Ionic Gradients at an Electrode above the Equilibrium Limit Current. 1. Concentration and Charge Gradients in an Immobile Electrolyte

Abstract

The charge and ionic concentration gradients next to an electrode where one member of a simple binary electrolyte is consumed is analyzed. It is shown that the thickness, profile, and formation time for a concentration gradient at the electrode can be expressed as a function of bulk voltage gradient and diffusivity of the consumed ion. When the concentration at the electrode approaches zero, this concentration gradient will move away from the electrode, becoming an equilibrium profile moving gradient layer, which moves away from the electrode at the speed of the counter ions in the bulk solution. The moving gradient layer will be followed by a thin, constant thickness transition layer and a growing unbalanced charge conduction layer. Conduction through this unbalanced charge layer will be by migration of a very low concentration of the consumed ion in the almost complete absence of the counterion. This layer is characterized by very high voltage gradients, high power dissipation (i.e., heating), and high stress gradients in the solvent.