This paper has picked up some coverage and headlines, not all of which are accurate. It’s from a group at Oak Ridge that’s looking at methods to use carbon dioxide as a chemical feedstock, and that’s a worthy goal. If we could usefully turn CO2 back into methanol, methane, or some other such material we would effectively be running combustion in reverse, with great benefits for our own atmosphere and probably the world’s economy as well.
This paper is unusual in that it has carbon dioxide being turned into ethanol, so there’s not only a reduction reaction going on, but a carbon-carbon coupling. It’s an electrochemical reaction, done with an exotic new material, copper nanoparticles deposited on the surface of a “carbon nanospike” electrode. This seems to have unusual properties, because what look like broadly similar electrode materials don’t accomplish the same result at all. The whole thing is a good illustration of what large changes can occur at the nanoscale – you’d think that copper is copper, and carbon is carbon, but that’s not true. Carbon can be diamond, graphite, or a zillion things in between, and copper (or any other metal) is a very different beast when it’s partitioned down into tiny particles. What size and what shape those particles can also make a vast difference. Nanotechnology is all about exploiting these differences, but the tricky parts are (1) getting the same materials reproducibly, (2) exploring their gigantic landscape of possibilities in some sort of productive way, and (3) trying to understand what’s going on and why once you find something useful. None of those steps are straightforward.
The headline writers should have read the conclusions section of the paper, however, where it says that “The overpotential. . .probably precludes economic viability for this catalyst“. Basically, you have to use more electric power to get ethanol this way than the resulting ethanol can possibly be worth. The authors suggest some ways that this might be overcome, but those will be matters for a lot of further experimentation. It’s worth noting (and the paper has many references to this effect) that if you just want to chew up carbon dioxide electrochemically, you can already do that with existing technology. The big issues are the cost of the electricity you need to run such a process, and where that electricity comes from. If the amount of carbon dioxide emitted to generate all that electricity is more than the amount you’re removing and turning back into reduced carbon feedstocks, the whole thing is as useful as a vacuum cleaner that sprays extra dirt out the back.
Since combustion itself gives off energy, you’re always going to be using energy to run it in reverse – you can’t escape thermodynamics. But that still gives you a lot of room, with one sort of catalyst or another, to do it in an efficient enough way that makes economic sense. After all, plants are quietly taking up carbon dioxide and converting it to reduced carbon compounds all around us, albeit at only a handful of carbon dioxide molecules per second per enzyme molecule, using sunlight as their energy source in the water-oxidation side of the process. It could well be that some sort of new electrolysis setup, as in this latest work, could do the trick. But as the Oak Ridge team and the others working in this area well know, scale is going to be a big problem here.
Carbon dioxide is found at its largest concentrations where it’s being emitted, but there’s often a fair amount of other gunk coming along with it. You not only have to be able to do your conversion on a gigantic scale, but your catalytic system also has to be able to operate for extended periods without fouling or being consumed by side reactions. (A number of such installations are already running to see how they perform under real-world conditions). If on the other hand you’re going to just quietly pull CO2 out of the ambient atmosphere, you’re going to be working at much lower concentrations of your starting material, so being able to run on a useful scale becomes difficult for that reason as well. The size, surface area, and flow characteristics of such an installation will be the subject of a lot of engineering expertise, and I think it’s fair to say that we’re not yet close to the efficiency needed to make something like this viable.
This latest work is certainly interesting and worth following up on – once you get up to the high electric potentials needed, it works at pretty good efficiency from an electron-use standpoint, and the production of ethanol (instead of methanol) could be a good feature, too. The amount we don’t know about nanoengineered electrode surfaces is vast, and there are surely a lot of useful things to be found in the field. But this isn’t the magic carbon-source solution that some of the stories are claiming. Not yet.