Effect of leakage currents on the primary current distribution in bipolar electrochemical reactors

The primary current distribution in a bipolar electrochemical reactor with outside inlet and outlet electrolyte manifolds was investigated by numerical solution of the Laplace equation and by experimental measurement in simulated cells made from conductive paper and segmented electrodes. The geometric parameters determining the distribution were the interelectrode gap, electrode length, transverse section and length of the electrolyte manifold. The effect of the number of electrodes in the bipolar stack was also analysed. Values obtained numerically have been compared with those obtained experimentally and a good agreement is observed between them. These results are useful for estimating the performance of the bipolar stack.     

Effect of leakage currents on the secondary current distribution in bipolar electrochemical reactors

The secondary current distribution in an electrochemical stack with one bipolar electrode was experimentally determined and compared with the theoretical prediction according to the Laplace equation. A close agreement between both results is reported. The parameters acting upon the current distribution were lumped into a dimensionless variable, called the bipolar Wagner number, and its effect on the current distribution and predictive suitability of the theoretical treatment is discussed.     

Modelling and calculation of the current density distribution evolution at vertical gas-evolving electrodes

During industrial electrolysis, for hydrogen, dichloride or aluminium production, there is bubbles creation at one or two electrodes which imply a great hydrodynamic acceleration but also a quite important electrical field disturbance. This disturbance can lead to the modification of the local current density and to anode effects for example. There is few works concerning the local modelling of coupled electro active species transport and electrochemical processes in a biphasic electrolyte. There are also few local experimental measurements in term of chemical composition, temperature or current density which would allow the numerical calculations validation. Nevertheless, effects like the anode effect, particularly expensive on the point of the process efficiency, should need a better understanding. Nowadays, the respective roles of the local temperature increases, the electro active specie composition or the transport properties modification due to bubbles are not known.The goal of the present work is the modelling and the numerical simulation of the vertical electrode configuration for a biphasic electrolysis process. Bubbles presence is supposed to modify the electrical properties, and then the electro active species diffusive transport and the current density. Bubbles are also motion sources for the electrolysis cell flow, and then hydrodynamic properties are strongly coupled with species transport and electrical field. The present work shows hydrodynamic and electrical properties in a laboratory scale electrolysis cell with a vertical electrode. The numerical algorithm used was the finite volume used in the computational fluid dynamic software Fluent^®.     

A factorial-design study of the variables affecting the electrochemical reduction of Cr(VI) at polyaniline-modified electrodes

An electrochemical process using reticulated vitreous carbon-supported polyaniline cathodes is proposed to reduce toxic hexavalent chromium, which is present in industrial wastewater. The effect of five variables was simultaneously studied: (1) flow velocity; (2) current density; (3) electrode thickness; (4) electrode porosity and (5) Cr(VI) concentration. Due to the number of variables, a Box–Behnken factorial-design was chosen in order to reduce the number of experiments required. The current efficiency, energy consumption and space–time yield were the variables evaluated. The process was also analyzed in terms of the polymer stability and the overpotential distribution inside the porous electrode.     

Electrochemical destruction of thiourea dioxide in an undivided parallel plate electrodes batch reactor

The paper presents the results of a study on the electrochemical destruction of pollutants present in a spent reducing bath of a textile factory. The investigations comprised the electro-oxidation of thiourea dioxide (TUD) (the main component of the reducing bath), sulphites and urea, which are formed during oxidation of TUD. The study performed in an undivided cell parallel plate electrodes reactor, using eight different anode materials under various hydrodynamic conditions, proved that electro-oxidation can be successfully applied for treatment of spent reducing baths. The best results of TUD and SO₃²⁻ electro-oxidation were obtained with a Ti/Pt electrode, which showed electrocatalytic effect for both the compounds, indicating a possibility of their direct electro-oxidation on the anode. Destruction of TUD and SO₃²⁻ proceeded also via indirect electro-oxidation, mediated by chlorine evolved on the anode. The process kinetics was mass transport controlled till Re=5000. No electrocatalytic effects were observed for urea with any of the tested anode materials. The elimination of urea resulted to proceed only by indirect electro-oxidation, mediated by chlorine. For elimination of urea a Ti/Pt–Ir electrode proved to be the best anode, probably due to its high efficiency in electro-oxidation of chlorides into chlorine.     

