Journal of Power Sources 131 (2004) 270 277
Relationship between electrochemical properties of SOFC cathode and
composition of oxide layer formed on metallic interconnects
a," b a a
K. Fujita , T. Hashimoto , K. Ogasawara , H. Kameda ,
a a
Y. Matsuzaki , T. Sakurai
a
Technical Research Institute, Tokyo Gas Co. Ltd., 16-25, Shibaura, 1-chome, Minato-ku, Tokyo 105-0023, Japan
b
Department of Applied Physics, College of Humanities and Sciences, Nihon University, Nihon, Japan
Received 3 October 2003; accepted 16 December 2003
Abstract
A Cr-poisoning of SOFC cathode was studied using half-cells with alloy separators of various Cr-content. The surfaces of the oxide
layer formed on the alloys were observed using energy dispersion X-ray analysis, and the precipitation of Cr at the interface between
YSZ electrolyte and the cathode was studied using electron probe micro analysis. All the Cr-containing alloys were found to promote
cathode degradation. However, the time dependencies of overvoltage in half-cell tests were different from one another in three kinds of
alloy materials. We found that the reason of the difference was in relation to the Cr2O3 and the MnCr2O4 in the oxide layer formed on the
alloy surface. Tests of single-cell stacks were also performed with these alloy separators. The degradation of stack voltage was large for
all the stacks, and that was consistent with the half-cell tests. The reason for the large degradation was found to be a formation of SrCrO4,
which was presumably synthesized by the reaction of Cr2O3 in the oxide layer on the alloy and Sr diffused from the cathode. Therefore, it
is necessary to develop a component in which the alloy does not contact the cathode directly.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Solid oxide fuel cell; Cr-poisoning; Metallic interconnect; Half-cell
1. Introduction 2. Experimental
The use of alloy materials for SOFC interconnects has 2.1. Preliminary evaluation of the oxide scale formed on
some advantages, such as reducing manufacturing costs, alloy surface
ease of handling the system and robustness to thermal
shock or thermal cycle [1,2]. We have been developing Three kinds of Cr-containing alloy materials, SUS430
anode-supported SOFCs and have attained high conver- (Nisshin Steel Co. Ltd., Japan), ZMG232 (Hitachi Met-
sion efficiency of 53% HHV [3,4] at 1023 K using the als Co. Ltd., Japan) and Inconel600 (Mitsubishi Material
Cr-containing alloy interconnects. However, long-term stack Co. Ltd., Japan), were annealed at 1073 and 1273 K in air
tests revealed that the use of Cr-containing alloy intercon- for 2 h to investigate the oxide layer formed on the alloy
nects degraded the performance of SOFCs [5], although an surface. Surfaces of the annealed alloys were analyzed by
excellent stability of single-cells without alloy separators X-ray diffractometry (XRD, BRUKER AXS Inc. M21X).
was confirmed [6]. Therefore, alloy separators are con- The chemical compositions of the alloy materials are listed
sidered to degrade the performance of SOFC [7,8]. Some in Table 1.
reports [9,10] have shown the deposited Cr at the interface
between electrolyte and cathode decreased the cathode per-
2.2. Half-cell tests
formance in half-cell tests. In this paper, we focused on
the relationship between the oxide layer formed on several
The degree of Cr-poisoning was investigated for each al-
alloys and the degree of Cr-poisoning.
loy using a half-cell measurement method. A schematic il-
lustration of the method is shown in Fig. 1. We used cylin-
drical pellets of electrolyte of 2 cm in diameter and 2 mm
"
in thickness. For the reference electrode, a platinum paste
Corresponding author.
E-mail address: k-fujita@tokyo-gas.co.jp (K. Fujita). of 0.3 mm in size was printed at the perimeter of the pel-
0378-7753/$ see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jpowsour.2003.12.050
K. Fujita et al. / Journal of Power Sources 131 (2004) 270 277 271
Table 1
Chemical compositions of the chromium-containing alloys used in this study
Chemical composition (wt.%) of the chromium-containing alloy
Cr Fe Ni Mn Si C P S Al La Zr Cu
SUS430 Nisshin Steel Co. Ltd. 16.03 82.27 0.29 0.91 0.42 0.06 0.018 0.001
ZMG232 Hitachi Metals Co. Ltd. 22.00 76.48 0.26 0.48 0.36 0.02 0.14 0.04 0.22
Inconnel600 Mitsubishi Material Co. Ltd. 14.00 6.00 77.80 1.00 0.50 0.20 0.5
let. For the cathode, La0.6Sr0.4MnO3 (LSM), which was
reported to easily lose activity by Cr-poisoning [11], was
screen-printed on the center of the electrolyte surface, and
sintered at 1423 K for 2 h. For the counter electrode, a plat-
inum paste was screen-printed on the other side of cathode,
and sintered at 1273 K for 10 h. Pt-mesh was set between the
cathode and the alloy separator. The flow-rate of air was set
2 l/min, in order to supply sufficient dry air into the half-cell.
