Estimating parameters from rotating ring disc electrode measurements

Rotating ring disc electrode (RRDE) experiments are a classic tool for investigating kinetics of electrochemical reactions. Several standardized methods exist for extracting transport parameters and reaction rate constants using RRDE measurements. In this work, we compare some approximate solutions to the convective diffusion used popularly in the literature to a rigorous numerical solution of the Nernst–Planck equations coupled to the three dimensional flow problem. In light of these computational advancements, we explore design aspects of the RRDE that will help improve sensitivity of our parameter estimation procedure to experimental data. We use the oxygen reduction in acidic media involving three charge transfer reactions and a chemical reaction as an example, and identify ways to isolate reaction currents for the individual processes in order to accurately estimate the exchange current densities.     

Theoretical analysis of catalytic current-potential curves: Part I. First-order regeneration mechanisms at different electrolysis regimes

The article describes a general treatment of the catalytic reaction mechanism at an electrode expanding in accord with any power law, the stationary electrode and the dropping mercury electrode being special cases. Reversible, irreversible and quasi-reversible electrode reactions are embraced by the treatment. A general expression for the current-voltage characteristic was derived, which was applied to different electrolysis regimes. The cases when the regeneration reaction will influence the catalytic wave characteristics are also discussed.     

二苯并18冠6促进钾离子在微液-液界面上的迁移反应

本文研制出一种用于构造互不相溶电解质界面 (ITIES)的两亲性微孔电极 ,并对发生于该界面上的二苯并 18冠 6 (DB18C6 )促进钾离子的迁移反应进行了研究 ,计算其相关的反应参数 .实验证明 ,该方法操作简便 ,是研究ITIES界面上电荷迁移反应的一种有效工具     

Cyclic chronopotentiometry with non-linear current-time functions at an SMDE Amalgamation and reversibility effects and experimental verification

The general analytical equations corresponding to the potential-time response obtained for a charge transfer reaction in cyclic chronopotentiometry with power current-time functions at a static mercury drop electrode are presented. The superposition principle can be applied in this case in spite of the fact that the successive applied currents are non-linear functions of time. These theoretical equations have been tested experimentally in the following cases: (a) for reversible systems in order to quantify the amalgamation product effects; (b) for non-reversible systems pointing out that a given electrode process becomes more irreversible the lower the power of time in the applied current; (c) to analyse the influence of the electrode kinetics on the potential-time curves by using solutions of Cd²⁺ with different concentrations of n-pentanol by determining kinetic parameters of tie charge transfer reaction.The sensitivity obtained in the characterisation of amalgamation processes is similar to that of voltammetric stripping techniques. Moreover, the analysis of the different cycles of the potential-time response allows us to obtain accurate values of kinetic parameters, as well as to detect possible complications in the charge transfer reaction.     

Kinetics of multiple electron transfer reactions at porous electrodes under stationary and flow conditions

The kinetics of consecutive two-electron transfer reactions at porous flooded electrodes are investigated under both stationary and flow conditions, where mass transfer is due respectively to diffusion and forced convection. The current-polarization relations were calculated for both modes of mass transfer as a function of the specific surface area of the electrode, the rates of the respective steps of the electron transfer reaction and the appropriate mass transfer coefficients. The computed solutions degenerate to the known limiting cases of single electron transfer control under conditions of very high or very low polarizations. Thus, at high anodic polarization, the electrochemical reaction is controlled by a single electron transfer step, the other step being too fast. Under conditions of 0.1<i/i_L<1, the overall reaction rate is controlled by both mass transfer and electrochemical activation. For flooded diffusion electrodes, the current-voltage curves follow the Tafel equation with a slope of double the normal value. This is attributed to mass transfer control in agreement with previous work. Experimental results, obtained on the porous flow-through electrode, agreed well with the theoretical predictions. The calculations presented here enable a quantitative evaluation of the relative influence of the rate of any step on the overall behaviour of the electrode under the appropriate experimental conditions.     

Modern Methods for the Study of Electrode Processes

The investigation of electrode processes calls for the use of methods that yield information on the reaction mechanism and the reaction rate, on the nature and stability of intermediates, and on adsorption processes at the electrode. It is no longer enough to record current-voltage curves or charging curves. In addition to the potentiostatic scan method, other methods that have proved useful for the investigation of electrode kinetics are the rotating disc electrode in particular, and to a smaller extent AC polarography. Intermediates can be identified with rotating double electrodes and by ESR spectroscopy or mass spectrometry. Adsorption processes on electrode surfaces can be studied by optical methods or by radioactive tracer methods.     

