Simulating bipolar electrodes, problems with coupling transport and electric currents

Hey,

I'm trying to simulate a system in 1D where a slab of metal surrounded from both sides by a liquid and subjected to electric field. I use Transport of Dillute Species in liquid phases, and tried both Electric Currents and Coefficient form PDE for the current.

The idea is that I apply an external electric field on the metal. Two electrode reactions take place at the metal-liquid boundary. The reaction rates are defined by Butler-Volmer kinetics, where the overpotential term is defined as V(metal)-V(liquid)-E0(standard potential of the redox couple). Similar model has been described here: www.electrochemsci.org/papers/vol6/6072609.pdf

So basically I define the dependence of current on potential on both metal boundaries, and potential should go to a value where the current is conserved (current in = current out). The model is solved and the solution looks ok, but relative error is high, ca. 0.2, so I get the warning message from the solver.

I've tried the system both with Electric currents, and Coefficient form PDE, where I basically just defined same governing equations as in Electric Currents, and have the same situation with both systems. Is this because the potential of the metal is not well defined? Or any other suggestions?

Best regards,

Pekka Peljo