Dynamic Octahedral Breathing in Oxygen-Deficient Ba0.9Co0.7Fe0

Mar 9, 2016 - XRD pattern of Ba0.9Co0.7Fe0.2Nb0.1O3-δ at room temperature. ..... suggests a dynamic breathing effect of the octahedra, driven by the ...
0 downloads 0 Views 8MB Size
Download PDF
Article pubs.acs.org/IC

Dynamic Octahedral Breathing in Oxygen-Deficient Ba0.9Co0.7Fe0.2Nb0.1O3‑δ Perovskite Performing as a Cathode in Intermediate-Temperature SOFC Yudong Gong,† Chunwen Sun,*,†,‡ Qiu-an Huang,§ Jose Antonio Alonso,*,∥ Maria Teresa Fernández-Díaz,⊥ and Liquan Chen† †

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ‡ Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, National Center for Nanoscience and Technology (NCNST), Beijing 100083, China § Physics and Electronic Technology, Hubei University, Wuhan, Hubei 430062, P. R. China ∥ Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain ⊥ Institut Laue Langevin, BP 156X, Grenoble Cedex, France ABSTRACT: Ba0.9Co0.7Fe0.2Nb0.1O3‑δ outperforms as a cathode in solid-oxide fuel cells (SOFC), at temperatures as low as 700−750 °C. The microscopical reason for this performance was investigated by temperature-dependent neutron powder diffraction (NPD) experiments. In the temperature range of 25−800 °C, Ba0.9Co0.7Fe0.2Nb0.1O3‑δ shows a perfectly cubic structure (a = a0), with a significant oxygen deficiency in a single oxygen site, that substantially increases at the working temperatures of a SOFC. The anisotropic thermal motion of oxygen atoms considerably rises with T, reaching Beq ≈ 5 Å2 at 800 °C, with prolate cigar-shaped, anisotropic vibration ellipsoids that suggest a dynamic breathing of the octahedra as oxygen ions diffuse across the structure by a vacancies mechanism, thus implying a significant ionic mobility that could be described as a molten oxygen sublattice. The test cell with a La0.8Sr0.2Ga0.83Mg0.17O3‑δ electrolyte (∼300 μm in thickness)-supported configuration yields a peak power density of 0.20 and 0.40 W cm−2 at temperatures of 700 and 750 °C, respectively, with pure H2 as fuel and ambient air as oxidant. The electrochemical impedance spectra (EIS) evolution with time of the symmetric cathode fuel cell measured at 750 °C shows that the Ba0.9Co0.7Fe0.2Nb0.1O3‑δ cathode possesses a superior ORR catalytic activity and long-term stability. The mixed electronic− ionic conduction properties of Ba0.9Co0.7Fe0.2Nb0.1O3‑δ account for its good performance as an oxygen-reduction catalyst.

1. INTRODUCTION Solid-oxide fuel cells (SOFCs) are the most attractive among all kinds of fuel cells due to the fuel flexibility and high energy conversion efficiency.1−3 In recent years, great efforts have been devoted to lowering the operating temperature to the intermediate 600−800 °C range or even lower. However, reducing the operating temperature decreases the electrode kinetics and results in large interfacial polarization resistances as well. This effect is most pronounced for the oxygen reduction reaction (ORR) at the cathode.3 Therefore, mixed ionic− electronic conductors (MIECs) operating at intermediate temperature with excellent performance are highly desired for designing high-performance SOFCs. Most MIECs are perovskite-type (ABO3) compounds with A = Ln (lanthanides), Sr, and Ba and B = Mn, Fe, Ni, and Co.3−5 Due to their mixed conductivity properties, cobalt oxides have drawn much attention recently.6 In terms of the high temperature and long-time stability, those BaCoO3‑δ-based MIECs with oxygen © 2016 American Chemical Society

