Evaluation of ORR active sites in nitrogen-doped carbon nanofibers by KOH post treatment

Oxygen reduction on N-doped carbon nanomaterials is believed to take place at either N-centered active sites (C-N-x) or Fe-centered active sites (Fe-N-x). In this work the origin of the oxygen reduction on nitrogen-doped carbon nanofibers (N-CNFs) is investigated by removing nitrogen and iron from the N-CNF surface using high temperature KOH treatment. The activities for the oxygen reduction reaction (ORR) in 0.5 M H2SO4 are correlated with the XPS results and discussed with respect to the contribution from C-N-x and Fe-N-x active sites. Increasing the time and temperature of the KOH treatment decreased the iron and nitrogen content at the N-CNF surface. The contribution from Fe-N-x active sites was found to be minor compared to the C-N-x active sites as the KOH-treated N-CNFs with no iron in the surface still showed considerable ORR activity. Furthermore, the activity was maintained when the fraction of pyridinic-N was greatly reduced compared to quaternary-N. Finally, even when no iron or nitrogen could be detected by XPS, 50% of the initial oxygen reduction activity of the N-CNFs persisted. It is therefore suggested that there are active sites not originating from iron or nitrogen atoms, but rather from a distinct carbon environment.