Concept of Electrochemical Cell Overpotential

The overpotential for galvanic and electrolytic cells can be defined as deviation of the cell potential from its equilibrium value, excluding the potential drop due to the cell internal resistance I ,  

It can be seen that with this interpretation, overpotential of a cell is always a negative value. Furthermore, the cell overpotential consists of contributions from both electrodes, and individual electrode contributions cannot be separated without using a third electrode. We will discuss, in the following section, how the overpotential of a single electrode can be defined. 

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