Active Site Identification in FeNC Catalysts and Their Assignment to the Oxygen Reduction Reaction Pathway by In Situ ⁵⁷Fe Mossbauer Spectroscopy

FeNC catalysts are the most promising substitutes for Pt-based catalysts for the oxygen reduction reaction in proton exchange fuel cells. However, it remains unclear which FeN4 moieties contribute to the reaction mechanism and in which way. The origin of this debate could lie in various preparation routes, and therefore the aim of this work is to identify whether the active site species differ in different preparation routes or not. To answer this question, three FeNC catalysts, related to the three main preparation routes, are prepared and thoroughly characterized. Three transitions A-C that are distinguished by a variation in the local environment of the deoxygenated state are defined. By insitu Fe-57 Mossbauer spectroscopy, it can be shown that all three catalysts exhibit a common spectral change assigned to one of the transitions that constitutes the dominant contribution to the direct electroreduction of oxygen. Moreover, the change in selectivity can be attributed to the presence of a variation within additional species. Density functional theory calculations help to explain the observed trends and enable concrete suggestions on the nature of nitrogen coordination in the two FeN4 moieties involved in the oxygen reduction reaction of FeNC catalysts.