Elucidation of the factors governing the oxygen evolution reaction in Ba0.5Sr0.5CoxFe1−xO3−δ catalysts via operando hard and soft X-ray absorption spectroscopy

Abstract

Ba_0.5Sr_0.5Co_xFe_1−xO_3−δ (BSCF) has garnered considerable attention as a promising catalyst for the alkaline oxygen evolution reaction (OER). However, the intrinsic roles of Co and Fe at the active sites during the OER process remain inadequately understood due to the limited studies integrating bulk-sensitive and surface-sensitive operando techniques. Prior studies on BSCF catalysts have either focused on surface transformations or employed bulk-sensitive techniques to probe redox dynamics. Our work uniquely integrates both bulk-sensitive (Co/Fe K-edge XAS) and surface-sensitive (Co/Fe L-edge) operando spectroscopy, which provides surface-sensitive and 3d orbital-specific information, and O K-edge XAS, which captures the dynamics of oxygen-containing active species. This comprehensive multi-edge approach directly correlates the bulk redox behavior with the surface electronic structure. The electrochemical performance is systematically evaluated for different Ba_0.5Sr_0.5Co_xFe_1−xO_3−δ. Further, intrinsic OER activity was evaluated by eliminating the bubbles' influence during OER by using the forced flow method. Our combined soft and hard XAS revealed that the superior OER activity of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF5582) is attributed to the structural and electronic features of the electrochemically active layer formed at the interface. Minor Fe doping (x = 0.2) predominantly occupies octahedral sites, modulating the electronic states of Co. Under increasing potentials, Co²⁺ in tetrahedral sites (T_d) transitions to Co³⁺ in octahedral sites (O_h), promoting the formation of the catalytically active CoOOH phase. In contrast, Ba_0.5Sr_0.5Co_0.2Fe_0.8O_3−δ (BSCF5528), with higher Fe doping, stabilizes Co³⁺ in O_h sites without substantial changes in oxidation state, leading to the predominance of FeOOH as the active phase and diminished catalytic performance. Operando O K-edge XAS results align closely with the observed behavior of the 3d transition metals, revealing that the formation of reducible Fe³⁺/Co³⁺–O(H) sites, particularly μ₂-O(H) bridges, significantly enhances OER activity. These findings provide a mechanical insight highlighting the critical role of Fe doping in optimizing the structural and electronic properties of BSCF catalysts to achieve superior OER performance.