Computation of Current Distributions using FEMLAB

An efficient method for the computation of current density and surface concentration distributions in electrochemical processes is analyzed using the commercial mathematical software FEMLAB. To illustrate the utility of the software, the procedure is applied to some realistic problems encountered in electrochemical engineering, such as current distribution in a continuous moving electrode, parallel plate electrode, hull cell, curvilinear hull cell, thin layer galvanic cell, through‐hole plating, and a recessed disc electrode. The model equations of the above cases are considered and their implementations into the software, FEMLAB, are analyzed. The technique is attractive because it involves a systematic way of coupling equations to perform case studies.     

A novel pulse shortcut strategy for simulating nano-second pulse electrochemical micro-machining

Nano-second pulse electrochemical micro-machining is essentially employed in high precision metal processing applications. Simulation wise, such a process involves heavy time-accurate calculations necessary to obtain the confined anodic current density distribution, crucial for the machining copy accuracy. A new alternative simulation method is presented, entitled the Pulse Shortcut Strategy (PSS), that avoids the entire time-accurate procedure and significantly reduces the computational effort and runtime. The PSS relies on calculating a current density Correction Factor (CF) that is based on the local electrolyte resistance, the interface polarization and double layer (DL) properties in combination with the nano-second pulse characteristics. The same confining effect is achieved by simulating a computationally cheap stationary current density distribution and altering it locally through the space-dependent CF. When the system has constant electrolyte resistivity, constant DL capacitance and linear polarization, the CF calculation reacts in full agreement with the time-accurate simulation results for any pulse on-time/off-time combination. The PSS accuracy is compared with full time-accurate simulations and represents a mutual validation method between the two. These results offer promising perspectives for the simulation of nano-second pulse electrochemical micro-machining, making it more attractive from a practical point of view once the general interest in the technology emerges. The approach can be generalized to other pulsating electrochemical systems.     

Multiple physical field coupling simulation of high current lithium electrolyzer in industry

The electric field, thermal field, and velocity field in a 60 kA diaphragmless lithium electrolyzer are studied by multiple physical field coupling simulations. The current density distribution, the balance of heat generation, heat dissipation, and the flow field distribution of the lithium electrolyzer are discussed. The velocity distribution is solved by the Euler–Euler two-phase flow model, and the turbulent flow of electrolyte is solved by the k–ε method. The results show that in an electric-heat field, there is a constant ratio of current loss, and the current density distribution is not uniform. The electrolyte temperature is evenly distributed, wherein thermal radiation plays a dominant role. In addition, it is found that the velocity distribution of the end-side interpolar space and the inside interpolar space is different. The influence of electrode distance in the range of 30~60 mm on the flow field is investigated, and the dominance of the electrode distance effect and the eddy current effect is analyzed. The optimal range of electrode distance is 40~50 mm.     

A primary, secondary and pseudo-tertiary mathematical model of a chlor-alkali membrane cell

A theoretical study of current density and potential at the anode, membrane and cathode, of a chlor-alkali membrane cell where the electrode blades are placed vertically, is presented. A representative unit cell is modelled in primary, secondary and pseudo-tertiary current distribution models. It is shown that electrolyte and membrane resistance has the greatest effect on current distribution. Furthermore, it is shown that there is a surprisingly small influence of mass transport on current distribution, on the assumption that the diffusion layer is of constant thickness. In converse to this, it is shown that mass transport affects the anode overpotential distribution to the extent that conclusions can be made about the occurrence of side-reactions and where they occur. Finally, it is shown that it is possible to estimate tertiary behaviour with a secondary current distribution model, by using an analytic expression at the anode surface.     

Placement of reference electrode in solid electrolyte cells

The placement of reference electrodes in solid state ionic conductors is not as flexible as in liquid state electrochemistry. This is in particular a problem when material from the gas phase is involved, as in solid oxide fuel cell. Many of the arrangements used are problematic: either they produce results that are very sensitive to electrode placement, change the potential distribution, do not provide a uniform current density and overpotential at the electrode or require delicate patterns liable to fail. We here present a new approach suitable for thin layer SOFC. It includes a calibration procedure derived from numerical simulations in combination with experiments. This allows the use of the common three-electrode arrangement on thin solid electrolyte (SEs) where the reference electrode is placed side by side with the working electrode, on an extension of the thin layer SE. This is so despite the sensitivity of that arrangement to both misalignment of the electrodes and to a difference in the impedance of the two current carrying electrodes. The misalignment tolerated, with the present method, may exceed the SE thickness. The allowed misalignment increases with the electrode/SE impedance ratio. The method copes also with the difference in the electrode impedance. Two special configurations are discussed in which the calibration is not required. However, these require a more accurate preparation technique of the cell.     

