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Electrochimica Acta 56 (2011) 5740–5745

Contents lists available at ScienceDirect

Electrochimica Acta

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igh temperature phase stabilities and electrochemical properties ofnBaCo4−xZnxO7 cathodes for intermediate temperature solid oxide fuel cells

ung-Hyun Kima,1, Young Nam Kimb, Zhonghe Bic, Arumugam Manthiramb,. Parans Paranthamanc, Ashfia Huqa,∗

Neutron Scattering Science Division, Spallation Neutron Source, Oak Ridge National Lab, 1 Bethel Valley Road, Oak Ridge, TN 37831, USAElectrochemical Energy Laboratory & Materials Science and Engineering Program, The University of Texas at Austin, Austin, TX 78712, USAChemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

r t i c l e i n f o

rticle history:eceived 4 March 2011eceived in revised form 14 April 2011ccepted 14 April 2011vailable online 27 April 2011

a b s t r a c t

InBaCo4−xZnxO7 oxides have been synthesized and characterized as cathode materials for intermediatetemperature solid oxide fuel cells (IT-SOFC). The effect of Zn substitution for Co on the structure, phasestability, thermal expansion, and electrochemical properties of the InBaCo4−xZnxO7 has been investigated.The increase in the Zn content from x = 1 to 1.5 improves the high temperature phase stability at 600 ◦Cand 700 ◦C for 100 h, and chemical stability against a Gd0.2Ce0.8O1.9 (GDC) electrolyte. Thermal expansion

−6

eywords:OFCathode

nBaCo4−xZnxO7

-ray diffractionlectrochemical properties

coefficient (TEC) values of the InBaCo4−xZnxO7 (x = 1, 1.5, 2) specimens were determined to be 8.6 × 10to 9.6 × 10−6/◦C in the range of 80–900 ◦C, which provides good thermal expansion compatibility with thestandard SOFC electrolyte materials. The InBaCo4−xZnxO7 + GDC (50:50 wt.%) composite cathodes exhibitimproved cathode performances compared to those obtained from the simple InBaCo4−xZnxO7 cathodesdue to the extended triple-phase boundary (TPB) and enhanced oxide-ion conductivity through the GDCportion in the composites.

. Introduction

The RBaM4O7 (R = Dy, Ho, Er, Tm, Yb, Lu, Y, Ca, In, and M = Fe, Co)xides containing mixed valences of transition metal M2+/3+ ionsave recently drawn attention due to their interesting structural,agnetic, and electrochemical properties. The crystal structure of

he RBaM4O7 can be represented in two different layers of corner-haring MO4 tetrahedra, so-called triangular and Kagomé planes,hat alternate along the c-axis [1]. The R3+ (or Ca2+) and Ba2+

ons occupy octahedral and anticuboctahedral sites, respectively.lthough the crystal structure of the RBaM4O7 had been suggesteds a hexagonal structure with a space group of P63mc, Huq et al.2] revealed that YbBaCo4O7 has a trigonal symmetry with a spaceroup of P31c at temperature >175 K using neutron powder diffrac-ion data. The Co2+/3+ ions in the CoO4 tetrahedra of the YbBaCo4O7re suggested to be present as high-spin state from their magnetic

nd thermoelectric properties [3].

The RBaCo4O7 exhibits interesting oxygen absorp-ion/desorption property that is strongly influenced by chemical

∗ Corresponding author. Tel.: +1 865 574 7923.E-mail address: huqa@ornl.gov (A. Huq).

1 Present address: Electrochemical Energy Research Laboratory, General Motors&D, Warren, MI 48090, USA.

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.04.047

© 2011 Elsevier Ltd. All rights reserved.

compositions, temperature, and oxygen partial pressures [4,5].This property has stimulated interest in their application as oxygenstorage materials. For instance, RBaCo4O7 absorbs extra oxygeninto the lattice (up to 1.5 oxygen atoms per formula unit) in anarrow temperature range (200 ≤ T ≤ 400 ◦C) and lose the extraoxygen rapidly at T > 400 ◦C [5]. Recent study on the crystal struc-ture of the oxygen-rich YBaCo4O8.1 revealed that the incorporationof extra oxygen in the lattice results in the partial transition ofCoO4 tetrahedra into the edge-sharing CoO6 octahedra, whichaccompanies structure distortion into an orthorhombic symmetrywith a space group of Pbc21 [1].

