A Hydrangea-Like Superstructure of Open Carbon Cages with Hierarchical Porosity and Highly Active Metal Sites

Carbon micro-/nanocages have attracted great attention owing to their wide potential applications. Herein, a self-templated strategy is presented for the synthesis of a hydrangea-like superstructure of open carbon cages through morphology-controlled thermal transformation of core@shell metal-organic frameworks (MOFs). Direct pyrolysis of core@shell zinc (Zn)@cobalt (Co)-MOFs produces well-defined open-wall nitrogen-doped carbon cages. By introducing guest iron (Fe) ions into the core@shell MOF precursor, the open carbon cages are self-assembled into a hydrangea-like 3D superstructure interconnected by carbon nanotubes, which are grown in situ on the Fe-Co alloy nanoparticles formed during the pyrolysis of Fe-introduced Zn@Co-MOFs. Taking advantage of such hierarchically porous superstructures with excellent accessibility, synergetic effects between the Fe and the Co, and the presence of catalytically active sites of both metal nanoparticles and metal-N_x species, this superstructure of open carbon cages exhibits efficient bifunctional catalysis for both oxygen evolution reaction and oxygen reduction reaction, achieving a great performance in Zn-air batteries.