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 MOF confinement, defect engineering, dual-solvent loading, and macrocyclic precursor mediation-are examined in detail. Then we summarize applications in oxygen reduction reaction (ORR) and CO₂ reduction reaction (CO₂RR) 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 (AC-STEM) with electron energy loss spectroscopy (EELS), synchrotron X-ray absorption spectroscopy (XAS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS)—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.