Definition and Physical Meaning of Overpotential

The standard potential for a given electrode reaction is related to the difference in Gibbs energy between reactants and products. 

Its dependence on concentrations in solution is expressed by the Nemst equation, which has the general form 

When at open circuit, a nonpolarizable electrode assumes its reversible potential, whereas a polarizable electrode may deviate from it significantly. In either case, the overpotential, t, is defined as the difference between the actual potential measured (or applied) and the reversible potential. 

This definition of overpotential is phenomenological and is always valid, irrespective of the reasons for the deviation of the potential from its reversible value. The overpotential is always defined with respect to a specific reaction, for which the reversible potential is known. When more than one reaction can occur simultaneously on the same electrode, there is a different overpotential with respect to each reaction, for any value of the measured potential. This situation is encountered most commonly during the corrosion of metals. When iron corrodes, for example, in a neutral solution, the overpotential may be + 0.4 V with respect to metal dissolution and -0.8 V with respect to oxygen reduction. During metal deposition, hydrogen evolution often occurs as a side reaction. At any given potential the overpotential with 

Physical Beclrochemistry Fundamentals, Techniques and Applications. Eliezer Gileadi Copyright 2011 WILEY-VCH Verlag GmbH Co. KGaA. Weinheim ISBN 978-3-527-31970-1 

---