Hydrogen evolution reaction, Tafel mechanism

Table 5.5 Constants a and b of the Tafel equation and the probable mechanism of the hydrogen evolution reaction at various electrodes with H30+ as electroactive species (aH3o+ ) (According to L. I. Krishtalik)... 

The Tafel slope for this mechanism is 2.3RT/PF, and this is one of the few cases offering good evidence that P = a, namely, that the experimentally measured transfer coefficient is equal to the symmetry factor. A plot of log i versus E for the hydrogen evolution reaction (h.e.r.), obtained on a dropping mercury electrode in a dilute acid solution is shown in Fig. 4F. The accuracy shown here is not common in electrode kinetics measurements, even when a DME is employed. On solid electrodes, one must accept an even lower level of accuracy and reproducibility. The best values of the symmetry factor obtained in this kind of experiment are close to, but not exactly equal to, 0.500. It should be noted, however, that the Tafel line is very straight that is, P is strictly independent of potential over 0.6-0.7 V, corresponding to five to six orders of magnitude of current density. 

Any combination of two or three elementary pathways will give the overall mechanism of the hydrogen evolution reaction. Thus, the electrochemical Volmer discharge of the proton, the electrodesorption Heyrovsky step, and the chemical Tafel recombination of the H adatoms can serve as a combination for the hydrogen evolution process. The electrochemical rate constants can be estimated through different experimental conditions, such as their exchange current densities 7o,i = 10 1, yo2 10 4 and jo 3 = 10 2 A cm-2 at V= 0 V where AGads = 0 with 0H 1/2 [7,48]. 

Recently, Ovchinnikov and Benderskii gave a quantum mechanical model of the hydrogen evolution reaction at a metal electrode. In this model, they have emphasized the importance of Gurney-based model rather than that of Marcus, Levich, and Dogonadze. However, they tried to combine the principal features of the two models. They have pointed out that transition along the reaction coordinate, rather than the solvent coordinate, was important to explain the Tafel behavior and the constancy of the transfer coefficient. 

The Tafel slope can be used to determine the rate-determining steps in electrochemical systems We describe Tafel slope plots and illustrate their use here. We start by analyzing the mechanisms for hydrogen evolution reactions by plotting the exchange current density as a function of overpotential. Two different mechanisms, labeled here as A and B, can be considered ... 

Table 7.2 Tafel slopes and reaction orders for four mechanisms of the hydrogen evolution reaction.

Figure C.20 illustrates some of the influences that can affect the rate of corrosion of a metal. Pure zinc (curve a), amalgamated zinc (curve b) and zinc containing inclusions of platinum (curve c) are compared. The presence of platinum greatly increases the corrosion rate, partly because of the high jo value for hydrogen at this low overpotential metal, and partly because of the change in slope of the Tafel line (explained by a different mechanism for the hydrogen evolution reaction q.v.).

Both schemes have been observed in various systems. We consider hydrogen evolution on platinum from an aqueous solution in greater detail. In this system the Volmer-Tafel mechanism operates, the Volmer reaction is fast, the Tafel reaction is slow and determines the rate. Let us denote the rate constant for the Volmer reaction as ki(rj), that of the back reaction as k i(rj). Since the Volmer reaction is fast and in quasiequilibrium, we have ... 

The two main reaction mechanisms are analogous to the mechanisms for hydrogen evolution. The equivalent scheme to the Volmer-Tafel mechanism is ... 

Thus, there are four possible mechanisms for the hydrogen evolution in an acid solution (1) CT is RDS, CD fast (2) CD is RDS, CT is fast (3) CT is RDS, ED is fast and (4) ED is RDS, CT is fast. Different paths and different mechanisms have different Tafel slopes. Readers are referred to Refs. 11, 15, 21-23, and 26 for determination of the reaction mechanisms. 

For instance, the Tafel mechanism for the evolution of hydrogen is an EC mechanism with reaction (25) occurring twice for each time reaction (26) takes place... 

Other results point to no electrocatalytic increment with amorphous metals. Heusler and Huerta [591] have investigated amorphous Co75B25 and Ni67B33 with respect to corrosion. For the reaction of hydrogen evolution, in the case of the Co alloy, Tafel slopes of 120 mV, along with lower exchange currents for the amorphous material have been reported. Thus, the mechanism is the same as for the crystalline metal. In the case of the Ni alloy, some decrease in the Tafel slope has been observed with heat treatment (which promotes crystallization). Similarly, the same Tafel slope of 120 mV and the same exchange current as for pure Fe have been measured with... 

The most reliable data are from studies of hydrogen evolution on mercury cathodes in acid solutions. This reaction has been studied most extensively over the years. The use of a renewable surface (a dropping mercury electrode, in which a new surface is formed every few seconds), our ability to purify the electrode by distillation, the long range of overpotentials over which the Tafel equation is applicable and the relatively simple mechanism of the reaction in this system all combine to give high credence to the conclusion that p = 0.5. This value has been used in almost all mechanistic studies in electrode kinetics and has led to consistent interpretations of the experimental behavior. It... 

A generalized Volmer-Heyrovsky-Tafel mechanism explained the problem of the intermediate and product adsorption with a nearly zero coverage at potentials approximately equal to that of the net hydrogen evolution process [51]. The electrochemical reaction rates of the individual processes are... 

Experimental investigations of hydrogen evolution at many cathodes have shown a wide variation in the Tafel slope, exchange current density and dependence of current on pH. This is typical of a reaction involving a specifically adsorbed intermediate. We shall therefore consider the values predicted by theory for mechanisms I and II. 

As in /l, the last term in Eq. (47) vanishes and c,Fe = 1 - h. From the measurements of hydrogen evolution kinetics in the same system, the transfer coefficient an was fotind to be 0.51. Hence, ac,Pe = 0.49 and the cathodic Tafel line would have a slope of 2RT/F (shown in Figure 10 in the form of the corrected Tafel plot), if the pH in the vicinity of the electrode were constant with changing current density. Considering this result, as well as the reaction order of 1 for OH ions, mechanism (38) is indicated for the deposition and dissolution of iron, with FeOH as the electroactive species and its discharge as the rds. 

In theory, all three cathodic hydrogen evolution mechanisms, i.e., normal, Mg -catalyzed and MgH2-catalyzed processes which all have the same overall hydrogen reactions, can result in the Tafel behavior. However, the latter two processes are relatively unlikely to dominate the cathodic process, since they have equilibrium potentials much more negative than the normal hydrogen equilibrium potential and since the four electron reactions (1.58) and (1.59) very rarely occur. Therefore, the most practical hydrogen evolution mechanism should be the normal hydrogen evolution process. 

Inhibitors may also retard the rate of hydrogen evolution on metals by affecting the mechanism of the reaction, as indicated by increases in the Tafel slopes of cathodic polarization curves. This effect has been observed on iron in the presence of inhibitors such as phenyl-thiourea, acetylenic hydrocarbons, aniline derivatives, benzaldehyde derivatives, and pyrihum salts. 

It should be emphasized that the independence of S on potential, together with other characteristics of the process (the dependence of the cathodic and the chemical reaction rate on the concentration of HsCl ions, the ij ] -effects, and the Tafel slope b - 120 mV), cannot be explained within the framework of traditional mechanisms of cathodic hydrogen evolution. A detailed analysis of this problem is given in [415]. Here, we shall confine ourselves to the salient features only. 

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