A. Hubin

Oxygen reduction at platinum modified gold electrodes

The reduction of oxygen has been studied on polycrystalline gold electrodes modified by platinum deposited spontaneously from an aqueous K2PtCl6 solution via the displacement of copper or lead adlayers. The change in the surface composition and morphology has been checked by XPS, AES and AFM data. The kinetic results have shown that such electrodes may present a higher catalytic activity compared to bulk platinum electrodes during a scan where the potential is made more positive and is thus clearly expressed by an hysteresis in the CV curves. The displacement of copper and lead deposits gave similar amplitudes of the hysteresis but the modified electrodes obtained via a lead deposit present a better stability upon cycling in acid solutions. The observed behaviour can be correlated to the modification of the MOH formation and reduction on noble metals.

Analytical solution for the steady-state diffusion and migration involving multiple reaction ions Application to the identification of Butler-Volmer kinetic parameters for the ferri-/ferrocyanide redox couple

Abstract An analytical solution for the one-dimensional steady-state transport of ions in an electrolyte towards a planar electrode is obtained. This electrolyte contains more than one electroactive species and any number of non-reacting species. The mass and charge transport equations give rise to an implicit form of a set of non-linear algebraic equations which must be solved numerically. The solution is generally applicable and can deal with any kind of overpotential relation at the electrode. The analytical solution is used to determine the mass and charge transport parameters for the Fe(CN) 6 3− 4− redox couple in a KCl solution for two different electrolyte concentrations (0.03 M Fe(CN) 6 3− 4− + 1.0 M KCl and 0.005 M Fe(CN) 6 3− 4− + 0.2 M KCl ) . The agreement between the experimental and analytical current densities is perfect for both electrolyte solutions under investigation. It is shown that, although an excess of supporting electrolyte is added, neglecting migration results in an under/overestimation of the diffusion coefficient of ferri-/ferrocyanide of 3 to 5% for the Fe(CN) 6 3− 4− / KCl ratios investigated. Furthermore, a mathematical background is given for the wide range of values found in the literature for the charge transfer coefficient and the rate constants of the ferri-/ferrocyanide redox system. The same approach can also be useful for other systems, as to parameter identification procedures.

Quasi-one-dimensional steady-state analysis of multi-ion electrochemical systems at a rotating disc electrode controlled by diffusion, migration, convection and homogeneous reactions

Abstract This article presents a new numerical method for the calculation of concentration, potential and current distributions in electrochemical cells controlled by diffusion, convection, migration and homogeneous reactions of ions. A multi-dimensional upwinding method, originating from the field of fluid dynamics, has been adapted in order to solve this non-linear system. The model developed is able to deal with two-dimensional electrochemical cells involving multiple ions. The electrolyte solutions are supposed to be dilute, at steady state and at a constant temperature. Numerical calculations for a quasi-one-dimensional test case at a rotating disc electrode are performed. These results are compared with both experimental and analytical data, the latter based on a chemical-electrochemical reaction model, for the reduction of silver in a nitrate-thiosulphate solution. The numerical data are found to be in good agreement with the experimental and analytical data. The calculation of concentration profiles provides an interesting insight into the degree of dis-equilibrium of a homogeneous reaction in a thin reaction layer near the electrode.

Morphology, Structure and Photoelectrocatalytic Activity of TiO2 / WO3 Coatings Obtained by Pulsed Electrodeposition onto Stainless Steel

