Improved phase stability and electrochemical performance of (Y,In,Ca)BaCo3ZnO7+δ cathodes for...
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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: [email protected]
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.
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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.
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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.
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 019728
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.
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