Axial engineering of bilayer single-atom catalysts for enhanced bifunctional oxygen electrocatalysis.

Axial ligand engineering is a promising strategy to enhance the performance of single-atom catalysts (SACs) in electrocatalysis. However, a single non-metallic axial coordination atom linked to monolayer SACs (MSACs) often exhibits insufficient stability. In this work, we designed a series of bilayer SACs (BSACs) with vertically stacked FeN₄ and MN₄ (M = Sc-Zn) layers bridged by axial non-metallic atoms (C, N, O, P, S, and Se). The bilayer structure stabilizes axial atom anchoring and redistributes electrons of dual-side metal atoms. As electrocatalysts for oxygen reduction (ORR) and evolution (OER) reactions, the introduction of axial ligands optimizes the binding strength of key intermediates and reduces the overpotentials. After high-throughput DFT screening of the ORR/OER pathways across 60 BSAC candidates, we found that FeN₄-P-MnN₄ (P-FeMn) and FeN₄-C-MnN₄ (C-FeMn) exhibit exceptional dual-sided bifunctional (ORR and OER) catalytic activity, with ORR overpotentials (Fe/Mn site) of 0.27/0.28 V and 0.37/0.29 V and OER overpotentials of 0.42/0.37 V and 0.31/0.42 V, respectively. Electronic structure analysis reveals that the axial P/C atoms induce a transition of the metal atoms from a high-spin state to a low-spin state, thereby shifting the d-band center and effectively weakening the metal-oxygen orbital hybridization (σ and π), leading to enhanced catalytic activity. This work advances axial ligand engineering in single-atom catalysts and offers new insights and strategies for designing stable and efficient bifunctional oxygen electrocatalysts.