Electrolysis occurs when the required decomposition voltage is exceeded. Theoretically, this voltage would have to be the same as the voltage which is calculated from the electrode potentials of the electrochemical standard scale. In practice, however, this voltage is not sufficient to cause reaction, the voltage has to be increased by a so-called overvoltage (over-potential).
Overvoltage is therefore the difference between the actually measured voltage (terminal voltage) and the thermo-dynamically calculated voltage from the potentials of the electrodes (redox potential). The reason for this can be found in the development of a layer of deposits on the electrode surface, which prevents electron exchange. To surpass this layer extra energy, i.e. overvoltage, is required. The amount of overvoltage is determined by many factors, among others, the electrode material, the electrolyte and current density.
Tin, lead, and quicksilver electrodes are well known for their high overvoltage use, however due to their short life span and the unavoidable emission of pollutants (heavy metals) in the electrolyte (such as pool water) they are nowadays practically not used. The boron-doped diamond electrode is the state-of-the-art alternative.