Insight into the different ORR catalytic activity of Fe/N/C between acidic and alkaline media: Protonation of pyridinic nitrogen

Abstract

Pyrolyzed Fe/N/C catalysts have been considered as the most promising non-precious metal electrocatalysts for oxygen reduction reaction (ORR) in fuel cells. Commonly, the ORR activity of the Fe/N/C and metal-free N-doped carbon catalysts is much higher in alkaline medium than in acidic medium. The exact reason for such pH-dependent activity is not very clear yet. Herein, through the XPS test of the mixture of a Fe/N/C catalyst and a solid acid—Nafion, we found that the pyridinic N that is important for ORR, has been protonated completely. The protonation of pyridinic N in acidic medium may reduce the charge density of adjacent carbon atoms, which further influence the catalytic activity. Through comparison of ORR behaviors of three Fe/N/C catalysts prepared from different nitrogen sources, we observed that although all samples hold the same rule of ORR activity in alkaline medium higher than in acidic medium, the order the catalytic activity depends on the proportion of pyridinic N: the higher percent of pyridinic N, the higher sensitivity to the solution acidity/alkalinity. This study indicates that the pyridinic N has high ORR activity in alkaline medium, but when it is protonated in acidic medium, the catalytic activity is greatly suppressed.