Boundary Condition for Electrochemical Growth of Metal Crystal Nucleus in the Mixed Kinetics Conditions

A formula derived for an ion–metal electrode yields the near-electrode concentration of ions-reactants, the concentration overvoltage, and the charge transfer overvoltage from the overvoltage and current density (at the known exchange current density) without explicit use of the diffusion coefficient or the limiting diffusion current.     

启动工况下质子交换膜燃料电池动态性能仿真分析

本文以等效电路模型为基础模型,结合动态气体压力模型和动态热传输模型建立了集总参数模型,在SIMULINK环境下,利用建立的模型模拟了电池启动过程,发现了电压的下冲现象,且电压的响应时间与电池温度的响应时间基本一致,说明启停过程中电池温度对电池的动态性能影响很大. 进一步从温度角度对模型中决定电池输出电压大小的热力学电动势、活化过电势、欧姆过电势和浓差过电势的动态响应情况进行了分析,发现启动过程电压的下冲现象主要由电池活化过电势和欧姆过电势的过冲引起;当以阶跃信号形式输入温度时,启动过程电池输出电压响应很快且未发生下冲现象,说明提高电池温度的响应速度能够改善电池的动态性能.     

Alloy electrodes with high hydrogen overvoltage for analytical use in voltammetry. Some preliminary results

Liquid mercury and liquid diluted mercury amalgams have been the major electrode systems employed in voltammetry and related methods. This is mainly due to their high overvoltage to hydrogen, which enables the determination of heavy metals (zinc, nickel, cobalt, etc.) and other species with high negative half-wave potentials; the toxicity of mercury and liquid diluted mercury leads to ever increasing restrictions in their use. The use of such systems may even be forbidden in the future, at least in online systems for work in the field. Recent work, carried out in our laboratory, has demonstrated that a non-toxic solid dental amalgam may be used as the electrode material, conveniently replacing mercury. An extension of this work has shown that electrode materials comprising a metal or a compound with low hydrogen overvoltage change their hydrogen overvoltage properties substantially when contaminated with even small amounts of metals or compounds which show high hydrogen overvoltage. This extends greatly the range of potentially available electrode systems and thereby analytical possibilities of voltammetry. This new discovery also makes it possible to produce solid electrodes that have high overvoltage to hydrogen without any use of mercury.     

Reversible Solid Oxide Fuel Cell for Power Accumulation and Generation

The anodic and cathodic polarization dependences for the oxygen electrode based on lanthanum-strontium manganite and the fuel Ni-cermet electrode are studied in the temperature range of 700–900°С in gas media that correspond to working conditions of a reversible fuel cell. The temporal behavior of these electrodes is studied in the course of periodic polarity changes of current with the density of 0.5 A/cm². The electrode overvoltage is shown to be about 0.1 V in modes of power generation and water electrolysis at 900°С and the current density of 0.5 A/cm². A single electrolyte supported tubular solid-oxide fuel cell was fabricated and tested in the fuel-cell and hydrogen-generation modes. It is found that at 900°С and overvoltage of 0.7 V, the cell generates the specific electric power of 0.4 W/cm² when the 50% H2 + 50% H₂O gas mixture is used as the fuel and air is used as the oxidizer. At the water electrolysis with the current density of 0.5 A/cm², which under normal conditions corresponds to generation of about 0.2 and 0.1 L/h of hydrogen and oxygen, respectively, the consumed power is about 0.55 W/cm². The efficiency of the conversion cycle electric power–hydrogen–electric power is 70–75%.     

对流扩散传质滞后的电极过程中之非Poisson涨落与非Nernst浓度极化

根据对流扩散传质滞后的恒稳电极过程中边界层的物理图像, 提出了该类电极过程的简化随机模型, 建立了相应的浓度极化的随机热力学理论, 揭示了非Nernst浓度极化来自于随电流密度增大电极化学反应体系涨落分布的非Poisson化与对中心极限律的偏离, 进一步阐明了与滞后的扩散步骤共存的对流传质对非Nernst浓度极化的效应及其规律. 同时, 给出了对流引起的非Nernst浓度极化的随机热力学算例.     

