Asymmetrically coordinated single-atom catalysts: from synthetic strategy to structure-activity relationship

Asymmetric coordination structures in single-atom catalysts (SACs) represent a frontier in electrocatalysis, offering tunable electronic environments and enhanced catalytic performance beyond traditional symmetric M–N4 motifs. This review first categorizes asymmetric SACs into four structural families: (1) single-metal asymmetric coordination, achieved by heteroatom substitution or axial ligand incorporation; (2) non-contact multi-metal sites, where adjacent but unbonded metal atoms synergize electronically; (3) directly bimetallic-bonded asymmetric coordination structures; and (4) bridged multi-metal constructs connected via non-metal linkers (e.g., O, N, S). Key synthetic strategies, including metal–organic framework confinement, defect engineering, dual-solvent loading, and macrocyclic precursor mediation, are examined in detail. Then we summarize applications in oxygen reduction reaction and CO2 reduction reaction catalysis, and highlight how asymmetric coordination tunes intermediate adsorption energies, breaks scaling relations, and enables tandem catalysis to improve activity, selectivity, and stability. Advanced characterization techniques - aberration-corrected scanning transmission electron microscopy with electron energy loss spectroscopy, synchrotron X-ray absorption spectroscopy, and time-of-flight secondary ion mass spectrometry - are discussed for their roles in resolving atomic dispersion, coordination environment, oxidation states, and dynamic evolution under operando conditions. Finally, challenges and future directions are outlined, including precise low-temperature assembly of heteronuclear sites, scalability, long-term stability under harsh reaction conditions, selective pathway control, and the integration of operando analyses with theoretical modeling to guide rational catalyst design.