Overpotential
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225686309
2008-07-14T22:10:01Z
Mild Bill Hiccup
5202324
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In [[electrochemistry]], '''overpotential''' is the difference in the [[electric potential]] of an electrode with no [[Electric current|current]] through it, at [[Chemical equilibrium|equilibrium]], and with a current. 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.
==Activation overpotential==
The potential difference above the equilibrium value required to produce a current.
==Reaction overpotential==
Reaction overpotential is caused by [[electrochemical reaction mechanism|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 [[catalysts#Electrocatalysts|electrocatalyst]]s. 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.
==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.
==Bubble overpotential==
Bubble overpotential is due to the evolution of gas at either the anode or cathode. This reduces the effective area for current 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.
[[Category:Electrochemistry]]
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