Fundamental Impact of Humidity on SOFC Cathode Degradation

Exposure of solid oxide fuel cell (SOFC) cathodes to atmospheric air contaminants, such as humidity, impacts the oxygen reduction reaction (ORR) mechanism and can result in long-term performance degradation issues. Therefore, a fundamental understanding of the interaction between water molecules and the cathode is essential to develop improved performance cathodes with enhanced durability. To study the effects of humidity on ORR, we used in-situ ¹⁸O isotope exchange techniques to probe the exchange of water with two of the most common SOFC cathode materials, (La_0.8Sr_0.2)_0.95MnO_3-x (LSM) and La_0.6Sr_0.4Co_0.2Fe_0.8O_3-x (LSCF). In these experiments, heavy water, D₂O (with a mass/charge ratio of m/z=20), was used to avoid the overlapping of the H₂O and the ¹⁸O₂ cracking fraction, which both have a peak at m/z=18. A series of 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 range, around 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 the LSM surface. For LSCF, H₂O has two different exchange mechanisms, each dominating in a different temperature region. The experimental data are summarized in a Temperature-P_O2 diagram to visualize the dominant reactions at each temperature and P_O2 for the two cathode materials.