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Improved phase stability and electrochemicalperformance of (Y,In,Ca)BaCo3ZnO7þd cathodes forintermediate temperature solid oxide fuel cells

Matthew West, Arumugam Manthiram*

McKetta Department of Chemical Engineering, University of Texas at Austin, Austin, TX 78712, USA

a r t i c l e i n f o

Article history:

Received 30 July 2014

Received in revised form

17 September 2014

Accepted 18 September 2014

Available online 16 October 2014

Keywords:

Solid oxide fuel cells

Cathodes

Phase stability

Polarization resistance

* Corresponding author. Tel.: þ1 512 471 179E-mail address: rmanth@mail.utexas.edu

http://dx.doi.org/10.1016/j.ijhydene.2014.09.00360-3199/Copyright © 2014, Hydrogen Energ

a b s t r a c t

With an aim to combine the performance-enhancing properties of Ca with the stability-

promoting properties of In in the swedenborgite YBaCo4O7þd-based cathodes for solid

oxide fuel cells (SOFC), cation-substituted Y1�x�yInxCayBaCo3ZnO7þd (0.2 � (x þ y) � 0.5)

oxides have been explored. All samples presented in this work are stable in air after 120 h

exposure to 600, 700, and 800 �C. Increasing In content shows a negligible impact on po-

larization resistances (Rp), but causes an increase in the activation energies (Ea) of (Y,In,Ca)

BaCo3ZnO7þd þ Gd0.2Ce0.8O1.9 (GDC) composite cathodes on 8 mol% yttria-stabilized zirco-

nia (8YSZ) electrolyte supported symmetric cells. Increasing Ca content shows a decrease

in Rp and an increase in Ea on similar electrochemical cells. All (Y,In,Ca)BaCo3ZnO7þd

samples investigated here show superior performance compared to the unsubstituted

YBaCo3ZnO7þd þ GDC cathode in the range of 400e800 �C. Especially, the

Y0.5In0.1Ca0.4BaCo3ZnO7þd þ GDC composite cathode exhibits good performance on GDC

electrolytes in the range of 400e600 �C. With superior phase stability and electrochemical

performance, the (Y,In,Ca)BaCo3ZnO7þd series of oxides are attractive cathode candidates

for intermediate temperature SOFCs.

Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

Introduction

Solid oxide fuel cells (SOFCs) have many advantages over

other fuel cell technologies such as higher efficiencies,

compatibility with a wide variety of fuels, and the ability to

operate without the use of precious metal catalysts [1,2].

However, SOFCs are still limited from widespread utilization

and commercialization due to materials challenges, primarily

due to their high operating temperatures. The traditional

operating temperature range (800e1000 �C) allows for internal

1; fax: þ1 512 471 7681.(A. Manthiram).91y Publications, LLC. Publ

reformation of fuels and excellent fuel utilization efficiencies,

but requires expensive specialized high-temperature mate-

rials to operate properly [1,2]. Reducing the operating tem-

perature to the intermediate-temperature (IT) range of

600e800 �C, or the low-temperature region below 600 �C,would allow for less expensive materials to be used in the

stack assembly. Unfortunately traditional SOFC cathode ma-

terials offer poor performance in these temperature regions

[1,2]. Therefore, one of the main areas of investigation in

SOFCs is to design and develop cathodes with superior per-

formance at these reduced temperatures [1e7].

ished by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 9 7 2 2e1 9 7 3 0 19723

Recently, a new series of SOFC cathodes based on the

swedenborgite-type RBa(Co,M)4O7þd (R ¼ Y, In, Ca; M ¼ Fe, Zn)

structure has shown favorable electrochemical performance

and excellent thermal expansion coefficient (TEC) matches to

traditional SOFC electrolytes [4e7]. However, these materials

suffer from a tendency to decompose after long-term expo-

sure (over 120 h) to operating temperatures in the range of

600e800 �C [4e7]. Previous work by our group has shown that

the stability and electrochemical performance of these ma-

terials depend on both the R and (Co,M)4 compositions and

tend to have an inverse relationship [4e7]; for example, a

larger R3þ ion promotes the reversible oxygen absorption of

these materials, but tends to suffer from increased phase

instability at higher temperatures [8,9]. Frequently, Co is

replaced with Zn, Al, or Ga, which help to stabilize the phase,

but also diminish the oxygen absorption and electrical con-

ductivity of these materials [4e8].

