Preparation Of High Performance Graphene-Based Films And Its Application In Supercapacitors

In recent years,graphene-based supercapacitors have attracted much attention from researchers because their long cycle life,high power density,green safety and other advantages.However,graphene electrode materials suffer from a decrease in specific surface area due to the agglomeration of lamellae caused by?-?bonding forces between the layers.Secondly,the specific capacitance of graphene electrode materials is relatively low,which also leads to the low energy density of the assembled graphene supercapacitor devices.Therefore,in order to overcome the drawback of low specific surface area of graphene electrode materials,this thesis prepared HPG/RGO thin film electrodes with tunable specific surface area by inserting hot pressed graphene(HPG)with folded surface as an intercalator into reduced graphene oxide(RGO)layers.In addition,in order to improve the specific capacitance of the graphene electrode material,the pseudocapacitance material Rhein was modified onto the graphene by non-covalent bonding to prepare a high specific capacitance Rhein/reduced graphene oxide(Rhein@RGO)thin film electrode.Finally,Rhein@RGO was assembled as the positive electrode material to form a zinc ion hybrid supercapacitor,which was used to further improve the energy density of the graphene-based supercapacitors.The main studies are as follows:(1)A flexible HPG/RGO composite film was prepared by mixing reduced graphene oxide(RGO)and graphene(HPG)prepared by hot pressing method.Under the hot-pressing condition,the surface folded HPG was formed due to the explosion of GO by the gas generated in the reduction process.The folded morphology of HPG was used to suppress the agglomeration between RGO layers,and the specific surface area of the composite film could be regulated by changing the addition amount of HPG.The electrochemical test results show that the HPG/RGO film electrode has a specific gravimetric capacity of 276.48 F g^-1(0.5 A g^-1),retains 54.21%of the initial capacitance at a current density of 50 A g^-1,and has an energy density of 9.61 Wh kg^-1(250 W kg^-1)in the case of assembled supercapacitors.Moreover,the capacitance retention rate is 90.14%after 10,000 charge/discharge cycles at a high current density of 20 A g^-1,and the coulomb efficiency is always sustained at 100%.In addition,the flexible supercapacitor based on HPG/RGO film maintains good stability under different bending conditions.(2)The graphene hydrogel thin film material with non-covalent modification of Rhein molecule was prepared by a simple hydrothermal method.During the hydrothermal process,the two-dimensional graphene sheets aggregate into porous graphene,and the porous structure of graphene enables rapid diffusion of ions,allowing Rhein molecules to undergo rapid and reversible redox reactions,providing additional pseudocapacitance for the supercapacitor.Electrochemical tests show that the Rhein@RGO film electrode has a specific gravimetric capacity of 374.51 F g^-1(0.5 A g^-1)and maintains 72.63%capacitance value even when the current density is increased to 50 A g^-1,demonstrating good rate performance.And compared with the RGO and the Rhein/RGO prepared by the mechanical mixing method thin film electrode,the mass specific capacitance of the Rhein@RGO film electrode is 1.5 times and 2 times then them,respectively.At the same time,the energy density of the supercapacitor assembled by Rhein@RGO is13.01 Wh kg^-1 at a power density of 125 W kg^-1,and the capacitance stays the same after10,000 cycles at a high current density of 30 A g^-1,demonstrating an outstanging cycling stability.(3)In order to enhance the energy density of the device,the Rhein@RGO hydrogel film as positive electrode,zinc sheet as negative electrode and 1 M Zn SO₄ as electrolyte were formed into a zinc ion hybrid supercapacitor based on the above study.The results show that the energy storage mechanism of the Rhein@RGO zinc ion hybrid supercapacitor is based on the coordination reaction of Zn²⁺with C=O on Rhein molecule on the positive electrode and deposition/detachment on the negative electrode of zinc foil.Electrochemical tests show that Rhein@RGO has a specific gravimetric capacity of201.41 m Ah g^-1(0.2 A g^-1),the assembled zinc ion hybrid supercapacitor has an energy density of 103.45 Wh kg^-1(102.45 W kg^-1).Moreover,the capacitance stays almost the same after 5000 charge/discharge cycles at 10 A g^-1 current density.In addition,the prepared coin cell has good series-parallel performance and certain practical value.