Sep 20 2022
Improving CO2 Conversion
Carbon is an extremely useful element. Carbon-containing compounds can be food, fuel, fertilizer, or building material. We also have an overabundance of carbon in the form of CO2 in the atmosphere, with industry producing over 34 billion tons per year. This is why one of the current technological “holy grails” is to develop a cost and energy efficient method of recapturing that carbon and feeding it into a useful production stream at industrial scale. This way a pollutant can be turned into product.
The problem is that CO2 is a stable molecule, and so it costs a lot of energy to break it apart – reversing reactions that produce the energy in the first place. Specifically, we need to split one oxygen off the CO2 to make CO (carbon monoxide). CO can be used in a variety of useful chemical reactions, making hydrocarbons, for example. The way to make reactions happen on useful industrial scales is with catalysts – a molecule that makes a reaction go faster (often by orders of magnitude). Of course the reaction also requires energy, because we want to go from a low energy molecule (CO2) to higher energy molecules (CO and O2). The challenge has been bringing all these elements together.
A new study introduces a new element into the equation – DNA. This may seem counterintuitive at first, but it makes sense when you put the whole picture together. Researchers at MIT were trying to crack this very specific problem – how do they bring together CO2 dissolved in liquid with a catalyst on the surface of an electrode that will be providing the energy? All these elements need to come together in the most efficient way. Further, catalysts can tend to break down with use, and we also need to get the old catalysts off the electrode and replace them with fresh catalyst. You can do this just by diffusing CO2 and catalyst in the liquid with the electrode and let randomness get it done, but this is highly inefficient, and efficiency is the game.
This is where the DNA comes in. The researchers engineered a single DNA strand (which is now really cheap to do and create) to bind to the electrode, which it nicely did. DNA comes in two complementary strands that bond together in the famous double helix. So they then created the complementary DNA strand and attached the catalyst to it. When that DNA was placed in the liquid it readily combined with its complementary strand, effectively trapping the catalyst next to the electrode. All they have to do then is dissolve CO2 in the liquid and power the electrode, and the conversion from CO2 to CO happens, and at greater speed and efficiency than older system. Further, the catalyst does not break down as quickly when bound to the DNA. And further still, when it does come time to replace the old catalyst with refreshed catalyst, all they have to do is heat up the liquid which causes the DNA strands to fall apart. Then can then remove the DNA with old catalyst and replace it with DNA with new catalyst.
There is another advantage here. This system allows the researchers to use molecular catalysts (vs solid state catalysts). These tend to be less efficient, but they have a critical advantage. They can be used to control the byproducts of the reaction. When electrically converting CO2 into other molecules, only some turns in to CO, while much of it turns into less useful molecules. With a molecular catalyst the researchers were able to increase the proportion of the reactions that produce CO.
Nothing in this system should prevent scaling up to industrial use, but as always we won’t know for sure until someone does it. There is still a lot of room for tweaking to optimize the efficiency of the system, but it is already an improvement in efficiency. The idea, of course, is to use an energy source that itself does not produce CO2 (wind, solar, nuclear, hydrothermal) to power the process. CO2 would have to be captured and then fed into this process to create CO for industrial use.
Realistically we’re not going to be able to capture 34 billion tons of carbon per year, at least not anytime soon. The idea, however, is that if we reduce our collective CO2 production by 80-90%, some type of carbon capture can take care of the rest (that last bit of CO2 production by industry that will be the hardest to get rid of). Ideally we will do something useful with that carbon, making the process cost-effective. If companies can make money capturing carbon, someone will do it. As always it’s hard to say if this specific breakthrough will be the one to get to industrial levels of CO2 conversion, but it does seem like a promising approach. There are many promising approaches, and we just need to keep pushing on multiple fronts to maximize the chance that something will work out.