Gas-Generating Porous Electrodes: Effect of the Porous Space Structure on Polarization Curves

The operation of gas-generating porous electrodes (GGPE) is studied qualitatively and quantitatively in the entire range of overvoltages. The range comprises a low-overvoltage region, a transition region, and a low-polarizability region. The first region extends to the instance when first gas pores form at the rear surface of GGPE. In the second region, the formation of a gas channel begins. This is the channel through which gas is removed from GGPE by filtration. The formation of the gas channel is completed when first gas pores emerge at the front surface of GGPE. In the third, gas is removed by a powerful process, specifically, by gas filtration in gas pores. The current in the last region increases very rapidly. The presented theory reveals basic parameters that determine the nature and principal features of polarization curves (PC). In all the regions, PC are calculated with constants close to those typical for chlorine generation in DSA. It is shown how the porous space structure (pore types, distribution of pores by size in the presence of micropores and macropores, maximum and minimum pore radii) and parameters that characterize outer-diffusion limitations (processes that occur in the electrolyte chamber) define all characteristics of GGPE and, which is more, define the shape of PC. Using results of this mathematical modeling of gas generation and removal in porous electrodes and electrolyte chamber, one can start looking for ways to purposefully alter the porous space structure of GGPE (in particular, DSA) in order to subsequently optimize characteristics of DSA and GGPE of other types.     

Gas-Generating Porous Electrodes: Calculating Complete Polarization Curves

A complete set of equations that allow one to estimate the maximum electrochemical activity of gas-generating porous electrodes (GGPE) is derived. The equations are valid if all stages that restrict the electrochemical process of gas generation are removed. The equations also allow one to calculate polarization curves throughout the entire range of variations in the overvoltage, provided no ohmic limitations are present in the system. The derivation is based on the lattice model describing the gas and electrolyte distributions over the GGPE thickness. The model was tested in previous works of the authors. Other basic characteristics of porous electrodes are calculated as well. These include, the overvoltage dependences of the oversaturation of electrolyte with gas, the electrolyte and gas porosities, the electrolyte-wetted specific surface area that takes part in the electrochemical gas evolution process, the average distance between neighboring pores, etc. The calculation makes use of constants close to those inherent in the chlorine generation process on DSA. The calculated polarization curves exhibit a clearly pronounced low-polarizability portion, which is the region of a rapid current increase in GGPE, which follows the linear Tafel portion. The presented theory gives one a chance to point out ways to change the structure of the porous space of GGPE and, in particular, DSA, in order to optimize characteristics of DSA and GGPE of other types in the future.     

Gas-Generating Porous Electrodes: The Nature of the Low-Polarizability Portion in the Polarization Curves

The phenomenon of a low-polarizability portion (LPP) is discussed in the paper. The phenomenon is responsible for the possible rapid increase in the current in polarization curves (PC) for the gas generation in gas-generating porous electrodes (GGPE). A fresh viewpoint on the nature of LPP is propounded. The inflection in PC, which follows the linear Tafel portion, owes its inception to the emergence, in the electrode pores, of a network of gas pores that are freed of electrolyte and are connected with each other. The effective diffusion coefficient for gas molecules in gas pores filled with water vapor is larger than the effective diffusion coefficient for the same gas molecules in liquid pores filled with an electrolyte solution by several orders of magnitude. The emergence of a gas phase in a GGPE leads to a rapid increase in the effective diffusion coefficient. This circumstance in turn is capable of rapidly intensifying processes of the formation and removal of gas, which leads to a considerable increase in the overall current. A method for obtaining an approximate solution of the problem (the notion on ideal porous electrode) is suggested. A system of equations is derived, which can sufficiently accurately, qualitatively and quantitatively describe the character of variations in a polarization curve in the initial part of a low-polarizability portion. Theoretical and experimental polarization curves for the chlorine evolution on a dimensionally stable anode are compared quantitatively.     

碱性燃料电池氧电极的研究：助催化剂的添加对Ag／C催化剂活性的影响

本文报道了一种新的用于碱性燃料电池氧电极的非贵金属催化剂．此种以化学还原法制备的Ａｇ－Ｎｉ－Ｂｉ－Ｈｇ－／Ｃ催化剂对氧电极的阴极还原具有较高的活性，当催化剂的组成为Ａｇ５０％－Ｎｉ２％－Ｂｉ３％－Ｈｇ３％－Ｃ４２％（ｗｔ％）时，催化剂的活性最佳．ＸＲＤ和ＳＥＭ证实，助催化剂的加入使银的结晶趋于无定型化，减小了银结晶的尺寸．经过５２００ｈ的寿命考察，催化剂活性没有明显的改变．     

Calculating Electrochemical Activity of Regular-Structure Porous Electrodes with an Immobilized Enzyme

