electrochemistry Electrochemistry is the branch of physical chemistry concerned with the relationship between Electric potential, electrical potential difference and identifiable chemical change. These reactions involve Electron, electrons moving via an electronic ...

, 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 a gas). In electrochemical cells, electrodes are essential parts that can consist of a varie ...

and

electrolyte An electrolyte is a substance that conducts electricity through the movement of ions, but not through the movement of electrons. This includes most soluble Salt (chemistry), salts, acids, and Base (chemistry), bases, dissolved in a polar solven ...

. These side-effects influence the

reaction mechanism In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical reaction occurs. A chemical mechanism is a theoretical conjecture that tries to describe in detail what takes place at each stage ...

s, 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 different from chemical thermodynamics, which deals with the direction in which a ...

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 engine ...

and metal deposition.. In a reaction, the attacking reagents can displace the bonding electrons. This electronic displacement in turn may be due to certain effects, some of which are permanent ( inductive and

mesomeric effect In chemistry, the mesomeric effect (or resonance effect) is a property of substituents or functional groups in a chemical compound. It is defined as the polarity produced in the molecule by the interaction of two pi bonds or between a pi bond an ...

s), 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 In chemistry and manufacturing, electrolysis is a technique that uses Direct current, direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of c ...

causes the elements in an electrolyte to be attracted towards one or the other pole 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 In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...

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 Ohm's law states that the electric current through a Electrical conductor, conductor between two Node (circuits), points is directly Proportionality (mathematics), proportional to the voltage across the two points. Introducing the constant of ...

, either

electromotive force In electromagnetism and electronics, electromotive force (also electromotance, abbreviated emf, denoted \mathcal) is an energy transfer to an electric circuit per unit of electric charge, measured in volts. Devices called electrical ''transducer ...

decreases and current increases, or vice versa. * the self-discharge rate increases in electrochemical cells. 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 A dendrite (from Ancient Greek language, Greek δένδρον ''déndron'', "tree") or dendron is a branched cytoplasmic process that extends from a nerve cell that propagates the neurotransmission, electrochemical stimulation received from oth ...

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 Thermal energy, energy that further increases temperature. Thermal runaway occurs in situations where an increase in temperature changes the cond ...

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

electropolishing Electropolishing principle: 1. Electrolyte 2. Cathode 3. Workpiece to polish (Anode) 4. Particle moving from the work-piece to the cathode 5. Surface before polishing 6. Surface after polishing Electropolishing, also known as electrochemical poli ...

and

electroplating Electroplating, also known as electrochemical deposition or electrodeposition, is a process for producing a metal coating on a solid substrate through the redox, reduction of cations of that metal by means of a direct current, direct electric cur ...

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.

References

* . Electrochemical concepts {{electrochem-stub

See also

References