Using light instead of heat, researchers at the Department of Energy’s Oak Ridge National Laboratory have found a new way to release carbon dioxide, or CO2, from a solvent used in direct air capture, or DAC, to trap this greenhouse gas. The novel approach paves the way for economically viable separation of CO2 from the atmosphere.
The on-demand release of carbon dioxide is possible because the long-lived excited state of a novel acid controls the solution’s proton concentration using ultraviolet light, creating conditions that lead to CO2’s energy-efficient release. By contrast, current DAC technologies filter air through an aqueous solution containing a sorbent material, such as an amino acid, that takes up atmospheric CO2 and holds it. Heating the solvent releases the CO2 and regenerates the amino acid for recycling. The CO2 can be either stored or converted into value-added products, such as ethanol, polymers or concrete.
“In the existing direct-air-capture technologies, CO2 release and sorbent regeneration are the most energy-intensive steps,” said ORNL chemist Yingzhong Ma, who led the study published in Angewandte Chemie International Edition with ORNL colleagues Radu Custelcean and Uvinduni Premadasa, both chemists. “The goal here is to use the amino acid sorbent, which is recyclable and has a lot of attractive properties, combined with a more energy-efficient approach to release the CO2 and regenerate the sorbent.”
The National Academy of Sciences concluded that DAC technologies have a role in removing billions of tons of CO2 from the atmosphere annually to help limit the rise in average global temperature to less than 2 degrees Celsius (about 4 degrees Fahrenheit). However, the intensive energy cost associated with sorbent regeneration and CO2 release at a scale that would mitigate climate change makes such a massive deployment a grand challenge necessitating the development of new DAC processes. The ORNL-led approach provided a proof of concept for using irradiation with ultraviolet light under ambient conditions instead of heating the solution to release the CO2 and regenerate the sorbent.
“Heating aqueous solutions is a common regeneration method, but it is extremely energy intensive,” said Custelcean, a pioneer in DAC. “We wanted to take heat out of the equation.”
Custelcean led a study in 2017 that proved a guanidine sorbent could directly capture CO2 from air. In 2018, he and colleagues demonstrated a practical, energy-efficient DAC method using solar heat to drive the release of the greenhouse gas from an amino-acid sorbent. This year, Knoxville-based startup Holocene licensed the technology to prepare it for industrial deployment.
In this new development, the key to releasing CO2 at ambient conditions is a photoacid, which is a molecule that becomes more acidic when it absorbs light. Shine a light on an acid such as vinegar and nothing happens. By contrast, expose a photoacid to ultraviolet or visible light, and a chemical group in the middle of the acid rotates from the opposite side of a bond to the same side. A subsequent reaction forms a ring, leading to transfer of a proton, or hydrogen ion, to the water solvent. This transfer dramatically increases the acidity of the solution, producing a change called a “pH swing.” The excess protons can now interact with bicarbonate, or HCO3-, which was made when CO2 reacted with the sorbent. The bicarbonate accepts a proton to become carbonic acid, or H2CO3, which is just one energetically favorable step away from carbon dioxide and water.
“This paper describes the first time where the macroscopic pH swing lasting from minutes to hours has been demonstrated using light as an external trigger to initiate the CO2 regeneration reaction,” said