# Electrolysis/fuel cell reversal question

twiz_

Before I ask, I once read that a fuel cell is not the reversal of an electrolytic reaction, and vise versa. Can someone please elaborate in the following question?

Anyway,
If I had two containers, both enclosed (pressure is out of the question), one with a mixture of hydrogen and oxygen (the infamous "HHO" mixture as they call it, for a lack of a better term), and the other simply with water containing the proper electrolytes for the reaction to take place, is it possible to essentially make a reverse reaction by submerging electrodes in each of the containers? In the perfect system taking place in my mind, I imagine the water would convert to HHO while the HHO in the other container would convert into water with perfect efficiency, but it would obviously be far less than perfect, that is, if it would even work.

If so, are there any examples? If not, I would like to know what's preventing the reaction from taking place.

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twiz_
I forgot to mention that no battery is present. In essence, the hydrogen fuel cell is the battery for the water electrolysis. I assume it would work since you would get the same amount of energy out as you would put in.

twiz_
Perhaps a small battery can compensate for energy loss in the system.

Mentor
Technically nothing wrong with the idea that you can use energy from hydrogen/oxygen fuel cell to electrolyze water. But it won't work for long. Voltage of both cells depends on the partial pressures of the gases involved (as given by the Nernst equation) and the process will stop the moment both voltages get equal (which is inevitable, as you stated both cells are "enclosed." - which I take to mean "not exchanging mass with the surroundings").

Most other fuel cells won't work this way - for example you can't produce ethanol just be passing current through the mixture of CO2 and H2O.

twiz_
Technically nothing wrong with the idea that you can use energy from hydrogen/oxygen fuel cell to electrolyze water. But it won't work for long. Voltage of both cells depends on the partial pressures of the gases involved (as given by the Nernst equation) and the process will stop the moment both voltages get equal (which is inevitable, as you stated both cells are "enclosed." - which I take to mean "not exchanging mass with the surroundings").

Most other fuel cells won't work this way - for example you can't produce ethanol just be passing current through the mixture of CO2 and H2O.

If I understood correctly, the notion that electrolysis is the reversal of a fuel cell is only applicable to certain fuels, in this case hydrogen. Am I correct in saying this? It implies that most other fuels, as you stated, won't work this way, which is why I shouldn't assume from the get go that it's simply a reversal. Squares are rectangles but rectangles aren't squares.

Also, with the addition of a battery as compensation for energy loss and pressure, I had the idea of using hydrogen and oxygen as a sort of working fluid in a system. It isn't technically a traditional working fluid, but it would make it possible to displace the energy without physically displacing the fluid as much (I hope), reducing friction and other sources of energy loss. I'll try to do some experiments with it, and maybe whatever I'm trying to say isn't pseudoscience.

Mentor
electrolysis is the reversal of a fuel cell is only applicable to certain fuels, in this case hydrogen. Am I correct in saying this?

Right.

I had the idea of using hydrogen and oxygen as a sort of working fluid in a system. It isn't technically a traditional working fluid, but it would make it possible to displace the energy without physically displacing the fluid as much (I hope), reducing friction and other sources of energy loss.

Nothing wrong with the idea on the thermodynamical level, I doubt it will be practical.

How are you going to keep the gases separated?

Working liquids have low compressibility, gases have high compressibility.

Electrolysis is rather slow (to be fast it requires kA currents).