Operation of an Oxygen Gas Diffusion Electrode in a Hybride Fuel Cell

The paper is devoted to the operation of a hybride fuel cell (i.e. in the course of simultaneous electrosynthesis of a chemical product), in particular, the polarization characteristic of an oxygen electrode and the ratio of the produced electric energy to the amount of the produced target product. An equation for the dependence of the polarization of an oxygen gas diffusion hydrophobized electrode of a hydrogen-oxygen fuel cell on the concentration of the produced electroactive soluble product H₂O₂ in an inner-kinetic regime is suggested. It is established that the polarization variation does not depend on its initial magnitude but does depend on the concentration of the target product and the ratio between currents of the side and target reactions. The ratio of the produced electric energy to the amount of the produced target product is found to depend on the concentration of the produced target product, the electrode polarization, and the ratio between exchange currents of the side and target reactions.     

Chlorine Evolution on Highly Porous Metal Oxide Anodes and the Origin of the Low-Polarizability Portion in Polarization Curves at Large Currents

Experimental and theoretical data on the chlorine evolution in the region of large anodic currents, where a low-polarizability portion emerges in polarization curves, are exhaustively analyzed. It is shown that this practically horizontal portion must emerge upon reaching an overvoltage at which practically all macropores, rather than only the largest of them, are filled with gas. The dramatic increase in the current, observed in this case, is connected with the chlorine evolution reaction penetrating through the entire depth of the coating and results from the formation of a unified system of gas channels in the porous space of the coating. Chlorine evolved in micropores moves via these channels at a high speed to the front side of the electrode, moving away from it in the form of gas bubbles.     

Electroreduction of molecular oxygen on dispersed copper in an ion-exchange matrix

Electrochemical reduction of molecular oxygen was studied on a [dispersed copper]-[macroporous KU-23 15/100S sulfocation exchanger with various metal concentrations] composite electrode. It was found that a high proton concentration in the ion-exchange matrix causes a decrease in the oxygen reaction overvoltage. The nanostructured state of copper particles causes stabilization of the intermediate product, i.e., hydrogen peroxide. Using the rotating disk electrode method, it was detected that the process is limited by external diffusion of oxygen to composite grains. The oxygen reaction is mostly concentrated on the grain surface and surface layers; oxygen is reduced in the bulk due to dispersed copper oxidation.     

Distribution of a Soluble Product During Its Electrosynthesis in the Porous Hydrophobic Electrode

The distribution of the concentration of a product, which is soluble in the electrolyte, inside a porous hydrophobic electrode is calculated. During the electrosynthesis under inner-kinetic conditions, the electrode obeys regularities observed in hydrophobic electrodes of fuel cells. The product distribution depends mainly on the polarization at the electrode front, the electrolyte conductivity, and the diffusion coefficient, and slightly depends on the electrode structure. An approximate equation describing the distribution of the product concentration inside the electrode is suggested.     

全钒液流电池高浓度下V(IV)/V(V)的电极过程研究

采用循环伏安、低速线性扫描和阻抗技术, 以石墨为电极, 研究了V(IV)/V(V)在较高浓度下的电极过程. 结果表明, 采用2.0 mol•L-1 的V(IV)溶液时, H2SO4浓度低于2 mol•L-1, V(IV)/V(V)反应极化大, 可逆性差, 表现为电化学和扩散混合控制; H2SO4浓度增至2 mol•L-1以上, V(IV)/V(V)反应的可逆性提高, 转为扩散控制, 且增加H2SO4浓度有利于阻抗的降低; 但H2SO4浓度超过3 mol•L-1, 溶液的粘度和传质阻力大, 阻抗反而增大. 在3 mol•L-1的H2SO4中, 随着V(IV)浓度的增加, 体系的可逆性和动力学改善, 阻抗减小; 但V(IV)浓度超过2.0 mol•L-1, 较高的溶液粘度导致溶液的传质阻力迅速增加, V(IV)/ V(V)的电化学性能衰减, 阻抗增大. 因此, 综合考虑电极反应动力学和电池的能量密度两因素, V(IV)溶液的最佳浓度为1.5～2.0 mol•L-1, H2SO4浓度为3 mol•L-1.     

Gas-Generating Porous Electrodes: Large Ohmic Resistances

It is shown how it becomes possible, in gas-generating porous electrodes (GGPE), to construct polarization curves (PC) that encompass also the region of high ohmic resistances up to an ultimate possible overvoltage that is reached at practically complete expulsion of the electrolyte out of the electrode pores near the front surface of the electrode, without restricting oneself, while taking into account ohmic limitations, by not-too-high values of overvoltages. As the true appearance of the dependence of the magnitude of an effective specific ionic electroconductivity of GGPE on the specific features of the structure of its porous space and the degree of occupation of its porous space by the electrolyte and gas is unknown, an attempt is undertaken to evaluate possible values of limiting ohmic limitations, specifically, to take into account the presence of both maximum and minimum ohmic losses. Calculations of PC and some other characteristics of GGPE are conducted using the constants that are close to the constants that take place for the process of formation of chlorine in dimensionally stable anodes. It is discovered that, if one selects a nonoptimum structure of the porous space of the electrode (this means that, with the emergence of gas pores in the electrode, ohmic losses would rapidly increase), then one fails to realize the values of the electrochemical activity of the electrode in excess of a few A cm^–2.     

