ANODE SUPPORTED SOLID OXIDE FUEL CELLS WITH
NANO ION-CONDUCTOR INFILTRATION
Çiğdem Timurkutluk1, Bora Timurkutluk 1, 2, Mahmut D. Mat1 and Yuksel Kaplan1
1
HYTEM, Nigde University, Mechanical Engineering Department, 51245, Nigde, Turkey
2
Vestel Savunma Sanayi A.Ş. Silikon Blok Zemin Kat ODTU Teknokent, 06531, Ankara, Turkey
ABSTRACT
A high performance anode supported solid oxide fuel cell (SOFC) was developed by low-cost tape
casting and co-sintering and nano-ion conductor infiltration techniques. NiO/ScSZ anode support,
NiO/ScSZ anode functional layer and ScSZ electrolyte are tape casted and laminated isostatically.
After co-firing anode supported electrolyte structure at 1350° for 4h, the cathode is painted by
screen printing and sintered at 1000°C for 2h. The mixture of gadolinium and cerium nitrate
solution is then infiltrated into both anode and cathode layers and fired at a temperature that
gadolinium nitrate and cerium nitrate undergoes solid state reaction and forms nano ion conductor
phase in both electrodes. The effect of molar concentrations and firing temperature of nano ion
conductor phase on the cell performance are investigated. The performance results show that nanosized ion conductor infiltration significantly improves the cell performance. The cell provides
1.718Wcm-2 maximum power density at an operation temperature of 750°C. The high performance
is attributed to increase in the oxide ion conductivity and three phase boundaries of both anode and
cathode layers by nano ion-conductor infiltration.
1. Introduction
Wet impregnation/infiltration is a low cost performance enhancement technique for SOFC, based
on placing a drop of a solution of oxides/metals on the top of porous electrodes (anode and cathode)
and subsequent penetration of solution inside of the electrodes by capillary action. The system is
then fired at a temperature where solution undergoes solid state reaction to form nano particles
around the main structure of electrode. The loading of oxides/metals can be controlled by repeating
the impregnation process. Although there have been many studies on the enhancement of SOFC
electrodes by microstructure optimization, few have concentrated on nano-particles deposition into
porous electrode backbone. The most frequent oxides in this aspect are CeO2, Gd doped CeO2
(GDC) and Sm doped CeO2 (SDC). The wet impregnation of CeO2 and doped CeO2 are usually
applied by using Ce(NO3)3 or mixture of Gd(NO3)3 or Sm(NO3)3 and Ce(NO3)3 solution. After the
heat treatment at a suitable temperature, the formation of GDC or SDC phase occurs improving the
electrode electrochemical activity due to reduced electrode resistance by increased ion conductivity
as well as increase in the three phase boundaries resulting in remarkable performance improvement.
The studies on the wet impregnation have been mostly focused on the wet impregnation of cathode
by metals/metal oxides/precious metals to improve the cathode activity since the biggest
performance loss is due to the cathode for SOFCs. Yamahara et al. [1] studied the performance of a
cell based on cobalt infiltrated LSM cathode support. Cobalt was impregnated into porous
LSM/SYSZ (Sc0.2Y0.02Zr0.89O2) cathode support with cobalt nitrate solution. The maximum power
density of the cell without cobalt impregnation was measured as 130Wcm2, where as the cell with
cobalt impregnated cathode provides 0.270 Wcm-2 maximum power density at 700°C. Jiang et al.
[2] showed that 5.8 mgcm-2 loading of nano-sized GDC infiltration into LSM cathode results in
substantial reduction in the electrode polarization resistance. The electrode polarization resistance
of GDC impregnated LSM cathode was measured as 0.21Ωcm2 which is considered as significantly
lower than that of LSM/YSZ (2.5 Ωcm2) or LSM/GDC (1.1 Ωcm2) composite cathodes. Li et al. [3]
have developed GDC impregnated La0.74Bi0.10Sr0.16MnO3-x cathode. It is found that the polarization
resistance significantly decreased by impregnating the GDC. The cell without impregnation shows
0.790 Wcm-2 maximum power density at 750°C whereas the cell with GDC impregnated cathode
exhibits 1.607 Wcm-2 maximum power density at the same operation temperature.
