Utz Retter

On the electrochemically driven formation of bilayered systems of solid Prussian-blue-type metal hexacyanoferrates: a model for Prussian blue ∣ cadmium hexacyanoferrate supported by finite difference simulations

Abstract The solid-state electrochemical formation of bilayered electrode structures of different metal hexacyanoferrates was studied. This formation occurs as a result of an insertion/substitution reaction during prolonged voltammetric cycling of microparticles immobilized on the surface of a graphite electrode in an electrolyte solution of the respective metal ions. A theoretical model is proposed for the cyclic voltammetric behavior for the case of the transformation of Prussian blue into cadmium hexacyanoferrate, using the finite difference simulation method. The simulated voltammetric curves and the assumed model are in good agreement with new experimental results which give clear evidence that the exchange reaction of this model reaction, i.e. the intramolecular substitution of high-spin iron by cadmium ions, takes place simultaneously with the electrochemical oxidation of Prussian blue (KFe III Fe II (CN) 6 ) to Berlin green (Fe III Fe III (CN) 6 ). The bilayered structures obtained with immobilized microparticles are discussed in comparison to those prepared by subsequent deposition as thin films.

On the transition from electrodesorption of a two-phase adsorbate to 2D hole nucleation in the dissolution of HgS layers at the mercury-electrolyte interface

Abstract The dissolution of anodic films of HgS was investigated by analysing current i -time t transients. The dissolution of the first monolayer and the bilayer can be theoretically well described by a new model which involves an exponential increase in the current followed by an exponential decrease in it. The model implies that the anodic layer consists of adclusters and adomlecules, that the adclusters dissolve at their rims into admolecules which diffuse across the surface and that only the admolecules can be reduced and desorbed (electrodesorption). Here the electrodesorption is assumed to be much slower than the dissolution of the adclusters and the surface diffusion of the admolecules. When all adculsters are dissolved, the electrodesorption of the admolecules follows a first-order process. The dissolution of an HgS multilayer proceeds via 2D progressive hole nucleation and growth.

On the adsorption and condensed film formation of dodecyl-, tetradecyl-, hexadecyl-, and octadecyltrimethylammonium bromides at the mercury/electrolyte interface.

The adsorption and condensed film formation of dodecyl (DTAB)-, tetradecyl (TTAB)-, hexadecyl (CTAB)-, and octadecyl (OTAB)-trimethylammonium bromides on the hanging mercury electrode is studied in KBr as supporting electrolyte, at various temperatures from 5 to 45 degrees C. A condensed film is formed at negative potentials and at room temperature only in the presence of CTAB. The decrease of the temperature favors the formation of the condensed film. A transition temperature is observed for the film formation. Capacity-time curves at the potentials where the film is formed show a nucleation and growth mechanism, with induction time depending not only on the final potential but also on the initial potential range, although it is in the desorption region. In this temperature range no film is observed for DTAB and TTAB. However, the film is observed for OTAB, but only at higher temperatures, and is more easily formed with increasing temperature. The film is formed in a certain potential region and the nucleation rate increases while moving toward more negative potentials. Hysteresis phenomena are observed during changes of scan direction. The capacity vs time curves for OTAB, where condensed film is formed, are treated using an Avrami plot formulation and have been explained as progressive one-dimensional nucleation with a decrease of the nucleation rate during the overall film formation. The results show a marked effect of the chain length of the alkyl chain on the film formation.

Investigation of the adsorption kinetics of 2,3-dimethylpyridine at the mercury/electrolyte interface

Abstract The influence of a potential jump from the desorption region with θf = 0 to the adsorption region at negative potentials on the formation of an organic layer was investigated by means of the time dependence of the double-layer capacity in the adsorption of 2,3-dimethylpyridine (2,3-DMP) at the mercury/electrolyte interface. The experimental time dependence of the double-layer capacity can be described theoretically as a function of the surfactant concentration by using either the model of adsorption inhibition or that of two-dimensional nucleation and growth.

