Fuel Cells; PEMFC; Cause of Proton Movement

[Menninger]

Lately I've started learning about fuel cells, particularly proton exchange membrane fuel cells (PEMFC), and a fairly basic question that has been bugging me is: Since the cathode is positive, it is clear why the electrons flow through the load and to the cathode, but why do the protons/hydrogen ions also move towards the cathode given that their charge ought to repel such movement?

From the very rudimentary knowledge I possess, the only thing I can think of is concentration gradients created by the continuous production of hydrogen on one end and consumption on the other. But I want to be sure - and I ask my question here because the answer has, for whatever reason (my incompetence or something else), been nontrivial to look up.

http://upload.wikimedia.org/wikipedia/en/5/5c/Fc_diagram_pem.gif
Thanks

 

[PWiz]

I'm not 100% sure, but from what I can remember from my chem classes is that hydrogen loses electrons at the electrode on the left in your diagram and this makes it negative. Consequently, this causes electrons to move to the electrode on the right, which intially makes this one negative and the left one positive. This causes the protons to move to the electrode on the right and accept these electrons. Hydrogen molecules are continuously losing electrons to the left electrode, and therefore this electrode (the source of the electrons) has a more negative electric potential (more electrons) than the right one at any particular instant (which are immediately transported away). Electrons are continuously accepted at the right electrode (electrons move towards it) and hence it has the more positive electric potential (fewer electrons) in the circuit at any instant (which are immediately accepted by the protons).

Remember that a current is being generated in the fuel cell, so the electron producing electrode is always negative and the accepting one positive. The accepting electrode only stays positive because any incoming electrons combine with the H+ ions and this maintains its positive potential (which would otherwise make the electrode become more negative over time till the potential difference reaches 0). I've not used the terms cathode and anode to avoid confusion here because contrary to a battery, current is generated here and the roles are reversed.

 

[Menninger]

Yes, thanks.

That is eventually what I found in a paper on another website. The movement is in accordance with an electrochemical gradient. The electric gradient is caused by what you say. The "chemical" gradient refers to the fact that random motion of the atoms tends towards equal concentrations on either side of the membrane - and of course the chemical reactions catalyzed ensure that equal concentrations never actually occur.