Northwestern University has developed a "humidity swing" technology for direct air carbon capture (DAC) that uses a series of ions to capture carbon dioxide at low humidity and release it at high humidity. This research increases understanding of DAC and provides a more energy-efficient method of carbon capture than traditional technologies.
As global society moves toward decarbonizing industrial production, it will be necessary not only to prevent the creation of new carbon in the atmosphere, but also to extract the carbon dioxide that is already there.
While traditional carbon capture focuses on collecting carbon dioxide from the point of emission in a carbon-heavy process, "direct air capture" (DAC) extracts carbon under normal atmospheric conditions. This approach is becoming increasingly important in the fight against climate change, especially as our reliance on fossil fuels decreases and the need to capture carbon at the source decreases. Using humidity technology, scientists have discovered several new ions that contribute to low-energy carbon sequestration.
New research from Northwestern University demonstrates a new way to capture carbon from ambient conditions, which examines the relationship between water and carbon dioxide in a system, informing "humidity swing" technology that captures carbon dioxide when humidity is low and releases it when humidity is high. This method combines innovative kinetic methods with a variety of ions to remove carbon from almost anywhere.
The research was recently published in the journal Environmental Science and Technology.
Vinayak P. Dravid of Northwestern University, the study's senior author, said: "Not only have we expanded and optimized the selection of carbon capture ions, we have also helped to reveal the fundamentals of complex fluid-surface interactions. This work advances our collective understanding of DAC, and our data and analysis provide strong motivation for theorists and experimentalists to further improve carbon capture under real-world conditions."
Dravid is the Abraham-Harris Professor in the Department of Materials Science and Engineering at Northwestern University's McCormick School of Engineering and director of global initiatives at the International Nanotechnology Institute. Doctoral students John Hegarty and Benjamin Shindel are the co-first authors of the paper.
Schindel said the idea behind the paper came from a desire to use environmental conditions to promote responses. "We like wet pendulum carbon capture because it has no explicit energy cost. Although humidifying a certain amount of air requires a certain amount of energy, ideally you get the humidity 'for free' and energetically rely on a natural reservoir of wet and dry air adjacent to the environment."
The research team also expanded the number of ions used to make the reaction possible.
"Not only have we doubled the number of ions that can achieve ideal humidity carbon capture, but we have also discovered the highest performing system to date," said John Hegarty.
In recent years, humidity swing capture technology has begun to emerge. Traditional carbon capture methods use adsorbents to capture carbon dioxide at a source location and then use heat or a created vacuum to release the carbon dioxide from the adsorbent. The energy cost of this approach is high.
Traditional carbon capture methods hold onto carbon dioxide, which means a lot of energy is required to release and reuse it. This approach doesn't work everywhere either. For example, agriculture, concrete and steel manufacturers are major sources of emissions, but their large footprints make it impossible to capture carbon from a single source. Wealthier countries should strive to reduce emissions below zero, while developing countries with a more carbon-based economy should reduce their production of carbon dioxide.
Another senior author, chemistry professor Omar Farha, has extensive experience exploring the role of metal oxide framework (MOF) structures in a variety of applications, including carbon dioxide capture and storage.
"DAC is a complex, multifaceted problem that requires an interdisciplinary approach," Farha said. "What I appreciate about this work is the detailed and careful measurements of complex parameters. Any proposed mechanism must account for these intricate observations."
Past researchers have focused on carbonate and phosphate ions to promote moisture swing trapping and have developed specific hypotheses as to why these specific ions are effective. But Dravid's team hopes to test a wider range of ions to see which ones work best. Overall, they found that the ions with the highest valence states—primarily phosphates—were the most effective, so they began looking for multivalent ions, excluding some, and found new ions that were effective for this application, including silicates and borates.
The team believes that future experiments coupled with computational modeling will help better explain why some ions are more effective than others.
There are already companies working to commercialize direct air carbon capture, using carbon credits to incentivize companies to offset emissions. Many companies are capturing carbon already captured through activities such as changing agricultural practices, whereas this approach could explicitly sequester carbon dioxide directly from the atmosphere, then concentrate it and ultimately store or reuse it.
Dravid's team plans to combine this carbon dioxide capture material with a porous sponge platform they developed earlier to remove environmental toxins including oil, phosphates and microplastics.