The dependence of the rate of electrochemical charge transfer on the electric field at the metal-electrolyte interface

Abstract

Electric fields at the metal-electrolyte interface are very high (of the order of 10⁷ V/cm) and one intuitively expects that these fields should have a profound influence on the movement of charged species such as ions and electrons at the interface. Qualitatively, such field effects manifest themselves as deviations from linearity of Tafel plots or as a dependence of the symmetry factor on electrode potential. It is shown that Gurney's potential energy curve representation of charge transfer reactions yields only small changes in β over a wide range of potential, with the anharmonic (Morse) curves showing smaller changes than the harmonic (parabolic) curves. Superposition of the double layer electric field on these potential energy curves increases the curvature of the Tafel plots, but the effect is still not very large, being within the limits of uncertainty in determining the correct form of the potential energy curves. The effect of electric field on electron transfer is considered both from the viewpoint of change in electron transfer distance arising from a dependence of coordinates of the activated state on potential and from the viewpoint of a direct effect on the electron transfer barrier (analogous to field electron emission). The field emission effects are found to be even less than the effects of the field on the proton transfer potential energy barrier.