The capture of CO2 from chimneys could make an important contribution to mitigating climate change, but there are two obstacles. One is that you have to store that CO2 somewhere (such as underground reservoirs). The other is that the capture process requires energy, so your power plant ends up producing less electricity per unit of fuel. This entails financial costs.
Attempts are being made to overcome both hurdles, but there are also other possible approaches. One that sounds obvious and appealing is to convert that CO2 into something useful and valuable, rather than just reservoir filler. The sticky wicket here is chemistry. Carbon dioxide is fairly stable and turning it into something else can require a large energy input.
However, Wajdi AlSadat and Lynden Archer of Cornell University are toying with a possible process that could convert CO2 to merchandise – and to generate electricity while you’re at it.
The process involves an electrochemical cell, where chemical reactions create potential differences 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 charge it, but to harvest both the electricity and the chemical products resulting from a supply of carbon dioxide and oxygen.
Others have experimented with similar processes using cathodes made of lithium, sodium or magnesium, but the Cornell researchers chose a cheaper metal that isn’t as highly reactive: aluminum. Opposite the aluminum anode is a cathode made of a fine stainless steel mesh. The electrolyte that links them together is a bit 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 takes place in the cell. Oxygen molecules gain an electron, giving them a negative charge. These charged oxygen molecules react with the CO2eventually building negatively charged molecules of C2O4-also known as oxalate. The oxalate interacts with positively charged aluminum atoms of the anode, and there you have it.
Now you could reverse that reaction and get your aluminum back by providing electrical energy, but you would also be releasing CO2 gas, which in this case would defeat the purpose. Instead, the researchers suggest draining that aluminum oxalate for conversion to oxalic acid, which is used for cleaning and as an input for some other industrial products. That conversion essentially swaps hydrogen for the aluminum, so you could close the loop and return “fresh” aluminum to the electrochemical cell.
To judge whether this is a good idea, the researchers tried to estimate the total impact of all this on CO2 emissions. First, they added up the emissions associated with the production of aluminum for the anode from raw ore, which amounts to about 7.9 kilograms of CO2 emitted per kilogram of aluminium. If the aluminum is recycled, it drops to just over 5.8 kilograms of CO2 for every kilogram of aluminum.
On the other hand, they combined the chimney CO2 captured by this process, the value of the electricity generated by the electrochemical cell (as measured by the emissions a natural gas plant would emit to generate the same amount of electricity), and the fact that their method of synthesizing oxalate is cleaner than the most used currently. All together that is 9.3 kilograms of CO2 emissions rescued per kilogram of aluminum involved. So while there are no free lunches, this appears to be a net reduction in greenhouse gas emissions.
Of course, the scalability of this technology would depend in part on having enough appetite for oxalic acid, but worrying about the size of an empty stall before the first calf is born is getting a bit ahead of the curve. There are several stables and far too few carbon-storing cows are working.
Open Access Scientific progress2016. DOI: 10.1126/sciadv.1600968 (About DOIs).