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

First-principles density functional theory (DFT) calculations were performed to explain the stability of catalytically active sites in Fe-Nx-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-Nx sites are downhill. Thus, a complete ORR is predicted to occur via a single site 4e(-) mechanism on graphitic Fe-Nx (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-Nx-C electrocatalysts.