Study On The Structure And Electrochemical Properties Of Graphene Aggregation Assisted By Electrolytes

Graphene is a promising kind of electrode material for supercapacitors due to its high theoretical specific surface area,high carrier mobility,high thermal conductivity as well as excellent mechanical strength.Oxidation-reduction is one of the main methods to prepare graphene on a large scale;however,during the reduction and drying process,graphene will appear uncontrollable restacking and agglomeration,which partly limits its application in the graphene-based supercapacitors.Besides,the performance of the graphenebased supercapacitor depends mainly on both the electron transfer rate and ion diffusion rate in the device,which is closely related to the electronic structure and micro-morphology of the electrode materials.Therefore,precise design and structure control of the graphene electrode materials under microscale is critical for obtaining high performance supercapacitors.Our previous studies found that when adding certain amount of electrolyte into the graphene oxide(GO)water dispersion system the GO surface charge density will be changed,and the reduction of graphene oxide(RGO)was obtained by hydrothermal reaction,through which the structure-activity relationship of the RGO obviously influence the performance of the supercapacitor.However,it is still not clear how the electrolyte and graphene interact with each other;especially,what is the role the electrolyte plays during the graphene gelation process,which need to be further studied.Based on the previous studies,the gelation process of GO under the action of electrolyte,the change of stability and surface charge distribution of the GO dispersion with electrolyte content,and the interaction mode between the electrolyte and GO tablets were investigated in this paper.The surface chemical composition,pore structure,hydrophilicity and electrochemical properties of graphene were controlled by changing the types of anions and cations of the electrolytes and controlling the heating and reduction modes in the reduction process of GO,and through the structure control of the related products,the surface chemical composition,pore structure,hydrophilicity and electrochemical properties of graphene were regulated.The relationship between the chemical composition and electrochemical properties is discussed,and the structure-activity relationship between the microstructure and electrochemical properties of graphene-based supercapacitors is revealed.The main research contents and results are as follows:(1)The effects of inorganic salts on the surface composition,aggregation structure and gelation process of graphene were studied.The surface charge distribution of GO tablets,the stability of GO aqueous dispersion and the microrheological behavior of GO tablets were investigated by adding NaCl into the GO dispersions.The results showed that the weak binding forces such as the van der Waals force and Coulomb force between NaCl salt and GO lead to the soft agglomeration of graphene,which is different from the hard agglomeration based on chemical bonding in the reduction and drying process of graphene.Based on this,the effects of different anionic sodium salts(neutral sodium salt and sodium ion salt with different redox activity)on the aggregation structure of graphene were investigated.The RGO was prepared by hydrothermal reduction method.The effects of different anionic salts on the aggregation structure,surface chemical composition,surface hydrophobicity and electrochemical properties of the RGO were investigated.The results showed that the graphene sample prepared by the NaCl salt-assisted hydrothermal method has improved electrochemical performance,and the specific capacity of graphene sample can reach as high as about 300 F/g at current density of 0.1 A/g.(2)The effect of heating mode on the aggregation structure and chemical composition of graphene was investigated.The heating rate of the reduction reaction of graphene oxide was accelerated by microwave heating,the aggregation structure and chemical composition of RGO were regulated,and its electrochemical properties were investigated as well.The results showed that the polymerization and reduction of graphene could be completed faster when the electrolyte NaCl was added into the GO water dispersion with the reaction system being heated by microwave.At the same time,the surface composition and aggregation structure of graphene have certain effect on the electrochemical performance,the oxygen content of the samples prepared by microwave assisted hydrothermal method is low,and the capacity decay of the sample at high rate is faster than that of the sample prepared by microwave assisted hydrothermal method.(3)Regulation of graphene aggregation structure by electrolyte-assisted spray drying-thermal reduction was done.The results showed that the spray drying method can be used to dry the samples on the second time scale,regulating the aggregation morphology of the samples and thus realizing instantaneous regulation of the aggregation structure of GO.On the one hand,adding electrolyte can improve the collection efficiency of GO in the spray drying process,and on the other,the electrolyte can act as a hard template,making the GO sheet aggregate into microsphere at the nozzle and then thermal reduction to obtain improved graphene material with better aggregation structure.The surface of RGO prepared by this method is rich in oxygen-containing functional groups,which residues help to increase the wettability of the material and provide additional pseudo-capacitance which is of benefit to increase the specific capacity of the material.