Fundamental Research On The Process Of Exfoliating Graphene By Supercritical CO₂ And Supercritical CO₂ Microemulsion Containing Ionic Liquid

The liquid phase exfoliation method has attracted much attention due to its simple operation and high quality of the product.While the common solvents used by exfoliation usually have high boiling points,high toxicity and expensive price,even external field strengthening methods like sonication usually consume too much energy,which increases exfoliation costs.In this paper,supercritical carbon dioxide（sc CO₂）and ionic liquids（ILs）-in-CO₂ microemulsion（ILs/CO₂）were used as exfoliation solvents to explore the regulation and internal mechanism of graphene exfoliation with green solvents.The main research contents were as follows:（1）Study on the phase behavior of IL/CO₂microemulsion.Using[Emim][Tf₂N]ionic liquid,Ls-mn（m=3,n=6:Ls-36;m=4,n=5:Ls-45;m=5,n=4:Ls-54）,Dynol-604 and TMN-6 surfactants,sc CO₂ was a continuous phase to construct IL/CO₂ microemulsion.The regulation rules of surfactant concentration,IL content and temperature on the formation of stable microemulsions was the key investigation objects.Based on that,molecular dynamics（MD）simulation was used to study the aggregation and assembly process of microemulsion as well as its microstructure.It was found that the number of EO groups and the number of PO groups of the surfactant were the key factors affecting the solubility of IL.By increasing the surfactant concentration,the solubility of IL in the microemulsion could be improved,and a higher system pressure was required at this time.The droplet size and hydraulic radius was measured by small angle X-ray scattering（SAXS）,and it was found that when the IL concentration was 0.5 mmol·L^-1,the gyration radius of different microemulsion agglomerates was related to the type of surfactants,and the radii were in the following sequence:Dynol-604<Ls-54<Ls-36<Ls-45<TMN-6.（2）The enhancement by co-solvents on the solubility of IL.In order to improve the solubility of IL in the microemulsion and try to reduce the cloud point pressure（CPP）of the system,H₂O and ethanol were added as co-solvents.Studies have found that with same amount of IL,the solubility enhancement effect of the co-solvents and the decrease of CPP were closely related to the types of surfactants and co-solvents.After adding a small amount of water,the cloud point pressure of the Ls-45 system was 0.5–1.0 MPa lower than that of the anhydrous system,and the solubility of IL at the same pressure was increased by about 20%.After adding water,the cloud point pressure of the Ls-36 system decreased by about 1.2 MPa.The co-solvent ethanol reduced the cloud point pressure better than water.After adding ethanol,the cloud point pressure of the Ls-54 system was reduced by about 3 MPa,the IL solubility of the Dynol-604system was increased by about 67%,and the IL solubility of the TMN-6 system was increased by about 45%.From the MD simulation results of the microemulsion constructed by Ls-45,it could be seen that the addition of ethanol to the system could speed up the self-assembly,and part of the ethanol could be dissolved in the polar core,which facilitated the solubilization of the IL.（3）Basic research on the process of IL/C microemulsion coupled jet impact to exfoliate graphene.Using[Emim][Tf₂N]/H₂O/Et OH/Ls-mn/CO₂ exfoliation agent,200 mesh natural graphite powder was exfoliated to prepare graphene.Focused on the influence of microemulsion composition（surfactant type,co-solvent type and addition amount）and operating conditions（pressure and time）on the exfoliation effect.On this basis,in order to recycle the high-pressure energy of the system,a self-developed jet impact device was used to perform secondary exfoliation of graphene to achieve process enhancement,and the exfoliated graphene was prepared into a supercapacitor to study its electrochemical performance.The best exfoliation operating parameters obtained were:1.7 m L Ls-45 surfactant,80μL[Emim][Tf₂N],0.5 m L water and ethanol co-solvent（volume ratio 1:1）,24 MPa,40°C,6 h,by which graphene with a concentration of 0.640 mg·m L^-1,a thickness of 3.5–5.5 nm,and a lateral size of 700–1100 nm was obtained.After the jet impact,the graphene concentration increased to 0.692mg·m L^-1,the sheet thickness decreased to 2.5–4.5 nm,and the lateral size decreased to 400–700 nm.The kinetic calculation results showed that jet impact could play a role in secondary exfoliation.The prepared graphene electrode had good charge and discharge reversibility,and the maximum specific capacitance was 197 F·g^-1 at a current density of 0.2 A·g^-1.（4）The regulation law of sc CO₂ exfoliation to prepare graphene.In order to reduce the exfoliation components,simplify the process and try to reduce the preparation cost,a process route with high-pressure supercritical CO₂（25–45 MPa）as the exfoliation solvent for preparing graphene was proposed.Focused on the study of the influence of pressure,time and temperature on the exfoliation effect,and by changing the centrifugal speed and time,explored the corresponding relationship between it and the thickness of the sheet.On this basis,the exfoliation and dispersion of graphene in supercritical CO₂ were calculated by the Hansen solubility parameter theory to provide theoretical guidance for actual exfoliation.Finally,the exfoliated graphene was prepared into a supercapacitor to study its electrochemical performance.The best exfoliation process parameters obtained were:under the conditions of45 MPa,48 h,and 50°C,the 3–6 layers of the prepared graphene accounted for 75%,and the lateral size of 0.2–1.2μm accounted for 85%.The highest graphene concentration could be obtained by centrifugation at 1000 rpm and 30 min,which was 1.625 mg·m L^-1.The graphene obtained by centrifugation at 5000 rpm for 5 min had 3–7 layers.The graphene obtained by centrifugation at 3000 rpm for 15 min had 2–3 layers.Hansen solubility parameter calculation showed that when the temperature was constant,high pressure was preferred in the exfoliation of graphene.The electrodes made of graphene obtained at the two centrifugal speeds had good charge and discharge reversibility,but the electrode made by centrifugation at 3000 rpm for 15min had a higher specific capacitance than that of 5000 rpm for 5 min.