I have a simple electrolysis setup which uses aqueous sodium chloride as electrolyte. The problem is that, the setup makes tons of chlorine gas along with hydrogen (I only want hydrogen). I didn't even realize it until I recognized that the odor smells like that of ozone. I am using a power supply of 24V @ 6A. Since, it's a pain to buy a new power supply, changing the supply isn't an option. Also, I have experimented with other electrolytes like aqueous sodium bicarbonate but they are far slower than the brine solution. So, the only other option is to consume the chlorine gas. I am not sure what to use here. I don't have a lot of reducing agents on hand. Any ideas?

  • $\begingroup$ If anyone is curious as to how I know what ozone smells like, I have experimented with lots of high voltage. It makes your throat feel dry. Chlorine does that too.. $\endgroup$ – AvZ Aug 24 '14 at 13:15
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    $\begingroup$ plainly keep your cell at temperature well above 50+C. In such conditions sodium chloride is oxidized to chlorate and later perchlorate. Another option is to try and use something else, like sodium hydroxide or magnesium sulfate. $\endgroup$ – permeakra Aug 24 '14 at 13:28

The issue is that oxidation of chloride to chlorine has a lower overpotential than oxidation of water to oxygen. You will always have to have an oxidation reaction going on while reducing water to hydrogen. The one with the lowest standard potential + overpotential will win. The reason the overpotential exists is that electrolysis is pushing the system away from equilibrium, so we need to provide additional electromotive force to get that to happen. The value of the overpotential is related to the activation energy of the redox event.

For example, with a graphite electrode, the overpotential for the oxidation of water is +0.95 V, while the overpotential for the oxidation of chloride is +0.12 V. When you compare the standard reduction potentials for these two reactions, we can see that chlorine is produced more readily (smaller overall voltage needed).

product    Red. Pot.    Overpot.     Total Pot.
O2         +1.30 V      +0.95 V      +2.25 V
Cl2        +1.36 V      +0.12 V      +1.48 V

To avoid chloride production, you need to get ride of chloride. Switch to sodium hydroxide. You have plenty of voltage in your power supply to overcome the overpotential of water/hydroxide.


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