Bulk Properties of the Oxygen Reduction Catalyst SrCo0.9Nb0.1O3âÎ´

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Bulk Properties of the Oxygen Reduction Catalyst SrCo Nb O

Robert E. Usiskin, Timothy C Davenport, Richard Y. Wang, Webster Guan, and Sossina M Haile Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04783 • Publication Date (Web): 01 Apr 2016 Downloaded from http://pubs.acs.org on April 2, 2016

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Chemistry of Materials

Bulk Properties of the Oxygen Reduction Catalyst SrCo0.9Nb0.1O3-δ Robert E. Usiskin1, Timothy C. Davenport1, Richard Y. Wang1, Webster Guan2, Sossina M. Haile1, 2*

1. Materials Science, California Institute of Technology, Pasadena, CA, United States. 2. Chemical Engineering, California Institute of Technology, Pasadena, CA, United States. *Corresponding author. Current address: Materials Science and Engineering, Northwestern University, Evanston, IL, United States. Email: [email protected]

Abstract The perovskite SrCo0.9Nb0.1O3-δ (SCN) has excellent electrochemical activity towards oxygen reduction, and it is also valuable as a possible model material for other state-of-theart perovskite catalysts based on strontium and cobalt, such as Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF). Here we report thermogravimetric, conductivity, and diffraction measurements from SCN. We find that the thermodynamic stability limits of SCN are slightly more favorable than those reported for BSCF, although both materials exhibit a slow oxidative partial decomposition under likely operating conditions. In SCN this decomposition is thermodynamically preferred when the average formal oxidation state of cobalt is greater than ~3.0+, but due to sluggish kinetics, metastable SCN with higher cobalt valence can be observed. The oxygen stoichiometry 3-δ varies from 2.45 to 2.70 under the conditions studied, 500 – 1000 °C and 10-4 – 1 bar O2, which encompass both stable and metastable behavior. The electronic conductivity is p-type and thermally activated, with a value at 600 °C in air of 250 S cm-1, comparable to that of La0.8Sr0.2MnO3-δ. The polaron migration enthalpy decreases linearly from 0.30 to 0.05 eV as 3-δ increases from 2.52 to 2.64. Thermal and chemical expansivities are also reported.

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Introduction The last several years have seen an explosion in activity aimed at developing high

activity air electrodes for solid oxide electrochemical cells that operate at 500 – 800 °C.1, 2 Such devices efficiently convert between chemical and electrical energy and could play a major role in a sustainable energy future. Mixed ionic and electronic conductors (MIECs) are desirable in such electrodes because they provide transport pathways such that electrochemical reactions are supported over the entire MIEC surface.3 By contrast, in composite electrodes formed of distinct ion- and electron-conducting phases, electrochemical reactions are limited to sites near the three-phase-boundary between the ion conductor, the electron conductor, and the gas phase.3 A particularly intriguing class of MIECs to have emerged in recent years encompasses derivatives of SrCoOx in which dopants are introduced on the Co site so as to stabilize the desirable cubic perovskite phase.4 In some cases, these materials display activity for oxygen electroreduction comparable to that of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF 5582), a benchmark material.5,

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Undoped SrCoOx is thermodynamically unstable below

~900 °C at ambient pressure,7-9 decomposing into Sr6Co5O15 and Co3O4.10 Under conditions where this decomposition is avoided, SrCoOx adopts either a cubic perovskite or an orthorhombic brownmillerite structure, where the latter is a derivative of the former resulting from ordering of the oxygen vacancies. The perovskite form displays several properties that are desirable in the air electrode of a solid oxide electrochemical cell, including high electronic conductivity,11 high oxygen ion diffusivity,7, 12, 13 and high activity towards both oxygen reduction12 and evolution.14 The properties of the brownmillerite are known to a lesser degree of certainty because of the convolution with decomposition effects, but given the ordered nature of the oxygen vacancies, it is believed to have a lower oxygen ion diffusivity.7 These observations have motivated ongoing efforts to stabilize the perovskite phase by chemical modification and thereby achieve favorable transport properties at lower temperatures. The stability of a perovskite with formula ABO3 can often be estimated from the Goldschmidt tolerance factor, a measure of the size compatibility between the two cations with respect to the perovskite structure.4 For the hypothetical stoichiometric perovskite 2


