Polymer microcapsules composed of liquid carbonate cores and highly permeable silicone shells are produced by microfluidic assembly. This motif couples the capacity and selectivity of liquid sorbents with high surface area to facilitate rapid and controlled ​carbon dioxide uptake and release over repeated cycles. While mass transport across the capsule shell is slightly lower relative to neat liquid sorbents, the surface area enhancement gained via encapsulation provides an order-of-magnitude increase in ​carbon dioxide absorption rates for a given sorbent mass. The microcapsules are stable under typical industrial operating conditions and may be used in supported packing and fluidized beds for large-scale carbon capture.

—Vericella et al.

Current carbon capture technology uses caustic amine-based solvents to separate CO₂ from the flue gas escaping a power plant’s smokestacks. But state-of-the-art processes are expensive, result in a significant reduction in a power plant’s output, and yield toxic byproducts.

The new technique employs an abundant and environmentally benign sorbent: sodium carbonate (the main ingredient in baking soda). The microencapsulated carbon sorbents (MECS) achieve an order-of-magnitude increase in CO₂ absorption rates compared to sorbents currently used in carbon capture. Another advantage is that while amines break down over time, carbonates have a virtually limitless shelf life.

Our method is a huge improvement in terms of environmental impacts because we are able to use simple baking soda—present in every kitchen—as the active chemical. Corrosiveness also is improved because the chemical is more benign and always is encapsulated. Putting the carbonate solution inside of the capsules allows it to be used for CO₂ capture without making direct contact with the surface of equipment in the power plant, as well as being able to move it between absorption and release towers easily, even when it absorbs so much CO₂ that it solidifies.

—Roger D. Aines, leader of the fuel cycle innovations program at Lawrence Livermore National Laboratory (LLNL) and a co-lead author

This schematic illustration shows the encapsulated liquid carbon capture process in which CO2 gas diffuses through a highly permeable silicone shell and is absorbed by a liquid carbonate core. The polymer microcapsules are then heated to release absorbed CO2 for subsequent collection. (Image courtesy of John Vericella, Chris Spadaccini, and Roger Aines, LLNL; James Hardin and Jennifer Lewis, Harvard University; and Nature.) Click to enlarge.

Researchers at LLNL and the U.S. Department of Energy’s National Energy Technology Lab are now working on enhancements to the capture process to bring the technology to scale.

The emission-scrubbing potential of CCS is not limited to the electric generation sector; Aines says that the MECS-based approach can also be tailored to industrial processes like steel and cement production, significant greenhouse gas sources.

These permeable silicone beads could be a “sliced-bread” breakthrough for CO₂ capture—efficient, easy-to-handle, minimal waste, and cheap to make. Durable, safe, and secure capsules containing solvents tailored to diverse applications can place CO₂ capture for CCS firmly onto the cost-reduction pathway.

—Stuart Haszeldine, professor of carbon capture and storage at the University of Edinburgh, who was not involved in the research

MECS are produced using a double capillary device in which the flow rates of three fluids—a carbonate solution combined with a catalyst for enhanced CO₂ absorption, a photocurable silicone that forms the capsule shell, and an aqueous solution—can be independently controlled.

Encapsulation allows you to combine the advantages of solid capture media and liquid capture media in the same platform. It is also quite flexible, in that both the core and shell chemistries can be independently modified and optimized.

—Jennifer A. Lewis, the Hansjörg Wyss Professor of Biologically Inspired Engineering at the Harvard School of Engineering and Applied Sciences (SEAS) and a co-lead author

Funding for the encapsulated liquid carbonates work was provided by the Innovative Materials and Processes for Advanced Carbon Capture Technology program of the US Department of Energy’s Advanced Research Projects Agency–Energy (ARPA-E).

Other authors who contributed to the Nature Communications article include: John Vericella, Sarah Baker, Joshuah Stolaroff, Eric Duoss, James Lewicki, William Floyd, Carlos Valdez, William Smith, Joe Satcher Jr., William Bourcier and Chris Spadaccini, all of LLNL; James O. Hardin IV of Harvard University; and Elizabeth Glogowski of the University of Illinois at Urbana-Champaign.

Resources

• John J. Vericella, Sarah E. Baker, Joshuah K. Stolaroff, Eric B. Duoss, James O. Hardin IV, James Lewicki, Elizabeth Glogowski, William C. Floyd, Carlos A. Valdez, William L. Smith, Joe H. Satcher Jr., William L. Bourcier, Christopher M. Spadaccini, Jennifer A. Lewis & Roger D. Aines (2015) “Encapsulated liquid sorbents for carbon dioxide capture” Nature Communications 6, Article number: 6124 doi: 10.1038/ncomms7124