Exchange Current Density
For a redox reaction written as a reduction
at the equilibrium potential the electrode/solution interface still has electron transfer processes going on in both directions. The cathodic current is balanced by the anodic current. When the potential is set more negative the cathodic current is greater than the anodic current. This ongoing current in both directions is called the exchange current density. Written as a reduction, cathodic current is positive. The net current density is the difference between the cathodic and anodic current density. Figure 1 is a graphic representation of the relation between the exchange current i0 and the net current. In the figure ic is cathodic current, ia is anodic current.
Figure 1.
A system with a high exchange current density has fast kinetics and can respond rapidly to a potential change. Figure 2 allows comparison of current density plots that vary only in exchange current density. Examination of the plots shows that at a given nonzero overpotential the magnitude of the current is greater with the higher the exchange current density.
Figure 2.