Effect of radial diffusion on the polarization at porous flow-through electrodes

The effect of radial diffusion on the polarization of porous flow-through electrodes has been investigated with the aid of a mathematical model. The proposed model takes into consideration the rates of mass transfer in the axial direction by convection and in the radial direction by diffusion as well as charge transport in the pore electrolyte and electron transfer kinetics at the electrode-electrolyte interface. Normalization of the variables gave rise to dimensionless groups pertinent to the kinetic, ohmic and radial diffusion effects. These are respectively,I the reversibility index, Δ the parameter of ohmic effect andψ the parameter of radial diffusion. The latter (ψ=2φ/Sh) is the ratio of two other dimensionless groups. With this formulation, larger values ofψ correspond to more predominant control of the electrode behaviour by radial diffusion. The same is also true for the parameter of ohmic effect Δ. Solutions have been obtained for two limiting cases: negligible and signifcant potential drop in the pore electrolyte. In both cases, equations have been derived which give the quantitative (highly non-linear) effect ofψ on the current-polarization relations. In the case of a significant ohmic potential drop in the pore electrolyte, it was found that the controlling parameter is the product Δψ The two variables seem to give a synergistic effect since, at large Δ values a certain change inψ has a more pronounced effect on the polarization than the corresponding change at lower Δ values. Qualitative and quantitative tests of some aspects of the model are reported using the electrochemical reduction of copper ions from acid copper sulphate solutions at a packed bed of copper particles. Satisfactory agreement was obtained.     

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

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.     

Effect of ohmic losses on potentiostatic transient response of a reversible redox system

The ohmic potential drop between the working and reference electrodes plays an important role in the early stage of the voltage-step transient process. The non-linear effect of the ohmic drop on the transient currents is calculated for large voltage steps, assuming reversible behaviour (negligible surface overpotential) of the redox couple O+ne=R.This paper was presented at the Workshop on Electrodiffusion Flow Diagnostics, CHISA, Prague, August 1990.     

Influence of perfluorinated and hydrogenated surfactants upon hydrogen evolution on gold electrodes

The influence of perfluorinated and hydrogenated surfactants on the mechanism of hydrogen evolution on gold electrodes in 1 M H₂SO₄ has been investigated from steady-state polarization curves and impedance data. The ohmic drop due to bubbles and the reaction inhibition, consequent on both the electrode surface masking by bubbles and the surfactant adsorption, have been separated and compared. The activity of surfactants decreases with the hydrophilic head in the order cationic> anionic> non-ionic. Perfluorinated surfactants produce a lower inhibition than the hydrogenated ones on the electrochemical reaction which occurs in a Tafel kinetic regime, independently of the presence of surfactants.     

Origin of ohmic losses at Co3O4/Ti electrodes

Co₃O₄/Ti and NiCo₂O₄/Ti electrodes were prepared in different ways to investigate the origin of the ohmic losses observed experimentally. In particular, titanium was pretreated in various ways including etching with a HF mixture, and reduction by cathodic hydrogen discharge prior to coating. Different types of commercial titanium and different concentrations of the precursors in solution were also tried. Some electrodes were prepared with a RuO₂ interlayer. Nickel and mild steel were also used as supports. Parameters to quantify the ohmic losses were the peak distance in voltammetric curves, and the initial slope and the deviation from linearity of current vs sweep rate plots. The experimental picture corroborates the view that the main component of the ohmic drop comes from the insulating barrier which forms at the support/oxide layer interface. The intrinsic conductivity of spinels does not appear to represent the main problem for thermal layers as usually prepared.Author to whom correspondence should be addressed.     

