Standard half-cell potential ( E
0
):
Normally E0 is an equilibrium value and assumes zero-current
across the interface.
When current flows, the half-cell potential, E0
, changes.
Overpotential ( Vp ):
Difference between non-zero current and zero-current half-cell
potentials; also called the polarization potential (Vp
).
Components of the overpotential ( Vp ):
Ohmic ( Vr ): Due to the resistance of the electrolyte (voltage drop
along the path of ionic flow).
Concentration ( Vc ): Due to a redistribution of the ions in the
vicinity of the electrode-electrolyte interface (concentration
changes).
Activation ( Va ): Due to metal ions going into solution (must
overcome an energy barrier, the activation energy) or due to metal
plating out of solution onto the electrode (a second activation
energy).