Pd seems to be the most promising precious metal for impregnation into the cathode. Liang et al. [4]
fabricated a novel nano structured Pd/YSZ composite cathode by wet impregnation. The impedance
measurements indicated that Pd impregnated cathodes present lower activation energy (105kj/mol)
and electrode polarization resistance compared to conventional perovskite based cathodes. Simner
et al. [5] investigated the effect of Pt and Pd infiltration on the performance of a cell with SDC
interlayered LSF/SDC composite cathode. Although Pt addition had no important change, the cell
with 2vol%Pd impregnated LSF/SDC cathode lead to 50% performance improvement at 0.7V and
700°C compared to that of the cell with bare LSF/SDC cathode. Similarly, Liang et al. [6] found
that at 750°C the cell with Pd impregnated LSM/YSZ cathode exhibits 1.42Wcm -2 maximum power
density which is seven times higher than that of a cell with conventional LSM/YSZ cathode at the
same operation temperature.
Cu is usually employed as anode catalyst when hydrocarbon fuels are used due to high catalyst
activity for hydrocarbon reforming with suppressed carbon deposition. However, due to low
melting point of copper and high sintering temperature of YSZ, the conventional manufacturing
methods for Ni/YSZ anode are suitable for Cu/YSZ anodes. Thus, the wet impregnation seems to be
an effective technique to solve this problem. In addition to Cu, ceria is a key component for the
anodes under the operation of carbon containing fuels due to its high catalytic activity. Without
ceria the power density of the cell with 40% Cu anode found to be only 0.047Wcm-2 at 800°C when
hydrogen was the fuel [2]. Wang et al. [7] investigated the activity and stability of GDC
impregnated Ni anodes under wet methane fuel. The results indicated that the impregnation of nano
sized GDC particles is very effective for re-oxidation of deposited carbon on Ni surface because of
the effective distribution and dispersion of oxygen ions as compared to GDC phase in the Ni/GDC
cermet anodes. Park et al. [8] have improved the Wang study by addition of both Cu and ceria into
the YSZ matrix by means of wet impregnation. The cell produced 0.12Wcm-2 maximum power
density under n-butane fuel at 700°C. Similarly, Lu et al. [9] found that 10% addition of ceria in
Cu-SDC anode resulted in more than three-fold increased power density from 0.022 to 0.081 Wcm-2
at 650°C. On the other hand, Jiang et al. [10] have impregnated Pd into La0.75Sr0.25Cr0.25Mn0.5O3
(LSCM)/YSZ composite anode and investigated the performance under methane and ethanol fuels.
The results show that the impregnation of Pd nano particles dramatically decrease the electrode
polarization and increase the power output under both methane and ethanol fuels.
Although there are some studies focused on the wet impregnation of electrode containing YSZ,
GDC or SDC, there is little consideration of the electrodes including ScSZ. Therefore, this paper
investigates the effects of nano-sized GDC impregnation into both anode and cathode layers of the
ScSZ based SOFC cells on the cell performance. The molar concentration and firing temperature of
the GDC wet impregnation are also optimized.
2. Experimental
2.1 Slurry preparation and tape casting
A high purity electrolyte material 10% scandium stabilized zirconia (ScSZ) is purchased from
Nextech Materials and ball milled for around 24h with an organic dispersant, plasticizer and binder
to form a tape casting slurry. Similarly, commercial NiO/ScSZ (wt% 50:50) anode support and
anode functional layer (AFL) slurries including different quantity of active carbon pore former are
prepared. The slurries then are casted by a laboratory scale tape casting equipment. The anode
support, AFL and electrolyte tapes are stacked together and laminated under 20MPa pressure for 20
min via a laboratory press at ambient temperature. After final shaping by a laser cutter, a two stage
of co-sintering procedure is applied to fabricate an anode supported cell with thin ScSZ electrolyte.