Current density distribution and gas volume fraction in the gap of lantern blade electrodes

Electrochemical processes involving gas evolution at appreciable rates have been optimized in their design for the sake of reduced energy consumption. The present investigation was conducted in view to reducing the energy demand of a pilot process for electrolytic reduction of hematite particles to iron metal; attention was paid at the design of the lantern blade anodes where oxygen evolution occurs. An experimental cell consisting on two facing anodes and two remote cathodes has been designed and used for investigation of the gas behavior and current density distribution at the anode blades. The model for prediction of secondary distributions was validated by measurement of the currents at the segmented anodes and the effects of the average current density and the anode gap could be observed. The model was finally applied to the pilot cell for iron production; as expected, larger gaps allow more uniform current distributions at the anode, however without reducing the cell voltage. In contrast, blade lengths in the order of 10–15 × 10⁻³ m only would allow visible reduction in the cell voltage.     

The influence of the local current density on the electrochemical exfoliation of graphite in lithium-ion battery negative electrodes

The local current density related to the exfoliation process of graphite negative electrodes in mixed ethylene carbonate/propylene carbonate electrolytes was controlled by a variation of the current applied to lithium half-cells containing either single type graphite electrodes or electrodes consisting of mixtures of an exfoliating and a non-exfoliating graphite. The partial local current density attributed to graphite passivation and its distribution within the volume of the electrode was found to be a key parameter to explain differences in the exfoliation behaviour observed for graphite electrodes. The local current density is related with a local overpotential which may suppress one of several possible electrochemical processes. In a negative electrode consisting of a mixture of a non-exfoliating and an exfoliating graphite component, the exfoliation of the exfoliation-sensitive graphite component can be completely suppressed when increasing the partial local current density attributed to the graphite exfoliation process above a certain threshold, by either decreasing the amount of exfoliating graphite particles in the electrode or by increasing the total current density, i.e., the specific current. The consideration of the local current density distribution for the electrochemical processes throughout the electrode leads to a more general concept for the graphite passivation behaviour during the first lithium insertion in lithium-ion batteries.     

Effect of a net on the limiting current distribution in a parallel plate electrochemical reactor. Part II: combined effects

In Part I, we used computational fluid dynamics (CFD) to solve the Navier-Stokes equations surrounding the inert net in a parallel plate channel and presented the individual effects of the transverse and longitudinal ribs on the limiting current density distribution. The type, location, spacing, and number of the ribs of the net were shown to affect the local and average current density distributions on each of the two electrodes. In Part II, we present the combined effects of the longitudinal and transverse ribs on the limiting current distribution. We calculated the enhancement factors for both the combined and individual effects and compared. The longitudinal ribs decreased the average current density whereas the transverse ribs increased the average current density relative to the no-net parallel plate reactor system. A maximum enhancement of 250% in the average current density for a spacing of 0.94 × 10⁻³ m was obtained with 16 transverse ribs. For the same spacing, a negative 70% enhancement was obtained with 14 longitudinal ribs and no transverse ribs. A maximum enhancement of 180% was observed for the entire net (14 longitudinal and 29 transverse ribs). The enhancements at each electrode are different for a given geometry.     

Effect of a net on the limiting current distribution in a parallel plate electrochemical reactor: part I. Individual effects

