materials
Article
Operation of Thin-Film Electrolyte Metal-Supported
Solid Oxide Fuel Cells in Lightweight and Stationary
Stacks: Material and Microstructural Aspects
Daniel Roehrens 1,3, *, Ute Packbier 2 , Qingping Fang 2 , Ludger Blum 2 , Doris Sebold 1 ,
Martin Bram 1,3 and Norbert Menzler 1
1
2
3
*
Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research,
IEK-1: Materials Synthesis and Processing, Jülich 52425, Germany; d.sebold@fz-juelich.de (D.S.);
m.bram@fz-juelich.de (M.B.); n.h.menzler@fz-juelich.de (N.M.)
Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research,
IEK-3: Electrochemical Process Engineering, Jülich 52425, Germany; u.packbier@fz-juelich.de (U.P.);
q.fang@fz-juelich.de (Q.F.); l.blum@fz-juelich.de (L.B.)
Christian Doppler Laboratory for Interfaces in Metal-Supported Electrochemical Energy Converters,
Jülich 52425, Germany
Correspondence: d.roehrens@fz-juelich.de; Tel.: +49-2461-61-96650
Academic Editor: Douglas Ivey
Received: 2 August 2016; Accepted: 2 September 2016; Published: 8 September 2016
Abstract: In this study we report on the development and operational data of a metal-supported
solid oxide fuel cell with a thin film electrolyte under varying conditions. The metal-ceramic structure
was developed for a mobile auxiliary power unit and offers power densities of 1 W/cm2 at 800 ◦ C,
as well as robustness under mechanical, thermal and chemical stresses. A dense and thin yttria-doped
zirconia layer was applied to a nanoporous nickel/zirconia anode using a scalable adapted gas-flow
sputter process, which allowed the homogeneous coating of areas up to 100 cm2 . The cell performance
is presented for single cells and for stack operation, both in lightweight and stationary stack designs.
The results from short-term operation indicate that this cell technology may be a very suitable
alternative for mobile applications.
Keywords: metal-supported solid oxide fuel cell; thin-film electrolyte; stack operation; gas-flow
sputtering; diffusion barrier layers
1. Introduction
Materials science and engineering has been a major contributor to the progress of fuel cell
technology [1]. Especially, solid oxide fuel cells (SOFCs) have attracted a great deal of interest because
of their fuel flexibility, versatility, and efficiency [2,3]. The advancement of oxide ion electrolytes and
the introduction of potent mixed ionic and electronic conducting (MIEC) electrodes has enabled a
reduction in operating temperatures [4–6]. This made it possible to incorporate metallic interconnects
into the cell design at substantially lower costs than for their ceramic counterparts [7,8]. Additionally,
the development of materials and microstructures led to significant improvements in terms of power
density and lifetime [9–12]. However, production costs for conventional ceramic SOFCs and limited
mechanical robustness remain limiting factors.
Recent advances in powder-metallurgy have led to the establishment of another SOFC concept: the
metal-supported SOFC (MSC) [13]. Here, the electrochemically active ceramic cell is constructed on top
of a porous, and usually highly corrosion-resistant, steel support. MSCs have been demonstrated to be
a promising technology for operation under non-stationary conditions because of their comparatively
high tolerance of thermal, mechanical, and chemical stresses [14,15]. Additionally, the incorporation of
Materials 2016, 9, 762; doi:10.3390/ma9090762
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Materials 2016, 9, 762
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Additionally,
the incorporation
of standardized
metal parts
as mass
a backbone
permits relatively
cheap
standardized metal
parts as a backbone
permits relatively
cheap
manufacturing,
which is crucial
mass
manufacturing,
which
is
crucial
in
terms
of
commercial
competitiveness
of
the
technology.
in terms of commercial competitiveness of the technology.
In
concepts have
have emerged
emerged enabling
enabling cheaper
cheaper manufacturing
manufacturing and
In recent
recent years,
years, many
many different
different concepts
and
assembly,
as
well
as
increased
mechanical
stability
[14,16–24].
Of
these,
the
Plansee
SE’s cell
design
assembly, as well as increased mechanical stability [14,16–24]. Of these, the Plansee
SE’s
cell
has
gained
attention,attention,
since it issince
manufactured
by a combined
deposition
design
has considerable
gained considerable
it is manufactured
by a sintering
combinedand
sintering
and
route,
leading
to leading
a very to
thin
electrolyte
membrane
being realized
[25–27].[25–27].
These These
cells have
deposition
route,
a very
thin electrolyte
membrane
being realized
cells been
have
manufactured
on
a
pilot
scale
and
achieve
power
densities
comparable
to
their
more
mature
been manufactured on a pilot scale and achieve power densities comparable to their moreceramic
mature
SOFC
counterparts
[28]. However,
this type this
of metal-supported
SOFC represents
a much younger
ceramic
SOFC counterparts
[28]. However,
type of metal-supported
SOFC represents
a much
technology
and
previous
electrochemical
characterizations
have
been
carried
out
mostly
a singleyounger technology and previous electrochemical characterizations have been carried out on
mostly
on a
cell
level or
foror
symmetrical
model
samples.
To our
knowledge,
results
single-cell
level
for symmetrical
model
samples.
To our
knowledge,
resultsofofelectrical
electrical performance
performance
characterization
published. In
characterization and
and long-term
long-term testing
testing in
in aa stack
stack environment
environment has
has so
so far
far not
not been
been published.
In this
this
study,
we
summarize
recent
results
with
the
Plansee
SE
MSC
both
in
terms
of
electrical
operation
study, we summarize recent results with the Plansee SE MSC both in terms of electrical operation for
for
single-cell
single-cell arrangements,
arrangements, as
as well
well as
as for
for lightweight
lightweight designs
designs and
and in
in stationary
stationary stacks.
stacks. Microstructural
Microstructural
features
dispersive
features before
before and
and after
after operation
operation were
were explored
explored with
with electron
electron microscopy
microscopy and
and energy
energy dispersive
X-ray
spectroscopy.
