Photoacids are molecules that release protons when exposed to light, allowing pH control in both space and time. These light-triggered pH switches are helpful for various applications, including capturing CO2. However, some photoacids, like protonated merocyanine, have drawbacks such as limited stability (less than 24 hours) against hydrolysis and low solubility, which restricts their effectiveness in buffered solutions like dissolved CO2.
In a new study, scientists at ETH Zurich introduced a simple pathway to dramatically increase the stability and solubility of photoacids. Through this, they are looking forward to removing CO2 from the atmosphere.
The new method uses light. Maria Lukatskaya, a Professor of Electrochemical Energy Systems, and her team are using the fact that in acidic liquids, CO2 exists as CO2, while in alkaline liquids, it forms carbonates. The liquid’s acidity controls this reversible liquid.
Scientists introduced photoacids—molecules that respond to light—into the liquid. When illuminated, the molecules make the liquid acidic; in the dark, they revert to the original state, making the liquid more alkaline. This process allows them to control the pH and influence the presence of CO2 or carbonates.
The scientists use a process to capture CO2 from the air. They pass the air through a liquid containing photoacids in the dark, making the liquid alkaline. In this state, the CO2 reacts and forms carbonates. Once enough carbonates accumulate, the researchers expose the liquid to light, turning it acidic.
This causes the carbonates to transform back into CO2, which bubbles out of the liquid, similar to how carbonation occurs in a cola bottle. The collected CO2 can then be stored in gas tanks. After most of the CO2 is removed, the researchers turn off the light, and the cycle begins anew, with the liquid ready to capture more CO2.
Anna de Vries, a doctoral student in Lukatskaya’s lab and the study’s lead author, said, “In practice, however, there was a problem: the photoacids used are unstable in water. During our earliest experiments, we realized that the molecules would decompose after one day.”
Scientists analyzed the decay of the molecule. They solved the problem by running their reaction not in water but in a mixture of water and an organic solvent. They determined the optimum ratio of the two liquids through laboratory experiments and explained their findings thanks to model calculations.
The scientists achieved two important outcomes with their mixture. Firstly, it allowed them to maintain the stability of the photoacid molecules in the solution for almost a month. Secondly, it facilitated the reversible switching of the solution between acidic and alkaline states using light. This success wouldn’t be possible if they used an organic solvent without water, as it would lead to an irreversible reaction.
Some carbon capture processes, like the established method using filters at ambient temperature, are also cyclical. In this method, CO2 molecules are collected by filters, but to release the captured CO2, the filters need to be heated to approximately 100 degrees Celsius. However, this heating and cooling process is energy-intensive and constitutes a significant portion of the energy required for the filter method.
Lukatskaya says, “In contrast, our process doesn’t require heating or cooling, so it requires much less energy.”
de Vries explains, “Another interesting aspect of our system is that we can go from alkaline to acidic within seconds and back to alkaline within minutes. That lets us switch between carbon capture and release much more quickly than in a temperature-driven system.”