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.     

Electrochemical study of electroless deposition of Fe-P alloys

Fe-P deposits were prepared by electroless deposition and the inducing effect of coupled aluminum on Fe-P deposition was studied. The partial reactions of reducer oxidation and metal ions reduction in electroless deposition Fe-P bath were investigated by electrochemical investigation. In situ mixed potentials of the process for electrolytes with different pH were measured. The plating rate of iron alloys was measured gravimetrically and calculated by the deposition current density. The results show that coupled aluminum induces the Fe-P deposition by shifting the potential negatively and decreasing the polarization resistance of anodic and cathodic reaction. The test of the mixed potential theory was performed by comparison between direct experimental values of the mixed potential and plating rate with those derived theoretically from the current-potential curves for partial reactions. Due to the hydrogen evolution, the plating rate determined by electrochemical measurement is higher than the average plating rate determined from gravimetrical measurements.     

Design and Modelling of a Dialysis Membrane Electrochemical Reactor (D-MER) for Oxidoreductase-Catalysed Synthesis

The study is the first step in a design of an electrochemical reactor able to process enzyme catalysed electrochemical synthesis. To develop economically efficient synthesis, the enzymes must be confined very close to the electrode surface. Here, the confinement was achieved with a dialysis membrane integrated in a classic filter-press electrochemical reactor. A simple electrochemical reaction, without enzymatic catalysis, was used to model the behaviour of the dialysis membrane electrochemical reactor (D-MER). The values of the physicochemical parameters were determined by independent experiments, and the thickness of the reaction layer in the reactor was the only parameter to be adjusted numerically. The conditions of this adjustment were discussed. The model gave a good fit to the experimental data and consequently proved that the current design of the reactor is ready to be used with more complex reactions as enzymatic synthesis. A first example was described, based on the glucose oxidase-catalysed synthesis of gluconic acid.     

A kinetic study of the electrochemical hydrogenation of ethylene

In this study, we have examined the kinetics of the electrochemical hydrogenation of ethylene in a PEM reactor. While in itself this reaction is of little industrial interest, this reaction can be looked upon as a model reaction for many of the important hydrogenation processes including the refining of heavy oils and the hydrogenation of vegetable oils.To study the electrochemical hydrogenation of ethylene, several experimental techniques have been used including polarization measurements, measurement of the composition of the exit gases and potential step, transient measurements. The results show that the hydrogenation reaction proceeds rapidly and essentially to completion. By fitting the experimental transient data to the results from a zero-dimensional mathematical model of the process, a set of kinetic parameters for the reactions has been obtained that give generally good agreement with the experimental results. It seems probable that similar experimental techniques could be used to study the electrochemical hydrogenation of other unsaturated organic molecules of more industrial significance.     

阴极极化对微生物在电极表面附着的影响

以天然海水作为电解质,利用荧光显微镜、扫描电镜等表征方法和开路电位、恒电位极化等电化学测试手段研究了生物膜附着与电极表面电位的相互影响。结果表明：（1）生物膜在电极表面的附着导致了开路电位正移;（2）阴极极化能有效抑制微生物附着,并且阴极氧还原反应是阴极极化抑制微生物附着的主要原因;（3）搅拌等能促进阴极反应、增大阴极极化电流密度的措施都会在一定程度上利于阴极极化对微生物附着的抑制作用。     

Electrochemically induced reactions in soils—a new approach to the in-situ remediation of contaminated soils?: Part 1: The microconductor principle

Electrochemical reactions can be induced in soils if the soil matrix contains particles or films with electronic conducting properties (‘microconductors’). In these cases, the wet soil may act as a ‘diluted’ electrochemical solid bed reactor. A discussion of this reaction principle within the soil matrix will be presented here. It will be shown, that under certain conditions, immobile organic contaminants may be converted.     

