Catalytic activity of Co–Nx/C electrocatalysts for oxygen reduction reaction: a density functional theory study

Abstract

First-principles DFT computations are performed to explain the origin and the mechanism of oxygen reduction reaction (ORR) on Co–N_x (x = 2, 4) based self-assembled carbon supported electrocatalysts in alkaline and acidic media. The results show that the formation of graphitic Co–N₄ defect is energetically more favorable than the formation of graphitic Co–N₂ defect. Furthermore graphitic Co–N₄ defects are predicted to be stable at all potentials (U = 0–1.23 V) in the present study while Co–N₂ defects are predicted to be unstable at high potentials. Therefore the Co–N₄ defect is predicted to be the dominant in-plane graphitic defect in Co–N_x/C electrocatalysts. O₂ chemisorbs to Co–N₄ and Co–N₂ defects indicating that both defect motifs are active for the reduction of O₂ to peroxide. However, the weak interaction between peroxide and Co–N₄ defect shows that this defect does not promote complete ORR and a second site for the reduction of peroxide is required, supporting a 2 × 2e⁻ dual site ORR mechanism independent of pH of the electrolyte. In contrast, the much stronger interaction between peroxide and Co–N₂ defect supports a 2 × 2e⁻ single site ORR mechanism in alkaline and acidic media.