(509an) First Principles Modeling of FeNx Clusters and Defects in Aqueous Acidic Media: Tying Structure to Site Stability and Activity for the Oxygen Reduction Reaction

Fe-N-C (iron-nitrogen-carbon) electrocatalysts have emerged as promising economic alternatives to precious group metal-based materials for the oxygen reduction reaction (ORR) in fuel cells in acidic media. The structure of active site in these catalysts, however, is not well understood, and their poor stability in an acidic environment poses a formidable challenge for their adoption in commercial fuel cells. FeN_xC moieties in a periodic graphene structure are proposed to be responsible for activity, but they may exist in a multiplicity of different configurations related to the nitrogen environment (pyridinic vs pyrrolic), site location (in-plane, edge, intrapore), site clustering, and nitrogen coordination (FeN₃, FeN₄). Further, it is unknown which of these structures are formed by pyrolytic synthesis, presenting difficulties in assigning intrinsic activity and stability to the various sites.

In this work, we use periodic Density Functional Theory to probe the equilibrium structure and stability of different site geometries by constructing high coverage phase diagrams with hydroxyl and epoxy groups populating the Fe-N-C surface. We consider H₂O and H₂O_2(aq) (the side product) as sources for the oxidizing groups. We find that single pyridinic sites at zigzag edge of graphene show the highest activity towards ORR among all FeN_x active sites described herein. In terms of stability, nitrogen at the bridge of iron atoms in clustered sites can oxidize to NO_(g) or NO_2(g). Likewise, H₂O₂ can contribute to or possibly accelerate oxidation of the carbon environment surrounding active sites if present in their vicinity. Ab initio molecular dynamic analysis, using potential of mean force for barrier estimation, shows kinetic feasibility for the chemical dissociation of H₂O₂ to form these oxidized structures. Finally, in some cases, the presence of oxidizing groups lowers the thermodynamic limiting potential for ORR at the iron atom, providing oxidation induced deactivation as a possible mode of catalyst degradation.