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

where n denotes the outward pointing normal of the boundary. The flux J0 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.

By default, the flux due to diffusion and migration in an electric field is prescribed. This is the appropriate flux condition when the relative velocity at the boundary is zero. When the fluid velocity is not equal to that of the boundary, it is often convenient to prescribe the total flux, including the convection. To do this select Include in the Convection section. When including the convection, the prescribed flux is defined as:

Specify the inward species flux for each species individually. Select the check box for the species to prescribe a flux for and enter a value or expression for the Inward flux J0. 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.