Identification of durable and non-durable FeNx sites in Fe-N-C materials for proton exchange membrane fuel cells

While Fe-N-C materials are a promising alternative to platinum for catalysing the oxygen reduction reaction in acidic polymer fuel cells, limited understanding of their operando degradation restricts rational approaches towards improved durability. Here we show that Fe-N-C catalysts initially comprising two distinct FeNx sites (S1 and S2) degrade via the transformation of S1 into iron oxides while the structure and number of S2 were unmodified. Structure-activity correlations drawn from end-of-test Fe-57 Mossbauer spectroscopy reveal that both sites initially contribute to the oxygen reduction reaction activity but only S2 substantially contributes after 50 h of operation. From in situ Fe-57 Mossbauer spectroscopy in inert gas coupled to calculations of the Mossbauer signature of FeNx moieties in different electronic states, we identify S1 to be a high-spin FeN4C12 moiety and S2 a low- or intermediate-spin FeN4C10 moiety. These insights lay the groundwork for rational approaches towards Fe-N-C cathodes with improved durability in acidic fuel cells.