Cascaded utilization of magnetite nanoparticles@onion-like carbons from wastewater purification to supercapacitive...

Developing high-performance carbon-based materials for environmental and energy-related applications produces solid waste with secondary pollution to the environment at the end of their service lives. It is still challenging to utilize these functional materials in a sustainable manner in different fields. In this study, we demonstrate a cascaded utilization of an Fe₃O₄@onion-like carbon (Fe₃O₄@OLC) structure from wastewater adsorbents to a supercapacitor electrode. The structure was formed by carbonizing Fe₃O₄@oleic acid monodisperse nanoparticles into interconnected Fe₃O₄@OLCs and subsequent insufficient acid etching. The hollow OLCs in the outside region of the hybrid structure provide high surface area and the encapsulated Fe₃O₄ nanoparticles in the inside region offer high ferromagnetism. The three-dimensionally interconnected graphitic layers are advantageous for efficient separation and high conductivity. As a result, the maximum saturation adsorption capacity of insufficiently etched interconnected Fe₃O₄@OLCs can reach up to 90.2 mg g^-1 and they can be efficiently separated under a magnetic field. Furthermore, the hybrid structure is thermally transformed into N-doped HOLCs, which are demonstrated to be a high-performance supercapacitor electrode with high specific capacitance and high electrochemical stability. The cascaded utilization of the hybrid structure in this study is meaningful for eco-friendly development of functional materials for environmental and energy storage applications.