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SrCoO3, the tolerance factor =

√( )

(where the ionic radii are rO = 1.40 Å, rCo = 0.53 Å,

and rSr = 1.44 Å for O2-, high spin Co4+, and Sr2+ with respective coordination numbers of 6, 6, and 12) is 1.04, substantially larger than the ideal value of 1.00. (Co4+ ions with low spin state would have a smaller ionic radius and make the tolerance factor even higher.) Such a high tolerance factor often leads to the formation of the hexagonal 2H phase (e.g., as adopted by BaTiO3).4 For SrCoO3 the instability produces a slightly different result as noted above: a disproportionation reaction to Co3O4 and Sr6Co5O15, where the latter has a structure similar to the hexagonal 2H type. This line of reasoning predicts that the tolerance factor in SrCoOx can be decreased (and thus perovskite stability enhanced) by increasing the average ionic radius of the B-site cation, which can be readily achieved by replacing some of the cobalt with a cation of larger radius. In addition, if the dopant has high valence, the Co can adopt the 3+ oxidation state (which is almost always thermodynamically more favorable than the 4+ oxidation state15) without requiring a large concentration of charge-compensating oxygen vacancies, a factor that can otherwise favor vacancy ordering. Such a strategy has indeed been successfully employed to create several apparently stable cubic perovskites with the formula SrCo1-yMyO3-δ, where M = Mo,16 Ta,17 Zr,18, 19 Sb,20 Ce,18 Sc,21 Nb,4, 19, 22, 23 and others,18, 19, 24 and y ~ 0.05 - 0.10, although the detailed electronic structure of the dopant may also play an important role in perovskite stabilization.18 Moreover, though not demonstrated, a potential advantage of B-site doped perovskites over A-site doped analogs is a decrease in the driving force for SrO segregation and exsolution, which has been implicated as a cause of fuel cell performance degradation in materials such as (La,Sr)CoO3 and (La,Sr)(Co,Fe)O3.2 Among the most interesting compositions in the SrCo1-yMyO3-δ family is SrCo0.9Nb0.1O3-δ (SCN). First synthesized in 2007,19 SCN exhibits extraordinary characteristics as a cathode material in solid oxide fuel cells and as an oxygen permeation membrane.22 Fuel cells using SCN have been reported with power densities as high as 1 W cm-2 at 600 °C, with an accompanying (symmetric cell) electrode polarization resistance at zero bias of just 0.09 Ω cm2.5 Furthermore, niobium in SCN has a fixed 5+ oxidation state (except under extremely reducing conductions, see Figure S1 and ref. 25), so SCN contains 3



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only one mixed-valent cation (cobalt), and consequently an analysis of its defect chemistry may be tractable, rendering SCN a possible model material for perovskites based on strontium and cobalt. Despite these favorable characteristics, several properties of SCN are poorly known. The thermodynamic stability is particularly relevant and unclear. A handful of studies suggest surprisingly good stability,6, 19, 22 but as has now been recognized from experience with BSCF, decomposition at moderate temperatures can be slow and easily overlooked.25 Such decomposition can pose a significant barrier to commercial application, and it can also hamper efforts to understand the fundamental materials chemistry. Thus, we examine here the stability limits of SCN. We also report several other properties, including redox thermodynamics (essential for understanding defect chemistry and transport behavior), electrical conductivity and polaron migration enthalpy (useful for understanding electronic structure and for predicting sheet resistance in future thin film studies), and thermochemical expansion behavior (helpful for predicting interfacial strain effects in both model systems and operating devices). Relative to a previous preliminary contribution,26 the current work includes several new results and substantially improved data collection procedures.

