10-10/11 :: October/November 2010
Stability of Ni–YSZ composites for solid oxide fuel cells during reduction and re-oxidation Mikko Pihlatie
ISBN: 978-951-38-7400-1 Technical Research Centre of Finland (VTT), June 2010, Paperback, Pages: 99 http://www.vtt.fi/inf/pdf/publications/2010/P740.pdf
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 have been the core of the present work.
The behaviour of Ni-YSZ cermets for SOFCs were characterised under conditions cyclically altered between reducing and oxidising (redox cycling). The main operating conditions that affect redox stability were shown to be temperature and humidity; both affect the growth of Ni particles through sintering. The temperature of re- oxidation also played a significant role in redox stability; a re-oxidation at a high temperature (850° C/1,562° F 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 (1,112° F) speed up the subsequent re-reduction significantly, indicating a change in microstructure due to the re-oxidation; also the electrical conductivity of the cermets improved on such a redox cycle. The redox strains during redox cycles below 700° C (1,292° F) were reversible, while cumulating strain and damage was created in the ceramic backbone at elevated temperatures.
NiO particle growth during oxidation, combined with low temperature pseudoplasticity was shown to be a decisive 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.