The Time Dependence of the Potential in Electrode Reactions

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Abstract
Changes in the potential of an electrode with time during electrolysis at constant current are controlled by two factors: (1) the charging and discharging of the double layer capacitor (causing ``capacitive'' changes) and (2) alteration of the composition and other properties of the electrode and the adjacent solution (causing ``non‐capacitive'' changes). Equations are derived for the capacitive changes in potential, assuming that the current‐voltage relation across the electrode surface is given by Tafel's law. An experimental study has been made of the potential accompanying the evolution of oxygen from 1N KOH at a platinum anode. When the current was held constant the potential increased slowly with time. When the current was interrupted the potential rapidly decreased, the decay being faster the higher the initial potential. The decay generally followed the above‐mentioned equations for a capacitive change in potential, thus making it possible to evaluate effective Tafel constants of the electrode at various stages during electrolysis. These constants were found to depend very much on the time and on the current density, indicating that the electrode surface undergoes important changes. The behavior of the potential on increasing or decreasing the current has also been studied. This too reflects changes in the electrode surface. Most of these variations can be understood if it is assumed that the electrode surface is non‐uniform, the current being carried by relatively few active centers. The non‐uniformity is similar to that encountered in ordinary catalysis, but is more complex since the same centers may not be active at all potentials. A simple graphical method of dealing with such a non‐uniformity has been developed. It is based on the assumption that Tafel's law is valid for each microscopic element of the surface, and allows the Tafel constants to vary from one element to another. The electrode as a whole may then no longer obey Tafel's law. The concepts of poisoning and promotion, so important in catalysis, are readily incorporated and explain most of the observed results in a direct manner.