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.
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