Microstructure Degradation of LSM: YSZ Cathodes for Solid Oxide Fuel Cells after Long Operation Time Using 3D Reconstructions By FIB Tomography and X Ray Fluorescence .pdf



Nom original: Microstructure Degradation of LSM: YSZ Cathodes for Solid Oxide Fuel Cells after Long Operation Time Using 3D Reconstructions By FIB Tomography and X- Ray Fluorescence.pdfTitre: Microstructure Degradation of LSM/ YSZ Cathodes for Solid Oxide Fuel Cells after Long Operation Time Using 3D Reconstructions By FIB Tomography and X- Ray FluorescenceAuteur: EHF Uni-Oldenburg

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Microstructure Effects
Tuesday, 30 May 2017: 13:40-17:00
Grand Salon B - Section 10 (Hilton New Orleans Riverside)
Chairs: Dagmar Gerthsen and Peter Vang Hendriksen
13:40
1577
Microstructure Degradation of LSM /YSZ Cathodes for Solid Oxide Fuel Cells after Long
Operation Time Using 3D Reconstructions By FIB Tomography and X-Ray Fluorescence
A. Zekri, M. A. Essafi, M. Knipper, T. Plaggenborg, and J. Parisi
(University of Oldenburg)

Solid oxide fuel cells (SOFCs) are getting more importance with their promising future,
due to their high-energy conversion efficiency , low- emissions and flexibility of usable
fuel type. The performance and the lifetime of SOFCs are keenly dependent on electrode
microstructure . In order to recognize the microstructural evolution and its degradation
kinetics in SOFC cermet cathodes during long exposure time (up to 20 000 h) under
realistic operating conditions ( T= 850 C, J= 190-250 ), investigations on porous LSM/
YSZ cathodes were conducted . The 3D- tomography technique (FIB /SEM ) offers
extensive data about the microstructures of various cathode aged during different
operating times (2 500 h, 15 000 h and 20 000 h), which allows an exact quantification of
particle size distribution, phase-connectivity, tortuosity factor and Triple Phase Boundary
Length (TPBL ). With the increasing operating time no significant 3D microstructural
changes in the cathode were noticed in the obtained data. However, additional qualitative
X-ray fluorescence measurement , indicate a clear presence of chrome contamination on
aged cathodes, which may be the main degradation mechanism in the SOFC cathode.
Keywords: chrome poisoning, long operating time, particle size distribution, SOFC,
triple phase boundary, SEM, XRF
Introduction
Solid oxide fuel cells (SOFCs) are electrochemical conversion devices that can be used to
provide electrical power properly and efficiently (1). As main components in the SOFC
system, the anode and the cathode play an important role for the system efficiency and
reliability. They have complex porous structures, which generally consist of three phases:
an electron-conductive phase (metal), an oxide-ion conductive phase (ceramic), and pores
for fuel transport. In Nickel (Ni) / Ceria Gadolinium Oxide (CGO) anodes, each phase is
a subject to one of the individual transport process mainly, oxide in CGO, electrons in Ni
or fuel in the pores. The electrochemical reactions take place in the so-called triple-phase
boundary (TPB), which presents the contact points between the three phases. Unlike
other types of fuel cells, SOFCs are able to operate by using different hydrocarbon as fuel
(CH4, CO, CO2 etc.) without using an external reformer, thanks to the high operating
temperature (up to 1000 °C). However, these relatively high operating temperatures can
lead to an accelerated degradation of the electrodes, and therefore may affect the
complete SOFC system negatively (2). Due to the harsh operating conditions, the SOFC
conversion efficiency steadily decreases by a few percent per 1000 h (3-5). One of the