Recently, electrochemical properties and high temperaturephase stability of the RBaCo4O7 has been explored for use as a cath-ode for intermediate temperature solid oxide fuel cell (IT-SOFC).Although the RBaCo4O7 suffer from phase decomposition at ele-vated temperatures of 700–800 ◦C [6,7], partial substitution of Znor Al for Co in YBaCo4O7 improved the high temperature phasestability [7,8]. In addition, the YBa(Co,Zn)4O7 has an importantadvantage of low thermal expansion coefficient (TEC) of 9.5 × 10−6

to 13 × 10−6/◦C in the range of 80–900 ◦C, which provides goodthermal expansion compatibility with the standard SOFC elec-

trolytes [7]. This low TEC value could be due to the absence ofspin-state transitions with the tetrahedral-site Co2+/3+ ions and rel-atively small amount of oxygen loss with increasing temperature. Inparticular, the YBaCo3ZnO7 + GDC composite (1:1 wt. ratio) cathode

ica Acta 56 (2011) 5740–5745 5741

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Table 1Room-temperature structure parameters of the InBaCo4−xZnxO7 oxides determinedby the Rietveld method based on the space group of P31c. The atomic positions areBa (2/3, 1/3, 1/2), In (2/3, 1/3, z), Co1/Zn1 (0, 0, z), Co2/Zn2 (x, y, z), O1 (x, y, z), O2 (0,0, z), and O3 (x, y, z).

InBaCo3ZnO7 InBaCo2.5Zn1.5O7 InBaCo2Zn2O7

a (A) 6.226 6.228 6.231c (A) 10.152 10.152 10.153Vol (A3) 340.777 341.004 341.416In (z) 0.875 0.876 0.876Co1/Zn1 (z) 0.435 (2) 0.436 (1) 0.433 (1)Co2/Zn2 (x) 0.175 (2) 0.175 (2) 0.177 (2)Co2/Zn2 (y) 0.831 (2) 0.830 (2) 0.834 (2)Co2/Zn2 (z) 0.690 (1) 0.691 (1) 0.692 (1)O1 (x) 0.530 (3) 0.524 (4) 0.522 (4)O1 (y) 0.524 (3) 0.515 (4) 0.518 (4)O1 (z) 0.753 (1) 0.753 (1) 0.749 (1)O2 (z) 0.246 (1) 0.244 (2) 0.250 (1)O3 (x) 0.128 (2) 0.135 (3) 0.133 (3)O3 (y) 0.801 (2) 0.807 (3) 0.801 (2)O3 (z) 0.498 (1) 0.492 (1) 0.499 (1)Rp 4.09 4.77 4.56

<0.4% weight loss in the temperature range of 20–700 ◦C in air.Therefore, the expansion of the unit cells with increasing tem-perature is mainly attributed to the normal expansion of the

J.-H. Kim et al. / Electrochim

ffered promising fuel cell performance superior to that obtainedrom the La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) cathode [7,9].

Encouraged by these earlier results, we have explored furtherhemical compositions to develop alternative cathode materials inhe RBa(Co,M)4O7 family. As a result, we present here a system-tic investigation of the InBaCo4−xZnxO7 system with various Znontents. To the best of our knowledge, high temperature phase sta-ility of InBaCo4−xZnxO7 and their potential as cathodes in IT-SOFCave not been reported before. The crystal structure, phase stability,hermal and electrochemical properties, and cathode performancesn IT-SOFC of the InBaCo4−xZnxO7 samples are discussed.

. Experimental

Samples of InBaCo4−xZnxO7 (x = 1, 1.5, and 2) were synthesizedy conventional solid-state reaction methods. Required amountsf In2O3 (Alfa Aesar, 99.9%), BaCO3 (Alfa Aesar, 99.8%), Co3O4 (Alfaesar, 99.7%), and ZnO (Alfa Aesar, 99.9%) were thoroughly mixedsing mortar and pestle, and calcined at 800 ◦C for 12 h in air. Thealcined powders were then ground, pressed into pellets, and sin-ered at 1100–1125 ◦C for 12 h in air. The InBaCo4−xZnxO7 + GDC50:50 wt.%) composite cathodes were prepared by ball-millingppropriate amounts of InBaCo4−xZnxO7 and GDC (Nextech, Microrade) in ethanol for 3 days. The GDC electrolyte disks were pre-ared by pelletizing and sintering required amounts of Gd2O3 andeO2 at 1600 ◦C for 10 h. The synthesized materials were charac-erized by X-ray diffraction (XRD), and the XRD data were refinedsing the Rietveld method with the GSAS program [10,11]. Theoom-temperature oxygen content values were determined byodometric titrations [12].

