development of mncoo coating with new aluminizing process...
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Development of MnCoO coating with new aluminizingprocess for planar SOFC stacks
Jung Pyung Choi*, K. Scott Weil, Y. Matt Chou, Jeffry W. Stevenson, Z. Gary Yang
Pacific Northwest National Laboratory, PO Box 999 Richland, WA 99352, USA
a r t i c l e i n f o
Article history:
Received 21 December 2009
Received in revised form
18 March 2010
Accepted 18 April 2010
Keywords:
SOFC
High temperature
MnCo
Aluminizing
Cr volatility
* Corresponding author. Tel.: þ1 509 376 3E-mail address: [email protected]
0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2010.04.110
a b s t r a c t
Chromia-forming ferritic stainless steels find widespread use as interconnect materials in
SOFCs at operating temperatures below 800 �C, because of their thermal expansion match
and low cost. However, volatile Cr-containing species originating from this scale can
poison the cathode material in the cells and subsequently cause power degradation in the
devices. To prevent this, a conductive manganese cobaltite spinel coating has been
developed, but unfortunately; this coating is not compatible with glass-based seals
between the interconnect or cell frame components and the ceramic cell due to reactions
between the coating and the glass. Thus, a new aluminizing process has been developed to
improve the stability of the sealing regions of these components, as well as for other
metallic stack and balance-of-plant components.
Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
1. Introduction components such as sealing glasses) [12,13], and increasing
Solid oxide fuel cells (SOFCs) are solid-state energy conversion
devices that produce electricity by electrochemical reaction of
fuel and air across an ionic conducting electrolyte membrane.
The operating temperature range of anode-supported SOFCs is
between 600w800 �C, making it possible to consider cost-
effective high temperature oxidation-resistant alloys as
replacements for conventional lanthanumchromate ceramics
for construction of interconnects in SOFC stacks [1e3]. The
chromia-forming ferritic stainlesssteelsare consideredamong
the most promising candidate materials for interconnect
applications due to their electrically conducting oxide scale,
appropriate thermal expansion behavior, and low cost [1e6].
For satisfactory long-term performance, however, there
remain several issues, which may include chromia-scale
evaporation [7,8] and subsequent cell poisoning [9e11], surface
instability (including oxidationand reactionswithneighboring
380; fax: þ1 509 376 2248.(J.P. Choi).2010, Hydrogen Energy P
electrical resistance due to scale growth on the metallic inter-
connects surface and potential spallation [14,15]. Newly
developed alloys, such as Crofer22 APU (manufactured by
ThyssenKrupp, Germany), are protected at elevated tempera-
turesvia formationof auniquescale comprisedof a (Mn,Cr)3O4
spinel top layer and a chromia or chromia-rich sub-layer
[14,16,17]. This spinel offers lowervolatility ofCr thanchromia.
However, this reduction is less than an order of magnitude.
Therefore, it appears that further improvement in long-term
scale stability is needed, particularly for SOFC stacks with an
operating temperature over 700 �C. For this purpose, protectiveinterconnect coatings are being developed which, when
applied to the cathode side, canhelp reduceCr volatility, as the
oxide scale growth occurs between the coating and the steel
substrate. These coatings can also minimize the decrease in
electrical conductivity caused by oxide scale growth by
reducing the oxidation kinetics of the steel. Sr doped
ublications, LLC. Published by Elsevier Ltd. All rights reserved.
Table 1 e The chemical composition of the ferriticstainless steels used in this study Wt%.
Fe Cr Mn Si C Ti P S La Nb
Crofer
22 APU
Bal 22.8 0.45 e 0.005 0.08 0.016 0.002 0.006 e
SS441 Bal 18.0 0.30 0.35 0.012 0.17 0.023 0.001 e 0.45
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lanthanummanganite, ferrite and chromite coating have been
reported to reduce the electrical resistance [18e20]. Although
Crdiffusion can occur through someperovskite coatings [8,21],
spinel protective coatings have shown very promising results.
