Probing active sites in iron-based catalysts for oxygen electro-reduction: A temperature-dependent 57Fe Mössbauer spectroscopy study

• Published in: Catalysis today Vol. 262; pp. 110 - 120
• Main Authors: Sougrati, Moulay Tahar, Goellner, Vincent, Schuppert, Anna K., Stievano, Lorenzo, Jaouen, Frédéric
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.03.2016; Elsevier
• Subjects: Carbon; Chemical Sciences; Electrocatalysis; Fuel cells; Iron catalysts; Material chemistry; Mössbauer spectroscopy; Oxygen reduction reaction; Electrocatalysis; Carbon; Fuel cells; Oxygen reduction reaction; Mössbauer spectroscopy; Iron catalysts
• ISSN: 0920-5861; 1873-4308
• DOI: 10.1016/j.cattod.2015.10.017

•A Fe-N-C catalyst almost exclusively containing isolated iron atoms was synthesized.•57Fe Mössbauer spectroscopy identifies two quadrupole doublets, D1 and D2.•D1 and D2, attributed to FeN4 moieties in N-doped carbon, do not vary down to 5K.•The Lamb-Mössbauer factors of D1 and D2 were evaluated for the first time.•A correlation between O2 reduction activity and the content of doublet D1 is found. Two Fe-N-C electrocatalysts for oxygen reduction were studied by 57Fe Mössbauer-spectroscopy between 300 and 5K. 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 5K. 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.