Recent work by our group has shown that while

YBaCo3ZnO7þd is unstable at both 600 and 700 �C, substitutionof a small amount of In in the R-site (Y0.9In0.1BaCo3ZnO7þd)

fully stabilizes the lattice at all temperatures of interest [10]. In

another work, it was shown that substitution of Ca2þ for Y3

significantly improves the performance by increasing the

oxidation state of cobalt, but requires a substitution of large

amount of Zn2þ for Co2þ/3þ to realize phase stability [7]. In this

work, we explore the tri-substituted Y1�x�yInxCayBaCo3ZnO7þd

(0.2 � (x þ y) � 0.5) series of cathodes, with an aim to combine

the stabilizing effect of In with the increased performance

effect of Ca.

Experimental methods

Materials synthesis

The (Y,In,Ca)BaCo3ZnO7þd samples were synthesized by con-

ventional solid-state reactions (SSR). Required amounts of

Y2O3, In2O3, CaCO3, BaCO3, Co3O4 and ZnO were mixed with

ethanol in an agate mortar and pestle for 1 h. Samples were

dried, pressed into pellets, and calcined at 1000 �C for 12 h

[4e7,10]. Powders were then ground, pressed into pellets

again, and sintered at 1200 �C for 24 h [4e7,10]. After sintering,

the pellets were annealed in air at 900 �C for 6 h and slowly

cooled to room temperature at a rate of 1 �C min�1 [10].

The Y0.2Ce0.8O1.9 (YDC) electrolyte powders were synthe-

sized by the glycine nitrate combustion (GNP) method [11,12].

Required amounts of Y(NO3)3$6H2O and Ce(NH4)2(NO3)6 were

dissolved in deionized water (DI water) with glycine

(NH2CH2COOH) and heated on a stir plate until a viscous gel

was formed. At this point, the stir bar was removed, and the

gel was heated until combustion. The resultant powder was

collected and then calcined at 600 �C for 2 h to ensure decar-

bonation [10].

The Gd0.2Ce0.8O1.9 (GDC) powder was synthesized by SSR.

Required amounts of CeO2 and Gd2O3 were ball-milled in

ethanol for 36e48 hwith 1mol% (metals basis) Ni(NO3)2 added

as a sintering aid [13,14]. Themixture was subsequently dried,

pressed into pellets, and sintered at 1550 �C for 24 h. These

pellets were then thoroughly ground by hand in an agate

mortar and pestle to produce a powder.

Material characterization of (Y,In,Ca)BaCo3ZnO7þd powders

After synthesis and annealing, the (Y,In,Ca)BaCo3ZnO7þd

powders were analyzed by X-ray diffraction (XRD) with Cu Ka

radiation. The XRD patterns were analyzed with the MDI Jade

program, and the data were refined to calculate lattice

parameters.

The long-term phase stabilities of the (Y,In,Ca)

BaCo3ZnO7þd samples were determined by long-term expo-

sure to traditional SOFC operating temperatures. The samples

were heated in alumina crucibles in a box furnace to high

temperatures at a rate of 2e3 �C min�1 and were allowed to

dwell for 120 h before cooling to room temperature at the

same rate. Three separate tests were conducted for 600, 700,

and 800 �C, and the resultant powders were characterized by

XRD [4e7,10].

The oxygen content and oxidation state of cobalt at room

temperature were determined by iodometric titration [15].

Thermogravimetric analysis (TGA) with a Netzsch STA 449 F3

thermal analysis systemwas used to determine the change in

oxygen content during thermal cycling. TGA data were

collected during two consecutive heating/cooling cycles at a

rate of 3 �C min�1 from 80 to 900 �C with a 15 h dwell at 900 �Cduring the first heating cycle to normalize oxygen contents [4].

