Oxygen Exchange Mechanisms on Solid Oxide Fuel Cell Cathodes in the Presence of Gas Phase Contaminants
Wachsman, Eric D
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Polarization losses associated with the oxygen reduction reaction (ORR) at the cathode and degradation of cathode materials remain as hurdles for high performance solid oxide fuel cells (SOFC). Rates of degradation depend significantly on the operating temperature and gas conditions, such as the presence of unwanted oxygen-containing compounds, namely H2O and CO2. In this study ORR fundamentals for the common cathode materials, La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) and (La0.8Sr0.2)0.95MnO3±δ (LSM), as well as their composites with Gd0.10Ce0.90O1.95 (GDC) and (Y2O3)0.08(ZrO2)0.92 (YSZ), respectively, are evaluated as a function of operating environment. A combination of electrochemical impedance spectroscopy (EIS) and gas phase oxygen isotope exchange is used to probe the kinetics of heterogeneous gas-solid reactions and electrode performance under a wide range of conditions. The results suggest that CO2 and H2O actively participate in the ORR and that the level of participation and governing mechanisms are dependent on the specific conditions. It was found that CO2 adsorbs readily on the surface of LSCF and leads to significant performance loss, while the affect of CO2 on LSM, an arguably similar material, is minimal. We propose that intrinsic material properties, such as vacancy concentration, will alter the contaminant interactions significantly, leading to specific contaminant-material relationships. The ORR on LSM is compared to that on LSM-YSZ composite, where the triple phase boundary (TPB), gas-electrode-electrolyte interface, plays a vital role. Further, the role of water in both the single phase, and composite materials is explored. A new in operando isotope exchange technique that couples electrochemical polarization with gas phase isotopic transient and steady state results is proposed, and initial results discussed. The development of in operando experiments is crucial to gain a full understanding of electrodes under real operating conditions, where chemical species are being driven by an electrochemical potential. The results contribute to the understanding of gas-solid reaction kinetics on ion conducting catalysts, and provide a basis for future experimental investigations.