Mass transfer in amperometric gas sensors

The mass transfer in amperometric gas sensors intended for atmosphere monitoring is studied theoretically and experimentally. External and internal constituents of the diffusion resistance (DR) of the sensors are determined. The external constituent is defined by conditions of convective diffusion of air under analysis relative to the sensor. The internal constituents are defined by parameters of construction of sensor elements, and electrolyte film, as well as the structure of the indicator electrode, and solubility of the gas under analysis in the electrolyte. Testing a chlorine sensor shows that the DR of the internal constituents is independent of the conditions of convective diffusion of the environment under analysis near the sensor. The similarity criterion of sensors of different types is shown to be the relative share of DRs of individual constituents in the overall DR of the sensor. The results obtained can be used for the development and design of sensors with required range and resolution.     

Mass transfer to tubular electrodes: ECE process

The mathematical model of mass transport for linear sweep voltammetry under hydro-dynamic conditions at tubular electrodes has been studied for ECE processes in which an irreversible chemical reaction is coupled between two reversible charge transfer reactions. The resulting boundary value problem is converted into system of two integral equations, which is solved numerically. The effects of axial flow rate, scan rate, potential difference, variation of chemical reaction rate and the effect of the ratio of number of electrons (n ₂/n ₁) involved in two charge transfer reactions on CV-voltammograms are investigated and shown graphically.     

Electric mass transfer of sodium chloride through cation-exchange membrane MK-40: A rotating membrane disk study

An experimental device consisting of a rotating membrane disk with horizontally positioned cation-exchange membrane MK-40 is described. The device’s design makes it possible to simultaneously obtain current-voltage curves (CVC) and dependences of effective transport numbers for ions of electrolyte and water dissociation products on the current density. Partial CVC are calculated and limiting current densities and diffusion layer thickness are determined at various disk rotation rates. At current densities below the limiting value, the disk’s CVC obey regularities of electrodiffusion kinetics. Upon raising the current density further, the salt ion fluxes increase due to a decrease in the effective diffusion layer thickness, which is caused by the emergence in the near-membrane region of a space charge and electroconvection. At high current densities there occur oscillations of the potential jump that are caused by hydrodynamic instability of the near-membrane solution layer.     

The Influence of Viscosity on Diffusion at the Stationary Electrode

Abstract

The influence of viscosity on diffusion at the stationary electrode is studied. Relations between the maximum electrolysis time attainable without any disturbance by natural convection, the thickness of the diffusion layer and solution viscosity are presented and discussed in the case of constant current electrolysis at shielded electrodes. A re-examination of data existing in literature is made.     

Are steady state diffusion layer thicknesses potential independent?

The required conditions are considered for the potential-independence of steady state diffusion layer thickness in an electrochemical experiment. Such conditions are illustrated by numerical solution of the mass transport equations for an electroactive species at the microdisc and tubular flow electrodes. Published in Russian in Elektrokhimiya, 2008, Vol. 44, No. 1, pp. 134–141. The text was submitted by the authors in English.     

Mass transport in gas channels of electrochemical cells with a solid-oxide electrolyte: Allowing for the electrode reaction rates in numerical calculations

The concentration fields, Nusselt number distributions along the electrode, and ratios of concentrations of electrochemically active components at the entrance into and exit out of an electrochemical cell are determined by numerical integration of the convective-diffusion equation for a broad range of rate constants of electrode reactions, Peclet numbers, and geometrical parameters of channels. Three regions of parameter values are revealed. At low reaction rates (K _l < 0.5), mass transfer is completely determined by electrode kinetics; at high rates (K _l > 20), it is completely determined by the diffusion in the gas phase; and in the intermediate region of values of K _l it is necessary to allow for both the diffusion processes and the electrode reactions.Translated from Elektrokhimiya, Vol. 41, No. 3, 2005, pp. 317–324.Original Russian Text Copyright © 2005 by Ezin, Somov.     

低信膜比条件下金膜电极电位溶出分析法研究

推导并验证了信号与金膜厚度的比值较小（即低信膜比）时金膜电极电位溶出分析法的过渡时间方程式和电位时间方程式。实验确定的理论式的应用条件为τ／ｌ２≤５８×１０１０ｓ／ｃｍ２，并发现当扩散路径约为扩散层厚度的３倍时，便可按半无限扩散条件处理扩散问题。从而解决了利用小信号进行金膜电极电位溶出基本问题，即信号与浓度的定量关系。     

Determination of the mass transfer coefficients in direct immersion solid‐phase microextraction

