The Electrochemical and thermal performances of Ca3Co4O9-δ as a cathode material
for IT-SOFCs
K. Nagasawaa, O. Mentreb, S. Daviero-Minaudb, N. Preuxb,
A. Rolleb, and H. NakatsugawaaaGraduate School of Engineering, Yokohama National University, 79-5 Tokiwadai,
Hodogaya Word, Yokohama, Kanagawa, 240-8501, Japanb
UCCS - Unite de Catalyse et de Chimie du Solide – UMR CNRS 8181, Ecole Nationale
Superieure de Chimie de Lille, Batiment C7a – BP.90, 108, 59652, Villeneuve d'Ascq
cedex, FranceWith the aim of application to cathode material in IT-SOFC, 2D
misfit layered calcium cobalt oxide Ca3Co4O9-δ was studied using
XRD, SEM, thermal dilatometory and impedance spectroscopy.
Chemical reactivity of Ca3Co4O9-δ with electrolyte indicated goodcompatibility with CGO against generating impunity phase
CaZrO3 with YSZ. Thermal expansion coefficient of pure
Ca3Co4O9-δ exhibited 10.0-11.0×10-6 K-1, which is similar to that of
CGO electrolyte. Indeed, we verified a good adherence between
electrode thin layer and the electrolyte. Electrochemical propertyof Ca3Co4O9-δ pellet showed good performances, i.e. ASR values
are 3.0 and 24.5 Ωcm2 at 700 and 600 ºC in ambient air
respectively, in our experimental conditions.
Introduction
Solid oxide fuel cells (SOFCs) have received considerable attention due to highconversion efficiency in several fuel cell types, without adding of catalytic agent toimprove the kinetics. Recently, the intermediate temperature SOFCs (IT-SOFCs)
operating at 600-800 ºC are of interest for improvement of temperature controllability
and longer operation time of materials. In the background, it put forward that gadoliniumdoped ceria (CGO) for electrolyte material shows higher ion conductivity at intermediate
temperature (under 800 ºC) in comparison with that of conventional yttria-stabilized
zirconia (YSZ) used in the high temperature region (900-1000 ºC) (1-2).
Previous studies on cathode materials for SOFC have already recently focused on
Perovskite cobalt-based oxide such as La1-xSrxMnO3 (LSM) (3-5), La1-xSrxCoO3 (LSC)(6-8) and Sm1-xSrxCoO3 (SSC) (9-11), showing promising performances. At the sametime, it is known that perovskite cobalt oxide has incompatible aspect with electrolytes
such as thermal expansion coefficient, and chemical reactivity with zirconia.
Layered calcium cobalt oxide Ca3Co4O9-δ is most known for thermoelectric material
due to the high figure of merit ZT (ZT=S2σT/κ where S, σ, T and κ are seebeckcoefficient, electrical conductivity, absolute temperature and thermal conductivity
respectively) (12-14). The crystal structure is constituted of stacking rock-salt type
[Ca2CoO3] layers and CdI2 type pseudo hexagonal CoO2 layers alternately. The
constitutional formula can be represented [Ca2CoO3]pCoO2 (p value represents bH/bRS,
where bH and bRS are b axis of pseudo hexagonal and rock-salt layer respectively with p~0.62) (15-16). Furthermore, the misfit between each subseat along the b axis results in

