Degradation Mechanisms of Platinum Group Metal-Free Oxygen Reduction Reaction Catalyst based on Iron Phthalocyanine

Platinum group metal-free catalysts have been considered the most promising alternative for platinum-based catalysts for the oxygen reduction reaction in fuel cells. Despite the significant advancement made in activity, their viability as fuel cell catalysts is still questionable due to their low durability. So far, deciphering the degradation mechanisms of this class of catalysts has been hampered by their undefined structure. Herein, we used a molecular model catalyst, iron-phthalocyanine, featuring Fe−N₄ active sites with resemblance to those in the more active Fe−N−C catalysts, and studied their degradation mechanisms. Based on X-ray photoelectron spectroscopy and the electrochemical measurements, three main demetallation processes were identified: at potentials higher than 0.65 V vs. RHE, where the metal center is Fe³⁺, an electrochemical oxidation of the ligand ring is occurring, between 0.6 and 0.2 V vs. RHE, Fenton reagents are produced and attack the catalyst and support, and at lower voltages, where peroxide is produced by the catalyst and the carbon support. The combination of the different iron oxidation states together with the oxygen species directs to different degradation mechanisms. The decay rates obtained in the stability measurements establish what is mainly causing the loss of activity. Thereby, this model molecule can aid in understanding the degradation mechanisms of other platinum group metal-free oxygen reduction reaction catalysts.