Electrochemical characterization of (Y,In,Ca)BaCo3ZnO7þd

Composite cathodes were created by ball-milling equal parts

by mass (Y,In,Ca)BaCo3ZnO7þd and GDC powders in ethanol

for 36e48 h [5e7,10]. As the RBa(Co,M)4O7þd oxides have his-

torically been shown to have low oxygen conductivity, com-

posite cathodes of (Y,In,Ca)BaCo3ZnO7þd and GDC were

utilized in a 1:1 mass ratio [4,5]. This ratio was fixed as the

previous work found it to be ideal in the case of YBaCo3ZnO7þd

composite cathodes [5]. Previouswork has also shown that the

performances of this class ofmaterials are strongly dependent

on the three-phase boundary (TPB) region, which is the sur-

face area where the electrolyte, cathode, and oxidant all touch

[5e7,10,16]. The powders were ball-milled to both increase the

homogeneity of the mixture and reduce the particle size,

thereby increasing the size of the TPB. The composite powders

after milling were then dried, and mixed with an organic

binder (Heraeus V006) in a 60:40 cathode: binder weight ratio

to create an ink [4e7,10]. YDC electrolyte ink was prepared

similarly, though the YDC powders were not ball milled prior

to mixing [10].

The polarization resistances (RP) of the composite cathodes

were measured on 8 mol% yttria-stabilized zirconia (8YSZ)

electrolyte supported symmetric cells in the range of

400e800 �C by AC impedance spectroscopy (Solartron 1260

FRA). The electrolyte thickness was 150e200 mm

(FuelCellMaterials.com). To prevent interfacial reactions be-

tween the cathode and electrolyte, an intermediate layer of

YDC was screen-printed on both sides of the electrolyte and

sintered at 1250 �C for 1 h [4,10,17,18]. The composite cathode

layers were then screen printed three times on each side, and

sintered for 3 h at 900 �C [4e7,10]. After sintering, silver cur-

rent collectors were applied using silver paste (FuelCellMate-

rials AgeI), mesh, andwires, and sintered at 800 �C for 1 hwith

1 �Cmin�1 heating and 2 �Cmin�1 cooling rates [10]. Silver was

Fig. 1 e Room-temperature XRD patterns of the as-synthesized (Y,In,Ca)BaCo3ZnO7þd samples before long-term stability

testing.

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used as the current collector due to its non-reactivity, high

electrical conductivity, and favorable oxygen reduction ki-

netics [19e21]. RP values of selected (Y,In,Ca)BaCo3ZnO7þd

were also measured on 250 mm thick GDC substrate (Fuel-

CellMaterials) for a comparison, without a YDC buffer layer.

Themicrostructures of all symmetric cells were observed by a

JEOL JSM-5610 scanning electron microscope after the elec-

trochemical impedance spectroscopy (EIS) measurement.

Table 1 e Unit cell parameters and activation energies ofthe (Y,In,Ca)BaCo3ZnO7þd oxides and compositecathodes.

Sample a (A) c (A) Volume (A3) a/c Ea (eV)a

High In

Y8In1Ca1 6.310 10.261 353.75 0.6149 1.223

Y7In2Ca1 6.305 10.256 353.05 0.6147 1.242

Y6In3Ca1 6.300 10.248 351.8 0.6144 1.257

Y5In4Ca1 6.286 10.234 350.23 0.6142 1.268

High Ca

Y8In1Ca1 6.310 10.261 353.75 0.6149 1.223

Y7In1Ca2 6.313 10.259 354.11 0.6154 1.259

Y6In1Ca3 6.318 10.254 354.43 0.6161 1.328

Y5In1Ca4 6.320 10.248 354.43 0.6167 1.320

a Activation energies calculated in the range of 400e750 �C.

Results and discussion

For ease of reference, the samples will be referenced by their

R-site composition; e.g. Y0.8In0.1Ca0.1BaCo3ZnO7þd will be

referred to as Y8In1Ca1, Y0.7In0.2Ca0.1BaCo3ZnO7þd will be

referred to as Y7In2Ca1, and so on. Similarly, the samples will

be grouped into two sub-series based on their calcium con-

tent; samples where y ¼ 0.1 will be labeled as “high-In,” and

samples where 0.2 � y � 0.4 will be labeled as “high-Ca,” with

Y8In1Ca1 belonging to both sub-series.

Phase stability and crystal chemistry of (Y,In,Ca)BaCo3ZnO7þd

The room-temperature XRD patterns of the (Y,In,Ca)

BaCo3ZnO7þd oxides, shown in Fig. 1, indicate that all samples

investigated were successfully synthesized as single-phase

materials with a P31c space group similar to

Y1�xInxBaCo3ZnO7þd (0 � x � 0.5) [10]. Unit cell parameters

obtained by refinements are listed in Table 1. The addition of

In decreases the unit cell volume, while the addition of cal-

cium slightly increases the unit cell volume. This is attributed

to the replacement of Y3þ (r ¼ 1.019 A), respectively, with In3þ

(r¼ 0.92 A) and Ca2þ (r¼ 1.12 A) [22]. In the high-In series, the a

and c lattice parameters both decrease with increasing In

content. In the high-Ca series, a increases and c decreases

with increasing Ca content, leading to a smaller change in

volume as compared to the In substitutions. This correlates

well with previous reports [7].

