Year after year, humans pump more carbon dioxide (CO2) into the atmosphere than nature can remove, fueling global warming. As the need to mitigate climate change becomes increasingly urgent, scientists are developing ways to actively remove CO2 from the atmosphere in addition to cutting emissions.
One of the biggest hurdles to scaling current carbon capture technologies is the vast amount of energy they consume, but what if there was an alternative that uses an abundant, cheap power source? A team of researchers at Harvard University recently took a major step toward that goal. Their technique, outlined in a Nature Chemistry study published August 13, harnesses sunlight to efficiently trap CO2.
They’re not talking about slapping solar panels on direct air capture systems that run on heat and electricity. This approach is based on specially designed molecules that use light to change their chemical state and reversibly trap CO2.
Harnessing the power of photochemistry
The methodology the researchers developed is a significant departure from leading direct air capture technologies. These systems tend to rely on chemical solvents or porous sorbents that readily bond to CO2, pulling it out of the air. But the ability to reuse those materials—and harness the carbon for practical use—requires a huge input of energy (usually heat) to release the trapped carbon into a container.
“If you want a practical way to pull carbon dioxide out of the atmosphere and then release it into a tank where you can use it, you need the solution—or whatever medium you’re going to use—to be able to both capture and release. That’s the key,” co-author Richard Liu, an assistant professor of chemistry and chemical biology at Harvard, told Gizmodo.
“Our innovation here is that we began thinking about whether you could use light directly to do that,” he explained.
To that end, Liu’s team synthesized organic molecules called “fluorenyl photobases” that do exactly that. When exposed to sunlight, they rapidly release hydroxide ions that capture CO2 from ambient air by chemically binding to it. In the absence of light, the reaction reverses, releasing the trapped CO2 and reverting the photobase back to its original state.
Scaling a new solution
Through a series of experiments, the researchers determined that the most effective fluorenyl photobase for CO2 capture was PBMeOH. This molecule showed no CO2 capture in the dark but the highest capture rate when exposed to light. What’s more, testing showed that a PBMeOH-based carbon capture system is stable and can complete many cycles with minimal loss of efficiency.
“They only fade about 1% per cycle, so you could imagine only replenishing every 100 cycles,” Liu explained.
This work demonstrates a reversible system for carbon capture that relies solely on sunlight as the direct energy input, highlighting photobases as a promising alternative to traditional sorbents.
The results are encouraging, but Liu and his colleagues will need to clear several hurdles before they can turn their framework into real-world technologies. They’re already working to address several challenges, such as the engineering aspects of how the system will expose these compounds to light and dark.
While the “best” approach is still unknown, photochemical systems present some key advantages over existing technologies, Liu said. Exploring new ways to remove CO2 from the atmosphere is more urgent than ever before. “Because we can’t get rid of every source in the short term, carbon capture from the atmosphere—and from point sources, especially—is going to be an important part of the solution.
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