Study On The Green Controlled Exfoliation Mechanism Of Graphene And Graphene Oxide And Its Electrochemical Application

Graphene,a monolayer of carbon atoms with unique two-dimensional structure,which exhibits amazing properties,lead to a series of unique performances.With the deepening research and the developing key technologies of graphene,increasing problem for preparation of high quality graphene is highlighted.Liquid phase exfoliation strategy of graphite attracted more attention owing to the advantages of abundant raw materials,low cost and large scale.However,the uncontrollable number of graphene layers,high energy consumption and serious pollution in liquid phase exfoliation strategy,the development of the applications of graphene is seriously restricted.Particularly in today's society with carbon peaking and carbon neutrality goals background,developing energy-saving,low pollution and controlled synthesis strategies for graphene becomes more important.In this paper,aiming at the problems of liquid phase exfoliation strategy of graphite,put forward the key scientific problems of restricted the exfoliation efficiency,designing the efficient driving forces to realize the controlled electrochemical and chemical interaction exfoliation methods.The reaction kinetics during the prepared process of graphene oxide(GO)were improved by the pre-intercalation strategy,demonstrating an efficient method for increasing the oxidation efficiency of GO.Furthermore,designing the graphene materials with rational layer spaces to optimize their electrochemical performance,and the applications of graphene materials in energy storage have been explored.The main research content includes the following four parts as follows:1.Because of the low decomposition voltage of water in the during the anodic electrochemical exfoliation process,the uncontrollable decomposition of water often provides excess driving forces,leading to the low ratio of single/few layered graphene nanosheets.Herein,we employed a sodium perchlorate-based water-in-salt(NaClO₄-WIS)electrolyte for the controllable EC exfoliation of graphite.Benefiting from the high salt-to-water ratio of the electrolyte,regulating the ratio of bound water and free water.According to the decomposition activity of different structure water molecule,the problem of water molecule decomposition activity has been solved,their thickness and oxidation extent of graphene nanosheets can be effectively controlled.Density-functional-theory-based molecular dynamics(DFT-MD)simulations exhibit a revolution from free water(hydrogen bonds)to bound water(covalent bonds)in WIS electrolytes as the salt-to-water ratio increases.The results further disclose that only at high concentration,free water molecules can be effectively restricted in WIS electrolyte,and they need to overcome more hindrance to arrive at the electrode surface than that in dilute electrolytes;therefore,the reaction rate of EC exfoliation can be controlled easily by reducing the amount of free water.2.Considering the high energy consumption and the low efficiency of exfoliation reagents in chemical intercalation exfoliation method,we developed a exfoliation reagent with selective oxidation and intercalation through the synergistic effect of H₂SO₄ and H₂O₂.This method controllable synthesis of graphene nanoplatelets(GNPs)with low-oxidation level and high-electrical conductivity.We also study the exfoliation process and propose a formation mechanism for the synthesis of the low-defect graphene.The results show that synergistic effect of H₂SO₄ and H₂O₂ plays an important role in the selective oxidation and low-defect exfoliation of graphite.Moreover,density functional theory(DFT)simulations exhibits that H₂O₂ would preferentially adsorb on the edge of graphite,and graphite edges be opened fast,leading to the exfoliation of graphite.In addition,a time-dependent temperature profile under an applied voltage of 5 V exhibits a steady-state temperature of up to 275^°C.3.In Chapter 3,we found that different ratios of H₂SO₄ and H₂O₂can trigger different reactions.Herein,by adjust the ratio of H₂O₂,the preparation of low stage graphite sulfate(H₂SO₄-GICs)was studied.As a key precursor for GO,H₂SO₄-GICs is a key factor to effect the reaction kinetics of GO.We added low amount of hydrogen peroxide(H₂O₂)in Hummers'method to assist in the formation of H₂SO₄-GIC to improve the reaction kinetics of GO.We studied the intercalation process and the structural evolution for the synthesis of GO.The results show that the amount of H₂O₂plays an important role in determining the stage of H₂SO₄-GICs.Confirming added low amount of H₂O₂ triggered the reaction of stage-I H₂SO₄-GICs,improving the exfoliation efficiency and oxidation degree of graphene oxide.Moreover,we also confirmed that this pretreatment method for graphite with a large size plays the same role,demonstrating the strong energy supply by adding minimal stoichiometric H₂O₂ to improve the oxidation degree of graphite oxide for any size.Because of the high degree and large interlayer spacing of pretreated GO(PGO),After reduction,As an anode in sodium ion batteries,RPGO shows good electrochemical performance shown excellent high rate performance,which can be ascribed the expanded interlayer spacing.4.Graphene has great application value and prospect in the field of electrochemical energy storage due to its excellent properties.Herein,by using the as-prepared GNPs and GO as main active materials,the GNPs/GO films were prepared by using a simple vacuum-filtration method.According to the structure characteristics and the different surface charge of GO and GNPs,the self-assembly of GNPs and GO was formed in the solution interface.Formed heterogeneous interface of GNPs/GO result in the different the interlayer spacing of GNPs/GO layer material.In whole process,GO show good dispersion own to abundant functional groups on the surface,which solved the problem of agglomerated GNPs,combining with the excellent electronic transmission characteristics of GNPs,the interlayer spacing of GNPS-GO heterogenous membranes can be precisely adjusted.After reduction,the graphene materials shows different rate performance in sodium ion batteries.Combined with large layer spacing and suitable pore structure,45%GNPs-RGO film display a high rate performance.Furthermore,we employ the porous carbon material(YP-50)as the cathode to couple with anodes of 45%GNPs-RGO film,building a Na ion hybrid capacitor,it shows a high energy density of 71 W kg^-1 at a high power density of 200 W kg^-1,displaying excellent electrochemical performance.