Nickel foam and carbon felt applications for sodium polysulfide/bromine redox flow battery electrodes

The first use of nickel foam (NF) as electrocatalytic negative electrode in a polysulfide/bromine battery (PSB) is described. The performance of a PSB employing NF and polyacrylonitrile (PAN)-based carbon felt (CF) as negative and positive electrode materials, respectively, was evaluated by constant current charge-discharge tests in a single cell. Charge/discharge curves of the cell, positive and negative electrodes show that the rapid fall in cell voltage is due to the drop of positive potential caused by depletion of Br₂ dissolved in the catholyte at the end of discharge. Cell voltage efficiency was limited by the relatively high internal ohmic resistance drop (iR drop). Polarization curves indicated that both NF and CF have excellent catalytic activity for the positive and negative redox reactions of PSB. The average energy efficiency of the single cell designed in this work could be as high as 77.2% at 40 mA cm⁻² during 48 charge-discharge cycles.     

Zur stromausbeute und kinetik elektrochemischer reaktionen bei hohen stromdichten unter stationären bedingungen

The investigation of the kinetics of electrochemical reactions at high current densities (20?300 A cm^?2) and high streaming velocities of the electrolyte, is difficult. Large ohmic drops occur which make the exact measurement of potential impossible. A method is reported for multiple electrodes in which the kinetic parameters are deduced, without measuring current-voltage curves, from the dependence of the current efficiency on total current density. The basis of the theory is discussed for a model system of two parallel Tafel type reactions. The anodic dissolution of iron with simultaneous oxygen evolution in KNO₃ and NaClO₃ solutions has been investigated experimentally.     

Polarization of carbon monoxide electrodes on platinum in a solid zirconialime electrolyte

The polarization of thin platinum electrodes has been investigated in a cell of the type Pt, O₂ • solid electrolyte 0·85 ZrO₂ 0·15 CaO • Pt, (CO + CO₂) by the static method, making a correction for the ohmic drop of potential, and with the help of a cathode ray oscilloscope. The results obtained by these two methods are in good agreement. The decay of the anodic overvoltage after switching off the polarization current has also been investigated.

Under the conditions investigated, the anodic polarization appears to be partly due to the slow interaction between carbon monoxide and oxygen, the evolution of which is probably the primary process.

The dependence of the anodic overvoltage on the current density at low concentrations of carbon monoxide in the gas phase points to concentration polarization due to the slow transport of CO to the reaction surface.     

Dynamic behaviour of an electrolyser with a two phase solid-liquid electrolyte Part II: Investigation of elementary phenomena and electrode modelling

The dynamic behaviour of an electrolyser with a two phase solid-liquid electrolyte was investigated. In a previous paper some results were obtained with high particle concentrations by means of spectral analysis of the electrolyte resistance fluctuations and of the potential fluctuations. Here low bead concentrations were used. This allows potential and electrolyte resistance transients to be well separated and to be studied in the time domain. This analysis gives a detailed view of the approach and residence of the beads near the collector, and a quantitative estimation of the ohmic drop effect for insulating and conductive beads. Ohmic drop fluctuations account for the potential fluctuations for insulating glass beads. Zinc beads behave as insulating particles in the low frequency range and generate a similar ohmic drop effect. They behave as conducting particles in the high frequency range and the fast charge exchange induces depolarizing pulses which favour the formation of compact zinc deposits on the current collector. From the analysis of both transients and PSDs of the electrolyte resistance and potential fluctuations, the mean percentage of the particles colliding with the collector has been estimated even for high bead concentrations.     

Influence of sample thickness and measurement set-up on the experimental evaluation of permeability of metallic foams

The pressure drop measured when a fluid flow through a porous medium is presumed a linear function of the medium thickness along the flow direction. Recent works have, however, reported that such linear dependence is not valid for highly porous foams. The deviations observed were attributed to the relative contribution of entrance and exit effects on the total pressure drop. In this work, the effect of the measurement set-up and of the specimen thickness on the pressure drop was investigated. Permeability measurements were carried out with nickel-chromium foams. Two different set-ups for holding the samples were tested. Pressure drop vs. velocity curves were obtained with water at room temperature. The results obtained showed that the normalized pressure drop was affected by the specimen thickness and the set-ups used. The size of the annular section supporting the specimens has a clear influence on the magnitude of the deviation from the expected linear behavior between the pressure drop and the medium thickness. This effect has been attributed to the fluid diffusion and flow expansion into the annular region covered by the specimen support. The magnitude of this effect is more important as the thickness of the specimens increased. No significant entrance or exit effect was observed when the supporting annular section was minimized. The dispersion of permeability constants caused by this effect was determined and discussed.     

