Studies on the electrochemistry and applications of conducting polymers

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%.