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Ferritic stainless-steel interconnect materials are used in intermediate or high-temperature SOFCs because of their oxidation resistance, high formability, and low cost. Their corrosion resistance is due to the formation of a protective chromium oxide scale during operation. However, the protective oxide layer produces Cr-containing volatile species at SOFC operating temperatures and conditions, which can cause cathode poisoning. Spinel coatings have been developed to prevent cathode poisoning while maintaining an electrically conductive pathway through SOFC stacks. Generally, Mn-Co-O spinel has been the material of choice, but PNNL has developed an Mn-Cu-O spinel protective coating to remove the costly Co and enhance the electrical conductivity. This paper will focus on the electrically conductive coating’s compatibility with LSCF in high-temperature operating environments.

Lanthanum strontium cobaltite (LSCo) is considered a good candidate as cathode contact materials for SOFC applications, due to its high electrical conductivity as compared to the standard LSM materials. However, it suffers very poor thermal cycle stability because large mismatch in CTE. To overcome the large residual stress, we proposed to use strong and short alumina fiber to enforcement the LSCo matrix. Alumina in vol% from 2.5 to 20% were formulated and prepared for sintering study, thermal expansion coefficient measurement, phase characterization, bulk strength, and contact strength evaluation before and after 10 deep thermal cycles. Results showed the presence of rigid alumina fiber did retard the densification of LSCo matrix substantially at high alumina content. Bulk strength showed increasing trend at low alumina content but decreased at high alumina v%, likely due to severe retardation in densifications. Contact strength on 1”x1” bilayer showed the strong bonding at low alumina v% while good thermal cycle stability was observed. The candidate composite was further tested in a stack fixture using 2”x2” cell. Results of impedance versus cycling and microstructure and fracture analysis will be discussed to assess the validity of fiber reinforced LSCo as contact material.

Ferritic stainless steel interconnect materials are used in intermediate or high-temperature SOFCs because of their oxidation resistance, high formability, and low cost. Their corrosion resistance is due to the formation of a protective chromium oxide scale during operation. However, the protective oxide layer produces Cr-containing volatile species at SOFC operating temperatures and conditions, which can cause cathode poisoning. Electrically conducting spinel coatings have been developed to prevent cathode poisoning and maintain an electrically conductive pathway through SOFC stacks. Generally, Mn-Co-O spinel is used for this protective coating in SOFCs. To remove the Co and enhance the electrical conductivity, PNNL has developed a Mn-Cu-O spinel for this protective coating. This paper will focus on the electrically conductive coating’s compatibility with LSCF in high temperature operating environments.

Cr poisoning has been identified as a leading cause for degradation of solid oxide fuel cells. Mitigation can be achieved by protective coatings such as Mn-Co spinel or aluminization on ferritic stainless steel, a leading candidate for metallic interconnect. This has been successfully demonstrated in short and medium term operation, although the long-term integrity of these coatings remains to be validated. In addition to the passive means for Cr mitigation via protective coatings, Pacific Northwest National Laboratory has teamed with University of Connecticut to adopt an active means to tackle the issue by including a novel Cr-getter material in the system. A novel Sr-Ni-oxide developed at University of Connecticut as Cr-getter has proven promising in small button cell testing. In this work, commercial YSZ/NiO anode-supported YSZ cells (50 mm x 50 mm) with either LSCF or LSM cathode were tested in a generic stack test fixture using humidified (~4.75%) air and diluted hydrogen. Two Cr-getter locations were investigated: one upstream of the cell and one “on cell.” Pre-oxidized AISI 441 metal stripes were used as the Cr source. In this study, we validated the candidate Cr-getter materials made either by a nitrate solution method or the solid-state reaction method. Candidate getter materials from nitrate solution approach were impregnated into a porous alumina form, while getter materials obtained by solid state reaction were sintered into small cylinders. For comparison, plain cells with and without a Cr source were also tested to provide baseline data. All cells were tested at constant current for ~1000h at 800oC. Cell performance and impedance were recorded, followed by post-mortem optical/electron microscopy and cathode microstructure/composition analysis. The results showed promising gettering performance with no Cr detected within cathode areas.

Pacific Northwest National Laboratory (PNNL), in collaboration with government agencies and industries, is actively engaged in the development, testing, and characterization of high efficiency, low cost modular solid oxide fuel cell power generation systems for stationary, automotive and military applications. Advanced SOFC systems are being developed which will offer ease of operation on a variety of gaseous liquid hydrocarbon and coal-derived fuels as well as "zero emissions" capability. SOFC R&D activities at PNNL continue in the areas of cell component materials, electrochemistry, cell design and modeling, high temperature corrosion, and fuel processing. Specific activities include development of optimized materials and cost effective fabrication techniques for high power density anode-supported cells operating at temperatures below 800 degrees C, characterization of processes responsible for high electrical performance and long term performance degradation, optimization and cell and stack designs using computational engineering models, and hydrocarbon fuel processing using micro technology.

Ferritic stainless steels are preferred to interconnect materials for intermediate temperature SOFCs because of their resistance to oxidation, high formability, and low cost. However, their protective oxide layer produces Cr-containing volatile species at SOFC operating temperatures and conditions, which can cause cathode poisoning. Electrically conducting spinel coatings have been developed to prevent cathode poisoning and to maintain an electrically conductive pathway through SOFC stacks. PNNL demonstrated the Mn-Co-O type of spinel coating on the interconnect part and delivered a great result. However, the industrial partners need the cost down of these coating materials. Hence, PNNL starts investigating next-generation coating materials. This paper will focus on the electrically conductive coating process.

Ferritic stainless steels are preferred interconnect materials for intermediate temperature SOFCs because of their resistance to oxidation, high formability and low cost. However, their protective oxide layer produces Cr-containing volatile species at SOFC operating temperatures and conditions, which can cause cathode poisoning. Electrically conducting spinel coatings have been developed to prevent cathode poisoning and to maintain an electrically conductive pathway through SOFC stacks. However, this coating is not compatible with the formation of stable, hermetic seals between the interconnect frame component and the ceramic cell. Thus, a new aluminizing process has been developed by PNNL to enable durable sealing, prevent Cr evaporation, and maintain electrical insulation between stack repeat units. Hence, two different types of coating need to have stable operation of SOFC stacks. This paper will focus on the electrically conductive coating process. Moreover, an advanced coating process, compatible with a non-electrically conductive coating will be shown.

An alkaline-earth silicate (Sr-Ca-Y-B-Si) sealing glass with varying K 2 O content (2-5 mol % ) was developed and evaluated for solid oxide fuel cell (SOFC) sealing applications. Two metallic interconnect plates were sealed with the glass and tested for electrical stability in a dual environment at elevated temperatures under a dc loading of 0.7 V. The metallic interconnect material, a ferritic stainless steel, was tested in the as-received state and after surface aluminization. The isothermal aging results showed stable electrical resistivity at 850°C for ~600 to 800 h for all the sealing glasses, with or without K 2 O. The electrical resistivities at 850°C were several orders of magnitude higher than the typical SOFC stack components. However, the glass with high alkali content (5%) showed excessive interfacial reaction, which resulted in debonding from both the as-received and the aluminized steel. Interfacial microstructures were characterized and possible reactions were discussed.