electrochemistry Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outco ...

, polarization is a collective term for certain mechanical side-effects (of an electrochemical process) by which isolating barriers develop at the interface between

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials d ...

and electrolyte. These side-effects influence the reaction mechanisms, as well as the

chemical kinetics Chemical kinetics, also known as reaction kinetics, is the branch of physical chemistry that is concerned with understanding the rates of chemical reactions. It is to be contrasted with chemical thermodynamics, which deals with the direction in ...

corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...

and metal deposition.. In a reaction we can displace the bonding electrons by attacking reagents. The electronic displacement in turn may be due to certain effects, some of which are permanent (inductive and mesomeric effects), and the others are temporary (electromeric effect). Those effects which are permanently operating in the molecule are known as polarization effects, and those effects which are brought into play by attacking reagent (and as the attacking reagent is removed, the electronic displacement disappears) are known as polarisability effects. The term 'polarization' derives from the early 19th-century discovery that electrolysis causes the elements in an electrolyte to be attracted towards one or the other

pole Pole may refer to: Astronomy *Celestial pole, the projection of the planet Earth's axis of rotation onto the celestial sphere; also applies to the axis of rotation of other planets *Pole star, a visible star that is approximately aligned with the ...

i.e. the gasses were '' polarized'' towards the electrodes. Thus, initially 'polarization' was essentially a description of electrolysis itself, and in the context of electrochemical cells used to describe the effects on the electrolyte (which was then called "polarization liquid"). In time, as more electrochemical processes were invented, the term 'polarization' evolved to denote any (potentially undesirable) mechanical side-effects that occur at the interface between electrolyte and electrodes. These mechanical side-effects are: * activation polarization: the accumulation of gasses (or other non- reagent products) at the interface between electrode and electrolyte. * concentration polarization: uneven depletion of reagents in the electrolyte cause concentration gradients in boundary layers. Both effects isolate the electrode from the electrolyte, impeding reaction and charge transfer between the two. The immediate consequences of these barriers are: * the

reduction potential Redox potential (also known as oxidation / reduction potential, ''ORP'', ''pe'', ''E_'', or E_) is a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respe ...

decreases, the reaction rate slows and eventually halts. * electric current is increasingly converted into heat rather than into desired electrochemical work. * as predicted by Ohm's law, either electromotive force decreases and current increases, or vice versa. * the self-discharge rate increases in

electrochemical cell An electrochemical cell is a device capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions. The electrochemical cells which generate an electric current are called voltaic o ...

s. Each of these immediate consequences has multiple secondary effects. For instance, heat affects the crystalline structure of the electrode material. This in turn can influence reaction rate, and/or accelerate

dendrite Dendrites (from Greek δένδρον ''déndron'', "tree"), also dendrons, are branched protoplasmic extensions of a nerve cell that propagate the electrochemical stimulation received from other neural cells to the cell body, or soma, of the ...

formation, and/or deform the plates, and/or precipitate

thermal runaway Thermal runaway describes a process that is accelerated by increased temperature, in turn releasing energy that further increases temperature. Thermal runaway occurs in situations where an increase in temperature changes the conditions in a way t ...

. The mechanical side-effects can be desirable in some electrochemical processes, for example, certain types of

electropolishing Electropolishing, also known as electrochemical polishing, anodic polishing, or electrolytic polishing (especially in the metallography field), is an electrochemical process that removes material from a metallic workpiece, reducing the surface roug ...

and electroplating take advantage of the fact that evolved gasses will first accumulate in the depressions of the plate. This feature can be used to reduce current in the depressions, and exposes ridges and edges to higher currents. Undesirable polarization can be suppressed by vigorous agitation of the electrolyte, or when agitation is impractical (such as in a stationary battery) with a

depolarizer A depolarizer or depolariser, in electrochemistry, according to an IUPAC definition, is a synonym of electroactive substance, i.e., a substance which changes its oxidation state, or partakes in a formation or breaking of chemical bonds, in a ch ...

References

* . Electrochemical concepts {{electrochem-stub

See also

References