Delving into Fe-content effects on surface reconstruction of Ba_0.50Sr_0.50Co_1-xFe_x O_3- for the oxygen evolution reaction

Surface reconstruction of cobalt-based oxides is recognized as a key to efficiently electrocatalyze the oxygen evolution reaction (OER) in alkaline environment. Identifying material features that promote surface reconstruction is crucial to rationally improve OER electrocatalysts. Here, the Fe-content effects on the surface reconstruction of flame-spray synthesized Ba0.50Sr0.50Co1-xFexO3-delta (BSCo1-xFex) is systematically investigated by gradually substituting Co with Fe (0 < x < 1). The electrochemical characterization reveals a volcano-shaped trend of the OER activity and stability as a function of the Fe-content, and identifies BSCo0.80Fe0.20 as the best performing electrocatalyst. This Fe-content dependent performance trend directly correlates with the extent of surface reconstruction, as unveiled by combining ex situ surface and operando bulk X-ray absorption spectroscopy. More specifically, the increasing electrocatalytic performance from x = 0.01 to 0.20 is explained by the ability of Fe to stabilize surface Co2+-atoms in the pristine material. This enhances the electrochemically triggered irreversible surface Co oxidation, leading to a more extensive formation of a Co- and Fe-based (oxyhydr)oxide layer that reaches deep into the electrochemically metastable bulk. The decreasing performance trend for x > 0.20 is related to the increasing oxygen content in the pristine material, leading to a stabilization of the bulk structure and preventing the (oxyhydr)oxide from growing into the bulk. Moreover, a high Fe-content (x > 0.40) stabilizes the surface Co2+-atoms in such an extent that the irreversible surface Co oxidation is increasingly suppressed, limiting the reconstruction process even on the surface. Overall, this study provides a fundamental understanding of the Fe-content effects on surface reconstruction in BSCo1-xFex and deciphers the highest electrocatalytic performance of BSCo0.80Fe0.20 as a combination of optimally, neither too weakly nor too strongly, stabilized surface Co2+-atoms and bulk structure, leading to the most extensive surface reconstruction.