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

摘要

Cobalt–nitrogen–carbon is hitherto considered as one of the most satisfactoryalternatives to precious metal catalysts for oxygen electrocatalysts. However,precisely tuning the local coordination of Co sites and thus engineering d-orbitalelectron configuration to optimize the binding energy of the intermediatesremains a huge challenge. Herein, a robust electrostatic self-assembly strategy isdeveloped to engineer penta-coordinated Co sites by introducing axial O ligandswith atomic-level precision to form CoN_4O_1 configurations on MXene nanosheets(CoN_4-O/MX). The optimized CoN_4-O/MX demonstrates outstanding bifunctionalelectrocatalytic performance with a small potential gap of 0.72 V, significantlyoutperforming the cobalt–nitrogen–carbon catalyst with plane-symmetricCoN_4 sites and precious metal counterparts. The Zn–air batteries integrated withCoN_4-O/MX provide an outstanding peak power density of 182.8 mW cm~(?2) and along-term cyclability for 250 h. Density functional theory calculations reveal thatCo-O coordination induces electronic delocalization to draw off partial electronsfrom the dz~2 orbital, which forms unsaturated orbital filling and lifts the energylevel, resulting in a stronger Lewis basicity to facilitate electron injection into theintermediate. The study presented here provides not only a novel methodologyto achieve precise control of heteroatom coordination, but also a fundamentalunderstanding about the structure–activity relationships of dz2 orbitals.