Researchers at ETH Zurich are developing a new method to remove CO2 from the atmosphere. They work with molecules that become acidic when exposed to light. The new process requires much less energy than conventional technologies.
In order to slow down global warming, we need to drastically reduce greenhouse gas emissions. Among other things, we need to do without fossil fuels and use more energy-efficient technologies. However, reducing emissions alone will not be enough to meet the climate targets. In addition, large quantities of the greenhouse gas CO2 must be captured from the atmosphere in order to store it permanently underground or to use it as a climate-neutral chemical raw material in industry. However, the CO2 capture technologies available today require a lot of energy and are correspondingly expensive.
Researchers at ETH Zurich are therefore developing a new method that works with light. In future, the sun will provide the energy for CO2 capture.
The scientists, led by Maria Lukatskaya, Professor of Electrochemical Energy Systems, are exploiting the fact that CO2 is present as CO2 in acidic, aqueous liquids, but reacts to form salts of carbonic acid in alkaline aqueous liquids. This chemical reaction is reversible. The level of acidity of a liquid determines whether it contains CO2 or a carbonic acid salt.
In order to influence the level of acidity of their liquid, the researchers added molecules known as photoacids, which react to light. If such a liquid is irradiated with light, the molecules make it acidic. In the dark, the molecules return to their original state, making the liquid more alkaline.
The ETH researchers' method works in detail as follows: the researchers separate CO2 from the air by passing the air through a liquid containing photoacids in the dark. Because this is alkaline, the CO2 reacts and forms salts of carbonic acid. As soon as these salts have strongly accumulated in the liquid, the researchers irradiate the liquid with light. This makes it acidic and the carbonic acid salts convert to CO2. Just like with a cola bottle, the CO2 bubbles out of the liquid. It can be collected in gas tanks. When there is hardly any CO2 left in the liquid, the researchers switch off the light and the cycle starts again.
In practice, however, there was a problem: the photoacids used are unstable in water. "Even in our first experiments, we realized that the molecules had decomposed after one day," says Anna de Vries, a doctoral student in Lukatskaya's group and first author of the study.
Lukatskaya, de Vries and their colleagues therefore analyzed the decay of the molecule and solved the problem by running their reaction not in water but in a mixture of water and an organic solvent. In laboratory experiments, the scientists determined the optimum ratio of the two liquids. Model calculations, which they carried out together with researchers from the Sorbonne Université in Paris, helped them to explain the results scientifically.
On the one hand, this enabled them to keep the photoacid molecules stable in the solution for almost a month. On the other hand, they ensured that light could be used to switch back and forth between an acidic and an alkaline solution. If the researchers used their organic solvent without water, the reaction would be irreversible.
Other methods of CO2 separation are also cyclic processes. One established method, for example, works with filters to which the CO2 molecules attach at ambient temperature. To remove the CO2 from the filters afterwards, they have to be heated to around 100 degrees Celsius. However, heating and cooling are energy-intensive: they account for most of the energy required by the filter method. "In our approach, there is no need for heating and cooling, which is why our method requires significantly less energy," explains Lukatskaya. Additionally, the new method developed by ETH researchers can be operated solely with sunlight.
"What's also interesting about our system is that we can switch from alkaline to acidic within seconds and back to alkaline within minutes. We can therefore switch between CO2 capture and release much more quickly than in a temperature-controlled system," says de Vries.
With this study, the researchers have demonstrated that photoacids can be used in the laboratory for the separation of CO2. On the way to market maturity, the researchers first want to further increase the stability of the photoacid molecule. The researchers also need to study the entire process even better in order to optimize it further.