Overpotential
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In electrochemistry, overpotential is the difference in the electric potential of an electrode with no current flowing through it, at equilibrium, and with a current flowing.
The measured overpotential represents the extra energy needed (an energy loss that appears as heat) to force the electrode reaction to proceed at a required rate (or its equivalent current density).
Consequently, the operating potential of an anode is always more positive than its equilibrium potential, while the operating potential of a cathode is always more negative than its equilibrium potential. The overpotential increases with increasing current density, see Tafel equation.
The value of the overpotential also depends on the "inherent speed" of the electrode reaction: a slow reaction (with small exchange current density) will require a larger overpotential for a given current density than a fast reaction (with large exchange current density).
An electrode reaction always occurs in more than one elementary step, and there is an overpotential associated with each step. Even for the simplest case, the overpotential is the sum of the concentration overpotential and the activation overpotential.
[edit] Activation Overpotential
The potential difference above the equilibrium value required to produce a current flow.
[edit] Concentration Overpotential
The potential difference caused by differences in concentration of the charge-carriers between bulk solution and on the electrode surface. It occurs when electrochemical reaction is sufficiently rapid to lower the surface concentration of the charge-carriers below that of bulk solution.
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