A density functional theory study of oxygen reduction reaction on non-PGM Fe–N_x–C electrocatalysts

First-principles density functional theory (DFT) calculations were performed to explain the stability of catalytically active sites in Fe–N_x–C electrocatalysts, their ORR activity and ORR mechanism. The results show that the formation of graphitic in-plane Fe–N₄ sites in a carbon matrix is energetically favorable over the formation of Fe–N₂ sites. Chemisorption of ORR species O₂, O, OH, OOH, and H₂O and O–O bond breaking in peroxide occur on both Fe–N₂ and Fe–N₄ sites. In addition to the favorable interaction of ORR species, the computed free energy diagrams show that elementary ORR reaction steps on Fe–N_x sites are downhill. Thus, a complete ORR is predicted to occur via a single site 4e⁻ mechanism on graphitic Fe–N_x (x = 2, 4) sites. Because of their higher stability and working potential for ORR, Fe–N₄ sites are predicted to be prime candidate sites for ORR in pyrolyzed Fe–N_x–C electrocatalysts.