Sandro Cattarin

Cyclic potential sweep electropolymerization of aniline: The role of anions in the polymerization mechanism

Abstract The cyclic potential sweep (CPS) method was applied to aniline electropolymerization in several strong acids (H 2 SO 4 , HNO 3 , HCl, HBF 4 , HClO 4 and CF 3 COOH). It is mainly the type of anion that determines the morphology of polyaniline (PANI) deposits, promoting either a compact (BF − 4 , ClO 4 − and CF 3 COO − , class 1 anions) or an open structure (SO 4 2− , NO 3 − and Cl − , class 2 anions), as suggested by the linear (class 1) or quadratic (class 2) dependence of deposition charges on number of cycles, scan rate and concentration of anilinium and anion, as well as by SEM analysis. From the potential dependence of deposition currents, PANI growth onto PANI is suggested to occur via adsorption of anilinium-anion ionic couples onto fully oxidized (pernigraniline) sites of PANI.

Electrosynthesis and properties of ring-substituted polyanilines☆

Abstract Electrochemical polymerization of ring-substituted anilines, with electron donor and acceptor substituents in the 2- and 3-positions, was investigated in 2.0 M sulfuric acid. Polymerization is mostly effective with sigma- and pi-donor substituents, the latter in the 2-position. Polymeric deposits were obtained from 2- and 3-methylaniline (toluidines), 2-methoxyaniline (anisidine) and 2-chloroaniline, and were characterized by cyclic voltammetry, elemental analysis, ir spectroscopy, uv—vis spectroelectrochemistry and electrical conductivity. Polytoluidine and polyanisidine are reversibly oxidized in two steps as polyaniline, whereas polycholoroaniline is oxidized in a single one-electron process. Polymerization was found to be more effective on polyaniline-modified electrodes.

Electrodeposition of polythiophene, polypyrrole and polyaniline by the cyclic potential sweep method

The cyclic potential sweep (CPS) technique was applied to deposition of electroactive polymers and tested on polythiophene (PT) and polypyrrole (PP), both in acetonitrile, and polyaniline (PANI) in sulfuric acid. The relationship between deposition charge Qi and the CPS parameters (number of cycles N, scan rate v and switching potential Eλ) has the simple form Qi ~ (N/v)n exp(aEλ), with n =1 for PT and PP and n = 2 for PANI. The dependence on monomer concentration is analogously square for PANI and linear for PT; for PP, the dependence is linear only in the presence of water, which acts as a scavenger of electrogenerated protons. The growth mechanism is reflected by n (over a substrate of constant area for n = 1 and expanding linearly in time for n = 2). The CPS relationships are explained satisfactorily by chronoamperometric results.

Synthesis and properties of film electrodes from n-substituted carbazoles in acid medium

Abstract Electroactive films were deposited on Au and glassy carbon electrodes by anodic oxidation of ethyl, phenyl and vinyl N-substituted carbazoles and were identified as C-C coupled polyetherocyclic sequences. In aqueous HClO4 the polyethylcarbazole filmed electrode exhibits two reversible oxidations which correspond respectively to 50 and 100% conversion into (cationic) polyiminic structures. An ac impedance investigation of the system outlined a behaviour typical of redox polymers. Polyphenylcarbazole films have similar characteristics. Conversely, oxidation of the N-viayl derivative leads to polycarbazole structures which have lost the pendant vinyl group owing to hydrolytic reactions.

Preparation of porous PbO2 electrodes by electrochemical deposition of composites

Porous PbO2 electrodes have been prepared by anodic codeposition of PbO2 particles with an electrochemically grown PbO2 matrix, with the aim of illustrating how composite deposition may be a suitable method for obtaining electrodes of large and controlled surface roughness. By exploiting the different crystal structures of PbO2, the composition of α-PbO2+β-PbO2 composites (i.e. composites with an α matrix and a β dispersed phase) has been determined by X-ray diffraction. It has been found that reliable indications on composition of deposits may be also obtained by measuring the current efficiency of the deposition process, which largely exceeds unity in the case of composites. This way, the dependence of the dispersed phase concentration in the deposits on current density and suspended particles concentration has been determined. The surface roughness of the composite electrodes has been estimated by electrochemical impedance spectroscopy. It was found to increase almost linearly with the deposition charge and reach limiting values as either the deposition current density or the suspended particles concentration are increased.

