Improving Electronic Conductivity and Oxygen Reduction Activity in Sr-Doped Lanthanum Cobaltite Thin Films: Cobalt Valence State and Electronic Band Structure Effects

Electronic conductivity and oxygen reduction activity are investigated in high quality La1–x Sr x CoO3‑δ (from x = 0 to x = 0.8) thin films with regard to their use as cathodes for intermediate-temperature solid oxide fuel cells. In our study, external interference from microstructure and crystallographic orientation is avoided and therefore we decouple the complex interaction among chemical composition, bulk electronic conductivity, and ORR activity. We observe that the electronic conductivity and the polarization resistance change together, both having the best values at x = 0.4 Sr doping concentration. The Co3+/Co4+ valence state and the higher overlap of the O 2p–Co 3d bands explain the electronic conductivity and the oxygen reduction activity improving with the Sr doping concentration from x = 0 up to x < 0.4. With further increasing the Sr doping content over 0.4, the shift of the O 2p band toward the Fermi level favors the oxygen vacancy formation process, demonstrated by the formation of Co2+ to balance the charge of the system. Therefore, the Co3+/Co4+ is no longer optimal for the surface oxygen reduction. The increased oxygen vacancy concentration provides more oxygen ion exchange sites for surface oxygen reduction reaction process. However, it also worsens the electronic conductivity and, thus, worsens the electronic charge transfer for the oxygen reduction process. Despite the enhanced formation of oxygen vacancies, the nonoptimal Co3+/Co4+ valence state and the poorer electronic conductivity cooperatively induce a decrease of the oxygen reduction activity in the case of heavily doped LSCO thin films. We show that the electronic conductivity and the oxygen reduction activity are in an intriguing correlation with the cobalt valence state and the electronic band structure.