▎ 摘　　要

Reducing the Pt utilization has become a desirable task in the electrocatalytic oxygen reduction reaction process. Here, we report that Pt single-atoms are successfully anchored on reduced graphene oxide sheets via facile chemical reduction. The resulting catalysts exhibit enhanced mass activity by loading tiny amounts of Pt atoms (0.48 wt%) and maximizing their atom-utilization efficiency. It is noticeable that Pt single-atom catalysts show a completely different trend in conventional Pt nanoparticle catalysts, for which H2O2 may propose as a main product due to the absence of adjacent sites for O-O breakage. A high mass activity is obtained with graphene-supported Pt single-atoms (3.10 A mg(Pt)(-1)), about 57 times that for commercial Pt/C at 0.8 V (vs. reversible hydrogen electrode). This value is also superior to state-of-the-art Pt-based catalysts for electrocatalytic oxygen reduction, indicating enhanced efficiency with high-dispersed Pt single-atoms and the stimulating metal-support interactions. The effects of particle size on the oxygen reduction process pathway are discussed through contrasting H2O2 selectivity of graphene-supported different Pt species (nanoparticle, cluster, and single-atom). Obviously, graphene-supported Pt single-atoms with isolated Pt sites will follow the two-electron pathway to generate H2O2, while graphene-supported Pt nanoparticles prefer to favor a complete reduction to form H2O.