Surface and bulk microstructures are critical factors in governing the catalytic activity of perovskite oxides for energy harvesting devices and chemical sensors. Herein, taking the La0.5Ba0.5CoO3-delta as a reference, we found both the surface and bulk chemistry can be optimized through control of its cation nonstoichiometry. Specifically, Ba-deficiency can suppress the formation of the segregation layer, stabilize the crystal structures, decrease the oxide ions diffusion barrier, and bring a considerable improvement in catalytic activity and durability for the oxygen reaction reactions of the perovskite catalyst. These advantages can provide essential guidance for energy material designs in solid oxide fuel cells and other devices with oxygen reduction reaction process. (C) 2019 Elsevier Inc. All rights reserved.