Fundamental Impact of Humidity on SOFC Cathode Degradation

Abstract

The durability of SOFCs under real working conditions is still an issue for commercial deployment. In particular cathode exposure to atmospheric air contaminants, such as humidity, can result in long-term performance degradation issues. Therefore, a fundamental understanding of the interaction between water molecules and cathodes is essential to resolve this issue and further enhance cathode durability. To study the effects of humidity on the ORR, we used insitu 18O isotope exchange techniques to probe the exchange of water with two of the most common SOFC cathode materials, LSM and LSCF. In this experiment, heavy water, D2O (mass/charge ratio of m/z=20), is used to avoid overlapping of H2O and 18O2 cracking fraction (m/z=18). Temperature programmed isotope exchange measurements were performed to comprehensively study the interaction of water with the cathode surface as a function of temperature, oxygen partial pressure, and water vapor concentration. The results suggest that H2O and O2 share the same surface exchange sites. Therefore, the presence of H2O competes with O2 for the available surface sites. Our findings show that H2O prefers to exchange with LSM and LSCF at a lower temperature, ~300-450°C. For LSM, O2 is more favorable than H2O to be adsorbed on LSM surface and the presence of O2 limits H2O exchange with LSM. H2O has two different exchange mechanisms to react with the LSCF surface, and each mechanism dominates in a different temperature region. The data are summarized to a Temperature-PO2 diagram to visualize the dominant reactions at temperature and PO2 for the two cathode materials.