Researchers use light

The details of the study, led by Maria Lukatskaya, Professor of Electrochemical Energy Systems at ETH Zurich, were published in the journal ACS. 

Acid switch

The team at ETH Zurich utilized the principle that CO2 exists in its gaseous form in acidic aqueous solutions. In contrast, in alkaline aqueous solutions, it undergoes a reaction to produce carbonates, referred to as salts of carbonic acid. This chemical transformation is reversible, and the acidity level of a liquid decides whether it contains CO2 or carbonates.

To manipulate the acidity of their liquid medium, the researchers introduced photoacids, molecules responsive to light. When this liquid is exposed to light, these molecules induce acidity. Conversely, in the absence of light, they revert to their original state, making the liquid more alkaline.

The process of isolating CO2 from the air starts with guiding the air through a liquid enriched with photoacids without light. Given the alkaline nature of the liquid, CO2 undergoes a reaction, forming carbonates. Once the accumulation of salts in the liquid reaches a notable level, the researchers expose the liquid to light. 

This light exposure induces acidity, prompting the conversion of carbonates back into CO2. Like the bubbles observed in a cola bottle, the CO2 is released from the liquid and can be gathered in gas tanks. Upon achieving minimal CO2 levels in the liquid, the researchers deactivate the light source, initiating a cyclic process. The liquid is then prepared to capture CO2 once more, according to the team.

In practice, a challenge arose as the photoacids used proved unstable in water, decomposing within a day. The team addressed this by shifting their reaction to a water and organic solvent blend. Through lab experiments and model calculations by Sorbonne University researchers, they optimized the liquid ratio. This mixture stabilized the photoacid molecules for nearly a month and facilitated the reversible switching between acidity and alkalinity using light. According to researchers, omitting water from the organic solvent would render the reaction irreversible. 

Sustainable process 

In contrast to other cyclical carbon capture methods, an established technique involves filters that trap CO2 molecules at ambient temperature. Subsequently, to release the captured CO2 from the filters, heating to approximately 100 degrees Celsius is necessary. However, the energy-intensive nature of heating and cooling constitutes a significant portion of the energy consumption in the filter-based method. 

"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," said Anna de Vries, a doctoral student in Lukatskaya's group and lead author of the study, in a statement. 

Researchers now aim to enhance the stability of photoacid molecules. Simultaneously, they explore the overall process parameters in-depth to achieve further optimization.