SOFC Interconnect

The identification and fabrication of interconnect materials are a challenge in the development of SOFCs. The primary function of interconnect is to carry the electrical current from the electrochemical cell to the external circuit. Interconnect can be either a metallic or a ceramic material that connects two individual cells. Interconnect must be extremely stable because it is exposed to oxidation and reduction on either side of the material. A generally used interconnect is La(ca)Cr03. The main disadvantage of this material is it degrades during long-term operation. 

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Lanthanum chromite is the most common base for SOFC interconnects, but chromites of other lanthanide elements have also been used [43, 45, 46, 48, 54, 55], Although the conductivity of calcium-doped gadolinium chromite for low calcium contents is in the upper range of conductivities for lanthanum chromite, other nonlanthanum chromites typically have lower conductivities. However, the use of other lanthanides provides benefits in controlling the phase transformation temperature and in potential cost savings [48],... 

Other categories of chromia forming alloys—including Ni(-Fe)-Cr base and Fe(-Ni)-Cr base alloys (e.g., austenitic stainless steels)—have a face-centered cubic (FCC) substrate structure. In comparison to the FSS, the FCC base alloys, in particular the Ni(-Fe)-Cr base alloys, are generally much stronger and potentially more oxidation resistant in the SOFC interconnect operating environment [6, 123-129], However, the FCC Ni(-Fe)-Cr base alloys with sufficient Cr for an appropriate... 

Traditional alloy design emphasizes surface and structural stability, but not the electrical conductivity of the scale formed during oxidation. In SOFC interconnect applications, the oxidation scale is part of the electrical circuit, so its conductivity is important. Thus, alloying practices used in the past may not be fully compatible with high-scale electrical conductivity. For example, Si, often a residual element in alloy substrates, leads to formation of a silica sublayer between scale and metal substrate. Immiscible with chromia and electrically insulating [112], the silica sublayer would increase electrical resistance, in particular if the subscale is continuous. 

With an emphasis on scale electrical conductivity (surface stability as well), a number of new alloys have been recently developed specifically for SOFC interconnect applications. The one that has received wide attention is Crofer 22 APU, an FSS developed by Quadakkers et al. [136, 137] at Julich and commercialized by Thyssen Krupp of Germany. Crofer 22 APU, which contains about 0.5% Mn, forms a unique scale, as shown in Figure 4.6, comprised of a (Mn,Cr)304 spinel top layer and a chromia sublayer [137-139], The electrical conductivity of (Mn,Cr)304 has been reported... 

Overall, the newly developed alloys, such as Crofer 22 APU and ZMG 232 (developed by Hitachi Metal, Ltd.) [142, 143], are promising candidate alloys for SOFC interconnect applications, but their stability during long-term operation at 650 to 800°C remains questionable. 

The oxidation and corrosion behavior of metals and alloys has been widely investigated in a range of environments for a myriad of applications. Recently, oxidation resistant alloys have been studied particularly for SOFC interconnect applications. 

Yang Z, Weil KS, Paxton DM, and Stevenson JW. Selection and evaluation of heat-resistant alloys for SOFC interconnect applications. J. Electrochem. Soc. 2003 150 A1188-A1201. 

Kim J-H, Peck D-H, Song R-H, Lee G-Y, Shin D-R, Hyun S-H, Wackerl J, and Hilpert K. Synthesis and sintering properties of (La Ca, 2 lSrx)Cr03 perovskite materials for SOFC interconnect. J. Electroceramics 2006 17 729-733. 

Ghosh S, Sharma AD, Basu RN, and Maiti HS. Synthesis of La07Ca03CrO3 SOFC interconnect using a chromium source. Electrochem. Solid-State Lett. 2006 9 A516-A519. 

Sammes NM and Ratnaraj R. High temperature mechanical properties of La SrjCrj y Coy03 for SOFC interconnect. J. Mater. Sci. 1995 30 4523M526. 

