1) Overvoltage 
In electrochemistry, overpotential is the difference in the electric potential of an electrode with no current through it, at equilibrium, and with a current.
In electrochemistry, overpotential is the potential difference (voltage) between a half-reaction`s thermodynamically determined reduction potential and the potential at which the redox event is experimentally observed. The term is directly related to a cell`s voltage efficiency. In an electrolytic cell the overpotential requires more energy than.....
(from the article `chemical industry`) At a mercury cathode, on the other hand, appreciable reduction of water requires a much higher voltage than that needed at an iron cathode. This ...
overvoltage.