In some aqueous-metal batteries or electrochemical parallel plate reactors, the spacing between the electrodes is controlled by a porous net. This net affects the limiting current distribution because it disrupts the parabolic laminar flow velocity distribution. Here, computational fluid dynamics (CFD) is used to solve the Navier-Stokes equations surrounding the inert net and the effect of the net geometry on the limiting current density is studied. The location, spacing, and number of the longitudinal and transverse ribs of the net are shown to affect the local and average current density distributions on each of the two electrodes. The effect of transverse ribs on the current distribution was found to be much higher than the longitudinal ribs. The results show that the longitudinal ribs decrease the local current density at the electrode which is not in contact and increase the current density the space between two adjacent longitudinal ribs at the electrode in contact. The transverse ribs on the other hand, increase the local current density to very high values at the electrode that is not in contact. The current density, however, falls along the flow direction as it exits the transverse-ribs region. These effects were observed to be mainly due to the changes in flow field distribution. A deviation of −40% was observed for a system of 4 longitudinal ribs and no transverse ribs at the non-dimensional axial position 0.06. For 2 transverse ribs, the deviation at the same axial position was approximately 250% of the local current density. All the results are for a net with a spacing of 0.94 × 10⁻³ m.     

Influence of experimental factors on the constant phase element behavior of Pt electrodes

Constant phase element is still an object of controversy in the literature. It replaces capacitance or pseudocapacitance at solid electrodes. In the present paper influence of experimental parameters as cell geometry, distance between the working electrode and the Luggin capillary, surface roughness, and adsorption on the CPE parameters was studied. The obtained results indicate that the main cause of such behavior is adsorption of trace impurities from solution.     

Mass transfer and current distribution on a metallic wire

The current and copper deposit distribution on a metallic fibre (stainless steel, diameter of 2 mm) was studied numerically and experimentally. The experimental copper deposit was measured with an optical microscope and the current distribution was deduced. The influence of electrolysis time on copper deposit distribution was also studied. A typical current tertiary distribution was observed. The experiment with a longer electrolysis time exhibited a larger current variation around the wire. A numerical study of this problem was also carried out. The simulation involved a laminar and turbulent flow solver together with a numerical model for the mass transfer of ionic species due to diffusion, migration and convection. A good correlation was found between simulated and experimental results for experiments with a short electrolysis time. This numerical model was then used to study the influence of the flow velocity and the diffusion coefficient on the current density and on the average mass transfer around a wire a few microns in diameter. The general relation: for 0.02 < Re_p < 14.22 and 1000 < Sc < 12,000 was obtained. Comparison with data available in the literature demonstrates good agreement between our model and previous results.     

Influence of pitch-based carbon foam current collectors on the electrochemical properties of lead acid battery negative electrodes

Pitch-based carbon foams prepared by a template method were used as negative current collectors to assemble flooded lead acid batteries, and the effect of the current collectors on the performance of the negative electrodes was investigated. Comparative galvanostatic charge–discharge tests show that the utilization efficiencies of the active material on the carbon foam negative current collector are 14% and 30% higher than those of the material on a lead grid one at 20 and 10 h discharge rates, respectively. Further EIS tests imply that there are also significant differences in the impedance spectra between the two electrodes. SEM images reveal that at the fully charged state, the particle size of the active material formed on the carbon foam current collector is much more uniform than that on the lead grid.     

Influence of the operating conditions on the electrochemical disinfection process of natural waters at BDD electrodes

Results are reported of an investigation of the oxidation products formed during electrolyses of aqueous solutions of NaCl, in which the initial concentration of chloride ions was maintained at levels normally present in natural waters. The Design of Experiments (DOE) technique has been used effectively to plan a framework of experiments, in which the main operative parameters: current density, electrolyte stirring rate, and cell geometry, were varied at two levels. Their effects, as well as their possible interactions have been evaluated on the yield of chloride oxidation and on the selectivity of the reaction towards the formation of hypochlorite.     

Nucleation on resistive substrates: Analysis of effect on film uniformity

When plating onto resistive substrates, potential drop in the electrode can cause non-uniformities in the current density distribution. Multiple studies of this so-called “terminal effect” have been conducted, assuming a layer-by-layer growth mechanism, as would apply, for example, to Cu deposition onto a thin Cu seed layer. However, when depositing Cu onto a material such as Ta or Ru, electrodeposition may occur by three-dimensional nucleation followed by growth. In such cases, a reduction in substrate resistance may not be realized prior to coalescence of the deposited film. Simulations show that nucleation can have a very significant impact on film-thickness uniformity. Results show a linear and significant increase in non-uniformity with the coalescence thickness of the depositing film. Simulations are extended to account for spatial variations of coalescence length. Implications of the model to wafer scale plating of Cu for interconnect applications are discussed.     