In planar SOFCs, individual cathode, anode, and electrolyte layers have been deposited by PS [109-111], as well as coatings on interconnect materials and full cells [108, 110, 112]. In addition to the interconnect layers themselves in tubular SOFCs, dense protective layers with good adhesion have also been deposited to protect planar SOFC interconnects from oxidation [110], and diffusion barriers to inhibit inter-diffusion between the interconnects and anodes have been produced by PS [113]. 

Additionally, LSFMand LSCM powders were synthesized with same synthesis route and organic carrier materials. In synthesis of LSFM and LSCM powders, the stoichiometry used in LSGM synthesis was kept to investigate the effects of different cations (Fe3+ or Cr3+ in place of Ga3+). Iron and chromium were chosen to replace gallium such that the new materials can be evaluated as candidates for SOFC interconnect and cathode materials. 

Corrosion of Oxidation-Resistant Alloys nnder SOFC Interconnect... 

CORROSION OF OXIDATION-RESISTANT ALLOYS UNDER SOFC INTERCONNECT EXPOSURE CONDITIONS... 

Fundamental research in the field of SOFCs started in Russia in the end of 1950s at the Institute of Electrochemistry in Sverdlovsk (now the Institute of High Temperature Electrochemistry, IHTE, Ekaterinburg). From the very outset, the works included study of solid oxide electrolytes (SOEs), electrode materials and electrode kinetics, other components of SOFC interconnects, seals, etc. 

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... 

E.M. Garcia et al, "Electrochemical Recycling of Cobalt from Spent Cathodes of Lithium-ion Batteries Its Application as Coating on SOFC Interconnects," Journal of Applied Electrochemistry, Vol 41, 11, 2011, 1373-1379. 

Horita T, Kishimoto H, Yamaji K, Xiong Y, Sakai N, Brito ME, Yokokawa H (2008) Evaluation of laves-phase forming Fe-Cr alloy for SOFC interconnects in reducing atmosphere. J Power Sources 176 54—61... 

Yasuda N, Uehara T, Tanaka S, Yamamura K (2011) Development of New alloys for SOFC interconnects with excellent oxidation resistance and reduced Cr-evaporation. ECS Trans 35 2437-2445... 

Yang Z, Xia GG, Maupin GD, Stevenson JW (2006) Evaluation of perovskite overlay coatings on ferritic stainless steels for SOFC interconnect applications. J Electrochem Soc 153 A852-A1858... 

Z. Yang, G. Xia, C. Wang, Z. Nie, J. Templeton, J. Stevenson, and P. Singh, Investigation of AISl 441 Ferritic Stainless Steel and Development of Spinel Coatings for SOFC Interconnect Applications, PNNL Report 17568, (2008). 

Z. Yang, G. -G. Xia, G. D. Maupin, and J. W. Stevenson, Conductive Protection Layers on Oxidation Resistant Alloys for SOFC Interconnect Applications, Surf. Coat.Tech., 201 4476-83... 

Broadly, interconnect materials for SOFC fall into two categories conductive ceramic (perovskite) materials for operation at high temperature (900 to 1000 °C) and metallic alloys for lower temperature operation. Though the shape of SOFC interconnects depends heavily on the cell and stack design, the materials choice is almost entirely determined by physical and chemical stability under operating conditions. 

Horita, T. (2009). LaCrOs-based perovskite for SOFC interconnects in T. Ishihara (ed.) Perovskite Oxide for Solid Oxide Fuel Cells. Springer. London. 285-286. 

To meet the foregoing requirements, the composition of LaCrOs was modified by doping of lower valence alkaline ions, such as Ca ", Mg +, and Sr, at the La " " or sites. The substitution of La site and Cr site with the other elements can decrease the sintering temperature and increase the electronic conductivity. So far, a number of papers have been published regarding the physical and chemical properties of doped LaCrOs. The present chapter describes the overview of LaCrOs-based ceramics for SOFC interconnects. The following topics are introduced and discussed ... 

M. Mori, Enhancing effect on densification and thermal expansion compatibility for Lao. 8Sro2Cro9Tio 1O3 based SOFC interconnect with B-site doping. J. Electrochem. Soc. 

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