Nitrogen-Doped Carbon Nanotube–Graphene Frameworks with Encapsulated Fe/Fe₃N Nanoparticles as Catalysts for Oxygen Reduction

Iron (Fe)- and nitrogen (N)-codoped carbon materials hold broad application prospects in the oxygen reduction reaction (ORR) because of their abundant reserves, low cost, and excellent catalytic activity. In this study, a N-doped carbon nanotube (CNT)–graphene framework with encapsulated Fe/Fe₃N nanoparticles (Fe–N–CNT@RGO) is designed and synthesized by annealing a mixture of iron acetylacetonate, dicyandiamide, and graphene oxide via a one-step calcination strategy. Fe–N–CNT@RGO has a better ORR catalytic activity than reduced graphene oxide (RGO), N-doped graphene, and N-doped CNTs with encapsulated Fe/Fe₃N nanoparticles with respect to the onset potential, limiting current density, and kinetic current density. Fe–N–CNT@RGO also has high stability and a high discharging cell voltage, which approaches those of platinum/carbon in zinc–air batteries. The relationship between the structure and activity of Fe–N–CNT@RGO demonstrates that the high density of Fe–N and pyridinic N sites, moderate wettability, and positive ζ potential promote exposure of the active sites, accelerate the transmission of hydrated oxygen, and enhance the adsorption of HO₂^– for the 4e^– ORR.