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.     

Simplified model to predict the effect of the leakage current on primary and secondary current distributions in electrochemical reactors with a bipolar electrode

A simplified mathematical model to calculate the current distributions in bipolar electrochemical reactors is proposed. The current distributions are deduced from a combination of the voltage balance in the reactor with a voltage balance including the electrolyte inlet and outlet. Thus, equations to predict the effect of geometric and operational variables on the current distributions at the electrodes are reported. The parameters acting upon the current distributions were lumped into two dimensionless variables and their effects on the current distributions are discussed. The primary current distributions are obtained as a limiting case. Comparisons between calculated and experimental primary current distributions are reported.     

Performance of a multipurpose research electrochemical reactor

This paper reports on a multipurpose research electrochemical reactor with an innovative design feature, which is based on a filter press arrangement with inclined segmented electrodes and under a modular assembly. Under bipolar connection, the fraction of leakage current is lower than 4%, depending on the bipolar Wagner number, and the current distribution is closely uniform. When a turbulence promoter is used, the local mass-transfer coefficient shows a variation of ±10% with respect to its mean value. The fluidodynamics of the reactor responds to the dispersion model with a Peclet number higher than 10. It is concluded that this reactor is convenient for laboratory research.     

Modeling current density distribution in electrochemical systems

A numerical method was developed for predicting current density distribution in electrochemical systems of several species at steady-state. The fundamental transport equation consisted a partial differential equation (PDE) involving linear terms of diffusion and laminar convection, and nonlinear terms of ionic migration. The boundary conditions (BCs) consisted also PDEs including flux conditions and involving nonlinear terms associated with exponential kinetics of heterogeneous electrode reactions. The method of finite-difference (FD) was used to approximate solution of the global PDE system in two dimensions using a nonuniform rectangular mesh of fine size near electrode edges where boundary layers and steeper gradients were developed. Diffusion and migration terms were discretized by central finite differences, while the convective term was discretized using an upwind differencing scheme in conjunction with introduction of a ‘minimal artificial viscosity’ to ensure convergence and prevent anomalies in solution at high velocities. The false boundaries technique treated flux BCs. The method was tested by application to a model and an experimental system placed between two equal size parallel-plate electrodes fixed in a channel's walls under laminar parabolic flow. Numerical results for local cathodic current density were compared to experimental measurements and analytical predictions for limiting conditions. Excellent agreement was demonstrated thus verifying applicability of this FD method to real situations.     

Evaluation of the hydrodynamic behaviour of turbulence promoters in parallel plate electrochemical reactors by means of the dispersion model

The hydrodynamic behaviour of electrochemical reactors with parallel plate electrodes is experimentally studied using the stimulus-response method either with an empty reactor or with different turbulence promoters. Theoretical results which are in accordance with the analytical and numerical resolution of the dispersion model for a closed system are compared with the classical relationships of the normalized outlet concentration for open systems and the validity range of the equations is discussed. The experimental results were well correlated with the dispersion model using glass beads or expanded plastic meshes as turbulence promoters, which have shown the most advantageous performance. The Peclet number was higher than 63. The dispersion coefficient was found to increase linearly with flow velocity in these cases.     

Influence of geometric variables on the current distribution uniformity at the edge of parallel plate electrodes

In electrochemical processes, the reaction is controlled by electrodes relative placement; to many respects parallel plates seems close to the ideal configuration. However, edge effect represents a detrimental phenomenon that can compromise overall process efficiency. Cell's enclosing insulating walls can attenuate extreme current densities occurring at the electrode edge if appropriately designed to modify the current flow paths. Configurations comprising a limited counter electrode width, an enclosing normal insulating wall, an enclosing oblique insulating wall and a thin parallel mask have been studied. Potential distribution and electrical current lines of these configurations are obtained from algebra operations of complex variables owing to conformal mapping method. It was found that current distribution non-uniformity is conveniently expressed as the absolute deviation from the prescribed value. The dependence of this parameter on geometric variables can be mapped. The picture given by these maps solved the engineering problem of deducing the cell geometry complying with a given current distribution uniformity. Furthermore, optimal parameters providing the best possible performance of each configuration have been identified. Among the geometric variables, the gap between electrodes is the governing parameter of uniformity; it scales the magnitude of edge effect.     