2

Experimental Procedures

2.1 Synthesis SrCo0.9Nb0.1O3-δ powder was synthesized by conventional solid state reaction. Stoichiometric quantities of SrCO3, Co3O4, and Nb2O5 (all from Alfa Aesar, >99.7% purity) were mixed, attritor-milled at 500 rpm for 30 min with 3 mm yttria-stabilized zirconia beads in acetone or isopropanol, removed from the milling media, dried at 110 °C, and calcined for 15 h in stagnant air at 1200 °C. This material was then attritor-milled again and annealed at 1200 °C for 10 h. The resulting powder is hereafter referred to as the “assynthesized” powder. Sintered compacts were prepared from powders that underwent an identical preparation, except without the additional 10 h anneal at 1200 °C. To these powders, 1.25 wt% polyvinylpyrrolidone dissolved in isopropanol was added to serve as a 4


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binder. The powders were placed in a 6 x 25 mm rectangular die and subjected to uniaxial pressure of 130 MPa for 10 min, followed by isostatic pressure under 350 MPa for 20 min. The resulting compact was sintered in a box furnace at 1100 °C for 10 h in stagnant air. A thin layer of excess powder was spread between the compact and the alumina support to avoid an interfacial reaction. Prior to further characterization, the compact surfaces were polished using successively smaller grit sizes down to a final grit size of 0.3 µm. While exploring synthesis conditions, the melting point of SCN was found to fall between 1300 and 1350 °C. Phase formation was evaluated by X-ray powder diffraction (Philips X'Pert Pro, Cu Kα). Electron microprobe analysis (JEOL JXA-8200) was employed for chemical analysis using SrTiO3, Co3O4, and Nb2O5 standards. Using a representative sample, six locations were evaluated and the results averaged. The relative cation amounts obtained from these averages were converted into a chemical formula by assuming no strontium vacancies or interstitials. The result was SrCo0.91±0.01Nb0.09±0.01O3-δ, within reasonable error of the nominal composition. Although X-ray powder diffraction analysis revealed small amounts of secondary phases as discussed below, no evidence for this was found in the electron microscopy studies. Image analysis (ImageJ) of micrographs collected using the same instrument indicated that the SCN compacts used in the conductivity measurements were approximately 30% porous. Accordingly, all measured bulk resistances of SCN were multiplied by a correction factor (1.43) to account for this porosity. 2.2 Gas control For all characterization experiments, premixed gases of O2 and either N2 or Ar (Air Liquide) were supplied to the relevant measurement chamber with a total (absolute) pressure of 1 bar and the desired oxygen partial pressure (pO2) in the range 10-4 – 1 bar. The pO2 values were verified using a zirconia-based oxygen sensor (Setnag). 2.3 X-ray powder diffraction X-ray powder diffraction (XRD, Philips X'Pert Pro, Cu Kα, equipped with a diffracted beam monochromator) was employed to confirm phase formation and also assess phase stability and thermal expansion behavior. Phase stability was explored by placing ~500 mg 5