deficiency, doped by Fe, Nb, or other high-valence metal ions, show better performance. For example, the Co valence can be tuned between Co3+ and Co4+ by double doping of Fe and Nb with the stoichiometric ratio7:2:1 for Co, Fe, and Nb.7,8BaCo0.7Fe0.2Nb0.1O3‑δ has been evaluated as an oxygen transport membrane material9 and a cathode material for ITSOFC with Ce0.9Gd0.1O1.95 (GDC) electrolyte.10 In this work, we have prepared perovskite Ba0.9Co0.7Fe0.2Nb0.1O3‑δ oxide by a solid-state reaction. The crystal structure of Ba0.9Co0.7Fe0.2Nb0.1O3‑δ and its temperaturedependent structural evolution were investigated by X-ray diffraction (XRD) and neutron powder diffraction (NPD) experiments. Its electrochemical performance as a cathode for SOFCs was examined in single SOFC cells and cathodic symmetrical cells. The relationship between the structure and Received: December 30, 2015 Published: March 9, 2016 3091

DOI: 10.1021/acs.inorgchem.5b03002 Inorg. Chem. 2016, 55, 3091−3097

Article

Inorganic Chemistry

reduced in situ from NiO-SDC to Ni-SDC cermet anodes, and then the cells were tested with humidified (3 wt % H2O) hydrogen as fuel in the anode side and ambient air as oxidant in the cathode side, using a SOLARTRON electrochemical workstation (1287A interface) in the temperature range of 650−700 °C.

properties of Ba0.9Co0.7Fe0.2Nb0.1O3‑δ as a cathode for SOFCs has been correlated, and the microscopical reason for the observed good performance has been unveiled.

2. EXPERIMENTAL SECTION

3. RESULTS AND DISCUSSION 3.1. Phase Identification. Figure 1 shows the XRD pattern of A-site deficient Ba0.9Co0.7Fe0.2Nb0.1O3‑δ. All the peaks can be

2.1. Preparation of Materials. Ba0.9Co0.7Fe0.2Nb0.1O3‑δ powder was prepared by a conventional solid-state reaction method. The stoichiometric amounts of BaCO3, Co3O4, Fe2O3, and Nb2O5 (Sinopharm Chemical Reagents Co. Ltd.) were mixed for 1 h with an agate mortar and then ball-milled for 12 h with ethyl alcohol as a medium. After that, the mixed slurry was dried in a vacuum oven for 6 h at 60 °C to get the precursors before they were transferred to a furnace and calcined at 1000 °C for 12 h in air with both heating and cooling rate of 3 °C/min. Then, another 12 h ball-milling procedure was carried out to get well-dispersed particles. The cathode slurry was prepared by mixing Ba0.9Co0.7Fe0.2Nb0.1O3‑δ powder and corn starch in a weight ratio of 80:20 with the addition of 6 wt % ethyl cellulose and α-terpinol solution for 30 min. 2.2. Structural Characterizations. Phase identification of the product was carried out by X-ray diffraction (XRD) in a Bruker-AXS D8 diffractometer with Cu Kα radiation in the 2θ range of 20−80°. Neutron powder diffraction (NPD) data were collected in the D2B diffractometer at the Institut Laue-Langevin, Grenoble with λ = 1.594 Å. For the RT acquisition, about 3 g of sample was contained in a vanadium can. For the high-temperature acquisitions, a quartz sample holder, open to the air atmosphere, was placed in the isothermal zone of a furnace with a vanadium resistor operating under vacuum (PO2 ≈ 10−6 Torr). The measurements were carried out in air at 200, 400, 600, and 800 °C. The collection time was 2 h per pattern. The diffraction data were all analyzed by the Rietveld method with the FULLPROF program18and the use of its internal tables for scattering lengths. The line shape of the diffraction peaks was generated by a pseudo-Voigt function. The irregular background coming from the furnace was extrapolated from points devoid of reflections. In the final run, the following parameters were refined: background points, zero shift, halfwidth, pseudo-Voigt, and asymmetry parameters for the peak shape, scale factor, and unit-cell parameters. Isotropic thermal factors for all the metal atoms and anisotropic for oxygen atoms were also refined for the NPD data. The coherent scattering lengths for Ba, Co, Fe, Nb, and O were 5.070, 2.49, 9.45, 7.054, and 5.803 fm, respectively. 2.3. Fabrication and Testing of Cells. Lanthanum strontium gallium magnesium oxide (LSGM) electrolyte-supported cells were fabricated to test the electrochemical performance and evaluate the stability of the cathode. La0.9Sr0.1Ga0.8Mg0.2O3‑δ (LSGM, 99.9%) powder was purchased from PRAXAIR Inc. and used without further treatment. Dense LSGM pellets were obtained after sintering LSGM disks at 1450 °C for 10 h. The obtained pellets were then polished with a diamond wheel to about 300 μm in thickness. For symmetric cells, the as-synthesized samples were first thoroughly mixed with Sm0.2Ce0.8O1.9, ethyl cellulose and α-terpinol to form composite cathode pastes. The cathode pastes were then screen printed onto both sides of the dense LSGM electrolyte and then calcined at 1050 °C for 3 h. For single cells, the anode slurry was first prepared by mixing NiO, Sm0.2Ce0.8O2−δ (SDC), and corn starch in a weight ratio of 65:35:20 with the addition of 6 wt % ethyl cellulose and α-terpinol solution. In order to avoid the reactions between Ni and LSGM electrolyte, a thin layer of Ce0.8La0.2O2‑δ (LDC) barrier layer was used as a buffer layer between the electrode and the electrolyte to prevent cation migration. The LDC layer was calcined at 1300 °C for 1 h. The anode slurry was then screen printed onto the LDC layer and followed by drying and calcining at 1250 °C for 2 h. The cathodes were fabricated using the same procedures as those described for the symmetric cells. The current−voltage (I−V) curves and electrochemical impedance spectroscopy (EIS) measurements were carried out with a SOLARTRON electrochemical workstation (1260A FRA). A symmetrical cell BCFN|LSGM|BCFN was used for EIS measurements. The stability of the cathodes in symmetric cells was monitored at 750 °C in air with different time intervals. For single cells, the green anodes were first

