(5i) Nitrogen-Containing Carbon Nanostructured (CNx) Catalysts for the Oxygen Reduction Reaction in PEM and Direct Methanol Fuel Cells

PEM and direct methanol fuel cells (DMFCs) have the potential to be used as portable, fast start-up energy conversion devices, however, there are some technical and economic problems that still need to be addressed before full-scale commercialization can occur. One component that still needs improvement is the oxygen reduction reaction (ORR) cathode catalyst. The ORR at the cathode is kinetically slow, limiting the performance of the fuel cells. Due to the slow kinetics, large quantities of platinum are used as the commercial catalyst at the cathode in both PEM and DMFCs, significantly increasing the cost. Alternate catalysts to platinum are limited due to the acidic and oxidizing environment of the fuel cell cathode. Carbon-nitrogen catalysts have been studied as alternatives to platinum. Early on, these catalysts were transition metal containing macrocycles, which were selected because of their similarity to the hemoglobin-type centers. While the macrocycles were active for ORR, they were not stable in the acidic environment. It was then found that heat treating, or pyrolyzing, the macrocycles at temperatures of 400-1000°C in an inert atmosphere increased the stability of the catalysts while maintaining high ORR activity. Studies on the pyrolyzed macrocycle catalysts showed that the macrocycle center did not remain intact after heat treatment, opening up the debate on the nature of ORR activity for these catalysts. The debate was further enflamed when it was found that ORR catalysts could also be produced using simple sources of nitrogen, carbon and a transition metal, like ammonia, carbon black, and iron acetate. Currently, there are two main arguments for the nature of activity for the carbon-nitrogen ORR catalysts: (i) metal (usually iron) stabilized by nitrogen is the active site and (ii) the active site is transition metal free. If the nature of ORR activity of these carbon-nitrogen catalysts can be definitely determined, active site concentration can be increased, thereby increasing the ORR activity to levels competitive to commercial platinum.

At Ohio State, we have grown nitrogen-containing carbon nanostructured (CNx) catalysts over oxide supports with less than 1 ppm transition metals contamination to study the nature of activity. Metal-free CNx catalysts were found to be active for ORR, supporting the argument for a transition metal-free active site. For comparison, catalysts were also grown using iron, nickel and cobalt impregnated on the supports. The support and metals were washed away after growth of the carbon nanostructures using acid and base washes, leaving the CNx. CNx catalysts grown over Fe- and Co- containing supports were more active for ORR than the transition metal-free catalyst, while those grown over Ni-containing supports were less active for ORR. Characterization of the catalysts through TEM and XPS found that the catalysts with higher quantities of stacked cup nanofibers and pyridinic-type nitrogen were more active for ORR.

Present work involves the involves investigation of the nature of the activity through control of both the nanostructure and nitrogen content, effect of heteroatom incorporation into the carbon beyond nitrogen on the activity, and the application of these catalysts in a DMFC environment.