Insights into the role of an Fe–N active site in the oxygen reduction reaction on carbon-supported supramolecular catalysts

In this study, a nitrogen-containing ligand supramolecule named PPYTZ was successfully synthesized using 2,6-pyridinedicarboxylic acid chloride and 3,5-diamino-1,2,4-triazole in order to carry out oxygen reduction reaction (ORR). Such a polymer provides abundant coordination sites for iron ions, and the PPYTZ-Fe/C composite catalyst was formed with the PPYTZ-Fe complex loading on the surface of Vulcan XC-72 carbon. The physical characteristics and ORR performance of the composite catalysts were characterized systematically <i>via</i> various relevant techniques, and their catalytic activity and reaction mechanism were evaluated and compared. The results showed that the catalytic activities and the reaction mechanism of ORR were highly dependent on the formation of an Fe-N unit. Accordingly, the PPYTZ-Fe/C catalyst containing Fe-N active sites exhibited high ORR catalytic activity (an onset potential of +0.86 V <i>vs.</i> RHE) in 0.1 M KOH. Such an Fe-N catalyst can accelerate the adsorption of O₂ and increase the limiting current density (from 2.49 mA cm^-2 to 4.98 mA cm^-2), optimizing the ORR catalytic process from a two-electron process to a four-electron process (an <i>n</i> value of 3.8).