Ligand Steric Hindrance‐Engineered Spatially Asymmetric Single‐Atom Catalysts: A New Platform for Electrochemical Oxygen Reduction

Single‐atom catalysts (SACs) have demonstrated tremendous potential in heterogeneous electrocatalysis due to their high atom utilization and well‐defined active sites. However, conventional SACs typically exhibit a symmetric electron density distribution, which leads to poor adsorption/desorption and activation of reaction intermediates and thus limits catalytic efficiency. In this study, we designed and synthesized a novel spatially asymmetric SACs through ligand steric hindrance engineering, which involves the post‑synthetic introduction of a bulky, conformationally constrained ligand of 2,2’:6’,2″‐terpyridine (Tpy), into a vinylene‐linked covalent organic framework pre‑anchored with single Co atoms, thereby creating a non‑planar, asymmetric coordination environment around the Co center to afford COF‑Co‑Tpy. Unlike SACs with planar or spatially symmetric structures, COF‐Co‐Tpy possesses a spatially asymmetric structure imparted by the terpyridine moiety, which not only induces a low‐spin state at the Co center, but also creates a unique reaction microenvironment where intermediates are adsorbed at the Co sites perpendicular to the mass transfer direction. The synergistic effect significantly reduces the adsorption energy barrier of the *OH intermediates. Consequently, COF‐Co‐Tpy exhibits remarkable oxygen reduction reaction (ORR) performance with a half‐wave potential up to 0.90 V, and its intrinsic catalytic activity is 22.46 and 6.15 times higher than those of planarly and spatially symmetric SACs, respectively.