Larring and Norby’s [21] work on Plansee Ducrolloy (Cr-5%Fe-
1%Y2O3), an interconnect alloy for high temperature
(900e1000 �C) SOFCs, indicated that a (Mn, Co)3O4 spinel layer
could be a promising barrier to chromium migration. Subse-
quently, Yang et al. [22,23] investigated thermal growth of
(Mn,Co)3O4 spinel layers with a nominal composition of
Mn1.5Co1.5O4 onto a number of ferritic stainless steels for
interconnect applications in intermediate temperature SOFCs.
Overall, it appears that the (Mn, Co)3O4 spinels are promising
coating materials to improve the surface stability of ferritic
stainless steel interconnects, minimize electrical resistance,
and reduce Cr volatility.
One of themain challenges associated with glass sealing of
ferritic stainless steel components is the seal glass material’s
reactivity with surface Cr-oxide, which can lead to formation
of alkaline earth chromate phase, which has a large thermal
expansion coefficient mismatch with the glass material and
the ferritic stainless steel. Therefore, authors did other sealing
research. [24,25] However, at this study, we will mention only
about glass sealing. MnCo spinel coatings can react with glass
sealants, potentially leading to weak bonding and poor seal-
ing. As an alternative, a new Reactive Air Aluminizing (RAA)
process is under development as a protective coating for
stabilizing sealing regions. This aluminized coating layer is
intended to prevent Cr-volatility/diffusion and to minimize
reactions with the glass sealant that decrease the sealing
performance and bonding strength. It also provides a non-
conductive layer to prevent shorting between adjacent inter-
connects in the SOFC stack. Typically, aluminizing layers are
prepared using vacuum heat treatment, reduction atmo-
sphere heat treatment, E-beam, or electro-plating methods.
All of these processes require controlled atmosphere heat
treatment, which increases production cost. On the other
hand, the RAA process under development by Choi et al.
Table 2 e The heat treatment combinations for processes AeD
Process Coating Heattreatment I
C
Spinel Al Re Ox Spin
A U e U e e
B e U e U U
C U U U e e
D U U e U e
Re, Reduction heat treatment; Ox, Oxidation heat treatment.
[26,27] simply requires heat treatment in air. This new tech-
nology simultaneously diffuses aluminum into the stainless
steel substrate and oxidizes aluminum on the surface. As
a result, the diffusion layer represents an aluminum reservoir
allowing for self-healing of any surface damage experienced
during stack assembly and operation.
The main theme of this paper is the application of these
two coatings (spinel and alumina) onto SOFC interconnects.
The primary challenge is the difference in the heat treatment
processes typically used for each coating. The (Mn, Co)3O4
spinel coating requires a preliminary reduction heat treat-
ment, followed by an oxidation heat treatment, which
densifies the coating via a reactive-sintering process in which
the spinel phase is re-formed. On the other hand, the RAA
process only requires an oxidation heat treatment. In this
paper, we discuss recent work performed to understand and
mitigate this challenge.
2. Experimental
The glycine-nitrate process (GNP), a proven means of obtain-
ing fine and homogeneous powders [28], was used for
preparing (Mn, Co)3O4 powders. Appropriate amounts of Mn
(NO3)3 and Co (NO3)3 solutions were mixed with glycine and
then slowly heated to the combustion point to form the
precursor ash. The ash was calcined in air at 800 �C for 4 h and
then attrition-milled. Pure Al powder (99.9%, Alfa Aesar) was
used for the aluminizing process.
Two different ferritic stainless steels were selected for the
dual coating development. The first steel, Crofer 22APU
(Thyssen Krupp VDM), was designed specifically for SOFC
interconnect applications. The second steel, AISI 441 is an
inexpensive stainless steel that is typically used for automo-
tive applications. However, recent studies indicate that it may
be a lower cost alternative for SOFC interconnects [29]. The
compositions of the steels are listed in Table 1. The coating
materials were applied to the steel surfaces in paste form.
