Lincoln Miara

Soft X-Ray Spectroscopic Study of Dense Strontium-Doped Lanthanum Manganite Cathodes for Solid Oxide Fuel Cell Applications

The evolution of the Mn charge state, chemical composition, and electronic structure of La{sub 0.8}Sr{sub 0.2}MnO{sub 3} (LSMO) cathodes during the catalytic activation of solid oxide fuel cell (SOFC) has been studies using X-ray spectroscopy of as-processed, exposed, and activated dense thin LSMO films. Comparison of O K-edge and Mn L{sub 3,2}-edge X-ray absorption spectra from the different stages of LSMO cathodes revealed that the largest change after the activation occurred in the Mn charge state with little change in the oxygen environment. Core-level X-ray photoemission spectroscopy and Mn L{sub 3} resonant photoemission spectroscopy studies of exposed and as-processed LSMO determined that the SOFC environment (800 C ambient pressure of O{sub 2}) alone results in La deficiency (severest near the surface with Sr doping >0.55) and a stronger Mn{sup 4+} contribution, leading to the increased insulating character of the cathode prior to activation. Meanwhile, O K-edge X-ray absorption measurements support Sr/La enrichment nearer the surface, along with the formation of mixed Sr{sub x}Mn{sub y}O{sub z} and/or passive MnO{sub x} and SrO species.

Polarization Resistance of La0.85Ca0.15MnO3 Cathodes for Solid Oxide Fuel Cells (SOFCs) Measured Using Patterned Electrodes

Patterned cathodes of calcium-doped lanthanum manganite (LCM) were fabricated on polycrystalline yttria-stabilized zirconia (YSZ) substrates by RF magnetron sputtering and photolithographic techniques. Samples were generated with a constant electrode/electrolyte contact area but different three phase boundary (TPB) lengths (lTPBs). Electrochemical impedance spectroscopy (EIS) was performed in the oxygen partial pressure (pO2) and temperature range of 10-3 atm to 1.00 atm and 600-800 C respectively. The area specific polarization resistance, Rp, was found to scale linearly with the inverse of lTPB, suggesting that the TPB is the active region for oxygen reduction. The resistivity decreased with increased temperature and pO2 and showed an activation energy of 1.17 +/- 0.03 eV.

SURFACE SEGREGATION STUDIES OF SOFC CATHODES: COMBINING SOFT X-RAYS AND ELECTROCHEMICAL IMPEDENCE SPECTROSCOPY

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 Sr 2+ and Mn 2+ , which readily form at elevated temperatures even in O 2 atmospheric pressures, is responsible for the improved cathodic performance upon application of a bias.