Overvoltage theories

Theory of overvoltages Analysis of ungrounded and grounded systems The necessity of grounding an electrical system Analysis of a grounded system ... 

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... 

In an earlier note (p. 9) we mentioned the occurrence of overvoltage in an electrolytic cell (and overpotentials at single electrodes), which means that often the breakthrough of current requires an Uappl = Eiecomp r] V higher than Ehack calculated by the Nernst equation as this phenomenon is connected with activation energy and/or sluggishness of diffusion we shall treat the subject under the kinetic treatment of the theory of electrolysis (Section 3.2). 

This description of the photoelectrochemical event was originally based on the use of an illuminated semiconductor electrode in a standard three electrode cell configuration. The theory can easily be extended, however, for practical applications to short-circuited cells prepared by deposition of inert metal with low overvoltage characteristics on a powdered semiconductor Such a metallized powder is shown in... 

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 theoretical model generally used for predicting the overvoltage-current function for metal/metal ion systems is based on the quasi-thermo-dynamic arguments of transition state theory. The anodic charge transfer process is considered to involve the rupture of the bond between an adatom - i.e. a metal atom in a favourable surface site - and the metal, followed by, or coincident with, the formation of electrostatic bonds between the newly formed ion and solvent or other complexing molecules. The cathodic charge transfer follows this mechanism in reverse ... 

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)) ... 

Tafel s theory leads to the conclusion that the slope of the straight line representing the dependence of the overvoltage on current density at constant... 

Activation less process — is an electrochemical reaction occurring with zero - activation energy. This behavior is predicted for the region of high - overvoltage which is rarely available in experiments because of - mass transport limitations. The prediction of such type of processes follows from the theory of - Levich and his school [i, ii]. For the diabatic - electron transfer processes the total current density j can be estimated by integrating over the energy levels of a metal electrode e ... 

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... 

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