Electrodeposition of PbO2 + CoOx composites by simultaneous oxidation of pb2+ and Co2+ and their use as anodes for O2 evolution

Abstract PbO2+CoOx composites have been deposited by anodisation of sulphamate solutions containing Pb2+ and Co2+ ions. The effect of experimental variables, like total Co2++Pb2+ concentration, Co2+/(Co2++Pb2+) ratio, potential, pH and angular speed of the anode, on the deposit composition has been investigated. The results are discussed according to the theories developed for the cathodic deposition of alloys, concluding that the system conforms to the trends typical of irregular deposition. XRD and XPS analyses have shown that the deposits are amorphous, but have not allowed to fully elucidate the nature of the Co containing compound(s) codeposited with PbO2. When used as anodes for oxygen evolution in aqueous NaOH, the PbO2+CoOx composites exhibit Tafel slopes of ca. 60 mV decade−1 and a reaction order of 1 with respect to OH−. Their overpotential is similar to that of PbO2+Co3O4 composites prepared by anodisation of suspensions. A marked activation of the PbO2+CoOx electrodes, during ca. 3 days of continuous oxygen evolution, is explained as the result of an increase of the Co surface concentration, as shown by XPS and impedance data.

Evidence of coupling between film growth and metal dissolution in passivation processes

Abstract In the anodisation of Nb in acidic fluoride solutions or warm aqueous alkali, and in that of Mo in concentrated phosphoric acid, impedance spectroscopy provides evidence that 3D film growth proceeds already during the active–passive transition. Indeed, linear potential dependences of both the inverse of the high-frequency capacitance and of the product of the high-frequency resistance times the steady-state current are observed already below the peak potential. Simultaneously, a pseudo-inductive loop at intermediate frequencies is detected in the impedance spectra. In the present paper, we propose a kinetic model for the interpretation of these data. The model assumes that the metal is covered with a non-stoichiometric oxide containing at least two oxidation states of the cation. The processes of oxidative dissolution of the lower valence cations and their transformation to cations of higher valence, leading to passivation, explain the shape of the I – E curve. These processes are limited by both charge transfer at the film–solution (F/S) interface and transport of cation vacancies through the film. Thickening of the oxide film is assumed to proceed simultaneously, and is limited by transport of oxygen vacancies accelerated by an interfacial charge of cation vacancies.

Nb Electrodissolution in Acid Fluoride Medium Steady-State and Impedance Investigations

The electrochemical behavior of Nb in acid fluoride media has been investigated by voltammetric, steady-state, and impedance techniques as a function of variables like c F (the total fluoride concentration, varied between 0 and 2 M), pH (0-7), and potential (-1 to 7 V vs. SCE). An active/passive transition is observed, followed by a wide current pseudoplateau where current depends on c F and pH in a way indicating that HF and related species dissolve the passivating oxide whereas F is inactive. Impedance diagrams obtained on this plateau can he reproduced by a literature model (surface charge approach), and the derived parameters may be used to calculate structural and kinetic quantities, as well as the formation ratio of the surface oxide.

Preparation of anodes for oxygen evolution by electrodeposition of composite Pb and Co oxides

Abstract The simultaneous anodic oxidation of Pb 2+ and Co 2+ in a sulphamate bath containing suspended Co 3 O 4 (or NiCo 2 O 4 ) particles has been studied as a method for preparing composite oxides in which the particles are dispersed in a matrix consisting of PbO 2 and hydrous Co oxides, CoO x with x 2 +CoO x )+Co 3 O 4 composites thus prepared are very rough and their Co concentration, expressed as X Co = N Co /( N Co + N Pb ), where N represents the number of atoms, approaches 0.5. As a result, they are more active catalysts in the oxygen evolution reaction than either PbO 2 +CoO x or PbO 2 +Co 3 O 4 prepared in the past by our group. For samples prepared in the presence of particles, the potential required to evolve oxygen at a fixed current density varies roughly linearly with X Co , irrespective of the fact that Co is present as Co 3 O 4 or CoO x .

Electrodissolution of Ti and p-Si in acidic fluoride media: formation ratio of oxide layers from electrochemical impedance spectroscopy

The electrodissolution of Ti and p-Si electrodes in acidic fluoride solutions has been investigated by electrochemical impedance spectroscopy as a function of potential E, fluoride concentration, pH and angular speed of the electrode, with the aim of characterising the oxide layers covering both materials. The impedance diagrams have been analysed to obtain the low frequency capacitance Clf, the high frequency capacitance Chf and the product RhfI (where Rhf is the high frequency resistance and I the steady-state current). In the potential domain where the oxide thickness x is proportional to E, the formation ratio dx/dE has been computed from Clf, Chf and RhfI. The calculated values have been found to be in good mutual agreement and close to the ones found in the literature.