Single-atom catalysts (SACs) have shown unexpected catalytic activity due to their unique electronic structure and coordination environment. Nonetheless, the synthesis of an atomically precise low-coordination single-atom catalyst remains a grand challenge. Herein, we report a coordinately unsaturated Ni-N₃ single-atom electrocatalyst using a metal-organic framework (MOF) derived N-C support with abundant exposed N for excellent electrochemical CO₂ reduction. The obtained Ni-N₃/NC active site exhibited highly efficient CO₂-to-CO conversion with a Faradaic efficiency of 94.6% at the current density of 100 mA/cm². In situ X-ray absorption spectroscopy (XAS) measurement suggested that the Ni atomic center with unsaturated coordination had the lower initial chemical state and higher charge transfer ability. In situ Fourier transform infrared (FT-IR) and theoretical calculation results revealed that the unsaturated catalytically active center could facilitate activation of CO₂ and thus heighten CO₂ electroreduction activity. These findings provided insights into the rational design of definitive coordination structure of SACs for boosting activity and selectivity.

Single-atom catalysts (SACs) with M-N₄ structure have drawn significant attention due to the facile preparation, maximum atom efficiency, unique electronic properties, uniform active sites, and excellent activity. Such catalysts integrated the merits of traditional homogeneous and heterogeneous catalysts effectively solve the cost, activity, and reuse problems. More importantly, the M-N₄ structure is flexible and other species like atoms, groups, and particles can be added to precisely control the local environment of M-N₄ to further improve the catalytic performance. Although unprecedented progress has been made, it remains difficulties in the rational design and controllable synthesis of a suitable SAC for a certain application. This review introduces the progress of M-N₄ catalysts and summarizes the strategies to modulate the M-N₄ structure, including changing the coordination number, tailoring the coordination structure, coordinating with groups, creating dual-atom catalysts (DACs), and coexisting of SAC with DAC and cluster. Special emphasis is placed on the preparation, structure characterization, and reaction mechanism of M-N₄-derived catalysts. Finally, the current challenges of these catalysts are also discussed to provide guidelines for the future design of efficient catalysts.