How to Catch Air
The advancement of society has been propelled by numerous examples of human ingenuity and innovation. The development of the steam engine led to the Industrial Revolution; the invention and mass production of automobiles revolutionized transportation; and the discovery of electricity and adoption of products from light bulbs to computers continue to drive humanity to new levels of discovery. Underlying all of these advancements was the burning of fuels, from wood to coal to oil, as the source of energy that made it all possible and the unfortunate production of carbon dioxide that comes with their burning.
Although research into solar, wind, and hydropower may provide future energy sources without the release of carbon dioxide, the burning of fossil fuels will continue to be an efficient, reliable source of energy without the availability concerns, such as when the sun does not shine or the wind does not blow. For that reason, researchers at the Center for Understanding and Control of Acid Gas-Induced Evolution of Materials for Energy (UNCAGE-ME) are investigating materials that can capture the carbon dioxide released from burning these fuels for storage in places other than the atmosphere, such as underground rock reservoirs.
To build their carbon capture devices, scientists at UNCAGE-ME use silica tubes with holes less than 50 nanometers in size, about 1/2,000 the width of a human hair! The tubes resemble a honeycomb with holes, or pores, that gases can flow in and out of easily. To capture carbon dioxide in the silica tubes, they coat the pores with a polymer called poly(ethylenimine) or PEI, a long molecule that is made up of repeating units like a chain made up of links, with sites on it that interact strongly with carbon dioxide. These sites pull molecules of carbon dioxide out of the gas stream as it passes through the tubes. The properties of the polymer coating are important, but those properties were difficult for scientists to observe until now because of the very small size of the silica tubes.
The scientists at UNCAGE-ME discovered a way to answer their questions about the properties of their materials. They did so by observing the polymer-filled pores using an experimental technique known as small angle neutron scattering (SANS). In SANS, a beam of neutrons, neutrally charged sub-atomic particles that make up atoms, is directed at a sample, and a detector measures how much the neutrons are scattered by the sample. One can imagine hitting a pool ball at the rail of a pool table and observing the angle it bounces off the rail. As the neutrons interact with different atoms in the sample, they deflect at different angles, which can be used to probe the atomic structure of materials.
In their study, the researchers analyzed the SANS patterns from different materials and determined the way in which the polymer filled the silica pores. Initially, they found that the polymer lined the walls of the pores; good for gas flow through the silica tubes but not for capturing carbon dioxide. The active sites of the polymer used to snatch the carbon dioxide from the gas stream also interact with the silica walls of the tubes, so that when the polymer lines the walls of the silica tubes most of the active sites interact with the walls and not the gas stream. As more polymer is added to the tubes, lumps of polymer form within the pores. These lumps begin to restrict the gas flow through the tubes, but create active sites on the polymer that no longer interact with the silica walls and are much better at capturing carbon dioxide from the gas stream.
With this new-found understanding of the ways in which polymer lines the silica pores, researchers can begin to envision the smart design of these materials by addressing the Goldilocks nature of the problem. With too little polymer added to the silica tubes, gas flows well through the silica tubes, but few active sites are available to catch the passing carbon dioxide. With too much polymer, the lumps within the pores may completely block the gas flow and prevent the carbon dioxide-laden air from reaching the available active sites. Future efforts by researchers at UNCAGE-ME will attempt to find the best polymer structure and loading that will be just right for allowing the flow of gases and capturing carbon dioxide.
The research was supported as part of UNCAGE-ME, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.
Holewinski A, MA Sakwa-Novak, and CW Jones. 2015. "Linking CO2 Sorption Performance to Polymer Morphology in Aminopolymer/Silica Composites through Neutron Scattering." Journal of the American Chemical Society 137:11749-11759. DOI: 10.1021/jacs.5b06823