X-ray spectroscopy.
2.
2. Results
Resultsand
andDiscussion
Discussion
Single-Cell Test
Test
2.1. Single-Cell
Single-cell MSCs
MSCs with small-area cathodes (1 cm22)) have been characterized
characterized under various
Single-cell
◦ C in a
conditions [26]. An example is given in Figure 1. This cell was activated in-situ for 10 h at 850 °C
dual gas atmosphere (air/3%
(air/3%humidified
humidifiedhydrogen)
hydrogen) in
in order
order to
to achieve
achieve densification
densification and adhesion of
Sr0.40.4
Co
O(LSCF)
cathode
to the
barrier
layer.
the green La0.58
Co
0.20.2
FeFe
0.8O
cathode
to the
barrier
layer.
0.58Sr
0.83-δ
3-δ (LSCF)
2
Figure
Figure 1.
1. I–V
I–V curve
curve of
of aa planar
planar 55 cm
cm by
by 55 cm
cm2single
singlecell
cellMSC
MSCwith
withaa44µm
µmthin
thinlayer
layer8YSZ
8YSZelectrolyte,
electrolyte,
◦
aa Ni/8YSZ
anode
and
an
LSCF
cathode
at
850
°C
in
3%
humidified
hydrogen
after
10
h
Ni/8YSZ anode and an LSCF cathode at 850 C in 3% humidified hydrogen after 10 h of
of in-situ
in-situ
activation.
For
more
details
about
the
manufacturing
refer
to
Section
3.
activation. For more details about the manufacturing refer to Section 3.
Although 10 h at 850
°C is not sufficient to ensure a sintering of the A-site-deficient LSCF
Although 10 h at 850 ◦ C is not sufficient to ensure a sintering of the A-site-deficient LSCF cathode,
cathode, the power densities obtained from single-cell tests with hydrogen are in a range that is
the power densities obtained from single-cell tests with hydrogen are in a range that is comparable to
comparable to full ceramic SOFCs
(1.5 A/cm2 at 0.8 V and 850 °C). When the fuel gas is switched to a
full ceramic SOFCs (1.5 A/cm2 at 0.8 V and 850 ◦ C). When the fuel gas is switched to a system-relevant
system-relevant simulated diesel reformate (50% N2, 15% H2 14% CO, 11% H2O, 10% CO2) and◦ a
simulated diesel reformate (50% N2 , 15% H2 14% CO, 11% H2 O, 10% CO2 ) and a temperature of 750 C
temperature of 750 °C cell performances in the range2 of 200 to 630 mW/cm2 at 0.7 V were recorded
cell performances in the range of 200 to 630 mW/cm at 0.7 V were recorded [26], which is sufficient
[26], which is sufficient for the application of this type of cell in a mobile APU.
for the application of this type of cell in a mobile APU.
2.2. Lightweight Cassette Stack
A two-layer lightweight stack was set up with cells welded into the interconnector frame. After
the glass sealant crystallized during the joining process for 100 h at 850 °C, gas tightness was
achieved. The fuel gas was then set to humidified hydrogen and galvanostatic stack operation
Materials 2016, 9, 762
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2.2. Lightweight Cassette Stack
A two-layer lightweight stack was set up with cells welded into the interconnector frame. After the
33 of
during the joining process for 100 h at 850 ◦ C, gas tightness was achieved.
of 12
12
The fuel gas was then set to humidified hydrogen and galvanostatic stack operation commencing at
22, and a fuel utilization of 20%. The resulting I–V and performance
◦ C, 0.3 A/cm
2 , and
commencing
at
°C,
A/cm
commencing
at 750
750
°C,a0.3
0.3
A/cm
, and aoffuel
utilization
of 20%.
I–V curves
and performance
750
fuel
utilization
20%.
The resulting
I–VThe
andresulting
performance
are shown
curves
are
curves
are2shown
shown
in Figures
Figures 22 and
and 3.
3.
in
Figures
and 3.in
Materials
2016,
762
glass
sealant
Materials
2016, 9,
9,crystallized
762
Figure
2.
Performance
two
metal-supported
lightweight
at
750
under
Figure 2.
2. Performance
Performance of
of two
two metal-supported
metal-supported cells
cells operated
operated in
in aaa lightweight
lightweight stack
stack at
at 750
750 ◦°C
°C
under
Figure
of
cells
operated
in
stack
C under
galvanostatic
conditions.
galvanostatic
conditions.
galvanostatic conditions.
Figure
3.
and
I-P-curves
of
cell
of
the
lightweight
MSC
stack
after
joining,
measured
at
800
Figure 3.
3. I-VI-V- and
and I-P-curves
I-P-curves of
of cell
cell 111of
ofthe
thelightweight
lightweight MSC
MSC stack
stack after
after joining,
joining, measured
measured at
at 800
800 ◦°C
°C
Figure
I-VC
(a)
and
750
°C
(b).
◦
(a)
and
750
°C
(b).
(a) and 750 C (b).
The
The measured
measured IV-curves
IV-curves show
show aa significantly
significantly reduced
reduced performance
performance of
of the
the MSC
MSC (300
(300 mA/cm
mA/cm22 at
at
The measured IV-curves show a significantly reduced performance of the MSC (300 mA/cm2
0.8
0.8 V
V and
and 800
800 °C)
°C) as
as part
part of
of the
the stack
stack compared
compared to
to the
the single-cell
single-cell measurements,
measurements, both
both in
in terms
terms of
of open
open
at 0.8 V and 800 ◦ C) as part of the stack compared to the single-cell measurements, both in terms of
circuit
circuit voltage
voltage (OCV)
(OCV) values
values and
and the
the slope
slope of
of the
the voltage
voltage curve,
curve, which
which translates
translates to
to aa higher
higher areaareaopen circuit voltage (OCV) values and the slope of the voltage curve, which translates to a higher
specific
resistance.
