Hydrogen evolution reaction kinetics

It was indicated earlier that the cathodic current was a poor indicator of adequate protection. Whilst, to a first approximation the protection potential is a function of the metal, the current required for protection is a function of the environment and, more particularly, of the cathodic kinetics it entails. From Fig. 10.4 it is apparent that any circumstance that causes the cathodic kinetics to increase will cause both the corrosion rate and the current required for full (/") or partial (1/ — /, ) protection to rise. For example, an increase in the limiting current in Fig. 10.5 produced by an increase in environmental oxygen concentration or in fluid flow rate will increase the corrosion rate and the cathodic protection current. Similarly, if the environment is made more acid the hydrogen evolution reaction is more likely to be involved in the corrosion reaction and it also becomes easier and faster this too produces an increased corrosion rate and cathodic current demand. 

For comparison we also show a cyclic voltammogram of a Au(lll) electrode (see Fig. 13.4). There is no detectable hydrogen adsorption region the hydrogen evolution reaction is kinetically hindered, and sets in with a measurable rate only at potentials well below the thermodynamic value. There is a much wider double-layer region in which other... 

This result represents the first use of FTIR measurements to obtain information about the hydrogen evolution reaction on iron. It also represents one of the first uses of FTIR to study the mechanism of the electrode kinetic reaction (14). 

Chen L, Lasia A (1991) Study of the kinetics of hydrogen evolution reaction on nickel-zinc Alloy electrodes. J Electrochem Soc 138 3321-3328... 

In the following discussion, an example is given that serves to show that introducing a Frumkin-Temkin isotherm does affect the kinetic relation between the current density, i, and the corresponding overpotential. The example chosen will use the hydrogen evolution reaction once more because it is relatively simple but at the same time involves consecutive steps and alternative pathways thus it has characteristics of many practical electrode reactions likely to be met in practice.68... 

The actual current passed / = 2F/4Jt,[H + ]exp[ — J pAE] since two electrons are transferred for every occurrence of reaction I. Equation (1.64) constitutes the fundamental kinetic equation for the hydrogen evolution reaction (her) under the conditions that the first reaction is rate limiting and that the reverse reaction can be neglected. From this equation, we can calculate the two main observables that can be measured in any electrochemical reaction. The first is the Tafel slope, defined for historical reasons as ... 

The importance of double layer structure on electrode kinetics was first shown by Frumkin for the hydrogen evolution reaction on mercury [44]. As a result of the structure of the electrochemical interface, the pre-electrode plane, i.e. the plane where the reactant undergoes electron transfer to become product, is such that the concentration of the reactant ion is different from that in the bulk solution and the corresponding potential difference with respect to the solution, (less than the applied electrode—solution potential difference ([Pg.34]

Only two general reviews [38, 39] entirely devoted to the hydrogen evolution reaction have appeared after the start of the development of cathode activation [40]. In several other cases, hydrogen evolution has been discussed within the general frame of electrocatalysis [4, 41-47] or kinetics of electrode reactions [48, 49]. However, only one of the two reviews mentioned above discusses electrocatalytic aspects with literature coverage up to the late 70 s, when the field of cathode activation was at the beginning of its development. 

Figure 19 Three-compartment cell for studies of the kinetics of the hydrogen evolution reaction on Pt. The lines between the chambers prevent mixing but allow ionic conduction.

Refs. [i] Koryta J, Dvorak J, Kavan L (1993) Principles of electrochemistry. Wiley, Chichester [ii] Sawyer DT, Sobkowiak A, Roberts ]L Jr (1995) Electrochemistry for chemists. Wiley, New York [iii] Calvo EJ (1986) Fundamentals. The basics of electrode reactions. In Bamford CH, Compton RG (eds) Comprehensive chemical kinetics, vol. 26. Elsevier, Amsterdam, pp 1-78 [iv] Conway BE (1999) Electrochemical processes involving H adsorbed at metal electrode surfaces. In Wieckowski A (ed) Interfacial electrochemistry, theory, experiment, and applications. Marcel Dekker, New York, pp 131-150 [v] Savadogo O (1999) Synergetic effects of surface active sites on the hydrogen evolution reaction. In Wieckowski A (ed) Interfacial electrochemistry, theory, experiment, and applications. Marcel Dekker, New York, pp 915-935... 

Lei us now return to the effect of pH on electrode kinetics, using concentrations instead of activities. Consider the hydrogen evolution reaction, and assume that it proceeds in the following two steps, with the second step being rate determining. 

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. 

Although many metal hydrides decompose at temperatures well below their melting points [1], there have been comparatively few studies of the kinetics and mechanisms of these hydrogen evolution reactions. Much of the interest in solid hydrides has been concerned with their thermodynamic properties, as hydrogen sources in fuel cells, or as reducing agents, or for technological applications in nuclear processes. 

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 work on the hydrogen evolution reaction at Hg from CF3S03 H30 (where the proton is present only as the unhydrated H30 ion) and from CF3SO3H in excess water (where the proton is present as the fully hydrated ion H9O4), Conway et directly derived the real entropies of activation for proton discharge at Hg by means of kinetic measurements at various temperatures employing a nonisothermal cell, i.e., with a reference electrode at... 

Examples of the complex plane plots obtained for fractal electrodes are presented in Fig. 33. With a decrease in parameter ([), the semicircles become deformed (skewed). The complex plane impedance plots obtained from Eq. (183) are formally similar to those found by Davidson and Cole " in their dielectric studies. Kinetic analysis of the hydrogen evolution reaction on surfaces displaying fractal ac impedance behavior was... 

The consequence of all these effects is that oxygen, or hydrogen, evolution reactions follow quite different kinetics than would be expected from thermodynamical considerations. For example, oxygen evolution on a platinum electrode starts at a potential significantly higher than that predicted by the Nernst equation. The reason for this is the formation of surface oxide and its associated dynamics as elucidated, besides others, by B. Conway [4]. 

The hydrogen evolution reaction is an example where its electrocatalytic character shows that it is necessary for both the description of the adsorption process and the knowledge of the kinetic parameters. Most analysis of the electrocatalytic properties involve correlations from the estimated exchange current densities with characteristic electric potentials, free energies of adsorption, enthalpy of sublimations for the metal electrode, etc. [60]. 

An electrochemical study of platinum and nickel deposition on silicon from fluoride solutions at the open circuit potential is presented. In the steady-state situation, the silicon oxidation current is balanced with a cathodic current such as to yield net zero current. In the case of platinum, the prevailing cathodic process is platinum deposition by hole injection into the valence band. In nickel solutions, a competition is established between nickel reduction and hydrogen evolution at pH=8 metal deposition is the prevailing reaction, either through a valence band process on p-type silicon or through a conduction band process on n-type. On the contrary, at pH<1 the hydrogen evolution reaction is kinetically faster and nickel deposition is not observed. The anodic and cathodic processes are coupled through the formation of silicon surface states. 

In general, the pH Increases at cathodic reaction sites. As the pH increases, the hydrogen evolution reaction becomes kinetically more difficult, and the oxygen reduction reaction more important. Oxygen transport is often rate limiting, especially at high rates of corrosion in aqueous solutions. 

The stoichiometric number of a reaction is the number of times the rate-determining step occurs during one act of over-all reaction (65). Stoichiometric numbers are sometimes useful in determining mechanisms as long as one knows the probable paths for the reaction. Stoichiometric numbers were first used in electrode kinetic equations by Bockris and Potter (42) to obtain information on the mechanism of the hydrogen evolution reaction. 

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. 

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