▎ 摘　　要

In recent years, 3d transition metals or alloys encapsulated by graphene layers (M@NG) have been emerging as prospective electrocatalysts especially for hydrogen evolution reactions (HERs) and oxygen evolution reactions (OERs). However, this strategy is limited in preparation of high-performance oxygen reduction reaction (ORR) catalysts. Herein we prepared Co@Ag bimetallic nanoparticles embedded in nitrogen rich graphene layers via pyrolysis metal organic frameworks (MOFs). The catalyst displays extraordinarily high ORR performance with a high onset potential of 0.989 V and a half-wave potential of 0.872 V in 0.1 M KOH. Moreover, it shows a superb long-term stability performance after 5000 cycles on account of the carbon layers that protect the material from corrosion as well as high methyl alcohol tolerance under methanol environments. Density functional theory calculations suggest that carbon atoms, which are adjacent to nitrogen dopants in Co@Ag@NC, are active sites for ORR Especially, Ag "mantle" in Co@Ag@NC contributes a great deal to tuning of electronic structure of carbon active sites, thereby promoting the activity of carbon atoms and enhancing ORR kinetics. This unique Co core/Ag mantle structure can boost the overall ORR activity of M@NG while keeping cost-effectiveness due to the very low requirement of Ag content, providing a new insight into design of other efficient electrocatalysts.