Oxygen Reduction and Evolution on Perovskite Thin Films Made with Pulsed Laser Deposition

The rates of oxygen reduction and evolution on thin films of La_0.6Ca_0.4CoO₃, La_0.6Ca_0.4MnO₃ and La_0.5Sr_0.5FeO₃, prepared with pulsed laser deposition, were measured with the RRDE technique in 0.1M KOH. The order of activity was La_0.6Ca_0.4MnO₃ > La_0.5Sr_0.5FeO₃ > La_0.6Ca_0.4CoO₃ with Tafel slopes of about 90 mV/decade. Oxygen reduction proceeds mostly through the direct 4e-pathway on La_0.6Ca_0.4CoO₃ with k₁=5.6 k₂. The order of activity for oxygen evolution was La_0.6Ca_0.4MnO₃ > La_0.6Ca_0.4CoO₃ > La_0.5Sr_0.5FeO₃ with Tafel slopes of 60mV/decade. The La_0.6Ca_0.4MnO₃ film showed a slow increase in porosity with measurement time and the La_0.5Sr_0.5FeO₃ film was unstable at E < -0.2 V vs. Hg/HgO. La_0.6Ca_0.4CoO₃ was active for H₂O₂ reduction and oxidation. A partially-carbon-coated La_0.6Ca_0.4CoO₃ film showed higher currents for oxygen reduction than the bare film, due to oxygen reduction on the carbon and further reaction on the neighboring La_0.6Ca_0.4CoO₃.