exchange current

of an electrode reaction
https://doi.org/10.1351/goldbook.E02238
The common value I0 of the anodic and cathodic partial currents when the reaction is at equilibrium \[I_{0}=I_{\text{a}}=- I_{\text{c}}\] 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 I0 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 (https://doi.org/10.1351/pac197437040499)