Showing papers on "Overpotential published in 1990"

Transition metal-based hydrogen electrodes in alkaline solution — electrocatalysis on nickel based binary alloy coatings

Abstract: Nickel-molybdenum, nickel-zinc, nickel-cobalt, nickel-tungsten, nickel-iron and nickel chromium binary alloy codeposits, obtained through electrodeposition methods 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 rank in the following order: Ni-Mo > Ni-Zn (after leaching Zn in KOH) > Ni-Co > Ni-W > Ni-Fe > Ni-Cr > Ni plated steel. Further investigations on the alloy electrocatalysts have revealed that the cathodic overpotential contribution to the electrolysis voltage can be brought down by 0.3 V when compared with conventional cathodes. The best and most stable hydrogen evolving cathode, based on nickel-molybdenum alloy, exhibited an overpotential of about 0.18 V for over 1500 h of continuous electrolysis in 6m KOH at 300 mA cm−2 and 353 K. The salient features of the coatings, such as physical characteristics, chemical composition, crystal structure of the alloy phases 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 nickel-molybdenum alloy cathodes were very encouraging. An attempt to identify the pathway for the hydrogen evolution reaction on these alloy coatings was made, in view of the very low apparent activation energy values obtained experimentally.

Electrochemical Studies on Protective Thin Co3 O 4 and NiCo2 O 4 Films Prepared on Titanium by Spray Pyrolysis for Oxygen Evolution

Abstract: Electrochemical studies were done on thin and films on Ti by cyclic voltammetric and potentiostatic polarization techniques. The films were quite conductive and prevented the underlying surface from playing a harmful role during measurements. The results indicated the formation of a surface quasi‐reversible redox couple, CoIV/CoIII, in solution. The reaction order with respect to OH− ion concentration was found to be nearly 2. The Tafel slopes decreased with increasing concentrations. was observed to evolve oxygen gas at a lower overpotential as compared to that of . Electrode kinetic parameters suggest similar mechanisms for the oxygen evolution for both the oxides.

Mass Transport of Leveling Agents in Plating: Steady‐State Model for Blocking Additives

Abstract: A quantitative model for the leveling effects of plating additives is presented. The fundamental assumption of this model is that the coverage of adsorbed additive is controlled by mass transport. Adsorbed additive is consumed through inclusion and electrochemical reaction and hence is replaced through transport and adsorption from the bulk solution. The primary effect of the adsorbed additive is assumed to be the blocking of the deposition current at the adsorption site. This steady‐state model reproduces the unique characteristics of additive polarization curves, including hysteresis and peaks in overpotential. Three dimensionless groups which characterize additive modified kinetics are identified. The model predicts the extent of either positive leveling or growing roughness on both the micro and macro scales. Sample plating maps are presented which indicate recommended operating windows for producing smooth and level deposits for various mass transfer boundary layer thicknesses and additive compositions.

The Effects of Separator Design on the Discharge Performance of a Starved Lead‐Acid Cell

Abstract: A mathematical model of a starved lead-acid cell has been developed to study the dynamic behavior of the cell during discharge. Concentrated binary electrolyte theory and a volume-averaging technique were used to model the transport of electrolyte. The model can be used to predict cell voltage and profile of: acid concentration, overpotential, porosity, reaction rate, and electrode capacity, as functions of time. The effects of separator thickness and its porosity were examined with respect to cold-cranking amperage and reserve capacity of the battery. The separator was found to be a significant factor governing performance. Starved lead-acid batteries are becoming increasingly popular in the secondary battery market due to many of their potential advantages: zero maintenance, operation in any position, internal gas pressure potentially usable as a built-in charge indicator, and low battery profile and weight, which is especially attractive in automotive SLI (starting-lighting-ignition) applications. A traditional flooded-type lead-acid battery contains excess acid to compensate for the water loss due to oxygen evolution on overcharge. A unit ceil of the battery is composed of a positive electrode, an acid reservoir, a separator, and a negative electrode. A starved lead-acid cell does not have an acid reservoir. Instead, a thicker and more porous separator is used to prevent physical contact of the positive and negative electrodes, serve as an electrolyte reservoir (this gives rise to the concept of an immobilized electrolyte), and promote oxygen gas transport from the positive electrode to the negative electrode for recombination on charging. The separator has always been thought to play a critical role in the operation of a starved lead-acid battery. Atlung and Fastrup (1), extending the work of Turner and Moseley (2), developed a mathematical model to study the effects of separator design on discharge rate and cell capacity. Their model, however, is of the separator alone, with constant reaction fluxes used as boundary conditions. We present here a detailed mathematical model of the whole starved lead-acid cell for discharge, which will allow the analysis of the total system and interactions between the components-positive electrode, separator, and negative electrode. This model is a modification of an earlier model by Gu et al. (3) for the flooded-type cell. With this model, the effects of porosity, tortuosity, thickness, and the level of electrolyte saturation of the separator, on the discharge performance of a starved lead-acid cell, can be studied, including any interactions between the separator and the electrodes. This paper is organized in the following manner. In the "Model Description" section, detailed formulation of the model is given. In the "Model Output" section, the model results are illustrated graphically using an example of a CCA calculation. Each plot is briefly discussed regarding its significance. In the latter section, the effects due to the separator on cold-cranking amperage (CCA) and reserve capacity (RC) are evaluated in greater detail.

