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.