Abstract
A critical view of interfacial electrochemistry in the past 50 years is discussed, with emphasis on tacit assumptions, which are sometimes hard to justify. The important role of the Tafel equation in studies of the mechanism of electrode reactions and in the development of electrode kinetics in the past century is recognized. However, it is shown that the validity of the ways it was implemented can be questioned, particularly in view of the uncertainty in the value of the symmetry factor commonly assumed. For example, the value of β pertinent to a species in the outer-Helmholtz plane cannot be the same as that applicable to a species already adsorbed on the surface. Three factors are involved in considering charge transfer to an adsorbed species: (a) The electrostatic field at the adsorption site is highly distorted; thus, the overpotential imposed may not apply at the point where the reaction takes place; (b) the effective charge on the adsorbed species may not equal the nominal charge assigned to it; and (c) the metal surface may already be modified by a monolayer of adsorbed species of the same kind, which is, however, inactive with respect to the reaction taking place. Similarly, in studies of the kinetics of metal deposition and dissolution, where charge is transferred across the interface by the ions, one cannot legitimately assume a value of β, although it can be measured experimentally. It is very risky to predict the future of interfacial electrochemistry, but one might extrapolate present trends. Thus, the importance of the fundamental aspects of the field may have declined in the past two or three decades, and this trend will probably continue. On the other hand, the importance of understanding interfacial electrochemistry as a basis for related fields such as nano-science, biology, micro- and nano-implanted biosensors, interaction of tissue with metal implants, materials science, as well as technologies such as corrosion and alloy plating is likely to increase.
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Notes
Politicians often like to use this logic when they lose an election, claiming that the minority may be right. They may indeed be right, but I have yet to see a politician who won the election adhering to this wise observation.
This does not apply to diffusion or solution resistance, for which the characteristic length may be a hundred micrometers or more.
The best advice for the experimentalist is to remove the tip of the capillary from the electrode surface far enough to eliminate the screening effect, since modern potentiostats enable electronic compensation for most of the residual solution resistance.
Note that this explanation is based on the tacit assumption that the Br− ion adsorbed maintains its charge. Even if this assumption is valid for the case of Hg, it may not be valid for other metals.
In the old literature, they were called “high-overpotential metals”. A better choice would be “low-exchange-current-density metals,” although this sounds rather cumbersome.
Removing the first water molecule would require more energy than the average because it represents transition from a stable form of a species to an unstable one, while removing the following water molecules represents changing from one unstable species to another.
This is a somewhat problematic statement because the hydrated ion could be considered as a complex, with water molecules as the ligands. The distinction can be justified by considering such “aqua-complexes” as the standard state because of their omnipresence in aqueous solutions.
Imagine what the number of citations of Prof. Tafel would be, if every paper in the past century mentioning his equation would cite the proper reference!
Take, for example, the discovery of the double-helix structure of DNA in 1954, which led to the routine use of sequencing in forensic laboratories, to identify a criminal by a very small sample of DNA he may have left behind.
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The author wishes to thank Prof. E. Kirowa-Eisner for useful comments and suggestions and Ms. D. Tzur for preparation of the figures.
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Gileadi, E. Problems in interfacial electrochemistry that have been swept under the carpet. J Solid State Electrochem 15, 1359–1371 (2011). https://doi.org/10.1007/s10008-011-1344-5
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DOI: https://doi.org/10.1007/s10008-011-1344-5