An overvoltage and ohmic resistance were measured under
a current density of 0.3 A/cm2 at 1073 K.
At first, a half-cell test was performed using La0.6Sr0.3CrO3
(LSC) ceramic as a separator, which was reported to be
Fig. 2. Schematic illustration of measurement method for a single-cell
innocuous for the cathode. After that, three kinds of alloy
stack module.
materials were used as separator. The interface between the
cathode and the electrolyte was examined by electron probe
micro analyzer (EPMA, Shimadzu Co. Ltd., EPMA-870)
The single-cell was an anode-supported type having a thin
after the tests. The surface and cross-section of the alloy
electrolyte for reduced-temperature operation. The details
were analyzed by scanning electron microscope and en-
of preparation process were written in a previous report
ergy dispersion X-ray analysis (SEM/EDX, JEOL Ltd.,
[12]. The stack operation was conducted at a current density
EX-23000BU).
of 0.2 A/cm2 at 1023 K using dry hydrogen and air.
2.3. Single-cell stack tests
3. Results and discussion
The degradation of single-cell stack performances was
examined using ZMG232 and SUS430 separators. A 3.1. Preliminary evaluation of the oxide scale formed on
schematic illustration of the measurement method for a the alloy surface
cell stack module is shown in Fig. 2. The cell was man-
ufactured by co-sintering of electrolyte anode bilayer at Cr-containing alloys were annealed at high tempera-
1773 K, followed by firing an interlayer of Ce0.8Sm0.2O2-´ tures formed various oxide layers on the surface. The
and a composite of La0.6Sr0.4Co0.2Fe0.8O3/SDC cathode. Cr-containing alloy materials used in this study were a
Fig. 1. Schematic illustration of measurement method for Cr-poisoning using a half-cell.
272 K. Fujita et al. / Journal of Power Sources 131 (2004) 270 277
annealed at 1000ºC
annealed at 1000ºC
3.0
3.0
2.5
2.5
2.0
2.0
annealed at 800ºC
annealed at 800ºC
1.5
1.5
1.0
1.0
Inconel 600
MnCr2O4
: MnCr2O4
ZMG232
0.5 0.5
Cr2O3
:
Cr2O3
0.0
0.0
20 40 60 80
20 40 60 80
2 (deg.) / Cu-
2 (deg.) / Cu-
Fig. 5. XRD patterns for Inconel600 alloy surface.
Fig. 3. XRD patterns for ZMG232 alloy surface.
MnCr2O4 were also detected on the surface of SUS430 and
nickel-based alloy (Inconel600) and ferrite system stainless
Inconel600 annealed at 1273 K. Since the peaks of these
steels (SUS430 and ZMG232). The difference in chemical
oxides of SUS430 were sharp even at 1073 K, the oxide
compositions of SUS430 and ZMG232 can be characterized
layer on SUS430 was considered to be generated easily as
mainly by the quantity of Cr and Mn and the presence of
compared with that on ZMG232 and Inconel600. These re-
Al and Zr. Generally, if aluminum is contained about 2%
sults do not conflict with the reports [13,14] that ZMG232
or more in the alloy, Al2O3 oxide layer will be formed on
and Inconel600 have high temperature resistance. These
the surface at high temperatures, and the alloy will have
results indicate that the oxide layer of MnCr2O4 and Cr2O3
high oxidation resistance. Even if the Al content is low in
were formed on the alloy materials, and these oxides would
the alloy, Al diffuses toward the surface and maybe finally
affect the performance of a cathode in a half-cell test and a
forms an Al2O3 layer. Therefore, the Al-containing alloy is
single-cell stack operation.