main causes for this power loss is the anode degradation after a long operating time under
severe conditions. This degradation includes the microstructural changes that negatively
affect ionic and electronic conductivities (6-10). For these reasons, understanding SOFC
degradation mechanisms in the microstructure is essential to enhance durability and
reliability of SOFC system. The performance of the two electrodes is mainly dependent
on three different parameters of the porous microstructure; the TPB density, the phase
connectivity, and the tortuosity: those key microstructural features are then considered
major parameters for optimizing electrode microstructures, for increasing the
performance of cells and for extending their durability.
In the present study, we were using FIB-SEM tomography (11-16) for our quantitative
analysis of the aging effect on the microstructure of Ni-CGO anodes. This aging process
was carried out under realistic operation-conditions and with an extra-long operation time
(up to 20 000 h). For this purpose, we have improved the methodology for
microstructural analysis by means of SEM and image analysis techniques. Another less
discussed aspect in previous studies, which can occur during aging, is the agglomeration
of CGO. In the present paper, this aspect is been recognized and discussed. The
influences of CGO agglomeration on the tortuosity as well as on the percolation and the
TPB (triple phase boundary) are analyzed. These parameters (porosity, tortuosity,
percolation, and TPB length) obtained by 3D-reconstruction, are then calculated. The
dependence of these parameters on aging time is also presented and discussed. Some of
the quantitative results are then combined with additional qualitative observations in
order to better understand the complex degradation phenomena in nickel based cermet
anodes.
Experimental Procedure
Typical electrolyte supported SOFC cells were taken from stacks and analysed. H.C.
Starck Ceramics GmbH, Selb, Germany, manufactured the cells. Sunfire GmbH, Dresden,
Germany manufactured the stacks and provided the aged cells. The cells consisted of a
contact layer (Ni/CGO) of around 10 µm thickness, a functional layer (Ni/CGO) of
around 15µm thickness, a partially Yttria-Stabilized Zirconia (YSZ) electrolyte layer of
around 100µm thickness, and a layer of a strontium-doped lanthanum manganite
perovskite (LSM) cathode of around 35µm thickness. The cell sample 2 was aged under
H2, N2 conditions and for 2 500 h. The stack of cell sample 3 was operated under a
different fuel gas (Catalytic Partial Oxidation) and for 20 000 h. The fuel utilization of all
stacks was 75%. The parameters are summarized in Table 1.
TABLE I: Parameters of the aged cell for samples 1, sample 2 and sample 3
Sample

Operationtime [h]

Temperature
[°C]

Current
density
[mA·cm ]

Fuelgas

1

0

-

-

-

2

2 500

850

230

H2, N2

3

20 000

850

90-200

H2, N2, CH4, CO2, CO,
H2 O

Sample Preparation and the 3D reconstruction
The samples were first impregnated by low viscosity epoxy resin under vacuum
conditions so that an improved contrast could be generated between pore and solid phases.
Subsequently, the cured anodes were cut and then mechanically ground with SiC
abrasives ranging from 500 1200 grit and polished with 9, 6 and 1 µm diamond
suspension using MetPrep-3 (Allied High Tech) to prepare them for the FIB-SEM (FIB,
Helios NanoLab 600i, FEI, USA) observation. This was done in order to minimize the
sample drift during milling and to improve the stability of the imaging under an electron
beam. After the deposition of a conducting gold thin layer of 100 nm on the top of the
sample, the sample was glued onto an aluminium sample holder. Prior to the FIB slicing
process, a Platinum (Pt) protective layer of 4 µm was deposited using an in-situ liquid
metal ion source (LMIS). This was done in order to protect the surface of interest from
accidental ion milling and/or from erosion during ion beam imaging. The details of the
preparation of the surface of interest have been introduced in a previous study (17). In
order to avoid the charging effect during imaging and to distinguish between Ni and CGO,
an Everhart-Thornley-Detector (ETD) for backscattered electrons (BSE) with a low
voltage of 3 kV was used. The milling and imaging procedure for 3D-reconstruction is
very time consuming and may cause, during operation, a drift of the electron beam, stage,
and sample. Microstructural parameters such as porosity, tortuosity, percolation and
particle size distributions were then calculated based on the 3D data. An as-acquired cell
was used as a reference sample for comparison with the aged cells.
The images were first cropped and then aligned using the Stackreg ImageJ plugin (18) to
correct the drift in x-y directions. The median filter (2pixels) was applied to remove the
noise and improve the segmentation of the surface of interest. Each phase was first
segmented independently using a single threshold value and then some morphological
operations such as the opening process (erosion then dilatation) were applied to eliminate
the transition zone between the CGO phase and pores so that the segmented image
matches well with the initial grayscale image.
Results and Discussion
Porosity
The porosities of different samples are given in the Table II. All along the aging process,
the porosity increases by increasing the operation time. In fact, during reduction of NiO
in Ni, additional pores are formed, which may be the reason for the increase of the
porosity after 2 500 h as was expected. However, the porosity continues to increase from
38 vol. % after 2 500 h up to 45 vol. % after 20 000 h. This increase may be attributed to
the reoxidation of nickel particles (17, 19-21). It is generally admitted that volume
expansion during reoxidation is not fully reversible during re-reduction (22), which leads
to an increase in porosity throughout the entire anode microstructure. This reoxidation
and its accompanied increase of porosity led to an increase of the length of the anodes.
This is strongly depended on the aging time (23). The length increase was also shown on
the same sample using EDX-analysis (17). Thus repeated redox cycling, which occurs
during a long operation time, causes a continual increase on porosity and then an increase

