Overvoltage and Factors Affecting Overvoltage

Overvoltage

The difference between the voltage at which a gas is actually evolved during electrolysis and the theoretical value at which it ought to have been evolved is known as overvoltage or overpotential.
Theoretically the voltage required for the decomposition of an acid solution is equal to the emf of the reversible cell with hydrogen at one atmosphere. However, it is found that the observed decomposition voltage is always higher than the theoretical (reversible) value.

Thus with platinum and lead electrodes, we require respectively a current of 1.7 and 2.2 volt for the electrolysis of dilute sulphuric acid against a theoretical value of 1.23 volt. This is explained by the fact that the passage of a small current is accompanied by relatively large polarisation which naturally depends on the nature of the electrodes.

When an aqueous solution of 1N H₂SO₄ is electrolyzed using Pt electrode at 25°C evolution of hydrogen and oxygen takes place at cathode and anode respectively. The reactions taking place at the electrodes are as follows-
At Cathode-
2H⁺ + 2e → H₂(g)
At Anode-
2OH⁻ → 1/2 O₂(g) + H₂O(l) + 2e
Net Reaction-
2H⁺ + 2OH⁻ → H₂ + 1/2 O₂ + H₂O
For the above electrolytic cell, the potential at which evolution of hydrogen and oxygen takes place place was found to be 1.7 Volts. This is called decomposition potential of 1N H₂SO₄ which is also the observed value of the potential.

Now if a galvanic cell is set up using hydrogen and oxygen as gas electrodes placed in 1N H₂SO₄ solution then following reactions takes place at respective electrodes at 1 atm pressure-
Anode (Oxidation): H₂(g) → 2H⁺ + 2e
Cathode (Reduction): 1/2 O₂(g) + H₂O(l) + 2e → 2OH^–

Net Reaction-
H₂(g) + 1/2 O₂(g) + H₂O(l) → 2H⁺ + 2OH^–
The reversible (theoretical) emf of the above cell is 1.23 volts calculated by Nernst's equation. Thus we find that theoretical reversible potential is greater than the actual decomposition potential by 1.7 – 1.23 = 0.47 Volts. This excess potential is termed as over voltage and is given by the formula-
η = E_d – E_r
where-
E_d is the decomposition potential of the electrolyte
E_r = Theoretical reversible potential of the cell

Factors Affecting the Overvoltage

Followings are the factors that affects the overvoltage-
1. Current Density(I)
2. Time
3. pH
4.Temperature
5. Pressure
6. Impurities

1. Current Density(I)

Overvoltage(w) increases with increases the current density.
w = a + b lnI
where a and b are constants.

2. Time

Overvoltage changes with time due to alternation in the nature of surface and sometimes due to traces of impurities which are deposited on the cathode as electrolysis is prolonged.

3. pH

Overvoltage changes in strong acidic or alkaline medium as the concentration of H⁺ or OH⁻ increases respectively. All the substances which change the electrokinetic potential at the surface of cathode affect the overvoltage.

4.Temperature

On increasing the temperature, overvoltage decreases as overvoltage is due to a sloe stage in the ionic discharge process.

5. Pressure

At low pressure, overvoltage increases readily on electrodes like Cu, Hg or Ni but at high pressure, the overvoltage is slightly affected.

6. Impurities

Overvoltage affected if impurities are present in cathode material.

What are the causes of overvoltage ?

Different theories were put forward to explain the phenomenon of over voltage. The main principle involved is as follows-
1. The discharge of the ions from the bulk of the solution to the layer on the electrode surface and diffusion across the layers.
2. Discharge of the ions to form atom on the electrode.
3. Conversion of atoms to normal start from the deposited substance. It is predicted that any one of the above process may be slow and hence might require excess of voltage.

On the basis of E^o values, oxygen gas should be liberated at anode but it is chlorine gas which is liberated in the electrolysis of aqueous NaCl.

Experimentally, it is observed that the actual voltage required for electrolysis is greater than that calculated from standard potentials. This extra voltage required is called overvoltage. It is required because the rate of transfer of electrons at the interface of electrode and solutions for both half reactions is slow. The overvoltage required for formation of oxygen is much larger than that required for the formation of chlorine. Hence, chlorine gas which is liberated in the electrolysis of aqueous NaCl.

 Share