In the first step, organic materials are burned with a slow heating schedule up to 800ºC. After this
stage, the half sintered electrolytes are transferred to the high temperature furnace. The fully
densified electrolyte is obtained after a relatively fast heating schedule (3ºC/min) up to 1400ºC.
2.2 Single cell preparation and testing
LSF-ScSZ (wt%, 50:50) cathode material is purchased from Nextech Materials and mixed with
ethyl cellulose and terpineol at a suitable ratio to prepare the screen printing paste. The paste is
them screen printed on the ScSZ electrolyte and sintered at 900ºC for 3h. The final cathode area is
1cm2 for performance test. The mixture of 10 mol % Gd(NO3)3 (99.9%, Aldrich) and 90 mol %
Ce(NO3)3 (99.9%, Aldrich) is prepared and wet impregnated into both porous anode and cathode
layers by a placing of a drop of the mixture five times. In order to investigate the effect of molar
ratio of the impregnation solution, five Gd(NO3)3 and Ce(NO3)3 impregnated anode supported
single cells are fabricated with different metal ion concentrations ranging from 0.5 to 2.5M. The
cells are then heated at various temperatures from 450 to 850°C to form GDC oxide phase and to
optimize the heat treatment temperature.
The anode supported single cells are then placed between two metallic interconnectors. Ag paste is
applied as a current collector and Pt wire as lead. The sample is placed in a temperature controlled
furnace with a push rod pressing capability. The cell performance was measured from 700 to 800°C
with hydrogen as a fuel and air as an oxidant. The I-V tests are conducted with a fuel cell test
station (ElectroChem Inc.) which have a temperature controlled furnace and push rod capability.
Microstructural investigation is conducted with SEM. Impedance measurements are performed with
an impedance analyzer (CHI Ins.) in a frequency range of 0.1Hz-250kHz.
3. Results and Discussion
3.1 Microstructure
Fig.1. shows the microstructure of the cross sections of the cell with impregnation and anode after
GDC impregnation. The microstructure of the cross section of the cell after impregnation is shown
in Fig. 1a. No cracking or delamination is observed. The electrodes show a typical porous structure
whereas the ScSZ electrolyte is seen to be uniform, continuous and dense. Fig.1b demonstrates the
cross section of the anode after 1.5M GDC impregnation. GDC particles appear to be discrete and
do not form a continuous network. In spite of the impregnation the anode has still porous structure
which allows hygrogen to reach all anode three phase boundaries.
Cathode
Electrolyte
GDC
Anode
(a)
(b)
Fig.1. SEM pictures of cross sections of (a) the cell before impregnation
and (b) cathode after impregnation
3.2 Effect of impregnation concentration
Fig.2. shows the performance results of the cells impregnated with different concentration of Gd
and Ce nitrate solutions at operating temperature of 700°C. The results of the cell without
impregnation is also given in the figure. The heat treatment of the impregnation is performed at
550°C for all cells tested. The current density and voltage behaviour of the cells are demonsrated in
Fig.2a. Except the 2.5M case, open circuit potentials are close to theoretical voltage indicating
ScSZ electrolyte has no cracks or pinholes. However, the cell impregnated with 2.5M solution
exhibits a lower a open circuit potential. The poor performance may be attributed to defects or
cracks in the electrolyte during the heat treatment of relatively high loading of the impregnation
solution due to mismatch of the thermal expension coeficient of GDC phase deposited on the ScSZ
electrolyte.
