Poly(o-aminophenol) Film Electrodes by Ricardo Tucceri

Author:Ricardo Tucceri
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

2.2.3 Effects of the Type and Concentration of Ions of the External Solution on the Charge-Transport Process at POAP Films

Effects of the type and concentration of ions of the supporting electrolyte on the electrochemical response of POAP films were studied in several papers [1, 11, 18, 31, 42]. Some voltammetric experiments were performed in Ref. [1] by varying the ClO4 − concentration between 0.1 and 1.7 M. While the anodic peak was not affected, the cathodic peak showed a positive variation. In this sense, a high concentration of perchlorate produces the same effect as a high proton concentration, making POAP more easily reducible. However, no noticeable changes were observed for anions such as SO4 2−, PO4 3− and Cl− or cations such as Li+, K+ and Cs+.

The participation of the supporting electrolyte anions in charge-transfer processes through POAP films was studied in LiClO4 and HClO4 solutions at different pH values and constant ionic strength (1 M) by employing CV and EQCM [11]. CV experiments with POAP-modified electrodes uncovered and covered with Nafion thin films were performed [11]. Nafion is known to hinder anionic permeability due to its own negative charge, so the films covered with this material should significantly change their electrical properties, as compared to the uncovered ones, when supporting electrolyte anions participate in the redox activity of the films. No significant effect of Nafion on the redox response of POAP was observed. This finding allowed the authors [11] to conclude that no perchlorate anions are involved in the redox reactions of POAP. In order to confirm this conclusion, EQCM measurements on POAP films were carried out in Ref. [11]. The microbalance frequency change in the presence of the POAP films was measured simultaneously with their current responses under the electrode potential cycling. The ratio, where (k is the constant of the Sauerbrey law) and is the charge consumed during the oxidation/reduction process, gave a mass number of about 3–5 in the main range of the cycling potential of 0.0–0.3 V (vs. Ag/AgCl). This mass number was considered very low, as compared with the molecular weight of ClO4 − anion. Then, it was concluded that an insignificant insertion of the anions into the POAP films occurs during the redox reaction of the polymer.

The ionic exchange of POAP in the presence of perchlorate anions was studied by using Probe Bean Deflection (PBD) [31]. During the reduction scan only a positive deflection of the PBD signal was observed, which was indicative of a simultaneous expulsion of perchlorate and insertion of protons, the latter process being dominant. To check this conclusion, a PBD profile was simulated by convolution of the current response using parameters reported in the literature [43] and considering that only protons are exchanged between the solution and POAP. The simulated profile fits reasonably well the negative scan but differs significantly in the positive direction, suggesting that not only protons but also perchlorate anions are exchanged during the positive scan.

The effect of the ionic strength of the solution on the transport properties of POAP was studied employing EIE [18].

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