Ferritic steel interconnects

J.P. Abelian, V. Shemet, E. Tietz, L. Singheiser, W.J. Quadakkers and A. Gil, Ferritic Steel Interconnect for Reduced Temperature SOFC, in Solid Oxide Fuel Cells VII, H. Yokokawa and S.C. Singhal, Editors, PV 2001-16, p. 811, The Electrochemical Society Proceedings Series, Pennington, NJ, 2001. 

The mechanical strength of the thin-film cell as well as the electrical and thermal conductivity are provided by an open porous metalfic substrate which also serves as a fuel gas distributor. The functional layers of the cell - anode, electrolyte, and cathode - are consecutively deposited onto this substrate by a multistep vacuum plasma spray process. The contact from the cathode to the ferritic steel interconnect (bipolar plate) is provided by a flexible and ductile perovskite coating. An... 

Piron-Abellan J, Shement V, Tietz F, Singheiser L, Quadakkers WJ, Gil A (2001) Ferritic steel interconnect for reduced temperature SOFC. In Yokokawa H (ed) Proceedings of the 7th international symposium on solid oxide fuel cells (SOFC-VII), Tsukuba, 3-8 Juni 2001 (Proceedings of the electrochemical society 2001-16, Pennington, 2001) p 811-819. ISBN 1-56677-322-9... 

Figure 7-7 Expansion of Typical Cell Components in a 10 cm x 10 cm Planar SOFC with Ni-YSZ anode, YSZ Electrolyte, LSM Cathode, and Ferritic Steel Interconnect.

Since the ROB is based on a SOC the operating temperature lies around 800 C. As mentioned above, the stagnant atmosphere surrounding the storage material consists of steam and hydrogen. During operation it is necessary to avoid local oxidation of nickel, which is part of the fuel electrode and also to avoid reduction of chromium oxide, which the main constituent of the protective layer on the surface of the ferritic steel interconnect. 

Furthermore, more detailed theoretical research needs to be done for gaining a better understanding of the degradation mechanisms. Here, gaseous transport of possibly volatile iron hydroxides [11] needs to be taken into consideration as well as the interaction between the storage material and the ferritic steel interconnect. 

Z. Yang et al., Chemical Compatibility of Barium-Calcium-Aluminosilicate-Based Sealing Glasses with the Ferritic Stainless Steel Interconnect in SOFCs, Journal of the Electrochemical Society, 150(8), pp. A1095-A1101 (2003). 

Stability of ferritic stainless steel interconnects, minimize contact resistance, and seal off chrominm in the metal substrates. 

Conductive/Protective Coatings for Ferritic Stainless Steel Interconnects... 

Key words Solid Oxide Fuel Cell/Interconnect/Ferritic Steel/High Temperature Conductivity... 

Metallic materials for be used as interconnects in SOFCs should fulfil a number of specific requirements [1, 2], Crucial properties of the materials are high oxidation resistance in both air and anode environment, low electrical resistance of the oxide scales formed on the alloy surface as well as good compatibility with the contact materials. Additionally, the value of the coefficient of thermal expansion (CTE) should match with those of the other cell components [3], These requirements can potentially be achieved with high chromium ferritic steels [4], however, previous studies [5] have shown that none of the commercially available ferritic steels seems to possess the suitable combination of properties required for long term reliable cell performance. 

Recently, a new class of FeCrMn(La/Ti) ferritic steels (see Table 1) has been developed to be used as construction materials for SOFC interconnects... 

Chemical compatibility of barium-calcium-aluminosilicate-based sealing glasses with the ferritic stainless steel interconnect in SOFCs. J. Electrochem. Soc., 150, A1095-A1101. 

Shaigan N, Qu W, Ivey DG, Chen W (2010) A review of recent progress in coatings, surface modificatirais and alloy developments for solid oxide fuel cell ferritic stainless steel interconnects. J Power Sources 195 1529-1542... 

Interconnect, cell frame, and metallic substrate are made from the ferritic steels CroFer22APU [55] and IT-11 [70],... 

Huczkowski, P., et al. Growth Rate and Electrical Conductivity of Oxide Scales on Ferritic Steels Proposed as Interconnect Materials for SOFC. in Sixth European Solid Oxide Fuel Cell Forum. 2004. Luzern, der Schweiz European Fuel Cell Forum. 

Evaluation of ferritic steels for use as interconnects and porous metal supports in IT-SOFCs, J. Power Sources, 151 (2005) 103-107. 

Malkow ft al. systematically investigated the thermal expansion and corrosion behaviour of commercial steels and model alloys [53]. The thermal expansion coefficient of ferritic steels decreases with increasing Cr content up to 20 wt% and increases with increasing Al content. By such alloying, the thermal expansion of ferritic steels can be adjusted and matched to the Ni/YSZ anode substrate, but not to the YSZ electrolyte. The oxidation of steels depends not only on the Cr content, but also on small amounts of alloyed elements, especially Al and Si. Once a compositional threshold is reached, alumina and silica layers arc formed instead of a chromia layer. This leads to a reduction in oxidation rate. However, such alumina and silica layers are insulating and have to be avoided when the steel interconnect is in contact with a contact or electrode material. In a comparative study [54] of commercially available ferritic steels with chromium contents between 12 and 28%. the contact resistance against... 

SOFC interconnect could be either ceramic or metals/alloys. Ceramic interconnects are normally used between 800°C and 1000°C, whereas metallic interconnects are preferred for 750°C and below. While doped LaCrOs is used as ceramic interconnects, the Cr-based alloys and ferritic steels are the choice for metallic interconnects. In a recent review article, Sakai et al. (2004) has summarized the trend of research activities in the area of SOFC interconnects (Fig. 21). Very recently possibility of using stainless steel has also been examined by Ishihara et al. (2003) and Bance (2004). For the Siemens-Westinghouse tubular SOFC, the interconnection is deposited in the form of a 85 pm thick, 9 mm wide strip along the air electrode tube length by plasma spraying. On the other hand, bipolar plates having channels on both sides are used for planar geometry. Several materials have been tested for SOFC interconnects, details of which are now available in the literature (Quadakkers 2003). 

Most of the initial initiatives to replace the ceramic interconnect with metallic interconnects including other alloys such as, Haynes 230, Inconel 600 have been found unsuccessful, particularly, in electrolyte-supported stacks where operation temperature is around 1000°C. In contrast, ferritic steel or some combinations of Cr-based alloys are being used by most of the developers who are testing their anode-supported structures at <750°C (Zhu et al. 2003, Anderson et al. 2003, Hilpert et al. 2003, Fergus 2005). Detailed list of commercially available ferritic steels which are found to be suitable for SOFC interconnect are given by Horita et al. (2002) Hilpert et al. (2003), Fergus (2003). This variety of steel is preferred to be used within... 

J. Froitzheim, G.H. Meier et al.. Development of high strength ferritic steel for interconnect application in SOFCs. J. Power Sources 178, 163-173 (2008)... 

X. Montero, F. Tietz et al., MnC01.9Feo.1O4 spinel protection layer on commercial ferritic steels for interconnect applications in solid oxide fuel cells. J. Power Sources 184, 172-179 (2008)... 

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