Use the Periodic Condition to define a periodic relation between two boundaries — for instance, in a model describing a repetitive unit cell.

Use the Apply for electrolyte phase checkbox to enable the periodic condition for the electrolyte phase potential and electrolyte salt concentration.

Use the Apply for electrode phase checkbox to enable the periodic condition for the electrode phase potential.

By specifying a Potential difference, an offset in potential (typically per unit cell) can be prescribed.

For more information, see Periodic Boundary Conditions in the COMSOL Multiphysics Reference Manual.