Axial-sulfur-ligand-induced coordination symmetry breaking in Co–N4 motifs for enhanced oxygen reduction reaction and durable rechargeable zinc–air batteries

Abstract

Precise control of the local atomic structure of single-atom catalysts represents a critical foundation for designing advanced electrocatalysts with specific activity and selectivity for energy conversion and storage. This work presents the facile and novel synthesis of a high-performance cobalt (Co) single atom catalyst (S–Co–N–C) featuring a dynamic, symmetry-broken S–Co–N₄ moiety, which is achieved through the controlled pyrolysis of a Co-impregnated γ-cyclodextrin metal–organic framework (Co-CDMOF). Experimental and density functional theory (DFT) analyses demonstrate that the axial sulfur (S) coordination induces an asymmetric electron distribution and optimizes the electronic nature of the Co site, thereby facilitating a rapid oxygen reduction reaction (ORR) pathway. The optimized S–Co–N–C demonstrates a high positive half-wave potential (E_1/2 of ∼0.79 V vs. RHE) and robust stability relative to Pt/C. Furthermore, the S–Co–N–C electrocatalyst performs effectively as an air-cathode in rechargeable zinc–air batteries (ZABs), delivering a high power density of ∼135 mW cm⁻² and excellent cycling stability over 200 h of operation. This investigation establishes a promising method for designing axial coordination configuration of single-atom catalysts (SACs) and highlights the promising potential of cyclodextrin-based MOFs as a highly versatile platform for fabricating advanced materials for energy conversion and storage.