Stability of Ni-YSZ composites for solid oxide fuel cells during reduction and re-oxidation
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Informaatio- ja luonnontieteiden tiedekunta |
Doctoral thesis (article-based)
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Date
2010
Major/Subject
Mcode
Degree programme
Language
en
Pages
Verkkokirja (2331 KB, 92 s.)
Series
VTT publications,
740
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.Description
Supervising professor
Lund, Peter, Prof.Mogensen, Mogens, Research Prof., Risoe National Laboratory for Sustainable Energy, Denmark
Kaiser, Andreas, Dr., Risoe National Laboratory for Sustainable Energy, Denmark
Keywords
fuel cell, SOFC, Ni-YSZ, Ni cermet, tech, redox stability, tech, thermomechanics, tech, sintering, tech, continuum mechanics, tech, creep, tech, viscoelastic, tech, NiO reduction, tech, Ni oxidation, tech, kinetics, tech
Other note
Parts
- [Publication 1]: M. Pihlatie, A. Kaiser, P. H. Larsen, M. Mogensen. Dimensional Behavior of Ni–YSZ Composites during Redox Cycling. Journal of The Electrochemical Society 156 (2009) B322-B329.
- [Publication 2]: M. Pihlatie, A. Kaiser, M. Mogensen. Mechanical properties of NiO/Ni–YSZ composites depending on temperature, porosity and redox cycling. Journal of the European Ceramic Society 29 (2009) 1657-1664.
- [Publication 3]: M. Pihlatie, T. Ramos, A. Kaiser. Testing and improving the redox stability of Ni-based solid oxide fuel cells. Journal of Power Sources 193 (2009) 322-330.
- [Publication 4]: M. Pihlatie, A. Kaiser, M. Mogensen. Redox stability of SOFC: Thermal analysis of Ni–YSZ composites. Solid State Ionics 180 (2009) 1100-1112.
- [Publication 5]: M. H. Pihlatie, H. L. Frandsen, A. Kaiser, M. Mogensen. Continuum mechanics simulations of NiO/Ni–YSZ composites during reduction and re-oxidation. Journal of Power Sources 195 (2010) 2677-2690.