Luc Heerman

Theory of the chronoamperometric transient for electrochemical nucleation with diffusion-controlled growth

Abstract The general theory of the chronoamperometric transient for the case of electrochemical nucleation with diffusion-controlled growth has been treated by Scharifker and Mostany [J. Electroanal. Chem. 177 (1984) 13] and Sluyters-Rehbach et al. [J. Electroanal. Chem. 236 (1987) 1]. These models, based on the concept of planar diffusion zones and the use of Avrami’s theorem to account for overlap, lead to conflicting results. This paper reports a new general equation for the current transient which is based on elements of both the existing theories. The theoretical predictions of this equation and their relevance for the analysis of experimental transients are discussed and compared with experimental observations.

Electrochemical nucleation with diffusion-limited growth. Properties and analysis of transients

A general equation for the potentiostatic transient for the case of electrochemical nucleation with diffusion-controlled growth has been published recently [L. Heerman, A. Tarallo, J. Electroanal. Chem. 470 (1999) 70]. This equation essentially is a correction to the model of Scharifker and Mostany [J. Electroanal. Chem. 177 (1984) 13] but leads to quite different predictions. This paper discusses the differences between these two models with special emphasis on the analysis of experimental transients. Numerical data are presented which allow the determination of the nucleation parameters (nucleation rate constant A and site density N0). Finally, it is shown that plots of I/Imax versus t/tmax, which are widely used in the literature for the graphical representation of experimental results, are not very sensitive and can easily lead to misinterpretation of experimental results.

Electrochemical nucleation on microelectrodes. Theory and experiment for diffusion-controlled growth

Abstract This paper describes a general theory for the study of electrochemical nucleation on microelectrodes for the case of diffusion-controlled growth. The starting point is the model of Sluyters-Rechbach (M. Sluyters-Rechbach et al., J. Electroanal. Chem. 236 (1987) 1) which is the most general available at present. This model is combined with the approximate solution of Shoup and Szabo (D. Shoup and A. Szabo, J. Electroanal. Chem. 140 (1982) 237) for a chronoamperometric transient on microelectrodes. In this way a single expression is obtained for the description of electrochemical nucleation and growth on microelectrodes for the case of diffusion-controlled overlap. The resulting equation is more general than the equations of Correia et al. (A.N. Correia et al., J. Electroanal. Chem. 407 (1996) 37) for the case of diffusion-controlled growth on inlaid disk electrodes. These authors presented separate equations for the short and long time limits and consider only the limiting cases of instantaneous and progressive nucleation. The limitations of this model and the conditions of its applicability are critically examined. The theory is illustrated with results obtained for the nucleation of copper from concentrated CuSO 4 +H 2 SO 4 solutions on platinum microelectrodes in the presence of an organic addition agent (gelatine). Analysis of the experimental transients was performed using a nonlinear least square fitting procedure based on the Levenberg–Marquardt algorithm with the nucleation site density and the nucleation rate constant A as fitting parameters.

The impact of concept mapping and visualization on the learning of secondary school chemistry students

The aim of this study was to examine whether the construction of integrated knowledge structures by students can be stimulated by concept mapping and by better visualization of concepts and their interrelationships. The investigation was carried out in regular teaching settings: chemistry courses in secondary schools in Flanders, in the domain of electrochemistry. A significant positive effect of extra attention to visualization on the learning achievement of students was found. However, significant effects of concept mapping as an instruction method could not be detected under the given research conditions.

Study of the Electrodeposition of Rhodium on Polycrystalline Gold Electrodes by Quartz Microbalance and Voltammetric Techniques

The electrodeposition of rhodium on different polycrystalline gold substrates from Na 3 RhCl 6 .12H 2 O + NaCI solutions was investigated by electrochemical quartz crystal microbalance and voltammetric techniques. A study of the electrodeposition of rhodium from the concentrated chloride solutions used in this work show several features that are associated with potentiostatic transients with growth of the clusters controlled by mixed kinetics, charge transfer and diffusion. The results in this paper offer a clear warning against the blind interpretation of potentiostatic transients with models based on simple diffusion controlled growth. At low overpotentials the electrodeposition of rhodium is characterized by very slow charge transfer kinetics and starts with the formation of a submonolayer. Even at more negative potentials current transients and massograms recorded at constant potential exhibit an apparent induction time, indicating that growth initially is controlled by mixed kinetics, charge transfer and diffusion. Bulk deposition of rhodium is shifted to more negative potentials compared with other solutions, e.g., H 2 SO 4 -based electrolytes, but the exact influence of rhodium speciation in the plating solutions remains unknown.

