Axially Modified Square-Pyramidal CoN₄-F₁ Sites Enabling High-Performance Zn-Air Batteries.

Abstract

Cobalt-nitrogen-carbon (Co-N-C) catalysts with a CoN₄ structure exhibit great potential for oxygen reduction reaction (ORR), but the imperfect adsorption energy toward oxygen species greatly limits their reduction efficiency and practical application potential. Here, F-coordinated Co-N-C catalysts with square-pyramidal CoN₄-F₁ configuration are successfully synthesized using F atoms to regulate the axial coordination of Co centers via hydrothermal and chemical vapor deposition methods. During the synthesis process, the geometry structure of the Co atom converts from six-coordinated Co-F₆ to square-pyramidal CoN₄-F₁ in the coordinatively unsaturated state, which provides an open binding site for the O₂. The introduction of axial F atoms into the CoN₄ plane alters the local atomic environment around Co, significantly improving the ORR activity and Zn-air batteries performance. <i>In situ</i> spectroscopy proves that CoN₄-F₁ sites strongly combine with the OOH* intermediate and facilitate the splitting of O-O bond, making OOH* readily decompose into O* and OH* via a dissociative pathway. Theoretical calculations confirm that the axial F atom effectively reduces the electronic density of the Co centers and facilitates the desorption of the OH* intermediate, efficiently accelerating the overall ORR kinetics. This work advances a feasible synthesis mechanism of axial ligands and provides a route to construct efficient high-coordination catalysts.