Showing papers on "Overpotential published in 1992"

Diode-like behaviour of a mitochondrial electron-transport enzyme.

Abstract: In mitochondria, electrons derived from the oxidation of succinate by the tricarboxylic acid cycle enzyme succinate-ubiquinone oxido-reductase are transferred directly to the quinone pool. Here we provide evidence that the soluble form of this enzyme (succinate dehydrogenase) behaves as a diode that essentially allows electron flow in one direction only. The gating effect is observed when electrons are exchanged rapidly and directly between fully active succinate dehydrogenase and a graphite electrode. Turnover is therefore measured under conditions of continuously variable electrochemical potential. The otherwise rapid and efficient reduction of fumarate (the reverse reaction) is severely retarded as the driving force (overpotential) is increased. Such behaviour can arise if a rate-limiting chemical step like substrate binding or product release depends on the oxidation state of a redox group on the enzyme. The observation provides, for a biological electron-transport system, a simple demonstration of directionality that is enforced by kinetics as opposed to that which is assumed from thermodynamics.

Kinetics of hydrogen evolution on Ni-Al alloy electrodes

Abstract: The kinetics of the hydrogen evolution reaction have been studied on Ni-Al alloy electrodes. The electrodes after leaching aluminium in alkaline solution, are very active despite their large Tafel slopes. The SEM studies show a formation of deep pores on the electrode surface. The kinetics were studied using an ac impedance technique. It was found that the impedance plots may be explained assuming a fractal model. The logarithm of the charge transfer resistance is a linear function of the overpotential. Using a nonlinear least square approximation it was found that the reaction proceeds through the Volmer-Heyrovsky mechanism and the kinetic parameters were estimated.

Transition metal-based cathodes for hydrogen evolution in alkaline solution : electrocatalysis on nickel-based ternary electrolytic codeposits

Abstract: Nickel-molybdenum-iron, nickel-molybdenum-copper, nickel-molybdenum-zinc, nickel-molybdenum-cobalt, nickel-molybdenum-tungsten and nickel-molybdenum-chromium ternary codeposits, obtained through electrodeposition on mild steel strips have been characterized with the objective of qualitatively comparing and assessing their electrocatalytic activities as hydrogen electrodes in alkaline solution. It has been concluded that their electrocatalytic effects for the hydrogen evolution reaction (h.e.r.) rank in the following order: Ni−Mo−Fe>Ni−Mo−Cu>Ni−Mo−Zn>Ni−Mo−Co∼ Ni−Mo−W>Ni−Mo−Cr>Ni-plated steel. Further investigations on these electrocatalysts have revealed that the cathodic overpotential contribution to the electrolysis voltage can be brought down by 0.3 V when compared with conventional steel cathodes. The best and most stable hydrogen evolving cathode of these, namely Ni−Mo−Fe, exhibited an overpotential of about 0.187 V for over 1500 h of continuous electrolysis in 6 M KOH at 300 mA cm−2 and 353 K. The salient features of the codeposits, such as physical characteristics, chemical composition, current-potential behaviour and the varying effects of the catalytic activation method were analysed with a view to correlating the micro-structural characteristics of the coatings with the hydrogen adsorption process. The stability under open circuit conditions, the tolerance to electrochemical corrosion and the long term stability of Ni−Mo−Fe codeposit cathodes were very encouraging. An attempt to identify the pathway for the h.e.r. on these codeposit cathodes was made, in view of the electrochemical parameters obtained experimentally.