Effect of mass transfer on the current distribution in monopolar and bipolar electrochemical reactors with a gas-evolving electrode

The current distribution in an electrochemical reactor with vertical parallel-plate electrodes was experimentally determined. The research was performed with monopolar and bipolar electrodes. The reactor has a gas-evolving electrode and at the counter electrode an electrochemical reaction with combined diffusion and charge-transfer kinetic control, takes place. Therefore the kinetics at the counter electrode are influenced by the bubble-induced convection and by the forced convection of the electrolyte. These reactors are found in many electrochemical processes, for example, electrowinning of metals and electrosynthesis. The test reactions were hydrogen evolution at the cathode and the anodic oxidation of sulphite to sulphate from basic solutions. The current distribution shows a minimum at a distance of approximately six times the equivalent diameter of the reactor from the inlet region. This minimum is a consequence of the interaction between forced convection and the bubble-induced convection, which shifts the mass transfer coefficient of the anodic reaction along the reactor. The effects of the total current, the volumetric electrolyte flow rate and the metal phase resistance on the current distribution are also analysed. The experimental current distribution data are compared with theoretical expectations and good agreement is found.     

Prediction of shunt currents in a bipolar electrolyzer stack by difference calculus

A model for predicting shunt/leakage currents in a bipolar electrolyzer stack with dual electrolyte inlets and significant amount of gases in the outlet ports and manifold is presented. Model includes electrolyte, manifold and membrane separator as resistance components in the electric circuit analog of the stack. Activation overvoltage associated with electrodes is taken as Tafel-like. Current balance and potential balance equations are applied to the stack and difference calculus is employed to reduce the problem to a set of linear difference equations with constant coefficients. The model is validated with published results and the effect of each resistance component and number of cells on leakage currents in the stack is presented. The U.S. Goverment’s right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged.     

A new technique for two-dimensional current distribution measurements in electrochemical cells

A new technique has been developed using a magnetic loop array to measure current distribution in electrochemical cells. The main advantage of this approach is the combination of high spatial and time resolution and stack integration with an easily handled measurement carried out independently of the cell operation. A polymer electrolyte fuel cell (PEFC) of technically relevant dimensions (about 600 cm² electrode area) with 5 × 8 current sensors has been constructed and operated, thus confirming the feasibility of the measurement technique.     

Corrosion behavior of two bipolar plate materials in simulated PEMFC environment by electrochemical noise technique

The corrosion behavior of 316L stainless steel (316L) and bulk amorphous Zr₇₅Ti₂₅ (AB) alloy was studied using electrochemical noise (EN) in simulated polymer electrolyte membrane fuel cell (PEMFC) conditions at 25 and 80 °C. At open circuit potential, under H₂ environment, AB exhibited a higher corrosion-resistance than 316L and the contrary was observed under O₂ environment. At imposed cathodic potential, under H₂ environment for PEMFC anodic simulation, the reduction mechanism of 316L at 80 °C gave the most important EN signals of all other conditions. At imposed anodic potential, under O₂ environment for cathodic simulation, the passive state was observed for 316L while severe localized corrosion for AB was obvious at 80 °C. It is concluded from the corrosion data of this work that in the anode environment of a PEMFC, the AB alloy could be a better candidate than 316L for bipolar plates. The contrary was observed in the simulated cathode environment.     

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.     

Effect of asymmetry in a bipolar membrane on water dissociation — a mathematical analysis

Based on a simple model established in the previous paper for water dissociation process on a bipolar membrane, this work is concentrated on analyzing the membrane asymmetry on the water dissociation process. Special attention is paid to thickness ratio, fixed group concentration ratio and the water diffusivity ratio of anion layer to cation layer on water concentration, proton or hydroxyl ions concentration in the depletion layer as well as the current density curves. The results suggest that for practical applications, an asymmetric bipolar membrane with the appropriate thickness, proper ion-exchange capacity and high permeability to water will receive more effective results than a symmetric one.     