A
lower
OCV
due
to
small
internal
leakages
may
be
attributed
to
variations
in
specific resistance. A lower OCV due to small internal leakages may be attributed to variations in the
the
area-specific resistance. A lower OCV due to small internal leakages may be attributed to variations in
production
production cycle;
cycle; the
the higher
higher ASR
ASR compared
compared to
to the
the single-cell
single-cell test
test is
is aa direct
direct result
result of
of the
the introduction
introduction
the production cycle; the higher ASR compared to the single-cell test is a direct result of the introduction
of
of several
several additional
additional components
components which
which are
are necessary
necessary for
for stack
stack operation,
operation, such
such as
as contacting
contacting oxide
oxide
of several additional components which are necessary for stack operation, such as contacting oxide
layers
and
or
interconnector
coatings.
Due
to
the
generally
larger
cell
sizes,
contact
geometry
layers and or interconnector coatings. Due to the generally larger cell sizes, contact geometry and
and
layers and or interconnector coatings. Due to the generally larger cell sizes, contact geometry and
gas-flow
gas-flow are
are different
different compared
compared to
to the
the single-cell
single-cell laboratory
laboratory experiment.
experiment. Additionally,
Additionally, neighboring
neighboring
gas-flow are different compared to the single-cell laboratory experiment. Additionally, neighboring
cells
cells influence
influence each
each other,
other, for
for example
example with
with respect
respect to
to temperature
temperature distribution
distribution [29].
[29]. These
These factors
factors
cells influence each other, for example with respect to temperature distribution [29]. These factors
may
may contribute
contribute to
to the
the earlier
earlier onset
onset of
of transport
transport limitation
limitation at
at aa current
current density
density of
of about
about 0.8
0.8 A/cm
A/cm22,, thus
thus
may contribute to the earlier onset of transport limitation at a current density of about 0.8 A/cm2 ,
explaining
explaining the
the difference
difference to
to the
the single
single cell
cell measurement.
measurement. However,
However, these
these issues
issues are
are common
common when
when
thus explaining the difference to the single cell measurement. However, these issues are common
comparing
comparing single-cell
single-cell experiments
experiments with
with stack
stack operation
operation and
and aa performance
performance loss
loss of
of 30%–50%
30%–50% is
is
observed
observed frequently
frequently [30].
[30].
In
In terms
terms of
of performance
performance degradation
degradation over
over time,
time, the
the data
data indicate
indicate different
different behavior
behavior comparing
comparing
cell
1
and
cell
2
in
Figure
2.
While
the
cell
voltage
for
cell
1
increases
by
5.2
mV
over
237
h
of
cell 1 and cell 2 in Figure 2. While the cell voltage for cell 1 increases by 5.2 mV over 237 h of operation,
operation,
which
which is
is probably
probably aa result
result of
of aa slight
slight improvement
improvement of
of the
the activity
activity at
at the
the cathode
cathode side,
side, cell
cell 22 shows
shows aa
progressive
progressive degradation
degradation of
of 18.62
18.62 mV
mV or
or 2.4%
2.4% over
over 237
237 h,
h, which
which equates
equates to
to 10.2%
10.2% over
over 1000
1000 h.
h. This
This was,
was,
Materials 2016, 9, 762
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when comparing single-cell experiments with stack operation and a performance loss of 30%–50% is
observed frequently [30].
In terms of performance degradation over time, the data indicate different behavior comparing
cell 1 and cell 2 in Figure 2. While the cell voltage for cell 1 increases by 5.2 mV over 237 h of operation,
which is probably a result of a slight improvement of the activity at the cathode side, cell 2 shows a
progressive
degradation of 18.62 mV or 2.4% over 237 h, which equates to 10.2% over 1000 h. This4 was,
Materials 2016, 9, 762
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however, not related to intrinsic defects on the cell level, since OCV values for both cells were similar
◦ C after the joining process, but rather were a direct result
at 1.011
1.011 V
V (cell
(cell 1)
1) and
and 1.015
1.015 V
V (cell
(cell 2)
2) at
at 750
750 °C
at
after the joining process, but rather were a direct result
of
an
external
leakage
and
subsequent
reoxidation
of cell
cell 22 that
during the
the course
course of
of the
of an external leakage and subsequent reoxidation of
that developed
developed during
the
experiment,
which
resulted
in
an
area
increase
in
dark
gray
oxidized
domains
and
some
cracks
in
experiment, which resulted in an area increase in dark gray oxidized domains and some cracks in the
the cathode
layer
in the
scanning
electron
microscopy
(SEM)analysis.
analysis.Cell
Cell1,1,on
onthe
theother
other hand,
hand, was
was
cathode
layer
in the
scanning
electron
microscopy
(SEM)
subjected
to
less
microstructural
degradation,
as
can
be
seen
in
Figure
4a.
A
cross-section
of
cell
is
subjected to less microstructural degradation, as can be seen in Figure 4a. A cross-section of cell 22 is
presented
in
Figure
4b.
presented in Figure 4b.
(a)
(b)
Figure
4. (a)
(a) SEM
SEM images
images of
of cross
cross sections
sections of
of cell
cell 11 after
after operation
operation at
at 750
750◦°C
under load
load (0.3
(0.3A/cm
A/cm22).).
Figure 4.
C under
The dark
dark gray
gray areas
areas within
within the
the metal
metal substrate
substrate ITM
ITM (intermediate
(intermediate temperature
The
temperature metal,
metal, Plansee
Plansee SE,
SE,
Reutte,
Austria)
are
comprised
of
chromium
and
iron
oxides;
and
(b)
SEM
images
of
cross-sections
Reutte, Austria) are comprised of chromium and iron oxides; and (b) SEM images of cross-sections of
of
cell
after operation
operation at
at 750
750 ◦°C
under load
load (0.3
(0.3 A/cm
A/cm22).).The
cell 22 after
C under
Thedark
darkgray
grayareas
areas within
within the
the metal
metal substrate
substrate
ITM
comprised of
of chromium
chromium and
and iron
iron oxides.
oxides.
