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The over voltage can be defined as “The difference between the potential of the electrode when gas evolution is actually observed and the theoretical reversible potential of the involved galvanic cell.” In case of evolution of hydrogen Gas the Over voltage is called as Hydrogen Over Voltage.
In 1905, Tafel defined the hydrogen over voltage as “The potential in excess than reversible potential required to discharge hydrogen ions at the surface of cathode”. Hydrogen over voltage depends on:
1) Current Density,
2) Chemical nature of the electrodes.
In the discharge of H+ ions at the cathode following stops are involved:
1) Diffusion of hydrogen (H3O+) ions to the electrode.
2) Transfer of these diffused ions to the electrode surface.
3) Neutralisation of charge on the hydrogen ions by electrons to form hydrogen atom.
4) Adsorption of hydrogen atoms on the electrode surface.
5) Combination of hydrogen atoms to form hydrogen molecules.
6) Evolution of hydrogen molecules in the form of gas bubbles.
The plot a graph of Hydrogen Over Voltage \((\eta)\) vs Current density (I A/cm2) is parabola, as current density increases, over voltage also increases.
The variation of hydrogen over voltage with current density is given by Tafel and the equation is known as Tafels equation.$$ \eta =a+b\log { \left( I \right) } $$where \((\eta)\) = over voltage a is constant but varies from electrode to electrode. The value of b is constant for all the elements used as electrodes.
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