Manuel Palomar-Pardavé

Electrochemical nucleation of cobalt onto glassy carbon electrode from ammonium chloride solutions

Abstract In the usual aqueous electrolytic baths, cobalt electrodeposition is accompanied by the hydrogen evolution reaction (HER), and therefore, in such circumstances, a quantitative evaluation of the kinetic parameters of the cobalt electrocrystallization process, is almost impossible. Here, we show a possibility to separate these two processes. The existence of a potential range where the cobalt deposition on glassy carbon electrode (GCE) occurs independently of HER, from an aqueous solution of 10 −2 M CoCl 2 , in 1 M NH 4 C1 ( pH = 4.66), was found, and the nucleation kinetic parameters, using the potentiostatic method and existing theoretical formalism, were determined. It was concluded that the mechanism of the cobalt deposition in 1 M NH 4 C1 occurs by multiple, progressive three-dimensional nucleation with growth controlled by mass transfer. Regarding the potential dependence of the steady-state nucleation rate, and atomistic theory of the electrolytic phase formation, it was shown that an active site plays the major role in critical nucleus formation over the entire potential range.

Nucleation and growth of cobalt onto different substrates: Part I. Underpotential deposition onto a gold electrode

Abstract The formation and stripping of a cobalt adlayer were studied by electrochemical techniques at underpotential conditions, monitoring the deposition/dissolution onto/from a polycrystalline gold electrode. The kinetics of the former process fit a model where adsorption and diffusion-controlled two-dimensional instantaneous nucleation of cobalt occurs simultaneously onto the gold electrode surface. The latter involves a complex mechanism where adsorption/desorption competes with two processes, an instantaneous nucleation and growth of two-dimensional holes under charge transfer control and a progressive one. However, it was found that the relative contribution of the instantaneous nucleation was the most important independently of the applied potential.

Detailed characterization of potentiostatic current transients with 2D-2D and 2D-3D nucleation transitions

Abstract Cyclic voltammetry, chronoamperometry and atomic force microscopy were used to study the kinetics of nucleation and crystal growth in the initial stages of silver overpotential deposition onto a vitreous carbon electrode from an aqueous solution of 10 −3 M Ag(I) in 1.6M NH 4 OH and 1M KNO 3 (pH 11). The silver deposition process was found to be complex and occurred via two different pathways as a function of the applied deposition potential. The identification and characterization of each pathway was based on a quantitative analysis of the current transient curves, which show that within a single potentiostatic current transient, initial progressive nucleation and two-dimensional (2D) growth was followed either by other 2D nucleation and growth processes (lower overpotentials) or by three-dimensional (3D) growth controlled by incorporation of adatoms into the lattice (higher overpotentials). In order to properly identify and interpret each step during the deposition process, the characteristic features of the current transient curve were treated separately using existing mathematical formalisms. After proper identification of all processes within the recorded transients, a general equation for the whole process including contributions from adequate 2D-2D or 2D-3D nucleation transitions and double-layer charging effects was derived. A comparison of theoretically calculated and experimentally obtained current transients via a non-linear fitting method showed that the different consecutive nucleation and growth processes actually overlap. AFM analysis, which was used to characterize the surface morphology of the silver deposit, confirmed the electrochemical findings. AFM images revealed a different morphology of the silver deposit in relation to the existence of 2D or 3D silver deposits, depending on the applied electrode potential. The enhanced growth of 3D silver clusters around surface imperfections on the vitreous carbon electrode surface observed during the early stages of deposition provides an explanation for the origin of the overlap of the nucleation processes.