The ratio of a/c is also listed in Table 1, where it can be seen

that an increasing In content slightly decreases this ratio,

while an increasing Ca content leads to an increasing ratio.

The naturally occurring swedenborgite (NaSbBe4O7) shows a

ratio of a/c ¼ 0.613, which may be used as a reference [23]. As

Ca content increases, the a/c ratio deviates further from the

swedenborgite ratio, whichmay lead to increased lattice strain.

The room-temperature oxygen contents of all (Y,In,Ca)

BaCo3ZnO7þd oxides determined by iodometric titration are

presented in Table 2. In the high-In series, the oxygen content

tends to increase with increasing In content, excepting an

initial decrease fromY8In1Ca1. This trendwas also observed in

previous work on Y1�xInxBaCo3ZnO7þd, where it was shown

that oxygen content initially increases with increasing In

content, then decreases once the unit cell volume is smaller

than approximately 350 A3 [10]. As the In content continues to

increase, the decreasing lattice volume reduces the available

interstitial volume and limits the oxygen content [6,8,10]. In

this work, with the addition of the larger Ca2þ ion, this

constraint is lessened: at the maximum In content investi-

gated, Y5In4Ca1, the unit cell has a volume of 350.23 A3.

For the high-Ca series, the room-temperature oxygen

content tends to decrease with increasing Ca content. The

decrease arises from the substitution of trivalent Y3þ ions by

divalent Ca2þ ions, which causes both an increase in Co

oxidation state and a decrease in oxygen content to maintain

Table 2 e Oxygen contents and Co oxidation states of the (Y,In,Ca)BaCo3ZnO7þd oxides.

Sample Room temperature TGA dwell Final cool

Oxygencontent(7 þ d)

AverageCo oxidation

statea

Oxygencontent(7 þ d)

AverageCo oxidation

statea

Oxygencontent(7 þ d)

AverageCo oxidation

statea

High In

Y8In1Ca1 7.167 2.478 7.248 2.532 7.373 2.615

Y7In2Ca1 7.128 2.452 7.237 2.524 7.363 2.609

Y6In3Ca1 7.158 2.472 7.271 2.547 7.434 2.656

Y5In4Ca1 7.170 2.480 7.288 2.558 7.412 2.642

High Ca

Y8In1Ca1 7.167 2.478 7.248 2.532 7.373 2.615

Y7In1Ca2 7.153 2.502 7.251 2.567 7.406 2.671

Y6In1Ca3 7.137 2.525 7.298 2.632 7.436 2.724

Y5In1Ca4 7.143 2.562 7.171 2.581 7.248 2.632

a The average Co oxidation state of the undoped YBaCo3ZnO7 would be 2.33.

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charge neutrality. This observation is supported by previous

work [6]. This leads to a trend in both series of an increasing

oxidation state with decreasing Y content at room

temperature.

The long-term phase stability XRD patterns are shown in

Fig. 2. All peaks match well with the XRD patterns shown in

Fig. 1, showing that all samples are stable after 120 h exposure

to all temperatures investigated here. As reported in our pre-

vious work, a small substitution of 0.1 In for Y results in the

fully stabilized Y0.9In0.1BaCo3ZnO7þd material due to increased

oxygen content and decreased lattice size [10]. Our group has

also previously found that in (Y,Ca)Ba(Co,Zn)4O7þd, a mini-

mum Zn content of 1.5 is necessary to stabilize a Ca substi-

tution of 0.5 [6]. The inclusion of Zn stabilizes the phase since

Zn2þ has a strong preference for tetrahedral coordination and

Co2þ/3þ ions are relatively unstable in tetrahedral coordina-

tion [4e8]. However, the inclusion of Zn negatively impacts

the performance of this material as a cathode in multiple

ways. Zn has a very stable electron configuration of 3d10, and

therefore cannot contribute to electrical conductivity, oxygen

absorption, or oxygen reduction. As can be observed in Fig. 2,

all investigated samples of (Y,In,Ca)BaCo3ZnO7þd are stable at

all temperatures of interest. When compared to the previous

work, where Y0.5Ca0.5BaCo3ZnO7þd is unstable at both 600 and

700 �C, this shows that the small substitution of In is once

again enough to stabilize the material at lower temperatures,

reducing the dependence on the content of the performance-

inhibiting Zn [7].

