Boosting the ORR/OER Activity of Cobalt-Based Nano-Catalysts by Co 3d Orbital Regulation

The transition metal oxides/sulfides are considered promising catalysts due to their abundant resources, facile synthesis, and reasonable electrocatalytic activity. Herein, a significantly improved intrinsic catalytic activity is achieved for constructing a Co-based nanocrystal (Co-S@NC) with the coordination of Co─S, Co─S─C, and Co─N_x─C. The calculational and experimental results demonstrate that the diversified chemical environment of Co-cations induces the transition of 3d orbitals to a high spin-state that exhibits the coexistence of Co²⁺ with fully occupied d_π orbitals and Co³⁺ with unpaired electrons in d_π orbitals. The diverse d_π orbitals occupation contributes to an elevated d-band center of Co ions, which accelerates oxygen reduction reaction and oxygen evolution reaction electrocatalytic kinetics of the Co-S@NC nanocrystal. Therefore, the Li-O₂ batteries with Co-S@NC as cathode catalyst exhibit 300 cycles at the current density of 500 mA g^-1 with a cut-off capacity of 1000 mAh g^-1. Moreover, the ultrahigh discharge specific capacity of 34 587 mAh g^-1 is obtained at a current density of 1000 mA g^-1, corresponding to the energy density 949 Wh kg^-1 of a prototype Li-O₂ battery. The study on 3d orbital regulation of nanocrystals provides an innovative strategy for bifunctional electrocatalysts toward the practical application of metal-air batteries.