Finite Volume model for diffusion- and activation-controlled pitting corrosion of stainless steel

Steel corrosion plays an adversary, but central role in different technological fields. Reasonable modeling of corrosion calls for a profound theoretical study of the underlying mechanisms. The present paper is concerned with mathematical modeling of (localized) pitting corrosion: We derive the mass conservation law of a dissolving body hosting a metal/solution interface which separates the solid metal electrode from the liquid electrolyte, and we complement the mass balance law by a thermally activated, potential-dependent electrochemical kinetics law for the dissolution reaction and by Fick’s law for ionic transport in the electrolyte solution. As long as the electrolyte solution adjacent to the electrode boundary does not reach its saturation level, the Arrhenius-type dissolution kinetics law governs the dissolution rate, and Fick’s law governs solely the concentration distribution in the electrolyte solution (activation-controlled corrosion mechanism). However, once the saturation level is reached at the electrode boundary, the pit depth evolution is governed by the diffusion of ions from the electrode boundary into the electrolyte-filled pit (diffusion-controlled corrosion mechanism). Corresponding mathematical solutions (time-dependent fields of concentrations) are obtained by means of the Finite Volume Method. For experimentally supported model input values (concerning dissolution activity, corrosion potential, transfer coefficient, metal charge number, ionic saturation concentration, and solid metal concentration), the influence of the overpotential on the corrosion characteristics (pit depth and shape evolution, current density, ionic concentrations in electrolyte) is shown by means of 1D and 2D simulations.     

Use of electrochemical microsensors for hydrodynamics study in crossing microchannels

The study of microfluidic systems is an important research challenge related to the design of microdevices for chemical processes. The understanding of physical phenomena, such as flow behaviour and heat and mass transfer performance is needed in order to develop these microsystems for industrial applications as mixers, reactors or heat exchangers. This work aims at characterizing two flow pattern behaviours, by using an electrochemical method, in a microdevice composed of crossing microchannels. A nonintrusive electrodiffusion method involving an electrochemical reaction of active species on an electrode flush-mounted into a wall is used to investigate wall shear stress. The measured limiting diffusion current is related to the wall shear rate in the vicinity of the electrode. The experimental cell consists of two crossing microchannels intersecting at right angle. Two channels sections are investigated, respectively 500 and 833 μm in hydraulic diameter. In each case, the influence of the crossing on the flow behaviour and on the mixing performance are characterized locally by using microelectrodes implemented at several positions on the wall of the channels located after the crossing. The experimental results are analyzed and a comparison with the results of CFD simulations using Fluent is performed.     

A reaction compiler for electrochemical kinetics

A computer code serving for an automatic translation of user-written source texts of electrochemical reaction mechanisms into corresponding target texts of mathematical equations that govern the kinetics of electrochemical systems under transient conditions is reported. The rules of the language enabling symbolic specification of the reaction mechanisms, the compiler options, and conventions regarding the target formulae are outlined and illustrated by examples. A considerable diversity of reaction mechanisms involving equilibrium, non-equilibrium reversible or irreversible reactions that can be electrochemical, heterogeneous non-electrochemical or homogeneous, is permitted. The reactions may involve bulk species (distributed in the electrolyte volume) and interfacial species (localized at the electrodes) of variable or constant concentrations, and electrons. The transient conditions may correspond to a number of electrochemical techniques, including potential-step method, linear potential scan voltammetry and chronopotentiometry. For kinetic problems in one-dimensional space geometry the generated governing equations take the general form of the reaction-advection-diffusion partial differential equations for the concentrations of bulk species (with initial and boundary conditions), optionally coupled with algebraic, ordinary differential or differential-algebraic equations for the concentrations of interfacial species. The governing equations can be obtained in the form of ELSIM problem definitions, enabling further solution by means of this simulation program.     

暂态电化学多组份气体传感器研究(I)多孔薄层电极库仑型气体传感器

常见的电化学气体传感器就其工作状态而言,大多属稳态类型,功能单一,传感器性能的提高受到“稳态”的制约,本文根据薄层电化学原理,提出并建立了一类全新的暂态电化学多组份气体传感器－多孔薄层电极库仑型气体传感器。这类以多孔薄层气体电极为核心元件构成的库仑型气体传感器,既有检测多种气体组份的功能,又有响应快、灵敏度高、低温度效应等优异性能。     

Modeling of redox electrochemical MHD and three-dimensional CFD simulations of transient phenomena in microfluidic cells

Computational fluid dynamics simulations of redox electrochemical magneto-hydrodynamics (MHD) covering a large parameter space have been done to establish the effects of redox species concentration, electrode area, and magnetic field strength and orientation on the limiting current. Simulations of milli-electrode and microelectrode systems show similarities and differences between the two. Potential sweep and potential step simulations using time-accurate solution algorithms enabled accurate capture of transient phenomena which is crucial to the development of lab-on-a-chip flow control applications. Power-law correlations reported in previous experimental investigations of milli-electrodes are found to be less accurate for microelectrodes and nano-electrodes suggesting possible effects of multidimensional diffusion. Further, the orientation of the magnetic field in relation to the electrode surface has a strong influence on the flow field. Interesting and complex flow features are observed in the post-processed still images and animations. This work advances redox MHD simulations by incorporating important aspects in the models that were not considered in previous models. Our simulations based on these new models provide new insight into redox electrochemical MHD.     

