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, whereas the operating potential of a cathode is always more negative than its equilibrium potential. The overpotential increases with increasing current density, as described by the 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.
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[edit] Activation Overpotential
The potential difference above the equilibrium value required to produce a current flow.
[edit] Reaction Overpotential
Reaction overpotential is caused by chemical kinetics in the boundary layer or at the electrode surface. The reaction overpotential can be reduced or eliminated with the use of homogeneous or heterogeneous electrocatalysts. The electrochemical reaction rate and related current density is dictated by the kinetics of the electrocatalyst and substrate concentration.
The platinum electrode common to much of electrochemistry is also electrocatalytically non-innocent for many reactions. For example, hydrogen is oxidized and protons are reduced readily at the platinum surface of a standard hydrogen electrode in aqueous solution. If electrocatalytically inert glassy carbon electrode is substituted for the platinum electrode than the result is irriversible reduction and oxidation peaks with large overpotentials.
[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.
[edit] Bubble Overpotential
Bubble overpotential is due to the evolution of gas at either the anode or cathode. This reduces the effective area for current flow and increases the local current density. An example would be the electrolysis of an aqueous sodium chloride solution - although oxygen should be produced at the anode based on its potential, bubble overpotential causes chlorine to be produced instead, which allows the easy industrial production of chlorine and sodium hydroxide by electrolysis.