The long-term phase stabilities of the InBaCo4−xZnxO7 pow-ers were assessed by heating the powder samples in platinumrucibles at 600, 700, or 800 ◦C, respectively, for 100 h, 100 h, or2 h, followed by quenching to room temperature. In addition, theigh temperature XRD of the x = 1.5 sample was measured with

ncreasing temperature after dwelling for 0.5 h at each temper-ture before recording. Thermal expansion data were collectedith a dilatometer (Linseis L75H) during three consecutive heat-

ng/cooling cycles at a rate of 3 ◦C min−1 between 80 ◦C and 900 ◦Cith an intermediate dwelling at 900 ◦C for 1 h in air. The electri-

al conductivity of the InBaCo4−xZnxO7 specimens was measuredith a four-probe dc method using a Van der Pauw configuration

n the temperature range of 200–900 ◦C [13]. The specimens for theigh temperature chemical stability tests were obtained by mixinghe InBaCo4−xZnxO7 and the GDC powders in a 1:1 weight ratio,ollowed by calcining in air at 900–1000 ◦C for 3 h.

The polarization resistances (Rp) of the InBaCo4−xZnxO7 andhe InBaCo4−xZnxO7 + GDC composite cathodes in contact withDC electrolytes were measured using symmetrical cells byC impedance spectroscopy (Solartron 1260 FRA and Princetonpplied Research VersaSTAT 4). All the cathode materials wereixed with an organic binder (Heraeus V006) to form slurry

nd then applied onto both the sides of dense GDC pellets bycreen printing. The parent and composite cathodes were heatedt 900–950 ◦C for 3 h. The microstructures of the cathodes werebserved with a scanning electron microscope (SEM, JEOL JSM-610).

. Results and discussion

.1. Crystal structure and phase stability of InBa(Co,Zn)4O7

The crystal structure of InBaCo4−xZnxO7 (x = 1, 1.5, and 2) haseen investigated. The room-temperature oxygen contents areetermined to be very close to ∼7.0 by the iodometric titration.

Rwp 5.22 6.02 5.76�2 4.47 4.39 4.62

Recent structure study reported that the RBaCo4O7 (R = Y and Yb)crystallizes in the trigonal P31c symmetry [1,2]. The XRD patternsof the InBaCo4−xZnxO7 (x = 1, 1.5, and 2) have been refined basedon the space group of P31c. The resulting structural parametersand the quality of refinements are listed in Table 1. In addition, therefinement profile of the x = 1.5 sample is illustrated in Fig. 1. Thelattice parameters and unit cell volume tend to increase with Zncontent as observed from the YBaCo4−xZnxO7 system [7].

High temperature XRD of the InBaCo2.5Zn1.5O7 sample has beenrecorded in a temperature range of 20–700 ◦C in air. Each XRD pat-tern was refined using the Rietveld method and the variation ofthe lattice parameters with temperature is plotted in Fig. 2. Thelattice parameters and unit cell volume of the InBaCo2.5Zn1.5O7increase monotonically with temperature. Fig. 3 also shows therelative change of the lattice parameter and the variation of TECin the InBaCo2.5Zn1.5O7 with temperature in air. The relative lat-tice parameter change has been calculated based on the dataobtained at 20 ◦C. Thermogravimetric analysis (TGA) revealed thatthe InBaCo4−xZnxO7 (x = 1, 1.5, and 2) powder samples only had

Fig. 1. Observed and calculated XRD profiles and the difference between them forthe InBaCo2.5Zn1.5O7 sample. The vertical bars show the calculated peak positions.

5742 J.-H. Kim et al. / Electrochimica Acta 56 (2011) 5740–5745

Fs

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Fig. 3. Variations of relative lattice parameter change and thermal expansion coef-

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ig. 2. Variations of lattice parameters and unit cell volume in the InBaCo2.5Zn1.5O7

ample measured by in situ XRD in air. Solid lines are guide to the eye.

etal–oxygen bonds, while slight reduction of Co3+ ions accom-anying with the oxygen loss will partly contribute as a chemicalxpansion. The relative lattice expansion is anisotropic with higherEC values along the a-axis at T ≥ 200 ◦C (Fig. 3). The average TECalues are determined to be 8.2–8.9 × 10−6/◦C in the temperatureange of 20–700 ◦C.

