In situ modulating coordination fields of single-atom cobalt catalyst for enhanced oxygen reduction reaction.

Single-atom catalysts, especially those with metal-N 4 moieties, hold great promise for facilitating the oxygen reduction reaction. However, the symmetrical distribution of electrons within the metal-N 4 moiety results in unsatisfactory adsorption strength of intermediates, thereby limiting their performance improvements. Herein, we present atomically coordination-regulated Co single-atom catalysts that comprise a symmetry-broken Cl-Co-N 4 moiety, which serves to break the symmetrical electron distribution. In situ characterizations reveal the dynamic evolution of the symmetry-broken Cl-Co-N 4 moiety into a coordination-reduced Cl-Co-N 2 structure, effectively optimizing the 3d electron filling of Co sites toward a reduced d-band electron occupancy (d 5.8  → d 5.28 ) under reaction conditions for a fast four-electron oxygen reduction reaction process. As a result, the coordination-regulated Co single-atom catalysts deliver a large half-potential of 0.93 V and a mass activity of 5480 A g metal -1 . Importantly, a Zn-air battery using the coordination-regulated Co single-atom catalysts as the cathode also exhibits a large power density and excellent stability.