Chemical Nature of Catalytic Active Sites for the Oxygen Reduction Reaction on Nitrogen-Doped Carbon-Supported Non-Noble Metal Catalysts

Much work has been devoted to synthesizing the non-noble metal catalyst such as nitrogen-doped carbon-supported transition metal catalysts (denoted as metal-N-C catalyst) for the oxygen reduction reaction (ORR). However, the catalytic mechanisms and precise chemical nature of the active sites in this kind of catalyst are still controversial, which hinders the development and commercialization of this novel ORR catalyst. The objective of this work is to study the nature of active sites for ORR in the Fe-N-C catalysts. We synthesized a new family of nitrogen-doped carbon with iron catalysts (denoted as Fe-N-C catalysts) by pyrolyzing the mixtures with various ratios of a nitrogen-Atom rich heterocycle compound, 1-ethyl-3-methylimidazolium dicyanamide (EMIM-dca), and iron chloride (FeCl₃). The ORR activity (J_K at 0.8 V vs RHE, in 0.1 M KOH solution) of a typical catalyst, Fe₁₅-N-C1000, in this family is 6.65 mA/mg, which is much higher than the values of the Fe-C (0.48 mA/mg) and N-C catalysts (0.25 mA/mg). The relationship between the ORR activity and the structures (the possible active sites in particular) of the catalysts was studied under different conditions. The active site in the catalyst is found to be the Fe-N species (most likely in the form of Fe₃N). Metallic iron (Fe) particles, Fe₃C species, and N-C species are not catalytically active sites, nor do these moieties interact with the Fe-N active sites during the catalysis of the ORR. High pyrolysis temperatures and increasing the Fe content during the synthesis favor the formation of the Fe-N active sites in the final catalyst. Our study opens up new synthetic control of parameters affecting the final structure and catalyst performance and allows modifying the unexplored avenues toward new multiply heteroatom doped nonprecious ORR catalysts.