Theory of Frequency-Dependent Polarization of General Planar Electrodes with Zeta Potentials of Arbitrary Magnitude in Ionic Media: 2. Applications and Results from Homogeneous and Array Systems of Electrodes

The expressions obtained in the previous paper for electrode polarization are applied to a homogeneous planar electrode and a planar array of electrodes used in the generation of nonuniform fields. The effective far field experienced outside the double layer is computed for both electrodes, and sample spectra are provided. The effective far field expression contains the electrode impedance and the effects of concentration polarization due to the static double layer on the electrode generated by the ζ potential. The effective far field results are compact and contain simple integrals that can be evaluated numerically.     

Electrochemical characterization of Ni-Co and Ni-Co-Fe for oxidation of methyl alcohol fuel w ith high energetic catalytic surface*简

 Non Pt based metals and alloys as electrode materials for methyl alcohol fuel cells have been investigated with an aim of finding high electrocatalytic surface property for the faster electrode reactions. Electrodes were fabricated by electrodeposition on pure Al foil, from an electrolyte of Ni, Co, Fe salts. The optimum condition of electrodeposition were found out by a series of experiments, varying the chemistry of the electrolyte, pH valve, temperature, current and cell potential. Polarization study of the coated Ni-Co or Ni-Co-Fe alloy on pure Al was found to exhibit high exchange current density, indicating an improved electro catalytic surface with faster charge-discharge reactions at anode and cathode and low overvoltage. Electrochemical impedance studies on coated and uncoated surface clearly showed that the polarization resistance and impedance were decreased by Ni-Co or Ni-Co-Fe coating. X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX) and atomic absorption spectroscopy (AAS) studies confirmed the presence of alloying elements and constituents of the alloy. The morphology of the deposits from scanning electron microscope (SEM) images indicated that the electrode surface was a three dimensional space which increased the effective surface area for the electrode reactions to take place.     

The influence of cathodic pretreatment on the kinetics of hydroxide ion oxidation on polycrystalline gold electrode

The electrochemical oxidation of the hydroxide ion was studied on a gold rotating disc electrode (RDE), in aqueous NaOH solutions in the presence of lithium perchlorate as a supporting electrolyte. By potentiodynamic polarization within the limits −1.6 V and +1.6 V vs. SCE, it was demonstrated that the overvoltage of the OH⁻ ion oxidation reaction may be significantly reduced with a 5 min long delay at the vertex cathodic potential of −1.6 V. This finding was explained in terms of the type of gold oxide formed on the gold surface under different experimental conditions.     

Adsorption,Thermodynamic, and Experimental Studies of Corrosion Inhibitor of Tin in 0.2 M Maleic Acid by Hydrogen Phosphate Ions (HPO42−)

The inhibition efficiency of H₂PO₄²⁻ ions against tin corrosion in 0.2 M maleic acid is studied using electrochemical methods, surface analytical methods, and thermodynamic analysis. The potentiodynamic polarization plots showed the presence of an active/passive transition state of the tin electrode. The EIS measurements confirmed that the inhibition efficiency of H₂PO₄²⁻ increased by increasing the concentration (η=81 % at C_inh=2.10⁻² M) and decreased by rising the temperature. The polarization tests demonstrated that the inhibitor performs as a cathodic-type. The adsorption of the inhibitor was spontaneous and followed the Langmuir adsorption isotherm. A model of the inhibition mechanism was suggested.     

Gas-Generating Porous Electrodes: Calculating Characteristics in the Low-Polarizability Portion

It is shown in the paper how specific features of the structure of the porous space in gas-generating porous electrodes (GGPE) create conditions for the emergence of a low-polarizability portion (LPP) in a polarization curve (PC). A set of equations is derived, which allows one to perform calculation of PC in the initial part of LPP. In the region of small overvoltages and in the initial part of a low-polarizability portion, calculations of basic characteristics of GGPE are performed. The calculations are carried out with the values of constants that are close to those that take place for the process of chlorine generation in dimensionally stable anodes. The shape of PC is established, together with the distribution of the oversaturation of electrolyte solution with gas over the GGPE thickness, the specific liquid surface area at which the electrochemical process of gas generation occurs, the amount of gas in the pores of the porous electrode, the overvoltage dependence of the oversaturation of electrolyte solution with gas at the front surface of GGPE, and the magnitude of the effective penetration depth of the process of gas formation into the porous electrode.     

