We report that the oxygen reduction reaction (ORR) activities of (001)-oriented manganese perovskite films decrease from 10 to 1 nm by more than an order of magnitude, which can be attributed to the barrier associated with interfacial band bending that impedes electron transfer to the electrolyte, and reduction of Mn3+ due to charge transfer from the Nb:SrTiO3 substrate. Furthermore, we show by substitution in La1−x(Ca,Sr)xMnO3 that Mn3+, not Mn4+, is the active valence state for ORR.
