High Quality Content by WIKIPEDIA articles! Overpotential is an electrochemical term which refers to the potential (voltage) difference 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 thermodynamically expected to drive a reaction. In a galvanic cell overpotential means less energy is recovered than thermodynamics would predict. In each case the extra or missing energy is lost as heat. Overpotential is specific to each cell design and will vary between cells and operational conditions even for the same reaction. Practically it is also useful to define the current density (typically small) at which the overpotential is measured.