The strategic regulation of the electronic properties in single-atom sites through integrating metal nanoparticles emerges as a promising route to enhance the oxygen reduction reaction (ORR) performance. Here, a symbiotic Co catalyst (CoSA/NP-NC) is successfully developed in which Co single atoms and Co nanoparticles are co-anchored within a nitrogen-doped carbon matrix. It demonstrates excellent ORR catalytic activity, with a half-wave po tential (E1/2) of 0.88 V relative to the reversible hydrogen electrode and a kinetic current density (Jk) of 20.67 mA cm− 2 at 0.80 V, surpassing the performance of standard commercial Pt/C catalysts. Advanced in situ syn chrotron infrared spectroscopy and X-ray absorption fine structure analysis unravel the tandem synergetic re action mechanism of SA and NP in symbiotic Co sites, that is, oxygen molecules first adsorbed on the Co SA site with low d-band electron occupancy to achieve rapid activation, and then transferred to the electron-rich Co NP for the reduction reaction. This symbiotic synergy promotes the O− O bond breaking in *OOH to form *O in termediates, thereby enhancing the durability and activity of the ORR.
