A density functional theory study of oxygen reduction reaction on Me-N-4 (Me = Fe, Co, or Ni) clusters between graphitic pores

In this study, we performed first-principles density functional theory (DFT) calculations to investigate the reaction pathway of oxygen reduction reaction (ORR) on Me-N-4 (Me = Fe, Co, or Ni) catalytic clusters formed between pores in graphene supports. Our DFT results indicate that formation of Me-N-4 clusters at the edges of graphitic pores is energetically feasible. The catalytic mechanism of the Me-N-4 clusters for ORR was elucidated by calculating the binding energies of ORR intermediates O-2, O, OH, OOH, HOOH, and H2O on these clusters. It was found that O-2 could be chemisorbed on the Co-N-4 and Fe-N-4 clusters but not on the Ni-N-4 clusters. Thus, the Co-N-4 and Fe-N-4 clusters are predicted to be active for catalyzing ORR. Moreover, we observed that the O-O bond in HOOH was broken in the lowest-energy adsorption configuration of HOOH on the Fe-N-4 and Co-N-4 clusters. This process of breaking the O-O bond suggests that the Fe-N-4 and Co-N-4 clusters between graphitic pores could promote 4e(-) reduction of O-2 with a single active site. Here, our theoretical study provides an explanation to the experimental findings of 4e(-) ORR on nitrogen-derived transition metal/carbon electrocatalysts. Furthermore, we illustrated that the ORR activity of the Me-N-4 clusters could be gauged by an electronic descriptor related to the d-electron orbitals of the transition metal atom.