Figure 1. XRD pattern of Ba0.9Co0.7Fe0.2Nb0.1O3‑δ at room temperature.

indexed in a perovskite phase BaCoO2.23 (JCPDS No. 750227), and no obvious impurities phases were detected. It was reported that the lattice shrinkage was caused by the A-site deficiency, and a pure phase Ba1−xCo0.7Fe0.2Nb0.1O3‑δ could be obtained only with a Ba-deficiency lower than 10 mol %.14 Furthermore, doping with high valence cations at B-sites not only decreases oxygen vacancies but also leads to a lattice expansion arising from the larger radii of Fe3+ (0.645 Å) and Nb5+ (0.64 Å) compared with that of Co3+ (0.61 Å). 3.2. Neutron Diffraction Characterization for Ba0.9Co0.7Fe0.2Nb0.1O3‑δ. A NPD study at RT (300 K) was essential to investigate the true symmetry of this oxide and microscopically determine the oxygen contents. The neutron pattern confirms the cubic symmetry with a = 4.058 48(8) Å; the structure was therefore defined in the cubic Pm-3m space group with Ba located at 1b Wyckoff site (1/2,1/2, 1/2); Co, Fe, and Nb distributed at random at 1a (0,0,0) site; and O1 at 3d (1/2,0,0). The Ba and O1 occupancies were refined, yielding a crystallographic formula at RT Ba0.950(8)Co0.7Fe0.2Nb0.1O2.43(2), showing a significant oxygen deficiency. The anisotropic displacement factors of O1 were also refined. The Ba deficiency is similar to that expected, and the Co, Fe, and Nb composition was fixed to the nominal one, since this triple occupancy of a single site cannot be determined from a single diffraction experiment. The final structural parameters and agreement factors are gathered in Table 1 for this perovskite oxide at RT. 3.3.Temperature-Dependent Structural Evolution for Ba0.9Co0.7Fe0.2Nb0.1O3‑δ. Single fuel cell studies, described below, demonstrate that Ba0.9Co0.7Fe0.2Nb0.1O3‑δ is an extraordinarily performing cathode for SOFC. A temperaturedependent NPD study appeared to be crucial to understand the crystal structure evolution of this material at the working temperatures of the IT-SOFC. NPD diagrams collected at 200, 400, 600, and 800 °C with λ = 1.594 Å showed the same crystal 3092

