Modulation of Ligand Fields in a Single-Atom Site by the Molten Salt Strategy for Enhanced Oxygen Bifunctional Activity for Zinc-Air Batteries

Achieving full utilization of active sites and optimization of the electronic structure of metal centers is the key to improving the intrinsic activity of single-atom catalysts (SACs) but still remains a challenge to date. Herein, a versatile molten salt-assisted pyrolysis strategy was developed to construct ultrathin, porous carbon nanosheets supported Co SACs. Molten salts are capable of inducing the formation of a Co single-atom and porous graphene-like carbon, which facilitates full exposure of the active center and simultaneously endows the Co SACs with abundant defective Co-N₄ configurations. The reported Co SACs deliver an excellent bifunctional activity and good stability for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Moreover, metal-air batteries (MABs) assembled with the Co SACs as air electrode also deliver excellent performance with high power densities of 160 mW·cm^-2, large capacities of 760 mAh·g^-1, and superior long-term charge/discharge stability, outperforming those of commercial Pt/C+RuO₂. DFT theoretical calculation results show that the defects in the second coordination shell (CS) of Co SACs promote desorption of the OH* intermediate for the ORR and facilitate deprotonation of OH* for the OER, which can serve as the favorable active site for oxygen bifunctional catalysts. Our work provides an efficient strategy for the preparation of SACs with fully exposed active centers and optimized electronic structures.