Active Site Structures in Nitrogen-Doped Carbon-Supported Cobalt Catalysts for the Oxygen Reduction Reaction

Abstract

The catalytic mechanism and the nature of active sites are revealed for the oxygen reduction reaction (ORR) with new non-noble-metal nitrogen-doped carbon-supported transition-metal catalysts (metal–N–C catalyst). Specifically, new nitrogen-doped carbon-supported cobalt catalysts (Co–N–C catalysts) are made by pyrolyzing various ratios of the nitrogen-atom rich heterocycle compound, 1-ethyl-3-methyl imidazolium dicyanamide (EMIM-dca) and cobalt salt (Co(NO 3 ) 2 ). The ORR activity ( J K at 0.8 V vs RHE, in 0.1 M KOH solution) of a typical catalyst in this family, Co 15 –N–C800, is 8.25 mA/mg, which is much higher than the ORR activity values of N–C catalysts (0.41 mA/mg). The active site in the catalyst is found to be the Co–N species, which is most likely in the form of Co 2 N. Metallic cobalt (Co) particles, Co 3 C species, and N–C species are not catalytically active sites, nor do these moieties interact with the Co–N active sites during the catalysis of the ORR. Increasing the Co salt content during the synthesis favors the formation of Co–N active sites in the final catalyst. Higher pyrolysis temperatures (e.g., a temperature higher than 800 °C) do not favor the formation of the Co–N active sites, but cause the formed Co–N active sites to decompose, which, therefore, leads to a lower catalytic activity. This reveals that the control of the parameters that affect the final structure is critical to catalyst performance and, therefore, the effective development of high-performance heteroatom-doped non-noble-metal ORR catalysts.