Influence of the coordination sphere on the mechanism of cobalt nucleation onto glassy carbon

Abstract Electrolytic phase formation of cobalt onto a glassy carbon electrode (GCE) was investigated using linear sweep voltammetry and the potential step technique in aqueous 10 −2 M CoCl 2 +1 M NH 4 Cl at pH 4.6 and 9.5. Thermodynamic, voltammetric and spectrophotometric analysis of the solutions showed that the predominant chemical species of Co(II) in solution were the Co(H 2 O) 6 2+ ion at pH 4.6 and the Co(NH 3 ) 5 2+ complex at pH 9.5. Voltammetric analysis showed that the experimental equilibrium potential of the Co(NH 3 ) 5 2+ /Co(0) system was more negative than the Co(H 2 O) 6 2+ /Co(0) couple. However the electrocrystallization overpotential for cobalt deposition on GCE from the aquo complex was higher than for the amine complex. Analysis of the current–time transients obtained at each pH, indicated that distinct mechanisms of nucleation are involved during the early stages of cobalt deposition. In the case of Co(H 2 O) 6 2+ reduction, the transients were described theoretically in terms of 3D nucleation with diffusion controlled growth. For cobalt deposition from the Co(NH 3 ) 5 2+ species, the transients were explained by a combination of three different kinds of parallel nucleation processes, 2D progressive nucleation, 2D instantaneous nucleation and 3D progressive nucleation, each of which was limited by lattice incorporation of cobalt ad-atoms.

Silver Electrocrystallization on Vitreous Carbon from Ammonium Hydroxide Solutions

ABSTRACT The early stages of the electrolytic deposition of silver onto vitreous carbon electrodes from ammonium hydroxide solutions have been investigated by the potential step technique. The analysis of the experimental current transients according to existing theories indicates that this process occurs by multiple three-dimensional nucleation, followed bydiffusion controlled growth of nuclei. It is shown that treatments that involve classifying the process as either instanta-neous or progressive nucleation are not always adequate for the quantitative analysis of electrochemical nucleation phe-nomena. The nucleation kinetics parameters A (nucleation rate constant per site) and No (number density of active siteson the substrate surface) were estimated separately from the current transient maxima by two different approaches. Bothquantities were found to vary with the potential and with the concentration of silver ions, except at very high overpo-tentials for silver deposition. The potential dependence of the nucleation rate A was interpreted according to the atom-istic theory and in all cases it was found that the number of atoms in the critical nucleus (nk) was one over the entirepotential range analyzed.Introduction

On the Theory of the Potentiostatic Current Transient for Diffusion‐Controlled Three‐Dimensional Electrocrystallization Processes

Several available theoretical descriptions of the potentiostatic current transient that allow the determination of the rates of the nucleation process, A, and the number densities of sites for nucleation, N0, with three-dimensional, diffusion-controlled growth of clusters are critically examined and compared with experimental and computer-simulated data. All these obtain the overall current f rom the material flux to free, noninteracting growing centers, considering the overlap of circular diffusion zones expanding around them. It is shown that even though when compared to experimental data the diverse theories appear almost indistinguishable, esse ntial differences among them arise from their distinct calculation of the growth rate of diffusion zones, yielding significantly different values for A and N0. Computer simulations of the process for which the values of A and N0 are accurately known allowed meaningful comparisons between theories and an assessment of the relative accuracy with which the different available formulations yield estimates of the kinetic parameters.

Chemical and electrochemical considerations on the removal process of hexavalent chromium from aqueous media

Two methods were used to remove Cr(VI) from industrial wastewater. Although both are based in the same general reaction: 3Fe(II)(aq) + Cr(VI)(aq) ⇌; 3Fe(III)(aq) + Cr(III)(aq) the way in which the required amount of Fe(II) is added to the wastewater is different for each method. In the chemical method, Fe(II)(aq) is supplied by dissolving FeSO4 · 7(H2O)(s) into the wastewater, while in the electrochemical process Fe(II)(aq) ions are formed directly in solution by anodic dissolution of an steel electrode. After this reduction process, the resulting Cr(III)(aq) and Fe(III)(aq) ions are precipitated as insoluble hydroxide species, in both cases, changing the pH (i.e., adding Ca(OH)2(s)). Based on the chemical and thermodynamic characteristics of the systems Cr(VI)–Cr(III)–H2O–e− and Fe(III)–Fe(II)–H2O–e− both processes were optimized. However we show that the electrochemical option, apart from providing a better form of control, generates significantly less sludge as compared with the chemical process. Furthermore, it is also shown that sludge ageing promotes the formation of soluble polynuclear species of Cr(III). Therefore, it is recommended to separate the chromium and iron-bearing phases once they are formed. We propose the optimum hydraulic conditions for the continuous reduction of Cr(VI) present in the aqueous media treated in a plug-flow reactor.

