Preparation And Capacitive Performance Of Carboxylated Graphene-based Composites

With the increasing population and rapid economic development,the consumption of traditional non-renewable energy sources is increasing and cannot meet the urgent requirements of human society.To achieve sustainable development in the future and reduce the use of fossil fuels,the development of green and environmentally friendly renewable energy sources and new high-efficiency energy storage systems has become an inevitable trend.The efficient conversion and storage of renewable energy is a key issue in developing new energy sources,thus developing high-performance energy storage components is particularly important.Due to the advantages of long service life,high power density,and environmentally friendly,supercapacitors can meet the requirements of many fields of application.The electrode material,which is one of the components of supercapacitors,largely determines the good performance of supercapacitors.Carboxylated graphene（CG）with high specific surface area,high mechanical strength,and good electronic conductivity can be composited with conducting polymers or transition metal oxides to achieve the complementary advantages of both,and then obtain composite materials with excellent electrochemical properties.This work focuses on the preparation and characterization of carboxylated graphene-based composites and their capacitive properties,aiming to obtain supercapacitor electrode materials with high specific capacitance and long cycle life.The main contents and results are as follows:1.Composite films of carboxylated graphene（CG）and poly（3,4-ethylenedioxythiophene）:poly（styrene sulfonate）（PEDOT:PSS）are prepared using a simple ultrasonic treatment and vacuum filtration.The structure and morphology of the composite film are characterized by Fourier Transform infrared spectroscopy（FT-IR）,Ultraviolet and visible spectrophotometry（UV-vis）,Raman spectroscopy（Raman）,X-ray photoelectron spectroscopy（XPS）,X-ray electron diffraction（XRD）,and Scanning electron microscopy（SEM）.The results show that CG is successfully composited with PEDOT:PSS,and the introduction of CG provides mechanical support for PEDOT:PSS,increases ion transport channels,and makes the composite more stable to improve capacitive performance.After being optimized,the prepared composite films exhibit high specific capacitance(192.7 F g^-1 at 0.5 A g^-1),good rate performance(up to 80.4%at 0.5 to 20 A g^-1),and favorable electrochemical stability（95.5%specific capacitance retention after 50,000 cycles）.To remove excess PSS and obtain film electrodes with higher specific capacitance,DMF,DMSO,and EG are selected for post-treatment of the composite films.The results show that the DMF post-treated PG₁ film electrodes show better capacitive properties than pristine electrodes,such as higher specific capacitance(204.3 F g^-1 at a current density of 0.5 A g^-1)and ultra-long cycling stability(104.6%retention of specific capacitance after 50,000 cycles at a high current density of 20 A g^-1).2.The two-step method（hydrothermal and calcination）is used to prepare NiO materials.The effects of hydrothermal time,hydrothermal temperature,raw material composition ratio,and calcination temperature on the capacitive properties of NiO are systematically investigated.According to the comparison of electrochemical performance,the preparation conditions are determined as follows:The molar ratio of urea to nickel sulfate is 1:1,and the mixture is magnetically stirred for 30 minutes and then hydrothermal at 180°C for 10 h.After washing and drying,the powder is calcined in a tube furnace at 450°C for 3 h at a heating rate of 3°C/min to finally obtain a black nickel oxide powder.The specific capacitance of NiO prepared under these conditions is 137.4 C g^-1 at a current density of 1 A g^-1,with a rate performance of 56%over a range of current densities from 1 to 10 A g^-1.The relationship between the electrochemical performance and the specific surface area and pore size of the NiO electrode material can be reasonably analyzed by N₂ isothermal adsorption/desorption measurements.3.The electrostatic self-assembly between the negatively charged carboxylated graphene after ionization and the positively charged nickel oxide on the surface is used to prepare CG/NiO composite electrodes with different ratios.The optimum electrochemical performance of the CN-3 electrode is found by electrochemical performance tests,with a specific capacitance of 175.5 C g^-1 at a current density of 1A g^-1,which is 38.1 C g^-1 higher than the specific capacitance of pure NiO.The cycling stability of the CN-3 and NiO electrodes was tested by GCD,and the results show that the specific capacitance retention of the CN-3 after 5000 cycles at a current density of5 A g^-1 was 55.4%,which is superior to that of the NiO electrode（48.4%specific capacitance retention after 1500 cycles）.Characterized by SEM,XRD,and nitrogen isothermal adsorption and desorption measurements,it is found that the enhanced electrochemical performance is due to the addition of CG,which provides mechanical support to the composite,enhances the electrical conductivity of NiO,and alleviates the volume expansion of the NiO electrode material during cycling,thereby enhancing the electrochemical performance of the CN-3 electrode.