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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)). The ORR activity (J at 0.8 V vs RHE, in 0.1 M KOH solution) of a typical catalyst in this family, Co-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 CoN. Metallic cobalt (Co) particles, CoC 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.

Much work has been devoted to synthesizing the non-noble metal catalyst such as nitrogen-doped carbonsupported 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 (EMIMdca), and iron chloride (FeCl 3 ). The ORR activity (J K at 0.8 V vs RHE, in 0.1 M KOH solution) of a typical catalyst, Fe 15 −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 Fe 3 N). Metallic iron (Fe) particles, Fe 3 C 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.

This work presents the synthesis of 1D aligned Pt–Ni nanostructures with ultrahigh aspect ratio (>1000) based on the anodic aluminum oxide (AAO) template and their composition- and aspect-ratio-dependent catalytic performances to methanol oxidation reaction (MOR). The 1D aligned Pt–Ni nanostructures were electrochemically deposited in the pores of the AAO template, and the diameter and length of the synthesized nanostructures are comparable with the diameter of pores and thickness of the used AAO. The aspect ratio of the 1D aligned nanostructures can be tuned merely by altering AAO templates with the appropriate aspect ratios. Voltammetric results show that the catalytic activities (both mass activities and specific activities) of the 1D aligned Pt–Ni nanostructures for MOR are composition dependent, and the highest electrocatalytic activity exhibits at a Pt/Ni molar ratio of 1:1 (PtNi). The mechanism of the promoting effect of Ni on Pt is explained based on modification of the electronic characteristics of the surface Pt atoms (Pt 4f) by Ni atoms due to the shift in electron transfer from Ni to Pt. Moreover, the catalytic activities of the 1D aligned PtNi nanostructures are in an aspect-ratio-dependent manner and increase in the order 0D PtNi nanoparticles, 1D aligned PtNi nanostructures at aspect ratios of ∼200, 500, and 1300 due to the preferential exposure of certain crystal facets and less surface defects of the 1D aligned nanostructures. This work is believed to open new and exciting possibilities for enhancing the performance of fuel cell catalysts. Our synthesized high-aspect-ratio 1D aligned nanostructures may also be useful as sensors and in other electrochemical applications.

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