerite-type structure, is currently one of the best oxygen-evolution-reaction (OER) catalysts. Identifying the Fe/Co occupancies at the octahedral (Oh) and tetrahedral (Td) sites in the structure is the foundation for the understanding of the role of cobalt in each site and the exploration of further improvement in the OER activity. Here, we investigate the Fe/Co distribution in Ca2FeCoO5 by means of atomic-resolution energy dispersive X-ray spectroscopy in scanning transmission electron microscopy and dynamical image simulations combined with systematic density functional theory calculations. Our careful microscopic study reveals the absence of long-range Fe/Co order within the transition-metal (TM) layers, but cobalt is slightly enriched at the Td and Oh sites in the as-synthesized (1100 °C) and 800 °C annealed for a month samples, respectively. The observed Co site preferences are interpretable from the viewpoints of TM ionic size effect and ligand field effect, which are competitive around a crossover point at a certain temperature between 800 and 1100 °C. We also elucidate that the as-synthesized sample with Co enrichment at the Td site shows the better OER activity, and the optimum annealing temperature for more OER active Ca2FeCoO5 should be higher than the crossover temperature.
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INTRODUCTION In recent years, metal-air batteries have attracted much attention as the next generation of rechargeable batteries, particularly owing to their extensively high energy densities.1−5 For instance, when lithium metal is used for the anode, the theoretical energy density of the metal-air battery reaches 5.2 kWh kg−1, which is an order of magnitude larger than the practical energy density of currently available lithium-ion batteries today (0.25 kWh kg−1).6 However, the lack of highly active and durable air-cathode catalysts is one of the critical issues to realize batteries with a sufficient charge/discharge efficiency and lifetime. Recently, Ca2FeCoO5 was discovered to exhibit a significant catalytic activity to oxygen evolution reaction (OER) in the charging process.7 Specifically, the OER current density is ten times larger than those of some typical OER catalysts such as RuO2 and Ba0.5Sr0.5Co0.8Fe0.2O3−δ. Ca2FeCoO5 also has long-term durability as well as better economic and productive efficiencies, making this oxide a promising cathode catalyst for the metal-air battery system. Ca2FeCoO5 crystallizes in a structure known as brownmillerite-type, which is suggested to be a possible key ingredient of the remarkable OER catalytic performance.7 This structure can be considered as an alternative layered arrangement of © XXXX American Chemical Society
tetrahedral (Td) and octahedral (Oh) sites for transition metals (TM) along the c-axis of the orthorhombic structure, where the TM atoms are surrounded by four- and six-coordinated O atoms, respectively.7−11 Particularly the Td sites with the smaller coordination number are expected to easily adsorb OH− and work as the major reaction centers.7 The importance of Td sites in the OER activity has been also recently analyzed in the similar system of YBaCo4O7.3 oxide.12 Another key to the OER performance of Ca2FeCoO5 is its higher Co compositional ratio. As reported in the literature, 7 Ca2Fe2−xCoxO5 catalysts were synthesized in the compositional range of 0 ≤ x ≤ 1, and their OER performance monotonically improved with the increasing Co content. This result clearly indicates that the high OER catalytic performance of Ca2FeCoO5 is also related to the high doping of Co. On the basis of the above two features, the Co contents at the Td/Oh sites should be essential for the OER activity of Ca 2 FeCoO 5 . Therefore, the identification of the Co occupancies at both sites is a fundamental requirement, which leads to deeper insight into the role of cobalt in each Received: May 9, 2019