ITM are
are comprised
Although the electrical performance of cell 1 did not show progressive degradation, an incipient
Although the electrical performance of cell 1 did not show progressive degradation, an incipient
corrosion of the metal substrate is visible as dark gray areas in Figure 4a (left). At the present time, it
corrosion of the metal substrate is visible as dark gray areas in Figure 4a (left). At the present time, it is
is unclear what the effect will be on the long term stability of the cell and the study of these issues is
unclear what the effect will be on the long term stability of the cell and the study of these issues is now
now part of research at the recently established Christian-Doppler Laboratory [31].
part of research at the recently established Christian-Doppler Laboratory [31].
The Ni/8YSZ, however, was not subjected to severe microstructural changes and the thin film
electrolyte was found to be crack-free for the entire surveyed area. Additionally, the LSCF-cathode
activated in situ shows good adherence to the electrolyte membrane and the porosity distribution
was found to be homogeneous.
2.3. Stationary Stack
In order to exclude influences characteristic of the lightweight stack design, an additional two-
Materials 2016, 9, 762
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The Ni/8YSZ, however, was not subjected to severe microstructural changes and the thin film
electrolyte was found to be crack-free for the entire surveyed area. Additionally, the LSCF-cathode
activated in situ shows good adherence to the electrolyte membrane and the porosity distribution was
found to be homogeneous.
2.3. Stationary Stack
In order to exclude influences characteristic of the lightweight stack design, an additional
two-layer stack was set up according to a well-known stationary stack design. For this test, a higher,
more system-relevant
current density of 0.5 A/cm2 was selected. Before galvanostatic operation,
Materials
2016, 9, 762
5 of an
12
in situ activation of the cathode was conducted at 850 ◦ C in dual gas conditions (air-side electrode:
air,
fuel-side air,
electrode:
stream)
forstream)
100 h.for
After
IV curves
were recorded
with
atmospheric
fuel-sideargon
electrode:
argon
100 activation,
h. After activation,
IV curves
were recorded
humidified
hydrogen
as
fuel
gas
at
different
temperatures
(see
Figure
5).
with humidified hydrogen as fuel gas at different temperatures (see Figure 5).
◦C
Figure
operation
in in
a stationary
stack
after
joining
at 800
°C (a)
Figure 5.
5. I-VI-V-and
andI-P-curves
I-P-curvesofofcell
cell1 1during
during
operation
a stationary
stack
after
joining
at 800
◦
and
750
°C
(b).
(a) and 750 C (b).
Both I-V curves show a significantly higher performance of the Plansee MSCs compared to the
Both I-V curves show a significantly higher performance of the Plansee MSCs compared to the
corresponding results from the lightweight stack (Figure 3). This is true for both cells from both stacks
corresponding results from the lightweight stack (Figure 3). This is true for both cells from both stacks
and can be explained by the improved electrical contact in the stationary design. Additionally, both
and can be explained by the improved electrical contact in the stationary design. Additionally, both
cells in the stationary design show a higher OCV-value because of improved internal and external
cells in the stationary design show a higher OCV-value because of improved internal and external
gas-tightness and a later onset of the gas-transport limitation, which is a result of a more efficient
gas-tightness and a later onset of the gas-transport limitation, which is a result of a more efficient
layout of the distribution manifolds and gas channels compared to the lightweight system.
layout of the distribution manifolds and gas channels compared to the lightweight system.
The initial cell characteristics, as shown in Figure 5, were very promising. However, during the
The initial cell characteristics, as shown in Figure 5, were very promising. However, during
operation of the stack elevated degradation was recorded (see Figure 6). Over the course of
the operation of the stack elevated degradation was recorded (see Figure 6). Over the course of
galvanostatic operation (43 h), cell 1 lost 31 mV or 4.15% of the initial value, while cell 2 degraded by
galvanostatic operation (43 h), cell 1 lost 31 mV or 4.15% of the initial value, while cell 2 degraded
61 mV (8.31%). Although the rate of performance loss in Figure 6 decreased considerably with
by 61 mV (8.31%). Although the rate of performance loss in Figure 6 decreased considerably
increasing time at 0.5 A/cm2, we2 decided to cool down the stack and investigate the cells’
with increasing time at 0.5 A/cm , we decided to cool down the stack and investigate the cells’
microstructure. Representative cross-sections are shown in Figure 7.
microstructure. Representative cross-sections are shown in Figure 7.
Although a slight internal leakage was detected after cooling down the stack, the thin 8YSZ
electrolyte membrane was intact along the analyzed cross-sections (see Figure 7). No significant
increase in local defect densities was observed compared to the initial state. Furthermore, the
cathode microstructure and contact to the electrolyte were well established. EDS analysis of the
cathode and electrolyte did not reveal any indications of a stoichiometric change in comparison to the
as-prepared state.
However, signs of progressive corrosion of the porous metal-support were found. The ITM
substrate exhibited a large number of dark gray areas (see Figure 7, left), which were identified as
Cr/Fe oxide phases by EDS analysis. These oxide areas are also found in direct contact with the
Figure 6. Performance of two MSC cells operated in a stationary stack at 750 °C under galvanostatic
2
cells in the stationary design show a higher OCV-value because of improved internal and external
gas-tightness and a later onset of the gas-transport limitation, which is a result of a more efficient
layout of the distribution manifolds and gas channels compared to the lightweight system.
