Simple Electrochemical Reactions

Conditionally, an electrochemical reaction will be called simple when the following conditions hold (at least as an approximation)  

The electron transfer step is the only reaction step, which means that other parallel or consecutive steps are absent. 

Neither the starting material nor the reaction product, nor any intermediates, are adsorbed on the electrode. 

During the reaction, chemical bonds are not broken, new chemical bonds are not formed, and the geometry of the reacting species remains unchanged. 

Electrochemical reactions only involving a change of charge of simple or complex ions but not any change in inner geometry are commonly called outer-sphere electron transfer reactions. For some time, the reduction and oxidation of simple and 

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The shapes of the polarization curves shown in Fig. 3, including those generally observed for other reducing agents, are invariably complex, and not representative of simple electrochemical reactions. From the inception of modern electroless deposition practice, a number of mechanisms have been advanced to describe the electroless deposition process, many of which dealt with Ni-P. These classical mechanisms include the following ... 

Electrochemical reactions consist of electron transfer at the electrode surface. These reactions mainly involve electrolyte resistance, adsorption of electroactive species, charge transfer at the electrode surface, and mass transfer from the bulk solution to the electrode surface. Each process can be considered as an electric component or a simple electric circuit. The whole reaction process can be represented by an electric circuit composed of resistance, capacitors, or constant phase elements combined in parallel or in series. The most popular electric circuit for a simple electrochemical reaction is the Randles-Ershler electric equivalent... 

KINETIC TRANSFER FUNCTION FOR SIMPLE ELECTROCHEMICAL REACTIONS... 

Kinetic Transfer Function for Simple Electrochemical Reactions... 

The experimental parameters include v, the kinematic viscosity, and to, the angular-disk velocity (given by 2n times the rotation frequency). The slope of the ij vs. a> plot can be used to evaluate n, the number of electrons transferred in the reaction. The product i to is constant at all rotation rates for a simple electrochemical reaction. A variation of i to with co is diagnostic of a more complex electrochemical reaction in which the time-scale of the experiment influences the apparent n value. 

It is important to obtain experimental information on the thermodynamics of electrode processes to ascertain the tendency of a particular reaction to occur under a given set of experimental conditions namely temperature, pressure, system com H)sition and electrode potential. Such information is provided by the standard- or formal-electrode potentials for the redox couple under consideration. Appropriate combinations of these potentials enable the thermodynamics of homogeneous redox processes to be determined accurately. However, such quantities often are subject to confusion and misinterpretation. It is, therefore, worthwhile to outline their significance for simple electrochemical reactions. This discussion provides background to the sections on electrochemical kinetics which follow. The evaluation of formal potentials for various types of electrode-reaction mechanisms is dealt with in 12.3.2.2. 

The theoretical treatment of electron transfer at metal electrodes has much in common with that for homogeneous electron transfer described in 12.2.3. The role of one of the reactants is taken by the electrode surface, which provides a rigid two-dimensional environment where reaction occurs. In some respects, electrode reactions represent a particularly simple class of electron-transfer reactions because only one redox center is required to be activated prior to electron transfer, and the proximity of the electrode surface often may yield only a weak, nonspecific influence on the activation energetics of the isolated reactant. As with homogeneous electron transfer, it is useful to consider that simple electrochemical reactions occur in two steps (1) formation from the bulk reactant of a precursor state with the reacting species located at a suitable site within the interphasial region where electron transfer can occur (2) thermal activation of the precursor species leading to electron transfer and subsequent deactivation to form the product successor state. 

Figure 1. Free-energy profile for simple electrochemical reaction + e - Y plotted against the nuclear-reaction coordinate. Fig. lA shows the overall electrochemical free-energy profile I, bulk reactant P, precursor state S, successor state 11, bulk product. Figs. IB and C show the components of the free-energy profile arising from the solution species (Y, Y ), and transferring electron, respectively.

Fig. 2 Schematic representation of the relationship between current and potential for a simple electrochemical reaction under activation control.

The above algorithm for simulating a simple electrochemical reaction can therefore be implemented by the following steps ... 

This section presents a selection of equations used in interpreting and predicting the experimental response of a simple electrochemical reaction to a step change in potential. For complete derivations and information on more complex systems, e.g. coupled chemical reactions, the reader should consult references (1-3). 

Through this reaction, each sulfur atom hosts two lithium atoms without the need of extra atoms to maintain a crystal structure that is required by Li-ion batteries using transition metal oxides or phosphates as the cathode materials. The simple electrochemical reaction represented in Equation 24.1 shows that every atom in the Li-S battery contributes to electrical energy storage. Therefore, for the same quantity of electrons transferred in the electrochemical reactions the weight of active materials in the Li-S battery is significantly reduced. Although Li-S batteries have an emf of about two-thirds of that offered by conventional cathode materials, sulfur... 

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