Nitrogen-Doped Large-Sized Graphene Tubes As an Active Support for a Hybrid Pt Electrocatalyst Towards Oxygen-Reduction

Currently, Pt remains the most effective catalysts to catalyze the oxygen reduction reaction (ORR) in both acidic and alkaline solutions for polymer electrolye fuel cells and meal-air batteries.(1) In order to significantly increase Pt utilization in the catalysts, Pt nanoparticles are supported on high-surface-area carbon blacks such as Vulcan VX-72 (Pt/C); however, a fast and significant loss of electrochemically active surface area (ECSA), and thus performance degradation, are often observed with traditional Pt/C catalysts over long periods of operation due to corrosion of the carbon supports and the Ostwald ripening and agglomeration of Pt nanoparticles.(2) To overcome activity and stability limitations, use of advanced carbon materials with ordered graphitic structures including carbon nanotubes (CNTs),(3) onion-like carbon,(4, 5) and graphene(6) is able to provide a new opportunity for Pt electrocatalysts with improved activity and durability.

In exploring carbon-based ORR transition metal-nitrogen-carbon (M-N-C) catalysts (M: Fe or Co),(7-11) in situ formation of various carbon nanostructures with nitrogen doping can be realized by catalyzing the decomposition of the nitrogen/carbon precursor at high temperatures (800-1000°C),(12) which was found to result in catalysts with superior activity for the ORR is comparison to other types of nonprecious metal catalysts (NPMCs). Importantly, we can control the formation of different nanostructures (e.g., CNT, onion-like carbon, graphene) during the catalyst synthesis by optimizing the transition metals, nitrogen/carbon precursors and templates.(12) The highly graphitized carbon nanostructures present in the Fe-N-C materials may serve as a matrix for hosting catalytically active nitrogen-metal moieties.(12) The presence of graphitized carbon appears to enhance the stability of the ORR catalysts.(8) In exploring such carbon nanostructure-rich catalysts for ORR, although substantial progress has been achieved in the synthesis and performance improvement, practical resolution regarding its long-term durability and high activity in fuel cells or other energy application is still far from effective.

Herein, we conceive a new method to prepare highly active and stable ORR catalysts by innovatively coupling Pt nanoparticles and highly active Fe-N-C materials in a unique hybrid configuration (Figure 1). In doing so, large-diameter nitrogen-doped graphene tubes (N-GTs) derived from dicyandiamide (DCDA), iron acetate, and metal organic frameworks (MOF), MIL-100(Fe), viaa high temperature method were prepared as novel supports for the development of Pt catalysts. The large size of N-GTs provides a better platform than common carbon nanotubes (with diameters usually less than 30 nm), thus favorably anchoring the deposited metal nanoparticles. While the as-prepared N-GTs were found to show good intrinsic ORR activity and stability in acidic electrolytes, by modification with well-dispersed Pt nanoparticles, the unique Pt/N-GT hybrid materials were found to provide excellent performance that is superior to commercial Pt/C catalyst. The observed enhancements are likely due to the complementary ORR active sites on N-GTs, their highly graphitized structure formed during the high-temeprature heat treatment process, and favourable interactions between the nitrogen dopant species and Pt nanoparticles.

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