Polypyrrole film capacitance

Figure 11.19 High frequency part of capacitance and resistance of a polypyrrole film as function of the potential. The film was prepared by anodic oxidation in a perchlorate electrolyte. Additionally, the cyclic voltammogram is shown. The film has metal-like properties at positive potentials (E> OV) and neutral state properties at negative potentials (E < -0.5 V).

Garcia-Belmonte, G., and J. Bisquert. 2002. Impedance analysis of galvanostatically synthesized polypyrrole films. Correlation of ionic diffusion and capacitance parameters with the electrode morphology. Electrochim Acta 47 (26) 4263. 

In the case of acetonitrile, where the solvation of ions is imlikely to occur, pyrrole itself can solvate anions. This may explain why polypyrrole films formed from acetonitrile with perchlorate counterions are less porous because, with pyrrole solvating the counterion during synthesis, a denser structure would be expected. This also results in polymers that are more conductive and have lower capacitance and greater electrochemical reversibility than those grown from water. Similar differences in conductivity were observed between acetonitrile and water when dodecyl-sulfate (DS) was used as the counterion, although the differences in conductivity were not so marked. The presence of DS probably provides some protection from the nucleophilic solvent. In other work, we have shown that pyrrole can be reversibly oxidized in surfactant-containing media (i.e., the surfactant stabilizes the free radical produced). This reversibility could not be detected in the absence of surfactants. 

The general situation at this time is that the field of electrode-kinetic measurements is in a state of infancy. One of the difficulties is illustrated by the work of Bull et al., who found (28) that polypyrrole-coated platinum appeared to be active because of the porosity of the polypyrrole film - the underlying substrate was in fact the active component. Correspondingly, Feldberg found (40) that polypyrrole on Pt is porous to solvent which results in an unexpectedly large surface area. From cyclic voltammograms he concludes (41) that significant proportions of the currents were taken up capacitively as the sweep rate increased. 

Recently supercapacitors are attracting much attention as new power sources complementary to secondary batteries. The term supercapacitors is used for both electrochemical double-layer capacitors (EDLCs) and pseudocapacitors. The EDLCs are based on the double-layer capacitance at carbon electrodes of high specific areas, while the pseudocapacitors are based on the pseudocapacitance of the films of redox oxides (Ru02, Ir02, etc.) or redox polymers (polypyrrole, polythiophene, etc.). 

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. 

Hughes, M., et al. 2002. Electrochemical capacitance of nanocomposite films formed by coating aligned arrays of carbon nanotubes with polypyrrole. Adv Mater 14 (5) 382. 

From the fitting, this has a value of 36 pF (slightly greater than obtained with the uncoated electrode). Rppy represents the in-plane resistance of the polypyrrole-palmitic acid film, which from the curve fitting is found to be 3030 Q. The capacitance of the polypyrrole multilayer coated interdigitated electrodes measured in vacuum was only a few pF has been ignored in the model. 

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