Modification Of Graphene And Its Application In Supercapacitors

Electrochemical supercapacitor(ESC)was widely studied and studied because it shows much higher energy storage capacity than traditional capacitors.The emerging of ESC has greatly promoted the application of capacitors in various fields,ESC's fast charging and discharging speed,long-lasting cycle life with excellent stability are much better than that of lithium-ion batteries.ESC will surpass the battery and become the main carrier of green energy once its ability to store charge is greatly improved.The energy storage performance of ESC is largely determined by its electrode materials,graphene(G)is a new type of nanocarbon material with two-dimensional structure build up by carbon atoms,its excellent electrical conductivity,large specific surface area make it a new electrode material that is very suitable for ESC.Chemical redox method is one of the main methods for the large-scale preparation of G,the chemical oxidative stripping can be called a classic process in the preparation process of G.However,the traditional chemical oxidation process is simple and rough,it has certain limitation on the degree of peeling of graphite,and the structure of graphene oxide(GO)obtained by the chemical oxidation method is generally difficult to maintain uniformity,and the structure between the sheet particles is also largely different.In view of this,we introduced a high-pressure homogenization process to further homogenize the GO prepared by the Hummer method,so that its morphology and homogeneity were improved,and its electrochemical performance was greatly improved after being reduced.In the current reduction method of GO,some poisonous reducing agents such as hydrazine hydrate and dimethylhydrazine are used,they have good reduction effect but cause environmental pollution and safety problems.The hydrothermal method is very popular in recent years as it is more environmentally friendly,as well as it can effectively reduce GO.Similarly,when the water is replaced by anhydrous ethanol,GO can also be effectively reduced and shows excellent electrochemical performance.Activated carbon(AC)has a very large specific surface area,the size and shape of the pore structure can be adjusted by controlling the activation process conditions,and its preparation process is simple and low in cost,making it one of the most widely used materials in ESC.As hollow tubular new carbon material and a highly practical porous carbon material respectively,Carbon nanotubes(CNT)and AC shows excellent electrical conductivity and huge specific surface area,whereas each of them has some inevitable shortages that limit their use in electrode materials.By combining G with AC or CNT,the advantages of various materials can be simultaneously exerted.In this paper,GO was further processed by high-pressure homogenizer to study the improvement brought by homogenization treatment.GO was reduced by absolute ethanol,and the difference between hydrothermal reduction and ethanol reduction was contrasted.The reduced GO product obtained from ethanol reduction is also combined with AC and used as ESC material to evaluate its capacitor performance,besides,the composite of G and CNT was constructed by the anhydrous ethanol method,and the energy storage application of the composite in ESC was studied.The main contents are as follows:1.GO was prepared by the modified Hummers method,and then the GO solution was treated by a high-pressure homogenizer,and the number of homogenization treatments is changed to obtain a GO solution which is treated to a different extent.The GO is formulated into a certain concentration of the solution in a closed reaction vessel,and reduced by hydrothermal method.Then,the structure and morphology of the materials before and after reduction are characterized and the reduced products are assembled into supercapacitors for electrochemical performance evaluation.The results show that the high pressure homogenization treatment can reduce the number of layers of GO,reduce the particle size,make the solution show better dispersibility and uniformity,and improve the defect degree of the obtained GO.After further reduction,its electrochemical energy storage performance is greatly improved(from 80 F/g to 168 F/g,organic electrolyte),indicating that high pressure homogenization can effectively improve the energy storage performance of reduced graphene oxide.2.GO is reduced in an autoclave with absolute ethanol as solvent(the product is marked as GE),without adding any additional reducing agent in the process to evaluate the difference in morphology and electrochemical performance of rGO obtained from anhydrous ethanol and thermal reduction(denoted as GW),respectively.The final reduced product was analyzed by Fourier transform infrared spectroscopy,Raman spectroscopy,X-ray diffraction,scanning electron microscopy and X-ray photoelectron spectroscopy,and finally assembled into a button-like ESC for electrochemical analysis.The results show that the anhydrous ethanol thermal method can effectively reduce GO,and the degree of reduction is different from that of the obtained material by the hydrothermal method.The GO has a higher carbon-oxygen ratio after hydrothermal reduction,while GE has lower defectivity and higher specific capacity than GW.On the basis of that,we further compounded GE and AC,and found that its structural advantages can be combined with AC that contains microporous structure to a certain extent.The electrochemical performance(especially the cycle performance)of the composite material in capacitors has been greatly improved.3.On the basis of the of sealed anhydrous ethanol reduction method,the carboxylated carbon nanotubes(CNTO)were introduced into GO to form a crosslinked network structure of G-CNT composites.Raman spectroscopy and scanning electron microscopy was used to investigate the effect on the structural properties of G by the introduction of CNTs in the composite.The CNT-G composite in different mass ratios of CNT and G was used to explore the optimal composite ratio.Finally,the composite materials were assembled into ESC electrode materials to evaluate its electrochemical performance,and we obtain composite materials with different mass ratios of CNT and G,which provided an applied preparation method for G-CNTs.