Formation of the active site structures during pyrolysis transformation of Fe-phthalocyanine into Fe-Nx-C electrocatalysts for the oxygen reduction reaction

Abstract

Fe-Nx-C electrocatalysts for the oxygen reduction reaction are typically fabricated via pyrolysis. However, the pyrolysis process is poorly understood. Therefore, a systematic investigation was initiated to elucidate the effects of the pyrolysis conditions (atmosphere and temperature) on the evolution of active sites starting from iron phthalocyanine supported over carbon black. The atomic level dispersion of Fe-Nx is sustained up to 600 °C and afterward, the growth of iron oxide nanoparticles is observed. Interestingly, the different X-ray absorption spectroscopy fingerprints acquired during in-situ and ex-situ experiments indicated the bonding of oxygen as a fifth ligand on the Fe sites when exposed to the open air. The ORR activities were analyzed in acidic and alkaline media. The best electrocatalytic activity was observed for the electrocatalysts pyrolyzed at 600 °C. Above this temperature, a reduction in the activity was observed. Surface-to-reactivity analysis was carried out identifying the relationship between surface chemistry/morphology and electrocatalytic activity.