Nonequilibrium open circuit potential

Two cases where equihbrium is lacking must be distinguished that which is unrelated to current flow and can be observed even for a nonworking electrode, and that which is related exclusively to the passage of current through the electrode. The former is the case of nonequilibrium open-circuit potentials (nonequihbrium OCP), and the latter is that of electrode polarization. 

Equilibrium potentials can be calculated thermodynamically (for more details, see Chapter 3) when the corresponding electrode reaction is known precisely, even when they cannot be reached experimentally (i.e., when the electrode potential is nonequilibrium despite the fact that the current is practically zero). The open-circuit voltage of any galvanic cell where at least one of the two electrodes has an nonequilibrium open-circuit potential will also be nonequilibrium. Particularly in thermodynamic calculations, the term EMF is often used for measured or calculated equilibrium OCV values. 

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