Theoretical and experimental study of electrochemical reactors with three-dimensional bipolar electrodes

A mathematical model to represent three-dimensional bipolar electrodes is proposed taken into account the leakage current. The model is analytically solved when the electrochemical reaction has a mass-transfer control at low overpotentials, which represents a limiting case of the general mathematical treatment. For this simplified situation, expressions are deduced to evaluate the current and potential distribution and to calculate the leakage current. The effect on the leakage current of kinetic, electrochemical and geometric variables, which are lumped into one dimensionless number, is discussed. The influence of the leakage current on the optimal bed depth under limiting current conditions is also analyzed. Likewise, experimental data, using the deposition–dissolution of copper as test reaction, are compared with the theoretical prediction of the general treatment achieving a good agreement between both.     

Influence of solid phase conductivity on spatial localization of electrochemical processes in flow-through porous electrodes Part II: Nonuniform porous matrix with a variable conductivity profile

The possibilities of improving the current distribution uniformity of the main reaction and of increasing porous electrode (PE) performance efficiency by using a porous matrix with a definite profile of conductivity s(x) have been theoretically analysed. A more general deduction of an equation for the s(x) profile providing the electrode with constant polarization is given. Peculiarities of this profile and ways of its approximate realization are discussed. The possibility of significant increase in the performance efficiency of electrodes of this type has been experimentally confirmed.     

The characterization of three-dimensional electrodes using an electrochemical tracer method

A new approach is suggested for the characterization of electrochemical reactors and is applied to three-dimensional electrodes. This approach permits the investigation of the fluid flow pattern through heterogeneous media and the overall reactivity of the bed. The fluid flow patterns have been derived by adapting the tracer method (well-known in chemical reaction engineering) for measurements on electrochemical reactors: auxiliary electrodes have been used both for the production and detection of concentration pulses. Experiments have been carried out on beds of glass beads, the size of the beads, height of the beds and flow rates being varied. The results are expressed as (Pe)-(Re) relationships. The reactivity of the beds has been determined using a new method, the mathematical background of which is due to be published. This method has been tested on electrochemically active beds of glass beads coated with copper and silver, the particle size and flow rates again being varied. The results are expressed ask=Sk _m(=SD/) relationships.List of symbols C concentration (mol cm^–3) - ¯D dispersion coefficient (cm² s^–1) - D diffusion coefficient (cm²s^–1) - diffusion layer thickness (cm) - d _p particle diameter (cm) - I(t) function defined by Equation 5 - K overall reactivity constant of the bed (s^–1) - k _m mass transfer coefficient (cm s^–1) - l distance along the length of the electrode (cm) - M _{1, 2} first and second moment of the distribution of residence times - fluid viscosity (g s^–1 cm^–1) - (Pe) Peclét number=UL/D - r electrochemical reaction rate (mol cm^–3 s^–1) - (Re) Reynolds number=Ud_p/. - fluid density (g cm^–3) - S specific surface area of the electrode (total surface/total volume) (cm^–1) - t time (s) - average residence time of the species entering the electrode (s) - U interstitial fluid velocity (cm s^–1) - v volumetric flow rate (cm³ s^–1) - free volume (cm³) - X the degree of a conversion - y ₁ (t) response of the three-dimensional electrode when the current is switched off - y ₂ (t) response of the three-dimensional electrode in the limiting current regime     

The effective thermal conductivity of three-dimensional reticulated foam materials

In this paper, the effective thermal conductivity k _eff of three-dimensional (3D) reticulated SiC foams were investigated through experimental and numerical methods. The results showed that the k _eff of SiC foams increases as the volume fraction f increases from 30% to 50%. However, there are no systematic changes detected in k _eff when the cell size of the foam varies at a fixed volume fraction. The k _eff of SiC foams as a function of f was obtained. Compared the experimental results with the calculated ones, it indicated that the outcome can be widely applied in estimating the effective thermal conductivity of other foam materials.     

Effluent treatment using a bipolar electrochemical reactor with rotating cylinder electrodes of woven wire meshes

BACKGROUND: The behaviour of a bipolar electrochemical reactor consisting of one or more rotating cylinder electrodes of woven wire meshes is reported using copper and cadmium deposition from dilute solutions as test reactions. RESULTS: The best performance related to electrode number was determined for copper deposition and was achieved by an arrangement with two bipolar electrodes, for which the conversion in a single pass was approximately 47%. The specific energy consumption was 3.27 kWh kg^?1 with a normalised space velocity of 23.05 h^?1. The copper powder obtained showed a nodular and dendritic surface morphology. This reactor configuration was also analysed for cadmium deposition, in which hydrogen evolution takes place simultaneously as a side cathodic reaction, considering the effect of flow rate and total current. The maximum conversion per pass for cadmium removal was 38.91%. In this case the reactor with two bipolar electrodes showed a performance similar to that of a monopolar reactor operated at a rotation speed three times higher. CONCLUSION: A continuous electrochemical reactor with two rotating bipolar electrodes of woven wire meshes presents a good performance for copper or cadmium removal from dilute solutions. Copyright © 2009 Society of Chemical Industry     

