National Synchrotron Light Source Lunch Time Seminar

Solid oxide fuel cells (SOFCs) are electrochemical devices characterized by a ceramic electrolyte - typically yttria stabilized zirconia (YSZ) - that enables the conduction of negative oxygen ions at elevated temperatures (i.e. > 800°C). At present the drive is to maintain high performance at intermediate operating temperatures (i.e. 500-700°C), which are more suitable for small-scale applications. As the temperature drops, reaction kinetics at conventional cathodes, such as strontium doped lanthanum manganite (La0.8Sr0.2MnO3), slow down considerably. This has led to various attempts to improve performance. The most notable being associated with the “burn-in” phenomenon or activation, where an applied bias improves the performance of the cathode over time. Various mechanisms have been proposed ranging from: Sr segregation; removal of Mn (Sr) oxide; nanopore formation; Mn2+ migration to the interface; and, La0.8Sr0.2MnO3/YSZ intermixing. However its origin still remains unclear.

In this talk, I will report our recent results regarding the chemical composition and electronic structure of LSMO films at various stages of operation. By rapidly quenching and sealing in vacuum, we were able to directly compare the pristine (as-fabricated) La0.8Sr0.2MnO3 with both "heat-treated" (800°C in air, and no bias) and "burnt-in" (800°C in air, -1 V bias) La0.8Sr0.2MnO3 cathodes. This circumvented issues regarding 1) surface preparation (i.e. Mn reduction) and 2) use of soft x-ray techniques (i.e. ultra-high vacuum). Using a combination of core-level X-ray photoemission spectroscopy, X-ray emission/absorption spectroscopy, resonant inelastic X-ray scattering and resonant photoemission spectroscopy, we observed La-deficiency (severest near the surface) and an increased Mn4+ contribution (i.e. corresponding to hole doping > 0.55) of the cathode prior to activation. In addition we also observed dramatic changes in the oxygen environment before the application of a