Challenges in the theory of electron transfer at passive interfaces

Electron transfer (ET) has been described as the simplest of chemical reactions, because it generally does not result in the change in identity of a chemical species, but merely results in a change in the oxidation state of a single atomic center. In principle, ET is the ideal vehicle within which to develop the theory of rate processes, because of the inherent simplicity of the reaction dynamics. However, the development of an effective theory that can predict reaction rates from first principles has proven to be elusive and scientists are no closer to that goal than they were three decades ago. However, starting with the groundbreaking work of Gurney in 1931, which was hailed as one of the first triumphs of quantum mechanics, QET (quantum electron transfer) theory has provided an unassailable explanation of Tafel’s law; the exponential dependence of the rate of a charge transfer reaction on the applied potential. This paper provides a brief review of QET theory with the particular goal of accounting for Tafel’s law on passive surfaces. The inverse Tafel’s law, where the current decreases exponentially with increasing voltage for an anodic (oxidation) reaction, is also discussed. In this case, the decrease in the current is due to the thickening of an oxide layer on the surface and the resulting decrease in the quantum mechanical tunneling probability of charge carriers across the film. Both phenomena (the normal and inverse Tafel’s law) occur in electrochemical and corrosion systems and, indeed, often occur in the same system, such as the reduction of a cathodic depolarizer on a passive surface. Finally, the use of the inverse law as a tool for probing the dynamics of formation of thin oxide films on metal surfaces is discussed with emphasis on the oxidation of hydrogen on oxidized platinum in alkaline solution at ambient and elevated temperatures.