Showing papers on "Overpotential published in 1982"

The Influence of Surface Recombination and Trapping on the Cathodic Photocurrent at p‐Type III‐V Electrodes

Abstract: The factors contributing to the large overpotential with respect to flatband potential required for hydrogen evolution and certain redox reactions at illuminated p-type electrodes of various III-V compounds have been investigated. Results of photoelectrochemical studies, involving minority carrier transfer to redox systems in the electrolyte, indicate that surface sites acting as recombination centers are important. Results of impedance measurements made with illuminated electrodes emphasize the importance of charge localization in surface states. A model based on surface recombination and electron trapping can explain the photocurrent-potential and impedance characteristics of the p-type electrodes. 35 refs.

Laser-enhanced plating and etching: mechanisms and applications

Abstract: We have developed experimental electroplating, electrodeless plating, and etching techniques that use a focused laser beam to define the localized plating or etching region. Enhancements in plating (etching) rates up to ≅1O3 to 1O4, compared to background rates, have been observed in the region of laser irradiation. A thermal model has been developed to describe the observed effects over the entire overpotential (polarization) curve. In the low overpotential region the enhancement is dominated by the increase in the local charge-transfer kinetics due to the local increase in temperature produced by absorption of the laser energy by the cathode (anode). At higher overpotentials, in the mass-transport-limited region, the main enhancement occurs due to hydrodynamic stirring caused by the the large local temperature gradients. Examples of gold, nickel, and copper electroplating are described to illustrate the value of this technique for micron-sized circuit personalization and repair. Additional examples of electroless laser-enhanced plating and exchange plating are also described.

Catalysis of Oxygen Cathodic Reduction by Adsorbed Iron(111)‐Tetra (N,N,N‐Trimethylanilinium) Porphyrin on Glassy Carbon Electrodes

Abstract: The adsorption of iron(111)‐tetra (N,N,N‐trimethylanilinium) porphyrin on glassy carbon was studied in aqueous solutions using cyclic voltammetry, differential pulse voltammetry, chronoamperometry, and the RRDE technique. The porphyrin was found to catalyze oxygen electroreduction; the overpotential is reduced by about 400 mV and the hydrogen peroxide yield is 5% compared to 25% in the absence of catalyst.

Kinetics of the Hydrogen Evolution Reaction on Mild Steel and Nickel Cathodes in Concentrated Sodium Hydroxide Solutions

Abstract: Data are reported at 288 to 358 K for both smooth and roughened electrode surfaces up to current densities of about 0.3 A/cm/sup 2/. Tafel plots for the nickel cathodes are linear over the whole current density range studied. The transfer coefficient, ..beta.., has an average value of 0.54 and is unaffected by varying either the caustic concentration, the temperature, or the surface roughness of the electrode. With the mild steel cathodes, the Tafel plots show two linear regions with the inflection point occurring at eta = 0.140 to -0.225 V. In the lower overpotential region, ..beta.. increases from about 0.5 to at least 1.4 with increasing caustic concentration and increasing temperature. This behavior is interpreted as being caused by a change in reaction mechanism and results from one of the reaction steps becoming reduced in rate. The ability of the caustic concentration or the temperature to affect the reaction mechanism is amplified by roughening the electrode surface. For the linear regions in the Tafel plots at high overpotentials, the slopes are almost invariant to changes in the caustic concentration, the temperature, or the surface roughness. The transition between the two Tafel regions appears to be caused by a changemore » in the electrode surface resulting from the reduction of a surface oxide, possibly FeOH. The dependence of the inflection point potential on the experimental variables is analyzed in terms of this potential being equivalent to the equilibrium potential of the inferred surface redox reaction.« less

The Electrocrystallization of Zinc from Alkaline Media

Abstract: The initial stages of the electrocrystallization of zinc from an alkaline electrolyte onto a polycrystalline silver substrate are investigated using the potential step method. In the overpotential region where the reaction is predominantly controlled by charge transfer, computer analysis of the experimental current‐time transients with detailed models of nucleation and growth is possible. This gives information about the kinetics of the reaction and the morphology of the crystal growth. It is concluded that the electrocrystallization of zinc proceeds via nucleation and growth, first of a thin layer of primary centers, having low angle of contact at low overpotentials, and subsequently by a layer, which starts to grow via secondary centers, formed at the sites where the primary centers coalesce.

