Fe/N/C Electrocatalysts for Oxygen Reduction Reaction in PEM Fuel Cells Using Nitrogen-Rich Ligand as Precursor

High temperature pyrolysis can significantly improve the activity and stability of Fe-based catalysts. However, unwanted iron nanoparticles, which are proven inactive to oxygen reduction reaction (ORR), will form under this procedure. Herein, a nitrogen-rich and hindrance multifunctional 6,7-di(pyridin-2-yl)pteridine-2,4-diamine (DPPD) monomer was deliberately designed and synthesized. High content of thermally stable nitrogen in DPPD can increase the degree of coordination with iron and provide a high content of active nitrogen after pyrolysis. Distorted nitrogen-rich ferrous complex polymers were successfully prepared to keep iron ions well separated and prevent them from aggregating during the heat treatment. Carbon-supported Fe-based catalysts with different initial iron loadings from 0.2 to 4.0 wt % were obtained. Transmission electron microscopy (TEM) revealed that there were no obvious nanocrystals observed, even the initial iron loading was up to 2.0 wt %. The electrochemical performance of the Fe-based catalysts was evaluated via cyclic voltammetry (CV) and linear sweep voltammetry (LSV). The result shows that an Fe-based catalyst with 2.0 wt % initial iron loading is the best ORR catalyst in acid media among all the iron loadings. Typically, in basic media, the catalyst with 2.0 wt % initial iron loading exhibits comparable electrocatalytic activity to commercial Pt/C material via an efficient four-electron-dominant ORR pathway coupled with better methanol tolerance as well as durability. XPS measurements confirmed that the outstanding activity of the catalyst with 2.0 wt % initial iron loading was likely attributed to higher content of pyridinic nitrogen, providing the highest density of active site structures.