Proc. of the 3rd European Solid Oxide Fuel Cell Forum, Nantes France, June 1998
INFLUENCE OF CURRENT DENSITY AND FUEL
UTILIZATION ON THE DEGRADATION OF THE
ANODE
Axel Müller°, André Weber*, Hans Jürgen Beie+, Albert Krügel*,
Dagmar Gerthsen°, Ellen Ivers-Tiffée*
* Institut für Werkstoffe der Elektrotechnik, University of Karlsruhe,
Adenauerring 20, 76131 Karlsruhe, Germany
° Laboratorium für Elektronenmikroskopie, University of Karlsruhe, Kaiserstraße
12, 76128 Karlsruhe, Germany
+
Siemens AG, Power Generation Group, Freyeslebenstr. 1, 91058 Erlangen,
Germany
Abstract
To investigate the efficiency and long term stability of Ni-YSZ-cermet anodes as
a function of current density and fuel utilization, various SOFC single cells were
operated under realistic working conditions for approximately 700h. The gas
composition present at different positions in a fuel cell stack, because of the
fuel utilization of the preceding cells, was simulated by adding a well known
amount of oxygen to the fuel (H2). After operation the microstructure of the
anodes was analyzed with optical microscopy and SEM. The nickel distribution
before and after operation was investigated by EDX and TEM. Nickel diffusion
into the electrolyte was studied by WDX. These investigations showed possible
degradation processes occurring in a Ni-YSZ-cermet anode and indicated the
limits of current density and fuel utilization, within which a degradation of the
anode may be tolerated.
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Proc. of the 3rd European Solid Oxide Fuel Cell Forum, Nantes France, June 1998
Introduction
Studies of power plants based on Solid Oxide Fuel Cells have shown that a fuel
utilization of at least 80 % is essential to achieve the required efficiency (1).
In the Siemens planar SOFC design a number of single cells are connected in
parallel in a layer, i.e. between two bipolar plates (2). The gas composition
varies as a function of location within each layer. This is because the single cell
at the gas inlet operates on nearly pure fuel, whereas the following cells
operate on mixtures of fuel and reaction products. As shown in fig.1, the
amount of fuel steadily decreases and the amount of water vapor (and CO2 in
the case of methane) increases from inlet to outlet. In the case of using
hydrogen as the fuel, the fuel utilization β is defined as:
β=
p(H 2 O)
p(H 2 ) + p(H 2 O)
single cells
O2-
O2-
O2-
O2-
cathode
electrolyte
anode
β
H2
H2+ H2O
fig.1 Local fuel utilization, i.e. amount of water vapor in a row of parallel single
cells. The first cell operates on pure fuel whereas the following cells operate on
a mixture of fuel and reaction products.
As a result of the changes in the effective oxygen partial pressure at the fuel
side, the Open Circuit Voltage (OCV) is a function of the local gas composition
in a layer, whereas all the cells in that layer are operating at the same voltage.
Therefore the voltage losses, and consequently the current density, depend on
the position of the cell in a layer.
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Proc. of the 3rd European Solid Oxide Fuel Cell Forum, Nantes France, June 1998
The aims of this study were firstly to determine the influence of current density
and fuel utilization on the efficiency and the long term stability of the anode, and
secondly to ascertain the limits of operation, i.e. values of current density and
fuel utilization which leads to a rapid failure of the anode.
Experimental
µ
The single cells consisted of a 150 µm thick electrolyte of yttria-stabilized
zirconia (YSZ: 8 mol% Y2O3-doped ZrO2) with a single layer La0.75Sr0.2MnO3
cathode and a Ni-YSZ-cermet anode (3). The electrodes were screen printed
onto the electrolyte substrate over an area of 4x4 cm² and exhibited a thickness
of about 30 µm after sintering at 1300 °C for 5 hours.
Pt mesh
furnace
air
cathode
electronic
load
DMM
anode
Ni mesh
H2
H2+ H2O
O2
β
β
fig.2 Schematic illustration of the measurement setup. The cell is situated in an
Al2O3-housing inside a furnace. The electrodes are contacted with a platinum
mesh (cathode) or a nickel mesh (anode). An electronic load provides the
current; the cell voltage is measured by a digital multimeter.