The effect of internal impellers on mixing in an electrochemical reactor with rotating rings electrodes

An electrochemical reactor with rotating electrodes has been used to to remove pollutants from aqueous media. Poor mixing and passivation of electrodes surface have been identified as the major drawbacks for the operation of this type of reactors because they adversely affect the critical reactions that take place in the liquid bulk. In this work, three different reactor configurations are proposed and their performance on reactor mixing time and process costs is evaluated. CFD simulations, based on previously validated models, were used to observe mixing inside the electrochemical reactors. Three different arrays were used for the rotating rings electrodes: (a) without impellers, (b) with four internal vertical fins and (c) with a pitched blade central impeller. Power consumption, torque, and parameters such as turbulent intensity, mixing time, among others, were evaluated for all configurations. The reactor with no impellers showed two separated zones of recirculation, reducing the reactor mixing and performance. The reactor with pitched blade impeller, showed no significant improvement due to its low central impeller pumping capacity at low rotational speeds (150 rpm). The array with 4 vertical fins operated at 130 rpm presented the highest flow/power ratio, and the lowest mixing time.     

A multi-region and open-source computational fluid dynamic tool for electrochemical systems with three-dimensional electrodes

In this work, a solver, implemented in OpenFOAM, is developed to investigate the mechanisms of ion transport through activated or resistive three-dimensional (3D) electrodes, and also leakage currents in the case of a bipolar connection. The transport model, which accounts for the balance of mass, momentum, species, and electric charge in the fluid, 3D and solid phases, was solved using a multi-region approach. The algorithm permits calculations for a fixed cell potential difference, and also for a current flowing through the cell under galvanostatic control. Analytical models and experimental results of the current and potential distribution are compared with the present tool, obtaining a close agreement between them. The model presented here is supplied as a free source code and can be employed to optimize both cell stack design and system operation conditions.     

Calculation of the oxygen potential profile across solid-state electrochemical cells

Although several methods of solving the charge and mass transport equations in solid electrolytes wired for device applications have been reported in the literature, there are several inconsistencies. Yuan and Pal have expressed some misgivings regarding the earlier treatment of Choudhury and Patterson. The recent analysis by Näfe is at variance with the analysis of Yuan and Pal. This paper presents a critical examination of various methods for solving transport equations in solid-state electrochemical devices based on mixed ionic-electronic conductors. A complete equivalence between the approaches of Yuan and Pal and Choudhury and Patterson is demonstrated. The oxygen chemical potential profiles generated by both models are identical for a variety of boundary conditions and circuit parameters for a typical cell configuration. The concept of fictitious conductivity introduced by Näfe is found to be inappropriate. It implies linear variation of the electrochemical potential of electrons inside the electrolyte, which is not generally valid for a mixed conductor. The oxygen chemical potential profiles inside the electrolyte calculated using Näfe's equation for different conditions differ substantially from those predicted by the other models. The flawed theoretical formulation is responsible for this mismatch. The methods of Choudhury and Patterson, Riess, and Yuan and Pal are essentially equivalent.     

Trickle bed reactor diluted with fine particles and coiled tubular flow-type reactor for kinetic measurements without external effects

Gas and liquid velocities in laboratory scale trickle bed reactors are one or two orders of magnitude lower than those in commercial reactors. Then, the kinetic data may include the external effects. This shortcoming of laboratory scale trickle bed reactor can be resolved by diluting the catalyst bed with fine inert particles. The catalyst bed dilution increases dynamic liquid holdup, pressure drop, gas–liquid mass transfer coefficient. Hydrogenation of 2-phenylpropene on Pd/Al₂O₃ was performed with the trickle bed reactor diluted with fine inert particles and the coiled tubular flow-type reactor to compare the kinetics with that of the basket type batch reactor. The trickle bed reactor diluted with fine inert particles is suitable to obtain the reaction rate without external effects even if the liquid velocity is low. The coiled tubular flow-type reactor should be used at high gas velocities.     

