Density Functional Theory Study of Ni-Nx/C Electrocatalyst for Oxygen Reduction in Alkaline and Acidic Media

Graphitic Ni-N-x (x = 2, 4) and Ni-N-2 edge defect motifs in Ni-N-x/C electrocatalyst and their ORR activity are studied using density functional theory. The results show that the formation of graphitic Ni-N-x and Ni-N-2 edge defects is energetically favorable. Furthermore, the computations predict that O-2 and peroxide both chemisorb to Ni-N-2 edge site but not to graphitic Ni-N-2 and Ni-N-4 sites. This indicates that ORR in Ni-N-x/C electrocatalyst occurs predominantly on edge sites via a sequential 2 X 2e(-) process in alkaline and acidic media. The free energy diagram for O-2 reduction on Ni-N-2 edge defect shows fewer uphill processes in alkaline medium than in acidic medium predicting that Ni-N-x/C ORR electrocatalyst is more active in alkaline medium, especially at high potentials. We find that the presence of magnetism in Ni-N-2 edge site decreases the adsorption energy of H2O2 by similar to 37% as compared to the nonmagnetic case; This significant effect of magnetism on the O-2 adsorption energy suggests that the magnetic state of the catalytic sites may provide an additional degree of freedom in designing efficient non-PGM ORR electrocatalysts.