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of SCN powder in an alumina boat inside a tube furnace (equipped with a quartz tube) and then holding the sample at 750 ˚C under stagnant air for 4 days, then at 750 ˚C for 6 more days, then at 1200 ˚C for 10 h. After each of these anneals, the sample was cooled by simply shutting the furnace off, and ex situ XRD patterns were then acquired. Data analysis was carried out by Rietveld refinement using the GSAS and EXPGUI software packages.27, 28 In a separate experiment, the expansion behavior was measured in situ using an Anton-Paar HTK 1200 chamber. Approximately 200 mg of SCN powder was supported in an alumina cup placed within the chamber, and diffraction data were collected upon equilibration in various environmental conditions over the temperature range 500 - 1000 °C and the oxygen partial pressure range 10-4 - 1 bar. The cubic lattice parameter a0 was then determined at each measurement condition. 2.4 Thermogravimetry Thermogravimetry (TG) measurements were performed on powder samples with a typical mass of 220 - 280 mg in a Netzsch STA 449C thermal analyzer using a platinum crucible. Based in part on the result of the stability behavior determined by XRD, two types of temperature sequences were used. At six oxygen partial pressures, pO2 = 8.0 × 10-1, 1.7 × 10-1, 3.5 × 10-2, 1.1 × 10-2, 5.8 × 10-3, and 1.4 × 10-3 bar, an “alternating sequence” was used, wherein the temperature was alternated between 1000 °C and each target temperature at a ramp rate of 20 °C/min and with a 1-2 h dwell at each condition. A similar measurement approach was previously employed to study BSCF.29 For three morereducing oxygen partial pressures, pO2 = 1.4 × 10-3, 4.1 × 10-4, and 1.7 × 10-4 bar, the kinetics of mass change were slow, and hence the alternating temperature approach was abandoned in favor of a "step sequence", wherein the temperature was simply increased from 500 to 1000 °C in 50 - 100 °C steps at 20 °C/min with a 1.5 – 4.0 h dwell at each step. The temperature was stepped up rather than down because oxygen excorporation was observed to be faster than incorporation at these partial pressures. Buoyancy- and driftcorrected mass changes were attributed to changes in oxygen content; thus the relative oxygen stoichiometry at all conditions was determined. Gravimetric data acquired during the day (but not at night) exhibit some random noise due to external electrical noise, 6


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mechanical vibrations, and/or pressure fluctuations. This random noise was assumed to be unbiased and contributed negligible variance to the averaged mass value at each test condition. In order to translate the relative oxygen stoichiometries implied by these measurements into absolute stoichiometries, mass changes were recorded upon holding a sample at a reference state of 1000 °C in 0.17 bar O2, and subsequently fully reducing it to SrO + Co + SrNbO3 by exposure to argon and then 7.5% H2, supplementary Figures S1-S3. The mass difference between the reference state and the reduction products provided the absolute oxygen stoichiometry of the reference state. This reduction measurement was performed twice to ensure reproducibility. 2.5 Conductivity A thin compact geometry was chosen for bulk electrical conductivity measurements to increase the resistance and thereby improve measurement accuracy. Specifically, a rectangular prism sample was polished to dimensions 23.5 × 5.4 × 0.25 mm and then cleaned by sonication in water to remove residual grit. Electrical contact was made to the distant ends of the sample using gold wires attached with silver paste (SPI 05063-AB). After the paste dried, the sample was installed in a quartz tube next to a zirconia-based oxygen sensor (Setnag) that also recorded the sample temperature. The sample resistance was measured at each temperature and oxygen partial pressure of interest using an impedance analyzer (Solartron 1260) at a fixed frequency of 100 Hz, a perturbation voltage of 10 - 40 mV, and no d.c. bias. (The complex component of the impedance was always zero at this frequency.) Data were collected at five distinct oxygen partial pressures (1.0, 0.2, 1.0 × 10-2, 1.0 × 10-3, and 1.0 × 10-4 bar) using a temperature profile in which the furnace temperature was initially held at 750 °C, then heated to 900 °C, then stepped down in 50 °C increments to 600 °C. Reproducibility and drift were checked by repeating the 750 °C measurement at the completion of the experiment. The ramp rate was 5 °C/min throughout, and the hold times at each condition ranged from 30 to 120 min, with longer times used for lower temperatures. The sample temperature matched the furnace temperature to within ~3 °C. To determine the resistance contribution of the experimental 7



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apparatus, a separate set of measurements was made in which the sample was replaced by a gold wire. The raw resistance values were corrected by subtracting the apparatus resistance and then multiplying by the porosity correction factor described above.

