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Is there a difference between how pem fuel cells work and typical batteries work?

  1. May 29, 2012 #1
    hi i've got to do a project for college and need some direction I want to know a few things more so if you can help let me know.

    some of the things include (if you could answer any it would be useful):

    why can't we make a longer electric current to harness all the electricity produced in a certain current (please see en.wikipedia.org/wiki/Fuel_cell and image 'scheme of proton...')
    what happens to the hydrogen if we don't allow it to mix with the oxygen (and instead forward it for a combustion process, can we do this? I mean can ionised hydrogen be lit, I know hydrogen and oxygen can be burned but how come this doesn't make water straight upon introduction to each other such as in a typical hydrogen fuel cell and instead after the combustion, it doesn't make sense)
    is electricity made on hydrogen and oxygen reacting together to make water or is it made through the electrons being passed through the circuit (or both)

    please don't mind if these questions don't make any sense I'm really trying to learn and ask the questions which don't make sense to me. thanks in advance
     
  2. jcsd
  3. May 30, 2012 #2

    chemisttree

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    We can.

    Ionized hydrogen is another name for acid. In this case it exists as the acid form of the membrane material itself. The ionized hydrogen is made at the same rate that it combines with oxygen to make water and the two processes cannot be decoupled, so this represents a slow type of combustion where the electron transfer between hydrogen and oxygen is harnessed to do work across some useful electric load rather than going entirely into heat where it is inefficiently used to do useful work.
    Electrons have to go through the circuit to make water, so the best answer is both.
     
  4. May 30, 2012 #3
    Thank you very much for your response, really appreciate it. So what I've understood so far from the replies of my question is that both current and combustion cannot happen as it is more or less the same thing (i.e. current is a form of slow combustion)
    However, I wanted to know more in the case of the harnessing electricity, basically I've appreciated that electrolysis (giving out electric into water ie using up) and combination of oxygen and hydrogen (giving back electric ie allows us to gain) equals 0 (0 being a simple term as I don't know the correct term for this), so there is no point of trying to make electricity like this; however, if we pass electrons through a circuit wouldn't this give us extra electricity to utilise? ie (combination + current passage) - (electrolysis) = 1 (again 1 being used just for illustration).
     
  5. May 31, 2012 #4

    chemisttree

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    I don't understand how you con conclude that both cannot happen after I told you that both do.

    You electrolyze water to produce hydrogen which only serves the purpose of storing the electrical energy as chemical energy. Using that hydrogen in a pem cell converts it back into electricity.
     
  6. Jun 1, 2012 #5
    sorry I'm still a little puzzled; what I meant from the first comment was that I know both can happen but not at the same time right? I mean we cannot pass it through the current first and do combustion after this.

    What I mean from the second comment is that basically the way you explained that electricity is produced, for me it sounded like the electricity is produced purely from combining both the hydrogen and oxygen which can directly be used as a source of electricity what i didn't know was that this still has to pass a circuit to produce a current and therefore use-able electricity.
     
  7. Jun 1, 2012 #6

    chemisttree

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    Now you have it exactly. During combustion, the electrons from atomic orbitals of hydrogen are 'donated' to a new molecular orbital that is part of both the hydrogen atom and that of oxygen. In effect, the electrons of hydrogen have flowed from an orbital exclusive to hydrogen to an orbital that includes oxygen. That is combustion described purely from the standpoint of what happens to the electrons in the process. The PEM cell intercepts those electrons, flows them through a load circuit (to do useful work) and they wind up in the same place... a molecular orbital of the new oxygen/hydrogen combination. The proton exchange membrane prevents the neutral hydrogen and oxygen from coming in contact but it allows the ionized form of hydrogen to pass through. So neutral hydrogen is ionized, the electron from that process is passed through a circiut and winds up on the oxygen side of the membrane along with the ionized hydrogen. For the process to work, the electron and the ionized hydrogen must find a way over to the oxygen side of the membrane. The electron's path is via a conductor and the proton's path is via the membrane. They both must arrive together on the other side of the membrane with the oxygen for the process to continue. If something stops either the ionized proton (membrane failure) or the the electron (open circuit or an infinite load), they cannot both arrive together and produce a molecule of water.
     
  8. Jun 2, 2012 #7
    thank you very much for your help, I completely understand it now, it should be easier for me to carry out my project. Thanks again
     
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