DOI: 10.1021/acs.inorgchem.5b03002 Inorg. Chem. 2016, 55, 3091−3097

Article

Inorganic Chemistry

Table 1. Unit-Cell, Positional, and Displacement Parameters for Ba0.9Co0.7Fe0.2Nb0.1O3‑δ in the Cubic Pm-3m (No. 221) Space Group, Z = 1, from NPD Data at 25, 200, 400, 600, and 800 °C, Collected with λ = 1.594 Å T (°C) a (Å) V ( Å3) Ba 1b (1/2,1/2, 1/2) B (Å2) focc Ba Co/Fe/Nb 1a (0,0,0) focc Co/Fe/Nb B (Å2) O1 3d (1/2,0,0) Beq (Å2)a focc β11 β33 reliability factors Rp (%) Rwp (%) Rexp (%) χ2 RBragg (%) a

25 4.0585(1) 66.848(2)

200 4.0757(1) 67.705(2)

400 4.0911(1) 68.471(2)

600 4.1085(1) 69.350(2)

800 4.1265(1) 70.266(3)

1.24(6) 0.95(1)

1.99(7) 0.95

2.17(7) 0.95

2.59(9) 0.95

3.02(9) 0.95

0.7/0.2/0.1 1.80(6)

2.22(8)

0.7/0.2/0.1 2.61(8)

0.7/0.2/0.1 3.47(9)

0.7/0.2/0.1 4.60(12)

1.495 0.770(6) 0.0150(8) 0.0380(16)

2.38 0.800(9) 0.030(1) 0.047(2)

3.022 0.780(8) 0.039(1) 0.057(2)

3.81 0.759(8) 0.049(1) 0.070(3)

4.45 0.721(8) 0.060(2) 0.076(3)

2.85 3.88 1.62 5.76 2.79

3.13 4.41 1.64 7.27 2.77

2.72 3.93 1.68 5.45 2.58

2.56 3.63 1.69 4.62 2.73

2.47 3.30 1.93 2.91 3.92

Anisotropic thermal factors β11 = β22 ≠ β33, β12 = β13 = β23 = 0.

symmetry but dramatic changes in the oxygen occupancy and magnitude and anisotropy of the displacement parameters. All the peaks could be indexed in the above-mentioned cubic Pm3m unit cell which is, thus, demonstrated to be stable at the working conditions of the cell. The access to a wide region of the reciprocal space allowed us the successful refinement of the anisotropic displacement factors for oxygen atoms, minimizing the correlation with the occupancy factor for these positions. At 25 °C a reliable oxygen occupancy of 2.43(2) was found, which gradually decreases upon heating, attaining the composition Ba0.9Co0.7Fe0.2Nb0.1O2.16(3) at 800 °C. The good agreement between observed and calculated profiles is displayed in Figure 2a and Figure 2b for the 200 and 800 °C patterns, respectively. Figure 3 illustrates the evolution of the cubic crystal structure displaying a remarkable anisotropy in the cigar-shaped (prolate) displacement ellipsoids. Table 1 contains the main structural parameters of the cubic structures at 200, 400, 600, and 800 °C. Figure 3a displays the thermal variation of the unit-cell parameter; Figures 3b and 3c show the evolution of the stoichiometry of the oxygen sublattice and Beq thermal displacement across the measured temperature range. For the cations (Ba, Co, Fe, and Nb) the thermal displacement parameters are constrained to be spherical. For O1 the anisotropy of the thermal ellipsoids is patent, with the largest thermal motions along the (Co, Fe, Nb)−O bonds. The magnitude of the thermal motions is monotonically enhanced with temperature, as shown in Table 1 and Figure 3c. In all the temperature regime, the O1 prolate ellipsoids are orientated along the [001] directions (Figure 4). At 800 °C the rootmean-square (rms) displacements of O1 are 0.22 Å perpendicular to the Co−Co distance and 0.26 Å parallel to it. As will be discussed below, this suggests a breathing of the (Co, Fe, Nb)O6 octahedra, which happens upon the migration of the oxygen vacancies across the solid, involving a dynamical variation of the valence of the Co and Fe ions, thus shortening and stretching the M−O distances along these directions. This anticipates a large oxide anion mobility in what could be described as a molten oxygen sublattice. Given the cubic

