Insights into oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) active sites for nitrogen-doped carbon nanostructures (CNx) in acidic media

[Display omitted] •CNx was shown as a potential catalyst for a regenerative fuel cell system.•The ORR activity of CNx was found to be much higher than Ir/C.•The OER activity of CNx was found to be significantly higher than Pt/C.•The ORR and OER activity of CNx increased with increase in pyridinic-N content. This study demonstrates promising bifunctionality of nitrogen-doped carbon nanostructures (CNx) for ORR and OER in acidic medium. Although CNx catalysts are not as active as Pt/C in ORR and Ir/C in OER, they exhibit significantly lower combined overpotential for ORR and OER relative to those shown by the two commercial catalysts, which are highly active only for one of the two reactions, but not both. The effect of various nitrogen functionalities on the ORR and OER activity of CNx catalysts was also studied. CNx samples with higher pyridinic-N site density exhibited higher ORR and OER activity.