(a)
(b)
Fig. 2. Effect of impregnation molarities on the cell performance (a) I-V and (b) I-P curves
The current and power density characteristics of the cells are given in Fig.1b. It is seen that up to
1.5M impregnation the performance of the cell tends to increase with the increasing solution
concentration due to increase in ionic conductivities of both electrodes together with increase in
three phase boundaries. The cell without impregnation provides a higher power density than that of
both cells impregnated with 2 and 2.5M of solutions. This may be due to deactivation of the
electrodes since the GDC phase covers the catalyst surfaces and fills the pores of the electrodes
limiting both gas flows and electrical conductivity. However, at 700°C the cell impregnated with
1.5M solution provides a promising maximum power density of 1.343Wcm-2 almost doubling that
of the cell without impregnation which shows only 0.783Wcm-2 maximum power density. Thus,
the effect of nano ion conductor phase by wet impregnation is very impressive. The concentration
of the Gd and Ce nitrate impregnation solution is optimized as 1.5M according to the performance
results.
The impedance measurement of both impregnated and normal MEA’s are show in Figure 3. It is
seen that resistivity electrode significantly decreases after impregnation. The performance
improvement is seen to be result of decreased resistivity of electrode due to mainly high ion
conductivity of GDC and increased three phase boundaries. It is seen that ionic conductivity of
impregnated cell relatively lower then normal MEA. This may be attributed small differences in the
thickness of electrolytes in both MEA’s and small ohmic resistance drop in infiltrated electrodes.
Impregnated
Un-impregnated
Fig. 3 Impedance responses of impregnated and un-impregnated membrane electrode assemblies
3.3 Effect of firing temperature
Fig.4. shows the effect of firing temperature of the 1.5M impregnation on the cell performance at
700°C. It is seen that all cells impregnated exhibit higher performance than the cell without
impregnation. After firing temperature of 650°C, dramatic drop in the performance is observed. The
maximum power densities obtained depending on sintering temperatures are plotted in figure 5. It is
seen that after 6500C heat treatment the performance significantly decreases. This result may be
attributed to grain growth of GDC particle at high firing temperature and blocking the catalytic
surface.
Fig.4. Effect of sintering temperature of impregnation on the cell performance
Fig.5. Points of maximum power densities
700°C
750°C
800°C
Fig.6. Performance of the cell impregnated with 1.5M solution and fired at 550°C
Temperature dependent performances of the cell impregnated with 1.5M solution and fired at
550°C are shown in Fig.6. The cell is seen to exhibits very good performances at all temperatures
tested. The maximum power densities obtained at 700, 750 and 800°C are 1.343, 1.556 and 1.897
Wcm-2, respectively, whereas the cell without impregnation provides only 0.773 Wcm-2 at 700°C.
The improvement of the cell performance is ascribed to increase in the conductivities of both
electrodes due to addition of nano-sized ion conductor GDC phases as well as increased number of
three phase boundaries.
Conclusion
The effects of wet impregnation on the performance of a SOFC membrane electrode assembly are
investigated. Wet impregnation is performed by mixing cerium nitrate with gadolinium nitrate and
infiltration of the solution in to both anode and cathode electrode by capillary action. The MEA is
then sintered in a range of temperature to obtain a nano GDC particles in both electrodes. SEM
investigations showed that a nano scale GDC particles forms in the porous shell of electrodes. The
performance of MEA significantly increased with GDC impregnation due to increased three phase
boundaries and high ionic conductivity of GDC. It is found that high sintering temperature
adversely affects the performance due to grain growth at high temperatures. The best performance
is obtained at 550oC firing temperature. The performance of impregnated MEA’s also affected by
molar concentration of infiltration solution. At high molarities, the GDC phase covers catalytic
surfaces and lowers the performance. The best performance is obtained at around 1.5 M
concentrations. The impedance results show that performance improvements in impregnation
mainly result of resistivity drop of the electrodes.
Acknowledgements:
Authors would like to thanks to TUBITAK of Turkey (Project # 105M095) and EU commission
(project# SSA 032308) for financial contribution.
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