Electrodeposition and Electrochemical Behavior of Palladium Particles at Polyaniline and Polypyrrole Films

Abstract: The electrodeposition of palladium particles at electronically insulating polyaniline and conducting polypyrrole films in aqueous solution has been investigated. The number density of particles and their overall surface area was calculated from current transients recorded during deposition at constant potential. Both were found to depend strongly on the palladium deposition overpotential. Hydrogen adsorption and desorption, as well as hydrogen evolution, were found to take place readily on the palladium particles at polyaniline in its reduced, electronically insulating, state. Hydrogen oxidation, however, was only observed concurrently with oxidation of polyaniline to its electronically conducting state. Oxygen reduction was found to occur on Pd-loaded polyaniline films at higher rates than similarly loaded vitreous carbon electrodes, due to larger real surface area of Pd obtained at polyaniline. Metal particles dispersed in conducting polymer films (1) have been described as possible etectrocatalysts for various electrode reactions (2-6). Given the current interest on this type of system (7), the present study was undertaken in order to describe the electrodeposition of palladium at polyaniline films on electrodes. In order to further understand the electrodeposition of metals at polymer films on electrodes, the deposition of palladium onto polypyrrole films was also studied. Finally, the electrochemical behavior and electrocatalytic properties of the polyaniline films thus modified with palladium particles was investigated.

Nodulation of electrodeposited copper in the presence of thiourea

Abstract: This paper considers the effect of thiourea concentration, temperature and current density on electrodeposited copper nodulation. Thiourea concentration was varied from 0 to 60 mg dm−3 in the range 30 to 60°C and at current densities of 21.5, 48.4, 75.3 and 129.2 mA cm−2. Thiourea was found to initiate nodulation and this effect was always associated with a large increase in overpotential i. e., >100 mV. This phenomenon occurred only when the applied current density was greater than a critical current densityi c. The critical current density was found to be insensitive to potential over a range of about 100 mV. Also, it decreased nonlinearly but smoothly with increase in thiourea concentration at a given temperature and increased similarly with temperature at a given thiourea concentration. Galvanostatic experiments were carried out to study this phenomenon and a qualitative explanation is given based on potentiodynamic measurements and suppositions as to the possible chemical reactions taking place at the cathode surface based on evidence in the literature.

Oxygen reduction on Ru-oxide pyrochlores bonded to a proton-exchange membrane

Abstract: Oxygen reduction at a gas-fed, porous, ruthenium-pyrochlore electrode attached to a Dow Developmental Fuel Cell Membrane was measured in solutions of various pH. Electrode assemblies containing high surface area Pb2Ru2−xPbxO7−y or Bi2Ru2−xBixO7−y with different amounts of Teflon content with/without the incorporation of Dow gel in the active part of the electrode with/without a CO2-treated Vulcan XC-72 carbon substrate were tested. The oxide pyrochlores were found to be chemically stable and to show their lowest overpotential if separated from a 2.5 M H2SO4 proton reservoir by the membrane. Interesting oxygen reduction activity at room temperature was obtained with the Pb2Ru1.74Pb0.26O7−y electrode bonded with 22% by weight Teflon and incorporating 5% by weight Dow gel. The performance of the oxides against B-site Pb concentration and a measurement of the surface charge on the particles indicate that, in this configuration, the active sites for the oxygen reduction reaction are OH− species at the O-site positions of the A2B2O6′O1−y pyrochlores, especially the bridging oxygen with one Ru and one Pb near neighbour, i.e. Pb−Ob−Ru. Evidence that oxide particles precipitated on CO2-treated carbon transfer electrons to the substrate is also presented.

Role of Overpotential on Texture, Morphology and Ductility of Electrodeposited Copper Foils for Printed Circuit Board Applications

Abstract: A study was made to correlate different electrodeposition parameters, like, e.g., cathodic overpotential, bath composition, and bath ageing, with characteristics as crystallographic texture and roughness, and the ductility of electrolytic copper foils. Copper foils with a low (220) preferred crystallographic orientation, and a smooth surface are obtained when depositing at 87 to 113 mV cathodic overpotential from copper sulfate solutions with a low chloride content. Under these plating conditions copper foils with the highest ductility were achieved

Aqueous polysulphide flow-through electrodes: effects of electrocatalyst and electrolyte composition on performance

Abstract: Porous electrodes are required to achieve satisfactory performance of the aqueous sulphide/polysulphide redox couple in energy conversion and storage applications. A flow cell for testing flow-through porous electrodes was constructed and operated. The effects of electrode material, temperature, flow rate, and electrolyte composition were studied. Catalytic electrode surface layers of Co and MoS2 demonstrated performances which were more than adequate to meet a design goal of 10–20 mA cm−2 at less than 50mV overpotential. Flow rate variation had only a small effect on the current density-overpotential behaviour, whereas raising the temperature and/or adding dimethylformamide to the electrolyte had much larger effects. These observations are consistent with steady-state results obtained on rotating disc electrodes.

