Achievements of NEDO Projects on SOFC Durability
Authors: Yokokawa Harumi 
Published: February 2020

The seven stacks have been examined for their durability/reliability in the NEDO SOFC projects. The main performance data of stacks have been analyzed into several contributions to voltage losses at selected time intervals, making it possible to derive degradation rates of respective parts. The current performance tests happen to be contaminated with sulfur in air supplied in CRIEPI sites, causing a more significant degradation in cathode in several stacks. Correspondingly, the ohmic loss exhibits similar degradations. Since sulfur content in CRIEPI air was higher than the average ones as confirmed by SIMS measurements by AIST and by S-TEM by Kyushu University, this situation provides weakly accelerated conditions of cathodes. Interestingly enough, this acceleration reveals differences in sensitivity of cathode against the sulfur poisoning effects. When the doped ceria interlayer was succeeded to be fabricated as dense films to prevent the Sr diffusion through doped ceria and following SrZrO3 formation as clarified by FIB-SEM and 3D reconstruction of microstructure by Kyoto University, sensitivity against Sr poisoning becomes large. Actual phenomena appearing as sulfur poisoning effects are also different from stacks to stacks; in particular, NGK INSULATOR cells exhibit peculiar behavior in the initial period of time, while Murata manufacturing cells exhibit strong dependence of air utilization, in other words, air flow rate. Kyocera cells exhibit more sensitive against sulfur after improving the ohmic parts. Although TOTO micro tubular cells do not use the doped ceria layer, a similar significant sulfur poisoning effects are recognized. For planar cells by NGK SPARK PLUG and DENSO, it can be seen effects of sulfur although other effects on the initial degradation make it difficult to extract sulfur effects alone. MHPS segment-in-series cells is free from sulfur poisoning due to their manganite-based cathode. A most plausible mechanism common to those sulfur related phenomena has been derived from considerations on all possible mechanisms of lowering or enhancing active cathodes like LSCF; this mechanism is focusing on balancing between the Sr depletion/volatilization from cathode and the Sr supplying from inside cathode by grain boundary/bulk diffusion; as a factor of enhancing the Sr depletion due to the volatilization in the form of Sr(OH)2, effects of protons or electrochemical potential of protons in the cathodes are considered as factors of controlling Sr behavior and of connecting the Sr volatilization and the microstructure of doped ceria or the thickness of the YSZ electrolyte. This new insight into the cathode degradation behavior caused by sulfur in air leads to the important understanding that cell performance durability appears as complicated interactions among cell components. This proves appropriateness of the present project of focusing on the simulation techniques for cell assemblies by Tohoku university as well as electrode performance based on 3D reconstructed microstructure by the University of Tokyo and Kyoto University. This situation also shows the appropriateness of the present project for investigating durability of those stacks which should be fabricated in a cost effectivity manner and should achieve high performance. It is hoped to show how efforts of establishing high durability can be consistent with cost reduction/high efficiency in the rest of project period of time. .