Mass and charge density effects on the saturation kinetics of polypyrrole doped with dodecylbenzene sulfonate

Abstract

In this article, the saturation kinetics model that describes chronoamperometric response of PPy(DBS) in our recently published work is extended to study the effect of mass and charge density on the step response of PPy(DBS). The saturation kinetics model is based on a mechanistic approach for charge storage in conducting polymers and leads to the development of structure-dependent input-output relationships to develop a cation concentration sensor. In this article, we demonstrate the use of poles and residues in the saturation kinetics model to deconstruct the chronoamperometric and chronocoulometric response by seperating the contributions from double layer charge accumulation and faradaic ion transport. We show that: (i) the number of redox sites, and therefore the number of ingressing ions at saturation, is directly proportional to the mass of the conducting polymer, (ii) the accessibility of these redox sites associated with ion ingress is inversely proportional to the conducting polymer charge density, (iii) the rate of ion ingress is found to be inversely proportional to mass and charge density, due to the decrease in the driving force per unit redox site and redox site accessibility, respectively. For lower charge densities, the mass has a dominant effect on saturation and rate of ion ingress, with charge density effects becoming apparent as it increases. The saturation charges obtained are consistent with the peak charges during cyclic voltammetry, thus validating the mechanistic interpretations. The findings of this article highlight the trade-offs between charge storage and transport properties for conducting polymer devices.