1. 
   Overpotential
   Overpotential is an electrochemical term which refers to 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. ...
   Found op http://en.wikipedia.org/wiki/Overpotential
   
   
2. 
   overpotential
   Overpotential (?) is a potential that must be applied in an electrolytic cell in addition to the theoretical potential required to liberate a given substance at an electrode. The value depends on the electrode material and on the current density.
   Found op http://www.ktf-split.hr/periodni/en/abc/o.html
   
   
3. 
   overpotential
   (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 ...
   Found op http://www.britannica.com/eb/a-z/o/42
   
   

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