Spinel or aluminum powder was mixed with a binder (organic
vehicle) and milled in a three-roll milling process to create
a homogeneouswell-dispersed paste. The pasteswere printed
onto the stainless steel substrates by screen printing, or
spraying. After application of the coating, the samples were
placed into a drying oven (80 �C) for about 1e2 h. In the case of
(Mn,Co)3O4 spinel coatings, the dried samples are typically
heat treated in a hydrogen furnace (pure H2 atmosphere) at
850 �C for 4 h, and then oxidized in air at 760e800 �C. RAA
.
oating Heattreatment II
Heattreatment III
el Al Re Ox Re Ox
U e U e e
e U e e U
e e U e e
e e e e e
0
250
500
750
1000
)stnuoC(ytisnetnI
18-0408> (Co,Mn)(Co,Mn)2O4 - Cobalt Manganese Oxide
10 20 30 40 50 60 70 80 90
2-Theta(°)
Fig. 1 e XRD result of the spinel precursor powder used in this study.
Fig. 2 e Cross sectional observation with SEM and EDS after 850 �C-4 h heat treatment in reduction atmosphere and 1000 �C-1 h oxidation heat treatment. (a) (Mn,Co)3O4 spinel layer, (b) EDX line scan of spinel coating.
Fig. 3 e Cross sectional SEM view of aluminized steel
surface; (a) chromium oxide layer present, (b) chromium
oxide layer not present.
Fig. 4 e SEM image and EDX results for spinel coating from
process B; (a) cross section of spinel coating. ‘a’ indicates
the EDX analyzed spot., (b) EDX result.
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coatings are typically heat treated in air at 1000 �C for 1 h. In
this study, a variety of heat treatment options were investi-
gated to gain insight towards the development of a satisfac-
torymethod for fabricating dual (spinel and alumina) coatings
onto SOFC interconnects. The various processing conditions
are shown in Table 2.
After the dual coating was processed, microstructural
analysis was done using scanning electron microscopy (SEM,
JEOL 5900LV) and energy dispersive X-ray spectroscopy (EDX).
The spinel precursor powder was analyzed by X-ray diffrac-
tion (XRD) prior to paste preparation.
To further evaluate the dual coating process, a coated
interconnect was also included in a single cell SOFC stack test.
3. Results and discussion
3.1. Structure and coating mechanism of (Mn, Co)3O4
and alumina
As mentioned above, the precursor for the (Mn, Co)3O4 coat-
ings was an oxide powder. XRD analysis of the precursor
powder is shown in Fig. 1. To obtain satisfactory coating
density at low sintering temperatures, it is necessary to
initially heat treat the coating in a reducing atmosphere.
During this heat treatment, the spinel powder decomposes
into multiple phases, as shown in equation (1).
4Mn1.5Co1.5O4 þ 5H2[ 0 6Co þ 6MnO þ 5H2O[ (1)
After this heat treatment, the coating consists of a porous
mixture of fine MnO and Co grains. During the subsequent
heat treatment in air, a reaction-sintering process occurs that
results in higher conductivity and density for the coating than
would be the case if the spinel powder were subjected to the
air heat treatment alone. The mechanism of oxidation is
shown in equation (2).
6Co þ 6MnO þ 5O2[ 0 4Mn1.5Co1.5O4 (2)
As noted above, the RAA process only requires heat treat-
ment in an oxidizing atmosphere such as air. Themain goal of
this heat treatment is to control the oxidation and diffusion
processes. When Al powder is exposed to air at high temper-
ature, the surface of the powder is easily oxidized. However,
the cores of the aluminum powders, which are not oxidized,
will melt once the temperature exceeds the melting point of
aluminum. The associated volume expansion fractures the
Fig. 5 e Cross section SEM and EDX analysis of aluminized
layer from process B; (a) cross section. ‘a’ is EDX analyzed
spot., (b) EDX result.
