(530e) First Principles Modeling of the Structure of Fe-N-C Defects in Acidic Media: Role of Oxidizing Functionalities on the Stability and Activity for ORR

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 the active site in these catalysts, however, is not well understood, and the poor stability of Fe-N-C electrocatalysts in an acidic environment poses a formidable challenge for the adoption of this technology in commercial fuel cells. While FeN_4, with Fe in a C bivacancy and N substituting four neighboring C atoms in a periodic graphene structure, has been widely proposed to be the active site in these catalysts, these and related species may exist in different configurations with possible variations in both the nitrogen environment (pyridinic vs pyrrolic) and the site location (in the bulk of a graphene layer, at its edge, or between graphitic pores), making it difficult to ascribe a particular mode of stability loss as the governing mechanism of degradation for the catalysts.

In this work, we use periodic Density Functional Theory to probe the equilibrium structure of different configurations of FeN₄ active sites by constructing high coverage phase diagrams with hydroxyl and epoxy groups populating the Fe-N-C surface. We consider H₂O and H₂O_2(aq) (side product of ORR) as two possible sources of these oxygenated intermediates. We find that spectator OH^* species are present on the Fe site under operating potentials for ORR, and that H₂O₂ can contribute to or possibly accelerate oxidation of the carbon environment surrounding the active site. A detailed ab initio molecular dynamic analysis, using potential of mean force methods for barrier estimation, also shows the kinetic feasibility for the chemical dissociation of H₂O₂ to form these oxidized structures. The presence of oxidizing groups lowers the thermodynamic limiting potential for ORR, thus providing oxidation induced deactivation as a possible mode of catalyst degradation.