3

Results and discussion

3.1 Phase Formation and Stability Ex situ diffraction patterns of the as-synthesized and annealed SCN powders are shown in Figure 1, and the results of Rietveld analyses of these patterns are summarized in Table 1, with refinement statistics provided in supplementary Table S1. The as-synthesized material is predominantly cubic perovskite. It also contains a small amount of the brownmillerite phase (6.7 ± 0.2 wt%) and a very small amount of CoO (1.5 ± 0.1 wt%). Significantly, there is no evidence of a Sr6Co5O15-type hexagonal phase. Annealing at 750 °C for 4 d results in a phase assemblage with an increased amount of perovskite, no detectable brownmillerite, and small concentrations of “Sr6Co5O15” (1.3 ± 0.1 wt%) and Co3O4 (2.0 ± 0.1 wt%). (Designation of the hexagonal phase as “Sr6Co5O15” reflects the fact that the refinements were carried out with no attempt to determine the precise composition of this phase, which may or may not include a small concentration of Nb. The cell volume determined here, 977.4 ± 0.4 Å3, is slightly larger than that reported10 for Sr6Co5O15, 969.62 ± 0.06 Å3, suggesting slight Nb incorporation.) Additional annealing at 750 °C increases the concentration of the two minor phases and lowers that of the perovskite phase. A high temperature anneal reverts the phase assemblage to the as-synthesized condition. The refined lattice constants of the cobalt oxide phases are consistent with literature values, and the presence of CoO after annealing at 1200 °C and of Co3O4 after annealing at 750 °C is consistent with the literature consensus that the stability boundary between CoO and Co3O4 occurs at approximately 950 °C under ambient atmosphere.30 (Oxidation of CoO during the cooling from 1200 °C is negligible due to sluggish kinetics.) The weight fraction of Co3O4 observed after the 750 °C anneals (2.0 and 2.5 wt%) exceeds that which would be expected (1.6 wt%) simply from oxidation of the CoO observed in the as-synthesized sample. 8


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In situ diffraction measurements, described below in the context of the lattice expansion behavior, did not reveal the presence of the brownmillerite phase at temperatures between 500 and 1000 °C. However, the incompatibility of the XRD furnace with the diffracted beam monochromator and the inherently lower signal-to-noise ratio of x-ray experiments at higher temperatures implies that the presence of the brownmillerite phase at the single digit percentage level cannot be ruled out. *

* brownmillerite + "Sr6Co5O15"

*

×

5000

× Co3O4

+ °

* brownmillerite + "Sr6Co5O15"

°

+

+

2000

° CoO

4000 (d) 3000 (c) 2000 (b)

Intensity / a.u.

6000

Intensity / a.u.

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* ×

× Co3O4 ° CoO +

°

°

+

(d) (c) 1000 (b)

1000 (a) 0 25 30 35 40 45 50 55 60 65 70 75 80 2θ / º

0 25

(a) 30

35 2θ / º

40

45

Figure 1. Left: X-ray diffraction patterns acquired from a sample of SrCo0.9Nb0.1O3-δ powder: (a) as-synthesized (annealed at 1200 °C in air), (b) after annealing at 750 ˚C in air for 4 d, (c) after annealing at 750 °C in air for 10 d, and (d) after a final anneal at 1200 °C in air for 5 h. Right: expanded view of the same data, overlaid with calculated patterns obtained by Rietveld refinement.

