Apparent negative activation energy in electrochemical insulating-to-conducting conversion at polymethylthiophene films

Abstract

An electrochemical conversion rate of poly(3-methylthiophene) films from the insulating state to the conducting state in dopant-rich solution decreased with an increase in temperature. The conversion rate defined here is the moving speed of the conducting front growing from the electrode toward the solution phase in response to the potential step. It was evaluated by time-variation of absorption at the growth experiment, in which the electrochemically synthesized poly(3-methylthiophene) film was peeled from the electrode and one end was connected to the electrode so that the conducting zone grew in the direction of the film length. The Arrhenius plot showed a linear line with a positive slope, indicating a negative value of the activation energy of the conversion. Although the values, which have potential-dependence, were extrapolated to a predicted value of the standard potential, the value was still negative. Correction of potential drop in the film and solution had little influence on the negative value. The anodic peak potential of cyclic voltammograms shifted in the positive direction as the temperature increased. The shift corresponds actually to applying a smaller overpotential to the film at the growth experiment. This explains the apparent negative value of the activation energy. The activation energy corrected for the smaller overpotential was 13 kJ mol −1, which agreed with the value obtained by temperature dependence of the voltammetric peak current. The temperature-dependence of the standard potential can be interpreted as an increase in chemical potential of the conducting species as a result of a decrease in the conductivity, according to the theory of stability of free electrons in metals.