Ultra-small Fe3O4 nanoparticles encapsulated in hollow porous carbon nanocapsules for high performance supercapacitors

A new nanoscale architecture of Fe₃O₄-carbon hybrid materials was developed by a vacuum incipient wetness procedure. The amount of Fe₃O₄ nanoparticles were controllably confined inside the cavity of the bowl-shaped hollow porous carbon nanocapsules (CNB). TEM images and TG curves proved that different loading of Fe₃O₄ small nanoparticles (NPs) with a diameter less than 50 nm were stored in CNB. Benefiting from the synergistic effect of the appropriate amount of uniformly dispersed Fe₃O₄ NPs and bowl-shaped carbon nano-capsules with high specific surface area, high conductivity and high amount of Nitrogen (N) and oxygen (O) elemental doping of Fe₃O₄@CNB, the new architecture provides good reversibility for the transport of electrolyte ions. When tested in supercapacitor devices, Fe₃O₄@CNB-2 (containing 40.3 wt% Fe₃O₄) exhibited the highest gravimetric (466 F g⁻¹) and volumetric capacitance (624 F cm⁻³). The supercapacitors based on these materials also showed excellent cycling stability (92.4% capacitance retention after 5000 cycles). This class of Fe₃O₄-carbon hybrid materials has excellent electrochemical properties, and its synthesis strategy can be extended to construct other hybrid materials for various applications, such as biomedicine, catalysis, energy harvest, energy storage and so on.