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DOI: 10.1021/acs.inorgchem.9b01356 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 1. (a) BF-TEM image and (b) electron diffraction pattern viewed along the [100] direction obtained from Ca2FeCoO5 synthesized at 1100 °C. The brightness and contrast in the right half of (b) are intentionally increased. (c) The unit cell and (d) the simulated electron diffraction pattern were calculated based on the crystallographic data in the literature.8 mol L−1 KOH aqueous solution at room temperature, using a potentiostat (Hokuto Denko Corp., HZ-7000) combined with a rotating disk electrode (RDE; ALS Co., Ltd., RRDE-3A). Details of the experimental procedures are given elsewhere.7 Electron-transparent thin specimens for S/TEM observations were prepared by gently crushing in ethanol and dispersing the samples on holey carbon amorphous films supported by Cu grids. Electron diffraction patterns and bright-field (BF) TEM images were taken by a JEM-2010 (JEOL Ltd.), operating at 200 kV. Atomic-resolution high-angle annular dark-field (HAADF) images were taken by ARM200CF and ARM300CF (JEOL Ltd.), operating accelerating voltages at 200 kV and 300 kV, respectively. Atomic-resolution elemental maps were obtained by STEM-EDX with the ARM200CF. First-principles calculations were performed using the projector augmented wave (PAW) method15,16 as implemented in the VASP code.17−19 We adopted the Perdew−Burke−Ernzerhof type exchangecorrelation interaction of electrons,20 and the cutoff energy for plane wave basis sets were set to be 500 eV. Valence electron configurations of the PAW potentials were 3s2 3p6 4s2 for Ca, 3s2 3p6 3d6 4s2 for Fe, 3s2 3p6 3d7 4s2 for Co, and 2s2 2p4 for O. The radial cut-offs of the PAW potentials of Ca, Fe, Co and O were 2.30, 1.90, 1.90, and 1.52 Å, respectively. The GGA+U approach21 was used to account for strong correlation effects of 3d orbitals with U set at 4.0 and 3.3 eV for Fe and Co, respectively.22 In order to identify energetically favorable Fe/Co configurations in the brownmillerite structure of Ca2FeCoO5, we systematically carried out series of total energy calculations with different Fe/Co configurations. One is a perfectly ordered Ca2FeCoO5 model based on the crystal structure reported by Ramezanipour et al.,8 and the others are random distribution models. Details of the model constructions are explained in the Supporting Information section. The k-point sampling meshes of 4 × 3 × 3 were used for the Ca2FeCoO5 conventional unit cell,8 which provided an energy convergence of less than 1 meV per formula unit. In the case of Ca2FeCoO5 supercell models, numbers of k-point sampling mesh
site and the further improvement in the OER activity. Although previous studies have dealt with the Co distribution in Ca2FeCoO5, the results are still controversial.8,13,14 In this study, we investigate the TM distribution in Ca2FeCoO5 by means of atomic-resolution energy dispersive X-ray spectroscopy (EDX) in scanning transmission electron microscopy (STEM) and theoretical image simulations combined with systematic density functional theory (DFT) calculations. Our careful observation on the as-synthesized Ca2FeCoO5 at 1100 °C revealed no signature of Fe/Co long-range order within each TM layer, but cobalt is slightly enriched at the Td site. Meanwhile, Co atoms rather prefer the Oh sites in the sample of long-term annealing at 800 °C, although both Fe and Co atoms are still distributed within the both sites. Noticeably, the OER activity of Ca2FeCoO5 seems to depend on the heating protocol, i.e., the as-synthesized sample shows higher OER activity than that of the 800 °C annealed sample. These results could be a guide for considering the best annealing condition to explore the highest OER activity in the Ca2FeCoO5 system.
