Preparation And Electrochemical Performance Of Graphene Based Material From Humic Acid And Potassium Humate

Graphene was prepared from humic acid by hydrothermal treatment combined with carbonization or direct carbonization with addition of MA in this thesis.Morphology,structure and performance of as-prepared graphenes were studied by Scanning electron microscope,Transmission electron microscopy,Atomic force microscopy,Raman spectroscopy,Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy and the synthesis mechanism of graphene was also discussed.Meanwhile,the electrochemical performance of as-prepared graphene in KOH electrolyte was studied by galvanostatic charge–discharge,cyclic voltammogram and electrochemical impedance spectroscopy,etc.The results indicate that substrate concentration,hydrothermal temperature and residence time have an important effect on the performance of graphene.The optimal graphene can be obtained by reduction of graphene oxide prepared from humic acid by hydrothermal carbonization at 190?for 10 h,followed by centrifugation and freeze-drying.The synthesis mechanism of graphene was discussed.Humic acid was oxided during the hydrothermal carbonization process and the content of carboxyl group was improved.Meanwhile,large scale aromatic sheet bonded by epoxy groups was cut into little aromatic sheets with plenty of functional groups new generated such as hydroxyl and carboxyl,which make graphene oxide produced soluble and dispersible in water.Meanwhile,the ratio of sp² hybridized carbon atom was raised.Graphene oxide prepared from humic acid exhibits transparent voile-like morphology with the thickness of 3.2 nm.Graphene prepared from graphene oxide by reduction also exhibits transparent voile-like morphology with the thickness of 4.7 nm.The specific surface area and total pore volume of graphene can achieve 239 m²·g^-1 and0.26 cm³·g^-1,respectively.As electrodes for supercapacitor,the areal capacitance reaches 28 uF·cm^-2 at the current density of 0.1 A·g^-1,suggesting that the specific surface area is made full use to store energy.The specific capacitance decrease gradually,yet the capacitance retention reachs 82.1%at a current density of 5 A·g^-1with the increase of current density.The cyclic voltammetry curve at high scan rate of500 mV·s^-1 still retains rectangular shape without obvious redox peaks,suggesting excellent electrochemical reversibility.The capacitance retention is up to 96.5%at the current density of 1 A·g^-1 after 2000 repeated charge-discharge cycles.The equivalent series resistance is about 0.15?Graphene can also be prepared by utilization of the solubility of potassium humate with the addition of MA to prevent the aromatic sheets from agglomerating,followed by carbonization,acid soaking and washing.The content of MA added has an important effect on the morphology and structure of the graphene.Graphene with the thickness of0.6-0.8 nm,exhibits plenty of wrinkles and three dimensional pores on its surface.With the increase of mass ratio,the specific surface area,total pore volume and mesopore volume pass through a maximum and then decrease.Meanwhile,the micropore volume decrease gradually and mesoporosity increase gradually.The specific surface area,total pore volume,mesopore volume and micropore volume can achieve 1267 m²·g^-1,1.883cm³·g^-1,1.670 cm³·g^-1 and 0.293 cm³·g^-1,respectively;Graphene with mass ratio of 3delivers a specific capacitance of 187 F·g^-1 at a current density of 0.0410 A·g^-1,and exhibits good cycling performance.