considered to be not good for the separator since an electric
conduction pass will be intercepted by the Al2O3 layer. The
3.2. Half-cell test
Al2O3 layer, however, was not formed in a short term test
on ZMG232 in which Al content is 0.14%. The XRD pat-
Half-cell tests were performed for measuring the degrada-
terns of the oxide layer formed on the Cr-containing alloy
tion of the cathode by Cr-poisoning. The results are shown
materials are shown in Figs. 3 5. Cr2O3 and MnCr2O4 were
in Fig. 6. At first, the half-cell test was carried out using
detected on ZMG232 surface after annealing at 1073 K,
a LSC ceramic separator. The absolute value of overvolt-
and the intensity was stronger at 1273 K. Other oxides,
age (|OV|) decreased gradually with operation time. The de-
such as Al2O3 and SiO2, were not detected. Cr2O3 and
crease seems to be due to a current effect . This effect is
considered to be caused by a change in the microstructure
of the electrode [15], but the details for the mechanism have
annealed at 1000ºC
not been clarified. Since the LSC ceramic is chemically sta-
3.0
ble, the cathode did not degrade by vapor species from the
LSC. In the case of the ZMG232 alloy separator, the |OV|
2.5
increased rapidly in the initial stage at 7 h. The increase of
the cathode polarization was considered to be caused by
2.0
the Cr-poisoning. However, the |OV| of ZMG232 had a ten-
annealed at 800ºC
1.5 dency to decrease at around 30 h of the operation time. The
|OV| of Inconel600 and SUS430 increased with operation
1.0
time over 100 h, and did not decrease. In order to examine
the influence of Ni and/or Al in the alloy on the cathode
: SUS430
MnCr2O4
0.5
degradation, the half-cell tests were carried out using Ni and
: Cr2O3
16Cr 3.3Al alloy as the separators. As a result, the electro-
0.0
chemical activity of the cathode was not affected by the ele-
20 40 60 80
ments. Based on these results, it can be concluded that the de-
2 (deg.) / Cu-
crease of |OV| is related to the Cr2O3 formation on the alloy
Fig. 4. XRD patterns for SUS430 alloy surface. surface.
Intensity (Arb. Unit)
Intensity (Arb. Unit)
Intensity (Arb. Unit)
K. Fujita et al. / Journal of Power Sources 131 (2004) 270 277 273
Fig. 6. The dependencies of overvoltage of half-cell test.
3.3. Cathode/electrolyte interface after the half-cell tests 3.4. Oxide layer on the alloy surface after the half-cell test
Cross-sectional SEM/EPMA images of cathode/electrolyte Cross-sectional SEM/EDX images of the oxide layer
interface after the half-cell tests using LSC, SUS430, In- formed on ZMG232 surface after a polarization for 23 and
conel600, and ZMG232 are shown in Figs. 7 and 8. The 90 h are shown in Fig. 9(a) and (b), respectively. Table 2
deposition of Cr was observed at the interface in the case of shows the quantitative analyses results of the alloy surface
Cr-containing alloy separators, while Cr deposition was not measured by EDX. The results show that MnCr2O4 layer
detected with the LSC ceramic separator. The deposition was formed on the ZMG232 alloy surface, and Al2O3 pre-
of Cr at the interface should cause the increase of |OV| in cipitated in the ZMG232 alloy at around 20 m from the
the half-cell test, which is considered in previous reports surface. The MnCr2O4 layer and Al inner oxides increased
[9 11]. In conclusion, the main element in the alloy which with the polarization time. In the case of the ZMG232 alloy
caused the degradation is chromium. separator, |OV| showed relatively high resistance against
Fig. 7. SEM/EPMA images of the interface between YSZ electrolyte and LSM cathode using SUS430 and Inconel600 alloy after half-cell tests.
274 K. Fujita et al. / Journal of Power Sources 131 (2004) 270 277
Fig. 8. SEM/EPMA images of the interface between YSZ electrolyte and LSM cathode using (La,Sr)CrO3 ceramics and ZMG232 alloy after 24 h in
half-cell tests.