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Table 1. Phase fraction weight percent determined by Rietveld refinement. Brownmillerite “Sr6Co5O15” Sr2(Co0.9Nb0.1)2O5

CoO

Co3O4

Sample

Perovskite

SCN as synthesized

91.79(4)

750 °C anneal for 4d

96.64(1)

1.3(1)

2.0(1)

750 °C anneal for 10 d

91.74(3)

5.8(1)

2.5(1)

1200 °C anneal

93.29(2)

6.7(2)

1.5(1)

5.2(2)

1.5(1)

Lattice Constants. Sample

Perovskite

Brownmillerite Sr2(Co0.9Nb0.1)2O5

SCN as synthesized

3.8798(2)

5.395(1), 15.618(4), 5.535(1)

750 °C anneal for 4d

3.8715(1)

9.520(3), 12.453(6)

8.084(1)

750 °C anneal for 10 d

3.8726(1)

9.531(1), 12.422(2)

8.083(1)

1200 °C anneal

3.8739(2)

“Sr6Co5O15”

CoO

Co3O4

4.262(1)

5.392(1), 15.572(4), 5.528(1)

4.262(1)

These diffraction results indicate that at high temperatures (1200 °C) under ambient pressures, the composition SrCo0.9Nb0.1O3-δ yields a phase assemblage that is dominated by a perovskite phase or a perovskite/brownmillerite mixture. (It is unclear from these ex situ measurements whether the brownmillerite forms at 1200 °C or during cooling to room temperature.) While the perovskite (or perovskite/brownmillerite) is thermodynamically stable at high temperatures, at lower temperatures it undergoes partial 10


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decomposition to “Sr6Co5O15” and Co3O4, in a manner analogous to the decomposition of neat SrCoOx into Sr6Co5O15 and Co3O4.10 The reversibility of the decomposition shows that the presence of “Sr6Co5O15” is driven by thermodynamic rather than kinetic considerations, despite the fact that the reactions that generate it are sluggish and may not be complete even after 10 d at 750 °C. Indeed, as a result of this sluggishness, the formation of “Sr6Co5O15” is avoided on cooling from 1200 °C directly to ambient temperatures. Given the presence of brownmillerite after annealing at 1200 °C, the absence of brownmillerite after annealing at 750 °C is somewhat surprising, since lower temperatures generally favor ordered structures. A plausible explanation is that the prolonged 750 °C anneal causes the perovskite to become enriched in Nb as a result of exsolution of Co3O4 and (likely Nb-deficient) “Sr6Co5O15”, adopting a composition that orders less readily into the brownmillerite phase. The slightly smaller lattice constant of the 750 °C annealed perovskite compared to that annealed at 1200 °C (Table 1) is largely a result of the higher average oxidation state of Co at 750 °C, and is not inconsistent with a higher Nb content. Hereafter we refer to the predominant phase encountered as SrCo0.9Nb0.1O3-δ or SCN while recognizing that it may be a mixture of perovskite and brownmillerite phases rather than perovskite alone, and that it is slightly deficient in Co relative to the measured global composition of SrCo0.91±0.01Nb0.09±0.01O3-δ. With this understanding of the phase behavior, we turn to the thermogravimetric results. A typical buoyancy-corrected thermogravimetric profile for an alternating sequence measurement (in which the sample was alternatively exposed to 1000 °C and a target measurement temperature) is shown in Figure 2. The example shown here is for an oxygen partial pressure of 0.17 bar. The profiles obtained for all six oxygen partial pressures evaluated by this method are presented in the supplementary material (Figure S4). Expanded views about T = 850 and 700 °C for the pO2 = 0.17 bar condition are presented in Figure 3. In all cases, cooling from 1000 °C induces a large gain in mass, as expected for a conventional redox reaction. At 850 °C, this fast uptake captures the entirety of the mass change (Figure 3a). In contrast, at 700 °C the fast weight gain is followed by a slow, quasilinear mass increase (Figure 3b), and the weight does not stabilize over the course of the 1 h hold. This slow mass gain is consistent with the decomposition behavior observed by 11



