Degradation Mechanisms of Metal-Supported SOFCs with Porous Stainless Steel Substrates

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Introduction Solid-oxide fuel cells (SOFCs) have attracted significant attention as an effective means for converting fuel into electricity. Recently, metal-supported SOFCs (MS-SOFCs) have been developed [1-3]. In comparison to ceramic-supported cells, MS-SOFCs demonstrate improved mechanical strength, which renders them well-suited for dynamic operations, including rapid start-up and redox cycles. Consequently, MS-SOFCs present a promising option for auxiliary power units utilized in range extenders for electric vehicles, unmanned aerial vehicles, and aircraft [4-6]. Additionally, metal-supported solid oxide electrolyzers have been suggested to intermittent renewable energy sources like solar and wind power [7, 8]. To create metal supports, powder metallurgy or laser drilling techniques are typically employed, with stainless steel being the material used. Porous metal supports show superior gas diffusion properties compared to those created by laser drilling, making them ideal for automotive applications that demand high power density. The oxidation behavior of porous stainless steel has widely been examined under air atmospheres. Earlier investigations have elucidated the temperature sensitivity of the lifespan of porous stainless steel, taking into account the impacts of varying porosities and chromium contents [9-11]. In the anode atmosphere, specifically in a humidified, reducing environment, iron oxides precipitate, resulting in breakaway oxidation. Various degradation mechanisms have been proposed to explain breakaway oxidation in humidified atmospheres [12]. The durability of MS-SOFCs utilizing porous stainless steel has been explored by multiple researchers [13-15]. While the degradation rate has been discussed at both the cell and stack levels, these studies have not mentioned each degradation phenomenon associated with changes in cell performance in depth. In this presentation, the steam oxidation behaviors of porous stainless steels with varying Cr concentrations are compared to elucidate the mitigation of steam oxidation effects, particularly concerning electrical resistance. Subsequently, a durability test for 8400 h is performed on an MS-SOFC constructed using the most suitable type of porous stainless steel, as identified through the steam oxidation tests, to assess the degradation factors. Experimental Procedure The oxidation behaviors of commercially available stainless steel powder including P434L (17Cr, Ametek, USA), 1C44Mo29 (21Cr, Sandvik, Sweden), and DAPXM27 (26Cr, Daido, Japan) are examined in this study. These materials possess varying Cr and Si contents. The mean particle diameter D50 for all powders measures approximately 20 μm. Following tape casting, the powders undergo sintering at 1250 °C for 1 h under 10% H 2 –90% Ar to produce porous stainless-steel plates with a thickness of about 200 μm. Test specimens are created by cutting the stainless-steel plates into sections measuring 5 mm × 40 mm to evaluate electrical resistance. Subsequently, the samples undergo preoxidation at 850 °C for 10 h in ambient air to develop an oxide scale on the alloy surface. The test specimens are subjected to a 50% H 2 –50% H 2 O mixture, simulating the anode atmosphere at 600 °C. Each sample is placed within a quartz tube and externally heated in an electric furnace. Hydrogen, bubbled through water, is introduced into the quartz tube. The bubbling temperature and flow rates of hydrogen are regulated using mass flow controllers, allowing for adjustments to the gas composition. The electrical resistance of the porous stainless steels is assessed using an impedance analyzer (HIOKI IM-3570). The in-plane resistance, which indicates the conductive path through the necking regions of the alloy particles, is continuously measured at 600 °C under 50% H 2 –50% H 2 O. The electrical resistivity ( ρ ) is determined using the following equation: ρ = R×S / L (1) where R is the impedance [Ω], S is the cross-sectional area along the conductive direction [cm], and L is the length [cm]. A metal-supported button cell is constructed utilizing 21Cr. A bonding layer, consisting of a mixture of fine 21Cr powder (D50: ~5 μm) and Sc- and Y-stabilized ZrO 2 (ScYSZ; Daiichi Kigenso, Japan), is placed between the metal support and the anode backbone to lower the interfacial resistance. Each material is combined with an organic binder, cast onto a Mylar tape, and laminated. The laminate undergoes debinding in air and is subsequently sintered at 1250 °C for 1 h under 10% H 2 –90% Ar. Following sintering, 1-μm gadolinia-doped ceria (GDC) layer is applied to the electrolyte surface using PVD, and then Ni/GDC catalyst is wet-infiltrated onto the anode backbone. The infiltration solutions are prepared by mixing nitrates of Ni, Gd, and Ce with Triton™-X 100 (Sigma Aldrich, USA) and ultrapure water. The resulting mixed solution is introduced into the anode backbone and calcined in air at 800 °C for 30 min. This procedure is followed by two additional infiltration and calcination cycles, each performed at 650 °C for 30 min in air. Ultimately, an LSC (DOWA Holdings, Japan) cathode is applied to the GDC barrier layer through screen printing. To assess its durability, the metal-supported button cell is positioned within an alumina tube. The edges of the cell are sealed with Pyrex glass. After the button cell is situated in the alumina tubes, the temperature is raised from room temperature to 800 °C and held for 6 h to sinter the cathode. The I-V curve and impedance are analyzed following the in-situ sintering of the cathode. Electrochemical impedance spectroscopy (EIS) measurements are conducted using a frequency response analyzer (VERSASTAT4-400, Princeton Applied Research) across an open-circuit voltage (OCV). Result and Discussion Fig. 1 shows the variation in electrical resistivity over time when an electric current is applied in the in-plane direction of the test piece. The electrical resistivity of 17Cr increased by 23%, while 21Cr showed a 5% increase until 500 h, while 26Cr increases double during the same period of time. Cross-sectional SEM images of the three samples are obtained to examine porous structure during steam oxidation. The structure of 21Cr consists of spherical particles that are sintered together, whereas 26Cr displays a jagged surface with thin necking between the particles. The results from the SEM observations indicate that having numerous necks and thicknesses above a certain threshold for maintaining the electron conduction pathway could explain why 21Cr showed the least increase in resistance during the oxidation test. In the case of 26Cr, despite having the lowest porosity, a smaller neck diameter is noted, indicating difficulty in sintering among the alloy particles with higher Cr content. The porous structure characterized by many thin necks between the particles may lead to the greatest increase in electrical resistance. To evaluate the change in electrical resistance, the microstructure of the porous stainless steel before and after the durability tests is analyzed using FIB-SEM and 3D reconstruction, with resistance assessed using a porous stainless steel model generated by software. The modeled microstructure is divided into three layers: alloy, oxide scale, and pores, each assigned distinct conductivities. Further details will be provided in the presentation. Given that 21Cr demonstrates the smallest and most stable resistance up to 500 h, a durability test is conducted using 21Cr under 78% H 2 –16% N 2 –6% H 2 O for 8400 h to confirm that the steam oxidation of the metal support remains insignificant. The results indicate a degradation rate of 5.2% per 1000 h. Throughout the durability test, no significant performance degradation is observed, thus avoiding critical issues such as pore blockage or delamination [16]. After the durability test, the microstructure is examined using FE-SEM. Although no notable changes are found in the electro

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