ECS Transactions, 7 (1) 1527-1531 (2007)
10.1149/1.2729258, © The Electrochemical Society
Microstructure and Effective Thickness of Cermet Anode for SOFC
Hiroshi Fukunagaa, Tomohito Ohnoa, Arai Chikaoa, Toru Takatsukaa, Koichi Yamadab
a
Department of Fine Materials Engineering
Shinshu University, Ueda, Nagano 386-8567, Japan
b
The University of Tokyo, Bunkyo-ku, Tokyo 113-8654, Japan
The effective thickness or the active thickness of Ni-SDC cermet
SOFC anode was investigated for two types of anode with different
microstructures. One was prepared by using large particles, and the
other by small particles. The performance of each cell was
measured for various anode thicknesses. The performance of the
anode increased with increasing anode thickness when large
particles were used; effective anode thickness was as large as 120
µm. On the other hand, the performance of anode prepared from
small particles showed little dependence on anode thickness. The
large effective thickness of the anode was achieved by sufficient
paths for ions, electrons and gas, due to high sintering temperature.
The high performance achieved by the large effective thickness
indicates an alternative method for the improvement of the anodic
performance of SOFCs.
Introduction
Solid oxide fuel cell (SOFC) is highly efficient due to its high operating temperature,
and therefore is expected as one of the important power generation systems of the future.
To develop a high performance electrode for SOFC, two approaches can be considered.
One is from the viewpoint of material, and the other from the viewpoint of morphological
structure. The anode of SOFC is generally made from a so-called cermet, which is a
composite of ceramic and metal. At present, a cermet of Ni and YSZ (yttria stabilized
zirconia) has been often used for SOFC anode material. This is because Ni-YSZ cermet
exhibits thermal expansion and chemical compatibility with YSZ electrolyte, which has
been widely used for SOFC. Recently, rare earth doped ceria, such as Sm doped ceria
(SDC) and Gd doped ceria (GDC), is attracting much attention as an alternative
electrolyte material due to its high ionic conductivity at intermediate temperature (1,2 ).
Furthermore, doped ceria has been studied as a material for cermet in the anode and has
shown high performance (3,4). From the viewpoint of electrode microstructure, factors
such as porosity, surface area, and a so-called triple phase boundary (TPB) among
electrolyte, electrode and pores are regarded to be important. It is well known that
electrochemical reaction takes place near the TPB, and an electrode with large TPB
length shows low overpotential (5). Studies on electrode structure have reported that the
active TPB is distributed in three dimensions when a cermet material is used as the
electrode. The extension of the effective reaction zone (ERZ) into the direction normal to
the electrode layer has been reported to be several to tens of micrometers for Ni-YSZ
cermet (6-8). Doped ceria exhibits higher ionic conductivity than YSZ and it also exhibits
electronic conductivity in a reducing atmosphere (2,9). Therefore, it is reasonable to
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ECS Transactions, 7 (1) 1527-1531 (2007)
expect the effective thickness of Ni-SDC cermet anode to be larger than that of Ni-YSZ,
provided that a three-dimensional network of electronic conduction, ionic conduction,
and gas diffusion are formed sufficiently in the electrode layer. We have reported that the
enlargement of the effective thickness for Ni-SDC could be as much as 100 µm (10,11),
although the structure of this anode may have not been a typical one. In this paper, two
types of Ni-SDC anode were studied using different size of particles. Cells with various
anode thicknesses were prepared and the thickness dependence of its overpotential was
measured. From the results, the relationship between the microstructure and the effective
thickness of the anode is investigated.
Experimental
SDC disk with the dimension of 9.5 mm in diameter and 400 µm in thickness was
prepared for the electrolyte. The disk was cleaned by ultrasonic bath using detergent,
distilled water, and hydrogen peroxide. Powders of NiO and SDC (Sm2O3:CeO2=2:8)
were used as the cermet material. Two types of anode were made. One was made by large
particles (NiO 7 µm, SDC 4 µm: NS7), and the other by small particles (NiO 0.5 µm,
SDC 1 µm: NS0.5). Powders were mixed in a ratio of 3:2 and were ground with an agate
mortar and a pestle to obtain homogeneous mixture to use as anode material. Alphaterpineol and ethyl-cellulose were mixed in a 4:1 weight ratio to be used as a binder. The
binder was added to the powder and they were mixed together. The mixture was pasted
on the disk using a doctor blade method. It was dried in an oven for 10 minutes at 100˚C.
Then it was sintered in an electric oven at 1450˚C for 3 hours. Cells with anode
thicknesses of 50, 80, and 120 µm were prepared by repeating this operation for once,
twice and three times, respectively. Sm0.5Sr0.5CoO3 was used as the cathode material. It
was pasted on the other side of the electrolyte and sintered at 1200˚C for 2 hours. A
thermocouple of Pt and Pt-Rh wire was attached using Pt paste on the side of the
electrolyte. It was then sintered at 1000˚C for one hour. The Pt wire was used as a lead
from the reference electrode during the overvoltage measurements. The anode and the
cathode chamber were supplied with H2 and O2, respectively. The gas flow rate was
adjusted by a mass flow controller. Current density vs. voltage was measured using a
galvanostat (HZ-3000, Hokuto Denko or 2000, Toho Giken). The overpotential was
measured by current interruption method using an oscilloscope.
