Overall Oxygen Electrocatalysis on Nitrogen-Modified Carbon Catalysts: Identification of Active Sites and In Situ Observation of Reactive Intermediates.

The recent mechanistic understanding of active sites, adsorbed intermediate products, and rate-determining steps (RDS) of nitrogen (N)-modified carbon catalysts in electrocatalytic oxygen reduction (ORR) and oxygen evolution reaction (OER) are still rife with controversy because of the inevitable coexistence of diverse N configurations and the technical limitations for the observation of formed intermediates. Herein, seven kinds of aromatic molecules with designated single N species are used as model structures to investigate the explicit role of each common N group in both ORR and OER. Specifically, dynamic evolution of active sites and key adsorbed intermediate products including O2 (ads), superoxide anion O2 - *, and OOH* are monitored with in situ spectroscopy. We propose that the formation of *OOH species from O2 - * (O2 - *+H2 O→OOH*+OH- ) is a possible RDS during the ORR process, whereas the generation of O2 from OOH* species is the most likely RDS during the OER process.