Optimized Co2+(Td)–O–Fe3+(Oh) electronic states in a spinel electrocatalyst for highly efficient oxygen evolution reaction performance

Abstract

Electronic configuration is crucial for enhancing the catalytic activities of spinels for the oxygen evolution reaction (OER). However, controlling the electronic spin state of materials is still a challenge. In this work, we synthesized Fe-doped meso-Co₃O₄via a nanocasting method. The merit of our method lies in high spin state Fe³⁺ (t_2g³e_g²) being controllably introduced into an octahedral site to regulate the valence states and configure the e_g electron of Co³⁺. The introduced Fe³⁺ prefers to occupy octahedral sites due to its lower formation energy. Then, Fe³⁺ doping enlarges the Co³⁺–O distance and decreases the lattice symmetry, leading to the splitting of the d-orbital in Co³⁺. Our density functional theory (DFT) calculations reveal that spin state optimized Co³⁺_(Oh) acts preferentially as an active site. Furthermore, CoFe-7.5 (Co_2.775Fe_0.225O₄), with its maximum Fe³⁺_(Oh) content, exhibits the best OER activity. Our work indicates that the introduction of Fe³⁺ enables an improvement in the electrocatalytic performance of Co₃O₄ by regulating the spin state of Co³⁺.