Discussion Forum

Hi Omar

1) It is due to electro-osmotic drag (some of the water moves to the cathode due to this effect, and its concentration decreases towards outlet) and in order to make the balance for H2, it gets increased at the outlet: w_H2= 1-w_H2O.

2) This is the charge balance for electrodes: i_a = i_c, which has to be satisfied, but there values are quite different.

By the way, the most important thing that is left in this model, is that the water produced must be considered in liquid form, and thus it completely ignores the two-phase effects which is not acceptable in an operating fuel cell.

I am wondering whether someone (who has access to Comsol 4.0) can inform us, whether this model has been updated to consider two-phase effect in the new version, or not?

Regards
Naveed

I am wondering whether someone (who has access to Comsol 4.0) can inform us, whether this model has been updated to consider two-phase effect in the new version, or not?

These are the physics included: cl.ly/11OD

Hi Naveed,

Thanks very much for your helpful hints. Regarding 2) the charge balance for the electrodes: Do they have a specific procedure to make this currents equal or slighly equal? For other cases, where the fuel is not Hydrogen neither the oxidant is Oxygen (for example: MeOH and H2O2) and i_a > i_c or viseversa, what would be a best approach to achieve this balance i_a = i_c ? Would one needs to make the anodic current density to match the cathodic one or the cathodic to match the anodic? Or a mergence between the two would be needed? I'm trying to calculate individual electrode current densities using B-V equations for anode and cathode but they are very different in values which makes the membrane potential distribution somewhat rare.

I will very much appreciate any input in this area.

Thanks,

-Omar

Hi Danial

Thanks for a brief look at PEMFC model in C4.0. It seems, still, two-phase mechanism is not included. However, I was expecting that this must be in this time. The model in its current form is very weak as it lacks many features of two-phase equations such as NS, energy and species for two-phases. Although, it could be possible to get help from the upcoming CFD module but still it would not have energy and species equations implementing two-fluid model approach.

Thanks anyway
Regards
Naveed

Hi Omar

The anodic and cathodic current densities are never equal, even in the current model we are discussing. Actually i_a > i_c (almost in all fuel cells, because cathodic reaction is a very slow reaction). The balance, i am referring to is the the sum of electronic and ionic current on a single electrode must be conserved.
You should select a higher i0a than i0c to start with.

By the way, how you are going to handle two-phase mechanism in PEMFC-as a model lacking this would be far from reality and not accepted in a well reputed Journal.

Regards
Naveed

Hi Naveed,

Thanks again for your respose. I would have expected them to improve this model as they have done for previous versions. In my case, I do not handle that specific situation as I am working with all-liquid microfluidic fuel cells. That simplifies my model to strictly Navier-Stokes, Diffusion-Convection and DC media application modes. In my anodic reaction, I do have CO2 production but very little at the current density range I am working with. Have you looked at recent publications regarding PEMFC? Maybe, Journal of Power Sources?

Again, thanks for the advice. I do have a last question. When working with B-V equation I have noticed people separate it into anodic and cathodic parts but for the exponential funtion of it ("exp(alpha*F*eta/RT)"), sometimes they do not use "n"= number of electrons. Are they assuming 1e- process? I am a bit confused.

-Omar

Hi Omar

Are you working with DMFC? or reformed fuel PEMFC? These must be the sources for CO2.
Yes, there are many good recent papers from M.M. Mench in JPS- I too have a couple on SOFC modelling.
Regarding your question: BV eqn has many forms, the most widely used is as listed by you (i.e. without n). In fact it comes after the simplification by replacing the fwd and backwards reaction rates with the exchange current density. You may refer to the book by F. Barbir or Colleen Spiegel on PEM. Various good research papers from the group of Prof. Djilali are worth to follow.

Comsol is launching a new add-in module "Fuel Cells and Batteries Module" expected in June 2010. Perhaps, by that time- the current PEMFC model may be extended to include the required two-phase interactions? As far as I know it is currently a major challenge to develop own PDEs for two-phase interactions with all NS, Energy and Species eqn.

Regards
Naveed

Hi Naveed,

Thanks very much for your inputs. They are really helpful. I am actually working with formic acid as a fuel and this is the source of CO2. In fact, it is a micro fuel cell with no membrane.

I also looked at one of your publications online (@ Int J Hydrogen Energ) and is very interesting to me the way you have approached such a problem. I also few questions there. My email address is: microfuelcells@gmail.com Hopefully we can keep in touch!

Thanks,

-Omar