The capture of CO 2 from smokestacks could make an important contribution to limiting climate change, but there are two obstacles. One is that you have to store that CO 2 somewhere (like underground reservoirs). The other is that the capture process requires energy, so your power plant ends up producing less electricity per unit of fuel. That comes with a financial cost.

There are efforts afoot to overcome both of those hurdles, but there are also other possible approaches. One that sounds obvious and attractive is to turn that CO 2 into something useful and valuable, rather than just reservoir filler. The sticky wicket here is chemistry. Carbon dioxide is pretty stable, and turning it into something else can require a large energy input.

Cornell University’s Wajdi AlSadat and Lynden Archer, however, are playing with one possible process that could convert CO 2 into a commodity—and generate electricity while you’re at it.

The process involves an electrochemical cell, where chemical reactions cause differences in potential that can move electrons between anode and cathode. Force electrons in the other direction and you can drive those reactions in reverse, like a rechargeable battery. In this case, the idea is not to recharge it, but to harvest both the electricity and chemical products that result from an input of carbon dioxide and oxygen.

Others have experimented with similar processes using cathodes made of lithium, sodium, or magnesium, but the Cornell researchers opted for a cheaper metal that isn’t so highly reactive—aluminum. Opposite the aluminum anode sits a cathode made of a fine stainless steel mesh. The electrolyte linking them is a little more unusual—an ionic liquid and some aluminum chloride salt.

The researchers sent a mixture of carbon dioxide with some oxygen (which was important) through that mesh cathode. A series of chemical reactions follow within the cell. Molecules of oxygen pick up an electron, giving them a negative charge. These charged oxygen molecules react with the CO 2 , eventually building negatively charged molecules of C 2 O 4 —otherwise known as oxalate. The oxalate partners up with positively charged atoms of aluminum from the anode, and there you have it.

Now, you could reverse that reaction and get your aluminum back by supplying electrical energy, but you would also release CO 2 gas, which would defeat the purpose in this case. Instead, the researchers propose carrying off that aluminum oxalate for conversion to oxalic acid, which is used for cleaning and as an input for some other industrial products. That conversion basically swaps hydrogen for the aluminum, so you could close the loop and return “fresh” aluminum to the electrochemical cell.

To come up with a verdict on whether this is a good idea, the researchers tried to estimate the total impact of all this on CO 2 emissions. First, they tallied up the emissions associated with producing aluminum for the anode from raw ore, coming up with about 7.9 kilograms of CO 2 emitted for each kilogram of aluminum. If the aluminum is recycled, that drops to something more like 5.8 kilograms of CO 2 for each kilogram of aluminum.

On the other side, they combined the smokestack CO 2 captured by this process, the value of the electricity generated by the electrochemical cell (as measured by the emissions a natural gas power plant would emit to generate the same amount of electricity), and the fact that their method of synthesizing oxalate is cleaner than the most common one currently used. All together, that’s 9.3 kilograms of CO­ 2 emissions saved per kilogram of aluminum involved. So while there are no free lunches, this would appear to be a net reduction of greenhouse gas emissions.

Of course, the scalability of this technology would partly rely on a sufficient appetite for oxalic acid, but worrying about the size of an empty barn before the first calf is born is getting a little ahead of oneself. There are multiple barns out there and far too few carbon-capturing cows at work.

Open Access at Science Advances, 2016. DOI: 10.1126/sciadv.1600968 (About DOIs).