Discussion Forum

Hello everybody,
I'm trying to simulate water electrolysis in an SOEC. In my stationary model I use SIEC physic, two TCS respectively for cathode and anode and two FP in the same manner. In my model I have defined equilibrium potentials described by Nernst equation for anode and cathode.
I'm simulating three different operative conditions at a constant temperature of 850°C. Endothermic with Phis=1.1V, termoneutral voltage with Phis=1.28V and exothermic with Phis=1.4V. I started using Butler Volmer equation for calculating the current. In the first condition when I define Nernst potential using the concentration of the reactants [mol/m^3], I have no problem with the solution, at least with the distribution of mole fraction, while the electrolyte current density vector in z direction, which is normal to the plane of reaction, is constant, and I would that it decreases from the inlet to the outlet of the cell according with the decrease of water concentration and the increase of hydrogen. I suppose it is because I have to use the concentrate dependent kinetic to calculate the current, but in this case the solution gives me a constant distribution of mole fraction, as if the physics are not coupled, but they are.
If I use mole fraction defining the Nernst potential, the solution don't converge.
Another problem is if I try to simulate the conditions with Phis=1.28V and 1.4V, in these cases the solutions never converge.
Can you help me to understand where I'm wrong?

Thanks

P.s. I have used the COMSOL Current Density Distribution in a Solid Oxide Fuel Cell tutor as support