New insights on Fe-N-C catalyst structure from valence-to-core X-ray emission and absorption spectroscopies

Various spectroscopic techniques have been extensively applied, both ex situ and in situ/operando, to study the structure of the Fe-N-C group of catalysts, promising materials for fuel cell applications. Valence-to-core X-ray emission spectroscopy (VtC XES), a powerful technique to monitor the changes in iron electronic state, is still novel for Fe-N-C characterization. In this work, we analyze the VtC XES spectra obtained on pyrolyzed Fe-N-C catalyst before and after accelerated stress tests in alkaline electrolyte under argon and oxygen atmospheres. By combining both experimental and theoretical spectra, we propose changes in the Fe local geometry after the stress protocols. The observed findings serve as an important contribution to the overall understanding of the FeNxCy active sites structures and demonstrate the capabilities of the VtC spectroscopy towards transition metal-based electrochemical systems analysis.