A systematic approach to the impedance of surface layers with mixed conductivity forming on electrodes

Electrode impedance can be evaluated on the basis of the electrode reaction kinetics in many systems, even for complicated electrode reactions. However, when a surface layer is present on the electrode surface, the theoretically well-established impedance model of the electrode reaction is often completed with phenomenological equivalent circuit elements in order to achieve the number of time constants as derived from the electrode impedance spectra measured. In these cases, the meaning of the phenomenological equivalent circuit elements are often unclear, though the presence of these elements is helpful to describe the system throughout the frequency domain used for the measurement. In the present work, an attempt will be shown to separate the effect of the electronic and ionic charge transfer in a surface layer and to identify the appropriate equivalent circuits. Examples are shown from the fields of lithium-ion batteries where a solid electrolyte interface as a surface layer is present at the negative electrode and the contribution of various charge carriers may be of importance.     

Mass transfer to tubular electrodes. Part 2: CE process

The kinetic equations for an electrochemical process consisting of a homogeneous first‐order chemical reaction followed by electron transfer at the electrode surface are solved numerically, for linear sweep voltammetry under hydrodynamical conditions in a tubular electrode. Models for both the cases involving reversible as well as irreversible electrode charge transfer reaction are investigated. The influence on current–potential voltammograms of the experimentally measurable parameters like the potential scan rate, axial flow rate and chemical equilibrium parameter is examined and depicted graphically. This revised version was published online in July 2006 with corrections to the Cover Date.     

Electron Correlation Effects for Adiabatic Electrochemical Reactions of Electron Transfer in a Model for an Electrode with an Infinitely Wide Conduction Band: Diagrams of Kinetic Modes

For adiabatic electrochemical reactions, within the framework of a model for an electrode with an infinitely wide conduction band, exact results are obtained for critical regions that correspond to different possible types of electron transfer processes (transfer of a single electron with and without an intermediate state, simultaneous transfer of two electrons) and regions that correspond to electroadsorption of the reactant in certain charge states. These regions form a diagram of kinetic modes (DKM) in the space of model parameters. Analytical expressions for outermost curves of DKM are obtained for some extreme cases. For the general case of a model for an electrode with an infinitely wide conduction band, a DKM is constructed and investigated with exact allowance for the effects of electron–electron correlations.     

Reactivity index based on orbital energies

This study shows that the chemical reactivities depend on the orbital energy gaps contributing to the reactions. In the process where a reaction only makes progress through charge transfer with the minimal structural transformation of the reactant, the orbital energy gap gradient (OEGG) between the electron‐donating and electron‐accepting orbitals is proven to be very low. Using this relation, a normalized reaction diagram is constructed by plotting the normalized orbital energy gap with respect to the normalized intrinsic reaction coordinate. Application of this reaction diagram to 43 fundamental reactions showed that the majority of the forward reactions provide small OEGGs in the initial stages, and therefore, the initial processes of the forward reactions are supposed to proceed only through charge transfer. Conversely, more than 60% of the backward reactions are found to give large OEGGs implying very slow reactions associated with considerable structural transformations. Focusing on the anti‐activation‐energy reactions, in which the forward reactions have higher barriers than those of the backward ones, most of these reactions are shown to give large OEGGs for the backward reactions. It is also found that the reactions providing large OEGGs in the forward directions inconsistent with the reaction rate constants are classified into S_N2, symmetric, and methyl radical reactions. Interestingly, several large‐OEGG reactions are experimentally established to get around the optimum pathways. This indicates that the reactions can take significantly different pathways from the optimum ones provided no charge transfer proceeds spontaneously without the structural transformations of the reactants. © 2014 Wiley Periodicals, Inc.     

