Simply speaking, at equilibrium, the rates of the anodic and cathodic processes are equal, and there is no net transfer of charge. The magnitude of current where the forward and reverse reactions are equal is defined as the exchange current. It can be expressed as

|i_a| = |i_c| = i_ex

where i_ex is called the exchange current density. It is usually represented by the symbol i_o.

Every reversible electrode reaction has its own exchange current density. Furthermore, an electrode will not achieve its reversible (equilibrium) electrode potential for Mⁿ⁺ + ne^- M; when its i_o value is much greater than the i_o value of any other reversible reaction in the system.

If the shift in potential is in the cathodic direction, then

I_net = |i_c| - i_a

If the shift in potential is in the anodic direction, then

I_net = |i_a| - i_c

The net current can be measured directly so the value of exchange current is significant. Thee would be a large change in the net current for a given change in potential. Consider a reaction such as  Cu Cu²⁺ + 2e^-. Although at equilibrium, current would not flow through the circuit, however, the interchange of atoms of Cu and ions of copper will take place at the electrode surface. Hence, there would be a current associated with anodic and cathodic partial reactions.

Exchange current density can be defined as the rate of oxidation and reduction at an equilibrium electrode.

The relationship between exchange reaction rate and current density can be directly derived from Faraday'slaw as

r_oxidation  = r_reduction = i_o / (n F)

Hence, i_o is the rate of oxidation and reduction reactions at an equilibrium expressed in terms of current density. The word exchange current density is a misnomer since there is no net current. It is just a convenient way of expressing the rate of oxidation and reduction at equilibrium.

Factors Affecting Exchange Current Density

A. Forward Reaction

As described above, only those atoms which are energetically at unfavorable positions, such as at grain boundaries, dislocations, half planes, are able to detain themselves and participate in the reaction. Atoms are more easily pulled from the kink sites than terrace sites. The number of surface atoms available (N_s) in a given area can be calculated.

B. Electrode Composition

  It depends upon the composition of electrode (see Table 3.1). The exchange current density for Pt is 10^-2 amp/cm², whereas for mercury Hg it is 10^-13 amp/cm².

C. Surface Roughness

 Large surface areas provide a high exchange current density.

D. Impurities

The exchange current density is reduced by presence of trace impurities, such as As, S, and Sb.

The following table gives the exchange current densities

Table 3.1. Exchange Current Densities.