Investigation of cathodic reaction mechanism in solid oxide fuel cells by operando X-ray absorption spectroscopy

The oxygen chemical potential is an essential indicator for the rate-limiting step of the oxygen reduction reaction in high-temperature electrochemical devices such as solid oxide fuel cells (SOFCs). However, standard electrochemical measurements cannot successfully analyze the oxygen potential profile. Herein, the relationship between oxygen deficiencies and the valence state was determined directly through operando X-ray absorption spectroscopy. To compare the rate-limiting reactions in SOFC cathodes, dense thin-film electrodes of La_0.6Sr_0.4CoO₃−_δ (LSC) on a Ce_0.9Gd_0.1O_1.95 (GDC) electrolyte, La_0.6Sr_0.4Co_0.8Fe_0.2O₃−_δ (LSCF) on a Y_0.1Ce_0.9O_1.95 (YDC) electrolyte, and La_0.9Sr_0.1MnO₃+_δ (LSM) on a Zr_0.92Y_0.08O_1.96 (YSZ) electrolyte were examined as model SOFC cathodes. Variations in the oxygen chemical potential of the electrodes with and without cathodic polarization were experimentally evaluated from the energy shift of the transition metal (Co, Fe, and Mn) K-edge X-ray absorption. It was found that the oxygen chemical potential of the LSC and LSCF electrodes was reduced by applying a cathodic potential and that this change in the oxygen chemical potential occurred mainly on the electrode surface. This result directly demonstrates that the electrochemical oxygen reduction at the cathode is rate-controlled by surface reactions. By contrast, the oxygen potential of LSM changes not at the electrode surface but inside the electrode, which demonstrates that oxide ion diffusion is the rate-determining step for the LSM/YSZ model electrode. This study directly reveals the different rate-determining steps of the electrode reaction for various SOFC cathodes.