Figure 2. Observed (crosses), calculated (line), and difference (bottom) neutron-diffraction patterns of Ba0.90Co0.7Fe0.2Nb0.1O3‑δ at (a) 200 °C and (b) 800 °C. 3093

DOI: 10.1021/acs.inorgchem.5b03002 Inorg. Chem. 2016, 55, 3091−3097

Article

Inorganic Chemistry

Figure 4. Crystal structures of Ba0.9Co0.7Fe0.2Nb0.1O3‑δ showing the prolate orientation of the O1 ellipsoids along the [001] directions at (a) 25 °C and (b) 800 °C.

high-frequency resistance is mainly related to charge-transfer processes and the low-frequency resistance is mainly related to diffusion processes, including the diffusion of oxygen as well as the intermediate oxygen species.13,14As shown in Figure 5a, the arc does not show any distinct difference at the high-frequency range, while they increase at medium and low frequency, which can be ascribed to the agglomeration of electrode particles with testing time at high temperature. The increasing of polarization resistance values exhibits two stages as shown in Figure 5c. It is obvious that the resistance increases rapidly in the first 30 h (fitted by the blue line) with a rate of ∼0.006 Ω cm2 h−1 and becomes linear after 50 h (fitted by the red line) with an increasing rate of ∼0.0007 Ω cm2 h−1. Figure 5d shows the evolution of the polarization resistance with the change of oxygen partial pressure (PO2). It can be seen that the Rp decreases with PO2 increasing, especially at the low-frequency range. It is similar to that of dense (La,Sr)MnO3‑δ films reported by Plonczak et al. in the PO2 range of 0.21−1.00 atm.15 3.5. Microstructure Evolution of the Electrode during the Durability Testing. The cross-sectional SEM images of the BCFN electrode show the microstructure evolution of the electrode before and after EIS testing, as shown in Figure 6. It is observed that the BCFN particles before EIS testing at 750 °C are uniform. The diameter of the particles is in the range of 100−300 nm. However, the particles tend to aggregate after the long-term durability testing. 3.6. Cell Performance. The electrochemical performance of Ba0.9Co0.7Fe0.2Nb0.1O3‑δ as a cathode material was evaluated in test cells Ni-SDC|LDC|LSGM|BCFN. Figure 7 shows the cell voltages and power densities, as a function of current density tested in 3 wt % H2O humidified H2 as fuel and ambient air as oxidant at the intermediate temperature range of 650−750 °C. The peak power density reached about 100, 200, and 400 mW cm−2 at 650, 700, and 750 °C, respectively.

Figure 3. Thermal variation of the unit-cell parameter (a), stoichiometry of the oxygen sublattice (b), and thermal displacement (Beq) of O1 (c).

symmetry, isotropic oxide diffusion is expected, favorable for engineering of the material in devices. 3.4. EIS of Symmetrical Cells. An effective measure of the catalytic activity of the fuel-cell cathode for oxygen reduction reactions is the area specific polarization resistance (Rp), which can be obtained from the electrochemical impedance spectroscopy (EIS) measurements on symmetric cathode fuel cells.11,12 The time-dependent cathodic polarization impedance spectra of Ba0.9Co0.7Fe0.2Nb0.1O3‑δ symmetrical cells are shown in Figure 5. It can be observed that the polarization resistance of the symmetrical cell is 0.028 Ω cm2 at the initial testing and increases to 0.067 Ω cm2 after 350 h test. The peak frequency of the arc tends to lower from 126 to 63.1 Hz from 0 to 350 h, respectively. The impedance spectroscopy contains several depressed arcs, which are not well separated usually. As shown in Figure 5b, the EIS data of Rp can be fitted well to equivalent circuit (RHC1)(RMC2)(RLC3), suggesting that there are at least three different processes of ORR, wherein Rx and CPEx (x = 1, 2, 3) are the resistance and capacitance at high, medium, and low frequency, respectively. It is generally accepted that the 3094

