Heterogeneous reaction, electrochemical

Transport Phenomena. Electrochemical reactions are heterogeneous and are governed by various transport phenomena, which are important features ia the desiga of a commercial electroorganic cell system. As for other heterogeneous reactions, the electrochemical reaction is impacted by heat and... 

The measurement of corrosion current has provided, as is well known, a quite useful electrochemical technique for determining corrosion rates. However, contrary to homogeneous corrosion, pitting corrosion is a typical heterogeneous reaction on a metal surface, so that it is difficult to estimate the actual corrosion state from the usual corrosion current data. 

Mediated electrolyses make use of electron transfer mediators PjQ that shuttle electrons between electrodes and substrates S, avoiding adverse effects encountered with the direct heterogeneous reaction of substrates at electrode surfaces (Scheme 6). In recent years this mode of electrochemical synthesis has been widely studio and it is becoming increasingly well understood. A review is given in vol 1 of the present electrochemistry series... 

Like other heterogeneous chemical reactions, electrochemical reactions are always multistep reactions. Some intermediate steps may involve the adsorption or chemisorption of reactants, intermediates, or products. Adsorption processes as a rule have decisive influence on the rates of electrochemical processes. 

As heterogeneous reactions at the interface electrode-electrolyte, electrochemical reactions are intrinsically more complex than typical (thermal) chemical transformations (Figure 1). We mostly neglect the exact structure of the interface in the following description. Transport of the educt... 

Selectivity in homogeneous reactions is treated in Refs. [2-6], while the selectivi-ties that are influenced in heterogeneous reactions by diffusion and adsorption are dealt with in Refs. [7, 8]. Kinetics and mechanisms of electrochemical reactions are covered in Chap. 1 of this volume and in Refs. [9-11]. 

The net result of a photochemical redox reaction often gives very little information on the quantum yield of the primary electron transfer reaction since this is in many cases compensated by reverse electron transfer between the primary reaction products. This is equally so in homogeneous as well as in heterogeneous reactions. While the reverse process in homogeneous reactions can only by suppressed by consecutive irreversible chemical steps, one has a chance of preventing the reverse reaction in heterogeneous electron transfer processes by applying suitable electric fields. We shall see that this can best be done with semiconductor or insulator electrodes and that there it is possible to study photochemical primary processes with the help of such electrochemical techniques 5-G>7>. 

A Simple Electrochemical Approach to Heterogeneous Reaction Kinetics 261... 

There is nothing unique about the determination of the mechanism of electrochemical reactions. Electrochemical kinetics is a parallel field to that of heterogeneous chemical kinetics and basically the mechanism tasks in the two related fields are the same. There are three goals that must be reached consecutively. 

An individual step may be qualified as electrochemical if it involves the transfer of an electron through the interface in order to change the oxidation state of a species Y + e Z. In addition, one can think of chemical steps, in which a species is converted into a more reactive form (or the reverse) with the same oxidation state, e.g. Y Y. A third possibility is that an intermediate species undergoes some kind of a dismutation reaction, e.g. 2Y= 0 + R. It is convenient to encode a mechanism by means of the letters E, C and D, placed in the order of the subsequent steps, e.g. EE, CEE, CED, etc. Note, however, that all the steps are considered here as heterogeneous reactions, i.e. taking place only at the interface. 

In heterogeneous solid state reactions, the phase boundaries move under the action of chemical (electrochemical) potential gradients. If the Gibbs energy of reaction is dissipated mainly at the interface, the reaction is named an interface controlled chemical reaction. Sometimes a thermodynamic pressure (AG/AK) is invoked to formalize the movement of the phase boundaries during heterogeneous reactions. This force, however, is a virtual thermodynamic force and must not be confused with mechanical (electrical) forces. 

In indirect electrochemical reactions [3], the heterogeneous reaction between the substrate and the electrode is replaced by a homogeneous redox reaction in... 

The numerical values for ki. .. k4 vary with RG. For instance, for RG = 10, the following values provide the analytical function Jfei = 0.40472, k2 = 1.60185, k3 = 0.58819, and k4 = -2.37294 [12]. The analytical approximations for hindered diffusion provide a way to determine d from experimental approach curves. For this purpose, one can use an irreversible reaction at the UME (often 02 reduction). In such a case, Fig. 37.2, curve 1 is obtained irrespective of the nature of the sample. Besides the mediator flux from the solution bulk, there might be a heterogeneous reaction at the sample surface during which the UME-generated species O is recycled to the mediator R. The regeneration process of the mediator might be (i) an electrochemical reaction (if the sample is an electrode itself) [9], (ii) an oxidation of the sample surface (if the sample is an insulator or semiconductor) [14], or (iii) the consumption of O as an electron acceptor in a reaction catalyzed by enzymes or other catalysts immobilized at the sample surface [15]. All these processes will increase (t above the values in curve 1 of Fig. 37.2. How much iT increases, depends on the kinetics of the reaction at the sample. If the reaction of the sample occurs with a rate that is controlled by the diffusion of O towards the sample, Fig. 37.2, curve 2 is recorded. If the sample is an electrode itself, such a curve is experimentally obtained if the sample potential... 

One can also evaluate the relative change in the rate of a heterogeneous reaction at the substrate by measuring the concentration of the reaction product at the tip. In this setup, the tip is positioned at a fixed distance from the substrate, and the time dependence of concentration is measured. This simpler approach is based on the proportionality between the heterogeneous reaction rate and the product concentration. It is most useful when the substrate flux cannot be measured directly (e.g., the substrate reaction is not an electrochemical process) [76-78]. 

Li H, Balaya P, Maier J. Li-storage via heterogeneous reaction in selected binary metal fluorides and oxides. J Electrochem Soc. 2004 151(ll) A1878-85. 

Figure 4.27a shows the equivalent circuit of a simple example of the ladder structure, for electrochemical systems that are known as Faradaic reactions involving one adsorbed species. This heterogeneous reaction occurs in two steps in the absence of diffusion limitation ... 

The particle size of the cathode material plays a very important role in their electrochemical behavior [113]. Smaller particles provide the performance of the heterogeneous reaction of intercalation/deintercalation in the kinetic regime. One of the problem is to enhance electronic conductivity of cathode materials. In this case, electronic additives (e.g., carbon) could be excluded. 

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