Effect of a polyethoxylate surfactant on the electrodeposition of tin

Abstract: The electrodeposition of tin in the presence of polyethoxylated additive TX-102 has been investigated in acid medium. The additive causes a substantial increase in the overpotential for the discharge of Sn2+ ions. Below 0.12 g dm−3 the additive has little effect on the quality of the tin deposits. Periodic oscillations in the voltage of the cell are observed, which are associated with the initial growth of whiskers. For higher concentrations of additive, the deposits are smooth and homogeneous, and no growth of whiskers and no voltage oscillations are observed. The effect of TX-102 for concentrations below the CMC has been interpreted in terms of blockage of high energy sites of preferential growth on the tin surface. For additive concentrations above the CMC, micelles are formed. These high molecular weight aggregates also block the surface, hindering the mass transport of Sn2+ through a membrane-like layer that forms on the tin.

Characterization of hydrogen evolution on cobalt electrodeposits in water electrolysis

Abstract: The hydrogen evolution reaction (HER) on cobalt electrodes has been investigated in both alkaline (30 wt% KOH) and acid (1m H2SO4) media at 25°C. The electrocatalytic cobalt materials were produced under different electrodeposition conditions, namely deposition in the absence or presence of bubbling oxygen or nitrogen gas with two gas flow rates (80 and 230 ml min−1) and at different current densities (50–800 A m−2) and deposition in a cobalt powder-containing bath. It has been shown that the electrocatalytic behaviour of the cobalt deposits can be significantly affected by deposition current density via a change of surface area of the cobalt deposits. A considerable HER overpotential decrease (up to 150mV) has been achieved on the highly porous and active cobalt electrodes deposited in the presence of bubbling oxygen and chloride ions in deposition solution. However, the HER overpotential was increased on the cobalt electrodes deposited with bubbling nitrogen in the bath.

A nickel molybdenite cathode for the hydrogen evolution reaction in alkaline media

Abstract: The characteristics and mechanism of the hydrogen evolution reaction on a nickel sulphide electrode incorporating molybdenite particles are described. At 25°C the overpotential for the reaction is 300 mV lower than that of the mild steel cathodes used in unipolar electrolysers. The material shows good stability under long term operation.

The Kinetics of Electrode Reactions: Part II—Mass Transfer and Mixed Control

Abstract: SUMMARYThe modes of mass transport to and from electrode surfaces are considered and it is seen that the rate of reactant supply is often a rate determining step in electrochemical reactions which occur in metal finishing. Complete mass transport control is characterised by the limiting current for a reaction and expressions are developed which relate this parameter to reactant concentration and electrode/electrolyte velocity. In many cases, reaction rates are governed partially by mass transfer and partially by electron transfer. This, “mixed control” case is also considered by means of current versus potential curves and the types of overpotential involved.

Comparability of redox reactions at n- and p-type semiconductor electrodes. 2. Electrochemical overpotential and recombination in view of the quasi-Fermi level concept

Abstract: In the present paper, majority carrier processes at p-type semiconductor electrodes and corresponding reaction mechanisms have been studied using classical electrochemical methods. Taking the Cu + /Cu 2+ redox system as an example, the corresponding current-potential curves obtained with p-type GaAs electrodes have been analyzed in detail. It is shown that the kinetics of the oxidation and reduction process are very fast, and the currents are entirely diffusion controlled