The initial cell characteristics, as shown in Figure 5, were very promising. However, during the
operation
of the stack elevated degradation was recorded (see Figure 6). Over the course
of
Materials 2016, 9, 762
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galvanostatic operation (43 h), cell 1 lost 31 mV or 4.15% of the initial value, while cell 2 degraded by
61 mV (8.31%). Although the rate of performance loss in Figure 6 decreased considerably with
2, we barrier,
Gd
CeOat2 (GDC)
diffusion
insufficient
inhibition
the cation transport
increasing
time
0.5 A/cm
decidedindicating
to cool down
the stack
and ofinvestigate
the cells’
2 O3 -doped
under
these
conditions
(see
Figure
8).
microstructure. Representative cross-sections are shown in Figure 7.
Materials 2016, 9, 762
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Figure6.6.Performance
Performance of
oftwo
twoMSC
MSCcells
cellsoperated
operatedin
inaastationary
stationarystack
stackat
at750
750◦°C
undergalvanostatic
galvanostatic
Figure
C under
22).
conditions
(0.5
A/cm
conditions
(0.5 A/cm ).
Materials
2016, 9, 762
6 of 12
Figure 7. Cross-section of MSC 2 (see Figure 6) after operation in a stationary design.
Although a slight internal leakage was detected after cooling down the stack, the thin 8YSZ
electrolyte membrane was intact along the analyzed cross-sections (see Figure 7). No significant
increase in local defect densities was observed compared to the initial state. Furthermore, the cathode
microstructure and contact to the electrolyte were well established. EDS analysis of the cathode and
electrolyte did not reveal any indications of a stoichiometric change in comparison to the as-prepared
state.
However, signs of progressive corrosion of the porous metal-support were found. The ITM
substrate exhibited a large number of dark gray areas (see Figure 7, left), which were identified as
Cr/Fe oxide phases by EDS analysis. These oxide areas are also found in direct contact with the Gd2O3Figure
7.diffusion
Cross-section
of MSC
MSC
(see Figure
Figure
6) after
after operation
operation
in aaofstationary
stationary
design.
Cross-section
of
22 (see
6)
in
design.
doped CeO2Figure
(GDC)7.
barrier,
indicating
insufficient
inhibition
the cation
transport under
these conditions (see Figure 8).
Although a slight internal leakage was detected after cooling down the stack, the thin 8YSZ
electrolyte membrane was intact along the analyzed cross-sections (see Figure 7). No significant
increase in local defect densities was observed compared to the initial state. Furthermore, the cathode
microstructure and contact to the electrolyte were well established. EDS analysis of the cathode and
electrolyte did not reveal any indications of a stoichiometric change in comparison to the as-prepared
state.
However, signs of progressive corrosion of the porous metal-support were found. The ITM
substrate exhibited a large number of dark gray areas (see Figure 7, left), which were identified as
Cr/Fe oxide phases by EDS analysis. These oxide areas are also found in direct contact with the Gd2O3doped CeO2 (GDC) diffusion barrier, indicating insufficient inhibition of the cation transport under
these conditions (see Figure 8).
Figure 8. Cross-section
Cross-section of
of MSC
MSC 22 after operation in a stationary design exhibiting interdiffusion
between substrate/DBL/anode.
substrate/DBL/anode.
phenomena at contact interface between
These interdiffusion phenomena may have been caused by insufficient stability of the barrier
These interdiffusion phenomena may have been caused by insufficient stability of the barrier
material GDC under the applied conditions. Indeed, it has been reported previously that doped
material GDC under the applied conditions. Indeed, it has been reported previously that doped cerates
cerates are prone to oxygen loss and phase transformation under highly reducing atmospheres,
are prone to oxygen loss and phase transformation under highly reducing atmospheres, which enables
which enables interaction with neighboring materials or even decomposition [32].
interaction with neighboring materials or even decomposition [32].
Higher performance degradation rates, compared to full ceramic SOFCs, are a relatively
common phenomenon for MSCs and have been reported for various cell concepts by other groups
[13,21,33–35]. Reasons for this are in part the presence of the porous steel support, which may oxidize
or coarsen during operation and can, at least in part, interact with the catalytically active centers of
Figure(usually
8. Cross-section
MSC 2 after
operation
a stationary
design barrier
exhibiting
interdiffusion
the anode
Ni). Byofapplying
a suitable
andin more
redox-stable
layer,
the expected
phenomena
at
contact
interface
between
substrate/DBL/anode.
lifetime of the MSC will increase significantly, while electrical properties remain largely unaffected.
Materials 2016, 9, 762
7 of 13
Higher performance degradation rates, compared to full ceramic SOFCs, are a relatively common
phenomenon for MSCs and have been reported for various cell concepts by other groups [13,21,33–35].
Reasons for this are in part the presence of the porous steel support, which may oxidize or coarsen
during operation and can, at least in part, interact with the catalytically active centers of the anode
(usually Ni). By applying a suitable and more redox-stable barrier layer, the expected lifetime of the
MSC will increase significantly, while electrical properties remain largely unaffected.
3. Materials and Methods
3.1. Cell Design and Manufacturing
The2016,
cell9,design
Materials
762
is presented schematically in Figure 9. The first step in the production cycle 7isofthe
12
manufacturing of the 0.8 mm thick porous metal support (Cr26-Fe, ITM) by a powder-metallurgical
metallurgical
process.
To avoidofinterdiffusion
Feanode,
and Crthe
into
the anode,
the substrate
coated
process. To avoid
interdiffusion
Fe and Cr intoofthe
substrate
is coated
with a 500isnm
thin
with
500 nmCeO
thin2 Gd
2O3-doped
CeO
2 (GDC) layer
by magnetron
sputtering.
the
second
step,
Gd2 Oa3 -doped
(GDC)
layer by
magnetron
sputtering.
In the second
step, In
a 40
µm
multilayer
anickel/8%-Y
40 µm multilayer
nickel/8%-Y
2O3-doped ZrO
2 (Ni/8YSZ)
anode
is applied
to by
thescreen-printing
substrate by screenZrO2 (Ni/8YSZ)
anode
is applied
to the
substrate
and
2 O3 -doped
printing
and subsequently
annealedatmosphere.
in hydrogenThe
atmosphere.
