Reaction mechanisms overpotential

In this notation, anodic current is positive, while cathodic current is negative. As the later section on oxygen reduction will show, the Tafel slope can change with overpotential. This is because the Butler-Volmer law only applies to outer-sphere reactions. Although it can describe electrode reactions, the equation does not account for repulsive interactions of the adsorbates or changes in the reaction mechanism as potential is changed. 

Figure 7.9 shows the activity for H2 evolution of three samples of Ni [27]. Smooth and sandblasted Ni exhibit the same reaction mechanism (same Tafel slope, b), but a higher current for the latter. This is clearly due to the rougher surface of sandblasted Ni, that is, to purely geometric effects. The third sample is Raney Ni. This is obtained from an alloy of Ni with Zn or A1 that are then leached away in alkaline solution [47-49]. This leaves a very porous solid with intrinsically very small particle size. The figure shows that Raney Ni, in addition to a much lower overpotential for H 2 evolution, also exhibits a lower Tafel slope. This is clear evidence for the occurrence of electronic eflects (diflerent mechanism) together presumably with important geometric effects. 

Rate determining step (cont.) electrocatalysis and, 1276 methanol oxidation, 1270 in multistep reactions, 1180 overpotential and, 1175 places where it can occur, 1260 pseudo-equilibrium, 1260 quasi equilibrium and, 1176 reaction mechanism and, 1260 steady state and, 1176 surface chemical reactions and, 1261 Real impedance, 1128, 1135 Reciprocal relation, the, 1250 Recombination reaction, 1168 Receiver states, 1494 Reddy, 1163... 

The passage of a net current through an electrode implies that the electrode is no longer at equilibrium and that a certain amount of overpotential is present at the electrode-electrolyte interface. Since the overpotential represents a loss of energy and a source of heat production, a quantitative model of the relationship between current density and overpotential is required in design calculations. A fundamental model of the current-overpotential relationship would proceed from a detailed knowledge of the electrode reaction mechanism however, mechanistic studies are complicated even for the simplest reactions. In addition, kinetic measurements are strongly influenced by electrode surface preparation, microstructure, contamination, and other factors. As a consequence, a current-overpotential relation is usually determined experimentally, and the data are often fitted to standard models. 

Given that electrochemical rate constants are usually extremely sensitive to the electrode potential, there has been longstanding interest in examining the nature of the rate-potential dependence. Broadly speaking, these examinations are of two types. Firstly, for multistep (especially multielectron) processes, the slope of the log kob-E plots (so-called "Tafel slopes ) can yield information on the reaction mechanism. Such treatments, although beyond the scope of the present discussion, are detailed elsewhere [13, 72]. Secondly, for single-electron processes, the functional form of log k-E plots has come under detailed scrutiny in connection with the prediction of electron-transfer models that the activation free energy should depend non-linearly upon the overpotential (Sect. 3.2). 

In this chapter we discnssed the reaction mechanisms of the most important electrochemical reactions involved in low temperature fuel cells. Apart from the hydrogen oxidation reaction, which is relatively easy, and does not lead to a large overpotential even at high current densities, the oxidation of low weight alcohols... 

Now, the next point to understand is the relation between the rate of the reaction (here measured uniformly for all perovskites studied at an overpotential of 0.3 V), and the strength of the OH bond to the transition metal. It is made clear from Figure 1.14 that the stronger the bond strength, the slower the reaction. This is a determinative piece of information that suggests that the reaction mechanism must involve OH in the r.d.s. The more difficult this becomes (the stronger the bonding), the more difficult it is for the reaction to occur. 

Equations 3.41 and 3.42 give the current density of the oxidation and reduction components of the interface electrochemical reaction as a function of the overpotential, r CT, with i0 M and the P s as kinetic parameters characterizing the reaction mechanism. To obtain the Tafel relationship (Eq 3.2),which expresses the overpotential as a function of the current density, Eq 3.41 and 3.42 are changed to make the current density the independent variable ... 

The equilibrium potential for a aingle cell, given by equation (11), for the cathodic and anodic reactions (5) and (8), is -406mV for a process gas containing 2000 ppm HgS and an anode product of pure sulfur vapor. To this must be added the overpotentials needed for both electrode reactions and ohmic loss. The electrode reactions have been studied in free electrolyte on graphite electrodes . Potential-step experiments showed very rapid kinetics, with exchange currents in both cathodic and anodic direction near 40 mA/cm . Cyclic voltammetry verified a catalytic reaction mechanism with disulfide as the electro-active species. At the cathode ... 

The value of b varies depending on the reaction mechanism. Commonly encountered values are 0.06-0.03. Thus, for a change in overpotential by 0.5 V, the reaction rate alters by four to eight orders of magnitude. 

The reduction of the overpotential in alkaline on chalcogenide ruthenium metal centers is certainly the result of the generation of H02 species, via the outer-sphere reaction mechanism, phenomentMi that also occurs on oxide-covered surfaces. This is apparently the rational of the kinetics facility in alkaline media in comparison to the acid medium. 

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