四电极电化学MEMS加速度传感器仿真

基于溶液相对于电极的运动会使电极附近因电势差作用而产生的离子浓度梯度发生改变,从而使电极电流发生变化等电化学原理,建立一种置于碘(I2)和碘化钾(KI)混合水溶液环境中的阳极—阴极—阴极—阳极(ACCA)四电极阵列传感单元模型。通过理论分析,该传感单元可以局部电解液为惯性质量,利用电解液中带电离子相对于电极的运动来产生并输出附加电流;进一步的建模仿真证明:在一定的加速度范围内(0a x0.149 m/s2),附加电流的大小同加速度呈正比关系,故可以将此电流放大输出,从而实现对加速度大小的表征。可利用MEMS工艺制作,并封装此传感单元成为加速度传感器。     

植酸钠修饰玻碳电极测定铅(Ⅱ)

采用涂覆法制备了植酸钠修饰电极,采用电化学方法研究了Pb2+在修饰玻碳电极上的电化学行为,对测定条件进行了优化。与裸玻碳电极相比较,Pb2+在修饰电极上的峰电流强度明显提高,该方法检出限低、分析速度快。在最佳实验条件下,Pb2+在1.2×10-6～1.2×10-5mol/L的浓度范围内与其峰电流呈良好的线性关系,检出限为8.0×10-7mol/L。该修饰电极制备简单,稳定性好     

PVP/Pd/IrO2电化学修饰电极的研究及其应用

本文研制了一种新型的 PVP/Pd/IrO_2电化学修饰电极。在 Pt 电极表面先后修饰 PVP(Poly VinylPyridine)及 Pd/IrO_2,该修饰电极对 HSO_3~-有良好的催化氧化作用;同时,在恒电位+0.6V 条件下,以该修饰电极与 Ag/AgCl 电极,Pt 对电极组成的气体传感器,对 SO_2也有良好的响应,预计在环境监测及环境保护等领域有应用前景。     

基于两电极体系的水质检测分析平台设计

为实现对水质的多参数检测,设计了一个基于两电极体系的水质检测分析平台。该检测平台通过最小化极化电流的思想,采用辅助电极面积远大于工作电极的电极参数,设计了基于两电极体系的集成电极和软硬件系统,摈弃了传统三电极体系的参比电极,同时克服了传统三电极体系容易污染样品的缺点,能实现电化学工作站的诸多功能。本研究在该检测平台上开展了水样的电导率(EC)、pH和重金属浓度的检测研究,获得了较好的结果,可用于实验室及工业现场的水质监测。     

Electrochemical properties of SWNT/ferritin composite for bioapplications

A functionalized single-wall carbon nanotube (SWNT), fabricated using an acid treatment, was used to prepare SWNT/ferritin composites. Different physical and chemical composites were synthesized on a glassy carbon electrode, and their structures and electrochemical properties were analyzed. The specific redox reaction due to ferritin appeared in both composites, but depended on the structure of the composite, which influenced the electrochemical properties. The redox reaction of ferritin was analyzed using fulfilled core, holoferritin, and coreless, apoferritin. From the electrochemical results, we confirmed that electron transfer through the ferritin shell is possible, and that the core of the ferritin facilitates electron transfer in the composites. The chemical composites showed a significant catalytic activity towards hydrogen peroxide, and the electrochemical results show that this type of composite has potential as biosensors and in bioapplications.     

Design of a highly sensitive and responsive electrode for particulate matter monitoring

In a previous sensor device based on a Pt|Sn_0.9In_0.1P₂O₇|Pt electrode system, electrochemical carbon oxidation was realized over the surface of a working electrode, enabling continuous carbon monitoring with self-regeneration of the sensor. Specific reactivity of the active oxygen species for carbon was demonstrated along with a high current efficiency. However, this reaction did not occur over the entire electrode layer, limiting the sensing properties and giving a low sensitivity and response speed. The main reason for carbon oxidation being limited to the surface region was the lack of proton conduction in the working electrode. To overcome this problem, we added proton-conducting Sn_0.9In_0.1P₂O₇ particles as an ionomer to the working electrode. The Sn_0.9In_0.1P₂O₇ ionomer was successfully distributed over the working electrode at the level of a few micrometers, providing reaction sites for carbon oxidation over the entire electrode layer. The resulting amperometric sensor provided a sensitivity to carbon that was 1.4 times greater than that of the ionomer-free working electrode, as determined by the current at which carbon oxidation had ceased. Moreover, carbon floating in a glass container could be transported to reaction sites present on the external surface of the working electrode, allowing for a large current and a rapid response.     