The long-term phase stabilities of the InBaCo4−xZnxO7 (x = 1,.5, and 2) powders have been assessed at different tempera-ures. The XRD patterns have been obtained after heating the purenBaCo4−xZnxO7 powder samples at 600, 700, or 800 ◦C for specifiedours, followed by quenching to room temperature. In Fig. 4a, sev-

ral decomposition peaks (marked as arrow) are observed for the= 1 sample at 700 and 800 ◦C. The decomposition product devel-ps after heating at 700 ◦C for 100 h corresponds to the BaCoO3−z

ig. 4. High temperature phase stability tests of InBaCo4−xZnxO7. The XRD patterns at therate of 2 ◦C min−1 with an intermediate dwelling at 900 ◦C for 3 h. The other XRD patter

pecified hours, followed by quenching to room temperature. While the unmarked refleormation of secondary phases.

ficient in the InBaCo2.5Zn1.5O7 sample measured by in situ XRD in air. The relativelattice parameter change (%) is calculated based on the value at 20 ◦C.

as observed from the YBaCo4O7 system [7]. In contrast, furtherincrease in Zn content to x = 1.5 and 2.0 improves the phase stabilityof the InBaCo4−xZnxO7 powders as demonstrated by the long-termstability tests at 600 and 700 ◦C for 100 h in Fig. 4b and c. How-ever, all the InBaCo4−xZnxO7 (x = 1, 1.5, and 2) samples have phasedecomposition problem at 800 ◦C with unknown decompositionproduct.

Although the IT-SOFC will be operated at T ≤ 700 ◦C, the phase

stability of cathode is required above the operation temperaturesfor the cell-fabrication process. In this regards, the InBaCo4−xZnxO7(x = 1, 1.5, and 2) powder samples have been cycled with a specific

top segment have been measured after two consecutive heating/cooling cycles atns have been measured after heating the powder samples at 600, 700, or 800 ◦C forctions belong to the InBaCo4−xZnxO7 phase, those marked with arrow indicate the

J.-H. Kim et al. / Electrochimica Acta 56 (2011) 5740–5745 5743

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ig. 5. Thermal expansion (dL/L0) curves of InBaCo4−xZnxO7 in the temperatureange of 80–900 ◦C in air.

emperature program that will be employed to attach cathode intohe electrolyte. The temperature program is a heating/cooling cyclen a temperature range of 20–900 ◦C at a rate of 2 ◦C min−1 with anntermediate dwelling at 900 ◦C for 3 h. After repeating two consec-tive heating/cooling cycles, the resulting XRD patterns indicatehat all the samples are stable during the temperature program.herefore, the decomposition problem of the InBaCo4−xZnxO7 sam-les at 800 ◦C will not restrict their application as cathodes for

T-SOFC operating below 800 ◦C.

.2. Thermal and electrical properties of InBa(Co,Zn)4O7

Fig. 5 shows the thermal expansion behavior of thenBaCo4−xZnxO7 (x = 1, 1.5, and 2) specimens measured byhe dilatometer in a temperature range of 80–900 ◦C in air. Theorresponding TEC values are calculated to be 8.97 (x = 1), 9.47x = 1.5), and 9.18 (x = 2) × 10−6/◦C at 700 ◦C, which is slightlyigher than that obtained with the high temperature XRD for= 1.5. At 900 ◦C, the TEC values are 9.17 (x = 1), 9.63 (x = 1.5), 9.28

x = 2) × 10−6/◦C, respectively. All the specimens exhibited low TECalues that will provide good thermal expansion compatibilityith the standard SOFC electrolyte materials like YSZ, GDC, and

SGM (10.0 × 10−6 to 12.5 × 10−6/◦C) [14,15]. The low TEC can bettributed to the absence of spin state transitions of Co2+/3+ ionsnd much diminished oxygen loss observed from TGA measure-ents. The Co2+/3+ ions with tetrahedral oxygen coordination in

BaCo3ZnO7 are in high-spin states and do not encounter anypin state transitions with increasing temperature unlike thectahedral-site Co3+ ions in perovskite oxides that undergo aow-spin to high-spin transition and an increase in ionic radius

ith increasing temperature [3,7].The total electrical conductivity of the InBaCo4−xZnxO7 (x = 1,