The influence of CO on the current density distribution of high temperature polymer electrolyte membrane fuel cells

In this work the poisoning effect of carbon monoxide (CO) on the performance of high temperature polymer electrolyte membrane (PEM) fuel cell is reported. The poisoning of the anode is assessed at 160 °C and 180 °C based on the transient behavior of the fuel cell potential and current density distribution. The current density distribution at similar cell potential and global current density is also critically compared for CO-free hydrogen feed and for CO-contaminated hydrogen feed. Furthermore, the current–cell potential (I–V) and power density curves and impedance spectra are obtained.The presence of CO causes a performance loss which is aggravated for higher CO concentrations and higher current densities and for lower temperatures. The transient behavior of the fuel cell potential and current density distribution show that the poisoning effect of carbon monoxide at the anode is very fast.The use of CO contaminated hydrogen at the anode yields an anisotropic distribution of carbon monoxide, which is accentuated for higher carbon monoxide concentrations and current densities.     

微细平行流通道换热器流量均布性试验研究

以水为工质,用试验的方法研究了不同进出口结构和Re数下微细平行流通道内流体的流动,使用高速摄影系统拍摄,得到流体在Z型和U型微细通道内的流动情况,并通过Image-pro plus软件提取各通道流体的流速。结果表明:在试验涵盖Re数下,Z型和U型结构换热器流量分配均匀性随着Re数的减小而变好,其中Z型换热器的整体流量分配均匀性提高了约80.4%,U型换热器的整体流量分配均匀性提高了约64.3%;进出口结构对微细平行流通道的流量分配和压降有很大的影响,Z型换热器整体均匀性比U型换热器高约2%—44%,Z型换热器进出口压降比U型减小了约1.2%—19.9%。因此Z型换热器的综合性能更好。     

Potential and current distribution at electrodes with local variations of the polarization parameter

Calculations are communicated in order to estimate the effect of local variations of polarization on the local single electrode potential and the local current density in cells with plane electrodes.     

Current distribution effects in AC impedance spectroscopy of electroceramic point contact and thin film model electrodes

The Finite-Element-Method (FEM) was used for the simulations of the effect of a changing current distribution during AC impedance spectrum recording on electroceramic point contact and thin film model electrodes. For pure electronic conducting point contact electrodes the transition from the primary current distribution to the DC current distribution restricted to the Three-Phase-Boundary (TPB) zone introduces an error in the determination of the reaction resistance, R_reac = Z(freq. → 0) − Z(freq. → ∞). The error is estimated for different width of the effective TPB zone and a rule of thumb regarding its significance is provided. The associated characteristic impedance spectrum shape change is simulated and its origin discussed. Furthermore, the characteristic shape of impedance spectra of thin electroceramic film electrodes with lateral ohmic resistance is studied as a function of an increasing lateral ohmic resistance. The characteristic change in the shape of the complex plane impedance plot with increasing lateral ohmic resistance is discussed in terms of extracting kinetic parameters such as electrolyte and reaction resistance.     

Influence of carbon black distribution on performance of oxide cathodes for Li ion batteries

The influence of carbon black content and carbon black distribution on performance of oxide-based cathodes, such as LiCoO₂ and LiMn₂O₄, is investigated. The electronic conductivity of oxide material/carbon black composites is compared with electrochemical characteristics of the same composites. Uniformity of carbon black distribution in cathode composites is achieved using novel coating technology in cathode preparation. In this technology, the active particles are first pretreated in a gelatin solution. The adsorbed gelatin then controls the deposition of carbon black so that carbon black particles are uniformly distributed in the final composite. The influence of various parameters, such as pH of gelatin, amount of gelatin and concentration of carbon black on the uniformity of carbon black distribution is investigated. It is shown that the conventional technology of cathode preparation yields quite non-uniform distribution of carbon black in cathode material. At the end, we demonstrate that uniformity of carbon black distribution has a crucial impact on reversible capacity, especially at high current densities.