Results and Discussion
Performance of SOFC with Different Anode Thickness
The thickness dependence of the performance of the cell with NS7 anode was
investigated. The terminal voltage and the power density of the cell with different anode
thickness at 900˚C using H2 as fuel are plotted against current density in Fig. 1 (a). The
maximum current density increased by increasing the anode thickness, indicating an
increase in the effective reaction area. The value of the maximum power density is
plotted against the anode thickness in Fig. 1 (b). It can be seen that the effective anode
thickness was as large as 110 µm or even larger. On the other hand, it has been reported
by other researchers that the thickness dependence of Ni-YSZ anode is much smaller. For
example, Ihara et al. reported that the maximum power density of Ni-YSZ electrode
saturated at several tens of µm (6). The large effective anode thickness of Ni-SDC anode
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ECS Transactions, 7 (1) 1527-1531 (2007)
should be due to the high electron and ionic conductivity of SDC, by which extended the
reaction area of Ni-SDC compared to Ni-YSZ.
(a)
(b)
Figure 1. (a) Terminal voltage and power density at 900˚C of NS7 for anode thicknesses
of 50(■), 80(◆), and 110(●) µm; (b) Anode thickness dependence of maximum power
density of NS7 (○) and Ni-YSZ (6) (●) anode at 900˚C
Thickness Dependence of the Overpotential of Anode with Different Particle Sizes
The anode thickness dependence of overpotential of NS7 and NS0.5 at various
temperatures was observed. Interfacial resistance Ri was calculated as the slope of the
current density vs. overpotential curve when the overpotential was below 50 mV. Ri vs.
anode thickness for NS7 and NS0.5 is shown in Fig. 2 (a) and (b), respectively. As can be
seen in Fig. 2 (a), Ri of NS7 decreased by increasing the anode thickness at each
operating temperature. The dependence was larger when the temperature was lower. On
the other hand, Ri of NS0.5 in Fig. 2 (b) shows little dependence on anode thickness,
regardless of the operating temperature. The thickness dependence of NS7 and NS0.5
was completely different. The different dependence of the two anodes indicates that rate
determining step of the two electrodes is different. This is ascribed to the different
microstructures of the two electrodes.
Microstructure of the Ni-SDC Cermet Anode
SEM images of the cross section of NS7 and NS0.5 are shown in Fig. 3 (a) and (b).
As shown in Fig. 3 (a), the particles of NS7 were well sintered together and the neck
between the particles was large. On the other hand, as seen in Fig. 3 (b), the particle of
NS0.5 was small and was not sintered as well as NS7. This difference in the
microstructure of the anode can be attributed to the difference in starting particle size and
sintering temperature, which was 1450˚C for NS7 and 1250˚C for NS0.5. This difference
in microstructure resulted in the difference in the thickness dependence of the
overpotential.
The electrode reaction of SOFC takes place at the TPB, which is the interface of gas
phase, metal phase (electron conductor), and ceramic phase (ionic conductor). An
electrode with large TPB length shows low overpotential. Electrochemical reaction which
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ECS Transactions, 7 (1) 1527-1531 (2007)
occurs at the anode involves many elementary steps, such as gas diffusion, adsorption
and desorption of reactant and product species, surface diffusion of adsorbed species,
electron transfer, conduction of electron and ion, etc. Metals, ceramics and pores must
form a three dimensional network and must be connected to the reaction site in order for
electron, ion and gas, respectively, to participate in the reaction.
(a)
(b)
Figure 2. Interfacial resistance of NS7 (a), and NS0.5 (b) vs. anode thickness at 700 (▲),
800(■), and 900 (◆)˚C.
(a)
(b)
Figure 3. SEM image of the cross section of NS7 (a), and NS0.5 (b).
It had been thought that the effective thickness of SOFC anode is less than several
tens of µm. However, from our studies, the effective thickness of Ni-SDC cermet can be
larger if the electrode structure exhibits sufficient electronic, ionic and gas diffusion paths
throughout the electrode (10,11). With these sufficient paths, the high ionic conductivity
of SDC resulted in the extension of the effective anode thickness. On the other hand,
NS0.5 did not show such dependence on thickness above 50 µm. The anode overpotential
of NS0.5 was indeed smaller than NS7, when the anode thickness was small. The
electrode with small size particles exhibits high surface area, and thus the reaction site
per unit area must be large. However, presumably due to the percolation, the connection
of the path to the reaction site was not sufficient. Therefore the decrease in overpotential
with the increase in the electrode thickness was not observed.
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ECS Transactions, 7 (1) 1527-1531 (2007)
Using a large particle size as in NS7 of this study may not be a general approach to
enhance the anodic reaction. However, the fact that high power density was achieved by
such a structure indicates that this can be an alternative approach to develop SOFC anode.
A benefit of this type of structure is expected as a tolerance to degradation by sintering,
since the particle size is large. Moreover, this structure provides sufficient gas diffusion
path, which is important in anode support SOFC. These benefits indicate an alternative
method for the improvement of the anodic performance of SOFC.
Conclusions
The effective thickness of SOFC anode was investigated for Ni-SDC cermet. Two
types of electrode with different microstructure was prepared using different particle size
of NiO and SDC, and the thickness dependence on anode overpotential was measured.
The thickness dependence varied according to the microstructure of the electrode. The
electrode prepared from large particle size and by high sintering temperature (NS7)
showed large thickness dependence. On the other hand, the electrode prepared from small
particle size and low sintering temperature (NS0.5) showed little dependence on the
anode thickness. The effective thickness of NS7 appeared to be larger than 100µm,
whereas that of NS0.5 was smaller than 50 µm. The temperature dependence of the two
electrodes was different, indicating that the rate determining step varied by the
microstructure of the electrode.
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