XRD (Table 1), in which the product phases have a higher average formal cobalt oxidation state than the reactant phase. Oxidation of CoO to Co3O4 may also contribute to the observed mass increase, but only to the later, slow gain as discussed below. On the other hand, a brownmillerite to perovskite transformation (generically ABO3-δ to A2B2O5) is ruled out as the cause of the mass gain, since such a phase change would be expected to occur either stoichiometrically (being purely an ordering reaction at a fixed concentration of vacancies) or in conjunction with reduction. Heating back to 1000 °C recovers the original phase assemblage, as indicated by a return in all cases to the original 1000 °C mass value (Figure 2). Moreover, the mass loss profile on reheating shows an inflection when the target temperature is moderate, Figure 3b, but no such inflection is evident when the target temperature is high. This behavior is further evidence of a reversible phase change. 1.5 1000 1.0 800 0.5 600

T / °C

∆m/mref / %

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0.0 400 250

500

750

1000

1250

Time / min

Figure

2.

Typical

buoyancy-corrected

thermogravimetric

data

obtained

from

SrCo0.9Nb0.1O3-δ (pO2 = 0.17 bar), where ∆ m / m ref is the change in sample mass relative to mref, the sample mass at 1000 °C.

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0.35

870

1.0

(a)

1000 0.8

(b)

0.25

0.6

800

0.4 0.2

600

840

T / °C

850

T / °C

0.30

∆m/mref / %

860 ∆m/mref / %

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


0.0

830

0.20 1100

1125

-0.2

400 700

1150

725

750

775

Time / min

Time / min

Figure 3. Expanded views of the thermogravimetric data shown in Figure 2 for (a) 850 °C and (b) 700 °C. The two dotted lines in (b) are linear fits to the data subsets highlighted in red. The intersection point of the two dotted lines was taken as the decomposition-free mass value at this condition.

On the basis of such thermogravimetric profiles, the stability limit of SCN at the oxygen partial pressures of measurement can be estimated. For example, at 0.17 bar, the profile measured upon cooling to 800 °C has an appearance similar to that at 700 °C (Figure 3b); a rapid initial weight gain is followed by a slow weight gain over time. As described above, the latter feature is absent upon cooling to 850 °C. Thus the stability limit at pO2 = 0.17 bar falls in the window between 800 and 850 °C. At higher pO2, this window was found to move to higher temperatures, whereas at lower pO2, the opposite was observed (Figure S4). At 5.8 × 10-3 bar O2 and lower, the oxidation kinetics were too gradual to distinguish a sharp initial weight gain from a later slow increase in mass, and thus the stability limit under these conditions could not be readily identified. The stability windows so determined are summarized in Figure 4. For reference the behavior of BSCF (as extracted from data reported by Mueller et al.29) is also presented, along with the CoO/Co3O4 phase boundary.30 That no “Sr6Co5O15” is obtained from the synthesis, which involved a final heat treatment at 1200 °C under air, is consistent with location of the boundary in combination with slow decomposition kinetics during cooling. Annealing at 750 °C under air, on the other hand, produces a mixture of perovskite and 13



“Sr6Co5O15”, which is similarly consistent with the proposed phase diagram. Separately, it is apparent that in some of the thermogravimetric measurements, the CoO/Co3O4 boundary was traversed on cooling from 1000 °C to the target condition. In these cases the slow mass gain observed may include a contribution from CoO oxidation. Indeed, for the specific condition of 750 °C and 0.17 bar O2, this is considered likely, because the magnitude of the slow mass gain over 1 hour in the TGA measurements (0.05 mg = 0.02%) is large relative to the mass gain from SCN decomposition and Co3O4 formation over 4 days implied by the phase analysis in Figure 1 (~0.07 mg = 0.03%). Given the possible role of CoO oxidation, the stability limit shown for SCN reflects a conservative evaluation. Furthermore, the unambiguous decomposition at 750 °C in air observed by x-ray diffraction in Figure 1 agrees with the stability limit obtained by thermogravimetry and suggests that the level of conservatism is

Bulk Properties of the Oxygen Reduction Catalyst SrCo0.9Nb0.1O3âÎ´

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Bulk Properties of the Oxygen Reduction Catalyst SrCo0.9Nb0.1O3âÎ´