Polarization

Polarization

The phenomenon of reverse emf takes place by the presence of product of electrolysis is called polarization. It results mainly due to slowness of one or more processes occurring at the electrode during discharge or formation of ions.
Consider the electrolysis of dilute H₂SO₄ solution using two platinum electrodes. In the initial stage the external emf applied is such that it just causes the evolution of hydrogen gas at cathode and oxygen gas at anode, but after some time, the evolution stops even though the current flows through the cell.

H₂SO₄ ⇌ 2H⁺ + SO₄⁻²
SO₄⁻² + H₂O = H₂SO₄ + O⁻²
At Cathode-
2H⁺ + 2e = H₂
At Anode-
2O⁻² = O₂ + 4e
This is accounted for the fact that the hydrogen and oxygen molecules get adsorbed on the surface of electrodes thereby giving rise to back emf which opposes the external emf applied. When the back emf (i.e. polarization emf) becomes equal to external emf, no current flows and the fuel cell is said to be completely polarized.


Polarization at a reversible electrode is mainly due to the disturbance caused in the equilibrium reaction taking place at that electrode. This is observed when the electric current flows through the electrode. Such a phenomenon is called electrolytic polarization and the electrode is said to be polarized.
Platinized electrodes are said to behave irreversibly due to slowness of one or more stages in the electrode process.
There are two types of polarization-
1. Concentration Polarization
2. Overvoltage

Concentration Polarization

Concentration polarization arises mainly due to a concentration change in the vicinity of the electrode i.e. due to slowness of diffusion of ions from or to the electrode and the bulk of the solution. It is mostly observed in metals.
Consider the dissolution of a metal anode giving cations in solution. If the cations formed do not travel rapidly in the solution, then there will be more concentration of the cations around the vicinity of the anode than in the bulk of the solution. This results in an increase in potential at the anode. The increase will be greater for higher current density. At cathode, the conditions are exactly opposite. The discharge of cations result in a decrease in concentration of the ions near the electrode and if this is not made up by diffusion, the concentration in the vicinity of the electrode becomes less than in the bulk of the solution.

Overvoltages

Decomposition voltage of electrolytes often differ from the theoretical reversible values due to polarization. For aqueous solutions of acids and bases where hydrogen and oxygen are evolved as a result of electrolysis, the extent of polarization is different for different electrodes. The theoretical value of the voltage in each case remains the same. The difference between the voltage actually necessary for gas evolution to take place and that theoretically required is called over voltage. It depends upon the nature of the electrode material.

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