A simple two-step pulsed electrodeposition/ electrosynthesis technique is employed for the preparation of a bicomponent photocatalyst, TiO2/WO3, onto metal substrates. TiO2 can be activated under UV light illumination and is well known for its water detoxification capabilities. The coupling between this wide band-gap semiconductor with a suitable narrow band-gap one, WO3, is used for effective separation of the photogenerated charge carriers. Besides the reduced surface recombinetion due to directional charge transfer, the combination with the visible light (Vis)-activated WO3 entails an extended photoactivity towards Vis wavelengths. In addition, the photocatalytic decomposition of organic water pollutants at TiO2/WO3 layers supported on conductive substrates can be further enhanced by applying a positive bias in an appropriate electrochemical cell. Drawing the electrons away from the surface through the external circuit reduces surface recombination rates of photogenerated electron-hole pairs. Recently, bilayer TiO2/WO3 photocatalysts were prepared onto stainless steel substrates by continuous cathodic electrodeposition of WO3 followed by TiO2 electrosynthesis [1, 2]. Their photoelectrocatalytic efficiency is very promising and superior to both their single-component counterparts. Also, there have been indications of the considerable impact of composition, morphology and structure on photoelectrocatalytic activity [3]. This implicates the necessity for appropriate monitoring and design of these factors. By applying a consecutive pulsed electrodeposition/electrosynthesis method for WO3 and TiO2, a favorable modification of the electronic properties at the TiO2-WO3 junction and an increased catalyst surface area has been sought. The morphology, structure and related composition distribution of the pulsed-deposited films onto metal substrates have been characterized by high resolution Field Emission SEM (FE SEM) (Fig. 1), SEMEDS and Raman spectroscopy. The photocurrents at photoanodes with various loadings, structure and morphology have been evaluated in the presence and absence of the model pollutants Na-oxalate and 4chlorophenol under UV and Vis light illumination. Similar to the case of continuous electrodeposition [3], a trend was observed of the impact and the need for optimization of the TiO2/WO3 loading ratio, surface morphology, structure and composition distribution to design high-performance photocatalysts. The performance for bulk photo-decomposition of 4-chlorophenol has been evaluated at photoanodes WO3 and TiO2/WO3 prepared by continuous electrodeposition and compared with that at pulsed-plated TiO2/WO3. Long-term photoelectrolysis at constant potential was applied, using spectrophotometry to monitor the variation of the pollutant concentration.

Pt-Ni carbon-supported catalysts for methanol oxidation prepared by Ni electroless deposition and its galvanic replacement by Pt

Pt–Ni particles supported on Vulcan XC72R carbon powder have been prepared by a combination of crystalline Ni electroless deposition and its subsequent partial galvanic replacement by Pt upon treatment of the Ni/C precursor by a solution of chloroplatinate ions. The Pt-to-Ni atomic ratio of the prepared catalyst has been confirmed by EDS analysis to be ca. 1.5:1. No shift of Pt XPS peaks has been observed, indicating no significant modification of its electronic properties, whereas the small shift of the corresponding X-ray diffraction (XRD) peaks indicates the formation of a Pt-rich alloy. No Ni XRD peaks have been observed in the XRD pattern, suggesting the existence of very small pockets of Ni in the core of the particles. The surface electrochemistry of electrodes prepared from the catalyst material suggests the existence of a Pt shell. A moderate increase in intrinsic catalytic activity towards methanol oxidation in acid has been observed with respect to a commercial Pt catalyst, but significant mass specific activity has been recorded as a result of Pt preferential confinement to the outer layers of the catalyst nanoparticles.

Study of the initial stages of silver electrocrystallisation from silver thiosulphate complexes: Part I: Modelling of the silver nuclei formation during the induction period

Abstract Chronoamperometry was used to investigate the initial non-stationary stages during the electrocrystallisation of silver thiosulphate complexes on a polycrystalline silver rotating disc electrode. At the beginning of the experiment a large current peak was always observed, followed by a current decrease after several seconds. A new electrochemical reaction model was introduced, suggesting the formation of 2D silver clusters as a reaction step which precedes the actual moment of 3D nucleation. By doing so, the nucleation phenomena observed during the so called induction period do not have to be considered as being a stochastic process consuming no current, as is often done in the literature. The short time transients are simulated using a numerical computational method, solving the basic mass transport equations with the boundary conditions as a function of the newly proposed model. Satisfying fitting results are obtained, thus allowing one to deduce values for the different electrochemical parameters involved.

