Characterization of Metallic Interconnects Extracted from Solid Oxide Fuel Cell Stacks Operated up to 20,000 h in Real Life Conditions: The Air Side

Abstract

Metallic interconnects represent the main component of a solid oxide fuel cell (SOFC) stack in terms of weight and volume. They are typically made of ferritic stainless steel (FSS) coated on the air side. At the stack operating conditions, the interconnect is exposed to a dual atmosphere: air at the cathode side; fuel (a hydrogen-rich mixture) at the anode side. The stacks considered in this study were field operated in reformed natural gas for 5000, 9000 and 20,000 h respectively. The analyzed interconnects are made from CROFER22APU and coated on the air side with Co-Mn base spinel. One interconnect has been studied for each stack by sampling and preparing cross section the inlet and outlet positions. The samples were characterized by SEM-EDXS in order to investigate the evolution of the interconnect at the air side. The interaction between the metal substrate and the coating is investigated highlighting the formation of chromia based thermal grown oxide (at the FSS/coating interface) and the solid-state diffusion of Cr and Fe from the metal into the coating. The microstructural features evolving as a function of time are also quantified.

Investigation of a Metallic Interconnect Extracted from an SOFC Stack after 40,000 h of Operation

“…Due to the high quality of the FSS in use, investigations need extremely long-lasting experiments and the reproduction of "real-life conditions" (i.e., temperature, atmospheres, and materials) to obtain a correct overview of the phenomena. The validation of laboratory tests and the refinement of degradation models can only occur by comparison with components extracted from operated stacks [21][22][23][24]. The project, Ad Astra, offers such an opportunity and several field-operated SOFC stacks over various periods have been studied.…”

“…The present paper refers to the longest period of operation. The post-mortem characterization of MIC samples extracted from a field stack operated in the fuel cell mode for 40,000 h completed a study carried on identical stacks operated in the range 5000-20,000 h [21,22]. The results are of fundamental importance for the modeling of the stack's performance evolution and for a better understanding of samples issued by accelerated tests.…”

Characterization of a metallic interconnect operated in stack during 40,000 hours in SOFC mode

“…Due to the high quality of the FSS used for the MICs the investigations need extremely long lasting experiments to have a correct vision of the phenomena and the reproduction of "real-life conditions" (i.e., temperature, atmospheres, materials). The validation of laboratory tests and the refining of degradation models can only occur by the comparison with components extracted from operated stacks [12][13][14]. The project Ad Astra offers such an opportunity SOFC stacks field operated for several thousand hours.…”

“…The present paper refers on the first results obtained by the post-mortem characterization of MIC samples extracted by a stack field operated for 40,000 hours in fuel cell mode and is an update of previous publications [12,13] referring of interconnects operated for up to 20,000 hours. The results are of fundamental importance for the modelling of the stack performances evolution and for a better understanding of samples issued by accelerated tests.…”

Surface Structure Analysis of Initial High-Temperature Oxidation of SS441 Stainless Steel

“…The temperature adjustment during initial Cr oxidation is a crucial parameter [ 23 , 24 ]. Several studies have performed oxidation at temperatures of 800–900 °C [ 25 ]. However, SOFC research is targeting lower operating temperatures to decelerate the degradation of cell components.…”