Diffusion of the analytes across the diffusion boundary layers and subsequently through the fiber coatings determines the extraction kinetics of solid‐phase microextraction in aqueous matrices. Besides, the matrix effects can distort the behaviors of the analytes transferring across the diffusion boundary layers. However, these processes were always studied via certain simplification, which often left the mass transfer through the fiber coatings unconsidered and the matrix effects partially investigated. Herein, a comprehensive study on the mass transfer processes in direct immersion solid‐phase microextraction was presented. Under different agitation speeds, it was determined that the mass transfer coefficients across the diffusion boundary layers were three to six orders larger than those through the fiber coatings. However, the mass transfer across the diffusion boundary layers was generally the major rate‐limiting step. In addition, the shuttle effect and the barrier effect, which were responsible for accelerating and retarding the extraction kinetics, respectively, were found to be the dominant matrix effect alternately under different agitation speeds. This study comprehensively illustrated the major rate‐limiting step and the dominant matrix effects through recording the mass transfer coefficients.     

Determining Solubility and Diffusivity by Using a Flow Cell Coupled to a Mass Spectrometer

One of the main challenges in metal–air batteries is the selection of a suitable electrolyte that is characterized by high oxygen solubility, low viscosity, a liquid state and low vapor pressure across a wide temperature range, and stability across a wide potential window. Herein, a new method based on a thin layer flow through cell coupled to a mass spectrometer through a porous Teflon membrane is described that allows the determination of the solubility of volatile species and their diffusion coefficients in aqueous and nonaqueous solutions. The method makes use of the fact that at low flow rates the rate of species entering the vacuum system, and thus the ion current, is proportional to the concentration times the flow rate (c?u) and independent of the diffusion coefficient. The limit at high flow rates is proportional to . Oxygen concentrations and diffusion coefficients in aqueous electrolytes that contain Li⁺ and K⁺ and organic solvents that contain Li⁺, K⁺, and Mg²⁺, such as propylene carbonate, dimethyl sulfoxide tetraglyme, and N‐methyl‐2‐pyrrolidone, have been determined by using different flow rates in the range of 0.1 to 80 μL s^?1. This method appears to be quite reliable, as can be seen by a comparison of the results obtained herein with available literature data. The solubility and diffusion coefficient values of O₂ decrease as the concentration of salt in the electrolyte was increased due to a “salting out” effect.     

T形微通道中气泡分散流的传质性能

在T形微通道中,以错流剪切的分散方式实现了微米级分散气泡的制备,并以NaOH水溶液吸收CO2为对象,考察了气．液微分散体系的分散规律和传质性能．通过考察两相流速对气泡分散尺寸的影响,建立了预测气泡形成尺寸的数学模型．根据气泡的初始分散尺寸、流动阶段的体积变化以及传质完成后的尺寸,首次测定和区分了气泡形成阶段和运动阶段的传质量,建立了原位测定气泡分散流传质系数札的方法,并考察了两相流量对札的影响．结果表明,由于微通道中气泡的形成时间很短,形成阶段的传质量在总传质量中所占的比例很低．气泡分散流的传质系数主要受液相流量的影响,气相流量的影响基本可以忽略．基于实验结果,建立了计算传质系数鼠的无因次准数关联,计算结果与实验结果符合良好．     

析气电极的传质过程研究

采用指示离子法研究了析气电极的传质过程。在Ｎｉ／ＫＯＨ体系中发现：ｌｇδ和ｌｇｉ的曲线斜率对于氢气泡为０．２８７，对于氧气泡则为０．５８３。用激光衍射法和激光多普勒法（ＬＤＡ）获得的气泡大小及气泡上升速度数据检测了析气电极传质过程的流体动力模型及渗透模型。结果表明，因析气效应产生的传质强化现象的机理一般不是单一的。     

Concentration changes in near-electrode layers during electrolysis of copper(II) sulfate and potassium dicyanoaurate: A spectrophotometric study

A spectrophotometric method is suggested for measuring concentration changes in near-electrode layers. The method is based on the interaction between a monochromatic light beam, traversing the electrochemical reaction zone, and metal ions. Concentration changes in the near-cathode layer during electrolysis of copper(II) in sulfate electrolytes and potassium dicyanoaurate in phosphoric electrolytes are studied.     

应用于锌-空气电池的新型碳载钴-聚噻吩复合催化剂

采用化学氧化聚合法合成了以碳为载体的钴-聚噻吩复合物(Co-PTh/C)作为气体扩散电极氧还原催化剂。 通过扫描电子显微镜-能量色散X射线能谱（SEM-EDX）、透射电子显微镜（TEM）等测试技术对催化剂进行表征。 结果表明,Co-PTh/C催化剂颗粒的粒径为10~30 nm,且分布均匀。 利用极化曲线、交流阻抗等电化学方法测试了其在碱性介质中(6 mol/L KOH)对氧还原的催化性能。 此催化剂在碱性介质中空气气氛条件下,电极电位在-0.20 V(vs.Hg/HgO)时电流密度达到0.152 A/cm2,催化性能高于质量分数5%Pt/C,显示出优越的氧还原电催化性能。 采取催化层/集流体/扩散层的排布方式,以纯锌为负极,6 mol/L的KOH为电解液,将气体扩散电极与锌负极组装成锌-空气电池。 电池以0.075 A/cm2进行恒流放电,放电电压为1.1 V且性能稳定。     