Theofcomposition
of the
is
subsequently
annealed in hydrogen
composition
the anode paste
is anode
variedpaste
in each
varied
in
each
step
(finer
particles,
lower
porosity)
to
ensure
a
graded
improvement
of
the
surface,
step (finer particles, lower porosity) to ensure a graded improvement of the surface, which is necessary
which
necessaryoffor
application
the 4 µm in
electrolyte
in step 3(GFS).
via gas-flow
for the is
application
thethe
4 µm
electrolyteofmembrane
step 3 viamembrane
gas-flow sputtering
Finally,
sputtering
(GFS).
Finally,
a 40 µm
LSCFon
cathode
screen printed on top500
of anm
magnetron-sputtered
a 40 µm LSCF
cathode
is screen
printed
top of aismagnetron-sputtered
thin GDC diffusion
500
nm thin
diffusion
barrier. in
Each
stepbelow.
will be discussed in detail below.
barrier.
EachGDC
step will
be discussed
detail
Figure
9. Schematic
composition of
Figure 9.
Schematic representation
representation of
of the
the Plansee
Plansee SE
SE MSC.
MSC. Ratios
Ratios given
given for
for the
the composition
of the
the
anode
in
(2)
are
in
weight
percent.
anode in (2) are in weight percent.
3.1.1. Substrate
3.1.1. Substrate
The substrate (ITM) consists of an oxide dispersion-strengthened (OSD) ferritic steel containing
The substrate (ITM) consists of an oxide dispersion-strengthened (OSD) ferritic steel containing
26% chromium, which is alloyed with small amounts of titanium, molybdenum, and Y2O3 additions.
26% chromium, which is alloyed with small amounts of titanium, molybdenum, and Y2 O3 additions.
The material was prepared by a powder-metallurgical process and exhibits porosities of about 40%,
The material was prepared by a powder-metallurgical process and exhibits porosities of about 40%,
good corrosion resistance and a thermal expansion coefficient that matches SOFC materials well and
good corrosion resistance and a thermal expansion coefficient that matches SOFC materials well
was found to be independent of the porosity [36–38]. ITM was formed into sheets of 0.8 mm in
and was found to be independent of the porosity [36–38]. ITM was formed into sheets of 0.8 mm
thickness and different areas from 25 cm2 2to 100 cm2,2 which can be welded to the interconnector
in thickness and different areas from 25 cm to 100 cm , which can be welded to the interconnector
frame. Figure 10 shows a cross-section and a fracture image, in which the oxide crystals along the
frame. Figure 10 shows a cross-section and a fracture image, in which the oxide crystals along the
grain boundaries are visible.
grain boundaries are visible.
The material was prepared by a powder-metallurgical process and exhibits porosities of about 40%,
good corrosion resistance and a thermal expansion coefficient that matches SOFC materials well and
was found to be independent of the porosity [36–38]. ITM was formed into sheets of 0.8 mm in
thickness and different areas from 25 cm2 to 100 cm2, which can be welded to the interconnector
frame. Figure
10 shows a cross-section and a fracture image, in which the oxide crystals along
Materials
2016, 9, 762
8 ofthe
13
grain boundaries are visible.
(a)
(b)
Figure 10.
10. Cross
Cross section
section of
of the
the Cr26-Fe
Cr26-Fe substrate
substrate material
material ITM
ITM (a)
(a) and
and fracture
fracture image
image (b).
(b). The
The bright
bright
Figure
crystals
along
the
grain
boundaries
were
identified
as
yttria
and
titania.
crystals along the grain boundaries were identified as yttria and titania.
Materials 2016, 9, 762
8 of 12
The interdiffusion of cations from the metal substrate into the anode, and vice versa, can be a
significant issue in the manufacturing and operation of MSCs and may lead to severe performance
performance
degradation [39]. Fe
Fe and
and Cr, once transported into the anode, form alloys with the Ni-catalyst
Ni-catalyst and,
thus, reduce cell performance. A number of possible protective coatings for different steel supports
displaying sufficient
stability,
a matching
thermal
expansion
coefficient
(TEC), and
electrical
sufficientchemical
chemical
stability,
a matching
thermal
expansion
coefficient
(TEC),
and
conductivity
have been
explored
in the literature
and shown
be effective
[39–41].[39–41].
In this In
study
electrical conductivity
have
been explored
in the literature
and to
shown
to be effective
this
we
decided
to produce
a thin (0.5–1.0
µm) GDC
magnetron
sputtering
on top of
study
we decided
to produce
a thin (0.5–1.0
µm)layer
GDCbylayer
by magnetron
sputtering
onthe
topporous
of the
substrate,
which iswhich
shown
Figure
porous substrate,
is in
shown
in11.
Figure 11.
Figure 11.
11. SEM
Figure
SEM image
image of
of aa cross-section
cross-section of
of an
an MSC
MSC with
with aa magnetron
magnetron sputtered
sputtered GDC
GDC diffusion
diffusion barrier
barrier
layer
covering
the
metal
substrate.
layer covering the metal substrate.
3.1.2. Fuel-Side
3.1.2.
Fuel-Side Electrode
Electrode
In order
order to
to achieve
achieve aa graded
gradedtransfer
transferfrom
fromthe
thelarge
largeparticle
particleand
andpore
poresizes
sizesofofthe
theITM
ITMsubstrate,
substrate,
In
a
a
3-step
screen-printing
process
was
developed.
