Abstract
We present in this study the electrochemical properties of poly(3,4-dimethylthiophene) (PDMT) in comparison with poly(3-methylthiophene) (P3MT). The electropolymerizations were carried out at constant current densities (j=+2.5mA/cm2) in 1-ethyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI) and N-butyl-N-methylpyrrolidiniumbis(trifluoromethanesulfonyl)imide (PYRTFSI). The electrochemical stability of PDMT has been considerably enhanced with the use of ionic liquids with a cyclability up to 1500 cycles in EMITFSI compared to only 100 cycles in acetonitrile and 0.1M tetraethylammoniumtetrafluoroborate (TBATFB). PDMT exhibits better electrochemical storage properties based on the measure of its specific capacitance (287F/g in EMITFSI) compared to P3MT (190F/g). The electrochemical reduction of doped PDMT is not complete as attested by UV–visible spectroscopic studies which showed the dual presence of transition bands of both neutral and polaronic species even after dedoping. This behaviour affects the doping/dedoping process of this polymer with loss of electroactivity whatever the ionic liquid used. Then, we performed the copolymerization at constant current densities from feed solutions containing the two monomers at different ratios. The evolution of the composition has been correlated to the continuous modulation of both electrochemical and optical properties. The oxidation potential of copolymers and the optical gap evolve according to the amount of DMT in the feed solution. Cyclic voltamperometry and UV–visible spectroscopy have confirmed that whatever the electrolyte (i) the copolymers have random composition and (ii) more DMT units are incorporated in the copolymer than DMT units initially present in the feed solution. The specific capacitance Cm of the copolymers increases with the amount of DMT from 190F/g to 287F/g in EMITFSI and from 230F/g to 279F/g in PYRTFSI at 30°C. The cyclability of copolymers is also improved compared to PDMT homopolymer. Therefore, this class of material is interesting in the field of electrochemical devices regarding the tunability of their electrochemical properties.
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