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September 2, 2004, 13:11 
Fuel cells: charge conservation equation

#1 
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I'm trying to modelling this equation with a UDS (us described in Journal of the Electrochemical Society, 147 (12) 44854493 (2000)). I've reduced to the most basic form my 3d mesh by representing anode and cathode as interfaces, instead of layers of finite thickness. My question is about the source terms in charge cons. equation, since these terms are volumetric how could they be referred to the relative interface???
Thanks in advance!!! Tim 

September 3, 2004, 04:50 
Re: Fuel cells: charge conservation equation

#2 
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I am working on fuel cells, although not hydrogen fuel cells, so I am not totally familiar with them, but I have a rudimentary understanding. When you talk of the charge cons. Equation are you referring to the catalyst layer? If this is modelled as a 3d zone then the source them is indeed volumetric. However, if you are modelling it as a 2D interface then the source terms of the boundary condition are the flux crossing that boundary. I actually have the paper you refer to, so you can mention specific equation/figure numbers if you wish.


September 3, 2004, 05:38 
Re: Fuel cells: charge conservation equation

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Hi!!!Thanks a lot for your reply.
When I talk of charge cons. equation( eq. [6] in the paper) I'm referring to the electrolyte membrane and the source terms ([13],[14]) are dependent on potential in different way. I'm modelling gas diffuser and catalyst layers with an unique interface (both for the cathode and the anode) and the volumetric source terms should be referred to them, but which expression could I use for the flux J in my UDS transport equation??? I've read the messages on this argument, but I'm still confused about it. 

September 3, 2004, 09:23 
Re: Fuel cells: charge conservation equation

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OK, I have read the part of the paper you refer to, although I have seen explained much better in other documents. S_thi is the source term for the charge cons. equation. i.e. the equation for calculating the potential distribution. The reaction produced or consumes electrons that results in a current. The current is the source term for the potential in the change cons. equation. In the catalyst layer, where the reaction is occurring, the source term is the current j, in the diffusion layer and membrane there is no reaction and so the source term is 0.
Is that helpful? 

September 3, 2004, 09:24 
Re: Fuel cells: charge conservation equation

#5 
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OK, I just read you original post again. If you are modeling you anode and cathode as interfaces, you must provide the current (j) as a UDS flux for your bounday condition. i.e. the potential flux.


September 3, 2004, 10:22 
Re: Fuel cells: charge conservation equation

#6 
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OK, but in (13) and (14) the surface overpotential eta (15) is dependent on the potential phi ... could this create some problem?
Is it correct to multiplying J by the catalyst layer's thickness and imposing these two boundary conditions with DEFINE_UDS_FLUX()? A final question: I've to refer these BCs to the relative surface (i.e. ID zone)by means of the macro DEFINE_ADJUST as for DEFINE_SOURCE??? THANK YOU VERY MUCH!!! 

September 6, 2004, 04:58 
Re: Fuel cells: charge conservation equation

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In answer to your first question, the only problem you will have is that of convergence to a final solution, but you will have to figure out how to solve this yourself.
Second question: the multiplying J by the thinkness sounds like a logical plan, although, I would do some calculations to varyify if this method is valid. If you are defining a surface boundary condition I would use a DEFINE_PROFILE, not DEFINE_UDS_FLUX. Third, Sorry, I don't really understand what the question is. 

September 9, 2004, 05:44 
Re: Fuel cells: charge conservation equation

#8 
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Thank you for your useful tips, I'm working on them and my error messagges are decreasing. Tim


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