▎ 摘　　要

Graphene-supported precious metal nanoparticle hybrids have received extensive attention in recent years because of the outstanding physical and chemical properties of graphene. In this work, we synthesize high-quality reduced graphene oxide (RGO) supported monodispersed Pt nanocrystal (Pt/RGO) hybrids with the assistance of polyethyleneimine (PEI), in which PEI serves as a multi-functional molecule for the coordination with K2PtCl4, anchorage of PtII precursors on the graphene oxide surface, and chemical functionalization of Pt nanocrystals. Then, we investigate in detail the effect of the molecular weight of PEI on the electrocatalytic activity of the resultant Pt/RGO hybrids for the formic acid oxidation reaction (FAOR). Electrochemical measurements show that PEI with high molecular weight (Mw = 10 000) between the two RGO sheets limits seriously the access of electrolytes to the Pt sites. In contrast, PEI with low molecular weight (Mw = 600) between the two RGO sheets allows electrolytes to access freely the Pt sites, and the resultant Pt/RGO hybrids show enhanced electrocatalytic activity and stability for the FAOR compared to the commercial Pt/C electrocatalyst due to the ensemble effect.