Efficient Oxygen Reduction Electrocatalysts Based on Gold Nanocluster-Graphene Composites

This paper describes the preparation and electrocatalytic activity of nanocomposites composed of reduced graphene oxide and Au25 clusters. Well‐defined nanocomposites are prepared by coating the surface of reduced graphene oxide with multiple layers of Au25 film, the thickness of which can be precisely controlled according to the preparation conditions. The electrocatalytic activity of the nanocomposites are examined for the reduction of [Ru(NH3)6]3+ and in the oxygen reduction reaction by chronoamperometry and electrochemical impedance spectroscopy as a function of Au25 thickness. Whereas the catalytic rate constants obtained for the reduction of [Ru(NH3)6]3+ are found to be rather constant with varying Au25 thickness, those for the oxygen reduction reaction increased drastically with an increasing number of Au25 layers. This increase can be ascribed to the porous structures generated in the nanocomposites. The porous channels generated in the nanocomposites offer confined space surrounded by electrified surface, greatly enhancing the electrocatalytic activity for the oxygen reduction reaction. Additional rotating disk electrode and rotating ring‐disk electrode voltammetry show that the nanocomposites support an efficient four‐electron reduction of oxygen. Cause and effect: The porous structures in gold nanocluster–graphene composite (Au25–rGO) electrodes provide a confined space and increase the residence time of O2 on the surface, facilitating the sluggish oxygen reduction reaction (ORR). The ORR at the Au25–rGO electrode proceeds by an efficient four‐electron transfer pathway to form H2O, whereas the two‐electron pathway to H2O2 is favored at a Au25‐modified electrode.