Effect of the bathing electrolyte concentration on the charge transport process at poly(o-aminophenol) modified electrodes. An ac impedance study

doi:10.1016/S0022-0728(97)00235-0

Journal of Electroanalytical Chemistry

Volume 434, Issues 1–2, 15 August 1997, Pages 83-91

https://doi.org/10.1016/S0022-0728(97)00235-0 Get rights and content

Abstract

The effects of ionic strength and film thickness on the transport properties of poly(o-aminophenol) (POAP) film electrodes were studied by impedance spectroscopy. The dependences of the characteristic impedance quantities (Warburg coefficient, resistance and redox capacitance at low frequency) on these external variables were interpreted on the basis of two models: a transmission line and a modified electron hopping model. Analysis of experimental data by means of the transmission line model allowed us to separate the charge transport process within the film in terms of electronic and ionic contributions represented by the diffusion constants. D_c and D_x, respectively. Application of the electron hopping model only gave an effective diffusion coefficient, D_cff. The diffusion coefficients D_c and D_x, in the first case, and D_eff in the second case, depend upon the ionic strength and film thickness. Moreover, D_eff is very close to D_c. This proves that charge propagation is dominated by electron transport in this polymer.

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In terms of the electron hopping model, the hysteresis phenomenon during the reactivation process could be explained in terms of a lack of relaxation of the configuration of redox sites of a film that was strongly deactivated (θc > 0.5). This lack of relaxation could be associated with the poor swelling effect [40] at the reduced state of the polymer. The high degree of swelling, which is a direct result of solvent ingress to compensate for changes in charge density within the film (proton exclusion and/or anion inclusion during oxidation), will be absent at the reduced state of a strongly deactivated POAP film.
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Redox mediation and permeation processes at deactivated poly(o-aminophenol) films. A study applying rotating disc electrode voltammetry and electrochemical impedance spectroscopy
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The objective of this work was to deactivate poly(o-aminophenol) (POAP) films and then study how their permeation and electron-transport processes are modified by the degree of deactivation. POAP films were deactivated by soaking in a ferric cation solution. The degree of deactivation (θ_c) was assessed by employing Cyclic Voltammetry within the potential region where POAP exhibits its maximal electroactivity (−0.2 V < E < 0.5 V vs. SCE). Rotating Disc Electrode Voltammetry (RDEV) and ac impedance measurements were applied in the interesting case where POAP films contact a redox active solution containing the p-benzoquinone/hydroquinone redox couple and a mediation reaction occurs at the polymer|electrolyte interface. RDEV applied at potential values E < 0.0 V vs. SCE allowed obtaining a diffusion coefficient value, D_e. It was observed that D_e decreases with increasing θ_c. This effect was attributed to the existence of inactive gaps within the redox site configuration of a deactivated film, which causes an increase in the hopping distance for electron conduction. The existence of inactive gaps seems to be related to the hindrance to accept protons by some redox sites in a deactivated film. This fact was proved by increasing the pH of the external solution contacting a non-deactivated POAP film. Impedance diagrams of deactivated films show an increase in the charge-transfer resistance at the metal|polymer interface, which is consistent with the presence of inactive zones within the redox site distribution of a deactivated film. Some deactivated POAP films were reactivated by treatment with an ammonium solution and it was observed that electron conduction can only be partially recovered. The permeation process at potential values where hydroquinone is able to diffuse through POAP was also analyzed on deactivated films. It was found that the rate of diffusion of hydroquinone decreases as the degree of deactivation increases. This effect was attributed to unfavourable partition equilibrium at the polymer|solution interface for the electroactive species in a deactivated film, as compared with a non-deactivated film. This work could be interesting due to the wide range of practical applications of POAP recently reported. In this sense, although in practical applications of POAP it is necessary to maintain its conducting properties unaltered, the polymer is subjected to extreme conditions that can cause its partial deactivation. Thus, it seems to be important to know at least how the charge-transport process changes at POAP films with their deactivation.
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These authors also observed a good correlation between the predicted and experimental results. Here, a principle difference of the impedance results reported in [24,25] and those of studies [1,2,7,21] should be emphasised. The data of the first works manifest the capacity dispersion in the region of low-ac frequencies, whereas direct indications on the dispersion are absent in the second studies.
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Effect of the bathing electrolyte concentration on the charge transport process at poly(o-aminophenol) modified electrodes. An ac impedance study

Abstract

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