Investigation of the mechanism of silver deposition from thiosulphate solutions by means of ac impedance measurements and surface-enhanced Raman spectroscopy

Abstract In order to optimize the reaction conditions for the process of silver deposition from thiosulphate solutions, the knowledge of the mechanism of the reduction reaction is required. In the present paper, the role played by NaNO 3 (added as supporting electrolyte) and Na 2 S 2 O 3 (complexing agent for Ag + ) on the reduction process is studied by means of ac impedance and surface-enhanced Raman spectroscopy (SERS). In a solution made up of 0.1 M NaNO 3 , 0.025 M Na 2 S 2 O 3 and 1 × 10 −3 M AgNO 3 ( pH = 12), with an equilibrium potential E 0 of − 215 mV/ sce , the reduction follows a CE mechanism, but divergences are found at overvoltages smaller than 30 mV. SERS measurements on a pretreated silver electrode showed that at potentials between E 0 and about − 250 mV/ sce , the adsorption of S 2 O 3 2− is stronger than at more cathodic potentials. The effect of the enhanced S 2 O 3 2− adsorption on the electrochemical behaviour is hidden in solutions containing more NO 3 − by an overall acceleration of the reduction rate.

Study of the initial stages of silver electrocrystallisation from silver thiosulphate complexes. Part II. Analysis of current transients

The initial, non-stationary stage of silver electrocrystallisation during the potentiostatic reduction of silver thiosulphate complexes was investigated in this work. Curve fitting based on known nucleation and growth laws does not give reliable results. It was suggested therefore that the formation of subcritical 2D silver clusters, preceding the formation of supercritical, stable 3D nuclei, and their growth must be seen as two simultaneously occurring phenomena, both consuming a certain amount of current. An analytical expression that takes this into account is introduced and evaluated with the experimental results. From the obtained fitting parameters the steady state nucleation rate and the total number of nuclei are determined as a function of the applied potential.

Silver reduction in a thiosulphate + nitrate solution : study of a CE reaction scheme by means of polarization curves and ac impedance

Abstract Silver electrodeposition from an electrolyte consisting of 0.1 M NaNo3 + 0.025 M Na2S2O3 + 1 × 10−3 M AgNO3 (pH 12) was investigated by means of polarization curves and ac impedance. In order to work under quasi-stationary conditions, measurements were performed on a rotating disk electrode, the reactive surface of which was covered with a fine silver pre-plating. It is shown that for cathodic overvoltages higher than 30 mV the reduction mechanism followed a CE reaction scheme. The rate constants of both the preceding homogeneous decomplexation reaction of Ag(S2O3)3−2 to Ag(S2O3)− and the heterogeneous electron transfer reaction were determined. The low-frequency limit of electrohydrodynamic impedance measurements was used to check the calculated chemical reaction parameters. In earlier publications in which higher nitrate/thiosulphate ratios were used, the influence of this decomplexation reaction could not be detected. Also, it was found that the lower NO − 3 S 2 O 2− 3 ratio used here caused a substantial difference from the current-overpotential equation at cathodic overvoltages lower than 30 mV. In this potential region, the low-frequency limit of the amplitude of the ac impedance increased with the overvoltage.

Influence of the sodium nitrate content on the rate of the electrodeposition of silver from thiosulphate solutions

Abstract On a pre-plated, polycrystalline silver rde , the silver electrodeposition process from solutions consisting of 1 × 10 −3 M AgNO 3 + x M NaNO 3 + 0.025 M Na 2 S 2 O 3 was studied by means of ac impedance spectroscopy, stationary polarization curves and low frequency electrohydrodynamic (EHD) impedance measurements. The sodium nitrate concentration ( x ) was varied between 0.05 and 0.6 M in order to investigate its influence on the kinetic parameters in a formerly proposed CE mechanism. A catalytic effect of NaNO 3 on both the chemical and charge transfer reaction rates was observed and quantified through the combined use of the different measurements. Thus, a continuous transition was monitored from a clearly noticeable chemical reaction (0.05 M NaNo 3 ) to one hard to detect (0.6 M NaNO 3 ), possibly explained through hydration phenomena. These results highlight the important consequences of adding NaNO 3 , originally intended to be a supporting electrolyte.