Researchers in California have produced a cheap plastic capable of removing large amounts of carbon dioxide (CO2) from the air. Down the road, the new material could enable the development of large-scale batteries and even form the basis of "artificial trees" that lower atmospheric concentrations of CO2 in an effort to stave off catastrophic climate change.
These long-term goals attracted the researchers, led by George Olah, a chemist at the University of Southern California (USC) in Los Angeles. Olah, who won the 1994 Nobel Prize in chemistry, has long envisioned future society relying primarily on fuel made from methanol, a simple liquid alcohol. As easily recoverable fossil fuels become scarce in the decades to come, he suggests that society could harvest atmospheric CO2 and combine it with hydrogen stripped from water to generate a methanol fuel for myriad uses.
Olah and his colleagues also work on making cheap, iron-based batteries that can store excess power generated by renewable energy sources and feed it into the electrical grid during times of peak demand. To function, the iron batteries grab oxygen from the air. But if even tiny amounts of CO2 get into the reaction, it kills the battery. In recent years, researchers have come up with good CO2 absorbers made from porous solids called zeolites and metal organic frameworks. But they're expensive. So Olah and his colleagues set out to find a cheaper alternative.
They turned to polyethylenimine (PEI), a cheap polymer that is a decent CO2 absorber. But it only grabs CO2 at its surface. To boost PEI's surface area, the USC team dissolved the polymer in a methanol solvent and spread it atop a batch of fumed silica, a cheap, industrially produced porous solid made from microscopic droplets of glass fused together. When the solvent evaporated, it left solid PEI with a high surface area.
When the researchers tested the new material's CO2-grabbing abilities, they found that in humid air—the kind present in most ambient conditions—each gram of the material sopped up an average of 1.72 nanomoles of CO2. That's well above the 1.44 nanomoles per gram absorbed by a recent rival made from aminosilica and among the highest levels of CO2 absorption from air ever tested, the team reported last month in the Journal of the American Chemical Society. Once saturated with CO2, the PEI-silica combo is easy to regenerate. The CO2 floats away after the polymer is heated to 85°C. Other commonly used solid CO2 absorbers must be heated to over 800°C to drive off the CO2.
"This is intriguing. It's nice that it works at low temperatures," says Klaus Lackner, a CO2 air-capture expert at Columbia University. That could make it useful for grabbing CO2 out of the air in addition to safeguarding batteries, says USC chemist and team member Surya Prakash. The polymer could be useful for building massive farms of artificial trees that would aim to reduce atmospheric concentrations of CO2 and prevent the worst ravages of climate change. But that's only if countries around the globe are willing to spend untold billions of dollars to rein in atmospheric CO2. The polymer also degrades at high temperatures, meaning it likely can't be used to snag CO2 from industrial smokestacks or automobile tailpipes, where the CO2 is often highly concentrated but typically comes out at high temperature. To overcome that limitation, Prakash says the USC team is now working to produce high surface-area PEIs that are better at taking the heat.