Conducting polymers electrode impedance

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. 

In situ electron transport measurements on conducting polymers are commonly made by using a pair of parallel-band electrodes bridged by the polymer [Fig. 9(A)].141142 Other dual-electrode techniques in which the polymer film is sandwiched between two electrodes [Fig. 9(B)],139,140 rotating-disk voltammetry [Fig. 9(C)],60,143 impedance spectroscopy,144,145 chronoamperometry,146 and chronopotentiometry147 have also been used. 

It usually takes place close to the melting temperature of the polymer when the pores collapse turning the porous ionically conductive polymer film into a nonporous insulating layer between the electrodes. At this temperature there is a significant increase in cell impedance and passage of current through the cell is restricted. This prevents further electrochemical activity in the cell, thereby shutting the cell down before an explosion can occur. 

The response to the applied perturbation, which is generally sinusoidal, can differ in phase and amplitude from the applied signal. Measurement of the phase difference and the amplitude (i.e. the impedance) permits analysis of the electrode process in relation to contributions from diffusion, kinetics, double layer, coupled homogeneous reactions, etc. There are important applications in studies of corrosion, membranes, ionic solids, solid electrolytes, conducting polymers, and liquid/liquid interfaces. 

Hillman et al. measured the quartz crystal impedance to determine changes in rigidity, swelling and ionic exchange in conducting polymer films [57, 58] and have also used dynamic quartz crystal impedance of modified electrodes during film growth and redox conversion [57] by qualitative analysis of the acoustic admittance-frequency peak width. 

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. 

Figure 18.2 A cutaway view displaying the coated electrode system with electrolyte, conducting polymer, metal electrodes, and substrate. The current path across the interface between the electrolyte and the polymer determines the impedance.

X. Cui and D.C. Martin, Fuzzy gold electrodes for lowering impedance and improving adhesion with electrodeposited conducting polymer films. Sens. Actuat. A Phys., 103(3), 384-394 (2003). 

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). 

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. 

Ever since Feldberg s original model separating faradaic and capacitance current associated with a conducting polymer that behaves like a porous metal, " there have been various attempts to rationalize experimentally the nature of the current at a conducting polymer. Apart from impedance models, there has not been so much effort placed on modeling the system. Experimental efforts to examine processes at conducting-polymer-modified electrodes consisted of voltam-metry, " impedance, " and quartz crystal microbalance. However the... 

Cai, H., Xu, Y, He, P.-G., and Fang, Y.-Z. (2003). Indicator free DNA hybridization detection by impedance measurement based on the DNA-doped conducting polymer film formed on the carbon nanotube modified electrode. Electroanalysis, 15, pp. 1864-1870. 

The bulk of EAP-based supercapacitor work to date has focused on Type I devices. Polypyrrole (PPy, Figure 9.4C) has been studied [147,151-153] for this application, with specific capacitance values ranging from 40 to 200 F/g. Garcia-Belmonte and Bisquert [151] electrochemically deposited PPy devices that exhibit specific capacitances of 100-200 F/cm with no apparent dependence on film thickness or porosity extensive modeling of impedance characteristics was used. Hashmi et aL [153] prepared PPy-based devices using proton and lithium-ion conducting polymer electrolytes. As is often observed, electrochemical performance suffered somewhat in polymeric electrolytes single electrode specific capacitances of 40-84 F/g were observed with stability of 1000 cycles over a 1 V window. 

Colloidal dispersions provide an alternative route for developing solvent-processible conducting polymers and can be produced chemically [129] or electrochemically [130] by the oxidation of monomer in the presence of a steric stabilizer. For electrochemically produced colloids, the steric stabilizer impedes polymer deposition on the electrode surface. Fine colloidal silica can also be used as a dispersant, and nanocomposite colloids of polypyrrole and polyaniline have been prepared using... 

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. 

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