Incorporation of Multinuclear Copper Active Sites into Nitrogen-Doped Graphene for Electrochemical Oxygen Reduction

Multinuclear metal active sites are widely used as catalytic reaction centers in metalloenzymes and generally show high catalytic activity. For example, laccases are known to catalyze the oxygen reduction reaction (ORR) to water at a multinuclear copper site with almost no energy loss. The ORR is an important reaction not only in oxygenic respiration but also in future energy generation devices such as polymer electrolyte fuel cells and metal-air batteries. For large-scale commercialization of these devices, there is a need to develop highly active ORR electrocatalysts based on nonprecious metals. Incorporation of multinuclear metal active sites in conductive materials such as carbon will allow us to develop highly active electrocatalysts like metalloenzymes. However, such methods had not been established yet. Herein, we report a copper-based ORR electrocatalyst with multinuclear copper active sites in nitrogen-doped graphene. The electrocatalyst was synthesized from the mixture of graphene oxide and a multinuclear copper complex in a short-period heating method. Electrochemical measurements revealed that the obtained electrocatalyst showed the highest electrocatalytic activity for the ORR in the Cu-based electrocatalysts in neutral aqueous solution. Physicochemical measurements including in situ X-ray absorption spectroscopy revealed the incorporation of multinuclear copper sites. Our synthetic approach will offer guidance for developing highly active electrocatalysts utilizing multinuclear metal sites not only for the ORR but also for other electrocatalytic reactions.