Nitrogen-containing carbon nanostructure (CN x ) catalysts developed by acetonitrile pyrolysis have been studied to better understand their role in the oxygen reduction reaction (ORR) in PEM and direct methanol fuel cell environments. Additional functionalization of the CN x catalysts with nitric acid has the ability to improve both the activity and selectivity towards ORR.

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The effect of catalyst loading levels on the oxygen reduction reaction (ORR) selectivity for water formation on both nitrogencontaining nanostructured carbon (CN x ) and commercial Pt/Vulcan carbon (VC) catalysts was examined using the rotating ring disk electrode (RRDE) method. Catalyst loadings from 142 to 1420 mg/cm 2 were studied. It was found that catalyst loading played a significant role in determining the selectivity for water formation, with lower loading levels showing a much higher selectivity for H 2 O 2 . The phenomenon was probed by examining the experimental collection efficiencies of the RRDE for each loading level and the catalyst thickness with scanning electron microscopy. The difference between experimental and theoretical collection efficiencies did not contribute significantly to the change in selectivity, although catalyst thickness did. It appears that a multistep mechanism where H 2 O 2 is an intermediate can account for this behavior. More of this H 2 O 2 intermediate can be detected when the reaction path length (catalyst thickness) is reduced. H 2 O 2 electroreduction experiments showed the CN x materials to have the ability to catalyze further reduction of hydrogen peroxide to water, supporting the possibility of a two-step mechanism.A significant hurdle still to be overcome in developing proton exchange membrane (PEM) and direct methanol fuel cells (DMFCs) is the cost and performance of the oxygen reduction reaction (ORR) cathode catalysts. Current commercial cathode catalysts are Pt and Pt alloys supported on carbon blacks. 1,2 Significant research is being done to reduce the amount of Pt through monometallic, 3,4 binary metallic, 5-7 and ternary metallic systems. 8,9 In addition to the Pt catalysts, there are also ruthenium, 10,11 chalcogenide, 11-13 and carbon-nitrogen 14-22 ORR catalysts being studied. Not only can Pt-free catalysts reduce the price of the catalyst, but they also have advantages in direct methanol 10-13,23,24 and mixed reactant 25,26 fuel cells, where they have superior selectivity to oxygen reduction over methanol oxidation.Typical ORR catalyst research involves a path where catalysts are first screened for ORR activity and selectivity for water formation in the rotating ring disk electrode (RRDE) half cell environment. Then the best performing RRDE catalysts (and sometimes a few poor ones for comparison) are manufactured into membrane electrode assemblies and tested in full fuel cells.The RRDE half cell technique is generally regarded as the best method for simultaneously testing the ORR activity and selectivity of the catalysts directly. In this technique, a cyclic voltammogram (CV) on a catalyst-coated disk is performed to monitor the activity of the catalyst, while a ring (typically Pt) surrounding the disk is held at a constant potential where hydrogen peroxide can be detected in an acidic environment to simulate the cathode side on the PEM or DMFC fuel cell. The reactions that may occur at the disk are the desired complete reduction to water 27and the unde...

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