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Questions on fuel cells

  1. Apr 5, 2004 #1
    Being fairly new to the whole fuel cell thing (or at least how it works), I have a couple questions.

    I have learned so far that Proton Exchange Membrane (PEM) fuel cells work using the following device and process:


    Why must the hydrogen be combined with oxygen at the end of the process? Why can't you recycle the hydrogen and do the process over again?

    Last edited: Apr 5, 2004
  2. jcsd
  3. Apr 5, 2004 #2
  4. Apr 5, 2004 #3
    I think you answered your own question. You can recycle the hydrogen but you have to put energy in to split the water molecules. You can't have a perpetual motion machine so the hydrogen must react in some way at the end. When hydrogen is burned it forms water as the energy is released.

    I am guessing here but is the driving force simply diffusion of the hydrogen from a high concentration to low concentration? If so, you must react the hydrogen in order to get a low conc. at the output. If you didn't react the H then diffusion would stop.
  5. Apr 5, 2004 #4
    It is probably better that the used hydrogen reacts with water to form H2O, because as mmwave said, it is probably too impractical to reuse the hydrogen.

    H2O emissions are generally a lot better to the environment than CO2 and other hydrocarbons :smile:
  6. Apr 6, 2004 #5


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    Staff: Mentor

    Quick clarification here: its not a matter of practicality, combining hydrogen with oxygen is how you get the energy - its the whole point of having a fuel cell.
    ...until you factor in where the energy required to make the hydrogen came from, of course.
  7. Apr 6, 2004 #6
    Hmm I must have misunderstood, then. I was thinking that the hydrogen broke up into an electron and a proton, with the proton traveling through the proton exchange membrane and the electron going through a seperate wire. I assumed that the electron could simply combine with the h again and the process could be repeated.

    So in order for the hydrogen proton and electron to recombine, they must combine with the O2 to form water?
  8. Apr 6, 2004 #7
    So the whole process could not occur without the water in the cathode. I see now. Doh! :biggrin:

    Thanks everyone.
  9. Apr 6, 2004 #8


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    Staff: Mentor

    The force propelling the proton through the membrane is the same force propelling the electron through the wire. This is almost exactly the same as how any other battery works: ions flow one direction through the battery and electrons flow the other direction through the wire, meeting back in the battery.
    Essentially yes.
  10. Jan 11, 2005 #9
    It could also be so that the consumer will buy more hydrogen. This hydrogen car is going to be just like a gasoline car. Big, bulky, amp-consuming machines are going to be needed in order to take the water and convert it into hydrogen, so that we the people may buy it for $5.00 a gallon. Of course, with proper research one day we might be able to make a small enough water to hydrogen converter, to where it could fit in the hydrogen car, and we can simply fill our cars up with free water, but they won't let that research happen.
  11. Jan 11, 2005 #10
    Not true. There is no real evidence that industry has stymied research in any way. In fact, both government and industry have worked hard for innovations in new fuel systems.
  12. Jan 11, 2005 #11
    I'm opening a parenthesis here. If all present car emissions were H20 instead of CO etc. wouldn't the humidity considerably rise in some regions? Are people starting to forecast what are the consequences of this or is it still too early for serious thought? I predict some new health problems.
  13. Jan 11, 2005 #12


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    Staff: Mentor

    An electrolysis machine can be made as big or small as you want it - the problem is energy. And no, conspiracy theory has nothing to do with it: just the laws of science.
    Car emissions are already mostly water and water already makes up a much larger fraction of our air than CO2 (in the summer and in temperate climates). More water in the air just means more fog and more rain (but an insignificant amount more) - the cycle is already closed. Not a big deal.
  14. Jan 11, 2005 #13


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    Science Advisor
    Gold Member


    Water is not a fuel. If you're going to use that idea, you need a supply of fuel onboard the car. (Batteries anyone?)
  15. Jan 11, 2005 #14
    Pedals. We're all too fat, and all too fast.
  16. Apr 18, 2006 #15
    Electrons and pem

    Physics cw and my basic understanding at stake here!

    please can someone explained to me why electrons are unable to travel through the menbrane and are forced to travel around creating an electrical circuit?
  17. Apr 18, 2006 #16


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    Staff: Mentor

    Ostensibly, the electric resistance of the conductor is less than the resistance through the membrane, and the voltage is such to attract the electrons through the conductor to the load.
  18. Apr 19, 2006 #17

    Thank you for your reply, kind of understand and i was just also wondering, why the protons travel through the membrane at all, when they are positively charged and yet on the other side of the membrane is another positively charged electrode, so why would they be attracted this way ? any wesites that you think i wuld find useful would be very much appreiciated. Because i have found most to be confusing because they seem to say that the anode can be negatively charged, can this happen ? i need help!! :)
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