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.     

The chemical engineering approach to some electrochemical processes

In recent years much research has been carried out in combining electrochemical processes with chemical engineering techniques, and in general, to apply principles of analysis and design of this discipline in search for new electrochemical technologies. This paper reviews seven major areas of activity in modern applied electrochemistry where the chemical engineering approach has been successful: Fluidized bed electrochemical reactors; Bipolar particulate reactors; Pulsed electrochemical reactors; Gas phase electrochemical reactors; Electrocrystallization and electrodissolution; Enhancement of heat and mass transfer in electric fields; dynamics, modelling and optimization of electrochemical reactors.     

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.     

The electrochemical behaviour of polycrystalline nickel electrodes in different carbonate-bicarbonate ion-containing solutions

The dissolution and passivation of polycrystalline nickel in carbonate-bicarbonate ion-containing solutions covering wide ranges of pH and electrolyte concentration were investigated by employing voltammetric, galvanostatic and potentiostatic transient techniques. Results obtained with a rotating disc electrode allow the competing reactions related to the active-passive transition to be distinguished through the influence of the potential sweep rate and the rotation speed on the electrochemical behaviour of the system at fixed concentrations of either carbonate or bicarbonate ion. The first oxidation level of nickel corresponds mainly to Ni(OH)₂ formation, the chemical dissolution of the surface layer and the precipitation of NiCO₃ and Ni(OH)₂. The partial removal of the prepassive layer is predominantly assisted by both the bicarbonate ion concentration and the electrode rotation. In the presence of chloride ions the formation of soluble Ni(II) species and NiCo₃ in the potential range of the first oxidation level appears to be enhanced. This effect can be interpreted by taking into account competitive adsorption processes at the base metal between Cl^– and OH^– ions.     

Kinetic study of the electrochemical processes of the bromine/bromine aqueous system on vitreous carbon electrodes

The electrochemical processes of the aqueous Br₂/Br⁻ system have been studied, by the method of the rotating disc electrode, on two different vitreous carbon electrodes: reticulated vitreous carbon RVC E.R.G. Inc., Lowell, Oakland, Calif.) and smooth vitreous carbon CVJ (Tacussel, Lyon France). On both electrodes the cathodic and the anodic processes involve two consecutive electrochemical steps:

The step (1) is rate-controlling in the cathodic process and the step (2) in the anodic process.The Br⁻₃ reduction occurs via formation of Br₂, with which Br⁻₃ is in rapid equilibrium, whereupon Br₂ is reduced according to mechanisms (1) and (2).     

The reaction engineering of electrochemical cells—I. Axial packed bed electrodes

It is shown that the graphical method of solving the coupled equations for an endothermic reaction in a plug-flow, non-isothermal reactor can be extended to electrochemical reactions in three-dimensional electrodes. The method of solution is described and as a first example, it has been applied to a simple reversible reaction in an axial packed bed electrode. The sensitivity to various parameters has been examined. The method is ideally suited to computer-aided design procedures.     

Fluctuation analysis in electrochemical engineering processes with two phase flows

Fluctuation analysis in electrochemical systems appears to be a suitable method for obtaining information on the system dynamic behaviour, especially when the voltage or current fluctuations are due to elementary events on the electrode at a semi-macroscopic scale, for example growth and detachment of bubbles on a gas-evolving electrode or contacts between charged particles in a fluidized or circulating bed reactor, or pits in localized corrosion. Therefore this situation is largely encountered in electrochemical engineering processes with two phase flows. By analysing the current (voltage) fluctuations at constant potential (current) and/or the electrolyte resistance fluctuations, this technique provides quantitative parameters which are often inaccessible by traditional deterministic techniques, steady state or not, which deal with time-averaged signals. The technique also leads to a better understanding of the elementary processes on the electrode. Two examples are given: the first concerning a gas-evolving electrode and the second a circulating bed electrode. This paper was presented at the International Workshop on Electrodiffusion Diagnosis of Flows held in Dourdan, France, May 1993.     

On the use of rotating paste (ring-) disc electrodes in mechanistic studies

The problems inherent in the use of paste electrodes for studying the electrocatalytic properties of porous materials are discussed on the basis of results obtained with a rotating ring-disc electrode, where a carbon-supported chelate paste (with Nujol as the pasting liquid) formed the disc, and O₂ reduction was the electrochemical reaction. It is shown that, in general, the rotating paste electrode is unsuitable for elucidating mechanisms pertaining to the pure catalysts, rather than to the paste itself.     

Optimisation of use of surplus night electricity in electrochemical processes

Cheaper electric energy is available during certain periods of the day, especially at night. Electrochemical plants could make use of the surplus electricity by applying a variable current density. An analysis of the problem is presented for an electrolysis operated at two different current densities. The optimum values of these current densities were calculated by balancing the investment against the energy (or fuel) costs. Dimensionless groups were introduced in order to reduce the large number of parameters and variables involved. As an extension of previous work the optimization was carried out by two methods: (a) by considering simultaneously the expenses for the electrolytic and the power plant; (b) by assuming that the energy is available at two different but fixed rates. The results depend on a number of parameters such as the specific investment cost for the electrolytic plant, the cell resistance etc. Their influence is shown and discussed in terms of the dimensionless groups used. Numerical values are presented for two examples: chlorine electrolysis and copper refining. In the first case the economically desirable ratio of the day and night current is very small, in the second case it is substantially larger. The optimization technique used is compared to the cash flow method. It is shown that the results obtained by the two procedures are the same.     