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EXPERIMENTAL SECTION
Polycrystalline Ca2FeCoO5 samples were synthesized by a solid-state reaction technique at 1100 °C for 12 h. CaCO3 (Wako Pure Chemical Corp., 99.99%), Fe2O3 (Wako Pure Chemical Corp., 99.9%), and Co3O4 (Rare Metallic Co. Ltd., 99.9%) were used as starting reagents.7 In addition to the as-synthesized sample, a part of the sample was further annealed at 800 °C for a month. Note that the sample needs to be annealed for a significantly long-term because the diffusivities of the TM atoms may be very small at the lower temperature of 800 °C. Power X-ray diffraction analysis indicated that both of the samples are of single phase with an orthorhombic brownmillerite-type structure. The OER activities were measured in 4 B
DOI: 10.1021/acs.inorgchem.9b01356 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 2. (a) Atomic-resolution HAADF STEM image and (b−f) STEM-EDX chemical maps obtained from the as-synthesized Ca2FeCoO5. The cyan and yellow circles in (a) indicate Ca and TM columns, respectively. The thick and thin yellow allows in (a), (d), and (e) indicate the Td and Oh TM sites. The red and green dashed ellipses in (d, e) indicate Fe-rich and Co-rich regions. The alternative red and green arrows in (f) indicate the local Fe/Co chemical order. were reduced according to the sizes of the supercells in comparison with the unit cell. As reference states for Ca2FeCoO5, total energies of Ca2Fe2O523 and Ca2Co2O523,24 crystals were also calculated. The sizes of k-point sampling meshes were 3 × 3 × 3 and 5 × 3 × 5 for Ca2Fe2O5 and Ca2Co2O5, respectively. Crystal structures were fully relaxed until that residual forces on all atoms become lower than 0.02 eV/Å.
TM sites can be visualized as brightest dot contrasts, and Ca atomic columns can be seen as slightly weaker contrasts. The Td and Oh TM sites are marked in the Fe and Co EDX chemical maps of Figure 2d and 2e, and both Fe and Co atoms occupy all the TM sites. This indicates that Fe/Co are chemically disordered at Td and Oh sites, which is consistent with the electron diffraction analysis of Figure 1b. A close inspection of EDX intensities evidences nonuniformity of Fe/ Co distributions, and this is highlighted by the (Fe + Co) map of Figure 2f. We also find the following features. (i) Red and green dashed ellipses are regions enriched by Fe and Co, respectively, and (ii) a Fe/Co partially ordered region can be observed at the Td sites, indicated by the alternative green and red arrows in Figure 2f. We note that these EDX intensity variations should be related to the Fe/Co fluctuations at each atomic column rather than an insufficiency of the X-ray counts (quantum noise), because the simultaneously recorded Ca atomic columns are considerably uniform in Figure 2b. To interpret whether the observed EDX intensity variations come from completely random occupation of TM or partial order of TM, we here carefully consider the following two issues. First, the sample thickness in STEM is as relatively thin as 25−30 nm, which introduces statistical variations in Fe/Co atomic columns along the binomial distribution, because the total number of atoms in projection is statistically poor as roughly 50 atoms. Second, the channeling effect of the electron probe (so-called dynamical scattering effect) must be taken into account,26−28 which may also affect the EDX intensity variations in experimental EDX maps. To investigate these issues, we compared the projected compositional maps and EDX maps calculated from the dynamical image simulations on the basis of the structural model with random Fe/Co distribution. We constructed a 6 × 4 × 50 supercell where Fe/ Co atoms are randomly distributed at the TM sites with the probabilities of 0.5/0.5. Figure 3a shows a histogram of the Co occupations per TM column, in which the Co distribution in our supercell obeys the binomial distribution as shown by the dashed line in Figure 3a. Using this structural model, projected compositional maps for Fe/Co and superimposed composition
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RESULTS AND DISCUSSION Figure 1a shows a BF-TEM image obtained from the assynthesized sample, demonstrating that the sample is homogeneous and has no precipitation. Figure 1b shows a selected area electron diffraction pattern viewed along the [100] direction obtained from the same area of Figure 1a, indicating high crystallinity and homogeneity of the sample as can be seen from the sharp Bragg reflections. Figure 1c shows a schematic illustration of the Ca2FeCoO5 lattice viewed along the [100] direction, where cyan, red, and green circles correspond to Ca, Fe, and Co atoms, respectively. The partially red- and green-colored circles indicated by the arrows stand for Fe/Co mixed sites, and the areas correspond to the occupancies. The colored polyhedra indicate the oxygen tetrahedron and octahedron at the TM sites. According to the structural model in the literature (Figure 1c),8,14 Fe/Co atomic ordering exists, which should provide extra 022 reflections in electron diffraction as indicated by the arrows in the simulated electron diffraction pattern of Figure 1d. In our diffraction experiments, however, no 022 reflections were observed that are most evident in the enhanced contrast in the right-side of Figure 1b. In addition to the absence of 022 reflections, there are no other superlattice reflections and diffuse streaks in the electron diffraction pattern, suggesting that the absence of both long-range and short-range Fe/Co ordering at Td and Oh sites in the present specimen. Figure 2 shows atomic-resolution HAADF STEM image and the corresponding EDX chemical maps viewed along the [100] orientation, obtained from the edges of O−K, Ca−K, Fe−K, and Co−K, respectively. In the HAADF STEM image, owing to the Z-contrast nature (Z is atomic number),25 the Td and Oh C
DOI: 10.1021/acs.inorgchem.9b01356 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 3. (a) Histogram of the Co occupation in the TM columns in the structure model with random Fe/Co distribution. The dashed curve in (a) indicates the ideal binomial distribution. (b−d) Composition maps and (e−g) simulated EDX maps from the structure model. Fe, Co, and superimposed chemical maps are in (b, e), (e, f), and (d, g), respectively. The intensity scales for (b) and (c) indicate the Fe and Co ratios, and those in (e) and (f) indicate the probabilities of ionization cross section.
