Electrochemical Polymerization Of 5-membered Heterocyclic Compounds In Ionic Liquids And Their Applications In Electrocatalysis

As a kind of new material,poly(hetrocyclic compounds)such as polyaniline, polythiophene,polypyrrole have many advantage physical and chemical properties and have considerable impact on both everyday life and the development of many fields of modern science and technology.These polymers have high flexibility and environmental stability,and also can transfer between nonconductor and conductor. Conducting polymers have been widely applied to the electrochemical catalytical areas,electrode materials,light-emitting diode,sensors and mesoporphyrin protection areas.5-membered cyclic compound polymers used to be prepared in organic or water solution.The conductivity of the polymer may be influenced by the existence of water. Ionic liquids are a kind of new solvents,which has no volatility,high conductivity, wide electrochemical potential windows and can dissolve many chemicals.They have been successfully used as solvents and electrolytes in many reactions.This thesis combine the conducting polymers and ionic liquids together and obtained several conducting polymers,such as poly(3-methylthiophene),polyaniline, polypyrrole and poly(3-methylthiophene-anline)copolymers.Besides the physical and chemical characterizations,these polymer modified electrodes were applied to the electrochemical catalysis of catechol,hydroquinone and the detection of ascorbic acid and dopamine.The electrochemical reduction of CO₂ was also investigated at polymer based electrodes.Modified electrodes show good electrocatalytical effects to the oxidation or reduction of above compounds.The detail researches are as follows:1,Electrochemial polymerization of 3-methylthiophene in ionic liquidPoly(3-methylthiophene)(PMT)was synthesized in the room temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate([BMIM]PF₆)by constant potential,constant current and cyclic voltammetry respectively.The structure, morphology and stability of PMT films were characterized by Fourier transform spectroscopy(FTIR),scanning electron micrographs(SEM).The undoped(reduction) and doped(oxidation)forms of PMT film prepared in ionic liquid were reversible and stable.PMT film has strong electrocatalytical effect to the redox reaction of catechol and hydroquinone.It also has obvioius electrocatalytical effect to the detection and separation of ascorbic acid and dopamine.Two methods of potential steps were used to observe the response time of the film and the film was found to have perfect electrochromic response.2,Electrochemical copolymerization of 3-methylthiophene and aniline in ionic liquidElectrochemical polymerization of aniline and 3-methylthiophene has been accomplished in 1-methylimidazium tetrafluoroborate([HMIM]BF₄)ionic liquid. Homopolymer and copolymers of aniline and 3-methylthiophene were obtained successfully.The copolymer was studied by cyclic voltammetry and electrochemical impedance spectroscopy.The formation of copolymer has been confirmed by FTIR and UV-vis spectra.The atomic force microscope(AFM)was used for morphology analysis.Both the homopolymer and the copolymer had the electrocatalytical activity for the hydroquinone,while the electrocatalytical effect at copolymer and polyaniline modified electrodes are better than that at PMT modified electrode.3,Electrochemical polymerization of 3-methylthiophene at TiO₂ electrode in ionic liquidTiO₂ electrode was prepared by sol-gel method,and then applied the electrode to the electropolymerization of 3-methylthiophene in[BMIM]PF6 ionc liquid.The properties of the composite film were studied by cyclic voltammetry,UV-vis absorption spectra,SEM and electrochemical impedance spectrum. Poly(3-methylthiophene)(PMT)film was obtained successfully and showed obvious doping an d dedoping process.The in situ UV-vis spectra showed the tran sition peaks of oxidation an d reduction states.In the reduction(dedoping)process,the electrode exhibited an absorption peak at 480 nm,while the peak disappeared in the oxidation(doping)process,replaced by an increasing visible light absorption.The p-n heterojunction existed in the TiO₂/PMT film electrode,which Can improve the photon-electron conversion efi ciency.The EIS spectrum presents the conductivity of the composite film.4,Electrochemical polymerization of pyrrole in ionic liquidPolypyrrole(PPy)films were electropolymerized on Pt and indium-tin-oxide (ITO)electrodes in both[BMIM]PF₆ and[BMIM]TFSI ionic liquid.The ionic liquid was used as both the growth medium and the supporting electrolyte.The cyclic voltammetry(CV)shows that the growth of polymer,prepared in ionic liquid was not similar to that prepared in traditional organic media.The current was affected by the viscosity of the solvents.The polymer was further characterized via AC impedance,in situ UV-Visible spectroscopy,SEM,Raman and FTIR.Both Raman and FTIR spectroscopy results show the characteristic peak of PPy.SEM shows the morphology differences between the both sides of the polymer.The electrocatalytic effect of the PPy modified electrode was investigated in hydroquinone solution and the result exhibited electrocatalytic properties for hydroquinone by the PPy modified electrode.5,Electrocatclytical effect to the detection of dopamine and ascorbic acid at PPy modified electrodePolypyrrole(PPy)was electropolymerized on a glassy carbon(GC)electrode in [BMIM]PF₆ ionic liquid and then was used for the detection of dopamine(DA)in the presence of ascorbic acid(AA)by differential pulse voltammetery(DPV).The results showed that PPy electrode displayed high electrocatalytic effect towards the oxidation of DA and AA with well separated anodic peaks(â–³E=200 mV at pH 6.2),negative shifted potentials and enhanced oxidation currents.The oxidation peak potentials and currents were affected by the pH valuation and the film thicknesses.6,Electrocatalytical reduction of CO₂ at PPy based electrodesPPy,cobalt phthalocyanine(CoPc),cobalt phmalocyanine/polypyrrole (CoPc/PPy)and polypyrrole/cobalt phthalocyanine-single walled carbonnanotube (PPy/CoPc-SWCNT)modified electrodes have been developed in the request for diminishment of the overpotential required for the electrochemical reduction of CO₂. Compared with pure PPy electrode,cyclic voltammograms(CV)at these modified electrodes showed clear and better electrocatalytic performances with some positive shifts of the decomposition potential for CO₂ reduction in 0.1 M LiClO₄ /acetonitrile(ACN)-H₂O electrolyte.The CoPc electrode was not very stable,while PPy/CoPc-SWCNT electrode showed highest stability.The mechanism study suggests that the pathway of the reduction of CO₂ is stepwise hydrogenation by H_ads,and H₂O is necessary in the whole process.The incorporation of CoPc onto PPy polymer matrix could enhance the catalytic activity for the formation of H_ads,providing extra advantages for the reduction of CO₂.