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I would like to build a small carbon dioxide generator for experimental purposes. There are two requirements that are set in stone:

  1. It has to be electrochemical. (Easy to switch on/off with electricity.)
  2. It has to generate only one gaseous product, which is carbon dioxide. (No need to separate the gases, which might prove difficult.)

I have thought and tried using oxalic acid in a cell with a proton exchange membrane, which did not work. (And I have not figured out why so far.) I kept generating oxygen instead of carbon dioxide. Also the byproduct was hydrogen, which is problematic in my application, and the PEM is very fragile, so I could not build up any pressure. There are a few requirements, that are debatable. For example if a reactant or product is highly toxic or extremely expensive, that would possibly be a dealbreaker. Also it would be nice to be able to enclose the cell and build up a bit of pressure with carbon dioxide (only 1-2 bar).

Unfortunately researching this is rather difficult, since the main topic of research is binding carbon dioxide, which are 99.9% of the publications that pop up with the keywords I have searched for. Are there other things I could try? I have not tried a lot and I am already running out of ideas, but please bear with me, I am a theoretician and have not been in the lab for quite a long time.

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    $\begingroup$ Maybe not to your spec but you can readily buy dry ice - could you not keep some of it around in a deep freeze and use as-needed? It'll still sublimate but might be enough of a solution to get you on your way. $\endgroup$ Jan 31, 2022 at 2:13
  • $\begingroup$ Carbon dioxide is easily liquefied at common temperatures. The most efficient way to release it from a cylinder is with a solenoid valve, which could use far less electricity than that required to produce the same amount by electrolysis. A solenoid toggle valve, e.g., smcpneumatics.com/MTV-3-M5.html , would use no power when static, i.e., open or closed, so would be more efficient, yet. Also, the raw material is pure CO2, so there is no waste to dispose, later. $\endgroup$ Jan 31, 2022 at 3:12
  • $\begingroup$ @ToddMinehardt Unfortunately that is not at all possible in my setup. While it would be relatively easy to get, storage really is the main problem there. I do not think the deep freeze would be sufficient for the timescales I had in mind and Dewars are really expensive (not even sure a Dewar would be much better than a styrofoam box in the deep freeze anyways). $\endgroup$
    – d.oelert
    Jan 31, 2022 at 4:12
  • $\begingroup$ @DrMoishePippik I am well aware of solenoid valves and CO2 in bottles. The ease of a solenoid valve comes with additional costs though. Pressure reducer, cylinder and all the other stuff (hell throw in a MFC for good measure, if I want to have any chance of accurate electronic control over the flow) quickly add up to an enormous cost. This question was not about how to get CO2 from a bottle, but how to generate it electrochemically. $\endgroup$
    – d.oelert
    Jan 31, 2022 at 4:17
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    $\begingroup$ Buy a pellet gun that uses CO2 cartridges. Do not load pellets. Stick a tube into the nozzle of the gun (pistol) and from there into your apparatus. 12 gram CO2 cartridges (800 psi) are available. A gun shop can probably help you. Or you could put a balloon onto the pistol - bang, bang - detach the balloon and attach to your apparatus (not much over 1 atm then, tho. $\endgroup$ Jan 31, 2022 at 15:05

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The Kolbe electrolysis may be helpful to you. It is an electrochemical decarboxylation process with free radicals as intermediates, with applications in syntheses of symmetric alkanes.
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$\ce{H2}$ is generated as a side product on the cathode, but using aprotic solvents or a substance with a higher reduction potential than that of $\ce{H2}$, like $\ce{CuSO4}$, should fix this. Alternatively, as suggested by Poutnik, consider separating the compartments with a porous glass.

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    $\begingroup$ There is always possible to arrange cathode + anode divided by salt bridge, diaphragm or porous glass to have both gases separated. $\endgroup$
    – Poutnik
    Feb 2, 2022 at 7:48

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