Studies On 2,5-dimethoxyaniline And Its Polymer For Graphene-based Supercapacitors

Supercapacitors（SCs）have become one of the most important electrochemical energy storage devices by virtue of their fast charging and discharging rates,high power density and long cycling life.However,the low energy density of SCs hinders their practical applications,so the fabrication of high-energy-density SCs has become one of the research priorities of scientists in recent years.Based on the mechanism of energy storage,SCs are usually classified into electrical double-layer capacitors（EDLCs）and pseudocapacitors.Generally speaking,the energy storage capacity of EDLCs mainly depends on the specific surface area of the electrode material,and the larger the specific surface area,the higher the double-layer capacitance.As we all know,graphene is one of the most representative EDLCs-type electrode materials.Well-dispersed graphene nanomaterials are supposed to have a large specific surface area.However,the specific surface area of graphene decreases dramatically due to the agglomeration that often occurs during the preparation and processing of graphene,resulting in a relatively low specific capacitance and energy density of graphene materials.Pseudocapacitive materials rely on rapid reversible redox reactions on the surface to store charge,and thus their specific capacitance is larger than double-layer capacitance,but such materials usually have low electrical conductivity and the structures are less stable during long electrochemical cycling.Under this background,combining double-layer capacitive and pseudocapacitive materials is the mainstream solution for the preparation of electrode materials with high specific capacitance and excellent cycling stability.Conducting polymers represented by polyaniline and redox-active small molecules represented by quinone molecules are commonly used as pseudocapacitive materials,so we envision whether we could design and prepare a new type of conducting polymer with the structure of both polyaniline and quinone molecules,and then use it to functionalize graphene to enhance the specific capacitance of graphene materials as much as possible.Using inverse synthetic analysis,we have carefully selected 2,5-dimethoxyaniline（DMA）,an ether-based organic small molecule,as the monomer for polymerization.It is a commercial and inexpensive organic intermediate that is almost electrically insulating.The general impression is that ether compounds have poor redox reversibility,so they are rarely used as electrode materials.During the synthesis of poly-2,5-dimethoxyaniline functionalized graphene,we stumble upon that the DMA molecule functionalized graphene alone also has a high specific capacitance.Therefore,this thesis will focus on the following two aspects.（1）The application of 2,5-dimethoxyaniline（DMA）functionalized graphene in supercapacitors is investigated.In this part,we prepare DMA functionalized graphene（DMA-r GO）material by hydrothermal reaction using graphene oxide（GO）and DMA as precursors.Elemental analysis and Raman spectroscopy show that GO is fully reduced.Infrared spectroscopy and X-ray photoelectron spectroscopy reveal the presence of amide and C-N bonds,demonstrating the formation of covalent bonds.Scanning electron microscopy displays that the material has a fluffy and porous lamellar structure.Electrochemical tests show that the material has a high specific capacitance of 523 F g^-1at a current density of 1 A g^-1,as well as good rate capability and cycling stability.We first assemble this material into an all-solid-state symmetric supercapacitor.Electrochemical tests show that the energy densities of the symmetrical device are up to 17 and 11.7 Wh kg^-1at power densities of 500 and25000 W kg^-1,respectively.Subsequently,in order to widen the voltage window to further enhance the energy density,we assemble an all-solid-state asymmetric supercapacitor using DMA-r GO and Ti₃C₂T_xMXene as positive and negative electrodes,respectively.The electrochemical tests show that the voltage window of the device is successfully widened from 1 V of symmetric structure to 1.6 V,and the energy densities of the asymmetric device are up to 32.4 and 25.1 Wh kg^-1at power densities of 400 and 8000 W kg^-1,respectively.In addition,the capacitance retention rate can reach 78%after 20,000 cycles at 5 A g^-1,indicating its good electrochemical stability.（2）The application of poly-2,5-dimethoxyaniline（PDMA）functionalized graphene in supercapacitors is investigated.In this part,we use GO and DMA as precursors,ammonium persulfate as initiator,and further restore the conductivity of the graphene network through subsequent hydrothermal reaction to prepare PDMA functionalized graphene（PDMA-r GO）material.Elemental analysis and Raman spectroscopy indicate that GO is fully reduced.Infrared spectroscopy and thermogravimetric analysis show that the polymerization of DMA molecules occurs successfully.Infrared spectroscopy and X-ray diffraction demonstrate that PDMA and graphene are connected by covalent bonds,and scanning electron microscopy shows that the material has a soft and fluffy lamellar structure.Electrochemical tests show that the material has a high specific capacitance of 685.4 F g^-1at a current density of 1A g^-1,and then the material is assembled into an aqueous all-solid-state symmetric supercapacitor.The device has an energy density of 21.6 and 12.3 Wh kg^-1at power densities of 300 and 6000 W kg^-1,respectively,and it achieves a capacitance retention rate of 85%after 20,000 cycles at 10 A g^-1.Subsequently,in order to widen the voltage window to further enhance the energy density,we assemble PDMA-r GO material into button symmetric supercapacitor using EMIMBF₄/AN solution as the organic electrolyte.Electrochemical tests show that the voltage window is successfully widened to 3.5 V and the device can provide energy densities up to 100.6and 47.6 Wh kg^-1at power densities of 1750 and 35000 W kg^-1,respectively.In addition,the capacitance loss is less than 20%after 10,000 cycles at 5 A g^-1,demonstrating its good cycling stability.