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Proc. of the 3rd European Solid Oxide Fuel Cell Forum, Nantes France, June 1998
β
The single cells were operated in the measurement setup shown schematically
in fig.2. The cathode was fed with air and the anode with a mixture of hydrogen
and water vapor (0.5 l/min each). The fuel utilization of the preceding cells in a
stack was simulated by injecting an appropriate amount of oxygen into the fuel
(fig.2). This approach allows gas compositions containing the desired amount
of water vapor to be fed to the anode. The measured OCV as a function of fuel
utilization i.e. water vapor partial pressure, shows good agreement with the
theoretical value calculated from the Nernst equation (fig.3).
1.4
theoretical data
measured data
cell voltage U / V
1.2
1.0
0.8
0.6
0.4
0.2
0.0
oxidant: air (80 % N2 + 20% O2, 0.5 l/min)
fuel: H2 + H2O (0.5 l/min)
temperature: 950 °C
0
20
40
60
80
100
fuel utilization β / %
fig.3
OCV as a function of (simulated) fuel utilization. The measured cell
voltage agrees well with the theoretical values calculated from the Nernst
equation.
To obtain information about the efficiency and the losses occurring at the
anode, impedance spectra and I/V-characteristics using reference electrodes
were measured as a function of fuel utilization. Several cells were operated
under constant conditions (current density and fuel utilization) for about 700 h
to investigate the long term stability and degradation of the anode.
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Proc. of the 3rd European Solid Oxide Fuel Cell Forum, Nantes France, June 1998
All cells were analyzed after operation by a variety of analytical techniques.
Scanning
Electron
Microscopy
(SEM)
was
used
to
investigate
the
microstructure of the anode. The nickel-distribution inside the anode layer was
studied
with
optical
microscopy;
additional
information
about
nickel
agglomeration was obtained from Energy Dispersive X-ray analysis (EDX) and
Wavelength Dispersive X-ray analysis (WDX). WDX was also employed to
investigate nickel diffusion into the electrolyte substrate. Structural changes in
the submicron range were analyzed by Transmission Electron Microscopy
(TEM).
Results and discussion
The dependence of the cell voltage on the current density and the fuel
utilization, which was obtained from I/V characteristics is shown in fig.4.
1.2
0.8
0.6
0.4
cell voltage / V
1.0
0.2
0
0
200
curre
nt den
s
400
ity / (m
600
A/cm
²)
800 80
60
40
20
tio
iliza
t
u
l
fue
n/%
fig.4 Three dimensional plot of the cell voltage as a function of current density
and fuel utilization at a temperature of 950 °C with air as the oxidant.
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Proc. of the 3rd European Solid Oxide Fuel Cell Forum, Nantes France, June 1998
From these data the dependence of the fuel composition and the current
density on the position within a layer can be calculated. Fig.5 shows the local
fuel utilization and current density for an overall fuel utilization of 80% at certain
cell voltages.
80
500
60
0.7 V
0.75 V
0.8 V
400
300
40
200
20
fuel utilization / %
current density / (mA/cm²)
600
100
fuel
0
0
position
fig.5
current density and fuel utilization as function of position in layer for
different cell voltages.
At a cell voltage of 0.75 V the current density decreases from 460 mA/cm² to 90
mA/cm² as the local fuel utilization increases from 0 to 80 %. Under these
conditions cells with a single layer cathode (4) would reach an average current
2
density of 230 mA/cm . At a cell voltage of 0.8 V the average current density
2
would only be 93 mA/cm .
Long term experiments with different operation conditions i.e. current density
2
between 200 and 1000 mA/cm and fuel utilization between 3 and 83 % were
carried out over periods of 700 – 1000 h to simulate the behavior of single cells
on different positions in a stack. The observed increase in the anode losses is
given in table 1.
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Proc. of the 3rd European Solid Oxide Fuel Cell Forum, Nantes France, June 1998
Current density / (mA/cm²)
200
300
300
400
480
520
600
Fuel utilization / %
83
20
60
76
60
38
9
∆UAnode / (mV/1000h)
32
9.3
4
47.6
99.4
57.1
33.3
table 1 Increase in the anode losses during the first 700 to 1000 h
While rapid failure of the anode was not seen in any of the investigated cells,
the degradation rate at high current densities and fuel utilization is definitely too
high (5). However, it should be noted that the degradation rate decreased
during operation. For example, the degradation rate of a cell, which was
operated at a current density of 480 mA/cm² and a fuel utilization of 60 %,
decreased from 183 mV/1000h to 60 mV/1000h during the first 350 h of
operation. Therefore the given values in table 1 cannot be seen as typical for
the whole lifetime of a cell.
