Examination of Catalyst Loading Effects on the Selectivity of CNx and Pt/VC ORR Catalysts Using RRDE

The effect of catalyst loading levels on the oxygen reduction reaction (ORR) selectivity for water formation on both nitrogen-containing 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 µg/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.