直接液流燃料电池的制备、活化及性能

采用热压方法制备阳极半膜电极（AHME）用于直接液流氧化还原燃料电池（DLRFC）单体中,研究了热压过程的条件参数对AHME性能的影响。为了使组装后的DLRFC达到最佳性能,采用高温水、恒电流放电以及变电流放电3种方式对其进行活化,研究了恒电流活化过程中活化条件对DLRFC电化学性能的影响。结果表明,最适宜热压条件是120 ℃,以200 kg/cm²的热压压力保压120 s左右。对于DLRFC单体而言,采用放电活化的方式优于高温水活化。恒电流活化方式中的最适宜活化时间、活化温度和活化电流密度分别是3 h、60 ℃和20 mA/cm²。     

Calculation of ohmic resistance effects in rectangular electrodes of finite thickness

The current distribution in electrochemical cells consisting of parallel rectangular plates is determined. The calculations involve the evaluation of the appropriate analytical solution of Laplace's equation within the electrodes and electrolyte, with boundary conditions corresponding to potential continuity (primary current distribution) or linear electrode kinetics (secondary current distribution) at the electrode-electrolyte interface, and do not make the usual assumption that current flow in the resistive electrode is one-dimensional. The current distributions are given in the form of Fourier series that allow the effects of electrode resistance and electrical contact geometry to be determined.     

Uniform Thin Film Electrodeposition onto Large Circular Wafer Substrates

A wafer-scale electrochemical flow cell specially suited for uniform thin-film electrodeposition is presented. The cell consists of a porous disk injector set parallel and coaxial to a working disk electrode with a narrow separation gap between them. The porous injector was designed to supply a uniform axial flow of electrolyte onto the surface of the working disk electrode. The mass transfer characteristics of the cell and the current distribution on the working disk electrode were studied for various operating conditions, geometric parameters and bath chemistries. The experimental results confirmed the simple scaling properties that were theoretically predicted. It has also been shown that this cell can provide nearly uniform current distribution on the working electrode under conditions of both migration and convective-diffusive mass transfer control. The results indicate that this cell is suitable for uniform thin-film electrodeposition onto large circular wafer substrates.     

Using electrochemical impedance spectroscopy to compensate for errors when measuring polarisation curves during three-electrode measurements of solid oxide fuel cell electrodes

The use of three-electrode techniques involving an independent reference electrode is invaluable in determining the overpotential losses at solid oxide fuel cell (SOFC) electrodes. However, there are numerous barriers to achieve the accurate measurement of such overpotentials in an SOFC. Furthermore electrochemical impedance spectroscopy (EIS) is commonly used to analyse the processes occurring on SOFC electrodes, and there has been considerable work in establishing viable three-electrode techniques for EIS experiments under open circuit conditions. However, the three-electrode techniques currently developed for EIS experiments are not well suited for conditions of high load, or changing gas compositions; either intentionally or under diffusion limiting conditions. This paper reports a solution for commonly used pellet cells, which mitigates these problems. The paper presents a method using EIS to correct for errors when measuring the working electrode overpotential during polarisation arising from a shift in the electrolyte current distribution from the primary to the secondary current distribution under load. This technique enables meaningful overpotentials to be calculated using experimentally simple cell geometries under conditions where they cannot normally be accurately measured.     

Determination of kinetic parameters for borohydride oxidation on a rotating Au disk electrode

Borohydride oxidation has been investigated using a rotating disk electrode technique. The parameters, such as apparent rate constant, Tafel slope, Levich slope, number of electrons exchanged and reaction order, have been determined. The borohydride ion is oxidised on the gold electrode with an electrochemical rate constant of around 1 cm s⁻¹ at intermediate potentials where side reactions had less effect. Influences of temperature, concentrations of borohydride and supporting electrolyte (NaOH) on the parameters were evaluated.     

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.     

A simple model for the vanadium redox battery

A two-dimensional stationary model, based on the universal conservation laws and coupled with electrochemical reactions, is applied to describe a single all-vanadium redox flow cell. Emphasis is placed on studying the effects of applied current density, electrode porosity and local mass transfer coefficient on the cell performance. The model results indicate that bulk reaction rate depends on the applied current density. The transfer current density and over-potential increase almost twice as the applied current density doubled. A decrease in electrode porosity leads to a more rapid depletion of the reactant concentration, a higher integral average value of the transfer current density and a more uniform distribution of the over-potential. The local mass transfer coefficient only affects the value of the over-potential.     

木质素基活性炭氮掺杂改性及其电化学性能

以磷酸法木质素基活性炭为原料, 三聚氰胺为氮源、KOH为活化剂, 采用同步掺杂方式制备了氮掺杂活性炭(NAC)。通过BET、XRD、拉曼光谱和XPS表征手段测试了改性后活性炭的结构及其组分, 并通过电化学表征手段, 测试了其作为超级电容器电极材料在几种不同性质电解液中的性能, 初步探究了电解液对电极材料电化学性能的影响机制。实验结果表明: 改性后的活性炭具有丰富的孔结构, 比表面积达到2 332 m²/g, 微孔孔容为1.37 cm³/g, 中孔孔容为0.74 cm³/g, 平均孔径为2.79 nm, 含氮元素7.5%, 其中类石墨型氮(N-Q)结构达到34.6%。丰富的孔结构和氮含量大幅提升了活性炭的电化学性能, 其在水系电解液中展现出了高比电容, 在1 A/g的电流密度下比电容最高可达424 F/g; 在有机系电解液中, 尽管其在1 A/g的电流密度下比电容最高仅为87 F/g, 由于其工作电压窗口更宽(0~2.5 V), 因此具备了更高的能量密度。对结果进行分析, 发现: 活性炭电极材料在水系电解液中的性能主要受电解液水合离子半径影响, 而在有机系电解液中的性能主要受电解液黏度的影响。     

