Revealing the optimal configuration for synergy effect of metal nanoparticles and MN4 sites for oxygen reduction reaction

Metal-nanoparticles (M-NPs) can adjust the electronic structure of the singly atomic MN4 to promote ORR reactivity effectively. However, the synergy effect of M-NPs and MN4 is influenced in a multifactorial way, and is difficult to investigate their separate effects on promoting ORR. Herein, we establish a new strategy to synthesize catalysts containing highly active MN4 and M-NPs composite site and study their separate effects on ORR specifically. The MN4 can be controlled in high-density, and meanwhile the M-NPs can be regulated precisely at very fine nanoscale. Our studies reveal that the MN4 density shall be high enough so that the ORR reactivity can depend on the M-NPs size. The catalyst with clusters, i.e. H-(MCs-(FeCo)N4), can show the optimal ORR activity (jm@0.85 V) ~1.5 times as high as commercial Pt/C. This work not only develop a high-performance ORR catalyst, but also propose a reliable strategy for the MN4/M-NPs composite site catalyst synthesis.