Fabrication of 3D macroporous Fe3O4–GO–Ni through a ‘nano-reinforced concrete’ method in the application of flexible supercapacitors

Aqueous flexible supercapacitors have promising potential in the application of wearable electronics but are limited by their low energy densities. Typically, thin nanostructured active materials are deposited on current collectors to achieve high specific capacitances based on active materials, yet the capacitance of total electrodes is sacrificed. The fabrication of 3D macroporous current collectors is a pioneering solution to retain the high specific capacitances of both active materials and electrodes, achieving supercapacitors with high energy density. In this work, through a ‘nano-reinforced concrete’ method, Fe3O4–GO–Ni with a 3D macroporous structure is synthesized on the surface of cotton threads. In the synthesis process, Ni, hollow Fe3O4 microspheres and graphene oxide (GO) function as the adhesive, fillers, and reinforced and structural materials, respectively. The resultant Fe3O4–GO–Ni@cotton exhibits ultrahigh specific capacitances of 4.71 and 1.85 F cm−2 as positive and negative electrodes, respectively. The electrodes with 3D macroporous structures have good compatibility with the volume change of active materials during the charge–discharge process, leading to excellent long-cycle performance up to 10 000 charge–discharge cycles. To demonstrate the potential of practical applications, a flexible symmetric supercapacitor is fabricated using Fe3O4–GO–Ni@cotton electrodes and shows an energy density of 19.64 mW h cm−3.