Intrinsic asymmetry, hysteresis, and conformational relaxation during redox switching in polypyrrole: A coulovoltametric study

Theoretical coulovoltammograms associated with the evolution of anodic charges when a polypyrrole film, which was previously compacted at a strong negative potential in a 0.1 M LiClO4 propylene carbonate solution, was submitted to anodic potential sweeps were compared with experimental responses. The electrochemically stimulated conformational relaxation (ESCR) model provides a good description of either the influence of the negative potential limit, the sweep rate, the temperature, or the electrolyte concentration on the rate of electrochemical oxidation, this being initiated through conformational relaxation processes and completed under counterions diffusion control. An analysis of both anodic and cathodic experimental branches, together with theoretical conformational relaxation curves, allows to detect four different structural rearrangement processes affecting electrochemical responses: the well-known conformational relaxation effect observed only in anodic branches, a hysteresis effect on cathodic curves at potentials lower than -600 mV vs SCE, an intrinsic asymmetry between anodic and cathodic branches, and a like-capacitive effect at the beginning of the anodic sweep. The combination of both hysteresis effects in cathodic branches and relaxation effects in anodic ones gives rise to a hysteresis cycle, whose integration yields the electrochemical energy required to expand the compacted polymer structure. On the other hand, the intrinsic asymmetry between anodic and cathodic branches has been related to the different values of coefficients z(r) and z(c) which, according to the ESCR interpretation, are associated with the electric charge required to open or close a mole of polymeric segments, respectively. This fact opens the possibility for a complete theoretical description of electrochemical responses of conducting polymers as a function of their structure.