Thermal properties and electrochemical performance of(Y,In,Ca)BaCo3ZnO7þd

The variations of oxygen content with temperature as

observed by TGA in air, with all mass gain and loss attributed

to absorption and desorption of oxygen, are shown in Fig. 3.

The full trace of oxygen content versus temperature for

Y8In1Ca1 is shown in Fig. 3a. Fig. 3a also defines three points

along the curve: room-temperature, TGA-dwell, and final-

cooling points. Table 2 lists the oxygen contents at these

points for all samples, along with the derived average Co

oxidation states. All samples show similar traces, gaining a

significant amount of oxygen during the 15 h annealing

period. For clarity, Fig. 3b and c only show the final cooling

curve of the high-In and high-Ca samples, respectively. To

determine the initial room-temperature oxygen content, all

samples were subjected to iodometric titrated prior to exper-

iments in the TGA.

As can be seen in Table 2, both the high-In and high-Ca

series show a trend of increasing oxygen content after

TGA annealing, with the exception of the Y5In1Ca4 sample.

This can be explained to be due to a reduction in oxygen

storage capacity; since Ca has a fixed valence state of Ca2þ

in this material, the substitution of Ca2þ for Y3þ leads to an

increase in the oxidation state of Co. As the reversible ox-

ygen absorption is directly related to the capability of the Co

ions to be oxidized, this increase in Ca content leads to a

decreased maximum oxygen content. This is supported by

previous work, which shows that an increased Ca content

results in a reduced oxygen absorption upon thermal

cycling [7].

Fig. 4 shows the Arrhenius plots of the polarization re-

sistances of the (Y,In,Ca)BaCo3ZnO7þd composite cathodes on

8YSZ electrolyte supported symmetric cells in the range of

400e800 �C. Since 8YSZ has insignificant electronic conduc-

tivity in this temperature range, the polarization resistances

of these cathodes were calculated by taking the difference

between the high- and low-frequency intercepts with the real

axis [24e26]. As shown in Fig. 4a, all high-In samples show

similar polarization resistances. This matches well with our

previous work in the (Y,In)BaCo3ZnO7þd series, where the In

content did not significantly impact the performance [10]. In

Fig. 4b, it can be seen that all high-Ca samples outperform the

Y8In1Ca1 samples. The activation energies obtained from

these plots in the range of 400e750 �C are given in Table 1. The

800 �C values are omitted from calculation because of the

increased errors in measurement at very low polarization re-

sistances due to the sensitivity limits of the AC-impedance

spectrometer. The high-In and high-Ca series both show a

general trend of increasing activation energy with increasing

In and Ca contents respectively, which correlates well to the

previous work [7,10]. The mechanism related to this increase

in activation energy is not yet well understood and should be

Fig. 2 e Room-temperature XRD patterns of the (Y,In,Ca)BaCo3ZnO7þd samples after 120 h exposure to 600, 700, and 800 �C.No impurity phases were detected in any sample after long-term exposure.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 9 7 2 2e1 9 7 3 019726

investigated further in future work. Fig. 4c compares the

performance of the Y8In1Ca1 composite cathode with a com-

posite cathode of YBaCo3ZnO7þd (Y10) þ GDC, where it can be

seen that Y8In1Ca1 slightly outperforms Y10 at all

temperatures.

Fig. 5 shows the Arrhenius plots of the polarization re-

sistances of Y8In1Ca1, Y5In4Ca1, and Y5In1Ca4 composite

cathodes onGDC electrolyte supported symmetric cells. These

cells were manufactured to better observe the effects of

composition on performance in the lower-temperature

Fig. 3 e TGA plots detailing the oxygen content variations

of the as-synthesized (Y,In,Ca)BaCo3ZnO7þd oxides: (a) full

curve of Y0.8In0.1Ca0.1BaCo3ZnO7þd as an example, (b) final

cooling curve of the high-In samples, and (c) final cooling

curve of the high-Ca samples. All samples were allowed to

anneal 15 h in air, as seen in (a). Fig. 3a also defines three

points along the curve that are used in Table 2: (1) is the

room-temperature point, (2) is the TGA-dwell point, and (3)

is the final-cool point.

