Towards understanding ORR activity and electron-transfer pathway of M-N_x/C electro-catalyst in acidic media

M-N_x/C (M = Fe, Co) type electro-catalyst as a promising alternative to Pt-based electro-catalyst for oxygen reduction reaction (ORR) in fuel cells has been studied for years. However, the mechanism of this four-electron process involving several successive steps and the dynamic intermediate (hydrogen peroxide) participation still remains obscure. In this study, a series of Co-N_x/C with varied densities of Co-N_x sites have been obtained for probing the ORR activity and pathway in acid media via adopting zinc ions as a size-comparable template to disperse and regulate Co-N_x sites. Importantly, it has been found that the half-wave potential can be positively correlated with the weight percentage of ionic cobalt species, which suggests the decisive role of the density of Co-N_x sites on ORR activity. More importantly, our results suggest that both Co-N_x/C and 20 wt% Pt/C catalyze ORR via two successive steps: ORR begins at the onset potential accompanying the generation of H₂O₂ intermediate, which is more easily to be adsorbed on platinum than on Co-N_x/C surface as detected by ring electrode; Subsequently at the half-wave potential and more negative, H₂O₂ is further reduced immediately and efficiently on platinum, but unfortunately at much lower rate on Co-N_x/C. Thus in addition to improving the initial ORR activity by maximizing the M-N_x coordination, it will be of great significance to endow the non-platinum catalysts with surface active sites capable of adsorbing and efficiently reducing the H₂O₂ intermediate for the complete oxygen reduction to water.