(617b) Interfacial Potential, Coverage and Solvation Effects on the in-Situ Structure, Stability and Activity of Fe-N-C Catalysts during ORR

Fe-N-C (iron-nitrogen-carbon) electrocatalysts have emerged as promising economic candidates for the oxygen reduction reaction (ORR) in fuel cells. 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 two dimensional graphene are proposed to be responsible for activity, but they may exist in a multiplicity of configurations related to the nitrogen environment (pyridinic, pyrrolic), site location (in-plane, edge, intrapore), site clustering, and/or nitrogen coordination.

In this work, density functional theory is employed to construct thermodynamic phase diagrams, and thereby determine potential dependent equilibrium structures for an extensive set of active sites. High coverage oxidation structures capturing co-adsorption effects are built using a graph theory algorithm, while stabilization by solvation is explicitly accounted in hydrated models using a simulated annealing inspired formalism. We show that the Fe-N-C catalyst surface gets occupied with hydroxyl or epoxy groups, resulting either from H₂O dissociation at high voltages, or from dissociation of the side product, H₂O_2(aq). Amongst all sites, pyridinic sites at zigzag edges of graphene, present in an isolated or clustered form, exhibit the highest ORR activity. However, carbon atoms neighboring them are over-oxidized with hydroxy groups within 0.1 V of ORR. Hence, high potential spikes during the fuel cell operation can restructure the active site and reduce its stability. The propensity for over-oxidation is observed for other sites as well, except ones present in the bulk, which only over-oxidize in presence of H₂O₂. These results point to active site oxidation with associated carbon/nitrogen corrosion, as important drivers of Fe-N-C activity loss during fuel cell operation. We close with evaluating electronic and mechanistic changes for active sites present in three dimensional graphitic stacks, and additionally due to nitrogen doping.