Molecules sensitive to light may enhance efficiency of carbon capture

Photosensitive molecules called photoacids have the potential to make the process of removing carbon dioxide from the atmosphere more energy efficient. Researchers are currently devising ways to make photoacids more practical.

This can be particularly beneficial for direct air capture (DAC) systems, which blow air over carbon-trapping materials called adsorbents. Existing systems require large amounts of energy to separate pure CO2 from the adsorbent for storage or use elsewhere. This is a major barrier to using DAC to remove billions of tons of CO2 from the atmosphere each year. “Every step I take is hitting a wall,” he says. Anna de Vries At ETH Zurich, Switzerland. “Direct air recovery companies everywhere are struggling and trying to create the most efficient process.”

Adding photoacid to the adsorbent may be effective. When exposed to light, each photoacid molecule changes shape and releases protons, making the solution more acidic. This “pH swing” releases CO2 from the adsorbent and photoacid mixture. When the light is turned off again, the photoacid and pH of the solution return to normal, allowing the adsorbent to absorb CO2 again. This cycle can then be repeated.

Typically, heat or pressure is used to release CO2, but using sunlight or lamps could potentially reduce the energy needed for this step, with the aim of halving the energy requirements of DACs. de Vries says. However, photoacids tend to be unstable and are not very soluble in water, which limits their efficiency in releasing CO2.

De Vries and colleagues added various solvents to the photoacid solution; found the mix This increases the solubility of the photoacid and extends its lifetime from just a few hours to nearly a month.

In another approach, Ubinduni Premadasa Oak Ridge National Laboratory in Tennessee and colleagues found Another photoacid, which can remain responsive to light for longer and produce more acid, allows CO2 to be released from solution more efficiently.

greg match Researchers at the University of Newcastle in the UK say these are an “elegant and innovative” solution. But larger systems can face challenges, such as loss of solvent through evaporation in the air, he says.

Although these researchers focused on capturing CO2 from the atmosphere, the first large-scale tests on photoacids may be conducted in water. A Washington state startup called Banyu Carbon uses photoacids to separate CO2 from seawater and plans to install a system capable of removing one tonne of CO2 per year in 2024.

In this system, when photoacids are exposed to light, the resulting acidity is temporarily transferred to seawater, and CO2 absorbed from the atmosphere is released from seawater. alex gagnonAccording to the company's co-founders, this reduces the energy needed to separate the CO2 and eliminates the need to power fans.