Conducting polymers for use in supercapacitors : an electrochemical and spectroelectrochemical investigation into low bandgap thiophene derivatives
The following low bandgap conducting polymers have been prepared by electrochemical polymerisation of the corresponding monomers: poly-4Hcyclopenta[2,1-b; 3,4-b']dithiophene-4-one, (PCDT), poly-4-dicyanomethylene-4Hcyclopenta[2,1-b: 3,4-b']dithiophene (PCDM) and 6,8-di(2-cyano-2(2- thienyl))ethenyl-4(ethylenedioxy)-4H-cyclopenta[2,1-b; 3,4-b' ]dithiophene (PCET). The polymers were characterised by cyclic voltammetry, spectroelectrochemical studies and impedance measurements in order to assay their suitability as active materials in high power supercapacitors. The redox switching of the polymers from neutral to doped states and the charge capacity of the films was studied by cyclic voltammetry. The results showed that only PCDM and PCET underwent efficient p- and n- doping which is a prerequirement for type III supercapacitors. The electrochemical switching is accompanied by changes to the optical properties of the polymers which has been examined by spectroelectrochemical measurements. In situ UV-visible spectroscopy indicated that the polymers possessed a low bandgap (< 2 eV) and suggested that they adopt a quinoid structure during doping.