Nitrogen-Doped Nanocarbon Supported Pt Electrocatalysts: Oxygen Reduction and Methanol Oxidation

Currently, Pt remains to be the most effective catalysts to catalyze oxygen reduction and methanol oxidation in both acidic and alkaline solution for energy conversion through these direct electrochemical reactions in fuel cell systems. In order to significantly increase the Pt utilization in the catalysts, Pt nanoparticles (NPs) are supported on high-surface-area carbon blacks. However, a fast and significant loss of electrochemically active surface area (ECSA) and thus performance degradation are often observed with the traditional Pt/C catalysts over time. Such phenomena can be mainly ascribed to the corrosion of carbon supports, Ostwald ripening and agglomeration of Pt NPs, and the resulting loss of collective electrocatalytic sites.¹ Thus, advanced carbon materials with ordered graphitic structure including carbon nanotubes (CNTs),^2,3 carbon nanofibers (CNFs), onion-like carbon,^4,5 and graphene⁶ have been extensively studied as supports in the Pt-based electrocatalysts due to their unique surface structure, high electric conductivity, corrosion resistance and large surface areas. Noteworthy, there has been increasing evidence showing that the electrochemical and physical properties of carbon materials are extremely sensitive to heteroatom (N, B, S and P) doping into the carbon structure.^4,5,7 In particular, nitrogen can dope into carbon planes and change their electronic and structural properties. Theoretical studies have shown that nitrogen atoms can be viewed as an n-type carbon dopant that disorders carbon lattices and donates electrons to carbon. Those resulting defects from nitrogen doping may serve as active sites for subsquent Pt doposition or oxygen adsorption, thus strengthening interactions between supports and Pt nanoparticles, and facilitaiting the ORR, respectivley.⁷

Meanwhile, during the development of non-precious metal catalysts, carbon-based ORR-active M-N-C catalysts (M: Fe or Co),^8-11 containing 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. Importantly, we can control the formation of different nanostructures (e.g., CNT, onion-like carbon, graphene) during the catalyst synthesis through optimizing the transition metals, nitrogen/carbon precursors, and templates.

Here, in order to combine Pt and non-precious metal catalysts with an aim of dramatically reducing the amount of Pt and increasing the overall catalytic activity, we are conceiving to develop a hybrid cathode catalyst consisting of Pt nanoparticles and the ORR-active N-doped nanocarbon as supporting materials (Fig. 1). The ORR-active carbon supports with nitrogen doping would not only offer a remarkable support effect by geometrically and electronically modifying the loaded Pt particles, but also provide a large amount of performing non-precious ORR active sites, thereby significantly improving ORR activity and durability of Pt catalysts. In addition, the promotional role of nitrogen doping for carbon supported Pt catalysts for methanol oxidation also will be discussed in this presentation.

Fig. 1. Schematic illustration of the formation of bamboo-like nitrogen-doped graphene tubes (N-GT) and Pt/N-GT nanostructures

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