F.E. Varela

Kinetics and mechanism of the electroreduction of anodic layers formed on lead in 5 M H2SO4 at 25°C

Abstract The potentiostatic electroreduction of anodic layers containing Pb(II)-species formed on lead has been investigated in 5 M H 2 SO 4 at 25°C. By properly adjusting both the anodic layer formation and the electroreduction conditions three well-defined processes could be distinguished. (i) The electroreduction of the primary PbSO 4 layer which involves mainly an instantaneous nucleation and three-dimensional growth mechanism under diffusion control. (ii) The electroreduction of the PbO layer, which builds up progressively beneath the initially grown PbSO 4 porous layer, consists of two current contributions; one term can be associated with an instantaneous nucleation and 2D growth under diffusion control and the other one, which is the main current contribution, can be related to a progressive nucleation and 3D growth mechanism under charge transfer control. The latter includes a death correction term for either the growing or the disappearing right circular cones phase centres. (iii) The electroreduction of the composite PbSO 4 -PbO layer which can be successfully described in accordance with a complex mechanism by taking into account the simultaneous influence of the two single surface layer constituents.

A study of thin anodic WO3 films by electrochemical impedance spectroscopy

The properties of thin anodic passive films potentiostatically formed (1 V ≤ Ef ≤ 5 V vs sce) on polycrystalline tungsten in aqueous H3PO4, H2SO4, HNO3 and HClO4 solutions (pH ≈ 1.3) were studied using electrochemical impedance spectroscopy at room temperature. The data were analysed with a transfer function using a non-linear fitting routine, assuming that the resistance of the film is coupled in series with the faradaic impedance of the W(0) → W(VI) reaction, with these in parallel with the capacitance of the metal/passive film/electrolyte system. The relative permittivity (er) of the films was found to depend on the acid solution used. Except for the case of films grown in HNO3, the values of er, are smaller than those of thicker films grown galvanostatically. The film grown in H3PO4 solutions presented the lowest value of er, which could be due to the incorporation of phosphate ions into the film. The number concentration of donors in the films (ND) was found to decrease with Ef (ie increasing film thickness) and to be quite dependent on the electrolyte used: ND(H3PO4) < ND(HClO4) < ND(HNO3) < ND(H2SO4). The values for ND(H3PO4) are, nevertheless, 103 times larger than the ones for much thicker WO3 films and are of the same order of magnitude as the ones for annealed WO3 films. The flat band potential of the films grown in H3PO4 was found to be more than 0.1 V smaller than those for films grown in the other solutions.

Characterization of passive layers formed on lead by electrochemical impedance spectroscopy

Abstract The corrosion process and characteristics of the surface layer formed on lead in concentrated sulphuric acid solutions were studied using electrochemical impedance spectroscopy (EIS) and rotating-disc electrodes. Data obtained under a wide variety of experimental conditions show that the surface films generated in the passive potential range consist of various Pb(II)-containing species. The inner barrier PbO layer, which builds up progressively beneath the initially grown PbSO 4 porous layer, behaves as a p-type semiconductor. The influence of its formation potential on the semiconducting properties and diffusion processes is discussed from impedance spectra by application of transfer-function analysis using non-linear fitting routines.

Kinetics and mechanism of PbSO4 electroformation on Pb electrodes in H2SO4 aqueous solutions

Abstract The electrochemical processes related to the passivation of Pb in 5 M H2SO4 at 25°C, have been investigated combining a rotating ring-disk electrode, linear potential sweep voltammetry and potentiostatic pulse techniques complemented with scanning electron microscopic imaging. The kinetics of PbSO4 anodic layer growth can be explained through the initial formation of a barrier layer acting as an ionic conductor, and the transference of Pb2+ cations from the substrate the conducting layer yielding Pb2+ ion in the solution. These ions further contribute to passivation through the precipitation of a PbSO4 passivating layer.

Electrochemical characterization of thin passive films on Nb electrodes in H3PO4 solutions

Utilizando-se a espectroscopia de impedância eletroquimica, foram estudadas as propriedades eletricas e semicondutoras de filmes anodicos passivos formados potenciostaticamente (1 V £ Ef £ 5 V vs. ecs) sobre eletrodos de niobio policristalino em solucoes aquosas de H3PO4 0,5 mol/L (pH 1,3), a temperatura ambiente. Os dados foram analisados com uma funcao de transferencia usando uma rotina de ajuste nao-linear, supondo que a resistencia do filme esta acoplada em serie com a impedância faradaica da reacao Nb(0) Nb(V) e estas em paralelo com a capacitância da interface filme/eletrolito. A permissividade relativa dos filmes foi estimada como cerca de 44. Encontrou-se que a concentracao em numero de doadores (ND) nos filmes decresce com Ef (isto e, a medida que a espessura do filme aumenta). A partir de graficos de Mott-Schottky, tambem obteve-se um valor de -0,72 V para o potencial de banda plana.

