A
half-cell is a structure that contains a conductive
electrode and a surrounding conductive
electrolyte separated by a naturally occurring
Helmholtz double layer. Chemical reactions within this layer momentarily pump
electric charges between the electrode and the electrolyte, resulting in a
potential difference between the electrode and the electrolyte. The typical anode reaction involves a metal atom in the electrode dissolved and transported as a positive ion across the double layer, causing the electrolyte to acquire a net positive charge while the electrode acquires a net negative charge. The growing potential difference creates an intense
electric field within the double layer, and the potential rises in value until the field halts the net charge-pumping reactions. This self-limiting action occurs almost instantly in an isolated half-cell; in applications two dissimilar half-cells are appropriately connected to constitute a
Galvanic cell.