Implementation of a Chloride‐selective Electrode Into a Closed Bipolar Electrode System with Fluorimetric Readout

Applicability of a bipolar electrode system was tested for arrangements containing a typical ion‐selective electrode (ISE) connected with an electrode coated by a conducting polymer characterized by electroluminescence. In this case a selective response of the ISE membrane at one pole of the bipolar electrode is transduced to a fluorimetric signal obtained by reduction of the conducting polymer at the second pole. This signal transformation mode was studied on example of a simple closed bipolar electrode system composed of all‐solid‐state chloride‐selective electrode with polypyrrole transducer as the sensing pole and the reporting pole represented by electrode coated by poly(3‐octylthiophene) (POT) layer characterized by fluorescence in the neutral state. In this system selective and linear dependences of fluorimetric signal on logarithm of chloride ions concentration in turn‐on mode were recorded for optimized external voltage applied. Alternatively, a concept of cascade bipolar electrode system with incorporation of additional bipolar electrode being a polarization source for the sensing bipolar electrode with ISE and POT layer was also tested. A significant advantage of the cascade system is its possibility to work spontaneously without external polarization. For this case also linear calibration plots of fluorimetric signal vs. logarithm of analyte concentration were recorded.     

Electrochemical characterization of MnO2 as electrocatalytic energy material for fuel cell electrode

 Development of inexpensive non Pt based high electrocatalytic energy materials is the need of the hour for fuel cell electrode to produce clean alternative green energy from synthesized bio alcohol using biomass. MnO₂, electro synthesized at different current density is found to be well performed electrocatalytic material, comparable to Pt, with higher current density, very low overvoltage for the electrochemical oxidation of methanol. From EIS study, the polarization resistance of the coated MnO₂ is found to be much low and electrical double layer capacitance is high, the effect increases with increase in current density of electro deposition. XRD, EDX and AAS analysis confirm the MnO₂ deposition. The morphology of SEM images exhibits an enhanced 3D effective substrate area, for electro oxidation of the fuel. A few nano structured grains of the deposited MnO₂ is also observed at higher current density. The fact supports that a high energetic inexpensive electro catalytic material has been found for fuel cell electrode to synthesis renewable energy from methanol fuel.     

Optimizing Weights of Platinum in the Active Layer of the Cathode of a Hydrogen–Oxygen Fuel Cell with a Solid Polymer Electrolyte

The active layer of the cathode of a hydrogen–oxygen fuel cell with a solid polymer electrolyte is computer simulated. The active mass of the electrode consists of substrate grains (agglomerates of carbon particles with Pt particles embedded into them) and grains of a solid polymer electrolyte (Nafion). The substrate grains presumably contain hydrophobic pores, which facilitate the oxygen penetration into the active mass. A calculation of characteristics of such an electrode focuses on the optimization of platinum weights. The principal parameters of the system are concentration and size of grains of substrate and Nafion, Pt concentration in substrate grains, average diameter of hydrophobic pores in substrate grains, and the electrode polarization. The optimum, at a given electrode polarization, electrochemical activity of the active layer, its thickness, and the platinum weight are calculated. A link between these quantities and principal parameters of the active layer is revealed.     

Detection of NADH and Ethanol at Titanium Containing MCM‐41 with Low Overpotential

Titanium‐containing MCM‐41 (Ti‐MCM‐41) modified glassy carbon electrode (GCE) can exhibit an excellent electrocatalytic activity towards the oxidation of β‐Nicotinamide adenine dinucleotide (NADH). A dramatic decrease in the overvoltage of NADH oxidation reaction is observed at 0.28 V vs. SCE. The application in the amperometric biosensing of ethanol using alcohol dehydrogenase enzyme (ADH) also has been demonstrated with this material. The proposed sensor shows a highly sensitivity, an acceptable reproducibility and a good stability. The linear range of ethanol is 25–1000 μM and the detection limit is 8.0 μM. Ti‐MCM‐41 modified electrode not only can be used to detect the concentration of NADH in biochemical reaction, but also as the potential matrix for the construction of dehydrogenases sensor.