Discussion Forum

Hello,
I am trying to simulate the pH (hydrogen ion) distribution of a PBS buffer solution (around pH 7) around two electrodes (Anode and Cathode) facing each other with current density of 4A/m^2 or voltage difference of 2V. Device geometry plot is attached in PDF file. Since the hydrogen ion is positively charged and OH- is negatively charged, when the current(or voltage) is forced on two electrodes, the pH around Cathode becomes higher(more basic, reaches pH 10 after a while experimentally) and the pH around Anode becomes low(more acidic, reaches pH 4 after a while experimentally).

So I basically follow the setting of the copper deposition example in the COMSOL model libray except I didn’t include the moving mesh module since I am only counting the H+ +e- = H and OH- - e- = OH reactions on the electrodes. I drew the geometry and selected Nernst Planck equation under the mass transport module. For the subdomain settings, I set the diffusion coefficient of both ions(H+ and OH-) to be 2E-9[m^2/s], Charge number to be +1/-1, mobility to be 8.07E-13 [s mol/kg], initial potential to be 0, initial H+ concentration to be 1E-4 mol/m^3(which is 1E-7 mol/L= pH 7). For the boundary setting of “current/potential”, I set current inflow value “0” to all the boundaries except the cathode and anode, I set the current inflow value of cathode and anode to scalar expressions based on the Butler-Volmer equation. And for the boundary setting of “C2”, I set all the boundaries to “insulation/symmetry”.

When I try to solve the problem, no matter which solver or no matter stationary or transient mode I use, I either get the error message like “Fail to find a solution: Returned solution has not converged”, “Failed to find consistent initial values. Last time step is not converged” or the pH value I get does not spread over a wide range (only 7.03 to 6.99) as I preferred. I am quite puzzled since I found good simulation result on the page 9 of an COMSOL invited presentation (www.comsol.com/papers/1789/) where the pH spread a wide range from 3 to 11. I am at a loss and desperately in need of some help from experts!

Thank you very much!
Yi