Tuning Carbon-Based Fuel Cell Catalyst Support Structures via Nitrogen Functionalization. I. Investigation of Structural and Compositional Modification of Highly Oriented Pyrolytic Graphite Model Catalyst Supports as a Function of Nitrogen Implantation Dose

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 catalyst–support 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 × 1014 to 9.6 × 1016 ions cm–2. 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 6–8 atomic % was achieved with a dosage equal to or greater than 2.5 × 1016 ions cm–2. The nitrogen implantation altered the initial pure sp2-hybridized graphitic carbon and resulted in the formation of sp3-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.