text: In electrochemistry, exchange current density is a parameter used in the Tafel equation, Butler–Volmer equation and other electrochemical kinetics expressions. The Tafel equation describes the dependence of current for an electrolytic process to overpotential.
  The exchange current density is the current in the absence of net electrolysis and at zero overpotential.  The exchange current can be thought of as a background current to which the net current observed at various overpotentials is normalized.  For a redox reaction written as a reduction at the equilibrium potential, electron transfer processes continue at electrode/solution interface in both directions. The cathodic current is balanced by the anodic current. This ongoing current in both directions is called the exchange current density. When the potential is set more negative than the formal potential, the cathodic current is greater than the anodic current. Written as a reduction, cathodic current is positive. The net current density is the difference between the cathodic and anodic current density. 
  Exchange current densities reflect intrinsic rates of electron transfer between an analyte and the electrode.  Such rates provide insights into the structure and bonding in the analyte and the electrode.  For example, the exchange current densities for platinum and mercury electrodes for reduction of protons differ by a factor of 10, indicative of the excellent catalytic properties of platinum.  Owing to this difference, mercury is the preferred electrode material at reducing (cathodic) potentials in aqueous solution.