Fig. 4 e Arrhenius plots of the (Y,In,Ca)BaCo3ZnO7þd þ GDC

symmetric cells on 8YSZ electrolyte in the range of

400e800 �C: (a) Arrhenius plots of the high-In samples, (b)

Arrhenius plots of the high-Ca samples, and (c) Arrhenius

plots comparing Y0.8In0.1Ca0.1BaCo3ZnO7þd with the

undoped YBaCo3ZnO7þd for a comparison.

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Fig. 5 e Arrhenius plots of

Y0.8In0.1Ca0.1BaCo3ZnO7þd þ GDC,

Y0.5In0.4Ca0.1BaCo3ZnO7þd þ GDC, and

Y0.5In0.1Ca0.4BaCo3ZnO7þd þ GDC symmetric cells on GDC

electrolyte in the range of 400e800 �C. The Y5In1Ca4sample shows improved performance at all temperatures,

particularly below 600 �C.

Fig. 6 e Nyquist plots of the (a)

Y0.8In0.1Ca0.1BaCo3ZnO7þd þ GDC, (b)

Y0.5In0.4Ca0.1BaCo3ZnO7þd þ GDC, and (c)

Y0.5In0.1Ca0.4BaCo3ZnO7þd þ GDC symmetric cells on GDC

electrolyte. Frequency markers are included for the

Y0.8In0.1Ca0.1BaCo3ZnO7þd þ GDC sample at all

temperatures.

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region, as the reduced ionic conductivity of 8YSZ at temper-

atures below 600 �C can limit performance [1]. Fig. 5 shows

that the polarization resistance of Y5In4Ca1 is similar to

Y8In1Ca1 at temperatures below 600 �C, but begins to show

reduced polarization resistance at higher temperatures.

Y5In1Ca4, on the other hand, outperforms Y8In1Ca1 at all

temperatures, notably including the 400e600 �C range. The

Nyquist plots of all three samples at 400, 500, and 600 �C are

presented in Fig. 6 to further show this comparison. At 400 �C(Fig. 6a), clear arc separation behavior is observed. According

to Adler et al. [26,27], the impedance at high-frequencies de-

rives from the charge transfer across the electrolyte, at me-

dium frequencies from the charge transfer at the electrode/

electrolyte interface, and at low frequencies from the oxygen

surface exchange. At 400 �C, the arc separation is clearest in

the Y8In1Ca1 sample, where three arcs are observed. As tem-

peratures increase to 500 and 600 �C, the separation of arcs

becomes harder to detect, with only the low-frequency oxy-

gen surface exchange arc remaining observable. After the

measurement, these cells were fractured and were observed

by SEM to determine the effect of microstructure on perfor-

mance. The cross-sectional images are presented in Fig. 7,

where it can be observed that all cells have negligible differ-

ences in microstructure.

The improved performance with increasing Ca content

may be understood by the effective change in the oxidation

state of the Co ions; by replacing Y3þ with Ca2þ, Co2þ is

oxidized to Co3þ. Table 2 lists the oxidation states of cobalt

determined after the 15 h dwell in TGA, where it shows that

samples with a larger Ca content tend to have a higher Co

oxidation state. Previous work on perovskite-based SOFC

cathodes has shown that the oxygen reduction reaction

(ORR) is strongly dependent upon the electronic structure

and covalency of the transition metaleoxygen bond

[28e31]. It has been shown that the ORR kinetics tends to

improve as the energy difference between the metal 3d and

Fig. 7 e SEM micrographs of the (Y,In,Ca)BaCo3ZnO7þd þ GDC symmetric cells on GDC electrolyte after testing (tests may be

seen in Figs. 5 and 6).

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 9 7 2 2e1 9 7 3 0 19729

oxygen 2p bands decreases and the metal-oxygen covalence

increases [29]. As the oxidation state of Co increases in

RBa(Co,M)4O7þd, the metal-oxygen covalency increases,

resulting in a reduction the area-specific resistance of the

cathode.

In our previous work, we showed that the composite Y10

cathode performed similarly to the well-studied pristine

Ba0.5Sr0.5Co0.8Fe0.2O3�d (BSCF) cathode at high temperatures,

and slightly outperformed BSCF in the 400e600 �C range [10].

