An in situ exploration of how Fe/N/C oxygen reduction catalysts evolve during synthesis under pyrolytic conditions

In pursuing cheap and effective oxygen reduction catalysts, the Fe/N/C system emerges as a promising candidate. Nevertheless, the structural transformations of starting materials into Fe- and N-doped carbon catalysts remains poorly characterized under pyrolytic conditions. Here, we explore the evolution of Fe species and track the formation of Fe-N₄ site development by employing diverse in-situ diagnostic techniques. In-situ heating microscopy reveals the initial formation of FeO_x nanoparticles and subsequent internal migration within the carbon matrix, which stops once FeO_x is fully reduced. The migration and decomposition of nanoparticles then leads to carbon layer reconstruction. Experimental and theoretical analysis reveals size-dependent behavior of FeO_x where nanoparticles below 7 nm readily release Fe atoms to form Fe-N₄ while nanoparticles with sizes >10 nm tend to coalesce and impede Fe-N₄ site formation. The work visualizes the pyrolysis process of Fe/N/C materials, providing theoretical guidance for the rational design of catalysts.