Concentration polarisation:

While ions are discharged and deposited on an electrode as  a result of passage of current, an area in the instant vicinity of the electrode is depleted of ions and the layer offers resistance to the passage of current.  This kind of   resistance because of changes within concentration of the electrolyte around the electrode is known as concentration polarisation.  Concentration polarisation occurs while the ions do not arrive at the cathode or the product species do not leave the anode fast sufficient to manage the desired current.

We have already looked which reactants are transported to an electrode surface through three mechanisms.  (1) Diffusion, (2) migration and (3) convection.  Concentration polarisation arises while the effects of migration, diffusion and convection are not sufficient to transport the ions at a rate which generates a current of desired magnitude. If the depleted area in the vicinity of the electrode is not supplied along with ions from the bulk of the solution through increased diffusion or through convection - agitation (stirring), the cathode will suppose more and more negative potential as the applied potential is increased.  Conclude, a back emf is set up and if the current has to flow, the applied emf must be suitably above the equilibrium potential to overcome the polarisation. Therefore, concentration polarisation needs applied potentials that are larger than the theoretical that is., calculated from Eq to manage a given current in an electrolytic cell.

For this purpose, vigorous stirring and heating are significant within electrodeposition to minimize the concentration polarisation.