A spatiotemporal model of interactions among metastable pits and the transition to pitting corrosion

A spatiotemporal model of metastable pitting at a homogeneous metal surface under potentiostatic conditions is presented. The model accounts for local changes adjacent to metastable pits and their effect on the nucleation of subsequent events. The local changes considered include concentration, potential drop and oxide film damage. The interactions among events can lead to the formation of clusters of metastable pits and to an explosive growth in the total number of pits. Effects of metastable pit generation rate, convective mass transport (e.g., solution stirring), solution conductivity and ohmic potential drop in solution on formation of corrosion patterns at the surface are presented. The effect of the presence of a stable pit is also considered.     

The vaporization of small binary drops on a flat plate at maximum heat flux

The vaporization times of small (12 mg.) drops of a binary liquid on a flat plate were measured in the maximum (critical) heat flux region of boiling heat transfer. The binary system studied was water-1-pentanol, the mixture previously shown by other workers to exhibit the most pronounced concentration effect on critical heat flux. Evaporation curves are reported in the critical region for pure water, pure pentanol and a number of mixtures in the concentration range 1.0 to 2.5 weight % pentanol in water. It was confirmed that drop lifetimes can be obtained in the critical heat flux region by comparatively simple techniques and that there is a concentration effect on drop lifetime that is very similar to that observed during critical heat flux boiling studies. A minimum drop lifetime is observed at 1.8 weight % pentanol in water, the same concentration at which maximum critical heat fluxes were obtained in pool boiling studies at one atmosphere by S. J. D. Van Stralen and co-workers.     

The anodic behaviour of zinc in aqueous KOH solution—I. Passivation experiments at very high current densities

The anodic behaviour of zinc in aqueous potassium hydroxide has been investigated at very high cds (1–10 A/cm²). Galvanostatic experiments have been made in aqueous KOH (1–12·8 M) for reaction times 0·1 s–0·2 ms.

The relationship between the cd and the passivation time follows a √ t relationship. The forms of the overpotential/time curves indicate changes in the ohmic component of the total polarization.

Within the time range investigated no evidence was found for a change in the mode of passivation.

The effect of an increase of temperature was to increase the passivation time: the magnitude of the increase was greater than expected from the increase in the diffusion coefficient alone.     

The effect of temperature on coalescence of liquid drops at liquid/liquid interfaces

Drop/interface coalescence times measured at 293–343K are reported for three oil/water systems (benzene, paraffin oil and 1,1,2,2-tetrabromoethane) with different interface ages. The anomalous coalescence behavior of water droplets is explained by considering the static electrical double-layer residing at the interface which influences the film thinning and the film rupture processes. Analyses using simplified coalescence models reveal that the incorporation of temperature dependence on the physical properties such as density difference between phases, viscosity of the continuous medium and interfacial tension, does not produce satisfactory agreement with the measured coalescence times. The effect of mutual saturation in contrast to unsaturated systems on coalescence times is illustrated. The reproducibility of the drop/interface coalescence times is examined and explanations are offered, relating the method and the conditions of the experimentation. Finally the importance of both the coalescence time and the film thickness to drop stability analysis is demonstrated.     

Contributions to the Ohmic Drop in the Electrolysis of ZnCl2 in a Molten Chloride Electrolyte

The electrowinning of zinc from zinc chloride with a molten chloride electrolyte was investigated. The electrolysis of zinc chloride shows ohmic limitations. The energy consumption is to a large extent determined by the anodic reaction, the evolution of chlorine. The chlorine gas plume was visualised in a see-through furnace and images were analysed to determine the plume angle. This parameter serves as an input parameter for a model to determine the ohmic contributions of the electrolyte, the plume and a layer of chlorine bubbles sticking to the anode, as well as the plume velocity. The results indicate that the major contribution (60%) to the ohmic drop is due to a stagnant layer of bubbles growing and sticking to the anode, thereby decreasing the effective anode area by coverage. The plume velocity influences coverage characteristics to some extent, which influences the ohmic drop.     

