Add the Electrolyte Potential node from Electrolyte submenus for boundaries, edges, and points to set a fixed potential at a position in the electrolyte. This node can be used to model half-cells, or to set the electrolyte potential at the position of, for example, a reference electrode.
l, to be equal to the Boundary electrolyte potential, 
l, bnd (SI unit: V).
) and select Advanced Physics Options in the Show More Options dialog box.