maps were constructed and presented in Figure 3b−d, where the images are generated by the convolution of Fe/Co
occupation with a two-dimensional Gaussian profile (for comparison). Even along the binomial distribution, Fe-rich D
DOI: 10.1021/acs.inorgchem.9b01356 Inorg. Chem. XXXX, XXX, XXX−XXX
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atures. The Fe/Co randomness is also evident by atomicresolution STEM-EDX analysis of Figure 4c; the features of the map are similar to those of the as-synthesized sample shown in Figure 4d. We further investigate the Co occupation at the Td sites by the EDX maps of Figure 4c and 4d. Figure 5 shows Co
(Co-poor) and Co-rich (Fe-poor) columns appear accidentally as indicated by red and green dashed ellipses. The alternate Fe/Co-rich arrangements are also found at the red and green arrows, which are similar to those in the experimental Fe and Co maps in Figure 2. For the further analysis, we performed EDX dynamical image simulations of Figure 3e−g employing the μSTEM code.29 Since the dynamical effect depends on the Fe/Co arrangement along the [100] direction, the resultant EDX intensity may differ from the projected composition maps of Figure 3b−d. However, the differences between the direct compositional and the simulated dynamical EDX maps are negligibly small, which leads to similar features of red and green ellipses and arrows in Figure 3e−g. This is because the present sample thickness is relatively thin and because the dynamical effect may not strongly appear for the intensity quantification. It is thus concluded that the experimentally observed EDX intensity variations originate from the nearly random distribution of Fe/Co at the Td and Oh sites, respectively. Since the Fe and Co atoms are next to each other in the periodic table, the nearly random distribution of Fe/Co is most likely related to the configurational entropy. From the viewpoint of thermodynamics, Gibbs free energy is given by G = H − TS (H is enthalpy, T is temperature, S is entropy), and therefore at lower annealing temperatures, we may suppress the random distribution of Fe/Co at the TM site and partial atomic order of Fe/Co in Ca2FeCoO5 may be grown. We then additionally postannealed the as-synthesized sample at 800 °C for a month. Figure 4a, 4b, and 4c respectively show the BF-TEM, selected area electron diffraction, and EDX superimposed Fe/Co map obtained from the sample annealed at 800 °C. The electron diffraction pattern is essentially the same as the as-synthesized sample, suggesting that Fe/Co long-range order does not take place even with significantly long-term annealing at lower temper-
Figure 5. Ratios of Co intensity to the sum of Fe and Co intensities at TM layers in the EDX maps in Figure 4. The letters Td and Oh denote the two kinds of TM layers. As seen in the profiles, cobalt atoms prefer the Oh sites in the 800 °C annealed sample, while they prefer the Td sites in the as-synthesized (1100 °C) sample.
occupation profiles at the TM layers obtained from Figure 4c and 4d, where Td and Oh denote the two kinds of TM layers. To minimize the effect of poor statistics of the Fe/Co distribution, the intensity of each column was averaged out along the [010] direction. Although Fe/Co atoms do not form long-range order within the TM layers, cobalt in the assynthesized sample is slightly enriched at the Td site. In contrast, cobalt atoms in the annealed sample at 800 °C rather prefer the Oh sites. We will discuss the temperature-dependent site preferences. Because the Co occupancy at the Td site was decreased by the lower temperature annealing, the OER activity is expected to be affected. We then measured linear sweep voltammograms for these samples. As shown in Figure 6, the as-synthesized sample with Co enrichment at the Td site shows higher OER performance than that for the annealed sample. The OER current density value of 210 mA cm2disc at 1.70 V vs RHE for the as-synthesized sample is indeed by a factor of 1.6 higher than 130 mA cm2disc for the annealed sample. We have checked the surface morphologies of these samples by SEM (see Figure S1), and there is no significant difference in surface area between these samples. This result suggests that the largely different OER activity cannot be attributed only to microstructural modifications but the importance of the higher Co content at the Td site for the OER activity of Ca2FeCoO5. This is consistent with the assumption that the Td sites are the dominant reaction centers, and the previous study indicating that the Co−OH bonding weaker than the Fe−OH bonding is more suitable for OER in perovskite materials.30
Figure 4. (a) BF-TEM image, (b) electron diffraction pattern, and (c) (Fe K + Co K) EDX map obtained from Ca2FeCoO5 annealed at 800 °C for a month. (d) (Fe K + Co K) map obtained from as-synthesized Ca2FeCoO5, which is the same as Figure 2f. E
DOI: 10.1021/acs.inorgchem.9b01356 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 7. Formation energies calculated by DFT for the perfect chemical ordered structure (black) and 16 different random distribution of Fe/Co configurations (red).
Figure 6. Linear sweep voltammograms for OER current obtained from Ca2FeCoO5. The black and red curves denote the data of the assynthesized (1100 °C) and 800 °C annealed samples, respectively.
discuss the observed Co site preferences in terms of the following two effects, i.e., (i) a TM ionic size effect and (ii) a chemical bonding effect between Fe/Co and O atoms known as the ligand field effect. On the basis of our DFT calculations, we identified that Fe3+ is in the high-spin state and Co3+ could be in both the high- and low-spin states even in the Fe/Co disordered structures. The ionic radii of six-coordinated highspin Fe3+, high-spin Co3+, and low-spin Co3+ are respectively 64.5, 61.0, and 54.5 pm,33 and therefore, the ionic radius of Co3+ is smaller than that of Fe3+ in any case. Co enrichment at the Td sites in the as-synthesized sample can be attributed to the ionic size difference, i.e., the smaller Co3+ may be stable at the narrower space of Td site.11 On the other hand, as previously pointed out,34,35 Co3+ prefers the Oh site in terms of the ligand field effect, and hence, the Co enrichment at the Oh site in the 800 °C annealed sample is reasonably understood. However, the ligand field effect becomes weaker at elevated temperatures because the energy gain has a dependence of a−5 (a is the distance between cation and oxide anion),36 and a becomes larger by the lattice expansion as the temperature increases.14 On the basis of the present experiments, these two effects could be competitive around a crossover point at a certain temperature between 800 and 1100 °C, and it is therefore that the Co content at the Td sites could be maximized at an annealing temperature above the crossover point. Since the linear sweep voltammograms of Figure 6 indicate that higher Co content at the Td sites seems to be favorable for the enhanced OER activity of Ca2FeCoO5, we propose that postannealing at the optimum temperature could improve the OER activity.
To examine the physical origin of Fe/Co intralayer mixing in Ca2FeCoO5, we performed systematic DFT calculations. All the calculations were performed under the condition of spinpolarized ferromagnetic states. Fe and Co ions are trivalent cations in Ca2FeCoO5, and the 3d electron configurations of Fe3+ and Co3+ are d5 and d6, respectively. In the past experimental study, the magnetic moment of Fe3+ is reported to be about 4.5−4.9 μB in the brownmillerite Ca2Fe2O5.31,32 Accordingly, we set spin configurations of Fe3+ to be in highspin states. In order to determine the most stable spin configurations of Co3+, we preliminarily calculated total energies of the ordered Ca2FeCoO5 model with different combinations of high-spin and low-spin at Td and Oh coordination sites. As results, the combination of high-spin for Co3+ at Td sites (e[↑↑↓]t2[↑↑↑]) and low-spin for Co3+ at Oh sites (t2g[↑↑↑↓↓↓]) are the most stable (see Table S1 in the Supporting Information). We also apply this spin configuration of Co3+ to all of the random distribution Ca2FeCoO5 models, which are 16 different Fe/Co configurations (the number of Fe and Co atoms are equal in each TM layer). Lattice constants of Ca2FeCoO5, Ca2Fe2O5, and Ca2Co2O5 obtained from our structure optimization calculations show good agreement with experimental literature data. These results are summarized in Tables S2 and S3 in the Supporting Information. We then evaluated the formation energies via ΔEfconfig(Ca2FeCoO5) = Efconfig(Ca2FeCoO5) − 0.5{Ef(Ca2Fe2O5) + Ef(Ca2Co2O5)} for each Fe/Co configuration. Figure 7 shows the calculated formation energies for the 16 random and the perfectly ordered Fe/Co configurations. The formation energies calculated for all the configurations are negative, and hence, TM mixing should be stable rather than the two-phase separation of Ca2Fe2O5 and Ca2Co2O 5. Although the formation energy gives the lowest value at the perfectly Fe/ Co ordered structure, the energy differences between the perfectly ordered and the random configurations are as small as 22−48 meV per formula unit, corresponding to the surrounding temperature of −18 to 284 °C in terms of kBT (kB, Boltzmann constant). It is therefore reasonable to assume that the Fe/Co mixing could easily form at the present annealing temperatures of 800 and 1100 °C. Besides the existence of Fe/Co mixing at the Oh and Td sites, temperature-dependent site preferences of Fe/Co were observed; cobalt atoms are slightly enriched at Oh and Td sites for respective temperatures of 800 and 1100 °C. We here
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CONCLUSIONS In summary, we have investigated the Fe/Co atomic distribution in a promising OER catalyst Ca2FeCoO5 with a brownmillerite-type structure, by means of atomic-resolution STEM-EDX and dynamical image simulations combined with systematic DFT calculations. Our careful microscopic study revealed a nearly random distribution of Fe and Co atoms within each TM layer (intralayer mixing), but cobalt atoms are slightly enriched at the Td and Oh sites in the as-synthesized (1100 °C) and 800 °C annealed samples, respectively. The systematic DFT calculations indicated that the perfectly ordered Fe/Co distribution is the energetically most stable structure, but the energy gain is negligibly small compared with Fe/Co random distribution structures, which can easily overcome at the present annealing temperatures. The Co site F
DOI: 10.1021/acs.inorgchem.9b01356 Inorg. Chem. XXXX, XXX, XXX−XXX
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(9) Hona, R. K.; Huq, A.; Ramezanipour, F. Unraveling the Role of Structural Order in the Transformation of Electrical Conductivity in Ca2FeCoO6‑δ, CaSrFeCoO6‑δ, and Sr2FeCoO6‑δ. Inorg. Chem. 2017, 56, 14494−14505. (10) Hona, R. K.; Huq, A.; Ramezanipour, F. Magnetic Structure of CaSrFeCoO6‑δ: Correlations with Structural Order. Mater. Res. Bull. 2018, 106, 131−136. (11) Grosvenor, A. P.; Greedan, J. E. Analysis of Metal Site Preference and Electronic Structure of Brownmillerite-Phase Oxides (A2B’xB2’xO5; A = Ca, Sr; B’/B = Al, Mn, Fe, Co) by X-Ray Absorption Near-Edge Spectroscopy. J. Phys. Chem. C 2009, 113, 11366−11372. (12) Kirsanova, M. A.; Okatenko, V. D.; Aksyonov, D. A.; Forslund, R. P.; Mefford, J. T.; Stevenson, K. J.; Abakumov, A. M. Bifunctional OER/ORR Catalytic Activity in the Tetrahedral YBaCo4O7.3 Oxide. J. Mater. Chem. A 2019, 7, 330−341. (13) Turner, S.; Verbeeck, J.; Ramezanipour, F.; Greedan, J. E.; Van Tendeloo, G.; Botton, G. A. Atomic Resolution Coordination Mapping in Ca2FeCoO5 Brownmillerite by Spatially Resolved Electron Energy-Loss Spectroscopy. Chem. Mater. 2012, 24, 1904− 1909. (14) Cascos, V.; Martínez-Coronado, R.; Alonso, J. A.; FernándezDíaz, M. T. Structural and Electrical Characterization of the CoDoped Ca2Fe2O5 Brownmillerite: Evaluation as SOFC -Cathode Materials. Int. J. Hydrogen Energy 2015, 40, 5456−5468. (15) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953−17979. (16) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758−1775. (17) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 558−561. (18) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (19) Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (20) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (21) Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P. Electron-Energy-Loss Spectra and the Structural Stability of Nickel Oxide: An LSDA+U Study. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 57, 1505−1509. (22) Wang, L.; Maxisch, T.; Ceder, G. Oxidation Energies of Transition Metal Oxides within the GGA+U Framework. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 195107. (23) Shaula, A.; Pivak, Y.; Waerenborgh, J.; Gaczynski, P.; Yaremchenko, A.; Kharton, V. Ionic Conductivity of Brownmillerite-Type Calcium Ferrite under Oxidizing Conditions. Solid State Ionics 2006, 177, 2923−2930. (24) Zhang, J.; Zheng, H.; Malliakas, C. D.; Allred, J. M.; Ren, Y.; Li, Q.; Han, T. H.; Mitchell, J. F. Brownmillerite Ca2Co2O5: Synthesis, Stability, and Re-Entrant Single Crystal to Single Crystal Structural Transitions. Chem. Mater. 2014, 26, 7172−7182. (25) Pennycook, S. J.; Boatner, L. A. Chemically Sensitive StructureImaging with a Scanning Transmission Electron Microscope. Nature 1988, 336, 565−567. (26) Lugg, N. R.; Kothleitner, G.; Shibata, N.; Ikuhara, Y. On the Quantitativeness of EDS STEM. Ultramicroscopy 2015, 151, 150− 159. (27) MacArthur, K. E.; Brown, H. G.; Findlay, S. D.; Allen, L. J. Probing the Effect of Electron Channelling on Atomic Resolution Energy Dispersive X-Ray Quantification. Ultramicroscopy 2017, 182, 264−275. (28) Chen, Z.; Taplin, D. J.; Weyland, M.; Allen, L. J.; Findlay, S. D. Composition Measurement in Substitutionally Disordered Materials by Atomic Resolution Energy Dispersive X-Ray Spectroscopy in
preferences observed for the both samples are interpretable from the viewpoints of TM ionic size effect and ligand field effect, where the two effects are competitive around a crossover point at a certain temperature between 800 and 1100 °C. We have also elucidated that cobalt enrichment at the Td site contributes to the enhanced OER activity, and hence, the best annealing temperature for more OER active Ca2FeCoO5 should be higher than the crossover temperature.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01356. SEM images of samples and details of DFT calculations (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kei Nakayama: 0000-0002-8403-0551 Ryo Ishikawa: 0000-0001-5801-0971 Shunsuke Kobayashi: 0000-0002-4933-8817 Teruki Motohashi: 0000-0002-4568-5600 Naoya Shibata: 0000-0003-3548-5952 Yuichi Ikuhara: 0000-0003-3886-005X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Kenta Suzuki of Kanagawa University for his help in the sample synthesis and characterization. This work was supported by the Research and Development Initiative for Scientific Innovation of New Generation Batteries (RISING2) project from the New Energy and Industrial Technology Development Organization (NEDO), Japan.
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REFERENCES
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