.5, and 2) specimens are compared in Fig. 6. The electricalonductivity increases with temperature for all the specimens,ndicating a thermally activated polaron behavior. Similarly, theBaCo4−xZnxO7 (0 ≤ x ≤ 2) samples are reported to be a p-type semi-

onductor [16]. From the temperature dependence of electricalonductivity data, the corresponding Arrhenius curves of log(�T)s. 1000/T are plotted in Fig. 6b. The activation energy (Ea) of thenBaCo4−xZnxO7 specimens indicating an energy barrier for the

Fig. 6. (a) Temperature dependence of total electrical conductivity and (b) its Arrhe-nius plot of InBaCo4−xZnxO7 in air.

hole conduction are calculated to be 0.20 eV for x = 1, 0.22 eV forx = 1.5, and 0.26 eV for x = 2, respectively. At a given temperature,the InBaCo4−xZnxO7 specimens show decrease in the electrical con-ductivity and increase in the activation energy Ea with increasingZn content from x = 1 to 2. This can be explained by a perturba-tion of the electron hopping pathway by Zn2+substitution having acompletely filled 3d orbital for Co2+/3+ ions.

3.3. Chemical stability of InBa(Co,Zn)4O7 against GDC electrolyte

The chemical stability of the InBaCo4−xZnxO7 against the GDCelectrolyte was assessed by heating the mixtures at high temper-atures. Fig. 7 shows the resulting XRD patterns with two differentphases marked individually. The x = 1.0 sample was stable againstGDC at 900 ◦C for 3 h, while it shows small amount of side-reactionproduct from XRD at 1000 ◦C for 3 h (not shown here). However,the x = 1.5 and 2.0 samples are stable in contact with GDC at both900 ◦C and 1000 ◦C for 3 h without any distinct side reaction productfrom their XRD. This result suggests that increasing Zn substitutionimproves the chemical stability of the InBaCo4−xZnxO7 powdersagainst the GDC electrolyte at high temperatures.

3.4. Electrochemical performances of InBa(Co,Zn)4O7

The cathode polarization resistances (Rp) of InBaCo4−xZnxO7(x = 1, 1.5, 2) were measured using symmetrical cells with a cell

configurations of cathode |GDC| cathode. Fig. 8a illustrates rep-resentative AC-impedance spectra of the parent InBaCo2.5Zn1.5O7cathode. Our earlier result revealed that a semicircle at a high fre-quency region originated from the grain-boundary resistance in

5744 J.-H. Kim et al. / Electrochimica Acta 56 (2011) 5740–5745

Fig. 7. XRD patterns of the InBaCo4−xZnxO7 + GDC mixtures after heating at 900 ◦Cor 1000 ◦C for 3 h.

Fig. 8. (a) Area-normalized AC impedance spectra for the InBaCo2.5Zn1.5O7 cath-ode at various temperatures in air and (b) variation of polarization resistances (Rp)of the simple InBaCo4−xZnxO7 cathodes (open symbols) and InBaCo4−xZnxO7 + GDC(50:50 wt.%) composite cathodes (closed symbols) with temperature in air.

Fig. 9. SEM images showing the cross sections of the InBaCo4−xZnxO7 + GDC compos-

ite cathode|GDC electrolyte portion in the symmetrical cells after the AC impedancemeasurements.

GDC electrolyte, which became hard to discern with increasingtemperature >650 ◦C due to the decreasing time constant beyondthe limit of the instrument [9,17,18]. Based on the results, theAC-impedance spectra are fitted using an electrical circuit modelconsisting of RS(RGB/CPEGB)(R1/CPE1)(R2/CPE2), where the RS isfrom the Pt wire and the bulk resistance of the GDC electrolyte,(RGB/CPEGB) is from the grain boundary resistance, and the twoconsecutive (R1/CPE1) and (R2/CPE2) circuits are from the cathode.Here, the lower frequency arc associated with the cathode polariza-tion resistance has been fitted using the (R1/CPE1) relating with thecharge transfer of oxide ions at the electrode–electrolyte interfaceand the (R2/CPE2) relating with the diffusion of oxygen. In Fig. 8a,the representative AC-impedance spectra of the InBaCo2.5Zn1.5O7cathode show good fitting based on the circuit model employed.Fig. 8b compares the Rp values of the InBaCo4−xZnxO7 (x = 1, 1.5, 2)cathodes in air. At a given temperature, Rp of the InBaCo4−xZnxO7cathodes decrease with increasing Zn content from x = 1 to 2. TheEa are calculated to be 0.91 eV (x = 1), 1.06 eV (x = 1.5), and 1.05 eV(x = 2), respectively.

