Permeation and electron conduction at de-activated poly(o-aminophenol) (POAP) films

R. TUCCERI

Szczyrk. Polonia

Workshop; Intenational Workshop on Electrochemistry of Electroactive materials WEEM-2009; 2009

WEEM

It is noteworthy that most basic results about POAP reviewed in the literature refer toconduction properties of POAP films with maximal electroactivity (freshly prepared)[1]. However, there is a small number of papers about conduction properties of POAPfilms which have lost part of their optimum electroactivity (de-activated POAP films).Considering the growing applications of POAP where the polymer being subjected torugged conditions (chemical environment, potential range, prolonged experiments,storage conditions, etc.) which could cause a decay of the electrochemical activity, nomuch efforts have been made to study the POAP de-activation [1]. In this sense, inpractical applications is important that the reversible redox response and conductingproperties of POAP films remain substantially unchanged before and after the films hadbeen used.The aim of the present work was to reduce the electroactivity (de-activation) ofpoly(o-aminophenol) (POAP) films by treatment with a 50 mM Fe2(SO4)3 solution, andthen study how are modified their permeation and electron transport processes with thedegree of de-activation. Different POAP films were immersed into a 50 mM Fe2(SO4)3solution during different time periods in order to obtain a different degree of deactivation.The degree of de-activation of each POAP film was assessed by comparingits voltammetric response after being subjected to a treatment with the ferric cationssolution and the response of the non-deactivated film. It was demonstrated that deactivationrestricts electron transport through the films, electron transfer at themetal|polymer interface and the permeation process of electroactive species across atthe polymer|solution interface.The effect of de-activation on the electron transport was demonstrated byRotating Disc Electrode Voltammetry, because it was observed a decreasing efficiencyof POAP to act as mediator in the presence of a redox active species (hydroquinone/pbenzoquinone)in solution [2]. This was explained on the basis of an increase of thehopping distance in the redox sites configuration of de-activated POAP films, whichleads to a reduction of the electron diffusion coefficient value (Electron HoppingModel). The increase of the hopping distance was attributed to the presence of inactiveredox sites that cannot accept protons. This was demonstrated by increasing the pH ofthe external solution contacting the polymer film, because the effect of increase the pHon the redox mediation, was similar to that observed with the increase of the degree ofde-activation. In this way a more expanded distribution of active redox sites is obtainedin a de-activated POAP film as compared with that of a non-deactivated film.Interfacial Resistance measurements on POAP coated gold film electrodes [3]seem to be consistent with Rotating Disc Electrode Votammetry experiments. Higherdistances among even active redox sites in de-activated POAP films, as compared withthe distance in the distribution of redox sites of a non-deactivated film, should cause amore diffuse reflection of the conduction electrons of the gold film at the goldfilm|poly(o-aminophenol) film interface. This effect was confirmed by analysing theslope of the linear dependence of the resistance of POAP coated gold film electrodeswith the degree of reduction of the polymer film. This slope increases with the degree ofde-activation of the polymer film. Specular diffusion of electrons at the poly(oaminophenol)|gold interface can be partially recovered by treatment of de-activatedPOAP films with an ammonium hydroxide solution. This would mean that a morecompact redox sites distribution was recovered with re-activation, as compared withthat of the initially de-activated film.Nyquist diagrams of POAP films show an increase of the charge-transferresistance at the gold |poly(o-aminophenol) film interface with increasing the deactivation.This is consistent with the presence of inactive polymeric zones at the gold|poly(o-aminophenol) film interface, which should occur leaving remnant distributionsof redox sites still active more expanded as the degree of de-activation increases.With regard to transport of electroactive species across POAP films, it wasfound that the rate of diffusion of hydroquinone decreases with increasing the degree ofde-activation. According to the Membrane Diffusion Theory, this effect was attributedto a unfavourable partition equilibrium at the film|solution interface for the electroactivespecies, which should be due to the presence of inactive sites into the redox sitesconfiguration of a de-activated film, that impedes a high degree of swelling ascompared with a non-deactivated film.The present work can help to gain further insights into the charge propagationprocess at POAP, in such a way of exploit more successfully the restrictive stabilityconditions of the polymer in its practical applications. However, it should be kept inmind that it is not the propose of this work to study the process of capture of iron ionsby POAP, but only employ it as a mean to de-activate the polymer. More work is inprogress to apply other de-activation methods (prolonged potential cycling, highpositive potentials, diffusion of species into the film, etc.).[1] R. Tucceri, Synthesis, transport properties and practical applications of poly(oaminophenol)films electrodes, Review Book, ISBN 978-81-308-0195-7, ResearchSignpost, 2008, in press.[2] A. Bonfranceschi, A. Pérez Córdoba, S. Keunchkarian, S. Zapata, R. Tucceri, J.Electroanal. Chem., 477 (1999) 1-13.[3] R. Tucceri, Surface Science Reports, 56 (2004) 85-157.