DOI: 10.1021/acs.inorgchem.5b03002 Inorg. Chem. 2016, 55, 3091−3097

Article

Inorganic Chemistry

superstructure reflections (Figure 2a) justify the departure of this symmetry to describe the crystal structure. It should be noted that the parent compound BaCoO3 exhibits a hexagonal 2H polytype where CoO6 octahedra share faces in columns along the crystallographic c direction.16This hexagonal polytype is useless as cathode given its poor electronic conductivity, arising from the face-sharing arrangement of the CoO6 octahedra. A similar topology can be found for BaFeO3, also crystallizing in a hexagonal polytype described in the P63/mmc space group, also exhibiting meager electronic conduction.17 At this point it is worth highlighting the role of Nb5+ ions into the stability of the wanted cubic phase: the presence of a highly charged cations destabilizes the columns of face-sharing octahedra in the hexagonal polytypes, thus leading to a 3Clike structure where the (Co, Fe, Nb)O6 octahedra are linked by the vertices, driving an excellent orbital overlap between Co and Fe 3d and O 2p orbitals and permitting the required conductivity for performing as cathode materials in SOFC. This cubic crystal structure of Ba0.9Co0.7Fe0.2Nb0.1O3‑δ is maintained in the temperature range between 25 and 800 °C. This perovskite is significantly oxygen deficient; the oxygen vacancies are fully disordered, and they noticeably increase in number with temperature, which is desirable for the mixed ionic and electronic conduction present in this MIEC material. The oxygen stoichiometry spans from 2.43(2) at 25 °C to 2.16(3) at 800 °C; assuming that Nb remains pentavalent and Ba is divalent, the average (Co, Fe) oxidation state varies accordingly between 2.84+ and 2.02+. The observed performance as a MIEC oxide of Ba0.9Co0.7Fe0.2Nb0.1O3‑δ is primarily linked to the successful stabilization of a 3C-type superstructure of perovskite just by substituting of 10% Co, Fe for Nb in the parent Ba(Co,Fe)O3‑δ polytype. Our present EIS data show excellent polarization resistances indicating that it is also a good catalyst for the oxygen reduction reaction. The random oxygen vacancy arrangement exhibited by this compound seems to be one of the reasons for this catalytic performance, perhaps because it favors the interaction with adsorbed O2 molecules prior to the reduction process. It is also worth mentioning that very similar systems such as SrCo1−xMxO3‑δ (M = Sb, Mo)18−20 exhibit at moderate temperatures (≈600−700 K) a phase transition from a tetragonal superstructure to the cubic aristotype, and it is this cubic phase that performs well as oxygen reduction catalysts. We conclude that the stabilization, via Nb doping, of a cubic perovskite structure, very disordered either at B sites and at the O sublattice, stable at the working temperatures of IT-SOFC, is responsible for the superior performance of Ba0.9Co0.7Fe0.2Nb0.1O3‑δ over related cubic phases, the strong delocalization and diffusion of the oxygen vacancies enhancing the catalytic process of oxygen reduction in an IT-SOFC. It is noteworthy the orientation of the anisotropic thermal displacements, along the (Co, Fe, Nb)−O directions, i.e., along the bonding direction within the octahedra. In cubic Pm-3m perovskites (aristotype), where BO6 octahedral tilting is not realized, there are only three normal modes Q1, Q2, Q3, according to the notation defined by Kanamori.21 Under the influence of mode Q1, the octahedral shape is kept regular; it only expands or shrinks as a whole, mimicking the movement of breathing. Q1 mode is thus sometimes called the breathing mode. Q2 and Q3 modes distort the octahedra, resulting in short and long B−O bonds; these modes are involved in Jahn− Teller-like distortions, as observed in Mn perovskites. This is not our case, where the octahedral breathing is related to the

Figure 5. (a) Comparison of impedance spectra of BCFN|LSGM| BCFN symmetrical cells tested at 750 °C and different time intervals, (b) experimental and fitted data of EIS for 0 and 350 h, (c) fitted line of polarization resistance at first 20 h and the following 330 h, and (d) impedance spectra tested at different PO2.