an incommensurate modulated structure between elements of each seat adequately
described using the 4D superspace formalism.
From the oxygen deficient nature of the rock-salt layers and from the excellent electrictransport of similar [CoO2] layers in superconducting sodium-cobalt bronzes (17),
Ca3Co4O9-δ could be considered as a natural intergrowth between electronic and potential
O2- conducting units. We surmise that this new candidate for IT-SOFC cathode is wellfrom the point of view of electrochemistry, thermal expansion and chemical stability.
Those several points constitute the purpose of this study.
Experimental
Polycrystalline Ca3Co4O9-δ was synthesized using the conventional solid state reaction.
The stoichiometric mixture sample was prepared from reagent grade CaCO3 and Co3O4
powders as precursors and calcined at 880 ºC for 12 h in air. The obtained sample waspelletized by uniaxial pressing at 10 MPa and sintered at 880 ºC for 24 h in air.
Impedance measurements were performed on symmetrical cells with CGO electrolyte.
CGO pellets were formed by uniaxial pressing at 2.5 MPa, and sintered at 1200 ºC for 2 h
in air. Ca3Co4O9-δ ink was deposited on both side of the pellet by brush and dried at room
temperature. The samples were heat treated in three steps: the step was heated up fromroom temperature to 500 ºC during 12 h, next was up to 700 ºC during 2 h and last was
down to room temperature at 6 h. The cylindrical cell after heat treating was 10.57 mm indiameter and 1.73 mm in thickness.
XRD measurements (Siemens, D8) were carried out the powder or pellet samples atthe condition of 2θ=6-70º and 0.02º step using Cu-Kα radiation. The lattice parametersrefinements were carried out using JANA2006 program (18).
Thermal dilatometory was performed for examining thermal expansion coefficient(TEC) of cathode materials. The measurements were carried out the temperature range
from 150 ºC to 820 ºC.
Cross-section scanning electron microscopy (SEM, Hitachi Ltd, S-4700) was
performed for observation of the electrode/electrolyte interface, at 15 kV.
Impedance spectroscopy measurements were performed using the Solartron SI 1260
analyzer. Here, the cells were slightly pressed between golden plates used as current
collectors. Measurement temperature and oxygen partial pressure conditions were carriedout from 760 to 540 ºC at 50-60 K step and from 1 to 3.7×10-5 atm respectively. The
applied AC-amplitude was 50mV, under zero DC polarization, in a frequency range from
1.0 mHz to 1.0 MHz.
Results and discussion
The XRD pattern of powder Ca3Co4O9-δ, which was refined by Rietveld analysis, isshown in Figure 1. The superspace group was supposed to be X2/m(0,p,0)s0, and cellparameters resulted in high fitting at a~4.8249 Å, b~2.8247 Å, c~10.831 Å, β~98.065 °and p~0.6182 which obtained good agreement of Rp~3.57 % and Rwp~5.22 %. About thetwo unidentified peaks at 2θ=12.9 and 18.0 °, Grebille et al. had mentioned that theycorrespond to new satellite reflections in RS layer (19). On the other hand, Ling et al.
suggested that they might be small-unidentified impurities (20).
Previous studies have already shown the incompatibility between zirconia-based
electrolyte and perovskite cobaltite/manganate, La1-xSrxCoO3 (LSC) or La1-xSrxMnO3(LSM), leading to the formation of low conductive La2Zr2O7 or SrZrO3 by reaction with

Intensity

20

30

40

50
2θ (°)

60

70

Figure 1. X-ray diffraction patterns of polycrystalline Ca3Co4O9-δ and calculated profile
by rietveld refinement. Filled circle marks indicate unrecognized peaks (see text).
YSZ (21). Then, we carried out preliminary chemical reactivity tests between layered
Ca3Co4O9-δ and several electrolytes. Ca3Co4O9-δ powders were mixed in 50 mass% with
CGO or YSZ, pelletized by uniaxial pressing at 10 MPa and heat-treated for 100 h at 750ºC. Figure 2 shows corresponding XRD patterns. In the case of YSZ electrolyte, XRD
pattern clearly showed CaZrO3 and Co3O4 impurity peaks in addition to Ca3Co4O9-δ andother cobaltites. In other words, the reaction with YSZ electrolyte leads to the

(a)

(b)

Ca3Co4O9
CGO

Intensity

Intensity

Ca3Co4O9YSZ
CaZrO3
Co3O4

35

40

45 50
2θ (°)

55

60

35

40

45 50
2θ (°)

55

60

Figure 2. X-ray diffraction pattern in reactivity tests between Ca3Co4O9-δ and YSZ (a) or
CGO (b).

