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Polarization

Dr. Dmitri Kopeliovich

Polarization is a deviation of the electrochemical process from equilibrium due to an electric current passing through the galvanic cell.

Polarization may occur either at the cathode (cathodic polarization) or at the anode (anodic polarization). Cathodic polarization is common.

There are three types of polarization:

Activation polarization

An electrochemical reaction proceeds through several successive steps.
The reaction rate is controlled by the slowest step (rate-determining step) of the process.
An energy barrier of should be overcome for proceeding the rate-determining step of the process. The activation energy required for overcoming the barrier changes the electrode potential.

Activation polarization is the overpotential (change of the electrode potential) caused by overcoming the energy barrier of the slowest step of the electrochemical reaction.

Common cause of cathodic activation polarization is the the reaction of Hydrogen formation and evolution at the cathode surface:

  • First step: reduction of the hydrogen ions resulted in formation of atomic hydrogen on the cathode surface.

H+ + e- = H

  • Second step: formation of molecules of gaseous hydrogen.

2H = H2

  • Third step: formation of hydrogen bubbles.

H2 + H2 + H2 +H2 + ... = nH2

The cathode is polarized by the hydrogen atoms producing a film covering the cathode surface. The film affects the process kinetic: it slows down the reaction between the electrons and hydrogen ions dissolved in the electrolyte.

At the equilibrium (non-polarized) state the rates of oxidation and reduction reactions proceeding at any electrode are equal. For example the ions of Cu are receiving electrons on the electrode surface and transfer from the electrolyte to the copper deposit. In parallel the same number of copper atoms give up their electrons and dissolve in the electrolyte.

Reduction:
Cu++ + 2e- = Cu

Oxidation:
Cu = Cu++ + 2e-

The processes produce two equal electric currents in opposite directions. The current passing through the electrode surface in the equilibrium (non-polarized) state at any direction is called exchange current.

The activation polarization overpotential may be calculated by the Tafel equation:

ηact =-(RT/αnF)ln(i/i0) = 2.3(RT/αnF)log(i/i0) for cathodic activation polarization
ηact = (RT/αnF)ln(i/i0) = 2.3(RT/αnF)log(i/i0) for anodic activation polarization

Where
R - gas constant R=8.3143 J/(mol*K);
T - temperature, K;
n - number of electrons transferred by one ion;
F - Faraday constant F=96500 C/mol (C-coulombs);
α - electron transfer coefficient (0<α<1);
i - current density;
i0 - current exchange density.

The factor β = 2.3(RT/αnF) is called Tafel slope and then activation polarization:

ηact = βlog(i/i0)

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Concentration polarization

Concentration polarization of an electrode is a result of formation of a Diffusion layer adjacent to the electrode surface where there is a gradient of the ion concentration.

Diffusion of the ions through the layers controls the electrochemical reaction (corrosion, Electroplating). The electrode potential may be calculated according to the Nernst equation:

E = E0 - (RT/nF)ln(Cion)

Where:
E0 - Standard electrode potential, V;
R - gas constant R=8.3143 J/(mol*K);
T - temperature, K;
n - number of electrons transferred by one ion;
F - Faraday constant F=96500 C/mol (C-coulombs);
Cion - molar activity (concentration) of ions.

The concentration within the diffusion layer changes from Cb (bulk concentration) to Cs (concentration at the electrode surface). Correspondingly the electrode potential changes within the diffusion layer:
from
Eb = E0 - (RT/nF)ln(Cb)
to
Es = E0 - (RT/nF)lnC(s)

The difference between Es and Eb is called concentration overpotential, which is the measure of the concentration polarization:

ηc = Es - Eb = (RT/nF)ln(Cs/Cb)

The concentration overpotential may be also expressed through the values of the electric current and limiting electric current passing through the diffusion layer:

ηc = Es - Eb = (RT/nF)ln(1-i/iL)

Concentration polarization may be lowered by increasing agitation and rising the electrolyte temperature.
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Resistance polarization

Resistance polarization refers to the potential drop due to either the high resistivity of the electrolyte surrounding the electrode or an insulation effect of the film on the electrode surface formed by the reaction products.

Resistance polarization may be expressed by the Ohm’s Law:

ηres = iR

Where
i - electric current; R - electrical resistivity.
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polarization.txt · Last modified: 2012/05/31 by dmitri_kopeliovich
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