(204f) Role of Graphitic Edge Plane Exposure in Nitrogen Containing Carbon Nanostructured (CNx) Catalysts During the Oxygen Reduction Reaction at PEM and DMFC Cathodes

Before PEM and direct methanol fuel cells (DMFCs) can be fully commercialized on a large scale, material costs must be reduced. The platinum cathode catalyst expense is one area where significant cost reductions will have to occur. Alternatives to the platinum oxygen reduction reaction (ORR) catalyst are limited due to the oxidizing and acidic environment of PEM and DMFCs. One class of alternative catalysts that has been studied are the carbon-nitrogen catalysts. These were first studied using macrocycles, mimicking the hemoglobin-type active site. Stability of the macrocycle ORR catalysts was significantly improved by pyrolyzing, or heat treating them in an inert atmosphere. Later, it was found that carbon-nitrogen catalysts could be produced with comparable activity and stability without the use of the expensive macrocycles. These could be produced by heat treating a carbon source, metal salt and a nitrogen source together at temperatures of 400 to 1000 °C.

The successful production of carbon-nitrogen ORR catalysts without using macrocycle precursors increased the debate in the literature as to what the nature of activity is for these carbon-nitrogen catalysts. Today, there are two main arguments in the literature as to what the nature of the active site is: an iron stabilized by nitrogen site, or a metal-free site. In our previous work, we prepared nitrogen containing carbon nanostructured (CNx) catalysts with less than 1 ppm transition metal contamination and found that they still have substantial ORR activity. Characterization of this catalyst, along with a complete set of catalysts prepared with transition metals showed that catalysts with more pyridinic-nitrogen content and a higher percentage of stacked cup nanofibers with more graphitic edge-plane exposure were more active for ORR. These findings supported the debate that a metal-free active site is part of the ORR CNx catalysts, but did not clearly define what the actual active site was or completely eliminate the possibility that there could be a second active site involving iron.

In an attempt to better understand and probe the nature of ORR activity in CNx catalysts, the carbon nanostructure has been controlled more precisely. Several nanofiber types including stacked platelets and ribbon nanofibers have been prepared and post-treated to incorporate nitrogen on the edge-planes. The role of graphitic edge-plane exposure and orientation, nitrogen types, and other characteristics on the ORR activity will be reported. Activity testing and characterization techniques used include rotating ring disk electrode (RRDE) half cell activity and selectivity testing, X-ray photoelectron Spectroscopy (XPS), Transmission Electron Microscopy (TEM) imaging, and temperature-programmed oxidation techniques.