decomposition of Ca3Co4O9-δ. On the other hand, no impurity phase was observed
between Ca3Co4O9-δ and CGO electrolyte, confirming a good compatibility between them.
After deposition of the electrodes on CGO using the method described in ref. (22), thecross section was analyzed. Then, Ca3Co4O9-δ layer deposited on CGO pellet were fixed
by resin after the heat treating and sliced. Typical SEM image of the cross section isshown on Figure 3. It was observed that the adherence between the electrode andelectrolyte is good without any evidence for surficial cracks. The porosity appears of
medium quality. The thickness of cathode cross section is estimated to about 30-40 µm.
The comparison of TEC between electrode and electrolyte is constitutive attributealong with electrochemical properties. Figure 4 is shown the linear thermal expansion
and TEC for Ca3Co4O9-δ. It yields TEC values of about 10.0-11.0×10-6 K-1 in theinvestigated region. On the other hand, Du et al. (23) and Kharton et al. (24) mentioned
TEC values for CGO electrolyte of 11.1, 11.5 and 12.0×10-6 K-1 at 400, 600 and 800 ˚Crespectively. In fact, this apparently good mechanical compatibility is a contributing
factor for IT-SOFC cathode application.
We carried out the impedance spectroscopy measurements for Ca3Co4O9-δ electrodedeposited CGO electrolyte. Figure 5 shows Nyquist plots in dependence of several
oxygen partial pressures or temperatures. The fitting analysis of impedance spectra wasused by Zview 3.1 program. Due to the symmetrical cells geometry, the area specificresistance (ASR) was calculated from the total polarization resistance Rp and electrodesurface area S using the following: ASR = (Rp*S) / 2. In ambient air, the resulting ASR
values are 3.0 and 24.5 Ωcm2 at 700 and 600 ºC respectively. It is remarkable that these
resistances values are obtained without any particular effort provided to optimize themicrostructure and porosity of the electrode layers. This result indicates that the layered
Ca3Co4O9-δ has the enough capability to candidate of new cathode material in IT-SOFC.
In addition, it was checked by XRD after polarization (Temperature: from room
temperature to 800°C; pO2: from 1 to 10-5 atm.; total experimental time ~ 1month) thatthe electrode remains unchanged.

resin

Ca3Co4O9

CGO

50.0 µm
Figure 3. Scanning electron micrograph of cutting cross section of Ca3Co4O9-δ deposited
CGO pellet.

0.01

20
3

4

9

0.008

∆L/L0 (-)

Thermal expansion coefficient (10 K )Ca Co O

15

0.006
10
0.004
5

0.002

-6
-1

0
0
100 200 300 400 500 600 700 800 900

Temperature (ºC)

Figure 4. Liner thermal expansion and thermal expansion coefficient of Ca3Co4O9.

-250

(a)

-400

-20

T=700 ºC-25

pO2=2E-1 atm

kensa-705-air-50-2402.z
kensabis2-703-10-3-50-d1-0403.z

pO
kensabis3-703-10-4-50-d1-1203.z
kensa-705-air-50-2402.z
2=2E-1 atm -15
kensabis2-701-10-5-50-d1-2403.z
pO
kensabis2-703-10-3-50-d1-0403.z
2=2E-3 atm
=3E-4
atm
pO
kensabis3-703-10-4-50-d1-1203.z
2
kensabis2-701-10-5-50-d1-2403.z
pO
2=7E-5 atm

-200

(b)

-20
kensa-705-air-50-2402.z
T=700ºC
kensa-605-air-50-2402.z
T=600ºC
kensa-505-air-50-2502.z
T=500
ºC

-300

kensa-705-air-50-2402.z
kensa-605-air-50-2402.z
kensa-505-air-50-2502.z

-15

-150
1E-1

-5

1E-2
1E-3

1E1
1E-1

-100
0
0

5

Z’’ Ωcm2

Z’’ Ωcm2

-10
-10

-200

1E2

-5

1E1
1E-1

1E-1

0
10

15

20

0

5

10

15

20

25

-100
-50

1E-3

1E-1 1E-2

1E-4

1E2

1E-4

1E1

0

1E-1

0
0

50

100

150
Z’ Ωcm2

200

250

0

100

200
Z’ Ωcm2

300

400

Figure 5. Oxygen partial pressure (a) or temperature (b) dependence of impedance
spectra of Ca3Co4O9-δ deposited CGO pellet.
Conclusions
Electrochemical and thermal properties of layered cobaltite Ca3Co4O9-δ were
investigated for applying to cathode material of IT-SOFC. Chemical compatibility with
CGO is good, however incompatible with YSZ as well as other perovskite cobaltite.
Polarization resistance of Ca3Co4O9-δ cathode showed comparable value to otherconventional cathode materials. It was understood that the background of striking

impedance spectra was considerable near TEC values between electrode and electrolyte.
From a comprehensive standpoint, we revealed that 2D misfit layered calcium cobalt
oxide Ca3Co4O9-δ inhered the possibility of cathode material in IT-SOFC.
Acknowledgment
This work was carried out as a collaborative project between Yokohama national
university and Ecole nationale superieure de chimie de lille by the program of theconsortium japonais du college doctoral franco-japonais (CDFJ) in Ministry of education,
culture, sports, science and technology and Embassy of France in Japan.
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