Tuning Carbon-Based Fuel Cell Catalyst Support Structures via Nitrogen Functionalization. II. Investigation of Durability of Pt-Ru Nanoparticles Supported on Highly Oriented Pyrolytic Graphite Model Catalyst Supports As a Function of Nitrogen Implantation Dose

Nitrogen functionalization of carbon-based support materials for low-temperature fuel cell catalysts has been shown to improve catalyst-support 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 Pt-Ru 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 Pt-Ru 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 x 10(16) ions cm(-2). 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.