two phase flow combined with porous media

Hi

I have a 3D model of a DMFC channel. The physics associated with this channel are:

1. Secondary current distribution

Cathode side:
2. Transport of concentrated species
3. Free and porous media flow


Anode side:
4. free and porous media flow
5. Transport of diluted species

I have methanol and water on the anode side while CO2 will be formed during the electrochemical reaction.
The model only considers one phase flow. This model works well.

Now I want to modify this model for a two phase flow. In such a condition CO2 will be formed as gas bubbles.
I presume I should use the multi phase, bubbly flow for that. But my question is that
1. how to connect all the physics on the anode side together (I already gave the velocity BC in free and porous media physics).
2. Which boundary conditions I need to define for the bubbly flow
3. How can I say CO2 is the only composition in the gas phase not for example H2O.

any help is really appreciated.