Ljj Jos Janssen

Electrochemical codeposition of inert particles in a metallic matrix

A survey on electrochemical codeposition of inert particles in a metallic matrix is given. Particles held in suspension in an electroplating bath are codeposited with the metal during electrodeposition. The particles used are inert to the bath and can be of different types, that is, pure metals, ceramics or organic materials. Combining this variety of types of particles with the different electrodeposited metals, electrochemical codeposition enables the production of a large range of composite materials with unique properties. Many experimental factors were found to influence the codeposition process, which led to some understanding of the mechanism. Models to predict the codeposition rate were developed, but were only partly successful.

The effect of electrolytically evolved gas bubbles on the thickness of the diffusion layer II

Abstract In order to elucidate the mechanism determining the mass transfer at a gas evolving electrode, the thickness σ of the Nernst diffusion layer was determined as a function of the current density for both hydrogen and oxygen evolving on horizontal and vertical electrodes in acid as well as in alkaline solutions. Also the diameter of the bubbles and the diameter of the bubble street were determined. It was found that both σ and the slope h of the log σ/log i curve strongly depend on the nature of the gas evolved and on the nature of the electrolyte and that the position of the electrode has only a small effect on σ and on h . At i 2 the diameter of free bubbles is practically independent of the current density. At i > 30 mA/cm 2 coalescence of bubbles occurs very often with exception of the hydrogen bubbles in alkaline solution. From the experimental results it was concluded that the mass transfer at a gas evolving electrode is determined by the flow of solution along the electrode surface, this flow being caused by detaching and rising bubbles and by coalescence of bubbles.

Applications of magnetoelectrolysis

A broad overview of research on the effects of imposed magnetic fields on electrolytic processes is given. As well as modelling of mass transfer in magnetoelectrolytic cells, the effect of magnetic fields on reaction kinetics is discussed. Interactions of an imposed magnetic field with cathodic crystallization and anodic dissolution behaviour of metals are also treated. These topics are described from a practical point of view.

The role of electrochemistry and electrochemical technology in environmental protection

For decades electrochemical technology has contributed successfully to environmental protection. In addition, environmental electrochemistry is capable of giving a huge contribution to further reach this goal by (1) application and improvement of existing technology, and (2) research, development and implementation of new technology. There is a tendency in the metal finishing and metal processing industries to redesign processes as closed loop systems. Therefore, we think especially the purification of dilute heavy metal ion-containing process liquids needs much of our attention. Electrochemical processes relevant to this purpose are considered and examples of (possible) industrial applications are discussed. Moreover, a critical evaluation of the usefulness of different types of electrodes is given. Generally, the two-dimensional electrode is the most favourable electrode configuration: the static flat electrode at high flow rates and in combination with mass transfer promoters, and the rotating disc electrode at high peripheral velocity. In both cases turbulent flow condition at the electrode are preferred to enhance the mass transfer. For redox systems where both the reactant and the product are soluble, the porous electrode is favourable due to its high specific surface area.

Formation of hypochlorite, chlorate and oxygen during NaCl electrolysis from alkaline solutions at an RuO2 (TiO2 anode

On-site electrolysis of a weak alkaline solution of NaCl has been applied to an increasing extent for disinfection. To optimize the electrolytic cell and the electrolysis conditions, the current efficiency for hypochlorite, chlorate and oxygen formation at a commercial RuO2/TiO2 anode were determined under various conditions. It was found that for solution with low NaCl concentrations, (lower than 200 mol m−3), and at 298 K, solution flow velocity of 0.075 ms−1 and high current density, (higher than 2 kA m−2), hypochlorite formation is determined by mass transfer of chloride. The formation of chlorate in weakly alkaline media at a chlorine and oxygen-evolving anode is ascribed to two reactions, namely, the direct oxidation of chloride to chlorate and the conversion of hypochlorite. This is suggested to split the well-known electrochemical Foerster reaction into a chemical reaction for the conversion of hypochlorite in chlorate and the electrochemical oxidation reaction of water. It is proposed that in an acidic reaction layer at the anode the mechanism of chlorate formation may be given by the following:

