N-doped porous carbon hollow microspheres encapsulated with iron-based nanocomposites as advanced bifunctional catalysts for rechargeable Zn-air battery

The design and development of low-cost, efficient, and stable bifunctional electrocatalysts for the oxygen reduction reaction(ORR) and oxygen evolution reaction(OER) are desirable for rechargeable metal-air batteries. In this work, N-doped porous hollow carbon spheres encapsulated with ultrafine Fe/FeOnanoparticles(FeO@N-PHCS) were fabricated by impregnation and subsequent pyrolysis, using melamine-formaldehyde resin spheres as self-sacrifice templates and polydopamine as N and C sources.The sufficient adsorption of Feon the polydopamine endowed the formation of Fe-Nxspecies upon high-temperature carbonization. The prepared FeO@N-PHCS has advanced features of large specific surface area, porous hollow structure, high content of N dopants, sufficient Fe-Nxspecies and ultrafine Fe Ox nanoparticles. These features endow FeO@N-PHCS with enhanced mass transfer and considerable active sites, leading to high activity and stability in catalyzing ORR and OER in alkaline electrolyte. Furthermore,the rechargeable Zn-air battery with FeO@N-PHCS as air cathode catalyst exhibits a large peak power density, narrow charge-discharge potential gap and robust cycling stability, demonstrating the potential of the fabricated FeO@N-PHCS as a promising electrode material for metal-air batteries. This new finding may open an avenue for rational design of bifunctional catalysts by integrating different active components within all-in-one catalyst for different electrochemical reactions.