Hydrogen overvoltage theories

Historically, electrocatalytic science developed from investigations into cathodic hydrogen evolution, a reaction that can be reahzed at many metals. It was found in a number of studies toward the end of the nineteenth century that at a given potential, the rate of this reaction differs by severaf orders of magnitude between metals. In one of the first theories of hydrogen evofution, the recombination theory of hydrogen overvoltage, the rate of this reaction was finked directfy to the rate of the catalytic... 

It next became interesting to investigate some chemical processes in which hydrogen overvoltage is intimately involved in order to see whether changes in these processes, produced by variations in the external pressure, are also in the direction predicted by the theory. Three such processes are (1) the corrosion of metals in acid solutions,... 

The theory that hydrogen overvoltage is due to more or less stable hydrides on the metal surfaces has been supported principally by New-bery.23 Although hydrides, and solid solutions of hydrogen, are undoubtedly formed on the surface of some metals during hydrogen evolution, they appear, to this writer at least, to be the result rather than a cause of overvoltage. 

Specific adsorption of anions must be accompanied by a sharp drop in the hydrogen overvoltage. This deduction from theory was for the first time tested by lofa, Kabanov, and co-workers and repeatedly confirmed later (see Figure 17). The value of r turns out to be extremely sensitive to anion adsorption a decrease in r is perceptible at such potentials at which the anion adsorption is not yet seen on the electrocapillary curves. 

Erdey-Gruz T, Volmer M. The theory of hydrogen overvoltage. Z Physik Chem 1930 A150 203-13. 

In addition to mass transport from the bulk of the electrolyte phase, electroactive material may also be supplied at the electrode surface by homogeneous or heterogeneous chemical reaction. For example, hydrogen ions required in an electrode process may be generated by the dissociation of a weak acid. As this is an uncommon mechanism so far as practical batteries are concerned (but not so for fuel cells), the theory of reaction overvoltage will not be further developed here. However, it may be noted that Tafel-like behaviour and the formation of limiting currents are possible in reaction controlled electrode processes. 

To use this formula, the assumption has been made that the fuel consists of a binary mixture of hydrogen and water, while the cathodic gas is a binary mixture of oxygen and nitrogen. The diffusion coefficient for binary mixtures D y eff is estimated by the equation proposed by Hirschfelder, Bird and Spotz [12], and the Knudsen diffusion coefficient for species i is given by free molecule flow theory [11], Finally, combining Equations (6.15-6.18) the anodic and the cathodic concentration overvoltages are given by (see also Equations (A3.20) and (A3.21)) ... 

II. Combination of Atoms as the Slow Process.—The view that the formation of molecules from hydrogen atoms, i.e., stage (4), was rate-determining, was proposed by Tafel and received much support at one time. According to this theory the high electrode potential which manifests itself as overvoltage is due to the accumulation of hydrogen atoms... 

Since the constant term has disappeared, it follows that according to the Tafel theory the overvoltage should be independent of the hydrogen ion concentration, in general agreement with experiment. Combination of... 

Dogonadze RR, Kuznetsov AM, Levich VG (1968) Theory of hydrogen-ion discharge on metals Case of high overvoltages, Electrochim Acta 13 1025-1044... 

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