Density Functional Theory Study of Ni–Nx/C Electrocatalyst for Oxygen Reduction in Alkaline and Acidic Media
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Abstract
Graphitic Ni–Nx (x = 2, 4) and Ni–N2 edge defect motifs in Ni–Nx/C electrocatalyst and their ORR activity are studied using density functional theory. The results show that the formation of graphitic Ni–Nx and Ni–N2 edge defects is energetically favorable. Furthermore, the computations predict that O2 and peroxide both chemisorb to Ni–N2 edge site but not to graphitic Ni–N2 and Ni–N4 sites. This indicates that ORR in Ni–Nx/C electrocatalyst occurs predominantly on edge sites via a sequential 2 × 2e– process in alkaline and acidic media. The free energy diagram for O2 reduction on Ni–N2 edge defect shows fewer uphill processes in alkaline medium than in acidic medium, predicting that Ni–Nx/C ORR electrocatalyst is more active in alkaline medium, especially at high potentials. We find that the presence of magnetism in Ni–N2 edge site decreases the adsorption energy of H2O2 by ∼37% as compared to the nonmagnetic case. This significant effect of magnetism on the O2 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.
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