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Electrochemistry of Stimulation Electrodes: Part I: Page 2

Electrode Electrolyte Interface

A model of the electrode-electrolyte interface is shown in this figure.
The surface of the electrode is hydrated, adsorbed water molecules covering the surface. Water is a polar molecule and can be oriented in response to localized charges on the surface of the electrode or ions in the medium. Ions are hydrated (shown here are cations) and anions can be adsorbed to the surface (they share an electron with the metal).
The center of the closest charge layer, the adsorbed layer, is defined as the inner Helmoltz plane (~0.2 nm thick) and the outer Helmoltz plane is defined by the center of the closest the hydrated ions can approach the surface (~0.5 to 0.7 nm). In the diffuse layer the ion concentration is greater than that found in the bulk of the electrolyte medium.

 

Application of Current.

When a cathodic current is applied to the metal electrode in an electrolyte medium the metal becomes charged giving rise to changes at the interface, that include desorbtion of adsorbed ions, reorientation of molecules and electron transfer to molecules in the electrolyte. In the case shown, electrons are transferred to water molecules to form hydrogen and hydroxyl ions, hydrogen gas is formed, hydrated cations move in the electrolyte medium toward the electrode, giving rise to a current in the medium, adsorbed anions are desorbed and electrons can be transferred to oxygen molecules to create free radicals that react destructively with molecules important to the life and function of a cell.

 

 
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