Activationless and Barrierless Electrode Processes

We have already mentioned a most important feature of electrode processes—the dependence of the activation energy on the potential, for example, in a cathodic process  

For a sufficiently strong shift of the potential in the negative direction to such a value of that a F — the activation energy of a cathodic reaction becomes zero. A similar result— zero activation energy of an anodic process—is obtained at a high positive potential. 

A negative value of the true ideal activation energy has no physical meaning. Therefore, it is natural to assume (this assumption will be corroborated in what follows) that a further shift of the potential, after W (or accordingly, Wa) has become zero, will not result in any further increase in i.e., varies monotonically with the potential. A constant value of equal to zero, formally corresponds to a = 0. 

Processes with = 0 and a = 0 may be referred to as activationless. The possibility of such processes has been discussed in the literature over many years. The first mention of such a situation can be found in a paper by lofa and Mikulin, who attributed to the activation energy becoming zero the fact that the extrapolated Tafel lines for different temperatures intersect at a single point. Audubert, who proceeded from the potential curve diagram, also arrived at the conclusion that the activation energy tends to zero at sufficiently high overpotentials. Later, various aspects of this problem were studied by a number of investigators (for a review see Ref. (70)). 

Initially, in the theory of barrierless and activationless processes, it was postulated that the activation energy varies monotonically with the potential after has decreased all the way to zero, its increase was considered impossible. Corroboration of this monotony principle can be found when the energy spectrum of electrons in a metal is taken into account. 

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