电沉积方法制备镍硫析氢电极及其电化学性能

采用恒电流电沉积方法制备Ni-S电极,通过极化曲线研究了硫脲质量浓度、电流密度、镀液温度、电沉积时间等对Ni-S电极析氢性能的影响,获得了较佳的制备工艺:NiSO4·6H2O187.2g/L,硫脲100g/L,H3BO340g/L,NaCl 20g/L,pH=4,电流密度30 mA/cm2,镀液温度55℃和电沉积时间1...     

Nano-structured composite cathodes for intermediate-temperature solid oxide fuel cells via an infiltration/impregnation technique

Solid oxide fuel cells (SOFCs) are high temperature energy conversion devices working efficiently and environmental friendly. SOFC requires a functional cathode with high electrocatalytic activity for the electrochemical reduction of oxygen. The electrode is often fabricated at high temperature to achieve good bonding between the electrode and electrolyte. The high temperature not only limits material choice but also results in coarse particles with low electrocatalytic activity. Nano-structured electrodes fabricated at low temperature by an infiltration/impregnation technique have shown many advantages including superior activity and wider range of material choices. The impregnation technique involves depositing nanoparticle catalysts into a pre-sintered electrode backbone. Two basic types of nano-structures are developed since the electrode is usually a composite consists of an electrolyte and an electrocatalyst. One is infiltrating electronically conducting nano-catalyst into a single phase ionic conducting backbone, while the other is infiltrating ionically conducting nanoparticles into a single phase electronically conducting backbone. In addition, nanoparticles of the electrocatalyst, electrolyte and other oxides have also been infiltrated into mixed conducting backbones. These nano-structured cathodes are reviewed here regarding the preparation methods, their electrochemical performance, and stability upon thermal cycling.     

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.     

A phenomenological approach to ionic mass transfer at rotating disc electrodes with a hanging column of electrolyte solution

Experimental data for the behaviour of a rotating disc electrode with a hanging electrolyte column of varying height are presented. A correlation involving the limiting current density, the rotation speed, and the physicochemical properties of the solution is established. The macroscopic effective copper electrodeposit thickness distribution is determined. Results obtained for different electrode designs are discussed in terms of an additional flow of reactant, and an apparent change in the effective electrode area caused by liquid column contraction. From the results the most suitable experimental conditions for the application of the rotating disc electrode with a hanging column of electrolyte to electrochemical kinetic studies can be found.     

Structure investigations of SOFC anode cermets Part I: Porosity investigations

The number of electrolyte/metal/pore three-phase boundaries in the SOFC anode cermet was determined in a cross-section photo according to the intercepted-segment method and given as the volume concentration of the electrochemically active phase boundary. The apparent geometrical exchange current density of hydrogen oxidation served as a relativizing measure of electrochemical activity. A direct correlation was found between the number of optically visible three-phase boundaries of electrolyte/nickel agglomerates/gas pores' in the unit volume of the electrode and the apparent electrochemical exchange current density. The electrode was also analysed porosimetrically by a standard porosimetry method (MSP) and the porosimetric curves were compared with the optical/electrochemical measurements of the same electrode. This comparison revealed that the sum of the integrated volume of nickel agglomerates and the single nickel particles in contact with the electrolyte give the total amount of the active nickel volume whose surface is directly proportional to the amount of optically visible three-phase boundaries in the unit volume.     

Efficient electrochemical synthesis of 2-arylsuccinic acids from CO2 and aryl-substituted alkenes with nickel as the cathode

A simple and efficient electrochemical route was developed by using non-noble metal nickel as the cathode and aluminum or magnesium as an anode for the synthesis of 2-arylsuccinic acids from carbon dioxide and aryl-substituted alkenes. The electrochemical dicarboxylation of aryl-substituted alkenes and carbon dioxide could be smoothly carried out in an undivided cell containing n-Bu₄NBr-DMF electrolyte with a constant current under 4 MPa pressure of CO₂ at room temperature in the absence of additional catalysts, and the corresponding 2-arylsuccinic acids were afforded in moderate to good yields (50-87%) and high selectivity (98%). The influence of some key factors (such as electrode materials, supporting electrolyte, substrate concentration, current density and CO₂ pressure) on the results of the electrochemical synthesis was investigated, and the electrochemical reaction mechanism was also briefly discussed.     

电氧化法脱除烟气中二氧化硫的研究

以Br2/Br-为氧化还原介质进行电化学法烟气脱硫,介绍了脱硫的原理及工艺流程,研究了电极材料、溴与二氧化硫的量比、电解液温度及电流密度等因素对脱硫过程的影响。结果表明,阳极材料宜选用DSA钌钛,阴极材料宜用镍基合金,最佳的n（Br2）：n（SO2）是1-1．5。电解液温度为55～65℃,电流密度对整个电解池的槽电压影响很大。