Is Fe the Most Active Site for Fe/N-Doped Graphdiyne?

We performed a systematic study on the activity of pristine, Fe-doped, N-doped, and Fe/N-codoped graphdiyne (GDY) for oxygen reduction reactions (ORRs). We found that the pristine GDY has a high overpotential because of the weak binding of the intermediates. The sp-hybridized N-doped GDY enhances the binding of the intermediates at the adjacent sp-hybridized C site, which greatly enhances its ORR activities with a low overpotential of 0.45 V. On the other hand, on Fe-doped GDY, the binding of the intermediates at the Fe site and its neighboring C sites becomes too strong, while the C site at the second nearest acetylene chain becomes the most active site with an overpotential of 0.43 V. In the case of Fe and N codoping, Fe and the C sites near Fe and N still bind the intermediates too strongly, and the most active site is located at the C with an optimal distance. The binding energy of OH* is an activity descriptor for Fe- and/or N-doped GDY. Based on the machine learning analysis of Delta G(OH*), both the properties of the active center (electronic and geometric properties) and its environment, especially the latter, play important roles in determining its activity. The scaling relation analysis and volcano plot suggest that Fe and N doping enhance the binding of the intermediates to different extents, and the C atom, which is bonded neither to N nor to Fe atom, with an optimal binding strength, becomes the most active site.