Investigations of the electroreduction process of anodic films grown on tin in carbonate-bicarbonate solutions

Combined voltammetry and potentiostatic current transient techniques have been employed to study the electroreduction process of composite surface layers anodically formed on high purity polycrystalline tin in carbonate-bicarbonate ion containing solutions. The dynamic system analysis performed by using parametric identification procedures and non-linear fit routines has demonstrated that data obtained under a wide variety of experimental conditions can be interpreted by taking into account the participation of various Sn(II)Sn(IV)-containing surface species. Thus, the electroreduction of passive layers formed at relatively low potentials can be explained through an instantaneous nucleation and 3D growth mechanism controlled by diffusion of OH− ions away from the reacting interface, whereas for surface layers produced at high positive potentials where the formation of SnO2 predominates, the electroreduction follows a progressive nucleation and 3D growth process under charge transfer control. The influence of anodizing time at different potentials as well as effects due to surface layer thickness can be discussed on the basis of electroreduction reaction models.

Influence of temperature on electroreduction of anodically formed passive films on lead electrodes in H2SO4 solutions Part I: Electroreduction of PbS04 layers

Combined voltammetry and potentiostatic current transient techniques have been employed to study the electroreduction processes of anodically formed lead sulphate layers on polycrystalline lead electrodes in concentrated sulphuric acid solutions in the 0–50° C temperature range. Data analysis using parametric identificiation procedures and nonlinear fit routines has demonstrated that the kinetics of the electroreduction processes of the Pb(II)-containing surface species can be interpreted in terms of complex nucleation and growth mechanisms. The electroreduction of the PbSO4 layer involves mainly an instantaneous nucleation and 3D growth mechanism under diffusion control. The activation energy of the different reaction stages can be calculated from Arrhenius plots derived from the corresponding kinetic parameters. This contributes to a better understanding of the whole cathodic reaction scheme as well as to the identification and characterization of the various stages taking place in the electroreduction processes.

Reaction and system modelling for Pb and PbO2 electrodes

Reaction models for the metal dissolution, active–passive transition and passive layer electro- reduction processes taking place during discharge/charge of the electrode active materials in the lead/sulphuric acid system are discussed. Reaction and system modelling can be evaluated on the base of results obtained from steady-state polarization, potentiostatic pulses, single (STPS) and repetitive (RTPS) triangular potential sweeps, modulated voltammetry, complex voltammetry combined with potentiostatic steps, and electrochemical impedance spectroscopy (EIS), using rotating ring Pb-disc Pb-electrode assemblies, as well as scanning electron microscopy (SEM). Data analysis derived from transient electrochemical techniques employing both parametric identification procedures, nonlinear fit routines and computer simulations has been interpreted in terms of appropriate nucleation and growth models for the formation of new phases, in which the characteristics and properties of the compounds generated on electrode surface exhibit a remarkable dependence on the corresponding operational potential.

A computer simulation of voltammograms describing the active-passive transition of Pb in sulphuric acid

Abstract A computer simulation of active-passive transition voltammograms of Pb in 5 M H 2 SO 4 is presented. The model implies alternative pathways leading to the formation of inner and outer structurally different PbSO 4 layers. The inner layer consists of a disordered crystalline conducting phase, which is formed following an instantaneous nucleation and two-dimensional growth mechanism under charge-transfer control. Electrodissolution of Pb proceeds by the diffusion of cations through the inner layer and, when Pb 2+ ion concentration in solution exceeds the critical supersaturation concentration, the precipitation of the outer highly crystalline PbSO 4 layer takes place, leading to passivation. Calculations are based on previously reported kinetic data. Good agreement was found between theory and practice.

Semiconductor properties of passive oxide layers on binary tin + indium alloys

Abstract The electrochemical behaviour and solid state properties of passive oxide layers formed on binary tin + indium alloys in alkaline aqueous solution were studied by using photocurrent spectroscopy (PCS) and electrochemical impedance spectroscopy (EIS). Results obtained from PCS and EIS measurements showed that the anodically formed tin oxides behaved as heavily doped, n-type amorphous semiconductors. From the photocurrent spectra the mobility gap energies of different passive layers were evaluated. The mobility gap exhibited a remarkable dependence on the alloy composition, shifting towards higher energies as the indium content in the alloy was increased. Impedance diagrams displayed a complex frequency response and the corresponding transfer function analysis employing both parametric identification procedures and non-linear fit routines, presented several contributions. The apparent flat-band potential obtained from Mott-Schottky plots depended on alloy composition, shifting more negatively with increasing indium content. On the basis of PCS and EIS data, a tentative scheme of the valence and conduction mobility edge energies for the different passive films was postulated and its correlation with the oxide layer composition critically discussed.