Abstract
An operating Ni-based SOFC can be severely damaged by
inadvertent oxidation of the nickel. A central way to
improve this Achilles' heel is to design and prepare a
dimensionally stable anode half cell that does not
overload the electrolyte upon re-oxidation. Understanding
the mechanisms that lead to the redox expansion, and
designing and manufacturing modified anode support
structures that improve stability is the core of the
present work.
The behaviour of Ni-YSZ cermets for SOFCs are
characterised under conditions cyclically altered between
reducing and oxidising (redox cycling). The main
operating conditions that affect redox stability are
shown to be temperature and humidity; both affect the
growth of Ni particles through sintering. The temperature
of re-oxidation also plays a significant role in redox
stability; a re-oxidation at a high temperature (850°C or
higher) leads to larger expansions.
The behaviour of the cermet under redox conditions is
highly dependent on microstructure; as porosity of the
composite increases, redox stability is improved. A redox
cycle at 600°C speeds up the subsequent re-reduction
significantly, indicating a change in microstructure due
to the re-oxidation; also the electrical conductivity of
the cermets improves on such a redox cycle. The redox
strains during redox cycles below 700°C are reversible,
while cumulating strain and damage is created in the
ceramic backbone at elevated temperatures.
NiO particle growth during oxidation, combined with low
temperature pseudoplasticity is suggested to be a
decisive internal factor for redox stability. Redox
cycling at high temperatures rapidly leads to
irreversible nonelastic strains (cracking, creep) in the
YSZ backbone that cause mechanical degradation.
The combination of mild operating conditions and
redox-improved cells appears to be a plausible solution
to circumvent redox failures. An intentional
low-temperature redox treatment could lead to an
improvement in performance. The durability and stability
of the anode can be improved by modifications in the
microstructure and the composition of the cermets.
Original language | English |
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Qualification | Doctor Degree |
Awarding Institution |
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Supervisors/Advisors |
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Award date | 11 Jun 2010 |
Place of Publication | Espoo |
Publisher | |
Print ISBNs | 978-951-38-7400-1 |
Electronic ISBNs | 978-951-38-7401-8 |
Publication status | Published - 2010 |
MoE publication type | G5 Doctoral dissertation (article) |
Keywords
- fuel cell
- SOFC
- Ni-YSZ
- Ni cermet
- redox stability
- thermomechanics
- sintering
- continuum mechanics
- creep
- viscoelastic
- NiO reduction
- Ni oxidation
- kinetics