Yolk–Shell-Structured Cu/Fe@γ-Fe₂O₃ Nanoparticles Loaded Graphitic Porous Carbon for the Oxygen Reduction Reaction

Core–shell Cu/γ-Fe₂O₃@C and yolk–shell-structured Cu/Fe@γ-Fe₂O₃@C particles are prepared by a facile synthesis method using copper oxide as template particles, resorcinol-formaldehyde as the carbon precursor, and iron nitrate solution as the iron source via pyrolysis. With increasing carbonization temperature and time, solid γ-Fe₂O₃ cores are formed and then transformed into Fe@γ-Fe₂O₃ yolk–shell-structured particles via Ostwald ripening under nitrogen gas flow. The composition variations are studied, and the formation mechanism is proposed for the generation of the hollow and yolk–shell-structured metal and metal oxides. Moreover, highly graphitic carbons can be obtained by etching the metal and metal oxide nanoparticles through an acid treatment. The electrocatalytic activity for oxygen reduction reaction is investigated on Cu/γ-Fe₂O₃@C, Cu/Fe@γ-Fe₂O₃@C, and graphitic carbons, indicating comparable or even superior performance to other Fe-based nanocatalysts.