Use a Separator node to define a current balance in the electrolyte and a material balance for the salt, here /. Both the material and current balances are defined for porous media.

The Separator node can be added from the Electrolyte submenu.

The Porosity in separator εsep (dimensionless) and the Exponent on porosity in electrolyte ex (dimensionless) parameters will define how the effective transport parameters of the pore electrolyte are calculated.

See also the Reservoir node.