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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: jungpyung.choi@pnl.gov

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.

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 6 ( 2 0 1 1 ) 4 5 4 9e4 5 5 6 4553

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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