Oxidation of hydrogen at platinum-polypyrrole electrodes

Polypyrrole, an electronic conducting polymer, is used as a matrix for the dispersion of Pt particles. These particles can be included by two methods, viz. (1) by electrochemical depositin of platinum particles on a polypyrrole covered glassy carbon disc and (2) by incorporation of Pt particles during the polymerization of pyrrole on a glassy carbon disc. As a model reaction the oxidation of hydrogen at these electrodes is studied. The polypyrrole electrodes prepared by method (1) exhibit a good catalytic behaviour. The other type of electrodes however show, despite the higher Pt loading, much less activity. Additionally, electrodes were prepared according to method (1) with poly(N-methylpyrrole) and polyaniline as the conducting polymer. These electrodes show a similar diffusion limited behaviour for the oxidation of hydrogen as polypyrrole-modified electrodes, however the oxidation starts at a much higher potential.

Diffusion coefficients of oxygen, hydrogen peroxide and glucose in a hydrogel

For the design of a new glucose sensor, a knowledge of the diffusion of all participating compounds is needed. A rotating disc electrode covered with hydrogel layer was used to determine the effective diffusion coefficients (Deff) of oxygen, hydrogen peroxide and hydroquinone in a hydrogel, which is used in the sensor. Measurements were carried out under steady-state conditions. The three compounds appeared to be slowed by the gel to the same extent. A comparison was made between the Deff values of glucose and hydroquinone by simultaneous diffusion through a hydrogel membrane. In this case glucose diffusion was slowed to a larger extent than hydroquinone diffusion. The effect, however, was independent of the degree of cross-linking of the hydrogel.

Oxygen reduction at polypyrrole electrodes - I.Theory and evaluation of the RRDE experiments

The rotating ring-disc electrode (rrde) is used to determine the heterogeneous reaction rate constants for the cathodic reduction of molecular oxygen at a polypyrrole electrode in 0.5 M H2SO4. During the rrde experiments, a substantial concentration of hydrogen peroxide is built up in the bulk electrolyte. The theory is adapted to these circumstances and from the results, the various reaction rate constants are calculated. In the modified theory, the bulk peroxide is used to determine the activity of the ring electrode and it is possible to correct the ring current in situations where no diffusion limitation of the peroxide oxidation occurs.

Reduction of nitric oxide at a platinum cathode in an acidic solution

Abstract The reduction of nitric oxide at a platinum electrode in 4 M H 2 SO 4 was investigated by the measurement of potential/current relations and by the determination of the current efficiencies for the hydroxylamine, nitrous oxide, ammonia, hydrazine and hydrogen formation at fixed potentials in the potential range from 0 to −400 mV vs sce . The potential/current curve for the reduction of nitric oxide has two waves, of which the limiting currents depend strongly on the potential and the time of the pretreatment of the electrode and on the direction of the potential change during the measurements. In the potential range of the first wave ( viz ϵ > −30 mV) nitric oxide is reduced only to nitrous oxide. In the whole potential range of the second wave ( viz from −50 to −250 mV) nitric oxide is reduced to hydroxylamine, ammonia and nitrous oxide. Formation of hydrazine has not been detected. From the literature and from the relations of the current efficiencies a possible mechanism is proposed for the reduction of nitric oxide.

The iodine/iodide redox couple at a platinum electrode☆

The I/iodide redox couple was studied on Pt in 0.5M H2SO4 by measuring the impedance as a function of frequency. From these measurements, the exchange c.d. j0 was derived according to Sluyters. The dependence of j0 on the reversible potential and the I and the iodide concns. was established. By comparing the theoretical slopes with the exptl. slopes, it is shown that the electrode process consists of the 3 reactions: I- -> Iad + e; Iad + I- -> I2 + e; I2 + I- -> I3-. The 2nd reaction det. the exchange c.d. The transfer coeff. is 0.5, the standard exchange c.d. is 400 ma./cm.2, and the energy of activation is 15 kcal./mole. [on SciFinder (R)]