Comparative Study of the Oxygen Reduction Reaction on Pyrolyzed FePc in Acidic and Alkaline Media

Transition-metal and nitrogen co-doped carbon materials (M/N/C) have received increasing attention as electrocatalysts for the oxygen reduction reaction (ORR). M/N/C catalysts usually exhibit distinct ORR catalytic activity between acidic and alkaline media. The origin of such pH-dependent activity is so far unclear. Herein, we investigate the dependence of ORR activity of FePc/C catalysts on the pyrolysis temperature (300-1000 degrees C) in both acidic and alkaline media to speculate the difference between active sites in the different media. The highest ORR activity is achieved at 800 degrees C and 500 degrees C in acidic and alkaline media, respectively. In particular, at 600-800 degrees C, the ORR activity increases greatly with increasing pyrolysis temperature in acidic medium, whereas it decreases in alkaline medium, indicating different active sites present in the two media. Thermal decomposition analysis of FePc suggests that highly active sites in acidic medium are formed through the decomposition of FePc or Fe-N-4 structures at 600-800 degrees C after releasing phthalonitrile (C6H5-C2N2), whereas intact FePc or Fe-N-4 incorporated in the carbon matrix are the main active sites in alkaline medium. XPS measurements show that there is a good relationship between the ORR activity and metal N in alkaline medium. This study is of significance to the development of highly active non-precious metal catalysts.