Characterization of a bipolar parallel-plate electrochemical reactor for water disinfection using low conductivity drinking water

A reactor was built, experimentally studied and modelled having six bipolar electrodes placed between the terminal ones, all of them were RuO₂/IrO₂ on a Ti sheet. The interelectrode gap was 1.5 mm. Plastic plates were optionally arranged in the inlet and outlet to the electrodes to produce entrance or exit regions. The Laplace equation was numerically solved for the solution phase to obtain the current and potential distribution. The calculations performed without entrance and exit regions show that the current distribution is pronounced at the thickness of the electrodes and at the electrode edges. The presence of the entrance and exit regions covers the electrode thickness, increases the current distribution and diminishes the leakage current. The hydrodynamics of the reactor was analyzed by the stimulus–response method and the best behaviour was obtained when the equipment was filled with glass beads. This reactor was tested analyzing the in-line production of sodium hypochlorite from drinking water. In a typical galvanostatic experiment, carried out at 10.3 A, with a volumetric flow rate of 2 dm³ min⁻¹ the hypochlorite concentration at the exit was 25 mg dm⁻³. In this case, the applied potential difference was 128 V and the current efficiency was 4.3%.     

Retracted:Effluent treatment using a bipolar electrochemical reactor with rotating cylinder electrodes of woven‐wire meshes

The following article from the Journal of Chemical Technology and Biotechnology , ‘Effluent treatment using a bipolar electrochemical reactor with rotating cylinder electrodes of woven‐wire meshes’ by Javier M. Grau and José M. Bisang (DOI: 10.1002/JCTB.2202), published online on 28 April 2009 in Wiley InterScience (www.interscience.wiley.com), has been retracted by agreement between the authors and John Wiley & Sons, Ltd. The retraction has been agreed because a duplicate version of the article was published on 6 May 2009 (DOI: 10.1002/JCTB.2199). This occurred due to a technical error. The publishers wish to apologise for this error.     

Modelling of three-dimensional bipolar electrodes with irreversible reactions

The behaviour of an electrochemical reactor with three-dimensional bipolar electrodes for irreversible reactions is analysed. Copper deposition at the cathodic side and oxygen evolution at the anodic one were adopted as test reactions at the bipolar electrode, from an electrolyte solution with a copper concentration lower than 1000 mg dm⁻³, pH 2 and 1 M Na₂SO₄ as supporting electrolyte. A mathematical model considering the leakage current is proposed, which can represent the tendency observed in the experimental data related to cathodic thickness and potential at both ends of the bipolar electrode. High values of leakage current were determined, which restricts the faradaic processes to small thicknesses at both ends of the bipolar electrode. Likewise, the performance of the bipolar electrochemical reactor for the treatment of effluents is experimentally and theoretically examined. In this case, the conversion for copper removal was 90.1% after 480 min of operation with one bipolar electrode and 94.8% after 300 min of operation with two bipolar electrodes at a total current of 3 A.     

Thermal conductivity of neutron-irradiated reactor graphites

The thermal conductivity of neutron-irradiated reactor-grade graphites made from needle coke (H-327), Gilsocoke (H-328), and isotropic petroleum coke (H-451) was measured between 22 and 800°C. Irradiation temperatures were between 600 and 1600°C, and fast neutron fluences ranged up to 10²² n/cm² (E>0·18 MeV). Irradiation reduces the conductivity toward a saturation level which increases with irradiation temperature. For given irradiation conditions, the reduction in conductivity is greater for the needle-coke graphites than for the isotropic graphites. The dependence of conductivity on measurement temperature is reduced by irradiation. The temperature dependence of the irradiation-induced increase in thermal resistivity is in fair agreement with that predicted from lattice dynamics theory for those vacancy loops which are small compared with phonon wave-lengths. The temperature dependence is not consistent with calculations which assume a temperature-independent phonon mean free path for irradiation-induced defects.     

Concatenation of electrochemical grafting with chemical or electrochemical modification for preparing electrodes with specific surface functionality

Surface modified electrodes are used in electro-analysis, electro-catalysis, sensors, biomedical applications, etc. and could also be used in batteries. The properties of modified electrodes are determined by the surface functionality. Therefore, the steps involved in the surface modification of the electrodes to obtain specific functionality are of prime importance. We illustrate here bridging of two routes of surface modifications namely electrochemical grafting, and chemical or electrochemical reduction. First, by electrochemical grafting an organic moiety is covalently immobilized on the surface. Then, either by chemical or by electrochemical route the terminal functional group of the grafted moiety is transformed. Using the former route we prepared lithium alkyl carbonate (–O(CH₂)₃OCO₂Li) modified carbon with potential applications in batteries, and employing the latter we prepared phenyl hydroxyl amine (–C₆H₄NHOH) modified carbon which may find application in biosensors. Benzyl alcohol (–C₆H₄CH₂OH) modified carbon was prepared by both chemical as well as electrochemical route. We report combinations of conjugating the two steps of surface modifications and show how the optimal route of terminal functional group modification depends on the chemical nature of the moiety attached to the surface in the electrochemical grafting step.     