The Concentration Overpotential of Gas Evolving Electrodes as a Multiple Problem of Mass Transfer

Abstract: The concentration overpotential due to dissolved gas generated at electrodes turns out to be the result of several mass‐transfer mechanisms acting simultaneously and with mutual interference An equation to predict the concentration over‐potential of gas evolving electrodes is given Numerical results are compared with experimental data and show satisfactory agreement

Simulation of Leveling in Electrodeposition

Abstract: This paper reports on a model of current distribution and electrode shape change for electrodeposition in the presence of diffusion-controlled leveling agents that have been developed. The system is treated as a special case of secondary current distribution, with the surface overpotential taken to depend on both the current density and the transport-limited flux of the leveling agent, according to an empirical relation adapted from polarization data measured at different conditions of agitation. The spatial variation of the leveling-agent flux is determined from a concentration field problem based on the assumption of a stagnant diffusion layer. The solution is obtained by the boundary element method, with a flexible moving-boundary algorithm for simulating the advancement of the electrode profile. To illustrate the model's performance, the evolution of a groove profile during deposition of nickel from a Watts-type bath containing coumarin is predicted and compared with measurements reported in the literature.

Properties of Ni-Sx electrodes for hydrogen evolution from alkaline medium

Abstract: Active layers for hydrogen evolution were prepared electrolytically from nickel-plating baths containing various amounts of thiourea. The sulphur content of the deposited layers, which is dependent on the thiourea content of the electroplating bath, ranged between 10 and 20 wt%. X-ray photoelectron spectroscopic (XPS) analysis revealed that sulphur was present in the layers not only in the form of sulphide, but also in the forms of thiourea and sulphate entrained from the nickel-plating bath. X-ray diffraction analysis showed that the coatings were poorly crystalline. Nickel sulphide, which produces an X-ray diffraction pattern, was present only in coatings deposited from baths with higher amounts of thiourea. The surface layers of such coatings contained a maximum of 10 at % nickel in the form of Ni3S2. When the Ni−Sx electrodes were under load in 1m KOH over a long period, the hydrogen overpotential was found to vary with time. On the basis of XPS measurements, the initial decrease in hydrogen overpotential was ascribed to the washing-out of adsorbed residues of thiourea. The subsequent increase in the overpotential was due to the deposition of iron on to the surface of the layers from the electrolyte used.

Material properties and processing in the production of fuel cell components: I. Hydrogen anodes from Raney nickel for lightweight alkaline fuel cells

Abstract: The production steps of Raney nickel based, PTFE bonded hydrogen anodes for alkaline fuel cells are examined. The Raney nickel catalyst has been made by leaching the nickel aluminium alloy and additional stabilization. The electrode is fabricated by mixing the catalyst with copper oxide for enhancing electronic conductivity and aqueous PTFE emulsion as a hydrophobic binder. Each process step, starting from the nickel aluminium alloy is described and the physical properties of catalyst and electrode are evaluated. At an overpotential of 100 mV the optimized hydrogen anode exhibits at negligable excess hydrogen pressure (1·02bars) a current density of nearly 400 mA cm−2 at 80°C in 30 wt% KOH. Long term performance test shows that electrode overpotential of more than 60 mV should be avoided. A life time of 5000 hrs at 50°C and a current density of 100 mA cm−2 has been proven.

Surface electrochemistry of the anodic Cl2 evolution reaction at Pt. Influence of co-deposition of surface oxide species on adsorption of the Cl˙ intermediate

Abstract: By means of analysis of potential relaxation transients, coupled with steady-state kinetic measurements at Pt, the effect of co-deposited OH and O species in the surface oxidation process on the extent of chemisorption of the Cl˙ intermediate in anodic Cl2 evolution has been determined. Experiments have been made on Pt electrodes that had been either freshly ‘reduced’ or ‘pre-oxidized’, in the presence of Cl– ions, for controlled times at various potentials. Coverage by surface oxide film species has a major effect upon the electrode kinetics of Cl2 evolution and this is associated with large changes in the adsorption pseudocapacitance and coverage of the ‘overpotential deposited’ Cl˙ intermediate. The relation of recombination-controlled kinetics to Cl˙ coverage and to the presence of co-deposited OH and/or O species at the Pt surface, is examined.

Ohmic drop and zones of reduced overpotential during the growth of small clusters

Abstract: The electrochemical growth of small clusters at constant external voltage has been studied theoretically. Expressions are derived here for the time dependence of the cluster size and the growth current accounting for the ohmic drop in the electrolyte. The potential distribution around the growing clusters is determined and the kinetics of spread of the zones of reduced overpotential are investigated.

