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

First-principles DFT computations are performed to explain the origin and the mechanism of oxygen reduction reaction (ORR) on Co–Nx (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–Nx/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.