Coordination-Dependent Oxygen Reduction Reaction Activity of Single Atom Co-Nx-C Electrocatalysts

The coordination number between the bound metal center and the supporting nitrogen atoms in single-metal-atom nitrogen-carbon catalysts (M-N-C) is an important structural parameter that can impact the catalytic activity. Understanding the structure-activity relationship between coordination number and catalytic activity is made challenging by difficulties in obtaining M-N-C catalysts with precisely controlled metal-nitrogen coordination environments. Herein, we address this challenge using heterogeneous molecular catalysts for the oxygen reduction reaction (ORR) constructed by depositing structure-defined organometallic molecules on a catalytically inert carbon nanotube substrate. The explicit metal-nitrogen coordination environments enabled us to establish accurate ORR activity-structure correlations for cobalt metal centers with first-shell Co-N coordination numbers of 3 to 5 (Co-Nx, where x = 3, 4, and 5). A good agreement between theoretical predictions and experimental ORR activity and selectivity was seen. Of note, Co atoms in an asymmetric coordination environment were found to exhibit higher activity for the two-electron (2e) ORR. Kinetic studies and operando spectroscopic measurements further revealed that the first-shell coordinating C or N atoms in asymmetric Co-N3 and Co-N5 centers could be protonated and participate in the ORR as a proton relay. The present study may aid in understanding the role of the coordination environment in other metal-based catalytic systems being applied to renewable energy conversion reactions.