D. Posadas

Optical properties of electropolymerized orthoaminophenol

Abstract The optical properties of a surface polymer formed by electropolymerization of ortoaminophenol (OAP) are studied by ellipsometry and in situ visible spectrophotometry. The polymer is opaque in the oxidized state and transparent in the reduced one. The complex refractive index of the oxidized and the reduced states were measured as a function of wavelength. At λ = 546.1 nm nr = 1.704 − i 0.02 and nox = 1.699 − i0.175.

Spectroelectrochemical study of poly-o-aminophenol

We report a Spectroelectrochemical study in the wavelength range 300 nm < λ < 800 nm, of poly (o-aminophenol) (POAP) in aqueous acid solutions of different pHs and with different supporting electrolytes. Four features were observed in the absorption spectra: the uv tail of the band of the aromatic ring structures, a weak absorption band at 340 nm corresponding to the polymer in the reduced state, a strong broad band at 440 nm corresponding to the oxidized state of the polymer and a maximum in absorbance at about 750 nm when going from the reduced to the oxidized states. Cycling the potential while holding λ at 750 nm yields a transient response in the absorbance peaking at the peak potential of the voltammetric response, which also is the potential of maximum conductivity of POAP. From the absorbance data the number of exchanged electrons during the oxidation cycle was 0.4 which agrees well with that estimated from the oxidation charge and the polymer thickness. This indicates that one in every four or five monomer units is being oxidized. The dependence of the absorbance on pH shows that as the pH increases the protonation of the amino groups decreases. The experimental results suggest that the imino groups in the oxidized state are not protonated. This is in agreement with the fact that POAP is easily oxidable and a very poor conductor. This is also in agreement with the increasing difficulty in reducing the polymer as the pH increases. Hysteresis effects were observed both in the voltammogram and in the absorbance during potential cycling. In the presence of anions bigger than perchlorate, such as benzenesulphonate, both the oxidation charge and the absorbance change decrease.

Impedance characteristics of poly-o-aminophenol electrodes

The impedance characteristics of poly-o-aminophenol film electrodes in NaClO4 0.4 M + HC1O4 0.1 M electrolyte have been measured in the frequency range 0.01 <f< 103Hz and at two different thicknesses. The experimental results can be interpreted through a simple charge transfer resistance in parallel with a constant, capacity at high frequencies and the redox capacity in series with the film resistance at low frequencies. The analysis of the impedance response allows to the electrochemical rate constant, the concentration of redox centers in the film, the film conductivity and the diffusion coefficient of the elections to be determined.

Voltammetric study of the reduction and relaxation of poly(o-toluidine). Effect of the polymer thickness and the external electrolyte nature and concentration

The reduction and relaxation of poly(o-toluidine) (POT) was studied as a function of the wait time at different waiting potentials near the reduction potential of the polymer. The influence of the film thickness, the acid concentration, and the ionic strength of the external electrolytic solution on these processes were also studied. Two types of electrolytes were employed: perchloric and sulfuric acid. Both the reduction and the relaxation times depend on the proton concentration of the external electrolyte media and on the film thickness. They are independent of the ionic strength and, in a limited range, of the waiting potential. The voltammetric response of fully reduced and relaxed polymers shows that, at low sweep rates, the kinetics are controlled by slow ionic movements within the polymer. Experiments with medium exchange show that, once the polymer is fully reduced and relaxed, its state is independent of the composition and concentration of the electrolyte in which this particular state was obtained. Furthermore, they also show that the shape of the voltammetric oxidation profile depends exclusively on the composition and concentration of the electrolyte in which the polymer is being oxidized. This means that the effect of the solution composition and concentration is manifested only through the participation of protons and anions in the mechanism of oxidation of the polymer.

IR response of poly-(o-toluidine) : spectral modifications upon redox state change

The IR spectra of poly(o-toluidine) in the reduced, half and fully oxidized states have been studied in both salt and base forms. The bands in the range 1500 to 1600cm−1 undergo important intensity changes as the degree of oxidation increases. The IR spectra show that during the electrochemical polymerization there is an influence of the nature of the anion present in the electrolyte. This shows up as conformational differences in the polymer backbone, but it does not affect the chemical structure. Also, spectral changes are observed when the polymer is oxidized in supporting electrolytes containing different anions. This has been interpreted by considering that the less hydrated anions can approach closer to the positive centres of the protonated amine groups.

