Impedance of conducting polymers

The film grown on an electrode surface has a duplex structure with a thin, compact first layer that is directly on the electrode surface and a porous second layer contacting the electrolyte. An equivalent circuit can be used to represent the electrical properties of this film. The components of an equivalent circuit can be determined by impedance spectroscopy. Therefore, this method has become one of the key methods for the characterization of conducting polymers. 

There has also been some discussion about the electron and ion transport contributions in the fihn. Popkirov et al. made experiments by growing a polymer film between two neighboring platinum electrodes. They were able to show that the electron conductivity dominated the charge transport and the conductivity in the polymer. The contribution by ion diffusion was only 5%. 

Another problem was that experimental results showed a dependence of the impedance data on the film thickness. For example, Komura et al., Albary et al., and Fletcher took 

In the diagram the cychc voltammogram of the polymer film is also shown. At positive potentials the film is metal conducting with high capacitance values and low resistance values. Capacitance decreases and resistance increases as the polymer becomes reduced. With the reduction the anions are expelled. The film becomes neutral state. 

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A similar equation hut containing the function coth was used hy Inzelt and L ng to descrihe the diffusional impedance of conducting polymers under reflective conditions [see Section III.6(ii) and Eq. (99)]. An electrical model containing this element accounted well for the impedance spectra, with a minimum number of free parameters. 

Deslouis, C., et al. 1995. Comparison of the AC impedance of conducting polymer films studied as electrode-supported and freestanding membranes. J Electrochem Soc 142 (6) 1902. 

At low frequency, the electrochemical impedance of conductive polymers is purely capacitive [102]. That is, the imaginary component of the impedance, Zi, is related to a capacitance Cl (farads/cm ) by... 

Through the combination of SPR with a - poten-tiostat, SPR can be measured in-situ during an electrochemical experiment (electrochemical surface plasmon resonace, ESPR). Respective setups are nowadays commercially available. Voltammetric methods, coupled to SPR, are advantageously utilized for investigations of - conducting polymers, thin film formation under influence of electric fields or potential variation, as well as - electropolymerization, or for development of -> biosensors and - modified electrodes. Further in-situ techniques, successfully used with SPR, include electrochemical - impedance measurements and -+ electrochemical quartz crystal microbalance. 

Conductive polymers are not used in fuel cells. However, the equivalent circuit of conductive polymers is similar to that of catalyst layers, which may help to understand impedance spectra in fuel cells. In general, the electric circuits of... 

Figure 4.38. Equivalent circuits of conducting polymers with a Randles circuit [12]. (Reprinted from Journal of Electroanalytical Chemistry, 420, Ren X, Pickup PG. An impedance study of electron transport and electron transfer in composite polypyrrole plus polystyrenesulphonate films, 251-7, 1997 with permission from Elsevier and from the authors.)...

Conducting polymers can be mixed with or electrochemically deposited onto transition metal oxides to achieve better performances as electrode materials. However, the wrapping of conducting polymers on metal oxides may reduce the effective contact area between metal oxides and electrolytes. This will further impede insertion of alkali ions into metal oxides and result in poor rate performance with low specific capacitance. A rational strategy is to use conducting polymers as active electrode materials directly and optimize their structures. To this end, a hierarchically nanostructured conductive polymer hydrogel was synthesized by a facile interfacial polymerization (Shi et al., 2014). 

G. Garcia-Behnonte, Analysis of conducting polymer electrode morphology from ion diffusion impedance measurements, in Progress in Electrochemistry Research, ed. by M. Nunez (Nova Science, New York, 2005), pp. 123-143... 

G. Popkirov, E. Barsoukov, and R. N. Schindler [1997] Investigation of Conducting Polymer Electrodes by Impedance Spectroscopy during Electropolymerization under Galvanostatic Conditions, J. Electroanal. Chem. 425, 209-216. 

Lange U, Mirsky VM. Separated analysis of bulk and contact resistance of conducting polymers comparison of simultaneous two-and four-point measurements with impedance measurements. J Electroanal Chem 2008 622 246-251. 

Among the various interesting and useful properties of conducting polymers, their switch-able electrical conductivity has proven the most attractive. The use of the conventional dc four-prove method or the ac impedance technique in a metal-polymer-metal sandwich structure for measurements of the conductivity of dry polymer samples is straightforward. 

Polymers that, to date, have been investigated for their electrical conducting properties include polypyrrole, polyaniline, polythiophene, and polymer nanotube composites. The advantages of conducting polymers include good conductivity, biocompatibility, good stability, low impedance ability to entrap molecules, efficient charge transfer, and ability to entrap biomolecules. 

Fletcher proposes adopting a porous electrode model, considering the conductive polymer film in contact with an aqueous electrolyte solution as consisting of a large number of identical, noninterconnected pores. The electrolyte solution is contained within the pores. The analysis then considers a single pore of uniform cross section. Three general impedance elements are considered the solution impedance x within the pore the interfacial impedance y between the solution within the pore and the pore wall and z, the internal impedance of the polymer. The latter quantities are assumed not to vary with distance inside the pore. 

Electrochemical impedance spectroscopy (EIS) has been used to characterize the ionic conductivity of conducting polymers in the electrode, separating this ionic resistance from the membrane and ohmic contact resistances at the interfaces [24— 31]. Such measurements are helpful in quantifying degradation of the ionomer in... 

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