Enhancement of electrocatalytic activity for oxygen reduction reaction in alkaline and acid media from electrospun nitrogen-doped carbon nanofibers by surface modification

Nitrogen-doped carbon nanomaterials show excellent electrocatalytic activity for oxygen reduction reaction (ORR) in alkaline solution while their activity in acid media is poor. This paper reports that the activity of electrospun nitrogen-doped carbon nanofibers (NCNFs) for ORR could be enhanced by three different surface modification methods, i.e. acidification with a H2SO4/HNO3 mixture, heat treatment in NH3, and the combined treatment of the above two processes. These three methods could improve the electrocatalytic activity of the pristine NCNFs to some degree, and particularly the combination of acidification and NH3 etching achieved the best results both in 0.1 M KOH and 0.5 M H2SO4 solutions, probably because of the improvement in wettability, the diameter reduction, porous structure, more exposed edge planes, and higher proportion of pyridinic-N. Further study found that electrochemical oxidation possibly led to the change of surface state and thus the activity deactivation of the NCNFs. The present approach of surface modification is of high potential in preparing metal-free nitrogen-doped carbon electrocatalysts with high ORR activity.