As Y5In1Ca4 shows greater thermal stability and electro-

chemical performance compared to Y10, this suggests that the

fully-stabilized Y0.5In0.1Ca0.4BaCo3ZnO7þd þ GDC composite

cathodemaymake an excellent candidate for intermediate-to

low-temperature SOFC applications.

Conclusions

The effects of R-site composition in the

Y1�x�yInxCayBaCo3ZnO7þd (0.2 � x þ y � 0.5) series of oxides

have been investigated. All samples investigated are stable

after 120 h exposure to 600, 700, and 800 �C, showing that a

small In content is able to sufficiently stabilize the phase.

Unit cell volume decreases with increasing In content and

increases with increasing Ca content. The oxidation state of

Co increases with increasing In and Ca contents. Increased

In content has minimal effect on the electrochemical per-

formance of the composite cathodes, while increased Ca

content improves the electrochemical performance. All

(Y,In,Ca)BaCo3ZnO7þd samples show improved electro-

chemical performance over the unsubstituted YBaCo3ZnO7þd

base composition. Y0.5In0.1Ca0.4BaCo3ZnO7þd has significantly

improved electrochemical performance compared to

Y0.8In0.1Ca0.1BaCo3ZnO7þd on GDC symmetric cells at low

temperatures. With good long-term stability at all tempera-

tures in the range of 600e800 �C and improved low-

temperature electrochemical performance, the high-Ca

content (Y,In,Ca)BaCo3ZnO7þd series of oxides are an attrac-

tive option for intermediate-to low-temperature SOFC

cathodes.

Acknowledgments

Financial support by the Welch Foundation grant F-1254 is

gratefully acknowledged. The authors thank Soa-Jin Sher and

Christina Ortiz for their assistance with some experiments.

r e f e r e n c e s

[1] Brett DJL, Atkinson A, Brandon NP, Skinner SJ. Intermediatetemperature solid oxide fuel cells. Chem Soc Rev2008;37:1568e78.

[2] Yamomoto O. Solid oxide fuel cells: fundamental aspectsand prospects. Electrochim Acta 2000;45:2423e35.

[3] West M, Manthiram A. Layered LnBa1�xSrxCoCuO5þd (Ln ¼ Ndand Gd) perovskite cathodes for intermediate temperaturesolid oxide fuel cells. Int J Hydrogen Energy 2013;38:3364e72.

[4] Kim JH, Manthiram A. Low Thermal expansion RBa(Co,M)4O7

cathode materials based on tetrahedral-site cobalt ions forsolid oxide fuel cells. Chem Mater 2010;22:822e31.

[5] Kim JH, Kim YN, Cho SM, Wang H, Manthiram A.Electrochemical characterization ofYBaCo3ZnO7 þ Gd0.2Ce0.8O1.9 composite cathodes forintermediate temperature solid oxide fuel cells. ElectrochimActa 2010;55:5312e7.

[6] Kim YN, Kim JH, Huq A, Paranthaman MP, Manthiram A.(Y0.5In0.5)Ba(Co,Zn)4O7 cathodes with superior high-temperature phase stability for solid oxide fuel cells. J PowerSources 2012;214:7e14.

[7] Kim YN, Kim JH, Manthiram A. Characterization of (Y1�xCax)BaCo4�yZnyO7 as cathodes for intermediate temperaturesolid oxide fuel cells. Int J Hydrogen Energy2011;36:15295e303.

[8] Parkkima O, Yamauchi H, Karppinen M. Oxygen storagecapacity and phase stability of variously substitutedYBaCo4O7þd. Chem Mater 2013;25:599e604.

[9] Kadota S, Karppinen M, Motohashi T, Yamauchi H. R-Sitesubstitution effect on the oxygen-storage capability ofRBaCo4O7þd. Chem Mater 2008;20:6378e81.

[10] West M, Sher SJ, Manthiram A. Effects of In substitution inY1�xInxBaCo3ZnO7þd (0 � x � 0.5) cathodes for intermediatetemperature solid oxide fuel cells. J Power Sources2014;271:252e61.

[11] Taguchi H, Matsuda D, Nagao M, Tanihata K, Miyamoto Y.Synthesis of perovskite-type (La1�xSrx)MnO3 (O � 0.3) at lowtemperature. J Am Ceram Soc 1992;75:201e2.

