PNNL

Abstract

Acceptor doped lanthanum chromite (LaCrO3) has long been the interconnect material of choice for high temperature SOFCs, typically operating at 1000?C, and in fact it is also viable at 800?C, since its conductivity is still 90% of that at 1000?C. However, lanthanum chromite is relatively expensive to prepare and fabricate. As the SOFC operating temperature is lowered, metal interconnects (e.g., ferritic steels or chromium alloys) become increasingly viable, but they have their own unique problems (including Cr-oxide formation and Cr volatilization), and it is therefore unlikely that bare metals can be used at temperatures greater than 700?C. For intermediate operating temperatures (700-800?C), development of cost-effective oxide coatings may allow use of metal interconnects, but the long-term performance of such coated metals remains in doubt. In the event that even coated metals prove unsatisfactory, lanthanum chromite may be the only viable choice, in which case it will be essential to continue to reduce chromite raw material and fabrication costs. In this study, the authors have investigated the practicality of replacing pure La in LaCrO3 with a less expensive mixed lanthanide (Ln) precursor, containing mainly La3+, but also significant proportions of Ce4+, Nd3+ and Pr3+. Typical compositions investigated were of the formula Ln0.85Sr0.15Cr1-yMyO3, where 0.02*y*0.1 and M = Co, Cu, Ni, and V. Samples were studied with respect to sinterability in air, thermal expansion, conductivity in air and at low pO2, phase stability, and dilation under reducing atmospheres. The most promising of the materials investigated was Ln0.85Sr0.15Cr0.95Cu0.05O3, which could be sintered to >90% theoretical density at 1250?C. This material exhibited conductivities >20 S/cm in air and >5 S/cm at pO2 = 10-16 atm, and indicated no phase change from 25-1000?C. The TEC (25-1000?C) of the material, 9.4 x 10-6 ?C-1, was somewhat lower than that of the YSZ electrolyte (10.5x 10-6 ?C-1).