A porous electrode of regular structure with an immobilized enzyme is studied. The electrode carcass, which consists of substrate particles, is a system of two sets of mutually perpendicular planes crossing one another (cellular structure). A monomolecular layer of enzyme molecules is deployed on the inner surface of such a porous substrate. In the center of each cell of the substrate gas pores, which are cylinders of porous grains of a hydrophobizing agent one grain thick, are situated. The rest of the cell space is filled by a solid polymer electrolyte. The ultimate goal of calculations is to estimate the electrochemical activity of such an electrode. The estimation is done for an oxygen electrode with an enzyme whose characteristics are close to those of laccase. The calculation assumes that active centers of enzyme molecules undergo a direct, i.e. without participation of mediators, reduction. It is shown that at an overvoltage of 30 mV, it would be possible to obtain a current density of 0.44 A cm^–2 in an electrode 16 m thick.     

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.     

Calculating the Electrochemical Activity of Porous Electrodes of a Filled-up Type with an Immobilized Enzyme

The electrochemical activity of porous electrodes of a filled-up type with an immobilized enzyme is calculated with the aid of computer-aided modeling. The percolation properties of two-component (a mixture of dispersed particles of the substrate and the enzyme) and three-component (a carrier of a gas reactant is added) models of a porous electrode are investigated. Taking into account specific features inherent in the forming macroscopic clusters (collections of particles of one sort or another connected with one another), i.e. an electron cluster and a gas cluster, makes it possible to determine the concentration of an active catalyst which is capable of taking part in the electrochemical process. The calculation of the electrochemical activity is performed in two-component structures, where the process of the current generation is limited by diffusion limitations, and in three-component structures, where the process of the current generation is limited by ohmic limitations. Estimates of the current are performed using an oxygen porous electrode with Nafion and an enzyme as an example. The electrochemical characteristics of this model electrode are close to those obtained on laccase. Introducing a hydrophobizing agent into the active layer of the porous electrode (passing from two-component structures to three-component structures) produces a positive effect. Although the number of active molecules of the enzyme drops in this case by an order of magnitude, the liquidation of diffusion limitations eventually raises the magnitude of the current by approximately threefold. Calculations show that it is quite feasible to obtain currents of 0.2 A cm^–2 on a porous electrode 16 m thick at an overvoltage of 30 mV.     

Gas-Generating Porous Electrodes: Allowing for the Ohmic Limitations

Traditionally, it is assumed, in calculations of gas-generating porous electrodes (GGPE), that ohmic limitations are infinitesimally small and can be ignored. This assumption is valid only at small overvoltages, when all the electrode's pores are filled with electrolyte. At sufficiently high overvoltages, gas pores start to form in GGPE, the amount of electrolyte in the porous space diminishes, and in principle, ohmic limitations can arise. In this communication, we obtain all formulas required for calculating polarization curves and other characteristics of GGPE throughout the entire range of overvoltages, including high overvoltages, at which ohmic limitations may arise. A particular calculation is performed with constants close to those typical for the chlorine generation in DSA. At high overvoltages, ohmic limitations substantially decrease the amount of gas generated in GGPE; the higher the overvoltage, the heavier the decrease.     

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.     

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.     

多孔电极电池的循环伏安法模拟

锂离子电池的全电池建模模拟对现代新能源领域的发展至关重要。伪二维（P2D）电化学模型是最常使用的全电池模拟模型,但一直被用于输入为电流,输出为电压的模拟中。本文基于P2D模型,通过对内电位、电极电位以及电池端电压的详细讨论,首次采用电压边界条件,利用COMSOL仿真软件完成了实验中常用的两电极体系和三电极体系的循环伏安法建模和模拟。并对比分析了两/三电极体系中扫描速率、颗粒半径、电极锂扩散速率以及最大嵌锂浓度这四个参数对循环伏安曲线形状的影响。结果表明,循环伏安测试时扫描速率越大,循环伏安曲线的峰值电流越大;固相锂扩散速率越大、电活性颗粒半径越小、最大嵌锂浓度越大,峰值电流越大。在相同的测试条件下,三电极体系比两电极体系的循环伏安图对称性更好,电流响应更大,并且颗粒半径、锂扩散速率及最大嵌锂浓度这三个参数对峰值电流的影响也更为明显。     

The Effect of Rare Earth on the Activity of Methanol Synthesis Catalyst

AlthoughtheCuO/ZnO/Al=O,cataIysthasbeenusedasasuccessfulmethan0lsynthesiscatalystfromCOhydrogenationformanyyears,itscatalyticactivityisnothighenoughf0rmethan0Isynthesisfr0mpureCO,hydrogenati0n.Manynewsynthesismethods,includingsol-gelco-precipitati0nmethodandultrasonicsynthesismeth0d,wereusedtopreparethemethanolsynthesiscatalyst"'.Intheliterature,manymetal0xidessuchasZrO,,Cr,O,,Ga,O,,Al,O,,SiO,andTiO,etc.wereaddedintocatalystt0improveitscatalyticactivityandmethanoIselectivity'~'.Howev…     

Electrodes of Synthetic Diamond Compacts Added with Platinum: The Platinum Effect on Electrochemical Activity

Russian Journal of Electrochemistry - Conducting diamond composite electrodes are prepared by converting graphite into diamond at high pressure and high temperature in two- and three-component...     