In
contrast
to
the
manufacturing
of
full
3-step screen-printing process was developed. In contrast to the manufacturing of full ceramicceramic
SOFCs,
SOFCs,relies
which
on atmospheric
of NiO,
screenpastes
printing
for theMSC
Plansee
MSC
which
onrelies
atmospheric
sinteringsintering
of NiO, the
screenthe
printing
forpastes
the Plansee
are based
aremetallic
based on
To avoid high-temperature
corrosion
of the metal
substrate,
on
Ni metallic
particles.Ni
To particles.
avoid high-temperature
corrosion of the
metal substrate,
sintering
has to
sintering
has
to
be
conducted
in
reducing
atmosphere
and
anode
porosities
have
to
be
defined
by the
be conducted in reducing atmosphere and anode porosities have to be defined by the organic content
organic
content
of the paste.
Sintering
is conducted
at 1180 atmosphere.
°C in hydrogen atmosphere.
◦ C in hydrogen
of
the paste.
Sintering
is conducted
at 1180
By gradually
of of
NiNi
in the
screen-printing
pastes
and adjusting
the solid
By
graduallyreducing
reducingthe
theparticle
particlesize
size
in the
screen-printing
pastes
and adjusting
the
load,
composition
and
paste
rheology
a
very
smooth
and
homogeneous
surface
was
achieved,
which
solid load, composition and paste rheology a very smooth and homogeneous surface was achieved,
is necessary
for the for
successful
application
of a thinof8YSZ
electrolyte
by gas-flow
sputtering.
A crosswhich
is necessary
the successful
application
a thin
8YSZ electrolyte
by gas-flow
sputtering.
section
image
of
a
sintered
3-layer
anode
is
presented
in
Figure
12.
A cross-section image of a sintered 3-layer anode is presented in Figure 12.
sintering has to be conducted in reducing atmosphere and anode porosities have to be defined by the
organic content of the paste. Sintering is conducted at 1180 °C in hydrogen atmosphere.
By gradually reducing the particle size of Ni in the screen-printing pastes and adjusting the solid
load, composition and paste rheology a very smooth and homogeneous surface was achieved, which
is necessary
Materials
2016, 9,for
762the successful application of a thin 8YSZ electrolyte by gas-flow sputtering. A cross9 of 13
section image of a sintered 3-layer anode is presented in Figure 12.
Figure 12. SEM-image of a cross section through the graded anode of an MSC before application of
the cathode.
3.1.3. Electrolyte
3.1.3.
Electrolyte Membrane
Membrane
Dense and
andthin
thin(4(4µm)
µm)
8YSZ
electrolytes
have
prepared
onoftop
the graded
Ni/8YSZ
Dense
8YSZ
electrolytes
have
beenbeen
prepared
on top
the of
graded
Ni/8YSZ
anode
9 of 12
2
2
anode
by means
of a gas-flow
for active
areas
of 84
upcm
to 84
cm . method
This method
allows
by
means
of a gas-flow
sputtersputter
processprocess
for active
areas of
up to
. This
allows
high
Materials 2016, 9, 762
deposition
rates rates
and aand
wide
rangerange
of possible
compounds
and isand
generally
considered
to be atomore
high
deposition
a wide
of possible
compounds
is generally
considered
be a
economic
physical
vaporvapor
deposition
(PVD)(PVD)
method
[42]. [42].
more
economic
physical
deposition
method
After GFS processing, the 8YSZ membrane shows a low defect density and leakage rates of less
3−1
−1 ·−2
−2 (∆p: 100 hPa) in air at room temperature have been obtained, which
3·s
10−4−4hPa·dm
hPa·dm
·s·cm
cm(Δp:
than 3 ×
× 10
100 hPa) in air at room temperature have been obtained, which is
is sufficient
the
electricaloperation
operationofofthe
thecell
cell[27].
[27].The
The8YSZ
8YSZlayer
layeradheres
adhereswell
wellto
tothe
the anode
anode surface
surface
sufficient
forfor
the
electrical
and is able to withstand small height
height variations
variations of
of the
the support
support even
even after
after operation
operation (see
(see Figure
Figure 13).
13).
Additionally,
that
thethe
thin
membrane
is flexible
withwith
respect
to mechanical
stressesstresses
which
Additionally,ititwas
wasfound
found
that
thin
membrane
is flexible
respect
to mechanical
may
arise
from
the
oxidation
of
the
anode
or
variations
in
thermal
expansion
coefficients
[43].
This
is a
which may arise from the oxidation of the anode or variations in thermal expansion coefficients [43].
phenomenon
which
is
characteristic
of
thin
membranes,
in
general,
and
was
recently
reported
for
full
This is a phenomenon which is characteristic of thin membranes, in general, and was recently
ceramic SOFCs
DueSOFCs
to the reactivity
of the reactivity
LSCF cathode
with zirconia,
densezirconia,
500 nm
reported
for full[44].
ceramic
[44]. Due to
of thematerial
LSCF cathode
materiala with
GDC
layer
is
deposited
by
magnetron
sputtering
on
top
of
the
electrolyte
in
a
second
step
to
avoid
the
a dense 500 nm GDC layer is deposited by magnetron sputtering on top of the electrolyte in a second
formation
of strontium
zirconates,
whichzirconates,
are detrimental
celldetrimental
performance
step
to avoid
the formation
of strontium
whichtoare
to [45].
cell performance [45].
Figure
13. SEM
SEMimage
image
a cross-section
through
anhighlighting
MSC highlighting
the GFS8YSZ
applied
8YSZ
Figure 13.
of aofcross-section
through
an MSC
the GFS applied
electrolyte
electrolyte
and
the
GDC
diffusion
barrier
after
operation.
The
cell
was
activated
in
situ
for
100
h
a
and the GDC diffusion barrier after operation. The cell was activated in situ for 100 h in a dual in
gas
◦
◦
dual
gas atmosphere
850operated
°C and operated
100 hC.at 750 °C.
atmosphere
at 850 Cat
and
for 100 h for
at 750
3.1.4.
3.1.4. Air-Side
Air-Side Electrode
Electrode
Cathodes
in thickness
thickness were
were applied
applied by
by screen
screen printing
printing on
on top
top of
of the
the electrolyte/diffusion
electrolyte/diffusion
Cathodes 40
40 µm
µm in
barrier
layer.
