Secondary Current Distribution
Now, start setting up the current distribution model. Change the selection of the entire physics interface to the orange domain only.
1
In the Model Builder window, click Secondary Current Distribution.
2
In the Settings window for Secondary Current Distribution , locate the Domain Selection section and choose Orange from the Selection list.
Electrolyte I
The selection is locked to all selected domains of the physics interface, which in this case is the orange only.
1
In the Model Builder window, under Component 1 > Secondary Current Distribution click Electrolyte 1 . The ‘D’ in the upper left corner of a node means it is a default node.
2
In the Settings window for Electrolyte, locate the Electrolyte section. Here the electrolyte conductivity can be set. From the σl list choose User defined and type sigma in the associated text field. sigma is defined in the Parameters node.
Electrode Surface I
Use Electrode Surface nodes to define both a metal electrode potential and an electrode electrolyte interface. Use a Butler–Volmer expression for the zinc electrode. The hydrogen kinetics are assumed to be very fast so that a linearized Butler–Volmer expression is applicable. Start with the Zinc nail (electrode).
1
In the Physics toolbar, click Boundaries  and choose Electrode Surface .
2
In the Settings window for Electrode Surface locate the Boundary Selection section and select Zinc nail from the Boundary Selection.
3
Click the Transparency button in the Graphics toolbar to enable transparency and inspect the active boundaries in the Graphics window.
Electrode Reaction I
1
In the Model Builder, expand the Electrode Surface 1 node and click Electrode Reaction 1 .
2
In the Settings window for Electrode Reaction, locate the Equilibrium Potential section.
3
From the Eeq list, choose Nernst equation.
4
In the Eeq, ref (T ) text field, type Eeqref_Zn.
5
In the CO text field, type c_Zn2/c_ref.
6
Locate the Stoichiometric Coefficients section. In the n text field, type 2.
7
Locate the Electrode Kinetics section. From the Kinetics expression type list, choose Butler–Volmer.
8
From the Exchange current density type list, choose From Nernst Equation.
9
In the i0,ref (T ) text field, type i0ref_Zn.
10
In the αa text field, type alpha_a_Zn.
Electrode Surface 2
Define the copper nail (electrode) in a similar way with an Electrode Surface.
1
In the Physics toolbar, Click Boundaries and choose Electrode Surface
2
In the Settings window for the Electrode Surface, locate the Boundary Selection section. In the Selection list there, choose Copper nail.
3
This cell is under galvanic control. Specify this in the Electrode Phase Potential Condition section by selecting Total current in the Boundary condition list and typing -i_app in the Il,total text field that appears. This parameter will also be used in the study to perform a galvanic polarization sweep over the cell.
Electrode Reaction 1
1
In the Model Builder window, expand the Electrode Surface 2 node, then click Electrode Reaction 1
2
In the Settings window for Electrode Reaction, locate the Equilibrium Potential section.
3
From the Eeq list, choose Nernst equation.
4
In the CO text field, type c_H0/c_ref.
5
Locate the Electrode Kinetics section. In the i0 text field, type i0_H2.
Initial Values I
We are using nonlinear kinetics in the model. Provide an initial value for the electrolyte potential in order to reduce solver time and improve convergence.
1
In the Model Builder window, under Secondary Current Distribution click Initial Values 1 .
2
In the Settings window for Initial Values, locate the Initial Values section.
3
As a rule of thumb one can often use the negative of the equilibrium potential of the grounded electrode as initial value for the electrolyte potential. Therefore, in the phil text field, type -E_eq_Zn0.
The interface node sequence in the Model Builder should now look like this: