Surface modification and doping of graphitic-carbon catalyst support materials in fuel cell systems, particularly via nitrogen functionalization, has been shown to improve catalyst performance and durability through the optimization of catalystÀsupport interactions. To ascertain the nature of these interactions, Raman and X-ray photoelectron spectroscopy were used to study the structural and chemical modifications that nitrogen ion beam implantation caused to highly oriented pyrolitic graphite (HOPG) model catalyst support systems. Ion implantation doses explored in this work ranged over 2 orders of magnitude from 9.0 Â 10 14 to 9.6 Â 10 16 ions cm À2 . Low doses of nitrogen result in a large amount of structural damage with little incorporation of nitrogen. However, it was found that with increasing dosage the incremental increase in structural damage was marginal, while the percentage of nitrogen on the HOPG surface continued to increase significantly until both the level of damage and amount of nitrogen incorporated into the graphitic structure reached saturation. A near-surface nitrogen saturation level of approximately 6À8 atomic % was achieved with a dosage equal to or greater than 2.5 Â 10 16 ions cm À2 . The nitrogen implantation altered the initial pure sp 2 -hybridized graphitic carbon and resulted in the formation of sp 3 -hybridized carbon while also incorporating nitrogen into the graphitic network in the graphitic, pyridinic, and pyrrolic form. This work sets the stage for understanding the effect of the amount and functionality of nitrogen on the durability of model carbon-supported fuel cell electrocatalysts, discussed in Part II (10.1021/ jp112236n) of this work.

Nitrogen functionalization of carbon-based support materials for lowtemperature fuel cell catalysts has been shown to improve catalystÀsupport interactions and therefore enhance both the performance and durability of the supported electrocatalyst. While previous work has focused on pure Pt electrocatalysts, this work focuses on understanding the role of nitrogen functionalization on the durability of PtÀRu alloy nanoparticle catalysts. A well-defined model catalyst system approach is used by employing highly oriented pyrolytic graphite (HOPG) as a model graphitic carbon support, nitrogen ion beam implantation as the doping route, and magnetron sputtering from a single alloyed PtÀRu target for nanoparticle catalyst deposition. A series of PtRu/HOPG substrates with different levels of doped nitrogen but very similar initial PtRu nanoparticle coverage, size, and composition were evaluated using TEM and XPS before and after potential cycling. As compared to an undoped support, supports doped with low nitrogen dosage levels (both oxygen and nitrogen sites are present, oxygen sites are predominant, and nitrogen concentration is relatively low) appear to have a negative effect on PtRu nanoparticle stability. However, higher nitrogen doses (both oxygen and nitrogen sites are present, nitrogen concentration is relatively high) have a positive effect on durability, reaching an optimum with an implantation dosage of 4.7 Â 10 16 ions cm À2 . The improvement in durability can be directly related to nitrogen levels and specifically to the amount of pyridinic nitrogen. It is shown that strong positive tethering effects are related to formation of clustered multinitrogen defects, i.e., pyridinic rings in which more than one carbon is replaced with nitrogen, a condition that is met only when the nitrogen dosage is sufficiently high.

No abstract

This study investigates the role of the nitrogen doping on the durability of Pt-Ru metal nanophase catalysts supported on highlyoriented pyrolytic graphite (HOPG) substrates. The effect of the ion dose during N 2 low ion energy implantation on the degree of the HOPG modification and levels of nitrogen doping are evaluated using X-ray photoelectron and Raman spectroscopy. PtRu catalyst metal was deposited onto unmodified and N-implanted HOPG substrates using microwave and magnetron sputtering deposition routes, resulting in deposition either with or without preferential nucleation in the defect-sites. The role of the nitrogen on improved bonding between catalyst and support is evident from the microscopic evaluation of substrates after potential cycling.

No abstract