To study the SOFC cathode oxygen reduction reaction (ORR) mechanism and the effects of ambient gas composition on ORR, we used in-situ ¹⁸O isotope exchange techniques to probe the exchange of H₂O and CO₂ 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). A series of temperature programmed isotope exchange measurements were performed to comprehensively study the interaction of H₂O and CO₂ with the cathode surface as a function of temperature, oxygen partial pressure, and contaminant gas concentration. 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.

We further report on the development of a novel in-operando ¹⁸O isotope exchange technique by modifying our heterogeneous catalysis system for operation and measurement under electrochemical polarization. The system allows ORR investigations under real time, real life, operating conditions. We will present our studies on the effect of cathodic polarization on LSM and LSCF. The studies show the effects of operating voltage, temperature, and oxygen partial pressure on the oxygen exchange coefficient. We will also present our model for the understanding and analysis of the isotope oxygen exchange profiles under in-operando conditions.