Probing the active site heterogeneity of Fe-N-C catalysts using integrated multimodal spectroscopy measurements and simulations

The single-atom Fe-N-C catalysts set their great promise with high oxygen reduction reaction (ORR) activity in replacing platinum-based ORR catalysts. Due to the high-temperature synthesis approach, Fe-N-C catalysts are highly heterogeneous and complex and understanding the active site structure is extremely challenging. In this article, we combine multimodal spectroscopy techniques, including X-ray absorption spectroscopy (XAS) and Mössbauer spectroscopy, with advanced spectroscopic simulations and electronic structure analysis to investigate the structures of an Fe-N-C catalyst immediately following pyrolysis and after air exposure. The site heterogeneity of the Fe center is identified, and for the first time, the second and third shell coordination environment of the Fe-N-C active sites, as well as the changes of the Fe oxidation state and coordination environment upon exposure to air are clearly demonstrated. This comprehensive study results in unprecedented insights into the molecular structures and the Fe local coordination environment of the Fe-N-C catalysts, providing molecular-level understanding that is critical for further advancements in both their activity and durability.