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Carbon Capture System Turns CO2 Into Electricity and Hydrogen Fuel (newatlas.com)
Researchers at Ulsan National Institute of Science and Technology (UNIST) and Georgia Tech have developed a new system that absorbs carbon dioxide and produces electricity and useable hydrogen fuel. New Atlas reports: The new device, which the team calls a Hybrid Na-CO2 System, is basically a big liquid battery. A sodium metal anode is placed in an organic electrolyte, while the cathode is contained in an aqueous solution. The two liquids are separated by a sodium Super Ionic Conductor (NASICON) membrane. When CO2 is injected into the aqueous electrolyte, it reacts with the cathode, turning the solution more acidic, which in turn generates electricity and creates hydrogen. In tests, the team reported a CO2 conversion efficiency of 50 percent, and the system was stable enough to run for over 1,000 hours without causing any damage to the electrodes. Unlike other designs, it doesn't release any CO2 as a gas during normal operation -- instead, the remaining half of the CO2 was recovered from the electrolyte as plain old baking soda. The research was published in the journal iScience.
the hydrogen is in in the "organic electrolyte". the potential energy is in the sodium metal and possibly the electrolyte.
It's not free energy. It's an ordinary and expensive chemical reaction.
“Common sense is not so common.” — Voltaire
The power has been put in upfront to produce the elementary sodium used as one of the electrodes. Not sure if you remember those nice and impressive basic experiments in school, when the teacher took out a tiny piece of elementary sodium, which much be kept away from water and is typical stored in some inert organic solvent. From the scheme that was posted, I would conclude that the sodium electrode is consumed during the process. Producing elementary sodium is a very energy intense process.
The paper can be downloaded (no paywall) from here. Equation 5 "Net equation" is
2Na + 2H+ -> 2Na+ + H2, E0 = 2.71V.
So yes, it works by consuming sodium metal. I am underwhelmed.
How much energy could we get from the metallic sodium if we didn't turn CO2 into NaHCO3 as a side reaction? What is our efficiency at making metallic Na? If these cells are sufficiently cheap, reliable, high power output, and efficient there might be potential for using this for grid scale energy storage in the form of metallic Na. Doing so would have an advantage of energy storage being limited only by your ability to store sodium, so it could work on seasonal timescales. Of course, then you'd have seasonal H2 production which would carry its own storage issues.
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If you check how sodium is produced, you will see that it's done by electrolysis of NaCl. With that you can forget about the production of electricity in the process.
The older production method is carbothermal reduction of sodium carbonate, which releases Carbon Monoxide, which will turn into CO2.
So the energy balance becomes
Na + H2O + CO2 => NaCO3 + H2
0 - 237.1 kJ/mol - 394.4 kJ/mol => -851.0 kJ/mol + 0
-631.5 kJ/mol => -851 kJ/mol
So it works out. The reaction is exothermic and a net 291.5 kJ of energy is released per mole.
But as you point out, the wild card is the Na. The above assumes you can get elemental sodium at zero energy cost. Sodium is extremely reactive and you just don't find it in its elemental state in nature. Depending on how much energy you had to use to refine some sodium compound to create the elemental Na needed for this reaction, it could be a net energy loss.
Carbon Dioxide is a very stable molecule, getting it to react requires a large amount of input energy. While TFA make it sound like we are gaining energy from this process, there is no free lunch.
The reason why this reaction produces energy is because it is consuming pure metallic sodium and converting it to sodium bicarbonate. Pure sodium does not exist in nature at all, because it is so reactive. Manufacturing metallic sodium is an extremely energy intensive process that involves splitting molten salt (Sodium Chloride) into sodium and chlorine gas using electrolysis. This is Downs' Process. The sodium bicarbonate that this process produces has industrial applications, some of them involve reactions that release the CO2 we just spent of ton of energy capturing back into the atomsphere, baking breads and cakes for example.
Any method that involves electrolysis is going to use a ton of energy. If we are going the electrolysis route, then might as well produce hydrocarbons using electrolysis to convert water and CO2 to syngas, which can then be used to produce hydrocarbons via the Fischer–Tropsch process. Hydrocarbons are way more useful from an industrial standpoint. The most obvious is we can burn them to power legacy Internal Combustion Engine vehicles, which closes the carbon feedback loop; but that is just one use. Hydrocarbons can be used as feedstock for all kinds of organic chemistry processes, we can make tons of plastics, polymers, lubricants, carbon fiber, etc. All of these things cannot be produced without oil mining today. The nice thing about hydrocarbon synthesis is that it can replace mined fossil fuels in all our existing petrochemical manufacturing processes. The same cannot be said about baking soda.