Recently, the YBaCo3ZnO7 cathode exhibited the optimum elec-trochemical performances by employing the YBaCo3ZnO7 + GDC(50:50 wt.%) composite as a cathode [7,9]. The composite cathodesconsisted of smaller particles with larger surface area comparedto the pure YBaCo3ZnO7 cathode, which is beneficial for providingextended Triple Phase Boundary (TPB) where the Oxygen Reduc-tion Reaction (ORR) occurs. In addition, it will provide higher ionicconduction through the GDC portion in the composite cathode andbetter TEC compatibility with GDC electrolyte [9]. Fig. 8b showsthat the InBaCo4−xZnxO7 + GDC (50:50 wt.%) composite cathodes

have lower Rp values compared to those obtained from the simpleInBaCo4−xZnxO7 cathodes. Similar to the simple cathodes, the Rp

values of the InBaCo4−xZnxO7 + GDC (50:50 wt.%) composite cath-ode increase with increasing Zn content. The Ea of the composite

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J.-H. Kim et al. / Electrochim

athodes are calculated to be 1.01 eV (x = 1), 0.97 eV (x = 1.5), and.07 eV (x = 2), respectively.

After the AC-impedance measurements, microstructures of theymmetrical cells were recorded with SEM. Fig. 9 shows the cross-ectional SEM images of the composite cathodes with x = 1 and 1.5ttached on top of the GDC electrolyte. While the bottom of theicrograph indicates the presence of dense GDC electrolyte, the

pper portion shows the layer of porous cathodes with approxi-ate thicknesses of ∼15 �m. From the SEM images, the composite

athode maintains the good porosity during the electrochemicalesting at high temperatures. The x = 1.5 cathode consists of largerarticles compared to that of the x = 1.0, which decreases the TPB

ength and subsequently deteriorates the cathode performance forRR.

. Conclusions

A new class of cathode materials InBaCo4−xZnxO7 based onetrahedral-site Co2+/3+ ions that crystallizes in the trigonal P31cymmetry has been investigated. The InBaCo4−xZnxO7 exhibitedow TEC values (8.6 × 10−6 to 9.6 × 10−6/◦C at 80–900 ◦C) that areompatible with standard electrolyte materials. This low TEC isttributed to the absence of spin state transitions of tetrahedral-ite Co2+/3+ ions and relatively small amounts of oxygen loss withncreasing temperature. The increase in the Zn content from x = 1o 1.5 improves the high temperature phase stability at 600 ◦C and00 ◦C for 100 h, and chemical stability against a GDC electrolyte.owever, the total electrical conductivity and cathode polarizationonductance (Rp

−1) decrease with increasing Zn content. The elec-rical performances of the InBaCo4−xZnxO7 cathodes are improvedy employing the InBaCo4−xZnxO7 + GDC (50:50 wt.%) compositeathodes. The enhanced catalytic activity for ORR of the compositeathode is attributed to the extended TPB and enhanced oxide-on conductivity through the GDC portion in the composites. Theomposite cathode with x = 1.5 offers a trade-off between electro-hemical performance and phase stability at high temperatures.

cknowledgements

This work was sponsored by the Laboratory Directed Researchnd Development (LDRD) Program of Oak Ridge National Labo-

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cta 56 (2011) 5740–5745 5745

ratory. A portion of the characterization effort was conducted atboth ORNL SHaRE user facility and Center for Nanophase Materi-als Sciences, Oak Ridge National Laboratory, which is sponsored bythe Scientific User Facilities Division, Office of Basic Energy Sci-ences, U.S. Department of Energy. Jung-Hyun Kim and ZhongheBi acknowledge the support of the ORISE postdoctoral fellowship.Ashfia Huq acknowledges Spallation Neutron Source for financialsupport which is supported by the Division of Scientific User Facil-ities, Office of Basic Energy Sciences, US Department of Energy,under contract DE-AC05-00OR22725 with UT-Battelle, LLC. Finan-cial support for the work done at the University of Texas at Austinby the Welch Foundation grant F-1254 is gratefully acknowledgedby Young Nam Kim and Arumugam Manthiram.

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