Fractals and Percolation in the Theory of Porous Electrodes

A computer-aided simulation of the structure of a porous electrode is performed using flat lattices of sites (they are capable of conducting electrons and are randomly distributed in the electrode) as an example. To adequately describe properties of a porous electrode, information about the degree of dispersion of the particles that make up the electrode (fractal dimensionality) must be complemented by that on their clusterization (presence of percolation clusters). These factors impart two properties to a porous electrode, specifically, a developed surface, on which an electrochemical process may proceed, and the possibility of a continuous supply of electrons to this surface. A percolation cluster may be dismembered to a trunk (it provides for the electron transport) and a crown (aggregate of particles that make a major contribution to the electrochemical process). The dismembering was performed via computer flow diagrams proposed by the authors. A computer-aided analysis of characteristics of a porous electrode points to the existence of an optimum structure in which the electrochemical activity is capable of reaching a maximum.     

对甲基苯酚在不同催化剂电极上的电氧化

在无隔膜电解槽中, 利用线性伏安法和恒电流电解法研究了Ti/PbO2、石墨和Pt电极对于对甲基苯酚电氧化的催化活性, 通过电解前后溶液中对甲基苯酚及其氧化中间产物的液相色谱测定, 比较了对甲基苯酚在3种电极上的转化和降解速度, 并讨论了对甲基苯酚电氧化降解的历程和速控步骤. 研究结果表明, Ti/PbO2电极能有效地催化氧化水溶液中的对甲基苯酚, 并将其彻底去除, 电极活性较高; 石墨电极也能够催化氧化水溶液中的对甲基苯酚, 但效果较差; Pt电极在3.0 h的电解时间内, 只能将对甲基苯酚转化成对苯二酚, 而不能将其完全矿化. 对甲基苯酚电氧化降解需要经过对羟基苯甲醇→对羟基苯甲醛→对羟基苯甲酸→对苯二酚→对苯醌→顺丁烯二酸→草酸, 最终生成二氧化碳和水的历程. 当以Ti/PbO2作阳极时, 对苯二酚转化为对苯醌和顺丁烯二酸转化为草酸两步反应为较慢的速控步骤; 当以石墨作为阳极时, 对苯二酚转化为对苯醌的反应为速控步骤.     

Effect of the Polymeric Matrix of a Catalyst on Its Activity in Hydrogenation

The swelling degree of ion-exchange resin AB-17-8-OH and palladium-containing catalysts based on this resin were studied. The effects of the diameter of polymeric support grains and temperature on the activity of the catalysts in model reactions of nitrobenzene and -nitronaphthalene hydrogenation were investigated. It was found that the catalyst preparation procedure and the particle size appreciably affect the rates of these reactions.     

Effect of Metal Inclusions on the Porous Structure and Photocatalytic Activity of Titanium Dioxide

A study was carried out on the effect of the inclusion of a series of transition metals (V, Cr, Mn, Fe, Ni, Cu, Zr, Mo) and also Al, Si, Zn, and Sn into titanium dioxide on its porous structure, anatase formation, and photocatalytic activity in the generation of molecular hydrogen from aqueous ethanol. An increase in the content of these metals leads to a decrease in photocatalytic activity, while an increase in the anatase content in titanium dioxide leads to an increase in such activity.     

基于活化多孔碳负载聚苯胺正极和活化多孔碳负极的有机非对称超级电容器

采用KOH活化法制得高比表面积的活化多孔碳(aHPC),借助原位化学氧化法制得疏松多孔的活化多孔碳负载聚苯胺纳米复合材料(aHPC@PANI),并分别以aHPC及aHPC@PANI为负极与正极,以四乙基氟硼酸-乙腈为电解液,构建有机非对称超级电容器。电化学测试结果显示:在1A/g电流密度下,aHPC@PANI正极与aHPC负极分别呈现256.7F/g(-0.6~0.8V)及152.4F/g(-2~-0.6 V)的比容量;所组装的有机非对称电容器呈现宽电位窗口(2.8V),高的能量密度(在0.75kW/kg功率密度下为56.2 W·h/kg)及优异的循环稳定性(循环5 000次后其比电容保持率高达92.4%)。