Rate of electrochemical reactions

Electrochemical macrokinetics deals with the combined effects of polarization characteristics and of ohmic and diffusion factors on the current distribution and overall rate of electrochemical reactions in systems with distributed parameters. The term macrokinetics is used (mainly in Russian scientific publications) to distinguish these effects conveniently from effects arising at the molecular level. 

The science of electrocatalysis provides the connection between the rates of electrochemical reactions and the bulk and surface properties of the electrodes on which these reactions proceed. 

Dependence of the electric field distribution in the double layer on particle size [Zhdanov and Kasemo, 2002 Chen and Kucemak, 2004a, b], which, according to Zhdanov and Kasemo, should result in an increase in the rates of electrochemical reactions on nanometer-sized metal particles. 

The film electrodeposition process was studied by means of linear sweep voltammetry. The rate of electrochemical reaction was determined from current density (current-potential curves). The film deposits were characterized by chemical analysis, IR - spectroscopy, XRD, TG, TGA and SEM methods. 

Both ion and electron transfer reactions entail the transfer of charge through the interface, which can be measured as the electric current. If only one charge transfer reaction takes place in the system, its rate is directly proportional to the current density, i.e. the current per unit area. This makes it possible to measure the rates of electrochemical reactions with greater ease and precision than the rates of chemical reactions occurring in the bulk of a phase. On the other hand, electrochemical reactions are usually quite sensitive to the state of the electrode surface. Impurities have an unfortunate tendency to aggregate at the interface. Therefore electrochemical studies require extremely pure system components. 

A particular important property of silicon electrodes (semiconductors in general) is the sensitivity of the rate of electrochemical reactions to the radius of curvature of the surface. Since an electric field is present in the space charge layer near the surface of a semiconductor, the vector of the field varies with the radius of surface curvature. The surface concentration of charge carriers and the rate of carrier supply, which are determined by the field vector, are thus affected by surface curvature. The situation is different on a metal surface. There exists no such a field inside the metal near the surface and all sites on a metal surface, whether it is curved not, is identical in this aspect. 

The fundamental reason for the uneven distribution of reactions is that the rate of electrochemical reactions on a semiconductor is sensitive to the radius of curvature of the surface. This sensitivity can either be associated with the thickness of the space charge layer or the resistance of the substrate. Thus, when the rate of the dissolution reactions depends on the thickness of the space charge layer, formation of pores can in principle occur on a semiconductor electrode. The specific porous structures are determined by the spatial and temporal distributions of reactions and their rates which are affected by the geometric elements in the system. Because of the intricate relations among the kinetic factors and geometric elements, the detail features of PS morphology and the mechanisms for their formation are complex and greatly vary with experimental conditions. 

The lack of knowledge of precise values of the roughness factor makes it difficult to compare data reported from different studies. This applies in particular to the double-layer capacity data, the values of surface concentration of the adsorbates, and the rates of electrochemical reactions. Therefore, the question of how to determine the real surface of the electrode is of cmcial importance. A survey of various methods for determining roughness was given by Trasatti and Petrii. For noble metal electrodes, the charges of hydrogen deposition and surface oxide formation can be utilized in real-surface determination." ... 

The rate of electrochemical reactions is given by the cell current, that is, in principle, it can be controlled independent of the temperature (the required overvoltages are influenced by the temperature, however). But usually, electroorganic conversions include chemical reaction steps and therefore the temperature influence, especially on reaction kinetics and selectivity, is frequently similar to that of pure chemical reactions. Consequently, a constant temperature is desirable to achieve clearly defined conditions for the investigations. 

Activation Polarization Activation polarization is present when the rate of an electrochemical reaction at an electrode surface is controlled by sluggish electrode kinetics. In other words, activation polarization is directly related to the rates of electrochemical reactions. There is a close similarity between electrochemical and chemical reactions in that both involve an activation barrier that must be overcome by the reacting species. In the case of an electrochemical reaction with riact> 50-100 mV, rjact is described by the general form of the Tafel equation (see Section 2.2.4) ... 

If current and the rate of electrochemical reaction are two sides of the same coin, then the question, What dictates the magnitude of I can he simplified, since we are, in effect, asking the related question, What dictates the rate of the electrochemical reaction occurring at an electrode . 

Rate of Electrochemical Reaction in Terms of Current. In this part of the derivation we start with a definition of the rate of reaction and the definition of the electric current. The rate of the reduction reaction v, reaction (6.6) from left to right, is defined as the number of moles m of Ox reacting per second and per unit area of the electrode surface ... 

Current-Potential Relationship for Partial Reactions, Partial i = /(A(/)) functions can be derived by joining equations expressing the rate of electrochemical reactions in terms of current [Eqs. (6.18) and (6.20)] and equations expressing the rate constant as a function of potential [Eqs. (6.31) and (6.32)]. Thus, the cathodic partial current density i is obtained from Eqs. (6.18) and (6.31) to yield... 

Electrocatalysis and the Rate of Electrochemical Reactions For a given electrochemical reaction A + ne B, which involves the transfer of n electrons at the electrode/ electrolyte interface, the equilibrium potential, called the electrode potential, is given by the Nernst law ... 

The formation of an electrical double layer at a metal-solution interface brings about a particular arrangement of atoms, ions and molecules in the region near the electrode surface, and an associated variation in electrical potential with distance from the interface. The double layer structure may significantly affect the rates of electrochemical reactions. 

Attempts to develop a model for the digital simulation of the cyclic voltammetric behaviour of PVF films on platinum62 electrodes required inclusion of the following features (a) environmentally distinct oxidized and reduced sites within the film (b) interconversion of the above sites and interaction between them (c) rate of electrochemical reactions to depend on the rate of interconversion of redox sites, the rate of heterogeneous electron transfer between film and substrate, intrafilm electron transfer and the rate of diffusion of counter ions and (d) dependence on the nature of the supporting electrolyte and the spacing of electroactive groups within the film. 

In all Figures electrochemical potentials possess clear maxima and minima. Such curve shapes conform to the shape for catalase and electrochemical reactions in the diffusion zone of the system. As mentioned above, molecular oxygen accumulated on the surface of the mimetic electrode during catalase reaction (8.1) diffuses through the adhesive and mimic layers to the electrode surface, where it is activated and interacts with H+. Anions OFT formed in this process may set the electrode surface free for the next portion of oxygen by diffusion only. Thus, the rate of electrochemical reaction (8.2) will be defined by the ratio of the rates of molecular oxygen diffusion to the electrode surface and reverse diffusion of OH- anions from the surface. 

The information required to predict electrochemical reaction rates (i.e., experimentally determined by Evans diagrams, electrochemical impedance, etc.) depends upon whether the reaction is controlled by the rate of charge transfer or by mass transport. Charge transfer controlled processes are usually not affected by solution velocity or agitation. On the other hand, mass transport controlled processes are strongly influenced by the solution velocity and agitation. The influence of fluid velocity on corrosion rates and/or the rates of electrochemical reactions is complex. To understand these effects requires an understanding of mixed potential theory in combination with hydrodynamic concepts. 

The rates of electrochemical reactions can be expressed in terms of concentration and potential. 

---