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

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. Dc and Dx, respectively. Application of the electron hopping model only gave an effective diffusion coefficient, Dcff. The diffusion coefficients Dc and Dx, in the first case, and Deff in the second case, depend upon the ionic strength and film thickness. Moreover, Deff is very close to Dc. This proves that charge propagation is dominated by electron transport in this polymer.

Apparent formal redox potential distribution in electroactive arylamine-derived polymers

Abstract The concept of distributed formal redox potentials is employed to analyse and study the i / E response of different arylamine-derived polymers, poly- o -aminophenol (POAP), poly- o -phenylenediamine (POPD) and polybenzidine (PBZ), and a conducting polymer, poly- o- toluidine (POT), in different electrolytic media. The analysis of the experimental data allows determining the formal redox potential distribution. The experimental formal redox potential distributions are interpreted on the basis of a simple statistical thermodynamic model that considers the possibility of expansions and contractions during the oxidation–reduction cycles applied to the polymer. It is considered that units having several redox centres constitute the polymers and that these units change in size during the oxidation–reduction cycles. The analysis of the experimental formal redox potential distribution allows obtaining the number of redox centres per polymer unit, the ratio of the internal partition function of the units and the formal redox potential of the centres. These results are in agreement with the previous studies on the electrochemical behaviour of these polymers.

The Effect of Temperature on the Impedance of Poly‐o‐toluidine in 3.7 M H 2 SO 4

The impedance response of poly(o-toluidine) (POT) electrochemically grown films was investigated in sulfuric acid 3.7 M, in the temperature range 218 to 293 K. The Nyquist diagrams as a function of temperature show that the transition potential from the nonconductive to the conductive state shifts in the positive potential direction as the temperature decreases. This implies that, at low temperatures, a greater field is needed to induce the transition. In the nonconductive state both the high-frequency and the low-frequency resistances increase as the temperature is decreased. The Arrhenius plots for both show a break at about 255 K. This is in agreement with the fact that the high-frequency resistance is controlled by ionic movements and that there is charge-transfer control at the polymer/solution interface. In the conductive state the low-frequency capacitance is independent of temperature, showing that the active site concentration does not depend on the temperature. The low-frequency resistance in this potential region also depends on the temperature. No break is observed in the Arrhenius plot for this quantity. However from its slope it is inferred that ionic motions within the polymer control the charge transport in this state.

Proton Exchange during the Redox Switching of Polyaniline Film Electrodes

Proton ejection/injection during the redox switching of polyaniline (PANI) have been qualitatively measured employing the rotating ring-disk electrode in the amperometric mode at the ring. The ring response shows that there is proton ejection during the oxidation of a fully relaxed polymer both from the leucoemeraldine to emeraldine forms and from it to the fully oxidized form. During the reduction of the polymer there is proton injection. However, this process is incomplete for the emeraldine/leucoemeraldine transition, indicating proton equilibration is a slow process. The ring response during the second positive half-potential sweep (nonrelaxed polymer) shows that the ring current is much smaller, indicating the polymer is not in protonic equilibrium with the external electrolytic phase.

Ellipsometry of nickel hydroxide electrodes formed by ex situ chemical precipitation. Potential routine and time effects

Abstract Ellipsometric and reflectivity data combined with voltammetric data have been obtained at 546.1 nm for different nickel hydroxide electrodes in 0.01 M NaOH at 25°C. The electrodes were prepared by ex situ chemical precipitation on a polished platinum substrate. The influence of the potential routine applied to the nickel hydroxide electrode on the corresponding optical parameters and time effects for both the oxidized and reduced forms of the active material kept at constant potential have been investigated. The nickel hydroxide layer is described as a mixture of nickel(II) hydroxide, nickel(III) hydroxide and electrolyte—water domains. The kinetic effects are discussed through a complex reaction pathway involving different nickel(II) and nickel(III) hydroxide species, the processes starting preferentially at the borders of the different domains.