Fuel cells are the direct energy conversion devices which convert the chemical energy of a
fuel to electrical energy with much greater efficiency than conventional devices. Solid Oxide
Fuel Cell (SOFC) is one of the various types of available fuel cells; wherein the major
components are made of inherently brittle ceramics. Planar SOFC have the advantages of
high power density and design flexibility over its counterpart tubular configuration.
However, structural integrity, mechanical reliability, and durability are of great concern for
commercial applications of these cells. The stress distribution in a cell is a function of
geometry of fuel cell, temperature distribution, external mechanical loading and a mismatch
of thermo-mechanical properties of the materials in contact. The mismatch of coefficient of
thermal expansion and elastic moduli of the materials in direct contact results in the
evolution of thermal stresses in the positive electrode/electrolyte/negative electrode (PEN)
assembly during manufacturing and operating conditions (repeated start up and shut down
steps) as well. It has long been realized and demonstrated that the durability and reliability of
SOFCs is not only determined by the degradation in electrochemical performance but also
by the ability of its component materials to withstand the thermal stresses.
In the present work, an attempt has been made to evaluate the thermal stresses as a function
of thermal and mechanical properties of the component materials assuming contribution
from other factors such as thermal gradient, mechanical loading and in-service loading
conditions is insignificant. Materials used in the present study include the state of art anode (Ni-YSZ), electrolyte(YSZ) and cathode materials(LM and LSM) of high temperature SOFC
and also the ones being suggested for intermediate temperature SOFC Ni-SCZ as an anode,
GDC and SCZ as electrolyte and LSCF as the cathode. Variation of thermo-mechanical
properties namely coefficient of thermal expansion, and elastic and shear moduli were
studied using thermo-mechanical analyzer and resonant ultrasound spectroscope respectively
in 25-900°C temperature range. A non-linear variation in elastic and shear moduli- indicative
of the structural changes in the studied temperature range was observed for most of the
above mentioned materials. Coefficient of thermal expansion (CTE) was also found to
increase non-linearly with temperature and sensitive to the phase transformations occurring
in the materials. Above a certain temperature (high temperature region- above 600°C), a
significant contribution from chemical expansion of the materials was also observed.
In order to determine thermal stress distribution in the positive electrode, electrolyte,
negative electrode (PEN) assembly, CTE and elastic and shear moduli of the component
materials were incorporated in finite element analysis at temperature of concern. For the
finite element analysis, anode supported configuration of PEN assembly (of 100mm x
100mm) was considered with 1mm thick anode, 10μm electrolyte and 30μm cathode. The
results have indicated that cathode and anode layer adjacent to cathode/electrolyte and
electrolyte/anode interface respectively are subjected to tensile stresses at the operating
temperature of HT-SOFC (900°C) and IT-SOFC (600°C). However, the magnitude of
stresses is much higher in the former case (500MPa tensile stress in cathode layer) when
compared with the stress level in IT-SOFC (178MPa tensile stress in cathode layer). These
high stresses might have been resulted from the higher CTE of cathode when compared with
the adjacent electrolyte. However, it is worth mentioning here that in the present work, we
have not considered any contribution from the residual stresses arising from fabrication and
the stress relaxation from softening of the glass sealant.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/86039 |
| Date | 10 October 2008 |
| Creators | Manisha, |
| Contributors | Radovic, Miladin |
| Publisher | Texas A&M University |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Thesis, text |
| Format | electronic, born digital |
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