Phase formation and acoustic emission during electrochemical incorporation of lithium into aluminium from molten LiCl—KCl

The formation of β-LiAl during electrochemical incorporation of lithium into aluminium from molten LiCl—KCl was investigated by chronoamperometric and acoustic emission techniques. The analysis of the potentiostatic current transients shows that the initial stages of formation of the phase β-LiAl may be described by an electrocrystallization model which includes instantaneous nucleation and three-dimensional growth controlled predominantly by the specific electrical conductivity of the electrolyte (the so-called ohmic control). It was found that upon electrochemical transformation of aluminium in β-LiAl a noticeable acoustic emission is generated which is strongly dependent on current density and time. The results of the acoustic emission measurements are interpreted in terms of the volume expansion which accompanies the transformation of aluminium into β-LiAl.     

The influence of electrode polarization on corrosion beneath paint

A Zn–Fe bi-electrode coated with different clear lacquers was exposed to NaCl solution, so that a well-defined galvanic cell was established with the current directly measured by a zero-resistance ammeter (ZRA). Electrochemical impedance spectroscopy (EIS) was periodically used to monitor the two coated electrodes individually or coupled together. Transient potentiostatic current measurements were also made after applying potential steps of different amplitudes to the individual metals, from which polarization curves were plotted. The ohmic potential drop in the paint was corrected using film resistance values determined by EIS. Evans diagrams were thus plotted from the iR-corrected polarization curves to make the first experimental determination of Evans diagrams as used by Mayne ¹ to illustrate his resistance inhibition model.     

Electrocatalysis and the oxygen evolution reaction

The oxygen evolution reaction has been investigated on a number of electrodes which are electrocatalysts for the chlorine evolution reaction, by making measurements in NaClO₄ solution. Steady state current—potential and impedance—potential measurements, obtained using automated equipment, have been used as the preferred experimental method. Analysis of the impedance has been undertaken by means of curve fitting, and the resulting parameter curves of the double-layer capacity, the charge transfer resistance, and the electrolyte ohmic resistance displayed as a function of potential. Some speculation is made about the interpretation of the parameter curves. The ohmic loss parameter, R_w, is used to correct the potentials to the true values. In comparison to some other gas evolution reactions, such as hydrogen and chlorine, R_w does not depend strongly on potential. The corrected log i—E curve is also considered. It is suggested that a more complete picture of the electrical performance of these electrodes can be obtained by using these electrochemical methods.     

Electrode a pate de carbone a liant electrolytique et composes electroactifs peu solubles incorpores: aspects quantitatifs

The behaviour of carbon paste electrodes containing insoluble electroactive compounds and an electrolytic pasting liquid has been studied. An investigation of some parameters which affect the characteristics of voltamperometric and chronoamperometric curves is presented. It is shown that such electrodes can be used for qualitative and quantitative analyses of solid compounds.     

Characterization of membrane-electrode assemblies for solid polymer electrolyte water electrolysis

The development of medium size SPE® (Solid Polymer Electrolyte) water electrolysers requires satisfactory membrane-electrode assemblies. Pt-Nafion®-Pt, Pt-Ir-Nafion®-Pt and Pt-Ru-Nafion®-Pt composites have been tested for such applications. Anodic overvoltage, cathodic over-voltage and ohmic drop across the SPE® have been measured in the temperature range of 20 to 80° C and in the current density range 0 to 1 A cm^–2 Kinetic parameters for the oxygen and hydrogen evolution reactions have been measured and compared to values obtained in aqueous acid solutions. Electrode structure has been investigated, before and after 2 500 h of continuous electrolysis at 1 A cm^–2, by scanning electron microscopy, in order to check electrode stability at high current density. The effect of membrane surface etching on the roughness factors of the electrodes has also been investigated. Life tests performed on bare membranes and SPE composites at various operating pressures are presented and discussed. Results on the poisoning effect of nickel ions added to the feed water of the cells are reported.