Corrosion properties of TiC films prepared by activated reactive evaporation

Abstract: Potentiodynamic measurements were used to test the corrosion properties of titanium carbide (TiC) films. The films were deposited by the activated reactive evaporation process which consists of electron beam evaporating titanium at a rate of 0.22 g min -1 in the presence of a d.c. discharge of C 2 H 2 . Pressure was maintained at 1 × 10 -3 Torr during deposition. The films were deposited onto quartz substrates held at 400 °C. By growing films on an insulating substrate the inherent corrosion properties of TiC films without any substrate contribution can be evaluated. The thickness and grain size of the films, which had a slight (111) texture, were 4 μm and 2 - 3 μm respectively. The TiC films were highly corrosion resistant. Activity in the polarization curve was obtained only in a strong electrolyte (1 M HCl). Neither sulphuric nor perchloric acid could activate the film. The passivation potential and critical current density (current density at the passivation potential) in 1 M HCl were 710 mV ( vs. the saturated calomel electrode (SCE)) and 60 μA cm -2 respectively. The passive potential and passive current density were 900 mV (SCE) and 20 μA cm -2 respectively. The passive potential range was 900 mV up to at least 6000 mV (SCE). The oxygen overpotential was high (more than 5000 mV) for the TiC film and comparable with that for titanium.

Electrochemical oxidation of triphenylphosphine in the presence of allylsilanes: Voltammetric studies and regioselective preparation of allyltriphenylphosphonium salts.

Abstract: Electrochemical oxidation of triphenylphosphine (1) in dichloromethane containing allylsilanes (2) in an undivided cell afforded allyltriphenylphosphonium salts (3). The addition of 1 to 2 took place exclusively at the γ-position of 2 and the reaction was extended to provide a convenient method for the synthesis of cyclic phosphonium salts (3) bearing β-exo-methylene. The yields of 3 depended on the cathode material : use of a lead cathode, which has a high hydrogen overpotential, gave favorable results. Based on the results of the voltammetric study, the process of formation of 3 is suggested to involve electrophilic attack of the triphenylphosphine radical cation on 2 as the key step.

The effect of the concentration of TBACl on the electrodeposition of zinc from chloride and perchlorate electrolytes

Abstract: The effect of the concentration (5 to 220 mg l−1) of tetrabutylammonium chloride (TBACl) on zinc deposition overpotential and on the degree of levelling of Zn electrodeposited on a Zn RDE was studied by cyclic voltammetry, galvanostatic measurements and SEM examination. The overpotential for Zn electrodeposition initially increases with increasing TBACl concentration until reaching a limiting value, at which point the surface becomes blocked by adsorbed TBA+. The optimum degree of deposit levelling is obtained for intermediate TBACl concentrations, whose value depends on the concentration (1 or 2.5m) of NaCl supporting electrolyte. When NaClO4 is substituted for NaCl in the bath, the overpotential for Zn electrodeposition markedly increases and consequently the levelling power of TBACl decreases.

Correction of Experimental Data for the Ohmic Potential Drop Corresponding to a Secondary Current Distribution on a Disk Electrode

Abstract: A numerical method is presented for adjusting experimental current-potential curves for the ohmic resistance corresponding to a secondary current distribution on a (rotating) disk electrode. The nonuniform current and potential distributions on the disk electrode cause the electrolyte resistance itself to be a function of measured current. The method described here is employed after the experiments are conducted and yields the Tafel slope as well as adjusted values for current density and surface overpotential that apply to the center of the disk. This facilitates the comparison of the experimental data to those obtained using mathematical models of the rotating disk electrode that, in the secondary current regime, apply strictly only to the center of the electrode. The Tafel slopes obtained agree to within 3 mV/decade with standard techniques for ohmic correction such as current interruption because, at the high current densities where the ohmic correction is most significant, the resistance correction approaches the primary resistance obtained by current interruption. The Tafel slope values for the two methods differ most for solutions of low conductivity. The major advantages of the ohmic correction method described here are that the experimental condition is never perturbed and that the method indicates the extent to which the current distribution is nonuniform.

Effect of nickel in zinc electrowinning under periodical reverse current

Abstract: The effects of nickel impurities were investigated during the electrowinning of zinc in acidic media using periodical reverse current (PRC) along with anodic potential current response and cathodic current efficiency. The chemical and structural characteristics of the cathode deposit were also determined using nickel concentrations of 3, 5 and 10 mg l−1 in sulphuric acid (200 gl−1 H2SO4) containing 60 gl−1 of zinc. The cathodic current efficiency drop caused by nickel was less detrimental in PRC than DC. It was found that the morphology of the deposit plays an important role in the redissolution process caused by impurities. The levelling effect caused by PRC decreases the rate of zinc redissolution, which results in an increase of current efficiency. However for concentrations equal to or greater than 10 mgl−1, the levelling effect caused by PRC is no longer dominant. On the anode nickel did not affect the overpotential of the lead-silver electrode in PRC or DC modes.