Studies on the electrochemistry and applications of conducting polymers

Huang, Huan (1998) Studies on the electrochemistry and applications of conducting polymers. Masters thesis, Memorial University of Newfoundland.

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    Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission.
    (Original Version)

Abstract

This thesis reports a study on electrochemistry and applications of some thiophene-based conducting polymers, which include polythiophene (poly-Th), polybithiophene (poly-BTh), polyterthiophene (poly-TTh), poly(4-dicyanomethylene-4H-cyclopenta[2, 1-b: 3, 4-b’]dithiophene) ( poly-CDM), O₂-modified poly-CDM, poly(3, 4-ethylenedioxythiophene) (poly-EDOT), poly-(CDM-co-EDOT), and poly[3-(p-fluorophenyl)thiophene] (poly-PFPT). -- Chapters 3, 4 and 6 focus on electrochemical and spectroscopic measurements of the band-gaps of the starting monomers and oligomers, and their polymers. As expected, increasing the number of thiophene units in the oligomer decreases the band-gap. There is a linear relationship between the band-gap and the inverse of the number of thiophene units in the oligomer, from which, a band-gap of ca. 1.9 eV is predicted for a perfect polymer. The band-gap of the polymers, however, decreases with increasing chain length of the starting oligomers, with the average effective conjugation length for poly-Th, poly-BTh, and poly-TTh found to be 16, 9 and 8 respectively. -- Poly-CDM has a significantly reduced band-gap because of the strong electron-withdrawing group, dicyanoethene, which decreases the LUMO energy level substantially. The small change of the electronic absorption upon n-doping reveals that n-type charge carriers to not delocalize well along the thiophene backbone. In situ conductivity measurements yielded maximum p- and n-type conductivities of 0.59 and 5.4x10⁻³ S cm⁻¹ respectively, and an intrinsic conductivity of 1.0x10⁻⁸ S cm⁻¹, in agreement with the band-gap of 0.8 eV. -- Reaction of n-doped poly-CDM with O₂ was found to produce a modified form of the polymer with a tunable and extremely low band-gap. The maximum p- and n-type conductivities both decrease upon reaction with O₂, but the minimum conductivity increases remarkably by several orders of magnitude. An intrinsic conductivity of 1.3x10⁻⁶ S cm⁻¹ measured for an O₂-modified polymer corresponds to a band-gap as low as 0.22 eV. It is proposed that this reduced band-gap is due to substitution of the polymer in the β positions with hydroxyl groups. -- Due to the strong electron-donating effect of dioxyethylene, poly-EDOT has a much lower oxidation potential compared to poly-Th. Poly-EDOT can be heavily p-doped and exhibits excellent cycling stability for p-doping. Maximum conductivities for p- and n-doping were obtained to be 0.60 and 5.7x10⁻³ S cm⁻¹, respectively. -- Compared to Th, CDM has a lower LUMO energy level, and EDOT has a higher HOMO energy level. Thus copolymerization of these two monomers has successfully produced copolymers with very low band-gaps and high intrinsic conductivities. A copolymer synthesized at 1.32 V had a band-gap of 0.19 eV and an intrinsic conductivity of 0.69 mS cm⁻¹. Strong evidence from voltammetric studies and Raman spectra supports the contention that poly-(CDM-co-EDOT) is a true copolymer. -- Poly-PFPT is used as the electrode active material for supercapacitors. For the polymer synthesized using constant current, after 1000 deep charge/discharge cycles p-type stability is close to or over 80%. This is much better than the n-type stability, for which the best figure is 47% after 1000 cycles. In order to improve the n-type stability, cyclic voltammetry mode has been employed as the synthesis method. Test results show that n-type stability has been improved to as high as 67%.

Item Type: Thesis (Masters)
URI: http://research.library.mun.ca/id/eprint/1017
Item ID: 1017
Additional Information: Bibliography: leaves 223-227.
Department(s): Science, Faculty of > Chemistry
Date: 1998
Date Type: Submission
Library of Congress Subject Heading: Conducting polymers; Polythiophenes; Electrochemistry

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