Effect of pyridine and its derivatives on the electrodeposition of nickel from aqueous sulfate solutions. Part II: Polarization behaviour

The electrochemical reactions occurring during electrodeposition of nickel from acidic sulfate solutions were examined by cyclic and linear sweep voltammetry techniques. The effect of pyridine, 2-picoline and 4-picoline on the electrode polarization behaviour and electron transfer parameter of the cathodic reduction process was also investigated. Strongest electrode polarization was seen with 4-picoline. The order of cathodic polarization was 4-picoline > 2-picoline > pyridine. Kinetic parameters such as Tafel slope, transfer coefficient and exchange current density were determined to analyse the nature of the electrode reactions. The exchange current density for nickel deposition on nickel and stainless steel substrates was in the order 4-picoline < 2-picoline < pyridine.     

Modeling of an industrial scale hydrodynamic cavitation multiphase reactor for Prileschajew epoxidation

The hydrodynamic cavitation multiphase reactor (HCMR) is emerging as a promising alternative for the intensification of liquid–liquid heterogeneous reactions, but research on HCMR modeling is lacking. In this article, an HCMR model was developed using Prileschajew epoxidation as the model system. First, based on experimental measurements of oil/water two-phase flow downstream of hydrodynamic cavitation devices, semiempirical correlations were proposed to describe the droplet size and droplet size distribution (DSD) as functions of flow conditions and geometry parameters. Then, with boundary conditions calculated by the DSD correlation, a droplet dynamics simulation in a reaction tank was performed by computational fluid dynamics coupled with population balance model to obtain the two-phase interfacial area. Finally, the acquired reactor model was substituted into an overall kinetic model, to simulate the epoxidation reaction in HCMR. Model predictions were verified by experimental results measured on an industrial scale HCMR.     

Selectivity analysis in electrochemical reactors. II. Engineering models of a batch reactor with a complex reaction sequence

This paper presents a mathematical model of a batch stirred-tank electrochemical reactor where a required cathodic reduction reaction is coupled with a complex reaction sequence between the reactant and the key product. The set of coupled, non-linear differential equations is solved numerically and simple dimensionless quantities characterizing the cell performance and selectivity are derived. The experimental results presented in Part I of this paper are found to be in excellent agreement with the model. In the particular case where the homogeneous chemical reactions may be neglected in the cathodic diffusion boundary layer, a simplified analytical expression of the process selectivity is proposed. This quantifies the effects of the operating conditions by means of a single dimensionless criterion.     

Development of an integrated process for electrochemical reaction and chromatographic SMB-separation

A new continuous process combining electrochemical reaction and chromatographic simulated-moving-bed (SMB) separation is presented. To demonstrate the potential of process integration, this reactor concept is applied to the direct electrochemical production of arabinose by means of simulation. Experimentally verified models of reactor and column units are combined with a model of the electrochemical SMB-reactor. These process units are characterized individually and their model parameters are discussed. The electrochemical reaction inside the microreactor can be described by a series reaction, which has an impact on the design of the integrated process: as the reaction should take place in areas of the SMB-process with maximum educt concentration, the reactors are switched on and off during operation. The integrated process shows interaction between the reaction and the separation to diminish side reactions. Case studies prove the theoretical feasibility of the integrated process. Compared to a conventional process with a reactor followed by a SMB-separation, higher yields can be obtained.     

Exploring single electrode reactions in polymer electrolyte fuel cells

Utilising a pseudo-reference electrode in polymer electrolyte fuel cells allows for the separation of anodic and cathodic contributions to the entire cell impedance. Modelling the impedance responses by using equivalent circuits inhibits the investigation of kinetic parameters of the basic electrochemical reactions, which take place at single electrode-electrolyte interfaces. Therefore, we evaluate single electrode impedance measurements by a kinetic model, which is based on specific reaction pathways, either for the oxygen reduction reaction (ORR) or the hydrogen oxidation reaction (HOR). As a consequence, it is possible to obtain kinetic parameters for the specific reaction of interest. Furthermore, the information gained from the single electrode impedance measurements and the kinetic model can give insight into single reactions steps. In particular, the ORR has to include a chemical step in the reaction pathway.     

The electrochemical fluorination of propene

A study of the electrochemical fluorination of propene under rigidly controlled conditions has led to the development of a number of techniques which improve the reproducibility and chemical yields of the process. It has been shown that controlled conditioning of the anode was critical to the reproducibility of the reactions. Control of the anode potential within specific limits during fluorination was essential to minimize breakdown of the organic compounds. The experimental data are discussed within the context of the possible reaction mechanisms.     