[12] Chick LA, Pederson LR, Maupin GD, Bates JL, Thomas LE,Exarhos GJ. Glycine-nitrate combustion synthesis of oxideceramic powders. Mat Lett 1990;10:6e12.

[13] Kleinlogel C, Gauckler LJ. Sintering of nanocrystalline CeO2

ceramics. Adv Mater 2001;13:1081e5.[14] Nicholas JD, De Jonghe LC. Prediction and evaluation of

sintering aids for cerium gadolinium oxide. Solid State Ionics2007;178:1187e94.

[15] Manthiram A, Swinnea JS, Sui ZT, Steinfink H,Goodenough JB. The influence of oxygen variation on thecrystal structure and phase composition of thesuperconductor yttrium barium copper oxide (YBa2Cu3O7�x).J Am Chem Soc 1987;109:6667e9.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 9 7 2 2e1 9 7 3 019730

[16] O'Hayre R, Barnett DM, Prinz FB. The triple phase boundary. JElectrochem Soc 2005;152:A439e44.

[17] Duan Z, Yang M, Yan A, Hou Z, Dong Y, Chong Y, et al.Ba0.5Sr0.5Co0.8Fe0.2O3�d as a cathode for IT-SOFCs with a GDCinterlayer. J Power Sources 2006;160:57e64.

[18] Tsoga A, Gupta A, Naoumidis A, Nikilopoulos P. Gadolinia-doped ceria and yttria stabilized zirconia interfaces:regarding their application for SOFC technology. Acta Mater2000;48:4709e14.

[19] Shim JH, Kim YB, Park JS, Prinz FB. Silver nanomesh as acathode for solid oxide fuel cells. ECS Trans 2011;35:2209e12.

[20] Huang H, Holme TP, Prinz FB. Oxygen reductioncharacteristics on Ag, Pt, and AgePt allows in low-temperature SOFCs. ECS Trans 2007;32:31e40.

[21] Guo Y, Zhou Y, Chen D, Shi H, Ran R, Shao Z. Significantimpact of the current collection material and methodon the performance of Ba0.5Sr0.5Co0.8Fe0.2O3�d electrodesin solid oxide fuel cells. J Power Sources2011;196:5511e9.

[22] Shannon RD. Revised effective ionic radii and systematicstudies of interatomic distances in halides andchalcogenides. Acta Crystallogr A 1976;32:751e67.

[23] Pauling L, Klug HP, Winchell AN. The crystal structure ofswedenborgite, NaBe4SbO7. Am Mineral 1935;20:492e501.

[24] Choi S, Yoo S, Kim J, Park S, Jun A, Sengodan S, et al. Highlyefficient and robust cathode materials for low-temperature

solid oxide fuel cells: PrBa0.5Sr0.5Co2�xFexO5þd. Sci Reports2013;3:2426.

[25] Liu M, Hu H. Effect of interfacial resistance on determinationof transport properties of mixed-conducting electrolytes. JElectrochem Soc 1996;143:L109e12.

[26] Adler SB, Chen XY, Wilson JR. Mechanisms and rate laws foroxygen exchange on mixed-conducting oxide surfaces. JCatal 2007;254:91e109.

[27] Adler SB. Factors governing oxygen reduction in solid oxidefuel cell cathodes. Chem Rev 2004;104:4791e843.

[28] Wang Z, Peng R, Zhang W, Wu X, Xia C, Lu Y. Oxygenreduction and transport on the La1�xSrxCo1�yFeyO3�d cathodein solid oxide fuel cells: a first-principles study. J Mater ChemA 2013;1:12932e40.

[29] Lee YL, Klies J, Rossmeisl J, Shao-Horn Y, Morgan D.Prediction of solid oxide fuel cell cathode activity with first-principles descriptors. Energy Environ Sci 2011;4:3966e70.

[30] Pavone M, Ritzmann AM, Carter EA. Quantum-mechanics-based design principles for solid oxide fuel cell cathodematerials. Energy Environ Sci 2011;4:4933e7.

[31] Suntvich J, Gasteiger HA, Yabuuchi N, Nakanishi H,Goodenough JB, Shao-Horn Y. Design principles for oxygen-reduction activity on perovskite oxide catalysts for fuel cellsand metal-air batteries. Nat Chem 2011;3:546e50.