Comprehensive Spectroscopic Elucidation of Atomically Dispersed FeN₄ Active Sites of Fe-N-C Electrocatalysts

To gain insights into single-atom active sites of iron, nitrogen-doped carbon (Fe-N-C), we performed comprehensive element-specific spectroscopies including Fe K-edge X-ray absorption, ⁵⁷Fe Mössbauer, and Fe Kβ_1,3 and valence-to-core (VtC) X-ray emission spectroscopy for two Fe-N-C electrocatalysts that exclusively have atomically dispersed FeN₄ active sites for the oxygen reduction reaction. Experimental VtC X-ray emission spectra (VtC-XES) coupled with spectral simulation of FeN₄ active site models allowed us to determine the dominant active site in each catalyst: FeN₄ active sites with 14-membered or 16-membered rings. Our study will open up possibilities to understand more details on active site structure-activity relationships not only for Fe-N-C electrocatalysts but also the other single-atom (electro)catalysts.