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INVITED TALK - Chemical Durability of SOFCs
Solid oxide fuel cell (SOFC) is the promising type of fuel cells with e.g. high electric efficiency and fuel flexibility. Durability and reliability are the most important technological issues on such an early commercialization stage. In our research group, we are focusing on the chemical degradation of SOFCs, determining the lifetime of SOFCs operating at high temperatures using various kinds of practical fuels. In this paper, various extrinsic and intrinsic degradation phenomena are systematically classified and their degradation mechanisms are discussed [1-3].
Extrinsic chemical degradation phenomena
While the advantage of SOFCs is their fuel flexibility, impurity species can often flow into the SOFC system, causing degradation phenomena. Due to their high operational temperatures, species with a high vapor pressure can also evaporate and flow into the SOFC stacks from system components. In addition, the use of low-purity raw materials could cause impurity poisoning by the contaminants in such raw materials. Possible extrinsic degradation phenomena are summarized in Fig. 1, including mechanisms associated with surface adsorption/desorption in the case of sulfur at a relatively low concentration, but associated with accumulation and reaction product formation for other impurities.
Intrinsic chemical degradation phenomena
External impurities can be, in principle, removed by using a getter as a desulfurization reactor or by using fuels with better purity. However, chemical degradation associated with diffusion from neighboring components may become important in long-term operation. As compiled in Fig. 2, various intrinsic degradation phenomena have been revealed by evaluating SOFC samples after long-term tests beyond one thousand or several thousand hours. Future perspectives are also presented including the importance of long-term / cycle testing followed by detailed microstructural analysis.
Acknowledgements: We thank for the financial support by METI to establish Next-Generation Fuel Cell Research Center. Financial supported by the NEDO SOFC project is gratefully acknowledged.
References
[1] K. Sasaki et al., J. Power Sources, 196[22], 9130 (2011).
[2] K. Sasaki et al., J. Electrochem. Soc., 153 [11], A2023 (2006).
[3] K. Sasaki et al., ECS Trans., 57 [1], 315 (2013).