SURFACE SEGREGATION STUDIES OF SOFC CATHODES: COMBINING SOFT X-RAYS AND ELECTROCHEMICAL IMPEDENCE SPECTROSCOPY
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1217-Y07-04
SURFACE SEGREGATION STUDIES OF SOFC CATHODES: COMBINING SOFT XRAYS AND ELECTROCHEMICAL IMPEDENCE SPECTROSCOPY Lincoln J. Miara1, L. F. J. Piper2, Jacob N. Davis1, Laxmikant Saraf3, Tiffany Kaspar3, Soumendra N. Basu1,4, K. E. Smith 1,2, Uday Pal1,4, and Srikanth Gopalan1,4 1
Materials Science Division, Boston University College of Engineering, Brookline, MA 02446 Department of Physics, Boston University, 590 Commonwealth Ave.,, Boston, MA 02215 3 Environmental and Molecular Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352 4 Department of Mechanical Engineering, Boston University College of Engineering, Boston, MA 02215 2
ABSTRACT A system to grow heteroepitaxial thin-films of solid oxide fuel cell (SOFC) cathodes on single crystal substrates was developed. The cathode composition investigated was 20% strontium-doped lanthanum manganite (LSM) grown by pulsed laser deposition (PLD) on single crystal (111) yttria-stabilized zirconia (YSZ) substrates. By combining electrochemical impedance spectroscopy (EIS) with x-ray photoemission spectroscopy (XPS) and x-ray absorption spectroscopy XAS measurements, we conclude that electrically driven cation migration away from the two-phase gas-cathode interface results in improved electrochemical performance. Our results provide support to the premise that the removal of surface passivating phases containing Sr2+ and Mn2+, which readily form at elevated temperatures even in O2 atmospheric pressures, is responsible for the improved cathodic performance upon application of a bias. INTRODUCTION As the demand for cleaner energy production grows, it is increasingly important to bring solid oxide fuel cells (SOFCs) to market. At present the drive is to maintain high performance at intermediate operating temperatures (i.e. 500-700°C) [1]. While at these temperatures the stack materials costs drop dramatically, performance also suffers. As the temperature drops, reaction kinetics at the cathode using conventional cathode materials such as strontium doped lanthanum manganite (LSM), slow down considerably. It has emerged in many studies that with an applied bias the performance of the cathode improves over time [2]. A pronounced effect, of about an order of magnitude in decreased polarization resistance, can be seen with a very small applied current [3]. Interestingly, this phenomenon occurs for both porous screen printed cathodes, and dense thin films [4, 5]. For a typical oxygen ion conducting electrolyte such as yttria-stabilized zirconia, the following oxygen reduction reaction (ORR) occurs at the cathode: (1) However, the exact intermediate reaction steps remain a mystery. It has been shown that for cathodes over a certain critical thickness the ORR occurs mainly at the triple phase boundary (TPB), and the reaction rate scales with TPB length [6]. However, work done on dense thin films
with small TPB lengths has shown that the ORR occurs via a combination of surface processes followed by bulk diffusion to the two phase cathode-electrolyte boundary
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