Gram-scale synthesis and unraveling the activity origin of atomically dispersed Co-N4O sites toward superior electrocatalytic oxygen reduction

Exploring highly efficient nonprecious metal-based single-atom catalysts (SACs) toward the electrocatalytic oxygen reduction reaction (ORR) is critical for the sustainable development of ORR-related energy conversion and storage systems. However, the scalable synthesis, delicate regulation of the coordination environment and molecular-level elucidation of the electrocatalytic mechanism remain challenging. Herein, we report a facile gram-scale synthesis of atomically dispersed Co sites anchored on N-doped carbon nanofibers (noted as Co-SA@N-CNFs) via a reliable predesigned phenolic resin-mediated strategy for efficient oxygen reduction electrocatalysis. The local coordination configuration of the single-atomic Co sites is proposed as the Co-N4O moiety with one O atom in the axial direction perpendicular to the Co-N4 plane. Theoretical calculations uncover that, compared with the common Co-N4 single sites, the formation of Co-N4O configuration is beneficial to reduce the reaction energy barrier, adjust the bond length between the metal sites and the intermediates, and also increase the electric conductivity. Therefore, the Co-SA@N-CNFs demonstrated distinguished ORR activity, outstanding electrochemical stability and methanol tolerance in KOH electrolyte. Furthermore, when assembled in liquid and flexible solid-state rechargeable zinc-air batteries (ZABs), the Co-SA@N-CNFs-equipped ZABs exhibited higher power densities, larger specific capacities and extraordinary cycling performance, compared with the Pt/C-based ZABs. The simple and robust methodology for the mass production of SACs and the engineered coordination environment for performance optimization inspire the future design of a wide range of SACs for energy devices.