A Study On The Preparation Of Electrochromic Conjugated Polymers,Composites And Supercapacitor Performance

With short charge-discharge time and high power density,supercapacitor becomes a new type of portable energy storage device which is the focus in recent years.In addition to the advantages of non-toxic,great flexibility and conductivity,conjugated polymers also have rich colors,simple synthesis method and controllable structure.It can obtain polymers or composites with specified functions by grafting functional groups,regulating molecular chain structure or blending with different materials.And they can become a potential supercapacitor electrode material.At the same time,the conjugated polymer electrode can exhibit a variety of color changes in different applied voltages,and reflect the energy storage level of the supercapacitor in real time.This function gives the electrochromic performance of supercapacitors,that can meet the increasing demand for devices in smart windows,portable electronic devices,smart wearables and military fields.This paper focused on improving the performance of conjugated polymers when used as electrode materials for electrochromic supercapacitors,and mainly developed the following aspects:(1)The application of polymers containing rigid twisted structures in electrochromic supercapacitors was studied,and the effects of different linking unit between rigid structural units on the electrochemical properties and electrochromic properties of the materials were systematically explored.First,three D-A conjugated polymers PI-IDT,PI-BT and PI-DPP with twisted spirobifluorene structure were synthesized.By using density functional theory(DFT),we found that the polymer exhibited a distinct HOMO/LUMO wave function distribution when the linking unit was electron-donors and electron-acceptors,respectively.PI-IDT with electron-donor dicyclopentadiene benzodithiophene(IDT)as the linking unit exhibited completely separated HOMO/LUMO wave functions,had independent p/n-dopable redox characteristics and excellent performance.During p-doping and n-doping processes,the area specific capacitance can reach 6.2 m F cm^-2 and 2.4 m F cm^-2,respectively.The other two polymers PI-BT and PI-DPP,which use electron-acceptors as the linking units,showed limited capacitance and poor cyclic stability.In addition,PI-IDT had rich color variations(orange,green,blue)and excellent coloring efficiency(155 cm² C^-1).The asymmetric supercapacitor PI-IDT//W_0.71Mo_0.21O_3-xwith PI-IDT as anode material showed discoloration from orange to blue,and delivered an energy density of 3.89 W h kg^-1.(2)In order to optimize the performance of the conjugated polymer electrodes,we tried to construct a 3D cross-linked network structure to effectively increase the specific surface area of the polymers,and explored the impact of porous cross-linked structure on the electrochemical performance and stability of materials.Four different cross-linking agents were mixed into the PCZBDTCZ,which can be electro-polymerize by side chain carbazole units.And seven new cross-linked polymers were synthesized by two electropolymerization methods,namely PPCZBDTCZ,PPCZBDTCZ-TCTA,PPCZBDTCZ-DTP,PPCZBDTCZ-TEPA,PPCZBDTCZ-D3P1-1,PPCZBDTCZ-D3P1-2 and PPCZBDTCZ-D3P1-3.We found that there was a certain relationship between the structure of the cross-linking agents and the morphology and performance of the cross-linked polymers.PPCZBDTCZ-D3P1-3 obtained by using a mixed cross-linking agent showed better performance in all aspects due to the advantages of combining two cross-linking agents.It attached loose cluster particles on the cross-linked network,making the film had a larger specific surface area,well optical contrast(?T=17.63%)and high coloring efficiency(190.39 cm²C^-1).And the area specific capacitance can reach 6.82m F cm^-2(the current density was 0.05 m A cm^-2),after 1000 cycles,the capacitance retention rate was 54%,which was significantly improved compared to the original polymer PPCZBDTCZ.(3)We designed a method that coating polymer conductive network on transition metal oxide to obtain higher conductivity and capacitance,and explored the organic-inorganic synergy effect on the electrochemical performance and electrochromic performance of polymer matrix composites.Using tri-branched TEPA and EDOT or DTP units for electrochemical monomer copolymerization,two conjugated polymers with main chain cross-linked structure similar to CMP were designed.And we deposited it on WO₃ film that grown in-situ on FTO by hydrothermal method(WO₃-FTO)to gain P(EDOT/TEPA)/WO₃ and P(DTP/TEPA)/WO₃composite thin film electrodes.Through a series of characterizations,we found that the two composite films showed better electrochemical performance than WO₃-FTO.Based on the redox properties of the two composite materials in different voltage ranges,the composite films showed a wide voltage window of 1.95 V and the area specific capacitance can reach 19.82 and12.31 m F cm^-2,respectively.The charge transfer resistance of the materials was reduced,proving that the conjugated polymer exists as a conductive network on the surface of WO₃-FTO.Among them,P(DTP/TEPA)/WO₃ exhibited better properties due to less surface polymer coating,could undergo four-color changes(dark-blue,purple-red,green and light blue),had improved electrochromic properties(?T=14.81%,coloring efficiency is 98.68 cm² C^-1)and smaller electrochemical impedance.