微带电极上同时受扩散、化学反应、电极反应速率控制的伏安方程

运用积分变换的方法推导了超微带电极上同时受扩散、化学反应、电化学反应动力学控制的伏安关系,得到了前行化学反应、平行化学反应和后行化学反应的准可逆伏安方程,并列出了计算所得的典型伏安曲线。     

宏观均相多孔电极电化学阻抗谱基础

电化学阻抗谱可用于诊断多孔电极内电荷转移反应,即界面电荷集聚和电荷传导,以及反应物质输运。本文采用复相量方法,在同态假设条件下,重新推演多孔电极阻抗谱模型,厘清传统多孔电极阻抗谱模型中的模糊性表述。(1) 定义多孔电极表征输入参数,包括电极基体电子电导率σ₁ 、电解质离子电导率σ₂、界面电荷传递电导率g_ct、单位面积界面电容C、固相扩散系数D、速度常数k、电极厚度d、特征孔深L_p 和单位体积表面积S_c;(2) 解析阻抗谱特征输出参数,包括场扩散常数K,特征频率ω₀、ω₁、ω₂、ω₃和 ω_max,它们分别相关于界面传导反应、有限场扩散、氧化还原反应、孔内扩散和最小特征孔尺寸,以及分别对应于从传导到扩散和从扩散到饱和的转折频率f_k1 和f_k2;(3) 当参数X和Z同时变化时(X = σ₁和Z = d,S_c,L_p,C,g_ct,D,k),通过阻抗谱特征参数的演变规律,分析了电荷转移反应中X和Ζ参数耦合竞争;(4)为深入分析电荷转移反应中参数X和Z的耦合竞争,引入了分叉频率f_XZ和f_ZX 。f_XZ和f_ZX所处位置可以用于表征参数X和Z影响电荷转移反应的深度和广度。当分叉频率f_XZ和f_ZX不存在时,表明电荷转移反应中参数X和Z在全频率范围内存在耦合竞争。总之,借助于特征频率和分叉频率,本文一方面研究了动力学参数和微观结构参数对多孔电极中电荷转移反应的影响,另一方面分析谱图的变化及其背后的阻抗谱特征演化规律。本文研究结果可为阻抗谱的系统仿真和辨识提供理论基础,可为多孔电极内电荷转移反应的竞争分析提供技术支撑,还可为电化学储能系统的优化设计提供诊断工具。     

Reaction electronic flux: a new concept to get insights into reaction mechanisms. Study of model symmetric nucleophilic substitutions

The present work is focused on studying chlorine and fluorine identity SN2 substitutions on a methyl center, within the framework of the newly introduced reaction electronic flux J(xi), that allows one to identify charge transfer and polarization mechanisms that take place along the reaction coordinate. The main results concern the discovery of different charge transfer mechanism, despite both reactions have the same energetic pattern with simultaneous bond breaking and formation. It turns out that the chlorine substitution is mainly driven by polarization effects and characterized by through bond interactions while intermolecular charge transfer dominates the fluorine exchange reaction, that is characterized by through space interactions.     

扬帆启航：电化学实验初步

电化学是研究电能和化学能相互转化的规律的科学。电能和化学能之间的相互转换,是通过电极/电解质溶液表界面的结构变化和电荷转移反应来实现的。以电化学能源器件为例,前者为超级电容器,能量存储和释放主要通过界面双电层的表界面结构变化来实现;后者为化学电源,能量存储和释放主要通过电极活性物质的表界面化学反应来实现。所以,研究一个电化学体系时,主要关注两个问题,即表界面结构和表界面电荷转移反应。而表界面结构直接影响表界面电荷转移反应的性质,因此,从电化学实验的角度,表界面的构筑是至关重要的,包括电极的制备和表征、电极修饰材料的制备及组装、溶剂和支持电解质的选择和优化、电化学实验环境等等。本文旨在向广大电化学初学者讲述电化学实验的准备工作以及对实验现场数据的基本判断,帮助大家在实验中及时发现问题,及早采取措施,高效率地获取可信的实验数据。     

A surface science approach to cathode/electrolyte interfaces in Li-ion batteries: Contact properties,charge transfer and reactions

Reactions and charge transfer at cathode/electrolyte interfaces affect the performance and the stability of Li-ion cells. Corrosion of active electrode material and decomposition of electrolyte are intimately coupled to charge transfer reactions at the electrode/electrolyte interfaces, which in turn depend on energy barriers for electrons and ions. Principally, energy barriers arise from energy level alignment at the interface and space charge layers near the interface, caused by changes of inner electric (Galvani) potential due to interfacial dipoles and concentration profiles of electronic and ionic charge carriers.In this contribution, we introduce our surface science oriented approach using photoemission (XPS, UPS) to investigate cathode/electrolyte interfaces in Li-ion batteries. After an overview of the processes at cathode/electrolyte interfaces as well as currently employed analysis methods, we present the fundamentals of contact potential formation and energy level alignment (electrons and ions) at interfaces and their analysis with photoemission. Subsequently, we demonstrate how interface analysis can be employed in Li-ion battery research, yielding new and valuable insights, and discuss future benefits.     