of anode thickness. Macroscopically, the increase of the pore volume fraction may result
in an overall swelling or irreversible expansion as shown in previous work (17).
TABLE II: Porosity for sample 1, sample 2, and sample 3 given in volume percent
Aging time [h]

Porosity [%]

1

0

33.4

2

2 500

38.4

3

20 000

45.2

Sample

Percolation
To calculate the percolation of the different phases in anodes,
AmiraTM was used. Voxels from the same phase in
contact were regrouped to form clusters (see Figure 1). The results of the percolation of
different phases in all samples are given in Table III. Independent of the aging time, the
percolated fraction of the pore phase was about 99.0% for the pores, which indicates that
this phase was almost fully percolated and was not affected by the aging time. For the
nickel phase, the percolated fraction was for all four samples above 99.0 % while it
decreases for CGO Phase from 98.0 % in sample1 down to 87.3 % in sample 3 after 20
000 h. The reason of the percolation loss in CGO phase may be the agglomeration of the
CGO particles as shown in Figure 2. In fact, the agglomeration leads to a heterogeneous
redistribution of the particles and then to the forming of isolated ones. This percolation
decrease in the CGO phase can negatively affect not only the ionic conductivity, but also
the electronic conductivity. It could be considered the main degradation mechanism in
the microstructure of the SOFC anodes.
a)

b)

c)

Figure 1: visualization of percolated and non-percolated cluster in 3D reconstructed
volume of sample 1 for each phase: a) for the CGO phase, b) for the nickel phase and c)
for the pore phase.

a)

b)

Figure 2: SEM cross-sections of sample 1 (Reference sample, oxidized nickel in anode)
(b) and of sample 2 after 20 000 h of operation. The images were taken using BSE
detector at 3 kV.
TABLE III: Summary of the percolation values for the three phases in the three different
aged samples
Percolation

Pore [%]

CGO [%]

Ni [%]

sample1

98.6

98.0

99.9

sample2

99.0

96.6

99.7

sample3

99.2

87.3

99.4

Tortuosity
The tortuosity of each phase was calculated using Amira
module (24-26).
particle to the corresponding straight and shortest distance L along the direction of flux.
The tortuosity of the CGO phase was analyzed because it can affect the ionic transport
within the CGO phase of the Ni CGO composite (27). On the other side, the Ni
tortuosity was not taken into consideration because it is not expected that the electron
transport may limit anode performance, based on the relatively high electronic
conductivity of nickel and the relatively low Ni CGO layer thickness. Given that the
tortuosity is depending on the microstructures, this value is always higher than one.
Moreover, it should be noted that for a volume of interest, the tortuosities of solid (CGO)
and pore phases might be different. In the current study both pore and solid tortuosities
were measured and the results were plotted in Figure 12 and then summarized in Table
IV. The CGO phase showed a rapid increase in the tortuosity value by increasing the
aging time. This tortuosity reached a quite high value of 2.49 after 20 000 h of operating
time with an increase of about 67% comparing to the reference sample 1. The long path
lengths required to circumnavigate the huge agglomerates of CGO formed during the
aging may explain this high value. The change in the particle shape and/or having
features that are more complex in the analyzed phase (due to the big agglomerate) led to