Microelectrodes for corrosion studies in microsystems

Electrochemical techniques using microcapillaries as electrochemical cells allow to study local processes on metal surfaces. Since the usual large-scale electrochemical techniques provide only average data integrating over a large surface area (mm²–cm²), they are inadequate to investigate local corrosion processes. Due to the enhanced current resolution of the microelectrochemical methods down to pA and fA, processes in the micro- and nanometer range can easily be studied. This method allows to perform local electrochemical investigations of single areas of special interest, such as grain boundaries, second-phase precipitates or inclusions. Therefore, the microcapillary technique is well suited to study the corrosion behavior of microsystems. Results obtained from studies of localized corrosion processes of stainless steels are presented and discussed.     

Real time mapping of corrosion activity under coatings

Current accelerated testing of aircraft coating systems for corrosion protection relies heavily on salt spray methods. Electrochemical techniques such as electrochemical impedance spectroscopy (EIS) and electrochemical noise methods (ENM) provide insight into the global properties of a coating system, and both techniques are being used on a limited basis. However, there is a need to investigate corrosion events with greater spatial resolution under coatings at the metal/coating interface. Such corrosion activity may be related to coating defects and variations in the surface chemistry of the underlying metal.

The scanning vibrating electrode technique (SVET) has been developed to allow high spatial resolution investigation of localized corrosion activity that may be associated with coating defects or galvanic coupled regions of the metal surface. The SVET offers high resolution in current measurements of the order of 0.5 μA/cm² and is able to detect in-situ initiation and progress of corrosion activity under a protective coating. Using the SVET, minute variations in d.c. current associated with localized corrosion activity are detected and used to map both anodic and cathodic corrosion activities in a localized area. The difference in initial corrosion activity under various coatings can be correlated to the performance life of the coatings. The application of SVET to aircraft coatings and corrosion is reported to demonstrate the utility of this important new electrochemical tool.

In the current study, the SVET was used to discriminate the corrosion protection performance of selected sol–gel based coating systems. Sol–gel based surface treatments are being developed as part of an environmentally compliant coating system alternative to the currently used chromate-based systems. The SVET results are compared with data obtained from chromium inhibition coating systems. The SVET analyses are compared with electrochemical impedance measurements. The comparison of such data will provide the basis to adopt SVET measurements as an early performance discriminator for newly developed coating systems.     

Cathodic reaction kinetics and its implication on flow-assisted corrosion of aluminum alloy in aqueous ethylene glycol solution

The cathodic reaction kinetics and anodic behavior of Al alloy 3003 in aerated ethylene glycol–water solution, under well-controlled hydrodynamic conditions, were investigated by various measurements using a rotating disk electrode (RDE). The transport and electrochemical parameters for cathodic oxygen reduction were fitted and determined. The results demonstrate that the cathodic reaction is a purely diffusion-controlled process within a certain potential region. The experimentally fitted value of diffusion coefficient of oxygen is 3.0 × 10⁻⁸ cm² s⁻¹. The dependence of cathodic current on rotation speed was in quantitative agreement with Levich equation. At potentials more positive than the diffusion controlled region, the cathodic process was controlled by both diffusion and electrochemical kinetics. The electrochemical reaction rate constant, k ₀, was determined to be 1.1 × 10⁻⁹ cm s⁻¹. There is little effect of electrode rotation on anodic behavior of Al alloy during stable pitting. However, fluid hydrodynamics play a significant role in formation of the oxide film and the Al alloy passivity. An enhanced electrode rotation would increase the mass-transfer rate of solution, and thus the oxygen diffusion towards the electrode surface for reduction reaction. The generated hydroxide ions are favorable to the formation of Al oxide film on electrode surface.     