Inherent emf relaxation of electrochemical cells with nanocrystalline Ag electrodes

The open circuit voltage of the electrochemical cell Ag (nano)|solid silver electrolyte|Ag (macro) is found to be inherently unstable. Even under conditions which support the morphological stability of the arrangement of nanocrystalline silver, the particles grow significantly as soon as they function as electrodes; i.e. when they are in contact with a silver electrolyte and connected electronically at the same time. The process is shown to be due to electrochemical Ostwald ripening with the interfacial transfer of Ag⁺ through the Ag/electrolyte interface being the rate limiting step. Its activation energy is 0.01 eV. The decay is in good agreement with modelling results.     

Application of diamond electrodes to electrochemical processes

Conducting diamond thin film is a new electrode material that has received great attention recently because it possesses several technologically important characteristics such as an inert surface with low adsorption properties, remarkable corrosion stability, even in strong acidic media, and an extremely wide potential window in aqueous and non-aqueous electrolytes. Thanks to these properties diamond electrodes meet the requirements for a wide range of electrochemical applications. The object of this article is to summarise and discuss the recent results available in the literature concerning the application of diamond electrodes to electrochemical processes such as water treatment and electro-synthesis of organic and inorganic compounds.     

Investigation of electrochemical behaviour of storage device prototypes with carbon electrodes

The electrochemical behavior of storage device prototypes with carbon electrodes and ionic liquids as electrolyte was investigated. The current-voltage dependences using different working electrodes and the dependences of specific capacitance, charge and discharge dynamics of supercapacitor cells on time were obtained. The optimal values of specific capacitance and specific energy of film-like supercapacitor samples at different voltages were obtained.     

The electrochemical impedance of metal hydride electrodes

The electrochemical impedance responses for different laboratory type metal hydride electrodes were successfully modeled and fitted to experimental data for AB₅ type hydrogen storage alloys as well as one MgNi type electrode. The models fitted the experimental data remarkably well. Several AC equivalent circuits have been proposed in the literature. The experimental data, however, could not always be satisfactorily approximated. The approximation model presented here exhibits smooth fit to the experimental results for all frequencies in the whole range from 10 kHz to 0.1 mHz. Equivalent circuits, explaining the experimental impedances in a wide frequency range for electrodes of hydride forming materials mixed with copper powder, were obtained. Both charge transfer and spherical diffusion of hydrogen in the particles are important sub processes that govern the total rate of the electrochemical hydrogen absorption/desorption reaction. To approximate the experimental data, equations describing the current distribution in porous electrodes were needed. Indications of one or more parallel reduction/oxidation processes competing with the electrochemical hydrogen absorption/desorption reaction were observed. The impedance analysis was found to be an efficient method for characterizing metal hydride electrodes in situ.     

Automated setup for impedance measurements of electrochemical cells with two electrodes

An automated setup which allows impedance measurements on cells with two electrodes over a wide frequency range from 1 mHz to 10 MHz combines two impedance analysers and a temperature regulator under the control of a personal computer. Impedance spectra are measured at a programmed set of constant temperatures between room temperature and 1100 K. Alternatively measurements can be made at constant temperature and at specified time intervals. Data are recorded on disk for subsequent analysis by least-squares fitting on a separate computer. Examples for polycrystalline beta-alumina and sol-gel materials are given.This paper is dedicated to Professor Brian E. Conway on the occasion of his 65th birthday and in recognition of his outstanding contribution to electrochemistry.     

Structure and electrochemical properties of nickel hydroxide electrodes with cobalt additives

In this article,cobalt additives are introduced into nickel hydroxide electrodes by two incorporation methods—co-precipitated cobalt hydroxide during the nickel hydroxide synthesis or post-added CoO with nickel hydroxide. The results of X-ray diffraction, cyclic voltammetry, electrochemical impedance spectroscopy, and charge–discharge tests indicate that (i) the diffraction peaks show a decrease in intensity and increase in the half peak breadths for Ni(OH)₂ with co-precipitated cobalt hydroxide; (ii) the electrochemical activity of nickel hydroxide can be improved by both incorporated cobalt and the effects of post-added CoO are more notable; (iii) CoOOH derived from post-added CoO is not stable in the KOH electrolyte when the potential of the Ni(OH)₂ electrode is lowered and its reduction product may be inactive, thus results in an irreversible capacity loss of nickel-metal-hydride battery after over-discharge-state storage.