Role of pyridinic nitrogen in the mechanism of the oxygen reduction reaction on carbon electrocatalysts.

The introduction of pyridinic nitrogen (pyri-N) into carbon-based electrocatalysts for the oxygen reduction reaction is considered to create new active sites. Herein, the role of pyri-N in such catalysts was investigated from a mechanistic viewpoint using carbon black (CB)-supported pyri-N-containing molecules as model catalysts, and the highest activity was observed for 1,10-phenanthroline/CB. X-ray photoemission spectroscopy showed that in acidic electrolytes, both pyri-N atoms of 1,10-phenanthroline could be protonated to form pyridinium ions (pyri-NH + ). In O 2 -saturated electrolytes, one of the pyri-NH + species was reduced to pyri-NH upon the application of a potential, whereas no such reduction was observed in N 2 -saturated electrolytes. This behavior was ascribed to the electrochemical reduction of pyri-NH + occurring simultaneously with the thermal adsorption of O 2 , as supported by density functional theory calculations. Moreover, according to these calculations, the coupled reduction was promoted by hydrophobic environments.