On the origin of frequency dispersion at the interface between mercury electrode and aqueous solutions of alkali halides

The origin of frequency dispersion of electrochemical impedance is investigated at the interface of mercury and aqueous solutions of single alkali halides. It is found that in the presence of each one of KI, CsI, CsF and CsBr salts, the interface presents certain potential regions where frequency dispersion effects are detected and others where the ideal capacitor behavior is closely approximated. Frequency dispersion effects are contributed by interfacial processes such as anion and cation adsorption, mercury halide film formation and dissolution and charge transfer reactions. The discrimination between frequency dispersion due to charge transfer processes occurring at the Hg/solution interface and that due to reactant adsorption itself is generally difficult and depends on the reaction mechanism, provided that a discrete adsorption step is anticipated.     

Synthesis of multi-unit protein hetero-complexes in the gas phase via ion-ion chemistry

The synthesis of protein hetero-complex ions via ion-ion reactions in the gas phase is demonstrated in a quadrupole ion trap. Bovine cytochrome c cations and bovine ubiquitin anions are used as reactant species in the stepwise construction of complexes containing as many as six protein sub-units. For any set of reactants, a series of competitive and consecutive reactions is possible. The yield of complex ions for any given sequence of reactions is primarily limited by the presence of competitive reactions. Proton transfer represents the most important competitive reaction that adversely affects protein complex synthesis. In the present data, proton transfer takes place most extensively in the first step of complex synthesis, when single protein sub-units are subjected to reaction with one another. Proton transfer is found to be less extensive when one of the reactants is a protein complex. The generation of hexameric hetero-complexes containing two cytochrome c molecules and four ubiquitin molecules is demonstrated with two different synthesis approaches. The first involved the initial reaction of several charge states of cytochrome c and several charges states of ubiquitin. The sequence of reactions in this example illustrates the array of possible competitive and consecutive reactions associated with even a relatively simple set of multiply charged reactants. The second approach involved the initial reaction of the 9(+) charge state of cytochrome c and the 5(-) charge state of ubiquitin. The latter approach highlights the utility of the multi-stage mass spectrometric (MS(n)) capabilities of the ion trap in defining reactant ion identities (i.e. charge states and polarities) so that synthesis reactions can be directed along a particular set of pathways.     

Reversibility of electrode reactions involving organic compounds

A general empirical approach allowing one to describe the kinetics and evaluate the mechanism of the electrode electron transfer reactions is offered. The approach is based on the electrode potentials, the vertical ionization potentials (oxidation), and the affinity to electron (reduction). An equation linking kinetic and thermodynamic parameters is derived. Electrode reactions involving organic compounds are discussed in polarographic terms. The conclusion is drawn that most electron transfer reactions involving organic compounds are reversible, and that the irreversibility of the net electrode reaction is due to the irreversibility of subsequent chemical and electrochemical stages. An experimental observation of the slow electron transfer is possible in the cases of a substantial reorganization of molecules in the presence of fast subsequent chemical and electrochemical reactions.     

Kinetics and mechanisms of charge transfer processes in photocatalytic systems: A review

Charge carrier transfer processes are very important and play a vital role in photocatalytic reactions. A fundamental understanding of the kinetics and mechanisms of these charge transfer processes is crucial from the viewpoint of developing efficient photocatalysis systems for large-scale industrialization. In this work, recent efforts concerning the understanding of the kinetics and the mechanisms of the charge transfer in photocatalytic processes have been reviewed. Fundamental aspects involved in these charge transfer processes, such as charge generation, charge trapping, charge recombination, and electron and hole transfer are primarily discussed. Moreover, some recent studies focusing on enhancing the photocatalytic efficiency by improving the charge transfer and separation are also reviewed.