Cr-poisoning. These results suggested that Cr-poisoning is
Table 3
reduced by the oxide layer of MnCr2O4 or Al2O3. Figs. 10 Chemical compositions of LSCF cathode surface in contact with ZMG232
alloy separator after a single-cell stack test
and 11 show the cross-sectional EDX images of Inconel600
and SUS430, and Table 2 shows the quantitative analyses
Elements Point A (mol.%) Point B (mol.%)
results. The oxide layer of Mn could hardly be observed
Cr K 7.23 50.07
in Inconel600, while the oxide layer containing Cr and Mn
Fe K 36.56 0.62
are observed in SUS430. These results agree with the XRD
Co K 8.63
patterns as shown in Fig. 5. The MnCr2O4 layer formed on Sr L 12.63 46.82
La L 34.95 2.5
SUS430 would be thicker than that on ZMG232 from XRD
results, and |OV| with SUS430 was smaller than ZMG232
The points are shown in Fig. 13.
before 30 h. These results suggest that the MnCr2O4 layer
is effective to reduce Cr-poisoning in the initial stage.
Al2O3 oxide layer, showed a constant value as shown in
Next, we will discuss the turn to a decrease in |OV| using
Fig. 6. It could be concluded that the recovery of |OV| of the
a ZMG232 separator at around 40 h after polarization. From
cathode with ZMG232 separator resulted from Al contained
Table 2, one can see that the amounts of Cr and Mn on
in ZMG232.
the ZMG232 surface layer after 90 h and that of on the
SUS430 after 150 h are almost comparable. This suggests 3.5. Single-cell stack test
that the formation of MnCr2O4 layer is not enough to prevent
Cr-poisoning completely. Figs. 9(b) and 10 show that the The results of single-cell stack tests using SUS430 and
Al inner oxide is peculiar to ZMG232. The |OV| of the ZMG232 separators are shown in Fig. 12. The cell voltage
cathode with the 16Cr 3.3Al separator, which formed the using the ZMG232 separator decreased rapidly and recov-
Table 2
Chemical compositions of the various Cr-containing alloy surfaces after polarization
Cr-containing Time for half-cell Chemical composition (mol.%)
alloy sample testing (h)
Mn Cr Fe Si Al Ni
ZMG232 23 7.13 21.25 7.58 1.01 0.13
ZMG232 90 15.46 17.21 2.69 0.03 0.24
SUS430 150 14.22 20.07 3.71 0.11
Inconnel600 100 2.20 27.16 1.03 0.16 4.10
K. Fujita et al. / Journal of Power Sources 131 (2004) 270 277 275
Fig. 9. (a) Cross-sectional SEM/EDX images of oxide layer formed on ZMG232 after a polarization for 23 h. (b) Cross-sectional SEM/EDX images of
oxide layer formed on ZMG232 surface after a polarization for 90 h.
Fig. 10. Cross-sectional SEM/EDX images of oxide layer formed on SUS430 after a polarization for 120 h.
276 K. Fujita et al. / Journal of Power Sources 131 (2004) 270 277
Fig. 11. Cross-sectional SEM/EDX images of oxide layer formed on Inconel600 after a polarization for 120 h.
ered after 30 h similarly to the half-cell test. The stack volt-
age using SUS430 decreased with the operating time, which
was similar to the half-cell test. These results indicate that
the main degradation of the stack occurred in the cathode,
and the half-cell test was proved to be useful to study the
degradation of the cell stack. However, the terminal output
voltage of the single-cell stack using ZMG232 decreased
slightly even after 40 h, although the |OV| was recovering.
The reason would lie in the direct contact of the alloy and
the cathode in the stack. The EDX images of the cathode
surface contacted with the ZMG232 are shown in Fig. 13,
and the quantitative analysis results are shown in Table 3.
Oxides of Sr and Cr covered the cathode surface partially.
The ratio of Sr to Cr in the oxide was almost unity. In or-
der to investigate the oxide, mixed powder of cathode and
Cr2O3 were sintered at 1173 K for 50 h in air. The XRD
pattern was measured and shown in Fig. 14. Formation of
Fig. 12. Time dependencies of the voltage of single-cell stacks using
SrCrO4 was detected, which indicates that the contact of
ZMG232 and SUS430 alloy separator operated at 0.2 A/cm2.
Fig. 13. SEM/EDX images of LSCF surface in contact with ZMG232 alloy.
K. Fujita et al. / Journal of Power Sources 131 (2004) 270 277 277
3
Acknowledgements
40x10
LSCF
Part of this work was performed as R&D program of
Cr2O3
SrCrO4
New Energy and Development Organization (NEDO). We
30
would like to thank NEDO and Ministry of Economy,
Trade and Industry (METI) for their advice and financial
support.
20
References
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Calculation
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