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I'm building an electrolysis cell myself, and got to wondering. On Earth, there is a significant force pushing bubbles of hydrogen and oxygen upward, so your electrodes still contact mainly water. However, in space, I imagine the bubbles would either diffuse around the electrode tank, or cluster around the electrode itself, and thus the speed of reaction would be significantly lowered.

Basically, I imagine that it would be hard to conduct electrolysis in zero gravity because you lose essentially all control of where the output gas goes. How do they solve this?

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I guess the generic answer for any mass separation system would be that you spin it, but mass separation isn't really effective for electrolysis since that requires larger separation between anode and cathode which leads to protonic ohmic losses, i.e. increased internal resistance. So a more optimal species separation is done not by their mass but by their size and charge, usually through a Proton Exchange Membrane (PEM).

PEM electrolysis allows for tighter electrode placement to reduce ohmic losses, allows for higher current and is tolerant to relatively high pressures. Water if fed on the anode end, and as current flows from cathode towards the anode, water is separated into hydrogen and oxygen, positively charged hydrogen is attracted through PEM towards the negatively charged cathode, and the negatively charged oxygen is repelled away from cathode towards the positively charged anode:

   enter image description here

  Diagram of PEM electrolyzer cell and the basic principles of operation. Animation source: Wikipedia on PEM electrolysis

PEM electrolysis is also main source of oxygen on the International Space Station, and they use perchlorate candles as backups (for emergencies or when the electrolysis machine is not operational):

The generator operates on a simple principle — electrolysis, the dissociation of oxygen and hydrogen from water through the application of electricity. What makes the generator safe? The device uses a proton-exchange membrane (PEM) to separate the generated hydrogen and oxygen.

In a PEM electrolyzer, water is oxidized at the oxygen electrode, or cell anode, to produce oxygen gas, releasing hydrogen ions (protons) and electrons. The hydrogen ions migrate from the cell anode to the cell cathode, or hydrogen electrode, under the effect of the electric field imposed across the cell, while the electrons are transferred by a dc power source. The protons and electrons recombine at the cell cathode to produce hydrogen. Liquid water is also released at the cathode due to a process called electro-osmotic drag. Oxygen and hydrogen are generated in a stoichiometric ratio—two volume units of hydrogen for every one of oxygen—at a rate proportional to the applied cell current.

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  • $\begingroup$ So basically the H2 and O2 are separated by electrical forces? $\endgroup$ – BWG Nov 16 '15 at 15:53
  • $\begingroup$ @BWG No, atomic hydrogen and oxygen ions ($\require{mhchem}\ce{H+}$ and $\ce{O-}$) are. Molecular hydrogen and oxygen ($\ce{H2}$ and $\ce{O2}$) are neutral to electric fields. $\endgroup$ – TildalWave Nov 16 '15 at 16:19

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