Fig. 6 e SEM image and EDX results for spinel coating from
process C; (a) cross section of spinel coating. ‘a’ indicate the
EDX analyzed spot., (b) EDX result.
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oxide layer, allowing molten aluminum to flow and wet the
surface of the steel substrate. The molten metal reacts with
the substrate, and diffusion of aluminum into the substrate
occurs. In addition, oxidation at the surface is promoted due
to the elevated temperature. By controlling these two reac-
tions, the desired alumina layer can be obtained, with the Al,
which diffused into the substrate, remaining available to
assist self-healing if regions of the alumina layer are damaged.
3.2. Dual coating investigation
3.2.1. Process AAs indicated in Table 2, process A began with application of
a (Mn, Co)3O4 spinel coating onto the stainless steel followed
by a reduction heat treatment at 850 �C for 4 h in a pure
hydrogen atmosphere. During this heat treatment, the
uncoated area of the stainless steel surface oxidized and
formed an oxide scale. It would be possible to grind away the
oxide layer prior to performing the RAA process, but of course,
this would complicate the overall process and increase cost.
Therefore, the RAA coating was applied to the stainless steel
surface without grinding off the oxidized area. After the
1000 �C oxidation heat treatment, cross sectional analysis was
performed with SEM. Fig. 2 shows the microstructure of the
spinel coating and EDX line scan analysis. As seen in the Fig. 2,
Cr diffuses out from the stainless steel during heat treatment
to form a chromium oxide scale between the spinel coating
and stainless steel substrate but did not penetrate the spinel
coating. The EDX results for the coating are indicative of
a homogeneous oxide spinel layer. Fig. 3 shows SEM images of
aluminized regions after process A.
As shown in Fig. 3 (a), during the reduction heat treatment,
a chromium oxide scale formed on the exposed surface of the
stainless steel, which was not covered by the spinel coating.
However, as shown in Fig. 3 (b), some spots were not covered
by the chromium oxide layer. In these regions, the aluminum
diffused extensively into the substrate, forming the observed
protrusion. Based on these results, process A is as expected
frompreviouswork, suitable for the spinel coating process but
not for the RAA process.
3.2.2. Process BIn this process, the alumina coating was fabricated before the
spinel coating. After the oxidizing heat treatment of the RAA
coating at 1000 �C, the non-aluminized surface, as expected,
was covered by a relatively thick chromium oxide scale. The
spinel slurry was then printed onto this chromium oxide
layer. Then the standard spinel coating process (reducing heat
treatment followed by oxidizing heat treatment) was done.
Fig. 7 e Cross sectional SEM and EDX analysis after process
C. (a) Aluminized layer, (b) EDX line scan of alumina
coating.
Fig. 8 e Cross sectional SEM observation 1000 �C-1 h
oxidation heat treatment only. (a) porous spinel layer, (b)
aluminized layer.
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Process B resulted in a good aluminized coating. The spinel
coating also appeared to be coherent, but, as shown in Fig. 4,
the spinel coating was separated from the oxide scale. EDX
analysis indicated that no diffusion layer was present
between the spinel and chromium oxide layer. This may have
caused the observed weak bonding to the chromium oxide
layer, which would likely cause high electrical resistance. The
RAA aluminized layer appeared to be dense and continuous,
as shown in Fig. 5.
3.2.3. Process CFor Process C, the spinel and aluminum slurries were both
printed onto the stainless steel surface prior to any heat
treatments. Then, the reduction heat treatment was per-
formed, followed by the oxidation heat treatment at 1000 �C.From the results of processes A and B, it was established that
the chromium oxide scale on the steel was not desirable.
Therefore, Process Cwas intended to allow both coatings to be
applied to an essentially scale-free stainless steel surface. As
shown in Fig. 6, the spinel coating appeared to be of good
quality and well bonded to the chromia scale that developed
during the heat treatments. The aluminized surface also
appeared to be satisfactory (Fig. 7).
