Electrode polarization

Electrochemical terminology

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Under irreversible conditions, when a current passes through the electrode, an irreversible electrode reaction occurs, and the electrode potential at this time is different from the reversible electrode potential. The phenomenon that the electrode potential displayed by the electrode when there is current passing through deviates from the reversible electrode potential is called electrode polarization. The characteristics of electrode polarization are that the cathode potential is more negative than the balance potential (cathode polarization), and the anode potential is corrected than the balance potential (anode polarization).

Chinese name: Electrode polarization
Foreign name: electrode polarization

Features: Cathode more negative, anode correction
Discipline: electrochemistry

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Basic concepts

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In the case of a reversible battery, the whole battery is in electrochemical equilibrium, and the two electrodes are also in equilibrium. The electrode potential is determined by Nernst equation What determines is the balanced electrode potential. At this time, the current passing through the electrode is zero, that is, the rate of electrode reaction is zero. To make a non-zero current pass through the electrode, the electrode potential must deviate Balanced electrode potential This phenomenon is called electrode polarization.

When the electronic conductor of electrode polarization contacts with the solution in the surrounding rock, a galvanic layer will be formed and a potential jump will occur. This potential jump is called the electrode potential when the electronic conductor contacts with the solution. When there is an external electric field, the relatively balanced electrode potential value will change. Generally, the difference between the electrode potential under the action of a constant current density and the relatively balanced electrode potential is called electrode polarization. The common ones are electrochemical polarization, concentration polarization, etc. The electromotive force caused by electrode polarization is called overvoltage^[1] 。

Irreversible condition

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The potential of the electrode when the electrode reaction occurs reversibly is called the reversible electrode potential. Reversible electrode potential is very useful for solving many electrochemical and thermodynamic problems. However, in many practical electrochemical processes, such as electrolytic operation or using chemical cells to do electrical work, it is not achieved in a reversible manner. When there is current passing through the electrode, irreversible electrode reaction must occur, and the electrode potential at this time j I And reversible electrode potential j r Obviously it will be different. The electrode potential displayed by the electrode when there is current passing through j I And reversible electrode potential j r The phenomenon of deviation is called "electrode polarization"^[2] 。 The magnitude (absolute value) of the deviation is called "overpotential" and recorded as h , i.e.:

h=|j r －j I |

According to the thermodynamic principle, for the primary battery, the terminal voltage of the two electrodes is the maximum during reversible discharge, which is its electromotive force E, and its value can be expressed by the reversible electrode potential E as:

E = j r (Positive) - j r (negative pole)= j r (cathode) - j r (Anode)

Terminal voltage of two electrodes when discharging under irreversible conditions E I It must be less than its electromotive force E, that is E I = E －△ E 。 The deviation is caused by the potential drop IR caused by the internal resistance R of the battery and the polarization of two electrodes under irreversible conditions. If the current passing through is not very large, the potential drop IR can be ignored without timing, and the size of △ E can be expressed as the sum of the over potential of the two electrodes, namely:

△E= h (cathode)+ h (Anode)

Therefore:

E I =E－DE= j r (cathode) - j r (Anode) -[ h (cathode)+ h (Anode)]

=（ j r －h ） r (cathode) -（ j r + h ）(Anode)

= j I (cathode) - j I (Anode)

For the electrolytic cell, the applied voltage required for the electrolytic reaction under the reversible condition is the minimum, which can be called "theoretical decomposition voltage". Its value is equal to the electromotive force E, and the reversible electrode potential can be used j r Expressed as:

E= j r (Positive) - j r (negative pole)= j r (Anode) - j r (cathode)

In case of irreversible electrolytic reaction, the applied voltage is V one Must be greater than electromotive force E, i.e. V one =E+△V。 Similarly, if the current passing through is not very large, the potential drop IR can be ignored without timing, and the size of △ V can also be expressed as the sum of the over potential of the two electrodes, namely:

DV= h (Anode) - h (Anode)

Therefore:

V I =E－DV

= j r (Anode) - j r (cathode)+[ h (Anode)+ h (Anode)]

=（ j r －h ） r (Anode) -（ j r －h ）(Anode)

= j I (cathode) - j I (Anode)

To sum up, whether it is a primary cell or an electrolytic cell, relative to the reversible electrode potential j r When there is current passing through the electrode, the anode potential will increase and the cathode potential will decrease due to the polarization of the electrode, namely:

j I (Anode) =j r +h

j I (cathode) =j r －h

Concentration polarization

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When there is current passing through the electrode, the ion concentration near the electrode surface is different from that in the bulk solution due to the slow diffusion of ions, thus making the electrode potential and j r The phenomenon of deviation is called "concentration polarization". When the concentration polarization occurs at the electrode, the cathode potential always becomes larger than j r Low, while the anode potential always becomes less than j r High. Electrode potential caused by concentration polarization j I And j r The absolute value of the difference is called "concentration overpotential". The concentration overpotential is a measure of the polarization degree of the electrode concentration. The value depends on the difference between the ion concentration on the electrode surface and that in the bulk solution. Therefore, all factors that can affect the concentration difference can affect the concentration overpotential value. For example, when it is necessary to reduce the concentration overpotential, the solution can be strongly stirred or the temperature can be raised to speed up the diffusion of ions; When it is necessary to cause concentration over potential, the disturbance to the solution should be avoided and the temperature should not be too high.

The rate of ion diffusion is closely related to the type and concentration of ions. Therefore, under the same conditions, the concentration polarization of different ions is different; The concentration polarization degree of the same ion at different concentrations is also different. Polarographic analysis is an electrochemical analysis method based on this principle, which can be used for qualitative and quantitative analysis of various metal ions in solution^[3] 。

Electrochemical polarization

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An electrode, under reversible condition, has a certain degree of electrification on the electrode, and establishes the corresponding electrode potential j r 。 When there is a current passing through the electrode, if the electrode reaction at the electrode solution interface is not fast enough, resulting in the change of electrode electrification, the electrode potential can also deviate j r 。 With electrode (Pt) H two (g)|H + For example, when reducing as cathode + Become H two If the speed of is not fast enough, the electrons that reach the cathode when there is current can not be consumed in time, so that the electrode has more negative electricity than the reversible case, thus making the electrode potential become less than j r Low, this low electric potential energy promotes the activation of reactants, that is, accelerates the H + Convert to H two 。 When (Pt) H two (g)|H + When oxidation occurs as anode, due to H two Become H + The rate of is not fast enough, and the degree of electron deficiency on the electrode due to the current passing through is more serious than that in the reversible case, resulting in the electrode carrying more positive electricity, thus the electrode potential becomes more than j r High. This higher potential is conducive to promoting the activation of reactants and accelerating the reaction of H two Change to H + 。 When this is extended to all electrodes, it can be concluded that when there is a current passing through, the electrode electrification degree is different from the reversible situation due to the slowness of the electrochemical reaction, which leads to the deviation of the electrode potential j r "Activation polarization" or "electrochemical polarization". The cathode potential always becomes larger than that of the concentration polarization when the electrode is activated j r Low, while the anode potential always becomes less than j r High. Electrode potential due to activated polarization j I And j r The absolute value of the difference is called "activation overvoltage". The magnitude of the activation overpotential is a measure of the activation polarization of the electrode^[3] 。

features

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one
The cathode potential is more negative than the balance potential (cathodic polarization).
two
Anode potential is corrected compared with balance potential (anode polarization).

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