Investigations of the anode microstructure showed, that the degradation of the
anode can be related to microstructural changes occurring during operation.
Fig.6 shows an anode prior to operation, i.e. the anode has been sintered and
the NiO subsequently reduced to metallic nickel. The nickel particles have a
mean diameter (d50) of about 0.5 µm, and are homogeneously distributed. This
results in a large amount of three phase boundary (TPB), and hence the initial
anode losses are comparatively low.
fig.6
SEM micrograph and nickel distribution determined by EDX (Kα line,
512x384 pixel) in an anode layer before operation.
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Proc. of the 3rd European Solid Oxide Fuel Cell Forum, Nantes France, June 1998
Fig.7 shows the typical microstructure of an anode after long term operation at
high current density and fuel utilization. Agglomeration of the nickel particles is
apparent and was observed in all anodes investigated. The agglomeration of
the nickel particles leads to a decrease of the amount of TPB, resulting in an
increase in the anode losses (6). If the agglomeration were to progress further,
the electrical conductivity of the anode layer ought to decrease due to lack of
connectivity between the nickel particles, resulting in rapid failure of the anode.
However, the decrease in the degradation rate observed may indicate that the
rate of agglomeration similarly slows with time.
fig.7 SEM micrograph and nickel distribution (EDX) after operation (1100 h at a
current density of 520 mA/cm² and fuel utilization of 38 %). An agglomeration of
the nickel particles is observed.
TEM analysis, in contrast to the results obtained from SEM and EDX, indicated
that the contact between nickel and YSZ particles in the submicron range is not
significantly affected. Fig.8 shows a TEM micrograph of a typical contact area
between nickel and YSZ particles in an anode layer after operation for about
700 h. For all the samples investigated a homogeneous distribution of nickel
and YSZ was observed.
In addition, areas with a diameter smaller than 1 µm, consisting of a mixture of
fine grained nickel and YSZ particles were found within the anode layer (fig.8).
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Proc. of the 3rd European Solid Oxide Fuel Cell Forum, Nantes France, June 1998
These grains exhibited a grain size smaller than 40 nm. The influence of these
areas on the electrochemical properties of the anode is at present not known.
Ni + YSZ
Ni
Ni
YSZ
YSZ
300 nm
300 nm
fig.8 TEM micrographs of an anode layer after operation (700 h, 600 mA/cm²),
showing good contact between nickel and YSZ particles (left) and an area of
fine grained nickel and YSZ particles (right).
WDX analysis revealed nickel diffusion of approximately 10 µm into the YSZ
electrolyte. The comparison of several cells operated for various periods
indicated no change in the diffusion profile with operating time, i.e. the nickel
diffusion occurring during operation is negligible. It is therefore assumed that
nickel diffusion into the electrolyte occurs during the sintering of the electrodes.
Conclusions
The investigation showed that the long term behavior of Ni-YSZ-cermet anodes
is significantly influenced by the operation conditions i.e. current density and
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Proc. of the 3rd European Solid Oxide Fuel Cell Forum, Nantes France, June 1998
fuel utilization. Operation at a high current density and fuel utilization results in
an increased degradation rate, but none of the anodes showed a rapid failure.
The measured values of degradation are not valid for the whole lifetime of a
cell, because the degradation rate decreases with time. Operation under
moderate conditions is expected to decrease the degradation to a tolerable
value. At present, the observed agglomeration of the nickel particles within the
anode layer is expected to be the main reason of the degradation. To obtain
further information relating the interaction between the microstructure of the
anode to its efficiency and long term behavior, additional TEM investigations
are essential.
Acknowledgments
We like to thank our partners and colleagues for their help, particularly Volker
Zibat and Mohammad Fotouhi for the WDX analysis and TEM investigations,
and also Carmen Boxler for the assistance in preparing single cells for analysis.
References
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(3) E. Ivers-Tiffée et al, Electroceramics IV: Electroceramics and Applications IIII. Hg. R. Waser. Augustinus Buchhandlung, p719, (1994)
th
(4) A. Hahn et al, Proceedings 5 Int. Symp. on SOFC, Aachen, p595, (1997)
th
(5) A. Gubner et al, Proceedings 5 Int. Symp. on SOFC, Aachen, p844, (1997)
(6) T. Iwata, J. Electrochem. Soc., 143, p1521, (1996)
362