Highly-Efficient Preparation And Control Of Chemically Peeled Graphene Materials

Graphene is regarded as one of the most promising super-materials in the 21st century.Because of its outstanding optical,thermal,electrical and mechanical properties,it is widely used in many fields of energy storage,environmental management,biomedicine,protective equipment,aerospace and so on.At present,the main methods to prepare graphene are mechanical peeling,epitaxy growth,chemical vapor deposition and chemical exfoliation.Among them,because its simple preparation conditions,low production equipment requirements and large-scale production ability,the chemical exfoliation method represented by Hummers method has been widely used.However,the chemical exfoliation method still has many problems,such as producing large amount of waste acid and wastewater,low production efficiency?oxidative exfoliation time>2.5 h,and tedious washing process?,high cost,difficult reduction,and low degree of graphene peeling?specific surface area<1000 m²/g?,and so on,which greatly limits the practical application of this method.Therefore,it is particularly important to develop more efficient methods to prepare high-quality graphene which can meet the needs of practical application.In order to prepare graphene more efficiently,based on Hummers method,this paper realizes the recovery and utilization of concentrated sulfuric acid,reducing the production of waste acid and wastewater,opens up a new way to prepare graphene oxide with ultra-high efficiency,solves the difficult problem of reducing graphene oxide efficiently,builds new methods to control the properties of reduced graphene oxide,and studys and optimizes the electrochemical performance of graphene materials.The main research contents are as follows:?1?A KMnO₄-based"Sanwu"oxidation method was developed to realize the efficient preparation of graphene oxide.This method not only significantly shortens the reaction time,but also completely eliminates the use of NaNO₃ and the high-temperature and low-temperature processes with high energy consumption,realizes the reaction at no more than 35?,and ensures that the whole oxidation reaction can be completed within 15 minutes.Thus,this method truly realizes the efficient preparation of graphene oxide.Compared with the conventional Hummers method,the graphene oxide prepared by this method has similar oxidation degree and lower defect degree.?2?The recovery and reuse of concentrated sulfuric acid were realized,and a new strategy for preparing graphene oxide was constructed.This strategy not only successfully solved the problem of recovery and reuse of concentrated sulfuric acid in the preparation of graphene oxide by Hummers methods,but also realized the preparation of graphene oxide.The recovery and reuse of concentrated sulfuric acid not only saves a lot of concentrated sulfuric acid,improves the effective utilization rate of concentrated sulfuric acid,but also reduces the production of waste acid and wastewater,thereby reducing the pressure of post-treatment,and greatly reducing costs.At the same time,through different post-treatment methods of graphite oxide filter cake,not only reduced graphene oxide can be prepared,but also KMnO₄ can be used to obtain reduced graphene oxide manganese oxide composites.?3?A discontinuous microwave-assisted stripping method of non-aqueous expanded graphite oxide was developed for the preparation of reduced graphene oxide.Firstly,a non-aqueous expanded graphite oxide was prepared by classical Hummers method,and then reduced graphene oxide was obtained by a discontinuous microwave process.It was found that the microwave stripping efficiency was greatly improved by this special microwave treatment method,which effectively solved the problem of low efficiency in traditional microwave process,and reduced graphene oxide with high specific surface area and high reductivity was obtained.At the same time,the most tedious washing and purification process is simplified,and the preparation efficiency of graphene is significantly improved.The specific surface area of the obtained reduced graphene oxide is 1337.1 m²/g and the atomic ratio of C/O is 19.0.The electrochemical results show that the reduced graphene oxide obtained has excellent supercapacitor performance,including high energy storage capacity?255.5 F/g at a current density of0.5 A/g?,good rate performance and ultra-long service life?with 100%capacitance retention after 100 000 charge-discharge cycles?.?4?A flame-assisted microwave stripping method was developed for the efficient reduction of graphene oxide.Firstly,the pre-reduced graphene oxide is obtained by flame pretreatment of graphite oxide prepared by classical Hummers method,and then highly reduced graphene oxide is obtained by microwave treatment.This unique approach greatly improves the reduction efficiency,so that the whole reduction process can be completed within 4 seconds.At the same time,the problem that the microwave process is affected by the properties of graphite oxide is effectively solved,which shows a high degree of reproducibility and operability.The results show that the reduced graphene oxide obtained possesses ultra-high reduction degree?C/O atom ratio is 31.1?and large specific surface area?1050 m²/g?.The electrochemical results indicate that it has excellent cycle stability.During the 50 000 charge-discharge cycles,the capacitance does not fluctuate significantly,and the capacitance remains 100%.Meanwhile,it has good conductivity?impedance<0.2??,high Coulomb efficiency?116.2%?,good specific capacity,and outstanding rate performance.?5?It is found that the properties of reduced graphene oxide can be modulated by controlling the porosity and thickness of graphite oxide precursors.The results show that in the process of handling graphite oxide by flame method,the higher the porosity of graphite oxide is,the higher the reduction degree of the product is,and the greater the defect degree is;the smaller the thickness is,the higher the specific surface area of the product is.It was found that the electrochemical properties of reduced graphene oxide were closely related to its specific surface area,defect degree and oxidation degree.After adjusting the properties of reduced graphene oxide,a ultra-high specific capacitance of 576 F/g can be obtained.At the same time,the functional modification of reduced graphene oxide was realized by flame treatment of graphene oxide modified with metal salts.And reduced graphene oxide composites uniformly modified by metal oxide nanoparticles were obtained,showing good electrochemical energy storage performance.?6?In order to solve the problem of poor cycling performance of reduced graphene oxide powder as a supercapacitor material,a layered nitrogen doped graphene oxide foam was prepared by flame method.The method first uses urea to modify graphene oxide and then flame treatment to obtain nitrogen doped graphene oxide foam.It was found that urea not only acted as a nitrogen source,but also reconstructs the accumulation of graphite oxide,and enhances the interaction between graphene oxide lamellae,effectively preventing the product reduced graphene oxide from being peeled off during flame treatment.The results demonstrate that the nitrogen doped graphene oxide foam showed obvious layered structure,and nitrogen doping amount reaches3.5-7.0 at.%.As a supercapacitor material,it has high specific capacity?323.7 F/g?,high Coulombic efficiency?125%?and high energy efficiency,as well as ultra-long cycle life.The specific capacitance increases by 9.2%after 50 000 cycles of charging and discharging at an ultra-high current density of 30 A/g.?7?In order to improve the mechanical properties of graphene assembly materials,we have achieved a compact package of reduced graphene oxide on the surface of Ni foam skeleton by electrostatic adsorption and flame induction.And a Ni skeleton foam material with reduced graphene oxide tightly wrapped was successfully constructed.As a result,the reduced graphene oxide modified Ni skeleton exhibits excellent supercapacitor performance.It not only has high specific capacitance and energy efficiency,but also exhibits excellent electrochemical cycle stability.At 40 A/g,the capacitance remains 94.4%after 20 000 cycles of galvanostatic charge/discharge.Unlike the Ni foam,the reduced graphene oxide-wrapped Ni foam was used as the growth substrate,and a dense layer of?-Fe₂O₃-modified porous material was obtained.The results exhibit that the electrochemical performance of the?-Fe₂O₃ electrode assembled by Ni foam encapsulated with reduced graphene oxide has been improved,showing higher specific capacitance and energy efficiency and better electrochemical cycling stability.At a current density of 1 mA/cm²,its areal capacitance reaches 1488.9mF/cm².This strategy provides a simple and feasible method for the construction of flexible electrodes.