Hydrogenation Tafel relations

Fig. 9.25. (e) The hydrogen evolution reaction overthe Tafel relation which is linear over eleven orders of magnitude (experimental points). Curved line Marcus expression with assumption of harmonic oscillators. (Reprinted from J. O M. Bockris and S. U. M. Khan, Quantum Electrochemistry, Plenum 1979, p. 228.)... 

We recall that the Tafel relation was originally observed for the hydrogen-evolution reaction on mercury. We should therefore have an equation similar to Eq. (5.26) to describe the current-potential relationship. For a single cathodic step we wrote ... 

Hydrogen evolution, the other reaction studied, is a classical reaction for electrochemical kinetic studies. It was this reaction that led Tafel (24) to formulate his semi-logarithmic relation between potential and current which is named for him and that later resulted in the derivation of the equation that today is called "Butler-Volmer-equation" (25,26). The influence of the electrode potential is considered to modify the activation barrier for the charge transfer step of the reaction at the interface. This results in an exponential dependence of the reaction rate on the electrode potential, the extent of which is given by the transfer coefficient, a. 

From experiments on the evolution of hydrogen at various metal cathodes in dilute sulphuric acid, Tafel in 1905 observed that an extra driving force was required to cause electrolysis to proceed at appreciable rates, expressed by the current density j [35]. The overpotential T is the difference between the working electrode potential and the reversible reaction potential and was related to current... 

In 1905, Julius Tafel found that the electrochemical evolution of hydrogen on different metals (Hg, Sn, Bi, Au, Cu, Ni, etc.) shows different kinetics which can be described by an empirical logarithmic relation... 

Many workers studied HER on various metal electrodes since Tafel s work. They attempted to obtain the correlation between the hydrogen overvoltage and the properties of the electrode material. During the course of these works, widely scattered voltage current relations were published by different workers. Kuhn et al. reviewed the previous works, and decided to lay down criteria to select the experimental data for their discussions. " They chose the data by only those workers who had used preelectrolysis for the purification of the electrolyte solution. Bockris described the importance of preelectrolysis of the electrolyte solutions in the study... 

Within the normal range of electrolyte concentrations (4.5-5.6 M H2SO4), the equilibrium potential, E, of the negative electrode is —0.330 to —0.345 V with respect to a standard hydrogen electrode (SHE). Deviations from this value during charge and discharge (i.e., the overpotential, tj) as a result of kinetic hindrances and resistive losses can be conveniently related to the current density, i, by the Tafel equation... 

There are 3 steps in scheme 7.270, two intermediates (adsorbed hydrogen and vacant sites), one balance equation which relates these two intermediates, and then respectively two independent routes according to the Horiuti-Temkin rule. Steps 1, 2 and 3 are usually referred as Volmer, Tafel and Heirovsky reactions respectively acknowledging the names of researches who emphasized the importance of these processes. 

In contrast with these cases, the m vs. 17 relation is significantly different in the absence or presence of reducible substance for the system butenediol-butanediol or allyl alcohol-propanol (Figure 19). Further, it was possible in these systems to observe the case where Th < 1, hence 172 > 0 at cathodic hydrogen overpotentials. This indicates that H(a), supplied through the Vol-mer step, is consumed by the electroreduction process which now exists as a reaction which competes with the Tafel step. The difference between the curves was the smaller the more negative was the electrode potential, namely. 

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