Oxidation of organic pollutants on BDD anodes using modulated current electrolysis

In this paper, a theoretical model is presented for organic pollutants mineralization at high current efficiency (close to 100%) and low energy consumption on boron-doped diamond electrodes. The model is formulated for a perfect mixed electrochemical reactor operated as a batch recirculation system under multiple current steps, in which the applied current is adjusted during the electrolysis to be close to the limiting value. An experimental validation with the anodic oxidation of 3,4,5-trihydroxybenzoic acid is also provided. The results have shown that multiple current steps electrolysis and continuous current control allowed obtaining high oxidation rate and current efficiency.     

Microstructure control to reduce leakage current of medium and high voltage ceramic varistors based on doped ZnO

The leakage current (I_f) and the non-linearity coefficient (α) are crucial parameters in varistors. This work, deals with the optimization of the electrical characteristics of medium and high voltage varistors based on the ZnO–Bi₂O₃–Sb₂O₃ system. First, the aim was to allow the formation and the stabilization of the spinel phase by reducing the heating rate. To do so, the right sintering temperature and dwell time have to be chosen to obtain a breakdown field suitable for industrial requirements. During cooling, the spinel phase can return to a pyrochlore phase, which contributes to the increase of leakage current. In a second part, this reaction was prevented by faster cooling in an appropriate temperature range. The fine-tuning of both the heating and cooling phases leads to a significant decrease of the leakage current. Moreover, the value of the non-linearity coefficient was increased by 80%, due to better and more homogeneous wetting of the ZnO grains by the Bi-rich phase.     

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.     

Study of the effect of electrode resistance on current density distribution in cylindrical electrochemical reactors

The distribution of current density in a cylindrical electrochemical reactor was determined experimentally, a thin Pt wire, 0.2 mm diameter and 500–600 mm long, being employed as central electrode. In these investigations two methods are used: (i) a reactor with a segmented counterelectrode; in this case, measurements of the current in each ring of the counterelectrode were made; (ii) a reactor with a bi-electrode probe; in this case, the distribution was obtained by measuring the ohmic drop in the solution phase, with the probe being positioned at different heights. A mathematical model to represent such reactors was developed assuming an axially constant electrolyte potential. The experimental and theoretical values are compared in order to determine the predictive suitability of the proposed model. Both the error in predicting the feeder overvoltage and a statistical parameter denote the agreement between the computed and the measured current density distributions. The parameters acting upon the current distribution were lumped in a single dimensionless variable, the so-called modified Wagner number, used to determine the applicability range of the proposed model. It was concluded that when this number exceeds 15×10^–3, for concentrated solutions, the model can be used to design this type of reaction.     

Effect of film structure on the electrochemical properties of gold electrodes for neural implants

The development of interfaces with low impedance is a prerequisite for long-term neural devices. A broad range of new materials has been developed for this purpose. Here we show how the performance of traditional gold electrodes can be improved by controlling the deposition parameters of the gold film. The morphology of the film was tuned from granular to columnar structure as shown by scanning electron microscopy of film cross sections. Electrochemical characterisation with impedance spectroscopy, chronoamperometry and cyclic voltammetry demonstrates that the dense columnar structure of the films effectively lowers the impedance of the interface and increases charge injection properties. The samples produced are also compared to titanium nitride films, a well-established electrode material with a columnar structure.     

The effect of metal and solution phase resistances on the current distribution in cylindrical electrochemical reactors

The mathematical modelling of cylindrical electrochemical reactors has been performed taking into account the resistance of the metal phase of the inner electrode and the resistance of the electrolyte. The model assumes that the external electrode is isopotential, the electrochemical reaction on the external electrode has a low polarization resistance (di/dη→∞) and at each axial position in the electrolyte the current is independent of the radial coordinate. The experimental and theoretical current density distributions are compared in order to determine the predictive suitability of the model and a good agreement is observed between them. Furthermore, a comparison is made between this model and a simpler earlier one and an important improvement in the prediction is observed.