Electropolymerization Of Conducting Polymers Via A Room Temperature Ionic Liquid And Its Application In An Electrochromic Device

Conjugated polymers stand for a family of important electrochromic materials. Among these conjugated polymers,Polythiophene and its derivatives are the most challenging class and gain popularity due to their unique advantages,such as low cost, good processibility,rapid response time,high optical contrast,flexibility for large-area application,and the ability to modify their structure to create multicolor electrochrome.They can be prepared by chemical or electrochemical methods. Among these methods,electrochemical techniques are particularly appropriate for controlled synthesis of these compounds and for the isolation of a well-defined oxidation state.The preparation,characterization and application of electrochemically active,electronically conjugated polymeric systems are still in the foreground of research activity in electrochemistry.Room temperature ionic liquids(RTIL)have been put in a wide range of synthetic application as green solvents during the past decade for their good chemical and physical properties.The use of ionic liquids is often initiated by their recyclable use and more environmentally benign,where the negligible volatility and non-flammability of ionic liquids make them ideal replacements for more toxic molecular solvents,importantly,overcome the problem of solvent evaporation that exists with the long-term use of volatile solvents in electrochemical applications. Rogers et al.predicted in 2003 Science that the next decade should see ionic liquids being used in many applications where conventional organic solvents are used today. Furthermore,ionic liquids will enable new applications that are not possible with conventional solvents.Especially air and moisture stable ionic liquids,with high electrical conductivities,excellent thermal and chemical stability,stable over a wide temperature range,good solvent transport properties,negligible vapor pressures and wide electrochemical windows(typically over 4V wide)and good solvents for a wide range of organic and inorganic materials,were expected to be a promising medium for electrochemical synthesis and application.In recent years,there has been great interest towards electrochromic applications and device fabrication,such as display panels,electrochromic mirrors and smart windows.Conducting polymers stand for a family of important electrochromic materials that have gained popularity due to their unique advantages,such as low cost, good processibility,flexibility for large-area application,and the ability to modify their structure to create multicolor electrochrome.Copolymerization by introducing new groups alters the existing structure and band gap,leading to absorption at different wavelengths and colors,which offers a means of controlling the electrochromic properties of conducting polymers.With the development of nanotechnology,conducting polymer/metal oxides nanocomposite films as a family of electrochromic materials has also gained extensive attention.In this thesis,electrochemical synthesis of homopolymer,copolymer and nanocomposite were carried out via potentiodynamic and galvanostat methods by using ionic liquid as the growth medium and the supporting electrolyte.The properties of these films were characterized by the means of the FT-IR spectrum,the scanning electron microscopy(SEM)thermogravimetry and differential thermal analysis (TG-DTA).The electrochemical and electrochromic properties were examined with cyclic voltammetry(CV),UV-Vis spectrum,chronoamperometry,and chronoabsorptometry.Based on this,electrochromic device was constructed and its electrochromic properties were also investigated.Totally,there are six chapters in this paper.The first chapterThis chapter has four parts,that is,ionic liquid,conduct polymer,electrochromic material and electrochromic device.The sort,speciality and preparation method of ionic liquid has been brief introduced,and its application in electrochemistry has also been reviewed in detail.Then conduct polymer's preparation method,various characterized technique and its application prospect has been discussed.The speciality and properties of electrochromic materials has also been reviewed.Lastly,we introduce the preparation,structure and mechanism of electrochromic device and review its application and prospect.The second chapterIn this chapter,stable ionic liquid likel-butyl-3-metyllimidazolium hexafluorophosphate[BMIM]PF₆ has been used as solvent and electrolyte for the electropolymerization of 3-bromothiophene by galvanostat and cyclic voltammetry. The structure and morphology of PBrT films were