exchange current of an electrode reaction

The common value $I 0$ of the anodic and cathodic partial currents when the reaction is at equilibrium

$I 0 = I a = − I c$

For an electrode at equilibrium at which only one reaction is significant $I = 0$ . When more than one reaction is significant at a given electrode, subscripts to $I 0$ may be used to distinguish exchange currents. $I$ is not usually zero when only one of these reactions is at equilibrium.

Source:

PAC, 1974, 37, 499 (Electrochemical nomenclature) on page 513