Surface Reduced Manganese States as a Source of Oxygen Reduction Activity in BaMnO3

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 (BaMnO3) is established for the oxygen reduction reaction (ORR). Electron energy loss spectroscopy (EELS) on the synthesized BaMnO3 find the rods to contain an amorphous surface layer with reduced Mn3+ states compared to Mn4+ states in the bulk. Consequently, the BaMnO3 rods show electrocatalytic activity for the ORR, which originates from the presence of Mn3+ at the rod surface. Furthermore, heating of the samples in air at 300 and 800 °C results in a decrease in the number of Mn3+ 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 Mn3+ state at the perovskite's surface, opening up possibilities for further surface engineering and tuning catalytic properties. BaMnO3 rods, synthesized via a low‐temperature low‐cost method, are characterized by the 2H‐perovskite structure. Despite their crystalline nature, the rods are coated in an amorphous surface layer. The rods display electrocatalytic activity for the oxygen reduction reaction, originating from the presence of reduced Mn states within this layer.