Design of Atomically Dispersed CoN Sites and Co Clusters for Synergistically Enhanced Oxygen Reduction Electrocatalysis.

Constructing dual-site catalysts consisting of atomically dispersed metal single atoms and metal atomic clusters (MACs) is a promising approach to further boost the catalytic activity for oxygen reduction reaction (ORR). Herein, a porous Co@SNC featuring the coexistence of Co single-atom sites (CoN) and S-coordinated Co atomic clusters (SCo) in S, N co-doped carbon substrate is successfully synthesized by using porphyrinic metal-organic framework (Co-TPyP MOF) as the precursor. The introduction of the sulfur source creates abundant microstructural defects to anchor Co metal clusters, thus modulating the electronic structure of its surrounding carbon substrate. The synergistic effect between the two types of active sites and structural advantages, in turn, results in high ORR performance of Co@SNC with half-wave potential (E) of 0.86 V and Tafel slope of 50.17 mV dec. Density functional theory (DFT) calculations also support the synergistic effect between CoN and SCo by detailing the catalytic mechanism for the improved ORR performance. The as-fabricated Zn-air battery (ZAB) using Co@SNC demonstrates impressive peak power density of 174.1 mW cm and charge/discharge durability for 148 h. This work provides a facile synthesis route for dual-site catalysts and can be extended to the development of other efficient atomically dispersed metal-based electrocatalysts.