Use the Ion-Exchange Membrane node to specify a domain with a permanent fixed space charge. The node is typically used for defining permselective membranes, used for various applications such as dialysis cells or flow batteries.

The Fixed space charge specifies the charge ions fixed in the membrane polymer matrix. Use negative space charges for cation selective membranes and positive charges for anion selective membranes, respectively.

Select the Apply Donnan Boundary Conditions checkbox to enable Donnan equilibrium conditions on all interior boundaries between the domain selected by the node and all adjacent domains selected by the interface (except Electrode nodes). The boundary conditions are applied for all species and the electrolyte potential dependent variable. This option is not available for the Poisson charge conservation model option.

The Add Donnan shift to initial values checkbox, when enabled, shifts the initial values (concentration and potential values specified on the Initial Values node) for the active selection of the ion-exchange membrane node in order to fulfill electroneutrality and compliance with the Donnan equilibria. It is assumed that the user-defined initial values represent the values for a bulk liquid electrolyte in equilibrium with the membrane. The shifted initial values are then used as initial values to the solver. This option facilitates model setup, since it typically excludes the need to sweep the fixed space charge of the membrane to a desired nonzero value using an additional study step. This option is only available for the Electroneutrality or the Water-based with electroneutrality charge-conservation model options.

For the remaining settings of this node, see the Electrolyte and Separator nodes.