Hydrogen Over-voltage

In some cathodic reactions, the reduction of the electro-active materials occurs in several steps. Polarization may occur if any of these steps is slow and irreversible. This polarization is termed as over-voltage and may take place at one or both the electrodes. Over-voltage is actually the extra voltage to be applied over and above the theoretical decomposition potential of an electrolyte to start the electrolysis of the electrolyte. There exist many parameters which affects the over-voltage. They are nature and physical state of the electrodes, nature of the material deposited, current density, temperature and nature of electrolyte. Hydrogen over-voltage plays an important role in the electrodeposition of metals.