Identification of a Robust and Durable FeN4Cx Catalyst for ORR in PEM Fuel Cells and the Role of the Fifth Ligand

Although recent studies have advanced the understanding of pyrolyzed Fe–N–C materials as oxygen reduction reaction (ORR) catalysts, the atomic and electronic structures of the active sites and their detailed reaction mechanisms still remain unknown. Here, based on first-principles density functional theory (DFT) computations, we discuss the electronic structures of three FeN4 catalytic centers with different local topologies of the surrounding C atoms with a focus on unraveling the mechanism of their ORR activity in acidic electrolytes. Our study brings back a forgotten, synthesized pyridinic Fe–N coordinate to the community’s attention, demonstrating that this catalyst can exhibit excellent activity for promoting direct four-electron ORR through the addition of a fifth ligand such as −NH2, −OH, and −SO4. We also identify sites with good stability properties through the combined use of our DFT calculations and Mössbauer spectroscopy data.