Srand
Fe-doped
lanthanum
cobaltites,
in
general,
and
A-site-deficient
LSCF, in
barrier layer. Sr- and Fe-doped lanthanum cobaltites, in general, and A-site-deficient LSCF, in particular,
particular, have been used for SOFCs of all kinds and lead to excellent cell performance [46,47].
However, due to the presence of the metal substrate, a conventional sintering route at temperatures
higher than 1000 °C in air is not possible for the MSC. Due to this, the cathode was kept in its green
state during stack assembly and the first heat-up of the cell. An in situ activation step (850 °C, 10 to
100 h) in dual gas conditions was applied in order to ensure sufficient adherence to the electrolyte
Materials 2016, 9, 762
10 of 13
have been used for SOFCs of all kinds and lead to excellent cell performance [46,47]. However, due
to the presence of the metal substrate, a conventional sintering route at temperatures higher than
1000 ◦ C in air is not possible for the MSC. Due to this, the cathode was kept in its green state during
stack assembly and the first heat-up of the cell. An in situ activation step (850 ◦ C, 10 to 100 h) in dual
gas conditions was applied in order to ensure sufficient adherence to the electrolyte and, thus, good
cell performance. The microstructure of the cathode after this activation step is shown in Figure 13
in Section 3.1.3.
3.1.5. Stack Assembly
Two MSCs with an active area of 84 cm2 each were assembled into a lightweight cassette design
that was developed for a mobile APU. The setup has been published earlier [26] and will only be
discussed briefly. The interconnector frame was made up of dense ITM and accommodates the
gas distribution manifolds. High temperature sealing was accomplished by using a conventional
glass-ceramic [48]. During the joining process, the two-layer short stack was heated to 850 ◦ C for 100 h
in a dual gas atmosphere under a mechanical load of 0.5 kN. After that, the temperature was set to
750 ◦ C and stack operation commenced under constant current conditions (0.3 A/cm2 ).
In addition to the lightweight setup, 10 × 10 cm2 MSCs were incorporated into a stack that was
developed for stationary applications. Details can be found in a previous publication [29]. The joining
process was identical to the lightweight-stack. Electrical operation was conducted galvanostatically
at a lower temperature (700 ◦ C) and under higher current density (0.5 A/cm2 ), compared to the
lightweight setup, in order to expose the MSCs to conditions that were more relevant to the system’s
point of operation.
For both stacks, humidified hydrogen was supplied to the fuel-side electrode (cassette stack
Uf = 20%, stationary-stack Uf = 40%).
3.1.6. Electron Microscopy
The SEM (scanning electron microscopy) images of polished cross sections were taken using a
Zeiss Ultra55 (Oberkochen, Germany) with INCA Energy 355 EDX (energy-dispersive X-ray) and
INCA Crystal EBSD (electron backscatter diffraction) detectors.
4. Conclusions
A metal-supported SOFC based on a porous steel substrate, a graded three-layer Ni/8YSZ
anode, and a thin film 8YSZ membrane was operated in two different stack designs. The cells
were characterized in terms of electrical performance and microstructural behavior. Single cell
IV-characterization of such cells with an LSCF cathode activated in situ displays power densities
of about 1 W/cm2 at 800 ◦ C and 1.5 A/cm2 . A two-layer lightweight MSC stack with an 84 cm2
active cell area was operated at 0.3 A/cm2 for more than 237 h at 750 ◦ C. IV characterization of the
cells activated in situ showed a reduction of the power densities obtained due to contacting and
gas-transport losses, which is in line with other studies. A further two layer MSC stack in a stationary
design was set up and operated at 0.5 A/cm2 for 43 h. For both stacks, one cell each showed increased
degradation rates, while the other one remained relatively stable. The increased degradation rate is
at least in part the result of setup related issues, which was confirmed by post-test microstructural
analysis of the cells. Even though cell degradation for this MSC concept was higher compared to
traditional full ceramic SOFCs, the first results from stack operation show performance data that are
sufficient for the operation of a mobile APU, while a future redesign of current diffusion barrier layers
will contribute greatly to extending cell lifetime.
Acknowledgments: The authors would like to thank the German Federal Ministry for Economic Affairs and
Energy (BMWi) for financial support as part of the MetAPU and NextGen MSC projects (contract No. 0327779F
and 0327867A); We would also like to thank Marco Brandner and Wolfgang Schafbauer, Innovation Services,
Plansee SE, Reutte Austria, for providing the MSCs, as well as for financial and technical support; Parts of the
Materials 2016, 9, 762
11 of 13
work were performed in cooperation with the Christian Doppler Laboratory for Interfaces in Electrochemical
Energy Converters. Equal shares in the funding of Christian Doppler Laboratories are borne by public authorities
and the companies directly involved in the laboratories. The most important source of funding from public
authorities is the Austrian Bundesministerium für Wissenschaft, Forschung und Wirtschaft (BMWFW). This
funding is gratefully acknowledged; Furthermore, the authors would like to thank Peter Batfalsky and Arnold
Cramer, Zentralinstitut für Engineering, Elektronik und Analytik (ZEA-1), Forschungszentrum Jülich GmbH,
Germany for assembly and post-test analysis of the MSC-stacks, as well as André Weber, Institut für Angewandte
Materialien–Werkstoffe der Elektrotechnik (IAM–WET), Karlsruher Institut für Technologie (KIT), Karlsruhe,
Germany for IV-characterization of MSC-single cells.
Author Contributions: Daniel Roehrens supported the stack-tests, SEM analysis and wrote the paper, Ute Packbier,
Qingping Fang and Ludger Blum carried out and supervised the electrical testing, Doris Sebold carried out the
SEM analysis, Martin Bram prepared visualizations and Norbert Menzler supervised the study.
Conflicts of Interest: The authors declare no conflict of interest.
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