The Flux node can be used to specify the total species flux across a boundary. The total species flux, for example at the surface of a electrode, is defined as:

where n denotes the normal unit vector to the boundary. Set the concentration of all species except the one computed from the electroneutrality condition, which ensures that the net charge is zero (see Species). The flux N0,i can be any function of concentration and electric potential (and indeed temperature if heat transfer is included in the model), often provided by a kinetic expression. In electrochemistry, this function usually depends on concentration and exponentially on the electric potential through either the Tafel equation or the Butler-Volmer equation.

Specify the inward species flux for each species individually. Select the check box for the species to prescribe a flux and enter a value or expression for the Inward flux N0,i (SI unit: mol/(m3·s)). To use another boundary condition for a specific species, click to clear the check box for that species’ flux. Use a positive value for an inward flux.