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Process takes CO₂ from the air, converts it to carbon nanotubes

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Carbon capture and storage involves the separation of carbon dioxide from other gases, after which it's pumped underground for storage. It's likely to be needed to reach our climate goals without simply shutting down many existing fossil fuel plants, and it will be essential if we overshoot our emissions goals by mid-century. But it also adds significant costs to building and operating fossil fuel plants, which explains why the process has never gotten past the point of sporadic demonstration projects.
An alternative to storage involves turning the carbon that's captured into a useful product—something the XPrize has made one of its challenges. Doing so requires two things: overcoming the chemical stability of CO2 and making a product that sells at a profit. We recently stumbled across a bit of creative chemistry that turns carbon dioxide from the air into a product that should be profitable: high-quality carbon nanotubes.

Something in the air

Our current methods for making carbon nanotubes typically rely on hydrocarbons. The chemistry of this source helps drive the tube-forming reactions, since it can be energetically favorable to remove the hydrogens from these molecules. Unfortunately, this doesn't get rid of CO2, and it's only good for emissions in the sense that some of the carbon ends up in nanotubes instead of the air.

But the new paper relies on some dramatically different chemistry to supply the carbon. It uses an electrochemical process to deposit carbon at one electrode and liberate oxygen at a second. And it relies on a molten salt that decomposes during the process and regenerates itself by sucking CO2 out of the air.

The salt in question is lithium carbonate, or Li2CO3, which is molten at a reaction temperature of 723°C. Decomposition produces dilithium oxide (Li2O, but not as dilithium crystals), and liberates both carbon and oxygen. The oxygen gets released in molecular form (O2) at the anode thanks to an aluminum oxide catalyst. And the carbon gets deposited on an iron catalyst on the cathode—under these conditions, it forms carbon nanotubes. The Li2O, meanwhile, reacts with carbon dioxide in the air and re-forms the lithium carbonate salt.

The research largely focused on controlling the growth of the nanotubes. All nanotubes are variations on a theme, with a hexagonal mesh of carbon atoms rolled into a cylinder. But the spacing between atoms allows nanotubes with different diameters. It's also possible to have a series of concentric tubes, with each wrapped around a smaller one—these are called "multiwalled." Smaller diameters and single-walled tubes are generally worth more, so the researchers focused on adopting the molten salt synthesis to produce the most valuable nanotubes.
Not in control

The gist of the research is that it's a challenge. Although the iron-based catalyst was initially layered evenly on top of the cathode, the high temperatures allowed it to move around. As a result, it formed little bubbles on the surface of the electrode, with the size of the bubbles being dependent upon the thickness of the layer that formed them. The size of those bubbles, in turn, helped determine the size of the nanotubes that formed on them. So, the best way to get a small-diameter nanotube is to use a very thin layer of catalyst to start with.

The problem was that this relationship didn't hold for long. If the reaction was stopped quickly—after three minutes—more of the nanotubes were narrow in diameter. But if the reaction was continued for a half-hour, the diameters gradually crept up. An examination of the electrode showed that, over time, diffusion of the catalyst drew it into larger and larger bubbles, and the diameter of the nanotubes grew accordingly. So, there's still some work to do with structuring the catalyst.

But the authors did some calculations that suggest doing so could be worth the effort. If you figure out the cost of the electricity needed to heat the salt up and drive the reactions, it costs half of what you can sell the nanotubes for.

That's not to suggest that this means we've figured out how to sequester carbon. The market for nanotubes is pretty small, so even making all of our carbon nanotubes this way wouldn't put a dent in our massive emissions. But it's a nice demonstration that some interesting things are possible when we step beyond the typical chemistry, and it hints that it might be possible to do similar things on much larger scales.

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