Flexible Micro-Supercapacitors Based On Fe₃O₄@CNOs/Graphene Hybrid Structure Self-Assembled Thin Film

Micro-supercapacitors（MSCs）with in-plane parallel electrode structure have become a significant energy storage unit for flexible wearable electronic devices due to the advantages of high power density,small size,and excellent mechanical bending stability.The electrode as the core of MSCs directly determines the performance of MSC devices.The fabrication of large-area thin film electrode is beneficial to prepare flexible energy storage units with large-scale series-parallel connection as well as achieve the performance consistency of energy storage units.In addition,the ultra-thin electrode preparation is expected to improve the device’s compactness,realize the light weight and high flexibility of wearable power source,and boost ion transfer on the electrode,thereby improving the practicability of the flexible wearable micro energy storage device.However,it is hard to accomplish the small thickness electrode preparation with large area and high volume capacitance at the same time according to the current research about MSCs,which limits the application potential of MSCs.In this paper,a composite thin film with large area and considerable specific capacitance was developed by self-assembled reduced graphene oxide（r GO）as substrate used to attach and transfer Fe₃O₄ nanoparticles encapsulated by oleic acid（Fe₃O₄@OA NPs）.r GO as the conductive substrate effectively decreases the contact resistance between nanoparticles and current collectors.Fe₃O₄@C hybrid structure was obtained by carbonization of Fe₃O₄@OA.The covalent connection between C and Fe₃O₄ nanoparticles boosts the electron transfer and improves the electrochemical activity of Fe₃O₄.The flexible MSC based on Fe₃O₄@C/r GO has an area specific capacitance of 9.32 m F cm^-2 at 20μA cm^-2 and a volume specific capacitance of 339 F cm^-3,which is better than most reported devices.The maximum volumetric energy density of 46.9 m Wh cm^-3 was obtained at 0.72 A cm^-3.The capacitance retention ratio of the device is still 96.2%after 5000 cycles,which fully demonstrates the potential of hybrid thin film for flexible MSCs.In order to further enhance the energy and power density of MSCs based on self-assembled film,we assembled an asymmetric MSC（AMSC）with Mn O@C/r GO as a positive electrode and Fe₃O₄@C/r GO as a negative electrode,respectively.The maximum volume energy density of 49.4 m Wh cm^-3 was obtained at 0.7 A cm^-3,which is better than the Fe₃O₄@C/r GO based symmetric device.The rate performance of AMSC is 10%better than that of symmetric MSC.The above results illustrate the superiorities of self-assembled Mn O@C/r GO and Fe₃O₄@C/r GO films used for fabricating high volume energy density flexible energy storage devices.The study presented in this paper indicates that the self-assembled film based on hybrid Fe₃O₄@OA NPs/Graphene structure as the electrode material can achieve ultra-thin preparation of MSCs.This self-assembled film has the advantages of high volume energy density,high volume power density,simple and large-area preparation,which will hopefully promote the development of MSCs with large-scale preparation and high performance.