A study of the electrochemical behaviour of electrodes in operating solid-state supercapacitors

The electrochemical behaviour of electrodes and of complete solid-state supercapacitors has been studied by cyclic voltammetry (CV) and galvanostatic charge/discharge (CD) measurements using two independent electrochemical equipments. The first one controlled the execution of the test and recorded the voltage and current values of the complete supercapacitor while the other one recorded the potential changes of the single electrodes. In this work, two different types of capacitors were studied: (a) a symmetric supercapacitor using carbon electrodes, and (b) a hybrid (asymmetric) supercapacitor with ruthenium oxide/carbon in the positive electrode and carbon in the negative electrode. The studies evidenced that in the symmetric capacitors the positive electrode controlled the capacitive performance and an optimal mass ratio from 1.2:1 to 1.3:1 between the positive and the negative electrodes was found in the investigated conditions. For the hybrid supercapacitor it was observed that the ruthenium-based positive electrode influenced the capacitive performance of carbon-based negative electrode and that an accurate balance of carbon loading in the negative electrode was necessary.     

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.     

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     

An electrochemical impedance spectroscopy study of fuel cell electrodes

Electrochemical impedance spectroscopy (EIS) was used to study the capacitance and ion transport properties of fuel cell catalyst layers. It was found that limiting capacitance correlates with active area. The capacitance per gram of catalyst was calculated and is proposed as a measure of catalyst utilization. Results obtained with catalyst layers immobilized on glassy carbon electrodes agree very well with results obtained with gas diffusion electrodes. EIS was also used to study ion conductivity and active area in fuel cell electrodes that contain the electroactive probe Os(bpy)₃²⁺. Together, these results validate the hypothesis that the non-ideal behavior of fuel cell electrodes is due to variations of conductivity across the layer, rather than variations in capacitance.     

An electrochemical study of reduced ilmenite carbon paste electrodes

The anodic dissolution of reduced ilmenite has been studied using the carbon paste electrode technique. To understand the polarization behaviour of reduced ilmenite, the polarization behaviour of iron oxide and iron powder carbon paste electrodes has also been investigated. It was shown that synthetic rutile and reduced ilmenite promoted the anodic dissolution of iron.     

Engineering concepts and technical performance of oxygen-reducing electrodes for batteries and electrochemical processes

PTFE-bonded carbon electrodes have been used successfully in alkaline and acidic fuel cell electrodes and as hydrogen depolarizing electrodes in chlor-alkaline systems. Multi-layered thin carbon electrodes can be produced by several manufacturing techniques: pressing, rolling and spraying methods can be applied. In all cases it is important that an optimal design of the diffusion-and working zone is achieved. Acetylene blacks, furnace blacks and graphites are used as carbon materials, mixed with PTFE and filler materials and evaluated in laboratory cells, before they are built into larger stacks. Compositions of the different mixtures for the fabrication of the electrodes are given and the construction of bipolar cells is discussed. The bipolar cell design is simple for mass production and the resulting fuel cell batteries should be of low cost.     

Non-uniform accessibility and the use of hydrodynamic electrodes for mechanistic studies: A comparison of wall-jet and rotating disc electrodes

The merits of non-uniformly accessible electrodes for discriminating between electrode reaction mechanisms are established. In particular a comparison of the theoretical behaviour of the uniformly accessible rotating disc electrode and the highly non-uniformly accessible wall-jet electrode towards a wide range of different types of electrode process shows that mechanistic resolution is better achieved with the latter electrode geometry.     

Kryptate complexes—adsorption at electrodes and their potential electrochemical use

The adsorption of the K⁺ complex of Kryptofix 222² or 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8,8,8]hexacosan, at the dropping mercury electrode was examined by ac polarography and drop-time measurements in 1 M potassium fluoride. The maximum surface coverage and diffusion coefficient of the complex were measured. A summary of potential uses in electrochemistry of the complex is given, and its usefulness as a cationic adsorbate demonstrated by its promotion, at rather low concentration, of the cathodic dissolution of lead as tetraethyl lead in a partly aqueous solution.     

A multiple-reflexion ellipsometric method for the study of electrochemical processes

The possibility of applying multiple-reflection ellipsometric measurements to the study of electrochemical processes occurring at a metal/electrolyte solution interface is discussed. Use is made of the relation between the ψ and δ parameters of elliptically polarized light after a single reflexion by a metal surface and the $\text{ψ}\text{?}$ and Δ parameters after repeated reflexions, for bare metal surfaces and those covered by a dielectric.