Preparation And Properties Of Electrochromic Polymers And Their Composites With Good Stability

With the increasing problems of energy crisis and environmental pollution,it is not only necessary to produce energy through clean means,but also urgent to develop energy storage devices for efficient utilization.Supercapacitors（SCs）are notable for their high power density,ultra-fast charge/discharge rate,long cycle lifeas well as excellent stability and safety.In addition,the rapid growth of intelligent electronic products has also triggered an urgent demand for smart energy storage systems.Designing innovative energy storage devices with intelligent functions to let people visually determine energy storage stage is of great significance for practical applications.During the process of charge insertion/extraction or chemical reduction/oxidation,some materials can change their color correspondingly.This phenomenon called electrochromism.The concept of incorporating electrochromicmaterials into supercapacitors that allow them to visually reflect energy storage levels through color changes has become feasible and an important trend in scientific and technological development.Conducting polymers（CPs）had the advantages of good conductivity,low cost as well as high flexibility and can be used in electrochromic supercapacitors.However,when they were used in SCs,relatively low mechanical stability and cycle life limited their performance.How to improve the stability of CPs is the focus of current research on electrochromic supercapacitor materials.Starting from the application of conducting polymer materials in electrochromic supercapacitors,this thesis focused on the following three parts:（1）A solution-processable PBDTC-MWCNT composite was designed and synthesized by Williamson reaction between the polymer with modified bromo group at the end of the side chain and the hydroxylated carbon nanotube.Compared with the traditional covalent modification method of carbon nanotubes,the one-step method can not only simplify the synthesis process,but also make the composite materials dispersed more evenly and can exist stably in solution for a long time.The effective charge transfer between the polymer and carbon nanotubes in the composite was also confirmed by a series of subsequent characterization such as UV-vis and photoluminescenceetc.A symmetricsupercapacitor（SC）based on PBDTC-MWCNT provides a 4.8 V operating potential window.As an energy storage device,the symmetric supercapacitor achieved maximum energy density of 174.7 W h kg^-11 at a power of4.8 kW kg^-1.In addition,color changes during charging/discharging of supercapacitor make it a smart device with both energy storage and electrochromic functions.（2）Based on carbazole and benzodithiophene（BDT）units,the thermal cross-linked polymer PCBDTP-X was successfully prepared by introducing 4-vinylbenzyl chloride groups into the side chain.This electroactive polymer has a moderate capacitance and good stability across a wide potential window of 1.2 V and is applied as a positive electrode material in all-polymer asymmetric supercapacitors.PCBDTP-X displayed a dual function of energy storage/electrochromism:it can visually monitor the charging/discharging of polymer electrode through color change and realize"smart"energy storage device.Then,PEDOT,a conducing polymer with wide potential window and high stability,was selected as the negative electrode to supplement p-doped PCBDTP-X in an all-polymer asymmetric supercapacitor（ASC）.PCBDTP-X//PEDOT electrochromic ASC can produce a high energy density of 19.68 W h kg^-11 and 600 W kg^-11 power density over a wide voltage range of 2.4 V,and can still maintain99.1%capacitance after 2000 cycles.（3）Firstly,a simple in-situ electrodeposition method was used to prepare PEDOT/GO hybrid nanostructures on ITO substrate.P（CZ-BT-CZ）was a kind of D-A polymer material whose ends of side chains were modifiedby carbazole group,and then P（CZ-BT-CZ）was further electrocpolymerized on the surface of ITO-PEDOT/GO by cyclic voltammetry to obtain cross-linked PP（CZ-BT-CZ）/PEDOT/GO composite electrode and its electrochemical performance was studied.Interestingly,besides the energy storage capacity,the composite electrode also showed distinct electrochromic properties.The sandwiched-type ASC which had a 2.0 V wide voltage window was then fabricated by assembling PP（CZ-BT-CZ）/PEDOT/GO and PEDOT/GO electrodes.ASC exhibited moderate specific capacitance(4.38 mF cm^-2),high energy density(24.3 W h kg^-2)and good cyclic performance（95.8%capacitance retention after2000 cycles）.This is mainly due to that GO provided a frame structure that allowed the composite film to have a large specific surface area and roughness,which facilitated the penetration of electrolyte during the electrode film.While,GO acted as a buffer layer,effectively preventing the volume change and mechanical damage of the polymer during redox.