Catalytic electrodes for the Redox Flow Cell energy storage device

Abstract: Several porous electrode materials have been studied for the redox couple Cr3+/Cr2+. The electrochemically active surface area, the hydrogen evolution overpotential and the limiting current of the chromium redox reactions were measured. Useful kinetic parameters can be deduced although their accuracies are limited due to the porous nature of the materials studied. The results presented will primarily be for one kind of graphite material and for ZrC. The performance of the ZrC electrode is promising and exhibits a significant improvement over carbon electrodes. The effects of lead chloride as a catalytic additive to the solution have been studied. Finally, the correlation between electrode performance and chromium complex formation will be discussed since the bridging mechanism is important in the Cr3+/Cr2+ redox reaction.

Controlling metal electro-deposition using electrolyte containing two polarizing agents

Abstract: A method for controlling the electro-deposition of metal from an aqueous electrolyte containing two organic polarizing agents is disclosed. The method requires measuring the nucleation overpotential, the plating overpotential, and the difference between these potentials. By relating these measurements to predetermined values, optimum process control can be obtained by changing the concentration of the various organic polarizing agents in the process electrolyte.

Polarisation phenomena occurring at Cu/semiconducting Te layer/electrolyte interfaces studied by rapid pulse techniques

Abstract: Tellurium films of different thickness are cathodically deposited on copper substrate using sodium tellurite solutions ofpOH 3.95. The kinetics of the primary processes and modes of conduction occurring at the Cu/Te layer/electrolyte interfaces are studied using rapid galvanostatic rectangular pulses. Approximate values for the specific conductance of the surface layer are calculated from the experimental resistance overpotential. Activation over-potentials are explained in view of charge transfer reactions associated with reactions occurring at the electrode/solution interface.

Special features of nickel-molybdenum alloy electrodeposition onto screen-type cathodes

Abstract: Electrolytic nickel-molybdenum alloy, which has a rather low hydrogen overpotential and high corrosion resistance, is of interest as cathode material in industrial electrolysis. Screen-type electrodes with a nickel-molybdenum coating can be used as nonconsumable cathodes in water-activated magnesium-alloy batteries.

Growth of zinc dendrites in acidic zinc chloride solutions

Abstract: The heights and widths of zinc dendrites were measured in pure zinc chloride solutions in the concentration and pH range prevailing in zinc-halogen load leveling batteries. At constant overpotential the height increases linearly with time, indicating a constant growth rate. The higher the concentration and the overpotential, the faster is the growth. The critical overpotential for initiation of dendrite growth is 14 mV independent on the concentration. The growth rate increases when the pH decreases. The influence of hydrogen gas evolved at low pH on the growth rate is discussed. Propagation rate is discussed in terms of an extended Barton-Bockris model. When the height of the dendrite is smaller than the diffusion layer thickness, mass transport to the tip is linear and the dendrite propagates in an exponential manner. When the height is larger than the diffusion layer, mass transport to the tip is spherical and the propagation is linear with time. At constant overpotential the widht is a linear function of the square root of the height, indicating that the dendrite has a shape which is preserved during the growth and the tip radius is maintained constant as predicted by the theory.

Effects of cadmium impurities on the electrowinning of zinc

Abstract: A study was carried out on the effect of cadmium, a major impurity in commercial zinc electrolytes, on the zinc plating current efficiency in acid sulfate baths. The synthetic electrolyte (200 g dm−3 Zn++ and 0–100 mg dm−3Cd++) was continuously circulated through the electrolysis cell. The electrolysis was carried out with a current density of 480–490 A m−2 at 37°C. Hydrogen overpotential, potential sweep, atomic absorption, x‐ray diffraction, and electron microscopy were used to determine the effect of cadmium on the behavior of the zinc plate. The results indicate that cadmium may either increase or decrease the zinc plating current efficiency. For Cd++ less than 20 mg dm−3, increasing cadmium concentrations in the electrolyte reduce the hydrogen overpotential and the zinc plating current efficiency. For Cd++greater than 20 mg dm−3, increasing cadmium concentrations cause a refinement in the structure of the zinc plate with subsequent increase in the hydrogen overpotential and in the zinc plating current efficiency. The incorporation of cadmium into the zinc plate neither significantly alters the crystal orientation of the deposit nor alters the hydrogen evolution mechanism.