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I understand in a fuel cell, that oxidation occurs at the anode where hydrogen gas is split into hydrogen ions and electrons, the former of which travel through the electrolyte. Then, at the cathode, oxygen combines with hydrogen ions and electrons and is reduced. What I don't understand is the point of the hydrogen ions. Some answers I've gotten from Google:

  • To complete the circuit (how? I'm not entirely sure how circuits work as I haven't studied physics, but I don't get how they complete the circuit, especially considering that the electrons and hydrogen ions are travelling in the same direction while having opposite charges?)
  • To combine with oxygen for reduction (this kinda makes sense; however, this might be a dumb question, but why can't the oxygen just be reduced into something else without using H+ ions?)

I guess the question I'm asking in a more broad sense is: why are ions important in fuel cells (and more broadly, galvanic cells). I'm only a high school student, so although I've searched quite a bit to try to find answers, I haven't been able to and hence why I'm asking here. Thanks in advance everyone!

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  • $\begingroup$ Keep in mind that every local element has to be in about balance of overall charge. Otherwise it would extremely quickly reach such a potential that would stop any current. Or it would cause the opposite current, electric sparcles, lightning or coulombic explosion. All depending on how much you would be determined to ignore such balance requirement. $\endgroup$
    – Poutnik
    Commented Mar 2, 2023 at 13:05

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None of your questions are dumb.

The electrons travel from the anode to the cathode when looking at the outside system (the wire who is connecting the two). The hydrogen ions move from the anode to the cathode. So you see that the negatively charged electrons move in the outside circuit to meet again with oxygen and protons at the cathode. By saying "completing the circuit" what is important to take away is that there is no charge accumulation, otherwise said the charge shouldn't pile up somewhere. This wouldn't work as charges repel each other and it would take more and more energy to keep piling them up. In a classical electric circuit electrons go around in loops, never piling up somewhere, as such "completing the circuit". (It's more nuanced than this, but not important for your question).

Oxygen can be reduced to something else without using H+, it can be reduced to the superoxide anion. It is a possibility, also reducing to hydrogen peroxide (with some hydrogen ions) can happen. But when reducing together with H+ it forms a very stable H2O and as such it is energetically better (get more 'electricity' for your buck).

Lastly, to finish the general remark at the end, ions are important in electrochemical reactions because we don't just need to make the reaction work. We also need to combat this charge buildup. We often add a "supporting electrolyte" to improve the conduction of the solution. The ions will then behave similar to electrons in a wire and will push each other slightly to reduce the charge build-up. If you did not have a lot of ions, then you will quickly get a lot of charge in one place and it will take a while for it to get from one electrode to the other.

Is this somewhat clear?

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  • $\begingroup$ Perfect, I think I understand; your remark about how completing the circuit is essentially just preventing charge accumulation makes a lot of sense $\endgroup$ Commented Mar 3, 2023 at 10:54
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TL;DR
It is both of what you found. To create water, you need hydrogen and oxygen in some form. The design of a fuel cell requires electrons to be moved and in exchange, some other charge has to move, here it is provided by the protons.

The lengthy explanation:
If we just combine oxygen and hydrogen and ignite it, it would (violently) react to water. It does this because it is an exothermic reaction. The interesting thing about a fuel cell is that we can control the flow of the energy that is set free during this reaction. We do not want to have just sudden heat. So at this point, a fuel cell is really just burning hydrogen with extra steps. But no matter how these steps look like, in the end you need both reaction partners. So yes, it is about "combine with oxygen for reduction".

Now to the "complete the circuit" part, and here it is important how the extra steps of burning hydrogen look like. In order to control the reaction, we now separate the gases with a membrane that can only be passed by hydrogen cations (protons). But molecular oxygen does not form a stable product with just protons, so we give the oxygen a few more electrons such that we have $O^{2-}$ and voilá, this is a nice reaction that can take place: $2 H^+ + O^{2-} \rightarrow H_2O$. Now we have to get the extra electrons for the oxygen from somewhere, and we have to put the spare electrons from the hydrogen somewhere, and again this is a perfect match. Since we blocked the direct way with the membrane, we use a wire to transfer the electrons from one place to another. We can plug in electrical load here which is driven by the energy that would normally be released by burning hydrogen (minus some losses). So you see, you need the proton to complete the circuit. No ions, no electric current. No electric current, no reaction.

About your other questions: Yes you can use many different ions. This is what happens in batteries. A fuel cell is simply speaking a battery with certain specifications. Here is a list with many actually used combinations: Electric battery - Wikipedia

In any galvanic cell, you are creating an imbalance of charge due to the electrons pushed from one to another cell. This imbalance causes the reaction to stop at a certain point (very fast). The charge difference is then so large that it outweighs the reaction enthalpy and nothing happens anymore. To compensate, charges have to be moved from one part of the cell to another. A classic is to move sulfate ions via a salt bridge, because they don't participate in the reaction but can compensate the charge.

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