Mechanical Characterization of Solid Oxide Fuel Cells and Sealants

Figure
 3-5
 shows the linear relationship between the 
thermal expansion coefficients of Ni-YSZ cermet and increasing Ni content [41]. The mismatch 
in thermal expansion coefficients results in stresses, which can cause cracking or delamination 
during fabrication and operation [41]. These stresses can be released through creep deformation, 

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which however also threatens the stability of the SOFCs. In this work, the effects of the anode 
composition on the mechanical properties, especially the creep behavior, was investigated.
Figure 3-5: Thermal expansion coefficient of cermet anode as a function of NiO / Ni content [41]. 
One of the main parameters defining the actual conductivity requirement is porosity, which 
controls the gas transport [32, 42], however, although high porosity increases the conductivity, it 
can reduce the mechanical robustness. 
3.2.2.Anode failures
The failure modes of SOFCs are complex and influenced by a number of factors. The origin of 
the failures are thermal or chemical stresses which arise during manufacturing and operation [43].
The formation of damage in SOFCs is summarized in 
Figure
 3-6
.  

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12 
Figure 3-6: A generalized scheme of formation of failures in SOFCs [43]. 
The internal stresses within the cell layers are mainly a result of thermal gradients, 
electrochemical reactions and mismatches of material behaviors (thermal expansion coefficient, 
etc.) under changing environments [44]. These stresses can lead to fracture, deformation and 
delamination of the cell layers. For example, the thermal expansion coefficient of Ni-8YSZ 
anode with 65% wt. Ni is around 13.1 ∙ 10
-6
K
-1
from RT to 800°C, while the value of dense 
8YSZ is around 10.5 10
-6
K
-1
[40, 41]. This thermal expansion difference results in residual 
stress in anode and electrolyte layers and leads to creep deformation at operation temperatures, 
which threatens the long-term reliability of SOFCs. Additional stress will be induced by the 
constraints imposed by the sealants and interconnect due to differences in thermal expansion, 
thermal gradients and simply mechanical loads.
Another reason that could cause stresses is the volume change due to the re-oxidation. Re-
oxidation may occur at high temperature due to sealant damage causing a lack of fuel gas. The 
rapid oxidation causes a volume expansion of Ni particles in the YSZ matrix, which leads a 
volume increase of the anode compared to the initial oxidized state [45, 46]. Associated tensile 
stress/strain induced in the electrolyte layer leads to cracking as illustrated 

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Figure 3-7: SOFC half-cell: (a) sintered in air (oxidized state); (b) reduced state; (c) re-
oxidized state; (d) cracked electrolyte layer after re-oxidation [47]. 
3.3.
Sealants 
3.3.1.
State of sealants for SOFC application 
There has been an increasing amount of work on sealant materials searching for enhanced 
properties leading to a large number of research works in recent years [48]. Today, efforts 
concentrate on improving performance under the rather extreme operation relevant conditions, 
where sealants need to be designed for high temperatures and high-stress applications [16, 49]. 
Joining dissimilar materials represents a great challenge due the differences in physical 
properties, such as melting points and thermal expansion coefficients (CTEs), undesirable 
interactions, robustness problems, etc. [50]. The understanding of materials, microstructure and 
mechanical properties of bonded or welded joints leads to the necessity to reassess joining 
techniques.
With the fast development of electrochemical devices (i.e. SOFCs) and high-temperature 
separation membrane reactors, various sealants have been developed for high-temperature 
applications [51, 52]. The high operating temperature necessary for solid-state electrochemical 
operation (650°C - 1000°C) considerably limits the variety of sealing options; in principle 
organic or polymer seal cannot be employed, while inorganic materials with high melting points 

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can be easily and widely used at high temperature above 600°C [53]. An overview of 
conventional high-temperature seals is given in