Role of Nitrogen Defects on the Activity of Fe-N-C Catalysts Towards the Oxygen Reduction Reaction

Hydrogen fuel cells are electricity-generating devices that produce clean energy with no SO_x or NO_x emissions. The oxygen reduction reaction (ORR) occurs at the cathode in hydrogen fuel cells and produces water as the major product. Currently platinum and platinum alloys are the best electrocatalyst for ORR but are expensive. Fe-N-C catalysts are a class of non-precious group materials that are promising for ORR as they show reasonable activity compared to platinum. They are synthesized by pyrolysis at about 850°C, which leads to production of several different moieties in the catalyst including iron nanoparticles in the form of nitrides, and carbides, FeN_x (x=1-4) moieties, and nitrogen moieties. A range of nitrogen defects have been identified experimentally, including pyrrolic, pyridinic, graphitic, protonated, and cyanide nitrogens with total concentrations typically ranging between 5-30 atomic %. While FeN_x sites are thought to show highest contribution to ORR activity, nitrogen defects have also been proposed to play a role in ORR. FeN_x sites are proposed to reduce O₂ to H₂O, graphitic nitrogens and protonated pyridinic nitrogens are proposed to reduce O₂ to H₂O₂, while pyridinic nitrogens are proposed to reduce H₂O₂ to H₂O. The aim of this study is to understand the role of these nitrogen moieties in ORR.

To begin, graphitic nitrogen and protonated pyridinic nitrogens were considered in varying concentrations and locations in a graphene framework. Then, they were considered in a graphene framework along with FeN₄ sites in the bulk of its sheet and its edge. First principles density functional theory (DFT) was used to model these defects and find energetics for nitrogen incorporation and that for ORR. Preliminary results for the graphene case show that protonated pyridinic nitrogens at the edges have lower nitrogen incorporation energies compared to graphitic nitrogens in the bulk, making edge sites more preferrable. These nitrogen defects may also influence where ORR intermediates bind on the surface due to change in the electronic structure of the system. Currently, we are probing this aspect and extending our analysis to the cases where FeN₄ sites are also present in the framework. This analysis will contribute towards improving the understanding of the complex structure of the Fe-N-C catalyst, which is important to improve ORR activity and catalyst stability for fuel cell applications.