Fuel Cell Electrons: Anion Movement, Protons, and DC

[Nim]

From what I understand about fuel cells, they generate power by influencing electrons to move themselves around an ion-conducting material to get to the other side. While the electrons make there way from one side to the other, they supply DC in the middle of there journey. But I am not quite sure why they are going to the other side.

Exactly how far does the electron actually stray away from the protons on the other side to get to them (that's what they are going for right)? Why do they leave the positive side (anode) to get to the negative side (cathode)? I know that they are attracted to protons, but the protons are farther away from them than the anode is and they are on the cathode side which should be repeling them.

 

[chroot]

In a proton-exchange membrane fuel cell, protons go right through the membrane from one side to the other. The electrons cannot pass the membrane (it's semipermeable), and, in order to reunite with the protons, must go around the electrical circuit.

- Warren

 

[jdavel]

Chroot,

There doesn't seem to be anything attracting the protons across the membrane to the cathode. Is it just entropy?

 

[chroot]

It's just a diffusion situation. There are no hydrogen ions on one side of the membrane, but many on the other. The hydrogen ions thus diffuse across the (quite permeable) membrane.

Proton-exchange membrane fuel cells are made using an organic membrane, like polyfluorosulfonic acid. The hyrogen ions on the acid groups are essentially free. Just like the free electrons in an electrical conductor allow electrons to move from one place to another, the free protons in the membrane allows protons to move easily from one place to another.

Here's a good page:

http://www.dodfuelcell.com/proton.html

- Warren

 

[Nim]

In a proton-exchange membrane fuel cell, protons go right through the membrane from one side to the other. The electrons cannot pass the membrane (it's semipermeable), and, in order to reunite with the protons, must go around the electrical circuit.

That's the part I understand. The part that I don't understand is why they would bother going around the membrane just to reach the protons when there is an anode right there infront of them. Plus the protons are on the negative side. I don't know how far they have to travel to get to the other side, but they are supposed to supply DC current. It's hard to imagine an electron going far out of its way to get to a proton, especially when it has to move farther away before it can start moving closer.

There are no hydrogen ions on one side of the membrane, but many on the other.

What happens to all of the excess electrons then? Are not all of the electrons recombining with the hydrogen ions on the other side?

 

[chroot]

The membrane is about as thick as saran wrap. The electrons do indeed recombine with the protons on the other side.

- Warren

 

[Nim]

Yeah, I know that.

 

[chroot]

Nim,

I'm not sure what you don't understand. The protons go through the membrane simply because of diffusion; the electrons cannot follow. They go through the circuit to come back and reunite with the protons. That's the whole story.

- Warren

 

[jdavel]

Nim said: "It's hard to imagine an electron going far out of its way to get to a proton, especially when it has to move farther away before it can start moving closer."

The same thing happens when you discharge a capacitor.

Fields in conductors are complicated, but the results they give are easy. If the net charge in a conductor is zero, the charge density at any point is zero. The electrons do whatever's needed to make that happen.

Edit: But the idea that a single electron goes all the way around to the other side is misleading. The whole sea of electrons in the conductor moves a tiny bit in that direction, and that's enough to neutralize a proton in the cathode.