the increase of the tortuosity values. In contrast to CGO, the pore phase shows a constant
tortuosity value of about 1.5 by increasing the aging time up to 20 000 h. This could be
explained by the further increase of the porosity (see above). There was always more
formed pore, which maintained the relatively good transport of fuel through the pore
phase.
TABLE IV: the tortuosity factors for the two phases CGO and pore calculated with
AmiraTM in the three aged samples
Sample

Tortuosity
Pore

Tortuosity
CGO

sample1

1.56

1.49

sample2

1.57

1.85

sample3

1.56

2.49

Triple Phase Boundary (TPB)
In our study, using the commercial image processing software AmiraTM, the centroid
length method was used for the evaluation of the TPB. Such method was introduced and
described by Shikazono et al. (28). Volumetric TPB densities for each sample are given
in Table V. TPB density for the reference sample was 3 µm.µm-3, these values is similar
to those obtained from FIB-SEM measurements of Ni-YSZ anodes using the same
method (28-30). It was shown that by increasing aging time from 0 h up to 20 000 h, the
TPB-density decreased gradually down to about 42 % after 20 000h of operating. This
decrease may be attributed to the CGO agglomeration and/or Ni-coarsening. In fact, due
to the formation of big CGO agglomerates, the particle distribution was negatively
affected and the CGO particles tended to agglomerate rather than to keep the contact with
nickel particles. This decrease in TPB density may be considered as an important factor
causing the performance loss of the cells.
a)

b

Figure 3: 3D visual representation of the three-phase boundary using AmiraTM software
a) in the reference sample 1 and b) in sample 3 aged after 20 000 h.

TABLE V: TPB densities of the three samples calculated using AmiraTM
Sample

TPB
[µm.µm-3]

sample1

3.08

sample2

2.21

sample3

1.78

Summary and Conclusions
The 3D microstructure of a SOFC anode aged up to 20 000h under real conditions was
successfully characterized with FIB/SEM tomography. The 3D volume was segmented
and then used to calculate several key parameters, such as porosity, particle size
distribution, tortuosity, three-phase boundary length, and phase connectivity. These
calculated parameters are critical for understanding the electrochemical conversion
efficiency, studying the electrode reliability, and improving manufacturing processes. At
present, the major drawback of the FIB/SEM tomography was that the whole electrode
could not be reconstructed because of the limitation of slicing method; instead, numerous
slices of the anode were imaged with a high z-resolution (up to 45 nm) to reconstruct an
adequate volume presentative for whole the electrode. The cell aging for very long time
up to 20 000 h under realistic condition allowed us to define the main degradation
mechanisms that may occur in the anode-microstructure. Some of them are listed as
follow:
CGO agglomeration: In addition to the nickel agglomeration, the CGO phase
tended to agglomerate too. This agglomeration had a negative impact on the
tortuosity and then on the ionic as well as the electronic conductivity. The
formation of big CGO agglomerates led to the connectivity loss of some particles.
The percolation of the ceramic part decreased due to the agglomeration from
about 98.0 % down to 87.3 % after 20 000h operating time.
Decrease on TPB density: Because of the nickel coarsening, as well as the CGO
agglomeration the number of the three phase boundary decreased dramatically
from 3.08 µm.µm-3! down to 1.78 µm.µm-3 after 20 000 h operating time. This
decrease had a direct negative impact on the performance of the complete cell and
could be the main degradation mechanism responsible for the performance losses.
Acknowledgement
The authors gratefully acknowledge the financial support from the German Ministry of
(Verbundvorhaben SOFC Degradation: Analyse der Ursachen und Entwicklung von
Gegenmaßnahmen) project (FKZ: 03SF0494E) and also from EWE AG, Oldenburg,
Germany. A part of the sample preparation was done at Jade Hochschule, Oldenburg

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