4. DISCUSSION Our present NPD data clearly show that Ba0.9Co0.7Fe0.2Nb0.1O3‑δ is cubic and the structure can be defined in the space group Pm-3m (perovskite aristotype). No 3095

DOI: 10.1021/acs.inorgchem.5b03002 Inorg. Chem. 2016, 55, 3091−3097

Article

Inorganic Chemistry

Figure 6. Cross-sectional SEM images of the BCFN electrode before (a) and after (b) long-term durability testing.

chemical bonds, suggests a dynamic breathing effect of the octahedra, driven by the motion of the oxygen vacancies (and the opposite diffusion of oxide ions) across the structure. This is concomitant to dynamic valence changes and octahedral size variations as the coordination of Co and Fe cations evolves when oxygen bonds are broken and restored upon diffusion. All these results suggest that Ba0.9Co0.7Fe0.2Nb0.1O3‑δ is a promising cathode for IT-SOFCs.

■

AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-10-82854648. Fax: +86-10-82854648. E-mail: [email protected]. *Tel.: +34-91-3349071. Fax: +34-91-3720623. E-mail: ja. [email protected].

Figure 7. Cell voltage (left) and power density (right) as a function of the current density.

Notes

dynamic change of the oxidation state of the Co and Fe ions as O diffuses across the solid. This observed arrangement of the thermal ellipsoids is not the one usually described in perovskites, where normally the thermal vibrations are permitted in perpendicular directions to the chemical bonds. A paradigmatic example is the cubic perovskite Ba0.87K0.13BiO3,22 whereas the thermal vibration parameters for the cations are constrained to be spherical (as observed in the present case); the oblate vibration ellipsoid for oxygen yields r.m.s. displacements of 0.258(5) Å perpendicular to the Bi−Bi distance and 0.12(1) Å parallel to it. A prolate ellipsoid for the oxygen anisotropic thermal displacement parameters would have indicated a softening of “breathing” modes at the bismuth atoms. Instead, an oblate spheroid locks the oxygen atoms at mid-bismuth positions with enhanced vibrations perpendicular to the Bi−Bi distance in Ba0.87K0.13BiO3. The present Ba0.9Co0.7Fe0.2Nb0.1O3‑δ cubic perovskite behaves in the opposite way: the prolate thermal ellipsoids oriented along the chemical bonds are indicative of a dynamic breathing effect of the octahedra, clearly correlated with a huge oxygen diffusivity in this material and with its outstanding performance as cathode in intermediate-temperature SOFC.

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (No. 51172275 and No. 51372271) and the National Key Basic Research Program of China (No. 2012CB215402). J.A.A. grateful acknowledges the Spanish Ministry of Economy and Competitivity for granting the project MAT2013-41099-R and ILL for making all facilities available for the neutron diffraction experiments.