Temperature dependence of the electronic factor in the nonadiabatic electron transfer at metal and semiconductor electrodes

The temperature dependence of the electronic contribution to the nonadiabatic electron transfer rate constant (k_ET) at metal electrodes is discussed. It is found in these calculations that this contribution is proportional to the absolute temperature T. A simple interpretation is given. We also consider the nonadiabatic rate constant for electron transfer at a semiconductor electrode. Under conditions for the maximum rate constant, the electronic contribution is also estimated to be proportional to T, but for different reasons than in the case of metals (Boltzmann statistics and transfer at the conduction band edge for the semiconductor versus Fermi–Dirac statistics and transfer at the Fermi level, which is far from the band edge, of the metal).     

Electrochemical reactions at the electrode/solution interface:Theory and applications to water electrolysis and oxygen reduction

Theoretical simulations on complex electrochemical processes have been developed on the basis of the understanding in electrochemistry,which has benefited from quantum mechanics calculations.This article reviews the recent progress on the theory and applications in electrocatalysis.Two representative reactions,namely water electrolysis and oxygen reduction,are selected to illustrate how the theoretical methods are applied to electrocatalytic reactions.The microscopic nature of these electrochemical reaction...     

微流控分析芯片的两种液相传质模式及其应用*

微流控分析芯片的微米级结构不仅显著增大内部流体的比界面积,同时缩短微通道内不同溶液间的传质距离,使传质效率相比于宏观体系有显著提高,从而可实现试样分析检测前的高效扩散分离和萃取富集等。本文综述了微流控分析芯片中两种液相传质模式——互溶液相间扩散分离分析、不互溶液相间萃取分离分析的研究进展,讨论了上述传质模式在微芯片装置和功能的集成化方面的应用;并讨论了相关研究的难点和发展趋势。     

Direct electron transfer reaction of glucose oxidase at bare silver electrodes and its application in analysis

The direct electron transfer reaction of glucose oxidase (GOx) at a bare silver electrode is verified. The electron transfer number n = 2, electron transfer coefficient α = 0.45 and rate constant of the electrochemical reaction K_s = 0.1 s⁻¹ are obtained. This communication presents a multimolecular adsorption model to explain the properties of the direct electron reaction between GOx and bare silver electrodes. The residual valence force may be an important factor to ensure a direct electron transfer reaction on the bare electrode. On the basis of the experimental fact that only biologically active GOx exhibits electrochemical activity in solution, a facile analytical method for analyzing the active GOx concentration is developed. The results determined correspond very well to that of a spectrometric method.     

Toward efficient catalysts for electrochemical CO2 conversion to C2 products

With impressive progress in carbon capture and renewable energy, carbon dioxide (CO₂) conversion into useful chemicals has become a potential tool against climate change. Electrochemical CO₂ conversion into C₂ products (ethylene and ethanol) is an especially economically promising approach and an active research area. Nonetheless, catalyst layer design for CO₂ conversion is challenging because of the complex CO₂-to-C₂ reaction pathways. In this review, we highlight key ideas in catalyst layer design for CO₂ conversion to C₂ in the past few years. We identify three fundamental principles to control catalyst selectivity—local CO₂ and CO concentration, local pH, and intermediate–catalyst interaction. To achieve these goals, we introduce design strategies for both catalytic materials and overall catalyst layer morphology.     

Diffusion boundary layers during electrodialysis

A laser interferometry method for direct measurements of thicknesses of diffusion boundary layers is described. The method is employed to explore electromembrane processes. Thicknesses of diffusion boundary layers at below and above limiting diffusion current densities are measured and compared with theoretical values. It is shown that the diffusion boundary layers in narrow channels of electrodialyzer sections overlap one another at high current densities. This radically alters traditional notions concerning the diffusion boundary layer. An interpretation for variations in the thickness of a diffusion boundary layer at current densities in excess of the limiting diffusion current density is given.     

活性氯在316L不锈钢电极表面的阴极行为研究

利用电化学方法研究了活性氯在3.5%NaCl溶液中316L不锈钢电极表面的阴极还原反应及其反应机制.实验表明,随着溶液中活性氯浓度的增加,316L不锈钢的自腐蚀电位正移,阴极反应的极限扩散电流明显增大,说明活性氯对316L不锈钢电极表面的两个阴极反应,即HClO还原和ClO-还原具有明显的促进作用.本文的研究确定了产生这两个还原反应的电位范围,并进一步探索了该还原反应速度的控制步骤.