Synergistically enhanced oxygen reduction electrocatalysis by atomically dispersed and nanoscaled Co species in three-dimensional mesoporous Co, N-codoped carbon nanosheets network

•3D mesoporous Co-N-C nanosheets network enwrapping Co nanohybrids were prepared.•The nanohybrids possess high content of Co-N4-type single-atom sites (>5 wt%).•The synergy of the two Co species lets the nanohybrids show superior ORR activity.•Such nanohybrids manifest better electrocatalytic stability than Pt/C catalyst.•The contributions of atomic and nanoscaled Co species were determined.