Characterizing Complex Gas–Solid Interfaces with in Situ Spectroscopy: Oxygen Adsorption Behavior on Fe–N–C Catalysts

Electrocatalysts for the oxygen reduction reaction within polymer electrolyte membrane fuel cells based on iron, nitrogen, and carbon elements (Fe–N–C) have received significant research attention as they offer an inexpensive alternative to catalysts based on platinum-group metals. Although both the performance and the fundamental understanding of Fe–N–C catalysts have improved over the past decade, there remains a need to differentiate the relative activity of different active sites. Toward this goal, our study is focused on characterizing the interactions between O₂ and a set of five structurally different Fe–N–C materials. Detailed characterization of the Fe speciation was performed with ⁵⁷Fe Mössbauer spectroscopy and soft X-ray absorption spectroscopy of the Fe L_3,2-edge, whereas nitrogen chemical states were investigated with X-ray photoelectron spectroscopy (XPS). In addition to initial sXAS and XPS measurements performed in ultra-high vacuum (UHV), measurements were also performed (at the identical location) in an atmosphere of 100 mTorr of O₂ at 80 °C (O₂-rich). XPS and sXAS results reveal the presence of several types of FeN_xC_y adsorption sites. FeN_xC_y sites that are proposed as the most active ones do not show significant change (based on the techniques used in this study) when their environment is changed from UHV to O₂-rich. Correlation with Mössbauer and sXAS results suggests that this is most likely due to the persistence of strongly adsorbed O₂ molecules from their previous exposure to air. However, other species do show spectroscopic changes from UHV conditions to O₂-rich. This implies that these sites have a weaker interaction with O₂ that results in their desorption in vacuum conditions and re-adsorption when exposed to the O₂-rich environment. The nature of these weakly and strongly O₂-adsorbing FeN_xC_y sites is discussed in the context of different synthetic and processing parameters employed to fabricate each of these five Fe–N–C materials.