Isolating CO2 in the electrolysis of Na2CO3 solution

[cafuffu]

TL;DR Summary

When doing the electrolysis of a water solution with Na2CO3 the CO2 will be released at the anode together with the O2, however i only want the CO2. What could be the best way to isolate it?

Hi all! This is my first question, i hope it does make sense :)

I'm trying to find a way to isolate CO2 gas when doing electrolysis of a water solution with Na2CO3. I understand that i can do that with two half cells connected by a salt bridge, and i will get NaOH plus H2 gas in the cathode half cell, and O2 plus CO2 gases in the anode half cell. However i don't care about the O2 so i want to find a way to avoid it mixing with the CO2. As far as i can see there are three ways that could possibly work:
1: Mix all gases together and make the O2 and H2 react, condensate and get rid of the water. This is maybe the easier way, but i would not like to spend a lot of energy to split the water for nothing.
2: Use an hydrogen depolarized anode. I'm not sure i understand exactly what this is, i get it would be a porous anode that could accept the H2 from the cathode and avoid making the O2 in the first place, is that correct? However i can't find where i could get one.
3: Use a three-compartment cell, and have the Na2CO3 in the middle compartment. The CO2 would then be released separately from the O2 and i could catch it alone. This would be my preferred option since i would like to also keep the H2, however I'm not actually sure it is possible.

What do you think is the best option?

 

[Borek]

CO₂ is not a product of the carbonate electrolysis. It can evolve due to some side reactions, but it is in no way efficient way of making it.

 

[cafuffu]

Are you sure about that? I found some material saying it should work: http://www.freepatentsonline.com/4337126.pdf https://pubs.rsc.org/en/content/articlelanding/2018/CC/C8CC00812D#!divAbstract (i don't have the full article).

If the CO₂ does not get released i think it would mean the CO₃ left from the Na₂CO₃ would end up as H₂CO₃ and HCO₃^-, are you saying they stay in water like that? I've seen a couple of youtube videos about this (I have yet to try it), and they claim that when all the carbonate is gone the pH of its half cell is neutral, not acidic.

 

[Borek]

At the anode you are decomposing water producing O₂ and H⁺, the latter can react further with carbonate, so the CO₂ is not a product of electrolysis itself.

When electrolysing stable salts of oxyacids the most common outcome is production of hydrogen and oxygen, salts are inert and present only to increase conductivity of the solution. Carbonates are a bit different, as they decompose in low pH, but a. their solution is always basic, b. produced H⁺ gets reduced at the cathode, so in general its amount in the solution - and the pH of the solution - doesn't change. Preventing mixing and transport can make the concentration of H⁺ to build up locally, to the levels at which CO₂ evolves.

That's a different case from - say - chlorides electrolysis, where chloride gets oxidized to gaseous chlorine, completely different situation.

Still, there are much cheaper ways of producing CO₂, electrolysis will be quite costly and slow.

 

[cafuffu]

Yes, i understand that CO₂ is not a direct product of the electrolysis, but rather the consequence of the fact that all the carbonate ions get left in the anode half cell while the Na+ ions leave it, and if i understand it correctly carbonic acid wants to stay at equilibrium with the CO₂ in the solution and in the atmosphere above, so if the carbonic acid increases, due to the CO₃^- ions binding with the free protons, some of it should evolve into CO₂ and exit the solution.

I know there are cheaper ways to produce CO₂, heating hydrogen carbonate being i guess the easiest, but i want to close the loop: bubble air through NaOH, get Na₂CO₃, extract the CO₂ and get NaOH back. Making Na₂CO₃ react with CO₂ to make NaHCO₃ is not so easy compared to making NaOH react with CO₂. Also, I'm not in a hurry :)

 

[willem2]

Yes, i understand that CO2 is not a direct product of the electrolysis, but rather the consequence of the fact that all the carbonate ions get left in the anode half cell while the Na+ ions leave it, and if i understand it correctly carbonic acid wants to stay at equilibrium with the CO2 in the solution and in the atmosphere above, so if the carbonic acid increases, due to the CO3- ions binding with the free protons, some of it should evolve into CO2 and exit the solution.

The Na+ doesn't really have to disappear. Some might go into the salt bridge, but you get diffusion in the opposite direction because of the difference in concentration. What happens is that the electrolysis of water removes OH^-, and this will make the solution less basic and more acidic. Adding acid to Na₂CO₃ is the easiest method of getting lots and lots of CO2.
Na₂CO₃ solution is pretty basic, so you probably need to run the electrolysis for a time before you get any CO2 out.

I know there are cheaper ways to produce CO2, heating hydrogen carbonate being i guess the easiest, but i want to close the loop: bubble air through NaOH, get Na2CO3, extract the CO2 and get NaOH back. Making Na2CO3 react with CO2 to make NaHCO3 is not so easy compared to making NaOH react with CO2. Also, I'm not in a hurry :)

I think it would work to concentrate CO2 from the air. It's probably quite wasteful if you don't win back the energy from the hydrogen and oxygen, and also because of the low concentration of CO2 in the air, and the fact that you need to separate the Oxygen and CO2.
Maybe you can separate the production of acid by electrolysis and adding the acid to Na₂CO₃ ?

 

[cafuffu]

The Na+ doesn't really have to disappear. Some might go into the salt bridge, but you get diffusion in the opposite direction because of the difference in concentration.

Well, but the Na+ does become NaOH so it can't diffuse back.

Maybe you can separate the production of acid by electrolysis and adding the acid to Na2CO3 ?

Do you mean to have another electrolytic cell that produces an acid and react that with the carbonate? I don't have enough experience with this stuff, would you have an example with which i could close the loop?

 

[cafuffu]

I've done some research and i guess i could use sodium phosphate, Na₃PO₄ which under electrolysis i think should produce NaOH in one half cell and phosporic acid, H₃PO₄, in the other half cell. I could then react the phosporic acid with the sodium carbonate and it should release CO₂, water and the original sodium phosphate. Now the question is whether that is more efficient, since i still lose energy splitting the water and also i guess when reacting the acid with the carbonate, since that is an exothermic reaction of which though i lose the heat.