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 × 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 H 2 O 2 by ∼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.