Development of a molecular imprinting thick film electrochemical sensor for cholesterol detection

A combined molecular imprinting and thick film electrochemical sensor for cholesterol concentration detection had been developed. The ferro-ferric cyanide coupled redox reaction was used as the means to quantify the cholesterol presented in the test medium. This electrochemical sensor employed a modified gold working electrode, a platinum counter electrode and an Ag/AgCl reference electrode. The alkanethiol was used to form the self-assembled monolayer (SAM) on the gold working electrode. The SAM was then used with the cholesterol as the template forming the molecular imprinting layer. This sensor prototype could detect cholesterol concentrations between 66 and 700 nM and only a 1 μL of the sample volume was required.     

Bioelectrochemical sensor based on PQQ-dependent glucose dehydrogenase

Bioelectrochemical responses of pyrroloquinoline quinone (PQQ) dependent glucose dehydrogenase (PQQ-GDH) have been studied with the use of two principal electrode configurations: (i) glassy carbon electrode, coated with Nafion layer, containing sorbed N-methylphenazonium (NMP), and overcoated by an enzyme layer, crosslinked by glutaraldehyde, and (ii) glassy carbon electrode, containing an electropolymerized layer of either Toluidine blue, or o-phenylenediamine, or pyrrole, and overcoated by a crosslinked enzyme layer. When operated within the potential window of 0.0–0.3 V, Nafion-coated and NMP-soaked bioelectrode shows an anodic current response to glucose without the presence of electron transfer mediator in solution. The current–concentration profile obtained resemble to that, typical for enzyme catalyzed reaction. Other configurations studied showed bioelectrochemical response to glucose only in the presence of soluble mediator NMP. Without any mediator, no electrochemical responses have been registered. It indicates that direct electron transfer between PQQ-GDH and all types of electrodes modified during current work is undetectable.     

基于二氧化锆纳米粒子固定乙酰胆碱酯酶的甲基对硫磷传感器

先在金电极表面电沉积二氧化锆纳米粒子并固定乙酰胆碱酯酶(AChE),将此电极浸入含有不同浓度的有机磷溶液中,根据电极在底物氯化乙酰巯基胆碱中电化学信号强度的大小来实现溶液中有机磷的定量检测.以甲基对硫磷为分析目标物,研究了传感器的主要响应特性、选择性及再生性能,考察了底物浓度、工作电位及溶液pH值对分析性能的影响.结果表明,该有机磷传感器在5.0×10-7～5.0×10-4 g/L浓度范围内对目标分析物有线性响应,检出限为1.0×10-7 g/L.该传感器灵敏度高,非特异性吸附小,再生性好,所用的二氧化锆纳米粒子层制备简单、操作方便,具有较大的应用潜力.     

Selective voltammetric determination of electroactive neuromodulating species in biological samples using iron(II) phthalocyanine modified multi-wall carbon nanotubes paste electrode

Iron(II) phthalocyanine (FePc) modified multi-wall carbon nanotubes paste electrodes (MWCNTPEs) were used as voltammetric sensors to selectively detect dopamine (DA) in the presence of serotonin (5-HT). The electrochemical behavior of DA at the new modified electrode was investigated using CV. The enhanced current response of DA indicates that FePc modification of the MWCNTPE surface results in a high catalytic activity for the redox reaction of DA. Differential pulse voltammetry was applied in detection of DA and 5-HT at FePc-MWCNTPE. The method parameters were optimized. Detection limit of 2.05 × 10⁻⁷ M was obtained for DA by using the electrocatalytic oxidation signal corresponding to the Fe^II/Fe^III redox process. The separation between the peak potentials of DA and 5-HT is 170 mV which is large enough for the simultaneously, selective determination of the two chemical species in their mixtures. There was no electrochemical response for ascorbic acid (AA) added in the sample. The monoamine neurotransmitter measuring method has been tested in analyzing deproteinized serum samples.     