Corrosion of dissimilar alloys: Electrochemical noise

A new approach for interpreting the electrochemical noise generated by freely corroding electrodes is presented. For a single electrode, with well-defined anodic and cathodic regions, it is proposed that the instantaneous values of the corrosion current and open circuit potential are functions of the time-invariant potential difference between the anodic and cathodic reactions and the time-dependent impedances associated with the anodic and cathodic reactions. The fluctuations in the values of anodic and cathodic impedances are determined by the interaction between the aggressive environment and the oxide, hydroxide or adsorbed surface layer covering the anodic and cathodic regions; the instantaneous values of impedances modulate the corrosion current. The mathematical implications of these assumptions are discussed and the results are compared with the traditional approach, based on time-dependent current sources acting on constant impedances. Subsequently, the findings for a single electrode are transferred to two dissimilar electrodes and an experimental technique that enables the estimation of the time evolution of anodic and cathodic resistances for each of the dissimilar electrodes is presented.     

A comparative electrochemical study of iron deposition onto n- and p-type porous silicon prepared from lightly doped substrates

Electrodeposition of iron from acidic sulfate solutions onto porous silicon (PS) prepared from n- and p-type (1 0 0) substrates is studied by electrochemical measurements. Results from current-potential curves show that deposition of iron on p-type PS can only be achieved under illumination and cathodic polarization, whereas the deposition is found to proceed on n-type even in the dark. The measurements of the cathodic current efficiency indicate that the fraction of current used for iron deposition decreases with the applied potential due to hydrogen evolution reaction which is a competing reaction to metal deposition. Scanning electron microscopy shows that very fine iron crystallites with an average size of 40-70 nm are formed under double potential step conditions. The energy band diagrams of silicon-solution interfaces determined by electrochemical impedance measurements reveal that the iron deposition mechanism on both substrates is electron transfer from the conduction band.     

Corrosion inhibition of amorphous FeBSiC alloy in 1 m HCl by 3-amino-1,2,4-triazole

The mode of corrosion inhibition of amorphous FeBSiC alloy due to 3-amino-1,2,4-triazole (ATA) in molar HCl is studied through weight loss and electrochemical steady-state and transient measurements. From the comparison of results with those obtained using other triazole type-organic compounds, it is shown that ATA is the best inhibitor. ATA acts on the cathodic reaction without changing the mechanism of the hydrogen evolution reaction. Impedance studies show that ATA acts by formation of a 3D compact and protective adsorbed inhibition film at the metal surface which is similar to a paint or polymer film. A correlation between the inhibition efficiency of ATA and its molecular structure is established. A schematic representation of surface coverage due to different adsorption modes of ATA is presented. The schematic representation proposed explains why the mechanism of hydrogen reduction is the same in the absence and in the presence of ATA.     

Tomographic imaging during reactive precipitation in a stirred vessel: Mixing with chemical reaction

Electrical resistance tomography (ERT) allows the user to non-invasively ‘see inside their process’ through the manipulation and measurement of electrical properties enabling a powerful real-time visualisation of the time evolving three-dimensional conductivity distributions within the process unit. A 4-plane 16-sensor ERT array retrofitted to a 7.5 l stirred vessel has been used to rigorously interrogate the single feed semi-batch precipitation of barium sulphate, providing over 1000 spatially varying data points per ‘captured’ frame. A variety of reactant concentrations and agitation intensities were investigated. The results obtained reflect both the hydrodynamics and complex reaction kinetics involved with reactions and detail a number of very distinct regions during the experimental runs. This is achieved through the direct visualisation of the induced feed plume, quantification of the homogeneity (‘mixedness’) within the vessel, time evolving conductivity trends and a further analysis into the rates of conductivity changes as the reaction proceeds. For some experimental runs the predicted conductivity trends for a perfectly mixed state have been calculated using a conductivity-concentration correlation. ERT offers many spatially varying data points as opposed to point wise measurements which offers a significant improvement for the validation of mathematical models which attempt to deal with reactive crystallisation. As well as data collection, specifically for model validation, ERT may offer the means to control the spatio-temporal distributions of reactants and phases within the reactor to aid the suppression of unwanted by-products for industrial processes whilst offering a means to monitor the process unit to ensure the required mixing intensity is always achieved. Also included is an analysis of the mean volume diameter of the precipitate for each experimental run with scanning electron microscope (SEM) images.