concentration overpotential

https://doi.org/10.1351/goldbook.C01230
The concentration
overpotential
of an
electrode reaction
at a given
electrode current density
(c.d.) is basically the difference in equilibrium potentials across the
diffusion layer
. More precisely, it is the potential of a
reference electrode
(of the same
electrode reaction
as the
working electrode
) with the interfacial concentrations which establish themselves at c.d., relative to the potential of a similar
reference electrode
with the concentrations of the bulk solution. From such a measured potential difference, with c.d. flowing, one needs to subtract the ohmic potential drop prevailing between the two electrodes.
Source:
PAC, 1981, 53, 1827. (Nomenclature for transport phenomena in electrolytic systems) on page 1838 [Terms] [Paper]