Nucleation and growth of cobalt onto different substrates: Part II. The upd-opd transition onto a gold electrode

A study of the electrochemical deposition of cobalt was carried out using electrochemical techniques. The formation kinetics and growth of cobalt nuclei onto a polycrystalline gold electrode were studied employing an aqueous 10 � 2 M CoCl2 1MN H 4Cl solution (pH 9.5). We obtain the potentiostatic j � /t plots when the potential step jumps from a potential value in the underpotential deposition zone to a final potential in the opd region. It was found that these current density transients can be described through a kinetic mechanism that involves three different contributions: (a) a Langmuir type adsorption process, (b) 2D diffusion-controlled instantaneous nucleation and (c) 3D nucleation limited by a mass transfer reaction. In order to describe the contribution due to 3D growth we test two different approaches by Scharifker and Mostany (J. Electroanal. Chem. 177 (1984) 13) and Heerman and Tarallo (J. Electroanal. Chem. 470 (1997) 70), the values of the experimental parameters A (nucleation rate constant), N0 (number of active nucleation sites) and D (diffusivity of the depositing ions) obtained in the two cases are quite close, however if the influence of the adsorption and 2D nucleation and growth processes is not considered, A and N0 are overestimated and underestimated, respectively. # 2003 Elsevier Science B.V. All rights reserved.

Identification of different silver nucleation processes on vitreous carbon surfaces from an ammonia electrolytic bath

We performed an electrochemical study of silver electrodeposition from an electrolytic bath containing 1 M NH4OH and 1 M KNO3 (pH=11), over a Ag(I) concentration range of 10−4 to 0.3 M, and identified silver nucleation processes on the vitreous carbon surface. Depending upon the silver concentration in the deposition bath, growth occurred either two-dimensionally (2D), controlled by adatom incorporation, or three-dimensionally (3D), controlled by diffusion or lattice incorporation. Qualitative and quantitative characterization of the observed nucleation processes were based on the results of cyclic voltammetry and chronoamperometry analysis. For quantitative characterization, different theoretical models related to electrocrystallization processes were used. Atomic force microscopy (AFM) was employed to probe the surface morphology of the silver deposit. AFM images revealed that silver deposits formed from ammonia baths with different concentrations of Ag(I) ion possess different morphological characteristics. Indeed, many of the surface characteristics clearly corroborated the mechanism of silver deposition proposed by electrochemical analysis.

Silver electrocrystallization from a nonpolluting aqueous leaching solution containing ammonia and chloride

Silver electrocrystallization from aqueous solutions at pH11, pC10 and pNH3 − 0.2, where Ag(NH3)2− is the dominant Ag(i) species, has been studied. In spite of the complexities of this medium, the experimental results can be satisfactorily described in terms of multiple nucleation and diffusion-controlled growth of hemispherical nuclei. Nucleation rates, A, and number densities of active sites on the electrode surface, N0, were determined from potentiostatic current transients as a function of overpotential. Saturation number densities of silver nuclei on the electrode surface obtained from the A and N0 values were found to be in excellent agreement with those obtained from the direct, microscopic observation of the electrode surface. Spatial distributions of nuclei were also analysed for silver electrodeposited at different potentials. It was found that nuclei were uniformly distributed when electrodeposited at low overpotentials, whereas inhibition of nucleation close to already established nuclei occurred at higher overpotentials. From the change of the true nucleation rate with overpotential, it was found that the critical nucleus is formed by a single atom within the −100 to −300 mV over-potential range.