Preparation Of Functional Graphene Via Ball-milling Assisted Supercritical CO₂ Exfoliation

Graphene has attracted worldwide attention for its excellent performance in light,electricity and heat,etc.With the development of graphene application research,graphene has gradually shown broad application prospects in military and civilian aspects.However,the difficulty of scale-preparation process of graphene has limited the application of graphene.The commonly used graphene production processes are low-yield and high-cost;and the quality of graphene as-prepared cannot be guaranteed.Therefore,developing a high-efficiency,low-cost,high-quality,batch-scale production of graphene is of great significance in the development of graphene from research to application.Inspired by liquid phase exfoliation,the method of preparing graphene via using supercritical fluid（SCF）as stripping medium has been studied.The efficiency of SCF exfoliation can be improved by different auxiliary means.The ball milling method has been widely used for its simplicity and processing capacity.This dissertation focuses on the preparation of functionalized graphene via ball-milling assisted supercritical CO₂ exfoliation.The main research contents are as follows.（1）B,N co-doped graphene-like carbon nanomaterials（BNC）were prepared by a two-step method and applied to the electrochemical performance of supercapacitor in an alkaline system.In the ball milling process,N atoms and defects were introduced into the original carbon matrix of graphite,so that the specific capacitance is significantly improved.But its performance at high current densities is poor.B atoms are introduced into the matrix by subsequent thermal treatment.As the temperature increases,the retention of nitrogen increases,which makes the BNC material have a higher initial capacitance,and the introduction of boron makes it perform well at high current densities.The retention rate of capacitance is increased up to 78.2%,which is far superior to pure graphene materials.Furthermore,the as-prepared supercapacitor has an excellent cycling stability.（2）Direct nonequilibrium molecular dynamics simulations were performed to investigate the mechanism of graphite exfoliation by supercritical CO₂ in the depressurization process.It was concluded that the graphite peeling rate and the graphene yield depended on the number of inserted CO₂ molecules in simulations,and the appropriate pressure or density of CO₂ is a prerequisite to achieve graphite exfoliation.This is confirmed by the experimental observations.Furthermore,the essential effect of the pressure or density of CO₂ was attributed to the competition between the van der Waals attraction in the graphite interlayer and repulsion between CO₂ and graphite.（3）Ball-milling assisted supercritical CO₂ exfoliation（BSCE）process was developed.The effects of temperature,pressure,rotating speed and time on the exfoliation efficiency were discussed to find out the optimal conditions of the BSCE process in exfoliating graphite to graphene.The graphene obtained under the optimal conditions has been characterized with morphology and structure,which proves that the BSCE process is highly efficient and the product is with high quality.Then,the graphene was applied to lithium ion battery anode material.Compared with commercial graphite,the graphene prepared by the BSCE process has a significant improvement in capacitance and cycle performance.（4）Under the optimal conditions of BSCE,a hydrophilic modifier（PVP）was added to the reaction system to simultaneously realize the exfoliation and functional modification of graphene.While maintaining the high efficiency and quality of graphite exfoliation,the modifier（PVP）has been introduced onto the edge or/and surface of the graphene resulted to hydrophilic graphene with little damage to the in-plane structure.It was proved by contact angle test that the graphene was greatly improved in hydrophilicity before and after exfoliation;and the graphene was dispersed in water,ethanol and other solvents to obtain dispersions.The concentration of graphene in water and ethanol can reach up to 0.854 mg/mL and 1.577 mg/mL,respectively.