ELSIM—a problem-solving environment for electrochemical kinetic simulations. Version 3.0-solution of governing equations associated with interfacial species,independent of spatial coordinates or in one-dimensional space geometry

A third version of the program ELSIM (for IBM compatible PCs) for the simulation of electrochemical transient methods has been elaborated upon. The program has been equipped with numerical algorithms that allow solution of kinetic problems characterized by the presence of interfacial species. Problems of this kind play an important role in electrochemical kinetic studies, among others in connection with electrocatalysis, modified electrodes or oscillatory systems. The user-written governing equations may take the form of differential—algebraic equations (for the concentrations of interfacial species) independent of spatial coordinates, or partial differential equations (for the concentrations of bulk species) in one-dimensional space geometry, coupled with differential—algebraic equations. Problem formulation has also been facilitated by the inclusion of a reaction compiler, which automatically generates the text of the above governing equations for user-written reaction mechanisms. Other new features include an option for the least-squares fitting of simulated transients to experimental curves, a hypertext help facility, and enhanced performance owing to the revised formula translation based on the three-address internal code generation, and on automatic differentiation.     

Parylene-based implantable Pt-black coated flexible 3-D hemispherical microelectrode arrays for improved neural interfaces

In implantable medical systems, low-impedance electrode-tissue interface is important for maintaining signal quality for recording and effective charge transfer for stimulation. In this paper, we propose a novel hemispherical biocompatible and flexible microelectrode arrays (MEAs) which were fabricated by the process of micro electrical mechanical system (MEMS). Compared with conventional planar microelectrodes, the interface impedance of hemispherical microelectrodes decreased due to their increased surface area. Parylene C thin film with good biocompatibility and flexibility was chemical vapor deposited as packaging material for decreasing nerve tissue damage. Pt-black coatings were electroplated by applying current pulses in H₂PtCl₆ solution on electrode sites for the further decrease of interface impedance. Moreover, the geometrical and electrical properties of these MEAs were demonstrated by using a scanning electron microscope (SEM) and an electrochemical workstation. Experimental results showed that the interface impedance decreased by about 34% compared with conventional planar microelectrodes, and significantly decreased by 84% with Pt-black coatings on electrode sites compared with those uncoated microelectrodes.     

一种光电滴定反应器的设计与应用

滴定反应器是滴定法高锰酸盐指数分析仪的核心部件,目前高锰酸盐指数分析仪大多使用电极法滴定反应器,电极需要浸入反应液,电极信号容易受反应液的干扰,电极液也需要定期更换,维护不方便.为了解决电极法滴定反应器缺点,用光电方法检测反应液颜色的变化,并基于该方法设计光电滴定反应器,由光源发射光线穿透反应液,接收器检测透光度的变化...     

Topology optimization for the design of flow fields in a redox flow battery

This paper presents topology optimization for the design of flow fields in vanadium redox flow batteries (VRFBs), which are large-scale storage systems for renewable energy resources such as solar and wind power. It is widely known that, in recent VRFB systems, one of the key factors in boosting charging or discharging efficiency is the design of the flow field around carbon fiber electrodes and in flow channels. In this study, topology optimization is applied in order to achieve optimized flow field designs. The optimization problem is formulated as a maximization problem for the generation rate of the vanadium species governed by a simplified electrochemical reaction model. A typical porous model is incorporated into the optimization problem for expressing the carbon fiber electrode; furthermore, a mass transfer coefficient that depends on local velocity is introduced. We investigate the dependencies of the optimized configuration with respect to the porosity of the porous electrode and the pressure loss. Results indicate that patterns of interdigitated flow fields are valid designs for VRFBs.     

Synergistic effects of micro/nano modifications on electrodes for microfluidic electrochemical ELISA

Microfluidic electrochemical sensing has been considered to be highly efficient. However, we showed, by using numerical simulations in this study, that a planar electrode formed on the bottom of a microchannel is exposed to only a small fraction of analytes in amperometric detection. We also showed that three-dimensional (3D) micropillar electrodes significantly improve the detection current. The practical performance was evaluated using 3D micropillar electrodes fabricated by photolithography. The output current increased as the diameters of the micropillars decreased, as predicted by the simulations. It is noteworthy that the current enhancements obtained with the 3D electrodes were larger than those expected from an increase in the surface area. Further increase in current was achieved by electrical deposition of nanoporous gold-black onto the surface of the 3D electrode: when a 3D electrode with micropillars 30 μm in diameter was used, the output current was approximately 20 times that obtained with a 2D electrode without modification. The applicability of the micropillar electrodes was demonstrated in electrochemical enzyme-linked immunosorbent assay (ELISA) of bone metabolic marker proteins. Although an increase in the surface area of the electrode leads to more noise in general, there is no significant difference in the signal-to-noise ratio between the modified 3D electrode and the 2D electrode without modification in the ELISA experiments. This nanoporous micropillar electrode could potentially be a useful component for the development of on-site diagnosis systems.