Résumé
Two Fe-N-C electrocatalysts for oxygen reduction were studied by 57Fe Mössbauer-spectroscopy between 300 and 5 K. The first catalyst contains almost exclusively FeNxCy moieties while the second contains additional crystalline phases, i.e. metallic iron and iron carbide. The Mössbauer parameters of two quadrupole doublets named D1 and D2, attributed to low and medium spin FeN4/C moieties, respectively, do not change with temperature down to 5 K. This indicates that such moieties do not undergo phase transition or magnetic ordering, supporting the view that the active sites are localized on isolated iron atoms. At room temperature, the Lamb-Mössbauer factors of doublets D1 and D2 are 0.46 and 0.52, smaller than those of α-Fe (0.67) and γ-Fe (0.78). These values allow for the first time a precise Mössbauer quantification of Fe species in Fe-N-C catalysts. The ORR activity is best correlated with the absolute content of the FeN4/C moiety associated with doublet D1, assigned to a FeIIN4/C moiety in low-spin state. The ORR turnover frequency of such moieties is however known to depend on chemical and electronic properties of the carbon matrix, which will require additional descriptor(s) than the site density in order to precisely interpret the ORR activity of such materials.