Hybrid flow cell system improves CO₂ direct air capture technology

Over the past decades, energy engineers have introduced a wide range of systems and approaches aimed at mitigating climate change and preserving the environment on Earth. Given that global dependence on fossil fuels is likely to continue for the foreseeable future, many of these technologies focus on capturing carbon dioxide (CO₂), one of the primary greenhouse gases contributing to climate change.

CO₂ capture technologies could be deployed at industrial sites, power plants, and other facilities that are known to emit large amounts of CO₂. One promising approach for the direct extraction of CO₂ from the air is known as pH-swing electrochemical capture.

This method, which could be powered using clean energy solutions, is designed to absorb and release CO₂ via a reversible reaction prompted by changes in acidity (pH). Despite their potential, pH-swing electrochemical capture solutions introduced so far have been found to deteriorate over time, which results in a reduced CO₂ capture capacity because of the oxidation of redox-active molecules by O₂.

Researchers at the University of Chinese Academy of Sciences, Harvard University and Westlake University have devised a new approach that could improve the reliability of pH-swing electrochemical capture technology.

This approach, outlined in a paper published in Nature Energy, relies on a hybrid flow cell system, an electrochemical device that could prevent undesirable chemical reactions between the molecules, enabling the extraction of CO₂ from the air, even in the presence of O₂.

“The build-up of atmospheric CO₂ concentrations has been happening so rapidly, and the world’s efforts to mitigate greenhouse gas emissions have been proceeding so slowly that, by mid-century or so, humanity may find it necessary to pull CO₂ out of the air,” Michael J. Aziz, co-senior author of the paper, told Tech Xplore.

“We have been developing methods of doing this that can be powered by clean, emissions-free electricity.”

For over five years, Aziz and his colleagues have been trying to develop a reliable electrochemically driven pH swing approach for the direct capture of CO₂ from the air. The main objective of their recent study was to overcome a key limitation of the system they have been working on.

“Some organic molecules can accept electrons and protons at the same time and remain stable before and after,” explained Aziz. “When we electrochemically drive electrons onto these molecules through an electrode, they pull protons (H⁺) off water (H₂O) molecules leaving hydroxide (OH^–) behind, thereby raising the pH of the solution.

“The hydroxide in the high-pH solution captures CO₂ by reversibly reacting with it. When we then electrochemically pull the electrons off the organic molecules, all these processes are reversed, causing the CO₂ to be released into a collection chamber for utilization or sequestration.”

The researchers have been assessing the viability of this approach for the capture of CO₂ for some time. Their earlier implementations, however, relied on organic molecules that were dissolved in water, which were found to prompt undesirable chemical reactions that hindered their system’s overall performance.

“The problem is that when this aqueous solution contacts air, or flue gas, the abundant oxygen (O₂) in the air or flue gas reversibly reacts with these dissolved molecules, messing with the CO₂ capture process,” said Aziz.

“The innovation reported in this paper is to build these organic molecules into a solid polymer instead of dissolving them. This way, the molecules inject hydroxide into the solution when they’re electrochemically driven to do so, and the hydroxide-bearing solution is sent out to capture CO₂ from the gas bearing CO₂ and O₂, but the organic molecules never come into contact with the O₂.”

The researchers evaluated their updated CO₂ capture system in a series of tests and found that it reliably separated the air from molecules that were reactive to O₂, capturing CO₂ with a coulombic efficiency of 99%.

These promising results highlight the potential of their approach, suggesting that it could enable the large-scale and energy-efficient capture of CO₂ directly from the air.

“Our proposed system could pave the way for the highly efficient capture of CO₂ from the air, despite the presence of O₂,” added Aziz.

“We are now continuing to explore methods of capturing CO₂ directly from the air, seeking the lowest-cost approaches that are safe and scalable.”

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