Electrochemical Impedance Spectroscopy

Non-steady-state measuring techniques are known to be extremely suitable for the investigation of the electrode kinetics of more complex electrochemical systems. Perturbation of the electrochemical system leads to a shift of the steady state. The rate at which it proceeds to a new steady state depends on characteristic parameters (reaction rate constants, diffusion coefficients, charge transfer resistance, double-layer capacity). Due to non-linearities caused by the electron transfer, low-amplitude perturbation signals are necessary. The small perturbation of the electrode state has the advantage that the solutions of relevant mathematical equations used are transformed in limiting forms that are normally linear. Impedance spectroscopy represents a powerful method for investigation of electrical properties of materials and interfaces of conducting electrodes. Relevant fields of application are the kinetics of charges in bulk or interfacial regions, the charge transfer of ionic or mixed ionic–ionic conductors, semiconducting electrodes, the corrosion inhibition of electrode processes, investigation of coatings on metals, characterisation of materials and solid electrolyte as well as solid-state devices.

Modelling of adsorption isotherms for condensed adsorption films at the metal-electrolyte interface using the Ising lattice gas model

Abstract Low temperature series expansions of the degree of coverage have been used to obtain approximate isotherms for the triangular, square and honeycomb Ising lattice gas models. These isotherms have been used to model the capacity-potential dependence of condensed adsorbed organic molecules at the metal-electrolyte interface.

Investigations on adsorption kinetics of sodium decyl sulphate at the mercury/electrolyte interface: Part II. Effect of bulk concentration

Abstract The electrosorption behaviour of sodium decyl sulphate at the negative charged mercury-electrolyte interface has been studied above and below the critical micellar concentration (CMC) by measuring the frequency dependence of the electrode admittance at the cathodic peak potential. From a series of possible adsorption models, a selection was taken by means of non-linear regression. From the concentration dependence of the relaxation times of the approved models, it follows that the diffusion control model possesses the highest probability within the whole concentration range investigated. For that model areas per adsorbed molecule were calculated from relaxation times. From the concentration dependence of surface areas, suppositions on the structure of the decyl sulphate adsorption layer are derived.

Investigations on adsorption kinetics of sodium decyl sulphate at the mercury/electrolyte interface: Part I. Examination of different models for the adsorption process by non-linear regression analysis

Abstract The frequency dependence of the electrode admittance of sodium decyl sulphate solutions above the critical micellar concentration was measured at the more negative ad/desorption potential. The following models, out of a series of possible models, could be selected by non-linear regression: one process with diffusion control; two diffusion-controlled processes occurring in parallel; two parallel processes, one of them diffusion-controlled the other adsorption-controlled.

Admittance measurements of the Tl+/Tl(Hg) electrode reaction in the presence of tribenzylamine adsorption and its coadsorption with polyethylene glycol

Abstract The frequency dispersion of interfacial admittance was investigated for the system Tl+/Tl(Hg) in the presence of adsorption of tribenzylamine (TBA) alone and coadsorption of tribenzylamine and polyethylene glycol (PEG). The interfacial admittance of the Tl+/Tl(Hg) reaction is not influenced by low TBA surface concentrations. At higher TBA degrees of coverage the diffusion admittance shows a type of frequency dependence, which can be explained theoretically by a hemispherical diffusion of the depolarizer caused by active and inactive centres on the surface having the dimensions of diffusional wavelength. The inactive centres can be attributed to two-dimensional condensed TBA-molecules. In coadsorption of TBA with PEG this effect appears at higher TBA concentrations.

Two-dimensional truncated nucleation in the anodic film formation of calomel on mercury

Abstract The anodic deposition of the first monolayer of calomel has been investigated by current-time and current-potential curves. A new theoretical model for two-dimensional nucleation and growth has been developed that considers a stop in nucleation caused by a drop in monomer supersaturation. Growth was assumed to proceed via surface diffusion. This mechanism explains the experimental current-time transients well. We determined data for the critical nucleus formation, such as Gibbs free energies, line tensions and nucleation rates. These change with overpotential in a sensible way.