Electronic Delocalization Regulates the Occupancy and Energy Level of Co 3d(z2) Orbitals to Enhance Bifunctional Oxygen Catalytic Activity

Cobalt-nitrogen-carbon is hitherto considered as one of the most satisfactory alternatives to precious metal catalysts for oxygen electrocatalysts. However, precisely tuning the local coordination of Co sites and thus engineering d-orbital electron configuration to optimize the binding energy of the intermediates remains a huge challenge. Herein, a robust electrostatic self-assembly strategy is developed to engineer penta-coordinated Co sites by introducing axial O ligands with atomic-level precision to form CoN4O1 configurations on MXene nanosheets (CoN4-O/MX). The optimized CoN4-O/MX demonstrates outstanding bifunctional electrocatalytic performance with a small potential gap of 0.72 V, significantly outperforming the cobalt-nitrogen-carbon catalyst with plane-symmetric CoN4 sites and precious metal counterparts. The Zn-air batteries integrated with CoN4-O/MX provide an outstanding peak power density of 182.8 mW cm(-2) and a long-term cyclability for 250 h. Density functional theory calculations reveal that Co-O coordination induces electronic delocalization to draw off partial electrons from the dz(2) orbital, which forms unsaturated orbital filling and lifts the energy level, resulting in a stronger Lewis basicity to facilitate electron injection into the intermediate. The work presented here provides not only a novel methodology to achieve precise control of heteroatom coordination, but also a fundamental understanding about the structure-activity relationships of d(z2) orbitals.