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/Fe 3 O 4 nanoparticles (FeO x @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 Fe 3+ on the polydopamine endowed the formation of Fe-N x species upon high-temperature carbonization. The prepared FeO x @N-PHCS has advanced features of large specific surface area, porous hollow structure, high content of N dopants, sufficient Fe-N x species and ultrafine FeO x nanoparticles. These features endow FeO x @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 x @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 x @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. N-doped porous hollow microspheres with encapsulated Fe-based nanocomposite show impressive activity and stability for ORR and OER, capable of serving as efficient cathode catalysts for the rechargeable Zn-air batteries.