Bottom-Up Construction of Active Sites in a Cu-N₄-C Catalyst for Highly Efficient Oxygen Reduction Reaction

Bottom-up construction of efficient active sites in transition metal-nitrogen-carbon (M-N-C) catalysts for oxygen reduction reaction (ORR) from single molecular building blocks remains one of the most difficult challenges. Herein, we report a bottom-up approach to produce a highly active Cu-N₄-C catalyst with well-defined Cu-N₄ coordination sites derived from a small molecular copper complex containing Cu-N₄ moieties. The Cu-N₄ moieties were found to be covalently integrated into graphene sheets to create the Cu-N₄ active sites for ORR. Furthermore, the activity was boosted by tuning the structure of active sites. We find that the high ORR activity of the Cu-N₄-C catalyst is related to the Cu-N₄ center linked to edges of the graphene sheets, where the electronic structure of the Cu center has the right symmetry for the degenerate π* orbital of the O₂ molecule. These findings point out the direction for the synthesis of the M-N-C catalysts at the molecular level.