Plato's Trousers
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I was trying to help an A level student with the hydrogen/oxygen fuel cell, but this really has me stumped! (I am sure I am missing something silly here).

This diagram shows the more negative half cell (hydrogen) as the cathode. Fair enough.

http://tinypic.com/view.php?pic=a1pso5&s=8#.U0v7jE3TWUl


But then when you google fuel cells, why do all the other examples of the hydrogen oxygen fuel cell show the hydrogen electrode as the anode? e.g this one

http://americanhistory.si.edu/fuelcells/basics.htm

How can the hydrogen side be the anode when it has the more negative half cell potential?

On the other hand, if it's the cathode, then why are the OH- ions migrating towards it?


I will rep the most satisfactory answer. (I shall also get the facepalm and getmecoat emoticons ready...)
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langlitz
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(Original post by Plato's Trousers)
I was trying to help an A level student with the hydrogen/oxygen fuel cell, but this really has me stumped! (I am sure I am missing something silly here).

This diagram shows the more negative half cell (hydrogen) as the cathode. Fair enough.

http://tinypic.com/view.php?pic=a1pso5&s=8#.U0v7jE3TWUl


But then when you google fuel cells, why do all the other examples of the hydrogen oxygen fuel cell show the hydrogen electrode as the anode? e.g this one

http://americanhistory.si.edu/fuelcells/basics.htm

How can the hydrogen side be the anode when it has the more negative half cell potential?

On the other hand, if it's the cathode, then why are the OH- ions migrating towards it?


I will rep the most satisfactory answer...
Umm the picture shows the hydrogen side as the anode in the first picture.
H2+2OH- -> 2H20+2e- Eo = -0.83 (oxidation => anode which is negative)
1/2O2+H2O+2e- -> 2OH- Eo = 0.40 (reduction => cathode which is positive )

Eo= 0.4-(-0.83) = 1.23 V

Not really seeing the problem here
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Plato's Trousers
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(Original post by langlitz)
Umm the picture shows the hydrogen side as the anode in the first picture.
H2+2OH- -> 2H20+2e- Eo = -0.83 (oxidation => anode which is negative)
1/2O2+H2O+2e- -> 2OH- Eo = 0.40 (reduction => cathode which is positive )

Eo= 0.4-(-0.83) = 1.23 V

Not really seeing the problem here
Ahh.. part of it is that I switched my anode/cathode terminology! Anode is the negative one. Duhh

alright, we agree that the hydrogen side should be the negative electrode. So why are the OH- ions migrating towards it?
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langlitz
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(Original post by Plato's Trousers)
Ahh.. part of it is that I switched my anode/cathode terminology! Anode is the negative one. Duhh

alright, we agree that the hydrogen side should be the negative electrode. So why are the OH- ions migrating towards it?
Alright I see where you're coming from now
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flyhigh99
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(Original post by Plato's Trousers)
I was trying to help an A level student with the hydrogen/oxygen fuel cell, but this really has me stumped! (I am sure I am missing something silly here).

This diagram shows the more negative half cell (hydrogen) as the cathode. Fair enough.

http://tinypic.com/view.php?pic=a1pso5&s=8#.U0v7jE3TWUl


But then when you google fuel cells, why do all the other examples of the hydrogen oxygen fuel cell show the hydrogen electrode as the anode? e.g this one

http://americanhistory.si.edu/fuelcells/basics.htm

How can the hydrogen side be the anode when it has the more negative half cell potential?

On the other hand, if it's the cathode, then why are the OH- ions migrating towards it?


I will rep the most satisfactory answer. (I shall also get the facepalm and getmecoat emoticons ready...)

Hmm if it helps,
I remember learning in physics how if you draw a circuit diagram, electrons travel from negative to positive right,

but within the battery it must move the opposite direction,
Perhaps to do with gaining potential energy? (going the opposite way to gain energy)
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flyhigh99
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(Original post by Plato's Trousers)
Ahh.. part of it is that I switched my anode/cathode terminology! Anode is the negative one. Duhh

alright, we agree that the hydrogen side should be the negative electrode. So why are the OH- ions migrating towards it?
Ah that's where my confusion lies!
electrons flow out of the anode but itself is positive and it is the negative terminal?
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langlitz
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http://chemistry.about.com/library/weekly/aa082003a.htm
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Plato's Trousers
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hang on, so anode and cathode can be either negative or positive?

Never knew that.

But I am still wondering why OH- ions seem to be moving towards the negative electrode (whatever it's called)

:confused:
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Borek
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Well migration due to the electric field can be ignored in typical setups, it is orders of magnitude slower than the diffusion.

If you look at every derivation of the function describing current vs applied potential in every electrochemical analytical method, be it polarography, voltammetry, CV, whatever, it is only diffusion that is taken into account. Migration is always ignored, as it doesn't matter.

It starts to being observable at very high potentials (Wien effect). But we are talking about gradients in the range of at least MV per cm.
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charco
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(Original post by Plato's Trousers)
hang on, so anode and cathode can be either negative or positive?

Never knew that.

But I am still wondering why OH- ions seem to be moving towards the negative electrode (whatever it's called)

:confused:
Anode is negative in electrochemical cells but positive in electrolytic cells. It is the electrode where oxidation occurs in all cells.
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flyhigh99
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(Original post by charco)
Anode is negative in electrochemical cells but positive in electrolytic cells. It is the electrode where oxidation occurs in all cells.
What is the difference between those two cells?

So in terms of fuel cells and half cells, which ones are they?

I thought all batteries were the same except some can be rechargeable if you reverse the direction of the equation.
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go-study-hard
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(Original post by Plato's Trousers)
I was trying to help an A level student with the hydrogen/oxygen fuel cell, but this really has me stumped! (I am sure I am missing something silly here).

This diagram shows the more negative half cell (hydrogen) as the cathode. Fair enough.

http://tinypic.com/view.php?pic=a1pso5&s=8#.U0v7jE3TWUl


But then when you google fuel cells, why do all the other examples of the hydrogen oxygen fuel cell show the hydrogen electrode as the anode? e.g this one

http://americanhistory.si.edu/fuelcells/basics.htm

How can the hydrogen side be the anode when it has the more negative half cell potential?

On the other hand, if it's the cathode, then why are the OH- ions migrating towards it?


I will rep the most satisfactory answer. (I shall also get the facepalm and getmecoat emoticons ready...)
Isn't it just because some diagrams show the fuel cell using an alkaline electrolyte and other diagrams use an acidic electrolyte?
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O.huxley
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Kinda out of my depth here but I'll give it a go.

The hydrogen and the oxygen each lose their electrons by different amounts, hence setting up a potential difference on either side of the electrolyte.

The electrons are unable to cross the electrolyte hence they have to flow through the wire.



I think the confusion comes from the fact that the first equation they have written for the "anode" - hydrogen electrode, they have grabbed straight from a data booklet. However when you are using that equation in the context of this question it has to be reversed because it is the hydrogen and hydroxide reaction that is happening forwards. This means that the E cell value has to be taken as the negative. i.e.

2H2O + 2e- --> H2 + 2 OH- E = -0.83v

Gets reversed making H2 the reactant to;

H2 + 2OH- --> 2H2O + 2e- E = +0.83v

Now the hydrogen electrode has the most positive E cell value (+0.83v > 0.4v) and hence why they have referred to the hydrogen electrode as the anode - the positive electrode.

Assuming all of the above is right, this would also explain the migration of the OH- ion - I think
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