PEM fuel cells, why the membrane?

[Topher925]

For those of you that arent very familiar with PEM fuel cells, their shortcomings basically revolve around the carbon/platinum membrane and is essentially the "heart" of the fuel cell. But why does a fuel cell need a membrane to function? If you think about it, just about all energy conversion devices can switch from generating to producing and vise versa just by changing the "effort" part of the power components. For example a motor can become a generator by raising and lowering the torque and voltage on the machine. A hydraulic gerotor can go from motor to pump by adjusting torque or pressure.

Now the reverse equivalent of a PEM fuel cells is an electrolysis cell. Almost identical in concept except for that pesky membrane. You can even reverse a PEM fuel cell to make it an electrolysis cell but you can't reverse an electrolysis cell to make a fuel cell unless you have that membrane. Now it just seems to me that their just has to be away to sort of "reverse" that electric field created in electrolysis cells to make the reaction happen the other way without the membrane. Theoretically, does this sound possible? Is there some law that will prove my theory wrong? If there is one I am thinking its going to be the second law of thermodynamics.

 

[Topher925]

Does no one have any insight or is this just a stupid question?

 

[Borek]

I would not call it a stupid question, but you seem to be missing the most important part of the whole process. Membrane is there to separate anode and electrode solutions, so that they HAVE to exchange charge through the external circuit. If you remove the membrane reaction happily goes in the solution ignoring the electrodes and you get nothing but some heat.

 

[Topher925]

Hm...I forgot about that. For some reason I was thinking the electrons wouldn't transfer through the electrolytic solution like during electrolysis(ion migration, hydronium and such). I guess technically the device does work both ways, just on the micro level and not the macro level unless you have that membrane.

 

[Cryptonic26]

Also consider that you actually can get a reverse reaction from a basic electrolytic cell. It's going to be a very weak power generator because you're getting diffusion of the gases across both electrodes (and releasing much of the gas as waste) however. It's brought down to the micro level because it's easier to dictate where the protons are going.. directing and concentrating the protons greatly improves the efficiencies of the process, making fuel cells practical as a chemical->electrical energy converter device.

 

[Topher925]

Its a good thing this forum is around. I was about the build one of these things to test some of these theories I have had. Yes good point about the concentration increasing the efficiency. Still, it just seems that there has to be a better way. I know the max efficiency is about 94.3%ish and the very best designs are no where near that.

 

[ekrim]

The membrane is only one costly component of a fuel cell. It's actually the electrodes which are made from carbon and platinum. The redox reaction kinetics at the electrodes are optimized with platinum as a catalyst (for a hydrogen-oxygen fuel cell with acidic membrane), so platinum is not replaceable.

The membrane is essentially Teflon, which has been phosporized to have a hydrophilic end that conducts protons. It is much more effective than suspending the electrodes in electrolyte solution. It is extremely thin and conducts the protons very effectively. It's ineffective to apply oxygen and hydrogen gas to electrodes in solution.

 

[mheslep]

Of course there are other fuel cell designs that do not require expensive precious metal for a membrane nor for the the electrodes.
http://en.wikipedia.org/wiki/Solid-oxide_fuel_cell
High temperatures replace the expensive PEMs catalyst, Nickel based anodes, LSM cathodes, and hydrocarbon fuels can be used directly in the anode instead of pure hydrogen.

 

[ekrim]

A professor at UIUC I think has been working on a membraneless fuel cell. Though this only works on a microscale and is not yet considered practical.