Abstract
| Original language | English |
|---|---|
| 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) |
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Keywords
- fuel cell
- SOFC
- Ni-YSZ
- Ni cermet
- redox stability
- thermomechanics
- sintering
- continuum mechanics
- creep
- viscoelastic
- NiO reduction
- Ni oxidation
- kinetics
Cite this
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Stability of Ni-YSZ composites for solid oxide fuel cells during reduction and re-oxidation : Dissertation. / Pihlatie, Mikko.
Espoo : VTT Technical Research Centre of Finland, 2010. 99 p.Research output: Thesis › Dissertation › Collection of Articles
TY - THES
T1 - Stability of Ni-YSZ composites for solid oxide fuel cells during reduction and re-oxidation
T2 - Dissertation
AU - Pihlatie, Mikko
N1 - Project code: 41902
PY - 2010
Y1 - 2010
N2 - 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.
AB - 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.
KW - fuel cell
KW - SOFC
KW - Ni-YSZ
KW - Ni cermet
KW - redox stability
KW - thermomechanics
KW - sintering
KW - continuum mechanics
KW - creep
KW - viscoelastic
KW - NiO reduction
KW - Ni oxidation
KW - kinetics
M3 - Dissertation
SN - 978-951-38-7400-1
T3 - VTT Publications
PB - VTT Technical Research Centre of Finland
CY - Espoo
ER -