Surface Reduced Manganese States as a Source of Oxygen Reduction Activity in BaMnO 3

Abstract In relation to perovskites, tweaking the oxidation state of the B‐site cation is fundamental to controlling the catalytic activity of these materials, thus necessitating a complete characterization of surface oxidation states. Herein, using a combination of atomic‐scale imaging and spectroscopic techniques, structure‐property correlation in barium manganese oxide (BaMnO 3 ) is established for the oxygen reduction reaction (ORR). Electron energy loss spectroscopy (EELS) on the synthesized BaMnO 3 find the rods to contain an amorphous surface layer with reduced Mn 3+ states compared to Mn 4+ states in the bulk. Consequently, the BaMnO 3 rods show electrocatalytic activity for the ORR, which originates from the presence of Mn 3+ at the rod surface. Furthermore, heating of the samples in air at 300 and 800 °C results in a decrease in the number of Mn 3+ states, and thus lowering of the ORR activity. This study represents a step‐stone study in understanding the mechanism of ORR activity and its association to the Mn 3+ state at the perovskite's surface, opening up possibilities for further surface engineering and tuning catalytic properties.