The concept of planar diffusion zones. Theory of the potentiostatic transient for multiple nucleation on active sites with diffusion-controlled growth

Most theoretical models used to describe the potentiostatic transient for the case of multiple nucleation on active sites with diffusion-controlled growth are based on the concept of planar diffusion. It is shown that the definition of a planar diffusion zone is based on an analogy with the Cottrell equation but does not involve a flux balance, as is usually stated in the literature. The current to a growing nucleus can be written in a form that presents this analogy but it remains a hemispherical current, i.e. there is no change in the diffusional regime or the associated boundary conditions. This is the key feature of the concept of planar diffusion zones because it provides a simple way to describe the transition of hemispherical diffusion to planar diffusion when the diffusion fields interfere. In essence, a model based on the concept of planar diffusion zones can only work just because it is not based on a flux balance. The concept of a planar diffusion zone usually is associated with the growth of a single nucleus but this notion is not really needed to derive the equation of the potentiostatic transient. It is better to apply this concept to the ensemble of growing nuclei as a whole since this leads in a natural way to the definition of a uniform diffusion layer, which is connected with the idea of a mean concentration field. It is this notion of a uniform diffusion layer that reduces the interference of hemispherical diffusion fields to a true two-dimensional problem so that Avramis equation can be used correctly to describe the overlap of the planar diffusion zones.

Electrochemistry of uranium(IV) in acidic AlCl3+N-(n-butyl)pyridinium chloride room-temperature molten salts

Abstract The electrochemistry of uranium in acidic AlCl 3 + N -(n-butyl)pyridinium chloride was investigated. In 2:1 melts, the reduction of U(IV) to U(III) on glassy carbon electrodes is irreversible (kinetic parameters for this electrode reaction are reported). Measurements of the formal standard potential of the U(IV)/U(III) redox couple as a function of the melt acidity indicate that U(III) exists as U 3+ while U(IV) is present as chlorocomplexes, UCl x (4− x )+ with 3≥ x ≥1 (in 2:1 melts, UCl 3+ is the major U(IV) species. The oxidation of U(IV) to U(V) on glassy carbon electrodes is irreversible and strongly dependent on the melt acidity. Spectroscopic measurements indicate that U(V) in those melts exists as a chlorocomplex with the metal atom in an almost regular octahedral field. In 2:1 melts, a second anodic wave for the oxidation of U(V) to U(VI) is observed at potentials more positive than the thermodynamic potential limit of the solvent; this wave merges with the chlorine evolution reaction if the acidity of the melt is decreased.

Potentiometric and spectroscopic study of uranium(IV)-uranium(III) in acidic AlCl3-N-(n-BUTYL)pyridinium chloride melts

Abstract The coordination and complex formation of uranium(IV)-uranium(III) in room temperature, acidic (AlCl 3 -rich) AlCl 3 - N -( n -butyl)pyridinium chloride melts has been investigated by potentiometric and spectroscopic methods. Uranium(III) exhibits octahedral coordination around the central metal ion; the species in the melt can be represented as U(S) n ( n = 2, 3) where S is either/or AlCl 4 − ion or Al 2 Cl 7 − ion (AlCl 4 − ion is the most probable counter-ion for all but the most acidic melt compositions). Uranium(IV) exists as a series of chlorocomplexes U(S) n Cl p ( p = 0–3) of relatively low symmetry (tetrahedral or even lower) which are characterized by their absorption spectra.

The electrochemical reduction of ruthenium(IV) in perchloric acid solutions on platinum electrodes

Two reversible waves are observed for the reduction of Ru(IV) on platinum electrodes. Each wave is the sum of two one-electron steps according to: Potentials (vs nhe) and species are given for 1.0 M HClO4 solutions; more hydrolysed products are formed in less acid solutions. The tetrameric Ru(III) ion slowly decomposes to a more stable Ru(III) species, probably a dimeric ion.

Electrochemistry of bismuth in a 67 mole % AlCl3-33 mole % N-(n-butyl)pyridinium chloride room temperature molten salt

The electrochemical reduction of Bi 3+ ion in a 2:1 AlCl 3 -N-(n-butyl) pyridinium chloride melt at glassy carbon electrodes results in the formation of Bi 5 3+ ion. The further reduction of this cluster ion to the metal is the sum of two processes: (i) the reversible two-electron reduction of the closo-trigonal bipyramid with the formation of a closed shell nido-square pyramid and (ii) the reduction of this unstable intermediate to the metal