■

REFERENCES

(1) Atkinson, A.; Barnett, S.; Gorte, R. J.; Irvine, J. T. S.; McEvoy, A. J.; Mogensen, M.; Singhal, S. C.; Vohs, J. Nat. Mater. 2004, 3, 17−27. (2) Sun, C. W.; Stimming, U. J. Power Sources 2007, 171, 247−260. (3) Sun, C. W.; Hui, R.; Roller, J. J. Solid State Electrochem. 2010, 14, 1125−1144. (4) Skinner, S. J. Int. J. Inorg. Mater. 2001, 3, 113−121. (5) Richter, J.; Holtappels, P.; Graule, T.; Nakamura, T.; Gauckler, L. J. Monatsh. Chem. 2009, 140, 985−999. (6) Raveau, B.; Seikh, M. Cobalt oxide: From crystal chemistry to physics; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2012; 148−175. (7) David, R.; Kabbour, H.; Bordet, P.; Pelloquin, D.; Leynaud, O.; Trentesaux, M.; Mentre, O. J. Mater. Chem. C 2014, 2, 9457−9466. (8) Ishihara, T.; Yamada, T.; Arikawa, H.; Nishiguchi, H.; Takita, Y. Solid State Ionics 2000, 135, 631−636. (9) Cheng, Y. F.; Zhao, H. L.; Teng, D. Q.; Li, F. L.; Lu, X.; Ding, W J. Membr. Sci. 2008, 322, 484−490. (10) Lu, S.; Ji, Y.; Meng, X.; Long, G.; Wei, T.; Zhang, Y.; Lv, T. Electrochem. Solid-State Lett. 2009, 12, B103−B105. (11) Yang, W.; Hong, T.; Li, S.; Ma, Z.; Sun, C.; Xia, C. R.; Chen, L. Q. ACS Appl. Mater. Interfaces 2013, 5, 1143−1148.

5. CONCLUSIONS Perovskite-type Ba0.9Co0.7Fe0.2Nb0.1O3‑δ oxide exhibits good long-term stability for IT-SOFCs operating at 750 °C. The peak power density reached about 100, 200, and 400 mW cm−2 at 650, 700, and 750 °C, respectively. After 350 h test, Ba0.9Co0.7Fe0.2Nb0.1O3‑δ keeps ORR activity with a slight increase in polarization of about ∼0.04 Ω cm2. This behavior is correlated with the structural features unveiled from a temperature-dependent NPD study: the orientation of the prolate thermal ellipsoids, directed along the (Co−Fe−Nb)−O 3096

DOI: 10.1021/acs.inorgchem.5b03002 Inorg. Chem. 2016, 55, 3091−3097

Article

Inorganic Chemistry (12) Yang, W.; Zhang, H. R.; Sun, C. W.; Liu, L. L.; Alonso, J. A.; Fernández-Díaz, M. T.; Chen, L. Q. Inorg. Chem. 2015, 54, 3477− 3484. (13) Zhou, W.; Ran, R.; Shao, Z. P.; Jin, W.; Xu, N. J. Power Sources 2008, 182, 24−31. (14) Jørgensen, M. J.; Mogensen, M. J. Electrochem. Soc. 2001, 148, A433−A442. (15) Plonczak, P.; Sørensen, D. R.; Søgaard, M.; Esposito, V.; Hendriksen, P. V. Solid State Ionics 2012, 217, 54−61. (16) Calle, C. de la; Alonso, J. A.; Fernandez-Diaz, M. T. Z. Naturforsch., B: Chem. Sci. 2008, 63, 647−654. (17) Ashima; Sanghi, S.; Agarwal, A.; Reetu. J. Alloys Compd. 2012, 513, 436−444. (18) Aguadero, A.; Calle, C. de la; Alonso, J. A.; Escudero, M. J.; Fernandez-Díaz, M. T.; Daza, L. Chem. Mater. 2007, 19, 6437−6444. (19) Aguadero, A.; Perez-Coll, D.; Alonso, J. A.; Skinner, S. J.; Kilner, J. Chem. Mater. 2012, 24, 2655−2663. (20) Aguadero, A.; Alonso, J. A.; Pérez-Coll, D.; De la Calle, C.; Fernández-Díaz, M. T.; Goodenough, J. B. Chem. Mater. 2010, 22, 789−798. (21) Kanamori, J. J. Appl. Phys. 1960, 31, S14−S23. (22) Wignacourt, J. P.; Swinnea, J. S.; Steinfink, H.; Goodenough, J. B. Appl. Phys. Lett. 1988, 53, 1753−1755.

3097

DOI: 10.1021/acs.inorgchem.5b03002 Inorg. Chem. 2016, 55, 3091−3097