Carbon capture could become practical with scalable, affordable materials

In a paper to be published on Thursday (April 3) in the journal Environmental Science & Technology, the Northwestern researchers present new, lower-cost materials to facilitate moisture-swing to catch and then release CO₂ depending on the local air's moisture content, calling it "one of the most promising approaches for CO₂ capture."

Atmospheric CO₂ continues to increase and, despite considerable worldwide efforts to cut down on carbon waste, is expected to rise more in coming decades. Exploring efficient and economical ideas for how to sequester excess CO₂ from air can help make up ground by offsetting emissions from delocalized sectors like aviation and agriculture, where emissions are particularly difficult to pinpoint and capture.

Moisture-swing direct air capture (DAC), which uses changes in humidity to catch carbon, will be central to global strategies to combat climate change, but its scalability has been limited due to the previously ubiquitous use of engineered polymer materials called ion exchange resins. The team found they could reduce both cost and energy use by employing sustainable, abundant and inexpensive materials -- often sourceable from organic waste or feedstock -- to make DAC technologies cheaper and more scalable.

"The study introduces and compares novel platform nanomaterials for moisture-swing carbon capture, specifically carbonaceous materials like activated carbon, nanostructured graphite, carbon nanotubes and flake graphite, and metal oxide nanoparticles including iron, aluminum and manganese oxides," said Northwestern materials science and engineering Ph.D. candidate John Hegarty, a co-author. "For the first time, we applied a structured experimental framework to identify the significant potential of different materials for CO₂ capture. Of these materials, the aluminum oxide and activated carbon had the fastest kinetics, while the iron oxide and nanostructured graphite could capture the most CO₂."

The paper demonstrates the significance of a material's pore size (pockets of space within porous materials where carbon dioxide can nestle) in predicting its power to capture carbon. The engineers argue this type of research will support the development of design principles to improve performance by modifying a material's structure.

Scaling carbon capture

Traditional methods to directly capture atmospheric CO₂ have failed to be competitive in many markets due to their high costs and technical complexity. More accessible and lower-cost DAC technologies could offset the emissions from agriculture, aviation, and concrete and steel manufacturing sectors that are challenging or impossible to decarbonize through renewable energy alone.

"The moisture-swing methodology allows for CO₂ to be sequestered at low humidity and released at high humidity, reducing or eliminating the energy costs associated with heating a sorbent material so it can be reused," said McCormick School of Engineering Ph.D. graduate Benjamin Shindel. According to Shindel and the study's other authors, the modality is appealing because it enables carbon removal from virtually anywhere and can leverage synergies to connect to other systems that will operate in a carbon utilization paradigm.

"If you design your system correctly, you can rely on natural gradients, for example, through a day-night cycle or through leveraging two volumes of air of which one is humid, and one is already dry in geographies where that makes sense," said materials engineering Professor Vinayak P. Dravid, who led the research.

Dravid is the Abraham Harris Professor of Materials Science and Engineering at McCormick and a faculty affiliate of the Paula M. Trienens Institute for Sustainability and Energy. He is also the founding director of the Northwestern University Atomic and Nanoscale Characterization (NUANCE) Center as well as the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource, and also serves as the associate director for global programs at the International Institute of Nanotechnology. Hegarty and Shindel share first authorship, and Weinberg College of Arts and Sciences Ph.D. student Michael L. Barsoum and his advisor, Northwestern chemistry chair and Professor Omar K. Farha, are also authors.

After the team assessed why ion exchange resins worked so well at facilitating capture -- a combination of ideal pore size and the presence of negatively charged ion groups on their surfaces that carbon dioxide can attach to -- they identified other platforms with more abundance and similar properties, with a focus on materials that would not put additional strain on the environment.

Previous literature tends to wrap together the mechanics of the entire system, making it difficult to assess the impact of individual components on performance. Hegarty said by looking systematically and specifically at each material, they found a "just right" middle range of pore size (around 50 to 150 Angstrom) with the highest swing capacity, finding a correlation between the amount of area within pores and the capacity the materials exhibited.

The team plans to increase their understanding of the new materials' life cycles that includes both overall cost and energy use of the platform, and hopes it inspires other researchers to think outside the box.

"Carbon capture is still in its nascent stages as a field," Shindel said. "The technology is only going to get cheaper and more efficient until it becomes a viable method for meeting emissions reductions goals for the globe. We'd like to see these materials tested at scale in pilot studies."

The paper, "Platform materials for moisture-swing carbon capture," was supported by the Department of Energy (DOE-BES DE-SC0022332), and all characterization and measurements were supported by the National Science Foundation's National Nanotechnology Coordinated Infrastructure Midwest network node, called the SHyNE Resource.