characterized by the means of the FT-IR spectrum and the scanning electron microscopy(SEM).The thermal stability of PBrT have been investigated through thermogravimetry and differential thermal analysis(TG-DTA).The electrochemical and electrochromic properties of PBrT flims were examined with UV-Vis spectrum,chronoamperometry,and chronoabsorptometry. The results showed the surface morphology of PBrT films that prepared in ionic liquid were denser and much smaller than that prepared in traditional solvent,and the PBrT films had good stability,reversibility,high electrochemical capacity and super thermal stability.Electrochromic performance of the PBrT films that prepared in ionic liquid showed a good color contrast and comparative response time.Simultaneously,ionic liquids exhibit wide electrochemical potential window,high ionic conductivity and recyclability.Therefore,they are promising to electropolymerization and so on.The third chapterIn this chapter,electrochemical homopolymerization of 3-chlorothiophene(ClT) and copolymerization with 3-methylthiophene(MeT)were carried out via potentiodynamic and galvanostat methods by using ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate,[BMIM]PF₆)as the growth medium and the supporting electrolyte.Both homopolymer(PClT)and copolymer P(ClT-co-MeT)were characterized via cyclic voltammetry(CV),Fourier transform infrared spectroscopy(FTIR),spectroelectrochemical analysis and kinetic study. Homopolymer revealed color changes between deep red and deep blue,whereas copolymer showed the most vivid change of color between bright red and greenish blue in fully reduced and oxidized states.Via kinetic studies,switching time and the maximum optical contrast%â–³T were found to be 2.4 s and 17%for PClT,0.9 s and 26%for P(ClT-co-MeT).Compared with PClT(580 times),the copolymer exhibited a long-term switching stability up to 2300 double switches.Results implied that copolymerization is a valuable approach to achieve the desired electrochromic properties. The fourth chapterIn this chapter,electrochemical polymerization in a room temperature ionic liquid,1-butyl-3-methylimidazolium hexafluorophosphate([BMIM]PF₆),has been used to prepare electrochromic poly(3-methylthiophene)(PMeT)and its more attractive derivatives:poly(3-hexylthiophene)(PHexT)and poly(3-octylthiophene) (POcT).Spectroelectrochemistry and electrochromic properties of the resulting polymers were characterized using various experiment techniques in [BMIM]PF6/CH3CN(1:1,v/v)solution.The thin films were bright red,orange red and orange yellow at its fully reduced state for PMeT,PHexT and POcT,respectively. After oxidization of these undoped polymers,the films underwent reversible change to the bright blue,blue or black blue form.These poly(3-alkylthiophene)s(PMeT, PHexT and POcT)films exhibit high chromatic contrast(46%,45%or 39%), comparative switching times(1.1,1.4 or 1.9 s),great electrochromic efficiency(250, 220,230 cm²C^-1)and long-term switching stability.High quality electrochromic polymers were provided for the use of commercially available thiophene monomers, avoiding the use of other custom synthesized monomers.The fifth chapterIn this chapter,poly(3-chlorothiophene)(PClT)was electrochemically synthesized on a nanoporous TiO₂ surface in the ionic liquid 1-butyl-3-metyllimidazolium hexafluorophosphate by galvanostat method to control charge density of 93 mC cm^-2.Spectroelectrochemical analysis presented that the PClT/TiO₂ nanocomposite film had an electronic bandgap of 1.80 eV with a bordeaux color in the reduced form and a dark blue color in the oxidized form.The switching ability of the PClT/TiO₂ nanocomposite film was monitored and the percent transmittance was measured as 12%at 555 nm and 25%at 770 nm.Moreover,a PClT/TiO₂ nanoeomposite film based electrochromic device was constructed and its electrochromic properties were also investigated in this paper.The sixth chapterAg nanoparticle film,which was fabricated by vacuum deposition method from high purity Ag wire onto cleaned ITO,was uniform and smooth.Ag/WO₃ composite film was prepared by cathodic electrodeposition of WO₃ onto the surface of Ag nanoparticle film.Moreover,Ag/WO₃ nanocomposite film based 5-layer electrochromic device(glass/ITO/Ag-WO₃ nanocomposite film/solid electrolyte/poly(3-methythiophene)/ITO/glass)was constructed and its electrochromic properties were also investigated in this paper.In comparison with a single component system(WO₃),a substantial enhancement in the electrochromic performance for the Ag embedded WO₃ composite system was clearly observed.The method of constructing this electrochromic device was simple and electrochromic properties appeared good.