Hydrogen evolution reaction, model

Figure 3.16 Volcano plot for the hydrogen evolution reaction (HER) for various pure metals and metal overlayers. Values are calculated at 1 barof H2 (298K) and at a surface hydrogen coverage of either 0.25 or 0.33 ML. The two curved lines correspond to the model (3.24), (3.25) transfer coefficients (not included in the indicated equations) of 0.5 and 1.0, respectively, have also been added to the model predictions in the figure. The current values for specific metals are taken from experimental data on polycrystalline pure metals, single-crystal pure metals, and single-crystal Pd overlayers on various substrates. Adapted from [Greeley et al., 2006a] see this reference for more details.

Medvedev IG. 2004. To a theory of electrocatalysis for the hydrogen evolution reaction The hydrogen chemisorption energy on the transition metal alloys within the Anderson-Newns model. Russ J Electrochem 40 1123-1131. 

Hammer-Nprskov d-band model, 70, 272-273, 327 Heme-copper oxidase, 610 High Throughput Synthesis of Nanoparticles, 572-574 Hydrogen (underpotential) adsorption, 60-63,254, 471-484, 526 Hydrogen evolution reaction (HER), 31, 79-87... 

The latter discussion confirms the results of the potential dependence of the current in that the activation barrier for the hydrogen evolution reaction is, at least on copper and silver, not affected by the electrode potential. This behavior is, on the other hand, connected with the observation of straight lines in a Tafel plot. It would be premature to come up with a comprehensive model that would explain this behavior more experimental work is necessary to substantiate and quantify the effects for a larger variety of systems and reactions. A few aspects, however, should be pointed out. 

In electrochemical proton transfer, such as may occur as a primary step in the hydrogen evolution reaction (h.e.r.) or as a secondary, followup step in organic electrode reactions or O2 reduction, the possibility exists that nonclassical transfer of the H particle may occur by quantum-mechanical tunneling. In homogeneous proton transfer reactions, the consequences of this possibility were investigated quantitatively by Bernal and Fowler and Bell, while Bawn and Ogden examined the H/D kinetic isotope effect that would arise, albeit on the basis of a primitive model, in electrochemical proton discharge and transfer in the h.e.r. 

In the present author s view, the Gaussian distribution function based on the solvent fluctuation model, which is developed for a simple redox couple, is used too often even when the basic assumption is not valid. For example, this type of distribution function is often drawn for the hydrogen evolution reaction where the oxidized state is H+ and reduced state is H2.105 Certainly the nature of the solvation is completely different between H+ and H2. Moreover, when one considers the kinetics of the hydrogen evolution reaction, one should consider not the energy level of H+/H2 but that of H+/H(a) as Gurney did. 

In addition to the primary reaction, the secondary reaction should also be modeled. Assuming that the hydrogen evolution reaction on the electrode under consideration is also of the Tafel type but zero order in hydrogen (because the solvent composition is practically constant during electrolysis), we can write... 

These reactions were postulated with an assumption that the alloy electrodeposition was always accompanied by the simultaneous hydrogen evolution (reaction (7.19)). This model has been confirmed by in situ surface Raman spectroscopic studies, by revealing existence of adsorbed intermediate [Ni(C6H507)Mo02]ads at the electrode surface [32]. 

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 effect of hydrogen entry on the kinetics of the hydrogen evolution reaction was taken into account in a mechanistic model developed for steady-state conditions.It was assumed that (i) hydrogen discharge involves only a single electron transfer reaction that is rate-determining, and (ii) desorption occurs by chemical recombination. Since only steady-state conditions were considered. 

C. Hitz, A. Lasia, Experimental study and modeling of impedance of the her (hydrogen evolution reaction) on porous Ni electrodes, J. Electroanal. Cheiti., 2001,500, pp. 213-222. 

Bockris and his colleagues essentially followed Gurney s model for the hydrogen evolution reaction, but they included the interaction between the electrode and hydrogen. They usually used y = 1 at > 0 and neglected the distribution at < as... 

It was shown in Chapter 4 (sections 4.3-4.5) that the relations characteristic of kinetic isotope effect in hydrogen evolution reactions are in good agreement with the predictions of the reorganization model, and in contradiction to the bond stretching model. The latter model, however, has so far been used to explain the main features of the kinetic isotope effect in homogeneous proton transfer reactions[256,443]. 

Most metals (other than the alkali and alkaline-earth metals) are corrosion resistant when cathodically polarized to the potentials of hydrogen evolution, so that this reaction can be realized at many of them. It has thus been the subject of innumerable studies, and became the fundamental model in the development of current kinetic concepts for electrochemical reactions. Many of the principles... 

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