3.2.4. Process DFor Process D, the spinel and aluminum slurries were again
printed onto the stainless steel surface prior to any heat
treatment. In this case, however, only the oxidation heat
treatment was performed. The purpose of this process was to
eliminate the reduction heat treatment, thereby following the
standard RAA process. If this process were successful, it
would save energy, production cost and time. However, as
shown in Fig. 8 (a), the spinel layer did not densify. This is
consistent with previous work, which found that the spinel
coating required the preliminary reduction heat treatment to
prepare the coating for a reaction-sintering process during the
oxidation heat treatment. The highly porous coating obtained
through oxidation only is unable to prevent Cr volatility and
reduce scale growth kinetics under the coating. As expected,
the aluminized layer was the same as typically obtained via
the standard, oxidation-only RAA process (Fig. 8) (b). PNNL is
currently developing approaches intended to allow for fabri-
cation of dense spinel coatings via a single oxidation heat
treatment, so that both coatings (spinel and RAA) can be
prepared simultaneously with a single heat treatment.
3.3. Stack cell test result
Based on themicrostructural analysis of the samples from the
four processes, process C was selected for application of dual
coatings to an AISI 441 steel cathode-side “interconnect” for
an in-house SOFC single cell stack test. For this cell test, the
Fig. 9 e Result of single cell stack test.
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fuel was 97%H2e3%H2O fuel, while the oxidant was air and
this test was done at constant voltage of 0.7 V. The operating
temperature was 800 �C. Results of this stack cell test are
shown in Fig. 9. Overall, the specific power was stable during
the 2325 h duration of the test. However, until 200 h, we can
see fast degradation. This is not clear until now. But, from
some other studies, after the stack test, Cr does not observed
at the cathode side or near the cathode/YSZ interface using
SEM and TEM. Therefore, Cr-poisoning is ruled out from this
fast degradation. One possible cause was the coarsening of Ni
particles at the anode side which likely occurred after reduc-
tion. However, furtherwork is needed to understand the cause
for the initial rapid degradation. The microstructures of the
spinel and aluminized coatings after completion of this test
are shown in Fig. 10 (a) and (b). In the case of the alumina
layer, which was in contact with the glass sealingmaterial, no
chromate formation was observed at the glass/interconnect
interface. This lack of interaction is expected to improve the
long-term stability of the seals during both isothermal and
thermal cyclic operation.
Fig. 10 e Cross sectional SEM image after 2325 h single cell
stack test at 800 �C. (a) aluminized layer in contact with
glass sealing material., (b) spinel coating in contact with
cathode contact paste.
4. Summary
(Mn, Co)3O4 spinel coatings exhibit good performance as
conductive layers on the cathode side of stainless steel
interconnect. Aluminized layers improve the stability of glass-
based seals to interconnects. The primary challenge related
to the use of both coatings on the same part is the difference in
the heat treatment processes typically used for each coating.
The (Mn, Co)3O4 spinel coating requires a preliminary reduc-
tion heat treatment followed by an oxidation heat treatment,
while the RAA process only requires an oxidation heat treat-
ment. The present study was performed to investigate pro-
cessing options for co-fabrication of the spinel and RAA
coatings. The best results were obtained when both coatings
were applied prior to any heat treatment and then subjected
to sequential reduction and oxidation heat treatments.
Promising coating microstructures were observed following
a single cell test, which incorporated a cathode-side; inter-
connect plate that had been coated using that method
(Process C).
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Acknowledgements
The authors would like to thank to Karl Mattlin for heat
treatment of the stack plates and Caleb Ellefson for metallo-
graphic sample preparation. The U.S. Department of Energy’s
National Energy Technology Laboratory (NETL) funded the
work summarized in this paper as part of the Solid-State
Energy Conversion Alliance (SECA) Core Technology Program.
The authors would like to acknowledge helpful discussions
with Wayne Surdoval and Briggs White of NETL. Battelle
Memorial Institute operates PNNL for the U.S. Department of
Energy under Contract DE-AC06-76RLO 1830.
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